Amorphous and crystalline thioacetamide ice: Infrared spectra as a probe for temperature and structure

Dedicated to Zo ﬁ a Mielke in recognition of her contributions to studies of molecular complexes in low-temperature matrices, as well as spectroscopic studies of hydrogen bonds and weak interactions


Introduction
Thioacetamide (CH 3 CSNH 2 ) has long been in the focus of research interest.Besides collecting the ultraviolet (UV) and infrared (IR) spectra of single crystals, in liquid phase or in mulls at room temperature [1e3], also Raman [4,5], and nuclear magnetic resonance (NMR) spectroscopy [6,7], calorimetry [8], as well as X-ray [9,10] and neutron-diffraction [11] techniques were applied in order to unravel its crystal structure and properties.Despite the extensive research, the low-temperature IR spectra of thioacetamide have only been reported for single crystals (at 77 K) and for the compound dispersed in KBr pellets (at 120 K) [12,13].Consequently, characterization of neat amorphous (glassy) or polycrystalline thioacetamide ices has never been reported so far.
Besides the fact that amorphous thioacetamide serves as an important source of sulfur for thin film deposition [14], lowtemperature thioacetamide ices may also be of interest from the astrophysical point of view.Despite sulfur is a ubiquitous element in space [15e18], there is a great disagreement of three orders of magnitude between the expected and the observed values in the interstellar dense molecular clouds and protoplanetary disks.This is generally explained by the formation of volatile icy mantles or refractory material on the dust grains [19,20].However, only a handful of candidate molecules have been investigated at relevant ice temperatures, such as methanethiol, ethanethiol, and propanethiol [21,22].Moreover, although an upper limit for thioacetamide in low-mass star-forming regions was given [23], its IR spectrum has not been collected under conditions similar to an astrophysical environment so far.Although mid-IR spectra of thioacetamide have been previously studied below room temperature [12,13], the temperatures used (77 and 120 K) were still significantly higher than the temperatures of 10e20 K prevalent in dense molecular clouds [24].The present study serves as a first step towards providing important data that may ultimately help observations with regard to sulfur-containing molecules.

Experimental methods
A commercial sample of thioacetamide obtained from Merck (99%) was used.The sample was placed in a small glass oven, connected to the vacuum chamber of the cryostat via a stainless steel gate valve.Before the experiments, the sample was pumped through the vacuum chamber at room temperature in order to eliminate air and other trapped volatile impurities.To increase vapor pressure and achieve the desired sublimation rate of thioacetamide, the sample was slightly heated (to ca.40 C) during deposition.A CsI window was used as optical substrate and was cooled down using a closed-cycle helium refrigerator with a DE-202 expander (Advanced Research Systems) with a base pressure of approximately 10 À7 mbar.The temperature of the cold window was measured directly by a silicon diode sensor connected to a LakeShore 331 digital temperature controller, which also enables the stabilization of the sample temperature with an accuracy of 0.1 K, by means of using a controlled heater element.The vapors of thioacetamide were condensed onto the optical substrate kept at 11 K.During and after the deposition of the sample, mid-IR spectra were collected (34 and 128 scans, respectively) using a Nicolet 6700 FT-IR spectrometer equipped with a Ge/KBr beamsplitter and a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen; the spectral resolution was 0.5 cm À1 .
The crystallization of thioacetamide was monitored as follows.After deposition, the amorphous sample prepared by fast condensation at 11 K was annealed to 200 K at a rate of 2 K min À1 , while periodically collecting the mid-IR spectra of the sample.The number of scans (34 scans) was chosen so that the collection of one mid-IR spectrum would take exactly 1 min, thus a time-dependent mid-IR spectrum of the sample was obtained at a resolution of 2 K.At certain reference temperatures (50, 100, 150, and 200 K) the annealing was stopped and the sample was kept at constant temperature in order to collect a longer mid-IR spectrum (128 scans).As it will be detailed below, after annealing to 200 K, a complete glass-to-crystal transition occurs.Until this point, the temperature stabilization was achieved by the simultaneous cooling and controlled heating of the sample.After reaching 200 K, the cooling was turned off and one last mid-IR spectrum was taken at the temperature of 260 K in order to confirm that all deposited compound evaporated from the substrate.
In a separate experiment, the temperature dependence of the mid-IR spectrum of the crystalline sample was studied.After producing the crystalline thioacetamide by depositing the compound at 200 K, the sample was cooled down at a rate of 4 K min À1 to 150 K then at 2 K min À1 to 10 K, while continuously monitoring the mid-IR spectrum of the sample (34 scans) and collect longer spectra (128 scans) at the reference temperatures of 150, 100, 50, and 10 K.In order to make sure that the changes in the spectra are completely reversible, the sample was again warmed up to 150 K at a ramp rate of 2 K min À1 to 50 K and then at 1 K min À1 to 100 K and then at 4 K min À1 to 150 K and continuously monitoring the spectral changes of the sample (34 scans).At the reference values of 50, 55, 60, 65, 70, 80, 90, 100, 125, and 150 K the sample was kept at constant temperature while taking a longer mid-IR spectrum (128 scans).Please note that the collection of figures in the main manuscript and in the Supplementary Material contains an indication to the method of deposition: when the figure captions refer to irreversible changes, this means type (i) of experiment, where samples were deposited at 11 K (for example Figs.An estimate for the thickness of the deposited samples can be given by using the approach reported in an earlier work [25].This reference discusses in detail how the thickness of the sample can be experimentally defined from the pattern of the interference fringes (see the second paragraph of the "4.Results and discussion" section, and Fig. 1), and arrives to the conclusion that the thickness (a very thin matrix in that work) was about 11.5 mm (having 10 fringes in the mid-infrared survey spectrum).In the very next sentence, it was concluded that: "This value is about one order of magnitude less than for an average sample studied in our setup."The same experimental setup was used to study the neat films in the present case.Note that in cryogenic matrices the thickness is defined by the  inert gas, not by the matrix-isolated compound itself.We consider that the amount of the studied compound is approximately the same in the matrix and in the neat film (which is justified by the fact that the intensities of the absorption bands are roughly in the same order of magnitude), and then simply make a correction for the thickness based on the matrix concentration (usually 1:1000).Then, in our case the estimate for the thickness of the neat film would be some 3 orders of magnitude less than that of the cryogenic matrix.This gives us an estimate for about 10e100 nm that would correspond to about one interference fringe over the entire mid-infrared range, which would be difficult to record.This is in agreement with the appearance of the survey spectra shown in Figs.S1 and S5, with no apparent fringes along the baseline.This thickness also means that the organic film is roughly in the order of 20e200 molecules deep (having an estimate of approximately 5 Å for the distance between thioacetamides in solid, see justification in the Supplementary Material), this should represent the bulk properties of such material reasonably well.

Theoretical computations
The quantum chemical computations were performed by using Becke's three parameter hybrid functional B3LYP, with the nonlocal and local correlation described by the LeeeYangeParr and the VoskoeWilkeNusair III functionals, respectively [26e28], with Pople's 6e311þþG(d,p) basis set [29] as implemented in Gaussian09 [30], The crystal structure of thioacetamide was taken from the Cambridge Crystallographic Data Centre (CCDC, ID: 1270356) [11] and was used to create a 2D sheet consisting of 12 thioacetamide molecules serving as a model dodecamer structure (Scheme 1).The Cartesian coordinates of the dodecamer can be    found in Table S1 of the Supplementary Material.In a single-crystal neutron diffraction study [11], it was demonstrated that a thioacetamide crystal consists of layers, and that there is no H-bonding between the adjacent layers, thus a 2D-sheet is a good approximation to simulate the real crystal.Furthermore, a previous ab initio study demonstrated the necessity of accounting for intermolecular hydrogen bonds in order to successfully reproduce the geometry of the thioacetamide individual units within the crystalline structure [31].In order to evaluate the importance of intermolecular hydrogen bonds, several different hexamer structures were constructed, with all of them having the same topology as the thioacetamide crystal [11].All of these hexamers were assumed to have at least a plane of symmetry, whereas all methyl groups were set to have the staggered geometry.Such orientation of the methyl groups in aggregates of thioacetamide was justified in detail by Ramondo and Bencivenni [31].Then the hexamer structures were fully optimized (within either C s or C 2h symmetry point group) and the optimizations were followed by harmonic vibrational computations at the B3LYP/6e311þþG(d,p) level of theory.The vibrational calculations on the hexamers were subjected to the vibrational mode automatic relevance determination (VMARD) as described by Teixeira and Cordeiro [32].Their structures, optimized geometries (Cartesian coordinates), relative zero-point corrected energies, and interpretation of selected vibrational modes for a chosen hexamer structure can be found in Scheme 2 as well as in the Supplementary Material (Schemes S1eS5 and Tables S2eS8).In order to assess the effect of non-covalent interactions, the geometry optimization of the hexamers was repeated at the same level of theory (B3LYP/6e311þþG(d,p) with the consideration of Grimme's empirical dispersion correction with the original D3 damping function [33].Harmonic vibrational frequency computations were also carried for the model dodecamer.The geometry of the dodecamer structure was taken as that reported by Jeffrey et al. [11]; and was used without further optimization.Only the vibrational modes with computed frequencies above 400 cm À1 were analyzed in this study.Uniform factors of 0.945 and 0.980 were used to scale computed vibrational frequencies of the dodecamer in the regions 3375e3000 cm À1 and 540e400 cm À1 , respectively, whereas a scaling factor of 0.975 was used for the rest of the mid-IR spectrum (3000e540 cm À1 ).For a graphical comparison of the theoretical spectra with the experimental ones, the simulated mid-IR spectra were obtained using the theoretical harmonic vibrational frequencies (scaled) and computed IR intensities (not scaled) of the dodecamer by convoluting them with Lorentzian functions with an   FWHM of 16 cm À1 (3375e3000 cm À1 ) and 4 cm À1 (below 3000 cm À1 ).

Results and discussion
4.1.Amorphous thioacetamide ice at 10 K When neat thioacetamide is deposited at low temperatures, the amorphous form of the ice can be observed (Figs.1e8 and Figs.S1eS4 in the Supplementary Material).By comparing the IR spectra with assignments of thioacetamide single crystals as well as those of thioacetamide dispersed in KBr pellets (at a concentration of 1:100) [12,13], a vibrational assignment can be safely made for the amorphous ice at 10 K. Hereafter, whenever the vibrational band positions in thioacetamide single crystal at 77 K or those of KBr pellets at 120 K are mentioned, the values are taken from Anthoni et al. [12] and Sankar Bala et al. [13]; respectively (Table 1).The band at 3285 cm À1 is due to the antisymmetric stretching of a free NH 2 group (n as NH 2 ), whereas the features at 3164 and 3000 cm À1 are owing to the symmetric stretching mode of the same group (n s NH 2 ) (Fig. 1, black line, black arrows).It should be noted that the band at intermediate position (at 3164 cm À1 ) can be due to decoupled NH 2 stretching modes, which occurs for instance in amino groups or nucleobases with non-equivalent protons e.g., when one proton participates in an intermolecular H-bond and the other proton is free [34e36], In comparison, n as NH 2 were found at 3250 in the single crystal at 77 K and at 3255 as well as 3205 cm À1 for thioacetamide dispersed in KBr pellets at 120 K, respectively.
The values for the other vibrational mode n s NH 2 are 3120 as well as 3090 (single crystal at 77 K), and 3050 cm À1 (KBr pellets at 120 K), respectively.
In the CH 3 stretching region the amorphous ice at 10 K has two very broad bands with low peak intensity (Fig. 2, black), that are due to the in-plane antisymmetric (n as,i.p.CH 3 , 2960 cm À1 ) and symmetric stretching vibrations (n s CH 3 , 2910 cm À1 ).The respective values for the single crystal are 2965 and 2910 cm À1 , whereas for KBr pellets (also called KBr disks) only one value is listed at 2940 cm À1 .In the NH 2 bending region (bNH 2 ), there is one intense and broad band at 1637 cm À1 making the assignment easy in this part of the vibrational spectrum (Fig. 3, black).The literature values for thioacetamide single crystal and low-temperature KBr disks are 1660 and 1650 cm À1 , respectively.Fig. 4 visualizes the amorphous ice spectrum in the IR region of 1550e1340 cm À1 at 10 K (black line).The bands at 1428 and 1395 cm À1 should represent the out-of-plane (o.p.) and in-plane (i.p.) antisymmetric bending modes of the CH 3 moiety (b as,o.p.CH 3 , b as,i.p.CH 3 ).The corresponding literature values are 1430 and 1400 cm À1 , respectively, in the study of thioacetamide single crystals, which mentions a strong coupling between the nCN stretching and antisymmetric methyl bending (b as,i.p.CH 3 ) modes [12].In contrast, the band at 1428 cm À1 is not explicitly assigned in Sankar Bala et al. [13]; even though it is listed in that work, whereas  the peak 1392 cm À1 is attributed to the first member of the Fermitriad consisting of b as,i.p.CH 3 , b s CH 3 , and the first overtone of the out-of-plane bending of the CS bond (2gCS).Moreover, two relatively sharp and intense infrared bands can be found at 1472 and 1366 cm À1 .The respective values for single crystal are 1490 and 1358 cm À1 , which attributes the former one to the nCN stretching mode coupled with b as,i.p.CH 3 and the latter one to the symmetric methyl bending (b s CH 3 ) [12].In the case of low-temperature KBr disks, the band positions are at 1490, 1480, and 1355 cm À1 and the bands were assigned to the nCN stretching mode and a member of the above-mentioned Fermi-triad (b s CH 3 ), respectively [13].The coupling between the nCN stretching and b as,i.p.CH 3 bending modes needs further elaboration and is discussed in Section 4.4.
Fig. 5 shows the NH 2 rocking band (rNH 2 , black line) at 1300 cm À1 , which is comparable to the previous findings for the single crystal and KBr disk (1308 and 1305 cm À1 , respectively).The IR region of 1075e925 cm À1 consists of two bands, a less intense at 1026 cm À1 and a more intense at 970 cm À1 (Fig. 6, black).They are due to the in-plane and out-of-plane rocking motions of the CH 3 group (r i.p.CH 3 , r o.p.CH 3 , overlapped at 10 K) and the CC stretching (nCC) vibrational modes, respectively.The respective values for single crystals are 1039 and 965 cm À1 .In the case of the work on KBr pellets the band at 1035 cm À1 was assigned to rCH 3 , whereas rNH 2 was assigned to a weak band at 1025 cm À1 .However, in the case of the peak listed at 970 cm À1 , no assignment was made for that value [13].
In the 825e675 cm À1 region, only one well-defined band can be observed at 10 K, that of the stretching vibrational mode of the CS bond (nCS) at 716 cm À1 (Fig. 7, black).The previous studies found this band at 722 cm À1 (single crystal) and at 715 cm À1 (KBr pellets), respectively.Finally, one broad band is observable in the region of 540e460 cm À1 , that of gCS centered at 517 cm À1 (Fig. 8, black).For single crystal, this band was found at 511 cm À1 , whereas for KBr disks at 508 cm À1 .

Crystallization, changes in vibrational spectrum
Crystallization can be induced if an amorphous thin layer of ice is slowly heated up (annealed), which has long been known and used phenomenon [37].Alternatively, the crystalline ice can be directly obtained if the neat compound is deposited at a relatively high temperature (i.e.above 190 K).Correspondingly, when the temperature of the amorphous thioacetamide ice is gradually increased, an irreversible change is observed throughout the spectral region, implying the crystallization of the sample.The sample remains amorphous at least up to 150 K (as can be seen in Figs.1e8).The effect of heating, resulting in crystallization, is the most prominent in the temperature range between 180 and 190 K (see Figs.   of the reported conversion temperature is due to the simultaneous scanning during the annealing ramp.All the new band positions listed in this section are those observable at 200 K.It should be noted that the IR spectra of the crystalline form are independent on the sample history, i.e. whether the sample was obtained by the heating of amorphous thioacetamide or the crystal was obtained by direct deposition at 200 K. In Fig. 1, it can be seen that the n as NH 2 band remains essentially unchanged up to 180 K, when the main peak shifts from 3284 cm À1 down to 3259 cm À1 with a shoulder emerging at 3227 cm À1 .These values are much more similar to the ones reported previously (3250 cm À1 for single crystal and 3255, 3205 cm À1 for KBr pellets) and also corroborate our theoretical findings.The frequency of symmetric stretching vibration of the amino group (n s NH 2 ) also moves to lower wavenumbers, from 3164 (with a shoulder at 3100 cm À1 ) to 3081 cm À1 .For the sake of comparison, the previous studies found these band positions to be at 3180, 3120 as well as 3090 (single crystal study), and at 3050 cm À1 (KBr pellets at 120 K), respectively.The downshift in frequency suggests an overall increase in the presence and strength of hydrogen bonds with the participation of the NH 2 group.Besides the change in position, both the n as NH 2 and n s NH 2 bands become narrower, and with higher peak intensities, as a result of crystallization and therefore the general increase in the orderliness within the sample.It is worth noting that two more shoulders become visible at 3138 and 3032 cm À1 and the latter may be assigned to the out-of-plane n as,o.p.CH 3 mode (Table 2).It should be also noted that the previous results assigned the band at 3320 cm À1 (and 3310 cm À1 in KBr pellets) as the 2bNH 2 mode, which is assigned by the present study as the n as NH 2 mode of the 'free' NH 2 moieties that do not take part in H-bonds in the crystal such as terminal amino groups or those at crystal defects.
Significant spectral changes can be observed in the CH 3 stretching region as well (Fig. 2).The broad and weak band owing to the in-plane n as,i.p.CH 3 mode at 2960 cm À1 becomes much stronger although its spectral position does not change much and the n s CH 3 band at 2910 cm À1 remains a shoulder.Furthermore, a new band appears at 2984 cm À1 , which is also assigned to the n as,i.p.CH 3 band.In the bNH 2 region, the broad band completely transforms upon annealing and a new sharp feature arises, centered at a somewhat higher frequency (1657 cm À1 , Fig. 3) making the agreement even better with the samples of single crystal or KBr pellets than in the case of the amorphous ice.The fact that the band becomes less broadened and more intense implies that the local environment across the sample becomes more homogeneous.Furthermore, the upshift in the position of the bending vibrational band also points to the formation of H-bonds with the participation of the NH 2 group.
The spectrum in the CH 3 bending region also undergoes fundamental changes when the amorphous sample is heated up to 200 K (Fig. 4).All the bands show an increase of the peak frequency (and intensity) in general upon crystallization, and appear at 1482,  1428, 1400, and 1369 cm À1 .The agreement with the literature values is remarkable, they are 1490, 1430, 1400, and 1358 cm À1 for single crystal and 1480, 1428, 1382, and 1355 cm À1 for KBr pellets.A detailed analysis of this spectral range and the band assignment is given in Section 4.4.
The rocking rNH 2 (Fig. 5) also displays what is expected for bands assigned to the H-bound NH 2 moiety: a considerable shift to higher frequency with a simultaneous narrowing of the band.The peak maximum after crystallization can be found at 1309 cm À1 (with a shoulder at 1315 cm À1 ).The agreement, unsurprisingly, becomes even better with the values obtained for single crystals (1308 cm À1 ) and for KBr pellets (1305 cm À1 ), when compared to the band position of the amorphous ice.In Fig. 6, the broad and featureless rCH 3 band slightly shifts towards higher wavenumbers and becomes a doublet (according to its in-plane and out-of-plane vibrations) with multiple shoulders between 1028 and 1043 cm À1 (Table 2), which is nicely reproduced by the simulated spectrum (discussed in Section 4.3).The same band was reported at 1039 cm À1 for single crystals and at 1035 cm À1 for KBr pellets (despite the contradictory assignments) in great accordance again with our crystalline sample.Furthermore, the nCC band also becomes higher in intensity due to the narrowing and moves to higher wavenumbers, to 976 cm À1 .The values for single crystals and for KBr disks are 965 cm À1 and 970 cm À1 (although we must point out to the varying spectral assignments), respectively.
The band due to nCS stretching becomes intense and narrow, with a peak at 712 cm À1 (Fig. 7).The values to compare are 712 cm À1 (single crystal) and 715 cm À1 (KBr pellets), respectively.In this region, two new bands arise and can be found at 774 and 724 cm À1 (as a shoulder), respectively, which belong to the torsional and wagging fundamental modes of the amino group (tNH 2 and uNH 2 ), respectively.These bands are relatively broad, also implying the participation of the respective moiety in H-bonds.In comparison, the values at 774 and 700 cm À1 were reported for single crystal, and at 770 cm À1 for KBr disk.Ultimately, Fig. 8 shows Scheme 1.The model structure of 12 thioacetamide molecules (dodecamer) used for the simulation of the infrared spectrum of the crystal.Dotted lines display intermolecular H-bonds; those highlighted in red show the central H-bonds within the centrosymmetric dimer sub-units, whereas those highlighted in blue visualize the lateral H-bonds participating in the formation of infinite chains.The structure of the dodecamer has the accurate geometry of the crystal reported by Jeffrey et al. [11]; and was used in vibrational calculations as is, without further optimization.
Scheme 2. The model structure 'I' of 6 thioacetamide molecules (hexamer, Hex-I) used for the normal mode analysis.The dotted lines display intermolecular H-bonds; those highlighted in red show the central H-bonds within the centrosymmetric dimer subunit, whereas those highlighted in blue visualize the lateral H-bonds participating in the formation of infinite chains.Note that this structure is a subset of the dodecamer shown in Scheme 1. the 540e460 cm À1 spectral region, where the broad gCS mode becomes narrower and more intense with the concurrent change its peak position to a higher wavenumber, at 522 cm À1 .Besides, two new peaks emerge, a more intense at 477 cm À1 and a weaker one at 462 cm À1 , which are assigned to the NCS bending mode (bNCS), with the lower one likely caused by the bNCS vibration involving CS groups that do not participate in H-bonding in the crystal (like the terminal ones or those at crystal defects).The corresponding values for single crystal are reported at 474 and 464 cm À1 , whereas for KBr pellets they are at 465 and 455 cm À1 .
The following general conclusions can be drawn for the crystallization of thioacetamide upon annealing.Since the randomly oriented molecules within the glassy sample (10 K) relax to an energetically more favorable, better ordered and more homogeneous structure (crystal at 200 K), less broadened and thus more intense bands are expected.Besides, the frequency shifts usually reflect the formation of H-bonds within the sample, which is also made possible by the higher orderliness of the crystal.New spectral features also become better discernible.Another important consequence of crystallization is the appearance of doublets owing to the two non-equivalent molecules in the crystal unit cell [9e11].

Temperature dependence of the vibrational spectrum of crystalline thioacetamide
Unlike in the case of amorphous thioacetamide, further considerable (but reversible) spectral changes can be induced by thermal treatment of the crystalline sample.The temperature between listed vibrational modes.The signs (þ) and (À) denote in-phase and in-opposite-phase couplings between the coordinates that participate in the resonance.
e Broadening due to the presence of n as,o.p. (CH 3 ) mode cannot be completely excluded.
dependence of the IR spectra of the crystalline thioacetamide ice is visualized in Figs.9e16 and Figs.S5eS19 in the Supplementary Material, as well as summarized in Table 3. Upon cooling from 200 K to 10 K, a ubiquitous downshift in the frequencies of the NH 2 stretching modes can be observed, with the relative increase of the peak intensity of the lower-frequency shoulders for both the n as NH 2 and n s NH 2 (Fig. 9).Furthermore, an overall narrowing and therefore increase in intensity can be observed when cooling down the sample, as a consequence of the reduction of thermal broadening effect.The decrease in the frequency of proton stretching modes can be explained by the strengthening of the H-bonds when lowering the temperature and agrees with the previous findings for low-temperature KBr disks.
While the position and intensity of the n as,o.p.CH 3 mode at 3032 cm À1 , it is almost unaffected by temperature (Fig. 9), the other CH 3 stretching vibrations change considerably.The bands at 2965 and 2940 cm À1 (at 50 K) undergo significant and reversible frequency shifts upon heating (Fig. 10).However, no temperaturedependent spectral changes due the intramolecular coupling between the n as CH 3 and the methyl torsional mode tCH 3 can be undoubtedly detected, which otherwise is a commonly observed phenomenon [38e40].
In Fig. 11, the band due to bNH 2 bending mode is shown.The increased frequency of this band (near 1660 cm À1 ) clearly indicates that the molecules of thioacetamide in crystal are involved in strong intermolecular interactions, which can be inferred from the comparison with the band due to bNH 2 bending mode of the monomeric (matrix-isolated) thioacetamide, appearing at 1597 cm À1 [41,42].
The assignment of the vibrations appearing in the 1550-1340 cm À1 range (Fig. 12) deserves special attention.This spectral region is characteristic of the nCN, b as,o.p.CH 3 , b as,i.p.CH 3 , and b s CH 3 vibrations.For the monomeric thioacetamide isolated in argon matrices, this range exhibits one strong, and in fact by far the strongest, infrared band in the entire vibrational spectrum (at 1346 cm À1 , with the IR intensity over 200 km mol À1 ).All remaining vibrations of the monomer in this range have infrared intensities lower by an order of magnitude [41].In the spectra of the crystals (Fig. 12), however, there are two bands of nearly equal infrared intensity, predicted as doublets near 1490/1486 cm À1 and 1419/ 1413 cm À1 (Fig. 12 top), and observed near 1482 and 1422 cm À1 (Fig. 12 bottom).Such change in the vibrational spectrum from the monomer to the crystal suggests that there is a resonance interaction between two vibrations which results in the exchange of infrared intensities.Indeed, the exchange of infrared intensities is known to occur in cases of 44] and Fermiresonances [44,45].In our case, the most probable candidates for the resonance interaction are the nCN and b as,i.p.CH 3 modes, a subject that is discussed in more detail in Section 4.4.
Interestingly, b as,o.p.CH 3 seemingly shows a reversible splitting when lowering the temperature (see Fig. 12 lower panel).At 200 K, there is only one inconspicuous broad peak centered at 1428 cm À1 (with a shoulder at 1434 cm À1 ).However, upon cooling down to 150 K, it gradually splits up to a prominent doublet at 1430 and 1421 cm À1 , which is not predicted by the quantum chemical computation (Fig. 12, upper panel).Such a thermally reversible splitting has not been reported so far to occur for methyl bending modes, although a similar phenomenon was observed in the case of methyl stretching modes in crystals of hydroxyacetone (see Fig. 10 of Sharma et al. [40].The thermally reversible splitting for hydroxyacetone occurs approximately at 90 K and over a wide temperature range for various alkane systems, and was explained in terms of an intramolecular coupling of the methyl stretching with the methyl torsion (tCH 3 ) mode [38e40].The explanation is as follows.Usually the barrier for internal torsion of the methyl group is on the order of 1e2 kJ mol À1 , and is associated with the tCH 3 fundamental modes on the order of a 100 cm À1 .At high enough temperatures, the low-frequency methyl torsional modes are easily populated to their higher vibrational states which are situated above the torsional barrier.In such situations, the methyl torsion becomes efficiently barrierless and all the CH bonds within the methyl group become equivalent.As a result, a division of vibrations of the methyl group for symmetric and antisymmetric modes is no more valid and the corresponding vibrational bands are  "smashed" over broad range of frequencies at higher temperatures.At lower temperatures, the methyl group torsional mode relaxes to its fundamental vibrational state, and the methyl group assumes either eclipsed or staggered orientation relatively to the adjacent functional groups.With cooling, the CH bonds within the same methyl group become non-equivalent, resulting in appearance of well-defined bands in the vibrational spectrum due to symmetric and antisymmetric modes.We checked if the energetic parameters of the methyl group in thioacetamide satisfy the above condition.For this purpose, the model hexamer Hex-I (Scheme 2) was constructed in two geometries.In the first geometry, the methyl groups in the two central molecules were set to have a staggered orientation with respect to the C]S group, while in the second geometry, these two methyl groups were switched to the eclipsed geometry.These two modifications of Hex-I were optimized within the C 2h point group.A more stable structure was that with the staggered orientation of the methyl group, with respective methyl torsion modes (tCH 3 ) having computed frequencies of 103.4 (A u ) and 104.9 (B g ) cm À1 .For the structure with the methyl groups switched to the eclipsed geometry, the respective vibrational modes became imaginary, 105.7(i) (A u ) and 105.1(i) (B g ) cm À1 .This model computation showed that the staggered geometry is indeed the most stable orientation of the methyl group of thioacetamide involved in intramolecular H-bond interaction, as indicated by previous works [31].Furthermore, the computed relative energies of the two structures were found to differ by 4.3 kJ mol À1 , meaning the torsional barrier within a single methyl group is near 2.2 kJ mol À1 .For such a barrier (about 2 kJ mol À1 ), and the vibrational mode on the order of 100 cm À1 , the fraction of molecules populating the second or higher excited states (i.e.those above the torsional barrier) is as high as 5% at 100 K and 14% at 150 K.Only at temperatures as low as 50 K or less, all the methyl torsional states (i.e., the ground and the first excited vibrational states) are below the barrier, and only at such temperatures the methyl torsion becomes hindered, resulting in vibrational splitting as shown in Fig. 12 (lower panel).It should be noted that if the dispersion correction is taken into account, the barrier height increases by some 60% to 7.1 kJ mol À1 (3.6 kJ mol À1 per monomer unit), confirming that the higher excited states are necessary to be populated for the virtually free rotation of the methyl group.
The thermal behavior of the band due to rNH 2 rocking of the amino group agrees well with that of the amino group bending as it slightly shifts towards higher wavenumbers when the temperature decreases (Fig. 13).Finally, the vibrational bands belonging to the rCH 3 rocking modes and the nCC and nCS stretching vibrations (rCH 3 and nCC in Fig. 14, nCS in Fig. 15) as well as the gCS and bNCS vibrational modes in Fig. 16 show only minor frequency shifts when changing the temperature.The notable exceptions are the peaks assigned to the torsional and wagging fundamentals of the amino group (tNH 2 and uNH 2 , respectively), which show significant upshifts by some 10 cm À1 along with the appearance of features (shoulders, doublets) implying an extensive participation in H-bonds, which strengthen at lower temperatures.In general, the following can be summarized regarding the temperature dependence of the IR spectrum of crystalline thioacetamide: upon cooling the sample, the bands become more intense and less broadened due to the reduction of thermal effect.In the case of the bands originated by the NH 2 moiety, extensive changes in the position of the peaks can be observed due to intermolecular H-bonds.

Resonance, bond lengths, stabilization energies
In order to confirm the hypothesis of the resonance between the nCN and b as,i.p.CH 3 modes, we constructed a model system of several thioacetamide units which affords vibrational calculations and the normal mode analysis.This model system consists of a hexamer, where the central two molecules form a centrosymmetric dimer, and the peripheral four molecules are added to provide the lateral hydrogen bonds for the free NH and C]S groups of the central dimer (Hex-I, Scheme 2).The vibrational calculations were performed on this system, after full geometry optimization, as described in Section 3. The vibrational output was subjected to the VMARD analysis using Bayesian regression, as described by Teixeira and Cordeiro [32].
Indeed, as expected, the two candidate modes, nCN and b as,i.p.CH 3 , of the central dimer participate in a resonance interaction, in almost equal weights.This interaction gives rise to two intense infrared bands, whose carriers can be described as nCN and b as,i.p.CH 3 modes coupled in symmetric and antisymmetric phases (modes 37 and 49 in Table S3 in the Supplementary Material).It can be reasonably assumed that the frequencies of these two noninteracting modes would appear near the position which is an average of the frequencies of the two strongest infrared bands in this range (i.e.near 1440 cm À1 depending on the temperature and whether the sample is amorphous/crystalline).This gives an unperturbed value of the nCN stretching mode in the crystal (at 1446 cm À1 at 10 K, Fig. 12), which is by a hundred wavenumbers higher than the nCN stretching frequency of the monomer (1346 cm À1 ) [41].Such an increase in the nCN frequency can be rationalized as considerable shortening of the CN bond length (r CN ), which becomes in the crystal much more polarized and shorter than in the monomer.Indeed, the geometry optimization of the model hexamer yields the r CN to be 132.3pm, as compared to 134.9 pm in the monomer.The former computed value (optimized for the hexamer) nicely approaches to the experimentally found r CN of 131.8 pm of the thioacetamide crystal [11], while the latter agrees with the experimental value of 135.6 pm found for CN bond of thioacetamide monomer in the gas phase [46].
Another evidence for the strong interaction can be found if the experiments are repeated with isotopically-labeled materials, especially if the samples are deuterated (where the largest isotope effect is expected), which results in the weakening or the complete disappearance of the resonance between the two vibrational modes [3e5].Therefore, the experiments were repeated with the deuterated derivative d 2 -N,N-thioacetamide under similar experimental conditions (Supplementary Material).As a consequence, these two modes decouple resulting in the upshift of the nCN stretching mode, which can be detected at 1514 cm À1 as a very strong band.In contrast, the bands due to the methyl bending modes all remain in their position compared to the non-deuterated thioacetamide, with that of b as,i.p.CH 3 at a maximum at 1439 cm À1 .Note that the latter value is very close to the aforementioned unperturbed value of the resonance system in the case of the nondeuterated derivative, and furthermore, that the agreement with the simulated spectrum is remarkable (Fig. S20 in the Supplementary Material).The findings are also confirmed by the repeated VMARD analysis of the Hex-I structure where the amino hydrogens are replaced with deuterium isotopes; Mode 31 accounts for the unperturbed nCN stretching vibration in the centrosymmetric unit and its mostly free of contributions from bending motions (Table S9 in the Supplementary Material).
It is interesting to note that the nCS stretching mode of the matrix-isolated monomer was observed at 735 cm À1 , while for the crystal it shifts to lower frequencies (near 712 cm À1 ).This decrease of vibrational nCS frequency upon crystallization is consistent with the lengthening of the CS bond.For the monomer, the CS bond length (r CS ) was computed to be 165.9pm, and the respective experimental value for the monomer in the gas phase is 164.7 pm [46].In the optimized hexamer (Hex-I), the computed r CS value increases to 169.0 pm (Table 4).This nicely compares with the experimental value for the CS bond in crystal equal to 168.6 pm [11].Such changes (and also the simultaneous shortening of the CN bond, discussed above) indicate that thioacetamide in crystals changes to the polar resonance structure (Scheme 3).Interestingly, the computed APT charge [47] on the sulfur atom in the hexamer (Hex-I) upon aggregation is À0.75 e, while in the monomer it is only À0.55 e, clearly confirming the increased polarization of the CS bond in the crystal.
The mean energy of the different H-bond types (here called "central" and "lateral", as denominated by Ramondo and Bencivenni [31] can also be assessed from the computational results.The mean stabilization energy (E stab ) provided by one "central" H-bond can be estimated by taking the half of the difference (since there are two central H-bonds in one centrosymmetric dimer unit) between the centrosymmetric dimer unit energy and twice the thioacetamide monomer energy.The mean E stab in the hexamers can be approximated by taking the energy of the hexamer minus six times the energy of the monomer, divided by the number of total H-bonds in the hexamer.All energy values used for the calculations are listed in Table S8, whereas the mean E stab are presented in Table 5.Based on the results, a few intriguing conclusions can be drawn.First, according to Table 5, the E stab values of the hexamers are very similar to each other (mean value 18.9 ± 0.4 kJ mol À1 ), furthermore, they are not significantly smaller than that of the central H-bonds in the dimer (19.3 kJ mol À1 ).Secondly, by having a look at the relative energy difference (DE ZPE , in kJ mol À1 ) between the hexamer structures, and the number of central and lateral H-bonds participating in them, one can conclude that the total number of H-bonds fundamentally determines the relative energy of the hexamer unit, i.e. the relatives energies of structures Hex-II, Hex-IV, and Hex-V having eight H-bonds have all similar DE ZPE values and are significantly lower than Hex-III, which in turn has seven H-bonds in total, whereas Hex-I having only six H-bonds has the highest relative energy among the hexamers.Briefly, not only the central H-bonds are important to stabilize the crystal structure but the lateral ones are also of elemental importance, because the experimentally determined crystal structure of thioacetamide contains the same numbers of the central and lateral H-bonds.
Similar qualitative conclusions can be drawn if the computational results with the dispersion correction are considered, however, there are remarkable differences.Most importantly, the mean H-bond strength increases by 85% to 35.7 kJ mol À1 in the dimer when compared to the uncorrected results.The effect is similarly strong for the hexamers, where the relative increment is 95% to a mean E stab value of 36.9 ± 1.4 kJ mol À1 .Nevertheless, the E stab value of the dimer and the mean value of the hexamers are still comparable to each other.An interesting outcome is that the relative stability of Hex-II is even more pronounced compared to the other hexamer structures, even Hex-IV and Hex-V are relatively less stable, possibly due to the different distribution of the central and lateral H-bonds.Namely, whereas Hex-II has 4 central and 4 lateral H-bonds, Hex-IV and Hex-V have 6 central and 2 lateral bonds.This result also suggests the fundamental importance of the presence of lateral H-bonds in the sample for the stabilization of the crystal.

Conclusion
Thin films of amorphous as well as of crystalline thioacetamide have been successfully deposited at 10 K and characterized by mid-IR spectroscopy.The irreversible amorphous-to-crystalline transformation was induced by heating (above 180 K) of the amorphous sample, which forms when the sample is deposited at cryogenic (10 K) temperature.The vibrational assignment of the experimental spectra was aided by results of quantum chemical analytical frequency computations on a model dodecamer (over a hundred atoms including 12 sulfurs), at the B3LYP/6e311þþG(d,p) level of theory, which allowed to revise some previous assignments, where the computations were carried out on smaller oligomers and at lower theory levels.The mean H-bond energies have also been estimated and it has also been pointed out that implementing the dispersion correction into the quantum chemical computations greatly enhances the obtained stabilization energies.Furthermore, the fundamental importance of the presence of lateral H-bonds besides the central ones in the centrosymmetric dimer in the    stabilization of the crystal structure has also been revealed.The spectral differences between the amorphous and the crystalline form of thioacetamide are significant and thus the results may help in the determination of the nature of thioacetamide in space, including its thermal history, if detected.In contrast to what has been found for the amorphous sample, upon repeated cooling and annealing, a reversible splitting of the antisymmetric out-ofplane methyl bending could be observed, which can be explained by its interaction with the methyl torsional mode and can only be observed for the crystalline sample.The methyl group can be found in its minimum-energy orientation only at low temperatures (10e50 K) meaning that the CH bonds in the CH 3 group remain non-identical at these temperatures and, in such cases, a usual separation between in-plane and out-of-plane vibrations of the methyl group.In contrast, owing to the low methyl torsional barrier (a few kJ mol À1 ), at temperatures high enough (above some 150 K), the excited states of methyl torsional mode are populated, therefore the methyl torsion becomes essentially barrierless which makes all the CH bonds indistinguishable within the methyl group.Thus the in-plane and out-of-plane vibrations are no longer distinguishable and the splitting disappears.This would allow for utilizing molecules with similar behavior as a molecular thermometer at cryogenic temperatures.Moreover, a strong resonance between the CN stretching and antisymmetric in-plane CH 3 bending has been shown to exist utilizing the VMARD analysis as well as the mid-IR spectrum of the N-deuterated thioacetamide derivative, where the interaction is considerably weaker, therefore making it easier to distinguish between the aforementioned two vibrational modes.
Regarding the astrophysical point of interest, this study can be considered as a first step towards a more realistic model, which should contain sulfurous compounds mixed with other, more abundant chemical compounds (such as water) making up the bulk of these astrophysically relevant ices.
1e8 and Figs.S1eS4).When the figure captions refer to reversible changes, this is type (ii) of the experiment, with deposition at 200 K (for example, Figs. 9e16 and Figs.S5eS19).The supplementary experiment with the deuterated derivative d 2 -N,N-thioacetamide (CH 3 CSND 2 ) was conducted in a similar way and is described in detail in the Supplementary Material.

Fig. 1 .
Fig. 1.Experimental mid-IR spectrum between 3375 and 3000 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 2 .
Fig. 2. Experimental mid-IR spectrum between 3000 and 2875 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 3 .
Fig. 3. Experimental mid-IR spectrum between 1700 and 1580 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 4 .
Fig. 4. Experimental mid-IR spectrum between 1550 and 1340 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).The resonance between n(CN) and b as,i.p. (CH 3 ) is discussed in detail in Section 4.4.Signs (þ) and (À) denote in-phase and in-opposite-phase couplings between the coordinates that participate in the resonance.

Fig. 5 .
Fig. 5. Experimental mid-IR spectrum between 1330 and 1265 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 6 .
Fig. 6.Experimental mid-IR spectrum between 1075 and 925 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 7 .
Fig. 7. Experimental mid-IR spectrum between 825 and 675 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 8 .
Fig.8.Experimental mid-IR spectrum between 540 and 440 cm À1 of the freshly deposited amorphous thioacetamide thin film at 10 K (bold black) and its irreversible change during annealing to 200 K (bold red).

Fig. 9 .
Fig. 9. Experimental mid-IR spectra of the crystalline thioacetamide between 3375 and 3000 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).

Fig. 10 .
Fig. 10.Experimental mid-IR spectra of the crystalline thioacetamide between 3000 and 2875 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K, and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).

Fig. 11 .
Fig. 11.Experimental mid-IR spectra of the crystalline thioacetamide between 1700 and 1580 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).
1e8 and Figs.S1eS4 in the Supplementary Material), and the glass-to-crystal transition is completed at 200 K.The inaccuracy

Fig. 12 .
Fig. 12. Experimental mid-IR spectra of the crystalline thioacetamide between 1550 and 1340 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).The resonance between n(CN) and b as,i.p. (CH 3 ) is discussed in detail in Section 4.4.Signs (þ) and (À) denote in-phase and in-opposite-phase couplings between the coordinates that participate in the resonance.The dashed line shows an approximate unperturbed position of the n(CN) mode at 10 K.

Fig. 13 .
Fig. 13.Experimental mid-IR spectra of the crystalline thioacetamide between 1330 and 1265 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).

Fig. 14 .
Fig. 14.Experimental mid-IR spectra of the crystalline thioacetamide between 1075 and 925 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).

Fig. 15 .
Fig. 15.Experimental mid-IR spectra of the crystalline thioacetamide between 825 and 675 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).

Fig. 16 .
Fig. 16.Experimental mid-IR spectra of the crystalline thioacetamide between 540 and 440 cm À1 (lower panel), showing reversible changes when cooling the sample down from 200 K (bold red) to 10 K (bold black), and their comparison with the simulated IR spectrum (upper panel) of the model dodecamer (shown in Scheme 1).
b n: stretching, b: bending, g: out-of-plane bending, r: rocking, u: wagging, s: symmetric, as: antisymmetric, i.p.: in-plane, o.p.: out-of-plane vibrations, &: resonance between listed vibrational modes.The signs (þ) and (À) denote in-phase and in-opposite-phase couplings between the coordinates that participate in the resonance.cBroadening due to the presence of n as,o.p. (CH 3 ) mode cannot be completely excluded.S. G obi et al. / Journal of Molecular Structure 1220 (2020) 128719

Scheme 3 .
Scheme 3. Resonance structures of thioacetamide.In crystalline state, the weight of the polarized structure (right) increases.
b C: central (in the dimer D), L: lateral (with respect to D). c The values show the mean E stab of an H-bond, regardless of its type (central or lateral).The values on the left are the uncorrected ones, whereas those on the right contain the empirical dispersion correction.

Table 1
Infrared spectrum of glassy thioacetamide.Normalized experimental band areas.The values were obtained by dividing the integrated area of a band by the sum of the experimental band areas.stretching, b: bending, g: out-of-plane bending, r: rocking, s: symmetric, as: antisymmetric, i.p.: in-plane, o.p.: out-of-plane vibrations, &: resonance between listed vibrational modes.The signs (þ) and (À) denote in-phase and in-opposite-phase couplings between the coordinates that participate in the resonance.
a At 10 K after deposition.bc n: a After annealing the amorphous ice, at 200 K. sh: shoulder.bAs obtained at the B3LYP/6e311þþG(d,p) level of theory.cNormalized experimental band areas at 200 K.The values were obtained by dividing the integrated area of a band in the experimental infrared spectrum by the sum of the experimental band areas.dn: stretching, b: bending, g: out-of-plane bending, r: rocking, u: wagging, s: symmetric, as: antisymmetric, i.p.: in-plane, o.p.: out-of-plane vibrations, &: resonance

Table 3
Temperature dependence of the infrared spectrum of crystalline thioacetamide.
a sh: shoulder.

Table 5
Calculated mean H-bond stabilization energies (E stab , in kJ mol À1 ) for selected hexamers that fit with the experimental crystal structure of thioacetamide.Zero-point corrected computed relative energy of the hexamer units (left: without dispersion correction, right: with empirical dispersion correction); absolute values of E ZPE for Hex-II is À3193.084152(uncorrected)andÀ3193.138457a.u.(dispersion corrected, TableS8in the Supplementary Material), as computed at the B3LYP/6e311þþG(d,p) level of theory without or with the GD3 empirical dispersion correction. a