Lattice Dynamics through the Structural Phase Transition in d

Article pubs.acs.org/JPCA

Lattice Dynamics through the Structural Phase Transition in D‑Amphetamine Sulfate Iwona Olejniczak* and Katarzyna Pogorzelec-Glaser Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland ABSTRACT: The polarized infrared and Raman spectra of the single-crystalline D-amphetamine sulfate have been measured as a function of temperature in the vicinity of the structural phase transition. Infrared and Raman-active modes are identified and assigned. Significant signatures of the structural phase transition are observed in the temperature dependence of infrared modes both of the D-amphetamine unit and the sulfate anion. The changes reflect differences in the unit cell between low- and high-temperature phases of the Damphetamine sulfate. Temperature dependence of the vibrational mode parameters displays pronounced hysteresis between 333 and 338 K that is extended over a smaller temperature range than 325−345 K found in the earlier DSC study.

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independent D-amphetamine molecule and one SO42− anion. The oxygen atoms of the sulfate anion are disordered, but the crystal structure displays similar alternating layers of Damphetamine cations and SO42−. D-Amphetamine sulfate in both phases aggregates into groups bonded by hydrogen N− H···O and C−H···π bonds. Almost all these interactions are weak, but one of the C−H···ring-molecule hydrogen bonds in the LT-phase that is very strong and involved in the T-shape arrangement.5,6 Taking this into account, the structural phase transition from the HT-phase to the LT-phase in D-amphetamine sulfate can be regarded as a result of ordering of SO42− anions, that allows the T-shape interaction within D-amphetamine layers. Vibrational spectroscopy is a well-known sensitive probe of bonding in solids. In particular, it provides a unique opportunity to study a local molecular structure. Amphetamine compounds were investigated mostly using Raman spectroscopy or surface-enhanced Raman spectroscopy (SERS) in order to identify samples of very low concentration.7−10 These studies were neither performed using crystalline samples nor focused on a detailed vibrational characterization of investigated materials. On the other hand, recently, Berg et al.3 published an extended review article on spectral studies of amphetamine compounds that included Raman spectra of solid D-amphetamine sulfate and also the respective ab initio density functional theory (DFT) calculations of the vibrational structure of the Damphetamine cation. In this work, temperature-dependent polarized infrared and Raman spectra for the first time measured in good quality single-crystalline samples are used to gain further insight into molecular structure modifications of D-amphetamine sulfate

INTRODUCTION Amphetamine, C9H13N, is a parent molecule for the group of amphetamines that are characterized by strong stimulating properties.1 Amphetamines are the most popular synthetic drugs and their abuse is considered to be a serious social problem in the contemporary world.2 For that reason most of the amphetamine research is focused on the detection and identification of samples from the point of view of forensic investigations (see ref 3 and references therein). The amphetamine molecule has two stereoisomers, levoamphetamine (L-amphetamine) and dextroamphetamine (D-amphetamine), and is usually synthesized as sulfate or phosphate salts. The D-amphetamine molecule (Figure 1) has a β-phenyl amine type skeleton that is characteristic for sympathomimetic amines that also include neurotransmitters. The crystal structure of D -amphetamine sulfate (C9H13N)2SO4 was determined at room temperature by Bergin and Carlström.4 The structure was later refined using singlecrystalline samples.5 X-ray crystallography, differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) studies revealed that D-amphetamine sulfate undergoes a structural phase transition from monoclinic P21 low-temperature phase (LT-phase) to monoclinic C2 high-temperature phase (HT-phase).5 This phase transition is of the first-order type and exhibits hysteresis; its temperature was determined to be 325/345 (by DSC), ∼325 (NMR), and 325 K (X-ray).5 In the LT-phase (Figure 2), the asymmetric unit of D-amphetamine sulfate consists of four nonequivalent D-amphetamine cations (C9H13N)+ and two sulfate SO42− groups, and the unit cell contains eight D-amphetamine molecules. The crystal packing the of LT-phase is characterized by two types of layers of the D-amphetamine donor molecules alternating along the caxis with the layers of SO42− anions.5 In the high-temperature HT-phase (Figure 3), the unit cell is reduced 2-fold along the caxis, the asymmetric unit contains one symmetrically © 2012 American Chemical Society

Received: May 18, 2012 Revised: September 7, 2012 Published: September 14, 2012 9854

dx.doi.org/10.1021/jp304843g | J. Phys. Chem. A 2012, 116, 9854−9862

The Journal of Physical Chemistry A

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Figure 1. D-Amphetamine cation in the conformation characteristic for the low-temperature phase.

Figure 2. Low-temperature crystal structure of D-amphetamine sulfate along the a axis (upper panel) and b axis (lower panel).

ature between the LT- and HT-phase. We compare signatures of the structural phase transition with results of earlier studies using different methods. We also report a detailed assignment

(C9H13N)2SO4 in the temperature range of the structural phase transition. In our discussion we focus our attention on the infrared spectra that display significant changes with temper9855



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Figure 3. High-temperature crystal structure of D-amphetamine sulfate along the a axis (upper panel) and b axis (lower panel).

LABRAM HR800 spectrometer equipped with a microscope. He−Ne (λ = 632.8 nm) and Ar (λ = 514.5 nm) laser lines were used with power reduced to about 0.1 mW to avoid sample overheating. The spectra were recorded with spectral resolution 2 cm−1. Both the IR and Raman spectra were measured for several temperatures between room temperature and about 360 K on heating and cooling, and the temperature was controlled using a standard THMS 600 water-cooled Linkam heating and freezing stage together with the Linkam TC 92 controller. The heating/cooling rate was about 1 K per minute.

of vibrational features observed both in infrared and Raman spectra of D-amphetamine sulfate. In our study we demonstrate that vibrational spectroscopy is a powerful method that allows to obtain precise information about structural modifications resulting in the first order phase transition in this material.

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EXPERIMENTAL SECTION Single crystals of D-amphetamine sulfate were crystallized using commercially available starting material according to a procedure described previously.5 Typical dimensions of samples chosen for spectral measurements were 0.6 × 0.2 × 0.1 mm3; they were in the shape of a parallelogram plate. The optical axes of the crystals were determined as those displaying the largest anisotropy at room temperature. Two directions within the best developed crystal phase were probed in the case of both infrared and Raman measurements: parallel, E∥c, and perpendicular, E⊥c, to the c axis. Polarized single-crystal infrared (IR) absorption spectra in the frequency range 600− 7000 cm−1 were measured using a Bruker Equinox 55 FT-IR spectrometer equipped with a Hyperion 1000 infrared microscope and a KRS5 polarizer, with the resolution 2 cm−1. Raman spectra of the single crystals for the electrical vector of the laser beam parallel and perpendicular to the c axis were measured in a backward scattering geometry with a Raman

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RESULTS AND DISCUSSION A. Vibrational Characteristics of D-Amphetamine Sulfate Single Crystal. Raman spectra of D-amphetamine sulfate (C9H13N)2SO4 were measured for the electric vector E parallel (E∥c) and perpendicular (E⊥c) to the c axis (see Figures 2, 3). The two spectra displayed rather weak polarization dependence, with slightly larger intensities of vibrational features for the perpendicular polarization. Therefore, we present the perpendicular Raman response only (E⊥c) for two excitation lines. Figure 4 displays Raman spectra measured at room temperature for excitation lines λ = 632.8 and 514.5 nm, in the frequency range 70−3300 cm−1. It is well9856



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to vibrations of the skeleton of the 2-aminopropane side chain.3,11 In particular, a characteristic very strong doublet that we attribute to wagging vibrations is present at 91/109 cm−1. Modes of the D-amphetamine cation involving ring deformations are found at 604, 624, and 739 cm−1. Two relatively strong bands related to vibrations of the carbon skeleton involving C−N stretching and NH bending are centered at 828 and 839 cm−1. In the frequency range 970−1040 cm−1, the Damphetamine ring vibrational features (1003, 1006, and 1031 cm−1) are observed, but also a relatively strong fingerprint symmetric stretching vibration of the SO42− sulfate anion appears at 978 cm−1. This is a distinct band found in aqueous solutions12 and solid samples13 of many sulfate salts at about 980−990 cm−1; based on theoretical calculations this band is expected to appear at 993 cm−1 in the spectrum of the SO42− sulfate anion.14 Another characteristic group of vibrational features is easily identified around 1200 cm−1. These three modes centered at 1161, 1180, and 1213 cm−1 are assigned to a number of D-amphetamine modes mostly involving ring and chain CH deformation. A distinct doublet 1587/1608 cm−1 is due to the two ring C−C stretching and in-plane bending Damphetamine modes. The strongest vibrational Raman features in the spectrum of the D-amphetamine sulfate appear between 2800 and 3100 cm−1. Based on DFT calculations by Berg et al.3 we identified in this frequency range a number of CH stretching modes of the D-amphetamine cation (see Table 1 for details). Polarized infrared spectra of D-amphetamine sulfate were measured for the electric vector E parallel (E∥c) and perpendicular (E⊥c) to the long crystal axis. The spectra recorded in the frequency range 600−3300 cm−1 at room temperature are shown in the upper panel of Figure 5. Similarly, as in the case of Raman spectra (Figure 4), they display vibrational features of both the D-amphetamine (C9H14N)+ cation and the SO42− sulfate anion. The strongest bands that appear around 1200 cm−1 and also in the frequency range 2500−3200 cm−1 for both polarizations are saturated in the measured sample. To avoid excessive saturation of vibrational features, a possibly thin single-crystalline sample of D-amphetamine sulfate was chosen for infrared measurements. Nevertheless, infrared spectra of the single-crystalline Damphetamine sulfate presented here are unique for two reasons. First, infrared spectroscopy is not a widely used method for characterization of amphetamine-related compounds, unlike Raman spectroscopy. Second, it is rarely used on single crystals because they are difficult to obtain in this class of materials. Here, we can compare our spectra with those measured for D-amphetamine hydrochloride and hydroiodide by J. S. Chappell15 in samples dispersed in KCl and KI discs (see Figure 3 in ref 15). In this standard method, vibrational features are well resolved but the information about band polarization is lost and also some of the spectral features depend on the matrix material. In case of low-symmetry molecules as the D-amphetamine cation in our study, theoretically calculated vibrational modes are usually both infrared- and Raman-active. On the other hand, there are significant differences between infrared and Raman intensities of most of the observed modes. Thus, in order to fully understand vibrational response of the material, it is important to discuss both Raman and IR response. Here, the band positions and their relative intensities in the polarized infrared spectra of D-amphetamine sulfate measured at room temperature (Figure 5a) are compared with those observed in

Figure 4. Room temperature single-crystal Raman spectra of Damphetamine sulfate for two excitation lines. The electrical vector of the laser beam was perpendicular to the c axis (in marked configuration y ⊥ c). Note that the spectra are offset for clarity.

known that relative intensities of Raman modes can be excitation wavelength sensitive. In the case of D-amphetamine sulfate, this effect is relatively small. Both spectra display similar features, with the spectrum measured for λ = 514.5 nm better resolved, in particular in the high frequency range above 2500 cm−1. So that in the following discussion we focus on the λ = 514.5 nm Raman response of the D-amphetamine sulfate that can be compared with that recorded by Berg et al. for the same excitation line.3 These authors present and analyze a number of both Raman and SERS spectra of various amphetamine species including D-amphetamine sulfate. Note that the Raman spectrum of D-amphetamine sulfate in3 was measured for commercial solid polycrystalline sample. Here we present the Raman response measured for a better quality recrystallized sample of D-amphetamine sulfate.5 In fact, Figure 4 displays the Raman spectrum with very well-resolved narrow vibrational features characteristic for a single-crystalline material. Most of observed bands are related to the D-amphetamine (C9H14N)+ cation, others are due to the SO42− anion. In our discussion of vibrational features we use results of DFT calculations performed for D-amphetamine (C9H14N)+ cation in the gas phase reported earlier by Berg et al.3 These authors use the hybrid B3LYP method together with the Hartree−Fock/ Kohn−Sham approach. A list of the band positions identified in the Raman spectrum together with the calculated frequencies and assignments based on Berg et al.3 is given in Table 1. Note that assignments of D-amphetamine vibrational features in this work are partly different than those in the original paper (compare columns 1 and 7 in Table 1). We believe that having very good quality both Raman and IR spectra we were able to assign vibrational modes in a more systematic manner. In general, our experimental Raman spectrum of Damphetamine sulfate (Figure 4) mostly displays vibrational features of the D-amphetamine cation (see calculated spectrum in3 for comparison). The D-amphetamine molecule itself consists of two almost planar, mutually perpendicular parts, the phenyl ring and the 2-aminopropane side chain, and its geometry is influenced by internal rotation of the side chain around the C7−C8 bond.5 At the low frequency range between 70 and 600 cm−1 we observe a number of Raman modes related 9857



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Table 1. Vibrational Band Positions (Frequency in cm−1)a R, E⊥c 292 K

IR, E∥c 298 K

IR, E⊥c 298 K

IR, E∥c 358 K

IR, E⊥c 358 K

91 vs 109 vs 250 m 262 m 339 w 366 w 435 w 467 m 501 vw 508 vw 604 m 624 m 637 w 700 vw 739 m 753 w 828 s 839 m 901 vw 912 vw 943 w 978 s 1003 s 1006 s 1031 m 1035 m

601 619 634 703 743

w m w s s

825 837 899 915

vw vw vw vw

976 w

603 619 635 700 739 751

w m w s s w

612 622 697 734

m m vw vw

696 s 733 s

910 w 976 w

836 vw 893 vw 911 vw 942 vw ∼969 vw ∼1005vw

∼972 vw

1161 w 1180 m

∼1060 vs 1150 m 1207 w

∼1060 vs 1150 w

1213 s

1231 m

1230 s

1260 vw 1343 w

1258 vw 1342 vw

1257 m 1338 w

1364 vw 1373 m 1395 m

1368 vw

1372 w 1395 w

1258 1335 1342 1364 1373 1395

1457 m

1457 m 1498 w

1455 m 1466 w 1497 vw

1455 w

1498 w

1456 m 1467 vw 1498 w

1537 m 1556 vw

1537 vw 1555 m

1548 vw

1548 vw

1585 vw

1585 vw

1584 vw

1642 s

1620 s 1643 s 1761 vw 1882 vw 1951 vw 2171 vs 2570 s ∼2660 vs ∼2900 vs

1617 s

1587 m 1608 s

2780 w 2883 s 2936 vs

w vw vw vw w m

1756 vw 1878 w 1947 w 2165 vs 2555 s ∼2660 vs ∼2900 vs

∼1100 vs ∼1158 w 1208 vw

909 w

∼1080 vs ∼1158 w

1227 s

1394 m

1755 w 1877 w 1944 w 2120 vs 2544 s ∼2640 vs ∼2900 vs

1256 vw 1338 vw

1373 m 1394 m

1497 w

1612 s

2124 vs 2566 s ∼2640 vs ∼2950 vs

2978 m 9858

R3

mode assignment3

97

C1−C7−C8 wag

241 245 299 345 380 414 420 503

C8−C9 twist

602 635

615 m

823 vw 837 vw

DFT3

716 746 781 826 872 903 935 941 959 1017 1024 1050 1071 1131 1190 1199 1207 1218 1262 1336 1345 1360 1385 1394 1426 1478 1488 1492

1500 1501 1529 1627 1642 1661 1664

255 m

C7−C8−N bend C2−C1−C7 i.p. bend CH3 rock ring twist N−C8−C9 bend; sulfate bend skeleton and ring def

342 w

435 w 466 w

∼502 w 604 w 623 m 740 w 826 m 838 m

914 vw 942 w

1002 1033 1066 1103 1158 1181 1210 1246 1258 1301 1334 1367 1396 1453 1583 1605

vs m vw vw w w m vw vw vw w w w w w m

2775 vw

3008

2881 w

3033 3052 3084

2937 m

N−C8−C7 and ring bend ring i.p. def sulfate bend ring o.o.p. def ring o.o.p. def and N−C8 str ring o.o.p. def and N−C8 str C1−C7 str and NH bend chain C−C and C−N str CH and NH def C7−C8 str chain CH ring CH o.o.p. def NH3 rock and CH3 rock NH3 and CH3 rock and C8−C9 str; sulfate sym str ring skeleton i.p. str ring CH o.o.p. def ring CCstr and CH i.p. def skeleton and CH def sulfate asym str ring CC str and CH and NH def ring CH i.p. def chain str and CH and NH def ring CH i.p. def C1−C7 str and ring CH i.p. def angle def and ring CH i.p. def ring and chain CC str and CH angle def chain CC str and ring CH angle def C7−C8−str and CH def CH3 umbrella and CH def CH3 umbrella and CH bend CH2 and CH3 def ring CC str and CH and NH bend NH3 umbrella and CH bend

CH3 def NH3 umbrella and CH bend ring i.p. CH bend ring CC str and ring i.p. bend ring CC str and ring i.p. bend NH3 bend NH3 bend overtone/combination bands

overtone/combination of sulfate asym str C7H2 sym str C9H3 sym str C7H2 asym str C8H str dx.doi.org/10.1021/jp304843g | J. Phys. Chem. A 2012, 116, 9854−9862


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Table 1. continued R, E⊥c 292 K

IR, E∥c 298 K

IR, E⊥c 298 K

IR, E∥c 358 K

IR, E⊥c 358 K

2984 m 3005 vw 3033 s

3055 3066 3168 3207

vs s w vw

3085 w

∼3085 vw

∼3133 vw

DFT3 3103 3131 3150 3162 3182 3191 3201 3394 3476

R3

mode assignment3 C8H and C9H3 str C9H3 asym str ring CH str

2980 w 3040 w 3058 vs 3069 s

ring CH str

3168 vw 3207 vw

NH3 sym str NH3 asym str

a

Observed at room temperature (low-temperature phase) and 358 K (high-temperature phase) in Raman (R) and IR spectra of single-crystalline Damphetamine sulfate together with the corresponding assignments based on earlier DFT calculations of D-amphetamine cation;3 in column 7 we cite vibrational bands in experimental Raman spectrum of D-amphetamine sulfate attributed to calculated modes by Berg et al.3 The following abbreviations are used: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; str, stretch; tw, twist; def, deformation; i.p., in-plane; o.o.p., out-of-plane.

cation are also expected in the same frequency range. Here we find the 1150 cm−1 mode mostly assigned to the ring C−C stretching. The following frequency range 1200−1700 cm−1 contains vibrational features of the D-amphetamine cation that display significant anisotropy at room temperature. In the E∥c spectrum, a characteristic triplet structure is observed, with maxima at 1207, 1231, and 1258 cm−1, related to ring in-plane deformation and chain stretching. This structure is displayed in the E⊥c spectrum as the strong middle mode centered at 1230 cm−1 with very weak other two components (Figure 5a). The doublet structure 1373/1395 cm−1 is another anisotropic feature. Here the 1373 cm−1 component is weak for E∥c and relatively strong in the E⊥c spectrum. The other mode related to CH2 and CH3 deformation is centered at about 1457 cm−1 for both polarizations. Another characteristic structure is observed as a broad band centered at 1537 cm−1 for the E∥c spectrum. This mode seems to be shifted to 1555 cm−1 in case of E⊥c polarization but this effect is rather related to different intensities of the broad mode components that are not easy to be identified. A strong feature centered at 1642 cm−1 in the E∥c spectrum appears as a doublet structure 1620/1643 cm−1 for E⊥c polarization. This mode is attributed to NH3 bending vibration. A very broad and intense band observed at ∼2170 cm−1 for both polarizations is probably an overtone/ combination mode. Such a feature was earlier observed in the infrared spectra of D-amphetamine hydrochloride.15 We suggest that this feature is mostly related to the asymmetric sulfate stretch that appears at ∼1070 cm−1 as the primary band. A broad and intense cluster of CH stretching modes of the Damphetamine cation is observed in the frequency range 2500− 3200 cm−1. Here strong modes centered at about 2570, 2660, and 2900 cm−1 are identified. B. Structural Phase Transition (325 K) from the Point of View of Lattice Dynamics. As it was confirmed by earlier DSC experiments, D-amphetamine sulfate undergoes structural phase transition in the temperature range 325 - 345 K.5 This phase transition from the low-temperature LT-phase to the high-temperature HT-phase results in the large distortion in the structure. Basically, the most important factors of this distortion are different patterns of hydrogen bonding in each phase, the disorder of the oxygen atoms of SO42− groups in the HT-phase, and different size of the unit cell that is doubled in the LTphase comparing the HT-phase.5 Vibrational spectroscopy is a perfect tool for detailed investigation of such structural changes. Let us now consider what kind of changes we can expect in the spectra of D-amphetamine sulfate from the point of view of the

Figure 5. Single-crystal absorption spectra of D-amphetamine sulfate measured for polarization parallel (E∥c) and perpendicular (E⊥c) to the long crystal axis (a) at low temperature phase (T = 298 K) and (b) at high temperature phase (T = 358 K). Note that the spectra in each panel are offset for clarity. The most prominent mode changes with temperature are marked with arrows.

Raman spectrum (Table 1). Band assignments are performed based on the calculated infrared spectrum of the D-amphetamine cation that is included in Berg et al.,3 but it is not discussed in their paper. In the low frequency range between 600 and 800 cm−1 we observe a couple of infrared modes mostly related to ring deformation of the D-amphetamine cation. A narrow mode assigned to ring in-plane deformation appears at 619 cm−1 for both polarizations at room temperature. Two relatively strong modes attributed to ring out-ofplane deformations are centered at 703 and 743 cm−1 for the E∥c spectrum and at 700 and 739 cm−1 for E⊥c. The two other characteristic lines display large anisotropy in the HT-phase and will be discussed in the next section. A broad very intense feature extending in the spectra between 1000 and 1200 cm−1 is most probably related to the SO42− asymmetric stretching vibration.13,14 On the other hand, modes of the D-amphetamine 9859



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Figure 6. Close-up views of temperature dependence of single-crystal absorption spectra of D-amphetamine sulfate for parallel polarization (E∥c) in the frequency range of (a) 1231 cm−1 mode (ring CH in-plane deformation), (b) 1642 cm−1 mode (NH3 bending), and (c) 2165 cm−1 mode (sulfate asymmetric stretching). (b, d, f) Respective temperature dependence of the band absorbance at maximum (b) or band position (d and f).

both Raman and polarized infrared spectra in the temperature range 290−360 K, on heating and cooling. These methods do not provide exactly the same information. While IR spectroscopy detects vibrations involving the electrical dipole moment changes, Raman spectroscopy is sensitive to vibrations with electrical polarizability changes. This implies that bonds that connect two identical parts of a molecule tend to be more active in Raman than in IR spectroscopy. On the other hand, vibrations of unsymmetrical, weakly polarizable bonds, for example, OH bonds, are very strong in IR but very weak in Raman. In case of vibrations of molecular groups involved in hydrogen bonding in our study (e.g., SO4, NH3) we should see more pronounced modifications for infrared features. In fact, the Raman spectra were almost identical in the whole temperature range (not shown). On the other hand, polarized IR spectra display significant changes between LT-phase (room

structural phase transition from the LT-phase to the HT-phase. First, the unit cell is 2-fold reduced so we can expect merging of some vibrational features in the HT-phase. Second, the disorder of the SO42− groups can result in changes of the sulfate modes including broadening and can also influence the NH3 modes because the latter are connected with SO42− anions through hydrogen bonding that changes at the structural phase transition. Here it was found that the disorder of the SO42− groups in the HT phase results in changes of both the N− H···O bond distances and angles.5 Third, modifications of the hydrogen bonding network of the C−H···π type that connects phenyl rings and the 2-aminopropane chains of the Damphetamine cations in the unit cell can alter a number of Damphetamine modes related to these two main parts. To identify signatures of the phase transition in the vibrational spectra of D-amphetamine sulfate we investigated 9860



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modifications of the spectra in a detailed manner and test for the effects of phase transition order. Figure 6a,c,e displays closeup views of temperature dependence of single-crystal polarized (E∥c) absorption spectra of D-amphetamine sulfate for three selected modes that show most pronounced changes at the phase transition, together with the respective temperature dependence of the selected band parameter: absorbance at maximum (b) or position (d and f). To characterize the 1231 cm−1 band (Figure 6a) attributed mostly to D-amphetamine ring CH in-plane deformation, we have chosen absorbance at maximum. Absorbance at maximum is a simple parameter that does not involve calculation. It can be used instead of band intensity to discuss temperature modifications if a feature displays only weak temperature dependence of the bandwidth. Absorbance of the 1231 cm−1 mode slightly decreases on heating until the temperature of 338 K is reached. Then it abruptly drops to very low values indicating the temperature of the structural phase transition. On cooling, the mode suddenly grows at 333 K, quickly reaches the top values, and saturates in the LT-phase. Thus, the 1231 cm−1 mode exhibits pronounced hysteresis of about 5 K (between 333 and 338 K) that is substantially narrower than 19 K (325−344 K) found in the earlier DSC study.5 Our results can also be compared with the temperature dependence of the unit cell parameters and results of NMR experiment that display discontinued jump on cooling at 325 K.5 Very similar behavior has been found in the temperature dependence of band frequency for two important modes. The position of the 1642 cm−1 mode assigned to the NH3 bending vibration of the D-amphetamine cation (Figure 6c,d) practically follows the same temperature dependence, as it was in the case of the absorbance of the 1231 cm−1 mode. Also, the broad combination mode centered at 2165 cm−1 that we attribute to SO42− asymmetric stretching vibration (Figure 6e,f) shifts to lower frequency in the HT-phase, with the hysteresis effect between 333 and 338 K. Very similar well pronounced hysteretic behavior is observed for other vibrational features that change their appearance between low-temperature LTphase and high-temperature HT-phase (not shown). Therefore, based on our IR experiment, we can confirm that the structural phase transition in the D-amphetamine sulfate is of the first order. The clear reversible effect characterized by sharp hysteresis loop, which we found in the IR spectra, is significantly narrower than in the DSC experiment that is a more macroscopic probe. On the other hand, the DSC experiment was performed with heating/cooling rate 10 K per min5 that was much different than 1 K per minute in our spectroscopic study. Nevertheless, our observation suggests that the same structural phase transition that is relatively narrow and well-defined from the point of view of local molecular structure, can be broadened if we take into account a long-range order.

temperature, Figure 5a) and HT-phase (358 K, Figure 5b) concerning many modes. In general, larger effects are observed for electric vector of light polarized in the direction parallel to the c axis (E∥c). In the LT-phase we observe more vibrational features, in the HT-phase modes are broadened as expected. Here, we closely examine the most important changes. A huge difference in intensity is observed in case of the two strong narrow modes centered at 703 and 743 cm−1 in the room temperature E∥c spectra (Figure 5a,b). These modes that are mostly attributed to the D-amphetamine ring out-of-plane deformation slightly shift to lower frequency and almost disappear in the HT-phase. On the other hand, they are only weakly modified in case of the E⊥c response. Such a prominent change in anisotropy of this doublet structure is probably related to different arrangement of the phenyl rings between the LT- and HT-phases.5 A significant change in width is observed in both polarizations in case of a broad saturated band at about 1100 cm−1, that is mostly related to SO42− asymmetric stretching vibration. We estimated the width of this band at absorption level 2.5 as 85 cm−1 in the LT-phase and 100 cm−1 in the HT-phase for both polarizations. The broadening of this sulfate mode in the HT-phase is most probably a signature of the sulfate group disorder. Another feature that changes its appearance when raising temperature from LT-phase to HTphase is the characteristic triplet structure mostly related to Damphetamine ring CH in-plane deformation (Figure 6a). This structure characterized by the strongest 1231 cm−1 middle component in the LT-phase transforms in HT-phase into almost a doublet structure with the strongest 1257 cm−1 component. It suggests that oscillator strength within this triplet structure is shifted at the phase transition from the middle component into the highest frequency component. This effect observed for the ring CH in-plane deformation mode most probably reflects changes in the C−H···π bonding. It is known that in both HT and LT phases almost all interactins of the C−H···π type are very weak except one in the LT phase that is very strong and allows a T-shape arrangement in the structure.5 So that we suggest that the strong 1231 cm−1 band observed in the LT-phase is related to the strongest C−H···π found in the low-temperature structure. Changes in intensity and the number of components are also observed in the frequency range 1330−1380 cm−1 where a number of Damphetamine stretching modes is observed (Figure 5a,b). A remarkable band attributed to the NH3 bending vibration of the −1 D-amphetamine cation appears at 1642 cm in the LT-phase (Figure 6c). This mode is significantly shifted to 1617 cm−1 in the HT-phase. Such a downshift is usually related to longer distances within molecular groups engaged in the vibration. In our case, this change in the mode position confirms a substantial change in the hydrogen N−H···O bonding pattern at the structural phase transition. More detailed discussion of the frequency changes of the NH3 bending mode is not possible because this broad feature obviously contains components related to multiple slightly different N−H···O bonding distances and angles found in both the LT and HT phases.5 A significant broadening together with the downshift from 2165 to 2120 cm−1 in case of the E∥c spectrum (from 2171 to 2124 cm−1 for E⊥c) is also observed for the combination mode involving SO42‑ asymmetric stretching (Figure 6e). This is another signature of the sulfate group disorder that appears in the HT-phase.5 The variable temperature IR experiment was performed in a closed temperature cycle. So that we were able to follow

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CONCLUSION Infrared and Raman spectroscopy were used to probe lattice dynamics of the single-crystalline D-amphetamine sulfate. Vibrational modes have been identified and attributed based on the earlier DFT calculation of the D-amphetamine cation. Polarized infrared spectra display modest polarization dependence in case of a few vibrational features. The temperature dependence of the polarized IR spectra on heating and cooling confirms that D-amphetamine sulfate undergoes a first order phase transition in the temperature range 333−338 K. Vibrational features that display most pronounced temperature modifications at the phase transition are related to molecular 9861



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groups engaged in hydrogen bonding formation that changes its pattern between the low-temperature and high-temperature phases. In particular, strong effects are observed in case of SO42− (stretching) and NH3 (bending) modes that are influenced by the N−H···O hydrogen bonds. These changes clearly indicate that the sulfate groups are disordered in the high-temperature phase. The sharp relatively narrow hysteresis comparing earlier DSC experiments, which characterizes temperature dependence of infrared active modes sensitive to the structural phase transition, suggests that the transition is well-defined from the point of view of local molecular structure, but its picture differs from what is observed at large scale.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Lattice Dynamics through the Structural Phase Transition in d

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