Infrared and Raman spectroscopy and DFT calculations of DL amino acids: Valine and lysine hydrochloride

Journal of Molecular Structure 1127 (2017) 419e426

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Infrared and Raman spectroscopy and DFT calculations of DL amino acids: Valine and lysine hydrochloride ^go a, J.A. Lima Jr. a, *, P.T.C. Freire a, F.E.A. Melo a, F.M. Paiva a, J.C. Batista a, F.S.C. Re J. Mendes Filho a, A.S. de Menezes b, C.E.S. Nogueira c , C.P. 6030, Campus do Pici, 60455-760, Fortaleza, CE, Brazil Departamento de Física, Universidade Federal do Ceara ~o, Campus do Bacanga, 65085-580, Sa ~o Luís, MA, Brazil Departamento de Física, CCET, Universidade Federal do Maranha c Departamento de Física, Universidade Regional do Cariri, CEP, 63010-970, Juazeiro do Norte, CE, Brazil a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 December 2015 Received in revised form 12 July 2016 Accepted 15 July 2016 Available online 18 July 2016

Single crystals of DL-valine and DL-lysine hydrochloride were grown by slow evaporation method and the crystallographic structure were confirmed by X-ray diffraction experiment and Rietveld method. These two crystals have been studied by Raman spectroscopy in the 25e3600 cm
1 spectral range and by infrared spectroscopy through the interval 375e4000 cm
1 at room temperature. Experimental and theoretical vibrational spectra were compared and a complete analysis of the modes was done in terms of the Potential Energy Distribution (PED). © 2016 Published by Elsevier B.V.

Keywords: Raman spectroscopy Infrared spectroscopy Crystal growth Vibrational assignments Racemic amino acids

1. Introduction Amino acids are the small molecules that compose proteins of all the human beings. Probably the first intriguing question about these compounds is: Why only the L-enantiomer forms proteins? The answer is closely related to the evolution of life and Abdus Salam has proposed a possible explanation [1]. In the crystalline state, amino acids molecules are mainly linked by hydrogen bonds, giving rise to a series of structures; so the polymorphism of this kind of material is another remarkable point. Even at ambient conditions some amino acids present polymorphism such as glycine [2e4], Lcysteine [5,6], DL-methionine [7], and DL-valine [8,9]. The possibility of undergoes phase transitions on varying pressure or temperature has also motivated researches to study amino acids extensively. Also related to hydrogen bonds another important point concerning amino acids is the comparison of pure enantiomer and racemic forms, DL, in which half of molecules are in the L-form and the other half in the D-form. L and DL counterparts are formed essentially by the same molecules, but the hydrogen

* Corresponding author. E-mail address: alves@fisica.ufc.br (J.A. Lima). http://dx.doi.org/10.1016/j.molstruc.2016.07.067 0022-2860/© 2016 Published by Elsevier B.V.

bonding network that stabilizes their structures can provide very different physical properties. In the last few years the number of papers reporting the vibrational and structural properties of racemic amino acids at ambient conditions [10e12], temperature- [13e18], and pressuredependent [19e21] studies has increased, but is still fewer in comparison with that on L-enantiomer [22e36]. Under pressure DL-serine maintains its atmospheric pressure structure stable up to 8.0 GPa [21]. DL-cysteine (DL-cysteine I) undergoes a phase transition to a new phase (DL-cysteine II) at low temperature that is the same phase of the polymorph obtained with increasing pressure at 0.1 GPa [37]. Such a behavior illustrates how complex can be a PT-diagram for this class of materials. DLcysteine II structure is preserved on cooling down to 3 K but under pressure it undergoes phase transitions at 1.55 and at 6.2 GPa. DL-alanine was investigated and results show its structure remains stable from ambient temperature down to 15 K and on compression up to 8.3 GPa [38]. Abagaro et al. [39] reported a phase transition undergone by DL-leucine at 2.4 GPa. Single-crystal X-ray diffraction experiments revealed that DL-norvaline undergoes two phase transitions on cooling. The first occurs at
80  C and the second at
100  C [40]. For DL-norleucine a reversible phase transition was reported on heating at 118  C with a very narrow hysteresis [41].

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Studies reporting vibrational properties of the L-enantiomers of valine [34], lysine hydrochloride [33], lysine hydrochloride dihydrated [35], glutamic acid monohydrated [36], and glutamic acid hydrochloride [42] have been reported, but a complete vibrational analysis of the racemate counterpart of these amino acids is still missing. In order to fill up this gap of information we present in this work an infrared and a Raman spectroscopic study in the spectral range from 25 cm
1 to 4000 cm
1 of DL-valine and DL-lysine hydrochloride. Additionally, vibrational frequencies obtained through DFT calculations were analyzed in terms of the Potential Energy Distribution (PED) and was used in conjunction with previous works on amino acid crystals to assign the experimental Raman and infrared bands.

VEDA software [45]. Theoretical Raman Intensities (RI) were derived from Raman Activities through the use of the following relationship obtained from basic Raman scattering theory:

2. Experimental details

in this expression, h, k, c, and T are, respectively: the Planck and Boltzmann constants, the speed of light and the temperature in Kelvin. Lorentzian band shapes with band width of 10 cm
1 were used for all calculated spectra plots.

2.1. Crystal growth Single crystals were grown from aqueous solution by the slow evaporation method at 297 K for both samples. Sigma Aldrich reagents (purity > 97%) were used for obtaining all the two materials. Colorless and elongated platelets single crystal of DL-valine were obtained after 6 weeks and small colorless crystals of DL-lysine hydrochloride were obtained after 4 weeks. 2.2. X-ray diffraction Powder X-ray diffractograms were measured at room temperature, using the Cu Ka radiation (l ¼ 1.5418 Å), in a conventional diffractometer D8 AdvanceeBruker, in the qe2q BraggeBrentano geometry. The 2q range was from 5 up to 150 , with increments of 0.02 and a counting time of 6 s per step. 2.3. Infrared Fourier Transform Infrared (FT-IR) spectra were recorded by using a Bruker VERTEX 70 FTIR/FT-Raman spectrometer (Bruker Optics Inc., Ettlingen, Karlsruhe, Germany). KBr pellets of solid samples were prepared from mixtures of KBr and the sample. Measurements were performed at room temperature. 2.4. Raman spectroscopy The room temperature Raman spectra were obtained with a triple-grating spectrometer (Jobin-Yvon T64000) equipped with a N2-cooled charge-coupled device (CCD) detection system. The 514 nm line of an argon ion laser was used for excitation. An Olympus microscope lens with a focal distance f D ¼ 20.5 mm and numerical aperture NA ¼ 0.35 was used to focus the laser beam on the sample surface. The spectrometer slits were set for a resolution of 2 cm
1.

RIi ¼ Cðv0
vi Þ4$v
1 i $Bi $Si; where C is a scaling constant (10
13), v0 is the argon laser excitation frequency, vi is the calculated frequency of the ith normal mode and Si is the corresponding activity for this mode. The Bi factor accounts for the contribution of excited vibrational states to the intensities:

Bi ¼ ð1
expð
h$vi $c=kTÞÞ

3. Results and discussion 3.1. Structural characterization X-ray diffraction experiments and Rietveld refinement were performed, allowing the confirmation of the structure of the two crystals (x-ray diffraction pattern and the refinement of the two amino acids can be seen in Figs. S2 and S4 of Supplementary Material). DL-valine crystallizes in the triclinic system p1 [8] with two molecules (i.e. 38 atoms) per unit cell and the cell parameters obtained in the refinement are a ¼ 5.2915(6)Å,   b ¼ 5.4228(6) Å, c ¼ 11.0785(4) Å, a ¼ 91.100(11) , b ¼ 92.554(11)  and g ¼ 109.943(15) . The crystalline structure is formed by chains where valine molecules are linked by hydrogen bonds and van der Walls interactions with all molecules in the trans-conformation [9]. Interesting enough, two hydrogen atoms form hydrogen bonds with short dimensions e 2.78 and 2.88 Å e and the third hydrogen participates of a bifurcated hydrogen bond. DL-lysine hydrochloride crystallizes in the monoclinic system (P21/c) with four molecules (i.e. 88 atoms) [46] per unit cell and the cell parameters are a ¼ 9.209(2)Å, b ¼ 11.247(2) Å, c ¼ 8.558(2)Å, b ¼ 105.7529 (4) . Figs. S1 and S3 of Supplementary Material show the molecules and the crystallographic projection of the two samples. Tables S1 and S2 show the crystallographic parameters refined. 3.2. Raman and IR spectroscopy and DFT calculations Fig. 1 presents the optimized molecular structure of DL-valine in (a) and DL- lysine in (b) with the atoms identification used in PED calculations. In the next sections it will be discussed the main modes observed for the two samples and the modes that were not discussed in the text can be found (as well as the PED attribution) in Tables 1 and 2.

2.5. DFT calculations Theoretical calculations were carried out with the Gaussian 09 program [43]. Geometrical coordinates of DL-Valine and DL-Lysine were optimized using the Hartree-Fock (HF) method with the 631 þ G(d,p) basis set and with the Polarizable Continuum Model (PCM) using water as solvent. Vibrational modes were calculated analytically by the software [44] and two scaling factors obtained through least-square fits were used. For frequencies below 2000 cm
1,scaling factors of 0.947 (DL-Valine) and 0.9642 (DLLysine) were used. For frequencies above 2000 cm
1, the scaling factor was 0.91 for both spectra. Vibrational modes contributions to the Potential Energy Distribution (PED) were analyzed using the

3.2.1. DL-valine Fig. 2 shows the Raman spectra (experimental and calculated) of DL-valine. IR spectra can be seen in Fig. 3. Our PED calculation obtained four Raman modes (scaled values: 61, 95, 129 and 180 cm
1) that were identified as vibrational combinations of deformations (d), out-of-plane deformations (g) and torsions (t) of CCN and CCO units. In the same spectral range were found seven modes in the DL-valine experimental Raman spectra (48, 60, 71, 106, 117, 135, and 175 cm
1). It is a tough work find the boundary for lattice modes but generally they are placed below 200 cm
1, but we believe that these complex vibrations also compose the lattice modes. Our PED indicated that the scaled mode associated with

F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

experimental one at 293 cm
1 can be identified as a torsion of CH3 unit, t(CH3). The experimental mode at 214 cm
1 was not associated with any scaled mode, but based in results obtained in Lima et al. [34] a reasonable assignment of this mode is a torsion of CH unit, t(CH). According to the PED results the experimental modes at 361, 370, and 420 cm
1 can be assigned to skeleton deformations what is also in good agrement with the results in L-valine [34]. Murli et al. [19] classified a Raman mode in DL-valine at 531 cm
1 as being a rocking of CO2, r(CO2). According to our PED calculation the mode observed at 529 cm
1 has not only the contribution of the deformation of the CO2, but also stretching of CC and CN units. The mode at 540 cm
1 that do not have any scaled mode associated with can be classified as librational mode of NOeH, lib(NOeH), because a mode at 544 cm
1 received the same classification according to Ref. [19]. Three experimental Raman modes at 685, 774, and 828 cm
1 in the Raman spectrum of DL-val can be classified as deformation, d(CO2), wagging, u(CO2), and out-of-plane deformation, g(CO2), respectively, in accordance with results for L-valine [34]. This classification is quite reasonable because the assignment done by PED to the respective scaled modes deformation of CO2 unit also contributes to the vibration. Generally stretching of CC and CN units respectively, n(CC) and n (CN), can be observed in the

421

850e1100 cm
1 spectral range. Based on results of our PED calculations the experimental modes (except the experimental Raman mode at 963 cm
1) in this region has great contributions of n(CC) and n(CN) vibrations. No scaled mode was related to the experimental Raman mode observed at 926 cm
1, but probably it can be assigned as a n(CC). Deformations of CH, CH3, and NH3 are expected between 1100 and 1400 cm
1. The mode observed at 1102 cm
1 in the experimental Raman spectra can be classified as rocking of NH3 in accordance with PED attribution and results of reference [34]. Two Raman modes observed at 1314 and 1323 cm
1 do not have any equivalent calculated mode, but a resonable classification for these modes is a deformation of CH, d(CH), or CH3, since Jarmelo et al. [47] found such a vibration on DL-serine at 1315 and 1321 cm
1 and, in L-Valine [34],it was observed at 1322 cm
1. At 1454 cm
1 (experimental Raman) a mode was classified as deformation of CH3 unit, d(CH3). The same vibration of NH3 unit, d(NH3), was atributed to a mode at 1678 cm
1. In L-Valine, L and DL-Alanine Raman spectra [10,22] the anti-symmetric deformation of NH3 was observed at 1650 and 1653 cm
1,respectively. Such a vibration was observed in the L conformer of Valine at 1633 cm
1, so we think that the experimental Raman mode observed at 1628 cm
1 can also be classified in the same way. CH and CH3 stretching vibrations are

Fig. 1. Optimized molecular structure of DL-valine (a) and DL-Lysine (b) molecules with the atoms identification used in PED.

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F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

Table 1 Raman calculated wavenumbers unscaled (ucalc) and scaled (uscal) by the dual scale factor 0.9642 (below 1800 cm
1) and 0.91 (above 1800 cm
1). Experimental Raman (uRaman) and FT-IR (uIR) modes and assignment of the vibrational modes for DL-Valine with PED. All wavenumbers are given in cm
1. Raman

ucalc

uscal

uRaman

65

61

100 136

95 129

190

180

247 263 279 342 374 449 506 566

234 249 264 324 355 425 479 536

706 831 886 948

669 787 839 898

1008 1017 1044 1066 1141 1176 1212 1278 1310

955 963 989 1009 1081 1114 1148 1211 1240

1420

1345

1470 1484 1507 1539 1556 1576

1392 1406 1427 1458 1474 1492

1398 1413 1420 1454 1471

1602 1608 1618 1622 1631

1517 1522 1532 1536 1544

1515

uIR

48 60 71 106 117 135 175 214 267 293 361; 370 420 473 529 540 685 774 828 890 926 950 963 1034 1070 1102 1135 1179; 1186 1266 1314 1323 1353

426 476 536 686 777 820 891 926 949

1035 1068 1106 1133 1180; 1188 1272 1318 1326 1359 1368 1390 1417 1459 1470 1503

1753 1802 1809 3171 3184 3191 3247 3248 3258 3281 3294

1661 1706 1713 2886 2898 2904 2955 2956 2964 2985 2997

3636 3737 3771

3309 3400 3431

1545

1628 1678

2880 2893 2924 2940 2961 2973 2985

1596 1625

2880

2940 2963 2975 2986 2998 3057; 3134

Lattice t(O1C7C8N3)[31] þ t(C12C10C8N3)[62] Lattice t(O1C7C8N3)[47] þ t(C12C10C8N3)[27] þ g(C7N3C10C8)[10] t(H4N3C8C10)[28] þ t(H5N3C8C10)[32] þ t(H6N3C8C10)[19] þ g(C7N3C10C8)[12] Lattice d(C7C8N3)[17] þ g(O2C8O1C7)[10] þ g(C7N3C10C8)[33] þ g(C16C8C12C10)[13] t(CH) t(H13C12C10C8)[11] þ t(H14C12C10C8)[29] þ t(H15C12C10C8)[30] d(C8C10C12)[30] þ d(C10C8N3)[28] t(H17C16C10C8)[27] þ t(H18C16C10C8)[18] þ t(H19C16C10C8)[22] d(O1C7C8)[28] þ d(C7C8N3)[39] d(O1C7C8)[11] þ d(C16C10C12)[32] þ d(C8C10C12)[25] d(O1C7C8)[12] þ d(C16C10C12)[26] þ d(C7C8N3)[10] þ d(C10C8N3)[18] v(C7C8)[10] þ g(C16C8C12C10)[48] v(C7C8)[13] þ v(N3C8)[21] þ d(O2C7O1)[16] lib(NOeH) v(C7C8)[11] þ d(O1C7C8)[22] þ g(O2C8O1C7)[11] v(C16C10)[10] þ v(C10C8)[20] þ d(O2C7O1)[30] v(C16C10)[17] þ g(O2C8O1C7)[48] þ g(C7N3C10C8)[11] v(C7C8)[18] þ v(N3C8)[11] þ d(O2C7O1)[16] þ t(H6N3C8C10)[11] v(CC) v(C12C10)[10] þ v(C7C8)[13] þ v(C10C8)[11] þ t(H18C16C10C8)[10] t(H15C12C10C8)[19] þ t(H18C16C10C8)[10] v(C12C10)[22] þ v(C16C10)[14] þ t(H13C12C10C8)[12] þ t(H17C16C10C8)[11] þ t(H19C16C10C8)[10] v(C10C8)[14] þ v(N3C8)[37] v(C12C10)[13] þ v(C16C10)[11] þ t(H5N3C8C10)[20] þ t(H9C8C7O1)[11] d(H4N3H6)[10] þ t(H4N3C8C10)[13] þ t(H6N3C8C10)[22] d(H9C8C7)[25] þ t(H17C16C10C8)[12] þ g(C16C8C12C10)[11] v(C12C10)[12] þ t(H13C12C10C8)[10] t(H14C12C10C8)[11] d(CH2)þ d(CH) d(CH2)þ d(CH) v(C10C8)[11] þ d(H9C8C7)[36] þ d(H11C10C16)[10] v(CC) þ d(CH2) d(H11C10C16)[22] þ t(H9C8C7O1)[12] þ g(C10C8C16H11)[27] v(O2C7)[13] þ v(O1C7)[15] þ d(H11C10C16)[16] þ g(C10C8C16H11)[29] t(H9C8C7O1)[53] d(H13C12H15)[20] þ d(H14C12H13)[20] þ d(H15C12H14)[21] d(H15C12H14)[16] þ d(H17C16H19)[20] þ d(H18C16H17)[15] þ d(H19C16H18)[28] v(O2C7)[10] þ v(O1C7)[29] þ v(C7C8)[11] þ g(C10C8C16H11)[11] d(NH3) d(H13C12H15)[16] þ d(H14C12H13)[19] þ d(H17C16H19)[30] þ d(H18C16H17)[12] d(H13C12H15)[23] þ d(H15C12H14)[33] þ d(H19C16H18)[13] þ t(H15C12C10C8)[10] d(H18C16H17)[31] þ d(H19C16H18)[28] þ t(H18C16C10C8)[11] d(H5N3H4)[11] þ d(H13C12H15)[11] þ d(H14C12H13)[25] þ d(H17C16H19)[16] d(H4N3H6)[10] þ d(H5N3H4)[37] þ d(H14C12H13)[10] d(NH3) d(NH3) d(H4N3H6)[12] þ d(H5N3H4)[27] þ d(H6N3H5)[40] v(O2C7)[17] þ d(H4N3H6)[30] þ d(H6N3H5)[25] þ t(H6N3C8C10)[10] v(O2C7)[30] þ v(O1C7)[21] þ d(H4N3H6)[22] v(C10H11)[93] v(C12H13)[12] þ v(C12H15)[11] þ v(C16H17)[28] þ v(C16H18)[20] þ v(C16H19)[24] v(C12H13)[31] þ v(C12H14)[13] þ v(C12H15)[24] þ v(C16H17)[10] þ v(C16H19)[11] v(C12H13)[30] þ v(C12H15)[32] þ v(C16H17)[23] þ v(C16H18)[12] v(C12H13)[16] þ v(C12H15)[20] þ v(C16H17)[37] þ v(C16H19)[17] v(C16H18)[52] þ v(C16H19)[44] v(C12H14)[80] þ v(C12H15)[10] v(C8H9)[99] v(NH3) v(N3H4)[64] þ v(N3H5)[20] þ v(N3H6)[16] v(N3H4)[35] þ v(N3H5)[22] þ v(N3H6)[43] v(N3H5)[57] þ v(N3H6)[41]

t- torsion, lib.- librational, n- stretching, d- deformation, g- out-of-plane deformation (Only PED values greater that 9% are given).

atributed to mode with high frequency (2800-3200 cm
1). Our calculations indicate that the Raman mode at 2880 cm
1 is practically compose only by the CH stretching, (CH), (93% of the PED). All the other modes (2893, 2924, 2940, 2961, 2973, and 2985 cm
1)

were associated with stretching vibrations of CH3, (CH3). Three IR modes with wavenumber higher than 2986 cm
1 do not have correspondent Raman modes and were atributed to stretching vibrations of NH3 unit.

F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

423

Table 2 Raman calculated wavenumbers unscaled (ucalc) and scaled (uscal) by the dual scale factor 0.9642 (below 1800 cm
1) and 0.91 (above 1800 cm
1). Experimental Raman (uRaman) and IR (uIR) modes and assignment of the vibrational modes for DL-Lysine with PED. All wavenumbers are given in cm
1. Raman

FT-IR

ucalc

uscal

43 56

41 54

87 98 119

84 95 115

178 194

172 187

254 289 298 330 350

245 279 288 318 338

408 548 566 713

393 529 546 688

805 839 874 913 947 974 1029 1036 1046 1077 1108 1150 1172 1202 1237 1275

776 809 843 880 913 939 992 999 1009 1038 1068 1109 1130 1159 1193 1229

1347

1298

1417 1428 1463 1474 1481 1500 1508 1546

1367 1377 1410 1422 1428 1446 1454 1490

1572 1577 1609 1616 1624 1632 1634 1667

1516 1521 1551 1558 1566 1574 1575 1608

1757 1790 1799 1801 1808

1694 1726 1734 1736 1744

3186

2899

uRaman

t(C10C9C8C7)[10] þ t(C9C8C7C6)[65] t(O1C5C6N3)[48] þ t(N3C6C7C8)[31]

53 73 79 84 95 111 126 139 147 162 175 199 215

Lattice Lattice d(C9C8C7)[12] þ t(N4C10C9C8)[54] t(O1C5C6N3)[17] þ t(C10C9C8C7)[41] t(N3C6C7C8)[45] Lattice Lattice Lattice Lattice t(H11N3C6C7)[19] þ t(H12N3C6C7)[35] þ t(H13N3C6C7)[32] d(C8C7C6)[26] þ d(N3C6C7)[12]

d(C9C8C7)[22] þ d(C5C6N3)[16] d(C10C9C8)[38] þ d(N4C10C9)[24] t(H23N4C10C9)[23] þ t(H24N4C10C9)[35] þ t(H25N4C10C9)[29] d(O1C5C6)[37] þ d(C5C6N3)[26] þ g(C5N3C7C6)[13] v(C9C8)[10] þ d(C8C7C6)[16] þ d(N4C10C9)[19] þ g(C5N3C7C6)[10]

265 299 347 405 515 535 674 758 787 806 867; 872 913 929 983 1006 1029 1055 1071 1113 1129 1145 1225 1275 1291 1310 1325 1344 1366

357 410 517 535 672 761 782 802 861; 866 911 927 983 1001 1028 1052 1069 1129 1145 1190 1226 1291 1307 1326; 1331 1345 1368

1423

1421

1456 1470

1456 1471 1481 1515 1530

1524 1556 1571 1595 1619

Assignment

uIR

1564 1577 1597 1614 1638

d(C9C8C7)[15] þ d(N3C6C7)[35] v(N3C6)[10] þ d(C10C9C8)[13] þ d(N4C10C9)[16] v(C5C6)[10] þ d(O2C5O1)[14] þ d(O1C5C6)[13] þ d(C9C8C7)[14] v(C5C6)[16] þ d(O2C5O1)[20] þ d(O1C5C6)[22] d(H15C7C8)[10] þ t(H17C8C9C10)[13] þ g(O2C6O1C5)[12] d(H19C9C10)[10] þ t(H19C9C10N4)[11] þ t(H21C10C9C8)[13] þ g(O2C6O1C5)[11] g(O2C6O1C5)[39] v(C5C6)[16] þ d(O2C5O1)[32] v(C10C9)[15] v(N3C6)[10] þ t(H24N4C10C9)[16] v(C7C6)[21] þ v(C9C8)[11] þ v(C5C6)[13] þ v(N4C10)[14] v(N4C10)[32] v(N3C6)[27] v(C10C9)[17] þ v(C8C7)[13] þ v(N4C10)[11] v(N3C6)[12] þ d(H17C8C9)[10] v(C8C7)[22] þ v(C9C8)[31] t(H12N3C6C7)[11] d(H14C6C7)[22] þ t(H13N3C6C7)[13] d(H14C6C7)[10] þ t(H20C9C10N4)[15] v(C7C6)[11]

d(H17C8C9)[20] d(CH2) d(CH2) d(CH2) d(H19C9C10)[25] þ t(H17C8C9C10)[13] d(H14C6C7)[17] þ t(H18C8C9C10)[26] d(H19C9C10)[22] þ t(H16C7C8C9)[18] d(H15C7C8)[14] þ d(H21C10N4)[16] d(H19C9C10)[10] þ d(H21C10N4)[37] þ t(H21C10C9C8)[11] d(H17C8C9)[11] þ t(H19C9C10N4)[15] þ t(H20C9C10N4)[20] t(H14C6C5O1)[45] v(O1C5)[15] þ t(H15C7C8C9)[11] d(H22C10H21)[13] þ t(H19C9C10N4)[14] þ t(H20C9C10N4)[15] þ t(H22C10C9C8)[19] v(O1C5)[21] þ d(H16C7H15)[23] d(H16C7H15)[13] þ d(H18C8H17)[49] d(H16C7H15)[12] þ d(H20C9H19)[44] þ d(H22C10H21)[24] d(H13N3H12)[17] þ d(H16C7H15)[29] þ d(H18C8H17)[24] d(H11N3H13)[12] þ d(H13N3H12)[33] þ d(H16C7H15)[12] d(H20C9H19)[24] þ d(H22C10H21)[42] þ t(H22C10C9C8)[17] d(H23N4H25)[34] þ d(H24N4H23)[32] þ d(H25N4H24)[32] d(H11N3H13)[19] þ d(H12N3H11)[31] þ d(H13N3H12)[28] d(H23N4H25)[52] þ d(H24N4H23)[13] þ d(H25N4H24)[15] þ t(H23N4C10C9)[10] d(H11N3H13)[33] þ d(H12N3H11)[37] þ t(H11N3C6C7)[12] d(H24N4H23)[41] þ d(H25N4H24)[38] þ t(H24N4C10C9)[13] v(O2C5)[42] þ v(O1C5)[29] þ d(H11N3H13)[10]

2867 2882

2882

v(CH2) v(C7H15)[30] þ v(C7H16)[64] (continued on next page)

424

F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

Table 2 (continued ) Raman

FT-IR

ucalc

uscal

uRaman

uIR

3194 3210 3227 3235 3262 3269 3300

2906 2921 2937 2944 2969 2975 3003

2909 2920 2937 2946 2962 2970 2991

2909

3333 3634 3652 3734 3751 3757 3763

3033 3307 3323 3398 3413 3419 3424

Assignment

v(C9H19)[40] þ v(C9H20)[47] v(C8H17)[24] þ v(C8H18)[64] þ v(C9H20)[10] v(C7H15)[60] þ v(C7H16)[26] þ v(C8H17)[10] v(C9H19)[54] þ v(C9H20)[36] v(C8H17)[60] þ v(C8H18)[21] v(C10H21)[53] þ v(C10H22)[39] v(C6H14)[98] v(NH3) v(C10H21)[41] þ v(C10H22)[57] v(N3H11)[19] þ v(N3H12)[22] þ v(N3H13)[58] v(N4H23)[34] þ v(N4H24)[29] þ v(N4H25)[37] v(N3H11)[38] þ v(N3H12)[21] þ v(N3H13)[41] v(N4H23)[38] þ v(N4H25)[58] v(N4H23)[28] þ v(N4H24)[67] v(N3H11)[42] þ v(N3H12)[57]

2938 2950 2973 3103; 3145

t- torsion, n- stretching, d- deformation, g- out-of-plane deformation (Only PED values greater that 9% are given).

modes between 1300 and 1350 cm
1 do not have scaled modes corresponding to them but they can be classified as CH2 deformations. A possible classification of an IR mode at 1368 cm
1 is a combination of CC stretching and CH2 deformation, based on the values of scaled Raman frequencies of our calculations. In the IR spectrum of a-L-Lysine.HCl a mode at 1361 cm
1 was ascribed as an wagging of CH2, u(CH2). An IR mode at 1638 cm
1does not have any correspondent in the Raman spectra. Petrosyan et al. [35] classified an IR mode in LLysine hydrochloride dihydrated observed at 1634 cm
1 as an antisymmetric stretching of CO2. Our calculations pointed to scaled mode at 1608 and 1694 cm
1 as being prevenient from deformations of NH3, so we will give the two possibilities for this mode. In high frequency region are expected stretching modes of CH, CH2 and CH3 unit. As DL-Lysine has five CH2 unit, only one CH and none CH3 the contribution in this region is practically due to CH2. According to our calculations only the last mode of Raman spectra (2991 cm
1) is attributed to a stretching of CH, n(CH). All other modes (2882, 2909, 2920, 2937, 2946, 2962, and 2970 cm
1)

3.2.2. DL-lysine.HCl Raman spectra of DL-lysine (experimental and calculated) are presented in Fig. 4 and the IR spectra can be seen in Fig. 3. Generally modes with wavenumber lower than 200 cm
1 are classified as lattice modes. According to our calculations some of these modes (53, 84 and 95 cm
1) are complexes ones and the vibrations associated with them have contributions of torsions of CCN and CCO units. For DL-Lysine the first mode not classified as lattice mode was observed at 175 cm
1. It was associated with a torsion of CCNH3 vibration. Skeleton vibrations were attributed to modes observed at 199 and 265 cm
1. The experimental mode at 215 cm
1 did not match any calculated frequency but we believe it can receive the same denomination. Results of references [22,34,35,48] show that Raman modes related to CO2 deformations (rocking, wagging) are observed between 500 and 900 cm
1. Our theoretical results are in accordance with these data and reveal that deformations of CH2, d(CH2) and stretching of CC, n CC), and CN, n(CN) also contribute to these modes. Between 867 and 1113 cm
1 the main contribution for the modes are stretching vibrations of CC and CN. Experimental

(a)

DL-valine

DL-valine

(b)

calculated (scaled)

Raman Intensity

Raman Intensity

calculated(scaled)

experimental

200

400

600

800

1000

1200 -1

Wavenumber (cm )

1400

1600

experimental

2900

3000 -1

Wavenumber (cm )

Fig. 2. Raman spectra of DL-valine (calculated and experimental) in the spectral range 20e1680 cm
1 in (a) and 2800e3200 cm
1 in (b).

F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

425

Absorbance

DL-Lysine.HCl

DL-valine

1000

2000

3000

4000

-1

Wavenumber (cm ) Fig. 3. IR spectra of DL-valine and DL- lysine crystals in the spectral range 400e4000 cm
1.

(a)

DL-lysine.HCl

DL-lysine.HCl

(b)

Raman Intensity

Raman Intensity

calculated(scaled)

calculated(scaled)

experimental experimental

200

400

600

800

1000

1200

1400

1600

-1

2900

3000 -1

Wavenumber (cm )

Wavenumber (cm )

Fig. 4. Raman spectra of DL- lysine (calculated and experimental) in the spectral range 20e1680 cm
1 in (a) and 2800e3200 cm
1 in (b).

were classified as stretching vibrations of CH2, n(CH2). The first mode of this region observed at 2867 cm
1 can also receive the same attribution. Modes in the IR spectra observed at 3103 and 3145 cm
1 can be assigned as stretching vibrations of NH3, since they are in agreement with values observed in the IR spectra of a and b form of L-Lysine hydrochloride [35].

frequencies of the vibrations. The optimized structures match the crystallographic ones. The assignment of the normal modes was done based in the PED as well as in previous articles reporting vibrational properties of amino acids. In conclusion, our work has contributed in a better understanding of the vibrational properties of racemic amino acids.

4. Conclusions

Acknowledgments

Single crystals of DL-Valine and DL-Lysine hydrochloride amino acids were grown and their structures were confirmed by X-ray diffraction experiments and Rietveld refinement. Raman and IR spectra were recorded at room temperature. Ab initio calculations were used to optimize the molecular structure and to obtain the

Authors acknowledge CNPq (Universal 454941/2014-5), and ~o Victor Barbosa and Tiago FUNCAP for financial support and Joa ~o Muniz for the refinement of the crystalline structure. P.T.C. Leita Freire thanks FUNCAP/CNPq for funding from PRONEX PR2-010100006.01.00/15. We also thank CENAPAD-SP for the use of the

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F.M. Paiva et al. / Journal of Molecular Structure 1127 (2017) 419e426

Gaussian 09 software package and for computational facilities made available through the project (proj373).

[27]

Appendix A. Supplementary data [28]

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2016.07.067.

[29]

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