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Tri-l -valine selenate — study of vibrational spectra and structural phase transition
Journal of Molecular Structure 563±564 (2001) 289±294
www.elsevier.nl/locate/molstruc
Tri-l-valine selenate Ð study of vibrational spectra and structural phase transition I. NeÏmec*, Z. MicÏka Department of Inorganic Chemistry, Charles University of Prague, Albertov 2030, 128 40 Prague 2, Czech Republic Received 31 August 2000; accepted 29 September 2000
Abstract The addition compound of l-valine and selenic acid was prepared and characterized as tri-l-valine selenate. FTIR and FT Raman spectra of natural and deuterated compounds were measured and interpreted. The results obtained lead to the conclusion that crystal structure is formed by selenate anions, l-valinium cations and l-valine zwitterions (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds. FTIR spectra were studied down to a temperature of 90 K. DSC measurements of natural and deuterated compounds were carried out in the temperature range 95±333 K. Mechanism of the ®rst-order type low temperature structural phase transition found at 138 K is discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectra; DSC measurements; Phase transition
1. Introduction The addition compounds of amino acids with inorganic oxyacids are a part of our research project focused on study of hydrogen bonded solids potentially exhibiting ferroelectric properties or proton conductivity. Amongst the members of this group that are so far known, only two addition compounds of valine were prepared, dl-valine nitrate [1] and bis(l-valine nitrate)[2]. Tri-l-valine selenate (TVSe) and di-l-valinium selenate monohydrate [3] are two compounds found in l-valine±selenic acid±water system. The submitted work is devoted to the study of TVSe by the methods of vibrational spectroscopy and calorimetry. FTIR measurements down to low temperatures and DSC measurements in a broad temperature * Corresponding author. Tel.: 1420-221-952-362; fax: 1420221-952-378. E-mail address: [email protected] (I. NeÏmec).
interval were carried out to study observed phase transition. 2. Experimental The substance was prepared by slow spontaneous evaporation of a solution of l-valine (99%, Aldrich) in selenic acid (Fluka, 2 mol l 21) (in a molar ratio of 1.5:1) at laboratory temperature. The colourless crystals obtained were collected under vacuum on an S4 frit, washed with ethanol and dried in the air. The deuterated compound ((CH3)2CHND31CHCOOD)2´ (CH3)2CHND31CHCOO 2´SeO422 was prepared by repeated recrystallization of natural TVSe from D2O (99%) in a desiccator over KOH. The contents of carbon (calculated, 36.29%; found, 35.6%), nitrogen (calculated, 8.46%; found, 8.4%), and hydrogen (calculated, 7.11%; found, 7.0%), were determined using a Perkin±Elmer 240 C elemental analyser.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00844-9
290
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
Fig. 1. FTIR (nujol mull) and FT Raman spectra of TVSe.
The infrared spectra of nujol and ¯uorolube mulls were recorded on a Mattson Genesis FTIR spectrometer (2 cm 21 resolution, Beer±Norton medium apodization) in the 400±4000 cm 21 region. Lowtemperature measurements were carried out by the nujol mull method in low-temperature cell with KBr windows in the 298±90 K interval. The temperature was controlled by a Fe±Const. thermocouple. The analogue signal was processed on a PC using the AX5232 temperature measurement board. The Raman spectra of polycrystalline samples were recorded on a Bruker RFS 100 FT Raman spectrometer (2 cm 21 resolution, Blackman±Harris 4-Term apodization, 1064 nm NdYAG laser excitation, 250 mW power at the sample) in the 50±4000 cm 21 region. The DSC measurements were carried out on a Perkin±Elmer DSC 7 power-compensated apparatus in the 95±333 K temperature region (helium or nitrogen atmosphere). A heating rate of 10 K min 21
was selected to measure approximately 10 mg of ®nely ground sample placed in aluminium capsule.
3. Results and discussion 3.1. Vibrational spectra of TVSe The vibrational spectra of TVSe are depicted in Fig. 1. The wavenumbers of the observed bands for natural and deuterated compounds are given in Tables 1 and 2, respectively. Detailed interpretation of the vibrational bands is based on previous vibrational study of l-valine [4±8] and selenate anion [9] and is also consistent with the results obtained for similar compound with sulphuric acid [10]. The results obtained together with the elemental analysis data enabled the conclusion that crystal structure of the compound is formed by selenate anions (SeO422), l-valinium cations
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
291
Table 1 FTIR and FT Raman spectra of TVSe (abbreviations: vs, very strong; s, strong; m, medium; w, weak; b, broad; sh, shoulder; n , stretching; d , deformation or in-plane bending; t , torsional; g , out-of-plane bending; r , rocking; v , wagging; s, symmetric; as, antisymmetric) Assignment
IR
Raman (peak intensity)
298 K
90 K
2n CyO, n N±H, n O±H n N±H¼O, n O±H¼O
3399m 3254m
n asCH3, n Ca H
3060m 2968s 2960s
3386m 3263m 3118m 3080m n.o. a n. o. a
2935s
n.o. a
2881s 2832s
n. o. a 2815m 2791m 2753m 2722m 2670m 2605s 2555s 2525m 2460m 2404m 2166w 1980m 1944m 1904w 1865w 1730m 1707s 1657w 1631m 1612m 1603m 1590m 1582m 1562w 1528s 1474m 1463m 1454m 1405m 1393w 1382w 1357m 1345sh 1329w 1302m 1295m 1281sh 1252s
n C±H n sCH3 ? n N±H¼O, n O±H¼O
2756s 2602m 2550m
? n O±H¼O
2461m 2400m 1960wb 1905wb
n CyO d as NH31
1728m 1705m 1620sh 1602m
n asCOO 2 ? d sNH31 d asCH3 n sCOO 2, d sCH3 n C±O d Ca H d sCH3 d CH d CH, d C±O±H
1585sh 1525s 1463m 1451m 1403w 1395sh 1380sh 1357w 1345sh 1328w 1295m 1249s
Assignment
r NH31, r CH3
2971(82) 2941sh 2933(82) 2904sh 2913(82) 2881(56)
g O±H¼O g OHO, n CC, n CN n C±C, n C±N
r NH31, n CC, n CN
2788(23) 2745(24)
n 3 SeO422 n 1 SeO422 ? d COO, g COH d COO
1730(15) 1704(17)
1604(15)
v COO
r COO
n 4 SeO422, d skelet.
1351(19) 1328(19) 1299(15) 1281(14) 1252(13)
90 K
1120w
1126m 1113w 1103w 1075w 1066w 1053sh 1045w 1028sh
1103w 1066w 1043w 1030sh 1019sh 1009w 976wb 961w 938w 906m 881s 861s 846s 806m 787m 747w
637w 623m 594w 567w
d skelet.
d skelet. n 2 SeO422
Raman (peak intensity)
298 K
685w 660w
1522(13) 1471(29) 1450(30) 1408(13)
IR
533w 508w 447m 424m 410m
1012w 986w 969w 960w 946w 940w 908m 883s 863s 847s 830w 817w 808m 792m 783sh 773w 748w 732w 726w 683w 670w 662w 643w 623m 594w 575w 565w 545w 534w 518w 485w 460m 444m 435m 429sh 423m 412m 402sh
1117(13) 1065(15) 1041(11) 1010(11) 964(12) 942(21) 906(20) 880(23) 858sh 842(100) 822(26) 807(33) 771(21) 744(19) 727(20) 687(7) 657(10) 642(10) 624(10) 565(10) 542(11)
446(18)
409(22) 368(24) 326(24)
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
292 Table 1 (continued) Assignment
r CH3
a
IR
Raman (peak intensity)
298 K
90 K
1193w 1177w 1158w 1138w
1195w 1179w 1161w 1142w
Assignment
IR 298 K
1193(16) 1177(12) 1158(11) 1138(14)
Raman (peak intensity) 90 K
g CH
278(13) 204(15) 95(43) 84(43)
external modes
n.o.: not observed due to nujol bands.
((CH3)2CHNH31CHCOOH) and l-valine zwitterions ((CH3)2CHNH31CHCOO 2) (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds (the similar arrangement was observed in crystal structure of tri-l-valine sulphate [10]). To de®ne
the ionic form l-valine in the crystals, there are the most important bands of stretching vibrations n CyO, n C±O and bending vibrations d C±O±H, g C±O±H (the case of l-valinium cation), and on the other side stretching symmetric and
Table 2 FTIR and FT Raman spectra of deuterated TVSe (abbreviations: vs, very strong; s, strong; m, medium; w, weak; b, broad; sh, shoulder; n , stretching; d , deformation or in-plane bending; t , torsional; g , out-of-plane bending; r , rocking; v , wagging; s, symmetric; as, antisymmetric) Assignment 2n CyO, resid. n N-H, n O±H n asCH3, n Ca H
n CH n sCH3 ? ? n N-D¼O, n O±D¼O n O±D¼O n CyO n asCOO ? d asCH3 n sCOO -, d sCH3 n C-O d Ca H d sCH3 d CH r CH3, d asND31 r CH3
IR 3390wb 2969m 2959m 2933m 2880m 2412m 2210mb 2090mb 1950mb 1725m 1701s 1582m 1525w 1463m 1399m 1380sh 1352m 1342sh 1322m 1289m 1244s 1193m 1176sh 1156m 1119w 1107w
Raman (peak intensity) 2973(83) 2936(65) 2914(77) 2881(46) 2790(8) 2745(10) 2410(5) 1960(4) 1730(8) 1703(8) 1593(3) 1475(23) 1451(27) 1402(9) 1380(6) 1347(11) 1329(15) 1296(9) 1279(7) 1253(7) 1189(9) 1141(6) 1126(6) 1110(4)
Assignment n C±C, n C±N,d sND31
n C±C, n C±N n 3 SeO42n 3 SeO42-, g C±O±D n 3 SeO42n 1 SeO42d COO, r ND31 v COO, g O±D¼O v COO, r ND31 r COO r COO, g C±O±D r COO d skelet. n 4 SeO42n 2 SeO42g CH external modes
IR 1080w 1062w 1029w 904m 888m 877sh 865s 847s 803sh 789s 740sh 730w 676w 637sh 609w 564w 521w 445s 421m 410m
Raman (peak intensity) 1073(2) 1025(5) 970(4) 940(4) 908(17) 895(16) 882(15) 869(16) 847(100) 810(14) 781(13) 747(16) 727(14) 638(5) 619(5) 524(9) 474(7) 447(9) 412(13) 369(17) 332(13) 210(3) 98(34) 86(49)
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
293
Fig. 2. FTIR spectra (nujol mull) of TVSe at different temperatures.
antisymmetric vibrations of COO 2 group (the case of l-valine zwitterion). Broad, medium to strong-intensity bands in the IR spectra in the 3400±2300 cm 21 region are character-
istic for stretching vibrations of N±H and O±H groups involved in H-bonds system. According to Novak et al. [11,12] the length of these hydrogen bonds can be Ê for O±H¼O type and estimated as 2.58±2.67 A
294
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
Ê for N±H¼O type. The well-localized 2.67±2.87 A vibrational manifestations of short O±H¼O bonds at ca. 1930 cm 21 correspond to O(H)¼O distance of Ê . In the IR spectrum of deuterated TVSe, the 2.53 A position of this band practically does not change and the n OH/n OD ratio equals 1. Such a behaviour of stretching vibrations, together with the fact that the ratio of the band ppositions of g OHO/g ODO is close to the value of 2; is characteristic for the `positive isotopic effect` [11], giving evidence that the potential energy curve of the short H-bonds is of the asymmetric two-minimum type.
terions (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds. The low-temperature structural phase transition was studied by the methods of FTIR spectroscopy and DSC. The minimal changes in the FTIR spectra observed on a decrease in the temperature together with the DSC measurements results lead to the conclusion that the TVSe phase transition mechanism is based mainly on minor changes of amino acid hydrocarbon skeleton geometry or ordering, i.e. without structural changes related to the polar groups and thus the system of hydrogen bonds.
3.2. Thermal behaviour of TVSe Crystals of TVSe are stable in the air up to a temperature of ca. 338 K, where they start to gradually decompose. The only phase transition was found by DSC method at low temperature DH 297:9 J mol21 ; heating Ð onset 138 K; cooling Ð onset 137 K). The phase transition is very likely to be of the ®rst-order type (according to peak shape). The fact that the deuterated sample exhibits only decrease of DH value DH 124:1 J mol21 ; heating Ð onset 140 K) indicates that the phase transition mechanism is not affected by H-bonds network. The low-temperature (below 140 K) spectra of TVSe (see Fig. 2) exhibit only partial separation of the bands connected with minimal shifting. These observations together with the DSC measurements results lead to the conclusion that TVSe phase transition is probably connected only with minor changes of amino acid hydrocarbon skeleton geometry or ordering. 4. Conclusions The study of TVSe vibrational spectra enabled to de®ne that crystal structure of compound is formed by selenate anions, l-valinium cations and l-valine zwit-
Acknowledgements This work was supported by the Grant Agency of Charles University of Prague (Grant No. 13/98/B CH) and the Grant Agency of the Czech Republic (Grant No. 203/98/1198). References [1] S.N. Rao, R. Parthasarathy, A.C.A. Spring (1974) 129. [2] N. Srinivasan, P.K. Rajaram, D.D. JebaRaj, Z. Kristallogr. 212 (4) (1997) 313. [3] I. NeÏmec, R. Gyepes, Z. MicÏka, submitted for publication. [4] U. Stahlberg, E. Steger, Spectrochem. Acta 23A (1967) 475. [5] A.R. Gargaro, L.D. Barron, L. Hecht, J. Raman Spectrosc. 24 (1993) 91. [6] A. Pawlukojc, L. Bobrowicz, I. Natkaniec, Spectrochem. Acta 51A (1995) 303. [7] L.A. Na®e, M.R. Oboodi, T.B. Freedman, J. Am. Chem. Soc. 105 (1983) 7449. [8] L. Burman, P. Tandon, V.D. Gupta, S. Rastogi, Polym. J 28 (6) (1996) 474. [9] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley, New York, 1986. [10] I. NeÏmec, I. CõÂsa ovaÂ, Z. MicÏka, submitted for publication. [11] A. Novak, Struct. Bonding 18 (1974) 177. [12] A. Lautie, F. Froment, A. Novak, Spectrosc. Lett. 9 (5) (1976) 289.
www.elsevier.nl/locate/molstruc
Tri-l-valine selenate Ð study of vibrational spectra and structural phase transition I. NeÏmec*, Z. MicÏka Department of Inorganic Chemistry, Charles University of Prague, Albertov 2030, 128 40 Prague 2, Czech Republic Received 31 August 2000; accepted 29 September 2000
Abstract The addition compound of l-valine and selenic acid was prepared and characterized as tri-l-valine selenate. FTIR and FT Raman spectra of natural and deuterated compounds were measured and interpreted. The results obtained lead to the conclusion that crystal structure is formed by selenate anions, l-valinium cations and l-valine zwitterions (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds. FTIR spectra were studied down to a temperature of 90 K. DSC measurements of natural and deuterated compounds were carried out in the temperature range 95±333 K. Mechanism of the ®rst-order type low temperature structural phase transition found at 138 K is discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectra; DSC measurements; Phase transition
1. Introduction The addition compounds of amino acids with inorganic oxyacids are a part of our research project focused on study of hydrogen bonded solids potentially exhibiting ferroelectric properties or proton conductivity. Amongst the members of this group that are so far known, only two addition compounds of valine were prepared, dl-valine nitrate [1] and bis(l-valine nitrate)[2]. Tri-l-valine selenate (TVSe) and di-l-valinium selenate monohydrate [3] are two compounds found in l-valine±selenic acid±water system. The submitted work is devoted to the study of TVSe by the methods of vibrational spectroscopy and calorimetry. FTIR measurements down to low temperatures and DSC measurements in a broad temperature * Corresponding author. Tel.: 1420-221-952-362; fax: 1420221-952-378. E-mail address: [email protected] (I. NeÏmec).
interval were carried out to study observed phase transition. 2. Experimental The substance was prepared by slow spontaneous evaporation of a solution of l-valine (99%, Aldrich) in selenic acid (Fluka, 2 mol l 21) (in a molar ratio of 1.5:1) at laboratory temperature. The colourless crystals obtained were collected under vacuum on an S4 frit, washed with ethanol and dried in the air. The deuterated compound ((CH3)2CHND31CHCOOD)2´ (CH3)2CHND31CHCOO 2´SeO422 was prepared by repeated recrystallization of natural TVSe from D2O (99%) in a desiccator over KOH. The contents of carbon (calculated, 36.29%; found, 35.6%), nitrogen (calculated, 8.46%; found, 8.4%), and hydrogen (calculated, 7.11%; found, 7.0%), were determined using a Perkin±Elmer 240 C elemental analyser.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00844-9
290
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
Fig. 1. FTIR (nujol mull) and FT Raman spectra of TVSe.
The infrared spectra of nujol and ¯uorolube mulls were recorded on a Mattson Genesis FTIR spectrometer (2 cm 21 resolution, Beer±Norton medium apodization) in the 400±4000 cm 21 region. Lowtemperature measurements were carried out by the nujol mull method in low-temperature cell with KBr windows in the 298±90 K interval. The temperature was controlled by a Fe±Const. thermocouple. The analogue signal was processed on a PC using the AX5232 temperature measurement board. The Raman spectra of polycrystalline samples were recorded on a Bruker RFS 100 FT Raman spectrometer (2 cm 21 resolution, Blackman±Harris 4-Term apodization, 1064 nm NdYAG laser excitation, 250 mW power at the sample) in the 50±4000 cm 21 region. The DSC measurements were carried out on a Perkin±Elmer DSC 7 power-compensated apparatus in the 95±333 K temperature region (helium or nitrogen atmosphere). A heating rate of 10 K min 21
was selected to measure approximately 10 mg of ®nely ground sample placed in aluminium capsule.
3. Results and discussion 3.1. Vibrational spectra of TVSe The vibrational spectra of TVSe are depicted in Fig. 1. The wavenumbers of the observed bands for natural and deuterated compounds are given in Tables 1 and 2, respectively. Detailed interpretation of the vibrational bands is based on previous vibrational study of l-valine [4±8] and selenate anion [9] and is also consistent with the results obtained for similar compound with sulphuric acid [10]. The results obtained together with the elemental analysis data enabled the conclusion that crystal structure of the compound is formed by selenate anions (SeO422), l-valinium cations
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
291
Table 1 FTIR and FT Raman spectra of TVSe (abbreviations: vs, very strong; s, strong; m, medium; w, weak; b, broad; sh, shoulder; n , stretching; d , deformation or in-plane bending; t , torsional; g , out-of-plane bending; r , rocking; v , wagging; s, symmetric; as, antisymmetric) Assignment
IR
Raman (peak intensity)
298 K
90 K
2n CyO, n N±H, n O±H n N±H¼O, n O±H¼O
3399m 3254m
n asCH3, n Ca H
3060m 2968s 2960s
3386m 3263m 3118m 3080m n.o. a n. o. a
2935s
n.o. a
2881s 2832s
n. o. a 2815m 2791m 2753m 2722m 2670m 2605s 2555s 2525m 2460m 2404m 2166w 1980m 1944m 1904w 1865w 1730m 1707s 1657w 1631m 1612m 1603m 1590m 1582m 1562w 1528s 1474m 1463m 1454m 1405m 1393w 1382w 1357m 1345sh 1329w 1302m 1295m 1281sh 1252s
n C±H n sCH3 ? n N±H¼O, n O±H¼O
2756s 2602m 2550m
? n O±H¼O
2461m 2400m 1960wb 1905wb
n CyO d as NH31
1728m 1705m 1620sh 1602m
n asCOO 2 ? d sNH31 d asCH3 n sCOO 2, d sCH3 n C±O d Ca H d sCH3 d CH d CH, d C±O±H
1585sh 1525s 1463m 1451m 1403w 1395sh 1380sh 1357w 1345sh 1328w 1295m 1249s
Assignment
r NH31, r CH3
2971(82) 2941sh 2933(82) 2904sh 2913(82) 2881(56)
g O±H¼O g OHO, n CC, n CN n C±C, n C±N
r NH31, n CC, n CN
2788(23) 2745(24)
n 3 SeO422 n 1 SeO422 ? d COO, g COH d COO
1730(15) 1704(17)
1604(15)
v COO
r COO
n 4 SeO422, d skelet.
1351(19) 1328(19) 1299(15) 1281(14) 1252(13)
90 K
1120w
1126m 1113w 1103w 1075w 1066w 1053sh 1045w 1028sh
1103w 1066w 1043w 1030sh 1019sh 1009w 976wb 961w 938w 906m 881s 861s 846s 806m 787m 747w
637w 623m 594w 567w
d skelet.
d skelet. n 2 SeO422
Raman (peak intensity)
298 K
685w 660w
1522(13) 1471(29) 1450(30) 1408(13)
IR
533w 508w 447m 424m 410m
1012w 986w 969w 960w 946w 940w 908m 883s 863s 847s 830w 817w 808m 792m 783sh 773w 748w 732w 726w 683w 670w 662w 643w 623m 594w 575w 565w 545w 534w 518w 485w 460m 444m 435m 429sh 423m 412m 402sh
1117(13) 1065(15) 1041(11) 1010(11) 964(12) 942(21) 906(20) 880(23) 858sh 842(100) 822(26) 807(33) 771(21) 744(19) 727(20) 687(7) 657(10) 642(10) 624(10) 565(10) 542(11)
446(18)
409(22) 368(24) 326(24)
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
292 Table 1 (continued) Assignment
r CH3
a
IR
Raman (peak intensity)
298 K
90 K
1193w 1177w 1158w 1138w
1195w 1179w 1161w 1142w
Assignment
IR 298 K
1193(16) 1177(12) 1158(11) 1138(14)
Raman (peak intensity) 90 K
g CH
278(13) 204(15) 95(43) 84(43)
external modes
n.o.: not observed due to nujol bands.
((CH3)2CHNH31CHCOOH) and l-valine zwitterions ((CH3)2CHNH31CHCOO 2) (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds (the similar arrangement was observed in crystal structure of tri-l-valine sulphate [10]). To de®ne
the ionic form l-valine in the crystals, there are the most important bands of stretching vibrations n CyO, n C±O and bending vibrations d C±O±H, g C±O±H (the case of l-valinium cation), and on the other side stretching symmetric and
Table 2 FTIR and FT Raman spectra of deuterated TVSe (abbreviations: vs, very strong; s, strong; m, medium; w, weak; b, broad; sh, shoulder; n , stretching; d , deformation or in-plane bending; t , torsional; g , out-of-plane bending; r , rocking; v , wagging; s, symmetric; as, antisymmetric) Assignment 2n CyO, resid. n N-H, n O±H n asCH3, n Ca H
n CH n sCH3 ? ? n N-D¼O, n O±D¼O n O±D¼O n CyO n asCOO ? d asCH3 n sCOO -, d sCH3 n C-O d Ca H d sCH3 d CH r CH3, d asND31 r CH3
IR 3390wb 2969m 2959m 2933m 2880m 2412m 2210mb 2090mb 1950mb 1725m 1701s 1582m 1525w 1463m 1399m 1380sh 1352m 1342sh 1322m 1289m 1244s 1193m 1176sh 1156m 1119w 1107w
Raman (peak intensity) 2973(83) 2936(65) 2914(77) 2881(46) 2790(8) 2745(10) 2410(5) 1960(4) 1730(8) 1703(8) 1593(3) 1475(23) 1451(27) 1402(9) 1380(6) 1347(11) 1329(15) 1296(9) 1279(7) 1253(7) 1189(9) 1141(6) 1126(6) 1110(4)
Assignment n C±C, n C±N,d sND31
n C±C, n C±N n 3 SeO42n 3 SeO42-, g C±O±D n 3 SeO42n 1 SeO42d COO, r ND31 v COO, g O±D¼O v COO, r ND31 r COO r COO, g C±O±D r COO d skelet. n 4 SeO42n 2 SeO42g CH external modes
IR 1080w 1062w 1029w 904m 888m 877sh 865s 847s 803sh 789s 740sh 730w 676w 637sh 609w 564w 521w 445s 421m 410m
Raman (peak intensity) 1073(2) 1025(5) 970(4) 940(4) 908(17) 895(16) 882(15) 869(16) 847(100) 810(14) 781(13) 747(16) 727(14) 638(5) 619(5) 524(9) 474(7) 447(9) 412(13) 369(17) 332(13) 210(3) 98(34) 86(49)
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
293
Fig. 2. FTIR spectra (nujol mull) of TVSe at different temperatures.
antisymmetric vibrations of COO 2 group (the case of l-valine zwitterion). Broad, medium to strong-intensity bands in the IR spectra in the 3400±2300 cm 21 region are character-
istic for stretching vibrations of N±H and O±H groups involved in H-bonds system. According to Novak et al. [11,12] the length of these hydrogen bonds can be Ê for O±H¼O type and estimated as 2.58±2.67 A
294
I. NeÏmec, Z. MicÏka / Journal of Molecular Structure 563±564 (2001) 289±294
Ê for N±H¼O type. The well-localized 2.67±2.87 A vibrational manifestations of short O±H¼O bonds at ca. 1930 cm 21 correspond to O(H)¼O distance of Ê . In the IR spectrum of deuterated TVSe, the 2.53 A position of this band practically does not change and the n OH/n OD ratio equals 1. Such a behaviour of stretching vibrations, together with the fact that the ratio of the band ppositions of g OHO/g ODO is close to the value of 2; is characteristic for the `positive isotopic effect` [11], giving evidence that the potential energy curve of the short H-bonds is of the asymmetric two-minimum type.
terions (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds. The low-temperature structural phase transition was studied by the methods of FTIR spectroscopy and DSC. The minimal changes in the FTIR spectra observed on a decrease in the temperature together with the DSC measurements results lead to the conclusion that the TVSe phase transition mechanism is based mainly on minor changes of amino acid hydrocarbon skeleton geometry or ordering, i.e. without structural changes related to the polar groups and thus the system of hydrogen bonds.
3.2. Thermal behaviour of TVSe Crystals of TVSe are stable in the air up to a temperature of ca. 338 K, where they start to gradually decompose. The only phase transition was found by DSC method at low temperature DH 297:9 J mol21 ; heating Ð onset 138 K; cooling Ð onset 137 K). The phase transition is very likely to be of the ®rst-order type (according to peak shape). The fact that the deuterated sample exhibits only decrease of DH value DH 124:1 J mol21 ; heating Ð onset 140 K) indicates that the phase transition mechanism is not affected by H-bonds network. The low-temperature (below 140 K) spectra of TVSe (see Fig. 2) exhibit only partial separation of the bands connected with minimal shifting. These observations together with the DSC measurements results lead to the conclusion that TVSe phase transition is probably connected only with minor changes of amino acid hydrocarbon skeleton geometry or ordering. 4. Conclusions The study of TVSe vibrational spectra enabled to de®ne that crystal structure of compound is formed by selenate anions, l-valinium cations and l-valine zwit-
Acknowledgements This work was supported by the Grant Agency of Charles University of Prague (Grant No. 13/98/B CH) and the Grant Agency of the Czech Republic (Grant No. 203/98/1198). References [1] S.N. Rao, R. Parthasarathy, A.C.A. Spring (1974) 129. [2] N. Srinivasan, P.K. Rajaram, D.D. JebaRaj, Z. Kristallogr. 212 (4) (1997) 313. [3] I. NeÏmec, R. Gyepes, Z. MicÏka, submitted for publication. [4] U. Stahlberg, E. Steger, Spectrochem. Acta 23A (1967) 475. [5] A.R. Gargaro, L.D. Barron, L. Hecht, J. Raman Spectrosc. 24 (1993) 91. [6] A. Pawlukojc, L. Bobrowicz, I. Natkaniec, Spectrochem. Acta 51A (1995) 303. [7] L.A. Na®e, M.R. Oboodi, T.B. Freedman, J. Am. Chem. Soc. 105 (1983) 7449. [8] L. Burman, P. Tandon, V.D. Gupta, S. Rastogi, Polym. J 28 (6) (1996) 474. [9] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley, New York, 1986. [10] I. NeÏmec, I. CõÂsa ovaÂ, Z. MicÏka, submitted for publication. [11] A. Novak, Struct. Bonding 18 (1974) 177. [12] A. Lautie, F. Froment, A. Novak, Spectrosc. Lett. 9 (5) (1976) 289.