Vibrational spectroscopic study of (NH2CH2COOH)2CdCl2 crystal

J o u r n a l of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 446 (1998) 109-113

Vibrational spectroscopic study of (NH2CH2COOH)2CdC12crystal Jianjun Liu a'*, Yue Wu a, Shifen Hu a, Guoxiang Lan a, Jimin Zheng b aDepartment of Physics, Nankai University, Tianjin 300071, People's Republic of China bDepartrnent of Chemistry, Nankai University, Tianjin 300071, People's Republic of China Received 7 August 1997; revised 25 November 1997; accepted 25 November 1997

Abstract The Raman and the infrared absorption spectra of diglycine cadmium chloride (NH2CH2COOH)2CdC12 crystal have been recorded. The observed vibrational bands are assigned to the internal vibrations of glycine molecules. The vibrational spectra show that the glycine molecule is in the form of the zwitterion in the crystal. © 1998 Elsevier Science B.V.

Keywords: Infrared absorption spectra; Glycine molecule; Mode assignment ; Raman spectra; Zwitterion

1. Introduction Organic optical materials have attracted much attention in recent years. Diglycine cadmium chloride (NH2CH2COOH)2CdC12 (DGCC) is a new organic single crystal grown by one of us. In this paper the Raman spectra of D G C C crystal at liquid nitrogen temperature ( 8 0 K ) and the infrared absorption spectra at room temperature are reported. The observed lines are assigned.

of an argon ion laser operating with an output of 200 mW. The sample was mounted inside the vacuum chamber of an Air Products refrigerator. The Raman spectra were measured over the wavenumber shift range 2 0 - 3 5 0 0 cm -~ with scanning steps of 2 cm -1 in the scattering configurations Y(xz)X, Y(xy)X corresponding to Ag and Bg modes, respectively. The infrared absorption spectra were recorded at room temperature on a Nicolet 710 F I ' - I R spectrometer in the range 4 0 0 - 4 0 0 0 cm -l using KBr pellets.

2. Experimental

3. Results and discussion

The DGCC crystal was grown by the cooling method in a saturated aqueous solution. The crystal structure was determined by X-ray diffraction. The polarized Raman spectra of D G C C were recorded with a SPEX-1403 double monochromator. The incident radiation was provided by the 488 nm line

The crystal structure of D G C C was determined by use of a four-circle diffractometer, and detailed results will be published elsewhere. The DGCC crystal is monoclinic and belongs to the space group P2~/n, There are four formula units (92 atoms in all) in the primitive cell. Therefore, a total of 276 normal modes of vibrations are expected for DGCC. The factor group analysis gives the following symmetry

* Corresponding author.

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S0022-2860(97)00403- 1

I l0

J. Liu et al./Journal of Molecular Structure 446 (1998) 109-113

Y(xz)X A~.~

. j_a

.F,I

J

~

, Y[xy)X Bg

~ 250

500

750

1000

1250

1500

Rarnan Shift (cm -1) Fig. I. Polarized Raman spectra of DGCC crystal at 80 K, 0-1700cm t. classification: r = 69Ag + 69Bg + 69A, + 6 9 B u.

Of these, A, + 2B, are three acoustic modes, all gerade optical modes are Raman active and all ungerade optical modes are IR active.

The polarized Raman spectra were recorded at room temperature and liquid nitrogen temperature (80 K), respectively. Figs. 1 and 2 are the polarized Raman spectra of DGCC crystal taken at 80 K. The Raman spectrum of DGCC at 80 K is identical to that at room temperature, which shows that no structural phase transition occurs in this temperature range. The infrared absorption spectra at room temperature is shown in Fig. 3. They are in agreement with the

Raman spectra of DGCC single crystal. The number of observed vibrational bands is less than predicted by group theory because of the weak intensity and overlapping of some lines. There exist eight glycine molecules and 12 monoatomic ions (four Cd 2+ ion and eight C1- ion) in DGCC crystal. It is reasonable to recognize that the interaction forces inside the glycine molecules are stronger than the forces between the glycine molecules and the monoatomic ions. The vibrational spectra of DGCC can be regarded as arising from the internal vibrations of the glycine molecules plus the external vibrations. The internal vibrations of the glycine molecule, relying on the common viewpoint of chemical structural analysis are contributed to

'7,

2750

2875 3000 3125 Rsman Shift (era -l)

3250

Fig. 2. Polarized Raman spectra of DGCC crystal at 80 K, 2750-3250 cm-I.

J. Liu et al./Journal of Molecular Structure 446 (1998) 109-113

111

N O0

8

r~ N

0

oo

1~ao

l~so

1t9o

~2o

~5o

~8o

Wavenumber(cm -1) Fig. 3. Infrared absorptionspectra of DGCC crystal at room temperature (KBr pellet). internal modes of CH2 and NH2 (or NH3 if glycine is in the zwitterion form) groups and by the glycine molecule skeleton itself. By comparing with the vibrational spectra of triglycine sulphate (TGS) and triglycine selenate (TGSe) [1], diglycine selenate (DGSe) [2], and the t~ form of glycine crystal [3], we assign all the observed high frequency lines to the vibrations of characteristic groups. The observed vibrational modes of DGCC are given in Table 3 together with the tentative assignments. Group theoretical analysis predicts that the CH2 isolated group has 12Ag + 12Bg Raman active modes in the DGCC crystal, and each of the vibrations of CH2 should split into two lines (Table 1). Most of the CH2 vibrations have been observed in the Raman spectra of the DGCC crystal. The two sharp and strong bands at 2964 and 2954 cm -I are attributed to the symmetric stretching vibration (Vs) and the

3000 cm -~ band is related to the asymmetric stretching vibrations (va) of the CH2 group. The Raman bands at 1454 and 1496 cm -~ in Ag mode and 1454 and 1504 cm -l in Bg mode are assigned to CH2 bending (6), the bands at 1334 and 1346 cm -1 to CH2 wagging (w) and the bands at 1308 and 1328 cm -j to CH2 twisting (t). The rocking modes (r) of CH2 appear at 908 and 942 cm -I. Glycine often crystallizes in the form of the zwitterion, e.g. in TGS [1] and the t~ form crystal of glycine [3]. Our structure analysis by X-ray diffraction cannot determine the form of glycine in the DGCC crystal. This problem can be solved by analysing the vibrational spectra of DGCC. If glycine is in the form of zwitterion, there should be an NH3 group in DGCC. Group theory predicts that there are 36 Raman active modes arising from NH3 group. The degenerate deformation vibration 6d(E), asymmetric

Table 1 Correlation of the intemal modes in DGCC with the modes of the isolated CH2 or NH2 group

Table 2 Correlation of the internal modes in DGCC with the modes of the isolated NH 3 group

Free group C2,.

Free group C3~.

Site C t

8V s

A I

86 8va 8w

AI B2 BI

8t

A2

8r

B2

A

Crystal Ceh Ag Be A, B,

2v,, 26, 2va, 2w, 2t, 2r 2vs, 26, 2v~, 2w, 2t, 2r 2v,, 26, 2Va,2w, 2t, 2r 2v~, 26, 2Va,2w, 2t, 2r

v = stretchingvibration,6 = bendingor deformationvibration,w = wagging vibration, r = rocking vibration, t = twisting vibration, s = symmetric, a = asymmetric.

Site C~

Crystal C2h

8Vs 8 6~

Ai AI

8 v~

E

8 6d

E

A.

8r

E

B.

8t

A2

Ae A

Be

2v~, 26~,4Va,46d, 4r, 2t 2v~,26~,4 Va,46d,4r, 2t 2v~, 26~ 4v~, 46d, 4r, 2t 2v~,26~, 4v~, 46d, 4r, 2t

v = stretching vibration, 6 = bending or deformation vibration, r = rocking vibration, t = twisting vibration, s = symmetric, a = asymmetric, d = degenerate.

J. Liu et aL/Journal o f Molecular Structure 446 (1998) 109-113

112

Table 3 Frequency (in c m -~) and a s s i g n m e n t s o f the vibrational bands for D G C C crystals Assignments h

W a v e n u m b e r (cm -~) and intensity a R a m a n lines

Infrared lines

Y(xz)X:A g m o d e s

Y(xy)X:B u modes

40 60 72 82

40 60 70 84

vs s s vw

96 w 100 w 124 s 154 w (sh) 162 s

172 w 184 v w 196 208 230 258

m w s s

308 w 344 s 496 514 530 594 678

w v w s w

s m w vw

94 s 100 vw (sh) 122 m 132 w 144m 162 w 174 v w

External modes

186 w 196 m 208 s 224 m 260 w 286 w 306 m 334 w 344w 494 w 538 w 588 w 686 w

513 531 590 674

s s m s

698 m 894 s 908 s 942 w 1030 w 1056 w 1110 w 1144 w 1174w 1308 s 1328 v w 1336 w 1426 s 1454 w 1496 1530 1548 1590 1602

m w w vw w

1618 v w 1630 v w

910w 1030 1056 1112 1140

m w w w

1308 w 1334 1346 1424 1434 1454

s m m w w

1504 w

910 m (sh) 935 m 1026 m 1043 w lll0s 1116 w 1133w 1153 1301 w 1330 s 1339 s (sh) 1410 s 1442 s 1473 s 1489 s 1532 w

1602 w

1570 s (sh) 1607 s

1640 v w 1672 v w

1652 s (sh) 1673 s(sh)

C-C C-C C-C COO COO COO COO COO COO C-C

6 6 r r /w

v

CH2 C-N C-N C-N

r

NH3 NH3 NH3

F

NH3 CH2

v v v

F r r

t

CH2 CH2

t

CH2 C-O

W

C-O CH2 CH2 CH2 COO COO NH3 NH3

V

NH~

5

NH3 C=O

V

W

v

b 6 V

5

J. Liu et al./Journal of Molecular Structure 446 (1998) 109-113

113

Table 3 (continued) Assignmentsb

Wavenumber (cm -~) and intensitya Raman lines

Infrared lines

Y(xz)X:A x modes

Y(xy)X:B e modes

2954 s 2964 s 3000 m

2954 w 2964 w 3000 s 3058 w 3078 w

3114 w

2957 w 2992 w 3004 w 3098 w 3151 w

CH2 CH2 CH2 NH3 NH3 NH3

v~ v~ va v v v

avs = very strong, s = strong, m = medium, w = weak, vw = very weak, sh = shoulder. bv = stretching, 6 = bending or deformation, w = wagging, r = rocking, t = twisting, s = symmetric, a = asymmetric. stretching vibration va(E) and rocking vibration r(E) of NH 3 group should split into four lines in the D G C C crystal (Table 1). Otherwise, there should be an NH2 group in DGCC. According to the group theoretical analysis (Table 2), all the internal modes of isolated the NH2 group should only split into two bands. In the absorption spectra of DGCC, the four bands at 1110, 1116, 1133 and 1159 c m -1 clearly come from the split of one broad absorption band. The rocking modes of NH3 group in the ot form crystal of glycine are observed in the same wavenumber region [3]. Thus, these four absorption bands are due to the rocking modes of the NH3 group. They appear at 1110, 1144 and 1174 cm -~ for the A g mode in the Raman spectrum. From these data one can draw the conclusion that glycine takes the zwitterionic structure in the D G C C crystal. The bending vibrations (6) o f the NH3 groups occur at 1590, 1602, 1616 and 1630 cm -~ for the A e mode and at 1602, 1618 and 1640 cm -1 for the B e mode. The band above 3 0 5 0 c m -~ is attributed to the N - H stretching vibration (v) of NH3 groups. The stretching vibration o f the C = O bond in a nonionized C O O H usually appears in the frequency region 1 7 5 5 - 1 8 0 0 c m -1 [4]. This vibration band is always found at lower wavenumber on ionization of the carboxyl group [5]. For the D G C C crystal them are no Raman bands in the 1 7 0 0 - 1 8 0 0 cm -I region. The Raman band at 1672 cm -l of the B e mode and 1673 cm -~ in infrared spectra can be assigned to the C O 0 - stretching vibrations. This is further evidence of the existence o f the glycine zwitterion in the D G C C crystal. The band at 1047 cm -1 in the Raman spectra of the

ot form crystal of glycine is attributed to the C - N stretching vibration [3]. Therefore, in the crystal investigated, the bands at 1030 and 1056 cm -l in the Raman spectra and the bands at 1026 and 1043 cm -~ in the infrared spectra can be assigned to the C - N stretching vibration. The bands at lower wavenumber i.e. in the region below 300 cm -l are assigned to the lattice variations of glycine, Cd + and C1- ions. Some of the remaining internal vibrational lines can be tentatively related to the characteristic vibrations of C O 0 , C - C . The assigmnents are reported in Table 3.

4. Conclusion The mid to high frequency vibrational lines of D G C C crystal can be essentially regarded as the internal vibrations of glycine molecules. The features of the Raman and infrared spectra show that the glycine molecule takes the zwitterionic struture in the D G C C crystal.

References [ 1] V. Winterfeldt, G. Schaack, A. Kloepperpieper, Ferroelectrics 15 (1977) 21. [2l G. Lan, H. Wang, J. Zheng, SpectrochimicaActa 46A (1990) 1211. [3] K. Machida, A. Kagayama, Y. Salto, Y. Kuroda, T. Uno, Spectrochimica Acta 33A (1977) 569. [4] J.F. Pearson, M.A. Slifkin, Spectrochimica Acta 28A (1972) 2403. [5] M. Drozdowski, M. Kozielski, S. Labuz, SpectrochimicaActa 42A (1986) 833.