Vibrational spectra of thiourea and N-methyl thiourea at low temperature: effect of hydrogen bonding and methylation

Journal of Molecular Structure, 176 (1988) 203-211

203

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

V I B R A T I O N A L S P E C T R A OF T H I O U R E A A N D N - M E T H Y L T H I O U R E A AT LOW T E M P E R A T U R E : E F F E C T OF H Y D R O G E N BONDING AND METHYLATION

PRABHAT K. PANJA*, SIB SANKAR BALA and PRADIP N. GHOSH**

Department of Physics, University of Calcutta, 92, A.P.C. Road, Calcutta 700009 (India) (Received 21 September 1987 )

ABSTRACT Study of IR spectra of thiourea and N-methylthiourea at various temperatures down to 120 K confirms the earlier finding that replacement of a hydrogen atom of the amide group of thiourea by a methyl group leads to hydrogen bonding. The change of lineshape on cooling shows that cooling leads to stronger hydrogen bonding but it does not affect the methyl torsional frequency.

INTRODUCTION

A recent study on the low temperature vibrational spectra of acetamide and thioacetamide demonstrated that the molecules containing both CH3 and NH2 groups show a number of interesting features [ 1 ]. The change in lineshape of the Fermi resonance (FR) bands upon cooling the sample shows the importance of the dynamic part of the interaction [ 2 ]. The vibrational spectrum of thiourea at room temperature shows a number of FR diads and triads, but the vibrational bands in strongly hydrogen bonded urea are very broad and they show a few hot bands [ 3 ]. In another recent investigation we have shown that on methylation thiourea shows hydrogen bonding [ 4 ]. Such hydrogen bonding arises from a hyperconjugative effect. Thus the presence of weak hydrogen bonding, a methyl torsional mode and the expected FR bands as found in the case of thiourea, makes the study of the vibrational spectra of N-methylthiourea at low temperature very interesting. In this work we report the IR spectra of thiourea and N-methylthiourea at five different temperatures. A study of the IR spectra of urea at low temperatures [5] does not show any additional feature except for a general sharpening of most of the bands. The vibrational spectra of thiourea and N-methylthiourea at room temper*Permanent Address: Department of Chemistry, Hooghly Mohsin College, Chinsurah, Hooghly ( West Bengal), India. **Career Awardee of the University Grants Commission, Government of India.

0022-2860/88/$03.50

© 1988 Elsevier Science Publishers B.V.

204

ature have been reported by a number of researchers [ 6-13 ]. Normal coordinate analysis of N-methylthiourea [ 11 ] shows that two NH stretching modes have a large deviation from the observed data while the CH stretching modes could be fitted well. This deviation may arise from the anharmonicity associated with hydrogen bonding. Our low temperature studies are particularly addressed to the nature of modification of lineshape associated with hydrogen bonding, Fermi resonance and methyl torsion. EXPERIMENTAL

The samples of thiourea and N-methylthiourea were obtained from BDH and Fluka respectively. The spectra were recorded on a Perkin-Elmer 580 IR spectrophotometer with a Specac 21000 liquid nitrogen cryostat and also by a Perkin-Elmer 599B IR spectrophotometer with a liquid nitrogen cryostat of our own design. The records were made on KBr pellets with a concentration 100

100

80

80

60

8

6O

E ~E 40

L 40 I.-

20

20

0 40001

L

L

I L 3600

i

i

~ ( c m -1 )

I L 3200

i

0

i 2800

1800

16(30

1400

1200

oJ ( c m -1)

Fig. 1. Infrared spectra of thiourea in the 4000-2800 c m - t region at different temperatures: (a) 300 K, (b) 270 K, (c) 220 K, (d) 170 K, (e) 120 K. Fig. 2. Infrared spectra of thiourea in the 1800-1200 c m - t region at different temperatures: (a) 300 K, (b) 270 K, (c) 220 K, (d) 170 K, (e) 120 K.

205 1oo

8O

60

e

~ 4o

1200

1000

800 w ( c m ~1 )

600

400

Fig. 3. Infrared spectra of thiourea in the 1200-350 cm -~ region at different temperatures: (a) 300K, (b) 270K, (c) 220K, (d) 170K, (e) 120K. 100

80

60

E 4O

20

o 4000

3400

3000

2600

w (cm -1)

Fig. 4. Infrared spectra of N-methylthiourea in the 4000-2600 cm-~ region at different temperatures: (a) 300 K, (b) 270 K, (c) 220 K, (d) 170 K, (e) 120 K.

206

1OC

80

8

60

~- 40

20

0 1800

1600

1400

1200

(cm -1)

Fig. 5. Infrared spectra of N-methylthiourea in the 1800-1200 era-1 region at different temperatures: (a) 300K, (b) 270K, (c) 220K, (d) 170K, (e) 120K. 100

80

6O E 40

20

0 1200

1000

800 w(cm -1)

600

400

Fig. 6. Infrared spectra of N - m e t h y l t h i o u r e a in t h e 1200-350 cm -1 region at different temperatures: (a) 300 K, (b) 270 K, (c) 220 K, (d) 170 K, (e) 120 K.

207 of 1:100. The recorded spectra were the same in both cases. The different regions of the spectra are presented in Figs. 1-6. OBSERVEDSPECTRAAND ANALYSIS In the NH stretching region thiourea shows sharpening of a few bands at lower temperatures {Fig. 1 ). The higher frequency bands/]1 (AI) and/]1o(B2) are resolved at lower temperature and are found at 3380 and 3350 cm-1. The Fermi triad (/]2, 2/]3 and 2/]12) analysed by us at room temperature [3] does not show any sharpening at lower temperature. The overtone bands are rather weak and broad, contrary to what is observed in the 1400-1600 cm -1 region (Fig. 2). The/]11 (Be) mode at 3250 cm -1 becomes sharp but does not show any additional splitting or shift at lower temperature. This confirms the earlier conclusion that the/]11 (B2) mode does not have any FR [ 3 ]. The combination mode/]3 (A1) +/]12 (Bz) expected in this region could not be identified. However several combination modes are expected in this region which may be responsible for the lineshape distortion of the FR bands 2/]3 and 2/]12. The NH2 deformation modes at 1590 and 1630 cm-1 become sharp at lower temperature. A small splitting of the 1630 c m - 1band into a doublet is observed (Fig. 2 ). An interesting feature of this region is the appearance of a sharp band at 1448 cm-1 at lower temperature. This appears on the low frequency side of the fundamental/]13 (Be) mode at 1470 cm-1. The noticeable shift in frequency of this fundamental on lowering the temperature (Fig. 2) shows that it has a Fermi resonance with the combination /]5(A1)+/]15(B2) mode. It is known from time independent perturbation theory that the FR frequencies are given by the expressions [ 3 ] A 1 '-{-A 2 = 0 ) 13 -{'- (2)5,15

1 A1A2 = O) 13 0)5,15 -- 4/~13515

(1) (2)

where 21 and 2e are the observed frequencies of/]la and (/]~+/]1~) modes and 0) la and 0)5,15are the unperturbed/]13 fundamental and (/]5 +/)15 ) combination frequencies; Kxas15is the anharmonic coupling coefficient. The intensity ratio is given by In T2 (21 - 2 2 ) - (0)la-0)sas) In T1 - (21 - 2 2 ) + (0)13-w~,15)

(3)

where T1 and 7'2 are the peak intensities [ 3 ]. Analysis of the FR pair with the help of the above equations leads to the anharmonie coupling coefficient and the unperturbed frequencies as shown in Table 1. In the lower frequency region the 630 em -1 band shows a splitting into three bands (Fig. 3 ), but this splitting cannot be explained at the moment.

208 TABLE 1 Data (cm -1 ) obtained from Fermi resonance calculations for t h e (v, 3, v5 + v,a) diad in thiourea at 120 K Parameter

Observed value

C o n s t a n t s calculated

value

v, v2 In T2 In T1

1470 1448 1.155

0),3 o)5,15 K,351~

1461 1457 21.63

TABLE 2 Vibrational bands (cm -~) of N - m e t h y l t h i o u r e a at five different temperatures Temperature ( K ) 300

270

220

170

120

3320 3240

3320 3250

3160

3160

3310 3260 3230 3160

3310 3260 3230 3160

3310 3270 3230 3160

Fig. 5

1630 1622 1615 1608

1630 1622 1615 1608

1632 1622 1615

1632 1622 1615 -

1632 1620 1615 -

Fig. 5

1400 1392 1385

1400 1392 1385

1400 1392

1400 1392

1400 1392

Fig. 6

1145 1120

1145 1120

1145 1120

1145 1125 1120

1145 1125 1118

Fig. 4

N-Methylthiourea has three N H and three CH stretching modes around 3000 cm-1 (Fig. 4). The vl N H stretching mode at 3320 cm-1 shows a small shift to 3310 cm-1 at 170 K (Table 2 ). A few high frequency shoulders also appear at lower temperature. These shoulders are similar to what we earlier reported [4] in the 1550 cm-1 band at room temperature. Such satellite lines are hot band transitions which can be ascribed to hydrogen bonding. On lowering the temperature the band at 3240 c m - ~splits into a 3270-3230 c m - 1pair which is assigned to a v2, 2 ~ FR diad. An important feature of the 92 and v3 NH stretching modes is that they are strongly affected by hydrogen bonding. In the normal coordinate calculation [ 11 ] those modes had shown large devia-

209

tion from the calculated values. This indicates strong anharmonicity associated with these modes. The strong va mode at 3160 cm-1 remains unshifted at lower temperature. This is clearly at a much higher frequency than the earlier IR measurement [ 11 ]. The (v2, 2v8) pair does not have the well defined lineshape necessary for a Fermi diad calculation. This is because of the satellite transitions as observed in the case of the Vl mode. Similar to the case of thioacetamide [ 1] all the CH stretching modes are weaker and do not show appreciable shift at lower temperature. A weak mode at 3020 c m - 1may be an overtone 2v> Another band at 2740 cm -1 becomes very sharp at lower temperature, although it does not show any frequency shift. This band is at much lower frequency than the CH stretching modes and hence has no FR. This band may arise from the overtone 2Vlo. The 1800-400 c m - 1 region indicates an interesting change in the lineshape at lower temperature (Figs. 5, 6). The band at 1630 cm -1 is an NH2 deformation v6 mode; it has three equally spaced bands on the lower frequency side (Table 2). At the lower temperature one of the low frequency bands (1608 cm -~) disappears and the other one (1615 cm -1) becomes weak. There is a distinct pair (1632-1620 cm -1) of bands at 170 and 120 K. The pair of low frequency bands (1615-1608 cm -1) cannot be an FR pair arising from combination modes because of the weakening of these two low frequency bands at lower temperature. Hence these bands may be hot bands associated with methyl torsion [ 14 ]. The relative intensities of 1630 and 1615 c m - 1bands at different low temperatures have been measured. These values are used to calculate the methyl torsional frequency (Table 3). These data show that torsional frequency is nearly independent of temperature (Table 3 ). The population in the excited methyl torsional levels is controlled by the Boltzmann distribution law, so with the decrease of temperature the population in the excited methyl torsional level decreases. This leads to weakening and disappearance of the hot bands. However, the 1630-1622 cm -1 pair widens to 1632-1620 c m - ' and remains sharp even at 120 K; this should be an FR pair (v6, v~2+v~7).Because of the low frequency hot bands the lineshape is not well defined for a calculaTABLE 3 Methyl torsional frequencies ( c m - ' ) from hot bands in N-methylthiourea at different temperatures Temperature (K)

300 270 220 170 120

Calculated frequencies ( c m - 1) from hot bands (1630-1615) cm -1

(1400-1392) cm -1

39 29 40 37 26

42 27 33 31 25

210 tion of the Fermi interaction. A similar feature appears for the vlO CH3 deformation mode at 1400 cm -1. On lowering the temperature, one of the low frequency hot bands (1385 c m - 1) disappears and the other one (1392 c m - 1) becomes weak (Table 2). The methyl torsional frequencies calculated from the relative intensities of 1400-1392 cm -1 bands are given in Table 3. These computations again show that the torsional frequency is independent of temperature. Another interesting feature is the reversal of intensity of the fundamental pair v12-v13 at 1145-1120 c m - 1 ( Fig. 6 ). The NH2 rocking mode v12 associated with hydrogen bonding becomes very sharp at lower temperature whereas the 1120 cm -~ (v13) mode loses intensity and splits into a 1125-1118 cm -~ pair. An FR interaction with the overtone 2v23 may be responsible for the loss of intensity. In the lower frequency region (Fig. 6) most of the bands in methylthiourea become very sharp. A particularly interesting case is the shift to higher frequency and the large gain in intensity of the out-of-plane modes v23 and v24. The v24 mode is associated with hydrogen bonding and it shifts from 550 to 570 c m - 1 at lower temperature. It is well known that the out-of-plane deformation modes usually have higher frequencies at lower temperature. In the case of hydrogen bonded systems, hydrogen bonding causes a shift of these modes to higher frequencies [15]. The hydrogen bond in N-methylthiourea becomes stronger at lower temperature; this imposes an additional potential on N - H . • •S deformation and a consequent increase in the frequency of the v24 NH mode. The relatively smaller increase of the frequency of the v23 mode at 590 c m - ~is an indication that it is not associated with hydrogen bonding. CONCLUSION On lowering the temperature the IR spectrum of N-methylthiourea shows more interesting features than that of thiourea. The NH2 deformation modes in thiourea show sharpening of the bands on lowering the temperature. In the case of N-methylthiourea the NH stretching modes show a small frequency shift and the appearance of a few band shoulders at low temperature. The FR bands are obscured to a greater extent by the hydrogen bonding effect and methyl torsional factors. This confirms our earlier prediction that thiourea becomes hydrogen bonded on methylation. A study of hot bands in the vibrational spectra of N-methylthiourea at different low temperatures leads to the methyl torsional frequency. It is generally expected that with decrease of temperature, the methyl torsional mode frequency will show an upward trend, but our measurements show that this frequency remains more or less constant over the temperature range used in this work and the relative intensity of the hot bands is dominated by thermal distribution law. Hence the change of methyl torsional frequency, if any, should be small.

211 ACKNOWLEDGEMENTS

The authors are grateful to Professor C.N.R. Rao of the Indian Institute of Science, Bangalore for extending to them the experimental facilities at Bangalore. P.K.P. and P.N.G. thank the University Grants Commission for financial help. S.S.B. thanks the Council of Scientific and Industrial Research for a Senior Research Fellowship.

REFERENCES 1 2 3 4 5 6

S.S. Bala, P.K. Panja and P.N. Ghosh, J. Mol. Struct., 157 (1987) 339. S.S. Bala, P.K. Panja and P.N. Ghosh, J. Mol. Struct. (Theochem), 151 (1987) 113. S.S. Bala and P.N. Ghosh, J. Mol. Struct., 101 (1983) 69. S.S. Bala, P.K. Panja and P.N. Ghosh, J. Mol. Struct., 131 (1985) 393. S.S. Bala, unpublished data. A. Yamaguchi, T. Miyazawa, T. Shimanouchi and S. Mizuschima, Spectrochim. Acta, 10A (1957) 170. 7 A. Yamaguchi, R.B. Penland, S. Mizushima, T.J. Lane, C. Curran and J.V. Quagliano, J. Am. Chem. Soc., 80 (1958) 527. 8 J.L. Duncan, Spectrochim Acta, Part A, 27 (1970) 1197. 9 G.B. Aitken, J.L. Duncan and G.P. McQuillan, J. Chem. Soc. A, (1971) 2695. 10 D. Hadzi, J. Kidric, Z.V. Knezevic and B. Barli c, Spectrochim Acta, Part A, 32 (1976) 693. 11 K. Dwarakanath and D.N. Sathynarayana, Bull. Chem. Soc. Jpn., 52 {1979) 2084. 12 K.A. Jensen and P.H. Nielsen, Acta Chem. Scand., 20 (1966) 597. 13 R.K. Ritchie, H. Spedding and D. Steele, Spectrochim. Acta, Part A, 27 (1971) 1597. 14 P.N. Ghosh and Hs.H. Gunthard, Spectrochim. Acta, Part A, 37 (1981) 347. 15 D. Hadzi and S. Bratos, in P. Schuster, G. Zundel and C. Sandhrfy (Eds.), The Hydrogen Bond, Vol. II, North-Holland, Amsterdam, 1976, pp. 567-606.