IR bandshapes of liquid trimethylsilyl cyanide

Received 18 August 1981

IR dispersion data wie obtained for liquid $hnethylsilyl byariide in the vs(CN) and ~ym-as(W.CI1~)3~region by the thin-film trmsm&s& technique. ihe correlation functions calculated from this data for both vibrations differ markedly. The results are discussedjn the framework of the collisional model of vibrational dephasing.

I. Introduction

The study of IR and Raman bandshapes with the aim to recognize the role of various relaxation mechanisms has evoked much interest in the last decade; for recent reviews, see refs. [l-3] .Several models of reoricntational and vibrational relaxation have been developed [ 1,2] , but experimental data are still rather Scarce [2,3], especially for pure Iiquids, and the role of various shaping mechnnisms is far from.clear, especially in the,irrfr;ired [3-51. In this \iv$c we report Ii bandshape studies of two fully sym~metr@I, paralleI$ypevibrations of Iiqnid trimethyleilyI cya&de,~(CH#iCN; the stretching vibration of the CN group, Y,(CN),and the symmetrical stretching vibration of tbe~Si(C!H,&group, V,(Si(CH&). Extensive IR and Raman bandshape studies on acetonitrile have been reported in a series of papers by Yarwood et aI. 141. (CIQSiCN has apparently not been studied in this respect hitherto.

2.Experimental

ic and finite-slit-width distortions of the bandsha.pe [ 71 . Several ceils of ticknesses ran&g from 4 to 13 r-en were assembled, using NaCl and I& windows. The spectia were recorded at 3 10 IL (CH,),SiCN was synthesized from (CH?)3SiCl and purified by fractional distillation; it was dried and stored over freshly prepared molecular sieves. The boiJingpoim: was 118.5-l 19°C.

3.Processing of spectra From the experimental thin-film transmission spectra, the spectrum of the complex refractive index was obtained, r?(v) = n@) + ik(v),

(1)

using a method described elsewhere [6j . .For the anchor point needed for the transformation, the value de. tcrmined for the sodium D line was taken. From this data the correlation functions were calculated according lo S(v) cos[2nc(u - Qf] dv II)

The IR transmission spectra were recorded on a Perkin-Rimer model 180 spectrophotometer, in a dou. ble-beam, qpen reference beani-,ho$e [6]. The scan speed w&-l ~5-2 qi-j/min @iti ti time ciuistant of 2-4 s; the ratio of thi spectralbIt widdthto the halfwidth ofthe,b+ was.less.than $1. Theae,set#gs re. ducid to a’;ninirnum any.pos&ble mechanical, electron0 009.2614/81/0000-0000/$02.75

0 1981 North-Holland

where the spectral density frrnction S(u) is &en by

‘(‘) =1 -

E”(U) exp(-hcv/kT)

and the imaginary component of the complex electric pcrmittivity [8] was calculated from 33 t

1 December 1981

CHEMICAL PHYSiCS LETTERS

Volume 84, number 2

E”(Y) = IZn(v)k(Q.

(4)

The co~e~ation times were calculated tegration of C(t):

by numerical

in-

m

initial moments pj of the bands were calculated for the k(p) curves by means of standard expressions. The

4. Results The v~(CN) band located at 2 189.7 cm-l was recorded in the 2240-2140 cm-l range and the symv,(Si(CH,),) band at 641.0 cm-1 in the 690-580 cm-1 region. The ~-f~ tr~s~~io~ spectra of

both

vibrations,

recorded

in the double-beam,

t-32

open

beam mode are presented in fig. 1. Only three runs are presented for each mode for clarity; six measurements at various ticknesses and window materials were made for each mode. Ln fig. 2 the spectrum of the complex refractive index is given for the v,(CN) reference

i-5

2220

22ixI

2lef3

ucm-:

I

Fig. 2. Spectrum of the compfex refractive index for the u&X)

band.

9

80.

60

i 660

640

620 Q[cm.

Fii_ 1. Double-beam, open reference beam spectra of (CH&SiCN. (A) Y&TN) band: (a) NaCl, 4.82 Mm; @) KBr, il.30 wm; (c) KBr. 13.13 Itm. (3) v&i(CH&) band: (d) UF, 4.45 pm;(e) mr, 10.93 gm; (f) KBr, 13.13 rm.

332

Fig. 3. Spectrum of the complex refractive index for the ~&MZH~:~) band.

CHEMICAL. pHYSICS

Volume 84, number 2

LETTERS

1 December 1981

retical secorid mdment, cakulated for a parallel vibration of a Cjv molecule [$I, equal to.56.7 cmm2 for (CH,),SiCN_ It should,+so be noted that; after Booth and Frankiss [IO] , the equilibrium mole fraction of the isocy&d& (CHS)3SiiC is only 0.0015 at 298 K; thus the weak vJNC) b%nd at 2095 cm-l does not disturb the measurements.

5. Discussion I

10

10

20

t [PSI

The overall .jnfrared correlation function for a nondegenerate normal mode Q of a polyatomic molecule, neglecting the correlation between vibration and rotation,isgivenby [ll]

Fig. 4. Experimental IR correlation functions -: (A) vJCN); (B) uJSi(CHs)s); - -free rotor correlation function.

vibration and in fig. 3 for the sym-vs(Si(CH3)3) vibration. The correlation functions averaged over six runs are presented for both vibrations in fig. 4; additionally, in fig. 4, the classical free-rotor correlation function, calculated from the formula for a parallel vibration of a C3v molecule [9], is shown; the moment of inertia

Gg(t)

= G$$t)G~~,(r),

(6)

where Crd(t) and G C-3 (t) are the vibrational and rotalmt

fiuuxions

tional correlation The comparison

for the normal

mode

a.

IR band profdes predicted by tfxe general theory of J_.eicknam, Guisani and Bratos [i I ] for parallel bands of a symmetric-top molecule, with the experimental band profiles for both modes, given in figs. 2’and 3, indicates that the reorientational parts are dominated by rotational diffusion. It is interesting to note that the correlation functions for both viirations coincide at Ml.3 ps (fig. 4); additionally, they coincide in this time interval with the chssical free-rotor correlation function. This behaviour is very shilar to

was taken

1O-4o gcm2 [lo]. as 1, =IY =426.0X The values of correlation functions at selected times are given in table 1; a rather slow decay can be noticed for both vibrations. In table 2 other spectral data are collected. The correlation times ‘O were obtained by integration of C(t) [eq. (S)] in the limits of O-6 ps. It is interesting to note that the experimental second moments for both vibrations roughly agree with the tbeo-

of

Table 1

G(t) values at selected times (with standard deviations) Mode

v,(CN) usWCH3)3)

G(r) t = 0.5 ps

r=1ps

r=2ps

t=4ps

0.84 = 0.02 f 0.02

0.68 + 0.02 0.54 f 0.01

0.40 f 0.01 0.26 f 0.01

0.113 f 0.008 0.047 f 0.005

0.80

Table 2 Spectral data fcr the studied bands (with standard deviations) Mode

u,(CN) ~&%
vcl

vrn (cm-’ 1

(m-1

2189.7 * 0.2 641.0

= 0.2

)

2189.1 f 0.2 640.4

r 0.3

fl2

TC

(cm-21

(PS)

58.1 * 8.9

2.1

5.5_7

1.6

t

10.4

333

Volume 84. number 2 that of Liquid trichloroacetonitrtie

f8] and it seems to indicate the possibility of large angular jumps of the C,, axis between collisions; Raman data, however, do not support this conclusion [13] A significant result of this study is the finding that the decay of the correlation functions for both vibrations of identical symmetry is considerably different. It is reasonable to assume that the reorientation4 parts of the correlation functions are identical for both vibrations; this does not have to be the case 11 I], but from Rantan data [13-l it follows that in this case this ~sumption is correct. Thus, the sequence of the vibrational correlation functions will be the same as the sequence of the overall IR correlation functions (fig. 4) whereas the values of G$L(r) wih be correspolld~~y somewhat higher than the values of Gpd(t). Thus, the differences in the decay of both modes are caused by different rates of vibrational relaxation. Of various chartrteJs of vibrational relaxation,

vibrational dephasing is usuahy the most efficient one [ 13, 14]_ The behaviour of the correlation functions can be understood in terms of the collisional model of vibrational dephasing of Fischer and Laubereau [ 14). From this mode1 it follows that for a given molecule the dephasing time is proportional to the square of the mode frequency: 7’“’ a fWW)2 , 2

(7)

Thus., the correlation function for the o,(CN) mode should decay more slowly than that for the v,(C(CH,),) mode, which is indeed observed (fig. 4). The ratio of the squares of the -mode frequencies differs considerably, however, from the ratio of the correlation times for both modes. This is most probably caused by the fact that in neat Liquids, other relaxation ~~~hanisnls can operate, mainly resonant energy tram+ fer and population relaxation. Studies are in progress

334

1 December

CHEMICAL PHYSICS LETTERS to evaluate

their contribution

as band-shaping

1981

mecha-

I-liSItX.

Acknowledgement This work was supported Sciences under the MR.I.9.

by the Polish Academy

of

Plato

References [ 11 S. Bratos. in: Vibrational spectroscopy of molecular liquids and solids, eds. S. Bratos and R.M. Pick (Plenum Press, New York, 1980) p. 43. [Z] W.A. Steele, in: Vibrational spectroscopy of molecular Liquidsand solids, eds. S. Bratos and R&f. Pick (Plenum Press, New York, 1980) p. 61. [ 3J J_ VincentCeisse. in: Vibrations sDec~oscouv of molecuiar liquids and solids, eds. S. Rratos and R.& Pick (Plenum Press, New York, 1980) p. 117. J. Yarwood, P.L. James, G. DGgeand R. Amdt, Faraday Discussions Chem. Sot. 66 (1978) 252, and references therein. J.P. Hawranek and R. Szostak, Actn Phys. Polon. A60 (1981) 666. J.P. Hawranek, P. Neefakantan, R.P. Young and R.N. Jones, Spectrochim. Acta 32A (1976) 75.85. KS. Seshadri and R.N. Jones, Spectrochhn. Acta 19 (i963) 1013. J.P. Hawranek and R. Szostak, Chem. Phys. Utters 69 (1980) 367. W.G. Rothschild, J. Chem. Phys. 57 (1972) 991. MR. Booth and S.G. Frankiss. Spectrochim. Acta 26A

(1970) 859.

J.C. Leicknam, Y. Guissani and S. Bratos, J. Chem. P&s. 68 (1978) 3380.

R. Szostak and J.P. Hawranek, in preparation. [ 131 R.hf. Lynden-Bell. Mol. Phys. 33 (19771907; 36 (1978) 1.529.

[14j S.F. Fischer nnd A. Laubereau, Chem. Phys. Letters 35 (197.5)

6.