Intensity in the Raman effect

JOURNAL

OF

MOLECULAR

SPECTROSCOPY

Intensity Part

16,

in

X. Some Absolute

378-385 (1965)

the

Raman

intensities

Effect

in Methanol

Vapor*

Z. KECKI~ Department

of Physical

Chemistry,

Warsaw University,

Warsaw,

Poland

AND H. J. BERNSTEIN Division

of Pure

Chemistry,

National

Research

Council,

Ottawa, Canada

The Raman spectrum of methanol vapor has been observed with a photoelectricalIy recording Raman spectrometer. Two new features have been observed, one of which supports the assignment of Margottin-Maclou and of Zerbi et al., and the other seems to be part of the vibration-rotation spectrum of the OH stretching fundamental. Absolute intensities and approximate depolarization ratios are given for five bands. Comparison of liquid and vapor phase intensities indicates that the intensity enhancement in the liquid is less than a factor of two. INTRODUCTION

The vibrational spectrum of methanol has been the subject of several papers recently (1-S) and some disagreement in the assignment has been pointed out (3). Among the many investigations’ only one was on the Raman spectrum of the vapor (4). Because of our continued interest in measurements of absolute Raman intensities in the vapor phase and because of the uncertainty in assignment, the present investigation of the Raman spectrum of methanol vapor was undertaken. EXPERIMENTAL The Raman photoelectric spectrometer and the gas Raman cell have been described previously (5-7). The six mercury low pressure arcs were cooled either with water or with a rhodamine aqueous solution to keep the temperature constant at 60°C. The electrodes were cooled with water to about 25°C. The gas Raman cell was warmed up to a temperature desired by passing either water or * National Research Council No. 8344. t Visiting scientist at National Research Council October, 1963-January, 1964. i See Reference 1 for a rather complete bibliography. 378

SOME ABSOLUTE INTENSITIES IN METHANOL VAPOR

379

a concentrated potassium nitrite solution (both thermostated) through a doublewalled cell jacket. All exposed parts of the gas cell and connecting tubes were protected by thermal insulation or heating tape to avoid the condensation of vapors. The spectra were recorded in most cases at 80°C. The investigated vapor was introduced into the evacuated gas cell by evaporating the liquid immersed in a thermostated bath kept 10”15°C lower than the temperature in the cell. The amount of evaporated liquid was estimated by weighing. In order to determine the vapor concentration in the cell the volume of the cell was measured by filling the evacuated cell with niOrogen from a container of a known volume. The pressure drop measured with a mercury manometer enabled one to determine the cell volume, according to Boyle’s law, to be 13.3 f 0.4 liters. The other details of the work with vapors were essentially as described in Reference 7. The conventional technique (8) was applied when recording the spectra of liquids. The spectra were excited at the temperature 25°C using a Toronto-type mercury arc. The instrument conditions used for the recording were: amplifier gain 5070 %I, response time 2.5 and 5.0 set, scanning speed 0.047 8/set and 0.094 w/se,, chart speed 0.9 mm/set and 0.12 mm/set, spectral slit width 5-9 cm-l. REAGENTS Methanol, reagent grade, from the Nichols Chemical Company, Montreal, was used without additional purification. The gas chromatography analysis proved that the impurities including water did not exceed 2%. Carbon tetrachloride, analytical reagent, from Mallinckrodt Chemical Works Ltd. Montreal was used as a standard in the measurements of t’he intensity and depolarization ratio for liquid phase. Hydrogen, extra dry, and methane, c.p., from Matheson of Canada, Ltd. were used as standards in the measurement’s for t,he gaseous phase. FREQUENCY The wavelength scale of the scanning device was calibrated with an emission spectrum of Argon-Neon lamp. The error limit in t,he frequency determination was &l en-l. INTENSITY The absolute intensity was determined in terms of gj(45ai2 + 7ri2), where gj is the degree of degeneracy of the jDh vibration, cri’ is the average and yj’ the anisotropy of the derivative of the polarisability tensor with respect to the normal coordinate at the equilibrium position. The intensity standards for the vapor phase were the intensity of the l-3 pure rotational transition of hydrogen (587 cm-l) and the intensity of the v1 vibrational band of methane (2914 cm-‘).

380

KECKI

AND BERNSTEIN

The latter, determined in References 6 and ‘7 was (190 f 2) X N X 10e3’ cm4 g-l. The experimentma procedure including the determination of instrumental correction factors was described in Reference 7. The Raman cell was wrapped with a Polaroid sheet with axis perpendicular to the length of the cell so that’ all intensities measured correspond to t.he J_ component (45~~‘~+ 7-r’“). The results were obtained using t,he following formulas. 1. The Gaseous Phase: (a) With respect, to the l-3 587 cm-l,

pure rotat’ional transition

(b) With respect to t’he v1 vibrational &(45&z + 7$)

band of hydrogen at

band of met,hane at 2914 en-l,

C X&XAX = sj &-4x(:;+4) 8 CH4 1Cj 1 _ e--hcvj/kr X 1 _ e_2914Xhc,kT x (190 zf 2) x N x 1o-32

b4

g-7.

2. The Liquid Phase (8) With respect t,o the 459-cm-l band of liquid Ccl4 the absolute intensity of which was determined in Reference 9 to be (56 f 15) X N X 10p3’ cm4 g-l,

x

(56 f

15 j X N X 1O-32 [em” g-l].

The symbols are: S-the planimetered area under the band contour (recorded wit,h Polaroid I); 6-instrumental correction factor including spectral sensitivit,y correction and speckal mirror efficiency for the gas phase (for liquids no mirror correction is required) ; C-concentrat.ion of t.he sample in mole/liter; M and cl-molecular weight and density, respectively; h, c, k, T-Plan& constant, speed of light, Boltzmann constant,, and absolute temperat)ure, respectively; B-rotational constant of Hz molecule; yo-anisotropy of Hz molecule frequencies of the exciting line equal to 2.1 X 1O-25 cm3 (IO); ~0 and vj-the and of the vibrat,ion, respect,ively; N-Avogadro number; n-refract.ive index; RPF-rotational partition fun&ion. (RPF)H,

= M xi

(2j + l)gi X e--j(j+l)kcB’kT,

SOME

ABSOLUTE

INTENSITIES

IN METHANOL

\‘APOR

381

wherej is the rotational quantun1 nun1ber and gj is the degree of degeueracy of the rotational level. In the case of the liquid phase the correction for reflection losses (8) R(n) was neglected (11). The procedure for the determining of the depolarization ratio was the san1c as in Reference 7. The vapor spectra were excited by bot’h Hg, aud Hgk lines and in all cases, where the overlapping with grating ghosts or weak mercury lines did not disturb the bands, t)hc results were proved to be consist& within the error limit RESULTS

The pronounced features observed in the Raruan spectrum of CH,OH vapor are shown in Figs. 1 and 2. There is a broad weak feature -1460 cm-l which arises from the CHB deformation modes but’ is unfortunately in a region where the instmn1eutal grating ghosts are prominent,. To resolve the problem of the origin of the 1071-cm-1 band this region was observed with (Fig. 1) and without, (,l’ig. 2) nit)rite filter solution. Further t’his band was observed with Hg, (4047 A) excitation, and the filter solution was efficient enough t,o ren1ove the 2914 band of methaue excited by Hg, . The presence of t#he Hg 5028 A line in t,he 3000~cm-1 region in Fig. 1 obscured the presence of t’he weak feature at 3036 cm-’ observed wit,h Hg, excitation (Fig. 2). The doublet’ appearance of the YOHband was confirmed by Hg, (4358 ii, escitation. The results of the frequency and intensit)y measurements are shown in Table I.

FIG. 1. Hg, excitation

of the Raman spectrum

of MeOH vapor with nitrit.e filt.er.

KECKI

382

AND

BERNSTEIN

FIG. 2. Hgk excitation of the Raman spectrum of MeOH vapor without filter. TABLE I RAMAN SPECTRUMOF METHANOL(INTENSITIESIN UNITS OF N X 1O-32cm4 g-l) Liquid

Vapor

1033 1071 -1460 2846 2927

2955 2999 3036 3683 3697

2.5 f 1.8 f weak, broad 120 f 14 f 66 & 8f3

0.7 0.3 ? 12 5 12

<0.35 <0.32

1032

<0.05
1033 a 1463

9f4

0.36 f

0.17

-

-

2845

2835

110 f

40

0.13 f

0.04

2955

2945

208 f

80

0.30 i

0.04

a

46 f

10

<0.25

not measured

S A corresponding band in the liquid seems to be overlapped. along with the results for the liquid obtained

with the same Raman spectrometer. The values for the depolarization ratios of the vapor bands are given as upper limits. The real values for Ptrue might be one-half the values listed. The Raman spectrum of CDsOH vapor under conditions similar to those for

SOME ABSOLUTE

INTENSITIES TABLE

IN METHANOL

VAPOR

383

II

VIBRATIONALFREQUENCIESIN METHANOLVAPOR” CDaOH

CHsOH Raman (cm-‘)

Infrared (cm-‘)

1034

1033

-

1071 -1460

Assignment

weak, broad 2846

1455 2845

CO stretching

Infrared (cm-*)

a’

Raman (cm-l)

988

984

C-O bending CH3 sym. bending a’ CH:, sym. st,retching a’ R branch of CD, sym. stretch.

2077 2098

2075

? (2 X 1075 CD3 asym. bend.)

2147

CH1 asym. stretching

2235

H ‘\

-

2927

2973

2105 2144

2268 2999

3036 3683 3697 a Infrared

3687

frequencies

OH stretching

and assignment,

a’

3690

3683 3694

taken from Reference? 1.

obtaining the spectrum of CHBOH is shown in Table II along with assignments for both molecules (see discussion). Because the features at 3697 cm-1 and 1071 cm-l have not been hitherto observed in t’he Raman spectrum of CHsOH vapor (4) we considered it important to establish whether these bands were to be associated with the monomeric species (at these pressures -800 mm Hg) or with some multimer. The intensity ratios of Y&Y~O~Zand v369~/~368~ were obtained at 60” and 90” and within the experimental error were found to be independent of temperature. It is reasonably well established therefore that the features at 1071 and 3697 cm-’ arise from the same molecular species responsible for the rest of the observed spectrum. The band at 1071 en+ although observed in the Raman spectrum at 800-mm pressure of methanol at 80°C has not been observed at 80-mm pressure and room temperature in the infrared (1). Because of the greater bandwidth in the 1030-cm-’ region in the infrared (-80 cm-l), compared with the Raman band width (-17 cml) it is apparent that this feature would be more pronounced in t#he Raman spectrum. Recently a new assignment (2) has been proposed for .H methanol

in which the band

at 1071 cnl-’

has been identified

with the

/ C \ “0

deformation

mode.

Presumably

this band

was observed

but

no spectra

were

KECKI

384

AND

BERNSTEIN

displayed (8). This assignment was found to be consistent with a Urey-Bradley treatment ((3) and is also consistent wit,h the present observations. From the normal coordinate analyses (2) it is evident that the modes with frequency 1033 and 1071 cm-l are highly mixed in CHSOH. However in CDSOH the CO stretching mode is pract’ically unmixed (2). In CDsOH this mode is apparently too weak to be observed but gains int,ensity in CHZOH by virtue of t,he mixing (see Table II). The band at 3036 cm-’ corresponds to ~~(~4~) in the assignment of MargottinMaclou (2). The feature at 3697 cn-l seems to be part of the vibration-rot)ation spectrum of t’he OH stretching fundamental. The absolute intensity in the CH region of CHSOH vapor is consistent with t,he results previously obt.ained (7) and the int,ensit,ies of the individual CH bands have been est.imat,ed by unsophisticated curve analysis. From the depolarization ratio of p $ 0.25 the value found for (&~/d&)~ is 20.72 X 10P3’ N g-l en? which corresponds bo a value for (&/&en) of about 0.9 x 10-l” cm2. (ran is the OH internuclear distance). VAPOR AND LIQUID INTENSITIES Because of t’he mixing of the CO stretching mode and the HCO bending mode the sum of the intensities in the vapor might be compared with that in the liquid in the 1033-en+ region. Wit,hin experimental error these are not different in the two phases and could be consistent with the proposal that hydrogenbonding in the liquid does not alter the CO intensity and frequency (12). In the gas, the depolarized features in the CHB deformation region are broad but in the liquid where & branches are expected to be more prominent this region should exhibit more pronounced bands. The intensities in t’he CH sketching region in vapor and liquid are about the same. The inversion of int,ensities of the two most intense CH bands in vapor and liquid is evidence of t’he fact that Fermi interactions account primarily for their intensities; the interactions in the vapor and liquid phase being different. By comparison with the enhancement (7) of the intensity of the 458-cm-l line in liquid CCL over the gas (about! a fact,or of 2) it seems that there is less enhancement for t,he liquid CHBOH lines in the spectral regions discussed above. ACKNOWLEDGEMENT We are indebted

to Mrs. I. Gajda

for technical

assistance.

RECEIVED July 6, 1964 REFERENCES 1. M. FOLK AND E. WIIALLEY, J. Chem. Phys. 34, 1554 (1961). 3. M. MARGOTTIN-MACLOU,J. Phys. Radium 21, 634 (1960). 3. G. ZERBI, J. OVEREND, AND B. CUWFORD, JR., J. Chem. Phys. 38, 122 (1963).

SOME

ABSOLUTE

INTENSITIES

IN METHANOL

\‘APOK

/f. J. R. NIELSEN AND N. E. W.ZRD, J. Chem. Phys. 10, 81 (1942). 5. J. U. WHITE, N. L. ALPERT, IND A. G. DEBELI,, J. Opt. Sm. Am. 46, lti (1955). 6. T. Y~SHIN~ AND H. J. BEWNSTEIN, J. Mol. Speclry. 2, 213 (1958). 7. H. W. SCHR~TTERAND H. J. BERNSTEIN, J. Mol. Spectry. 12, 1 (1904). 8. H. J. BERNSTEIN AND G. ALLEN, J. Opt. Sot. .4rn. 45, 237 (1955). 9. .J. BR.INDM~~LLER AND H. %HRijTmR, 2. Physik. 146, 131 (19%‘); H. W. SCHR~~TTER, 2. Elektrochem. 64, 853 (1900). 10. IL. P. BELL, Trans. Faraday Sot. 38, 422 (1942). 11. A. I. SOK~LOVSKX~ :IND S. G. RAUTLIN, Opt. Spectry. (USSR) 6, 29 (1959). 12. Z. KECKI, Speclrochim. Acta 18, 1155, 1165 (19G2).

:r18r,