Infrared intensities and optical constants of crystalline C2H4 and C2D4

Specrrorhimrca Acre. Vol. 44A, No. I, PP. 27-31. Pruned I” Great Br~tam.

Infrared intensities

05X4-8539.88

1988

Pergamon

and optical constants

of crystalline

13.00 + 0.00 Journals

Ltd.

C2H4 and

CA G. ZHAO, M. J. OSPINA Department

of Chemistry

and Biochemistry,

University

(Received 12 January 1987; in$na/form

and

R. K. KHANNA

of Maryland,

College Park, Maryland

20742, U.S.A.

20 February 1987; accepted 20 February 1987)

Abstract-Infrared absorption spectra of several thin films at - 0.6 cm ’ resolution. The integrated band intensities obtained by a linear fit of the integrated absorbances vs film of the absorption data was carried out to obtain the complex in the regions of absorption bands.

of C2H4 and CzD4 at - 55 K were investigated of the infrared active fundamental modes were thickness. An iterative Kramers-Kronig analysis refractive indices ofcrystalline CzH4 and CZD4

slit width employed was - 0.6 cm - 1 throughout the 4COO450 cm- ’ range. The line absorption intensities of each of the bands werecomputed by averaging the data for three or more film thicknesses for which transmission was in the 2&70 y0 range. Infrared spectra of typical films of crystalline C2H4 and C2D4 are shown in Fig. 1. Channel fringes in the background indicate uniform film thickness; also, the films are practically non-scattering, as no noticeable background extinction is observed in the 4000 cm-’ region. Figures 2 and 3 display integrated absorbance B( = j In (1,/r) d3) for each band as a function of film thickness. A least squares fit of the data gives the integrated extinction coefficient, A( = ljdjln (lo/I)d3).

INTRODUCTION

Absolute i.r. intensities and peak frequencies are two essential parameters for proper identification of chemical species in a complex environment [l, 21. Interpretation of much remote sensing data from planetary atmospheres is based on this information [3,4]. Also, for several simple molecules, the intensity data have been utilized to evaluate the dipole moment derivatives and other pertinent parameters for use by theoreticians to check the reliability of their calculations [Ml. We have undertaken the task of obtaining laboratory data on absolute intensities of several simple hydrocarbons in their condensed phases because of their importance in planetary systems. We present here some of the results on CzH4 and C2D4. Earlier spectroscopic studies of C2H4 and C2D4 include their i.r. and Raman spectra in the vapor and condensed phases [9-171, which have resulted in unambiguous assignments of the normal modes. Effects of the crystal field on the vibrational frequencies of the molecules are, however, less well understood [ll, 13, 171. Further, absolute intensities of the i.r. active modes have been investigated in detail only for the vapor phases [18-201. To our knowledge, there is no report in the literature on the absolute intensities of the i.r. bands of condensed phases of C2H4 or any of its deuterated derivatives. We report here absolute intensity data on crystalline C2H4 and C2D4. These data have been subjected to Kramers-Kronig analysis to determine complex refractive indices which are also presented here. These data may be of direct use in the modeling of Titan’s atmosphere, where C2H, has been detected in the vapor phase [ 181 and condensation of several species is conjectured in this cold environment [19].

DISCUSSION Structural

elucidation

The crystal structure of CzH4 has been reported to be monoclinic [22] (space group C&J, with two molecular units in the primitive cell. The coupling of the vibrational modes of the two molecules gives factor group splitting of each mode into 2 components; however, the mutual exclusion of the modes into Raman and i.r. is retained, as shown in the correlation table (Table 1). The first detailed study of the i.r. spectra of crystalline CzH4 by BRECHER and HALFORD [lo] indicated consistency of the experimental data with the monoclinic crystal structure. The magnitude of the crystal field splittings of the normal modes indicates

Table 1. Correlation

EXPERIMENTAL

The experimental details are similar to those contained in an earlier report on crystalline C2NZ [21]. The i.r. spectra of several films of crystalline C2H4 and CZD., were recorded at 55 K on a Perkin-Elmer 1800 FT-i.r. instrument. The spectral 21

of CIH,/C,D, unit cell modes

molecular

modes

to

28

G. ZHAO

2 5 ‘Z .I?

E

et al.

so-

60

-

40

-

20

-

z 0 I-’

I

I

I

3000

I

2000

I

1500

I

1000

Wavenumber

Fig. 1. Infrared absorption spectra of (a) C2H4 solid at 55 K (2.36 pm thick sample) (b) C,D, solid at 55 K (3.32 pm thick sample). The undulating baselines are channel fringes caused by coherent internal reflection within the thin films.

100

25 -6

T E u

60

m 6

:

m IO

I

2 Thickness

3 (pm)

I

3

2 Thickness

+-rd

Fig. 2. Integrated absorbance vs film thickness for i.r. active fundamental modes of crystalline C2H4.

Fig. 3. Integrated absorbance vs film thickness for i.r. active fundamental modes of crystalline C2D4.

that hydrogen-hydrogen repulsion is the most significant intermolecular interaction [13]. Subsequent analysis of the i.r. spectra of CzH4 by JACOX [ll], under higher resolution, revealed several anomalies (for example, the splittings of 5r2 N 1440 cm-’ and bands into three components each) 210 N 820 cm-r which could not be explained on the basis of two formula units per primitive cell. JACOX[l l] analysed her results in the light of a possible phase transition in the 4-53 K region. Recent work by RYTTER and GRUEN [ 171 is also suggestive of a phase transition in the 50 K region. On the other hand, the calorimetric data [23] on solid CzH4 exhibit no anomaly until the sample approaches the melting point. Finally, the

Raman data on solid C2H4 at 79 K [12] exhibit factor group splittings as expected on the basis of two formula units in the primitive cell. Our investigations of the i.r. spectra of a film of CzH4 deposited at 8 K revealed structureless bands in the regions of active fundamentals. Warming the sample resulted in sharpening of the bands; by w 40 K 3io and Or2 exhibited three components each as was found by JACOX[l 11,while the 3, band exhibited four components. Further heating to 70 K resulted in sublimation of the sample as was also observed by JACOX.In view of the sample loss at 70 K the results of annealing studies by RYTTERand GRUEN [17] may be questionable. Thus, the multiplet structure of some of

Optical

constants

of CzH4 and CzD.,

tained from (a) individual spectra and (b) the ratio of transmission spectra for two different thicknesses (which generally largely cancels out the reflection effects). Table 2 gives fundamental frequency and integrated extinction coefficient data for solid CzH4 and C,D,; we believe these to be accurate to within 10 %. Corresponding values for the gas phase [19,20] are also given. Even though the extinction coefficients are drastically altered on condensation, the ratios Aso,,d/Agas for corresponding modes of C,H, and C2D4 are of similar magnitudes. Kramers-Kronig analysis of the extinction data obtained from (a) the individual spectra and (b) the ratio of the spectra of two samples gave the refractive indices (Figs 4,5) which agree within 1076, confirming the above conclusion on the reflection losses. The highest dispersion is in the region of a7, which is the strongest band in the thermal infrared region for both C2H4 and C2D4. The complex refractive indices obtained in this study were utilized to calculate the transmission spectrum of samples of known thicknesses. In all cases the agreement with the laboratory data was within 10 %. The k values are reliable only in the regions of absorption bands. The strongest, and therefore most diagnostic features of C,H, and C,D,,

the bands indicates crystalline character of the film and is not compatible with the monoclinic or any other centrosymmetric structure with two molecular units in the primitive cell. One should examine the possibility of pseudo-symmetry with two equivalent structures; for example, two orientations of CH2 units which give, effectively, more symmetric structure. (It is to be noted that the extra multiplicity has been observed only for the modes associated with the CH2 group vibrations.) Intensities

and optical constunts

The integrated absorbance, B, for each of the bands of CzH4 and CzD4 was evaluated from the transmission spectra for different sample thicknesses. As wasdone for CzNz [21], a baseline, Ia, was constructed by appropriate scaling of the transmission spectrum of the blank so as to coincide with the spectrum of the film in the nonabsorbing regions. The slopes of the plots of B vs film thickness for different bands gave the integrated extinction coefficients, A. Remarkable linearity of the plots with deviations less than 10% in all cases suggests that the reflection losses within the bands are not large. This is confirmed by comparisons of the line extinction

Table 2. Vibrational

GH,* Infrared:55

K

coefficient,

frequencies

(cm- ‘) and integrated

G&t

GDA*

R:79K

Infrared:55K 2338.5 2328.5

3092.2 3088.0 3068.3 3065.9 3006.0 2993.5

1348.0 1328.5 1226.7 1222.4

*Present work. tRef. [12]. $ Ref. [ZO]. §Ref. [19],

GH.+ Assignment i, (CH/CD)st

hohi

and C,D,

CzD., A~as

A%,,*

A~as

2.8 x lo4

6.9 x 10”:

1.3 x IO4

3.6 x 104$

i,, (CH/CD)st

1.6 x 10“

3.8 x 104:

8.7 x IO3

1.9 x 104:

i,, (CH,/CD,)def

4.1 x lo4

2.8 x 104$

2.1 x lo4

1.4 x 104$

2.5 x lo5

2.2 x 1051

1.3 x lo5

1.3 x 105$

1.4 x lo3

1.4 x 1035

6.7 x 10’

i, (CHI def) i, (CH,

rock)

734.0 737.0

i, (CH,/CD,)

725.0 722.0

a, (CHJCD,)wag

twist

720.5 951.5 941.6

825.8 822.5 819.5

C,H,

a* (C-C) St 1074.2 1071.4 1068.9

1041.9 1036.0 953.0 948.9 941.1 937.0

(cm- ‘) of crystalline

i, (CHst)

1621.7 1614.6 1440.2 1436.0 1434.0

i.r. band intensities

i, (CHst)

2188.9 2185.3

2972.9 1965.7

, ob-

o!

29

h (CH, 591.2 588.0

wag)

i,, (CH,/CD,)rock

30

G. ZHAOet al.

0.6 r 3.4

1.6 c I .4

2000 Wavenumber

Fig. 4. Complex refractive indices of crystalline C2H4 in the i.r. region.

6-

0.4

,0.3 3

,111

I 3000

I

ILL 2000

,3.2 3.

I

LIO L

Wavenumber

Fig. 5. Complex refractive indices of crystalline C,D, in the i.r. region.

Optical constants of C2H4 and C2D4 are the 950 cm - ’ and - 750 cm - ’ bands respectively. The optical constants also available in tabular form at 0.3 cm - ’ intervals can be utilized in radiative transfer calculations scattering

of

emission

flux,

absorption

and

by these species in a given environment.

191

31

B. L. CRAWFORD, J. E. LANCASTERand R. G. INSKEEP, J.

them. Phys. 21,678 (1953). [lo] C. BRECHERand R. S. HALFORD,

J. them. Phys.

[ll] i!f?i.)iAcOX,J. them. Phys. 36 140 (1962). [12] G. R. ELLIOT~~~ G. E. LEROI, ;. them. Phys.

351109 59, 1217

(1973). Acknowledgement-This

work

was

supported

by

NASA

Grant NAGW-637. REFERENCES

VI

G. HERZBERG, Infrared and Raman Spectra of Pofyatomic Molecules. Van Nostrand, Princeton (1945). E. B. WILSON, J. C. DECIUS and P. C. CROSS, 7’he Theory of Infrared and Raman Spectra. Dover, New York (1980). c31 R. L. BYER and M. GARBUNY, Appl. Optics 12, 1436

PI

(1973). [41 J. PEARL, R. HANEL, V. KLNDE, W. MAGUIRE, K. Fox, S. GUFTA, C. PONNAMPERUMA and F. RAULIN. Nature 280, 755 (1979).

[51 M. UYEMURA, S. MAEDA. Bull them. Sot. Japan 45,

1081 (1972). M. UYEMURA, S. DEGLJCHI, Y. NAKADA and T. ONADA, Bull. them. Sot. Japan 55, 384 (1982). 171 K. KING and W. T. KING, J. them. Phys. 80,974 (1984). PI R. D. AMOS, Chem. Phys. Lett. 114, 10 (1985).

PI

[13] D. A. DOWS, J. them. Phys. 36, 2836 (1962). [14] H. J. BECHER and A. ADRIAN, J. molec. Strut. 6, 479 (19701. [15]

D. VAN LERGERGHE, I. J. WRIGHTand J. L. DUNCAN, J. molec. Spectrosc. 42, 1251 (1972). [ 161 J. L. DUNCAN, D. C. MCLEAN and P. D. MALLINSON, J. molec. Spectrosc. 45, 221 (1973). [17] E. RYTTER and D. M. GRUEN, Spectrochim. Acta 35A, 199 (1979). [18] A. M. THORNDIKE, A. J. WELLS and E. B. WILSON, J. them. Phys. 15, 157 (1946). [19] R. C. GOLIKE, 1. M. MILLS, W. B. PERSON and B. CRAWFORD, JR., J. them. Phvs. 25. 1266 (1956). [ZO] T. TAKANAGA, S. KOND~ and S. SAEKI, J: them. Phys. 70,247 1 (1979). [21] M. J. OSPINA, G. ZHAoand R. K. KHANNA, Spectrochim. Acta, this issue. [22] G. J. H. VANNES and A. Vos, Acta crystallogr. B35, 2593 (1979). [23] C. J. EGAN and J. D. KEMP, J. Am. them. Sot. 59, 1264 (1937).