J. Phys. Chem. lQ00, 84, 3587-3591 (7) R. Chiarizia. P. f3. Danesi, and C. Dornenichini, J. Inorg. Nucl. Chem., 40, 1409 (1978). (8) G. R. Dyrkacz, G. F. Vandegrift, M. W. Thornsen, and E. P. Horwltz, J. Phys. Chem., 83, 670 (1979). (9) G. 6. Honaker aind H. Freiser, J. Phys. Chem., 66, 127 (1962). (10) 8. E. McClellan (and H. Freiser, Ana/. Chem., 36, 2262 (1966). (11) J. S. Oh and H. I-reiser, Anal. Chem., 30, 295 (1967). (12) P. R. Subbaraman, M. Cordes, and H. Freiser, Anal. Chem., 41, 1878 (1969). (13) 0. Colovos, A. Yokoyama, and H. Freiser, Anal. Chem., 41, 1878 (1969).
3587
(14) T. Seklne and Y. Hasegawa, "Solvent Extraction Chemistry", Marceii Dekker, 1977, Chapter 5. (15) R. Chiarlzia, iP. R. Danesl, and S. Fornarini, J. Inorg. Nucl. Chem., 41, 1465 (1979). (16) P. R. Danesi and R. Chaizia, CRC Crit. Rev. Anal. Chem., in press. (17) T, s. Laxami,,laraganau, s. K, patil, and H. D, J. Nucl, Chem., 26, 1001 (1966). (18) P. R. Danesi,,R. Chiarlzla, and G. F. Vandegrift, J . Phys. Chem., 84, 3455 (1080). (19) D. Kalina and T. Muscatello, prlvate communication.
Low-Frequency Raman Spectrum and Asymmetric Potentiial Functfon for Internal Rotation of Giaseous n-Butanet D. A. C. Compton, $. Montero,t and W. F. Murphy' Division of Chemistry, Natlonal Research Council of Canada, Ottawa, Ontario, Can,ada K1A OR6 (Received:June 19, 1980; In Final Form: September 17, 1980)
The Raman spectrum of gaseous n-butane has been recorded below 350 cm-l by using higher sensitivity than in previous work. Two regions with complex band structure were recorded near 250 and 110 cm-'; both of these regions were assigned to the asymmetric torsional modes of n-butane. When the observed data were used to determine the potential function to internal rotation for the asymmetric torsion, coefficients of Vl = 395 f 2, 'V3 = 1166 f 5, V, = 26 f 1, and V s = -34 f 2 cm-l were found. The resulting potential function has an enthalpy difference between the conformers of 311 f 10 cm-l(3.72 f 0.12 kJ mol-l) and a gauche dihedral angle of L18 f lo. It is shown that the results are consistent with experimental heat capacity and entropy data.
Introduction The nature of the possible conformations of n-butane has been of interest for many years because this molecule is the simplest alhane which may exist as a mixture of rotational conformers. Many experimental techniques have been applied to obtain information about the conformers; experimental results'-7 for gaseous n-butane are summarized in Table I. Examination of these results shows a poor level of consistency, with AH' values ranging from 2.08 to 4.04 IrJ mol-l. Two of the present authors have been involved in previous spectroscopic studies of this compound wherein relatively high values, 4.044 and 3.717 kJ mol-l, were obtained for AH'. In the first study: the relative temperature dependence of pairs of bands in the Raman spectrum was monitored to measure the change in relative concentration of the conformers with temperature, and thus obtain a value of AHo. More recent res u l t indicate ~ ~ ~ ~that the spectrum is more complicated than had been believed, so that members of the chosen band pairs cannot be attributed solely to ,a given conformer; the results of that study are thus placed in doubt. In a study of the low-frequency infrared and Raman spectra,'I the conclusions were limited because olf the low intensity of the observed spectra. Other recent estiimates for AHo of' gaseous n-butane are significantly lower than these spectroscopic values. Calculations have been carried out to fit experimental heat capacity and entropy data by use of the methods of statistical mechanic^,^ but the low-frequency vibrational data available to these authors were incomplete. A very low value of AHo, 2.08 kJ mol-l, may be estimated from the Issued as NRCC No. 18748. t NRCC Visiting Resiearch Officer from Instituto de Estrudura de
la Materia, C.S.I.C., Serrano 119, Madrid1 (6), Spain. 0022-36!54/80/2084-3587$01 .OO/O
results of an eljectron diffraction study.6 The present situdy was undertaken when it was realized that h a n data obtained for n-butane by using the higher sensitivity now available to us would allow measurement of more accurate torsional data and thus a better definition of the asymmetric potential function. From the resulting function, values for AH' and the barriers to internal rotation can be obtained.
Experimental Section The Raman spectrum of n-butane was recorded below 350 cm-l with a Spex 14018 double monochromator equipped with an RCA-C31034 photomultiplier and photon-counting dletection. A Coherent Radiation CR-12 argon ion laser was used at 514.5 nm with a power of up to 8 w. Research grade n-butane (Phillips Petroleum Co., 99.94 mol 9% purity) was studied by using a Brewster angle cell with the multipass intensity enhancement system described previously.1° Spectra at elevated temperatures were obtained by heating the cell in a stream of hot air. The only impurity observed was air, which was removed by condensing tlhe sample into a sidearm and pumping off the residual gases. Results The higher instrumental sensitivity available to us allowed weak features in the spectrum of gaseous n-butane to be recorded with better resolution than in the previous s t ~ d y .The ~ spectral regions between 130 and 90 and between 270 and 210 cm-' are shown in Figures 1 and 2, respectively, and the observed bands are listed in Table 11. All of these observed bands are polarized. Previously, only two polarized Q branches at 116 and 107 cm-l were observed in the lower frequency region, and 0 1980 American Chemical Soclety
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Compton et al.
'ABLE I: Experimentally Determined Valuesa for the Asymmetric Potential Function of Gaseous n-Butane
technique
AH
thermodynamic thermodynamic electron diffraction Raman (temperature dependence) thermodynamic electron diffraction Raman/infraredc (torsional analysis) Raman (torsional analysis)
* *
barrier s-trans/ gauche/ gauche gauche gauche dihedral angle
ref
3.35 15.1 1 2.9 15.5 22.2 2 2.6 i 1.5 117 f 8 3 4.04 f 0.23 4 14.6 3.18 5 115 f 6 2.08 0.92b 6 3.71 9.2 8.9 118 7 3.72 0.12 15.2 15.2 118 f 1 this work Units: AH" and barriers in kJ mol-', dihedral angle in degrees. Conversion factors used: 1 kJ mol-' = 239.0 cal mol-' = 3.59 cm-'. Reference 6 gives the value of A G O for the equilibrium between the s-trans and one of the equivalent gauche mformers.8 If we assume that AS" for this equilibrium is negligible compared to the estimated error, the value for AH" sted here is obtained. Subsequent results from our statistical thermodynamics calculations (see below) predict a small A S " mtribution which has the effect of raising this tabulated value of AH" by 0.3 kJ mol-'. These values were obtained from i approximate potential function based on only three pieces of torsional data. TABLE 11: Observed Raman Spectrum of Gaseous n-Butane below 300 cm-' freq, cm-'
re1 inta
assignmentb
obsd calcdC
254.gd m t 248.2e w, sh t 244.2d S -0.4 t24-0 239.1d m t3+l 0.6 23 2.0e t44-2 -0.1 m 230. 5e t vw, sh 224.ge W -0.1 t54-3 123 vw, br 116.6d S g 1' 4- of 0.1 114.1d g2*+11' 0.0 m 111.3d g 3 f 4- 2f -0.1 m 105.6e W g a s, strong; m, medium; w, weak; sh, shoulder; br, broad; v, very. All bands listed here are very weak compared with the higher frequency fundamentals. t, s-trans; g, gauche. Bands not assigned are discussed in text. Using the potential coefficients listed under model I1 in Table 111. Estimated frequency uncertainty t0.5 cm''. e Estimated frequency uncertainty * 1.0 cm-',
1 I30
110
I20
100
90
igure 1, iaman spectrum of gaseous n-butane between 130 and 0 cm-'; the observed bands were assigned to the asymmetric torsion f the gauche conformer. Experimental conditions: laser power, 6 J; spectral slit width, 0.7 cm-'; time constant, 100 s; 100 counts per econd full scale; sample pressure, 900 torr. I l lr 4 5 5 z t 4 y 2
260
250
240
230
220
210
I
Vgure 2. Raman spectrum of gaseous n-butane between 270 and 110 cm-'; the observed bands were assigned to the overtones of the isymmetric torsion of the s-trans conformer. Experimentalconditions vere the same as for Figure 1.
hese were assigned to asymmetric torsional transitions of ,he gauche c ~ n f o r m e r .Under ~ the present experimental :onditions, several more bands could be resolved in this 'egion. It is agreed that these bands arise from asymmetric orsional transitions of the gauche conformer; however, the letailed assignment and analysis of these bands must now )e revised. In addition, we have observed a very weak, )road depolarized band near 123 cm-l, which is not ap-
parent in Figure 2. This band corresponds in frequency to a similarly featureless absorption in the far-infrared spectrum which was assigned to the asymmetric torsion of the s-trans c ~ n f o r m e r .Since ~ the Raman band cannot be assigned to the same mode because of the mutual exclusion rule, it is assigned to the overlapping, depolarized R branches of the gauche asymmetric torsion. The spectral region 270-210 cm-l, shown in Figure 2, is more complicated. These bands were recorded at a number of temperatures along with the stronger band at 319 cm-l, which is known to be a bending mode of the gauche c o n f ~ r m e r . ~To ? ~allow for changes in the relative populations of the torsional levels at different temperatures, the integrated intensity of the entire region between 270 and 200 cm-l was measured at each temperature. It was clear that the intensity of the 319-cm-l band increased markedly with increasing temperature relative t o the intensities of the bands in the 240-cm-l region, thus indicating t h a t the latter bands are due to A transitions of the s-trans conformer, in contrast with t i e previous ass i g n m e n t ~At ~ higher temperatures these bands became broad and relatively featureless. However, we were able to determine that the relative intensity of the bands at 255 and 244 cm-l was approximately independent of temperature, indicating that both of these bands arise from the s-trans conformer. In particular, it was considered unlikely that any of these features are due to gauche methyl torsions in contrast to the previous as~ignment.~ The relative
The Journal of Physlcal Chemistty, Vol. 84, No. 26, 1980 3589
Raman Spectrum of n-Butane
TABLE 111: Calculated Values for the Coefficients of the Asymmetric Potential Function of n-Butanea model I1 model I
-
value
dispersion
VI
741.3
3.1
v2
b
coeff
v3 v 4
v,
V6 AH rJc
1142.4 b b -20.6 54 2 0.67 116
9.3
-
dis,, value persion
395.1 b 1166.0
2oool
--
1 \
/ I
1.9 5.3
b
26.3 4.2 -34.0 311 0.28 118
1.2 2.1
gauche dihedral angle a All values are in CUI-'. The dependence of the reduced rotational constant, F,on the dihedral angle is takenihom ref 7. b' These coefficients were found to he insignificant and were fixed to zero in the final calculaStandard deviation of the frequency fit. tions.
intensity measurements of the 240- and 319-cm-l regions were not precise enough to accurately determine AH", but a value in the range 3-5 kJ mol-l would be required to explain the observed temperature behavior. Since previous vibrational a s s i g n m e n t ~ ~ agree ~ ~ Jthat ~ the lowest frequency s-trans 4 fundamental is at 431 emW1, the bands in the 240-cm-' region must be overtones of the s-trans asymmetric torsion, which has been observed7 at approximately 121 cm-l in the infrared spectrum. The strongest band (24.4.2 cm-l) appears to be the first of a fairly regular series of bands proceeding to lower frequency; assignments of this series to overtones of the s-trans asymmetric torsion are given in Table 11. The somewhat weaker band at 255 cm-l does not seem to be part of this series, as the spacing between it and the 244-cm-l band is much larger than the spacings between the subsequent bands in the series. I t is considered that this band and the weak shoulderri at 248.2 and 230.5 cm-l form part of a much weaker series of bands arising from asymmetric torsional transitions of molecules in excited methyl torsional states. The methyl torsions observed near 200 cm-l in the far-infrared spectrum7each show a series of excited state bands, which indicates that some interaction exists between the A, symmetry methyl and asymmetric torsional modes of this molecule. A preliminary study of the temperature behavior of the entire vibrational spectrum indicates that almost all observed bands include overlapping contributions from both conformers. This observation reinforces previous doubts7 regarding the AHo measurement made previously in this l a b ~ r a t o r y .However, ~ a detailed consideration of these data is beyond the scope of the present article. Asymmetric Potential Function. The asymmetric potential function for the internal rlotation about the CHZ-CH, bond in rib-butane was calculated in the same manner as previo~sly,~ using the present assignments (Table 11). The spectral data were fit to the usual expression for the potential function in terms of the dihedral angle a
V(a) = 1/22cV,(1- cos i a ) I
using coefficients V1 to v6. AH"was introduced into the calculations as a forbidden transition, and given a low weighting, in order to fix the relative energies of the gauche and s-trans minima. The value of AHo was then varied in order to obtain the best fit. I t became apparent during the calculations that the available data, as assigned in Table I][,could be fit by using
I +
I80
I
0
1
I
-180
DIHEDRAL ANGLE
Flgure 3. The asyrnmetric potential function of n-butane, showing the observed torsional itransitions. The dihedral angle of zero corresponds to the s-trans conformer. The gauche levels are doubly degenerate.
two potential functions, listed in Table I11 as models I and 11. Neither of the models used was able to adequately fit the band at 105.6 cm-l, so it was dropped out in later calculations. It is likely that some perturbation is occurring to affect the frequency of this band. The first fit of the data (model I in Table 111)was made by using only the coefficients Vl, V3,and V6,since in an earlier study of the asymmetric potential function of the isoelectronic compound, ethyl methyl ether,12only these three coefficients were found to be significant. For the n-butane data, the resulting potential function had a AH" of 542 cm-l (6.5 kJ mol-l), much larger than any of the previous estimates. Because of this problem, attempts were made to fit the data without restricting any coefficients during the initial stages of the procedure. In this case a small value of V5 was found to be significant and have a relatively large effect on the calculated results. The resulting fit (model I1 in Table 111) was not only better than model I in terms of dispersions and standard deviation, but also resulted in a value for AH" which was considered to be pliysically more realistic. The potential calculated for the asymmetric internal rotation of n-butane is shown Figure 3. The function is dominated by the Vl and V3 terms, with V5 and v6 acting as corrections tlo accurately fit the data. The results were found to be very sensitive to the value of AH" used, sinice a change in AHo of only a few cm-l from the final value results in a significant increase in the dispersions and standard deviation of the fit. Thus we estimate a precision for AHo of f10 cm-l, and so the final value for AHo is given as 311 f 10 cm-' (3.72 f 0.12 kJ mol-I). However, it must be pointed out that the actual error in the calculated AHo value depends on various assumptions present in the mathematical model, so a more detailed error analysis is not possible. The significance of the potential coefficient V, in the final fit requires some comment. A positive V5 value contributes maxima to the potential function at f36, f108, and f180". The angle of 36" is sufficiently removed from the s-trans minimum to have only a small effect on the lower torsional levels in this well. The maxima at &108", however, are close to the gauche minima at f120°; V, therefore acts essentially as a correction term to the fit of the gauche data. The value of 118 f loobtained for the dihedral angle of the gauche conformer from the potential function agrees with the electron diffraction result: 115 f 6", to within the experimental error. Thermodynamic Functions. In a recent statistical mechanical calculation of the thermodynamic functions
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The Journal of Physical Chemktty, Vol. 84, No. 26, 1980
Compton et al.
TABLE IV: Comparison between Values Derived Experimentally'for the Heat Capacity and Entropy of n-Butane as an Ideal Gas, and Those Values Calculated by Using Statistical Methods in This Work Cpo/JK-' mol-' S"/JK-' mol-' calcda T/K 272.66
I
calcda I1
exptlb
I
I1
exptl
297.9
301.2
301.5 5 0.P 300.7d
344.9 113.2 110.2 110.6 f 1.1 451.6 139.9 137.1 138.0 fr 1.4 a This work. I and I1 refer to model I and model I1 in Table 111, where A H = 542 and 311 cm-l, respectively. ence 13. Reference 14. Reference 1 5 , no error estimate given.
of gaseous n-butane? the three barriers to internal rotation and AH' were used as variable parameters to fit experimental values of the heat capacity13and entropy.14J5 As a check of the current values for the barriers to methyl rotation7 and AH', a direct calculation of the heat capacity and entropy was made by using a similar method. The method used for calculating the the thermodynamic functions for an ideal gas existing as a mixture of conformers has been detailed previously by one of the present authors;16it is essentially the same as that used by Chen et al.5 The s-trans and gauche conformers of n-butane were treated as separate pure compounds and the functions were calculated for each. The functions of the mixture were then obtained from these by using the experimental value for AH'. The principal moments of inertia of each conformer were calculated from the structure proposed in the recent electron diffraction study,6the gauche dihedral angle was taken to be 118' as found from the asymmetric potential function. The fundamental vibrational frequencies above 500 cm-' were taken from tabulated data,ll those below 500 cm-' from the recent low-frequency vibrational study7and from the results obtained in this work. The contributions from the methyl torsional modes were calculated from tabled7 by using a barrier height of 13.43 kJ mol-l for s-trans-n-b~tane;~ this value was assumed for the gauche conformer as well. The thermodynamic functions were calculated by using the two different values of AHo found for models I and 11, and each set of results is compared in Table IV with at sethe best experimental values for CP0l3and S014J5 lected temperatures. It can be seen that the calculated statistical values match the experimental values within the range of quoted experimental uncertainty only for model 11; it is thus concluded that only the value of AH' obtained with this model is physically reasonable. Due to the uncertainty in the barrier height for the gauche methyl torsions, we did not feel justified in tabulating the values for the thermodynamic functions over a range of temperatures. However, we used the method described to calculate the concentration of the gauche conformer at 298.16 and 1000 K to be 32 and 57%, respectively, slightly different from the previous estimates5 of 35 and 55%. The room temperature value differs significantly from the previous estimate of 46 f 9% obtained from electron diffraction results.6 Conclusions The observation of the low-frequency Raman spectrum of n-butane by using higher sensitivity than was previously available has required a reevaluation of the torsional potential functions. The temperature dependence of the bands near 250 cm-l relative to that of the known gauche fundamental at 318 cm-l shows that the former bands are due to the s-trans conformer. The value of V , obtained for n-butane, 1166 cm-l (13.95 kJ mol-l), is very close to the threefold barrier height
Refer-
calculated for propane of 1139 f 10 cm-118or 1152 cm-l l9 from vibrational and microwave spectroscopic techniques, respectively. The actual barriers to internal rotation in n-butane are different from V3due to the contributions from other terms, being 1272 cm-l (s-trans/gauche and gauche/gauche) and 957 cm-l (gauche/s-trans). The barrier for s-trans-n-butane is presumably increased from that in propane by the same intramolecular forces which increase the CCC bond angles from 112 f 1' in propane20 to 113.8 f 0.4' in n-butane. A further interesting comparison may be made between the values of Vl and AH' of n-butane and the isoelectronic molecule ethyl methyl ether,12the latter molecule having a AHoof 390 cm-l and Vl of 562 cm-l l2 (or 535 cm-l for CD30C2H5,wherein the coefficients were better defined). The value of AH' for the ether was attributed to steric hindrance between the methyl tops in the gauche position, resulting in a positive value for Vl and a gauche dihedral angle of 116'. The lower AH' and the somewhat larger dihedral angle of 118" to be found in n-butane suggests that the steric interaction is smaller in this case. This is not unreasonable since the skeletal bond lengths and angles are significantly smaller in s-trans-ethyl methyl ether,21 which indicates that the methyl tops will be closer together in the gauche conformer of this compound relative to n-butane. The variation and accuracy of the available results for AH' of gaseous n-butane as given in Table I deserves further comment. The present result has the advantage that it has been derived from data obtained for each of the two conformers, while other methods rely on fitting data observed for the overall system by using a model which includes as a parameter the relative abundance of the conformers. The fact that our results reproduce the observed thermodynamic functions13-15so well in the statistical thermodynamics calculations reinforces our confidence in them. Indeed, the problem with the previous determinations of AH" from the thermodynamic funct i o n ~originates ~ ~ ~ , ~in the use of poor estimates for the torsional vibrational frequencies, which were only recently measured e~perimentally.~ The AH' results attributed to electron diffraction measurements in Table I were estimated by us from the reported free energy difference, AGO (see footnote b of Table I). The free energy difference was obtained from the relative concentration of the two conformers entering as a parameter in the fit of the reduced molecular intensity curve.6 Shrinkage corrections used in this analysis were estimated from vibrational frequencies obtained from normal coordinate calculations. The torsional frequencies used22were an average of 10% lower than subsequently determined experimental value^.^ The effect of this factor combined with other necessary assumptions made in the electron diffraction data analysis could give rise to an error in the AGO result which is even larger than that predicted.6 Thus, we feel that present results are not incompatible with those obtained previ-
J. PhyS. Ghem. 1980, 8 4 , 3591-3592
ously; however, the method used here yields a value of AH" which has a much higher precision than was previously possible. Two recent experimental determinations of AH" for liquid n-butane have recently been reported, which were obtained from the temperaturez3iz4and pressurez3dependence of the Raman spectrum. The resulting values of AH", 2.26 f 0.4223and 2.33 f 0.05z4kJ mol-l, agree well with each other, but they are significantly different from our value of 3.72 d: 0.12 kJ mol-l for the gaseous phase. A difference between the AH"values for the two phases is to be expected, but the magnitude of the difference is surprising. Statistical mechanical calculation^^^ predict that steric effects iin the liquid would lower AHo by about 0.7 kJ mol-' from the gaseous phase value. Other contributions to the observed lowering could arise from dipole or dispersive interactions, but a detailed quantitative explanation of the difference, which we consider to be real, remains to be given.
References and Notes (1) K. S. Pitzer, J . Chem. Phys., 8, 711 (1940). (2) K. Ito, J. Am. Chem. Soc., 75, 2430 (1953). (3) R. A. Bonham and L. S. Bartell, J . Am. Chem. Soc., 81, 3491 I1959). (4) A. L. Verma, W. F. Murphy, and H. J. Bernstein, J. Chem. Phys. 60, 1540 (1974).
359 1
S. S. Chen, R. C. Wilhoit, and B. J. Zwolinski, J. Phys. Chem. Ref. Data, 4, 859 (1975). W. F. Bradford, S.Fitzwater, and L. S. Bartell, J. Mol. Struct., 38, 185 (1977). J. R. Durig anld D. A. C. Compton, J. Phys. Chem., 83, 265 (1979). L. S. Bartell ,and D. A, Kohl, J. Chem. Phys,, 39, 3097 (1963). I. Harada, H. Takeuchi, M. Sakaklbara, H. Matsuura, and T. Shlmanouchl, Btdl. Chem. SOC.Jpn., 50, 102 (1977). W. Kiefer, H. J. Bernstein, H. Wieser, and M. Danyluk, J. Mol. Spectrosc., 43,393 (1972). T. Shimanouohi, H. Matsuura, Y. Ogawa, and I. Harada, J. Phys. Chem. Ref. IDafa, 7 , 1323 (1978). J. R. Durlg and D. A. C. Compton, J. Chem. Phys., 69, 4713 (1978). 8. P. Dailey and W. A. Felslng, J. Am. Chem. Soc., 65, 44 (1943). J. G. Aston and C. H. Messerly, J . Am. Chem. Soc., 62, 1917 (1940). T. R. Das and N. R. Kuloor, Indian J . Techno/., 5 , 33 (1967). D. A. C. Comlpton, J . Chem. Soc., Perkin Trans. 2 , 1307 (1977). D. R. Stull, E. F. Westrum, and G. C. Slnke, "Chemical Thermodynamics of Organic Compounds", Wiley, New York, 1969. J. R. Durig, P. Groner, and M. G. Griffin, J. Chem. Phys., 66, 3061 (1977). A. Trlnkaus, PI. Dreizler, and H. D. Rudolph, Z. Naturforsch. A , 28, 750 (1973). T. Iijima, Bull. Chem. SOC. Jpn., 45, 1291 (1972). M. Hayashi and K. Kuwada, J . Mol. Struct., 28, 147 (1975). Supplementary Publication to ref 6, available as SUP 26054 from British Library Lending Division, Boston Spa, Wetherby, Yorkshlre, LS23 7BQ. J. Devaure and J. Lascombe, Nouv. J. Chim., 3, 579 (1979). S. Kint, J. R. Zscherer, and R. G. Snyder, J. Chem. Phys., 73, 2599 (1980). L. R. Pratt, C. S.Hsu, and 0. Chandler, J. Chem. Phys., 68, 4202 (1978).
Selective Photoinduced Condensation of a Semiconductive Film from Gaseous Reactants J. F. Glullani and A. Auerbach" Optlcal Sciences Division, Naval Research Laboratory, Washington, D.C. 20375 (Received: June 30, 1980; In Fhal Form: September IO, 1980)
Selective photoinduced condensation of a semiconductivefilm froim the chemionization reaction of antimony pentafluoride and benzyl chloride has been detected. This film is deposited only in the direct path of a beam of light on the inner surface of the reaction vessel window. The electrical conductivity of this film increased by as much as four orders of magnitude with time of deposition. The film is both photosensitive and luminescent.
We report what is believed to be a specific photocatalyzed effect which was observed while investigating the absorption spectra of gaseous ions, resulting from the chemionization reaction between colorless antimony pentafluoride (SbFJ, and benzyl chloride (C7H7Cl).' When a gently focussed l-cm diameter beam of light from a 150-W high-pressure unfiltered xenon cw lamp was directed through a fused quartz window into the reaction zone, a film formed directly in the path of the light beam on the inner surface of the transmitting window. This film exhibited a number of interesting properties. It appeared greyish in color, and began to luminescence strongly in the blue under the xenon light as the film developed with time. It also exhibited an electrical conductivity which increased by four orders of m,agnitude with time of deposition. An additional increase in electrical conductivity was measured upon the direct exposure to the xenon exciting light itself. A schematic of the experimental apparatus in which this phenomenon was detected is shown in Figure 1. The reactants enter the reaction zone through gas inlet sources
* NRC-NRL
Postdoctoral Fellow.
positioned 90" with respect to one another. The benzyl chloride source was equipped with a heater, which was kept at around 350 "C during the experiment. A 150-W Bausch and Lomb xenon lamp output beam was focussed by a 10-cm fused quartz lens system through the quartz window, between two silver-coated contacts painted on the inside window face, into the reaction zone. The contacts were electrically connected to a Keithley Model 600B electrometer via vacuum feedthroughs. The reaction vessel was pumped by a liquid nitrogen cold trap and a 5-cm diffusion pump. The reaction zone pressure was monitored with a thermocosple gauge and capacitance monometer. The reactants were cooled to 0 "C, and their vapors were mixed in the flow tube. The total system pressure was generally below 10 pm, with approximately 5 pm contributed by each gas. As mentioned above, prior to entering the reaction zone, the benzyl chloride vapor was heated to 350 "C. Shortly after the reactants entered the flow tube, a condensate began to form on that part of the cell window exposed to the light beam: ils size and shape coincided with the beam spot. This condensate formed only in the presence of both
This article not subject to US. Copyright. Published 1980 by the American Chemical Society
