1220
J . Phys. Chem. 1991, 95, 1220-1223
distance of 3.079 A in the case of T2. Therefore, the abnormally short P-0 distance (2.722 A) in the case of the T2 structure of H20,H4P+ might be attributable to an inclination toward a pentavalcnt complcxation due to the high formal charge on the phosphorus atom. Wc note also that with regard to the protontransfer transition states low barriers are associated with the large positive charge on the transferring H atom.3' I n the case of (H,P- -H- -pH,)+, the transferring H atom carries a near-zero charge (Table I ) , which is exceptionally different from other proton-transfer systems we have examined. The low protonic charactcr of thc H atoms in PH4+ is in keeping with other qualitative fcaturcs of P-H bonds. Phosphorus is generally classeds4as having an electronegativity less than hydrogen, while carbon, nitrogen, and oxygen are progressively greater. Sulfur and carbon are nearly the same. Therefore, one can class P-H bonds in phosphinic compounds as more hydridic than protonic. (54) Allcn, L. C. J . Aril. Chem. SOC.1989, 111, 9003.
Conclusion
It must be first stressed that the work presented here was largely directed toward sensitizing both the theoretical and experimental communities to the mechanistic complexity of what appear to be simple ion-molecule systems. Our studies are largely semiquantitative with the view that if detailed kinetic treatments are undertaken in the future, investigators will generate their own theoretical parameters for whatever dynamic model is used. Our goal here is suggestive. We think certain systems presented above merit detailed experimental study. In general, any system that has potentially exceptional dynamical behavior due to the usual energic position of the critical configuration allows a combined experimental and theoretical analysis. A successful combined analysis gives a double pleasure. The theoretician finds satisfaction in rationalizing physical events, and the experimentalist understands what is occurring at the detailed molecular level. Acknowledgment. We thank the computing staffs at Paris and Orsay for their help, in particular J.-P. Gauthier.
Cotton-Mouton Effects, Magnetic Hyperpolarizabilities, and Magnetic Anisotropies of the Methyl Halides. Comparison with Molecular Zeeman and High-Field NMR Spectroscopic Results Michael H. Coonan and Geoffrey L. D. Ritchie*q' Department of Chemistry, University of New England, Armidale, New South Wales 2351, Australia (Received: June 27, 1990; In Final Form: August 21, 1990)
Measurements of the vapor-phase Cotton-Mouton effects of methyl fluoride, methyl bromide, and methyl iodide are reported. Analysis of the results, in conjunction with those of an earlier study of methane and methyl chloride, shows that in the series CH,X ( X = H, F, CI, Br, I ) the magnetic hyperpolarizability anisotropy, which is related to the quadratic response of the molecular polarizability to B magnetic field, is positive in sign and roughly proportional in magnitude to the mean polarizability. Thc magnetic anisotropies ( 1029Ax/JT-2) found for methyl chloride, methyl bromide, and methyl iodide (CHJI, -I 5.0 i 1.3; CH,Br, -15.1 & 0.8; CH31,-18.0 & I . 5 ) are compared with values obtained by the molecular Zeeman and high-field 2tl N M R spcctroscopic methods.
Introduction
The magnetic anisotropy of a diamagnetic molecule is a fundamental descriptor of the electronic charge distribution and its interaction with an applied magnetic field, and it is closely related to several other basic electric and magnetic properties2 There are three main experimental routes to this quantity: the Cotton-Mouton effect (magnetic-field-induced birefringen~e),~-~ presently under investigation by two groups, one in Ulm, West Germany, and the other in Armidale, Australia; the molecular Zeeman effect in beam-maser or microwave spectroscopy, the latter developed by Flygare and his collaborator^^^^ to become by far the most prolific source of reliable magnetic anisotropies; and, ( I ) The experimental work described here was performed in the School of Chemistry, University of Sydney, New South Wales 2006, Australia. (2) For a summary of some relevant equations see: Lukins, P. B.; Laver, D. R.; Buckingham. A. D.; Ritchie, G. L. D. J . Phys. Chem. 1985. 89, 1309-1 3 12. (3) Cotton, A.; Mouton, H . C.R. Hebd. Seances Acud. Sci. 1905, 141, 317-319. 349-351, (4) Buckingham, A . D.; Pople, J . A. Proc. Phys. Soc.. London, Sect. B 1956. 69. I 1 33-1 138. ( 5 ) Buckingham, A . D.; Prichard, W. H.; Whiffen, D. H . Trans. Faraday SOC.1967. 63. 1057-1064. (6) Hiittner. W.; Flygare, W. H. J . Chem. Phys. 1967, 47, 4137-4145. (7) Hiittner. W.; Lo. M.-K.; Flygare, W. H . J . Chem. Phys. 1968, 48, 1206- I 220.
more recently, high-field NMR spectroscopy, first exploited for this purpose by Lohman and MacLean8 and subsequently refined by others. The position in relation to the magnetic anisotropies of the methyl halides is that in 1970 and 1974 the anisotropies of CH,CI, CH,Br, CH31,9aand C H , P b were derived in applications of the microwave Zeeman method and in 1978 that of CH3CI was much more precisely specified by the beam-maser Zeeman method;'" in 1988 a value for CH3CI was obtained from measurements of the temperature dependence of the vapor-phase Cotton-Mouton effect;" and, also in 1988, the magnetic anisotropies of CH3CI, CH3Br, and CH,I were deduced from the ?HN M R spectra of the deuterated compounds.I2 In view of the conclusion in the report of the NMR investigation that the magnetic anisotropy of CH31 as determined by the microwave Zeeman method is about 14% too high, we were prompted to complement our earlier study" of CH4 and CH,CI with measurements of the Cotton-Mouton effects of CH3F, CH,Br, and (8) Lohman, J . A. B.; MacLean, C. Chem. Phys. 1978, 35, 269-274. (9) (a) Vanderhart, D. L.; Flygare, W. H. Mol. Phys. 1970, 18, 77-93. (b) Norris, C. L.; Pearson. E. F.; Flygare, W. H . J . Chem. Phys. 1974, 60, 1758-1 760.
(IO) Ellenbroek, A. W.; Dymanus, A. Chem. Phys. 1978, 35, 227-237. ( I I ) Lukins. P. B.; Ritchie, G.L. D. J . Phys. Chem. 1988, 92, 2013-2015. (12) Van Zijl, P C. M.; Bothner-By, A A . J . Magn. Reson. 1988, 79, 439-447.
0022-3654/9l/2095-l220%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 9.5, No. 3, 1991 1221
Cotton-Mouton Effects of Methyl Halides TABLE I: Cotton-Mouton Effects of Methyl Fluoride, Methyl
- 0.~1
Bromide and Methyl Iodide at 632.8 nm
TIK
no. of Dress.
296.2 296. I
7 9
421.7 4 10.2 402.6 389. I 378. I 351.4 334. I 3 17.4 304.8 294.2
IO 9
466.8 448.6 442.8 407.9 405.8 390.2 358.3 334.6 3 15.5 31 1 . 1 291.2 295.3
IO 6 II 5 5 9 9 9
9 9
IO 9 6 IO 9 9
11
8 9 7
max I 06B/ nIkPa m 3 mol-' Methyl Fluoride 980 -209 930 -209
1027,CJ mJ A-* mol-' -0. I95 f 0.007 -0.198 f 0.006
Methyl Bromide 229 -250 I78 -260 21 I -278 227 -283 188 -296 208 -346 171 -390 203 -448 I69 -502 I68 -560
- I .30 f 0.08 -1.34 f 0.04 -1.34 f 0.05 -1.40 f 0.05 -1.51 f 0.04 -1.58 f 0.03 -1.72 f 0.05 -1.80 f 0.02 -I .87 I: 0.04 -1.96 f 0.05
Methyl Iodide 90 -360 I I9 -370 I I8 -370 IiI -380 35 -380 35 -400 95 -470 95 -560 30 -700 41 -700 37 -800 30 -800
-1.81 -2.00 -1.92 -1.97 -2.07 -2.35 -2.51 -2.55 -2.90 -2.81 -3.08 -3.13
f 0.08 f 0.10 f 0.07 f 0.07 f 0.1 3 f 0.10 f 0.06 f 0.05 f 0.08 f 0.06 f 0.10 f 0.10
" Dcnsity sccond viriill cocfricicnts from ref 16. CH31, so as to establish a basis for comparison of results from the three methods. In addition, the necessity to examine the temperature dependence of the effect, in order to separate the orientation and distortion contributions, provided information about the magnetic hyperpolarizability anisotropy, another observable molecular property that is currently of interest.I3 Experimental Section
Apparatus and procedures for observations of the magneticfield-induced birefringences of the vapors were as previously d e ~ c r i b e d ,cxccpt '~ that the microcomputer and chart recorder were replaced by an Apple Ile microcomputer fitted with a Data Translation DT2832 analog input module; this modification permitted routinc mcasurements of retardances as small as 5 X rad, with a limiting sensitivity of f 2 X rad, and gave improved ease of data acquisition and manipulation. I n the case of methyl fluoride, the smallness of the effect and of the available gas sample precluded a temperature-dependence study, and the measurements were confined to duplicate runs, over pressure ranges up to 980 kPa, at two temperatures (296.2, 296.1 K ) in the vicinity of room temperature. However, for methyl bromide and methyl iodide (and, as previously reported," methyl chloride), mcasurcmcnts were possible over ranges of pressure and temperature (CH,Br, IO temperatures, 294-422 K, maximum pressure 229 kPa; CH31, 12 temperatures, 295-467 K, maximum pressure 1 19 kPa). and the temperature dependences were adequately determined. The samples were as follows: methyl fluoride (Matheson, >99.0%) and methyl bromide (Fluka, >99%),used without further treatment; methyl iodide (Fluka. >99%), washed successively with dilute sodium thiosulfate, water, dilute sodium carbonate, and water, then dried with calcium chloride, distilled from phosphorus pentoxide, and stored with silver Gas-Chromatographic (13) Fowler, P. W.; Buckingham, A. D. Mol. Phys. 1989, 67, 681-691. (14) (a) Lukins, P. B.; Buckingham. A. D.; Ritchie, G. L. D. J . Phys. Chem. 1984.88, 2414-2418. (b) Craven, I . E.; Haling, M. R.; Laver, D. R.; Lukins, P. B.; Ritchie, G. L. D.; Vrbancich, J . J . Phys. Chem. 1989, 93, 627-63 I . (IS) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laburutory Chrmicals, 2nd ed.; Pergamon: Oxford, U. K., 1980; p 335.
CH3 I I
I
2.0
2.5
I
3.0
I
3.5
Figure 1. Temperature dependence of the vapor-state Cotton-Mouton effects of CH3Br and CH31.
analyses confirmed the purities of all three as 299.5%. In addition, methyl iodide, a liquid, was subjected to several freeze-pump thaw-distill cycles in the vapor-handling system immediately prior to the commencement of measurements. The definition of the molar Cotton-Mouton constant in terms of experimental observables has been given elsewhere.",'4 Recorded density second (and, in the case of methyl fluoride, third) virial coefficients16 were used to calculate the number densities from the measured pressures; Cotton-Mouton second virial coefficients were not detectable under the conditions of these experiments. The results are summarized in Table I, where the uncertainties shown are based on the standard deviations derived from the least-squares straight lines. Except for a preliminary investigation of methyl fluoride, made well over 20 years ago with a less sensitive version of the present equipment,I7 the vapor-phase Cotton-Mouton constants of these three molecules have not previously been determined. A value of the solution-phase Cotton-Mouton constant of methyl iodide in carbon tetrachloride at 298 K has been reported,'* but the extraction of the magnetic anisotropy of the solute from this quantity is not straightforward. Discussion
The low-density molar Cotton-Mouton constant, ,C, of a diamagnetic and axially symmetric species is, in SI units,I9 ,C = 2nV,,,r2[3(n2 2)21-l[(nll- n , ) B 2 ]
+
(la)
= (NApo2/270to)[Aq+ (2/3kT)AaAx]
(Ib)
where eq la is a measure of the refractive index difference, nll-n,, induced in the gas by the magnetic induction Band eq Ib is the theoretical relationship4 between the observed birefringence and fundamental molecular properties; Aq ( =t)a8,a8 - I/3qpa,8& A a ( = a z r- CY,,), and Ax ( q Z z- x,,) are the anisotropies i n the magnetic hyperpolarizability, the optical-frequency electric polarizability, and the magnetizability, respectively. It follows, therefore, that the Cotton-Mouton constants of the methyl halides should exhibit a linear dependence on the reciprocal of the temperature; and, if the effect is measured over a range of temperature such that a plot of ,C against T I can be reliably extrapolated to T I = 0. the intercept determines Aq and the slope Ax, provided of course that P a is independently known. As already noted, a temperature-dependence study was precluded for methyl fluoride; (16) Dymond, J . H.; Smith, E. B. The Virial Coefficients of Pure Gases and Mixtures; Clarendon Press: Oxford, U . K., 1980.
(17) Corfield, M. G. Ph.D. Thesis, University of Bristol, 1969. (18) Battaglia, M . R.; Ritchie, G.L. D. Mol. Phys. 1976, 3 l , 1283-1286. (19) See ref 2 for a summary of the algebraic and numerical factors required to convert the relevant electric and magnetic properties from SI to cgs units.
Coonan and Ritchie
1222 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE II: Analysis of the Cotton-Mouton Effects of the Methyl Halides
102'(intercept)/m5A-2 mol-'
CH2F
CH3CI
CH,Br
CH3I
0.225 f 0.033 2 f 13 -14.1 f 1.0' -29.6 -39.0 -24.9
0.19 f 0.13 -0.492 f 0.039 1.705 f 0.054 47 i 33 -15.0 f 1.3 -53. I -63. I -48. I
0.26 f 0.06 -0.654 f 0.021 2.253 f 0.045 6 6 f 15 -15.1 f 0.8 -71.1 -8 I .2 -66.1
0.39 f 0.19 -1.018 f 0.066 2.946 f 0.059 99 f 47 -1 8.0 f 1.5 -95.0 -107.0 -89.0
(I
10?4(slopc)/m5A-2 K mol-'" I0 4 n ~ t U ni?/ ~ * IO'"Aq/C m 2 V-l T-2 10!9Ax/J T-2 1 0 ? 9 ~ /~. 1- 2 d I 0?9~,,/.1T-2 1 0 ? 9 ~ , , / .~1 - 2
Equation I b. hOptical-frequency(632.8 n m ) polarizability anisotropies from ref 20 and 21 (see text). "Reference 9b. dMean molecular magiictizabilitics, x = (x:: + 2x,,)/3. from ref 29 150
TABLE Ill: Comparison of Experimental Values of the Magnetic Anisotropies" of the Methyl Halides
N
'I-
CHIF
100
1
I
E
. 3
50
CI
0
0
v)
0
"IF I
I
2
4 40
10
l
6
l
8
2
a l C m V
I
1
0
-1
Figure 2. Variation of the magnetic hyperpolarizability anisotropy with the mean molecular polarizability in the series CH3X ( X = H, F, CI. Br, I).
however, Figure I shows the relevant plots of ,C against T'for methyl bromide and methyl iodide. Table I I contains a complete analysis of thc Cotton-Mouton effects of these three molecules, and for comparison methyl chloride, in terms of the magnetic hyperpolarizability, thc magnetic anisotropy, and the individual componcnts of thc magnetizability. In the case of CH3F,the estimate of AT shown in Table 11 was derived from the single-temperature data in Table I, together with the known values of Aa and A x (see below) for this molecule (eq I ) ; for CH,CI, CH,Br, and CH,I, this quantity was obtained directly from the T'= 0 intercept of the appropriate least-squares straight line. Because of the long extrapolations that are required or, for CH,F, othcr uncertainties, the values of AT that emerge, the first for thcsc molcculcs, are of rather low precision. For all four, thc proportions of the relatively small Cotton-Mouton constants at 298 K that originate in the temperature-independent contributions arc minor (CH3F, ==-5%; CH3CI, CH3Br, CH31, ==-I 3%). and furthcr improvement of the apparatus would be needed for any significant increase in the precision of the intercepts. As cxpcctcd from the earlier results for CH4 and CH3CI," AT is found to be positive in sign for CH3F, CH3Br, and CH31, and, despitc thc largc unccrtainties, to increase in magnitude across the complctc serics as the size of the substituent increases. Since the magnctic hypcrpolarizability tensor describes the quadratic responsc of thc clcctric polarizability tensor to a magnetic A7 might reasonably be predicted, at least in molecules of the same symmctry, to bc proportional to the mean polarizability, a,which is, of coursc, a mcasurc of the molecular size. The relevant data",2n.2'(substituent, 1040a/C m2 VI,1050Aq/C m2 V-' T-2) for the molecules of intcrest (H, 2.89, 1 I .3 f I .5; F, 2.90, 2 13; CI, 5.04, 47 f 33; Br, 6.22, 66 f 15; I, 8.41, 99 f 47) are displayed in Figure 2 and appear to support a relationship of this kind. Two points deserve mention: first, of the experimental values of A?, that for methane is far more precisely determined than those
*
(20) Bogaard, M . P.;Buckingham, A. D.; Pierens, R. K.; White, A. H. J. Chmi. Soc.. Faruday Trans. I 1978, 74, 3008-301 5. ( 2 1 ) Miller. C. K.; Orr, B. J.; Ward, J . F. J . Chem. Phys. 1981, 74. 4858-4871,
CHI1
Expressed as 1029Ax/J T-2. "Cotton-Mouton effect, vapor state, ref 1 I (CH3CI) and present work (CH3Br, CHJ). 'Molecular Zeeman effect, vapor state, refs 9 and IO. d95.2-MHz 2H N M R spectra, dilute-solutionstate, ref 12.
7-
-50
CH,Br
-15.0 f 1.3 -15.1 f 0.8 -18.0 dz 1.5 -14.1 f 1.0 - 1 3 . 2 f 0 . 7 -14.1 f 0 . 7 - 1 8 . 2 f 0 . 8 - I 3.90 f 0.03 2H N M R d -12.5 -13.8 -15.9
IBr
>
N
CHIC1
CME~ M ZE'
r
for the halomethanes, because in this particular case the normally dominant orientation contribution to the Cotton-Mouton constant is absent, and the effect arises entirely from the magnetic hyperpolarizability; and second, methane and methyl fluoride fortuitously have almost identical mean polarizabilities. Experimental values of AT are now available for a range of small molecules,22 but few, if any, trends have previously been identified. Reliable ab initio computations of this property are limited to the recent study by Buckingham and FowlerI3 of two-electron systems (H2, D,, and others) at the near-Hartree-Fock level, with C H F and finite-field methods as implemented in the CADPAC program. Clearly, there is scope for further work in this area. I n order to deduce the magnetic anisotropies, A x , from the slopes of the plots of ,C against T I (or, in the case of CH3F, to determine AT), reliable optical-frequency polarizability anisotropies, A a , at 632.8 nm, usually obtained from vapor-phase Rayleigh depolarization ratios and refractivities,20are required. However, for CH3F,a very weakly anisotropic molecule, it is now well recognized2' that the depolarization ratio, and therefore the value of Aa, obtained by the conventional m e t h ~ dare ~ ~over.~~ estimates, because of the inclusion of a spurious and significant contribution from the depolarized vibrational Raman spectrum. Fortunately, CH3F has been examined by the electrooptical Kerr and the Raman spectroscopic method,26both of which avoid this difficulty; and a reliable (and more than 30% smaller) experimental value of ALY,subsequently confirmed by ab initio molecular-orbital c a l c ~ l a t i o n s is , ~ now ~ ~ ~available ~ for this molecule. By contrast, CH3CI, CH,Br, and CH31are much more anisotropic, and the depolarization ratios20 for these molecules are sufficiently large to be little affected by inclusion or exclusion of the vibrational Raman spectrum. The values of A x that emerge from the Cotton-Mouton effects of CH3CI, CH,Br, and CH31, and also the previously reported microwave Zeeman spectroscopic result for CH3F, are shown in Table I I . Combination of these with the known mean magneti~abilities~~ yields the individual (22) See ref 1 1 for a summary of relevant references. (23) Baas, F.; V a n Den Hout, K. D. Physic0 1979, 95A, 597-601. (24) Buckingham, A. D.; Orr, B. J . Trans Faraday Soc. 1969, 65, 673-68 I . (25) Orr, B. J. Hyperpolarizabilities of Halogenated Methane Molecules: A Critical Survey. In Nonlinear Behavior of Molecules, Atoms and Ions in Electric, Magnetic and Electromagnetic Fields; NEel, L., Ed.; Elsevier: Amsterdam, 1979; pp 227-235. (26) Bogaard, M. P.; Orr, B. J.; Murphy, W. F.; Srinivasan, K.; Buckingham, A. D. Unpublished results cited in ref 21. (27) Amos, R. D. Chem. Phys. Lett. 1982.87, 23-26. (28) (a) Spackman. M. A. J . Phys. Chem. 1989, 93, 7594-7603. (b) Spackman, M . A. Unpublished results.
1223
J. Phys. Chem. 1991, 95, 1223-1227 components, xzzand x , ~of, the magnetizability. Obviously, the direction of maximum (negative) magnetizability is parallel rather than pcrpcndicular to the C-X axis; and, since IAxl increases only slightly through thc series, replacement of F by the larger halogens CI, Br, and I augments xz: and xr., to about the same extent. It is of intcrest to compare the present values of A x for CH3CI, CH3Br, and CH31 derived from measurements of the CottonMouton effects of these species in the vapor state with those previously obtained from observations of the molecular Zeeman effect in the vapor phase”.I0 and others from a recently developed method that exploits the electric quadrupole splittings that appear in the high-ficld *H NMR spectra of deuterated methyl halides in the dilutc-solution state.’* From the summary in Table Ill it can be sccn that the results from the Cotton-Mouton effect and the molecular Zccman effect are in good agreement, insofar as in each casc the difference between the values is less than the sum of the unccrtainties. Unfortunately, the report of the NMR spectroscopic study does not explicitly state the likely errors, although an earlier investigation30by the same authors estimated these as 3-7%. However, for all three molecules the results obtained by the NMR method appear to be somewhat lower than those from both the Cotton-Mouton and Zeeman methods, especially the former. Moreover, in view of the concordance between the Cotton-Mouton and Zeeman values for CHJ, the suggestion that thc lattcr result for this molecule is perhaps 14% too high cannot be supported. Finally, it is relevant to consider some important features of the three methods that have been used to obtain the magnetic anisotropies of the diamagnetic molecules of interest here. The Cotton-Mouton effect is applicable to dipolar and nondipolar species and is unaffected by the presence or absence of quadrupolar nuclei: the microwave Zeeman effect requires that the molecule possess a permanent electric dipole moment, even if only a small moment introduced by judicious isotopic substitution; and the NMR method exploits the small quadrupolar splitting that appears as a consequence of molecular orientation at high fields (14.6 T) in the 2H NMR spectra of the monodeuterated compounds. As can be seen from the data in Table I, which are typical of weakly anisotropic aliphatic compounds that may be liquids under normal (29) Landolr-Bornstein Numerical Dara and Functional Relationships in Science and Technology, Vol. 16. Diamagnetic Susceptibility; Springer: Berlin, 1986. (30) Bothner-By, A. A.; Dadok, J.; Mishra, P. K.; Van Zijl, P. C. M. J . Am. Chem. Soc. 1987. 109, 41 80-41 84.
conditions, it is necessary to peform the measurements of the Cotton-Mouton effect over rather a wide range of pressure (in some cases up to ==IO00kPa, so as to increase the magnitude of the effect) and temperature ( ~ 2 9 0 - 4 7 0 K, so as to separate the temperature-independent and temperature-dependent contributions to the effect). Of course, the microwave spectroscopicobservations are made at very low pressures ( = I Pa), an obvious advantage for involatile or reactive species; and the NMR spectra are obtained from dilute solutidns ( 4 - 1 0 mol %) of the deuterated solute in solvents such as cyclohexane or diethyl ether. Provided the requirements noted above in relation to pressure and temperature are met, measurements of the Cotton-Mouton effect are straightforward, and the only additional parameter needed to evaluate the magnetic anisotropy is the optical-frequency polarizability anisotropy, normally measurable to high precision. The analysis of the microwave Zeeman spectra of molecules that contain heavier halogen atoms (CI, Br, and I ) is complicated by nuclear quadrupolar interactions, while difficulties in the interpretation of the quadrupolar splitting in the NMR spectra seem to arise in relation to the value of the quadrupole coupling constant to be used and’the effect of the solvent on the apparent molecular properties. For these reasons, the three methods to which reference is here made must be considered as complementary, and further comparisons of the present kind are highly desirable. Summary
In conjunction with an earlier study of CH, and CH3CI, the present investigation of the vapor-state Cotton-Mouton effects of CH3F, CH3Br, and CHJ has established that in the series CH3X (X = H, F, CI, Br, I) the magnetic hyperpolarizability anisotropy is positive in sign and roughly proportional to the mean molecular polarizability. The measurements have also yielded reliable free-molecule magnetic anisotropies for CH3CI, CH3Br, and CH31, and these have been compared with values obtained by the molecular Zeeman and high-field 2H NMR methods. Acknowledgment. A Commonwealth Postgraduate Research Award (to M.H.C.), financial support from the Australian Research Council (to G.L.D.R.), technical assistance from Dr. D. R. Laver (University of Sydney), a sample of methyl fluoride from Dr. R. K. Pierens (University of Sydney), and helpful discussions with Dr. M. A. Spackman (University of New England) are gratefully acknowledged. Registry No. CH3F, 593-53-3; CH,Br, 74-83-9; CH31, 74-88-3; CHACI. 74-87-3.
Unimolecular Decomposition of SiH,, SiH,F, and SiHpF, at High Temperatures Mitsuo Koshi,* Shin Kato, and Hiroyuki Matsui Department of Reaction Chemistry, University of Tokyo, Hongo. Bunkyo-ku, Tokyo 113, Japan (Received: July 3, 1989; In Final Form: August 3, 1990)
The thermal decomposition of SiH,, SiH3F, and SiH2F2diluted in Ar was studied behind incident shock waves by monitoring IR emission from these reactant molecules. The rate constants of the unimolecular decomposition for all of three molecules wcrc round to bc in thc pressure falloff region over the present experimental conditions ( T = 1190-21 50 K and P = 0.2-1.6 atm). The activation energies of measured rate constants were compared with heats of reactions for the various possible pathways for the thermal decomposition of these molecules and it was concluded that thrce-centcr H2 climination reactions wcrc thc dominant pathways for unimolecular reactions of SiH4-,F, (n = 0. I , and 2). These rate constants decrease with increasing number of fluorine atoms, n, in SiH,-,F,. RRKM falloff calculations have been performed to examine the effect of fluorine substitution on the unimolecular decomposition rates of these molecules.
Introduction
Recently silane and its derivatives have received considerable attention because of their importance in the semiconductor industry. For example, fluorosilanes are expected to be new source gases which can modify the deposition rate and properties of
amorphous silicon alloys.’-2 However, very little information is available for the elementary chemical processes of these molecules. ( 1 ) Matsuda, A.; Yagii,
K.;Kaga, T.: Tanaka, K. Jpn. J . Appl. Phys. 1984,
L576,23.
0022-3654/9l/2095-l223$02.50/0 0 1991 American Chemical Society
