PHOTOCHEMISTRY OF MONOFLUOROBENZENE
995
Spectrofluorometric Studies. IV. Some New Aspects of the Photochemistry of Monofluorobenzene by M. E. MacBeath, G. P. Semeluk, and I. Unger Department of Chemistry, University of N e w Brunswick, Fredericton, N e w Brunswick, Canada
(Received J u n e 80, 1068)
The photochemistry of monofluorobenzene in the vapor phase has been examined over a range of wavelengths extending from 2350 to 2710 b. The sensitized emission of biacetyl was used to determine triplet-state yields for exciting wavelength of 2470, 2550, 2590, and 2670 d. The effect of cyclohexane on the fluorescence yield and on the sensitized emission yield has been examined. @triElet = 0.67 at A,, = 2470 this sum is 0.75 at A,, = 2550 b, 0.93 at ,A, = 2590 and 0.82 at ,A, = 2670 A. A mechanism consistent with the experimental observations is presented.
a,
Introduction Monofluorobenzene (FB) lends itself very well to photochemical investigation for several reasons: the molecule does not, dissociate when irradiat)ed in the 2500-A spectral region;’ its absorption in this region is relatively strong;2 it exhibits fluorescent emission very similar to, and in the same region as, that of benzene;a its triplet state lies very close to that of b e n ~ e n e ; ~ it has four distinct absorption maxima between 2400 and 2700 A,2 which makes it possible to examine differences in photochemical behavior due to transition into four relatively closely spaced vibrational levels; and the presence of a heavy atom has a tendency to increase the lifetime of the excited state.s Unger8 applied the sensitized emission of biacetyl (BiA) techniqueeJ to FB excited at 2537 A and reported a triplet yield of about 0.90 while the sum of triplet plus singlet yield was given as 1.13. The fact that the total quantum yield exceeded unity combined with a possible error due to the “around the corner effectJJ8prompted Phillipsg to reexamine the photochemistry of FB using the Cundall technique’091’ as a diagnostic for the triplet state. The total quantum yield of singlet and triplet was given as 1.05 f 0.09 by Phillips, still distressingly higher than unity. We felt that it was of interest systematically to investigate the photochemical behavior of FB as a function of exciting wavelength and to try to resolve some of the anomalies reported in the two previous investigation^.^*^ Experimental Section Materials. The monofluorobenzene used was obtained from the Aldrich Chemical Co. Vpc analysis showed this materia1 to be essentially pure. The biacetyl was also obtained from Aldrich, stored in a dark bulb, and purified just prior to use by bulb-to-bulb distillation “in vacuo.” The benzene was fluorometric grade obtained from Hartman-Leddon Co. and used without further purification. The cyclohexane, obtained from the same company, was also fluorometric grade.
+
a;
The high-vacuum line employed in this study was a grease- and mercury-free line using Hoke packless diaphragm metal valves exclusively. Pressure measurements were performed by means of a spiral gauge constructed at the National Research Council in Ottawa. Pressures below 3 Torr were measured by allowing a relatively large pressure into the gauge and then expanding into the previously calibrated line containing the cell and mixing pump, and also into a large calibrated flask when necessary. A flowthrough cell, cuvette type, 1 X 1 X 4.5 em high and polished on four sides, was used. Mixing was accomplished by means of an allglass circulating pump constructed a t the National Research Council in Ottawa. Emission measurements were performed on a modified12Aminco-Bowman spectrophotofluorometer (SPF), obtained from the American Instrument Co. Inc. The entrance slit of the excitation monochromator was set a t 0.5 mm and the exit slits were set at 1.0,0.5, and 1.0 mm. This combination of slits allowed a band pass of approximately 70 which, although fairly large, was necessary in order to obtain sufficient, light intensity. Two entrance slits were used on the emission monochromator, both set at 1.0 mm and the exit slit on this monochromator was also 1.0 mm. These slit arrangements were (1) 0.P. Semenova and G. S . Tsikunov, Zh. Fiz. Khim., 18, 311 (1944). (2) W. F. Forbes, Can. J. Chem., 37, 1977 (1959); 8. H. Wollman, J . Chem. Phys., 14, 123 (1946). (3) I. Unger, J. Phys. Chem., 69, 4284 (1965). (4) D. F. Evans, J. Chem. Soc., 2753 (1959). (5) E. C. Wu and 0.K. Rice, J. Phys. Chem., 72, 542 (1968). (6) H. Iahikawa and W. A. Noyes, Jr., J. Amer. Chem. Soc., 84, 1502 (1962). (7) H. Ishikawa and W. A. Noyes, Jr., J. Chem. Phys., 37,583 (1962). (8) W. A. Noyes, Jr., and C. 8. Burton, International Conference on Photochemistry, Munich, Sept 6-9, 1967. (9) D. Phillips, J . Phys. Chem., 71, 1839 (1967). (10) R. B. Cundall, F. G. Fletcher, and D. G. Milne, Trans. Faraday Soc., 60, 1146 (1964). (11) R. B. Cundall and A. S. Davis, i b i d . , 62, 1151 (1966). (12) F. W. Ayer, F. Grein, G. P. Semeluk, and I. Unger, Ber. Bunsenges. Physik. Chem., 72, 282 (1968). Volume 78, Number 4 April 1960
M. E. MACBEATH, G. P. SEMELTJK, AND I. UNGER
996 found to give a minimum of scattered light as well as good reproducibility. Experimental details concerning the light source, emission, and absorption measurements have been given previously.12 To obtain the quantum yields of fluorescence of FB and the sensitized emission of BiA from the data obtained in our system, a relative method was used and has been described elsewhere.a To use this method it is necessary to calibrate the instrument with a reference material whose quantum yield of fluorescence has been accurately determined and whose spectral range is close to that of FB. Benzene satisfied both of these requirements very well. Thus all fluorescent yields reported in this paper are based on a fluorescent quantum yield of benzene being 0.18313and all triplet yields are based on a triplet yield of benzene of 0.63.14 The extinction coefficient for FB varies from 8 to 700 over the wavelength range in this study. At the higher values, even in the small cell used in this study, there is a possibility of error due to the “around the corner effect.” I n view of this, Beer’s law was used to calculate the light absorbed in the central portion of the cell from which emission was viewed.16 Since the extinction coefficient also varies with pressure, it was determined at every pressure used. The phosphorescent emission from biacetyl falls in the same region as the second-order light from the fluorescence of FB and, unless compensated for, could lead to artificially high sensitized emission yields. Blank runswere carried out withFBand thepercentageof fluorescence transmitted as second orderwas determined. Then using the figure obtained above, for any mixture of FB and BiA, this second-order contribution was calculated and subtracted from the total emission.
Results Fluorescence Measurements. The variation of fluorescent quantum yield of FB as a function of exciting wavelength is shown graphically in Figure 1. The yield is
.d
0-d
4
nfA--A.’
t-
2300
I
2400
2500
2600
2700
2800
x (AI
Figure 1. Variation of fluorescence quantum yield as a function of exciting wavelength: 0, 5 Torr of FB; A, 40 Torr of FB. The Journal of Physical Chemistry
Table I: Variation of 4 and l/Qfas a Function of FB Concentration a t Various Exciting Wavelengths sxaitation,
2390
Equation of line
f
Qr = 0.015
+ 0.011 X 103[FB]
Press. range of FB, Torr
5.5-48
f 2 8 . 3 % , max 33.2% 2430
Qr = 0.043
+ 0.008 X 103[FB]
3.5-49
f 6 . 4 5 % , max 12.8% 2470
l/@ = 5.84
+ 0.004 X 10SIFB]
3.6-49
f 3 . 0 2 % , max 10.4% 2510
l/*f = 5.05
+ 0.261 X 103[FB]
3.6-48
&2.18Y0, max 5.67’ 2550
1/*r = 5.10
+ 0.298 X 103[FB]
3.4-48
f 1 . 5 4 % , max 2.78% 2590
l/Qf = 2.96
2630
1/@r= 3.87
+ 0.988 X 103[FB]
3.4-48
f l . 0 2 7 5 , max 2.09%
+ 0.298 X 103[FB]
3.0-48
f 0 . 6 4 , max 1.86% 2670
l/Qf = 3.93
2710
l/@t
+ 0.447 X 103[FB]
3.0-48
&0.87Y0, max 2.8070 =
3.89
+ 0.429 X 103[FB]
2.7-48
rt3.127,, max 3.56%
A,
=O at 2350 it rises to a value of about 0.20 at 2500 A (depending on pressure), and it varies little from this point until 2710 Several other features are worth the fluorescence yield is pressure noting. At 2470 independent; below this wavelength plots of @I vs. [FBI are linear with positive slope; see Table I. Above 2470 plots of l/% vs. [FBI are linear and exhibit a positive slope; see Table I. (The errors reported in the tables are for any individual run; day to day reproducibility is of the order of 15%.) At low pressure a maximum in fluorescent yield occurs at 2590 A. It should be emphasized here that the self-quenching shown above 2470 A is not an artifact since only the light absorbed in that portion of the cell being viewed by the emission monochromator was considered in calculating fluorescent yields. The fluorescence of FB is quenched more strongly by BiA than by itself; these data are summarized in Table 11. I n all cases plots of 1/+r vs. lBiA], at constant IFB 1, are linear. Addition of cyclohexane t o FB increases the fluorescent yield when A,, is 2470 A but does not affect this yield for any of the other exciting wavelengths
A. A
A
(13) W.A. Noyes, Jr., W. A. Mulac, and D. A. Harter, J . Chem. Phgs., 44, 2100 (1966). (14) W.A. Noyes, Jr., and D. A. Harter, ibid., 46, 674 (1967). (15) J. L. Durham, G. P. Semeluk, and I. Unger, Can. J. Chem., 46, 3177 (1968).
PHOTOCHEMISTRY OF MOKOFLUOROBENZENE
T
Table 11: Variation of l/@f as a Function of BiA Concentration, at, Constant F B Concentration, for Various Exciting Wavelengths
2470
1/@r = 6.02
7
Press. range of BIA, Torr
Equation of line”
Licitation.
997
+ 7.72 X 10a[BiA]
0.1-29
rt2.38%, max 6.20% 2550
l/@f= 5.49
:
+ 7.74 X 10a[BiA]
0.1-20
1-1.3970, max 5.8% l/@f= 2.49
2590
+ 4.51 X 10a[BiA]
0 * 1-20
&2.537$, max 6.72% 2670
l/@f= 3.64
+ 7.33 X 10a[BiA]
:
10
I0
20
0.1-20
f1.4570, max 2. 85y0
40
30
PRESSURE FB (torr)
Figure 2. Variation of fluorescence yield of F B as a function of F B concentration, 0 , and as a function of FB concentration in the presence of 50 Torr of cyclohexane, 0; Xeloitation = 2550 A.
FB pressure constant at 5 Torr.
examined; typical data obtained are shown in Figure 2 and in Table 111. Sensitized Emission 1Measurements. The triplet yield of F B excited at 2470, 2550, 2590, and 2670 A was determined using the sensitized emission of biacetyl technique. Three types of experiments were performed at the four wavelengths mentioned above. (a) The F B pressure was held constant at 5 Torr and the BiA pressure was varied. Plots of 1/apvs. [BiA] go through a minimum at about 10 Torr of BiA and then become linear. The data are summarized graphically in Figure 3. (b) The BiA pressure was held constant at 5 Torr t
Table 111: Effect of Cyclohexane on the Fluorescence Yield and Sensitized Emission Yield of FB in BiA-BF Mixtures for Various Exciting Wavelengths
Xercitstion,
2470
A
*f
*”
7 -
‘a.
-... -.__. ._.... ..-A.”
~
I
.A*--.
I
I
I
I
1
Pressure, Torr---
CH
FB
BiA
0.108 0.117 0.127 0.127 0.127
0.23 0.28 0.24 0.32 0.33
0.0 5.14 10.28 20.56 30.07
10.0 10.0 10.0 10.0 10.0
5.0 5.0 5.0 5.0 5.0
2550
0.118 0.120 0.116 0.122
0.37 0.36 0.38 0.40
5.42 10.42 20.49 30.49
10.0 10.0 10.0 10.0
5.0 5.0 5.0 5.0
2590
0.237 0.239 0.237 0.241
0.44 0.46 0.46 0.44
4.72 9.79 19.79 29.93
10.0 10.0 10.0 10.0
5.0 5.0 5.0 5.0
0.178 0.179 0.178 0.176 0.181
0.40 0.39 0.42 0.39 0.40
0.0 5.21 10.21 20.07 30.07
2670
2-
9.93 9.93 9.93 9.93 9.93
5.0 5.0 5.0 5.0 5.0
and the FB pressure varied. Plots of 1/9, vs. pressureof F B go through a minimum at about 10 Torr of FB and then become linear; see Table IV. (c) The F B pressure was held constant at 10 Torr, the BiA pressure was held constant at 5 Torr, and the sensitized emission yield was measured as a function of increasing cyclohexane (CH) pressure; 9pincreases with increasing CH pressure at A,, = 2470 A but is not affected by CH at the other exciting wavelengths; see Table 111.
Discussion Our experimental observations combined with the known photochemical behavior of biacetyl16 lead us to (16) W.A.Noyes, Jr., and I. Ungsr, Advan. Photochem., 4,49 (1966), and references therein. Volume 7% Number 4 April 1969
M. E. MACBEATH, G. P. SEMELUK, AND I. UNGER
998 postulate the following mechanism, for exciting wavelength greater than 2470 A. FB
+ h~
'FBI
'FBI
FB
--+
'FBI (+M) 'FBI
----+
+ hv
'FBI ( + M )
--+
+ FB
'FBI
--+
2FB
isomerization
-4
aFB1(+M) --+FB(+M)
+ BiA --+ aFB1+ BiA
'FBI
+ FB aBiAI* + FB
1BiAII
-4
sibly being bimolecular; it is difficult to conceive of these reactions occurring without collisions. However, at the pressures used here collisions may not be rate determining. Reactions 5 and 15 are included since it has been established that benzene and benzene derivatives may undergo phototransposition of ring carbon atoms when excited in the neighborhood of 2500 A. 17-19 Reaction 10 very likely does not occur.2o For simplicity only one mechanism is given for wavelengths above 2470 A, but the various rate constants will have values dependent on wavelength and some rate constants may have negligible values at some exciting wavelengths. Since at shorter wavelengths the possibility of dissociation must be taken into account, the following steps and reactions 2, 3, and 5 describe the situation below 2470 A.
3BiA1*-+ dissociation
+M
aBiA1*
--+
8BiAI
FB
+M
'FBI*(+M)
lBiAII --+ dissociation 'BiAII
+ M -+
+ hv
'FBI"
+M
--+
+ FB
'FBI*
'FBI"
--+
3FB'*(+M)
(17)
+ FB
(18)
'FBI
-4
--+
(16)
dissociation
(19)
isomerization
(20)
1BiAI --+ aBiA1 'BiAI
--3
aBiA1--+
aBiAI--+ aFBI--+
'FBI*
+ hvr BiA + hv, BiA
'FBI* --+ hv
BiA
isomerization
I n the above mechanism FB and BiA represent groundstate monofluorobenzeneand biacetyl molecules, respectively. The Arabic numerals represent spin multiplicities and the Roman numerals are used to indicate first or second excited states. 31 represents any quenching molecule, and the asterisk denotes a vibrationally excited molecule. Reactions 3 and 6 are shown as pos-
Table IV: Variation of I/@, as a Function of Pressure of FB, ast Constant Pressure of BiA, for Various Exciting Wavelengths hemitation.
Equation of line'
2470
I/$, =- 3.97 - 0,149 X 10'JFBI =t1.75%, max 2.927,
2550
l/@,
If kq < (k2 reduces t o of IrB, Torr
=
2.62
+ 0.112 X 10SIFB]
17-46.4 20-50
f2.80%, max 4.127,
I/@D
=
1.77
-+ 0.261 X 10SIFB]
10-49
=!~3.057~, max 4.88% 2670
+
kzkis[FB] 1czi(k2 (k2 4- 1 ~ 3 h[FB]
a* = -
l& = 2.11
+ 0.242 X 10a[FB]
f2.68%, max 5.46y0 BiA pressure constant at 5 Torr. ~~
The Jouriiul of Physacal Chemislrg
(21)
=
+ + hCFRI + + ks) ( h+ k i s [ F B T + h e + kzo + kz1)
+ ka + kj) and
Pressure range
a
+ FB
Here the asterisk is used to indicate a molecule that is very highly vibrationally excited. In our mechanism we have allowed only for two vibrational levels of the first singlet to be taking part in the various reactions. I n reality, a great many vibrational levels must be involved, each with its own rate constant. A detailed treatment would give much more complex expressions. We do not feel that our data warrant. such a detailed treatment. The fluorescent quantum yield for exciting wavelengths below 2470 A would be given by
at
2590
-4
kzkis[FB] (kl? kl9 4
+
k5)
k3
k18
< ( l c ~+ kzo), then eq 22
+ kzi(1c2 + + + + ks + k3
k20
(22)
hl) (k2
(23) k6)
Equation 23 predicts that plots of $f us. [FBI should be linear and have a positive slope and is consistent with experimental observation; see Table I. Returning now to the case above 2470 A, steady-state (17) K. E. WiLbach, A. L. Harkness, and L. Kaplan, J. Amer.
12-5 2
Chem. Soc., 90, 1116 (1968), and references therein. (18) H. R. Ward and J. S. Wishnok, ibid., 90, 1085 (1968).
(19) D.Bryce-Smith and H. C. Longuet-Higgins, Chem. Commun., 593 (1966). (20) J. Lemaire, J. Phys. Chem., 71, 2653 (1967).
PHOTOCHEMISTRY OF MONOFLUOROBENZENE
999
Table V: Values of Rate Constants and Quenching Cross Sections at Various Exciting Wavelengths Xeroitatian,
A
Kdv mol-’
Kz~ sec-I
5.10 x 4.41 X 4.48 X 2.9 x 3.44 x 3.54 X 3.61 X
2470 2510 2550 2590 2630 2670 2710
Ka9sec-1
2.47 X 1.79 X 1.84 X 5.68 X 9.87 x 1.04 X 1.04 X
107 107 107 107 107 107 10’
108 108 108 107 107 108 108
treatments yields
Equation 24 predicts that plots of 1/9f us. [FBI at zero [BiA] should be linear with intercept = (IC2 k3 k5)/kZand slope = kd/lcz while plots of l/@
+ +
us. [BiA] a t constant [FBI should be linear with slope = k,/kz. These predictions are confirmed experi-
mentally. Using the integrated molar absorptivity for F B given in the literature,21 the radiative lifetime TO can be determined. Fluorescent yields can now be used to obtain the mean radiation lifetime T , and ~ ~ hence kz a t the various exciting wavelengths. Making now the reasonable assumption that ks is either zero or negligible,23k3, kq, k7, and quenching cross sections can be evaluated, which are summarized in Table V. Assuming (10) and (1 1) to be unimportant, the steadystate treatment for sensitized emission gives (k13
*
-1
=
+ha)
(ha
+ ksb[M])
(he
+
~ I K
+k~[BiA]/(kz + + kd[FB] + LS + k&BiA]) k3
k1&sbksk3[M][BiA]
(25) a t moderate [BiA], ks[BiA] > (k5 duces to (k13
+
kl4) ( h a
+ ksb[M])
+ k16) and eq 25 re(k2
+
k3
h[FB]
Again a t moderate pressures of either FB or BiA, and particularly in the presence of CH, ksb[M] > ksa and eq 26 becomes qJP--1
=
(hi3
+
ki4) (k2
+ ka + h[FB] + + h[BiA])
1. sec-l
2.04 X 1.15 X 1.36 X 2.87 X 1.02 x 1.58 X 1.55 X
108 1010
1010 1010 1010
1010 10’0
u2 of FB, A2
0.03 1.67 1.98 4.17 1.48 2.29 2.25
u2 of BIA,
K7, mol-’ 1.
sec-1
3.94 X
loll
55.7
3.47 X 10’’ 1.31 X 1011
49.0 18.7
2.59 X 10”
36.6
deed found to be the case experimentally. Equation 27 also predicts that CH should have no effect on the sensitized emission yield. This prediction is confirmed for A,, of 2550, 2590, and 2670 A. However, at 2470 A the sensitized emission yield does increase with addition of CH, which is probably due to quenching of very highly vibrationally excited FB molecules, from dissociating levels. Table 111, which summarizes these results, shows that at lower pressures of CH, for x, = 2470 A the per cent increase in cPp is seen to be approximately the same as the per cent increase in The sum of a t i n p l e t @triplet is found to be 0.67 for A, = 2470 A, 0.75 for A, = 2550 A, 0.93 for A, = 2590 A, and 0.82 for A, = 2670 A. These figures were arrived at by adding the unquenched fluorescence yield to the triplet yield. The true triplet yield is equal to the triplet yield determined experimentally multiplied by the ratio 9f (unquenched)/@f (quenched). The reason for this has been eloquently stated by Phillipsg and is simply based on the fact that in determining triplet yields by the biacetyl technique some of the singlet, and thus the triplet, is quenched. It is very difficult to estimate the errors associated with the values given above. In Tables I, 11, and IV the reproducibility of the data is given. One would have to add to this the error in Almy and Gillette’s2* work ( ~ ~ * 1 2 0 1 ,on ) which our data are based, to obtain an estimate of the error in our work. Finally, it should be pointed out that the use of truly monochromatic light is necessary if one is to reach any firm conclusions in studies of this type. I n the present instance the band pass was approximately 70 A; this is much too large and absorption may occur into different vibrational levels of the first electronic state. Each of these has a different extinction coefficient, different fluorescent rate, intersystem crossover rate, etc. Simultaneous excitation into two or more vibrational levels will tend to obscure any distinct reactions occurring
+
kid&
(27) Equation 27 predicts that a t constant [FBI plots of 1/ap2)s. [BiA] should be linear with a positive slope; similarly at constant [BiA], plots of 1/ap us. [FBI should be linear and have a positive slope. This is in-
(21) M. Ballester, J. Palau, and J. Riera, J . Quant. Spectry. Radiative
Transfer, 4, 819 (1964).
(22) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” John Wiley and Sons, Inc., New York, N. Y., 1966, p 174. (23) K. E. Wilabach, A. L. Harkness, and L. Kaplan, International Conference on Photochemistry, Munich, Sept 6-9, 1967. (24) G. M. Almy and P. R . Gillette, J. Chenz. Phys., 11, 188 (1943). Volume YS, Number 4
April 1969
S. J. STECK,G. A. PRESSLEY, JR., AND F. E. STAFFORD
1000
from a specific level and errors due to the “around the corner” effect may be introduced if the extinction coefficient for one vibrational level is much larger than that of its neighbors. The relatively large band pass used here was the best that our instrumentation could provide.
Acknowledgments. The authors gratefully acknowledge a grant from the Associated Alumni of the University of New Brunswick toward the purchase of the instrumentation used in this study and other financial assistance from the National Research Council of Canada.
Mass Spectrometric Investigation of the High-Temperature Reaction of Hydrogen with Boron Carbide1 by Sara J. Steck,2 George A. Pressley, Jr., and Fred E. Stafford Department of Chemistry and the Materials Research Center, Northwestern University, Evanston. Illinois 60201 (Received June 8 4 , 1 9 6 8 )
The boron-containing gaseous molecules BH, BH3, HBC2, and BH2CzHz2+1(x = 1 and 2) have been identified as products of a high-temperature reaction of hydrogen with boron carbide; higher molecular weight boroncarbon-hydrogen molecules may have been present, but they could not be observed because of interference from hydrocarbons formed in the reaction. Evidence for BH2 was not found. Concentrations of BH3 and the two alkylboranes far exceeded the predicted equilibrium quantities. This phenomenon is thought to result from a steady-state reaction in the crucible, and a mechanism is suggested. The other molecules, BH and HBC2, appeared to be present in near-equilibrium concentrations. Values for Do(BH-H) and D’(BH2- H) are discussed; revisions are suggested. Secondary electron multiplier gains determined for H, Hz, and llB are in the ratio 1:3.5: 19.2 for 6.5-kV ions. D (H-BC2) is determined to be 100 f 10 kcal/mol.
Introduction Tabulated thermodynamic data3 suggest that the high-temperature reaction of hydrogen with boron carbide should yield such relatively simple molecules as BH, BHZ, and BHZ. A11 three of these species have been identified as transient intermediates, and various properties have been reported: for BH, both the uv4 and vacuum uv6spectra have been subject to rotational analyses and the ionization potential was evaluated;6for BH2, an electron impact ionization potentials has been obtained, and rotational analysis of the electronic spectrum has indicated a bent geometry in the ground state;’ and for BH3, the ionization potentials-8 and the mass spectrumg have been measured. Because of the small number of electrons in these molecules, data for these species are particularly valuable for comparison with results of theoretical c a l c ~ l a t i o n s . ~ ~Some - ~ ~ discrepancies appear to exist between the limited experimental data and various theoretical treatments; for instance, the available thermodynamic data3 for these hydrides do not agree with arguments used to explain the boron fluoride bond dissociation energies.lS Especially since the carboranes show greater thermal stability than do the boron hydrides, the high-temperature reaction of boron carbide with hydrogen might The Journal of Physical Chemistru
be expected also to yield boron-carbon-hydrogen species. In particular, the molecules BC2 and BzC (1) Supported by the Advanced Research Projects Agency through t h e Materials Research Center a t Northwestern University and by t h e U. 8 . Atomic Energy Commission, Document No. COO-1147-27. The acquisition and maintenance of the mass spectrometer facility have been made possible by support from the Materials Research Center, the Atomic Energy Commission, and the university. (2) Recipient of Public Health Service Fellowships 1-F1-GM-29, 815-01A1, and 5-F1-GM-29, 815-02 from the National Institute of General Medical Sciences. 1966-1968. (3) D. R. Stull, Ed., “JANAF Thermochemical Tables,” Cleariughouse for Federal Scientiflc and Technical Information, Springfield, Va., April 1968, Document No. PB-168,370. (4) J. W. C.Johns, F. A. Grimm, and R . F. Porter, J. Mol. Spectrosc., 2 2 , 435 (1967). (5) 8 . H. Bauer, G. Hereberg, and J. W. C. Johns, ibid., 13, 256 (1964). (6) T. P. Fehlner and W. S. Koski, J . Amer. Chem. Soc.. 86, 2733 (1964). (7) G. Hereberg and J. W. 0. Johns, Proc. Roy. Soc., A298, 142 (1967). (8) J. H. Wilson and H. A. McGee, Jr., J . Chem. Phys., 46, 1444 (1967). (9) A. B. Baylis, G . A. Pressley, Jr., and F. E . Stafford, J. Amer. Chem. Soc., 8 8 , 2428 (1966). (10) P. C. H. Jordan and H. C. Longuet-Higgins, Mol. Phys., 5 , 121 (1962). (11) W. 0. Price, T. R. Passmore, and D. M. Roessler, Discussions Faraday Soc., 3 5 , 201 (1963). (12) A. C. Hurley, Proc. Roy. Soc., A261, 237 (1961). (13) F. 0. Ellison, J. Chem. Phys., 4 3 , 3654 (1965).
