J . Phys. Chem. 1984, 88, 5039-5042 between points 8 and 12 in Figure 5. Problems of accurate measurement of BDE values in hydroaromatic systems notwithstanding, it would seem to leave the simple NBMO approach (like the correlations of Steinsb and Herndon1'3l2) as an empirically useful correlation which can be used as a predictor of proven or known reliability for BDE values which are hard to measure, or for rates measured at high temperature (e.g., in shock-tube experiments) where the reduction to BDE values (=AH,' at room temperature) is uncertain due to unknown heat capacities.
Conclusions We have shown that the decompositions of the title compounds proceed, more or less equally, via two pathways, a C-C scission and a C-H bond break. Taking the CyH bond energy as the unknown, we have measured Mfo2gs(Ph2CH,g) = 69 kcal mol-'. This value indicates that the extra stabilization energy associated with replacing the. C H 3 group in PhCHCH3 with a second Ph group to form Ph2CH is -4 kcal mol-'. We have also measured the respective bond dissociation energies in the N-substituted compounds, but cannot report a radical heat of formation because data are lacking for heats of formation of the parent compounds.
5039
Our value of AiYf0298(Ph2CH,g)agrees within less than 1 kcal mol-' with that from the data of Robaugh and Stein,8 suggesting that the A factor for the toluene C-H bond dissociation process, contrary to recent reports,' is comparable to those for all other benzyl-X dissociation. Acknowledgment. We thank Dr. R. Patrick of our group for his kind help in adapting the RRKM program to the large number of oscillators used in the present study and for his assistance in performing the falloff calculations. This work is supported in part by the Department of Energy, contract no. DE-AC0379ER10483. Supplementary Material Available: A table of B / R ratios and rate constants at various temperatures for unimolecular decomposition of diphenylmethane and 2-benzyl- and 4-benzylpyridine in small- and large-aperture reactors; a figure illustrating the changes in the mass spectrum of diphenylmetqane at 900 OC at m l e 77, 78, 91, 92, and 165-168; and Tables A-1-A-3 listing molecular and transition-state parameters for the decomposition of diphenylmethane, 1,1-diphenylethane, and 2,2-dipheriylpropane (6 pages). Ordering information is given on any current masthead page.
Spectral Analysis of the Chemiluminescence Generated by H,B*X iO(3P) P.M. Jefferst and S.H. Bauer* Department of Chemistry, Cornell University, Ithaca, New York 14853 (Received: January 9, 1984)
In a flow tube reactor, intense chemiluminescence is generated when low pressures of H3B.N(CHJ3 or other borane adducts are rapidly mixed with O(3P) in a He carrier. Superposed on the extended a-BO system, there are a few P-system bands, moderate intensities of OH*, NH*, CH*, and BH*, and a very strong CN*. No single vibrational temperature exists for BO(A211); the rotational temperature is 350 K.
Introduction In a rapid-mixing fast-flow tube reactor, Anderson and Bauer' measured the rates of attack on B2H6and H 3 B C 0 by 0 and N atoms, as well as by N 2 0 , NO, NO2, and 02.The products of reaction were analyzed by a direct sampling T O F mass spectrometer. In a quartz flow tube reactor, the emitted chemiluminescence was recorded photographically. From these data both the stoichiometry and the exoergic pathways required to generate the highest recorded frequencies were determined. To account for the observations, two initiation mechanisms (which operate concurrently) were required: mechanism 1: 0 + H3BCO OH + [H2BCO]* --.* O H + BH2 + C O HzO [HBCO]* H2O BH + C O mechanism 2: 0 H3B.CO [H,B*O]* C O BO + H2 + H + C O other compounds, with oxygen attached to boron
-
+
+
+
+
-
+
+
+
-+
The highly exoergic steps which generate BO* are presumed to arise from the attack by OOP) on BH or on BH2. In a subsequent investigation Jeffers and Bauer2 measured the oxidation rate of the amine-borane adducts H3B-N(CHJ3 and H3B.N(C2H5), by 0 atoms. These rates are about 2 orders of magnitude faster than for borane carbonyl and 4 orders of magnitude faster than for B2H6. Intense chemiluminescence was again observed; when molecular O2was present, the flame was 'Permanent address: Department of Chemistry, SUNY at Cortland, Cortland, NY 13045.
0022-3654/84/2088-5039$01.50/0
blue-green in color, while a cool blue flame resulted with only 0 atoms in the flow. The BO-CYbands are the most prominent features, upon which broad fuzzy bands of BO2 are superposed when mixtures of 0 / 0 2are used. In limited respects our results are similar to the reaction among BCl, + 0 + N, which generated B O ( X 2 P ) and BO2 when O2was added, reported by Llewellyn et al.3 and Sharma and Joshi who investigated 0 + B(C2HS)3.4a In this paper we report on detailed spectral studies of 0 H 3 B ~ N ( C H J 3and of 0 N(CH,)3. We also recorded photographically spectra of the chemiluminescenceemitted when O(3P) atoms were mixed, at low pressures, with borane adducts of pyridine, tetrahydrofurane, tert-butylamine, and dimethyl sulfide. These spectra have many features in common, associated with the BO-CYband system, but each showed some unique aspects which we have not yet investigated. The emission intensities generated by the amine and the amine-borane adduct were photoelectrically recorded from 2650 to 6500 A. The vibrational populations for the BO*(A211)are clearly non-Boltzmannian, with some striking discontinuities in a log N(v) vs. v plot (portions have
+
+
(1) G. K. Anderson and S. H. Bauer, J . Phys. Chem., 81, 1146 (1977). (2) P. M.Jeffers and S. H. Bauer, Chem. Phys. Lett., 80, 29 (1981). (3) I. P. Llewellyn, A. Fontijn, and M. A. A. Clyne, Chem. Phys. Lett., 84, 504 (1981). (4) (a) A. Sharma and J. C. Joshi, J. Quant. Spectrosc. Radiat. Transfer, 12, 1073 (1972);(b) The integrated intensity of the (4,O)band was chosen for normalizing the entire spectrum because it is the most intense and required the least amplification. Inspection of the records shows that all the bands drifted together. For example, after (4,O)decreased to 51%, (10,l) decreased to 57% and after (4,O)decreased to 85%, (2,l) decreased to 84%. In this respect (4,O)is not unique.
0 1984 American Chemical Society
5040 The Journal of Physical Chemistry, Vol. 88, No. 21, 1984
TABLE I: Relative (Integrated) Vibrational Band Intensities
H3B,X + H e
d/
Jeffers and Bauer
(shielded1 pw generator A
i
5
small a
monochro :
.
P(O,3) P(l,4) P(2,5) N, (2nd')?
1 , 10. In addition to BO*, emissions were observed from OH(A22++X211), NH(A311+ X32), CN(B22++X22+), CH(A2A-XZII), and BH(A'II+ X'B+).We looked for but found no indications of electronically excited NO, CO, C2, or BN. The a-band intensities were corrected for their Franck-Condon factor^,^ and the relative populations in the various vibrational levels for the A state were estimated. Although the data from many runs scatter within 50%, the relative populations within a run always showed the same characteristic features. The variations, as a function of the upper vibrational state, match quite well (Figure 2). These results are essentially independent of pressure over the range 2-20 torr. It appears that no single vibrational temperature exists. For example, the populations of the states u ' = 4-7 can be fitted by 2940 K (7/30/82) and by 3800 K (8/3/82), but the breaks between 3 C u ' C 4 and 7 < v' < 8 are unmistakable in each of these runs. The rotational temperatures were estimated by comparing computed band contours with those recorded. A typical example
-
(6) R. S . Mulliken, Phvs. Rev.. 25, 259 (1925). (7) We used the currently most reliable potential energy curves for the various electronic states of BO, the associated Franck-Condon factors, Einstein coefficients, etc. These were computed by Dr. H. H. Michels and co-workers (Drivate communications) An early reference to this work is H. H. Michels ei al., "Electronic Transition Lasers"; J. Steinfeld, Ed., MIT Press, Cambridge, MA, 1976, p 2781. A. P. Walvekar, Indian J. Phys., 43B,742 (1969); A. V. Nemukhin et al., Chem. Phys., 57, 197 (1981).
Chemiluminescence Generated by H3B.X
+ O(3P)
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5041 TABLE I1 EMISSION INTENSITY
A
" 0
+
E
8n
CN
-
__
1
38.90
4010
85.0 6.8
,;:;
;:;
,
* x2:+(2,1)
a2z*
(0,O)
...........................................................
f
6.2
417
-
X'L+(O.O)
Yt
Figure 2. Reduced band intensities vs. upper vibrational quantum number: ( O ) , 8/3/82 data; (e),7/30/82 data. The subscripts to the indi-
vidual points indicate the lower vibrational state quantum number.
,lp'
3525
I+
3526
Figure 3. A typical scan of one band, BO-a!(4,O).
of a strong BO band is shown in Figure 3. All the bands have similar rotational line distributions and closely similar temperatures, 2350 K. Rotational equilibration is anticipated since at the ambient pressure in the flame, the excited species are subjected to at least 25 collisions per microsecond even at the lowest pressure, while the lifetime of BO* is -1.7 /.LS.~ To determine whether population inversions are generated in the production of BO (r,(X) = 1.2049 A; r,(A) = 1.3524 A), the branching ratio for the partition of the nascent BO between the X and A states must be estimated. An attempt to use a line reversal measurement at the (4,O) band head was not successful. Even with a Xe lamp intensity 100 times as great as the flame and with the narrowest possible slit settings, no absorption was observed; the total intensity remained the sum of the lamp plus the flame across the entire band. (8) M. A. A. Clyne and M. C.
Heaven, Chem. Phys., 51, 299 (1980).
~~
~
(9) (a) M. A. A. Clyne, B. A. Thrush, and R. P. Wayne, Trnas. Faraday SOC.,60,359 (1964); (b) A. Fontijn and H. I. Schiff in "Chemical Reactions in the Lower and Upper Atmosphere", Interscience, New York, 1961, pp 239-254.
J . Phys. Chem. 1984, 88, 5042-5048
5042
-
react with a variety of oxygen carriers, for example B
+ H20
or B
+ C3H60
BO(A211)
+ H2(1Z:g+)10
BO(A211)
+ products”
where C 3 H 6 0 = 1,2-epoxypropane. Those chemiluminescent emissions were recorded with much lower spectral resolution in molecular beam experiments, so that the rotational temperatures could not be ascertained. While all of our experiments were performed under multiple-collision conditions (note the relaxed rotational temperature), vibrational relaxation was not achieved even in the presence of excess oxygen atoms and many free radicals. In view of the observed low rotational temperature (hence low translational temperature), it remains for highly exoergic reactions to be the direct source of the excited electronic and vibrational emitters. Only two reactions involving B/H species have sufficient energy to account for the observed u’ = 10 in the A state:12 0
+
BH
-
LO-B-HI
-
OBx
+
HD
and
(Were BO produced in the X state, the enthalpy releases at 400 K would be 113.26 kcal/mol for the first and 107.53 kcal/mol for the second reaction.) However, the latter is less plausible for generating high states of vibrational excitation in BO*, since it (10) (a) J. L. Gole and S.A. Pace, J . Phys. Chem., 85,2651 (1981);(b) A. W. Hanner and J. L. Gole, J . Chem. Phys., 73, 5025 (1980). (1 1) P. Davidovitz et al., Chem. Phys. Lett., 86,491 (1982),and previous
communications.
(12) For estimates of enthalpies of boron-containing species, refer to M. J. S.Dewar and M. L. McKee, J . Am. Chem. Soc., 99,5231 (1977),and the JANAF Tables.
is unlikely that the ejected H2 would leave without significant internal energy. Further, with respect to the three-atom reaction, one could argue that the electronically excited species are generated directly, rather than by resonant transfer, from very high vibrational levels in the 22+state. The intermediate, presumably linear structure 0-B-H13 must incorporate the electronic orbital angular momentum from the oxygen, which would readily correlate with the n-state of the product BO. For the production of BO(A211) by reaction of boron atoms with oxygen carriers, Hanner and Golelob proposed that resonant transfers from the X state (u” = 17) contribute significantly to enhance the population of the u’ = 4 level. Also, at the crossing of the X2B+ and A211potential curves ( u ” = 21; u’ = 9), another close coincidence appears, which leads to a second perturbation. Further investigations are needed to determine the origin of the stepwise features in the vibrational-state populations. No mechanisms have yet been established which account for the production of BO in the @system. One possible explanation is the occurrence of extensive energy pooling among the various excited species. Thus, if oxygen atoms attack BH(AIII), which is present in significant levels, then BO(B2Z+) could be produced.
Acknowledgment. This study was supported by the ARO under Grant No. DAAG29-81-K-0037. We thank Professor T. A. Cool for a loan of his standard black-body source and Professor R. F. Porter for the use of his optical pyrometer. We greatly appreciate the revised potential energy curves and detailed calculations of vibrational states sent to us by Dr. J. H. Michels (United Technologies Research Center). Registry No. H,B.N(CH,),,75-22-9;N(CH,),,75-50-3;0,, 778244-7;pyridine borane adduct, 110-51-0;tetrahydrofuran borane adduct, 14044-65-6; tert-butylamineborane adduct, 7337-45-3; dimethyl sulfide borane adduct, 13292-87-0. (13) IR spectrum of matrix-isolated HBO was reported by E. R. Lory and R. F.Porter, J . Am. Chem. SOC.,93, 6301 (1971). In the gas phase, the spectrum of the analogous halogenated species ClBO was analyzed by K. Kawaguchi, Y . Endo, and E. Hirota, J. Mol. Spectrosc., 93, 381 (1982).
Oscillating Convective Effects in SF6-Ar Laser-Heated Mixtures S. Ruschint and S.
H.Bauer*
Department of Chemistry, Cornell University, Zthaca, New York 14853 (Received: February 14, 1984)
Periodic temperature variations were observed in vertical absorption cells which were filled with mixtures of SF6 and Ar. The cells were irradiated from below with a continuous C02laser beam of moderate intensity (3-15 W). The onset of oscillations and their dependence on mixture composition and laser irradiation intensity were measured. A qualitative model is proposed to account for this periodic behavior. Allowance for temperature oscillationsmust be made in developing thermochemical or kinetic parameters from laser-powered homogeneous pyrolysis experiments.
Introduction The nonlinear behavior of absorption of infrared radiation by molecular gases has been the cause of a wide range of effects. When the absorbing gas is mixed with a suitable amount of inert gas so as to raise the overall pressure to several torr or higher, thermalization occurs in less than a millisecond and a unique local temperature can be defined for all molecular degrees of freedom. As a consequence, mixtures of gases can be heated by laser radiation to temperatures which are determined by the composition of the species present. One practical application of this type of heating is laser-powered homogeneous pyrolysis.’ Thus, by collisional energy transfer, chemical reactions which require high ?.Permanent address: Faculty of Engineering, Department of Electron Devices, Tel-Aviv University, Ramat-Aviv 69978,Tel-Aviv, Israel.
temperatures can be investigated for reactants which do not absorb at the available laser wavelengths, avoiding perturbations due to heterogeneous reactions which may take place on the hot w a l k 2 Detailed information and analyses of the temporal and spatial temperature distributions in laser-heated absorption cells are therefore of considerable interest. When the absorption cross section is high most of the incident radiation is deposited close to the cell entrance window. Then large temperature gradients are induced which, in vertical configuration, lead to convective flows. Such flows cause mixing, for in addition to diffusion the mass of the hot gas rises to the Bauer, I n f . J . Chem. Kine?.,7,509 (1975). (2) M. R. Berman et al., J. Am. Chem. SOC.102,5692(1980);K. E. Lewis et al., J . Phys. Chem., 84,226 (1980). (1) W. M. Shaub and S. H.
0022-3654/84/2088-5042$01.50/00 1984 American Chemical Society
