J . Phys. Chem. 1990, 94, 3291-3294
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Temperature Dependence of the Collisional Quenching of NH(a ’A) by N2, 02,CO, and Xe H. H. Nelson,* J. R. McDonald, Chemistry DivisionlCode 61 10, Naval Research Laboratory, Washington, D. C. 20375-5000
and Millard H. Alexander Department of Chemistry, University of Maryland, College Park, Maryland 20742 (Received: October 30, 1989; In Final Form: January 23, 1990)
We have measured the temperature dependenceof the collision-induced quenching rate constant for NH(a ‘A) with the collision cm3 s-I and an activation partners N2, CO, 02,and Xe. For collisions with N2 we derive an A factor of (7.06 1.03) X energy of (1.26 f 0.12) kcal/mol. This activation energy is consistent with recent theoretical and experimental work that suggested a barrier on the HN-N2 singlet surface. For collisions with oxygen, we derive an Arrhenius expression: k ( 0 2 ) = (1.80 f 0.10) X 10-12exp[(-2.07 0.05) kcal/mol/RT] cm3 s-I. As expected, quenching by CO and Xe is temperature independent with quenching cross sections of 1.9 and 1.5 A2, respectively.
*
*
Introduction Over the past two decades, there have been many studies of the production and subsequent reactions of singlet N H radicals. Most of these studies have used ultraviolet photodissociation of precursors such as HN,, HNCO, and NH, as the source of the singlet NH.I-l3 Recently, Foy et aI.I4 and Stephenson et aI.l5 have shown that IRMPD of HN, and DN, produces both NH(a !A) and, in a spin-forbidden process, NH(X ,Z-). This suggests that there is a finite probability of crossing between the singlet and triplet surfaces at large HN-N2 distances. In their work on DN3, they found that the translational energy of the ND(a ]A) fragment did not peak at zero as might be expected. Instead, the N D translational energy was characterized by a distribution function that resulted from an average kinetic energy of the two fragments of 1460 cm-I. They postulated that the kinetic energy of the fragments results from an exit channel barrier on the HN, singlet surface. In a companion paper, Alexander, Werner, and DagdigianI6 report ab initio MCSCF-CI and CASSCF calculations on the HN, surface. Their calculations also suggest a barrier on the singlet surface at large HN-N2 separations as pictorially represented in Figure I . This exit channel barrier on the HN, singlet surface has been cited by Freitag, Rohrer, and StuhlI2 as the reason for the large
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( I ) McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chem. Phys. Leu. 1977, 51, 57. (2) Baronavski, A. P.; Miller, R. G.; McDonald, J. R. Chem. Phys. 1978, 30, 119. (3) Donnelly, V . M.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. 1979, 43, 27 I . (4) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. Leu. 1979, 64, 421. ( 5 ) Piper, L. G.;Kreck, R. H.; Taylor, R. L. J . Chem. Phys. 1980,73,791. (6) Rohrer, F.; Stuhl, F. Chem. Phys. Ler!. 1984, 1 1 1 , 234. Nelson, H. H.; McDonald, J. R. Chem. Phys. 1985, 96, (7) Cox, J. W.; 175. ( 8 ) Rohrer, F.; Stuhl, F. J . Chem. Phys. 1987, 86, 226. (9) Bower, R. D.; Jacoby. M. T.: Blauer, J. A. J . Chem. Phys. 1987,86, 1954. (IO) Kenner, R. D.; Rohrer, F.; Stuhl, F. J. Chem. Phys. 1987,86, 2036. ( I I ) Rohrer, F.; Stuhl, F. J. Chem. Phys. 1988, 88, 4788. (12) Freitag. F.; Rohrer. F.; Stuhl, F. J. Phys. Chem. 1989, 93, 3170. (13) Hack, W.; Wilms, A. J . Phys. Chem. 1989, 93, 3540. (14) Foy, B. R.; Cassassa, M. P.; Stephenson, J. C. King, D. S . J . Chem. Phys. 1988. 89. 608. ( 1 5 ) Stephenson, J. C.; Cassassa, M. P.; King, D. S . J . Chem. Phys. 1988, 89, 1378. (16) Alexander, M. H.;Werner, H.-J.; Dagdigian, P. J. J. Chem. Phys. 1988.89, 1388.
0022-3654/90/2094-329 1 $02.50/0
difference they observe for the room temperature rate constant for collisional quenching of NH(a IA) by N2 and CO. They report a 200-fold increase in the observed rate when the collision partner is changed from N 2 to CO. In order to determine if the relative inefficiency of the reaction with N2 is due to an entrance channel barrier on the HN-N2 surface and, if so, to determine the barrier height, we have measured the temperature dependence of the collision-induced quenching rate constant for NH(a IA). We report here our results for collisions with N2, CO, 02,and Xe.
Experimental Section NH(a IA) radicals are produced by the photolysis of hydrazoic acid (HN,) at 266 or 193 nm. The HN3 absorption cross section at 193 nm is approximately 40 times larger than at 248 or 266 n ~ n , ~but ’ photolysis at 193 nm produces a significant population of triplet N H by both onell and twoIS photon processes. This prompt appearance of triplet N H complicates the measurement of NH(X ,2-) appearance rates due to collisional processes. Therefore, several experiments with O2as the collision partner were performed at room temperature using 266-nm photolysis of HN, to ensure that the rate constant obtained for NH(a IA) removal was equal to that for NH(X ,Z-) appearance. We have experimentally confirmed the equivalence of these two rates for collisions with 02. The experiments are described in detail in a separate paper.19 For the temperature-dependent measurements of the disappearance rate of NH(a IA) reported here, the photolysis laser is an ArF excimer laser (Lumonics Model 861) apertured to 0.7-cm diameter with pulse energy of 5-10 mJ. NH(a !A) radicals are probed by use of laser-induced fluoresence on single rotational lines of the NH(c Ill-a !A) 0-0 transition near 326 nm. ‘The pressure of buffer gas in the system is sufficient to maintain a thermal rotational distribution in the NH(a ‘A) reactant. The probe laser is an excimer pumped dye laser (Lambda Physik Model FL2002E) with pulse energy of 10-100 FJ.
The pump and probe laser beams counterpropagate through the reaction cell which consists of a 5-cm-diameter stainless steel cross with long glass sidearms. The cell is contained in a convection oven equipped with ports for the sidearms and light collection optics. The oven temperature is measured with a (17) McDonald, J. R.; Rabalais, J. W.; McGlynn, S. P. J . Chem. Phys. 1970, 52, 1332. (18) Hofzumahaus, A.; Stuhl, F. J . Chem. Phys. 1985, 82, 3152. (19) Adams, J. S.; Pasternack, L. Manuscript in preparation.
0 1990 American Chemical Society
3292 The Journal of Physical Chemistry, Vol. 94, No, 8, 19913
Nelson et ai.
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I
I
I
Y
0
0 0
I
r (HN
- Ne)
Figure 1. Schematic representation of the HN, potential energy surfaces leading to NH(a ‘A) and NH(X %-) formation by fission of the internal N-N bond. The energy of the singlet-triplet surface crossing has not been precisely fixed by experiment or calculation but likely lies between I2 500 and I5 200 cm’l. The barrier to dissociation on the singlet surface, labeled AEa, is measured in this work to be 450 cm-I.
thermocouple and is constant to f l K. Fluorescence emission, collected at right angles to the laser beam axis, is collimated by a lens, separated from scattered excimer laser light by a solution filter and detected by an RCA 31000 photomultipler tube. The resulting signal is captured by a gated integrator (SRS Model 235) and digitized and stored by a laboratory microcomputer. The delay time between the pump and probe laser pulses is varied by the computer from -100 ps (to obtain a signal base line) to +IO ms. NH(a ] A ) disappareance profiles are collected after a minimum 5-gs delay from the photolysis laser pulse to avoid any effects due to scattered photolysis light, vibrational relaxation of the NH(a [A), or prompt emission of photoproducts. To obtain the decay profile, 500 points, each one being the average of 2-10 laser shots, are collected from the minimum delay ( 5 ps) to the longest time that NH(a ‘A) is still detectable. Experiments are performed under pseudo-first-order conditions with the molecular collision partner in large excess over NH(a ‘A). In the absence of added reagent, the NH(a ‘A) disappearane profiles are well approximated by single exponentials resulting from diffusion of the NH from the probe region and the known, fast reaction of NH(a lA) with the precursor HN3.1*5-7920When the reactant is added, the NH(a IA) decay profile is well represented by a single exponential decay for 4 to 6 lifetimes. HN3 is prepared by a standard method from the reaction of sodium azide and molten stearic acid2’ and stored at less than 300-Torr pressure in 10-Lglass bulbs. Prior to use, the HN3 is diluted 1:lOOOwith He in a high-pressure steel tank. This mixture is stable for several days. The HN,/He mixture, additional He used as buffer gas, and the reactant are combined, mixed, and flow slowly through the reaction cell. Typical total flow rates are 500 sccm as measured by Tylan mass-flow meters. This flow rate is sufficient to exchange the active region between laser shots. Most experiments are performed at a total pressure of 20 Torr. He (Air Products Industrial Grade, 99.995%), N2 (Air Products Industrial Grade, 99.998%), 0,(Matheson UHP, 99.98%), CO (Matheson CP,99.5%), and Xe (Spectra Gases Research Grade, 99.99%) are used as received. Results and Discussion We have measured the rate constants for collisional quenching of NH(a ‘A) with the collision partners N2, CO, O,,and Xe over the temperature range 298-619 K. First-order rate constants are obtained for the measured NH(a ]A) disappearance at a variety (20) Paur, R.J.; Bair, E. J. fnt. J . Chem. Kinet. 1986, 8, 139. ( 2 1 ) Krakow, B.;Lord, R. C.;Neely, G.0.J . Mol. Specrrosc. 1968, 27, 148.
2
6
4
E
10
12
(Torr1 po2
Figure 2. Plot of the pseudo-first-order disappearance rate constants for NH(a IA) as a function of O2pressure at 302 and 476 K. The symbols are measured decay constants, and the solid lines are the results of linear least-squares fits to the data.
TABLE I: Measured Collision-Induced Intersystem-Crossing Rate Constants, k , and Derived Cross Sections, on. for NH(a ‘A) collision Lra partner T, K kt“ f lo 1o-Ipcm2 N2 306 (7.93f 0.66)X 0.0097
0 2
co
Xe ‘uQ
370 415 473 596 298 302 370 433 476 301 382 433 523 619 298 476
(1.35 f 0.06)X (1.56f 0.05)X (1.80f 0.08)X (2.39f 0.19)X (5.65f 0.15)X (5.35f 0.41)X (1.00f 0.03)X (1.66f 0.07)X (2.03f 0.03)X (1.64f 0.08)X (1.66f 0.10)X (1.84f 0.02)X (2.01f 0.06) X (2.24f 0.03) X (1.09f 0.04)X (1.30f 0.03)X
IO-”
IO-’)
IO-” IO-” IO‘” IO-” IO-” IO-” IO-” IO‘”
0.0151 0.0165 0.0178 0.0210 0.0072 0.0068 0.0114 0.0175 0.0204 2.03 1.82 1.90 1.89 1.93 I .59 I .50
= k/(c).
of collision partner partial pressures. These first-order rate constants are plotted against collision partner pressure to obtain the second-order rate constant. Typical results for the reaction with O2 at 302 and 476 K are shown in Figure 2. All the measured bimolecular rate constants are listed in Table I along with the calculated quenching cross section, aQ = k Q / ( u ) . This removes the reduced mass and relative velocity dependence from the measured rate constants. As can be seen from the data in Table I, the removal cross sections for collisions with CO and Xe are not temperature dependent while those for N2 and 0, vary with temperature. The rate constants for reaction with N2 and 0, are plotted as a function of temperature in Figure 3. When these data are fit to an Arrhenius expression, the resulting parameters are k(N,) = (7.06 exp[(-1.26 f 0.12) kcal/mol/RT] cm3 s-I and f 1.03)X k ( 0 , ) = (1.80 f 0.10) X exp[(-2.07 f 0.05) kcal/mol/RT] cm3 s-l. The uncertainties are f l u and are statistical only. There are several points to consider that result from our measurements with N2 as the collision partner. We confirm the prediction by Stephenson et aI.l5 and the speculation by Freitag et al.I2 that there is a significant entrance channel barrier on the HN-N2 surface. Our measured activation energy, -450 cm-’, and hence the height of the barrier on the singlet HN-N, surface, is somewhat smaller than the value derived from kinetic energy measurements on the NH(a ‘A) product by Stephenson et al., 1460 cm-I. There are several possible explanations for this difference. We do not directly measure the temperature dependence of the
Collisional Quenching of NH(a ’A) by N2, 02,CO, and Xe
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. I
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Lo
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The Journal of Physical Chemistry, Vol. 94, No. 8, 1990 3293 The results of our measurements on the NH(a ‘A)-CO system are qualitatively different. In this case, we observe an Arrhenius A factor 50 times greater than for the N2 system and no temperature dependence of the intersystem-crossing rate. This pronounced difference in the ease of quenching of NH(a ‘A) by N2 and C O can be understood in terms of the behavior of the two degenerate A orbitals of N H during the approach of the quenching partner. In C, geometry these two orbitals of N H are labelled IOa’(r,) and 2 a ” ( ~ ~ ) Both . ’ ~ of these orbitals are nonbonding in the isolated N H radical. In general, as the N H and N2(CO) partners approach, the energy of the in-plane loa’ orbital will increase, due to Pauli repulsion between this orbital and the highest u lone-pair oribital of the partner. By contrast, the energy of the 2a” orbital will decrease, since this orbital eventually correlates with the weak 7r bond in the ‘A’ electronic ground state of HN, or HNCO. In a simplistic analysis the bonding in HN3 or HNCO is due to the stabilization of this 2a” orbital. Asymptotically, at large HN-M distances, the wave function of the NH(a IA) N2(CO) system is doubly degenerate. In C,geometry the component of this degenerate state which is symmetric (A’) with respect to reflection of the electronic spatial coordinates in the plane of the molecule corresponds to the electron occupancy ...( - ...(2a’’)2, while the component which is antisymmetric (A”) corresponds to the electron occupancy ...1Oa’2a”. Because of the unfavorable energetics of the loa’ orbital, the A” state is repulsive and correlates at shorter range with an excited electronic state of HN, or HNCO. In the case of the A’ state, the Pauli repulsion can be minimized by orienting the N H fragment perpendicular to the N-N (C-0) bond and by confining the two nonbonding N H A electrons to the out-of-plane 2a“ orbital. This qualitative picture is, at least for HN,, supported by elaborate multireference, configuration interaction calculation^.'^^^^^^^ As the N 2 collision partner approaches, the confinement of the two N H A electrons, which asymptotically can occupy either the lOa’(a,) or 2a”(7rY)orbitals, to a single orbital (2a”) leads to a slight increase in the electronic energy before the eventual A bonding involving the 2a” orbital causes the overall energy to decrease. It is this slight increase in energy at Ion range (the maximum occurs for HN-N2 distances of -2.4 that leads to the activation energy in the collision-induced intersystem crossing of NH(a ‘A) by N2 observed here. Since the dissociation energy of HNCO is considerably larger than that of HN,, the lowering in energy of the 2a” orbital as the HN-CO distance decreases is more pronounced than in the case of HN-N2. This lowering overcomes the increase in energy due to the orbital confinement of the N H A electrons, so that no long-range barrier occurs in the A’ state of HNCO. Thus, the collision-induced intersystem crossing of NH(a ‘A) by CO occurs without any activation energy. In order to confirm this qualitative analysis, further ab initio calculations on HNCO, of sophistication comparable to those recently reported for HN3,1692435 are necessary. We have also made measurements with O2 and Xe as the collision partner. As expected, we find no temperature dependence for the intersystem-crossing rate with Xe. For the case of 02, we observe an appreciable activation energy, 2.07 kcal/mol. By analogy to the N2 system, we attribute this to a barrier in the entrance channel for complex formation. This complex has been observed by Freitag et al.I2 to dissociate to 0 2 ( b and we also observe NH(X ,E-)containing multiple quanta of vibrational energy as a primary product. The details of these product studies are contained in a separate paper.I9
+
2
m
E
u
10-13 2 8
Y
6 4
1.5
2.0 2.5 3.0 3 UT (10-3 K - ~ I Figure 3. Arrhenius plots for the reaction of NH(a ’A) with N2 (upper panel) and O2 (lower panel). The solid symbols are measured bimolecular rate constants with 2a statistical uncertainties and the solid lines are the results of weighted linear least-squares fits to the data. formation of [HNJ’ complex, which would directly give the barrier height, but the product of complex formation and crossing to the triplet surface. If there were a significant temperature (energy) dependence of this surface-crossing rate, this would result in a measured activation energy smaller than the actual barrier. The singlet-triplet crossing has been reported to occur at 12 500 cm-’ above the HN, singlet well based on experimental measurements of HN, decompositionZ2and preliminary calculations.I6 This would place the crossing 4000-5000 cm-’ below the NH(a ‘ A ) N 2 asymptote.I6 More recent measurements of the dissociation lifetimes of overtone excited H N b Foy et aL2, and preliminary calculations by Alexander et and Y a r k ~ n y ? ~ however, indicate that the singlet-triplet crossing may occur as little as 2000 cm-’ below the asymptote. Small changes in the thermal energy on top of this are unlikely to have a large effect on the crossing rate. It is also possible that Stephenson et al. have underestimated their up-pumping rate or overestimated the DN3 dissociation rate and are therefore observing ND(a ’A) radicals with more than the minimum kinetic energy. This, coupled with a small mismatch between the barrier height and an even multiple of the C 0 2laser photon energy may explain the difference between our two measurements. From our measured Arrhenius A factor, we can estimate the singlet-triplet crossing probability for the HN3 system. Our A factor, 7 X IO-!,, is 100-1000 times lower than the thermal collision rate for these collision partners. On this basis, we estimate that the crossing probability is -0.003, in excellent agreement with the crossing probability derived by similar reasoning from the measured A factor for HN, thermal decomposition to NH(X
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+
3 T ) .22
(22) Kajimoto, 0.;Yamamoto, T.; Fueno, T. J . Pfiys. Cfiem. 1979, 83, 429. (23) Foy, B. R.; Cassassa. M . P.: Stephenson, J . C.; King, D. S. J . Cfiem. Pfiys. 1989, 90, 7037. (24) Alexander, M . H . ; Hemmer, T. E.; Werner, H.-J.; Knowles, P. J . To be published. (25) Yarkony. D. R . To be published.
1)2492s
Summary
We have measured the temperature dependence of the collisional quenching rate constants for NH(a IA) with N,, CO, 02, and Xe. In agreement with recent work on the IRMPD of DN, and a theoretical investigation of the HN3 potential energy surfaces, we find a small barrier on the singlet surface at (presumably) large HN-N2 separations. This observation confirms the speculation of Freitag et al. about the difference in collisional quenching efficiency between N2 and CO. We also measure a
J . Phys. Chem. 1990, 94, 3294-3297
3294
87-NC-119. We are grateful to Paul Dagdigian and David Yarkony for helpful discussions.
significant activation energy for the reaction with O2and ascribe this to a barrier to complex formation. Acknowledgment. The work of M.H.A. was partially supported by the US.Air Force Office of Scientific Research under Grant
Registry No. N H , 13774-92-0; 7782-44-7; Xe, 7440-63-3.
N2,7727-37-9; CO, 630-08-0; 02.
Time-Resolved Studies of the Temperature Dependence of Gas-Phase Insertion Reactions of Phenylsllylene with Si-H Bonds Mark A. Blitz, H. Monty Frey, Fred D. Tabbutt,*-+and Robin Walsh* Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 ZAD, U.K (Received: October 6, 1989; In Final Form: January 17. 1990)
The 193-nm laser flash photolysis of gas-phase phenylsilane has been found to generate a broad-band absorption in the wavelength range 460-600 nm, which is plausibly shown to be due to the transient species phenylsilylene, PhSiH. Time-resolved measurements of its decay have been carried out in the presence of the added silanes, SiH4, MeSiH3, Me2SiH2,and Me,SiH. Second-order rate constants have been obtained for each added gas within the temperature range 291-564 K. Rate constants are generally 2 I O - ’ l cm3molecule-l s-l, indicating a high reactivity for PhSiH. Negative activation energies are found ranging as high (in magnitude) as -I 5 kJ mol-’. Possible explanations, including intermediate complex formation, are considered for these reactions. There appears to be little or no electronic interaction between the phenyl group and interacting (frontier) orbitals on the silicon atom in PhSiH
Introduction In contrast to silylene, SiH2, whose spectrum is known’,2and for which a number of rates of reaction have been m e a ~ u r e d , ~ - ~ relatively little is known about phenylsilylene, PhSiH. Yet phenylsilane, PhSiH,, the common precursor to SiH2, is also the precursor of PhSiH. End-product studiesI0 of the photolysis of PhSiHl at 193 nm have shown the existence of two primary processes C6H,SiH3 hv C6H6 + SiHz (1)
+
--
H2 + C6H5SiH (2) Quantitative analysis shows that channel 2 is favored over channel 1 by a factor of ca. 6, and so under these experimental conditions it appears that PhSiH3 is an even more abundant source of PhSiH than of SiH,. One of the characteristic silylene reactions is insertion into Si-H bonds. Time-resolved gas-phase studies have been made of the insertion both of SiH:v9 and of SiMe2Il into the Si-H bonds of SiH,, Si2H6,and the methylsilanes. These studies, carried out at room temperature, have shown a reactivity pattern in which methyl substitution deactivates the transient silylene but activates the substrate silane. The results may be interpreted in terms of a two-stage process consisting of a combination of electrophilic and nucleophilic interaction^.^ This ambiphilic behavior was first p r o p o ~ e d l ~for , ’ ~the reaction of SiH2 with H, by analogy with the equivalent reaction of CH2(’Al). Recent measurements in our laboratory of some of these reaction rate constants over a temperature rangel, show a decrease in the rate constants with increase in temperature. The negative activation energies for these insertion processes may be explained by invoking the formation of an intermediate. weakly bound complex which may either revert to reactants or proceed to products, viz. X2Si + H-SiY, z X2Si-SiHY, X2HSiSiY3 (A) This mechanism receives some support from kinetic studies on the reverse decomposition reactions of methyl-substituted di~i1anes.I~ Given the development of a mechanism to explain Si-H insertions by substituted silylenes and a potentially rich source of
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‘Permanent address: The Evergreen State College, Olympia, WA 98505.
0022-3654/90/2094-3294$02.50/0
PhSiH, it becomes possible to explore the effect of an aromatic substituent on the silylene reactivity and thereby deepen our understanding of the mechanism. A hint that phenyl may be a more deactivating substituent than methyl comes from time-resolved studies in solution by Gaspar et which suggest that phenylmethylsilylene, PhSiMe, is less reactive than %Me2. However, some doubt is cast on the values of the rate constants obtained by Gaspar et a1.I6 by the recent solution studies of Shizuka et al.,” who obtained significantly higher values when employing a different SiMe2 precursor. These latter values were much closer to those we have measured for similar reactions in the gas phase.” We therefore describe here a time-resolved, temperature-dependent kinetic study of the gas-phase insertion reactions of an aromatic-substituted silylene.
Experimental Section Apparatus and Chemicals. The apparatus for these experiments has been described in detail elsewhere.8JlJ8 Briefly, a photolysis ( I ) Dubois, 1.; Herzberg, G.; Verma, R. D. J . Chem. Phys. 1977,47,4262. ( 2 ) Dubois, 1. Can. J . Phys. 1968, 46, 2485. (3) Inoue, G.; Suzuki, M. Chem. Phys. Left.1985, 122, 361. (4) Jasinski, J. M. J. Phys. Chem. 1986, 90, 5 5 5 . ( 5 ) Chu, J. 0.;Beach, D. B.; Jasinski, J. M. J . Phys. Chem. 1987,91,5340. ( 6 ) Chu, J. 0.;Jasinski, J . M. J . Chem. Phys. 1988,88, 1678. Beach, D. B.; Estes, R. D.; Jasinski, J. M. Chem. Phys. Lett. (7) Chu, J. 0.; 1988, 143, 135. (8) Baggott, J. E.; Frey, H. M.; King, K. D.; Lightfoot, P. D.; Walsh, R.; Watts, I . M. J . Phys. Chem. 1988, 92, 4025. (9) Baggott, J. E.; Frey, H. M.;Lightfoot, P. D.; Walsh, R.; Watts, I. M. J . Chem. SOC.,Faraday Trans. 1990.86, 27.
(IO) Baggott, J. E.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. Chem. Phys. Lerr. 1986. 125. 22. ( I I ) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R Chem. Phys. Left. 1987, 135, 39. (12) Sax, A.; Olbrich, G. J . Am. Chem. SOC.1985, 107, 4868. (13) Gordon, M. S.; Gano, D. R. J. Am. Chem. SOC.1984, 106, 5421. (14) Baggott, .~ J. E.; Blitz, M. A.; Frey, H . M.; Walsh, R. Unpublished 1
~
1
~
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results.
( I 5 ) Becerra. R.; Bertram, J. S.; Walsh, R.;Watts, 1. M. J . Chem. Soc., Faraday Trans. 2 1989. 85, 1837. (16) Gaspar, P. P.; Holten, D.; Konieczny, S.; Corey, J. Y.Acc. Chem. Res. 1987, 20. 329. (17) Shizuka. H.; Tanaka, H.; Tonokura, K.; Murata, K.; Hiratsuka, H.; Ohshita, J.; Ishikawa, M. Chem. Phys. Lett. 1988, 143, 225.
0 I990 American Chemical Society
