- Email: [email protected]
Kinetics of reactions of BH with NO and C2H4
Volume 150, number 3,4
KINETICS
OF REACTIONS
CHEMICAL PHYSICS LETTERS
16 September 1988
OF BH WITH NO AND CzHl
J.A. HARRISON, R.F. MEADS and L.F. PHILLIPS Chemistry Department, University of Canterbury, Christchurch, New Zealand
Received 7 June 1988; in final form 28 June 1988
Excimer laser photolysis of diborane and laser-induced fluorescence of BH have been used to measure rate constants for the reactions of BH with NO and C:H, at total pressures near 1 Torr, over the temperature range 250-350 K. Upper limits were obtained for the rate constants for reactions of BH with 01, CO, CHI, and C2H6.Experiments with different buffer gases show no effects that can be attributed to the buffer gas acting as a third-body during adduct formation. For the BH t NO reaction the results are consistent with a model in which an intermediate HBON*, formed by capture over the centrifugal barrier in a dipoledipole potential, rearranges in a time of the order of a picosecond to give the products BO and NH.
1. Introduction
2. Experimental
Boron hydrides have long been of interest as highenergy fuels [ 1] but, despite several studies of the properties of diborane/air and diborane/nitric oxide flames [ 21, virtually nothing is known about the rates of fundamental processes involving boron hydride radicals. Pasternak et al. [ 31 have measured room-temperature rate constants over the pressure range from 6 to 620 Torr for reactions of BH3 with CO, NO and CzH4, and found upper limits for the rate constants for reactions with O2 and HzO. Recently, we gave the results of a preliminary spectroscopic study of the prompt emission observed during 193 nm photolysis of diborane [ 41. We now report rate constants for reactions of BH with NO and C2H4,and upper limits for rate constants for reactions with CH4, C2Hs, CO and 02, at pressures near 1 Torr of various buffer gases, at temperatures between 250 and 350 K. The measurements reported here were performed using laser-induced fluorescence to monitor the BH concentration via the Q(O,O) branch of the A++Xsystem at 433.4 nm. For the reaction of BH with NO the results are consistent with a theoretical model in which capture over the centrifugal barrier in a dipole-dipole potential [ 51 is followed by a fast rearrangement of the resulting HBON* complex [ 6,7] to give the products BO and NH.
The apparatus and procedures were similar to those used previously to measure rate constants for reactions of the NH radical [ 81. Gas flows were measured with calibrated Tylan mass flowmeters and mixed before entering the cell. Typical concenrration ratios buffer: diborane : reactant were 2000 : 1.O : O-7, at a cell pressure near 1 Torr. The single-photon absorption cross section of diborane at 193 nm is only 4.4~ 10m2’cm* [ 91, the measured absorption of 193 nm radiation in the cell was always less than 1Oh,and all [ BH ] decays obeyed pseudo-firstorder kinetics. Temperatures were controlled by circulating liquid from a thermostat bath through the cell’s outer jacket and measured with a copper-constantan thermocouple located in the gas stream just below the viewing region. The total pressure in the cell was measured with a type 222CA O-10 Torr MKS Baratron gauge. Pressures very different from 1 Torr either required reactant flows that were too small to measure accurately (below 0.3 Torr), or resulted in quenching of the fluorescence by which [BH] was being monitored (above 2.5 Torr). For this reason the possible effect of interactions with the buffer gas were investigated by varying the nature rather than the pressure of the buffer. Diborane was prepared by the method described by Jeffers [ lo] and kept frozen at 77 K until re-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
299
Volume 150,number 3,4
CHEMICALPHYSICSLETTERS 3
quired. Helium and argon were passed through a column of reduced BTS catalyst at 160 “C followed by a silica gel trap cooled with liquid nitrogen (He) or a solid-COJethanol bath (Ar). SF6 (Air Products instrument grade), N2 (NZIG oxygen-free grade),
O2 (Matheson UHP grade), CO (Matheson CP grade), CH, (NZIG UHP grade), CzH4 (NZIG CP grade), and C2H6 (Matheson CP grade) were used without further purification. NO (Matheson CP grade) was passed through a trap containing silica gel cooled with solid-COJethanol. Diborane was photolysed with the focused output (lens focal length 50 cm) of a Lumonics TE-86 1 extimer laser operating at 193 nm with a pulse repetition frequency of 17 Hz. The low repetition frequency ensured that the contents of the cell were completely swept out between pulses. BH fluorescence was excited with an AVCO C5000 nitrogenpumped dye laser (dye: stilbene 420) and observed through Corning 7-69 and 3-73 filters (combined bandpass 410-490 nm, peak transmission 37% at 430 nm) with an EM1 9813QA photomultiplier. The output from the photomultiplier was taken through a preamplifier to a computer-controlled PAR model 160 boxcar integrator.
3. Results and discussion
Ln
16 September 1988
r
I
-3 0
90
t i me/v3
I 80
Fig, 1,Representativefirst-orderdecay plots for BH in 1Torr He at room temperature. [NO] (in lOI molecule cm-‘): open circles, 0, filled circles, 0.45; diamonds, 0.92, half-shaded diamonds, I. 12; squares, 1.51.
ing the nature of the buffer gas. For both reactants there is no evidence for third-body effects of the sort which would cause the reaction rate to increase in the presence of a buffer with a large number of internal degrees of freedom. However, for the reaction of BH with NO there is evidence, in the form of lower rate constant values being obtained in the presence of Ar and especially SF6, of the reaction rate being
3.1. The experimental data
Typical pseudo-first-order decay plots for BH in the presence of varying amounts of NO are shown in fig. 1, Signals from the first 40 ps after the photolysis pulse were omitted from the kinetic data because of a significant background arising from both prompt BH emission and window fluorescence. Graphs of the first-order decay rates versus reactant pressure were used to determine second-order rate constants for the BH+NO and BH+C2H4 reactions over a range of temperatures as shown in fig. 2. Both rate constants are very large, and both have a small, negative temperature dependence. The error bars in fig. 2 represent the 95% confidence limits of the slopes of the plots of first-order decay rate versus concentration, plus an additional uncertainty of 6% arising from the measurements of gas pressure and time. The results given in table 1 show the effect of vary300
IO"
k
15
350 Fig, 2. Observed variation of rate constants with temperature. Squares: BH+ C2H4;circles:BH+ NO.
Table 1 Variation of measured rate constant with buffer gas at room temperature and 1 Torr total pressure. (Units of k: 10-i’ cm3 molecule-’ s-‘) Buffer
k(BH+NO)
k(BH+ethylene)
He Ar
15.6t2.3 10.55 1.2 8.9k 1.7 6.5 22.0
14.44 14.2+ 17.1 f 13.3+
N2
SF6
16 September 1988
CHEMICAL PHYSICS LETTERS
Volume 150, number 3,4
1.7 1.7 1.9 1.3
partly limited by the rate of diffusion of NO in the buffer gas. The effect is not apparent with ethylene, and we conclude that nitric oxide is depleted in the viewing region by a chain reaction involving such species as atomic boron, diborane fragments, NH, N and 0. Measured rate constants for the BH + NO reaction in a helium buffer are independent of helium pressure between 0.3 and 2.0 Torr, from which we conclude that the rate constant values in fig. 2 are not limited by diffusion of NO into the viewing region. Upper limits (95% confidence limits) were obtained for the rate constants of the room temperature reactions of BH with CH, (9.4x lo-‘” cm’ molecule- ’ s- ’ ), C2H6 (1.6x 10-12), CO (1.3x10-13) and O2 (1,1x10-“). We also attempted to measure the rate constant for the reaction of BH with NO*, but this proved impossible because of strong chemiluminescence that was transmitted by the Coming filters and which decayed on the same timescale as the [ BH] decay in the absence of added reactant. An attempt to use N20 as a buffer gas was also thwarted by intense chemiluminescence. Chemiluminescence was a problem with the OZ reaction, but was not sufficiently intense to entirely prevent data collection. All of these interferences may be attributed to formation of BO* or BO’; by reactions of B(‘P) with the oxygen-containing species, as described by Davidovits and others [ ll-131.
of the relatively large number of atoms in this species and the absence of detectable third-body effects on rate of the BH+CzH4 reaction rate at pressures near 1 Torr, it seems likely that this adduct may be a stable product. At the very least it evidently survives for a time longer than the 100 us time scale of the present experiments. For the BH + NO system, in contrast, the results of quantum-chemical calculations [ 61 are available to show that two bound complexes HBON and HBNO can be formed in principle, but only the former is capable of dissociating to give products that are exothermic relative to the original reactants. At the energy of BH+NO, HBON* can dissociate exothermically to give BO+NH or HBO+N. The reaction to yield HBO +N ( 4S) is spin forbidden, while that to give HBO + N ( 2D ) is barely exothermic, and we therefore conclude that the most likely products are BO and NH. The bimolecular association to give HBON* occurs as the end result of a dipole-dipole attraction, and therefore can be calculated [ 5 1 as the capture rate in a potential comprising the dipole-dipole potential (dipole moments 1.27 and 0.16 D, polarizabilities 3.3~ 1O-24 and 1.68~ 1O-24 cm3, for BH and NO, respectively) plus the Morse parameters (r,=1.364>( lo-* cm, L&=132.5 kJ mol-‘, w, = 1184 cm-’ ) of the HB-ON bond from ref. [ 6 1. The results of carrying out this calculation (which has no adjustable parameters) over a range of temperatures are shown as the unfilled points in fig. 3, which also includes the experimental data of fig. 2. The predicted capture rates are seen to be essentially 30
1O1l k
15
3.2. The theoretical model
c 0
At present there are no structural data available for the BH-C2H4 complex that we assume to be involved in the reaction with C2H4, and we can only speculate as to its stability and ultimate fate. In view
zoo
400
Temperature/K
Fig. 3. Comparisonof theoreticalcapturerates (tilled circles) and overall reaction rates (open circles) with experimental data (squares) for the BH +NO reaction.
301
Volume 150, number 3,4
CHEMICAL PHYSICS LETTERS
independent of temperature between 200 and 400 K, and are equal to or slightly greater than the measured overall rate constants. The situation is not quite the same as for the reaction of NH with NO [ 81, where the theoretical capture rates and the measured rate constants essentially coincide; here it appears that there may be some limitation of the overall rate by processes occurring on the HBON potential surface. Results of calculating an overall rate constant by incorporating the rates of rearrangement and dissociation of HBON*, in essentially the same manner as was done for the more complex NH*NO* system [ 71, are shown as the filled circles in fig. 3. These values have a small, negative temperature coefficient, and are equal to or slightly smaller than the experimental results, which mostly lie between the calculated capture rates and overall rates. Thus there is no doubt that the experimental results are consistent with this model. However, the following points must be noted: (1) The purely statistical calculation of the rate constants k(E) for dissociation and rearrangement is probably not reliable for a system as small as HBON*, in which dynamical effects are likely to play an important part. The calculated lifetimes of the HBON* complex are of the order of 1 ps, which seems barely long enough to allow statistical redistribution of energy from the newly formed B-O bond. (2) The results obtained for the overall rate constant with such a small system are very strongly dependent on the nature of the activated complex assumed for the process of dissociation back to reactants. In the present instance, if a very tight transition state is assumed, with no internal rotations and vibration frequencies equal to the five highest frequencies of ground-state HBON (or even to the three highest and two lowest), the rate of dissociation back to reactants becomes negligible and the overall rate is equal to the capture rate. If, on the other hand, a looser transition state is assumed, with two one-di-
302
16 September 1988
mensional free rotors and the same bond angles ( E 60” ) as in ground-state HBON, dissociation back to reactants becomes very fast and the calculated overall reaction rates are too small by an order of magnitude or more. The results in fig. 3 were obtained by assuming a transition state with two onedimensional free rotors, with the bond angles opened out to 15O_ This is physically plausible, but the results can no longer be regarded as a prediction. It appears that more complex reaction systems, such as BH, + NO, may provide a better testing ground for the theory.
Acknowledgement This work was supported by the New Zealand Universities Research Committee.
References [ 1] A.G. Gaydon and H.G. Wolfhard, Flames, their structure, radiation and temperature, 2nd Ed. (Chapman and Hall, London, 1970). [2] W. Roth, J. Chem. Phys. 28 (1958) 668. [ 3] L. Pastemak, R.J. Balla and H.H. Nelson, J. Phys. Chem. 92 (1988) 1200. [4] J.A. Harrison, R.F. Meads and L.F. Phillips, Chem. Phys. Letters 148 (1988) 125. [ 51 L.F. Phillips, J. Chem. Sot. Faraday Trans. II 83 (1987) 857. [ 61 J.A. Harrison and R.G.A.R. Maclagan, Chem. Phys. Letters 146 (1988) 243. [ 71 L.F. Phillips, Chem. Phys. Letters 135 ( 1987) 269. [8] J.A. Harrison, A.R. Whyte and L.F. Phillips, Chem. Phys. Letters 129 ( 1986) 346. [ 91 M.P. Irionand K.L. Kompa, J. Chem. Phys. 76 ( 1982) 2338. [ 10 ] W. Jeffers, Chem. Ind. (April 8, 1961). [ 111J. DeHaven, M.T. O’Connor and P. Davidovits, J. Chem. Phys. 75 (1981) 1741. [ 121 A. Brzychcy, J. DeHaven, A.T. Prengel and P. Davidovits, Chem. Phys. Letters 60 (1978) 102. [ 131 G.J. Green and J.L. Cole, Chem. Phys. Letters 69 (1980) 45.
KINETICS
OF REACTIONS
CHEMICAL PHYSICS LETTERS
16 September 1988
OF BH WITH NO AND CzHl
J.A. HARRISON, R.F. MEADS and L.F. PHILLIPS Chemistry Department, University of Canterbury, Christchurch, New Zealand
Received 7 June 1988; in final form 28 June 1988
Excimer laser photolysis of diborane and laser-induced fluorescence of BH have been used to measure rate constants for the reactions of BH with NO and C:H, at total pressures near 1 Torr, over the temperature range 250-350 K. Upper limits were obtained for the rate constants for reactions of BH with 01, CO, CHI, and C2H6.Experiments with different buffer gases show no effects that can be attributed to the buffer gas acting as a third-body during adduct formation. For the BH t NO reaction the results are consistent with a model in which an intermediate HBON*, formed by capture over the centrifugal barrier in a dipoledipole potential, rearranges in a time of the order of a picosecond to give the products BO and NH.
1. Introduction
2. Experimental
Boron hydrides have long been of interest as highenergy fuels [ 1] but, despite several studies of the properties of diborane/air and diborane/nitric oxide flames [ 21, virtually nothing is known about the rates of fundamental processes involving boron hydride radicals. Pasternak et al. [ 31 have measured room-temperature rate constants over the pressure range from 6 to 620 Torr for reactions of BH3 with CO, NO and CzH4, and found upper limits for the rate constants for reactions with O2 and HzO. Recently, we gave the results of a preliminary spectroscopic study of the prompt emission observed during 193 nm photolysis of diborane [ 41. We now report rate constants for reactions of BH with NO and C2H4,and upper limits for rate constants for reactions with CH4, C2Hs, CO and 02, at pressures near 1 Torr of various buffer gases, at temperatures between 250 and 350 K. The measurements reported here were performed using laser-induced fluorescence to monitor the BH concentration via the Q(O,O) branch of the A++Xsystem at 433.4 nm. For the reaction of BH with NO the results are consistent with a theoretical model in which capture over the centrifugal barrier in a dipole-dipole potential [ 51 is followed by a fast rearrangement of the resulting HBON* complex [ 6,7] to give the products BO and NH.
The apparatus and procedures were similar to those used previously to measure rate constants for reactions of the NH radical [ 81. Gas flows were measured with calibrated Tylan mass flowmeters and mixed before entering the cell. Typical concenrration ratios buffer: diborane : reactant were 2000 : 1.O : O-7, at a cell pressure near 1 Torr. The single-photon absorption cross section of diborane at 193 nm is only 4.4~ 10m2’cm* [ 91, the measured absorption of 193 nm radiation in the cell was always less than 1Oh,and all [ BH ] decays obeyed pseudo-firstorder kinetics. Temperatures were controlled by circulating liquid from a thermostat bath through the cell’s outer jacket and measured with a copper-constantan thermocouple located in the gas stream just below the viewing region. The total pressure in the cell was measured with a type 222CA O-10 Torr MKS Baratron gauge. Pressures very different from 1 Torr either required reactant flows that were too small to measure accurately (below 0.3 Torr), or resulted in quenching of the fluorescence by which [BH] was being monitored (above 2.5 Torr). For this reason the possible effect of interactions with the buffer gas were investigated by varying the nature rather than the pressure of the buffer. Diborane was prepared by the method described by Jeffers [ lo] and kept frozen at 77 K until re-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
299
Volume 150,number 3,4
CHEMICALPHYSICSLETTERS 3
quired. Helium and argon were passed through a column of reduced BTS catalyst at 160 “C followed by a silica gel trap cooled with liquid nitrogen (He) or a solid-COJethanol bath (Ar). SF6 (Air Products instrument grade), N2 (NZIG oxygen-free grade),
O2 (Matheson UHP grade), CO (Matheson CP grade), CH, (NZIG UHP grade), CzH4 (NZIG CP grade), and C2H6 (Matheson CP grade) were used without further purification. NO (Matheson CP grade) was passed through a trap containing silica gel cooled with solid-COJethanol. Diborane was photolysed with the focused output (lens focal length 50 cm) of a Lumonics TE-86 1 extimer laser operating at 193 nm with a pulse repetition frequency of 17 Hz. The low repetition frequency ensured that the contents of the cell were completely swept out between pulses. BH fluorescence was excited with an AVCO C5000 nitrogenpumped dye laser (dye: stilbene 420) and observed through Corning 7-69 and 3-73 filters (combined bandpass 410-490 nm, peak transmission 37% at 430 nm) with an EM1 9813QA photomultiplier. The output from the photomultiplier was taken through a preamplifier to a computer-controlled PAR model 160 boxcar integrator.
3. Results and discussion
Ln
16 September 1988
r
I
-3 0
90
t i me/v3
I 80
Fig, 1,Representativefirst-orderdecay plots for BH in 1Torr He at room temperature. [NO] (in lOI molecule cm-‘): open circles, 0, filled circles, 0.45; diamonds, 0.92, half-shaded diamonds, I. 12; squares, 1.51.
ing the nature of the buffer gas. For both reactants there is no evidence for third-body effects of the sort which would cause the reaction rate to increase in the presence of a buffer with a large number of internal degrees of freedom. However, for the reaction of BH with NO there is evidence, in the form of lower rate constant values being obtained in the presence of Ar and especially SF6, of the reaction rate being
3.1. The experimental data
Typical pseudo-first-order decay plots for BH in the presence of varying amounts of NO are shown in fig. 1, Signals from the first 40 ps after the photolysis pulse were omitted from the kinetic data because of a significant background arising from both prompt BH emission and window fluorescence. Graphs of the first-order decay rates versus reactant pressure were used to determine second-order rate constants for the BH+NO and BH+C2H4 reactions over a range of temperatures as shown in fig. 2. Both rate constants are very large, and both have a small, negative temperature dependence. The error bars in fig. 2 represent the 95% confidence limits of the slopes of the plots of first-order decay rate versus concentration, plus an additional uncertainty of 6% arising from the measurements of gas pressure and time. The results given in table 1 show the effect of vary300
IO"
k
15
350 Fig, 2. Observed variation of rate constants with temperature. Squares: BH+ C2H4;circles:BH+ NO.
Table 1 Variation of measured rate constant with buffer gas at room temperature and 1 Torr total pressure. (Units of k: 10-i’ cm3 molecule-’ s-‘) Buffer
k(BH+NO)
k(BH+ethylene)
He Ar
15.6t2.3 10.55 1.2 8.9k 1.7 6.5 22.0
14.44 14.2+ 17.1 f 13.3+
N2
SF6
16 September 1988
CHEMICAL PHYSICS LETTERS
Volume 150, number 3,4
1.7 1.7 1.9 1.3
partly limited by the rate of diffusion of NO in the buffer gas. The effect is not apparent with ethylene, and we conclude that nitric oxide is depleted in the viewing region by a chain reaction involving such species as atomic boron, diborane fragments, NH, N and 0. Measured rate constants for the BH + NO reaction in a helium buffer are independent of helium pressure between 0.3 and 2.0 Torr, from which we conclude that the rate constant values in fig. 2 are not limited by diffusion of NO into the viewing region. Upper limits (95% confidence limits) were obtained for the rate constants of the room temperature reactions of BH with CH, (9.4x lo-‘” cm’ molecule- ’ s- ’ ), C2H6 (1.6x 10-12), CO (1.3x10-13) and O2 (1,1x10-“). We also attempted to measure the rate constant for the reaction of BH with NO*, but this proved impossible because of strong chemiluminescence that was transmitted by the Coming filters and which decayed on the same timescale as the [ BH] decay in the absence of added reactant. An attempt to use N20 as a buffer gas was also thwarted by intense chemiluminescence. Chemiluminescence was a problem with the OZ reaction, but was not sufficiently intense to entirely prevent data collection. All of these interferences may be attributed to formation of BO* or BO’; by reactions of B(‘P) with the oxygen-containing species, as described by Davidovits and others [ ll-131.
of the relatively large number of atoms in this species and the absence of detectable third-body effects on rate of the BH+CzH4 reaction rate at pressures near 1 Torr, it seems likely that this adduct may be a stable product. At the very least it evidently survives for a time longer than the 100 us time scale of the present experiments. For the BH + NO system, in contrast, the results of quantum-chemical calculations [ 61 are available to show that two bound complexes HBON and HBNO can be formed in principle, but only the former is capable of dissociating to give products that are exothermic relative to the original reactants. At the energy of BH+NO, HBON* can dissociate exothermically to give BO+NH or HBO+N. The reaction to yield HBO +N ( 4S) is spin forbidden, while that to give HBO + N ( 2D ) is barely exothermic, and we therefore conclude that the most likely products are BO and NH. The bimolecular association to give HBON* occurs as the end result of a dipole-dipole attraction, and therefore can be calculated [ 5 1 as the capture rate in a potential comprising the dipole-dipole potential (dipole moments 1.27 and 0.16 D, polarizabilities 3.3~ 1O-24 and 1.68~ 1O-24 cm3, for BH and NO, respectively) plus the Morse parameters (r,=1.364>( lo-* cm, L&=132.5 kJ mol-‘, w, = 1184 cm-’ ) of the HB-ON bond from ref. [ 6 1. The results of carrying out this calculation (which has no adjustable parameters) over a range of temperatures are shown as the unfilled points in fig. 3, which also includes the experimental data of fig. 2. The predicted capture rates are seen to be essentially 30
1O1l k
15
3.2. The theoretical model
c 0
At present there are no structural data available for the BH-C2H4 complex that we assume to be involved in the reaction with C2H4, and we can only speculate as to its stability and ultimate fate. In view
zoo
400
Temperature/K
Fig. 3. Comparisonof theoreticalcapturerates (tilled circles) and overall reaction rates (open circles) with experimental data (squares) for the BH +NO reaction.
301
Volume 150, number 3,4
CHEMICAL PHYSICS LETTERS
independent of temperature between 200 and 400 K, and are equal to or slightly greater than the measured overall rate constants. The situation is not quite the same as for the reaction of NH with NO [ 81, where the theoretical capture rates and the measured rate constants essentially coincide; here it appears that there may be some limitation of the overall rate by processes occurring on the HBON potential surface. Results of calculating an overall rate constant by incorporating the rates of rearrangement and dissociation of HBON*, in essentially the same manner as was done for the more complex NH*NO* system [ 71, are shown as the filled circles in fig. 3. These values have a small, negative temperature coefficient, and are equal to or slightly smaller than the experimental results, which mostly lie between the calculated capture rates and overall rates. Thus there is no doubt that the experimental results are consistent with this model. However, the following points must be noted: (1) The purely statistical calculation of the rate constants k(E) for dissociation and rearrangement is probably not reliable for a system as small as HBON*, in which dynamical effects are likely to play an important part. The calculated lifetimes of the HBON* complex are of the order of 1 ps, which seems barely long enough to allow statistical redistribution of energy from the newly formed B-O bond. (2) The results obtained for the overall rate constant with such a small system are very strongly dependent on the nature of the activated complex assumed for the process of dissociation back to reactants. In the present instance, if a very tight transition state is assumed, with no internal rotations and vibration frequencies equal to the five highest frequencies of ground-state HBON (or even to the three highest and two lowest), the rate of dissociation back to reactants becomes negligible and the overall rate is equal to the capture rate. If, on the other hand, a looser transition state is assumed, with two one-di-
302
16 September 1988
mensional free rotors and the same bond angles ( E 60” ) as in ground-state HBON, dissociation back to reactants becomes very fast and the calculated overall reaction rates are too small by an order of magnitude or more. The results in fig. 3 were obtained by assuming a transition state with two onedimensional free rotors, with the bond angles opened out to 15O_ This is physically plausible, but the results can no longer be regarded as a prediction. It appears that more complex reaction systems, such as BH, + NO, may provide a better testing ground for the theory.
Acknowledgement This work was supported by the New Zealand Universities Research Committee.
References [ 1] A.G. Gaydon and H.G. Wolfhard, Flames, their structure, radiation and temperature, 2nd Ed. (Chapman and Hall, London, 1970). [2] W. Roth, J. Chem. Phys. 28 (1958) 668. [ 3] L. Pastemak, R.J. Balla and H.H. Nelson, J. Phys. Chem. 92 (1988) 1200. [4] J.A. Harrison, R.F. Meads and L.F. Phillips, Chem. Phys. Letters 148 (1988) 125. [ 51 L.F. Phillips, J. Chem. Sot. Faraday Trans. II 83 (1987) 857. [ 61 J.A. Harrison and R.G.A.R. Maclagan, Chem. Phys. Letters 146 (1988) 243. [ 71 L.F. Phillips, Chem. Phys. Letters 135 ( 1987) 269. [8] J.A. Harrison, A.R. Whyte and L.F. Phillips, Chem. Phys. Letters 129 ( 1986) 346. [ 91 M.P. Irionand K.L. Kompa, J. Chem. Phys. 76 ( 1982) 2338. [ 10 ] W. Jeffers, Chem. Ind. (April 8, 1961). [ 111J. DeHaven, M.T. O’Connor and P. Davidovits, J. Chem. Phys. 75 (1981) 1741. [ 121 A. Brzychcy, J. DeHaven, A.T. Prengel and P. Davidovits, Chem. Phys. Letters 60 (1978) 102. [ 131 G.J. Green and J.L. Cole, Chem. Phys. Letters 69 (1980) 45.