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The chemiluminescent reaction between H3B-N(CH3)3 and O(3P) atoms; relative rates
CHEMICAL
Volume80,numberl
THE CHEMILUMINESCENT RELATIVE
REACTION
PHYSICS LEX-TERS
BETWEEN H3B-N(CH&
15 May1981
AND O(3P) ATOMS;
RATES
Peter M . JEFFEBS * and S .H. BAUER Department of Chemistry. Cornell University, Ithaca. New York I4853, USA Received 20 November 1980; in f-1
form 10 February 1981
At ambient temperatures, oxygen atoms attack bomne-trhnethylamine initially at the HsB moiety to di@ace the amine. The subsequent attack on BHz and BH generates highly excited BO(A *II, v < 11) which is the source of chemltuminescence. The rates of destruction of H3BNMe3 and H3BNEt3 are approximately two orders of magnitude faster than the comparable rate for H,BCO.
1_ Introduction
BH2 +O+BH+OH.
In a previous report [l] we described our study of the mechanism of attack by oxygen and mtrogen atoms on diborane and borane carbonyl. In particular, we measured in a flow tube reactor (TOF MS detector) at room temperature the bimolecular rate constant for H3BC0 + 0 and found it to be 3.9 X 101r mole-l cm3 s-l. The measured stoicbiometric ratio and the observation of strong chemiluminescence in the visible (Boa and BO*) as well as in the UV (BO
:d
/-H-
CO+[H
BO]*+CO 31
1
l
BO, HBO, etc.
(1)
and O&H-B-CO I H --f OH + [H2BCO]* l-
(3)
The high exoergicity required to populate BO(A 211, u < 11) is available from either radical, BH2 (BH) + 0 + BO- + Hz (H), Go0
= -106
(-113)
kcal/mole*,
(4)
The chemiluminescent spectra recorded for the carbony1 gave no indication of OH(A 2Z+-_X 211i) or BH(A IIl-X lx*) emissions_ Here we report on the kinetics of reaction between oxygen atoms and borane trimethylamine using an improved experimental unit. In this case there are two additional centers for possible attack by oxygen, abstraction of an H atom from the methyl groups, or attachment at the amine nitrogen to generate an energized amine oxide as an intermediate. The mass spectra showed that the adduct species was destroyed and we were able to follow the decay of the released trimethylamine due to the oxidative attack. We also recorded the very strong chemiluminescence spectrum generated in this process.
2. Experimental
BH2 (BH+H)+
CO,
’ Permanent address: Department of Chemistry, SUNYCortland, New York 13045, USA.
(2)
Since the reaction rates are very fast the flow tube
* Based on heats of formation at 0 K listed in JANAF rabies. 29
i Volume 80, number 1
CHEMICAL
PHYSICS LETTERS
diameter was decreased from 2 to 1 cm and a larger pump was installed so that we could obtain the desired flow rate. Currently we can measure, to an estimated upper limit for bimolecular reactions, constants =5 X 1013 mole-l cm3 s-l. New sampling orifices were installed following rhe optimum design parameters calculated by Anderson [2] ; a fast jet ejector pump now maintains the desired low pressure in the region between the sampling orifice and the skimmer. Their separation can be varied externally to optimize the S/N ratio. An Extranuclear QPMS is the detecting instrument. The microwave discharge urut for generating either N or 0 atoms can be modulated so that we could obtain either incremental mass spectra “locked onto” the presence of 0 atoms in the flow tube, or steady-state changes in the reactant concentrations- In these experiments, as previously, we operated under pseudo-first-order conditions, with the excess oxygen atom concentratron determrned by titration, either at the side tube prior to entrance of the (He, Oz, 0) stream into the flow tube, or at a point downstream from the injection position so as to obtain an estimate of the losses of oxygen due to recombinations. The required low pressure of H,B -N(CH3)3 was obtained by a slow flow of helium over the clear white crystals of the adduct, maintained at room temperature; we estimated rts effecfive vapor pressure to be 0.08 Torr. (At room temperature, the equilibrium vapor pressure of the adduct is ~0 -46 Torr-) We could introduce into the center tube trimethylamine or the adduct to permit comparison of their mass spectra without changes in the other parameters, with and without excitdtion by the microwave generator, as a function of injector position..
3. Results Test runs for the destruction of ethylene by excess oxygen atoms gave a bimolecular rate constant: k(C,H, + 0) = 2.5 X 10” mole-l cm3 s-l (typical magnitudes are listed in table 1); with our first flow reactor [l] an average value 2.87~~:~~ X lOr1 was obtained. Hampson’s compilation [3] lists for the combined rate constant (4 CH3 +HCO and + CH,CQ + HZ): (4-5) X 10” at 300 K, with activation energies which range from 1130 to 1690 Cal/mole; no recommended value is grven. Since we are here concerned 30
15
May 1981
with relative rates for closely related reactions, which range over four orders of magnitude, no effort was made to determine a preferred rate constant for C,H, + 0. We are certain, however, that our low values are not due to oxygen depletion along the flow reactor due to recombination on walls or via third bodies. We then measured the bimolecular rate constant for the reaction of trimethylamine with oxygen atoms, at ambient temperatures, by observing the decay in the intensity of the ion peak at m/e = 58, which is the most prominent peak in the mass spectrum of the amine_ Experiments were run with O/O,/He mixtures, produced by the discharge, and wrth O,-free gases generated by discharging He/N2 mixtures and titrating with NO. Under the latter conditions 7 runs gave values k(NMe3 + 0) = (4.5 2 1) X lOI* (table 1). For comparison, Atkinson and Pitts [4] reported k(NMe, + 0) = 13 X 101* mole-l cm3 s-l (with a negative activation energy of 415 cal/mole). Our initial experiments with borane-trimethylamine indicated that this adduct was destroyed extremely rapidly_ Three different rate constants were obtained, depending on which mass peak was monitored_ At m/e = 58 six experiments gave kbi = (3 f 1) X 1Ol2 _ The fact that k(H3B-NMe3 + Q),, IS only slightly less than k(NMe, + O),, indicates that the initial attack on the adduct is even faster, thereby releasing the amine which was subsequently oxidized_ Confirmation that the primary attack on the adduct is at the H,B end, rather than at the NMe3 moiety, was provided by the very high rate of disappearance of the peaks at 68-71 (parent ion peaks for the adduct). The intensity of these peaks declined to the background level as soon as the microwave generator was turned on for all positions of the injection nozzle beyond 6 cm from the sampling orifice_ Measurements with injector distances of 2-6 cm yielded a rate constant of 1.4 X 1013 mole-l cm3 s-l , which should be considered a lower limit, since such close spacings do not allow for complete mixing. A few runs with H,B -NEt, showed that its rate of attack is as fast or faster than that for H3B -NMe3 _ Further details of the oxidation reaction can be gleaned from the changes in the spectra recorded under modulation conditions. A reasonable account of the product distribution generated in the reaction NMe3 + 0 was proposed by Slagle et al. [IS] based on the mass spectra peaks they recorded with a photo-
Volume 80, number 1
CHEMICAL PHYSICS LETTERS
1.5May 1981
Table 1 Summaryof experimentalparameters Run no.
Reactant
p CTorr) (total)
Reagentflow (more/s)
(X 105)
N2
NO (X 106)
He (X 105)
02
5.75 5.75
11.1 15.5
3.43 2.52
72.0 72.0
-
1.80 1.95 1.90 1.87 1.97 1.93 1.93
11.1 8.9 8.9 8.9 11.0 16.6
5.20 4.46 2.78 4.04 2.98 3.10 3.05
72.0 66.7 66.7 66.7 79.0 79.0 69.7
1.2 -
1.95 1.93 1.50 1.70
11.1 8.9 9.5 -
3.70 3.01 2.90 3.90
1.95
11.1
3.70
Linear flow rate (cm s-r )
(X 105) -
Injector position
kbi a)
(cm)
(m/e= 27) 8-lla S-llb
C2H4
3380 3580
5-20 5-20
0.26 0 23
1
9600 9350 9360 9520 10400 10900 10400
4-+ 10 6-14 6-+ 15 5-15 6* 15 6-17 s-+15
4.2 3.2 5.1 3.7 5.0 5.0 56
66.7 66.7 60.0 66.0
1.0
9320 9200 10900 9800
5-+14 6-20 5+ 12 4-+ 13
2.6 2.7 36 3.1
66.7
-
(In/e= 58) 7-24 8-18a 8-18b 8-18c S-15a 8-15b S-14
NMes
(m/e= 58) 8-18d 8-18e 7-30 7-24
HsBNMes
(m/e= 71) 8-18f
HaBNMes
~___-
9320
2-6
(13.5)b)
a) In units of lo’* mole-r cm3 s-I_ b) This is typical of a number of runs It is a lower bound since it is bkely that mixing of the reagentswas incomplete.
iomzation mass spectrometer. The mass spectra we obtained with our electron impact ion source closely parallel theirs, with a few exceptions. We found a negligible peak at m/e = 43, but definite peaks at 29 (NCH3); 27 (HCN; possibly llB0); 16 and 17, which they did not detect. Of the entire spectrum only the peak at 44 (NC2H6) they ascrr%ed to a fragment derived from an oxygen containing icn, (CH,),NO. Inspection of their proposed oxidation scheme shows that the latter is the end product of a major branch resulting from the fragmentation of (CH,),NO. Our ac mass spectra, with the ion currents locked onto the modulated microwave discharge, showed the largest decrement occurred at m/e = 58, and sizeable decrements at m/e = 59 (parent peak) and at m/e = 42 (NCzH&, as well as a large increment at m/e = 16, as expected. Also, there were significant increases in peak heights at m/e = 44 (NC2H&; m/e = 30
(NCH4); m/e = 29 (NCH3). The first of these confirms the proposal by Slagle et al. [S] that oxygen atoms lead to the production of (CH3)2NO. Interpretation of the enhancement at m/e = 29 and 30 is somewhat less certain but can be fitted into therr scheme, assuming that oxygen atom attack favors the production of (CH3),NCH2 and CH3N(CH2)2 radicals, r
OH?
P3Q3NO*--f + H,O +CH3N(CH2)2 I
CH3N(CH2);+NC$
+C2H+ (2% (5) 31
Volume 80, number 1
(H&NO*
CHEMICAL PHYSICS LETTERS
+
+ OH +(CH,),NCH,
-+ HNCHf -I-C2H4. (30)
(6)
As stated above. the addition of oxygen atoms to H3BN(CH3)3 totally destroys the peaks at m/e = 6871, the peaks at nr/e =41,42,43 decrease. while m/e = 30 increases. As anticipated m/e = 13 (BH$) and 15 (CHZ) decrease to about half their magnitudes. A final interesting note. When the rate of attack by oxygen atoms on either NMe3 or H,BNMe3 was measured by recordmg the change in intensity wrth posltion of the injector at m/e = 42 (NCZH4), we found a low brmolecular rate constant, k2 = 4 X 1010 mole-l cm3 s-l -, i.e. two orders of magnitude lower than when we monitored the principal ion. This suggests that the oxygen atom attack on NMe3, via loss of OH and subsequent loss of CH4 (thermal process), generates a substantial level of
H 2-rN\CH
2_
This relarively stable species is subsequently oxidized by the excess reagent at a slower rate than the initial very rapid assocration which generates the energized amine oxide. No ions which definitely can be assigned
to oxidation products of the H3B moiety have been detected. Chemiluminescence, which appeared in the flow tube when the adduct was injected into the rapidly flowing stream of 0, 02, and He, was sufficiently bright to be clearly visible in a well lighted room; it was bluegreen in color. With 0 atoms provided by discharging N2 and titrating with NO, so that no molecular oxygen was present, the flame was a pure, cool blue. The length of the flame was determined by the flow velocity of the carrier and its shape indicated that the rate was somewhat diffusion limited. 32
15 May 1981
Preliminary spectra of this emission were recorded with a $ m monochromator converted to a spectrometer for recording the dispersed light on 107 Polaroid film. As in the case of the H,BCO + 0 pair, the BO cr bands were prominent, showing vibrational populations up to u = 11 of the A *II state which is = 103 kcal/mole above the ground state X 2Z+(u =O). With O/O2 mixtures the broad somewhat fuzzy bands of BO, were present (BO + 0, + BOZ + 0). In addition we found characteristic OH band heads at 306.4 (RI); 307.8 (Q1); 308.9 (Q2) nm, (A *F-X 211i). Also, what appears to be a weak band head (degraded to the red) at 433 5 nm may be the (0,O) Q head of the III-1 Z+ transition of BH. The OH and BO, features were not observed when molecular 0, was absent_
4. Discussion The above observations indicate that: (a) The cross sections for reactions analogous to (1) and (2), for the H3B-NMe3 + 0 and H,B-NEt, + 0 pairs are very large, being = l/10 of kinetic theory values. (b) The large numbers of BH, (BH) thus generated are rapidly removed by subsequent encounters with 0 atoms in hot reactions [(3) and (4)] _(c) There was no evidence of either OH* or BH* in the spectra emitted by the pair H,BCO + 0. We believe that for H,BNMe, f 0 the much faster rate for (2) generates considerably higher concentrations of BH, (BH), so that secondary steps become evident _It is not clear which reactions are invalved . The highly exoergic step, BH2+O+HBO+H,
A%oo
= -103 k&/mole,
(7)
may generate excited HBO, as may also the direct production of HBO rn (1). These could transfer their excitation to the abundant OH and BH species. The intriguing question remains: why is the rate of attack on the borane moiety nearly two orders of magnitude faster when it is attached to NMe3, compared withco(k2=39x1011 mole-l cm3 s-l), which is another two orders of magnitude faster than when it is attached to another BH, unit (k2 = 2 -7 X log) ? The corresponding enthalpies for adduct dissociation are 379,24 and 28 kcal/mole. Of these H,BNMe,
has the highest thermal stability. Another significant difference between H,BCO and H3BNMe3 is their reactivity toward N atoms. Anderson [l] observed ap-
Volume 80, number 1
CHEMICAL
PHYSICS LETTERS
proximately the same rate constant for 0 and N atom attack on the carbonyl; both generated intense bluegreen luminescence. In contrast, the amine adduct is essentially unreactive toward N atoms, on the time scale of our experiments. Since the major attack is due to abstraction of a H atom from H,B, it was proposed [6] that the potential barrier is lowest for this abstraction from H3B -NMe, because the resulting radical, although short lived, is somewhat stabilized by bonding of B to N, in contrast to the lack of such stabilization in H,BCO, or in B2H6 _It appears that, contrary to our initial expectations, the rates of attack by O(3P) on the H3B moiety are sensitively dependent on the nature of the bases to which it is attached. With trimethylamine we found an exceptionally fast rate for generating BH, (BH) radicals, which may prove useful for developing a “chemical” electronic transition BO* laser. Note that re(X 2Z+) = 1.205 A, I-,=(A 2l$) = 1.352 A. A calculated value for the radiative lifetime of the A state is 1.8 + 0.2 J.S [7] _We have yet to establish whether the direct precursor of BO* is BH2 or BH. The latter seems more reasonable; for the former, one may anticipate significant vibrational energy would be carried away by the eliminated H2, so
15 May 1981
that the residue would be insufficient to generate BO* inu= 11.
Acknowledgement This investigation was supported, in part, by the AFOSR, under grant No _804046 _
References [l] [2] [3] [4] [5 ] [6]
[7]
G.K. Anderson and S.H. Bauer, J. Phys. Chem. 81 (1977) 1146. GX. Anderson, PhD_ Dissertation, Cornell University, August, 1975. R.F. Hampson, National Bureau of Standards Report No. FAA-EE-80-17 (1980). R. Atkinson and J.N. Pitts Jr., J. Chem. Phys. 68 (1978) 911. 1-R. Slagle, J-F_ Dudlich and D. Gutman, J. Phys. Chem. 83 (1979) 3065. B-K_ Carpenter, private communication_ H.H. Michels, R. Harris and J. Lillis, in: Electronic transition lasers, ed. J. Steinfeld (MIT Press, Cambridge, 1976) p_ 278.
Volume80,numberl
THE CHEMILUMINESCENT RELATIVE
REACTION
PHYSICS LEX-TERS
BETWEEN H3B-N(CH&
15 May1981
AND O(3P) ATOMS;
RATES
Peter M . JEFFEBS * and S .H. BAUER Department of Chemistry. Cornell University, Ithaca. New York I4853, USA Received 20 November 1980; in f-1
form 10 February 1981
At ambient temperatures, oxygen atoms attack bomne-trhnethylamine initially at the HsB moiety to di@ace the amine. The subsequent attack on BHz and BH generates highly excited BO(A *II, v < 11) which is the source of chemltuminescence. The rates of destruction of H3BNMe3 and H3BNEt3 are approximately two orders of magnitude faster than the comparable rate for H,BCO.
1_ Introduction
BH2 +O+BH+OH.
In a previous report [l] we described our study of the mechanism of attack by oxygen and mtrogen atoms on diborane and borane carbonyl. In particular, we measured in a flow tube reactor (TOF MS detector) at room temperature the bimolecular rate constant for H3BC0 + 0 and found it to be 3.9 X 101r mole-l cm3 s-l. The measured stoicbiometric ratio and the observation of strong chemiluminescence in the visible (Boa and BO*) as well as in the UV (BO
:d
/-H-
CO+[H
BO]*+CO 31
1
l
BO, HBO, etc.
(1)
and O&H-B-CO I H --f OH + [H2BCO]* l-
(3)
The high exoergicity required to populate BO(A 211, u < 11) is available from either radical, BH2 (BH) + 0 + BO- + Hz (H), Go0
= -106
(-113)
kcal/mole*,
(4)
The chemiluminescent spectra recorded for the carbony1 gave no indication of OH(A 2Z+-_X 211i) or BH(A IIl-X lx*) emissions_ Here we report on the kinetics of reaction between oxygen atoms and borane trimethylamine using an improved experimental unit. In this case there are two additional centers for possible attack by oxygen, abstraction of an H atom from the methyl groups, or attachment at the amine nitrogen to generate an energized amine oxide as an intermediate. The mass spectra showed that the adduct species was destroyed and we were able to follow the decay of the released trimethylamine due to the oxidative attack. We also recorded the very strong chemiluminescence spectrum generated in this process.
2. Experimental
BH2 (BH+H)+
CO,
’ Permanent address: Department of Chemistry, SUNYCortland, New York 13045, USA.
(2)
Since the reaction rates are very fast the flow tube
* Based on heats of formation at 0 K listed in JANAF rabies. 29
i Volume 80, number 1
CHEMICAL
PHYSICS LETTERS
diameter was decreased from 2 to 1 cm and a larger pump was installed so that we could obtain the desired flow rate. Currently we can measure, to an estimated upper limit for bimolecular reactions, constants =5 X 1013 mole-l cm3 s-l. New sampling orifices were installed following rhe optimum design parameters calculated by Anderson [2] ; a fast jet ejector pump now maintains the desired low pressure in the region between the sampling orifice and the skimmer. Their separation can be varied externally to optimize the S/N ratio. An Extranuclear QPMS is the detecting instrument. The microwave discharge urut for generating either N or 0 atoms can be modulated so that we could obtain either incremental mass spectra “locked onto” the presence of 0 atoms in the flow tube, or steady-state changes in the reactant concentrations- In these experiments, as previously, we operated under pseudo-first-order conditions, with the excess oxygen atom concentratron determrned by titration, either at the side tube prior to entrance of the (He, Oz, 0) stream into the flow tube, or at a point downstream from the injection position so as to obtain an estimate of the losses of oxygen due to recombinations. The required low pressure of H,B -N(CH3)3 was obtained by a slow flow of helium over the clear white crystals of the adduct, maintained at room temperature; we estimated rts effecfive vapor pressure to be 0.08 Torr. (At room temperature, the equilibrium vapor pressure of the adduct is ~0 -46 Torr-) We could introduce into the center tube trimethylamine or the adduct to permit comparison of their mass spectra without changes in the other parameters, with and without excitdtion by the microwave generator, as a function of injector position..
3. Results Test runs for the destruction of ethylene by excess oxygen atoms gave a bimolecular rate constant: k(C,H, + 0) = 2.5 X 10” mole-l cm3 s-l (typical magnitudes are listed in table 1); with our first flow reactor [l] an average value 2.87~~:~~ X lOr1 was obtained. Hampson’s compilation [3] lists for the combined rate constant (4 CH3 +HCO and + CH,CQ + HZ): (4-5) X 10” at 300 K, with activation energies which range from 1130 to 1690 Cal/mole; no recommended value is grven. Since we are here concerned 30
15
May 1981
with relative rates for closely related reactions, which range over four orders of magnitude, no effort was made to determine a preferred rate constant for C,H, + 0. We are certain, however, that our low values are not due to oxygen depletion along the flow reactor due to recombination on walls or via third bodies. We then measured the bimolecular rate constant for the reaction of trimethylamine with oxygen atoms, at ambient temperatures, by observing the decay in the intensity of the ion peak at m/e = 58, which is the most prominent peak in the mass spectrum of the amine_ Experiments were run with O/O,/He mixtures, produced by the discharge, and wrth O,-free gases generated by discharging He/N2 mixtures and titrating with NO. Under the latter conditions 7 runs gave values k(NMe3 + 0) = (4.5 2 1) X lOI* (table 1). For comparison, Atkinson and Pitts [4] reported k(NMe, + 0) = 13 X 101* mole-l cm3 s-l (with a negative activation energy of 415 cal/mole). Our initial experiments with borane-trimethylamine indicated that this adduct was destroyed extremely rapidly_ Three different rate constants were obtained, depending on which mass peak was monitored_ At m/e = 58 six experiments gave kbi = (3 f 1) X 1Ol2 _ The fact that k(H3B-NMe3 + Q),, IS only slightly less than k(NMe, + O),, indicates that the initial attack on the adduct is even faster, thereby releasing the amine which was subsequently oxidized_ Confirmation that the primary attack on the adduct is at the H,B end, rather than at the NMe3 moiety, was provided by the very high rate of disappearance of the peaks at 68-71 (parent ion peaks for the adduct). The intensity of these peaks declined to the background level as soon as the microwave generator was turned on for all positions of the injection nozzle beyond 6 cm from the sampling orifice_ Measurements with injector distances of 2-6 cm yielded a rate constant of 1.4 X 1013 mole-l cm3 s-l , which should be considered a lower limit, since such close spacings do not allow for complete mixing. A few runs with H,B -NEt, showed that its rate of attack is as fast or faster than that for H3B -NMe3 _ Further details of the oxidation reaction can be gleaned from the changes in the spectra recorded under modulation conditions. A reasonable account of the product distribution generated in the reaction NMe3 + 0 was proposed by Slagle et al. [IS] based on the mass spectra peaks they recorded with a photo-
Volume 80, number 1
CHEMICAL PHYSICS LETTERS
1.5May 1981
Table 1 Summaryof experimentalparameters Run no.
Reactant
p CTorr) (total)
Reagentflow (more/s)
(X 105)
N2
NO (X 106)
He (X 105)
02
5.75 5.75
11.1 15.5
3.43 2.52
72.0 72.0
-
1.80 1.95 1.90 1.87 1.97 1.93 1.93
11.1 8.9 8.9 8.9 11.0 16.6
5.20 4.46 2.78 4.04 2.98 3.10 3.05
72.0 66.7 66.7 66.7 79.0 79.0 69.7
1.2 -
1.95 1.93 1.50 1.70
11.1 8.9 9.5 -
3.70 3.01 2.90 3.90
1.95
11.1
3.70
Linear flow rate (cm s-r )
(X 105) -
Injector position
kbi a)
(cm)
(m/e= 27) 8-lla S-llb
C2H4
3380 3580
5-20 5-20
0.26 0 23
1
9600 9350 9360 9520 10400 10900 10400
4-+ 10 6-14 6-+ 15 5-15 6* 15 6-17 s-+15
4.2 3.2 5.1 3.7 5.0 5.0 56
66.7 66.7 60.0 66.0
1.0
9320 9200 10900 9800
5-+14 6-20 5+ 12 4-+ 13
2.6 2.7 36 3.1
66.7
-
(In/e= 58) 7-24 8-18a 8-18b 8-18c S-15a 8-15b S-14
NMes
(m/e= 58) 8-18d 8-18e 7-30 7-24
HsBNMes
(m/e= 71) 8-18f
HaBNMes
~___-
9320
2-6
(13.5)b)
a) In units of lo’* mole-r cm3 s-I_ b) This is typical of a number of runs It is a lower bound since it is bkely that mixing of the reagentswas incomplete.
iomzation mass spectrometer. The mass spectra we obtained with our electron impact ion source closely parallel theirs, with a few exceptions. We found a negligible peak at m/e = 43, but definite peaks at 29 (NCH3); 27 (HCN; possibly llB0); 16 and 17, which they did not detect. Of the entire spectrum only the peak at 44 (NC2H6) they ascrr%ed to a fragment derived from an oxygen containing icn, (CH,),NO. Inspection of their proposed oxidation scheme shows that the latter is the end product of a major branch resulting from the fragmentation of (CH,),NO. Our ac mass spectra, with the ion currents locked onto the modulated microwave discharge, showed the largest decrement occurred at m/e = 58, and sizeable decrements at m/e = 59 (parent peak) and at m/e = 42 (NCzH&, as well as a large increment at m/e = 16, as expected. Also, there were significant increases in peak heights at m/e = 44 (NC2H&; m/e = 30
(NCH4); m/e = 29 (NCH3). The first of these confirms the proposal by Slagle et al. [S] that oxygen atoms lead to the production of (CH3)2NO. Interpretation of the enhancement at m/e = 29 and 30 is somewhat less certain but can be fitted into therr scheme, assuming that oxygen atom attack favors the production of (CH3),NCH2 and CH3N(CH2)2 radicals, r
OH?
P3Q3NO*--f + H,O +CH3N(CH2)2 I
CH3N(CH2);+NC$
+C2H+ (2% (5) 31
Volume 80, number 1
(H&NO*
CHEMICAL PHYSICS LETTERS
+
+ OH +(CH,),NCH,
-+ HNCHf -I-C2H4. (30)
(6)
As stated above. the addition of oxygen atoms to H3BN(CH3)3 totally destroys the peaks at m/e = 6871, the peaks at nr/e =41,42,43 decrease. while m/e = 30 increases. As anticipated m/e = 13 (BH$) and 15 (CHZ) decrease to about half their magnitudes. A final interesting note. When the rate of attack by oxygen atoms on either NMe3 or H,BNMe3 was measured by recordmg the change in intensity wrth posltion of the injector at m/e = 42 (NCZH4), we found a low brmolecular rate constant, k2 = 4 X 1010 mole-l cm3 s-l -, i.e. two orders of magnitude lower than when we monitored the principal ion. This suggests that the oxygen atom attack on NMe3, via loss of OH and subsequent loss of CH4 (thermal process), generates a substantial level of
H 2-rN\CH
2_
This relarively stable species is subsequently oxidized by the excess reagent at a slower rate than the initial very rapid assocration which generates the energized amine oxide. No ions which definitely can be assigned
to oxidation products of the H3B moiety have been detected. Chemiluminescence, which appeared in the flow tube when the adduct was injected into the rapidly flowing stream of 0, 02, and He, was sufficiently bright to be clearly visible in a well lighted room; it was bluegreen in color. With 0 atoms provided by discharging N2 and titrating with NO, so that no molecular oxygen was present, the flame was a pure, cool blue. The length of the flame was determined by the flow velocity of the carrier and its shape indicated that the rate was somewhat diffusion limited. 32
15 May 1981
Preliminary spectra of this emission were recorded with a $ m monochromator converted to a spectrometer for recording the dispersed light on 107 Polaroid film. As in the case of the H,BCO + 0 pair, the BO cr bands were prominent, showing vibrational populations up to u = 11 of the A *II state which is = 103 kcal/mole above the ground state X 2Z+(u =O). With O/O2 mixtures the broad somewhat fuzzy bands of BO, were present (BO + 0, + BOZ + 0). In addition we found characteristic OH band heads at 306.4 (RI); 307.8 (Q1); 308.9 (Q2) nm, (A *F-X 211i). Also, what appears to be a weak band head (degraded to the red) at 433 5 nm may be the (0,O) Q head of the III-1 Z+ transition of BH. The OH and BO, features were not observed when molecular 0, was absent_
4. Discussion The above observations indicate that: (a) The cross sections for reactions analogous to (1) and (2), for the H3B-NMe3 + 0 and H,B-NEt, + 0 pairs are very large, being = l/10 of kinetic theory values. (b) The large numbers of BH, (BH) thus generated are rapidly removed by subsequent encounters with 0 atoms in hot reactions [(3) and (4)] _(c) There was no evidence of either OH* or BH* in the spectra emitted by the pair H,BCO + 0. We believe that for H,BNMe, f 0 the much faster rate for (2) generates considerably higher concentrations of BH, (BH), so that secondary steps become evident _It is not clear which reactions are invalved . The highly exoergic step, BH2+O+HBO+H,
A%oo
= -103 k&/mole,
(7)
may generate excited HBO, as may also the direct production of HBO rn (1). These could transfer their excitation to the abundant OH and BH species. The intriguing question remains: why is the rate of attack on the borane moiety nearly two orders of magnitude faster when it is attached to NMe3, compared withco(k2=39x1011 mole-l cm3 s-l), which is another two orders of magnitude faster than when it is attached to another BH, unit (k2 = 2 -7 X log) ? The corresponding enthalpies for adduct dissociation are 379,24 and 28 kcal/mole. Of these H,BNMe,
has the highest thermal stability. Another significant difference between H,BCO and H3BNMe3 is their reactivity toward N atoms. Anderson [l] observed ap-
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PHYSICS LETTERS
proximately the same rate constant for 0 and N atom attack on the carbonyl; both generated intense bluegreen luminescence. In contrast, the amine adduct is essentially unreactive toward N atoms, on the time scale of our experiments. Since the major attack is due to abstraction of a H atom from H,B, it was proposed [6] that the potential barrier is lowest for this abstraction from H3B -NMe, because the resulting radical, although short lived, is somewhat stabilized by bonding of B to N, in contrast to the lack of such stabilization in H,BCO, or in B2H6 _It appears that, contrary to our initial expectations, the rates of attack by O(3P) on the H3B moiety are sensitively dependent on the nature of the bases to which it is attached. With trimethylamine we found an exceptionally fast rate for generating BH, (BH) radicals, which may prove useful for developing a “chemical” electronic transition BO* laser. Note that re(X 2Z+) = 1.205 A, I-,=(A 2l$) = 1.352 A. A calculated value for the radiative lifetime of the A state is 1.8 + 0.2 J.S [7] _We have yet to establish whether the direct precursor of BO* is BH2 or BH. The latter seems more reasonable; for the former, one may anticipate significant vibrational energy would be carried away by the eliminated H2, so
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that the residue would be insufficient to generate BO* inu= 11.
Acknowledgement This investigation was supported, in part, by the AFOSR, under grant No _804046 _
References [l] [2] [3] [4] [5 ] [6]
[7]
G.K. Anderson and S.H. Bauer, J. Phys. Chem. 81 (1977) 1146. GX. Anderson, PhD_ Dissertation, Cornell University, August, 1975. R.F. Hampson, National Bureau of Standards Report No. FAA-EE-80-17 (1980). R. Atkinson and J.N. Pitts Jr., J. Chem. Phys. 68 (1978) 911. 1-R. Slagle, J-F_ Dudlich and D. Gutman, J. Phys. Chem. 83 (1979) 3065. B-K_ Carpenter, private communication_ H.H. Michels, R. Harris and J. Lillis, in: Electronic transition lasers, ed. J. Steinfeld (MIT Press, Cambridge, 1976) p_ 278.