J . Phys. Chem. 1987, 91, 4379-4383
1000KIT
Figure 3. Arrhenius plot of measured values of CH, + 0 - *H2C0 + H rate constant (k,)obtained in the current study (closed circles and solid line) and in prior investigations: open circle, Slagle, Pruss, and Gutman, ref 12; open triangle, Washida, ref 13; open square, Plumb and Ryan, ref 14; dotted line, Bhaskaran, Frank, and Just, ref 3; dashed line;
Washida and Bayes, ref 1 1. of oxygen atoms was recorded, and the data were used to obtain kl. Over the range of temperatures covered (294-900 K), kl was found to be independent of temperature (see Table I). A similar temperature independence of kl was observed in two other studies. An extrapolation of the constant value of kl obtained in the current cm3 molecule-' s-l) to the 1700-2300 K temstudy (1.4 x perature range covered by Bhaskaran, Frank, and Just yields a value virtually identical with the one they obtained throughout this temperature range, 1.3 X cm3 molecule-' s - I . ~ Their high-temperature study of the kinetics of reaction 1 was essentially the thermal equivalent of the photochemical experiment used in the current investigation. CH, and 0 were produced suddenly at low concentrations by shock-heating precursor molecules (N20 and C2H6)highly diluted in an inert gas. In these experiments, concentrations of H and 0 were monitored by using atomic absorption. Conditions were chosen to yield a high degree of sensitivity of these measurements to the rate constant of reaction 1. In the only other investigation of the temperature dependence of k , , Washida and Bayes also found none in their study conducted near ambient temperature, 259-341 K.]' The rate constants for reaction 1 obtained in the current study and in the prior direct investigations of this reaction are plotted in Figure 3. The close agreement among the results of these investigations increases confidence that the kinetics of this reaction
4379
is established quantitatively for most conditions encountered in combustion processes. In a data evaluation of prior investigations of reaction 1, Warnatz had suggested a consensus value of kl = 1.2 X 10-Io cm3 molecule-] s-] which is independent of temperature (300-2500 K) for use in combustion modeling.* This average value of earlier direct and indirect determinations of kl is only slightly below our measured values. SO2 as a Source of Oxygen Atoms for Kinetic Studies. The 193-nm photolysis of SO2 has proven to be an excellent source of ground-state oxygen atoms for kinetic studies. Decomposition using unfocused excimer laser photolysis is extensive enough (2-5%) to be accurately measured, thus providing the means to determine oxygen atom concentrations quantitatively. The procedures described in this paper for studying the kinetics of the CH3 + 0 reaction can be used to investigate the kinetics and mechanisms of many additional reactions of oxygen atoms with free radicals. The low reactivity of SO2 with free radicals simplifies data analysis. No reactions of 0 or CH3 with SO, could be detected. For this reason the range of conditions which could be used in these kinetic studies was not limited by the presence of SO2 in the system but was determined essentially completely by the conditions which can be produced and maintained in the tubular reactor. The increasing importance of the heterogeneous loss of oxygen atoms above 600 K can be taken into account with confidence by the data analysis described in this study even if the loss is significant (up to 30%during the testing time). Since the 0-atom decay rate can be measured (in the absence of the second reactant) and is not altered by the reaction under study (because the oxygen atom concentration is always maintained in large excess), the data obtained in these high-temperature experiments can be analyzed in a straightforward manner to obtain the desired quantitative information on the kinetics of the reaction under study.
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy, under Contract DE-AC02-78ER14953. We thank James Bernhardt for his help in conducting these experiments, Paul F. Sawyer for his continued development and improvement of our data analysis computer codes, and Leslie Garland and Kyle D. Bayes for numerous useful comments and suggestions regarding this research. Registry No. CH,, 2229-07-4; 0, 17778-80-2; SO2, 7446-09-5; CH3COCH3, 67-64-1.
Isomerization Kinetics of Partly Delocalized Radicals Observed by Muon Spin Rotation Walter Strub, Emil Roduner,* and Hanns Fischer Physikalisch- Chemisches Institut der Uniuersitat Zurich, CH-8057 Zurich, Switzerland (Received: December 23, 1986; In Final Form: April 16, 1987)
Barriers to isomerization by rotation about a partial double bond in a-carbonyl-, a-carboxyl-, and a-carbamide-substituted alkyl radicals have been determined by means of the technique of muon spin rotation. They agree with those observed previously by electron spin resonance but are higher than the thermochemically determined stabilization energies of similar species.
1. Introduction
Resonance stabilization of organic radicals has been a topic of interest in recent years. Well-known examples are allyl and benzyl radicals including substituted derivatives,'-6 and further0022-3654/87/2091-4379$01.50/0
more, a-carbonyl- and a-alkoxycarbonylalkyl radicals.'-12 Usually, resonance energies are derived from thermochemical (1) Benson, S. W. In Thermochemical Kinetics; Wiley: New York, 1976.
0 1987 American Chemical Society
4380 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 measurements by comparison of the energies required to break chemical bonds to give stabilized and nonstabilized radicals. An alternative approach is based on the determination of the barrier to internal rotation about the partial double bond at the radical center by means of ESR, assuming that the barrier arises mostly from the loss of resonance energy in the transition state. The experimental results are quite scattered, with the clear trend that the thermochemical methods yield lower values. Thus, zero resonance stabilization was inferred for the acetonyl radical, H2CCOCH3, from H abstraction reactions from acetone and 2-propan01,~whereas C-C bond dissociation lead to a value of 27 f 4 kJ.mol-' for stabilization by -COR and of 15 & 4 kJ.mo1-l for -COOCH3.2 ESR yielded 39 f 2 kJ-mol-' for acetonyl' and similar values for other a-carbonyl radicals,8 and 47 & 2 kJ.mol-' was obtained for a -COOR-substituted case." In a plot of bond dissociation energies D(R-H) vs. barriers to internal rotation of the resulting radicals, acetonyl did not correlate with the majority of examples.12 A new determination of D[CH3COCH2-HI gave 381 f 6 kJ-mol-', 25 kJ-mol-' less than what was used previou~ly.'~ With this new value one derives a barrier of 32 kJ-mo1-l from Nonhebel and Walton's correlation, which is close to but still lower than the value derived from ESR. In view of such uncertainties it appears desirable to check some of the data with an independent method. We have measured isomerization barriers using the technique of muon spin rotation ( F S R ) . ~ ~ -The ' ~ method has recently proven successful for the determination of absolute rate constants for various fast radical reactionsI7 including temperature-dependent studies of radical rearrangement^.'^,'^ The experimental technique rests on the following principles: Spin-polarized positive muons are stopped in the targets of interest in transverse magnetic fields. The subsequent time evolution of spin polarization, the analogue of a free induction decay in pulsed magnetic resonance, is monitored by means of a single particle counting technique which takes advantage of the anisotropy of the muon decay (k+ lifetime: 2.2 b s ) into a positron and two neutrinos. In unsaturated organic compounds free radicals form by addition of muonium (Mu @+e-),which is chemically a light isotope of hydrogen.I4*l7They are identified by their muon-electron hyperfine interactions.16 Rate constants are extracted from lifetime broadening in the Fourier transform spectra.Is Kinetic isotope effects are too small to be detectable since Mu sits two bonds away from the reactive radical center, and it is usually not directly involved in the reaction. l 7
(2) Ruchardt, C.; Beckhaus, H. D. Top. Curr. Chem. 1985, 130, 1. (3) Korth, H. G.; Trill, H.; Sustmann, R. J . Am. Chem. Sot. 1981,103, 4483. (4) Korth, H. G.; Lommes, P.; Sustmann, R. J . Am. Chem. SOC.1984,106, 663. ( 5 ) Conradi, M . S.; Zeldes, H.; Livingston, R. J . Phys. Chem. 1979, 83, 2160. (6) Golden, D. M.; Benson, S. W. Chem. Reu. 1969. 69, 125. (7) Golde, G.; Mobius, K.; Kaminski, W. Z . Naturforsch. A 1969, 24, 1214. (8) Camaioni, D. M.; Walter, H. F.; Jordan, J. E.; Pratt, D. W. J . Am. Chem. Sot. 1973, 95, 1978. (9) Brauman, J. I. In Frontiers of Free Radical Chemistry; Academic: New York, 1980. ( I O ) Solly, R. K.; Golden, D. M.; Benson, S. W. Int. J. Chem. Kinet. 1970, 2, 381. (11) Lung-min, Wu; Fischer, H. Helu. Chim. Acra 1983, 66, 138. (12) Nonhebel, D. C.; Walton, J. C. J . Chem. Soc., Chem. Commun. 1984, 731. (13) Holmes, J. L.; Lossing, F. P.; Terlouw, J. K. J. Am. Chem. Soc. 1986, 108, 1086. (14) Walker, D. C. In Muon and Muonium Chemistry; Cambridge University Press: London, 1984. (15) Webster, B. Annu. Rep.-R. SOC.Chem., Sect. C 1984, 3. (16) Roduner, E.; Fischer, H. Chem. Phys. 1981, 54, 261. (17) Roduner, E. Prog. React. Kinet. 1986, 14, 1. ( 1 8) Burkhard, P.;Roduner, E.; Hochmann, J.; Fischer, H. J. Phys. Chem. 1984. 88, 773. (19) Burkhard, P.; Rcduner, E.; Fischer, H. Int. J . Chem. Kiner. 1985, 27, 89.
Strub et al.
-
100
_ _
_ -
150
3L3 K
200 M P z
Figure 1. pSR Fourier power spectra obtained with ethyl methacrylate a t 2 kG. The muon label sits in the methyl group trans (A) and cis (B) to C=O in the radical MuCH~C(CH,)COOC,H,.
The present work concentrates on the kinetics of isomerization reactions of the type
with R, = H, CH,, and R, = OC2Hs,N(CH3),, and CH,. The asterisk denotes the Mu-labeled methyl group. Preliminary results were reported previously.20 In high transverse fields each radical isomer gives rise to one pair of lines in the p S R spectrum, corresponding to electron spin a and @, respectively. Hence, if both isomers are observed, this leads to four radical lines. The isomerization reaction causes an exchange between lines which correspond to the same electron spin quantum number. The resulting effect on the line-shape function is the same as in other magnetic resonance type techniques where a free induction decay is observed. The situation of a one-spin molecule undergoing a reversible first-order chemical reaction in a nonequilibrium situation has been treated theoretically by Ernst et aLzl In the limit of slow exchange (resolved isomers) the line shapes are Lorentzian, and the initial amplitudes are proportional to the initial concentrations ( z muon polarizations in our case).
2. Experimental and Data Analysis The radicals were generated from the a-0-unsaturated parent compounds ethyl methacrylate, ethyl acrylate, N,N-dimethylacrylamide, methyl vinyl ketone, and isopropenyl vinyl ketone. The samples were distilled to remove stabilizer, degassed by freeze-pump-thaw cycles, and sealed in spherical glass bulbs of 35 mm diameter. They were used as plain liquids, except for one case, where a 1:l mixture of ethyl methacrylate and ethyl isobutyrate was employed. A cryostat served to hold the temperature constant to 10.2 K in time and over the sample. All NSR experiments were carried out at the muon channels of the Swiss Institute for Nuclear Research (SIN) in Villigen, using a standard pSR assemblyI6 with a magnetic field of 2 kG transverse to the muon spin polarization. For each experimental point (60-90) X lo6 good events were accumulated in about 6 h and four counter telescopes. This yielded four independent histograms with 2K channels each and a time resolution of 0.86 ns per channel. (20) Strub, W.; Roduner, E.; Fischer, H. Proceedings of rhe ZZnd Congress AmpPre on Magnetic Resonance and Related Phenomena; University of Zurich: Zurich, 1984. (21) Kiihne, R. 0.;Schaffhauser, T.; Wokaun, A.; Ernst, R. R. J . Magn. Reson. 1979, 35, 39.
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4381
Kinetics of Partly Delocalized Radicals
'1
R
200
250
350 K
300
Figure 2. Line-width parameter for the cis (filled circles) and trans (open circles) isomers of MuCH2C(CH3)COOC2H5.
The experiments were confined to the slow exchange region, where the lines corresponding to different isomers were separated in the spectrum. Quantitative analysis was by a x2fit of a modified Lorentzian line-shape function as described elsewhere.'* The resulting frequencies yield directly the muon-electron hyperfine coupling constants.16 From the initial amplitudes we determine the relative yields of the radical isomers, and from the relaxation rate A, the Arrhenius parameters A and E, are obtained by a X2-fit to X = XR + A exp(-E,/RT) Here, XR represents a residual line width due to physical relaxation processes, whereas the second term is the rate constant of the isomerization. The errors given correspond to one standard deviation. They allow for possible correlation of parameters; Le., a value X f AX corresponds to xmin2 + 1 with the other parameters relaxed. This is important since Arrhenius parameters are often strongly correlated.
3. Results 3.1. a-(Ethoxycarbonyl)-2-propylRadical. The pSR spectrum observed with ethyl methacrylate is displayed in Figure 1. The signal due to muons in diamagnetic environments (v = 27 MHz) is not of interest here and therefore not displayed. In agreement with previous work22 the spectra reveal the presence of two muonated radicals. The minor one, with a reduced muon-electron hyperfine coupling constant A,' = A p P / p U=r 84.8 MHz at room temperature, was assigned** to the cis isomer 3B, and the major one, with A,' = 86.4 MHz, to the trans form 3A:
3A
3B
The rates of Mu addition to vinyl monomers are controlled by diffusionz3and probably the same for the two isomers of the parent compounds. The relative yields of the radicals should therefore correspond to the isomer distribution of the parent molecule. In our experiments (10 temperatures) P A / P Branges from 2.2 (3) to 208 K to 1.3 (2) at 353 K. From these data, we derive AH(s-cis-s-trans) = 1.9 (3) kJ.mol-' and A S = 3 (1) J.mol-'.K-'. These values agree with literature data on methyl acrylate in CS2 s o l ~ t i o n . ~This ~ , ~corroborates ~ the assignment given above, and since the product radicals are energetically degenerate here, it (22) Rcduner, E.; Strub, W.; Burkhard, P.; Hochmann, J.; Percival, P. W.; Fischer, H.; Ramos, M.; Webster, 8.C . Chem. Phys. 1982, 67, 275. (23) Stadlbauer, J. M.; Ng, B. W.; Walker, D. C.;Jean, Y . C.; Jto, Y . Can. J. Chem. 1981, 59, 3261. (24) George, W. 0.; Hassid, D. V.; Maddams, W. F. J. Chem. Soc., Perkin Trans. 2 1972, 400. (25) Jones, G . I. L.; Owen, N. L. J . Mol. Srruct. 1973, 18, 1.
demonstrates that one observes a nonequilibrium situation. It is obvious from Figure 1 that the radical lines broaden at higher temperature. The quantitative data for the relaxation rates are given in Figure 2. As expected for a degenerate case, the two isomers show the same temperature dependence. The line corresponds to the best fit of eq 2 to the combined set of data, with log (k3/~-')= (13.2 f 0.4) - (45 f :)/2.3RT and XR = 0.53 (1) ps-l, where RT is in kJ-mol-'. Six temperatures over the range of Figure 2 were repeated with a 1:l mixture of ethyl methacrylate with ethyl isobutyrate. Within experimental error, the widths of the radical lines are the same as in the pure methacrylate. This absence of a concentration dependence indicates that there is no significant contribution of a second-order reaction, as, e.g., for the onset of polymerization. 3.2. a-(Ethoxycarbony1)ethylRadical. Three radicals were observed with ethyl acrylate. The highest coupling constant, A,,' = 106.3 MHz at 284 K, belongs to a minor species and is typical for primary radicals.'8s22 Hence, it is attributed to CH2CHMuCOOCH2CH,. The dominant radical has A,' = 97.8 MHz at 298 K, and the third species has A,' N 99.8 MHz at the same temperature. The magnitudes of A,' suggest secondary radicals CH2MuCHCOOCH2CH3,but in contrast to the case of methacrylate the dominant species now has the smaller coupling. For the assignment to the two isomers we rely on the radical yields. Spectroscopic information reveals that the s-trans form of methyl acrylate is by 1.36 f 0.08 kJ-mol-' more stable than the s-cis isomer, and a similar behavior is found for the ethyl acrylate.24 We therefore assign the major radical to the trans form 4A
4A 4B and the minor to 4B. This does not necessarily conflict with the assignment of the H-analogous radical" where the methyl protons are assumed to have a slightly larger coupling in the trans compared with the cis group. Unequal barriers for methyl rotation might well lead to a cross-over behavior at higher temperatures in the muonated case. Determination of the kinetic parameters was possible for the major isomer only. The fit of eq 2 to the line widths yields
log
( k 4 / ~ - ' )=
(13.9 f 8:g) - (44 f !)/2.3RT
with XR = 0.81 (3) ps-l. This agrees well with the ESR data" which gave log ( A / & ) = 14.0 ( 5 ) and E, = 47 (2) kJ-mol-1 for i4 and about 45 kJ-mol-' for k4. 3.3. a-(N,N-Dimethylamidocarbony1)ethyl Radical. With N,N-dimethylacrylamide, a weak feature was observed at 260 K with A,' = 110.5 MHz. It belongs to the primary radical. A dominant species was observed up to 373 K and had A,' = 96.6 M H z at 293 K. N M R shows that the parent molecule exists predominantly in the s-cis conformation.26 We therefore assume that the observed dominant radical is also the cis isomer, 5B.
A?
k5
7
q
(5)
N(CH,), 5A
5B
Analysis of the temperature-dependent line widths gave log
( E ~ / s - ' ) = 12.7(f i:)
- (48 f !)/2.3RT
and XR = 0.73 (3) ps-l. 3.4. 1-Methylacetonyl Radical. The pSR spectrum obtained with methyl vinyl ketone shows two clear radicals, a major one with A,' = 94.7 M H z and a minor one with A,,' = 87.9 MHz at (26) Kruk, C.; Spaargaren, K. Spectrochim. Acta 1971, 27A, 7 7 .
4382
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
li;
o/
Strub et al. an increase by 0.4 pus-'. This contrasts with the case of the trans- 1-methylacetony1 radical which exhibits line broadening already around 260 K. Quantitative determination of Arrhenius parameters for 1,l-dimethylacetonyl is not possible, but by comparison with 1-methylacetonyl it can be concluded that the activation energy of the former should be considerably higher if the corresponding frequency factor is not even lower than in the latter case. With log (Ais-') = 13.2 the observed line broadening is compatible with E, E 50 kJ-mol-I.
I 61 ps'
4. Discussion The XR values in pSR experiments range from 0.53 ps-' for O!
c
250
300
350 K
Figure 3. Line-width parameter X for the cis (filled circles) and trans (open circles) isomers of MUCH~CHCOCH,.
293 K. The lines are thus separated more than for the radicals derived from the esters. An assignment with the larger coupling for the trans isomer 6A as inferred from INDO calculationss agrees with the radical yields, since the trans form of the parent molecule is more stable than the cis isomer by about 2 kJ-mol-' in CS2 solution27and in the pure liquid.28 From the temperature dependence of the relative radical yields, which range from P A / P B = 3.0 (3) at 243 K to 1.7 (3) at 363 K, we derive an enthalpy difference of 2.1 (7) kJ-mol-' for the two isomers of the ketone. This agrees well with the literature value.27 The temperature dependence of the line widths is quite different for the two isomers (Figure 3). The trans form broadens more rapidly. We obtain for the rate constants of the isomerization 0
L
XR
= 1.09 ( 5 )
log
o
6B
6A
with
*
ps-I,
and
(L~/s-')= (11.1 f i:;)
radical 3 to 1.09 ys-l for radical 6A. Of this, 0.4 ps-I is attributed to an unresolved multiplet pattern.l6 The rest should be due to electron relaxation mostly, although small contributions due to temperature and magnetic field inhomogeneities are not excluded. In ESR, the XR values of 1.8 ps-l (104 mG) for 6A and 1.4 ps-' (80 mG) for 6B have a contribution of 1.26 ps-l from unresolved side bands due to 100 kHz modulation. The remainder is of the same origin as in pSR, and it is gratifying that it is of the same magnitude in both techniques. Transition-state theory predicts log (Ais-') = 13.2 for reactions with zero activation entropy near room temperature. Within error, the experimental values for the systems 3-5 agree with expectation. For the same radicals the activation energy is (46 f 2) kJSmol-'; Le., it is not significantly affected by methyl substitution at the radical center nor by replacement of the ester group by a dimethylamido group. The 1,l-dimethylacetonyl radical shows a behavior compatible with the above systems, but its monomethyl derivative gives exceptionally low values for both frequency factor and activation energy, by pSR and by ESR. The pSR data for k , are good enough to allow a more detailed analysis: The best fit with three free parameters and 13 data points gives x2 = 16. A forced fit using log (Ais-') = 13.2 results in XR = 1.26 (3) ps-' and E, = 45 (1) kl-mol-', at the cost of an unacceptably high value of x 2 = 70, however. Satisfactory agreement with the data is obtained when two parallel reactions are assumed. With fixed values of log (Ais-') = 13.2 and E, = 46 kJ.mol-' for k6 it yields log ( k x / ~ - ' = ) (8.3 f 0.3) - (13 f 2)/2.3RT
- (33 f ;)/2.3RT
with XR = 0.80 (5) ps-l. Quite unexpectedly, the frequency factors of this reaction come out considerably smaller than for the others. We therefore decided to also study the kinetics of the H-analogous radicals by means of ESR. The species were produced by photochemical CI elimination in a 20% solution of 3-chloro-2-butanone in methanol. The kinetic parameters were determined from alternating line-width effects by simultation of a group of lines at the low-field end of the spectrum, using the program ESREXN by H e i n ~ e r .Experiment ~~ and analysis were performed as described in more detail elsewhere.'] The cis isomer 6B was lower in energy by 2.6 kJ.mol-I. The resulting rate constants log ( k s / s - ' ) = 10.4 (2) - 23 (1)/2.3RT log (L,5/s-') = 10.4 (8) - 25 (4)/2.3RT agree with those obtained by pSR within error, but the trend is to even lower Arrhenius parameters. 3.5. 1,I-Dimethylacetonyl Radical. Irradiation of isopropenyl methyl ketone with muons leads to only one observable radical, which is however very strong. It shows A,' = 8 1.6 MHz at 298 K. Since the parent ketone exists predominantly in the s-trans f ~ r m , ~we ~ . conclude ~' that the observed radical has the same conformation. Its pSR line width is constant (0.96 ps-') between 210 and 330 K. Only the highest point measured (349 K) showed (27) Bowles, A. J.; George, W. 0.; Maddams, W. F. J . Chem. Sac., B 1969. 810. (28) Durig, J. R.; Little, T. S . J. Chem. Phys. 1981,75, 3660. (29) Heinzer, J. Mol. Phys. 1971,22, 167. (30) Hobson, R. F.; Reeves, L. W. J . Magn. Reson. 1973,IO, 243. (31) Liljefors, T.; Allinger, N. L. J . Am. Chem. Sac. 1976, 98, 3745.
with XR = 0.95 (12) ps-l and x2 = 10. About the nature of this possible parallel reaction X one can only speculate. From the pSR data alone, addition of the radical to the vinyl monomer is not excluded, although it was shown in section 3.1 to be unimportant for system 3. In ESR, the alternating line-width effect is specific for reaction 6. The fact that ESR gives the same result as pSR should allow to exclude a parallel reaction. The closest example for comparison in literature is acetonyl, which is distinguished from system 6 only by the missing methyl group at the radical center. Golde et aL7 report log (Ais-') = 12.9 f 0.3 for the frequency factor, Le., a perfectly normal value. Their activation energy (39 f 2 kJ-mol-I) is significantly below our data for the systems 3-5 (46 & 2 kJ.mol-I) and the result by Lung-min Wu and Fischer" for the ester derivative (47 f 2 kJ-mol-'). The most recent value of 32 kJ-mol-' for acetonyl derived from HolmesI3 dissociation energy using Nonhebel and Walton's relation'* is still lower and agrees quite well with our pSR data on system 6. There is no obvious reason why substitution of the ester or dimethylamido group at the a-carbon by a methyl group should decrease the activation energy or even the frequency factor, but the experimental data, although somewhat scattered, seem to suggest this. The present work shows that there is a considerable barrier to internal rotation about the partial double bond between an alkyl radical center and a carbonyl, carboxyl, or carbamide substituent. Earlier values from ESR work are confirmed with the pSR techniques, supporting previous doubts concerning some thermochemical data.'* It should, however, be stressed that the radical stabilization energies derived from thermochemistry relate to equilibrium structures, whereas the dynamics of the internal rotation may not permit a fully relaxed transition state. Along the minimum energy path from C=C-;O to [C-C=O]l the rotation is correlated with a significant increase in the C-C and a decrease
4383
J . Phys. Chem. 1987, 91, 4383-4388 in the C-0 bond lengths. This is a motion perpendicular to the reaction coordinate, and it is questioned that it is sufficiently concerted with the rotation. The effect could thus account for somewhat higher barrier values obtained from ESR and wSR.
Acknowledgment. Support by the Swiss National Foundation for Scientific Research and by S I N is gratefully acknowledged.
We thank Prof. C. Ruchardt and one of the referees for drawing our attention to ref 13. Registry No. a-(Ethoxycarbonyl)-2-propylradical, 37999-09-0; a(ethoxycarbony1)ethyl radical, 37999-08-9; a-[(dimethylamino)carbonyllethyl radical, 97 165-76-9; 1-methylacetony1 radical, 1555291-7; 1 ,I-dimethylacetonyl radical, 50636-94-7.
Collision-Induced Intersystem Crossing of NH by O2 Nathan Presser,+Yi-Fei Zhu, and Robert J. Gordon* Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60680 (Received: December 24, 1986; In Final Form: April 7 , 1987)
Fluorescence from NH(A3n) was detected when a mixture of HNCO, 02,and He was excited with a vacuum-UV flash lamp. This emission had a decay time of up to 1 ms, indicating that the NH(A311)state was generated by long-lived metastable precursors. From a number of photochemical tests we deduced that the most likely candidates are NH(b'Z+) + 02(a'Ag), which pool their energy to produce NH(A311) + 02(X3Z,-).
Introduction The N H radical has six known bound electronic states.' Two of them, including the ground state, are triplets (X32- and A311), which correlate with the isoelectronic O(,P) atom, while the remainder are singlets (a' A, b'Z+, c'n, and d'Z+) correlating with O(lD) and O(IS). An energy level diagram of these states is given in Figure 1. Two of these, the a'A and b'2' states, are metastable because of spin and orbital angular momentum selection rules. This paper is concerned with the interconversion of singlet and triplet N H states by collisional processes. Singlet and triplet N H can be formed by a number of methods. In the case of photodissociation, the spin-selection rule is strictly obeyed. For example, single-photon dissociation of NH32 and HNC03.4at 193 nm produces only singlet N H states. At much shorter wavelengths two triplet fragments may be formed; e.g. photodissociation of HN, at 121.6 nm produces NH(X3Z-) N2(B3JI,) and NH(X32-) N2(B'3Z;),5 while photodissociation of H N C O at 132 nm yields NH(X%) + CO(a311).6 In single-photon dissociation of N H 3 between 106 and 200 nm only singlet N H fluorescence was o b ~ e r v e d , ~while - ' ~ triplet NH(A311) emission was produced by a spin-allowed two-photon (1 93 nm) process12-'6 involving an electronically excited N H 2 intermediate.I6*l7 Single-photon dissociation of larger imino compounds also obey the spin conservation rule.'* Another method of producing triplet N H is by sensitized decomposition. An example is the collision of N H 3 with N 2 metastables to produce NH(A311) + N2(X'Z,).5 This type of process is not restricted by spin conser~ation.'~For example, collisions of NH3,lF2' HN3,21HNC0,21and ethylenimineIs with metastable triplet rare gas atoms produce both spin-allowed NH(A311) and spin-forbidden NH(clII). Still another way of producing triplet N H is by electron bombardment. It was reported that collision of NH, with electrons produces both singlet and triplet N H , presumably via low-energy electron exchange.22 Once singlet N H is formed, it may cross to the triplet manifold by various processes. The simplest is by emission of a photon. X and a X) have Such spin-forbidden transitions (e.g. b very long (17.8 ms2, and 3.3 s24)radiative lifetimes. Intersystem crossing (ISC) can also be induced by particle collisions. For example, ISC of NH(c) NH(A) induced by collisions with Xe has a greater than 10% efficiency.l8 Another possibility is resonant spin exchange with a triplet molecule. Such a spin-allowed process
+
+
-
-
-
'Present address: Chemical Physics Division, The Aerospace Corp., Los Angeles, CA 90009.
-
occurs in a collision of NH(a) with ground-state O2 NH(a'A)
+ 02(X3Z,+)
NH(X3Z:-)
+ O,(a'A)
(1)
which also has 10% collision effi~iency.~ Similarly, spin exchange of NH(clJI) with 02(X3Zg+)has been found to yield NH(A311) 02(a1A)with unit e f f i ~ i e n c y . ~ ~ In this paper we report a new mechanism for singlet-to-triplet conversion of N H . When a mixture of H N C O and He was irradiated with pulses from a broad-band UV flashlamp, only singlet N H emission was observed, as expected. However, when O2was added to the gas mixture, emission from NH(A311) was detected, lasting for as long as 1 ms. This very long lifetime implies N
+
(1) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand: New York, 1979. (2) Donnelly, V. M.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. 1979. 43. 27 1. (3) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lett. 1979, 64, 421. (4) Spiglanin, T.S.; Perry, R. A,; Chandler, D. W. J . Phys. Chem. 1986, 90, 6184. (5) Maruyama, Y.; Hikida, T.;Mori, Y. Chem. Phys. Lett. 1985,116, 371. (6) Okabe, H. J . Chem. Phys. 1970,53, 3507. (7) Suto, M.; Lee, L. C. J . Chem. Phys. 1983, 78, 4515. (8) Masanet, J.; Gilles, A.; Vermeil, C. J . Photochem. 1974/75, 3,417. (9) Xuan, C. N.; Di Stefano, G.; Lenzi, M.; Margani, A. J . Chem. Phys. 1981, 74, 6219. (IO) Washida, N.; Inoue, G.; Suzuki, M.; Kajimoto, 0. Chem. Phys. Left. 1985, 114, 274. (11) Ni, T.; Yu, S.; Ma, X.; Kong, F. Chem. Phys. Lett. 1986, 126, 413. (12) Haak, H. K.; Stuhl, F. J . Phys. Chem. 1984, 88, 2201. (13) Hofzumahaus, A,; Stuhl, F. .I. Chem. Phys. 1985, 82, 3152, 5519. (14) Hellner, L.; Grattan, K. T.V.; Hutchinson, M. R. J . Chem. Phys. 1984, 81, 4389. (15) Halpern, J. B.; Jackson, W. M.; McCrary, V. Appl. Opt. 1979, 18, 590. (16) Kenner, R. D.; Rohrer, F.; Stuhl, F. Chem. Phys. Lett. 1985,116,374. (17) Zetzsch, C.; Stuhl, F. Eer. Bunsen-Ges. Phys. Chem. 1981,85, 564. In this study ground-state NH(X'Z-) was observed for X < 147 nm. The authors speculate that the primary, spin-allowed products are NH(X) H H. (18) Kawasaki, M.; Iwasaki, M.; Tanaka, I. J . Chem. Phys. 1973, 59, 6328. Kawasaki, M.; Tanaka, I. J . Phys. Chem. 1974, 78, 1784. (19) Sekiya, H.; Nishiyama, N.; Tsuji, M.; Nishimura, Y. J . Chem. Phys. 1987, 86, 163. (20) Tabayashi, K.; Shobatake, K. J . Chem. Phys. 1986, 84, 4930. (21) Stedman, D. H. J . Chem. Phys. 1970, 52, 3966. (22) Fukui, K.; Fujita, I.; Kuwata, K. J . Phys. Chem. 1977, 81, 1252. (23) Gelernt, B.; Filseth, S. V.; Carrington, T. Chem. Phys. Lett. 1975, 36, 238. (24) Esser, H.; Langen, J.; Schurath, U. Eer. Bunsen-Ges. Phys. Chem. 1983, 87, 636. (25) Rohrer, F.; Stuhl, F. J . Chem. Phys. 1987, 86, 226.
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0022-3654/87/2091-4383$01 .50/0 0 1987 American Chemical Society
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