J. Phys. Chem. 1996, 100, 4873-4878
4873
Visible-Light-Induced Oxygen Atom Transfer from NO2 to (CH3)2NH in a Cryogenic Argon Matrix Nobuaki Tanaka,† Junichi Oike, and Kazuhiko Shibuya* Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro, Tokyo 152, Japan
Munetaka Nakata Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo UniVersity of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183, Japan ReceiVed: October 25, 1995; In Final Form: January 3, 1996X
Visible-light-induced oxygen atom transfer from NO2 to dimethylamine has been investigated in a lowtemperature Ar matrix. The bimolecular reaction with the threshold wavelength of 585 nm occurred to form dimethylamine N-oxide ((CH3)2NHO) and NO. Unstable dimethylamine N-oxide was detected for the first time as an intermediate using an FTIR spectrometer. Further irradiation at a shorter wavelength (514.5 nm) caused the photodecomposition of dimethylamine N-oxide into N-methylmethyleneimine (CH2dNCH3) and water, which interacted with each other through the hydrogen bond. The reaction mechanism is discussed in comparison with that of a CH3NH2/NO2 system.
Introduction In a series of studies on oxygen atom transfers using photoexcited NO2 as an oxidizing reagent, we have provided information on potential energy surfaces of the reaction of visible-light-absorbed NO2 with unsaturated hydrocarbons.1-12 The initial step of the reactions is bimolecular oxygen atom transfer to form a transient oxirane biradical. The oxirane biradical is stabilized by three competing paths: (1) recombination with a NO cage partner to produce a nitrite radical, (2) ring closure to oxirane, and (3) hydrogen or methyl group migration to aldehyde. These reactions proceed on the singlet surface of the oxirane biradical13 and contrast with the reaction of O(3P) with an unsaturated hydrocarbon on the triplet surface of the oxirane biradical.14 The singlet surface of the oxirane biradical is located 20 kcal mol-1 below the triplet,13 and the reaction can be controlled selectively in the unsaturated hydrocarbon/NO2 photochemical systems. For example, in a trans-2-butene/NO2 system, stereochemical retention of oxirane is complete.2 In a 1,3-butadiene/NO2 system, an oxygen atom transferred site-selectively to the end carbon of 1,3-butadiene with the aid of stabilization due to allylic conjugation of the butenyloxirane biradical.12 The rates to final products, NO + oxirane or NO + aldehyde, depend on the excess energy of the oxirane biradical, which can be controlled by the wavelength of the visible light. Recently, we have also carried out experiments on the visiblelight-induced reaction of NO2 with amine characterized by a lone electron pair and reported the photoinduced dehydrogenation reaction of methylamine by NO2.15 In this photochemical system, the oxidized products were only water and imine. Analogous to the mechanism in the unsaturated hydrocarbon/ NO2 system, oxygen atom transfer from photoexcited NO2 to methylamine was proposed as the initial step of the bimolecular reaction. However, we could not detect the primary photoproduct, methylamine N-oxide. Furthermore, in our recent work on a cyclohexadiene/NO2 system, we have found evidence of † Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4873$12.00/0
simultaneous occurrence of two reaction paths: (1) oxygen atom transfer followed by recombination with NO to form the corresponding nitrite radical and (2) hydrogen atom transfer to form HNO2 and the cyclohexadienyl radical.16 As a result, one question was proposed, whether the dehydrogenation of amine occurred via oxygen or hydrogen atom transfer between the amine and photoexcited NO2. With the purpose of elucidating the photochemical mechanism of the amine/NO2 system, we have chosen dimethylamine as an amine, where the potential energy surface responsible for the amine/NO2 reaction is proposed. Experimental Section Light irradiation was conducted with an Ar+ laser (Spectra Physics, Stabilite 2016) or an Ar+ laser pumped dye laser (Spectra Physics, Model 375 B). The Ar+ laser was operated in all- or single-line mode. Rhodamine 6G was used as laser dye. IR spectra were measured in the 4600-600-cm-1 range with 0.5-cm-1 resolution by a JASCO 8000S Fourier transform infrared spectrometer with a liquid-nitrogen-cooled MCT detector. Each spectrum was obtained by scanning over 100 times. A closed-cycle helium cryostat (Cryogenic Technology Inc., Model 21 Cryodyne) was used to control the temperature of the Ar matrix. (CH3)2NH (Kodak) was used after trap-to-trap distillation at 77 K. Ar (Takachiho, 99.9999% purity) was used without further purification. NO2 (Sumitomo Seika, pure grade) was purified by freeze-pump-thaw cycles at 77 and 193 K. An isotopic NO2 mixture was prepared by oxidation of N16O (Takachiho) by 18O2 (Matheson, 99 atom % 18O). The isotope ratio of N16O2/N16O18O/N18O2 was 4/5/2. Other experimental details were reported elsewhere.12 Results Prior to the laser irradiation upon (CH3)2NH/NO2/Ar mixtures, several kinds of experiments were carried out. A mixture of NO2/Ar or (CH3)2NH/Ar was deposited on a CsI window at 16 K without mixing with each other, warmed up to 30 K, and © 1996 American Chemical Society
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Figure 1. Infrared difference spectrum upon 580-nm (200 mW cm-2) irradiation of the matrix (CH3)2NH/NO2/Ar ()2/1/400) for 5 h. The positive and negative bands indicate the growth and depletion, respectively, during this irradiation period. The decreasing bands due to the reactants are indicated by “O”. The product band at 1873 cm-1 is due to NO. The absorption changes by N2O4 production (dimerization of NO2) and CsNOx production are indicated by “×”. In the 1700-1400-cm-1 region the spectrum is disturbed by absorption of atmospheric H2O.
TABLE 1: IR Absorptions, Relative Intensities, and Assignments of Dimethylamine N-Oxide in an Ar Matrix
TABLE 2: Assignments and Absorptions of CH2dNCH3‚‚‚H216O in an Ar Matrix
ν/cm-1 (CH3)2NH16O 2780 1489 1472 1405 1202 935 854
ν/cm-1
(CH3)2NH18Ob
intensitya
assignment
2780 1489 1472 1405 1202 935 849
ndc
CH3 stretch CH3 deform CH3 deform CH3 deform CH3 rock C-N strech N-O stretch
m m m m vw s
a s, strong; m, medium; vw, very weak. b Isotope-labeled species was measured using an isotope mixture of N16O2/N18O16O/N18O2 ()4/5/2) as the NO2 reactant. c Not determined because of overlapping with dimethylamine depletion.
isolated assignments
this work
H2O ν3 H2O ν1 CH2dNCH3 (CdN stretch) CH2dNCH3 (CH2 scissors) CH2dNCH3 (CH3 deform) CH2dNCH3 (CH3 rock) CH2dNCH3 (CH3 wag) CH2dNCH3 (CH3 wag) CH2dNCH3 (CH2 wag) CH2dNCH3 (C-N stretch)
3702 3387, 3360 1663 1473 1445 1231 1108 1061 1037 953
a
recooled down to 16 K. In these annealing experiments, dimerization of NO2 or aggregation of dimethylamine was observed to occur, while no other chemical reaction took place. When the NO2/Ar matrix was irradiated with Ar+ laser light, photoisomerization of trace N2O4 and dimerization of NO2 were recognized to occur.17 Although the corresponding experiments on (CH3)2NH/Ar were performed, no spectral change indicating the occurrence of reaction was observed by FTIR spectroscopy. Mixtures of NO2/Ar and (CH3)2NH/Ar were codeposited on a CsI window with a ratio of (CH3)2NH/NO2/Ar ) 2/1/400 or 1/1/400. In the infrared spectra obtained after the codeposition, strong bands due to NO2 and (CH3)2NH were observed.17,18 The spectrum also contained additional weak bands due to N2O4 and N2O3. The threshold wavelength to induce the photochemical reaction was found to be 585 nm. Figure 1 shows the infrared difference spectrum obtained upon 580-nm irradiation of the matrix (CH3)2NH/NO2/Ar ) 2/1/400 at 200 mW cm-2 for 5 h. The irradiation resulted in the appearance of new bands, which were not assigned to NxOy, with a decrease in intensities of the bands due to reactants, (CH3)2NH and NO2. Dimerization of NO2 was also observed. Five clear bands at 1489, 1472, 1405, 1202, and 854 cm-1 grew with maintaining the same relative intensities during the irradiation period, and they were attributed to a single molecule we call species A. Table 1 displays the frequencies of species A. Isotopic frequencies of species A obtained upon 580-nm irradiation of the matrix (CH3)2NH/isotope-labeled NO2/Ar ) 2/1/400 are also listed in
Ar matrix 3734.3a 3638.0a 1658b 1472b 1439b 1218b 1098b 1045b 1023b 948b
gas
1659c 1470c 1441c 1221c 1100c
1661c 1475c 1444c 1220c
1026c 950c
1026c 952c
Reference 21. b Reference 19. c Reference 20.
Table 1. The band at 854 cm-1 clearly shows the isotopic shift to 849 cm-1. Product bands listed in Table 1 were not observed when the (CH3)2NH/NO2/Ar matrix was annealed without photoirradiation. A product responsible for the 1873-cm-1 band, whose isotopic counterpart appeared at 1823 cm-1, was readily assigned to NO, judging from previous works on photolytic NO2 systems in cryogenic matrices.2 Figure 2 shows the spectral change observed upon further irradiation of the matrix containing species A with 457.9-514.5nm light (all-line operation) at 100 mW cm-2 for 30 min. The bands due to species A decreased, while new product bands appeared. Eight product bands at 1663, 1473, 1445, 1231, 1108, 1061, 1037, and 953 cm-1 could be attributed to a single product we call species B. Three OH bands at 3702, 3387, and 3360 cm-1 shown in Figure 2 were also due to a single product we call species C, though the base line in the 3500-3000-cm-1 region was not flat due to interference caused by amorphous water accumulated on the matrix surface. Frequencies of the product bands due to species B and C are displayed in the second column of Table 2. The 3387- and 3360-cm-1 bands were slightly broader than the 3702-cm-1 band. In the experiment using the isotope-labeled NO2, their isotopic counterparts were found to appear at 3380 and 3350 cm-1 as shoulder bands. Photoisomerization of N2O4 from the asym-form (1828, 1644, 1290, 904, and 787 cm-1) to the sym-form (1750, 1730, 1257, and 745 cm-1) was also observed to occur.17
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J. Phys. Chem., Vol. 100, No. 12, 1996 4875
Figure 2. Infrared difference spectrum obtained upon irradiation with 457.9-514.5-nm light of an Ar+ laser (100 mW cm-2) for 30 min subsequent to 580-nm irradiation for 5 h. The decreasing bands due to the reactants are indicated by “O”. In the 3800-3600-cm-1 region the spectrum is disturbed by absorption of atmospheric H2O.
TABLE 3: Comparison of ν3 and ν1 Absorptions (cm-1) for Hydrogen-Bonded Water in Ar Matrices ν3 species CH2dNCH3‚‚‚water CH2dNH‚‚‚watera HCONH2‚‚‚waterb NH3‚‚‚waterc uncomplexed waterb a
Figure 3. Absorbance change of dimethylamine N-oxide (O, monitored at 1202 cm-1) and N-methylmethyleneimine (9, monitored at 1108 cm-1) yields with irradiation of the matrix (CH3)2NH/NO2/Ar ()2/1/ 400) at 514.5 nm (200 mW cm-2). Solid lines present simulated curves (see text for details).
When the matrix (CH3)2NH/NO2/Ar ) 2/1/400 was irradiated with 514.5-nm light (single-line operation, 200 mW cm-2), the bands due to species A reached a maximum at the irradiation time around 30 min and then started to decrease upon continued irradiation. The other product bands continued to increase at least for 3 h. The growth and decay behavior of typical bands of species A (1202 cm-1) and species B (1108 cm-1) is shown in Figure 3. Discussion Identification of Final Products. Species B (Table 2) was identified as chemically unstable CH2dNCH3 on the basis of the matrix isolation spectrum obtained by Hinze and Carl.19 The vibrational frequencies measured were consistent with their values within 16 cm-1. Hinze and Carl measured the CH2dNCH3 frequencies under isolated conditions, which are close to the values measured in the gas phase.20 This indicates that the photoproduct of CH2dNCH3 obtained in this study is perturbed by some chemical species existing around it in the matrix cage. Unfortunately, the CH3 and CH2 asymmetric and symmetric stretching vibrational bands could not be confirmed in the present study since they were overlapped with the dimethylamine depletion bands.
16O
H2
3702 3702 3706.7 3702.2 3734.3
ν1 18O
H2
3720.1
16O
H2
3387, 3360 3420, 3397 3430.5 3434.9 3638.0
H218O 3380, 3350 3421.0 3629.5
Reference 15. b Reference 21. c Reference 22.
The absorption bands of species C were measured at 3702, 3387, and 3360 cm-1. The 3387- and 3360-cm-1 bands shift to 3380 and 3350 cm-1, respectively, for the 18O isotopic counterparts. These bands are likely to be characteristic H2O fundamentals in hydrogen-bonded complexes: For example, the water ν1 fundamentals observed for water‚‚‚formamide21 are 3430.5 cm-1 for H216O and 3421.0 cm-1 for H218O. The H216O ν1 and ν3 fundamentals observed for water‚‚‚methyleneimine15 are 3420 and 3397 cm-1, and 3702 cm-1, respectively. In the case of the water‚‚‚ammonia complex, the ν1 and ν3 fundamentals of water are 3434.9 and 3702.2 cm-1, respectively.22,23 On the basis of these observations, species C is identified as hydrogen-bonded water: The 3702-cm-1 band is assigned to the ν3 fundamental. The 3387- and 3360-cm-1 bands are assigned to the ν1 fundamentals at the different sites in the matrix, while the 3380- and 3350-cm-1 bands are those of H218O at the corresponding different sites. We cannot specify these sites at this moment. Judging from the fact that the infrared absorption bands due to N-methylmethyleneimine and water are both shifted from the isolated ones, the N-methylmethyleneimine product seems to be hydrogen-bonded with the water product in the matrix cage (CH2dNCH3‚‚‚H2O). Table 3 shows the ν3 and ν1 values of water and water hydrogen-bonded with CH2dNCH3 and CH2dNH15 for comparison. The frequency shift of the water ν1 band of the CH2dNCH3‚‚‚H2O complex is found to be larger than that of the CH2dNH‚‚‚H2O complex. This tendency can be explained by stronger electron-donating ability of a CH3 group than a H group and larger proton affinity24 of CH2dNCH3 (210 kcal mol-1) than CH2dNH (204 kcal mol-1). The
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Figure 4. Absorbance change of dimethylamine N-oxide (O, monitored at 1202 cm-1) and NO (0, monitored at 1873 cm-1) yields with irradiation of the matrix (CH3)2NH/NO2/Ar ()2/1/400) at 580 nm (200 mW cm-2). Solid lines present simulated curves (see text for details).
hydrogen bond strength is thus in the increasing order of CH2dNH and CH2dNCH3. The hydrogen atom of water will make a bond with the nitrogen atom of the imines as shown below. This structure is also supported by an ab initio calculation.25 CH3 CH2
N H
H O
Identification of the Reaction Intermediate (Species A). To clarify the reaction mechanism of photoexcited NO2 with (CH3)2NH, the product growth was monitored during the irradiation period. The growth behavior of species A (1202 cm-1) and NO (1873 cm-1) upon 580-nm irradiation is shown in Figure 4. It should be mentioned here that the final products, CH2dNCH3 (species B) and H2O (species C), were not observed with the irradiation at 580 nm. Both of the 1202- and 1873cm-1 bands exhibited single-exponential growth and no induction period, which indicates that species A and NO are the primary reaction products. A least-squares fit of each growth curve was employed assuming a pseudo-first-order reaction: reactant pair + hν f species A + NO. The rate equation gives the following integration form:
AI ) A0
I (1 - e-k1t) R
or N,N-dimethylhydroxylamine. The possibility of N,N-dimethylhydroxylamine has been ruled out by the fact that the strong bands ν(OH), ν(N-O), and νsym(CNC) around 3600, 950, and 800 cm-1, respectively,26 were not observed. Another possibility of dimethylamine N-oxide is considered below. The deformation mode, δ(CH3), is expected to be observed in the region between 1500 and 1400 cm-1. Three bands measured at 1489, 1472, and 1405 cm-1 were assigned to the deformation mode of the CH3 group. The 1202-cm-1 band is probably due to the CH3 rocking mode. The doublet splitting of the 850cm-1 band observed in the experiment using N16O2/N18O16O/ N18O2 indicates the presence of a N-O bond. The 2780-cm-1 band, which was disturbed by the depletion band of dimethylamine, belongs to the symmetric CH3 stretching mode. It is thus concluded that species A is dimethylamine N-oxide. Photochemical Reaction Mechanism. In the dimethylamine/NO2 system, we have found that oxygen atom transfer from NO2 to dimethylamine is caused by the irradiation of visible light longer than 398 nm, corresponding to the first dissociation limit of NO2. The bimolecular reaction between dimethylamine and photoexcited NO2 (NO2*) certainly occurs. It is supposed for the (CH3)2NH/NO2 system that the oxygen atom of NO2* may attack lone-pair electrons on the N atom of dimethylamine, which gives dimethylamine N-oxide (species A). This N-oxide undergoes further photolysis at 457.9-514.5 nm to yield N-methylmethyleneimine (CH2dNCH3) and water. We have thus elucidated the photochemical dehydrogenation mechanism of amine by NO2: It occurs via not hydrogen but oxygen atom transfer between amine and NO2*. The intermediate amine N-oxide could be isolated as (CH3)2NHO in the present (CH3)2NH/NO2 system, while it could not be stabilized as CH3NH2O in the CH3NH2/NO2 system.15 The reaction scheme for the (CH3)2NH/NO2 system is described below. NO2 + hν
Here, A and denote absorbance and absorption coefficient, respectively. Subscripts R and I refer to reactant pair and species A (or NO), respectively. A0 denotes the absorbance of the reactant pair before irradiation. The rate constants (k1), in units of 10-3 min-1, were determined to be 12 ( 3.1 and 11 ( 2.6 for species A and NO, respectively. Both products were formed at essentially the same rates with the irradiation at 580 nm. The absorbance difference in the two plots in Figure 4 corresponds to the difference between the absorption coefficients of NO and species A (species A/NO ) 0.94). Considering that (1) upon 580-nm irradiation species A and NO were initially formed as concurrent products and (2) species A was secondarily photodecomposed at 457.9-514.5 nm into CH2dNCH3 and H2O, species A is expected to contain an oxygen atom. In the CH3NH2/NO2 system studied previously,15 we have indicated the possibility that the dehydrogenation reaction, CH3NH2 + NO2 + hν f CH2dNH + H2O + NO, proceeded through transient formation of methylamine N-oxide and N-methylhydroxylamine, but we could not detect these intermediate species. Therefore, we consider species A to be the corresponding intermediate species: dimethylamine N-oxide
(2) CH3
NO2* + (CH3)2NH
NO + H3C
N+ H (species A)
(3)
O– CH3 H3C
N+ H + hν O–
(1)
NO2*
CH3
H3C N H3C
OH
CH2
(4)
N H
H O
Figure 5 shows the potential energy diagram. In the present (CH3)2NH/NO2 system, the threshold wavelength (λ ) 585 nm) to induce oxygen atom transfer was longer than that (λ ) 514.5-575 nm) observed in the CH3NH2/NO2 system. The values of ionization potentials of CH3NH2 and (CH3)2NH are 8.97 and 8.24 eV, respectively. The threshold wavelength becomes longer as the ionization potential decreases. The initially produced intermediate, amine N-oxide, is an ion-pair molecule, so that the ionization potential of amine controls the threshold photon energy: The ion-pair potential surface affects the barrier height of oxygen atom transfer from NO2 to amine. Triplet amine N-oxide is believed to readily isomerize and/ or dissociate. Taking as an example O(3P) + NH3(X1A1), it was calculated that NH3O lies on a repulsive surface.27 Withnall and Andrews28 proposed NH3O as an intermediate to generate NH2OH in the photolysis of NH3/O3 in an Ar matrix. For aliphatic amine N-oxide, Atkinson and Pitts29 and Slagle et al.30,31 assumed the N-oxide formation by O(3P) oxidation of amine as an initial step, followed by isomerization. Theoretically,32 triplet N-oxide is reported to be unstable. Triplet N-oxide cannot be prepared with the 580-nm irradiation of the present (CH3)2NH/NO2 system from an energetical reason (see
Visible-Light-Induced Oxygen Atom Transfer
J. Phys. Chem., Vol. 100, No. 12, 1996 4877 SCHEME 1
Figure 5. Potential energy diagram for the photochemical reactions of NO2 and dimethylamine. The energy levels of singlet and triplet (CH3)2NHO are drawn on the basis of calculated energies relative to (CH3)2NH + O(3P).32
Figure 5). Upon 580-nm irradiation, we observed the intermediate of (CH3)2NHO but did not detect the final products (CH2dNCH3 and H2O). Singlet N-oxide is calculated to rearrange to hydroxylamine with a high energy barrier (44 kcal mol-1).32 Therefore, the singlet amine N-oxide produced does not possess enough available energy to overcome the barrier and could not isomerize to hydroxylamine. The result may also imply that (CH3)2NHO does not absorb visible light at 580 nm. The threshold wavelength for the photodissociation of (CH3)2NHO was determined to be 514.5 nm as the lower limit. Possibly the photodissociation into CH2dNCH3 and H2O may take place by the indirect dissociation via hydroxylamine or by the direct dissociation. Upon 514.5-nm irradiation the absorbance change of N-oxide showed growth and decay behavior typical of a reaction intermediate, as shown in Figure 3. The time behavior contrasts with that observed in the CH3NH2/NO2 system,15 where the N-oxide of CH3NH2O was not isolated due to the decomposition into CH2dNH and H2O. We could only fit the absorbance changes of (CH3)2NHO and CH2dNCH3 (Figure 3) well by a consecutive mechanism: reactant pair f N-oxide f CH2dNCH3. The final product, CH2dNCH3, is likely to be formed via two reaction paths (one- and two-photon paths), which is described in our previous paper.12 In the one-photon path, initially generated excited N-oxide exceeds the potential barrier to form the final products (CH2dNCH3 and H2O). In the two-photon path, the energetic N-oxide is first stabilized as an intermediate in the Ar matrix and then absorbs another photon to decompose into CH2dNCH3 and H2O. Rate constants are analyzed as the pseudo-first-order reactions shown in Scheme 1. Symbols R, I, and P correspond to the reaction pair, N-oxide, and CH2dNCH3, respectively, in the present system. By a nonlinear least-squares fitting of eqs 1 and 2 in ref 12 to the experimental data obtained upon 514.5-nm irradiation, the kinetic parameters k1, k2, and k3 were determined to be 52 ( 19, 12 ( 3, and 13 ( 7 in units of 10-3 min-1, respectively. The ratio of the absorption coef-
ficients, I/P, was also derived to be 1.7 ( 0.3. The result suggests that a direct path opens upon 514.5-nm irradiation, although the contribution is minor (k3/(k1 + k3) ) 0.2). We estimate the upper limit of the barrier height corresponding to N-oxide forming N-methylmethyleneimine and water to be 12 kcal mol-1, which is much smaller than the calculated value (44 kcal mol-1 32). The final products are observed to be strongly hydrogenbonded in the matrix cage. In the CD3NH2/NO2 system previously reported,15 only HDO among the three isotopic waters (H2O, D2O, HDO) was formed as the photochemical product. This isotope experiment indicates that, when water is formed as the product, two hydrogen atoms are eliminated from methylamine where one hydrogen atom originates from the methyl group and the other from the amino group. For the (CH3)2NH/NO2 system two hydrogen atoms are expected to be eliminated from dimethylamine N-oxide where one hydrogen atom originates from the methyl group and the other from the amino group. A similar mechanism was proposed for the reactions of an oxygen atom with methylamine in the gas phase.30,31 Conclusions The photochemical reaction of NO2 with dimethylamine in an argon matrix was studied by FTIR, where three reaction products, N-methylmethyleneimine (CH2dNCH3), water, and NO, were generated. Photoexcited NO2 cannot decompose unimolecularly into NO and O, but it plays an important role in the photochemical reaction. The initial step of the bimolecular reaction is explained by oxygen atom transfer from photoexcited NO2 to methylamine to form dimethylamine N-oxide. It undergoes secondary photolysis at a shorter wavelength to decompose into the dehydrogenated product, N-methylmethyleneimine, and water. These photochemical reactions contrast with those observed in the unsaturated hydrocarbon/NO2 systems, where the oxirane biradical generated by oxygen atom transfer undergoes ring closure, recombination with NO, and/or intramolecular hydrogen atom transfer. Acknowledgment. We are grateful to Professor K. Obi (Tokyo Institute of Technology) for his encouragement throughout this work. We thank Associate Professor Y. Kajii (The University of Tokyo) for his helpful discussions. This work was partly supported by the Grant-in-Aid for Scientific Research No. 06453018 and 06239218 from the Ministry of Education, Science, Sports, and Culture. References and Notes (1) Nakata, M.; Shibuya, K.; Frei, H. J. Phys. Chem. 1990, 94, 8168. (2) Nakata, M.; Frei, H. J. Am. Chem. Soc. 1989, 111, 5240. (3) Nakata, M.; Frei, H. J. Phys. Chem. 1989, 93, 7670. (4) Nakata, M.; Frei, H. J. Chem. Soc. Jpn. 1989, 1412. (5) Fitzmaurice, D. J.; Frei, H. J. Phys. Chem. 1991, 95, 2652. (6) Fitzmaurice, D. J.; Frei, H. Chem. Phys. Lett. 1992, 192, 166. (7) Nakata, M.; Frei, H. J. Am. Chem. Soc. 1992, 114, 1363. (8) Frei, H. Chimia 1991, 45, 175. (9) Fitzmaurice, D. J.; Frei, H. J. Phys. Chem. 1992, 96, 10308. (10) Nakata, M. Spectrochim. Acta 1994, 50A, 1455. (11) Frei, H. Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1992; Vol. 20, p 1.
4878 J. Phys. Chem., Vol. 100, No. 12, 1996 (12) Tanaka, N.; Kajii, Y.; Shibuya, K.; Nakata, M. J. Phys. Chem. 1993, 97, 7048. (13) Fueno, T.; Takahara, Y.; Yamaguchi, K. Chem. Phys. Lett. 1990, 167, 291. (14) Schmoltner, A. M.; Chu, P. M.; Brudzynski, R. J.; Lee, Y. T. J. Chem. Phys. 1989, 91, 6926. (15) Tanaka, N.; Oike, J.; Kajii, Y.; Shibuya, K.; Nakata, M. Chem. Phys. Lett. 1995, 232, 109. (16) Tanaka, N.; Shibuya, K.; Nakata, M. Unpublished results. (17) Bandow, H.; Akimoto, H.; Akiyama, S.; Tezuka, T. Chem. Phys. Lett. 1984, 111, 496. (18) Dellepiane, G.; Zerbi, G. J. Chem. Phys. 1968, 48, 3573. (19) Hinze, J.; Curl, R. F., Jr. J. Am. Chem. Soc. 1964, 86, 5068. (20) Stolkin, I.; Ha, T. -K.; Gu¨nthard, Hs. H. Chem. Phys. 1977, 21, 327. (21) Engdahl, A.; Nelander, B.; Astrand, P.-O. J. Chem. Phys. 1993, 99, 4894. (22) Nelander, B.; Nord, L. J. Phys. Chem. 1982, 86, 4375.
Tanaka et al. (23) Engdahl, A.; Nelander, B. J. Chem. Phys. 1989, 91, 6604. (24) Peerboom, R. A. L.; Ingemann, S.; Nibbering, N. M. M.; Liebman, J. F. J. Chem. Soc., Perkin Trans. 2 1990, 1825. (25) Migchels, P.; Zeegers-Huyskens, T.; Peeters, D. J. Phys. Chem. 1991, 95, 7599. (26) Bohlig, V. H.; Franke, S.; Fruwert, J. Z. Phys. Chemie, Leipzig 1987, 268, 355. (27) Hart, B. T. Aust. J. Chem. 1976, 29, 231. (28) Withnall, R.; Andrews, L. J. Phys. Chem. 1988, 92, 2155. (29) Atkinson, R.; Pitts, J. N., Jr. J. Chem. Phys. 1978, 68, 911. (30) Slagle, I. R.; Dudich, J. F.; Gutman, D. Chem. Phys. Lett. 1979, 61, 620. (31) Slagle, I. R.; Dudich, J. F.; Gutman, D. J. Phys. Chem. 1979, 83, 3065. (32) Goldblum, A.; Loew, G. H. J. Am. Chem. Soc. 1985, 107, 4265.
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