J. Phys. Chem. 1996, 100, 15815-15820
15815
Visible Light-Induced Reaction of NO2 with Propene in Low-Temperature Argon and Xenon Matrices Munetaka Nakata,* Yukie Somura, and Masao Takayanagi Graduate School of Bio-Applications and Systems Engineering, Tokyo UniVersity of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183, Japan
Nobuaki Tanaka† and Kazuhiko Shibuya Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro, Tokyo 152, Japan
Tadafumi Uchimaru and Kazutoshi Tanabe Department of Physical Chemistry, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan ReceiVed: March 26, 1996; In Final Form: June 20, 1996X
Visible light-induced oxygen atom transfer from NO2 to propene has been investigated in low-temperature argon and xenon matrices. The reaction intermediate was propyl nitrite radical, and the final products were methyloxirane and NO, which was confirmed by FT-IR spectroscopy. Conformational structure of propyl nitrite radical was determined by the vibrational analysis of NdO and NsO stretching modes for the normal and 18O-isotope-substituted species with the aid of ab initio calculations, where geometrical optimization was carried out by using the DFT method with the 6-31G* basis set. From the analysis of absorbance growth behavior of the infrared bands for propyl nitrite radical and methyloxirane, first-order rate constants were determined by least-squares fittings. The photoreaction of propene and NO2 in xenon matrices was found to occur more rapidly than in argon matrices. The wavelength dependence of the rate constants is also discussed.
Introduction Matrix-isolation infrared spectroscopy is an ideal technique for studies on photoinitiated bimolecular reactions. In lowtemperature surroundings made of inert rare-gas atoms, photoreaction intermediates are stabilized by relaxation of the excess energy and monitored by infrared spectroscopy. Vibrational analyses of the infrared spectra of reactants, intermediates, and final products provide important information on reaction pathways. Recently we applied the matrix-isolation technique to studies on visible light-induced photooxidation of alkenes,1-8 dienes,9-11 alkynes,9,12,13 and alkylamines14,15 by NO2 and explicated the photoreaction mechanisms. In the present study, the visible light-induced photoreaction mechanism of NO2 with propene in low-temperature argon and xenon matrices has been examined. There are two possible conformations for the propyl nitrite radical, which is produced from methyloxirane biradical by recombination with NO in lowtemperature matrices: a secondary radical of CH3•CHCH2ONO and a primary radical of CH3CH(ONO)•CH2. To determine the conformation of the propyl nitrite radical, we have performed vibrational analysis of the normal and 18O-isotope-substituted species. The conformational specificity is discussed in terms of stabilities of reaction intermediates with the aid of ab initio calculations. Experimental Section Propene and NO2, purchased from Sumitomo Seika Co., were diluted with argon gas (Takachiho Co., 99.9999% purity) or † Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00912-4 CCC: $12.00
xenon gas (Nippon Sanso Co., 99.995% purity) after purification.5 The mixing ratio of propene/NO2/(Ar or Xe) was 10/1/ 100. Oxygen-isotope-substituted NO2 was prepared by mixing NO with 18O2 in a glass cylinder. The gas mixture was irradiated for 1 h with the UV light from a high-pressure mercury lamp and used after purification. The isotope ratio was N16O2/N16O18O/N18O2 ) 1/2/1, which was confirmed from the relative intensities of the infrared bands. We call this isotope-labeled NO2 mixture “isotopic NO2” to distinguish it from “normal NO2” composed of only N16O2. Other experimental details are reported elsewhere.5 Results Measurements of Infrared Spectra. We focus our interest on the spectral region between 2000 and 700 cm-1, because this region contains all of the information necessary to characterize the present photoreaction. An infrared spectrum of an argon matrix sample recorded immediately after deposition shows bands of propene (1824, 1650, 1455, 1438, 1416, 1375, 1044, 991, 935, and 910 cm-1)16 and those of NO2 (1608 and 749 cm-1),2 except for weak bands of N2O4 and traces of N2O3. The matrix sample was irradiated for 2 h at 582 nm with a CW dye laser. An infrared difference spectrum between before and after the irradiation is shown in Figure 1. Except for the spectral changes due to photoisomerizations of N2O4 and N2O3, intense bands of propyl nitrite radical, denoted as I, were newly detected at 1650 and 770 cm-1. They were easily assigned to the wellknown NdO and NsO stretching modes of general nitrite organic compounds.16 The other infrared bands for a final product appeared at 1459, 1446, 1409, 1369, 1146, 1133, 1107, © 1996 American Chemical Society
15816 J. Phys. Chem., Vol. 100, No. 39, 1996
Nakata et al.
Figure 1. Infrared difference spectrum obtained upon 582 nm irradiation of propene/NO2 in an argon matrix for 2 h. Symbols I and F denote a reaction intermediate (propyl nitrite radical) and a final reaction product (methyloxirane), respectively.
Figure 2. Infrared difference spectrum obtained upon Ar+ laser (all lines of operation) irradiation of propyl nitrite radical in an argon matrix for 2 min. The propyl nitrite radical was prepared by the 582 nm irradiation of propene/NO2 for 2 h (Figure 1).
1026, 953, and 832 cm-1, denoted as F. They could be readily assigned to methyloxirane by comparison with infrared spectra available from the literature.17 Another increasing peak at 1872 cm-1 is due to dissociated NO.2 When the matrix sample was subsequently irradiated with an Ar+ laser (all lines) for 1 min after recording Figure 1, the band intensities of the propyl nitrite radical decreased, as shown in Figure 2, while those of the methyloxirane and NO increased. In addition to the matrix sample of propene/normal NO2, the matrix sample using isotopic NO2 (N16O2/N16O18O/N18O2 ) 1/2/1) was also irradiated first at 582 nm with the dye laser and then with the Ar+ laser (all lines). The infrared difference spectra of the NdO and NsO stretching regions for isotopelabeled propyl nitrile radical are enlarged and compared with those of the normal species (C3H616ON16O) in Figure 3. Two bands with nearly equal intensities were observed at 1650 and 1610 cm-1 in the NdO stretching region. The 40 cm-1 isotope shift must be reasonable for the NdO stretching mode of the propyl nitrite radical. One broad band was observed at 762 cm-1 in the NsO stretching region, which implies that the band is composed of some overlapped peaks for isotope species, as will be described in Discussion. The bands at 832 and 813 cm-1 were easily assigned to normal species and 18O-isotopesubstituted species for a final product, methyloxirane, respectively. Determination of Rate Constants. To determine the rate constants for photoreaction of NO2 with propene, the absorbance growth behavior of the 770 cm-1 band for propyl nitrite radical and the 832 cm-1 band for methyloxirane was measured. The dependencies of the absorbances on irradiation time are illustrated in Figure 4. By analogy with other alkene-NO2
Figure 3. Infrared spectra of propyl nitrite radical in argon matrices: (a) NdO stretching region in the spectrum obtained by using a propene/ N16O2 sample; (b) NsO stretching region in the spectrum obtained by using a propene/N16O2 sample; (c) NdO stretching region for the 50% 18 O-isotope-substituted sample; (d) NsO stretching region for the 50% 18 O-isotope-substituted sample.
photoreaction systems reported in our previous papers,1-7 we assumed the reaction mechanism in Scheme 1, which consists of one-photon and two-photon pathways. One-photon pathway means that methyloxirane is produced directly from propene and photoexcited nitrogen dioxide. On the other hand, twophoton pathway means that propyl nitrite radical is produced from propene and photoexcited nitrogen dioxide, and then methyloxirane is produced by the secondary photolysis of the propyl nitrite radical. The absorbances of the 770 cm-1 band for propyl nitrite radical and the 832 cm-1 band for methyloxirane can be represented by resolving the rate equations of Scheme 1 as
AI ) A∞P(I/P){k1/(k2 - k1 - k3)}{e-(k1+k3)t - e-k2t}
(1)
AP ) A∞P[1 + {1/(k2 - k1 - k3)}{k1e-k2t (k2 - k3)e-(k1+k3)t}] (2) where AI and I represent the absorbance and extinction coefficient of the 770 cm-1 band for propyl nitrite radical, respectively, and AP, A∞P, and P represent the absorbance, the absorbance at infinite irradiation time, and extinction coefficient of the 832 cm-1 band for methyloxirane. The details on the formula derivations are described elsewhere.2
Visible Light-Induced Reaction of NO2 with Propene
J. Phys. Chem., Vol. 100, No. 39, 1996 15817
Figure 4. Absorbance growth behavior of propyl nitrite radical (squares) and methyloxirane (circles): (a) 582 nm irradiation in argon; (b) 592 nm irradiation in argon; (c) 582 nm irradiation in xenon; (d) 592 nm irradiation in xenon. Solid lines represent calculated curves using the rate constants listed in Table 1.
SCHEME 1
TABLE 2: Vibrational Frequencies of NdO and NsO Stretching Modes for Alkyl Nitrite Radicals (in cm-1)
•C H ONO 3 6 k1
reactant
k2
C3H6 + NO2
k3
CH2 + NO
CH3CH O
TABLE 1: Dependencies of Rate Constants (h-1) on Wavelength and Matrix 582 nm k1 k2 k3
592 nm
Ar
Xe
Ar
Xe
0.059 ( 0.013 0.35 ( 0.07 0.055 ( 0.023
0.088 ( 0.018 0.59 ( 0.15 0.14 ( 0.06
0.013 ( 0.018 0.34 ( 0.09 0.012 ( 0.021
0.031 ( 0.019 0.62 ( 0.18 0.039 ( 0.035
By least-squares fits using the formulae 1 and 2, the rate constants, k1, k2, and k3, were determined to be 0.059 ( 0.013, 0.35 ( 0.07, and 0.055 ( 0.023 h-1, respectively. The calculated absorbances, which are illustrated in the solid lines of Figure 4a, reproduced the observed growth behavior well. Similar analysis of the absorbance growth behavior was performed to examine the wavelength dependence of the reaction mechanism when the matrix sample was irradiated at 592 nm with CW dye laser (Figure 4b). The rate constants determined are summarized in Table 1. In particular, the k1 and k3 values upon 592 nm irradiation are smaller than the corresponding values upon 582 nm irradiation. This is a general trend observed for the visible light-induced reactions of alkeneNO2 systems in argom matrices.5 We also determined the rate constants in a xenon matrix. The 770 cm-1 band of propyl nitrite radical in argon shifted to 762 cm-1 in xenon, while the 832 cm-1 band of methyloxirane in argon shifted to 830 cm-1 in xenon. The growth behavior of these absorbances is illustrated in Figure 4c,d, where all of the experimental conditions in xenon matrices were the same as those in argon matrices. The rate constants determined by leastsquares fits are summarized in Table 1, together with those
NdO stretch NsO stretch reference
CH2dCH2 CH3CHdCH2 cis-CH3CHdCHCH3 trans-CH3CHdCHCH3 (CH3)2CdCH2 (CH3)2CdC(CH3)2
1664 1650 1652 1645 1644 1634
773 770 772 755 754 744
1 this work 3 2 4 5
CH2dCHCHdCH2 CH2dC(CH3)CHdCH2 CH2dC(CH3)C(CH3)dCH2
1661 1657 1657
806, 781 787, 774 775
10 10 10
determined in argon matrices. The rate constants in xenon matrices are 2-3 times larger than those in argon matrices. Discussion Conformational Analysis of Propyl Nitrite Radical. As discussed in our previous papers, alkyl nitrite radicals play an important role in alkene-NO2 photoreactions in low-temperature matrices.1-8 The vibrational frequencies of NdO and NsO stretching modes for some alkyl nitrite radicals derived from alkenes and dienes are summarized in Table 2. They decrease systematically as the number of methyl groups increases, except for cis-2-butyl nitrite radical, which has a strong strain due to repulsion between two methyl groups. This trend was also found in some closed-shell nitrite compounds reported by Tarte.16 In the case of propyl nitrite radical, there are two possible isomers: a secondary radical, CH3•CHCH2ONO, and a primary radical, CH3CH(ONO)•CH2. However, we observed only two infrared bands corresponding to the NdO and NsO stretching modes of one isomer, as shown in Figure 3a. Generally speaking, the secondary radical, which has an unpaired electron of •CH(CH3), is expected to be more stable than the primary radical, which has an unpaired electron of •CH2. Steric effects on the bimolecular reaction may also support NO2 approaching more easily to the end carbon (dCH2) than to the center carbon
15818 J. Phys. Chem., Vol. 100, No. 39, 1996
Nakata et al. TABLE 3: Scaled Vibrational Frequencies of Isotope Species for Propyl Nitrite Radicals (cm-1)a secondary radical
primary radical
isotope
NdO stretch
NsO stretch
NdO stretch
NsO stretch
C3H616ON16O C3H616ON18O C3H618ON16O C3H618ON18O
1654 1613 1653 1612
768 765 756 753
1653 1613 1652 1611
757 757 736 736
a Scale factor is tentatively estimated to be 0.94 from ethyl nitrite radical; see the text.
Figure 5. B3-LYP/6-31G*-optimized structures of propyl nitrite radicals. Bond lengths (in angstroms), bond angles (in degrees), and torsion angles (in degrees) are given.
(dCH(CH3)) because of the disturbance due to the methyl group. Therefore, we assume that the secondary radical, CH3•CHCH2ONO, is stable in low-temperature matrices. Ab Initio Calculations. To confirm the above-mentioned assignment, we carried out ab initio calculations with the Gaussian 94 suite of computer programs.18 The hybrid density functional,19 in combination with the Lee, Yang, and Parr correlation functional20 (B3-LYP), was used to optimize the geometries of secondary and primary radicals. This method has been found to be remarkably successful in reproducing molecular structures, vibrational force fields, and frequencies.21-24 The optimized structures are shown in Figure 5. The absolute energies were -322.972 12 and -322.969 42 au for the secondary and primary radicals, respectively. The absolute energies were also calculated by a complete basis set (CBS) extrapolation, which has been successfully applied for the prediction of heats of formation, bond dissociation energies, gas phase acidities, and proton affinities for various chemical species, including radicals.25 The calculated values of CBS-4 energy26 at 0 K were -322.499 13 and -322.495 07 au for the secondary and primary radicals, respectively. Consequently, the former is more stable than the latter with an energy difference of 2.5 kcal/mol, as we expected. The calculated vibrational frequencies of NdO and NsO stretching modes are 1760 and 817 cm-1 for the secondary radical and 1759 and 806 cm-1 for the primary radical. They seem to be a little higher than the observed values. We assumed a tentative scale factor of 0.94, which was derived from the calculated value, 1769 cm-1, and the observed value, 1664 cm-1, of the NdO stretching mode for ethyl nitrite radical.1 The
scaled frequencies are listed in Table 3 together with those for 18O-isotope-substituted species. The scaled frequency of the NdO stretching mode for the secondary radical is 1654 cm-1, which is the same as that for the primary radical. On the other hand, the scaled frequencies of the NsO stretching mode are 768 and 757 cm-1 for the secondary and primary radicals, respectively. The former is closer to the observed value, 770 cm-1, than the latter. We assumed that the propyl nitrite radical observed is the secondary radical. The 18O isotope shift of the NdO stretching mode, shown in Figure 2, can be explained in the case of either the secondary or the primary radical. The observed 1650 cm-1 band is due to •C3H616ON16O and •C3H618ON16O species, and the 1610 cm-1 band is due to •C3H616ON18O and •C3H618ON18O species. On the other hand, the 18O isotope shift of the NsO stretching mode cannot be explained by the primary radical, but can be by the secondary radical. In the case of the primary radical, a doublet with 21 cm-1 isotope shift must be observed as well as the transbutyl nitrite radical case.2 However, we did not observe it in our spectrum. Therefore, we concluded that the nitrite radical is the secondary radical. The peaks of the four isotope species, whose scaled frequencies are 768, 765, 756, and 753 cm-1, probably overlapped each other and one broad peak appeared at 762 cm-1. Selectivity of Final Products. In the bimolecular reaction of NO2 and ethylene,1 both oxirane and acetaldehyde were produced. In the present bimolecular reaction, the final product was only methyloxirane, where hydrogen migration did not occur and no propanal was formed. A schematic reaction diagram is shown in Figure 6, which was modified from the result of ab initio calculations for oxirane biradical reported by Fueno.27 We suppose that the barrier from methyloxirane biradical to propanal is too high to allow the formation of propanal. Contrary to this hydrogen migration, the ring closure occurs with almost no barrier to form methyloxirane. If the matrix sample is irradiated with shorter wavelength laser light, hydrogen migration may occur through high vibrational levels of the 1σσ state of the methyloxirane biradical. Propanal will be also produced through intersystem crossing from 3σπ to 1σσ, if the excitation energy is large enough to pass the barrier. Recently, Laboy and Ault studied the 193 nm excimer laserinduced photooxidation of propene by oxygen in argon matrices.28 They observed propanal as the major product, although the overall yields were low. It is interesting to note that the final products depend on the symmetry of alkene reactants: In the case of symmetric reactants such as ethylene (H2CdCH2), 2-methylpropene ((CH3)2dCH2), and 2,3-dimethyl-2-butene ((CH3)2dC(CH3)2), hydrogen or methyl group migration of the corresponding oxirane biradical occurs to produce the corresponding carbonyl compounds (acetaldehyde,1 2-methylpropanal,4 and 3,3-dimethyl-2-butanone,5 respectively) in addition to the corresponding oxiranes. On the other hand, in the case of asymmetric reactants (such as
Visible Light-Induced Reaction of NO2 with Propene
J. Phys. Chem., Vol. 100, No. 39, 1996 15819
Figure 6. Energy level diagram for the propene and NO2 system. This is a modification of the reaction scheme employed for the ethylene and NO2 system.27
propene, trans-2-butene,2 and cis-2-butene3), no migration occurs and the final products are only the corresponding oxiranes, except for the secondary photolysis of cis-2-butyl nitrite radical which shows abnormal vibrational frequencies for NdO and NsO stretching modes, as shown in Table 2. The electronic characters of σσ and σπ states for the asymmetric oxirane biradicals may be indistinguishable from each other, while they are completely different for the symmetric oxirane biradicals. The mixing of the electric characters may accelerate ring closure to yield oxirane from the both σσ and σπ states, although we have no experimental evidence to explain it at this moment. Matrix Effects on Reaction Rate Constants. We also found that the reaction rate constants in xenon matrices are larger than those in argon matrices. To understand this result, we consider three factors of rare-gas matrices: (1) The cage size of xenon is larger than that of argon, where the lattice constants of xenon and argon are 6.13 and 5.28 Å, respectively.29 In the larger space environment, the photoreaction may occur more easily. (2) Interaction between a bimolecular reaction pair and xenon atoms is larger than in the case of argon, where polarizabilities of xenon and argon are 4.04 × 10-24 and 1.64 × 10-24 cm3, respectively.30 The stronger interaction may accelerate the photoreaction rates. (3) Absorption coefficients of NO2 and propyl nitrite radical may change in xenon and argon matrices. The rate constants, which we determined in the present study, should be proportional to the absorption coefficients. Knudsen and Pimentel studied quantum yields and matrix effects on rotational isomerization of 2,3-difluoropropene.31 They found that the interconversion rates between the cis and gauche forms were faster in Kr than in Ar. Rasanen et al.32 also found the matrix effect on infrared-induced rotational isomerization and explained it in terms of a vibration-rotation relaxation process, which follows the order kXe > kKr > kAr. Our result may suggest that similar processes play an important role in visible light-induced photoreaction between propene and NO2. Acknowledgment. The present work is partly defrayed by a Grant-in-Aid on Priority-Area-Research on “Photoreaction
Dynamics” (No. 07228215) and that for Scientific Research No. 06453018 from the Ministry of Education, Science, Sports, and Culture of Japan. 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) Nakata, M. Spectrochim. Acta 1994, 50A, 1455. (6) Fitzmaurice, D. J.; Frei, H. J. Phys. Chem. 1992, 96, 10308. (7) Fitzmaurice, D. J.; Frei, H. Chem. Phys. Lett. 1992, 192, 166. (8) Blatter, F.; Frei, H. J. Phys. Chem. 1993, 97, 3266. (9) Nakata, M.; Frei, H. J. Am. Chem. Soc. 1992, 114, 1363. (10) Tanaka, N.; Kajii, Y.; Shibuya, K.; Nakata, M. J. Phys. Chem. 1993, 97, 7048. (11) Fitzmaurice, D. J.; Frei, H. J. Phys. Chem. 1991, 95, 2652. (12) Harrison, J. A.; Frei, H. J. Phys. Chem. 1994, 98, 12142. (13) Harrison, J. A.; Frei, H. J. Phys. Chem. 1994, 98, 12152. (14) Tanaka, N.; Oike, J.; Kajii, Y.; Shibuya, K.; Nakata, M. Chem. Phys. Lett. 1995, 232, 109. (15) Tanaka, N.; Oike, J.; Shibuya, K.; Nakata, M. J. Phys. Chem. 1996, 100, 4873. (16) Tarte, P. J. Chem. Phys. 1952, 20, 1570. (17) Kirchner, H. H. Zeit. Phys. Chem. 1963, 39, 273. (18) GAUSSIAN 94 was employed for the calculations. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN94; Gaussian, Inc.: Pittsburgh, PA, 1995. (19) Becke, A. D. J. Chem. Phys., 1993, 98, 5648. (20) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (21) Stephens, P. J.; Devkin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (22) Rauhut, G.; Pulay, P. J. J. Phys. Chem. 1995, 99, 3093. (23) Ricca, A.; Bauschlicher, C. W. Theor. Chim. Acta 1995, 92, 123. (24) Tsai, H.-H.; Hamilton, T. P.; Tsai, J.-H. M.; Harrison, J. G.; Beckman, J. S. J. Phys. Chem. 1996, 100, 6942.
15820 J. Phys. Chem., Vol. 100, No. 39, 1996 (25) Ochterski, J. W.; Petersson, G. A.; Wiberg, K. B. J. Am. Chem. Soc. 1995, 117, 11299. (26) Ochterski, J. W.; Petersson, G. A.; Montgomery, J. A. J. Chem. Phys. 1996, 104, 2598. (27) Fueno, T.; Takahara, Y.; Yamaguchi, K. Chem. Phys. Lett. 1990, 167, 291. (28) Laboy, J.; Ault, B. S. J. Photochem. Photobiol. A: Chem. 1991, 59, 19.
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