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h From Atkinson (1994) and Atkinson et al. (1997a).

'Recommended temperature dependence is k = 4.88 X 10" ,lij2e-22li2/' cm1 molecule"' s"' from 290 to 523 K.

'' Dlugokencky and Howard (1989) report a curved Arrhenius plot that can be matched from 204 to 378 K by k = (1.78 + 0.36) x 10"12 exp[(-530 + 100)/T] + (1.28 + 0.26) x 10"14 exp[(570 + 110)/71; Atkinson (1997a) recommends k = 1.22 X 10"1 xe3s2/cm3 molecule"1 s" 1 from 204 to 378 K.

' From Chew et al. (1998). ' From Martinez et al. (1998).

h From Atkinson (1994) and Atkinson et al. (1997a).

'Recommended temperature dependence is k = 4.88 X 10" ,lij2e-22li2/' cm1 molecule"' s"' from 290 to 523 K.

'' Dlugokencky and Howard (1989) report a curved Arrhenius plot that can be matched from 204 to 378 K by k = (1.78 + 0.36) x 10"12 exp[(-530 + 100)/T] + (1.28 + 0.26) x 10"14 exp[(570 + 110)/71; Atkinson (1997a) recommends k = 1.22 X 10"1 xe3s2/cm3 molecule"1 s" 1 from 204 to 378 K.

' From Chew et al. (1998). ' From Martinez et al. (1998).

that N03 undergoes electrophilic addition to the double bond in much the same fashion as OH:

ono2

However, there is no pressure dependence of the rate constants over the range from about f Torr up to 1 atm, suggesting that the adduct does not decompose significantly back to reactants under atmospheric conditions.

This mechanism is consistent with the observation of significant yields of epoxide products and N02 for some alkenes (Olzmann et al., 1994). For example, Fig. 6.7 shows the infrared spectrum of the minor products from the reaction of N03 with 2,3-dimethyl-2-butene at

740 Torr in air, after the spectra of the reactants and the major product, acetone, as well as of N02 and HN03 have been subtracted out. Bands due to the epoxide (in ~ 17% yield) are clearly seen, as well as bands due to 2,3-dinitroxy-2,3-dimethyl-2-butane. Together with acetone, these compounds account for 75% of the reacted alkene (Skov et al., 1994).

Figure 6.8 summarizes the mechanism of the reaction of N03 with 2,3-dimethyl-2-butene (Skov et al., 1994; Olzmann et al., 1994). The addition of the N03 to a double bond is about 20 kcal mol~' exothermic, so that the initially formed adduct is excited and either can be stabilized or decompose by breaking the weaker C0-N02 bond to form the epoxide and N02. The stabilized adduct can also decompose in a similar manner, in competition with its reaction with 02. Under ra a> cc

700 1000 1300 1600 Wavenumber (cm"1)

FIGURE 6.7 Infrared spectrum of products of the reaction of 2,3-dimethyl-2-butene with NO, (spectra of acetone, N02, and HNO, have been subtracted out) (adapted from Skov et al., 1994).

700 1000 1300 1600 Wavenumber (cm"1)

FIGURE 6.7 Infrared spectrum of products of the reaction of 2,3-dimethyl-2-butene with NO, (spectra of acetone, N02, and HNO, have been subtracted out) (adapted from Skov et al., 1994).

surface-level tropospheric conditions, 1 atm total pressure in air, the epoxide yields are small but not insignificant. For example, epoxide yields of 7, 12, f 8, and 28% from the reactions of isobutene, trans-2-butene, f-butene, and propene, respectively, have been reported (Berndt and Boge, 1995).

The lifetime of the excited N03-alkene adducts is sufficiently long that rotation about the C-C bond leads to the same yields of trans- and c/s-epoxides regardless of the configuration of the reactant alkene; for example, the reactions of both cis- and trans-2-butene give about 80% of the trans form of the product epoxide and 20% of the cis form (Benter et al., 1994).

The peroxy radical formed in Fig. 6.8 is expected to react in air as discussed earlier, i.e., with NO, H02, R02, or N02. Since N03 itself reacts rapidly with NO, large concentrations of NO will not be present at the

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