Nitromethane photolysis

Environmental Concerns. Few data on the environmental effects of the nitroparaffins are available. However, they are known to be of low toxicity to the fathead minnow (109). Based on their uv spectra, the nitroparaffins would be expected to undergo photolysis in the atmosphere. The estimated half-life of 2-nitropropane in the atmosphere is 3.36 h (110). Various values have been determined for the half-life of nitromethane, but it is similar to 2-nitropropane in persistence (111). Reviews of the available data on the environmental effects of nitromethane and 2-nitropropane have been pubhshed by the U.S. Environmental Protection Agency (112,113). 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

In 1958, Dalby114 measured the absorption spectra of HNO and DNO from 6500 to 7500 A. He proposed the transition 1A <- 1A performed a rotational analysis, and obtained the bond angles and lengths for both spectroscopic states. He produced HNO from the flash photolysis of nitromethane, nitroethane, isoamyl nitrite, or mixtures of ammonia and nitric oxide. 

Lenhardt et al. [30] conducted the first direct study of the reaction of butyl radicals with 02, reporting room-temperature rate coefficients for n-butyl, s-butyl, t-butyl, and 3-hydroxy s-butyl, where the radicals were prepared by broadband flash photolysis of the iodides. The bimolecular rate coefficients were independent of pressure over the range 1 to 4 torr, showing that these association reactions are in the high-pressure limit. The rate coefficients increased in the order n-butyl < s-butyl < f-butyl < 5-hydroxy s-butyl. On the other hand, the CH3 + 02 association has been shown to be well into the unimolecular falloff at pressures from 0.5 to 6 torr at room temperature [62]. Falloff behavior is not unexpected for the smaller CH3 radical, in contrast with C4H9 radicals. Methyl was generated by 193-nm photolysis of nitromethane. 

On photolysis, nitromethane yielded NO2 and methyl (Bielski and Timmons, 1964) as in its mass spectrum and, unlike aromatic nitro compounds, showed no significant loss of NO (Aplin et al., 1965). The mass spectra of many aromatic nitro compounds show evidence for... 

Early work on the photolysis of nitromethane showed that the major products are CH2O, Nj, NO, HjO and CO. It was suggested that the primary process produced isomerization followed by decomposition, viz. 

Nicholson (1961) concluded from his analysis of products of full arc photolysis of nitromethane that process (I) was the dominant primary photolysis mode. Also Rebbert... 

Greenblatt et al. (1987) observed the formation of OH (X H) radicals in the low-pressure gas-phase, 282 nm photolysis of nitroethane, 1- and 2-nitropropane, and 2-nitro-2-methylpropane. They did not detect OH in similar experiments with nitromethane. The mechanism is suggested to involve a 5- or 6-membered ring intermediate that leads to the OH product ... 

Schoen, RE., M.J. Marrone, J.M. Schnur, and L.S. Goldberg (1982), Picosecond UV photolysis and laser-induced fluorescence probing of gas-phase nitromethane, Chem. Phys. Lett., 90, 272-276. 

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