Protonation sites in methyl nitrate and the formation of transient CH 4NO 3 radicals. a neutralization–reionization mass spectrometric and computational study

Abstract

Protonation sites in methyl nitrate ( 1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728–738 kJ mol −1 depending on the optimization method used to calculate the equilibrium geometry of the CH 3O(H)–NO 2 + ion ( 2 +). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol −1 for the formation of the syn–syn, anti–syn, and syn–anti ion rotamers 3a +, 3b +, and 3c +, respectively. Ion 2 + was prepared by an ion–molecule reaction of NO 2 + with methanol and used to generate the transient CH 3O(H)–NO 2 · radical ( 2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a +– 3c + with 2 + that was used to generate transient radicals 3a– 3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO 2. Radicals 3a– 3c were calculated to be weakly bound. When formed by vertical neutralization, 3a– 3c dissociated completely on the 4.2 μs time scale of the experiment. The main dissociations of 3a– 3c were formations of CH 3O · + HONO and CH 3ONO + OH ·. The gas-phase chemistry of radicals 3a– 3c and their dissociation products, as studied by neutralization–reionization mass spectrometry, was dominated by Franck–Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a– 3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.