Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization-reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at t he Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the mo st 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 CH3O(H)-NO2+ ion (2(+)). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinit ies were 725, 722, and 712 kJ mol(-1) for the formation of the syn-syn, ant i-syn, and syn-anti ion rotamers 3a(+), 3b(+), and 3c(+), respectively. Ion 2(+) was prepared by an ion-molecule reaction of NO2+ with methanol and us ed to generate the transient CH3O(H)-NO2. radical (2) by femtosecond collis ional electron transfer. Exothermic protonation of 1 produced a mixture of 3a(+)-3c(+) with 2(+) that was used to generate transient radicals 3a-3c. R adical 2 was found to be unbound and dissociated without barrier to methano l and NO2. Radicals 3a-3c were calculated to be weakly bound. When formed b y vertical neutralization, 3a-3c dissociated completely on the 4.2 mu s tim e scale of the experiment. The main dissociations of 3a-3c were formations of CH3O. + HONO and CH3ONO + 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 n eutralization and reionization. The adiabatic ionization energies of 3a-3c were calculated as 7.54, 7.57, and 7.66 eV, respectively. (C) 2000 American Society for Mass Spectrometry.