Ab initio studies of the protonation of CO, N2 and NO+: Calculation of the minimum energy reaction paths

Dissociative Protonation Sites: Reactive Centers in Protonated Molecules Leading to Fragmentation in Mass Spectrometry

It is often found in mass spectrometry that when a molecule is protonated at the thermodynamically most favorable site, no fragmentation occurs, but a major reaction is observed when the proton migrates to a different position. For benzophenones, acetophenones, and dibenzyl ether, which are all preferentially protonated at the oxygen, deacylation or dealkylation was observed in the collision-induced dissociation of the protonated molecules. For para-monosubstituted benzophenones, electron-withdrawing substituents favor the formation of RC6H4CO+ (R = substituent), whereas electron-releasing groups favor the competing reaction leading to C6H5CO+. The ln[(RC6H4CO+)/(C6H5CO+)] values are well-correlated with the sigmap+ substituent constants. In the fragmentation of protonated acetophenones, deacetylation proceeds to give an intermediate proton-bound dimeric complex of ketene and benzene. The distribution of the product ions was found to depend on the proton affinities of ketene and substituted benzenes, and the kinetic method was applied in identifying the reaction intermediate. Protonated dibenzyl ether loses formaldehyde upon dealkylation, via an ion-neutral complex of the benzyloxymethyl cation and neutral benzene. These gas-phase retro-Friedel-Crafts reactions occurred as a result of the attack of the proton at the carbon atom to which the carbonyl or the methylene group is attached on the aromatic ring, which is described as the dissociative protonation site.

A high-precision ab initio determination of the equilibrium geometry and force field of HOC+

Ab initio molecular orbital theory is used to determine if the molecular ion HOC+ is linear (as are the isoelectronic species HCN, HNC, HCO+, etc.), or if it is quasilinear. Near the Hartree-Fock limit the molecule is either linear, or very close to linear. Electron correlation favors the linear geometry, leading to the unequivocal prediction of a linear molecule. Detailed comparisons between HOC+ and isoelectronic HNC show an apparent lack of convergence in the bending potential for the former which is remedied by the addition of f functions to the basis set. The HOC+ potential energy surface is computed in the bending and bend-stretch coordinates and fit to an analytical function. Use of this function to compute the rotation-vibration energies results in improved agreement with experiment relative to previous potentials by nearly two orders of magnitude, as documented in the accompanying paper.