Proton affinity of peroxyacetyl nitrate. A computational study of topical proton affinities

The structure and energetics of the peroxyacetyl nitrate conformers syn- an d anti-PAN and several cations formed by PAN protonation were investigated by a combination of density functional theory and ab initio calculations, s yn-PAN is the more stable conformer that is predicted to predominate in gas -phase equilibria, The acetyl carbonyl oxygen was found to be the most basi c site in PAN, the oxygen atoms of the peroxide and NO2 groups being less b asic. The 298 K proton affinity of syn-PAN was calculated as 759-763 kJ mol (-1) by effective QCISD(T)/6-311+G(3df,2p) and 771-773 kJ mol(-1) by B3-MP2 /6-311+G(3df,2p). The calculated values are 25-39 kJ mol-l lower than the p revious estimate by Srinivasan et al, (Rapid Commun. Mass Spectrom. 1998; 1 2: 328) that was based on competitive dissociations of proton-bound dimers (the kinetic method). The calculated threshold dissociation energies predic ted the formation of CH3CO+ + syn -HOONO2 and CH3COOOH + NO2+ to be the mos t favorable fragmentations of protonated PAN that required 83 and 89 kJ mol (-1) at the respective thermochemical thresholds at 298 K. The previously o bserved dissociation to CH3COOH + NO3+ was calculated by effective QCISD(T) /6-311+G(3df,2p) to require 320 kJ mol(-1). The disagreement between the ex perimental data and calculated energetics is discussed. Copyright (C) 2000 John Wiley & Sons, Ltd.