Proton affinity of peroxyacetyl nitrate. A computational study of topical proton affinities.
The structure and energetics of the peroxyacetyl nitrate conformers syn- and anti-PAN and several cations formed by PAN protonation were investigated by a combination of density functional theory and ab initio calculations. syn-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 basic site in PAN, the oxygen atoms of the peroxide and NO(2) groups being less basic. 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(-1) lower than the previous estimate by Srinivasan et al. (Rapid Commun. Mass Spectrom. 1998; 12: 328) that was based on competitive dissociations of proton-bound dimers (the kinetic method). The calculated threshold dissociation energies predicted the formation of CH(3)CO(+) + syn - HOONO(2) and CH(3)COOOH + NO(2)(+) to be the most favorable fragmentations of protonated PAN that required 83 and 89 kJ mol(-1) at the respective thermochemical thresholds at 298 K. The previously observed dissociation to CH(3)COOH + NO(3)(+) was calculated by effective QCISD(T)/6-311 + G(3df,2p) to require 320 kJ mol(-1). The disagreement between the experimental data and calculated energetics is discussed.
Turecek
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Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, Washington 98195-1700, USA. turecek@chem.washington.edu
D000393:Air Pollutants / Q000737:chemistry* / Q000378:metabolism; D002412:Cations; D007536:Isomerism; D013058:Mass Spectrometry; D008968:Molecular Conformation; D015394:Molecular Structure; D010463:Peracetic Acid / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D011522:Protons; D012905:Smog; D013816:Thermodynamics
D000393:Air Pollutants; D002412:Cations; D011522:Protons; D012905:Smog; D010463:Peracetic Acid; C004185:peroxyacetyl nitrate