Ab initio and DFT study of the molecular conformations and the thermochemistry of the CH 2[dbnd]CHC(O)OONO 2 (APAN) atmospheric molecule and of the CH 2[dbnd]CHC(O)OO and CH 2[dbnd]CHC(O)O radicals

Abstract

The molecular structures and conformational mobilities of the atmospheric peroxyacryloyl nitrate, CH 2 CHC(O)OONO 2 (APAN) molecule and its radical decomposition products CH 2 CHC(O)OO and CH 2 CHC(O)O were studied by ab initio and density functional methods. The potential energy curves for the internal rotations were calculated using the B3LYP hybrid functional with the 6-311++G(d,p) basis set. The equilibrium conformation of APAN, determined at B3LYP/6-311++G(3df,3pd) level, is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in almost perpendicular planes, τ(COON) = 86.4°. A quantum statistical analysis showed that only a 7% of the internal rotors of the above molecules can freely rotate at room temperature. From isodesmic energies calculated at the G3MP2//B3LYP/6-311++G(3df,3pd) level of theory, standard enthalpies of formation at 298 K for APAN, CH 2 CHC(O)OO and CH 2 CHC(O)O are predicted to be −34.8, −8.9, and −16.0 kcal mol −1. The resulting O N and O O bond dissociation enthalpies for the channels CH 2 CHC(O)OONO 2 → CH 2 CHC(O)OO + NO 2 and CH 2 CHC(O)OONO 2 → CH 2 CHC(O)O + NO 3 of 34.1 and 36.4 kcal mol −1 are significant larger than those reported for similar processes of the atmospherically relevant PAN species.