Ab initio and DFT study of the molecular conformations and the thermochemistry of the CH2CHC(O)OONO2 (APAN) atmospheric molecule and of the CH2CHC(O)OO and CH2CHC(O)O radicals

The molecular structures and conformational mobilities of the atmospheric peroxyacryloyl nitrate, CH2CHC(O)OONO2 (APAN) molecule and its radical decomposition products CH2CHC(O)OO and CH2CHC(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, CH2CHC(O)OO and CH2CHC(O)O are predicted to be −34.8, −8.9, and −16.0 kcal mol−1. The resulting ON and OO bond dissociation enthalpies for the channels CH2CHC(O)OONO2 → CH2CHC(O)OO + NO2 and CH2CHC(O)OONO2 → CH2CHC(O)O + NO3 of 34.1 and 36.4 kcal mol−1 are significant larger than those reported for similar processes of the atmospherically relevant PAN species.