The protonation of N2O reexamined - A case study on the reliability of various electron correlation methods for minima and transition states

摘要: The protonation of N2O and the intramolecular proton transfer in N2OH+ have been studied using large basis sets conjunction with second‐order many‐body perturbation theory (MP2), singles doubles coupled cluster (CCSD), augmented method [CCSD(T)], complete active space self‐consistent field (CASSCF) methods. It is shown that MP2 inadequate even for HNNO+, which has a minor nondynamical correlation effect; transition state only CCSD(T) produces reliable geometry due to serious effects. Harmonic frequencies accurate 50 cm−1 or better are predicted both protonated species. affinity at 298.15 K found be 137.6 kcal/mol, excellent agreement recent experimental redetermination 137.3±1 kcal/mol; HNNO+ isomer 4.4 kcal/mol above HONN+ isomer, an interconversion barrier ∼89 herewith confirming evidence species occur together energy difference 6±1.5 kcal/mol. Comparison traditional double‐zeta plus polarization (DZP) newer consistent polarized valence double zeta (cc‐pVDZ) set appears indicate latter might lead more geometries harmonic frequencies, although detailed investigation would needed before any definitive conclusions.