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

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