Higher Level Theoretical Binding Energies of Methyldiazonium Ion. Is an Experimental Reinvestigation Warranted?

Several experimental values were reported for the binding energy of methyldiazonium ion, and they differ by 13 kcal/mol. Previous theoretical investigations also were associated with considerable uncertainty. The methyl cation affinity (MCA) of N ₂ plays a crucial role as the anchor point for the absolute gas phase methyl cation affinity scale, and an accurate value is thus especially pertinent. To resolve this longstanding controversy, the reaction energy for the process CH ₃ ⁺ + N ₂ → CH ₃ N ₂ ⁺ has been determined using full fourth-order Møller-Plesset perturbation theory, CI theory (CID, CISD), quadratic CI theory (QCISD, QCISD(T)), Gaussian-1 (G1) and Gaussian-2 (G2) theory, and coupled cluster methods (CCD, CCSD, ST4CCD, CCSD(T)) with large basis sets. The proton affinity (PA) of diazomethane, CH ₂ N ₂ + H ⁺ → CH ₃ N ₂ ⁺ , also was determined at these theoretical levels. The best calculations all point to an MCA(N ₂ ) of 44.1 kcal/mol and to a PA(CH ₂ N ₂ ) = 211.4 kcal/mol. While the PA(CH ₂ N ₂ ) value falls within the experimental range, the calculated binding energy of methyldiazonium ion is more than 4.2 kcal/mol lower than the latest experimental value and 5.3 kcal/mol above the earliest experimental value. These results strongly reaffirm that an experimental reevaluation of the methyl cation affinity is warranted.