Important role of the Beckmann rearrangement in the gas phase chemistry of protonated formaldehyde oximes and their [CH4NO]+ isomers

Abstract

The [CH₄NO]⁺ potential energy surface has been (re)examined in detail using ab initio molecular orbital calculations. Geometries of the stationary points were optimized at the MP2/6-31 G(d,p) level. On the basis of MP4SDTQ/6-311 ++ G(2d,2p) electronic energies with zero-point corrections, the following heats of formation at 0 K could be proposed: Δ_fH^o₀ in kJ mol^–1: H₂CNOH: 31 ± 12; H₃C–NOH⁺: 903 ± 12; H₂CNH–OH⁺: 759 ± 12; H₂CN–OH₂⁺: 838 ± 12; H₃C–NHO⁺: 840 ± 12 and cyclic H₂[graphic omitted]H⁺: 918 ± 12. The proton affinities could also be evaluated: E_PA(H₂CNOH)= 799 ± 12 kJ mol^–1 and E_PA(CH₃–NO)= 763 ± 12 kJ mol^–1. Energies of the transition structures of several unimolecular rearrangements and fragmentations obtained using MP4SDTQ/6-311 ++ G(d,p)+ ZPE calculations suggest that the following sequence of transformations is the most energetically favoured route: protonation of formaldehyde oxime →N-protonated oxime →O-protonated oxime → fragmentation products (HCN + H₃O⁺). The 1,2-H-shift connecting both protonated forms constitutes the rate-controlling step. The classical Beckmann rearrangement of the O-protonated formaldehyde oxime is the most facile reaction of all the paths considered and should thus play an important role in the gas phase unimolecular chemistry of the [CH₄NO]⁺ ion isomers. The CH₄+ NO⁺ reaction has also been examined as a simple model for the electrophilic substitution of aliphatic hydrocarbons. While the insertion of NO⁺ into a C–H bond can be established, the evidence recently reported for preferential attack of NO⁺ on the carbon atom could not be confirmed.