Isotope labeling and infrared multiple-photon photodissociation investigation of product ions generated by dissociation of [ZnNO 3 (CH 3 OH) 2 ] + : Conversion of methanol to formaldehyde

Electrospray ionization was used to generate species such as [ZnNO₃(CH₃OH)₂]⁺ from Zn(NO₃)₂•XH₂O dissolved in a mixture of CH₃OH and H₂O. Collision-induced dissociation of [ZnNO₃(CH₃OH)₂]⁺ causes elimination of CH₃OH to form [ZnNO₃(CH₃OH)]⁺. Subsequent collision-induced dissociation of [ZnNO₃(CH₃OH)]⁺ causes elimination of 47 mass units (u), consistent with ejection of HNO₂. The neutral loss shifts to 48 u for collision-induced dissociation of [ZnNO₃(CD₃OH)]⁺, demonstrating the ejection of HNO₂ involves intra-complex transfer of H from the methyl group methanol ligand. Subsequent collision-induced dissociation causes the elimination of 30 u (32 u for the complex with CD₃OH), suggesting the elimination of formaldehyde (CH₂ = O). The product ion is [ZnOH]⁺. Collision-induced dissociation of a precursor complex created using CH₃-¹⁸OH shows the isotope label is retained in CH₂ = O. Density functional theory calculations suggested that the “rearranged” product, ZnOH with bound HNO₂ and formaldehyde is significantly lower in energy than ZnNO₃ with bound methanol. We therefore used infrared multiple-photon photodissociation spectroscopy to determine the structures of both [ZnNO₃(CH₃OH)₂]⁺ and [ZnNO₃(CH₃OH)]⁺. The infrared spectra clearly show that both ions contain intact nitrate and methanol ligands, which suggests that rearrangement occurs during collision-induced dissociation of [ZnNO₃(CH₃OH)]⁺. Based on the density functional theory calculations, we propose that transfer of H, from the methyl group of the CH₃OH ligand to nitrate, occurs in concert with the formation of a Zn–C bond. After dissociation to release HNO₂, the product rearranges with the insertion of the remaining O atom into the Zn–C bond. Subsequent C–O bond cleavage, with H transfer, produces an ion–molecule complex composed of [ZnOH]⁺ and O = CH₂.