The gas-phase ion chemistry of methyl and ethyl borates

The fragmentation of alkyl borates appears to be controlled by the ability of the oxygen to stabilize the positive charge of the fragment ions. Medium-resolution mass spectrometry suggests structural assignments which are consistent with this assumption. For example, the ions of m/z 59 and m/z 43 for methyl borate, are assigned to be CH₃OBOH⁺ and CH₃OBH⁺, respectively, and not BO⁺₃ and BO⁺₂ as previously reported. Several additional experimental procedures were used to further verify the structural assignments. Collision-induced dissociation was utilized to investigate the fragmentation pathways. Ion/molecule reactions indicated that many of these fragment ions contain a -BO⁺ unit which is susceptible to nucleophilic attack. Proton affinity bracketing methods were used to obtain thermodynamic values for doubly bonded boron molecules. For CH₃OBO, a proton affinity of 183 ± 3 kcal mol^-1 and a heat of formation of -110 ± 26 kcal mol^-1 were determined. For CH₃ -BCH₂, a previously reported proton affinity value of 220.5 ± 2 kcal mol^-1 suggests a heat of formation of 31 ± 6 kcal mol^-1. These values, as well as bond strengths, are compared with similar carbon compounds.