Rearrangements of Transient Neutral Molecules in the Gas Phase. Does the Conversion of CCCHO to HCCCO Involve Oxygen or Hydrogen Migration?

Abstract

Stable (CC13CHO)− may be formed in the chemical ionisation ion source of a VG ZAB 2HF mass spectrometer by the SN2(Si) reaction between Me3SiC≡C13CHO and F−. Vertical (Franck–Condon) one-electron oxidation of (CC13CHO)− in the first of the tandem collision cells of a VG ZAB 2HF mass spectrometer gives CC13CHO. Some of these neutrals have sufficient excess energy to effect rearrangement to HCC13CO, some of which are energised and decompose to HCC• and 13CO. Thus, the neutral rearrangement exclusively involves H migration: no products from O migration are detected. The corresponding two-electron oxidation of (CC13CHO)− gives mainly unrearranged (CC13CHO)+. A minority of these cations are energised and rearrange by H and O migration to yield (HCC13CO)+ and (OCC13CH)+, respectively. All experimental observations are backed up by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory.