ARTICLE IN PRESS FOCUS: MCLAFFERTY REARRANGEMENT Photoionization of 2,3-Dimethyl-2-Butanol (Thexyl Alcohol): Interaction Between the Charged and Expelled Fragments

Photoionization studies of (CH3)2CHC(CH3)2OH (tert-hexyl alcohol, also called thexyl alcohol) exhibit four fragmentations below 10 eV. As with other tertiary alcohols, no molecular ion is detected. The only ion observed at threshold corresponds to propane loss. Examination of a deuterated analogue, (CH3)2CHC(CD3)2OH, shows only loss of C3H7D, implying that the fragment ion has the structure of ionized acetone enol. There is no evidence for reversible deuterium transposition, as has been reported for isotopomers of the homologous secondary alcohol (CH3)2CHCH(CH3)OH. Propane loss from thexyl alcohol is attributed to intermediacy of ion–neutral complexes containing isopropyl radical and O-protonated acetone. Simple cleavage to give O-protonated acetone has an appearance energy 18 kJ mol 1 higher than that of propane loss. Thermochemical estimates and ab initio calculations both predict that methyl loss should have a lower threshold than the fragmentation leading to isopropyl loss, but experiments show the appearance energy to be 6 kJ mol 1 higher. This is consistent with previous reports of reverse activation barriers for methyl cleavages. Finally, formation of tert-hexyl cation, (CH3)2CHC(CH3)2 , is observed with an appearance energy comparable to that of methyl loss, substantially below that predicted for OH radical expulsion from the molecular ion. The comparatively low threshold of this fragmentation is ascribed to ion-pair formation (concomitant with hydroxide ion) directly from an electronically excited neutral. Interactions between charged and neutral fragments (including energetics, bond orders 1, and electrical charges on molecular fragments) are explored using a combination of DFT and ab initio methods, along with topological analysis using the Atoms in Molecules approach. (J Am Soc Mass Spectrom 2004, 15, 989–997) © 2004 American Society for Mass Spectrometry