Temperature Dependence of the Isobutane Chemical Ionization Mass Spectra of Open-chain and Cyclic Alcohols; Structural,Stereochemical and Molecular-Size Effects. A reevaluation of the isobutane chemical ionization of alcohols

The temperature dependence of the isobutane chemical ionization (CI.) mass spectra of 54 open-chain, cyclic and unsaturated C₅- to C₁₀-alcohols was studied at temperatures ranging from 60 to 250°, and enthalpy changes were calculated for the corresponding main reactions of typical alcohols. The CI. reactivity is controlled by the temperature and the substrate structure as usual, and in addition, by the molecular size. The combination of thermal, structural and substrate-size effects leads to the following main conclusions. At low-reactivity conditions, i.e. at 150° or less, the alcohols with less than 11 C-atoms give four distinct types of spectra, with (M – OH)⁺ usually as the base peak. The characteristic ions are MC₄H₉⁺ and (M – H)⁺ for primary, MH⁺ and (MC₄H₉ – H₂O)⁺ for secondary, (MC₄H₉ – H₂O)⁺ for tertiary and allyl-type alcohols. Configurational assignments of stereoisomeric alcohols are also possible, by means of steric compression and shielding effects. The MH⁺/(M – OH)⁺ ratio in the spectra of epimeric methylcyclohexanols is at least 3 to 4 times higher for the isomers with mainly axial OH-group conformation compared to the equatorial isomers. Stereospecific (M - H)⁺ ions are apparently formed from trans-2-methylcyclopentanol and endo-norbornan-2-ol by a favorable abstraction of the unshielded H(α)-atoms versus normal behavior of the other epimers. While the spectra recorded at 200° show almost exclusively (M – OH)⁺ ions, those at 250° give nevertheless some C-skeleton information through the temperature dependent decomposition of the (M – OH)⁺ ions.