ISOMERIZATION AND ISOTOPE EFFECTS IN STERICALLY CONGESTED CLUSTER IONS

The association reactions of methylated acetone with acetone and dimet hyl ether were studied over an extended temperature range (30-250-degr ees-C) by high-pressure mass spectrometry. In addition, the associatio n reactions of various deuterium-substituted analogues of both the rea ctant ion and neutral molecules were investigated. Two distinct isomer ic forms are apparent from the examination of the behaviour of the ass ociation equilibrium constant as a function of reaction temperature: ( i) a low-temperature, covalently bound isomer characterized by larger -DELTAH-degrees and -DELTAS-degrees values and significant equilibrium isotope effects on the entropy of association; and (ii) a high-temper ature, electrostatically bound isomer of lower -DELTAH-degrees and -DE LTAS-degrees values and with insignificant equilibrium isotope effects . The equilibrium isotope effects and larger -DELTAS-degrees values ar e more pronounced for the methylated acetone-dimethyl ether adduct tha n for the less sterically congested methylated acetone-acetone adduct. Molecular models of the low-temperature isomers show severe restricti on or complete loss of internal rotations and is the origin of the obs erved equilibrium isotope effects. No such phenomenon is expected or o bserved in the more loosely bound high-temperature, electrostatic isom ers. Potential energy surfaces for the transfer of methyl cation from methylated acetone to a second acetone molecule and to dimethyl ether are proposed.