Intermolecular forces in organic clusters

Abstract

The effects of alkyl groups on the binding energies of cluster ions were investigated. The data were obtained by variable-temperature pulsed high-pressure mass spectrometric equilibrium measurements on protonated clusters A{sub n}H{sup +} (A = MeOH, n-PrOH, n-AmOH, MeCOOH, MeNH{sub 2}, n-PrNH{sub 2}, and Me{sub 2}NH). Inner shell binding energies are not affected by alkylation, suggesting pure hydrogen-bonding interactions with the core ions. Shell-filling effects are also not affected. Thus, going to the second shell causes an equal drop of 17 {plus_minus} 4 kJ/mol (4 {plus_minus} 1 kcal/mol) in all the observed clusters, despite the variation in the number of hydrogen-bonding sites and charge densities at the core ions. This suggests opposing effects of intramolecular and intermolecular solvation at the shell-filling step. However, the alkyl groups do affect the outer shells. The bonding energies in the outer shells of the alcohol clusters approach the respective bulk condensation energies within {plus_minus} 6 kJ/mol ({plus_minus}1.5 kcal/mol.) This suggests liquidlike hydrophobic alkyl-alkyl interactions in the outer shells which may contribute 12-25 kJ/mol (3-6 kcal/mol) to the bonding energies. 70 refs., 12 figs., 2 tabs.