INCORPORATION OF HYDRATION EFFECTS WITHIN THE SEMIEMPIRICAL MOLECULAR-ORBITAL FRAMEWORK-AM1 AND MNDO RESULTS FOR NEUTRAL MOLECULES, CATIONS, ANIONS, AND REACTING SYSTEMS

A recently developed continuum model for the incorporation of hydration effects into the AM1 and MNDO semiempirical molecular orbital Hamiltonians is evaluated by comparison with representative experimental data. The new method gives quantitatively correct predictions for the aqueous-phase conformational behavior of H3N+CH2CH2CO2- and the aqueous-phase activation energetics of the S(N)2 reaction of Cl- and CH3Cl. The dissociation energetics of t-BuCl are correctly predicted, while the computed dissociation pathway includes a shallow energetic minimum corresponding to the expected tight ion pair. The absolute hydration energies of neutral molecules and cationic and anionic species are generally predicted satisfactorily. Systematic deviations from the experimental data due to the intervention of hydrophobic effects (not currently allowed for) as well as those traceable to the simplified integral treatment are noted. Contrary to intuition, continuum methods apparently can give quantitatively reliable hydration energies for species in which specific hydrogen-bonding effects are generally considered to be important. This was true for protonated amines, although only partially so for protonated oxygen bases.