Solvation Effects on Zwitterion Formation

Several methods for determining solvation free energies are compared for the formation of a zwitterionic intermediate between methylamine and formaldehyde. While in the gas phase no minimum exists for C−N separations less than that found in the van der Waals complex, all methods examined here for determining the solvation free energy predict a stable zwitterionic minimum. The most stable minimum exists using the polarized continuum model (PCM). The finite difference Poisson−Boltzmann method yields comparable results when parameters from a solvent interaction potential are used. A stable zwitterion was also found when two explicit molecules of water were included.