| model molecular system: methylamine + proton + methylamine (MPM), in which two methylamines are fixed at predetermined positions, and only the proton is allowed to move, in the presence of electric external fields, has been extensively explored in our work. The potential energy surfaces (PES) under different fields in each nuclear configuration are set up by extensive ab initio calculation using the Gaussian 94 package. Then taking this surface as background, the properties of the 3D molecular system have been solved by using a 3D Fourier Grid Hamiltonian (FGH) formalism.;It is a fairly realistic model of systems which are of importance (e.g., amino acids), and small enough to calculate in a reasonable time period with our computer. The proton is transferred, with the aid of an external electric field, from a potential well approximately 1A from one methylamine nitrogen to a well neighboring the other when the two nitrogens are constrained to remain either 3.2A or 3.6A apart. When the methylamines are allowed to optimize without constraint they form a potential surface for the proton such that the proton is shared between the two methylamines, so that transfer is not a relevant possibility. Under conditions in which the methylamines are constrained to be further apart than optimal, two wells are formed which localize the proton in one or the other. We have found that a field shift of less than... |
