An ab initio study of the betaine anion-dipole-bound anionic state of a model zwitterion system

The electron binding to a model zwitterionic molecule, betaine, is studied at the coupled cluster level of theory with single, double, and noniterativ e triple excitations as well as at the density functional theory level. Our results indicate that there is only one stable conformer of betaine with a dipole moment of 11.5 D. This dipole moment supports a relatively strongly bound anionic state and the vertical electron detachment energy was found to be 2261 cm(-1). Furthermore, we predict the excess electron binding ener gy for the betaine analog with a blocked canonical structure (N,N-dimethylg lycine methyl ester) to be less than 100 cm(-1). This significant differenc e in electron binding energies suggests that the gas phase instability of z witterions of some common naturally occurring amino acids with respect to t heir canonical forms may be reversed by the excess electron attachment. The calculated Franck-Condon factors for the betaine's anion/neutral pair sugg est that the photoelectron spectrum of the anion will display only a very w eak vibrational structure. (C) 2001 American Institute of Physics.