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 noniterative 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 energy for the betaine analog with a blocked canonical structure (N,N-dimethylglycine methyl ester) to be less than 100 cm−1. This significant difference in electron binding energies suggests that the gas phase instability of zwitterions of some common naturally occurring amino acids with respect to their canonical forms may be reversed by the excess electron attachment. The calculated Franck–Condon factors for the betaine’s anion/neutral pair suggest that the photoelectron spectrum of the anion will display only a very weak vibrational structure.