Electric field effects on the geometry and vibrations of charged molecules: the hydroxide ion case

We demonstrate how different possible geometry optimization constraints influence the computed OH− geometry and stretching vibrational frequency variation with electric field strength and how these constraints simulate various OH− bonding environments found in solid hydroxide compounds. The unbound OH− is best modeled using the center of coordinate optimization constraint which downshifts the OH− stretching frequency with increasing electric field strengths. The fixed O atom optimization constraint causes upshifts of the OH− stretching frequency with increasing electric field strength and simulates the metal-oxygen and XH⋯OH− bonding situations. Larger downshifts in the OH− stretching frequency are obtained with the fixed H atom constraint to simulate OH−⋯X bonding.