“…Time-resolved IR spectroscopy of molecular anions provides an alternative approach to characterizing CIP equilibria in aqueous solution for molecular ions. The IR spectra of a variety of anions shift measurably when they form a CIP with alkali and alkaline earth metal cations in polar solvents. − Of these molecular anions, thiocyanate (SCN¯), isocyanate (OCN¯), and azide (N 3 ¯) provide the most tractable IR spectroscopy, particularly for time-resolved measurements. ,,,,− We have used the NCS – anion to study contact ion pairing in water because of the long excited-state lifetime of the CN-stretch vibration. , The sensitivity of the nitrile stretch frequency to variations in the nitrile chemical and electrostatic environment has been used in a variety of prior experiments. , Under certain circumstances, the CN-stretch frequency provides a probe of the local electrostatic environment. , Stronger intermolecular interactions, such as CIP formation or hydrogen-bond formation to the nitrile group, lead to shifts in the CN-stretch frequency that cannot be accounted for with the Stark effect. , For the specific case of CIP formation between Mg 2+ or Ca 2+ and NCS – , bonding of the M 2+ cation to the nitrogen atom in the NCS – anion induces an electronic polarization that shifts electron density from the CS bond to the CN bond, leading to an increase in the CN-stretch frequency . This electronic polarization induced by CIP stabilizes the CN triple-bond resonance structure of thiocyanate, an alternative means of explaining the increase in the CN-stretch frequency.…”