Nitrile and thiocyanate IR probes: Quantum chemistry calculation studies and multivariate least-square fitting analysis

Hydration effects on the CN stretching mode frequencies of MeCN and MeSCN are investigated by carrying out ab initio calculations for a number of MeCN-water and MeSCN-water complexes with varying number of water molecules. It is found that the CN frequency shift induced by the hydrogen-bonding interactions with water molecules originate from two different ways to form hydrogen bonds with the nitrogen atom of the CN group. Considering the MeCN- and MeSCN-water cluster calculation results as databases, we first examined the validity of vibrational Stark effect relationship between the CN frequency and the electric field component parallel to the CN bond and found no strong correlation between the two. However, taking into account of additional electric field vector components is a simple way to generalize the vibrational Stark theory for the nitrile chromophore. Also, the electrostatic potential calculation method has been proposed and examined in detail. It turned out that the interactions of water molecules with nitrogen atom's lone pair orbital and with nitrile π orbitals can be well described by the electrostatic potential calculation method. The present computational results will be of use to quantitatively simulate various linear and nonlinear vibrational spectra of nitrile compounds in solutions.