Structure and dipole moments of the two distinct solvated forms of p-nitroaniline in acetonitrile/CCl4 as studied by infrared electroabsorption spectroscopy

Structure and dipole moments of the two distinct solvated forms of p-nitroaniline (pNA) in acetonitrile/CCl4 have been studied by infrared electroabsorption spectroscopy. We measured a series of infrared electroabsorption spectra of pNA dissolved in an acetonitrile/CCl4 mixed solvent by altering the angle chi between the external electric field and the electric field vector of the incident infrared light. A singular value decomposition analysis has revealed that the observed infrared electroabsorption spectra are decomposed into two major components: the chi-dependent and chi-independent components. The decomposed spectral components as well as the infrared absorption spectrum are well explained in terms of two distinct solvated forms of pNA that exist in equilibrium in the mixed solvent. These solvated forms can be assigned to the 1:1 and 1:2 species, which have one and two acetonitrile molecule(s), respectively, associated with pNA. From a least-squares fitting analysis of the chi-dependent spectral component, it is shown that, for both the 1:1 and 1:2 species, a head-to-tail structure accounts well for the experimental results. On the other hand, the chi-independent component is likely to arise from the population change between the two solvated forms. This electric-field-induced population change of solvated forms may lead to the control of dielectric environments in solution by an external electric field.