Linear and nonlinear electric properties and their dependence on the conformation and intramolecular H-bonding: A model study

The 3-aminoacroleine molecule is used as a model to study the dependence of linear and nonlinear molecular electric properties on the molecular conformation. Among four conformers of this molecule one of them has the intramolecular hydrogen bond, and thus, permits the study of its effect on electric properties. It has been established on the basis of MP2 calculations that the most elongated conformer of 3-aminoacroleine has the largest values of the dipole moment, dipole polarizability, and first hyperpolarizability. They were found to significantly depend on the conformation of the molecule. Also the electron correlation effect on the first hyperpolarizability is found to be very large. The results calculated for four conformers permit also to estimate the effect of the intramolecular hydrogen bond. The incremental lowering of the dipole polarizability is negligible whereas that for the first hyperpolarizability is of the order of −120 a.u. Considerable attention has been paid to the calculation of vibration corrections to electric properties. For the dipole polarizability and first hyperpolarizability the so-called pure vibrational corrections are found to be unusually large. These large values arise from the use of the harmonic zeroth-order approximation in the perturbation treatment of contributions due low-frequency modes. It is concluded that the perturbation method for the evaluation of pure vibrational corrections to electric properties should be used with great care and possibly preceded by the analysis of low frequency modes.