Structural and dipole (hyper)polarizabilities of the ground and excited singlet states of urea and thiourea: A gas phase CIS(D) theoretical study

Abstract

A study of the ground and the first five electronic excited singlet states properties of urea and thiourea molecules is reported. Our interest lies on structural features (bond distances and angles), electronic behavior (energy, dipole moment, charges), and the linear (polarizability) and nonlinear (first hyperpolarizability) optical properties of these relevant molecules within each excited state and how different they are from the ground state. Ground state properties were evaluated using ab initio and Density Functional Theory methods at HF, MP2, BLYP, and B3LYP levels on the corresponding optimized geometries, while excited states properties were determined through the CIS and CIS(D) procedures. The standard 6-31+G(d,p) and 6-311++G(3d,3p) basis sets were employed for all these calculations. The results of the ground state properties of urea and thiourea excellently agree with most of the available data in the literature. Moreover, excited state properties appear to be strongly affected within each state, and in general, they are quite different from the ground state values. Particularly, the first hyperpolarizability β represents the most affected property, and some interesting trends are proposed and discussed. Finally, based on our results, we suggest that a study of the excited state properties of both urea and thiourea derivatives can be useful as a guide for designing new nonlinear optical materials.