Abstract
We present a computational study of polarizabilities and hyperpolarizabilities of organic molecules in aqueous solutions, focusing on solute–water interactions and the way they affect a molecule’s linear and non-linear electric response properties. We employ a polarizable quantum mechanics/molecular mechanics (QM/MM) computational model that treats the solute at the QM level while the solvent is treated classically using a force field that includes polarizable charges and dipoles, which dynamically respond to the solute’s quantum-mechanical electron density. Quantum confinement effects are also treated by means of a recently implemented method that endows solvent molecules with a parametric electron density, which exerts Pauli repulsion forces upon the solute. By applying the method to a set of aromatic molecules in solution we show that, for both polarizabilities and first hyperpolarizabilities, observed solution values are the result of a delicate balance between electrostatics, hydrogen-bonding, and non-electrostatic solute solvent interactions.
Highlights
The investigation of non-linear optical properties of molecular systems has for long been of particular interest owing to the peculiar optical behavior of materials that possess a high nonlinear response, which have found applications in fields such as signal processing and telecommunications.[1]
The problem of accurately simulating electric response properties of molecular systems in solution has been the object of many studies over the years, with research effort focusing on increasing the accuracy of the quantum mechanics (QM) methods employed for the simulation of the light−matter interaction, which is at the origin of the response, as well as investigating different strategies to incorporate environmental effects into the calculation, in the case of molecules in liquid solutions.[2−9]
Changes in the molecular orbitals (MO) caused by repulsion can be appreciated by plotting the matrix J that relates one set of MOs into the other: J = C†repSCnorep where Crep is the MO coefficient matrix calculated at the QM/ FQFμ level with Pauli repulsion, S is the atomic orbital overlap matrix, and Cnorep is the MO matrix calculated at the same level without Pauli repulsion
Summary
The investigation of non-linear optical properties of molecular systems has for long been of particular interest owing to the peculiar optical behavior of materials that possess a high nonlinear response, which have found applications in fields such as signal processing and telecommunications.[1]. We have selected six organic molecules (Figure 1) from ref 72, for which experimental measurements of their first hyperpolarizability values in aqueous solutions exist.[71] All QM and QM/MM calculations were performed using a locally modified version of Gaussian[16] computational chemistry package[78] and employed the B3LYP,[79−81] CAM-B3LYP,[82] and M06-2X83 density functionals in combination with the 6311++G(d,p) basis set.
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