Anisole–Water and Anisole–Ammonia Complexes\nin Ground and Excited (S<sub>1</sub>) States: A Multiconfigurational\nSymmetry-Adapted Perturbation Theory (SAPT) Study

Binary complexes\nof anisole have long been considered paradigm\nsystems for studying microsolvation in both the ground and electronically\nexcited states. We report a symmetry-adapted perturbation theory (SAPT)\nanalysis of intermolecular interactions in anisole–water and\nanisole–ammonia complexes within the framework of the multireference\nSAPT(CAS) method. Upon the S<sub>1</sub> ← S<sub>0</sub> electronic\ntransition, the hydrogen bond in the anisole–water dimer is\nweakened, which SAPT(CAS) shows to be determined by changes in the\nelectrostatic energy. As a result, the water complex becomes less\nstable in the relaxed S<sub>1</sub> state despite decreased Pauli\nrepulsion. Stronger binding of the anisole–ammonia complex\nfollowing electronic excitation is more nuanced and results from counteracting\nshifts in the repulsive (exchange) and attractive (electrostatic,\ninduction and dispersion) forces. In particular, we show that the\nformation of additional binding N–H···π\ncontacts in the relaxed S<sub>1</sub> geometry is possible due to\nreduced Pauli repulsion in the excited state. The SAPT(CAS) interaction\nenergies have been validated against the coupled cluster (CC) results\nand experimentally determined shifts of the S<sub>1</sub> ←\nS<sub>0</sub> anisole band. While for the hydrogen-bonded anisole–water\ndimer SAPT(CAS) and CC shifts are in excellent agreement, for ammonia\nSAPT(CAS) is only qualitatively correct.