Anisole-Water and Anisole-Ammonia Complexes in Ground and Excited (S1) States: A Multiconfigurational Symmetry-Adapted Perturbation Theory (SAPT) Study

Abstract

Binary complexes of anisole have long been considered paradigm systems for studying microsolvation in both the ground and electronically excited states. We report a symmetry-adapted perturbation theory (SAPT) analysis of intermolecular interactions in anisole-water and anisole-ammonia complexes within the framework of the multireference SAPT(CAS) method. Upon the S₁ ← S₀ electronic transition, the hydrogen bond in the anisole-water dimer is weakened, which SAPT(CAS) shows to be determined by changes in the electrostatic energy. As a result, the water complex becomes less stable in the relaxed S₁ state despite decreased Pauli repulsion. Stronger binding of the anisole-ammonia complex following electronic excitation is more nuanced and results from counteracting shifts in the repulsive (exchange) and attractive (electrostatic, induction and dispersion) forces. In particular, we show that the formation of additional binding N-H···π contacts in the relaxed S₁ geometry is possible due to reduced Pauli repulsion in the excited state. The SAPT(CAS) interaction energies have been validated against the coupled cluster (CC) results and experimentally determined shifts of the S₁ ← S₀ anisole band. While for the hydrogen-bonded anisole-water dimer SAPT(CAS) and CC shifts are in excellent agreement, for ammonia SAPT(CAS) is only qualitatively correct.

Figure 1

Schematic representation of vertical and adiabatic electronic excitations depicted on the example of anisole–H₂O complex with differences between ground and excited state (blue color coded) geometries for anisole–H₂O and anisole–NH₃ complexes. Distances in Å.

Figure 2

Difference of the dispersion energy density ΔQ^AB(r) between vertical excited-state and ground-state ΔQ^AB(r) = Q^A*B(r) – Q^AB(r) for anisole–H₂O (a) and for anisole–NH₃ (b) complexes. The presented isosurfaces encompass 50%, 40%, 30%, 20%, 10%, and 1% of the integrated dispersion energy density difference ΔQ^AB(r). Values on the color scale are reported in mE_h·Å^–3. Positive values correspond to regions where the dispersion energy density in the excited state is depleted (less negative) compared to the ground state. Basis set is aug-cc-pVTZ.