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Accurate potential energy surfaces for benzene·M complexes (M = Li+, Na+, K+, and NH4+) are obtained using coupled-cluster theory through perturbative triple excitations, CCSD(T). Our computations show that off-axis cation−π interactions, where the cation is not directly above the aromatic ring, can be favorable and may influence molecular recognition. Even perpendicular, side-on interactions retain 18−32% of their π-face interaction energy in the gas phase, making their bond strengths comparable to hydrogen bonds in the gas phase. Solvent effects have been explored for each complex using the polarizable continuum model. [ABSTRACT FROM AUTHOR]

Ground-state structures with side-on nitrosyl (η 2-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO) 3(NO), a model transition-metal−NO compound. Surprisingly, we have observed no evidence for ON and η 2-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO) 3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure. [ABSTRACT FROM AUTHOR]