Multipole-bound states of succinonitrile and other dicarbonitriles

Very recently two anionic states of succinonitrile have been observed, and these two states have been interpreted as a dipole-bound state of the gauche and a quadrupole-bound state of the anti conformer. Here we study the electron binding properties of succinonitrile using high-level ab initio methods. While the dipole-bound state can be investigated using well established approaches, studying the quadrupole-bound state is more challenging owing to the multiconfiguration character of its wave function. The standard methods typically applied to dipole-bound anions fail, and we employ direct electron propagator based and equation-of-motion coupled-cluster methods. Since there is no experience with this type of quadrupole-bound state, various basis set related and methodological aspects are examined in detail. According to our results the quadrupole moment as such plays only a minor role in binding the extra electron, whereas electron correlation effects are decisive. Our best fixed-nuclei electron binding energy is 11 meV. In view of the small binding energy the influence of the nuclear motion on the electron binding properties is examined, in particular, the torsional motion around the central carbon-carbon bond, since it is a very soft mode and the dipole and quadrupole moment depend strongly on it. Our results provide a firm basis to interpret the experimental findings and support the experimental assignments. Moreover, we discuss molecules that possess only a quadrupole-bound state, and preliminary results for dicarbonitriles of bicyclopentane and cubane are presented.