Rydberg Electron Stabilizes the Charge Localized State of the Diamine Cation

Abstract

The interpretation of Rydberg spectra of gaseous dimethylpiperidine (DMP) as showing the co-existence of a localized and delocalized mixed-valent DMP+ radical cation has generated a controversial discussion about the performance of various electronic-structure methods. We revisit this discussion by high-level computations. An apparent barrier separating localized and delocalized DMP+ minima at MRCI+Q and a few other levels reflects unphysical curve crossings of the reference wave functions. These discontinuities on the surface are removed in state-averaged MRCI+Q calculations, as well as at IP-EOM-CCSD or CCSD(T) levels. This has prompted a high-level computational study of the actually observed Rydberg state of neutral DMP. Indeed, the 3s-type Rydberg electron binds more strongly to a localized rather than to a delocalized positive charge distribution. This stabilizes the localized structure to the extent where it becomes a local minimum for the DMP∗ Rydberg state, in contrast to DMP+, explaining the experimental observations. This appears to be the first case where interactions of a Rydberg electron with the underlying cationic core alter molecular structure in such a fundamental way.