More evidence for bent ro-vibrationally averaged structures of linear triatomic molecules: The non-zero b axis component of the dipole moment and the [formula omitted] structure in electron diffraction

Abstract

We have previously demonstrated theoretically, by computing expectation values over the bending wavefunction, and experimentally, by re-interpreting observed values of the rotational constant B0, that a linear triatomic molecule will necessarily be observed as being bent on ro-vibrational average. We present here further evidence for this assertion: (1) the non-zero b axis component of the electric dipole moment 〈μb〉 for the linear molecules CO2, HCN, and HCO+, and (2) the internuclear distances rg in electron diffraction (ED) for CO2 and HCN when re-interpreted without an a priori assumption of the linear ro-vibrationally-averaged structure. In either case, the theoretical values calculated as the expectation values over the DVR3D wavefunctions show good agreement with the corresponding experimental values. If, as in a conventional paradigm, the r0 structure of a linear molecule were observed as linear, the 〈μb〉 would be of zero-value and the shrinkage effects in ED (claimed in the 1960s for the observation that the distance between terminal atoms was shorter than the sum of the constituent bond lengths) would become inevitable. Hence, the non-zero value of 〈μb〉 and no shrinkage effect in the reasonable, re-interpreted terminal-atom distance in ED, definitely show the bent ro-vibrationally averaged structure on observation even for the ro-vibrational ground state. Thus, we have additional pieces of indisputable evidence supporting the assertion that the ro-vibrationally averaged structure of a linear molecule is “bent” on observation.