Equilibrium structure of protonated cyanogen, HNCCN+

The cubic force field of protonated cyanogen, HNCCN+, has been calculated at the CCSD(T) level of theory employing correlation consistent bases of quadruple-zeta quality. Semi-experimental equilibrium structures have then been derived from the experimental ground-state rotational constants available for various isotopologues and the corresponding vibrational corrections calculated from the theoretical force fields. While a good agreement has been found with the pure theoretical best estimate of equilibrium geometry, computed at the CCSD(T) level of theory accounting for basis set truncation as well as including core correlation corrections, large discrepancies have been noted with the experimental substitution, rs, as well as effective, r0, structures.