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 struc- tures have then been derived from the experimental ground-state rotational constants available for var- ious 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 includ- ing core correlation corrections, large discrepancies have been noted with the experimental substitution, rs, as well as effective, r0, structures