The equilibrium structure and rotational constant of HCC+

Using ab initio electronic structure techniques, the equilibrium structure of HCC+ has been predicted and the resulting equilibrium rotational constant (Be) is 44.3 GHz. Through comparison of the experimentally determined J=1→0 transition frequency with a similarly calculated Be for HNN+, the J=1→0 transition in HCC+ is predicted to occur at 88.2 GHz. Isotopic substitution gives a rotational constant (Be) for DCC+ of 36.3 GHz. An estimate of the hyperfine interaction leads to the prediction that the rotational spectrum of DCC+ will consist of three lines around 72.6 GHz with separations of 0.09 and 0.06 MHz. While error limits from theoretical determinations are not clear cut, the use of a large basis set and careful treatment of electron correlation suggest that an experimentally determined transition frequency for HCC+ is most likely to be within ±0.3 GHz of 88.2 GHz.