HIGHLY ACCURATE QUANTUM-CHEMICAL CALCULATIONS FOR THE INTERSTELLAR MOLECULES C3 AND l-C3H+

Composite potential energy surfaces with coupled-cluster contributions up to CCSDTQP were constructed for C3 and l-C3H and used in the calculation of spectroscopic properties. The use of very large AO basis sets and the consideration of higher-order correlation beyond CCSD(T) is of utmost importance for C3 in order to arrive at quantitative spectroscopic data. The first detection of l-C3H in the interstellar medium was reported by Pety et al.,a who attributed 9 radio lines observed in the horsehead photodissociation region to that species. That assignment was questioned by the recent theoretical work of Huang et al.b However, our more accurate calculations are well in support of the original assignment. The calculated ground-state rotational constant is B0 = 11248 MHz, only 0.03% off from the radio astronomical value of 11244.9512 ± 0.0015 MHz. The ratio of centrifugal distortion constants D0(exp.)/De(theor.) of 1.8 is quite large, but reasonable in comparison with C3O and C3.