Candidates for U-lines at 1377 and 1394 MHz in IRC+10216: ab initio molecular orbital study

The candidates for the U-lines at 1377 and 1394 MHz observed in IRC+10216 are surveyed by using the RCCSD(T)/aug-cc-pVTZ and/or RCCSD(T)/cc-pVTZ methods. The C6H− anion and the C6H(2Σ+) radical are proposed as the probable candidates, respectively. The scaling arguments used to improve the predictions of rotational constants are reasonable and represent only a small (within ca. 0.1%) correction to the computed values.