Millimeter-wave spectroscopy of HC5N in vibrationally excited states below 500 cm-1.

The rotational spectrum of the semistable HC5N molecule has been investigated in the millimeter-wave region for eight excited bending states which lie below 500 cm-1, namely $v_{11} =1,\,2,\,3$, and 4, $v_{10}=1$, $v_9=1$, ($v_{10} = 1$, $v_{11} = 1$), and ($v_{10} = 1$, $v_{11} = 2$). Transitions up to $J = 101 \leftarrow 100$ were measured, reaching a frequency as high as 270 GHz. The set of spectroscopic constants determined in the present work makes it possible to predict with much improved accuracy the transition frequencies of millimeter-wave lines produced by vibrationally excited HC5N in hot core molecular sources and circumstellar shells. In addition, results of extensive CCSD(T) calculations are presented, which yield accurate term values for low-lying bending vibrational states