Vibrationally excited states of HC5N: millimeter-wave spectroscopy and coupled cluster calculations

The rotational spectrum of HC₅N has been investigated in the millimeter-wave region, from 60 to 290 GHz, for 15 vibrationally excited states which lie approximately between 500 and 860 cm⁻¹, namely (v₆ v₇ v₈ v₉ v₁₀ v₁₁) = (000005), (000006), (000007), (000008), (000020), (000030), (001000), (010000), (100000), (000021), (000101), (001001), (010001), (000110), and (001010). Gas-phase copyrolysis of pyridine and phosphorus trichloride or, alternatively, a dc discharge in a gaseous mixture of vinyl cyanide and acetylene were used to produce the semi-stable HC₅N molecule. A large number of vibrational and rovibrational interactions has been taken into account to fit properly the measured transition frequencies of the states investigated. The most important perturbations are caused by the high-order Coriolis resonances observed between the (v₈, v₁₀) and (v₈ − 1, v₁₀ + 2) states, and between the (v₇, v₁₀, v₁₁) and (v₇ − 1, v₁₀ + 3, v₁₁ − 1) states, and by the cubic anharmonic interactions which mix the (v₆, v₈, v₁₁) states with the (v₆ − 1, v₈ + 1, v₁₁ + 1) states, and the (v₆, v₁₀) states with the (v₆ − 1, v₁₀ + 2) states. The analysis of the spectra was facilitated by CCSD(T) calculations with the cc-pVQZ basis, which provided accurate predictions of a large variety of spectroscopic constants like harmonic vibrational wavenumbers, vibration–rotation coupling constants, l-type doubling constants, and normal-coordinate cubic force constants. Excellent agreement between experiment and theory was generally observed, even when the experimental data were strongly perturbed by resonance effects.