Theoretical spectra and energetics for c-C3HC2H, l-C5H2, and bipyramidal D3h C5H2

The recent astronomical detection of c -C 3 HC 2 H and l -C 5 H 2 has led to increased interest in C 5 H 2 isomers and their relative stability. The present work provides the first complete list of anharmonic vibrational spectral data with infrared intensities for three such isomers as well as including the first set of rotational data for the bipyramidal C 5 H 2 isomer allowing for these molecules to serve as potential tracers of interstellar carbon. All three isomers have fundamental vibrational frequencies with at least one notably intense fundamental frequency. The l -C 5 H 2 isomer has, by far, the highest intensities out of the three isomers at 2076.3 cm −1 (738 km mol −1 ) and 1887.5 cm −1 (182 km mol −1 ). The c -C 3 HC 2 H isomer has one intense peak at 3460.6 cm −1 (84 km mol −1 ), and the bipyramidal C 5 H 2 isomer has one intense peak at 489.3 cm −1 (78 km mol −1 ). The relative intensities highlight that while l -C 5 H 2 is not the lowest energy isomer, its notable intensities should make it more detectable in the infrared than the lower energy c -C 3 HC 2 H form. The bipyramidal isomer is firmly established here to lie 44.98 kcal mol −1 above the cyclic form. The explicitly correlated coupled cluster rovibrational spectral data presented herein should assist with future laboratory studies of these C 5 H 2 isomers and aid in detection in astronomical environments especially through the newly operational James Webb Space Telescope.

Abstract licence: CC BY

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