Structure of the CCCN and CCCCH radicals: Isotopic substitution and ab initio theory

Abstract

The millimeter-wave rotational spectra of the 13 C isotopic species of the CCCCH and CCCN radicals and CCC 15 N were measured and the rotational, centrifugal distortion, and spin-rotation constants determined, as previously done for the normal isotopic species ͓Gottlieb et al., Astrophys. J. 275, 916 ͑1983͔͒. Substitution ͑r s ͒ structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration-rotation coupling constants calculated ab initio was compared with a large-scale coupled cluster RCCSD͑T͒ calculation. The calculated vibration-rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD͑T͒ calculations are reported here for CCCCH. The equilibrium geometries, excitation energies ͑T e ͒, and dipole moments of the A 2 ⌸ excited electronic state in CCCN and CCCCH were also calculated. We estimate that T e ϭ2400Ϯ50 cm Ϫ1 in CCCN, but in CCCCH the excitation energy is very small (T e ϭ100Ϯ50 cm Ϫ1 ͒. Owing to a large Fermi contact interaction at the terminal carbon, hyperfine structure was resolved in 13 CCCCH. Measurements of the fundamental Nϭ0→1 rotational transition of CCCCH with a Fourier transform spectrometer described in the accompanying paper by Chen et al., yielded precise values of the Fermi contact and dipole-dipole hyperfine coupling constants in all four 13 C species. The Fermi contact interaction is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant b F in 13 CCCN and C 13 CCN from the millimeter-wave rotational spectra.

Structure of the Cumulene Carbene Butatrienylidene: H2CCCC

“…The 13 C enriched Li 2 C 2 was prepared at the NIH Stable Isotope Resource, Los Alamos National Laboratory. The lines of the four 13 C isotopic species were comparable in strength and about 20 times weaker than the normal species ( see Fig. 2 for a sample spectrum) .…”

“…2 for a sample spectrum) . The implication is that insertion of the 13 C occurs more or less randomly in the discharge.…”

“…The carbon-13 isotopomers of H 2 CCCC were observed in a discharge through normal (FTM) spectrometer (9). Briefly, gas pulses produced by diacetylene and a statistical mixture of HCCH, H 13 CCH, and H 13 C 13 CH which was produced by the hydrolysis of Li 2 C 2 . The 13 C enriched Li 2 C 2 was prepared at the NIH Stable Isotope Resource, Los Alamos National Laboratory.…”

On theoretical predictions of noble-gas hydrides

“…Indeed, high-level ab initio methods are required to treat accurately open-shell species ͑such as C 3 N and C 4 H͒ appearing after dissociation of noble-gas hydrides. 10 In particular, the dissociation energy of HY precursors is usually overestimated with the MP2 method. This trend is demonstrated in Table I for a number of selected species.…”

Organic synthesis applied to space sciences

“…The millimeter-wave rotational spectra of the CCCCH and CCCN radicals and their isotopic species were measured and the rotational, centrifugal distortion, and spin-rotation constants determined [64]. Free CCCCH was produced in a DC glow discharge (∼0.6 A) through helium and acetylene at a temperature of 120 K and pressure of 30-40 mTorr.…”