Structures and Stabilities of C₅H₂Isomers:  Quantum Chemical Studies

Abstract

Five isomers of the carbon-rich molecule C5H2 are investigated computationally, using methods based on the coupled-cluster approximation. All of these structures are related to isomers of C3H2 via substitution of hydrogen by ethynyl or attachment of a C2 fragment to a carbene center. The two most stable forms of C5H2 are linear triplet pentadiynylidene (4) and singlet ethynylcyclopropenylidene (6). Both of these isomers have been observed in the laboratory, as has a thirdthe cumulene carbene pentatetraenylidene (5)which is predicted to lie about 15 kcal/mol above the linear triplet. Two other isomers are also studied: ethynylpropadienylidene (7) and 3-(didehydrovinylidene)cyclopropene (8). Both are found to lie less than 25 kcal/mol above the most stable form of C5H2 and to possess rather large dipole moments. Predictions for the harmonic vibrational frequencies of 12C and mono-13C isotopomers, infrared intensities, and rotational constants are also presented. These should assist efforts to identify these molecules in the laboratory and in the interstellar medium.

Higher energy electronic transitions of HC2n+1H+ (n=2–7) and HC2n+1H (n=4–7) in neon matrices

“…20,24,33 Such behavior can be expected also for cations of polyacetylenes, where both odd and even members have doublet ground state resulting from the unpaired electron. 20,24,33 Therefore, it is worthwhile to compare electronic transitions of even and odd members of the polyacetylene cations with each other. Electronic transitions from the ground to higher excited electronic states of the even HC 2n H + ͑n =2-7͒ polyacetylene cation series have been studied in neon matrices.…”

“…19 They were also studied by ab initio methods. [20][21][22][23][24] Much less is known about the HC 2n+1 H + ions. Apart from a few theoretical investigations on the smaller members of the HC 2n+1 H + homologous series ͑n =2-4͒ ͑Refs.…”

Electronic spectra of linear HC5H and cumulene carbene H2C5

“…1 were selected for a computational study, whereby structures A, B, C, D, and F have been investigated by coupled cluster theory before. 20 As starting point for high-level calculations using a multi-configurational second order perturbation theory (CASPT2), equilibrium geometries were first optimized at DFT (density functional theory) level with the M06-2X functional 28 and the cc-pVTZ basis 29,30 set of the Gaussian09 software. 31 The thus obtained structures were then refined with the CASPT2 method of the MOLCAS program 32 along with the same basis set.…”

“…1) have been found within 90 kJ/mol. 20 Rotational spectra were recorded for the four lowest singlet isomers possessing a permanent dipole moment, i.e., structures B, C, D, and F, [23][24][25] whereas the HC 5 H triplet was studied in solid matrices only. 11,26 The first matrix investigation was realized by co-authors of this paper.…”

Laboratory Detection of the Ring‐Chain Carbenes HC4N and HC6N

“…These are of interest to both chemists and astronomers : as possible intermediates in combustion and other processes and as molecules with intense rotational spectra closely related to known interstellar species, and so they are plausible candidates for detection with large radio telescopes. There are few laboratory data on the stability of the ring chains relative to their isomers, but ab initio calculations indicate that the and ring chains are the C 5 H 2 C 7 H 2 most stable isomers with the respective elemental formulae (Seburg et al 1997 ;Aoki & Ikuta 1994).…”