Stability and properties of C4 isomers

Abstract

The relative electronic energies of the 1Ag rhombus and the 3Σ−g linear isomers of C4 have been computed using a 5s3p1d basis at various levels of coupled-cluster and many-body perturbation theory. At the highest level performed, CCSD+T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C4 is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.

Infrared laser spectroscopy of jet-cooled carbon clusters

“…The scaling factor is found to be almost exactly the same for both clusters, e.g. Vexpt/Vcalc = 0.975 and 0.976 for C4 and C6, respectively, for the MP2/6-31G* level of calculations [28,29], and it is 0.890 and 0.895 for C4 and C6, respectively, for the UHF/6-31G* level [25,28]. Using the scaling factor and the recent ab initio result for C8 [29] for the MP2/6-31G* level, one can predict the highest frequency antisymmetric stretch fundamental of C8 to be 2067 cm -1.…”

“…Recent experiments [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] as well as high level ab initio calculations [19][20][21][22][23][24][25][26][27][28][29][30] have largely vindicated the description originally put forth by Pitzer and Clementi [31] in 1959. Clusters with odd numbers of carbons possess linear singlet ground states, whereas even numbered clusters have two low-lying isomers, a triplet linear form and a singlet cyclic form.…”

Infrared observation of linear GeC3 trapped in solid Ar

“…4 and 5͒ and then in the gas phase. 6,7 Coupled cluster calculations by Bartlett and coworkers predicted 8,9 that a cyclic isomer with transannular bond is nearly isoenergetic with the linear structure; however, the cyclic isomer has so far eluded spectroscopic detection.…”

Vibrational spectra of tetra-atomic silicon–carbon clusters. II. Si2C2 in Ar at 10 K

“…In the case of C 4 , it has not been definitely determined whether the linear form ͑ 3 ⌺ g Ϫ state͒, or the rhombic structure ͑ 1 A g state͒ with two tricoordinate carbons lies lower in energy. Ab initio calculations at the MBPT͑2͒ level 3 and more recently, a study with coupled-cluster methods 11 indicate that the two isomers are nearly isoenergetic, while direct spectroscopic measurements from infrared [12][13][14][15] and electron paramagnetic resonance 16 -18 ͑EPR͒ spectra have identified only the linear form. The SiC 3 structure ͑ 1 A 1 state͒ in which the molecule is a rhomboidal, four-membered ring, with a transannular ͑cross-ring͒ carbon-carbon bond, is predicted 5 by a configuration interaction calculation (CISD ϩQ/TZ2P) to be 4.1 kcal/mol more stable than a second rhomboidal structure with a carbon-silicon transannular bond, and 4.3 kcal/mol more stable than the linear Si-C-C-C structure ͑ 3 ⌺ Ϫ state͒.…”