Coupled Cluster Study of the Linear Carbon Chains C_2n+1 (n = 5−9)

Abstract

Making use of the coupled cluster variant CCSD(T) in conjunction with Dunning's cc-pVTZ basis set, equilibrium structures and complete harmonic force fields have been calculated for the linear carbon chains of type C2n+1 with n = 5-9. With the exception of C3, which is a well-known "floppy" molecule with an extremely shallow bending potential, all members of the C2n+1 series up to C19 appear to behave like fairly normal semirigid molecules. The IR active bending vibrations of lowest wavenumber for C17 and C19, which may be of interest to forthcoming far-infrared astronomy, are predicted to occur at 13.1 and 11.1 cm(-1), respectively, with corresponding absolute IR intensities of 6.6 and 5.9 km mol-1. Huge IR intensities are calculated for one antisymmetric stretching vibration per chain (19,948, 29,632, and 30,040 km mol(-1) for C15, C17, and C19, respectively). A quantitative description of these vibrations may require the explicit consideration of anharmonicity effects and electronic structure calculations going beyond CCSD(T).

Linear Cn Clusters: Are They Acetylenic or Cumulenic?

“…Multiconfiguration reference methods or coupled cluster methods are needed for the correct prediction of the ground state of C 2 . , Since we are aiming at the evolution of the properties of linear C n with size n , and the connection between C n and C ∞ , this deficiency does not affect our study. Figure shows that for the ground state geometry of C n ( n = 2−8), B3LYP/6-31G* predictions agree well with our presented and with earlier CCSD(T)/cc-pVTZ calculations and available experimental data. , This justifies our selection.…”

Evidence for Solution-State Nonlinearity of sp-Carbon Chains Based on IR and Raman Spectroscopy: Violation of Mutual Exclusion

“…The computational procedure started from an initially bent polyyne skeleton, and for longer chains ( C12−C20 ), the geometry optimization converged to a slightly bent structure as a minimum. Previous experimental , and theoretical − studies agree that the bending potential of related linear carbon structures is shallow. As for the presence of stable minima for bent structures, recent work has shown that this seems to be the case in carbon suboxide , and Pt-end-capped polyynes .…”

“…7 Other sp-carbon chains, such as, for example, carbon suboxide 8 and carbon clusters, 9 have been shown experimentally to possess shallow bending potentials in the gas phase. Additionally, the existence of a nonlinear equilibrium structure or shallow bending potential has also been computationally predicted for some cumulenes, 10 small carbon-clusters, 9,11 and carbon suboxide. 12 From an experimental point of view, gas-phase bending (static and/or dynamic) of carbon chains and clusters is also expected to play a dominant role in fullerene formation.…”

Molecular Structure of the Octatetranyl Anion, C8H−: A Computational Study

“…Among the single-reference based post-HF methods, coupled-cluster theory appears to be a reasonable choice for the quantum mechanical treatment of a closed-shell species such as C 8 H − . For instance, Botschwina has pioneered the theoretical study of carbon chain clusters and other astrophysically relevant molecules using coupled-cluster theory. , Second, the molecule is negatively charged, meaning that diffuse functions are to be included into the basis set. Extended basis sets are therefore necessary to accurately determine the geometry of C 8 H − , but the size of this molecular anion foresees the need for very large computational resources particularly if one wants to use highly correlated post-HF methods.…”