ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The ground electronic states of linear and rhombic C4 have been studied by high level ab initio quantum chemical techniques. Geometries, harmonic vibrational frequencies, infrared intensities, and other quantities have been determined using 4s3p2d1f correlation consistent basis sets and coupled-cluster methods including triple excitations. The linear–rhombic isomer energy difference has been investigated with a range of basis sets, including a 5s4p3d2f1g correlation consistent set. The linear–rhombic energy difference is influenced significantly by basis set, presence of triple excitations, and the choice of reference function for the open-shell linear isomer. The effect of basis set variation is complex, but once a reasonable quality of basis set has been achieved, further extensions favor the rhombic isomer. The inclusion of triple excitations also favors the rhombic isomer. The use of a restricted Hartree–Fock reference function for the linear isomer yields higher energies at the coupled-cluster level than if an unrestricted Hartree–Fock reference function is used, thereby again favoring the rhombic isomer. The most complete calculations of this study [coupled-cluster singles and doubles with noniterative triples (CCSD(T)) with a 5s4p3d2f1g basis set] indicate that the rhombic isomer is preferred by about 1 kcal mol−1. The coupled-cluster vibrational frequencies of the linear isomer are all real, in agreement with previous work, indicating that this isomer is not bent in the gas phase. The infrared intensities of linear C4 obtained in this work differ significantly from those obtained previously with smaller basis sets and either self-consistent field theory or second-order perturbation theory. The present calculations give a dissociation energy of C4 of 433 kcal mol−1, which is close to a previous value obtained with the aid of an empirical correction, and implies that several experimental estimates of the heat of formation of C4 are unreliable. Electron detachment energies of linear C4− and electron affinities of C4 are computed with larger basis sets than previously and are in very good agreement with recent anion photoelectron data.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.463407
