IS $C_{4}$ BENT?

Abstract

Cheung and Graham have recently obtained high resolution ESR spectra of the $C_{4}$ molecule in rare gas matricies which show splitting of the perpendicular lines into separate x and y $components.^{1}$ Their interpretation of these spectra is that triplet $C_{4}$ is not linear, but is slightly bent. This is contrary to several recent ab initio calculations. Using larger basis sets than have previously been employed for $C_{4}$, TZ2P and TZ2PF, the structure of this molecule has been reinvestigated via ab initio calculations which include electron correlation to second order in the many-body perturbation scheme. Linear, several forms of alinear, and rhombic $C_{4}$ were studied. Within the levels of theory employed here, triplet $C_{4}$ is not bent, but rather is linear. The amount of energy required to bend linear $C_{4}$ is quite small, however. This may account for the molecule being slightly bent in a matrix. The inclusion of f functions in the basis set lowers the energy of rhombic $C_{4}$ about 10 kJ/mol more than that of linear $C_{4}$. $^{1}$H. M. Cheung and W. R. M. Graham, J. Chem. Phys. 91, 6664 (1989).