IS C4 BENT

Cheung and Graham have recently obtained high resolution ESR spectra of the C4 molecule which show splittings of the perpendicular lines into separate x and y components [1]. Their interpretation of these new spectra is that triplet C4 is not linear, but is slightly bent. This is contrary to numerous ab initio calculations. Using larger basis sets than have previously been employed for C4, triple-zeta plus two sets of (d) polarization functions, denoted TZ (2d), and triple-zeta plus two sets of d functions and one set of f functions, denoted TZ (2df), the structure of this molecule has been reinvestigated via ab initio calculations which included electron correlation to second order in the many-body perturbation theory (MP2). Linear, several forms of alinear, and rhombic C4 were studied. Within the levels of theory used here, triplet C4 is found not to be bent, but rather is linear. The inclusion of f functions in the basis set lowers the energy of rhombic C4 over that of the linear isomer by about 10 kJ/mol.