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 separatex andy 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 ofd functions and one set off functions, denoted TZ (2df), the structure of this molecule has been reinvestigated via ab initio calculations which include 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 off functions in the basis set lowers the energy of rhombic C₄ over that of the linear isomer by about 10 kJ/mol.