A b i n i t i o study of the structure, infrared spectra, and heat of formation of C4

The lowest-lying 1Ag and 3Σ−g states of C4 have been studied using basis sets including f functions, quadratic configuration interaction, and full fourth-order Mo/ller–Plesset perturbation theory with multiple spin projection. Basis set effects on the linear-cyclic separation have been found to be fairly significant. Use of the 6-311G* and [5s3p1d] basis sets leads to qualitatively different conclusions; this problem is remedied by expansion of the polarization space. At the highest level of theory considered, the cyclic structure lies about 1 kcal/mol below the linear structure. A G1-type estimate of its heat of formation at 0 K of 249.6 kcal/mol is proposed. This value is in disagreement with the generally quoted experimental values, but in good agreement with third-law values by Drowart et al. [J. Chem. Phys. 31, 1131 (1959)] corrected for improved partition functions. Geometries and harmonic frequencies are reported. It is tentatively suggested that a matrix ir band at 1284 cm−1 may belong to cyclic C4; to aid experimentalists in confirming or rejecting this assignment, theoretical isotope shifts are reported.