A THEORETICAL-STUDY ON THE ASSIGNMENT OF FUNDAMENTAL FREQUENCIES OF ORTHO-BENZYNE

The geometry and quadratic force field of o-benzyne are calculated by the recently developed unrestricted Hartree-Fock natural orbital complete active space (UNO-CAS) method. The force field, after an empirical scaling by scale factors of the UNO-CAS force field of benzene, satisfactorily reproduces the matrix IR frequencies of o-benzyne and its deuterium and C-13 isotopomers. On the basis of the calculations, earlier assignments of the fundamentals of o-benzyne, o-benzyne-d4, and (1,2-C2C4H4)-C-13 are discussed. The conclusion that the frequency of the formal triple bond stretching is at around 1860 cm-1 instead of 2080 cm-1 is confirmed. The matrix IR frequencies at 1627, 1607, 1598, 15%, 1307, 1271, 1055, and 838 cm-1 are concluded to be not fundamentals of o-benzyne, those at 1411, 1198, 1112, 1029, 679 and 616 cm-1 are concluded to be not fundamentals of o-benzyne-d4, and those at 1298, 1266, 1040, and 835 cm-1 are concluded to be not fundamentals of (1,2-C2C4H4)-C-13. Reassignments on some of the normal modes are proposed.