VIOLATIONS OF HUND RULE IN MOLECULES - WHERE TO LOOK FOR THEM AND HOWTO IDENTIFY THEM

Violations of Hund's rule are predicted to be found in D-4h cyclobutad iene (CBD), D-8h cyclooctatetraene (COT), and in non-Kekule hydrocarbo n diradicals that have disjoint NBMOs. In D-4h CBD the singlet is calc ulated to lie below the triplet by -Delta E-ST = 6.3 kcal mol(-1) at t he CAS-SDCI + (Q)/6-31G level of theory. This value is bracketed by t he CASSCF and CASPT2N values of, respectively, -Delta E-ST = 10.6 and 4.0 kcal mol(-1). For D-8h COT CASSCF, CASPT2N and MR-(pi-SD, sigma-S) CI + (Q)/6-31G calculations give, respectively, -Delta E-ST = 15.8, 6 .7 and 8.5 kcal mol(-1). In both [4n]annulenes the geometry of highest symmetry is the equilibrium geometry of the tripler but the transitio n state for bond shifting in the singlet. Consequently, the ''bond-alt ernation'' vibration, which shortens half the C-C bonds and lengthens the other half, has a relatively high frequency in the triplet but an imaginary frequency in the singlet. As a result, zero-point vibrationa l energy corrections increase -Delta E-ST by ca. 2 kcal mol(-1) in bot h antiaromatic annulenes. The role of calculations in assigning the el ectron photodetachment spectrum of COT.(-) and thereby confirming expe rimentally that D-8h COT really does violate Hund's rule is discussed. CASPT2N calculations of UV spectra are shown to be useful for decidin g whether the singlet or tripler state is responsible for the UV-Vis s pectrum of non-Kekule hydrocarbon diradicals. Comparison of the spectr a calculated for the singlet and the triplet states of 1,2,4,5-tetrame thylenebenzene (TMB), 4,5-dimethylenecyclopentane-1,3-diyl (DMCPD) and 2,3-dimethylenecyclohexane-1,3-diyl (DMCHD) with those observed indic ates that TMB has a singlet ground state but that DMCPD and DMCHD both have triplet ground states. (C) 1997 Elsevier Science B.V.