Time-resolved IR spectroscopy of 1,3-dicyanophenylcyclopentane-1,3-diyl diradicals: CN stretching wavenumber as a vibrational signature of radical character.

CN stretch bands of singlet and triplet cyclopentane-1,3-diyl diradicals (1,3-di(4-cyanophenyl)-2,2-dimethoxyoctahydropentalene-1,3-diyl and 1,3-di(4-cyanophenyl)-2,2-dimethyloctahydropentalene-1,3-diyl) were observed by time-resolved IR spectroscopy. CN stretching wavenumbers of the singlet and triplet diradicals were downshifted by 10 and 19 cm(-1) from those of their corresponding ring-closed compounds, which are in closed-shell electronic states, respectively. The observed downshifts are attributed to the bond-order decrease in the CN bonds due to the contribution from a resonance structure that has cumulative double bonds (C ═ C ═ N(•)) at the para-positions of the radical carbons. This resonance structure is only possible when the molecules have unpaired electrons; thus, the wavenumber downshift can be regarded as an experimental manifestation of the radical character of the diradicals. The observed CN stretching wavenumbers indicate that the radical character of the singlet diradical is less significant than that of the triplet. The smaller radical character is ascribed to the contribution from the zwitterionic and π-single bonding resonance structures in the singlet diradical. Unrestricted DFT calculations at the B3LYP level of theory with the 6-31G(d) basis set reproduced the small/large relationship between the wavenumber downshifts of the singlet and triplet diradicals; however, the shift of the singlet diradical was overestimated.