Substituted effects on bonding characteristics of cyclopentane-1,3-diyl diradicals monitored by time-resolved infrared spectroscopy

Cyclopentane-1,3-diyl diradicals (DRs) provide excellent opportunities to study the properties of diradicals because their lifetimes can be significantly lengthened to up to milliseconds with the introduction of proper substituents. This study investigated the bonding characteristics of singlet and triplet DRs having C=O and p-cyanophenyl groups (S-DR3 and T-DR3) by monitoring the photo-induced formation of the diradicals from their precursor azo compounds using time-resolved IR (TR-IR) spectroscopy. Upon the formation of S-DR3, a C=O stretching wavenumber was upshifted by 22 cm⁻¹, whereas a C≡N stretching one was downshifted by 12 cm⁻¹. The observed shifts indicate that the unpaired electrons increase and decrease the C=O and C≡N bond orders, respectively. The effects of the unpaired electrons in S-DR3 were similar to those observed in our previous TR-IR studies on a singlet cyclopentane-1,3-diyl diradical having C=O but no C≡N groups (S-DR2) and on that having C≡N but no C=O groups (S-DR1), respectively. Contrastingly, upon the formation of T-DR3, the C=O wavenumber was downshifted by 16 cm⁻¹, indicating that the unpaired electrons decrease the C=O bond order. More notably, no detectable shifts were observed in the C≡N stretching wavenumber. These observations are not clearly explained by a model suggested in the previous studies on S-DRs. Here, we discuss and propose a more elaborated resonance hybrid of DRs that can explain the directions and relative magnitudes of the observed wavenumber shifts irrespective of spin multiplicities. We expect that the findings and suggestions presented here will stimulate research in both organic and theoretical chemistry.