Manabu Abe, Waldemar Adam, Weston Thatcher Borden, Masanori Hattori, David A. Hrovat, Masatomo Nojima, Koichi, Nozaki, and Jakob Wirz, "Effects of Spiroconjugation on the Calculated Singlet-Triplet Energy Gap in 2,2-Dialkoxycyclopentane-1,3-diyls and on the Experimental Electronic Absorption Spectra of Singlet 1,3-Diphenyl Derivatives. Assignment of the Lowest-Energy Electronic Transitions of Singlet Cyclopentane-1,3-diyls", J. Am. Chem. Soc. 2004, 126, 574-582.

Abstract: The effect of a 2,2-ethylene-ketal on the singlet-triplet energy gap (DEST) and on the first electronic transition in singlet cyclopentane-1,3-diyls (1) has been investigated. UDFT calculations predict a significant increase in the preference for a singlet-ground-state in the diradical with the cyclic ketal at C2 (1g; DEST = -6.6 kcal/mol in C2 symmetry and -7.6 kcal/mol in C2v symmetry), compared to the 2,2-dihydroxy and 2,2-dimethoxy disubstituted diradicals (1d; DEST = -3.6 kcal/mol in C2 symmetry and 1e; DEST = -3.4 kcal/mol in C2 symmetry ). Spiroconjugation is shown to be responsible for the larger calculated value of abs(DEST) in 1g, relative to 1d and 1e. A strong correlation between the calculated values of DEST and the computed electronic excitation energies of the singlet diradicals is found for diradicals 1d, e, and g and for 2,2-difluorocyclopentane-1,3-diyl (1c). A similar correlation between DEST and lambda(calc) is predicted for the corresponding 1,3-diphenylcyclopentane-1,3-diyls 3, and the predicted blue shift in the spectrum of 3g, relative to 3e, has been confirmed by experimental comparisons of the electronic absorption spectra of the annelated derivatives 2c, e, and g in a glass at 77 K. The wavelength of the first absorption band in the singlet diradicals is increasingly blue-shifted in the order 2e (lambda(onset) = 650 nm) > 2g (lambda(onset) = 590 nm) > 2c (lambda(onset) = 580 nm). The combination of these computational and experimental results provides a sound basis for reassignment of the first electronic absorption band in singlet diradicals 2c, e, and g to excitation of an electron from the HOMO to the LUMO of these 2.2-disubstituted derivatives of cyclopentane-1,3-diyl.