Deuterium-induced isotope effects of a C-H---Se 'hydrogen bond' on the IR and NMR spectra of 6H,12H-dibenzo[b,f][1,5]diselenocin

Deuterium-induced isotope effects of the C-H---Se nonbonded interaction of 6H,12H-dibenzo[b,f][1,5]diselenocin (1) were investigated. The IR spectroscopic behavior of 6H,12H-dibenzo[b,f][1,5]diselenocin-6,6,12,12-d4 (1-d4) was compared with that of the reference compound (benzyl-α,α-d2 phenyl selenide (2-d2)). The results for both boat (1B) and chair (1C) conformers of 1 indicated that the C-H---Se interaction causes the decrease in the low wavenumber shift of ν1 (Δν1 = -2 cm-1 for 1B-d4 and -12 cm-1 for 1C-d4, whereas Δν1 = -15 cm-1 for 1B and -52 cm-1 for 1C). The 77Se NMR showed the upfield isotope shift due to the through-space C-H---Se interaction (ΔΔδ = 0.44 ppm for 1B and 0.25 ppm for 1C). These findings clearly suggested that the interaction is attractive in nature: MO calculations at the HF/3-21G* level and NBO analysis showed the importance of the electron delocalization from the selenium lone pair to the antibonding orbital of the C-H bond. These observations strongly suggested that the interaction be called a C-H---Se hydrogen bond. (author)