Transient resonance Raman and density functional theory study of the 7-bromo-2-fluorenylnitrenium cation

Abstract

We report the first transient resonance Raman spectra for a halogen-substituted arylnitrenium ion, the 7-bromo-2-fluorenylnitrenium ion. The Raman band vibrational frequencies were found to be in good agreement with those predicted from density functional theory calculations for the singlet state of the 7-bromo-2-fluorenylnitrenium ion but not very well for the triplet state, particularly for the diagnostic 1530 cm−1 to 1650 cm−1 aromatic C=C stretch modes. This and the predicted 23.7 kcal/mol singlet-triplet energy gap with the singlet state being more stable indicates the transient resonance Raman spectra are due mainly to the singlet 7-bromo-2-fluorenylnitrenium ion. Comparison of the structures and properties of the 7-bromo-2-fluorenylnitrenium ion to the closely related 2-fluorenylnitrenium ion previously studied shows the 7-bromo substituent does not significantly perturb the iminocyclohexadienyl character of the 2-fluorenyl moiety and the structure of the first phenyl ring. However, the 7-bromo substituent does noticeably influence the structure of the second phenyl ring to which it is attached and also the normal mode character of a number of lower frequency modes with relatively delocalized character.