Carbon-13 Nuclear Magnetic Resonance Studies of Benzocycloalkenes and Fluorobenzocycloalkenes

The C NMR spectra of o-xylene, tetralin, indan, benzocyclobutene, and o-di-tert-butylbenzene and (excluding the latter) their 3- and 4-fluoro analogues have been obtained and assigned. In the hydrocarbon cases, aromatic carbon assignments were confirmed by examination of tactically deuterated compounds, while considerations of C–F couplings and F contributions to carbon screenings permitted complete assignments of all aryl and essentially all aliphatic carbon signals in the fluorobenzocycloalkenes. It is concluded that fluoro substitution in aromatic systems is manifested by sufficiently regular trends in C–F couplings and contributions to carbon screenings as to be a useful strategy for spectral assignment of the unfluorinated aromatic. Trends in chemical shifts with the size of the fused ring (“ring stain”) are discussed in the light of existing chemical shift theory, and available quantum chemical calculations for these systems. Some examination of the H NMR spectra of the deuterated benzocycloalkenes and C satellite spectra have been conducted and while confirming the previous proton assignments for indan demand reversal of the assignments for o-di-tert-butylbenzene and benzocyclobutene. Preliminary studies of relaxation phenomena in these systems (T measurements) indicate a useful basis for spectral assignments, as C in all cases examined has a longer T than C. A qualitative explanation in terms of preferred modes of molecular rotation is advanced