Proton NMR Spectra of Cyclopentadiene, 1,3-Cyclohexadiene, 1,3-Cyclooctadiene, and 1,2-Dihydronaphthalene

The proton NMR coupling constants between vinyl protons in cyclopentadiene (I), 1,3‐cyclohexadiene (II), and 1,3‐cyclooctadiene (III) have been obtained by analyses of the spectra of the latter protons when the methylene protons were decoupled. An analysis of the full spectrum of I has also been achieved. Analysis of the spectrum of 1,2‐dihydronaphthalene (IV) was undertaken both to obtain a value of ${}^3{J}_{\mathrm{HH}}$ across the olefinic double bond, and to compare certain of its coupling constants with those from our previous work on indene. The long‐range coupling constants between vinyl protons in the planar molecule I and nearly planar II are significantly different from those in the nonplanar III. The differences are discussed in terms of the conformations of these molecules, and the importance of these results for testing theories of long‐range coupling constants is pointed out.