A complete analysis of the1H nmr spectrum of 2-fluorotoluene yieldsthe spin–spin coupling constant between19F and the methyl protons, as 1.99 Hz. Analysis of nmr spectra of 21 other derivatives of 2-fluorotoluene shows thatcan vary between 1.69 and 2.55 Hz. This strong dependence on substitution contrasts with the near invariance of other long-range couplings such as,,The substituent dependence is discussed in terms of coupling mechanisms. INDO MO FPT calculations ofare inadequate. By means of appropriate model compounds, an adequate empirical conformational dependence is deduced forwhich can be used to reproduce some observed couplings. INDO MO FPT computations, in which certain off-diagonal Fock matrix elements are suppressed, are used to show that spin polarization via interacton of the methyl hydrogen orbitals is a major source of the discrepancy between theory and experiment. Some STO 3G MO calculations are reported for 2-fluorotoluene conformations.