Application of Low-Field NMR to the Unambiguous Assignment of Spin-Coupling Parameters. I. Pentafluoro-Iodobenzene

Abstract

A low-frequency internal-lock spectrometer system, obtained by substitution of a 6.14-MHz rf unit and probe in a Varian HA−100 spectrometer, makes it possible to record high-quality precalibrated 1H and 19F spectra at about 1.5 kG. Since at such field strengths, chemical shifts are reduced by about one order of magnitude compared to usual conditions, determinations of relative signs of coupling constants (J's) and assignment of J's to particular pairs of nuclei can be made in many cases in which this is not possible at 14 or 23 kG due to ``first-order'' characteristics of spectra. In this way, the relative signs of the three meta JFF's in C6F5I were determined, and the values of −4.95 and −1.22 Hz were unambiguously assigned to J26 and J35, respectively. (These signs are based on ortho JFF<0, as found in a preliminary analysis of the 6.14-MHz spectrum of C6HF5.) In general, low-frequency (∼6 MHz) field-lock NMR spectroscopy yields detailed information on spin coupling constants, when ``non-first-order'' conditions can thus be created.