Accurate structures from combined gas electron diffraction and liquid crystal NMR data; the importance of anisotropy of indirect couplings for 1,4-difluorobenzene

Abstract

Gas-phase electron-diffraction scattering data and dipolar couplings from NMR experiments in four different liquid crystal solvents have been combined to give a high-accuracy molecular structure of 1,4-difluorobenzene. The anisotropic components of the CF and FF indirect couplings have been deduced directly from the experimental data. The resultant structure has standard deviations of around 0.2 pm for interatomic distances and less than 0.20 for inter-bond angles.

The indirect through-space F–F coupling in peri-difluoronaphthalene: is it anisotropic?

“…

The 1 H, 19 F and 13 C spectra have been obtained of a sample of peri-difluoronaphthalene dissolved in the nematic liquid crystalline solvent ZLI 1695. The 13 C satellite spectra from the six, single-13 C isotopomers at natural abundance in both the 1 H and 19 F spectra were identified and analysed to yield a set of residual total, anisotropic spin-spin couplings, T ij .

…”

Anisotropy of indirect couplings and accurate molecular structures of 1,2- and 1,3-difluorobenzenes by combined analysis of gas electron diffraction, rotational spectroscopy and liquid crystal NMR data

“…Our calculated corrections to dipolar coupling constants are not identical to those used in earlier work [18,19]. We have shown that uncertainties of terms calculated in this way are roughly proportional to their magnitudes, and that 10% of the magnitude, with a lower limit of 0.1 Hz, is a (a) 19 reasonable estimate [17]. Allowing for these realistic uncertainties results in the weights given to the dipolar couplings in our analyses being lower than those used in the earlier work.…”

“…As was noted in our earlier study of 1,4-difluorobenzene [17], these apparent discrepancies could be attributed to anisotropic components of one or more of the indirect coupling constants involving these atoms. The model used for refinement was then amended to include factors that allow for the anisotropy of these CF and FF indirect (J) couplings.…”

“…We have studied the three isomers of difluorobenzene. Combined analysis of electron-diffraction data and dipolar coupling constants for 1,4-difluorobenzene have already been published [17]. In that case, the absence of a dipole moment precluded the measurement of rotation constants by microwave spectroscopy.…”

Comparison of the molecular structures of five benzene derivatives as determined independently by gas-phase electron diffraction in two different laboratories: a perspective

“…Two other parameter representations, r h0 and r h1 , have been used by the Edinburgh group in recent publications, including Refs. [2] and [5]. The r hn notation, originally introduced by Sipachev [23], is described as follows by Rankin and co-workers [24].…”