Proton and 19F Magnetic Resonance of Partially Oriented Fluoromethanes

Abstract

It is shown that the absolute sign of the indirect proton–fluorine nuclear spin–spin coupling constant JHF is positive in the fluoromethanes from their spectra in the nematic liquid crystal p,p′-di-n-hexyloxyaz-oxybenzene and the knowledge of the previously determined absolute signs of the 19F magnetic shielding anisotropies in CH3F and CHF3. Values of the 19F and proton magnetic shielding anisotropies are determined and the problems of shift referencing are discussed. If it is assumed that the contributions to magnetic shielding of the H and F nuclei are axially symmetric about the respective C–H and C–F bonds, the experimental values can be transformed into magnetic shielding anisotropies along bond directions.

Magnetizabilities and Nuclear Shielding Constants of the Fluoromethanes in the Gas Phase and Solution

“…As for the isotropic shielding, the basis set effects are larger than for Δσ C and it is difficult to see any trends with respect to increasing size of the basis set. As for the anisotropy of the carbon shielding, comparison with experiment ,,,− is difficult since they differ from each other and have rather large error bars. Especially the second anisotropy of the CH 2 F 2 molecule differs substantially from experiment …”

Anisotropy of the 1J(PSe) spin-spin coupling. X-ray and liquid crystal NMR study of trimethylphosphine selenide

“…0 -2(3) 0 C(l) 7735 (25) 2500 5608 (23) 379(49) 188(30) 268 (43) 0 -22 (38) 0 C'(D 7755(17) 654(17) 1909(18) 265(26) 269 (28) 449 (40) -66 (24) -5 (27)135(28) …”

On Measuring Nuclear Magnetic Shielding Anisotropies in Liquid Crystal Solvents

“…The single-phase measurement yielded a value (-124 ppm) 21 percent smaller than the original two-phase measurement value (-157 ppm). However, this two-phase value was calculated without correcting for the temperature dependence of the isotropic chemical shift [18]. If such a correction is made to the original data, using the value for do i so/ dT shown in Table 1, we obtain Llcrl' 120 ppm, in good agreement with the single-phase result.…”

“…This shielding anisotropy had previously been measured by several workers using both two-phase [1,18] and single-phase [11] techniques. The results are all in agreement, but are consistently higher (by 0.8 to 1.5 ppm) when CH 4 , rather than TMS, is used as the internal reference.…”