Signs of ¹⁹F¹H and ¹⁹F¹³C spin–spin coupling constants mainly transmitted through space

Abstract

The details of two‐dimensional 13C1H correlated spectra have provided information about the relative signs of J(C, F) and J(F, H) for several molecules in which C, F and H are mutually coupled. Unlike signs are found for J(C, F) and J(F, H) when the coupling pathways lie approximately in the plane, but serious deviations from planarity can bring about changes in sign of J(F, H) and J(C, F).

1hJFHcoupling in 2‐fluorophenol revisited: Is intramolecular hydrogen bond responsible for this long‐range coupling?

“…Because the overlapping of the latter two at the site of the F nucleus yields a negative value , it is expected that contributions originating in the O 1 ‐H/C 2 ‐F 2 overlap yields a positive contribution to the FC term of 4TS J FH SSCC while that originating in the O 1 ‐H/LP 1 (F) overlap a negative one. This observation might provide an interesting rationalization for the experimentally reported sometimes positive and sometimes negative values for proximate J FH SSCCs regardless of the number of formal bonds separating the coupling nuclei . As a final remark, it is important to point out that proximate J FH transmitted through hydrogen bonds were studied few years ago from both experimental and theoretical points of view …”

“…Apparently, the competition between these two types of contributions yields a small FC term for 1h J FH SSCC in compounds 1 and 2 , although the F 2 –H (OH) distance is only 2.21 Å whereas the sum of their respective van der Waals radii is (1.47 + 1.20) = 2.67 Å. This competition could be the rationalization why J FH SSCCs originating in proximate molecular fragments were experimentally reported sometimes as positive and sometimes as negative …”

1H and 13C nuclear magnetic resonance studies of the conformational equilibrium of 2-(diphenylphosphino)benzaldehyde in polar and nonpolar solutions. Signs of the proximate coupling constants, 4J(CHO, 31P) and 3(13CHO, 31P)

“…Thls result is reminiscent of a recent set of relative sign determinations (12), from which it appears that the negative proximate J(H, F), for 4 = 0, can become positive for sufficiently large 4 values (see 5). An assessment of the critical 4 awaits improvements in the theory of coupling constants and (or) reliable computations or measurements of 4 for the molecules in question.…”

“…A recent discussion (12) of proximate 'v~J(H, F) and 4 7 5~(~, F), for which the nuclei and protons exist in the same C-H bond, points out the importance of sign determinations and also that, while the theoretical treatments are promising, they do not agree very well with experiment. Nevertheless, it is clear (12) that the orientation of the C-H bond with respect to the inplane lone-pair on fluorine, the one in the plane defined by coplanar C-F and C-H vectors, will likely determine the sign of 5 3 6~(~, F). In DPPB, the lone-pair sits on phosphorus and, in the 0-trans conformer, a proximate coupling pathway can be represented by P...H-C=O .…”

Infrared and Nuclear Magnetic Resonance Spectroscopic Study of Secondary Amide Hydrogen Bonding in Benzoyl PABA Derivatives (Retinoids)

“…There have been extensive spectroscopic studies on the subject of hydrogen bonding of small molecules containing the secondary amide group using infrared (IR) spectroscopy [1][2][3][4][5][6][7][8][9][10] and nuclear magnetic resonance (NMR). [11][12][13][14][15][16][17][18] Hydrogen bonding characteristics of drug molecules have important implications for physical properties such as solubility and bioavailability and for predicting interactions with other molecules that can affect biological activity. Using solution infrared data and varying concentration, intramolecular Hbonds involving the secondary amide N-H group can be differentiated from intermolecular H-bonds because the former are not dependent on concentration.…”