Effect of the orientation of an α‐substituent on vicinal ¹³C–;¹H spin–spin coupling constants

Abstract

The magnitude of the NMR spin-spin coupling constant, 3J(CH), between a vicinal I3C-'H pair depends, inter afia, on the value of the torsion angle CD, #CX-C-H) and is influenced by the presence of an electronegative substituent located on the coupling "C nucleus. The form and magnitude of the effect of the orientation Yxc of such an a-substituent were examined. The coupling constant between C-1 and a hydrogen atom located on C-3 in a series of a-substituted propanes were studied by means of the semi-empirical INDO method. In the calculations both CD and Y(X-13C-C-C)were systematically varied in steps of 30". These calculations reveal that the variation of Y at a constant CD has a pronounced effect on the calculated coupling constant JCalc. The magnitude of this effect is shown to be strongly dependent on the electronegativity x of the a-substituent. Thus, it is shown that Jealc depends on CD and Y , in addition to 2. The resulting set of two-dimensional Karplus-type surfaces can be described by an equation that contains only nine adjustable parameters. Measurement of 3J(CH) in cis-and trans-2,2,6,6-tetradeuterio-4-tert-butylcyclohexanol confirmed some of the theoretical predictions. In the cis compound (mCH = MOO, Y, = 60") 3J(C-1,H-3eq) is 7.1 Hz, whereas in the trans compound (CDCH = 180", I,, = 180O) 3J(CH) equals 10.4 Hz, in qualitative agreement with the INDO calculations.

NMR analysis of the trisaccharide 2′‐fucosyllactose by heteronuclear trans‐glycosidic coupling constants and molecular simulations

“…18,20,21 In addition, one must not over-interpret data since 3 J(C,H) in 1 H-C-C-13 C-OH fragments have been shown to differ significantly (3.3 Hz) simply by changing the configuration of the hydroxyl group substituting the coupling pathway of interest. 22 One of the most useful applications of 3 J(C,H) in conjunction with Karplus relationships is, besides determination of stereochemistry, to reveal conformational trends in similar compounds, e.g. in glucosyl trisaccharides where differences in 3 J(C,H) values correlated with results from molecular simulations.…”

Conformational study of a guaiacyl β‐O‐4 lignin model compound by NMR. Examination of intramolecular hydrogen bonding interactions and conformational flexibility in solution

“…To the best of our knowledge, no Karplus relationship considering simultaneously in one equation the effect of electronegative substituents located on the three carbons along the coupling pathway has been reported yet for long-range proton-carbon coupling constants. We used a hybrid approach by combining into a single equation the relationship proposed by van Beuzekom et al, 50 which considers the influence of the torsion angle C -Cˇ-C˛-H˛and the electronegativity and orientation of the substituent located on the -carbon, in conjunction with the empirical correction terms proposed by Morvai et al 51 for hydroxyl substituents located on theˇ-and˛-carbons of the coupling pathway. The final equation used has the following form: 13 C spin-lattice relaxation experiment 13 C spin-lattice relaxation times T 1 were measured using the classical inversion-recovery with gate decoupling pulse sequence.…”

Effect of oxygen substituents on two- and three-bond carbon-proton spin-spin coupling constants

“…The measured coupling constants of iridoids 1-12 provide such data for hydroxyl substituents. Theoretical calculations 20,21 and experimental studies 22 -24 both indicate that the effect of substituents depends on the dihedral angle between coupled atoms and the type or orientation of substituents at˛-,ˇ-and -positions. To describe this we use the dihedral angle between coupled atoms ( ) and, furthermore, the dihedral angle between a substituent and the atoms of the coupling pathway ( ), namely O-13 C˛-Cˇ-C , O-Cˇ-C -1 H and 13 C˛-Cˇ-C -O dihedral angles for˛-,ˇ-andsubstitution, respectively.…”