Vicinal proton-proton coupling constants

Abstract

An assessment of the effect upon 3JHH of individual substituents with second row e-atoms has been carried out both theoretically, at ab initio and semiempirical levels of approximation, and empirically, by parametrization of Karplus type equations. The Fermi-contact contributions calculated with a 6-31G** basis set to determine substituent effects AJ x in monosubstituted ethanes CH3CHzX (X = CH3, NH2, OH and F), show a decay with the relative electronegativity of substituent A)~x towards a constant value like that presented by the experimental data. Similar quadratic or exponential decays are calculated with the same basis set for the two largest parameters in the Karplus equation, i.e. for the angle independent term Co and for the coefficient C2 of cos 2q5. By contrast, for the coefficient $2 of sin 2~b a proportional dependence on AZx is calculated. These calculated dependences upon Azx for Co, C2 and S 2 have been confirmed by empirical parametrization of the corresponding Karplus type equations using an appropriate dataset of experimental 3JnH couplings.

The effect of carbonyl group in the asymmetry of 3, 4JCH coupling constants in norbornanones

“…Nevertheless, it should be noted the strong σ C 3 -C 4 → σ * C 2 O hyperconjugative interaction involves the σ C 3 -C 4 bond orbital which contains the C 4 coupling nucleus of the 3,4 J C 4 H 1 coupling. It should also be noted that in norbonanones (1)(2)(3)(4)(5) (Table 2). These two last interactions, σ C 1 -C 7 → σ * C 2 O and σ C 3 -C 4 → σ * C 2 O , define an additional coupling pathway for 3,4 J C 4 H 1 but not for 3,4 J C 1 H 4 , providing an adequate rationalization to explain the observed difference between the 3,4 J C 1 H 4 and 3,4 J C 4 H 1 couplings in norbornanone derivatives.…”

“…Many experimental and theoretical works have been published in which several effects affecting the original Karplus relationship have been discussed: [4] for instance, substituent effects, [5,6] substituent orientation, [7] bond angles along the coupling pathway, [8,9] the presence of a heteroatom along the coupling pathway [10,11] and hyperconjugative interactions involving bonding or antibonding orbitals belonging to the coupling pathway. [12,13] Karplus-like relationships describing dihedral angle dependence of 3 J CH coupling have also been established.…”

Analytical calculation of nuclear magnetic resonance indirect spin–spin coupling constants at the generalized gradient approximation and hybrid levels of density-functional theory

“…For comparison, we have included the empirical Durette-Horton curve. 46 We note, however, that several ͑similar͒ Karplus curves exist in the literature; [47][48][49] Refs. 48 and 49 also contain Karplus curves calculated at the Hartree-Fock level.…”

On the molecular and electronic structure of matrine-type alkaloids

“…The theoretical difference Dd(H eq /H ax ) ranges from 2.9 in Table 4 Experimental and theoretical 13 Concerning the substituent and stereoelectronic effects on the indirect nuclear spin-spin coupling constants, owing to the large variety of n J(XY)s, we restrict the interest on a selection of vicinal and geminal 1 H-1 H and one-bond 13 C-1 H coupling constants. Such interproton couplings, involving lactamic and bridgehead H atoms of the matrine framework, display a well-documented conformational sensitivity [54][55][56]. On the other hand, an interesting manifestation of stereoelectronic factors is provided by the Perlin effect, that is the significant difference in 1 J(CH) presented by methylenic C-H bonds adjacent to the N or O LPO in a six-membered ring [57][58][59][60].…”