Molecular orbital calculations of substituent effects on directly bonded¹³C-H coupling constants

Abstract

The effect of substituents on 13C-H coupling constants is discussed, within the framework of the Pople-Santry MO theory of nuclear spin coupling, by making a theoretical analysis of the contributions due to differences of coulomb and resonance integrals as well as due to the presence of electron lone-pairs.Earlier treatments based on correlations of dcrt with s characters are shown to be less justified mainly because the description of CH bonds in terms of localized MO's is not valid, and no physical meaning can be rigorously attributed to concepts such as rehybridization.

Structural Dependencies of Interresidue Scalar Coupling h3JNC‘ and Donor 1H Chemical Shifts in the Hydrogen Bonding Regions of Proteins

“…However, for θ 2 ) F ) 90°the overlap integrals with all three hybrid orbitals are equal, and the overlap with the 2p π AO on oxygen assumes the maximum value. The Pople-Santry MO formulation [49][50][51][52] implies a quadratic dependence of the coupling constant on S(ho 1 ′) in eq 3a. This should lead to the dominant contribution to h3 J NC′ since it involves a direct (electron-mediated) mechanism between the n-h and o 1 ′-c 1 ′ bond pairs in Figure 2.…”

“…48 These usually lead to smaller coupling contributions than direct mechanisms since they arise in higher order perturbation theory. 48,50,51 Consider the situation for trans-H-bond coupling in Figure 2. In many H-bonding situations the donor hydrogen will interact very effectively with the lone pairs o 2 ′and o 3 ′, and the latter by angularly independent interactions with o 1 ′, and c 1 ′-c 3 ′.…”

“…The computed values are strongly dependent on both quantities. The Pople-Santry formulation [49][50][51][52] implies a quadratic dependence on the overlap integrals S hσ′ and S hs′ in eq 3a. Since the latter are dominated by an exponential dependence on internuclear separation, 57 the computed quantities were fit to a form combining the cos 2 θ 2 and the exponential dependencies, Equation 5 has a standard deviation 0.12 Hz and correlation coefficient r 2 ) 0.898.…”

“…Early studies of scalar coupling made use of semiempirical valence-bond − (VB) and molecular orbital (MO) methods . Perturbation theory was used to obtain expressions relating coupling constants to VB exchange integrals 46-48 or MO resonance integrals. − In semiempirical MO theory the latter are assumed to be proportional to overlap integrals between orbitals on different centers. The exchange integrals of simple VB theory contain other terms, but the overlap integrals also play a major role…”

The effect of substituents on proton coupling constants: Simple theoretical correlations

“…He assumes that the carbon hybridization considered appropriate to a localized orbital approach to the 13C-H coupling constants is also adequate for the calculation of JHHvic; this, without a proper justification, which seems in fact difficult to find as is suggested by recent work on the interpretation of 13C-H coupling constants [5,6]. The tentative explanation we offer for the observed decrease of Jttti vie in vinyl derivatives, for example,…”