Pople coupling

The first theoretical handling of the weak R-T combined with the spin-orbit coupling was carried out by Pople [71]. It represents a generalization of the perturbative approaches by Renner and PL-H. The basis functions are assumed as products of (42) with the eigenfunctions of the spin operator conesponding to values E = 1/2. The spin-orbit contribution to the model Hamiltonian was taken in the phenomenological form (16). It was assumed that both interactions are small compared to the bending vibrational frequency and that both the... 

The fact that the absolute magnitudes of these two sets of coupling constants are different is also expected on the basis of the Pople, Bothner-By treatment (48). It would clearly be of some considerable interest if a general study were to be made of the stereospecificity of this geminal dependence since this may lead to another method for determining carbohydrate conformations. 

A theoretical value for the magnitude of dJ/dEz was obtained using the delocalized molecular orbital approach of Gil and Teixeira-Dias 17> who calculated substituent effects on. The Pople expression for the contact contribution to the coupling constant 1 c-h1 °f a methyl group can be written... 

Pople, J. A., Santry, D. P. . Molecular Orbital Theory of Nuclear Spin Coupling Constants. Mol. Phys. 8, 1 (1964). 

Pople,J. A., Bothner-By, A.A. Nuclear Spin Coupling between Geminal Hydrogen Atoms. J. Chem. Phys. 42, 1339 (1965). 

To account for this behaviour these authors proposed an extension of the theory, given by Pople and Santry, which predicted that the coupling constant depends upon the energies of the s relative to those of the p, d, etc. electrons of the coupled nuclei and additionally upon the resonance integral between the outer-shell r electrons of the two atoms. It was considered that the first factor remained fairly constant for the P-P bonds and the observed variation was qualitatively explained by the dependence of the second factor upon the electronegativity and the bulkiness of the substituent. 

The first two terms describe the chemical shifts for nuclei i and j and the last, the nuclear spin-spin interaction. Nuclear spins are coupled via intervening electrons. The coupling constant A, sometimes represented by J in the literature, is expressed in cycles per second (cps). Analyses of observed spectra for S,-, 5,- and A,-, are quite straightforward in situations where po 5i — 5, Aij but can become quite tedious where i 0 5 — 5,- and A,-,-are comparable. va is the resonance frequency expressed in cps. The latter case is the so-called intermediate coupling situation and is discussed in detail by Pople et al. (108). 

In the absence of X-ray crystallographic data, 31P and 195Pt NMR spectroscopy is a useful method to investigate the nature of the products in the solution. In addition to earlier references to these topics, the Pople-Santry theory has been used to calculate the signs and relative magnitudes of the coupling constants for the type of phosphorus-bridged complexes covered in this section.1502... 

Microwave spectra, Stark effect144 CNDO/2139 (Pople and Segal) and quadrupole (14N) coupling constants... 

NH4+ has received rather little attention recently. Pople and co-workers studied the geometry with the 4-31G basis set,67 and Hopkinson and Csizmadia58 reported calculations with a (8,3/3) ->[2,1/1] basis. In a rather differently motivated study, basis sets of ca. DZ quality were used by Claxton et al.59 who varied 7 (NH) and the proton exponent in calculations of the deuteron quadrupole coupling constants. These... 

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