Spin dipolar term

We have seen that the dependence of Bn(R) on the vibrational coordinate causes a mixing of the vibrational level of interest with neighbouring levels. This mixing results in centrifugal distortion corrections to all the various parameters Xn(R) in the perturbation Hamiltonian 3C when combined in a cross term. The operator has the same form as in the original term, for example, (2/3) /6T 0(S, S) for the spin spin dipolar term, multiplied by N2. The coefficient which qualifies this term has the general form... 

There are three separate contributions to the total magnetic hyperfine interaction, namely, the Fermi contact term, the orbital hyperfine term, and the electron spin-nuclear spin dipolar term ... 

Beer and Grinter used finite perturbation theory to calculate J(Si-H), J(Si-H), (161) and J(Si-C) (143) as well as the analogous phosphorus couplings. The best results are obtained for J(Si-Q (Table XXI) for which the correlation coefficient of the best fit of calculated to observed values is 0-985. The various calculated contributions to J(Si-C) are in Table XXI. Clearly, the orbital and spin-dipolar terms, which are small and also of opposite sign, have little effect on the calculated value of J. It is concluded that the Fermi contact term is probably sufficient for the calculation of J(Si-Q and that inclusion of silicon d orbitals is not required. 

Blizzard and Santry (16, 17) also examined C-C coupling constants using the INDO-FPT method. They included the orbital and spin dipolar terms as well as the contact term. Of the 15 molecules considered none were cyclic hydrocarbons. The contact term was found to be the most important, followed by the orbital and spin dipolar terms. The authors reported that the contact term completely dominates in the case of C-C single bonds, while the other terms become important when the two carbons are joined by multiple bonds. 

It has been shown by Peralta et that a spin-dipolar term might play an important role in the spin-spin F-F coupling across three and five bonds. 

The situation for carbon-nitrogen couplings is complicated (636). Concerning one-bond carbon-nitrogen couplings it has been suggested that the Fermi contact term is dominant for all CN systems for which isosteric CC systems exist (156). In particular, in most CN systems with formal lone-pairs apart from the Fermi contact term the orbital and spin dipolar terms are operative and non-negligible (636,156). 

The spin-dipolar term in the Pople-Santry treatment may in... 

One of the unique spectroscopic features of flie T1 site, as mentioned above, is the small compared to that of the normal Cu(II) centers. There are three contributions to account for the metal hyperfine coupling, which are flie Fermi contact (. f), the spin dipolar (.4s), and the orbital dipolar (4l) terms. The Fermi contact term is isotropic (A = 1) and associated wifli unpaired electron spin density at flie nucleus. In a Cu(II) center, this contribution involves the unpaired 3d electron spin polarizing the inner Is, 2s, and 3s core electron pairs (mostly 2s) to produce a net negative spin density at the nucleus. The spin dipolar term is anisotropic (A = -1/24 ) and involves the electron spin, averaged over flie shape of its 3d orbital... 

PSn) and A (PPb) for certain formal hybridizations on P, Sn, and Pb show that these estimated noncontact terms are under 10% of typical experimental C(PSn) and. r(PPb) values. In transition metal complexes which could have substantial n back-bonding between the coupled metal and ligand atoms, the dipolar and orbital contributions may have to be considered. Upper limits for the spin-dipolar term for one-bond couplings A (XC) and X(XF) in the absence of multiple bonds have been calculated for elements X = Li to Bi and displayed in Periodic Table form. ... 

C-N and N-N couplings have been analyzed by coupled Hartree-Fock calculations, and the conclusions can help to explain couplings to H, P, Si, metals etc. If neither of the coupled atoms bears a lone pair with 5 character, A (NX) like X(CC) is positive and increases with 5 character in the bond orbitals (e.g. 5/ <5/7 <5/ ). The Fc term normally dominates, but the orbital and spin-dipolar terms increase with n bond order and become relatively large for triple bonds. These terms depend on and so increase with effective nuclear charge of the... 

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