NMR J-coupling analysis

FINITE-FIELD PERTURBATION THEORY OF FERMI-CONTACT INTERACTIONS 

The J-coupling can be evaluated from the small energy lowering and UHF-type orbital distortions that accompany response to the FC spin perturbation. The contribution of a given NBO or NLMO to Jab coupling requires calculation of the spin polarization response to perturbations at each nucleus, and J-couphngs among N nuclei therefore require N separate UHF-type finite-field calculations. 

Note that accurate evaluation of J-coupling constants requires highly flexible description of core orbitals, so the 6-311-1— -G basis set provides only a qualitative description for present illustrative purposes. 

NIC analysis requires a complex interactive partnership with a host ESS program that is capable of finite-field FC perturbation calculations, and is currently implemented only in NB05.9-linked versions of the Gaussian-09 program. Because separate FC perturbations are required for each coupled nucleus of interest, overall NIC analysis involves more Ga 55ia/J-specific job input detail than is required for other NBO keylist options (cf. Sidebar 7.1). For further details of NIC algorithms and input syntax, see the NBO Manual, p. B-147ff. 

As a simple illustration of J-coupling calculations and analysis, let us return to the ethanol example (1/0-7.1) to focus on proton spin-spin couplings that dominate the experimental H-NMR spectmm. An input file to compute Jjjjj, couplings and perform NIC analysis for all distinct H, H pairs is shown in FO-7.4. The overall job consists of six chained calculations (with Linkl— separators) for successive 

---