Spin coupling constants direct observations

A systematic development of relativistic molecular Hamiltonians and various non-relativistic approximations are presented. Our starting point is the Dirac one-fermion Hamiltonian in the presence of an external electromagnetic field. The problems associated with generalizing Dirac s one-fermion theory smoothly to more than one fermion are discussed. The description of many-fermion systems within the framework of quantum electrodynamics (QED) will lead to Hamiltonians which do not suffer from the problems associated with the direct extension of Dirac s one-fermion theory to many-fermion system. An exhaustive discussion of the recent QED developments in the relevant area is not presented, except for cursory remarks for completeness. The non-relativistic form (NRF) of the many-electron relativistic Hamiltonian is developed as the working Hamiltonian. It is used to extract operators for the observables, which represent the response of a molecule to an external electromagnetic radiation field. In this study, our focus is mainly on the operators which eventually were used to calculate the nuclear magnetic resonance (NMR) chemical shifts and indirect nuclear spin-spin coupling constants. 

Turning now to lithium, we have two nuclides available for NMR measurements Li and Li. Both are quadrupolar nuclei with spin quantum number / of 1 and 3/2, respectively. The natural abundance of Li (92.6%) provides enough NMR sensitivity for direct measurements, but also Li (7.4%) can easily be observed without enrichment. However, isotopic enrichment poses no practical problem and is advantageous if sensitivity is important, as for measurements of spin-spin coupling constants in solution and of quadrupole coupling constants in the sohd state. 

The quantitative theory of CIDNP " is developed to a state where the intensity ratios of CIDNP spectra can be computed on the basis of reaction and relaxation rates and the characteristic parameters of the radical pair (initial spin multiplicity, T) the individual radicals (electron g factors, hfcs, a) and the products (spin-spin coupling constants, J). On the other hand, the patterns of signal directions and intensities observed for different nuclei of a reaction product can be interpreted in terms of hfcs of the same nuclei in the radical cation intermediate. 

Frequencies and intensities of bands in the IR spectra of 1,2,4-triazines have been calculated by the 4-31G method50 and by ab initio Hartree-Fock level with 6-31G, 6-31G, U-9 and 3-21G methods.51 The shifts for the protons in the parent 1,2,4-triazine have been predicted and are in reasonable agreement with the observed values.52 The shifts of the 13C NMR signals for 3,5,6-trichloro-l, 2,4-triazine have been calculated by the first and second order SCS method and compared with the experimental values.53 The nature of lone pair effects of heteroatoms on direct 13C — H spin coupling constants has been calculated by the AMI method54 and the nuclear shielding tensors of 15N and 1 C nuclei by the SOLO (second-order corrected localized orbital-local origin method) ab initio method.55 14N Shifts have been predicted.75 The electron distribution of 1,2,4-triazines has been estimated from the observed NMR shifts.76... 

Considerations similar to those made about electric dipole moments apply to other one-electron properties, for instance the nuclear spin-spin coupling constants between non-bonded hydrogen atoms in molecules like methane. These quantities are approximately equal to zero in the simple molecular orbital theory, as it is easily proved by using equivalent orbitals corresponding to the CH bonds instead of the usual delocalized MO s (34). Actually, the nuclear spins of protons cannot interact wta the electrons, since a localized MO cannot be large on two hydrogens at the same time, and correlation should be primarily responsible for all coupling constants, except perhaps for those observed for directly bonded atoms (see Sec. 4). 

The shifts 6( H) and 6(3ip) and spin-spin coupling constants J(3ip,iH) were derived from the H NMR doublets (splitting by ip with 1=1/2) and the ip NMR triplets (splitting by two protons), which were observed in the experimental systems described above, and are listed in the table below regardless of the difficulties in assigning the spectra. 6( H) is referred to TMS and is positive for low-field shifts see a compilation of chemical shifts of protons directly bonded to P [10]. is referred to 85% H3PO4 (also positive for low-field shifts) see a review on ip NMR spectra [11]. For details and supplemental information, see the remarks below the table ... 

The only compounds where direct NMR observations have been employed to deduce a halogen spin coupling constant are FCIO, CIF ,... 

N spectrum is observed without decoupling for an appropriate crosspolarization time T, the spin coupling constants can be directly measured provided that an additional tt/2 pulse is conveniently applied at the end of the cross-polarization sequence in order to remove the phase anomalies (C 25). 

Spin densities help to predict the observed coupling constants in electron spin resonance (ESR) spectroscopy. From spin density plots you can predict a direct relationship between the spin density on a carbon atom and the coupling constant associated with an adjacent hydrogen. 

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