Hyperfine predicting

In this exercise, we will predict the isotropic hyperfine splitting in HNCN radical at a... 

Using different DFT functionals and basis sets (Focsan et al. 2008, Lawrence et al. 2008) it was confirmed that the isotropic ()-methyl proton hyperfine couplings do not exceed 9MHz for the carotenoid radical cation, Car-. DFT calculations of neutral carotenoid radicals, Car formed by proton loss (indicated by ) from the radical cation, predicted isotropic P-methyl proton couplings up to 16 MHz, a fact that explained the large isotropic couplings observed by ENDOR measurements for methyl protons in UV irradiated carotenoids supported on silica gel, Nafion films, silica-alumina matrices, or incorporated in molecular sieves (Piekara-Sady et al. 1991, 1995, Wu et al. 

O Malley, P., and S. J. Collins. 1996. Density functional studies of free radicals accurate geometry and hyperfine coupling prediction for semiquinone anions. Chem. Phys. Lett. 259, 296. 

Once a hyperfine pattern has been recognized, the line position information can be summarized by the spin Hamiltonian parameters, g and at. These parameters can be extracted from spectra by a linear least-squares fit of experimental line positions to eqn (2.3). However, for high-spin nuclei and/or large couplings, one soon finds that the lines are not evenly spaced as predicted by eqn (2.3) and second-order corrections must be made. Solving the spin Hamiltonian, eqn (2.1), to second order in perturbation theory, eqn (2.3) becomes 4... 

For aromatic hydrocarbon radical anions, this approach works pretty well. Figure 2.7 shows a correlation plot of observed hyperfine splitting versus the spin density calculated from Hiickel MO theory. It also correctly predicts the negative sign of aH for protons attached to n systems. 

The NO2 molecule offers an example which illustrates this point. The spectrum of N02 molecules rigidly held on MgO at —196° is characterized by gxx = 2.005, gyv = 1.991, and gzz = 2.002 (29). If this molecule were rapidly tumbling, one would expect a value of Qa.v — 1 999. The spectrum of NO2 absorbed in a 13X molecular sieve indicates an isotropic gzv = 2.003 (.80), which is within experimental error of the predicted value for NO2 on MgO. The hyperfine constants confirm that NO2 is rapidly tumbling or undergoing a significant libration about some equilibrium position in the molecular sieve (81). 

Gardner 45) has observed the spectrum of Cl atoms adsorbed on a silica-gel surface at 77°K. The experimental results indicate that the orbital degeneracy of the 3p atomic orbital has been removed as a result of the electrostatic interaction with the surface. From the occupancy of the atomic orbitals one would predict that gx >011 — 2.00 and indeed the experimental g values are = 2.012 with g = 2.003. The hyperfine coupling indicates that the impaired electron is highly localized in the 3p orbitals. 

Recent calculations of hyperfine parameters using pseudopotential-density-functional theory, when combined with the ability to generate accurate total-energy surfaces, establish this technique as a powerful tool for the study of defects in semiconductors. One area in which theory is not yet able to make accurate predictions is for positions of defect levels in the band structure. Methods that go beyond the one-particle description are available but presently too computationally demanding. Increasing computer power and/or the development of simplified schemes will hopefully... 

The 14N hf and quadrupole parameters observed in Co(acacen) by Rudin et al.59 are the first magnetic data reported on equatorial nitrogen ligand nuclei in a low-spin Co(II) complex. Only two of the four predicted AmN = 1 ENDOR transitions (3.9) were observed for each nitrogen nucleus. A numerical calculation of the transition probabilities shows that the corresponding transitions in the other ms-state are at least ten times less intense (hyperfine enhancement). 

The effect of the sulfone dipole on the ESR spectrum of the dibenzothiophene 5,5-dioxide anion has been examined, modified Hiickel calculations which ignored d-orbital conjugation predicting spin distributions which agreed with experimental data. Proton hyperfine splitting constants have also been obtained for the sulfone. Correlation with MO calculations shows that the sulfone group contributes a vacant symmetric orbital to the conjugated system. ... 

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