14N hyperfine couplings

Isotropic Proton and 14N Hyperfine Coupling Constants for the Nitrobenzene Anion and some of its Derivatives (G)... 

Delocalization of the spin density onto the R2 group is particularly efficient if R2 = aryl, alkoxy or alkylthio. Delocalization of the unpaired spin to an aryl ring is consistent with the lower g-values and a(14N) hyperfine couplings observed for N-arylsulphonamidyls. Moreover, the interpolated order for the proton hyperfine couplings, viz. para > ortho... 

In order to identify organic free - radicals present at quantifiable concentrations during the sonication of PCBs, we employed Electron Spin Resonance (ESR) with a spin trap, N-t-butyl-a-phenyl-nitrone (PBN). PBN reacts with the reactive free - radicals to form more stable spin-adducts, which are then detected by ESR. The ESR spectrum of a PBN spin adduct exhibits hyperfine coupling of the unpaired election with the 14N and the (3-H nuclei which leads to a triplet of doublets. The combination of the spin-adduct peak position and peak interval uniquely identifies the structure of a free-radical. 

The three-pulse electron spin-echo envelope modulation (ESEEM) technique is particularly sensitive for detecting hyperfine couplings to nuclei with a weak nuclear moment, such as 14N. It has been used to probe the coordination state of nickel in two hydrogenases from M. tkermoautotrophicum, strain AH (56). One of these enzymes contains FAD and catalyzes the reduction of F420 (7,8-dimethyl-8-hydroxy-5-deazaflavin), while the other contains no FAD and has so far only been shown to reduce artificial redox agents such as methyl viologen. 

The importance of the different nodal properties of the two types of SOMO shown in Fig. 16 lies in their influence upon the nitrogen hyperfine coupling A(14N) in the electron spin resonance spectra in... 

These mononuclear species give ESR spectra characterized in every case by hyperfine coupling to two 14N nuclei and to the a hydrogens, when present, of the SR groups (23, 55) (Table IV). If base is added... 

When [Fe4S3(NO)7] reacts with RS in DMF solution, then, for a range of substituents R, the products are (23) [Fe(NO)2(SR)2] , as formed from [Fe2(SR)2(NO)4], together with [Fe(NO)(SR)3] in this latter series (Table IV), although the A(14N) value depends upon R, no hyperfine coupling to the a-hydrogen atoms in R was resolved, so that all the spectra of [Fe(NO)(SR)3] comprise three lines only. The mono-nitrosyl complexes are fairly short lived, and within 2 days only the dinitrosyl complexes were detectable. Since the formal oxidation states of iron in [Fe(NO)2(SR)2] and [Fe(NO)(SR)3] are Fe(-I) and Fe(I), respectively, it is likely that [Fe(NO)2(SR)2] arises from the basal iron atoms in [Fe4S3(NO)7] , and that [Fe(NO)(SR)3] arises from the apical iron [Eq. (17)]. 

Pople et al 27) developed the INDO method specifically to account for hyperfine coupling constants in terms of spin densities. Pople, Beveridge and Dobosh have calculated the spin densities of a large number of compounds and found a good agreement with the experimental isotropic hyperfine coupling constants of 1H, 13C, 14N, 170, and 19F (Tables 32 to 35). 

The microwave spectrum of isothiazole shows that the molecule is planar, and enables rotational constants and NQR hyperfine coupling constants to be determined (67MI41700). The total dipole moment was estimated to be 2.4 0.2 D, which agrees with dielectric measurements. Asymmetry parameters and 14N NQR coupling constants show small differences between the solid and gaseous states (79ZN(A)220>, and the principal dipole moment axis approximately bisects the S—N and C(4)—C(5) bonds. 

Bond lengths and angles, dipole moments (Table 2, Section 4.01.3.2) and 14N nuclear coupling have been determined and the 14N hyperfine structure has been analyzed for several 1,3,4-oxadiazoles (74PMH(6)53). Observed 44N quadrupole coupling constants have been compared with values obtained from microwave spectra (74JCP(6l)l494). 

Table 7. Values ofe2QqQ. For 14N the value is derived from a variety of theoretical studies on simple molecules, the remainder come from direct measurements of atomic hyperfine coupling constants...

The most important examples of 2S states to be described in this book are CO+, where there is no nuclear hyperfine coupling in the main isotopomer, CN, which has 14N hyperfine interaction, and the Hj ion. A number of different 3E states are described, with and without hyperfine coupling. A particularly important and interesting example is N2 in its A 3ZU excited state, studied by De Santis, Lurio, Miller and Freund [19] using molecular beam magnetic resonance. The details are described in chapter 8 the only aspect to be mentioned here is that in a homonuclear molecule like N2, the individual nuclear spins (1 = 1 for 14N) are coupled to form a total spin, It, which in this case takes the values 2, 1 and 0. The hyperfine Hamiltonian terms are then written in terms of the appropriate value of h As we have already mentioned, the presence of one or more quadrupolar nuclei will give rise to electric quadrupole hyperfine interaction the theory is essentially the same as that already presented for1 + states. 

The theory of the magnetic hyperfine interactions in NCI is essentially the same as that already described for the PF radical in the previous section, except that the nuclear spins / are 1 for 14N and 3/2 for 35C1. The form of the effective Hamiltonian for the quadrupole interaction and its matrix elements for two different quadrupolar nuclei was described in some detail in chapter 8 when we discussed the electric resonance spectra of CsF and LiBr. We now use the same case (b) hyperfine-coupled basis set as was used for PF. The quadrupole Hamiltonian for the two nuclei can be written as the sum of two independent terms as follows ... 

Table 19 Isotropic hyperfine coupling constants (MHz) for the atoms UB(2P), 13C(3P), 14N(4S) and 170(3P) computed using various DFT functionals. All calculations are done using the (18s,13p,4d,3f) basis set. FYom Ref [113].

Fig. 3. First derivative electron spin resonance spectra. (A) ESR spectrum of an unpaired electron. (B) ESR spectrum of an unpaired electron interacting with a nitroxide resulting in a nitrogen hyperfine coupling constant aN. (C) ESR spectrum of an unpaired electron interacting with a H nucleus and a l4N nucleus as is typical for PBN radical adducts. (D) ESR spectrum of an unpaired electron interacting with the l3C nucleus, the H nucleus and the 14N nucleus of the trichloromethyl radical adduct of PBN, where the carbon tetrachloride was labeled with 13C.

Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the exact cancellation limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state. 

Fig. 3.7. First derivative liquid-phase ESR spectra for nitroxide radicals formed by the spin-trapping method, (a) the CC13 adduct of PBN, showing hyperfine coupling to the 14N and H of the parent nitrone (b) the same species formed from l1-CClj radicals, showing the extra large doublet splitting from l3C (The /Mines are from an extraneous radical species.) (c) the hydroxyl radical adduct of DMPO, showing the characteristic 1 2 2 1 quartet, generated because of the fortuitous equality of the 1H and l4N splittings.

Contact shifts for 19F (and H, I3C, and 14N) have been measured for the complexes Ni(acac)2(An)2 (acac = acetylacetonate, An = ortho-, meta-, and para-fluoroaniline, 2,4,6-trifluoroaniline, and pentafluoro-aniline). (204) The hyperfine coupling constants derived from these measurements are in good agreement with those obtained by ESR, confirming that the unpaired spin density on the aromatic ring is delocalized in a manner similar to that of aromatic -electron radicals. 

---