Hyperfine couplings and ¹³C relaxation in alkylamines coordinated to Ni(II) acetylacetonate

Abstract

The conformational dependence of the proton and carbon-13 hyperfine coupling constants in aliphatic amines Coordinated to Ni(AA)~ has been investigated by N.M.R. contact shift measurements. It is shown that the hyperfine coupling constants ac/~ and ari B are given by a~ (O) = px*'(B: cos O+ B~ cos 2 0), with B: H = 10.5 G, B2 H = 10.6 G, B: c = -4.9 G, B~ c = 13.1 G where 0 is the torsion angle about the C1-N bond. The experimental values of aH/~ and ac/~ in the ethylamine-Ni(AA)z complex have been compared with those calculated from the above expression and the potential function computed by INDO for the rotation of C~H5 about the N-C1 bond in EtI~IH~ +. Good agreement is obtained for a potential barrier Vo = 15.5 kJ mol-:.The :3C paramagnetic relaxation rates T:M-: and T2M-: in the complexes have also been determined. For n-butyl and n-hexylamine complexes, the dipolar longitudinal relaxation rates have been calculated from Monte Carlo simulations of random fluctuations of the magnetic field due to internal and overall motions and to electron spin transitions. Satisfactory agreement with experiment has been obtained from C3 to the terminal methyl carbon. On the other hand T:M-: of C1 and C2 are significantly larger than expected from Monte.Carlo calculations because of local 2s and 2p spin densities. The latter contribution has been estimated by calculating the hyperfine dipolar tensor from the average C-Ni distances and the C1 or C2 spin density matrices obtained from INDO calculations on related alkylamine cation radicals. The validity of this semi-quantitative treatment has been checked on the simplest example of the ethylamine-Ni(AA)2 complex.

Paramagnetic constraints: An aid for quick solution structure determination of paramagnetic metalloproteins

“…11(A)], then the coupling depends on cos 2 . This is the case when D is an amino nitrogen, because the only sizable spin density delocalization occurs on the bond between the metal ion and the nitrogen donor (85,86). The relationship is described by the general Karplus equation (84):…”

Proton and carbon-13 NMR studies of electron spin delocalization in nickel(II) bis(alkyl xanthate) complexes of nitrogenous bases and comparison with nickel-nitroxyl interactions in analogous spin-labeled complexes

“…Detectable 13C isotropic shifts have also been observed for the 7-and ¿-carbons of alkylamines coordinated to Ni(acac)2. 25 The and 13C shifts for Ni(acac)2(RNH2) were interpreted in terms of predominantly a delocalization with some contribution of spin polarization at the a-carbon. 25 Comparison of NMR Isotropic Shifts with EPR Data.…”

“…25 The and 13C shifts for Ni(acac)2(RNH2) were interpreted in terms of predominantly a delocalization with some contribution of spin polarization at the a-carbon. 25 Comparison of NMR Isotropic Shifts with EPR Data. The electron-electron spin-spin interaction in spin-labeled nickel complexes is expected to depend on the magnitude of the overlap of the orbitals containing the two unpaired electrons.…”