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)₂ has been investigated by N.M.R. contact shift measurements. It is shown that the hyperfine coupling constants a _C ^β and a _H ^β are given by

with B ₁ ^H=10·5 G, B ₂ ^H=10·6 G, B ₁ ^C=-4·9 G, B ₂ ^C=13·1 G where ϑ is the torsion angle about the C1-N bond. The experimental values of a _H ^β and a _C ^β in the ethylamine-Ni(AA)₂ complex have been compared with those calculated from the above expression and the potential function computed by INDO for the rotation of C₂H₅ about the N-C1 bond in EtNH₂ ⁺. Good agreement is obtained for a potential barrier V ₀ = 15·5 kJ mol^-1.

The ¹³C paramagnetic relaxation rates T _1M ^-1 and T _2M ^-1 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 _1M ^-1 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.