Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculations

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Abstract

The P2<sub>1</sub>/c form of the nitronyl nitroxide 2-phenyl-4, 4, 5, 5-tetramethyl-4, 5-dihydro-1 H-imidazole-1-oxyl 3-oxide (NitPh) was investigated by conventional and polarized neutron diffraction. Model-free reconstruction of the spin distribution from the experimental structure factors using the recently developed 3D maximum of entropy technique gives the first direct experimental evidence for the p-shape of the spin density in the vicinity of the major spin carriers in nitroxide free radicals. Using other reconstruction methods gives individual atomic spin populations. Most of the spin density is equally shared between the four atoms of the two NO groups. In addition, the bridging sp<sup>2</sup>carbon atom carries a significant negative spin density. Derealization of the unpaired spin onto the phenyl fragment is weak; sign alternation on the ring is observed and agrees with previous NMR experiments. The experimental results are compared to those obtained by various theoretical ab initio calculations. Unrestricted Hartree-Fock (UHF), second order Moller-Plesset (MP2), and complete configuration interaction (CI) calculations predict correctly the signs of individual atomic spin populations, but as long as the magnitudes are concerned, the agreement with experiment is rather poor. Nevertheless, the results are improved significantly in going from UHF to MP2 and CI as the correlation energy is better accounted for. On the contrary, density functional theory (DFT) calculations give good results for both the signs and magnitudes of spin populations and require much less computation time and system resources. The role of the basis set was also studied. UHF is improved by using a larger basis set. The choice of a suitable basis is thus crucial. Unlike the case of UHF, DF results are quite stable with respect to the basis set used, and a rather limited one is sufficient. © 1994, American Chemical Society. All rights reserved.

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