SPIN-DENSITY IN A NITRONYL NITROXIDE FREE-RADICAL - POLARIZED NEUTRON-DIFFRACTION INVESTIGATION AND AB-INITIO CALCULATIONS

The P2(1)/c form of the nitronyl nitroxide ,4,5,5-tetramethyl-4,5-dihy dro-1H-imidazole-1-oxyl 3-oxide (NitPh) was investigated by convention al and polarized neutron diffraction. Model-free reconstruction of the spin distribution from the experimental structure factors using the r ecently developed 3D maximum of entropy technique gives the first dire ct experimental evidence for the p-shape of the spin density in the vi cinity of the major spin carriers in nitroxide free radicals. Using ot her reconstruction methods gives individual atomic spin populations. M ost of the spin density is equally shared between the four atoms of th e two NO groups. In addition, the bridging sp(2) carbon atom carries a significant negative spin density. Delocalization of the unpaired spi n onto the phenyl fragment is weak; sign alternation on the ring is ob served and agrees with previous NMR experiments. The experimental resu lts are compared to those obtained by various theoretical ab initio ca lculations. Unrestricted Hartree-Fock (UHF), second order Moller-Pless et (MP2), and complete configuration interaction (CI) calculations pre dict correctly the signs of individual atomic spin populations, but as long as the magnitudes are concerned, the agreement with experiment i s rather poor. Nevertheless, the results are improved significantly in going from UHF to MP2 and CI as the correlation energy is better acco unted for. On the contrary, density functional theory (DFT) calculatio ns give good results for both the signs and magnitudes of spin populat ions and require much less computation time and system resources. The role of the basis set was also studied. UHF is improved by using a lar ger 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.