Computational study of the geometric and electronic structures of MN₂ (M = Mo or U)

Abstract

The geometric and electronic structures of singlet, triplet, quintet and septet MN₂ (M = Mo or U) have been calculated using quasi-relativistic, non-local density functional theory. The distribution of the MoN₂ structures over the spin states agrees well with previous theoretical data, as do the relative energies and vibrational wavenumbers of the minima. Six true minimum energy stationary point UN₂ structures have been located. Whereas all of the MoN₂ structures are less stable than the Mo + N₂ asymptote, all of the UN₂ minima are stable with respect to dissociation to metal + dinitrogen. Singlet linear NUN is found to be the most stable UN₂ structure at the scalar relativistic level, and the inclusion of spin–orbit coupling does not significantly affect the NUN energy with respect to the U + N₂ asymptote. The bonding in all of the UN₂ structures has been analysed. The U–N₂ interaction in the quintet and septet narrow angle side-on triangular geometries is a mixture of U 5f→N₂ π_g δ and π backbonding, with the latter component being much the more significant. U→N₂ π backbonding is also found to be the principal U–N₂ interaction in triplet and septet linear end-on UN₂. The U–N Mulliken overlap populations are largest for the wide angle triangular and linear NUN structures, consistent with the much shorter U–N bond lengths in these geometries in comparison with the narrow angle side-on and linear end-on minima. The agreement between the calculated and experimental vibrational wavenumbers for linear NUN is very good, and is superior to previous theoretical studies. The relevance of the present work to previous computational investigations of the U–N₂–U bonding in [{(NH₂)₃(NH₃)U}₂(µ-η²∶η²-N₂)] is discussed.

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