7Li solid-state NMR of lithium amides: structural information from7Li quadrupolar coupling constants

Abstract

Lithium-7 quadrupolar coupling constants, χ(7Li), were measured for a number of microcrystalline lithium amides by solid-state 7Li magic angle spinning (MAS) NMR. A linear correlation between χ(7Li) and the structural N—Li—N angle known from x-ray diffraction work was found: χ(7Li)=(4.1±0.5)∢N—Li—N -(110.8±69.0). On this basis, structural information for new systems becomes available. For the polymer material [H2C(CH2)3NLi]n a ladder structure is proposed. The π-flip of the five-membered metallacycle formed by the coordination of tetramethylethylenediamine (TMEDA) with lithium in [(H2C(CH2)3NLi)2·TMEDA]2 was detected by 13C cross-polarization/MAS DNMR spectroscopy. From the analysis of the temperature-dependent lineshape of the methyl signals, a barrier of ΔG≠(293)=66 kJ mol-1 was derived for this rotational process in the solid, which compares favourably with data derived for related compounds. © 1998 John Wiley & Sons, Ltd.