Understanding Trends in 27Al Chemical Shifts and Quadrupolar Coupling Constants in Chloroalkyl Aluminum [AlClx(Me)3-x]n=1 or 2 Compounds

The usage of alkyl aluminum compounds and related structures as co-catalyst finds a broad range of application in homogeneous and heterogeneous catalysis. While understanding the nature of the aluminum species in solution or in solids can be a challenge, Al-27 solid state NMR is a powerful tool to understand the structures of Al species, but their assignment remains mostly empirical, typically by comparing chemical shifts with known compounds. In this work, the observed trends in Al-27-NMR parameters - chemical shift and quadrupolar coupling constant - of chloroalkyl aluminum compounds, a prototypical class of important Lewis activators, are traced back to their frontier orbitals and electron polarization through a natural localized molecular orbital analysis. This study thus provides guidelines to understand the nature of chemical shift and thereby assignment of possible structure.