Ethylene coordination, insertion, and chain transfer at a cationic aluminum center: A comparative study withAb Initio correlated level and density functional methods

Abstract

The performance of correlated ab initio methods and DFT methods was compared for the propagation and chain transfer steps of ethylene polymerization by a model aluminum–amidinate system, [{HC(NH)2}AlCH2CH3]+. All methods agree that the main chain transfer mechanism is β‐hydrogen transfer to the monomer (BHT), and that this is substantially easier than propagation; implications for the real Jordan system are discussed briefly. Counterpoise corrections are necessary to obtain reasonable olefin complexation energies. Activation energies are consistently lower at DFT (BP86, B3LYP) than at ab initio levels [MP2, MP3, MP4, CI, CCSD(T)]; the differences are particularly large (16 kcal/mol) for the BHT reaction. This is suggested to be related to the known problem of DFT in describing hydrogen bridged systems. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 398–410, 2000

Improving “difficult” reaction barriers with self-interaction corrected density functional theory

“…For many activation barriers in this category, gradient-corrected XC functionals offer little advantage over the simple local density approxi-mation ͑LDA͒ functionals, 7,9,11 while an admixture of exact exchange may lead to only partial improvements. 9,10,13,14,16,17,20,29 A persistent error, across three generations of approximate XC functionals, appears to indicate a common, fundamental deficiency. Specifically adjusting approximate XC functionals, to reproduce the difficult reaction barriers, 17,20 leads to an improved description, and provides a pragmatic way of studying such reactions with DFT.…”

On Ethylene Polymerisation with Aluminium and Scandium Complexes: DFT and Post‐HF Calculations

“…Table 1 comprises the results obtained in the form of energy barrier values for both ethylene insertion as well as termination by β ‐hydrogen transfer (BHT). The results shown have been obtained with the type of methods previously employed to study the aluminium based systems 11–20. The following observations are immediately obvious.…”

“…This ratio is indicative of whether a potentially catalytic species is or is not expected to yield a polymer, with oligomerisation being the result when the ratio is not large but still favours propagation. Previous computational studies11 revealed that β ‐hydrogen transfer is by far the most likely termination mechanism. This termination process and the propagation process are illustrated in Scheme .…”

“…Aluminium has an 3p 1 electronic configuration and has neither d‐ nor f‐electrons, which makes it attractive for theoretical studies 11–20. The simplicity of the electronic configuration makes it tempting to believe that computations will readily lead to a straightforward understanding of the activity of these complexes in olefin polymerisation.…”

“…However, this is not the case. Firstly, from a theoretical point of view, for ( 1 ) with R = R′ = H, the BP86 density functional often applied in organometallic chemistry was said to lead to unrealistically low barriers for the β ‐hydrogen transfer which leads to chain termination 11,14,15. When the CCSD(T) level of theory was invoked, the barrier value increased by some 150% to 25 kcal · mol −1 .…”