The active catalytic species and its isomerisation in the catalytic carbonylation of methanol - a density functional study

The Monsanto acetic acid process is one of the most effective ways to produ ce acetic acid industrially. This process has been studied experimentally b ut theoretical investigations are so far sparse. In the current work the ac tive catalytic species [Rh(CO)(2)I-2](-) (1) and its isomerisation has been studied theoretically using the hybrid B3LYP exchange and correlation func tional. Similar calculations has been performed for the iridium complex [Ir (CO)(2)I-2](-) (2) that also is catalytically active in the methanol carbon ylation. Experimental work has confirmed the existence of the cis forms of the active catalytic species, but they do not rule out the possibility of t he trans isomers. Our gas phase results show that cis-1 has 4.95 kcal/mol l ower free energy than trans-1, and cis-2 has 10.39 kcal/mol lower free ener gy than trans-2, In the case of rhodium, trans-1 can take part to the catal ytic cycle but in case of iridium this is not very likely. We have also inv estigated the possible mechanisms of the cis to trans conversions, The liga nd association mechanism gave free energy barrier of 13.7 kcal/mol for the rhodium complex and 19.8 kcal/mol for iridium, Thus the conversion for the rhodium complex is feasible whereas for iridium it is unlikely, (C) 2001 El sevier Science B.V. All rights reserved.