Using density functional theory the metathesis reactions of 1-octene in the presence of Grubbs 1 [RuCl2(PCy3)(2)(=CHPh)] is investigated. At the GGA-PW91/DNP level, the complete geometry optimization and the activation energy of various activation steps and catalytic cycles in the dissociative mechanism are performed. The formation of the catalytically active heptylidene species is kinetically and thermodynamically favoured, while the formation of trans-tetradecene is thermodynamically favoured. The computational results are in agreement with the experimental results obtained with NMR and GC/MSD experiments. Grubbs 1 is active for the metathesis of 1-octene at 25 degrees C yielding trans-7-tetradecene as the major product. The formation of by-products, i.e. styrene isomers and PCy3, is consistent with the dissociative metal carbene mechanism. The rapid formation of the heptylidene derivative of Grubbs 1 is observed by in situ NMR. (c) 2006 Elsevier B.V. All rights reserved.