Competition between agostic WCH2+ and HWCH+:: A joint experimental and theoretical study

We present both theoretical and experimental photodissociation results on the products of the methane dehydrogenation by W+ in the gas phase. We show that the reaction may lead to two isomers: whereas only the methylidenetungsten WCH2+ had been proposed, we show that the hydridomethylidynetungsten HWCH+ can also be formed. Both density functional and highly correlated ab initio quantum chemical calculations have been performed using a relativistic core potential for W+. Spin-orbit couplings have been evaluated semiempirically. We found the HWCH+ and the WCH2+ isomers to be nearly degenerate, the latter structure exhibiting a strong agostic distortion. Photodissociation of the mass selected [W,C,2H](+) product of the reaction has been carried out in a Fourier transform ion cyclotron resonance mass spectrometer. The H+WCH+ channel has been observed as the major photofragmentation channel and a photodissociation threshold of 2.5+/-0.1 eV has been derived. This low-energy value is in good agreement with the thermodynamic threshold determined theoretically. These results suggest a very large bond energy associated with the triple WC bond in WCH+ (about 158 kcal/mol). (C) 2001 American Institute of Physics.