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

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