Theoretical study on the gas-phase reaction mechanism between rhodium monoxide cation and methane

Abstract

The gas-phase reaction mechanism between rhodium monoxide cation and methane has been investigated on the singlet and triplet state potential energy surfaces at the CCSD(T)/6-311+G(2d,2p), SDD//B3LYP/6-311+G(2d,2p), SDD level. Over the 300–1100 K temperature range, the branching ratios of Rh+ + CH3OH and RhCH2 + + H2O are 83.8–52.6% and 16.2–47.4%, respectively, whereas the branching ratio of CH2ORh+ + H2 is so small to be negligible. For the main products (Rh+ + CH3OH and RhCH2 + + H2O) formation, the minimum energy reaction pathway involves singlet–triplet spin inversion, and both b-RhCH3OH+ and H2ORhCH2 + are the energetically preferred intermediates. Alternatively, in the CH2ORh+ + H2 reaction, both b-RhCH3OH+ and H2RhOCH2 + are the energetically favorable intermediates, and the main products are Rh+ + CH3OH. In the RhCH2 + + H2O reaction, the main products are Rh+ + CH3OH with the energetically predominant intermediate b-RhCH3OH+. In the reaction of Rh+ + CH3OH, both b-RhCH3OH+ and H2RhOCH2 + are the energetically preferable intermediates, and the main products are CH2ORh+ + H2. Besides, toward methane activation, the cation RhO+ exhibits higher reaction efficiency than the cation Rh+, the neutral RhO, and its first-row congener CoO+, and it exhibits lower methanol branching ratio and higher water branching ratio than RhO and CoO+.