Mechanism and Energetics for Dehydrogenation of Methane by Gaseous Iridium Ions
Ir^+ has been observed as the most efficient transition-metal ion for dehydrogenation of CH_4 in the gas phase. We carried out theoretical studies of the complete reaction profile for this process and find three salient factors responsible for the high reactivity: (i) the ability of Ir^+ to change spin easily, (ii) the strength of the Ir-C and Ir-H bonds, and (iii) the ability of Ir^+ to form up to four covalent bonds. We show that among transition-metal ions Ir^+ is unique in best possessing all three characteristics. The combination of these factors leads to a global minimum for the singlet Ir(H)_2(CH_2)^+ structure, which plays an important role in the activation. On the basis of these results, we suggest solution-phase analogues that may also activate CH_4.
© 1994 American Chemical Society. Received July 1, 1993. We wish to thank Professor J. L. Beauchamp for pointing out the special property of Ir^+ for forming IrCH_2^+ from CH_4. We also thank Dr. K. K. Irikura and Mr. E. P. Bierwagen for helpful discussions. J.K.P. acknowledges a graduate fellowship from BP America (Dr. Jim Burrington). This research was supported by the NSF (Grant No. CH91-100284). We also want to thank the NSF-Pittsburgh Supercomputing Center for a Cray allocation. The facilities of the MSC used in these studies are supported by the NSF-GCAG, DOE-AICD, Allied-Signal, Asahi Chemical, Asahi Glass, BF Goodrich, BP America, Chevron, Teijin Ltd., Vestar, Xerox, Hughes, and the Beckman Institute.