Why is Pt₄⁺ the Least Efficient Cationic Cluster in Activating the C−H Bond in Methane? Two-State Reaction Computational Investigation

Abstract

The two-state reaction mechanism of Pt4 + with CH4 on the quartet and doublet potential energy surfaces has been investigated at the B3LYP level. Crossing points between the potential energy surfaces are located using different methods, and possible spin inversion processes are discussed by means of spin−orbit coupling (SOC) calculations. As a result, after the C−H insertion intermediates, two distinct H2 elimination reaction paths have been found, and the second C−H bond broken is regarded as the rate-determining step for two reaction paths. For the first pathway, there is a minimal energy crossing point (MECP) between the two surfaces, where the reacting system will change its spin multiplicities from the quartet state to the doublet state near this crossing region because the magnitude of the spin multiplicity mixing increases in a small energy gap between high- and low-spin states and will greatly enhance the probability of the intersystem crossing. The single P1 ISC and double P2 ISC passes estimated at MECP (SOC = 201.25 cm−1) are approximately 0.51 and 0.76, respectively. Not only will the reaction overcome a spin-change-induced barrier (ca. 6 kcal/mol) but it also will overcome an adiabatic barrier (ca. 24.7−20.8 kcal/mol), which will greatly reduced the efficiency of the reaction system. As for the second reaction path, similarly, there is a crossing seam between CP-2 and CP-3, the spin-change-induced barrier is very high, about 15.86−18.86 kcal/mol, which also indicates a less favorable process kinetically. Therefore, the lack of a thermodynamic driving force is an important factor contributing to the low efficiency of the reaction system. These conclusions are consistent with the experimental observations.

Activated Methane on Small Cationic Platinum Clusters

“…The calculated structures and spectra, with assignments, that provide the best match to the experiment are shown for the Pt n CH 4 + complexes. The calculated vibrational frequencies are scaled by a factor of 0.97. desorption is consistent with the calculations of Lv et al, [13] who reported the transition state (TS) for Pt 4 + insertion into a C À H bond to be higher in energy than the separated species, in contrast to Pt 3 + , where this TS lies slightly lower in energy than the separated molecules. Entropic factors may also be important, given that the number of pathways for dissociation are presumably much greater than for dehydrogenation.…”

“…[7] In the case of small ionic clusters reacting with CH 4 under single collision conditions, Pt n [C,2H] + complexes were found to be the favored products. [8][9][10] There have been a number of computational studies of the interactions of platinum clusters [11][12][13] and surfaces [5,14,15] with methane, primarily using density functional theory (DFT). Such calculations are challenging, owing to the large system size, number of electrons and possible paths, and the fact that several electronic states and crossings between them may need to be treated.…”

“…Such calculations are challenging, owing to the large system size, number of electrons and possible paths, and the fact that several electronic states and crossings between them may need to be treated. [13] Experimental spectroscopic characterization of these species, particularly the reaction intermediates, can therefore provide important information about their structures and benchmark data for theory.…”

“…The structures we identify are similar to those predicted for Pt 3 CH 4 + [11] and Pt 4 CH 4 + . [13] To investigate the degree of activation of the CÀH bonds in the complexes, we compared the changes of the C À H bond lengths with those of small hydrocarbons. The most activated, tertiary, C À H bond in isobutane has a bond length of 1.122 , while free CH 4 has a bond length of 1.087 , a change of the order of 0.03 .…”

“…The PtÀ CH 4 binding fits into the picture of an agostic bond, with interaction between a filled CÀH sp 3 orbital and a Pt d orbital. [13,14,25] The dissociation channels for the different cluster sizes after IR excitation can also provide useful information. Figure 3 shows the changes in mass-spectral intensity for the important species involved in the IRMPD process for n = 4,5.…”