Site Isolation Leads to Stable Photocatalytic Reduction of CO2 over a Rhenium-Based Catalyst.

A porous organic polymer incorporating [(α-diimine)Re(CO)3Cl] moieties was produced and tested in the photocatalytic reduction of CO2, with NEt3 as a sacrificial donor. The catalyst generated both H2 and CO, although the Re moiety was not required for H2 generation. After an induction period, the Re-containing porous organic polymer produced CO at a stable rate, unless soluble [(bpy)Re(CO)3Cl] (bpy=2,2'-bipyridine) was added. This provides the strongest evidence to date that [(α-diimine)Re(CO)3Cl] catalysts for photocatalytic CO2 reduction decompose through a bimetallic pathway.

Theoretical study of reactions in the multiple well H2/S2 system.

The potential energy surface of the H(2)/S(2) system has been characterized at the full valence MRCI+Davidson/aug-cc-pV(Q+d)Z level of theory using geometries optimized at the MRCI/aug-cc-pVTZ level. The analysis includes channels occurring entirely on either the singlet or the triplet surface as well as those involving an intersystem crossing. RRKM-based multiple well calculations allow the prediction of rate constants in the temperature range of 300-2000 K between 0.1 and 10 bar. Of the SH recombined on the singlet surface, the stabilization of the rovibrationally excited adduct HSSH is at the low-pressure limit at 1 bar, but it has a rate comparable to that forming another major set of products H(2)S + S (via an intersystem crossing) at temperatures below 1000 K; at higher temperatures, HSS + H becomes the dominant product. For the reaction H(2)S + S, the presence of an intersystem crossing allows the formation of the singlet excited adduct H(2)SS, most of which rearranges and stabilizes as HSSH under atmospheric conditions. At high temperatures, the majority of excited HSSH dissociates to SH + SH and HSS + H. Compared to reported shock tube measurements of the reaction H(2)S + S, most of the S atom consumption can be described by the triplet abstraction route H(2)S + S --> SH + SH, especially at high temperatures, but inclusion of the singlet insertion channel provides a better description of the experimental data. The reaction HSS + H was found to proceed predominantly on the singlet surface without a chemical barrier. The formation of the major product channel SH + SH is very fast at room temperature (approximately 4 x 10(15) cm(3) mol(-1) s(-1)). While the formation of H(2)S + S or S(2) + H(2) via an isomerization or an intersystem crossing, respectively, are minor product channels, their rates are significantly higher than those of the corresponding direct triplet channels, except at elevated temperatures. Finally, due to the relatively shallow nature of its well, the stabilization of H(2)SS is negligible under conditions of likely interest.

Computational study of the reaction SH + O2.

The reaction of SH + O2 has been characterized using multireference methods, with geometries and vibrational frequencies determined at the CASSCF/cc-pVTZ level and single-point energies calculated at the MRCI/aug-cc-pV(Q+d)Z level. The dominant product channels are found to be SO + OH and HSO + O. Whereas the formation of SO + OH has a barrier of approximately 81 kJ mol-1, it is energetically more favorable than the formation of HSO + O, which is barrierless beyond the endothermicity of approximately 89 kJ mol-1 at 0 K. Thus, the reaction SH + O2 --> SO + OH is 2 orders of magnitude faster than the reaction SH + O2 --> HSO + O at room temperature, revealing that the atmospheric oxidation of SH leads directly to the formation of SO + OH with the rate coefficient of approximately 1.0 x 10(-2) cm3 mol-1 s-1. At temperatures above 1000 K, however, the rates of the two channels become comparable. This may be attributed to the entropy effects leading to the higher pre-exponential factor for the channel (forming HSO + O) via a more loose transition state than that (forming SO + OH) entailing a four-centered transition state. Whereas the hydrogen abstraction reaction producing S + HO2 is found to proceed on the quartet surface, the substantial barrier of approximately 165 kJ mol-1 means that it occurs as a minor product channel. Finally, the formation of possible products SO2 + H is prohibited due to the lack of a transition state for the direct sulfur insertion.

Theoretical study of hydrogen abstraction and sulfur insertion in the reaction H2S + S.

The reaction of H2S + S has been characterized at the multireference configuration interaction level with the geometries optimized using the aug-cc-pVTZ basis set and the single-point energy calculated using the aug-cc-pV(Q+d)Z basis set. As in the analogous reaction of H2 + S, the presence of an intersystem crossing enables products (SH + SH) to be formed on the singlet surface through S insertion, which bypasses the triplet barrier (19.1 kJ mol-1 relative to SH + SH) of the H abstraction route. This provides theoretical evidence for SH + SH formation without barrier beyond endothermicity at sufficiently low temperatures. The H abstraction route, however, is expected to be competitive at higher temperatures due to a much higher Arrhenius pre-exponential factor (6.9 x 10(14) cm3 mol-1 s-1 derived from TST calculation) than that of S insertion channel (3.7 x 10(13) cm3 mol-1 s-1, derived by a least-squares fit to the measurements). With a slightly higher transition-state barrier than that of the H abstraction channel, the production of S2 + H2 is less favored due to proceeding via intersystem crossing and insertion. While the formation of HSS + H is energetically unfavorable relative to SH + SH, recombination channels producing H2SS or the more stable HSSH are expected to occur under collisional stabilization conditions at high pressures.