The relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms is discussed and is illustrated by multiconfigurational electronic structure calculations on the ArOH + R(*) --> ArO(*) + RH reactions. The key topographic features of the Born-Oppenheimer potential energy surfaces that determine the predominant reaction mechanism are the conical intersection seam of the two lowest states and reaction saddle points located on the shoulders of this seam. The saddle point corresponds to a crossing of two interacting valence bond states corresponding to the reactant and product bonding patterns, and the conical intersection corresponds to the noninteracting intersection of the same two diabatic states. The locations of mechanistically relevant conical intersection structures and relevant saddle point structures are presented for the reactions between phenol and the N- and O-centered radicals, (*)NH2 and (*)OOCH3. Points on the conical intersection of the ground doublet D0 and first excited doublet D1 states are found to be in close geometric and energetic proximity to the reaction saddle points. In such systems, either the HAT mechanism or both the HAT mechanism and the proton-coupled electron transfer (PCET) mechanism can take place, depending on the relative energetic accessibility of the reaction saddle points and the D0/D1 conical intersection seams. The discussion shows how the two mechanisms are related and how they blend into each other along intermediate reaction paths. The recognition that the saddle point governing the HAT mechanism is on the shoulder of the conical intersection governing the PCET mechanism is used to provide a unified view of the competition between the two mechanisms (and the blending of the two mechanisms) in terms of the prominent and connected features of the potential energy surface, namely the saddle point and the conical intersection. The character of the dual mechanism may be understood in terms of the dominant valence bond configurations of the intersecting states, which are zero-order approximations to the diabatic states.

The HCCO + N O reaction (r5) was investigated in C2H2/0/NO systems at a pressure of 2 Torr (He bath gas) using discharge flow-molecular beam mass spectrometry techniques (DF-MBMS). The first rate coefficient data at temperatures > 300 K are presented. The coefficient was measured relative to the known k(HCCO+O) from the changeof steady-state HCCO signals upon adding increasing amounts of NO. Thus, in the temperature range from 290 to 670 K, the k(HCCO+NO) coefficient was found to exhibit a slight but significant temperature dependence: k(T) = (1.0 f 0.3) X 10-lo e-(350*150)/(TIQ cm3 molecule-' (T = 290-670 K). The product distribution was determined at 700 K. The experiments relied on the fact that all reaction pathways yield either CO or C02. Ketenyl radicals, generated by quantitative reaction of a known amount of 0 atoms with C2H2 in high excess, were reacted with a large excess of NO, ensuring quantitative conversion into the products C02 and CO. The product distribution was essentially deduced from the ratios [CO2]fomd/[O]input and [CO]for,d/ [O]inPut. Formation of CO together with CHz(3B1) in a minor (25 f 15%) channel of the C2H2 + 0 reaction was taken into account. Small corrections for secondary reactions such as HCCO + 0 -H + 2 CO were made by kinetic modeling. Thus, the following yields were obtained: for HCCO + N O -(CHNO) + CO, 77 f 9%; for HCCO + N O -(CHN) + C02, 23 f 9%. Strong product signals were also observed at m / e = 43 and m / e = 27, confirming that CHNO isomers and C H N isomers are formed along with CO and COz, respectively.Theoretical predictions regarding the CO/CO2 yield ratio, presented in a companion paper in this issue, can be reconciled with the experimental product distribution only when an as yet unidentified entrance pathway to the formyl isocyanate intermediate is assumed to exist and to be thermally accessible.

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The germylone dimNHCGe (5, dimNHC = diimino Nheterocyclic carbene) was successfully prepared via the reduction of the germanium cation [dimNHCGeCl] + with KC 8 . The molecular structure of 5 was unambiguously established by both NMR spectroscopy and single-crystal X-ray diffraction. The reactivity of 5 was investigated, revealing that it undergoes oxidative addition of HCl, CH 3 I, and PhI, accompanied by an unusual migration of the H, Me, and Ph groups from germanium to the carbene ligand. Related chemistry was also observed with C 5 F 5 N, which results in the migration of the fluorinated pyridine moiety to the carbene ligand. Compound 5 also undergoes cycloaddition with tetrachloro-o-benzoquinone to afford a Ge(IV) adduct.