Quintet pathway

NHg and OH ligands, [Fe(IV)(0)(NH3)5]2+, [Fe(IV)(0)(0H)(axial) (NH3)4]+, and [FedV)(0)(OH)2(eq)(NH3)3] (49). For these systems, the o- and n- pathways were investigated on both the triplet and quintet surfaces. The 5a and pathways were similar to those calculated for TauD, with 5c the lowest in energy of the four alternatives. The -pathways were next in energy, where the triplet and quintet surfaces were found to have comparable barriers for H-atom abstraction. The triplet a pathway could only be located for [Fe(IV)(0)(NH3)5]2+ and had the highest barrier at... 

Quintet m-phenylenedinitrenes have been generated in low-temperature matrices by photolysis of the corresponding diazide precursors, and their secondary photochemical transformations studied. Two competing ring-opening pathways were identified, and it was also concluded that quintet dinitrenes are much more photochemically reactive than triplet nitrenes. An EPR study of radical intermediates formed in the photooxidation of 4,4 -diazidodiphenyl in benzene and toluene has also been published The radicals apparently arise by abstraction of H atoms from the solvent by a triplet nitrene-02 complexes. 

The bold values show the free energies of the proposed reaction pathway s singlet, t triplet, q quintet structures... 

Fig. 25 Potential energy surfaces for the most feasible two-state reaction pathways for ethylene dimerization catalyzed by Cr(II)OH (If), via either a Cr-carbene mechanism or a metallacycle mechanism determined at the M06 level of theory. Also shown are the crossing points optimized at CASSCF level. The triplet metallacycle reaction pathway is depicted in blue, and the triplet Cr-carbene reaction pathway is shown in dark red. The quintet parts are in black. Energies are in kcal moP and relative to il. Bond lengths are in angstroms. Angles are in degrees...

The ground spin state of chromium (acetylene) adducts is known to be of quintet, and the most plausible reaction pathway on quintet surface needs to overcome two activation barriers to finish a single catalytic cycle, as depicted in Scheme 3.4. However, the reaction on quintet surface is prohibited by presenting a ffee-energy barrier of 31.1 kcal/mol that transforms two coordinated acetylene into the key intermediate 4D. Thus, the turnover of frequency (TOP) for the catalytic cycle on the quintet surface is 1.36 X 10 h, which rules out the quintet reaction mechanism for acetylene cyclotrimerization by Cr(II)/Si02 model catalyst. 

Figure 3.12 Gibbs free energy profiles for acetylene cyclotrimerization by Crfllj/SiOj cluster model. The Gibbs free energies are calculated at 298.15 K, 1 atm as default in Gaussian09. Also shown are the total energies in parentheses. The triplet reaction pathway is depicted in gray, while the quintet parts are in black. Energies are in kcal/mol and relative to 1C plus the corresponding number of acetylenes.

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