Direct dynamics study on mechanism and kinetics of the biradical self‐reaction of HOO

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of the biradical hydroperoxy radical has been presented at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) level of theory. Our theoretical calculations suppose a stepwise mechanism involving the formation of a postreactant complex in the triplet and singlet entrance channels. Four transition states of the six‐membered chain complexes (³TS1 and ¹TS1) and six‐membered ring complexes (³TS2 and ¹TS2) are located at the high dual level CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) method. The rate constants of Path 1 ～ Path 4 at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G (d,p) level are calculated by means of the conventional transition state theory (TST) and canonical variational TST without and with small‐curvature tunneling (SCT) correction within the temperature range of 200–2,500 K. The calculated results show that the triplet channel is the dominating reaction channel and Path 2 is found to be the most favorable pathway. The rate constants of Path 2 are in good agreement with the experimental values at the experimentally measured temperatures. Moreover, the variational effect is not obvious in the low temperature range but is not neglectable in the high temperature range. The SCT plays an important role particularly in the low temperature range. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Unexpected electron transfer in cryptochrome identified by time-resolved EPR spectroscopy

Subtle differences in the local sequence and conformation of amino acids can result in diversity and specificity in electron transfer (ET) in proteins, despite structural conservation of the redox partners. For individual ET steps, distance is not necessarily the decisive parameter; orientation and solvent accessibility of the ET partners, and thus the stabilization of the charge-separated states, contribute substantially.     

Kinetic Magnetic‐Field Effect Involving the Small Biologically Relevant Inorganic Radicals NO and O2.−

Oxidation of dihydrorhodamine 123 (DHR) to rhodamine 123 (RH) by oxoperoxonitrite (ONOO^?), formed through recombination of NO and O₂^.? radicals resulting from thermal decomposition of 3‐morpholinosydnonimine (SIN‐1) in buffered aerated aqueous solution at pH 7.6, represents a kinetic model system of the reactivity of NO and O₂^.? in biochemical systems. A magnetic‐field effect (MFE) on the yield of RH detected in this system is explored in the full range of fields between 0 and 18 T. It is found to increase in a nearly linear fashion up to a value of 5.5±1.6 % at 18 T and 23 °C (3.1±0.7 % at 40 °C). A theoretical framework to analyze the MFE in terms of the magnetic‐field‐enhanced recombination rate constant k_rec of NO and O₂^.? due to magnetic mixing of T₀ and S spin states of the radical pair by the Δg mechanism is developed, including estimation of magnetic properties (g tensor and spin relaxation times) of NO and O₂^.? in aqueous solution, and calculation of the MFE on k_rec using the theoretical formalism of Gorelik at al. The factor with which the MFE on k_rec is translated to the MFE on the yield of ONOO^? and RH is derived for various kinetic scenarios representing possible sink channels for NO and O₂^.?. With reasonable assumptions for the values of some unknown kinetic parameters, the theoretical predictions account well for the observed MFE.     

[Re(CO)3Cl]‐Chelation‐Mediated Electronic Coupling between Two Amine Redox Sites through the 5,5′‐Positions of 2,2′‐Bipyridine

The electronic coupling between two amine redox sites bridged through the 5,5′‐positions of the [Re(CO)₃Cl]‐chelated 2,2′‐bipyridine was studied by the electrochemical, spectroscopic, and EPR analysis. Interestingly, multiple near‐infrared bands were observed in this new organic mixed‐valent system. The results are interpreted with the aid of DFT and TDDFT calculations.     

Improving Physical Properties via C?H Oxidation: Chemical and Enzymatic Approaches

Physicochemical properties constitute a key factor for the success of a drug candidate. Whereas many strategies to improve the physicochemical properties of small heterocycle‐type leads exist, complex hydrocarbon skeletons are more challenging to derivatize because of the absence of functional groups. A variety of C H oxidation methods have been explored on the betulin skeleton to improve the solubility of this very bioactive, yet poorly water‐soluble, natural product. Capitalizing on the innate reactivity of the molecule, as well as the few molecular handles present on the core, allowed oxidations at different positions across the pentacyclic structure. Enzymatic oxidations afforded several orthogonal oxidations to chemical methods. Solubility measurements showed an enhancement for many of the synthesized compounds.     

Structures and stabilities of CBHz (z ≤ 8) and CxB3-xHz (x = 1, 2, z ≤ 14): prediction of novel organo-boron radicals

Hydrogenated boron-carbon clusters, i.e. organo-borons, have received considerable attention both theoretically and experimentally. Herein, using a topology searching strategy, we systematically explore the structures and stabilities of small organo-borons with CBH_z (z ≤ 8) and C_xB_3-xH_z (x = 1, 2,z≤ 14) stoichiometry, with particular interests in the intrinsic stabilities of the organo-boron radicals. At the CCSD(T)/aug-cc-pVQZ//B3LYP/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVTZ levels, the stabilities of these global minimum organo-boron species were evaluated by considering dissociation pathways and the binding energy per atom. Aside from the five already studied radicals (CBH₂, CBH₄, C₂BH₂, C₂BH₄ and CB₂H₃), we predict six novel radicals, i.e. CBH₆, C₂BH₆, C₂BH₈, CB₂H, CB₂H₅ and CB₂H₇, which could be detected under suitable circumstances. However, observation of the highly hydrogenated CB₂H₉ radical is much less likely due to its minute stability towards H-extrusion. The computationally determined stable/meta-stable maximum hydrogenation numbers for CB, C₂B and CB₂ (6, 8 and 8, respectively) are in excellent agreement with a simple electron-counting model for C_xB_y chains. The newly predicted organo-boron radicals await future laboratory verification.