A simple theoretical approach to bond energies

The linear combination of fragment configurations (LCFC) method is used to study factors which control the relative strengths of bonds. Trends in bond strengths and the relative stability of structural isomers are predicted for a variety of organic molecules. It is argued that complex interactions within large organic molecules can be simplified to the interaction of the two electrons of a single bond. A compilation of experimental data is presented to support the proposed theoretical model.     

Fluorinated alcohols enable olefin epoxidation by H2O2: template catalysis

Experimental observations show that direct olefin epoxidation by H(2)O(2), which is extremely sluggish otherwise, occurs in fluorinated alcohol (R(f)OH) solutions under mild conditions requiring no additional catalysts. Theoretical calculations of ethene and propene epoxidation by H(2)O(2) in the gas phase and in the presence of methanol and of two fluorinated alcohols, presented in this paper, show that the fluoro alcohol itself acts as a catalyst for the reaction by providing a template that stabilizes specifically the transition state (TS) of the reaction. Thus, much like an enzyme, the fluoro alcohol provides a complementary charge template that leads to the reduction of the barrier by 5-8 kcal mol(-)(1). Additionally, the fluoro alcohol template keeps the departing OH and hydroxyalkenyl moieties in close proximity and, by polarizing them, facilitates the hydrogen migration from the latter to form water and the epoxide product. The reduced activation energy and structural confinement of the TS over the fluoro alcohol template render the epoxidation reaction observable under mild synthetic conditions.     

MD simulations and QM/MM calculations reveal the key mechanistic elements which are responsible for the efficient C–H amination reaction performed by a bioengineered P450 enzyme

An enzyme which is capable of catalyzing C–H amination reactions is considered to be a dream tool for chemists due to its pharmaceutical potential and greener approach. Recently, the Arnold group achieved this feat using an engineered CYP411 enzyme, which further undergoes a random directed evolution which increases its efficiency and selectivity. The present study provides mechanistic insight and the root cause of the success of these mutations to enhance the reactivity and selectivity of the mutant enzyme. This is achieved by means of comprehensive MD simulations and hybrid QM/MM calculations. The study shows that the efficient C–H amination by the engineered CYP411 is a combined outcome of electronic and steric effects. The mutation of the axial cysteine ligand to serine relays electron density to the Fe ion in the heme, and thereby enhances the bonding capability of the heme-iron to the nitrogen atom of the tosyl azide. In comparison, the native cysteine-ligated P450 cannot bind the tosyl azide. Additionally, the A78V and A82L mutations in P411 provide ‘bulk’ to the active site which increases the enantioselectivity via a steric effect. At the same time, the QM/MM calculations elucidate the C–H amination by the iron nitrenoid, revealing a mechanism analogous to Compound I in the native C–H hydroxylation by P450.

Computer simulation method reveals the mechanism of C–H amination reaction due to a single site mutation.     

Using Valence Bond Theory to Understand Electronic Excited States:Application to the Hidden Excited State of Linear Polyenes

A VB method is presented and applied to calculate the hidden excited states, 2¹A_g and other covalent excited states of polyenes from C₄H₆ to C₂₈H₃₀. The ground rules needed to understand the results are qualitatively outlined and used to discuss the asymptotic behavior of these molecules as n goes to infinity. The theory enables to understand in a coherent and lucid manner excited state properties, such as the make-up of the various states, their energies, geometries, the puzzling increase of the C=C frequency in the excited state, the opposite bond alternation properties of the ground and excited, isomerization patterns, soliton characters, etc.