Molecular mechanics internal energy barrier

A number of theoretical and experimental approaches toward conformational analysis of crown thioethers have been developed. A molecular mechanics force field for thiols and thioethers, developed from crystal structures and experimental data <87JST(I59)I37> reproduces molecular structures and provides reasonable fits for both the energy differences between conformers and barriers to internal rotation. Thioethers with gauche placement at carbon-sulfur bonds were calculated to be 0.69 kJ mol more stable than those with anti placement, in agreement with earlier SCF-MO calculations <85MP33l). 

Quantum chemistry applies quantum mechanics to problems in chemistry. The influence of quantum chemistry is evident in all branches of chemistry. Physical chemists use quantum mechanics to calculate (with the aid of statistical mechanics) thermodynamic properties (for example, entropy, heat capacity) of gases to interpret molecular spectra, thereby allowing experimental determination of molecular properties (for example, molecular geometries, dipole moments, barriers to internal rotation, energy differences between conformational isomers) to calculate molecular properties theoretically to calculate properties of transition states in chemical reactions, thereby allowing estimation of rate constants to understand intermolecular forces and to deal with bonding in solids. 

Use a molecular-mechanics program to calculate the barrier to internal rotation in ethane. Use a molecular-mechanics program to find the geometries of the gauche and anti conformations of butane and the energy difference between them. 

Theoretical analysis of kinetics of a chemical reactions, whether it is done with the aid of the theory of transition states or by chemical dynamics methods, rests on the classical notion of the necessity to overcome an activation barrier, i.e. the saddle point on the PES of a molecular system. Meanwhile, a sizeable contribution to the total rate of some reactions is made by underbarrier trajectories—this is a purely quantum mechanical effect of the system oozing through the energy barrier so that there exists a certain probability of a transition from the reactants to the products even when the internal energy of the system is... 

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