Empirical force field calculations, hydrogen

Hydrogen bonding is included in empirical force field calculations in two ways. In the MM series (27). bond dipoles are placed at the centers of the bonds and the Jeans equation is used ... 

Intramolecular hydrogen bonding in sucrose may be retained in aqueous solution. Empirical force field calculations suggest that the molecule of sucrose retains its crystal structure conformation in aqueous solution with the loss of one of the... 

Emsley and coworkers125 have made an X-ray crystallographic and empirical force-field study of a simple non-hydrogen-bonded 2-acetylenamine (50). The molecule has the EZ configuration in the crystal, but the calculations (MM2 and MMP2) predict the ZZ form to be more stable by 0.7 kcal mol - Ab initio calculations on 3-aminoacrolein also favor the ZZ form, but here the intramolecular hydrogen bond plays a decisive role. 

Another well-known example is D-glucopyranose, the anomeric equilibrium mixture of which is made up of 36% of the axial a anomer and 64% of the equatorial j3 anomer at 20 °C in water as a polar solvent sr = 78), with AG a j3) = —1.4 kJ/ mol [82], This is in agreement with free-energy simulations with an empirical force field [285] and quantum-chemical calculations [286], showing that the preference of d-glucopyranose for the y9-anomer in water is mainly due to electrostatic solvation and hydrogen-bonding effects, which stabilize the more dipolar y9-anomer better than the a-anomer. 

CFF "9-6-1 and MCMS (Momany et al., 1974 Dunfield et al., 1978). a semi-empirical force field which uses a special function for the hydrogen bond, and theoretical values for dispersion parameters and partial charges. The figures are a concise summary of calculated vs. experimental data for the individual molecules. 

The various types of successful approaches can be classified into two groups empirical model calculations based on molecular force fields and quantum mechanical approximations. In the first class of methods experimental data are used to evaluate the parameters which appear in the model. The shape of the potential surfaces in turn is described by expressions which were found to be appropriate by semiclassicala> or quantum mechanical methods. Most calculations of this type are based upon the electrostatic model. Another more general approach, the "consistent force field method, was recently applied to the forces in hydrogen-bonded crystals 48> 49>. 

Calculations using conventional TST and the Bigeleisen-Wolfsberg [16] treatment for isotope effects have demonstrated that thd = 1-44 is a useful benchmark for primary hydrogen isotope effects. Using empirical harmonic force fields and various reactant-state and transition-state geometries, More O Ferrall and Kouba [30] found, for proton-transfer models, that the exponents were within 2% of the 1.44 value, and similar computational approaches gave Xhd = 1.43-1.45 (343 K)... 

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