The Important Role of Lone-Pairs in Force Field (MM4) Calculations on Hydrogen Bonding in Alcohols

Abstract

An expanded treatment of hydrogen bonding has been developed for MM4 force field calculations, which is an extension from the traditional van der Waals-electrostatic model. It adds explicit hydrogen-bond angularity by the inclusion of lone-pair directionality. The vectors that account for this directionality are placed along the hydrogen acceptor and its chemically intuitive electron pairs. No physical lone-pairs are used in the calculations. Instead, an H-bond angularity function, and a lone-pair directionality function, are incorporated into the hydrogen-bond term. The inclusion of the lone-pair directionality results in improved accuracy in hydrogen-bonded geometries and interaction energies. In this work is described hydrogen bonding in alcohols, and also in water and hydrogen fluoride dimer. The extension to other compounds such as aldehydes, ketones, amides, and so on is straightforward and will be discussed in future work. The conformational energies of ethylene glycol are discussed.

Estimation of the thermodynamic parameters of hydrogen bonding in alcohol solutions by the method of infrared spectroscopy

“…In other words, despite significant progress in computer modeling of complex associated molecular systems aimed at calculations of the force constants and intramolecular vibrational frequencies, a system analysis of the results of IR spectroscopic measurements with application to the simplest alcohol-containing compounds associated by hydrogen bonds still remains an urgent problem both in scientific and applied aspects. In the applied aspect, the importance of the above-indicated problem is caused primarily by the role hydrogen compounds play in the formation of a variety of structural isomers of biological objects [3]. Relatively weak hydrogen bonds (3-5 kcal/mole) are extremely important for functioning of alive systems.…”

Importance of explicit smeared lone-pairs in anisotropic polarizable molecular mechanics. Torture track angular tests for exchange-repulsion and charge transfer contributions

“…The importance of lone pairs to improve directionality in H-bonded complexes was demonstrated in the context of classical, non-polarizable force-fields by Lii and Allinger [ [77] and Refs. therein].…”

Efficient Transition State Modeling Using Molecular Mechanics Force Fields for the Everyday Chemist

“…[3][4][5][6][7] MM is usually taught in secondary and postsecondary education as a ball and spring model where atoms "feel" classical forces between them. Explicit electron considerations are omitted in this model (in more advanced versions, polarization may be considered, 8 and electron lone pairs may be introduced for directionality in hydrogen bonding 9 ), and the potential energy surface (PES) is described by functions that characterize spring stretching, bending, dihedral (torsional) angles, and more. These functions are parameterized and critiqued based on experimental data such as X-ray diffraction and NMR data [10][11][12][13][14][15][16] or the more accurate computational techniques that do take into consideration electronic effects and more complex phenomena.…”

“…[9], the rate should be defined by the total free energy difference between reactant and transition structure. Thus all contributors, such as all motions (vibrations, rotations, translations), should be considered, but these properties can be difficult to obtain.…”