In some force fields, the hydrogen bond is described not only by the electrostatic term, but the angle one. Does this have any justification other than the crystallographic statistics?
Nice question :-) .
When it comes to force fields, the justification for each mathematical term come from a) it's attempts to account for part of the quantum mechanical (QM) nature of an intramolecular or intermolecular interactions, and b) it reproduces known experimental or QM results. Historically at one point in force-field development, hydrogen bond terms were added in an attempt to better account for the overlap between molecular orbitals that are involved in the interaction. Since the MO have spatial direction, then the interaction would also have a directionality. However, many force fields have found that the electrostatics and the Lennard-Jones potentials do an adequate jobs at reproducing these types of bonds. Hydrogen bonds are difficult to model using classical idea because part of their stability is due to a small percentage of electron transfer, which is purely quantum in nature.
Take a look at the following article for further information:
Lii, J-H and Allinger, N L, Directional Hydrogen Bondign in the MM3 Force Field. 1. J. Phys. Org. Chem. 7, 1994, 591-609.
Best Regards,
Karl
Nice question :-) .
When it comes to force fields, the justification for each mathematical term come from a) it's attempts to account for part of the quantum mechanical (QM) nature of an intramolecular or intermolecular interactions, and b) it reproduces known experimental or QM results. Historically at one point in force-field development, hydrogen bond terms were added in an attempt to better account for the overlap between molecular orbitals that are involved in the interaction. Since the MO have spatial direction, then the interaction would also have a directionality. However, many force fields have found that the electrostatics and the Lennard-Jones potentials do an adequate jobs at reproducing these types of bonds. Hydrogen bonds are difficult to model using classical idea because part of their stability is due to a small percentage of electron transfer, which is purely quantum in nature.
Take a look at the following article for further information:
Lii, J-H and Allinger, N L, Directional Hydrogen Bondign in the MM3 Force Field. 1. J. Phys. Org. Chem. 7, 1994, 591-609.
Best Regards,
Karl
This article contains references to the history of the directionality:
http://onlinelibrary.wiley.com/doi/10.1002/cphc.201100540/abstract
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