By Steve Scheiner
Content material: 1. Quantum Chemical Framework -- 2. Geometries and Energetics -- three. Vibrational Spectra -- four. prolonged areas of capability strength floor -- five. Cooperative Phenomena -- 6. vulnerable Interactions, Ionic H-Bonds, and Ion Pairs
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Content material: 1. Quantum Chemical Framework -- 2. Geometries and Energetics -- three. Vibrational Spectra -- four. prolonged areas of power power floor -- five. Cooperative Phenomena -- 6. vulnerable Interactions, Ionic H-Bonds, and Ion Pairs
Additional resources for Hydrogen Bonding: A Theoretical Perspective (Topics in Physical Chemistry)
4), each contribution is computed as the difference in energy between the complex on one hand and the sum of monomers on the other. 2 that E tends to be of similar magnitude to its two constituent terms for translational, rotational, and vibrational energies. Such is not the case, however, for electronic energies. These quantities represent the energy released upon forming a given molecule from an assortment of isolated nuclei and individual electrons and are hence very large in magnitude. Nearly the same energy is released whether these components are assembled into a pair of isolated monomers or into a H-bonded complex; the difference between these two options (representing Eelec) is many times smaller than the energy of assembly in either case.
This issue was first raised by a number of research groups in the late 1970s and early 1980s170~174 and is related to the change in properties of each monomer associated with the ghost orbitals of its partner. Consider as a simple example a spherically symmetric atom like Ar. An atom-centered basis set would correctly reflect that Ar has no dipole (or higher) moment. Suppose now that an additional species is added to the system, another Ar atom for example. Within the context of the basis set of the pair, the spherical symmetry of the first Ar atom is lost; consequently each atom has associated with it a nonzero permanent dipole moment.
The final result is thus not fully optimized but the earlier restraint can then be released and the geometry now optimized over all parameters, intramolecular as well as intermolecular. A second issue concerns the landscape of the potential energy surface. As will be detailed in a number of examples, the surface of many H-bonded complexes is rather flat. However, the surface is far from featureless, containing a number of different local minima, connected by stationary points of various orders.