Hydrogen bond inner shell

Terms for the electrostatic interactions [Eq. (2.13)] for the region outside the first solvation shell, and an appropriate one for the inner region, must be added to Eq. (2.12) for each ion of an electrolyte B, for the evaluation of AsoIyGb. Since cations do not accept hydrogen bonds and anions do not donate them, except when protonated, like HSOi, the term in a of the solvent becomes unimportant for cations and that in P of the solvent for anions. 

In the calculations of the energy of hydration of metal complexes in the inner coordination sphere, one must consider hydrogen bond formation between the first-shell water molecules and those in bulk water, which leads to chains of hydrogen-bonded water molecules. Such hydrogen-bonded chains of ethanol molecules attached to the central metal ion have been found as a result of DFT B3LYP calculations on ethanol adducts to nickel acetylacetonate, where the calculated energy of hydrogen bonds correlated well with experimental data [90]. 

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

It is not possible to determine k for a hydrogen atom directly from experimental X-ray data, because its value correlates strongly with the temperature parameter due to the absence of unperturbed inner-shell electrons. The use of neutron temperature parameters provides an alternative. Combined analysis of X-ray and neutron data on glycylglycine and sulfamic acid suggests that for X—H (X = C, N) groups, the H atom is more contracted than for the H2 molecule, with a k value as large as 1.4 for both C—H and N—H bonds (Coppens et al. 1979). 

Thus, according to these calculations the formation of the hydrogen bond is accounted for by the decrease of repulsion between A—H and B and by the presence of some donor-acceptor interaction H B besides by the purely electrostatic interaction. It is found that the peculiarity of the hydrogen atom in the formation of interxnoleeular (or intramolecular) bonds is, first, that it has no inner electron shell and second, that it has rather high an ionization potential. 

The hydrated ion may be pictured as having a small number — possibly four or six — of water molecules firmly held in contact with the ion and constituting an inner shell, and a larger, less well defined, number more loosely held in an outer shell. Round a cation the inner shell water molecules are probably bonded by the strong ion-dipole force which operates when the water molecule is held in some such position as is indicated in formula (8). Anions are usually less hydrated than cations. The inner shell water molecules may not fit so well. Probably they are hydrogen bonded as shown in (9). In all cases, the outer shell water molecules are supposed to be hydrogen bonded to those of the inner shell. 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

The shapes of the orbits and the determination of how many electrons actually can occupy a shell are very complex. At the most basic level, the bonding of atoms into molecules is based on a tendency of any atom to try to fill a shell to the maximum number of electrons by sharing electrons. Thus, a carbon atom, which has two electrons in the inner shell (the maximum possible) and four electrons in the outer (looking for four more), will bond with four hydrogen atoms, each of which has one in its only shell and is thus looking for one more to reach its maximum. 

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