Specific solute-solvent interactions, supermolecule approach

Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule may be obtained by the use of molecular mechanics simulations. At the semi-empirical MO level this approach has been successfully used to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. 

Concerning the best way of treating the solvent molecules, there is still plenty to do before this question can be answered. The supermolecule approach is quite appropriate for describing specific or short-range solvation effects. On the other hand, long-range effects require a large number of solvent molecules and the use of sophisticated ab initio methods to properly describe the solvent-solvent and solute-solvent interactions becomes prohibitive. Hybrid methods (QM/MM), where part of the solvent structure is represented by a MM force field, have not been extensively tested. In... 

The simplest discrete approach is the solvaton method 65) which calculates above all the electrostatic interaction between the molecule and the solvent. The solvent is represented by a Active molecule built up from so-called solvatones. The most sophisticated discrete model is the supermolecule approach 661 in which the solvent molecules are included in the quantum chemical calculation as individual molecules. Here, information about the structure of the solvent cage and about the specific interactions between solvent and solute can be obtained. But this approach is connected with a great effort, because a lot of optimizations of geometry with ab initio calculations should be completed 67). A very simple supermolecule (CH3+ + 2 solvent molecules) was calculated with a semiempirical method in Ref.15). 

It is also possible to combine the supermolecule and continuum approaches by using specific solvent molecules to capture the short-range effects (i.e., those involving specific noncovalent interactions between solute and solvent) and a reaction field to treat longer range effects.33-35 Alternatively, structures along the gas phase reaction coordinate can be immersed in a box of hundreds (or more) of explicit solvent molecules that are treated using force field approaches.36,37 Each type of method - the SCRF, solvent box, and supermolecule approaches - tests the importance of particular features of the solvent on the reactivity of the solute dielectric constant, multiple specific classical electrostatic interactions, and specific local directional noncovalent interactions, respectively. 

The effects of solvents on the nuclear shielding of solute molecules may be categorised into specific and nonspecific classes. The specific interactions include hydrogen-bonding, protonation, etc. From the theoretical standpoint these may be considered by the supermolecule approach. 

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