PCM model

Ire boundary element method of Kashin is similar in spirit to the polarisable continuum model, lut the surface of the cavity is taken to be the molecular surface of the solute [Kashin and lamboodiri 1987 Kashin 1990]. This cavity surface is divided into small boimdary elements, he solute is modelled as a set of atoms with point polarisabilities. The electric field induces 1 dipole proportional to its polarisability. The electric field at an atom has contributions from lipoles on other atoms in the molecule, from polarisation charges on the boundary, and where appropriate) from the charges of electrolytes in the solution. The charge density is issumed to be constant within each boundary element but is not reduced to a single )oint as in the PCM model. A set of linear equations can be set up to describe the electrostatic nteractions within the system. The solutions to these equations give the boundary element harge distribution and the induced dipoles, from which thermodynamic quantities can be letermined. 

The original PCM method uses a cavity made of spherical regions around each atom. The isodensity PCM model (IPCM) uses a cavity that is defined by an isosurface of the electron density. This is defined iteratively by running SCF calculations with the cavity until a convergence is reached. The self-consistent isodensity PCM model (SCI-PCM) is similar to IPCM in theory, but different in implementation. SCI-PCM calculations embed the cavity calculation in the SCF procedure to account for coupling between the two parts of the calculation. 

The energy and geometry data listed in Table 2.5 show that the effect of solvent bulk (computed for cyclohexane, toluene, diethylether chloroform, THF, and methanol by PCM model), decreases the activation energy, increasing asynchronicity for the [4 + 2] cycloaddition reactions. 

For the small system involved in the water exchange on [Be(H20)4]2+, we evaluated the effect of an implicit and approximated explicit treatment of the bulk water while investigating water exchange on [Be(H20)4]2+. For the implicit treatment, the CPCM and PCM models were applied as implemented in Gaussian, and geometry optimizations and... 

While the transition states could all be confirmed as transition states, only the precursor in the gas phase pc, and for the water cluster approach pc-wc, was confirmed as a local minimum, and despite intensive search no minima could be found within the CPCM and PCM model approximation. 

Table II. PCM modelization of 110 efg tensors of water molecule and of some...

Table III. PCM modelization of 170 efg tensors in diopside CaMgSi206 and forsterite Mg2Si04.Units for Vaa are V.A"2 and 1 - yj = 6.3 as for the corundum structure.

PCM model. AIM calculations were also used to investigate hydrogen bonding in each intermediate. The experimental and theoretical results are in excellent agreement. The calculations also indicate the reaction will be much faster in acetonitrile than in the gas phase, as one would expect for a process forming ionic products. 

The, S n2 reaction between lithium isothiocyanate ion pair and methyl fluoride has been calculated at the MP2(full)/6-31H-G //HF/6-311- -G level of theory in the gas phase and in acetone using the PCM model.108 Both the gas phase and acetone reactions occur with inversion, rather than retention, of configuration. However, the transition states and products are different in the gas phase and in solution methyl thiocyanate is formed in the gas phase by transition state (72) whereas methyl isothiocyanate is formed much more slowly in acetone via the looser transition state (73). 

We have now achieved a situation in which dielectric continuum solvation models in general, and especially COSMO, are quite well established for SCF ground-state calculations of organic molecules. Many of the methods, tools, and properties available for gas-phase calculation can also be performed in a dielectric continuum solvation model. The PCM model including C-PCM provides the greatest breadth of implemented functionality. 

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