Steric
A Program to Calculate the Steric Size of Molecules about a Point in
terms of their Cone Angles and Solid Angles, as well as their total
Volumes and Projected Areas
This program was originally written to calculate the
solid angle represented
by a molecule as seen from a point perspective according to a
semi-analytical algorithm developed by White et al. (1993).
Since
then, not only has the semi-analytical algorithm undergone major changes
(Taverner (1996))
to incorporate atomic overlaps of order
greater than two, but several other calculations have been added. The
original Cone Angle concept by Tolman
has been incorporated.
Vertex angle and Solid angle of bonded
and non-bonded regions of overlap
have been included. A fully numerical Solid angle
can also be calculated,
together with a fully numerical molecular volume.
The molecular volume is not
the only parameter, however, that is not dependent on a point of
perspective. The new fully analytical molecular projection
area is not
dependant on a point perspective, but is dependant on a directional
perspective. All the other calculations are applicable only to situations
where the steric size of the molecule about a point is of interest.
Examples of such situations are:
- Ligands about a metal atom.
- Comparisons of different reaction sites in catalysts and small
enzymes.
- Steric effects in reaction rates for point reactions.
- Steric effects in stereoselective synthesis.
Examples where the molecular volume is of interest:
- Steric effects in molecular packing in the solid state.
- Order-disorder phase transitions in crystals.
- Host-Guest Inclusion Compounds.
Examples where the molecular orthogonal projection
area is of interest:
- Molecule-Surface interactions.
- pi-Stacking interactions.
Further extensions to the above calculations were added with the radial
profile calculation and the conformer average calculation. The radial
profile calculation calculates the solid angle (or alternative steric
parameter) of the intersection between the molecule and a sphere of a
particular radius about the point of perspective. This is performed for a
range of radii normally encompassing the entire molecule. Two examples of
this are given here, in which two solid angle calculations are compared:
Example Plots from Steric Calculations
Clicking on an image gives a medium sized blow-up
Clicking on a lable gives a large blow-up
| 
| 
|
| Contour Plot of CH3
| Perpsective CH3
| Orthogonal Benzene
|
| 
| 
|
| Radial Profiles
| Multiple Overlap versus
Double Overlap Solid Angles
|
| Close up of PPh2Me
| PPh2Me
|
|
The conformer average involves taking the molecular conformer data from a
large set of conformers generated using another program and calculating
their steric sizes. The conformer average itself is obtained from a
weighted average, the weights being calculated from the energies obtained by
the conformer program. This calculation can be performed on all steric
parameters, including numerical volume, whereas the radial profile cannot be
performed on the volume, for obvious reasons.
Steric is available in binary for Linux (1.2.8), IRIX
(5.2) and 32-bit extended DOS (gjgpp)
and in source code for other unix platforms on the
Structural Chemistry FTP
Site
This program was written by
Brian Craig Taverner,
The last paper to be accepted is on the multiple overlap algorithm. It is
available in HTML and
other formats
