Subject: Molecular radius for organic solvents

From: Gerry Tanoury <[email-masked]>
Date: Thu, 11 Oct 2001 09:51:30 -0400

Dear CCLers,

I am performing some calculations using the COSMO solvation model.  One of
the parameters defines the solvent's molecular radius.  Is there a place
where I can find the molecular radii of common organic solvents?  I am
especially interested in toluene.

Kind regards,

Gerald J. Tanoury, Ph.D.
Principal Research Chemist
Sepracor Inc.
111 Locke Dr.
Marlborough, MA  01752
Ph: (508) 357-7442
Fax: (508) 357-7808
E-mail: [email-masked]

---

From: Craig Burkhart <[email-masked]>
Date: Thu, 11 Oct 2001 15:18:05 -0400

Gerry:

A common method of computing the effective radius of a solvent (also works
for solids, by the way) is by computing the effective volume of the solvent
sphere:

4 Pi r_eff^3 = cnv 3 amu occ / rho,

> from which it follows that

r_eff = (r_eff^3 / (4 Pi))^(1/3),

where r is the effective radius of the molecule in Angstroms, cnv is the
conversion from cm^3 to Angstroms^3 (= 10^24 Ang^3/cm^3), amu is the atomic
mass of the molecule (it's molar mass divided by Avagadro's number), occ is
the packing density (assuming random close packing of spheres, this would be
~0.58; for a crystalline material this you might want to use the bcc
occupancy, 0.680), and rho is the molecular density in gm/cc.

If you use the above formula for water, you will note that it does not
correspond (1.60 A for occ = 0.58; 1.53 for occ = 0.50) to the "reported"
radius of 1.40 Angstroms. The reason why there is a discrepancy, to the best
of my knowledge, is that the 1.40 A value is a "best fit" from water
solvation simulations.

In the absence of a "best fit" situation, I would suggest that you follow
the above formula. For typical organic liquids, I use 0.58 as the occupation
factor.

Hope this helps.

Craig Burkhart
Goodyear Chemical R&D