# Theoretical Calculations of Properties of Molecules for Separation Lab

by students in Physical Chemistry Lab II (Spring 2001): Dustyn Sawall, Stacy Mundschau, Eric Kiefer

### The molecules and their Lewis structures

The primary property that will be used for separation is molecular polarity.  In general molecules that have polar bonds based on a large difference in electronegativity between atoms on either side of the bond will be polar.  However, you must be aware that sometimes the bonds may be arranged so that the polarity cancels out.

Here is a table that summerizes the relative polarity of the above molecules.
 Molecule Relative Polarity and Comments water very polar --the electronegative oxygen diethyl ether very polar --the electronegative oxygen hexane non-polar cyclohexane non-polar toluene non-polar m-xylene non-polar benzoic acid very polar --two electronegative oxygens m-toulic acid very polar --two electronegative oxygens m-nitrobenzoic acid somewhat polar --N and O partially cancel each other

With the links below, you can learn more about the advanced calculations that can be performed to get more precise estimates of relative polarity and get a chance to view images of the electron distributions and the molecular orbitals that lead to the charge separations that produce polarity.

### Types of calculations performed

The dipole moment for the above chemicals can be determined by experimentation; however, good approximations of the dipole moment will be sufficient for the purpose of separation. To obtain good estimates of the dipole moment, without doing the actual experiment, we used quantum mechanics. As a little background, quantum mechanics can be thought of as a set of mathematical equations that can be used to position all of the atoms and electrons so that the molecule has the lowest possible energy. Because these computations are very time consuming and very repetitive, a software package called CACHE Workshop was used to do the mathematics. Included in this package are three programs named after the respective calculation it performs: Extended Huckel, ZINDO, and MOPAC. Although they all perform quantum mechanics, they each make different assumptions. For Extended Huckel, it assumes that the structure of the molecule is determined by molecular mechanics. This assumption, among others, works well for aromatic or conjugated systems. Both ZINDO and MOPAC are semi-empirical in that they incorporate results from spectroscopy. ZINDO calculations are good for predicting spectra, and MOPAC generally works the best for any general molecule since it makes the fewest approximations.

Have questions about these molecules? We have answers. The links listed here will take you to the results of the calculations. They will show how the polarity was calculated.

### The results

Based on the calculations done, the order of relative polarity is:
 Molecules benzoic acid m-toluic acid water diethyl ether m-xylene m-nitrobenzoic acid toluene cyclohexane hexane