Non polar molecule with polar bonds?

[DrClaude]

Maybe looking up the Mulliken population analysis will help you.

 

[neilparker62]

We seem to be going in circles. It makes as much sense as you want it to. Using CCl4 as an example, it’s clear that electron density is nonuniform and higher near the Cl nuclei than the C nucleus. So treating each C-Cl bond by itself (and excluding the other Cl’s), there is clearly a dipole moment, although making this quantitative is a lot of work (see Bader’s atoms in molecules). It serves as a useful approximation/heuristic, especially in certain applications. To take another example: the normal modes of CH3Cl (chloromethane) that involve Cl and C can be expected to have a much higher oscillator strength than those that mainly involve only C and H, which is reflected in the IR spectrum. But again, this is an approximation, as any normal mode will likely contain contributions from all the atoms in the molecule.

Thanks for your patience - greatly appreciated. I think I am generally out of my depth here but scouring some (intimidating) references on Mullikan populations, I came across the following set of graphs which (if (big if!) I interpret correctly) is quite telling in respect of carbon tetrachloride. +- 0 charge on Carbon atom ?!

 

[neilparker62]

Maybe looking up the Mulliken population analysis will help you.

Thanks - way out of depth here but did some "cherry picking" - see graphs above.

 

[TeethWhitener]

This is probably as close as you’re going to get for what you want. Bader’s AIM might also be worth a look. Mulliken population analysis looks specifically at the populations of the atomic orbitals in the basis set used to build up the molecular orbitals, so again, this is a particular way of divvying up which electron density belongs to which atom (i.e., it’s up to you to decide what the nature of those atomic orbitals is and how many of them to include). As I said, it’s an approximation, since in reality all the electrons “belong” to all the atoms (atomic orbitals don’t really exist by themselves in a molecule), and different approximations will give different answers.

 

[DrDu]

If measurement of the polarity of single bonds is effectively impossible, under what pretext can we claim that there are "polar covalent bonds in a non polar molecule" ? Even in $CCl_4$ let alone $CH_4$. Other than by recourse to subtraction of electronegativities which may not validly represent the physics of the shared electron pairs in the molecule as a whole.

Today, it is possible to map the electron distribution in molecules precisely using highly resolved X-ray diffraction methods. NMR shifts also yield information about electronic density on the individual atoms. Polarity of the bonds in symmetric molecules gives rise to non-vanishing higher multipole moments, e.g. a high quadrupole moment in CO2. We are not living in the nineteenth century.