I am currently trying the determine the dipole moment of a specific bond in a molecule. The bond is part of an alkyl chain coming off of a substituted benzene ring. I have been able to calculate the dipole moment of the whole molecule, but I would like to see how different substituents on the benzene ring change the dipole moment of the alkyl chain.
Any help is greatly appreciated!
Hi!
Actually, this "simple" question is quite hard to answer. To calculate such a dipole moment, you first need to define correctly the chemical bond you are interested in. In standard DFT (or wavefunction) approaches, neither the atoms or the bonds are unequivocally defined*.
There exist some partition schemes (such as Mulliken's or Hirshfeld's), which permit to define atomic properties such as charge, but they all are arbitrary. The fact is, usually this is not a big issue (but you may find people who think it is).
In your case, I would then simply advise you to look at the atomic charges calculated by Gaussian and, basing on the charges and the distances you may access a dipole moment. Avoid Mulliken's charges (which are often completely wrong). You can also
Hoping that helped!
Fred
*Actually, there exists a theory whose goal is to define and characterise atoms (and bonds) in molecule - its name is Atoms In Molecule (AIM). If you are interested, this scheme permits a unambiguous definition of atoms and bonds properties - noticeably dipole moments.
Hi!
Actually, this "simple" question is quite hard to answer. To calculate such a dipole moment, you first need to define correctly the chemical bond you are interested in. In standard DFT (or wavefunction) approaches, neither the atoms or the bonds are unequivocally defined*.
There exist some partition schemes (such as Mulliken's or Hirshfeld's), which permit to define atomic properties such as charge, but they all are arbitrary. The fact is, usually this is not a big issue (but you may find people who think it is).
In your case, I would then simply advise you to look at the atomic charges calculated by Gaussian and, basing on the charges and the distances you may access a dipole moment. Avoid Mulliken's charges (which are often completely wrong). You can also
Hoping that helped!
Fred
*Actually, there exists a theory whose goal is to define and characterise atoms (and bonds) in molecule - its name is Atoms In Molecule (AIM). If you are interested, this scheme permits a unambiguous definition of atoms and bonds properties - noticeably dipole moments.
Frederic's answer is extremely accurate. I just want to add a few comments:
1- For the calculation of dipole moments, I'd recommend calculation the atomic charges using non-overlapping atoms-in-molecules schemes. This means, you could use Bader's proposal, and you may want to avoid Hirshfeld-like partitions.
2- A more thorough test, albeit a more demanding, will be to try several partition schemes and see which one gives you dipole moments that correlate better with whatever property/phenomena you are trying to study.
3- Finally, I think it would be nice to look at atomic charges derived using potential-fitting algorithms.
Dear Zachary,
A better approach to the calculation of dipole moments is to take into
account the locations and magnitudes of the partial charges on the concerned
atoms. These partial charges are included in the output of many molecular
structure software packages. To calculate the x-component, for instance, we
need to know the partial charge on each atom and the atom’s x-coordinate
relative to a point in the molecule; the same for y and z coordinates, and form the sum (x, y and z).
AM1, PM3, DFT and other proframs can be used to calculate the partial charges.
The following links demonstrates the approach for calculating the dipole moment of an amide bond (see pages 5-7):
http://www.as.utexas.edu/astronomy/education/spring07/scalo/secure/AtkinsNoncovalent.pdf
Hoping this will be fruitful,
Rafik
I hate to interject a cloud over the previous comments, but there are two problems with assigning a dipole moment to a bond: (1) Unless the two atoms have the same and equal charges, the dipole moment is undefined, just as it is for a charged molecule. (2) The Mulliken and other calculations are ways of partitioning electron density into point charges, which again is a no-no in quantum mechanics. There are no point charges when static point charges are Classical Mechanics not Quantum Mechanics. That being said, of course there are some programs that seem to have a certain intuitive feel to them that will yield a value you are looking for. Just as there are many brands of DFT methods that all claim to have better results - none of them are follow the Variational Theorem. Does that make HF better than DFT - not necessarily. Find one of the aforementioned programs that give reasonable results it terms of your expectations based on many compounds. I will leave it to you to decide what is reasonable.
Cheers!
Shannon
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