[simonwei]

Hi, all:

Is there any experimental methodology to determine the direction of an organic molecule's ground-state dipole moment? The molecule I'm interested in is a rod-like liquid crystal, and I'm wondering if I can characterize the angle between the dipole moment and long molecular axis.
By the way, I guess quantum mechanical software package like Guassian can predict this angle. Does anyone know the typical accuracy of the quantum mechanical calculation?
Thank you very much for your time.

simonwei

[tamim83]

Yes, you could use Gaussian to get the dipole moment.  You can even get the direction of the dipole moment vector and determine the angle between it and the molecular axis.  However, many of the methods do a really poor job with the magnitude of the dipole moment (not sure how they do with direction, my guess is probably a bit better).  I don't have any of my books with me to look and see what would give you a good answer; I can follow up on that when I get back to campus after the holiday.  Since your molecule is large, I would suggest optimizing the geometry with a low level of theory and small basis set (since its "organic", you may be able to get away with a semi-empirical  calculation).  Then, you can use a higher level of theory and a larger basis set for a single point energy calculation.  This would give you a pretty good value for the dipole moment. 

Hope this helps you some. 

[tamim83]

I looked at one of my computational books today.  Looks like the best thing is to add diffuse functions to your basis set and don't use HF (it gives answers that are way to high).  DFT should work fine, which is good since you have such a large system.  B3LYP with a aug-pc-1 basis set (or similar) seems to give decent results. 

I got this from Introduction to Computational Chemistry 2nd ed by Frank Jenson, just in case you need a good reference for stuff like this. 

[simonwei]

Thank you very much, Tamin83, for your detailed reply.
There seems no "one-step" experimentation approach to obtain the orientation of dipole moment according to my literature search. I guess first I have to obtain the molecular structure details, such as bond angles, bond lengths, and dihedral angles, from diffraction analysis or quantum mechanical calculation (or other methods?), and then I can get the molecular dipole moment, the vector sum of individual bond dipoles. Am I right?

[tamim83]

You are right, but using a quantum chemistry program is probably the easiest way to get started.  In the output for a Gaussian calculation, you are given the x, y, and z components for the dipole moment (along with the magnitude).  If you compare this with the standard orientation of the molecule (also given in x,y,z coordinates in the same file), you can get the direction and angle of the dipole moment from the molecular axis.  Gaussview, the graphical interface software for Gaussian (only for Windows as far as I know) will even show you this as well.  

I don't know much about experimentally determining dipole moments but you are probably right about there being no one step method for figuring this out.  

Good luck, and happy to help.  

[San Yad]

Hi !

I use Praseodymium (Pr) Atom interaction with one of the tyrosine hydrogen. I am able to  make  input file (.gjf) with one of the oxygen of Pr interact with Tyrosine hydrogen. But when I submit job  for optimization. I did not get the RHF value? and the output file quickly runs and finish the jobs in few seconds. This suggest that there is no interaction or I am missing some parameter in input file?

Anyone can help me to find out what I am missing in my input file?

Rgds,
San