I am trying to simulate IR and Raman of a catalyst and trying to see the effect of Na+ on the frequency. For the catalyst I am getting a very good match for Raman (after converting to intensity), but for IR some strong experimentally observed modes (like S=O str.) are appearing as medium or weak band in theory. I am taking a dimer (obtained from crystal structure and then optimized) in gas phase to simulate the solid. I know IR intensity strongly depends upon intermolecular interaction which is not described properly by taking dimer. How to match the intensity better?
Then I tried to add Na+ cation to the catalyst to see the shift in frequency. Shift in frequency is qualitatively changing as observed in experiment. Now I want to add a counter ion (BPh4), to get the contribution from it as seen in experiment. Probably since I am doing the calculation in gas phase adding BPh4- close to Na+ causing the Na to go little distant from catalyst and shift in catalyst frequency is diminished. The crystal structure of the complex is not known, so I do not have any idea of where to add it. I do see interaction of BPh4 with the catalyst, 4 Phenyl ring breathing mode is split in the complex. Could you suggest how to solve the problem?
Thanks and regards.
Partha
I think structure is very important for getting useful information on IR spectra. Could you guess it or via structure prediction program, such as USPEX code.
I think USPEX, Minima Hopping, Swarm or any other structural search are useful when you do not have any info, of course you can impose constraints but you should know something more about your system. I saw a paper no long ago in Nature where they have combined data mining with experimental information. But I think you are only reporting IR, do you have XRD? have you tried to fit the structure of XRD? do you have Raman? having a single experiment is not enough... all these search methods are also useful for small unit cells but as the number of atoms increase, it becomes a NP like of problem.. it is hard.. you should get more info from experiment to try to direct the search. An addtional point, do you have weak interactions? you have to be sure that theory will provide useful information.... this can not be a blindly search!
Dear Romeo,
Thanks for your response.
I do not have any XRD data as it could not be crystallized, that's a problem. But I do have Raman data, so I am trying to use both Raman and IR to match with theory. I see a change in BPh4 peak when it is binding to the catalyst, so there is some interaction.
I do not think, this is enough, it is hard to give any advice if you do not provide more details of what you are doing, but XRD is a normal method to do structural characterizations.. otherwise, the lack information will get into your results.
Dear Dr. Romeo,
The crystal structure of this compound is available. But when it binds to NaBPh4 I do not know how the arrangement is in space. Basically I am interested in the binding of Na with the compound, BPh4 may be perturbing the spectra to some extent. So I am trying to predict the structure through Raman and IR together with calculation. I am assuming some structure and calculating its vibrational frequency to check the validity. I have some results which is matching with the experiment. I know this is not a very accurate method, but my goal is to predict the binding of Na+ with the compound.
Do I understand your question correctly ? You appear to be saying that you want to predict spectra in the vapour phase which look like spectra recorded of crystals in the solid state. Why ?
Dear Dr. Geoffrey,
You are correct. The reason is I do not know any other software than Gaussian, so trying to predict the structure with it.
Ah.. that is easy, there is Abinit and Quantum Espresso, both free. With those, you can calculate even the Raman spectra but in both you can calculate the whole phonon spectra. Vasp/Phonopy will allow also to calculate the phonon spectra but you will have to buy Vasp. After you get the phonon spectra or at least the gamma point, you can obtain the symmetry group representations and identify which is is Raman or infrared (you can use the Bilbao crystallographic for such purpose). Go and search for solid state results in this field and you will find what you need...of course with some work.
Dear Dr. Romeo,
Actually I am little busy in finishing my Ph. D. Won't be able to learn at this time, but thanks for your suggestion. Will try those after finishing thesis.
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