Spectra of small molecule and computed spectra

In summary, the speaker is working on calculating the spectra of a small molecule with a nitrogen group attached using DFT B3LYP with TVZ as the basis set. They have experimental data for the molecule's IR and raman, but there are differences between the calculated and experimental modes. The speaker wonders if increasing the size of the basis set would help and how big of a deviation is expected. They also mention that the calculation is in gas while the experiment is in water, which may affect the location of some modes. The speaker is seeking advice on determining the best functionals and basis set size for optimizing the calculation.
  • #1
greisen
76
0
Hi,

I am calculating the spectra of a small molecule - benzen with nitrogen group attach. I have some experimental data of the molecule IR and raman.
I use DFT B3LYP with TVZ as basis set when I compare the modes and are a lot of differences between the two.

1. Would it help to increase the size of the basis sets?
2. How big a deviation should one expect?

The calculation is in gas whereas the experiment is in water - that should change the location of some of the modes.

Any comments appreciated thanks in advance
 
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  • #2
I assume that you are comparing the calculated frequencies with the experimental ones? If so, it is very likely that the calculated frequencies are somewhat higher than the experimental ones. Is this the difference you are seeing?
 
  • #3
yes it is this difference. Is there a way to determine if one is going to use DFT which functionals to use and the size of the basis set in order to optimise the calculation?
 

1. What are small molecules?

Small molecules are organic or inorganic compounds that have a low molecular weight and are typically composed of less than 100 atoms. They are the building blocks of larger molecules and play a crucial role in biological processes, chemical reactions, and drug development.

2. How are spectra of small molecules measured?

The spectra of small molecules can be measured using various spectroscopic techniques such as infrared spectroscopy, nuclear magnetic resonance spectroscopy, and mass spectrometry. These techniques analyze the interaction of molecules with different forms of energy, such as light or magnetic fields, to determine their chemical and structural properties.

3. What is the significance of studying computed spectra?

Computed spectra refer to the theoretical spectra that are generated using computational methods, such as quantum mechanics calculations. These spectra provide valuable insights into the electronic and vibrational properties of molecules and can aid in the interpretation and prediction of experimental spectra.

4. How do small molecules and computed spectra contribute to drug discovery?

Small molecules are essential for drug discovery as they can be synthesized and modified to target specific biological processes or disease-causing molecules. Computed spectra can also be utilized in the design and optimization of drug candidates by predicting their properties, such as solubility and binding affinity.

5. Can spectra of small molecules be used for identification purposes?

Yes, spectra of small molecules are commonly used for identification purposes in fields such as forensics, environmental analysis, and quality control in industries such as pharmaceuticals and food. The unique spectral signatures of molecules can be compared to databases to determine their identity and purity.

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