Predicting Vibrational Shifts

[knghrts17]

Hello all,


We are currently in the middle of a study involving adsorbates on the surface of CNT using vibrational spectroscopy. I don't have much experience with this, but I was wondering if it is possible to predict the peaks using first-principles calculations? If so would anyone be willing to share this information (i.e a source)?

Thanks in advance

 

[alxm]

Well, you can calculate anything from first principles.. in principle! :)

But in general, QM calculations don't do a terrific job of predicting shifts. Which isn't to say they can't be useful though. Depends also entirely on how you do it. Most quantum-chemical software can calculate vibrational spectra through a variety of methods, the simplest being a time-independent solution to the equilibrium position, and then calculate the second-order derivatives of the energy w.r.t. atomic coordinates (a harmonic oscillator approximation). I think there are also various approximate methods for getting anharmonic corrections. The best way is of course to work with the time-dependent equation. It also depends on the nature of the vibrational bonds in question. I.e. DFT methods tend not to reproduce van der Waals forces correctly - which may be a big issue if you're talking about nanotubes.

There's a lot of development in this within theoretical chemistry. In short, it's still pretty far from being a 'black box' method where you can just insert a model and get good values. If you're serious about doing it though, I'd suggest considering a collaboration with one of the many who study this stuff full-time.

 

[Modey3]

alxm,

One does not need to work with the time-dependent S.E to solve for the natural vibration frequencies of the absorbate. Using the harmonic approximation, one can predict the natural vibration frequency shift of the absorbate on the CNT surface. The simplest assumption is to assume that the absorbate vibrates independently of the CNT.

modey3