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Calculation of infrared spectra of acidic groups in zeolites
This project was finished with a set of publications, and a successful Ph.D. thesis defence.
Infrared spectroscopy of the acidic group (Al-OH-Si) in zeolites is one
of the most important methods to study zeolite acidity. Without adsorbates
there is a sharp peak at 3600 wavenumbers of the O-H stretch vibration.
When methanol is adsorbed on HZSM-5 experimentally three broad bands are
observed: 2900-2700 cm-1, 2400-2360 cm-1 and 1730-1680
cm-1, denoted as ABC triplet. This triad is accompanied by
a band at 3560-3500 cm-1. The ABC triplet has been interpret
as a Fermi resonance of the OH stretch with the overtones of the in-plane
and out-of-plane bending and the remaining band at 3500 cm-1
as methanol OH stretch.
Calculation of the spectra consists of two parts. The first part is calculation
of a potential-energy surface (PES) and of a dipole surface (for the intensities).
For this we use DFT calculations, in particularly B3LYP exchange-correlation
functional, on a cluster containing 8T atoms, which represents an 8T ring
of chabazite crystal structure. The potential energy surface is obtained
by scanning the configuration space. The second part is the calculation
of the vibrational frequencies, wavefunctions and intensities. For this
part we are using at the moment our own code AnharmND, written by E. L.
Meijer, which uses a multivariate polynomial to represent the PES and
the dipole surface.
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