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Significance
Over the past two decades, studies involving noble gases have attracted significant interests amongst researchers. Recently, theoretical simulations for the experimentally observed noble gas compounds such as Ar-NiCO, have been conducted for anharmonic vibrational state. For example, the QCISD(T) method used for all electron calculation of Ar-PdCO and Ar-PtCO predicted a binding energy of 5.3 and 8.2 kcal mol-1 and with a bending frequency of 10% just like in the case of Ar-NiCO.
However, anharmonic vibrational state calculations for noble gas compounds such as Ar-PdCO and Ar-PtCO have not been explored in the previous studies. This is due to the available limited computational program, which has also hindered theoretical studies on their infrared intensities, fundamental frequencies and overtone band. Although there are several methods for computing infrared intensities and vibrational frequencies of Ar-PtCO, they however still experience difficulties such as in treating floppy molecules.
Dr. Yuriko Ono and Professor Tetsuya Taketsugu from Hokkaido University in collaboration with Dr. Kiyoshi Yagi from RIKEN and Professor Toshiyuki Takayanagi from Saitama University in Japan developed a novel technique for calculating infrared intensities and vibrational frequencies of Ar-PtCO. They purposed to investigate the anomalous effects of the noble gas on the vibrational and infrared spectra of the PtCO from the calculations. The work is currently published in the research journal, Physical Chemistry Chemical Physics.
Briefly, the research team employed a vibrational configuration interaction (VCI) method to calculate the IR intensities and vibrational frequencies. It was based on the CCSD(T) energies and CCSD dipole moments at thousands of grid points. The vibrational frequencies comprised of overtone, fundamental frequencies, and combinational bands.
The authors observed that the Ar and PtCO were bound by a strong van der Waal interaction forces which resulted in a 10% increase in the bending frequency of the Pt-C-O. The van der Waal interaction was also responsible for the drastic loss of the infrared intensity in the Pt-C-O fundamental level and a corresponding increase in the infrared intensity in the overtone band. Also, the research team found a disappearance of the Pt-C-O bending fundamental level in the infrared (IR) spectrum. This was because in Ar-PtCO, the infrared intensity in the fundamental level of the bending vibration of the Pt-C-O reduced significantly to a point of becoming almost negligible.
The study successfully presented a novel VCI based method for computing the vibrational frequencies and infrared intensities of Ar-PtCO. This interesting work has shown that the anomalous effects of Ar binding on the intensities of the Pt-C-O bending modes were due to drastic changes in the dipole moments of Ar-PtCO along the Pt-C-O bending normal coordinates. The large intensity of the overtone level and the decrease in the intensity of the fundamental level were the main cause of the misalignment of the Pt-C-O fundamental level in the Ar- matrix. The computation resolved the consistency between the matrix-isolation and the gas-phase experiments for PtCO.
Reference
Ono, Y., Yagi, K., Takayanagi, T., & Taketsugu, T. (2018). Fundamental peak disappears upon binding of a noble gas: a case of the vibrational spectrum of PtCO in an argon matrix. Physical Chemistry Chemical Physics, 20(5), 3296-3302.
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