Solving the mystery in vibrational spectrum of PtCO in argon matrix

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.

Mystery in vibrational spectrum of PtCO in argon matrix has been solved by highly sophisticated electronic and vibrational quantum-chemical computations.. Advances in Engineering

About the author

Yuriko Ono received her Ph.D. degree from Tokyo Institute of Technology in 2004 and worked as a postdoc researcher one year at the Institute for Molecular Science. In 2005, she moved to Hokkaido University, and has continued working as an academic researcher in quantum chemical laboratory at the Faculty of Science, Hokkaido University.

She has done a state-of-the-art work on the theoretical prediction of noble gas compounds by carrying out highly sophisticated electronic structure and vibrational state computations. She is now developing a parallel program code of the automated reaction path search method combined with quantum chamical computations.

About the author

Kiyoshi Yagi received his Ph.D. degree in quantum chemistry from the University of Tokyo in 2004. He became an assistant professor at the University of Tokyo in 2005, a lecturer at the University of Yamanashi in 2009, and a research assistant professor at the University of Illinois at Urbana-Champaign in 2011. Then, he joined Theoretical Molecular Science Laboratory in RIKEN as a research scientist in 2012 and has been a senior scientist since 2015.

He has developed methods for anharmonic vibrational calculations; the generation of anharmonic potential energy surface and a general solver of vibrational Schrödinger equation. He is the main author of SINDO program for vibrational analysis, and one of the author of GENESIS program for molecular dynamics.

About the author

Toshiyuki Takayanagi received his Ph.D. degree from Tokyo Institute of Technology in 1990 and became a researcher at Japan Atomic Energy Research Institute. He worked with Prof. George C. Schatz at Northwestern University in 1995-1996 as a visiting scholar. In 2004, he moved to Saitama University as an associate professor and he became a professor in Saitama University in 2007.

His main interest is nuclear quantum effects in chemical reaction dynamics and he has published over 180 scientific papers.

About the author

Tetsuya Taketsugu received his Ph.D. degree in theoretical chemistry from the University of Tokyo in 1994 and became an assistant professor at the University of Tokyo in 1995. In 1999, he moved to Ochanomizu University as an associate professor and in 2005, he has become a professor of quantum chemistry group in Hokkaido University. He has established key methodology in quantum chemical calculations for real molecular system by interfacing electronic structure theory with dynamics and with vibrational theory.

He has done a pioneering work on the reaction-path bifurcation in multi-dimensional coordinate space, providing insights to reaction dynamics beyond the static reaction pathway. He developed ab initio molecular dynamics code in the electronic structure program GAMESS in 1995, and extended it to the excited-state dynamics with a surface hopping scheme in 2002.

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 matrixPhysical Chemistry Chemical Physics20(5), 3296-3302.

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