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Quantum Chemistry I

  • Akira Terasaki, Professor
  • Masashi Arakawa, Assistant Professor
Atomic and molecular clusters offer a unique opportunity to elucidate how physical and chemical properties of a matter emerge as atoms and molecules associate together one by one. We are interested, for example, in how ferromagnetism emerges for iron, cobalt, and nickel among 3d transition metals, whereas other elements do not exhibit it. For another example, silver nanoparticles are known to show strong photoabsorption, so-called surface-plasmon resonance, which is technologically important in coupling light with materials. However, one does not know how this picture works for small particles less than 1 nm in diameter. Furthermore, clusters provide a model of catalysts to gain molecular-level insights into mechanism of chemical reactions. We tackle these problems in materials science with single-atom precision toward advanced nano-science and technology.

I-1. Spectroscopy and chemical probe of size-selected clusters

Our state-of-the-art experimental technique is based on mass spectrometry enabling size-selection of clusters in single-atom precision. Figure I-1 shows one of the experimental setups in our laboratory. Metal cluster ions are produced in a magnetron-sputtering cluster ion source, guided by ion guides and deflectors, and size-selected by a quadrupole mass filter. Cluster ions thus size-selected are stored in an ion trap; this is the key part of our technique to accumulate dilute sample clusters in a high density for high S/N measurement. Optionally, the stored ions are cooled down either by liquid N2 or by liquid He, and are applied with a magnetic field.

Figure I-1
Figure I-1. Experimental setup for spectroscopy and chemical reaction of size-selected cluster ions.

The stored size-selected cluster ions are interrogated either by laser or by X-ray spectroscopy. Laser spectroscopy probes valence electrons to measure optical absorption and magneto-optical effects. The optical responses are measured via photofragmentation and cavity-enhanced methods. X-ray probes inner-shell electrons: X-ray absorption spectroscopy (XAS) provides chemical analysis of constituent atoms and X-ray magnetic circular dichroism (XMCD) reveals magnetism. The cluster ions are interrogated by foreign molecules introduced into the ion trap as well. Chemical reactivity and reaction pathways are measured by employing a mass analyzer for product analysis.

I-2. Study of chemical-reaction pathways step by step

Figure I-2
Figure I-2. Mass spectrum of product ions upon sequential reactions of AlN+ clusters with O2 and H2O molecules.

One of the advantages of our experimental technique is that we are able to elucidate elementary processes of sequential chemical reactions step by step. We have demonstrated this for formation of hydrogenated hydrated-alumina cluster ions, Al2O3(H2O)NH+, from aluminum cluster ions, AlN+, in the presence of oxygen and water molecules representing a natural atmospheric environment. Figure I-2 shows a mass spectrum of product ions upon reaction of AlN+ with O2 and H2O. We were able to disentangle the reaction pathway by means of step-by-step experiment as follows:
Equation I-1
The predominant products at 139, 157, and 175 amu were identified as Al2O4H3(H2O)1+, Al2O4H3(H2O)2+, and Al2O4H3(H2O)3+ with one, two, and three H2O molecules remaining intact, respectively, with the aid of collision-induced dissociation experiment to obtain further structural information. The chemical composition of these products is similar to that of bauxite, which is a form of bulk aluminum abundant naturally. This study thus models oxidation, hydroxylation, and hydration steps on an aluminum surface in natural environments. It was revealed that reactions with O2 and H2O to form alumina, Al2O3, at initial steps are followed by successive hydroxylation and hydration reactions.

Quantum Chemistry II

  • Kensuke Harada, Lecturer
Our group studies molecular structure and dynamics of transient molecules and molecular complexes with millimeter wave spectroscopy. We have developed a sensitive detection technique with multi-reflection optical path in millimeter wave region. We are observing rotational transitions as well as proton tunneling transitions of transient molecules and internal rotation transitions of molecular complexes in the millimeter wave region. Molecular structures as well as fine and hyperfine structures of radical species and intermolecular potential functions of molecular complexes have been determined by observed spectra. Recent research topics are as follows.

II-1. Fast ortho–para conversion in vinyl radical

Figure II-1
Figure II-1. The nuclear spin conversion of H2CCD.

Nuclear spin state is the smallest size molecular memory, although nuclear spin conversion rate is usually too slow to use it as a practical memory. The conversion rate is around 1 hr−1 Torr−1 for the usual closed shell molecules. Recently, we found out very fast nuclear spin conversion for the vinyl-d radical.

The ground vibronic state are split into two components 0+, 0 by the tunneling motion of the α-deuteron (Fig. II-1). The observed tunneling-rotation spectra between the 0+ and 0 states were significantly perturbed by the ortho–para mixing interaction due to the electron spin-nuclear spin interaction, which is several thousand times larger than the nuclear spin-rotation interaction that cause the nuclear spin conversion for the usual closed shell molecules.

The rate constant of the ortho–para conversion is predicted to be 1.2 × 105 s−1 Torr−1, suggesting extremely rapid ortho–para conversion, which is more than 109 times faster compared with that in the closed shell molecules such as H2CO (Mol. Phys. 108, 2289 (2010) Invited Article).

In the analysis of the chemical reaction in the interstellar space, the ortho–para conversion has been neglected because of the very slow conversion rate. The present result shows that the ortho–para conversion should be taken into account for radical species.

II-2. Parity conservation in dissociation process

Figure II-2
Figure II-2. Energy levels and observed transitions of He–HCN.

If a molecule has energy more than dissociation energy, is the molecule dissociated? Answer is not simple and it depends on the symmetry of the molecule. Recently we have observed some typical example for the He–HCN complex.

The He–HCN complex is a weakly bound complex with a binding energy of 9 cm−1. The energy levels of He–HCN is shown in Fig. II-2, where arrows show the observed internal rotation transitions. Several upper levels of the observed transitions are located above the dissociation energy and are stable bound states, since these levels can dissociate only to the rotational excited state of HCN due to the parity conservation. These parity conservation rule should be taken into account for the calculation of reaction rates.

II-3. Recent publications

  1. “Millimeter-wave spectroscopy of the FeCO radical in the ν2 and ν3 vibrationally excited states.”
    Keiichi Tanaka, Mitsuhiro Nakamura, Mitsuaki Shirasaka, Ai Sakamoto, Kensuke Harada, and Takehiko Tanaka,
    J. Chem. Phys, 143, 014303 (2015).
  2. “Fourier-transform microwave spectroscopy of the H2–H2O complex.”
    Kensuke Harada, Keiichi Tanaka, Hirofumi Kubota, and Toshiaki Okabayashi,
    Chem. Phys. Lett., 605–606, 67–70 (2014).
  3. “Ortho–para mixing hyperfine interaction in the H2O+ ion and nuclear spin equilibration.”
    Keiichi Tanaka, Kensuke Harada, and Takeshi Oka,
    J. Phys. Chem. A, 117, 9584–9592 (2013).
  4. “Fourier-transform microwave spectroscopy of the H2–HCN complex.”
    M. Ishiguro, K. Harada, K. Tanaka, T. Tanaka, Y. Sumiyoshi, and Y. Endo,
    Chem. Phys. Lett, 554, 33–36 (2012).
  5. “Millimeter-wave spectroscopy of H2C=CD: Tunneling splitting and ortho–para mixing interaction.”
    M. Hayashi, K. Harada, R. Lavrich, T. Tanaka, and K. Tanaka,
    J. Chem. Phys., 133, 154303 (2010).
  6. “Millimeter-wave spectroscopy of deuterated vinyl radicals, observation of the ortho–para mixing interaction and predicton of the fast ortho–para conversion rates.”
    K. Tanaka, M. Hayashi, M. Ohtsuki, K. Harada, and T. Tanaka,
    Mol. Phys., 108, 2289 (2010). (Invited Article)
  7. Ortho–para mixing interaction in the vinyl radical detected by millimeter-wave spectroscopy.”
    K. Tanaka, M. Hayashi, M. Ohtsuki, K. Harada, and T. Tanaka,
    J. Chem. Phys., 131, 111101(1–4) (2009).
  8. “Millimeter-wave spectroscopy of CoNO produced by UV laser photolysis of Co(CO)3NO.”
    A. Sakamoto, M. Hayashi, K. Harada, T. Tanaka, and K. Tanaka,
    J. Chem. Phys. 129, 134303 (2008).
  9. “Millimeter-wave spectroscopy of the internal-rotation band of the He–HCN complex and the intermolecular potential energy surface.”
    K. Harada, K. Tanaka, T. Tanaka, S. Nanbu, and M. Aoyagi,
    J. Chem. Phys., 117, 7041–7050 (2002).