Shape Discrimination Observation of Molecular Orbital in Solution for the First Time in the World - Direct observation of molecules in solution using polarization characteristics of soft X-ray synchrotron radiation (Press Release)
- Release Date
- 21 Jun, 2010
- BL17SU (RIKEN Coherent Soft X-ray Spectroscopy)
RIKEN
Key research findings
• Observing unexpectedly strong polarization dependence in soft X-ray emission spectra of molecules in solution
• Demonstration of the presence of the molecular orbital in fluid liquid, with a shape similar to that in gas
• Observation technique serving as a new method in research on mutual interactions in solution and in applied research on, for example, chelating resin and resin with molecular recognition properties
|
Scientists at RIKEN (Ryoji Noyori, President) succeeded in establishing an observation technique for clearly discriminating the shape of molecular orbitals in solution using polarized*1 soft X-rays*2 at SPring-8. This was achieved by a joint research team consisting of the following members: Shik Shin, the team leader (also a professor at The Institute for Solid State Physics, The University of Tokyo), Yuka Horikawa, a junior research associate (JRA),*3 Takashi Tokushima, a research scientist of the Excitation Order Research Team, Quantum Order Research Group at RIKEN SPring-8 Center (Tetsuya Ishikawa, Director), and Atsunari Hiraya, a professor at the Faculty of Science, Hiroshima University. A molecule consists of a nucleus and surrounding electrons. Therefore, the characteristics of a molecule largely change depending on the electron state*4 in the molecule. Thus far, various types of research have been carried out on solid and gas molecules targeting the electron state. However, for molecules of the liquid state and in solution, research on the electron state has rarely been carried out. This is because most of the methods used for the direct observation of electrons involve observation in vacuum; liquid samples are vaporized in vacuum and it is difficult to discriminate the signals of various molecules contained in a solution sample. The scientists of the Excitation Order Research Team at RIKEN SPring-8 Center considered the research on the electron state of molecules of the liquid state and in solution to be important. They developed an exclusive apparatus using soft X-ray emission spectroscopy, which enabled direct observation of the electron state in a molecule, ahead of the other countries, and actively carried out research on the electron state of molecules of liquid and in solution samples under atmospheric pressure. The research group succeeded in clearly discriminating the shape of the molecular orbital, where electrons are distributed, by observation utilizing the polarization characteristics of soft X-rays as electromagnetic waves. The results revealed that the shapes of the molecular orbital of all the molecules in liquid are not always disturbed even when the molecules flow randomly. Such a method that enables the direct observation of the electron state and the mutual interaction of molecules of the liquid state and in solution is expected to contribute to fundamental science research, such as that on mutual interaction between biomolecules and ions in aqueous solution samples and on chemical reactions in solution samples. Moreover, this observation technique will also be used in metal analysis and applied research on the mutual interaction between metal ions and molecules in chelating resin and resin with molecular recognition properties, both of which are used for the recovery of rare metals. The results of this research were published on 21 June 2010 as an advance article in the online version, prior to in the printed version, of the British scientific journal Physical Chemistry Chemical Physics. Publication: |
<Figure>
Fig. 1 Schematic of soft X-ray emission spectroscopy
The lines represent molecular-orbital energy levels, and filled and dotted circles on the lines indicate electrons and holes (the absence of an electron in an electron orbital), respectively. When soft X-rays with a specific energy are irradiated onto a material, an electron in the core receives its energy and jumps to an unoccupied molecular orbital (soft X-ray absorption), or an electron in the core is expelled from the atom, and a hole is formed in the core. The hole is unstable and a valence electron falls into the hole to achieve a stable condition within a very short duration of several femtoseconds (1 femtosecond = 10-15 s). Soft X-ray emission spectroscopy is a method of observing the X-rays emitted upon the transition of the electron in order to understand the electron state and related information.
Fig. 2 Shape of molecular orbital of acetic acid and its energy level
Because electrons in a molecule, basically, successively occupy the molecular band with lower energy, the orbitals with lower and higher energies become occupied and unoccupied orbitals, respectively.
Fig. 3 Selection of molecular direction using polarized soft X-rays
A: Change in soft X-ray absorption of π* orbital, which is actually used in the experiment, among various molecular orbitals of acetic acid.
B: Schematic illustrating the selection of the molecular direction upon irradiation of polarized soft X-rays. The luminous molecules indicate those which absorbed soft X-rays. The arrangement of the molecules in the left and right figures is the same; the directions of the molecules that absorb soft X-rays are selected depending on the direction of the electric field (E) indicated by double-headed arrows.
Fig. 5 Change in soft X-ray emission spectra of acetic-acid molecules in
acetonitrile depending on the oscillation direction of electric field
The soft X-ray emission induced by acetic-acid molecules in acetonitrile was measured using linearly polarized light with vertical and horizontal electric-field vectors. The molecular models in the figure show the shapes of molecular orbitals predicted in the previous study. The molecular orbitals circled with red lines indicate molecular orbitals classified as out-of-plane orbitals.
<Glossary>
*1 Polarization
It is known that light has properties of both particles and waves. Not only visible light but also X-rays and soft X-rays have characteristics of electromagnetic waves. Electromagnetic waves propagate by oscillation of electric and magnetic fields. Polarization is the state in which the oscillation directions of electric and magnetic fields are aligned. For visible light, polarized light can be produced using an optical device called a polarizer; however, it is difficult to produce polarized light for soft X-rays because the mutual interaction between soft X-rays and a material is too strong. Therefore, the position of the experimental setup itself is changed during measurement to observe the change in intensity with respect to the change in the oscillation direction of the electric field (polarization dependence) using soft X-rays. However, some factors, such as the deviation of the position of the experimental setup, have adverse effects on the measurement results, and high-precision measurement is difficult. In contrast, at large-scale synchrotron radiation facilities, such as SPring-8, measurement is carried out by an easier method, because synchrotron radiation can be produced by bending the electron beam in a storage ring. For BL17SU RIKEN beamlines, synchrotron radiation (soft X-ray) is produced by making electron beams meander using an undulator, a light source in which magnetic poles are alternately aligned. The oscillation direction of the electric field of soft X-rays can be aligned vertically or horizontally by changing the plane direction of the meandering. Accordingly, by merely changing the setting of the light source, it is possible to measure the polarization dependence at high precision.
*2 Soft X-rays
Soft X-rays are light in the energy region of approximately 100-2,000 eV. Unlike general X-rays of high energy and high permeability used for medical purposes, soft X-rays are low in permeability and easily absorbed by various atoms and molecules. Therefore, when soft X-rays are irradiated onto a material, various responses, such as electron emission, luminescence, and ion generation, are induced. Using these characteristics of soft X-rays, soft X-ray emission obtained by irradiation on a material is observed in soft X-ray emission spectroscopy. The obtained soft X-ray emission spectra fairly accurately reflect the condition of valence electrons related to the material properties, and the valence-electron condition of the molecule can be examined by observing the soft X-ray emission spectra.
*3 Junior research associate (JRA)
The JRA system is RIKEN's original career system in the research field; RIKEN hires young researchers working toward their Ph.D. as part-time research scientists to promote cooperation between experienced researchers with extensive practical knowledge and research experience and young researchers with flexible ideas and enthusiasm.
*4 Electron state
Electrons are trapped on a molecular orbital produced by the positive electric charges of the nucleus, and rotate around the molecule. The shape and energy of the molecular orbital differ depending on the type of molecules (Fig. 2). Some electrons existing in the atoms and molecules are deeply involved in chemical bonding and material properties, whereas the involvement of others is negligible. From the results of calculation of the molecular orbital of acetic acid, it was found that there are two types of molecular orbital; one is a molecular orbital localized in the vicinity of the nucleus, and the other is a molecular orbital extended over the molecule. The former is the molecular orbital for core electrons that do not contribute to material properties or chemical bonding, whereas the latter is the molecular orbital for valence electrons, which are related to material properties and chemical bonding. The molecular orbital occupied by electrons is called an occupied orbital, and that without electrons is called an unoccupied orbital. Because the electrons, basically, successively occupy the molecular orbital with lower energy, the orbitals with lower and higher energies become occupied and unoccupied orbitals, respectively.
|
For more information, please contact: Dr. Takashi Tokushima (RIKEN) Dr. Yuka Horikawa (RIKEN) Dr. Masaki Oura (RIKEN) |
- Previous Article
- New Avenues for Function Control and Application Development of Fullerenes (Press Release)
- Current article
- Shape Discrimination Observation of Molecular Orbital in Solution for the First Time in the World - Direct observation of molecules in solution using polarization characteristics of soft X-ray synchrotron radiation (Press Release)