S2
Photochemistry and spectroscopy of simple molecules related to atmospheric
chemistry
(Tokyo Institute of Technology)
Shibuya, Kazuhiko
The Earth’s atmosphere can be considered to be a large photochemical reaction vessel interfacing
with ocean and land in addition to biosphere and human habitation. The light source is the sun and the
reactants are many molecules with natural and anthropogenic origins, which are emitted into the
atmosphere. Trace constituents, such as nitrogen dioxide and ozone, serve as optical absorbers and can
initiate photochemical cyclic reactions, where reaction intermediates (free radicals) are generated
transiently. They always play central roles in the molecular transformation.
The reaction conditions in the atmosphere are rather moderate. The thermal energy is not large
enough to promote reactions and contributes the mixing of the atmospheric constituents through
thermal diffusion. The collision frequency is typically in the 109~1010 collisions/s and the dominant
collision partners are N2 and O2. Thus, the atmospheric chemistry is dominated by free radical
reactions under the thermal equilibrium conditions not so far from the standard conditions [1].
Atmospheric free radicals like OH and HO2 are paramagnetic species, which have spin: Most free
radicals are in the doublet spin-state or have odd numbers of electrons. OH, one of the most important
free radicals, is sometimes called as “Atmospheric Cleanser”. Suppose that methane is emitted from
the ground. The OH radical attacks methane
CH4 + OH → CH3 + H2O
(1)
though it takes long time. The residence time of methane in the atmosphere is more than 10 years, and
the long reaction time is mainly controlled by the extremely-low concentration of the “Atmospheric
Cleanser”. It has recently been recognized that some OH reactions are enhanced upon the
water-hydroxyl radical complex formation. We prepared photochemically water-hydroxyl radical
complexes and investigated the infrared spectra using matrix isolation techniques. I will present the
rigorous spectroscopic assignment for the OH-H2O band [2]. Also we developed an IR-UV double
resonance LIF method to detect OH.
The OH radical is generated by ozone and water vapor as follows.
O3 + hvUV → O(1D) + O2
O(1D) + H2O → OH + OH
(2)
(3)
The UV light below 340 nm is thought to be important in the O(1D) production from an ozone
monomer in the atmosphere [3]. Additionally, the ozone-water complex, O3-H2O, has been proposed to
be a possible source for OH in the troposphere. The OH radical might be produced efficiently through
the photolysis of the O3-H2O complex. Information on the binding energy and rotational/vibrational
structure of the O3-H2O complex are required to be obtained. We recently investigated the complex
and evaluated the contribution of the complex in the atmosphere [4].
The chemical processes following (1) are a series of oxidation reactions leading to formaldehyde.
CH3 + O2 + M → CH3O2 + M
(4)
CH3O2 + NO → CH3O + NO2
(5)
CH3O + O2 → CH2O + HO2
(6)
It wondered long time why the reactant pairs in the reaction sequence (4)-(6) were all paramagnetic
molecules (O2 and NO). At the present, I understand that these paramagnetic molecules serve
occasionally as free radicals and are sometimes called as “Radical Scavenger”.
I will introduce the photochemistry of formaldehyde, which is “isoelectronic” with oxygen. These
two molecules have the same 12 valence electrons but the electrons are orbiting under the different
Coulomb potential prepared by different positive charge distribution. We can recognize certain
electronic state correlation between isoelctronic molecules in general. The chemical fate of
formaldehyde is described as follows.
CH2O + hvUV → CO + H2
(7a)
CH2O + hvUV → HCO + H
(7b)
HCO + O2 → CO + HO2
(8)
Formaldehyde is not a free radical but photochemically-active under the sun-light irradiation
conditions. The paths (7a) and (7b) have different functions in the reaction chains. Finally, methane is
oxidized into carbon dioxide, which is inert in the atmosphere, through the oxidation of carbon
monoxide.
I will present our research on the spectroscopy and photochemistry of nitrogen dioxide [5] and nitric
oxide, the fundamental characterization of singlet oxygen [6,7] and oxygen dimol [8] and the
spectroscopy of HOOCX (X=Cl or Br ) [9].
[1] 「フリーラジカルの科学」廣田榮治編、学会出版センター (1998)
[2] K. Tsuji and K. Shibuya, J. Phys. Chem. A 113, 9945–9951 (2009) and unpublished results
[3] A. R. Ravishankara, G. Hancock, M. Kawasaki, and Y. Matsumi, Science 280, 60-61 (1998)
[4] M. Tsuge, K. Tsuji, A. Kawai, and K. Shibuya, J. Phys. Chem. A 111, 3540-3547 (2007) and
unpublished results
[5] K. Tsuji, M. Ikeda, J. Awamura, A. Kawai, and K. Shibuya, Chem. Phys. Letters 374, 601-607
(2003) and others
[6] A. Kawai and K. Shibuya, J. Photochem. Photobio. C: Photochemistry Reviews 7, 89-103
(2006)
[7] E. Furui, N. Akai, A. Ida, A. Kawai, and K. Shibuya, Chem. Phys. Letters 471, 45-49 (2009)
[8] A. Ida, E. Furui, N. Akai, A. Kawai, and K. Shibuya, Chem. Phys. Letters 488, 130-134 (2010)
and unpublished results
[9]
T. Yoshinobu, N. Akai, A. Kawai, and K. Shibuya, Chem. Phys. Letters 477, 70–74 (2009) and
unpublished results