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Infrared Spectroscopy of CH2Cl in Solid Parahydrogen
Yuki Miyamoto and Takamasa Momose University of British Columbia T
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CH2Cl (Chloromethyl Radical)
An important radical in atmospheric/environmental chemistry Source of Chlorine atom Ozone destroyer Spectroscopic studies Matrix isolation Jacox and Milligan, J. Chem. Phys. 53, 2688 (1970). Andrews and Smith, J. Chem. Phys. 53, 2956 (1970). High resolution in gas phase MW: Endo et al. Can. J. Phys. 62, 1347 (1984). MW: Bailleux et al. J. Mol. Spectrosc. 229, 140. (2005) IR: Whitney et al. J. Chem. Phys. 125, (2006). A planar structure (C2v) with the ground electronic state of 2B1
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CH2Cl (Chloromethyl Radical)
a source of 1CH2 and 3CH2 Dribinski et al. J. Chem. Phys. 115, 7474 (2001). CH2Cl + UV ( nm) → CH2 (a1A1) + Cl (2P3/2,1/2) CH2Cl + UV ( nm) → CH2 (X3B1) + Cl (2P1/2)
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Parahydrogen Matrix 1) Quantized rotation of small dopants
2) Small intermolecular interaction CD4 / p-H2 0.007 cm-1 = 200 MHz hexagonal close packed structure 3) Small “cage effect” in situ photolysis in solid Ne in solid p-H2
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Radicals in Parahydrogen Matrix
1. Methyl radical (CH3, CD3) J. Chem. Phys. 103, 1400 (1995); 108, 7334 (1998); 2. Ethyl radical (C2H5) J. Phys. Chem. A101, 522 (1997). 3. Carbon chain Cn (n=3, 5, 7, ...) J. Phys. Chem. 100, (1996).
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Experiments in situ photolysis of CH2ClI in solid parahydrogen
Senapatiet al. J. Phys. Chem. A106, 8479 (2002). CH2ClI + UV (>310 nm) → CH2Cl + I Xe lamp with a UVD36A filter ( nm) n-s*(C-I) CH2Cl + UV ( nm) → CH2 (a1A1) + Cl (2P3/2,1/2) CH2Cl + UV ( nm) → CH2 (X3B1) + Cl (2P1/2) 1CH2 3CH2
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Experiments ortho concentration <0.05% sample concentration
< 10 ppm temperature 3.5 K
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Photolysis of CH2ClI before UV after UV after annealing
The photolysis occurs efficiently no other products CH2ClI + UV (>310 nm) → CH2Cl + I CH2, CH3, CH4, CH2I etc.
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Assignments ν2 (CH2 scissors) ν1 (CH2 symmetric-stretch)
ν3 (CCl stretch) unstable form of CH2Cl ? after UV CH2Cl after annealing (5K, 5 min) after 10 h after 24 h
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Nuclear Spin Conversion of CH2Cl
CH2Cl has ortho-para spin modifications A B C Ka = 0, I = 1 ortho electronic state of 2B1 Ka = 1, I = 0 para 37Cl : 35Cl ~ 1 : 3 37CH2Cl 35CH2Cl conversion rate k = K CH4 I=1(J=1) I=2 (J=0) k(CH4) = K J.Chem. Phys. 128, (2008) after annealing after 10 h after 24 h Ka = 0, I = 1 Ka = 1, I = 0
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Rotational Structure 35 37
CH2 sym.-stretch (a1) A-type transition (Ka = 0) Uniaxial rotation J’ = 2 1 b In gas In p-H2 c J = 1 a Ka = Ka = Hrot= AJa2+BJb2 +CJc2 Hrot= A Ja2 B,C → 0 35 P,Q,R branches no P,R branches 37 35Cl Ka = 1 = A’ – A” 35Cl Ka = 0 p-H2 gas 37Cl Ka = 0 35 37Cl Ka = 1 0.22 cm-1 0.11 cm-1 37 0.19 cm-1 0.14 cm-1
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Concentrated Samples 10 ppm 100 ppm ka=1 ka=0 t = 0 t = 0 t = 10 h
slower nuclear spin conversion decay of the radicals over a few days
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Conclusions The CH2Cl radical was successfully produced by the in-situ photolysis of CH2ClI. Nuclear spin conversion of CH2Cl was observed. In a concentrated sample (100 ppm), the nuclear spin conversion was almost suppressed. The CH2Cl radical probably exhibits uni-axial rotation in the matrix.
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