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Dynamics of Chemical Reactions and Photochemical ProcessesPowerPoint Presentation

Dynamics of Chemical Reactions and Photochemical Processes

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Dynamics of Chemical Reactions and Photochemical Processes

Yuan T. Lee

Academia Sinica, Taipei, Taiwan

C2H5NO2≠

C2H5 + NO2

HCN +HONO + NO2

+ N2O + H2CO + ‧ ‧

‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧

N2 + CO + H2O

Questions

1. Dissociation Mechanism

Unimolecular vs. biomolecular

Primary and secondary dissociations

2.Dissociation Dynamics

Modes of Energy Release

X. Zhao

E. Hintsa

Hexahydro-1,3,5-trinitro-1,3,5-triazineRDX3CO + 3H2O + 3N2

High Temperature Combustion

Concerted Steps

HCN + HONO

(3×) CH2=N-NO2

N2O + H2CO

RDXQuantum chemistry is developing in at least two directions.

First, very large systems can now be studied using conventional methods, namely Hartree-Fock theory, density functional theory, and second-order perturbation theory. Structural optimizations including all geometrical degrees of freedom can now be completed for molecules with as many as 200 atoms. Frozen geometry computations (usually not very useful) can be carried out for systems of 1000 atoms. This work opens up a vast new expanse of chemistry for theoretical studies.

F. Schaefer

Secondly, more and more rigorous new methods are emerging every year. These can be applied to smaller systems (perhaps up to the size of benzene) to yield what I call sub-chemical accuracy, reliability to 0.5 kcal/mole or better. As you know well, such energetic quantities are critical to combustion and environmental studies and in some cases are very difficult to determine from experiment. Among the newer methods, coupled cluster theory with all single, double, triple, and quadruple excitations, CCSDTQ, is becoming a viable technique.

F. Schaefer

Especially important is the development of methods that explicitly include the inter-electronic coordinates R12. These ideas have been around since the famous work of Hylleraas on the He atom in 1928. However, it is only in the past five years that such methods have become useful for studying chemical systems. Also encouraging is that most of the work in this R12 area is being done by young people, for example Wim Klopper (Karlsruhe), Fred Manby (Bristol), and Edward Valeev (Virginia Tech).

F. Schaefer

Ortho-Benzyne decomposition, Simmonett, Allen, and Schaefer (2006)

CCSD(T) / cc-pVTZ transition state geometry

Fully optimized geometries of 2’-deoxyriboadenosine 2’-deoxyribothymidine pair.

J. Gu, Y. Xie, and H.F. Schaefer (2006)

m 2’-deoxyribothymidine pair4, u4

m1, u1

m2, u2

m3, u3

M=m1+m2=m3+m4

m1u1 =m2u2 or m2 /m1 =u1 /u2

m3u3=m4u4 orm4 /m3=u3 /u4

Photodissociation of C 2’-deoxyribothymidine pair6H5CD3

@ 193 nm

J. Am. Chem. Soc. 124, 4068 ( 2002 )

Velocity Axis

m/e 15 CH3

m/e 16 CH2D

m/e 17 CHD2

m/e 18 CD3

Mass Axis

m/e 76

m/e 77 C6H5

m/e 78 C6H4D

m/e 79 C6H3D2

m/e 80 C6H2D3

m/e 93 C6H5CD2

m/e 94 C6H4DCD2

m/e 95 C6H5CD3

m/e 96

Energy diagram of isomers and photoproducts of C 2’-deoxyribothymidine pair6H5CH3

C6H5CH2 + H

DFT

CCSD

Early Discovery: Ring Permutation (in 1960s) 2’-deoxyribothymidine pair

hn

New Observation: Seven-Membered Ring Pathway

C6H5 + CD3

C6H4D+CD2H

Also

C6H3D2+CDH2

C6H2D3+CH3

C6H5+CD3

hn193nm

C6H5 + 13CH3

hn193nm

C513CH5+CH3

Also C6H5+13CH3

Comparison of Photoisomerization Mechanisms
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