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Dynamics of Chemical Reactions and
Photochemical Processes
Yuan T. LeeAcademia Sinica, Taipei, Taiwan
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m1u1 = m2u2 or m2 / m1 = u1 / u2
m4, u4
m2, u2
m1, u1
m3, u3
M=m1+m2=m3+m4
m3u3 = m4u4 or m4 / m3 = u3 / u4
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C2H5NO2≠
HONO + CH2=CH2
C C
H
H
H
H
H
N
OO
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C2H5NO2≠
C2H5 + NO2
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Hexahydro-1,3,5-trinitro-1,3,5-triazine RDX
NH2C
NCH2
N
CH2
NO2
NO2O2N
HCN + HONO + NO2
+ N2O + H2CO + ‧ ‧
‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧
Δ
N2 + CO + H2O
Questions
1. Dissociation Mechanism
Unimolecular vs. biomolecular
Primary and secondary dissociations
2.Dissociation Dynamics
Modes of Energy Release
X. ZhaoE. Hintsa
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RDX
NH2C
NCH2
N
CH2
NO2
NO2O2N
3CO + 3H2O + 3N2
(3×) CH2=N-NO2
HCN + HONO
N2O + H2CO
Concerted Steps High Temperature Combustion
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Quantum chemistry is developing in at least two
directions. First, very large systems can now be studied using conven
tional methods, namely Hartree-Fock theory, density func
tional theory, and second-order perturbation theory. Stru
ctural optimizations including all geometrical degrees of f
reedom can now be completed for molecules with as ma
ny as 200 atoms. Frozen geometry computations (usually
not very useful) can be carried out for systems of 1000 ato
ms. This work opens up a vast new expanse of chemistry f
or theoretical studies.
F. Schaefer
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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
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Especially important is the development of methods that
explicitly include the inter-electronic coordinates R12. Th
ese ideas have been around since the famous work of Hyll
eraas on the He atom in 1928. However, it is only in the pa
st five years that such methods have become useful for st
udying chemical systems. Also encouraging is that most o
f the work in this R12 area is being done by young people,
for example Wim Klopper (Karlsruhe), Fred Manby (Bristo
l), and Edward Valeev (Virginia Tech).
F. Schaefer
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Basis SetLowest a1 mode /
cm-1
Lowest b2 mode / cm-1
cc-pVDZ 615i 93i
cc-pVTZ 621i 40
CCSD(T) / cc-pVTZ transition state geometry
Ortho-Benzyne decomposition, Simmonett, Allen, and Schaefer (2006)
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Fully optimized geometries of 2’-deoxyriboadenosine 2’-deoxyribothymidine pair.
J. Gu, Y. Xie, and H.F. Schaefer (2006)
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m1u1 = m2u2 or m2 / m1 = u1 / u2
m4, u4
m2, u2
m1, u1
m3, u3
M=m1+m2=m3+m4
m3u3 = m4u4 or m4 / m3 = u3 / u4
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mv=Fmv=F△△t=mat=ma△△tt =Constant=P=Constant=P
E=E=
m Em E-1
m
P
2
2
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Toluene
CH3
CH3CH3*
I.C.
CH2
+ H
+ CH3
193nm
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-20
0
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40
60
80
100
120
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160
Reaction Coordinate
Rela
tiv
e E
nerg
y (
kca
l/m
ol)
C6H5• + CH3•
C6H5CH2• + H•
C7H7• + H•
TS1
TS2
TS3
toulene
bicyclo[4.1.0]heptadiene
cycloheptatriene
193 nm
104
90
m/e 15 CH3
m/e 16 CH2D
m/e 17 CHD2
m/e 18 CD3
m/e 76 m/e 77 C6H5
m/e 78 C6H4Dm/e 79 C6H3D2
m/e 80 C6H2D3
m/e 93 C6H5CD2
m/e 94 C6H4DCD2
m/e 95 C6H5CD3
m/e 96
Velocity Axis
Mas
s A
xis
Photodissociation of C6H5CD3 @ 193 nm
J. Am. Chem. Soc. 124, 4068 ( 2002 )
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-20
0
20
40
60
80
100
120
140
160
Reaction Coordinate
Rel
ativ
e E
nerg
y (k
cal/m
ol) C6H5• + CH3•
C6H5CH2• + H•
C7H7• + H•
TS1
TS2
TS3
toulene
bicyclo[4.1.0]heptadienecycloheptatriene
193 nm
104
90
Exp. Value
Energy diagram of isomers and photoproducts of C6H5CH3
DFTCCSD
C6H5CH2 + H
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Comparison of Photoisomerization Comparison of Photoisomerization MechanismsMechanisms
CH3
CH31
2
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5
6
CH3
CH31
2
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5
6
CH31
2
3
4
5
6
CH3
Early Discovery: Ring Permutation (in 1960s)
h
DDD
H
HH
H
HHD
D
D
HH
H
H
CD3
H
H
H
H
H
*
*
* *
h193nm
h193nm
CD2H
H
H
H
H
D
New Observation: Seven-Membered Ring Pathway
C6H5 + 13CH3
C6H5 + CD3
C6H4D+CD2H AlsoC6H3D2+CDH2
C6H2D3+CH3
C6H5+CD3
C513CH5+CH3
Also C6H5+13CH3
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NH2 NH2
N
CH3
H
H H
H
N
CH3
NH2
H
H
H
H
H N
H
HH
H
H H
H
N
CH3
H
H H
H
h193nm
h193nm
H, NH2
H, CH3
CH3
NH2
C6NH7