1 Dynamics of Chemical Reactions and Photochemical Processes Yuan T. Lee Academia Sinica, Taipei,...

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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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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