Transcript 1‧以電腦為工具探討化學反應。

實驗九. 預測化學反應途徑與反應速率
組員名單：
段振斌
49712048
目的、原理(一)
李厚寬
49712025
原理(二) 、(三)
簡 薇
49712016
原理(四) 、步驟
實驗目的
1‧以電腦為工具探討化學反應。
2‧解分子的 electronic Schrödinger equation
藉以預測：分子結構、反應途徑、過渡態、中
間產物、產物
3‧熟悉
分子記算程式：Gaussian 03、Gauss view
分子繪圖程式：Chem Draw
4‧計算反應速率常數，並得出產量。
實驗原理
一. The Born-Oppenheimer appoximation
二. The Hartree-Fock equation
三. 6-31G basis set
四.計算反應速率常數 (Eyring equation)
B ORN -O PPENHEIMER A PPROXIMATION
Molecular Hamiltonian：
First
term: The kinetic energy of the nuclei
Second term: The kinetic energy of the electrons
Third
term: The potential energy of the repulsions between
the nuclei
Fourth term: The potential energy of the attractions between
the electrons and nuclei
Fifth
term: The potential energy of the repulsions between
the electrons
資料來源：Levine Quantum Theory 第五版 13.1節
A S AN EXAMPLE , CONSIDER
H2
ˆ 
2
 -
2
2
2m p
 -
2
2
2m p
2m e
 2
1
2
2m e

2
2
e e e e e e

- - 
r r1 r1 r2 r2 r12
2
2
2
2
2
2
α、 β為兩氫原子之原子核，下標1、2
為電子1和電子2，mp為質子之質量
資料來源：Levine Quantum Theory 第五版 13.1節
利用S CHRÖDINGER 方程式找能量
:電子
:原子核
Purely electronic Hamiltonian:
資料來源：Levine Quantum Theory 第五版 13.1節
：
The nuclear repulsion
Purely electronic energy:
ˆ  E 

el el
el el
資料來源：Levine Quantum Theory 第五版 13.1節
Re：位能達最低點時之平衡距離
De：達平衡時之解離能
D0：分子的振動能階之最低能量(零點能量)
ν=0：zero point energy=1/2hν
Purely nuclear Hamiltonian:
ˆ   E

N N
N
將電子的運動與原子核的運動用數學方法來進行分離
的過程及稱為Born-Oppenheimer approximation
因此我們將wave function寫成：
  qi ,q    el  qi ;q  N  q 
但B.O approxmation在計算
時，並不考慮核之間的作用力
資料來源：Levine Quantum Theory 第五版 13.1節
H ARTREE - FOCK
APPROXIMATION
◎ Consider a simpler
N-el system:
(neglect the el-el repulsion)
N
H   h(i )
i 1
Recall:
→ where h(i) is the operator describing the K.E
and P.E of electron i.
Now we can write:
◎ Hartree product:
because H is the sum of one-el Hamiltonians ,
a wave function which is a simple product of
the wave functions for each electron .
ψHP(x1,x2,…,xN) = χi(x1) χj(x2)˙˙˙χk(xN)
An eigenfunction of H :
Such a many-electron wave function is termed
a “Hartree product”.
NOTE:
The Hartree product does not satisfy
the “antisymmetry principle” .
◎If we put electron-one in χi and electron-two
in χj ,we have:
ψHP12(x1,x2)= χi(x1) χj(x2)---------------(1)
ψHP21(x1,x2)= χi(x2) χj(x1)---------------(2)
We can obtain a wave function to satisfy the
antisymmetry principle by taking the appropri-
ate linear combination of these two HP.
It can be rewritten as a determinant and is
called
a“slater determinant”.
i ( x1) j ( x1)
i ( x 2) j ( x 2)
1/ 2
 ( x1, x 2)  2
-1/2
ψ(x1, x2,..., xN) = (N!)
i ( x1)
j ( x1)
 k ( x1)
i ( x 2 )
j ( x 2)
 k ( x 2 )
i ( xN )
j ( xN )
 k ( xN )
Clearly,
ψ(x1,x2) = -ψ(x2,x1)
(fermion)
◎Hartree-Fock equation
where f(i) is an effective one-el operator
called the Fock operator
1 2 M ZA
f (i )   i    HF (i )
2
A1 riA
Where νHF is the average potential experienced by the i-th electron due to the presence
of the other electrons.
The potential energy of interaction between
Q1Q 2
Point charges Q1 and Q2 is V 12 
4 0 r12
Q1  2
V 12 
dv 2

4 0 r 12
 2  e s 2
Q1  e
2
V 12  e'
2

s2
2
r 12
2
e
e'2 
4 0
dv 2
Adding in the interactions w/ the other el’
we have:
n
V 12  V 13  ...  V 1n   e'
j 2
2
sj
r
2
1j
dvj
 The
Born-Oppenheimer approximation is inherently
assumed.
 Relativistic
effects are completely neglected.
 The
variational solution is assumed to be a
linear combination of a finite number of basis
functions.
 Each
energy eigenfunction is assumed to be
describable by a single Slater determinant.
 The
mean field approximation is implied.
6-31G
◎basis
BASIS SET
set: a mathematical description of orb-
itals of a system, which is used for approxim-
ate theoretical calculation or modeling. It is
a set of basic functional building blocks can
be stacked or added to have the features we
need.
  a1 1  a 2 2  ...  ann
   curgu
u
χ稱為收斂高斯函數(contracted Gaussian)
g為初始高斯函數( primitive Gaussian)
→ 例如STO- 3G：
就是以3個初始高斯函數( primitive Gaussian)來組
合成一個基底函數組(basis set)
◎6-31G
→內殼層（inner shell）：每個原子軌域(AO)以一個
基底函數來表示，此基底函數是由6個初始函數線性組
合而成。
→價殼層（valence shell）：由兩個基底函數組合而
成每一的基底函數則分別是由3及1個初始函數所構成。
“＊” 號代表極化函數(polarized functions)，第一個*號表
示重原子中加入更高階的角動量函數，以苯為例，重原子為碳
原子，因此加入六個d形式的基底函數。第二個*號代表再每一
個氫原子中加入三個p形式的基底函數。“＋” 號表加入擴散
函數(diffuse function)，因此每一個碳原子中需加入三個p
及一個s形式的基底函數；另外，第二個+號表示每一個H原子
中加入一個s形式函數。
Eyring equation

k P
A  B  C 
d [ P]
V
 k  [C  ]
dt

C ：activated complex or transition state

1. C is in pre  equilibriu m with A、B
if gases ,

P C  / P
P C  P
RT [C  ] P
[C  ]
P



 
( PA / P )( PB / P ) PAPB RT [ A]RT [ B ] RT [ A][ B]
nA
(ideal gas : PV  nRT  PA  RT  [A]RT )
V
RT
 [C  ]     [ A][ B]
P

d[P]
RT
 k  [C ]  φ k  Κ  [A][B]  k2[A][B]
dt
P
RT
 k2  φ k  Κ   to find k2: get (1) k  (2) Κ 
P
2. V 
(1) to get k ：

k  κ κ : transmiss ion coefficien t
(2) to get Κ  ： (equilibri um constant for A  B  C )

φ
q J, m νJ 
 ΔrE 0 /RT
recall, K  (
) e
J
NA


NA qφc  ΔrE /RT
0
Κ  φ φ e
qA qB

where ΔrE 0  E 0 (C ) - E 0 (A) - E 0 (B)
φ
q J  standard molar partition function
vibrationa l mode
q
1
1  e  h / kT

h
1
kT
利用泰勒展開式
x2
h
e  1 x 
  , now that x  
2!
kT
1
1
kT
q


h
h
1  (1 
 ) 1  (1 
) h
kT
kT
x





qC   qC  ,T qC  , qC  ,R qC  , E
kT 
( qC  ,T qC  , R qC  ,E qC  , )
h
kT 

代入 
qC 
h

 
kT 

qC 
N
q
kT
kT 
A C   rE0 / RT
 rE0 / RT
h
e

e


 
 
h q A q B
h
q A qB
NA
RT   RT
kT 
已知k 2   k    

h
P
P
kT RT 



h P
kT 

C
為Eyring equation
h
以上資料來源 : Atkins CH24.4
實驗步驟
點選gview.exe