Computational Quantum Chemistry Part I: Obtaining Properties
MolecularThermodynamics Quantum Part Atkins9e
Transcript of MolecularThermodynamics Quantum Part Atkins9e
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7Quantum theory: introduction and principle
Postulates of quantum mechanics:
The wavefunction
ψ has all the information.
ψ can be obtained from the Schrodinger equation, ^ H ψ = Eψ .
The Born interpretation
⌈ ψ ⌉2 is the probability density for finding a particle.
Acceptable wavefunctions
ψ must be continuous, a continuous first derivative, be single-valued, and be square-
integrable.
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Observables
Ω → Ω̂ built from the position and momentum operators of the form
^ x= x × ̂p x=ℏid
dx
The eisenberg uncertainty principle
[Ω̂1 , Ω̂2 ]=Ω̂1 Ω̂2−Ω̂2 Ω̂1≠0 ! ∆ Ω1 ∆ Ω2 ≥1
2|⟨ [Ω̂1 , Ω̂2 ] ⟩|
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8Quantum theory: techniques and applications
Translational motion
Free motion in one dimension
−ℏ2
2m
d2
ψ
d x2= Eψ
The solutions of this equation have the form
ψ = A eikx +B e−ikx E=k
2ℏ
2
2 m where A and B are constants.
! all values of " are possible
! the translational energy of a free particle is not quanti#ed
$n either case of eikx or e−i kx , |ψ |2 is independent of %
! the position of the particle is completely unpredictable
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A particle in a box
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(a) The acceptable solutions
The Schrodinger equation for the region where & ' ( is the same as for a free
particle, so the general solutions are given as)
ψ k = A eikx +B e−ikx= A (cos kx+isin kx )+B (coskx+isin kx )=( A+B ) coskx+ ( A−B ) isin kx
Then, with new coefficients
ψ k =C coskx+ D sin kx E k =k
2ℏ
2
2 m
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oundary conditions
The wavefunction must be #ero where & is infinite.
ψ k ( x )=0 for the regions of x ≤ 0 and x ≥ L
! by the requirement of wavefunction continuity
ψ k (0)=0 and ψ k ( L)=0
! C =0 andkL=¿0
ψ k ( L )= Dsin¿
!kL=nπ n=1,2,…
! ψ n ( x )= D sin(nπx/ L)n=1,2, …
! En=n2h2/8 m L2
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!onclusion:
The need to satisfy boundary conditions implies that only certain
"a#efunctions are acceptable$ and hence restricts obser#ables to discrete
#alues%
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(b) &ormali'ation
The normali#ation condition)
∫0
L
ψ ¿ψ d x=C 2∫0
L
sin2 nπx L
d x=C 2 × L2=1 , so C =(
2 L )
1 /2
The complete solution is
En= n
2h
2
8m L2 n=1,2,…
ψ n ( x )=(2
L )1/2
sin( nπx L )
The energies and wavefunctions are labeled with the *quantum number+ n .
A quantum number is an integer in some cases, as we shall see, a half-integer
that labels the state of the system.
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c The properties of the solutions
n ↑ ! the number of nodes ↑
ψ n has n - nodes
the number of nodes ↑ ! the average curvature of the wavefunction
! "inetic energy of the particle ↑
/ach wavefunction is a superposition of momentum eigenfunctions)
ψ n ( x )=( 2 L )1/2
sin(nπx
L )=( 2 L )
1
2 ( eikx−e−i kx ) k =nπ L
! equal probability of travel to both directions
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Because n cannot be #ero, the possible lowest energy is
E1= h
2
8 m L2
This lowest, irremovable energy is called the 'eropoint enery.
The physical origin of the #ero-point energy can be e%plained)
. The uncertainty principle
! a confined particle some information for the position
! the momentum cannot be #ero precise value
0. Acceptable wavefunctions
! must be #ero at both ends and need to be survived
! must have curvature
! non#ero "inetic energy
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The separation between ad1acent energy levels
En+1− En=(n+1)2 h2
8 m L2 −
n2
h2
8 m L2=(2 n+1)
h2
8 m L2
Separation ↓ as L↑
Separation → 0 as L→ ∞ ! not quanti#ed li"e a free particle
Separation ↓ as m↑
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!orrespondin principle
2lassical mechanics emerges from quantum mechanics as high quantum
numbers are reached.
(d) *rthoonality
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3avefunctions corresponding to different energies are orthogonal)
Suppose for two different energies of En and Em
^
H ψ n= En ψ n ^
H ψ m= Em ψ m
Then ∫ψ m¿ ^ H ψ n d x= En∫ψ m
¿ψ n d x ∫ψ n
¿ ^ H ψ m d x= Em∫ψ n¿
ψ m d x
4ow consider noting that energies are real
∫ψ m¿ ^ H ψ n d x−{∫ψ n¿ ^ H ψ md x }¿= En∫ψ m¿ ψ nd x− Em∫ψ nψ m¿ d x
5sing hermiticity, the sum on the left is #ero, E
(¿¿n− Em)∫ψ m¿ ψ n d x0=¿
owever, the two energies are different6 therefore the integral on the right
must be #ero)
∫ψ m¿ ψ n d x=0
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+otion in t"o and more dimensions
Schrodinger equation for a particle in a bo% in two dimensions)
−ℏ2
2 m (∂2ψ
∂ x2 +
∂2ψ
∂ 2 )= Eψ
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(a) ,eparation of #ariables
,eparation of #ariables technique
7ivides the equation into two or more ordinary differential equations, one for
each variable
3rite the wavefunction as a product of functions)ψ ( x , )= ! ( x )" ( )
3ith this substitution to the equation)
−ℏ2
2m
d2 !
d x2 = E ! !
−ℏ2
2 m
d2
"
d 2 = E" " E= E ! + E"
ψ n1 ,n2 ( x , )=2
( L1 L2 )1 /2 sin(
n1 πx
L1)sin(
n2 π
L2)
En1
,n2=( n1
2
L12+
n22
L22 ) h
2
8 m
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$n same way, the solutions for a particle in a bo% in three dimensions)
ψ n1 ,n2 ,n3 ( x , , # )=(8
L1 L2 L3 )1/2
sin(n1 πx
L1)sin(
n2 π
L2)sin (
n3 π#
L3)
En1
,n2
, n3=( n1
2
L12+
n22
L22+
n32
L32 ) h
2
8 m
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(b) -eeneracy
3hen L1= L2= L in two-dimensional case6
ψ n1 ,n2 ( x )=2 L sin (n
1πx
L )sin (n
2π
L )
En1 ,n2=(n12+n2
2) h2
8 m L2
2onsider the cases n1=1,n2=2 and n1=2,n2=1 )
ψ 1,2 ( x )=2
Lsin(
πx
L )sin(
2 π
L ) E1,2=
5h2
8m L2
ψ 2,1 ( x )= 2 Lsin (2 πx
L )sin( π
L ) E2,1= 5h
2
8m L2
! Although the wavefunctions are different, they are deenerate.
! The occurrence of degeneracy is related to the symmetry of the system.
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.ibrational motion
armonic oscillation if the potential function is
$ =1
2 kx2
with restoring force % =−kx from % =−d$ /dx
! The Schrodinger equation becomes
−ℏ2
2 m
d2
ψ
d x2+
1
2kx
2ψ = Eψ
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The enery le#els
The permitted energy levels are
E&=(&+ 12 )ℏ' '=( k m )1 /2
&=0,1,2, …
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armonic oscillator has #ero-point energy6
E0=1
2ℏ '
! Because the particle is confined, its position is not completely uncertain.
! Therefore, its momentum cannot be e%actly #ero.
! The particle fluctuates incessantly around its equilibrium position6 though
classical mechanics would allow the particle to be perfectly still.
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The "a#efunctions
(a) The form of the "a#efunctions
The wavefunctions for a harmonic oscillator,ψ ( x )= ( × (polynomial in x ) ×(bell-shaped Gaussian function )
. ψ → 0as x →) ∞
0. *heexp+nen 2 is proportional to x2× (mk )1 /2 , so it decays more rapidly for large masses
and stiff springs.
8.$t spread over wider range as& increases .
! 2orresponding principle wor"s at high & .
ψ & ( x )= ( & H & ( )e− 2 /2
= x-
- =( ℏ2
mk )1/4
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/otational motion
/otation in three dimensions: the particle on a sphere
A particle of mass m that is free to move anywhere on the surface of a sphere of
radius r .
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a The Schrodinger equation
The Schrodinger equation is
−ℏ
2
2 m ∇2 ψ = Eψ
9rom :ustification ;.< on p8(
1
.2 /
2ψ =
−2mEℏ
2 ψ /
2= 1
sin20
∂2
∂ϕ2+
1
sin0
∂
∂0 sin 0
∂
∂ 0
/2
ψ =− ψ ϵ ϵ=2 1E
ℏ2
2e3456e 1 =m. 2
*7+854n5mn5m2e.6,9∧m9
9=0,1,2, … m9=9 , 9-1,…, - 9 +1,- 9
Angular momentum quantum number 9
=agnetic quantum number m9
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The normali#ed wavefunctions and energies are
" 9, m9 (0 ,ϕ ) :spherical harmonics !able "#3 on p302$
E=9 (9+1 ) :2
2 1 9=0,1,2, …
! 854ni#ed∧independen +; m9
! (29+1 ) -foldde%enerate
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(b) Anular momentum
2omparing with the classical rotational energy, E=ni5de +; 4n>594. m+men5m={9 (9+1 ) }1/2 : 9=0,1,2,…
&-co mp+nen +; 4n>594.m+men5m=m9 : m9=9 ,9 -1,… , - 9
The number of nodes in " 9, m9 (0 ,ϕ ) increases with 9
! higher angular momentum implies higher "inetic energy
! a high "inetic energy arises from the motion parallel to the equator
because curvature is greatest in that direction
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(c) ,pace quanti'ation
9or each 9 , there are only 29+1 possible m9 values)
The orientation of a rotating body is quanti#ed) space quanti'ation
The plane of rotation of a particle can ta"e only a discrete range of
orientations.
*tto ,tern and 0alther 1elach (in 2342)
5xperiment: Shot a beam of silver atoms through an inhomogeneous
magnetic field
6dea: A rotating, charged body behaves li"e a magnet and interacts with the
applied field
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5xpectation:
2lassical mechanics)
Angular momentum can have all orientations)
! the associated magnet can ta"e any orientation
! a broad band of atoms is e%pected
>uantum mechanics)
The angular momentum is quanti#ed)
! the associated magnet lies in a number of discrete orientations
! several sharp bands of atoms are e%pected
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/esults:
*bser#ed discrete band$ and so confirmed the quantum mechanics%
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(d) The #ector model
9 x ,9 , and 9 # are complementary observables)
[ ̂9 x ,
^9 ]=i :
^9 # [
^9 ,
^9 # ]=i :
^9 x [
^9 # ,
^9 x ]=i :
^9
And
[ 9̂2 ,9̂8 ]=0 8= x , , and #
! Although we can specify the magnitude of the angular momentum and any
of its component, if 9 # is "nown, then it is impossible to ascribe values to
the other two components.
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8%2 ,pin
Stern and ?elach observed two bands of Ag atoms in e%periment)2onflict with the fact that 9 must be an integer.
! from spin , not due to orbital angular momentum
,pin:
,pin quantum number: 6
+anitude: { 6 (6+1 ) }1/2ℏ
,pin manetic quantum number: m6=6 , 6 -1,… , -6
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5lectrons ha#e spin:
6=1
2
+anitude: {12 (
12+1)}1 /2
: =(34 )
1/2
:
m6=+12
(denoted as - or ↑ )
m6=−12
(denoted as ? or ↓ )
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The ground state configuration of Ag atom)
[ @. ] 4 d10 5 61 , a single unpaired electron outside a closed shell
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Particles:
Fermions: with half-integer spin
/lectron, proton, neutron,@osons: with integer spin including (
photon
+atter is an assembly of fermions held toether by forces con#eyed by bosons%
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3Atomic structure and atomic spectra
The structure of hydroenic atoms
amiltonian for hydrogenic atom
^ H =−ℏ2
2 me∇e
2− ℏ
2
2m ( ∇ (
2 − e2
4 π 0.
(a) The separation of #ariables
7ivide Schrodinger equation into two parts)
.The motion of the atom as a whole
0.The motion of electron relative to the nucleus
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Schrodinger equation becomes)
−ℏ2
2 C ∇2 ψ −
A e2
4 π 0 . ψ = E ψ
1
C=
1
me+ 1
m ( C :.ed53edm466
Because potential energy is centrosymmetric independent of angle,
we can write
ψ (. , 0 ,ϕ )= D (. ) " (0 ,ϕ)
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The equation does separate, we have to solve
/2
" =−9 (9+1 ) "
spherical harmonics: 'uantum number 9 , m9
−ℏ2
2 C
d2
5
d .2+$ e;; 5= E55=.D
7he.e $ e;; = − e2
4 π 0. +
9 (9+1 ) ℏ2
2 .2
! a particle of mass in one-dimensional region where the potential
energy is &eff ) radial "a#e equation
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b The radial solutions
$ e;; =− e2
4 π B0 .+
9 (9+1 )ℏ2
2 .2
The st term)
2oulomb potential energy
The 0nd term)
stems from centrifugal force from the angular momentum of electron
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$ e;; = − e2
4 π 0 .+
9 (9+1 ) ℏ2
2 .2
0hen 9=0
/lectron has no angular momentum
ure 2oulombic and attractive at all radii
0hen 9 ≠ 0
The centrifugal term is positive repulsive
As . →0 , this repulsive term dominates the 2oulombic term.
! $ e;; is repulsive
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The allowed energies
En= − 2 e4
32 π 2
B 02ℏ
2n
2 n=1,2,…
adial wavefunctions have the form D ( . )=( polynomial in . ) ×(decayin% e(ponential in . )
adial wavefunctions depend on both n and 9
Dn, 9 (. )= ( n, 9 9 Ln+1
2 9+1 ( ) e− /2
7he.e =2 A.
n 4040=
4 π 0ℏ2
me e2 =52#" pm(B+h. .4di56)
L ( ):466+3i4ed L4>5e..e p+9n+mi49
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6nterpretation of the radial "a#efunction$ D (. ) :
./%ponential factor ensures that D (. )→0 as . →∞ .
0.The factor
9
ensures that provided9>0
D ( . )=0
at the nucleus.8. L ( ) oscillates and accounts for the presence of radial nodes.
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3%2 Atomic orbitals and their eneries
Atomic orbital:A one-electron wavefunction for an electron in an atom
7efined by three quantum numbers) n , 9 , m9
ψ n , 9 , m9 F|n , 9 , m9 ⟩
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>uantum numbers)
rincipal quantum number) n
n=1,2,3, …
Dee.minehe ene.> +; 4 hd.+>eni34+m
En= − 2 e4
32π 2
02
ℏ2
n2
Angular momentum quantum number) 9
9=0,1,2,… , n−1
=4>ni5de +; 4n>594. m+men5m={9 (9+1 ) }1/2ℏ
=agnetic quantum number) m9
m9=−9 ,−9+1,… ,9−1, 9
&-co mp+nen +; 4n>594. m+men5m=m9 ℏ
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The value of the principal quantum number, n ,
controls the ma%imum value of 9
9
controls the range of values ofm
9
/lectron has its own spin) 6=1
2m6=)
1
2
! 4eed four quantum numbers (n , 9 , m9 , m6) to specify
the state of an electron in a hydro)enic atom
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(a) The enery le#els
/nergy levels of hydrogenic atoms) En=
− 2 e4
32 π 2
B 02ℏ
2n
2
2+5nd 64e6 : E0 → n+854ni#ed )
E ∝ 2
ydberg constant)
h3 D H =
H e4
32 π 2 02 ℏ2 H i6 he .ed53ed m466 ;+. hd.+>enG
D H =− H
me D D=
me e4
8 π 2
02
h3
3
! The only discrepancies arise from the neglect of relativistic corrections
the increase of mass with speed.
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(b) Atomic orbitals
The ground state) ψ 1,0,0 F|1,0,0 ⟩
ψ =1
(π 403 )1 /2 e−. /40(;+. =1)
1 6 +.2i49 i6 6phe.i3499 6mme.i349G
m4xim5m495e+; ψ =1
(π 403 )1/2
(4 .=0)∧exp+neni499 de346
2ontributions of the potential and "inetic energy
otential energy prefers to sit on the top of the nucleus.
Cinetic energy li"es to be spread out uniformly.
! A compromise between the two e%tremes)
! The wavefunction spread away from the nucleus and has a reasonably
low average curvature.
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3%4 ,pectroscopic transitions and selection rules
Transition
ψ i → ψ I ∆ E=h J
,election rule) a statement about which transitions are allowed
*allowed+ or *forbidden+
!onser#ation of anular momentum:
A photon has an intrinsic spin angular momentum) 6=1 .
! ∆ 9=) 1 ;+.he .4n6ii+n +; ψ i → ψ I
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Quantum mechanical #ie": (ustification 2%9 on p;)
#, ;i=∫ψ ; ¿ # ψ i d x=⟨ ; | #|i ⟩;+. +ne - e9e3.+n4+m
=−e . )ith components: x=−e x , =−e , #=−e#
After calculating ;i for all three components
! ;i ≠ 0 )hen∆ 9=) 1∧∆ m9=0,) 1
! 4o restrictions for n , because it does not relate directly to angular
momentum
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1rotrian diaram:
Summari#es the energies of the states and the transitions between them
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The structures of manyelectron atoms
The Schrodinger equation for a many-electron atom is highly complicated and no
analytical e%pression for the orbitals and energies can be given.
! 3e are forced to ma"e appro%imations.
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Pauli principle:
3hen the labels of any two identical fermions are e%changed, the total
wavefunction changes sign6 when the labels of any two identical bosons are
e%changed, the total wavefunction retains the same sign.
auli principle for two-electron total wavefunction) K (1,2 )
K (2,1 )=−K (1,2 )
9or two electrons, there are four possible spin states)
- (1 ) - (2)- (1) ?(2) ? (1)- (2) ? (1) ? (2)
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Because we cannot tell which electron is which or there must be symmetry
−¿(1,2) ? (1) ? (2)+¿(1,2) ¿
- (1 )- (2) ¿
3here ) (1,2)=(1/2)1 /2 {- (1 ) ? (2)) ?(1)- (2)}
−¿(1,2)−¿ (2,1)=− ¿+¿ (1,2 ) ¿+¿ (2,1 )= ¿
¿
9our possible total wavefunctions for the same special function ψ )
ψ (1 ) ψ (2)- (1 ) - (2)
ψ (1 ) ψ (2) ? (1) ? (2)
+¿ψ (1 ) ψ (2) ¿
−¿ψ (1 ) ψ (2) ¿
! Only the last one satisfies the auli principle.
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Any acceptable wavefunction for closedshell species)
,later determinant form:
K (1,2,… , ( )= 1( ( M )1/2
[ψ 4 (1) - (1 ) ⋯ ψ 4 ( ( ) - ( ( )
⋮ ⋱ ⋮
ψ # (1) - (1 ) ⋯ ψ # ( ( ) - ( ( ) ]! Antisymmetric under any e%change of any pair of electrons
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(a) Penetration and shieldin
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5ffecti#e nuclear chare:
e;; = −
7he.e i6he 6hie9din>3+n64n
The shielding constant is different for 6 and p electrons.6 electron has a greater penetration than a p electron.9ig (.0( on
p8D(
6 electron has less shielding than a p electron.
! The energies of subshells depend on 9 , but not on m9
6
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.alence electrons:
/lectrons in the outermost shell of an atom in its ground state
! esponsible for the chemical bonds that the atom forms
(b) The buildinup principle
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Building-up principle or Aufbau principle )
9or an atom with atom number )
9eed into the orbitals the electrons in succession.
The order of occupation is)
1 62 6 2 p 3 63 p 4 63 d 4 p 5 6 4 d * 6
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und+s ma%imum multiplicity rule)
An atom in its ground state adopts a configuration with the greatest number
of unpaired electrons
Spin correlation)
/lectrons with parallel spins behave as if they have a tendency to stay well
apart, and hence repel each other less.
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2onsider to satisfy the auli principle
+¿(1,2) 4(1,2)−¿(1,2) p(1,2)K 2(1,2)=ψ ¿
K 1(1,2)=ψ ¿
7he.e ψ ) (1,2 )=(1/2 )1/2 {ψ 4 (1) ψ 2(2))ψ 2 (1 ) ψ 4(2) }
p (1,2 ) is parallel spin state
4 (1,2) is paired spin state
3hen two electrons occupy at the same position, .1=.2 :
−¿ψ ¿
vanishes while +¿ψ ¿ survives
−¿ (1,2)=(1/2 )1 /2
{ψ 4 (1 )ψ 2 (2 )−ψ 2 (1 )ψ 4(2)}ψ ¿
! spin correlation
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5lectron confiurations:
On average, 3 d electrons are closer to nucleus than 4 6 electrons.
! 3 d electrons repel each other more strongly than the two 4 6
electrons.
! Sc has [ A. ] 3 d1 4 62
2r and 2u have [ A. ] 3 d5 4 61 and [ A. ] 3 d10 4 61 , respectively.
The subtle shades of energy differences and electron-electron repulsion
! The rich-comple%ity of inorganic d- me49 chemistry
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2alculations show that the energy difference between [ A. ] 3 d3 and
[ A. ] 3 d1 4 62 depends on e;;
! As A e;; increases, transfer of a 4 6 electron to a 3 d orbital becomes
more favorable because electron-electron repulsions are compensated
by attractive interactions.
!2+¿ : [ A. ] 3d3
$ ¿ , the observed [ A. ] 4 60 3 dn configurations for
2+¿ =
¿ cations of
Sc through En
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The spectra of complex atoms
The actual energy levels are not given solely by the energies of the orbitals,
because the electrons interact with one another in various ways.
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3% ,inlet and triplet states
,inlet:
−¿ (1,2)=(1/2 )1 /2 {- (1 ) ? (2 )− ?(1)- (2)} ¿
Triplet:
- (1 ) - (2 )
+¿ (1,2)=(1
2 )1
2 {- (1 ) ? (2 )+ ? (1 )- (2 ) } ¿
? (1) ? (2)
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9or states arising from the same configuration, the triplet state is generally lies
lower than the singlet state. The two states of 1 61 2 61 He differ by FD0 cm-
corresponding to (.G e&.
T"o simple features in the spectrum of atomic
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3%9 ,pinorbit couplin
,pinorbit couplin:
$nteraction of the spin magnet moment with the magnetic field arising from the
orbital angular momentum
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(a) The total anular momentum
The energies of the levels with quantum numbers 6 , 9 , I )
E9 , 6 , I=1
2
h3A { I ( I+1 )−9 (9+1 )−6 ( 6+1 ) }
The strength of the spin-orbit coupling depends on the nuclear charge.
! The greater the nuclear charge, the stronger the spin-orbit interaction.
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(b) Fine structure
Fine structure:
The structure in a spectrum due to spin-orbit coupling
$n 4a, the spin-orbit coupling affects the energies by about < cm-
3%;Term symbols and selection rules
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a The total orbital angular momentum
b The multiplicity
c The total angular momentum
Term symbol:
L❑2 N+1
594. m+men5m854n5mn5m2e. , L
m4>ni5de: { L ( L+1 ) }1 /2ℏ
495e : L=91 +92 ,9 1 +92−1,… ,∨91−92∨¿ L=012345* …
L=N OD %P H 1 …
4 39+6ed 6he99h46 4 #e.+ +.2i49 4n>594. m+men5mG
+496pin 854n5m n5m2e. , N
495e : N=61+62, 61+62−1,… ,∨61−62∨¿
m59ip9i3i=2 N+1
+494n>594. m+men5m 854n5mn5m2e. , 63heme :
f the spin-orbit couplin% is )ea, then it is effectie
only )hen all the orbital momenta are operatin% cooperatiely
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495e :
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== couplin
ussell-Saunders coupling fails when the spin-orbit coupling is large in
heavy atoms. $n that case, the individual spin and orbital momenta of the
electrons are coupled into individual 1 values6 then these momenta are
combined into a grand total : .
!orrelation diaram )
The labels derived by using the ussell-Saunders scheme can be used to
label the states of the 11 -coupling scheme.
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(d) ,election rules
,election rules:
∆ N=0 ∆ L=0,) 1 ∆9=)1 ∆ 2e3+me6 m+.e4pp.+p.i4e
! .4n6ii+n62e7een 6in>9e ∧.ip9e 64e6 (;+. 7hi3h∆ N=)1 ) ,
7hi9e;+.2idden∈9i>h 4+m6, 4.e499+7ed∈he4 4+m6G
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