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CH103 – Physical Chemistry:Introduction to Bonding
Understanding Atomic and Molecular Structure using
Quantum ChemistryInstructor for D3: Prof. Arindam Chowdhury,
Rutherford beamed alpha particles (++) through gold foil and detectedthem as flashes of light on a screen. The gold foil was only 0.00004 cm thick, meaning on a few hundreds of atoms thick
1871-1937
Planetary model loggerheads with classical electromagnetic theory
Classical EM theory can not explain Blackbody Radiation
Theories based on classical physics unable to explain temperaturedependence of emitted radiation (radiant energy density)
Sun, stars…hot iron rodAll classical theories led to the so called “Ultraviolet Catastrophe”
23
8( ) ; bk T cT d dcν
πρ ν ν ν νλ
= =
Max Planck assumed energies of oscillators are discontinuous
3
/( )1v bv t
aT de
νρ ν =−
Assumption: Energy of electronic oscillators were discrete;Proportional to integral multiple of frequencies
E = Energy of electronic oscillatorsv = frequency of electronic oscillatorsh = Planck’s constant = 6.626 x e-34 joule-secNote: h came in as a fitting parameter
OscE nhν=
1858-1947
Planck never believed his theory was right, since he was a classical physicist
• Electrons rotate in circular orbits around a central (massive) nucleus, and obeys the laws of classical mechanics.
• Allowed orbits are those for which the electron’s angular momentum equals an integral multiple of h/2π. i. e. mevr = n h/2π
• Energy of H-atom can only take certain discrete values: “Stationary States”
• The Atom in a stationary state does not emit electromagnetic radiation
• When an atom makes a transition from one stationary state of energy Ea toanother of energy Eb, it emits or absorbsa photon of light: Ea – Eb = hv
1885-1962
Bohr’s model explains atomic spectra n=1,2,3,...
2(2 )
nhmvr
r nπ
π λ
=
=
4
2 2 2 2
1 1 , 1, 2,3,...8
ei f
i f
m eE h n nh n n
νε
∆ = − = =
Spectral Transitions: ∆E=hv
Further details in Tutorials as problems!Explains Rydberg’s Formula
4
2 2 20
1.8
en
m eEh nε
= −
Photoelectric Effect
1. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy!
2. Red light will not cause the ejection of electrons, no matter what the intensity!
3. Weak violet light will eject only a few electrons! But their maximum kinetic energies are greater than those for veryintense light of longer (red) wavelengths
0
20
( )Wave Energy relatedto
of
E E Sin kx t
Intensity EIndependent
ω
ω
= −
∝
Photodetectors, Photovoltaics, Elevator sensor, smoke detectors
Experimental Observations
Einstein: light can behave like particles
2
0
Energy to remove e' from surface
12
0
P M
M
E hv KE mv
KE hv
φ φ
φφ
=
= = + = +
= − ≥
Borrowing Planck’s idea that ∆E=hv, Einstein further proposed that radiation itself existed as small packets of energy (Quanta), now known as PHOTONS
PE hν=
1879-1955; Nobel prizeFor explanation of Photoelectric effect
Energy of photonfrequency dependent
Light is EM Radiation: Waves
Diffraction or Interference Pattern can be possible only if light is a wave
10
( )( )
~ 3 10 / sec
m
m
m
m
E E Sin kx tB B Sin kx tE c cmB
ωω
= −= −
= ×
Wave-Particle Duality of Light
Light has both wave-like andparticle-like propertieseven though
not observed by us (humans)
simultaneously
De Broglie Hypothesis: Matter Waves
h hp mv
λ = =
Since Nature likes symmetry, particles also should have wave-like nature
Wave-Packet1892-1987
-3410
-31 6
6.6x10 J s 7 109.1x10 Kg 1x10 m/s
h mmv
λ −= = = ××
Electron-wave moving @ 106 m/s λ too small for MACROSCOPIC
OBJECTS
Electrons are well known “particles” with a negative charge
J.J. Thompson, Nobel Prize in 1906FOR SHOWING ELECTRONS ARE PARTICLES
Essentially how a CRTMonitor or standard TVworks
Electron show interference patterns WavesDavisson and Germer 1927
J.J’s son, George Thompson, Nobel Prize 1937For showing ELECTRONS are WAVES
Light can have Particle or Wave like propertiesElectron/matter can have Particle or Wave-
like properties, but we do not know what they really are!!
Electrons or photons show both wave and particle nature “WAVICLE”:
Best suited to be called some form of “Energy”
Wave-Particle Duality
Uncertainty Principle - Heisenberg
. 4xhx pπ
∆ ∆ ≥
IMPORTANT FOR SUB-MICROSCOPIC SYSTEMS
h = 6.626 x 10-34 J s
1901-1976
Further details in Tutorials as problems!
Uncertainty Principle: ∆X.∆Px~ h
Wave-Packet
Specifying position accurately would require many waves, each having a fixed value of momentum (k),
P becomes completely uncertain.
If only one wave with known k (momentum), the position becomes completely uncertain,since 1 wave spreads indefinitely in space.
The nature of the world within the submicroscopic atoms is not directly observable –
we deduce it on the basis of highly indirect evidence, and then proceed to describe it……in terms/quantities which we know from our
experience of the macroscopic world
However, the analogies may not always be good ones
Classical quantities such as position and momentum have decreased suitability for describing the subatomic world.
Inherently, “nature” at the atomic scaleis not deterministic, and their exact behaviors
can not be predicted – Probabilistic
Bohr Model with a De Broglie Touch
Constructive InterferenceOf the electron-waves Can result in stationaryStates (Bohr orbits); If wavelength don’t match,There can not be any Energy Level (state)
2 n=1,2,3,...
n=1,2,3,...2
r nh
mvnhmvr
π λ
λ
π
=
=
=
Need a new theory for dynamics of electrons, atoms or molecules
• Probabilistic, not deterministic (non-newtonian)• Wavelike equation for describing sub/atomic systems
Schrodinger 1925!!!
Mathematical Tools Required
1
0
4, , , , , , /( ) ( ) : dSQR CUBE Sin new functions numbersdx y
Af x g x ∂⇒
∂= ∫
1. Operators: A symbol that tells you to do something to whatever follows it. Operators can be real or complex, and can be represented as matrices. An operatorEssentially perturbs a function or system, as if a measurement is being done!
2. Solve 2nd order partial differential equations with multiple variables:
2 2
2 2( ) ( ) ( ) ( ) ( ) ( ) 0f f f f mx y x y f x nf y
x yx y+ +
∂ ∂ ∂ ∂+ + + =∂ ∂∂ ∂
2
2( ) ( )fd x kf x C
dx+ =
2 2
2 2( ) ( ) ( ) ( )( ) ( )f f f fmd x d x d y d yf x nf y const
dx dydx dy+ + = + + =
[ ] 1 1 2 2 1 1 2 2
3
:
( ) ( ) ( ) ( ) : ;
but or , : Operators
Linear OperatorsdA c f x c f x c Af x c Af x Lineardx
SQR or Sin Cos Nonlinear
+ = + ⇒
Separation of Variables…
2 2 2
1 1: ( ) ( ) and ( ) ( )
n n
j j j j j jj j
Mean E x x x P x E x x x P x= =
= = = =∑ ∑
4. Expectation Values or Average or Mean values
, ( ), ( ) ( ) : Given A find the function x such that A x a x Eigenvalue Equationφ φ φ=
3. Eigenfunctions and Eigenvalues:
Operator operating on a function results in re-generating the same function (which is called eigenfunction) multiplied by a number (eigenvalue)
; ? ?( ) n
xn
d Can or Sin x Eigenvaluesdx
A x be eα αφ=
If P(x) is the probability
(#) for , , but (#) for , n
n
d Sin x Sin x n even it is Sin x n odd not eigenfucntiondx
α α α= ⊗ = ≠ ⊗ =
! :n
x n x nn
d Yes EVdx
e eα αα α= ⇒
LAWS of Quantum Mechanics The state of a system is completely specified by thewavefunction ψ(x,y,z,t). It’s square gives probability density…
To every observable in classical mechanics, there corresponds a linear operator in quantum mechanics
In measurement of observable associated with operator A, only values that will be observed are the eigenvalues of A
The average value of the observable corresponding to A is
Ψ(x,y,z,t) of a system evolves according to time-dependent Schrodinger Eqn:
ˆ*a A dυ= Ψ Ψ∫
1. Born Interpretation of Wavefunction1. The state of a QM system is completely specified by a wave function Ψ(x,y,z,t) & all possible information can be derived from Ψ
Ψ can be real or complex
2. Ψ*. Ψ dv is the probability that the particle lies in volume element dv, so Ψ∗Ψ is the probability density at that point
1882-1970
y
x
z
xa
ya
za
dxdy
dz
*
2
1 3 : ( , , , ')Pr( , , )
( , , , '). ( , , , ')
( , , , ')
a a a a a a
a a a a a a
a a a
Particle System in D x y z tx x x dx y y y dy z z z dz
x y z t x y z t dxdydz
x y z t dv
Ψ≤ ≤ + ≤ ≤ + ≤ ≤ +
= Ψ Ψ
= Ψ
*
2* 2 2
:
( )( )
can be complex f ig and f ig
f ig f ig f g
Ψ Ψ = + Ψ = −
∴ Ψ Ψ = − + = + = Ψ
1a. Corollary: Normalization
* *
( , , ). ( , , ) 1all space all space
x y z x y z dxdydz dτΨ Ψ = Ψ Ψ = Ψ Ψ =∫∫∫ ∫
Normalization of Wave function: Since Ψ∗Ψdv is the probability, the total probability of finding the particle somewhere in space has to be unity.
If integration diverges, i.e infinity: Ψ can not be normalized, and therefore can NEVER be an acceptable wave function. But if it is a constant A ≠1, then it is OK
Work on problems related to normalization of wave functions
Automatically, a condition has been imposed on the nature of wave function Ψ:Ψ must vanish at ± infinity, or rather at the boundaries! Also Ψ has to be finite!!!
X
Ψ
X
Ψ
The state of a QM system is completely specified by a wavefunctionwhich is ψ(x,y,z,t). Square gives probability density…
To every observable in classical mechanics, there correspondsa linear operator in quantum mechanics
In measurement of observable associated with operator A, only values that will be observed are the eigenvalues of A
The average value of the observable corresponding to A is
Ψ(x,y,z,t) of a system evolves according to time-dependent Schrodinger Eqn:
ˆ*a A dυ= Ψ Ψ∫
LAWS of Quantum Mechanics
2. Operator FormalismTo every observable in classical mechanics, there corresponds
a linear operator (real or complex) in quantum mechanics
Classical quantity Quantum Mechanical operator
Position, x x̂
Momentum, px dxdi−
Kinetic energy in 1-D, 2
2x
xpKm
= 2
22
2 dxd
m
−
Kinetic energy in 3-D,
( )2 2 212 x y zK p p p
m= + +
∂∂
+∂∂
+
∂∂
− 2
2
2
2
2
22
2 zyxm
Potential Energy, V(x) Multiply by V(x)
Total Energy, E= K+V
∂∂
+∂∂
+
∂∂
− 2
2
2
2
2
22
2 zyxm
+ V(x)
Find out QM operators corresponding to angular momentum, and KE for rotating systems
The state of a QM system is completely specified by a wavefunctionwhich is ψ(x,y,z,t). Square gives probability density…
To every observable in classical mechanics, there corresponds a linear operator in quantum mechanics
In measurement of observable associated with operator A, the only values that will be experimentally measured are the eigenvalues of A, (which can not be imaginary)
The average value of the observable corresponding to A is
Ψ(x,y,z,t) of a system evolves according to time-dependent Schrodinger Eqn:
ˆ*a A dυ= Ψ Ψ∫
LAWS of Quantum Mechanics
3. Real (observable) eigenvalues
ˆ n n nA aΨ = Ψ
•In any measurement of abservable associated with operator A, the only values thatwill be ever observed are the eigenvalues an, which satisfy the eigenvalue equation:
Only real eigenvalues will be observed, which will specify a number corresponding to the classical variable for the particular eigenfunction
All the eigenfunctions of Quantum Mechanical operators are “Orthogonal”
* ( ) ( ) 0 for m n m nx x dx m nψ ψ ψ ψ+∞
−∞
= = ≠∫
Ψn corresponds to the eigenfunctions or eigenstates of the system.
Eigenvalues represented by an are observables
If the system is in state Ψk, a measurement on the system will yield an eigenvalue ak
There may be, and typically are, many Eigen-f(n)s for the same QM operator!
The state of a QM system is completely specified by a wavefunctionwhich is ψ(x,y,z,t). Square gives probability density…
To every observable in classical mechanics, there corresponds a linear operator in quantum mechanics
In measurement of observable associated with operator A, only values that will be observed are the eigenvalues of A
The average value of the observable corresponding to A is
Ψ(x,y,z,t) of a system evolves according to time-dependent Schrodinger Eqn:
ˆ*a A dυ= Ψ Ψ∫
LAWS of Quantum Mechanics
4. Average or Expectation value
2 *( ) ~ . ( 1 )P x dx dx in DΨ = Ψ Ψ
Prescription for obtaining the average or mean value of a classical observable
<a> corresponds to the average value of a classical physical quantity or observable, and A represents the corresponding Quantum mechanical operator
ˆ ˆ*all space
a A dv A= Ψ Ψ = Ψ Ψ∫
nd 2 2( ) ( ) and 2 moment ( )E x x xP x dx x x P x dx+∞ +∞
−∞ −∞
= = =∫ ∫
2 *
ˆ 1 . ( ) = . ~all space
In D a A P x dx A dx A dx+∞ +∞
−∞ −∞
= Ψ Ψ Ψ∫ ∫ ∫
The state of a QM system is completely specified by a wavefunctionwhich is ψ(x,y,z,t). Square gives probability density…
To every observable in classical mechanics, there corresponds a linear operator in quantum mechanics
In measurement of observable associated with operator A, only values that will be observed are the eigenvalues of A
The average value of the observable corresponding to A is
Ψ(x,y,z,t) of a system evolves according to time-dependent Schrodinger Equation:
ˆ*a A dυ= Ψ Ψ∫
( , ) ( , )i r t H r tt
∂Ψ = Ψ
∂
LAWS of Quantum Mechanics
5: Schrodinger Equation (SE)
1887-1961
The SE equation is somewhat equivalent to Newton’s laws, but applicable to atomic/molecular and subatomic systems
2 2 2
2
(1 )
2 2
x
x
P D ix
PKm m x
∂→ −
∂
∂= = −
∂
Predicts the probability of finding the particle at a specified volume element at future time.. counterpart to Newton’s Equations of motion.
2 2
For ( , ),
( , ) ( , ) ( , )2
x
x t Schrodinger Equation
i x t V x t x tt m
Ψ
∂Ψ = − ∇ + Ψ ∂
2 2
22
2
( )
( , )2
, 1
x
x
Hamiltonian Energy Operator
H V x tm
where in Dx
⇒ = − ∇ +
∂∇ =
∂
2nd order partial differential equation
Time evolution of the wavefunction is somehowrelated to the total energy of the system/particle
Classical quantity Quantum Mechanical operator
Position, x x̂
Momentum, px dxdi−
Kinetic energy in 1-D, 2
2x
xpKm
= 2
22
2 dxd
m
−
Kinetic energy in 3-D,
( )2 2 212 x y zK p p p
m= + +
∂∂
+∂∂
+
∂∂
− 2
2
2
2
2
22
2 zyxm
Potential Energy, V(x) Multiply by V(x)
Total Energy, E= K+V
∂∂
+∂∂
+
∂∂
− 2
2
2
2
2
22
2 zyxm
+ V(x)
Time-Dependent Schrödinger Equation
1 2 1 2( , ,... , ) ( ). ( , ,... )n nx x x t f t x x xψΨ =Very often, V(x,t) = V(x): solutions to the TDSE has the form:
i.e. The space part and the time part can be separated out in the solutions!
Maths: Separation of variables to solve linear 2nd order differential equations
22
1 2
( , ) ( , ) ( , ) ( , )2
( , ) ( , ,... , )
x
n
i x t x t V x t x tt m
x t x x x t
∂Ψ = − ∇ Ψ + Ψ
∂Ψ = Ψ
2 22 2
2
( )
( , ) , 12
x x
Hamiltonian Energy Operator
H V x t where in Dm x
∂⇒ = − ∇ + ∇ =
∂
Separation of space and time variables
:1 ( ) ( ) ( ) ( )
dt
TIME DEPENDENT PART of solutiondf t df t iEE
i f t dt f t
−
− = ⇒ = −
[ ] [ ]
2 2
2
1 1 ( ) ( ) ( ) 2 ( ) ( )
LHS F(x) = RHS F(t) :
f tV x E constm x x i f t t
E independent of x and t
ψψ
∂ ∂− + = − =
∂ ∂
2 2
2 2
Let us consider the solutions for TDSE that have the form:
( ) ( )( ) ( )
( , ) ( ). ( )f t xx and f t
t t x x
x t f t xψψ
ψ∂Ψ ∂ ∂ Ψ ∂
= =∂ ∂ ∂ ∂
Ψ =
For a one-particle, 1-D, If V(x,t)=V(x):2 2
2: ( )2
TDSE V xm x i t
∂ Ψ ∂Ψ− + Ψ = −
∂ ∂
Substitute & divide by ( ) ( )f t xψΨ =
/ ( ) ~ iEtf t e−
Time Independent Schrödinger Equation 2 2 2 2
2 2
Rearranging ( ) :
1 ( ) ( ) ( ) ( ) ( )2 ( ) 2
time INDEPENDENT SPACE part
d x dV x E V x x E xm x dx m dx
ψ ψ ψψ
−
− + = ⇒ − + =
2 2
2
1 ( ) ( ) dimension (Energy), 2 ( )
( !!!)
d x V x E E has same as Vm x dx
this const is the ENERGY of the system energy comes automatically
ψψ
− + =
2 2 2 2
2 2 21
1 (3 ) :
( , , )2
N
i i i i i
For particle D system solve TISE
V x y z Em x y z
ψ ψ ψ=
∂ ∂ ∂⇒ − + + + = ∂ ∂ ∂
∑
2 2
2
: ( )
( )2
( ) ( )Time Independent Schrodinger Equation Stationaty States
dwhere H V x T Vm dx
H x E xψ ψ−
≡ − + = +=
We have suddenly come up with another EIGENVALUE PROBLEM to solve!!!
N
HΨn=EnΨn : Energies and Wavefunctions
( ) ( )( )( )
2 2 *
/ * /
20 *
. ( )
iEt iEt
f f f
e e
eF t
ψ ψ ψ
ψ ψ
ψ ψ ψ
−
Ψ = =
=
= =
≠
For Stationary states (i.e. for a particular solution): Ψ2 and Energy Const in time
2 2
2, ( ) 2
( ) ( )n n n where H V x Total Energym x
H x E xψ ψ ∂≡ − + ⇒
∂=
MANY solutions to TISE possible different Ψ with different energies
All energies are allowed unless there are some special conditions (just like a wave when restricted at the ends – Stretched string)
Does not mean that the system is at rest (recap Bohr’s stationary state), but energies and the probability do not change with time.
Well-Behaved or valid wavefunctions1. Ψ must be normalizable: Ψ must be finite and 0 at boundaries/ ±infinity2. Ψ must be a solution of the Schrodinger equation:
A. Must be a continuous function of x,y,z.B. The slope of the function i.e. dψ/dq must be continuous in x,y,z.
The essence of Quantum Mechanics• Not deterministic: Can not precisely determine many parameters in the
system, but Ψ can provide all the information (spatio-temporal)
• Only average values and probabilities can be obtained for classical variables, now in new form of “operators”.
• Total energy is conserved, but quantization/discretization of energy levels come spontaneously from restriction on wave function or boundary condition
• Final outputs tally very well with experimental results –that’s why QM is still out there! Does not violate Classical mechanics for large value of mass.
Applications- 1. Free Particle
2 2
2
:
( ) ( ) ( ) ( )2
( ) ( )
Time Independent Schrodinger Equation
x V x x E xm x
H x E x ψ ψ ψψ ψ
−
∂− + =∂
= ⇒
2 2 2 22
2 : : ( ) sin cos( ) cos sin
2( ) ( sin cos ) ( )
ndSolve the Order Linear DE Trial Solution x A kx B kxD x A kx B kx
mED x k A kx B kx k x k
ψψ
ψ ψ
= += −
= − + = − =
2 2 22 22 2
2 2 2
:( ) 0 :" "
( ) 2( ) ( ) ( ) 0 2
For a free particleV x as there are no external potential forces free
x mEE x D x k x where D and km x x
ψ ψ ψ ψ
=
∂ ∂⇒ − = ⇒ + = = =
∂ ∂
2 2
; 2
2 2 ( ) sin cos
kE No restrictions on k Energies of free particle continuousm
mE mEAnd x A x B xψ
∴ = ⇒
= +
No Quantization:All energies allowed
2. Particle in 1-D Potential Well
2 22
2 : ( ) ( ) ( ) 0 ( ) ( )
: ( ) 0
mFor regions I and III D x E V x D x x
Normalization condition not satisfied Only solution x
ψ ψ ψ ψ
ψ
+ − = ⇒ = ∞
⇒ ⇒ =
2 2 2 22 2
2 2 : ( ) ( ) 0 ( ) ( ) 0
Similar to Free Particle solution,
: ( ) sin cos
mE mEFor regions II D x x D x k x k
but now boundary conditions present
Trial Solution x A kx B kx
ψ ψ ψ ψ
ψ
+ = ⇒ + = =
⇒
= +
Boundary ConditionsΨ(0)=0 and Ψ(L)=0
V=0V=∞ V=∞
Region IRegion II
Region III
XX=0 X=L
: 0, ( ) 0 0 ( ) sin , ( ) 0 sin 0 0 sin 0
0, ( ) 0 !! sin 0 1, 2,3, 4,...
Boundary Conditions x x B x A kxAlso at x L x A kL either A or kLIf A x everywhere or kL kL n n
ψ ψψ
ψ π
= = ⇒ = ⇒ == = ⇒ = ⇒ = =
= = = ⇒ = =
x
V x x L
x L
0
( ) 0 0
∞ <= ≤ ≤∞ >
Particle in Box (PIB) Energies are Quantized: Certain Levels Allowed
2 2 2 2 2 22
2 2 2
2
:sin 0 for 1, 2,3, 4,...
22 8
, n=1,2,3,4,... ,
n
n
From Boundary ConditionskL kL n n
m n n n hE k EL mL mL
or E nAutomatically energy is no longer continuousbut discrete or quantize
( )[ ]1 1 1 11 2 1 1 1 1 2 2 2 2 1 2( , , ) ( , , )e e e e
He He HeH r r E Eε ε θ φ θ φ ε εΨ = + Ψ Ψ = Ψ ⇒ = +
1 11 21 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2 1 1 2 2
12
1( , , ) ( , , ) ( , , ) ( , , ) ( ) ( )e e
HeH H r r r H r r rr
θ φ θ φ θ φ θ φ Ψ = Ψ Ψ + Ψ Ψ + Ψ Ψ
2 4 2
1 2 1 22 2 21 1
1 2
( ) 13.6 4( 13.6) 108.8
. : 24.6 54.4IE IE
Z e ZE Theory eV eV E E E E eVn n
Exp Ionization Energies E eV E eV
µ= − = − = − = ⇔ = + = −
= − = −
Most serious assumption of “orbital approximation” is drastic ! Electrons stay out of each other’s way by undergoing “correlated”
Motion to ensure electronic repulsion is minimum
Need to use other (numerical) approximate methods such as Variational Methodor Perturbation Theory without using orbital approximation
Effective Nuclear ChargeLet us find out what the ionization energies come out to be:
For He: Z=2, n=1 (GS), say a=a0
Electrons must be shielding each other!!!Outer electrons feel that the net charge on the nucleus
is less than what is expected: Screening of Z“Effective Nuclear Charge”
1 1 2 212
1 ( ) ( ) , !r r not small cannot be neglectedr
Ψ Ψ ⇒
+2
Electronic Shielding: Effective Nuclear Charge
Effective nuclear charge is same for electrons in the same orbital, but greatly varies for electrons of Different orbitals (s,p,d,f) and n. Zeff determines
several chemical properties of multi-electron systems
+2
3 32 2
0 0
1 20 01 (1)1 (2) .eff eff
He
eff effZ r Z ra aZ Z
s sa a
e e− −
Ψ ≈ ≈
σ
σ=
= −
−= ⋅
∑
1
eff
n
Hatomi i
Z Z
ZE E
n
See slides of Prof. Naresh Patwari (details provided)
σ= − =
= − ×
2
, , , , ...
( , ') ( 13.6 )
i
ieff
Hi
Z where i s p d f
Zavg per e IE eV
n
Multi-electronics: Consequences of Zeff
• How do we get 2p energy higher than 2S?• How does Radial distributions change? • How does Zeff affect atomic properties?
Read up on electronic
configuration of multi-
electronic atoms: Hund’s Rule
& Aufbau Principle
Quantization of Spatial (Orbital) Angular Momentum of Electrons in the atom
L=2ml+1
Na/Ag atoms: ns1: l=0: Zero Orbital Momentum For 1S electrons
If electrons are classical, "spinning" particles, then distribution of their spin angular momentum vectors is taken to be truly random and each particle wouldbe deflected up or down by a different amount, producing an even distribution.But electrons are deflected either up or down by a specific amount.
Intrinsic angular momentum or “Spin” of an electron is quantized
This can only mean that spin angular momentum is quantized (i.e., it can only take ondiscrete values). There is not a continuous distribution of possible angular momenta.
Electrons are spin-1⁄2 particles. These have only two possible spin angular momentum values, called “spin-up” (or α) and “spin-down” (or β)
The exact value in the z direction is ms= +ħ/2 or −ħ/2. If this value arises as a result of the rotating particles, then they would have to be spinning impossibly fast (GREATER THAN THE SPEED OF LIGHT).
Spin S(ω) where w is unknown (internal, if exists) coordinate (NON-CLASSICAL)
Implication of “Spin” on Hydrogen 1-e wavefunctions: Spin Orbitals
Introduction of spin results in the 4th quantum numberFor each electron: 3 spatial, and one for spin (4th coordinate)
Introduce a “spin” component to each of the 1e WFs along with spatial components. This doubles the degeneracy of each level little effect on energy.
1e WFs are now called SPIN ORBITALS.
2e Spin orbitals and Exclusion Principle
The complete wavefunction (both spatial and spin coordinates) of system of identical fermions (i.e. electrons) must be anti-symmetric with respect to interchange of all the coordinates (spatial and spin) of any two particles.
(Pauli’s?) Exclusion Principle: (by Dirac and Heisenberg)
Wavefunctions must reflect the indistinguishibility of electrons: No known experiments can distinguish between the last two functions, which clearly distinguishes between electrons – CAN NOT BE ALLOWED
(1, 2) (2,1)( )
Indistinguishibility
Exchange OperatorΨ = ±Ψ Symmetric:
Anti-symmetric:
Postulate 6: Complete wavefunctions describing a many electron system must be antisymmetric under the exchange of any two electrons.
For He atom ground state (both electron is 1s):
Space part is SYMMETRIC: 1s(1).1s(2) since both e in same orbital
We must therefore multiply this with an ANTI-SYMMETRIC Spin function
Spin Part:(1,2)(2,1)
6th LAW of Quantum Mechanics
Total approximate Ψ of He (GS): 1s(1)1s(2) * 2-1/2 [α(1)β(2)-β (1)α(2)]
Which is also anti-symmetric w.r.t exchange of electronic coordinates
Also, two electrons is 1s orbitals have OPPOSITTE Spins!!!
Ground and excited states of HeGround state: Both electrons are in same orbital (1s): 1s(1)1s(2)Symmetric
If they are in different orbitals: spatial part: 1s(1)2s(2) or 1s(2)2s(1)Both these are neither symmetric nor anti-symmetric.
Spatial Part can only be: 1s(1)2s(2) + 1s(2)2s(1) (SYMMETRIC)
or 1s(1)2s(2) - 1s(2)2s(1) (ANTI-SYMMETRIC)
Complete (spatial & spin) wavefunctions for an excited state of He(Note: Spatial*Spin part is antisymmetric)
Molecular Electronic Structureand Bonding in Diatomic and
Polyatomic Molecules
Lec 7 CH103 AC
Simplest Molecules: H2+ and H2
For H2 Molecule (2 Nuclei + 2 Electrons):
R
r1Ar1B
r2A r2B
r12
( ) ( )2 2 2 21 2
1 2 1 2 122H
1 1 1 1 1 1~ A B e eA A B B AB
Hr r r r R r
−∇ − ∇ + −∇ − ∇ + − − − − + +
( ) ( )2 2 21
1 12H
1 1 1~ A B eA B AB
Hr r R
+
−∇ − ∇ + −∇ + − − +
H+2 Molecule Ion (2 Nuclei + 1 Electron)
Born-Oppenheimer Approximation
+2
2H (1 )
1 1
1 1 1 1 1~ H e
e A AB e B AB eB
H Hr R r R r
−∇ − + − = + −
Nuclei are stationary wrt electronic motion
( ) ( )2 2 21
1 12H
1 1 1~ A B eA B AB
Hr r R
+
−∇ − ∇ + −∇ + − − +
CONSTANT
Model 1: Valence Bond Theory (VBT)1 electron in H1 2 electron in H2
Inclusion of ionic terms “Resonance”
Model 2: Molecular Orbital Theory
Both VBT and MOT are used frequently – each ofthem work good for certain systems, respectively,and often are not so good for other systems!
Molecular Orbital Theory: LCAO Linear Combination of Atomic Orbitals (LCAO)
1 1 2 1 1 1 2 2
2 2 2 2 21 1 2 2 1 2 1 22
A BMO Approx S S
MO
c c c c
c c c c
ϕ ϕ ψ ψ ψ ψ
ϕ ψ ψ ψ ψ
= = + = +
= + +2 2 2
1 2 1 2 1 22 2 2
2 1 1 2 1 2
, ( ) ( ) ~ 1 1
, ( ) ( ) ~ 1 1b b b b b A B
a a a a a A B
for c c c c c s sfor c c c c c s s
ϕ ψ ψ ϕ ψ ψ
ϕ ψ ψ ϕ ψ ψ
= = = + ⇒ = ± + +
= − = − = − ⇒ = ± − −
2 21 2 1 2: , symmetry c c c c= ⇒ = ±
Importance of Overlap Integral2 2
1 2 1 2 1 1 2 2 1 2 2 1
? !
1 ( ) ( )
b a
b b b b
How to find out c and c Normalization
c cϕ ϕ ψ ψ ψ ψ ψ ψ ψ ψ ψ ψ ψ ψ = = + + = + + +
( ) ( )1 2 1 21 1;
2 2 2 2b aS Sϕ ψ ψ ϕ ψ ψ= + = −
+ −
S can be positive, zero or negativeS is a qualitative measure of bond strength
1 2 2 11 1 where
2 2 2 2b ac and c S Overlap IntegralS S
ψ ψ ψ ψ= = = = ⇔+ −
1 1
( ) ( )1 1 1 1/
1 12 2 2 2A B A Bb b S S S Sb
E H HS S
ϕ ϕ ψ ψ ψ ψ+
= = + ++ +
1 1 1 1
/ /
Let
and 1 1
S S S Si i jiii ij
ii ij ii ijb a
ij ij
H H and H H
H H H HE E
S S
ψ ψ ψ ψ
+ −
= =
+ −= =
+ −
H2+Bonding and Anti-bonding Orbitals
( )
( )
1 1
1 1
12 2
12 2
A B
A B
a S S
b S S
S
S
ϕ ψ ψ
ϕ ψ ψ
= −−
= ++
1 1 1 1 1 1 1 1/
1 =2(1 ) A A B B A B B AS S S S S S S Sb
E H H H HS
ψ ψ ψ ψ ψ ψ ψ ψ+
+ + + +
(1 )1 1 1 1 1 1 1 1.
1 1( ) +( )B A B A B A B A
H eij BA S S S S S S S SAB eBconst
H or H H HR r
ψ ψ ψ ψ ψ ψ ψ ψ
= = −
(1 )1 1 1 1 1 1 1 1
.
1 1( ) +( )A A A A A A A A
H eii AA S S S S S S S SAB eBconst
H or H H HR r
ψ ψ ψ ψ ψ ψ ψ ψ
= = −
2(1 )
1 1 1 1 1~ H e
eA AB eB AB eB
H Hr R r R r
−∇ − + − = + −
1 (1 ) 1 11 1( ) where
A Aii AA BB S e S SAB eB
H or H H E J J Coulomb IntegralR r
ψ ψ= = + − = ⇒
1 1 (1 ) 1 1 1 1 11 1
B A B A B Aij S S e S S S S SAB eB
H ER r
ψ ψ ψ ψ ψ ψ= + −
Stabilization Energy due to Bond Formation
1 (1 ) 1 11, where
Re : .
B Aij S e S SAB eB
SH E S K KR r
K Exchange Integral or sonance IntegralLowering of energy that leads to bond formation
ψ ψ= + − =
⇒
K is purely quantum-mechanical concept
1
S
Interaction energy viewed as a -ve charge cloud on atoms with +ve charged nucleus
E1s(1e)
E1s(1e)*S
Bonding/antibonding energy depends on RAB
H2+
Represents the energies ofbonding and anti-bonding levels at equilibrium RAB
Electron Densities and Energy: f(RAB)
H2+ : MO, Electron Density and Energy
σ Bond formation with 1S-orbitals!
φ+
φ − φ−2
φ+2
Electron Density of Sigma Bonds
Note the signs, symmetries and nodes
LCAO-MO of p-orbitals
σ -bonding using p-orbitals
π-bonding involving p-orbitals
( ) ( ) ( )( ) ( ) ( )
:
( , , ) ( , , )
:
:
Inversion Operation I
I x y z x y z
r r r Symmetric
r r r Anti sym
⇒ + + + → − − −
ΙΨ = Ψ − = +Ψ
ΙΨ = Ψ − = −Ψ −
Symmetry and Nomenclature of MOsGerade (g) (symmetric) and Ungerade (u) (antisymmetric) wrt inversion operation!
Hydrogen molecule ion:
“b” Bonding using s: symmetricσg
“a” antibonding using s: antisymmetric σu*
Types of Covalent Bonds
•What are the symmetries?•Where are the nodes?•What are the relative energies?•How S changes with RAB? •What are the Ψ(MO)?
δ bonds
π bonds
σ bonds
Dihydrogen Molecule (H2)
R
r1Ar1B
r2A r2Br12
Place 2e with opp. spin in bonding orbital of H2+ [ ]1 1 1 1
MOT overemphasizes ionic terms in a covalent bond! VBT better?
Dihydrogen Molecule: One more electron goes to bonding orbital
Bond strength increases: Bond order=1
Effective nuclear charge changes the absolute Energy levels and the orbitals!
Matching of energies of AO important for LCAO-MOIf energies are not close to each other, they wouldNot interact to form MOs.
*
Energies of H2+, H2, He2
+, He2
He2+
He2
Matching of AO energies for MO
Due to large difference in energy of 1s(H) and 1s(F), LCAO-MO for both 1S is not feasible in HF. Rather, 2Pz(F) and 1S(H) form a sigma bond.
Both symmetry and energyMatching is required for MO.
Valence electrons are most important
Electron Density Maps/Contours
H2 Li2: core 1σ
Li2: core 1σ∗ Li2: Valence 2σ
Li2: Total
MO Contours show electron density maps
O2 molecule
Total
2σ and 2σ∗
3σ and 1π
1π∗
Total Electron Density Mapsof 1st row Homonuclear Diatomics
Expected MO and Energies for DinitrogenAre these MO and correct energy level diagram for N2?
There is a problem! Spectroscopy says NO!
Actual MO and Energy Diagram for N2
Mixing of 2S and 2P orbital occur because of small energy gap between them.2s and 2p electrons feels not so different effective nuclear charge.
Note how the MO of 2sσ have π-type looks, while π-levels are clean
Nature 2004 vol 432 867
S-P Mixing: Hybridization of MO
Mixing of 2S and 2P orbital occur because of small energy gap between them.2s and 2p electrons feels not so different effective nuclear charge.
S-P Mixing: B2 confirms it!
Boron is paramagnetic. This can only happen if the two electrons with parallel spin are in the π-orbitals π-bonding energies lower than σ*?
Incorrect!
MO Energy Diagram for F2
Less mixing of S and P orbital because of higher energyGap between 2S and 2P levels in Oxygen and Fluorine!2s and 2p electrons feels very different nuclear charge
MO Energy Diagram (Homo- Diatomics)
Bond Order = ½ (# of bonding electrons - # of antibonding electrons)Bond order = 0 molecular can not exist – No bond formationBond order higher stronger bond, shorter bond length
Bond order, strength, length......magnetic properties