Download Lecture 5: Spectroscopy and Photochemistry I

Lecture 5: Spectroscopy and
Photochemistry I
Required Reading: FP Chapter 3
Suggested Reading: SP Chapter 3
Atmospheric Chemistry
CHEM-5151 / ATOC-5151
Spring 2005
Maggie Tolbert & Jose-Luis Jimenez
Outline of Next Two Lectures
• Today
– Importance of spectroscopy & photochemistry
– Nature of light, EM spectrum
– Molecular spectroscopy
• Thursday
– The Sun as a radiation source
– Light absorption
– Atmospheric photochemistry
1
Importance of Spectroscopy and Photochemistry I
• Most chemical processes in the atmosphere are initiated by
photons
– Photolysis of O3 generates OH – the most important atmospheric oxidizer:
O3 + hv → O2 + O(1D)
O(1D) + H2O → 2 OH
– Solar photodissociation of many atmospheric molecules is often much faster
than any other chemical reactions involving them:
CF2Cl2 + hv → CF2Cl + Cl
(photolysis of CFCs in the stratosphere)
HONO + hv → OH + NO
(source of OH in the troposphere)
(source of O3 in the troposphere)
NO2 + hv → O + NO
NO3 + hv → O2 + NO or O + NO2 (removal of NO3 generated at night)
Cl2 + hv → Cl + Cl
(source of Cl atoms)
H2CO + hv → H2 + CO or H + HCO (important step of hydrocarbon
oxidation)
etc.
Importance of Spectroscopy and Photochemistry II
• Absorption of solar and earth radiation by
atmospheric molecules directly influences the
energy balance of the planet
– Greenhouse effect (CO2, H2O, N2O, CFCs)
– Stratospheric temperature inversion (O3 photochemistry)
• Spectroscopy of atmospheric molecules is used to
detect them in situ
– OH is detected via its electronic transition at 310 nm
– NH3 is detected via its fundamental vibrational transition at
1065 cm-1, etc.
2
Solar Radiation: Initiator of Atmos. Reactions
Average thermal energy of collisions:
~ RT = 8.3 J mol-1 K-1 x T
RT = 2.5 kJ mol-1 @ 300 K
Energy of photons (E = hv):
300 nm photon = 380 kJ mol-1
600 nm photon = 190 kJ mol-1
Typical bond strengths:
D0(O2) = 495 kJ mol-1
D0(Cl2) = 243 kJ mol-1
C-H, O-H, C-O ~ 400 kJ mol-1
Atmospheric chemistry on Earth is driven by
photolysis, not by thermal excitation!!!
From S. Nidkorodov
What is light?
• Dual nature
– Photon: as particle
• Energy but no
mass
– As wave: electric
and magnetic
fields oscillating in
space and time
From F-P&P
• Wavelength,
frequency
• c ~ 3 x 109 m/s
Discuss in class: at a fundamental physical level,
why are molecules capable of absorbing light?
3
The Electromagnetic Spectrum
• Units used for photon energies and wavelengths:
– 1 eV = 8065.54 cm-1 = 96.4853 kJ/mol = 23.0605 kcal/mol =
11604.4 K
– 1 Å = 0.1 nm = 10-10 m; micron = 10-6 m = 1000 nm
• Solve in class: Calculate the energy, frequency, and
wavenumber of a green photon (λ = 530 nm).
c
λ=
ν
E = hν
v=
1
λ
(wavenumber)
Types of radiation important in lower atmosphere
• Ultraviolet and visible radiation (λ = 100-800 nm)
– Excites bonding electrons in molecules
– Capable of breaking bonds in molecules (⇒
photodissociation)
– Ultraviolet photons (λ = 100-300 nm) have most energy, can
break more and stronger bonds. We will pay special attention
to them.
• Infrared radiation (λ = 0.8 - 300 µm)
– Excites vibrational motions in molecules
– With a very few exceptions, infrared radiation is not energetic
enough to break molecules or initiate photochemical
processes
• Microwave radiation (λ = 0.5 - 300 mm)
– Excites rotational motions in molecules
4
Fundamentals of Spectroscopy
• Molecules have energy in translation,
vibration, rotation, and electronic state
– Translation (= T) cannot be changed directly
with light
– We will focus on the other 3 energy types
– The photon energy matches the energy
spacing between molecule’s quantum levels
– Optical transition between these quantum
levels is allowed by “selection rules”
– “Forbidden” transitions can occur but are
weaker
Ephoton
• Molecule can absorb radiation efficiently
if:
v', J', …
v", J", …
Vibrational Energy & Transitions
• Bonds can be
viewed as
“springs”
• Energy levels are
quantized,
– Ev = hvvib(v+1/2)
– vvib is constant
dependent on
molecule
– v = 0, 1, 2… is
vibrational
quantum number
From F-P&P
5
Vibrational Energy Levels
• Ideally: Harmonic Oscillator
– Restoration force of “spring”
follows Hooke’s law: F= k ∆x
– Ev = hvvib(v+1/2), v = 0, 1, 2…
– Energy levels are equally spaced
• Really: Anharmonic oscillator
– Restauration force rises sharply
at small r, bond breaks at large r
(
)
(
)
2
(
)
3
Evib = hν v + 1 − hνxe v + 1 + hνye v + 1 + ...
2
2
2
– Vibrational quantum levels are
more closely spaced as v
increases
From F-P&P
Vibrational Selection Rules
• For ideal harmonic oscillator
– ∆v = ±1
• For anharmonic oscillator
– ∆v = ±2, ±3 weaker “overtone” transitions can occur
• At room T most molecules at v = 0
– Energy spacing of levels is large (~1000 cm-1)
– v'‘ = 0 → v‘ = 1 is by far strongest
• For purely vibrational transition
– Absorption of light can occur if dipole moment
changes during vibration. E.g. HCl, CO, NO
– Homonuclear diatomics, e.g. O2, N2 don’t have v.t.
6
Infrared Active and Inactive Modes
• Only vibrational modes
that change the dipole
moment can interact
with light and lead to
absorption
• CO2 is infrared active,
but not all of its modes
are
Rotational Energy and Transitions
• If molecule has permanent dipole
– Rotation in space produces oscillating electric field
– Can interact with light’s fields and result in absorption
– Only heteronuclear molecules
• Rigid rotor
– No simultaneous vibration
– Allowed energy levels:
Erot = BJ ( J + 1) cm -1
B=
h2
8π I
2
where I =
m1m2
R2
m1 + m2
• Nonrigid rotor
Erot = BJ ( J + 1) − DJ 2 ( J + 1) 2 + ...
• Spacing increases with J
• Spacing between levels small, many levels are populated
7
Example: Ground Electronic State of HF
Etotal = Erot + Evib
Erot ≈ BJ ( J + 1)
Evib ≈ hνv
Rotational level manifolds for
different vibrational quanta
overlap with each other
HF molecular constants
9 Bv=0 = 20.557 cm-1 (rotational constant)
9 ν = 4138.32 cm-1 (harmonic frequency)
9 νxe = 89.88 cm-1 (anharmonicity)
Possible
rovibrational
transition:
v=0 → v=1
J=14 → J=15
From S. Nidkorodov
Vibration-rotation of HCl
• Molecules vibrate and rotate
simultaneously
From F-P&P
8
Electronic Energy and Transitions
• Several additional quantum numbers
– Λ: related to electronic angular momentum
– S: spin number
•
•
•
•
Multiplicity = (2S + 1)
Mult = 1, 2, 3 are referred to as singlet, doublet, triplet
Most stable molecules have singlet ground states
O2 has triplet ground state, important exception
From F-P&P
– Ω = | Λ+ Σ|
• Σ = +S, S-1, …. , -S
– “g” or “u” states
– “+” or “-” states of Σ
• More complex selection
rules involving these numbers:
Electronic Transitions (ETs)
• Molecules can undergo an
ET upon absorption of an
appropriate photon
– Simultaneous vibrational
and rotational transitions
– No restriction on ∆v, many
vib. trans. can occur
– ∆J = -1, 0, +1
• P, Q, and R branches
• Frank-Condon principle
– Time for ET so short (10-15
s) that internuclear distance
cannot change
– “vertical” transitions
From F-P&P
9
Potential Energy Curves for an ET
• At room T, v''=0
• Prob of transition
proportional to
product of vib.
wavefucntions
– Transition to v'=4
in upper
electronic state
most intense
From F-P&P
Repulsive States
• No minima in PE
vs r curves
• Dissociation
occurs
immediately after
absorption of light
From F-P&P
10
More complex case & Predissociation
From F-P&P
• Some repulsive and some
non-repulsive upper elec.
states
• Example
– Trans. to R causes
immediate dissociation
– Trans. to E can lead to
dissociation if cross over to
state R occurs
• “Predissociation”
– If high enough energy,
trans. to E can yield
A + B*
From S. Nidkorodov
Polyatomic Molecules
1. Number of vibrations increases to s = 3N-6 (s = 3N-5 for linear molecules),
where N is the number of atoms in the molecule:
H2O:
C6H6:
C60:
N=3
N = 12
N = 60
⇒s=3
⇒ s = 30
⇒ s = 174
2. Three independent axes of rotation, each characterized by its own rotational
constant (A, B, C):
Asymmetric tops A ≠ B ≠ C
H2O molecule, meat grinder
Prolate symmetric tops A < B = C
CH3F molecule; a pencil
b
a
c
Oblate symmetric tops A = B < C
CH3 radical, planet Earth
c
b
c
a
a
b
3. Complexity of the absorption spectrum increases very quickly with N. New
types of bands become possible:
9 Sequence bands: one vibration excited while maintaining excitation in another vibration (allowed)
9 Combination bands: two different vibrations excited simultaneously (forbidden in harmonic approximation)
9 Overtone bands are also possible, just like for diatomic molecules (forbidden in harmonic approximation)
11
Example: Vibrational Spectrum of H2O
• Water has s = 3
vibrations:
v1 = 1595 cm-1
v2 = 3652 cm-1
v3 = 3756 cm-1
• It is a strongly
asymmetric top:
A = 27.9 cm-1
B = 14.5 cm-1
C = 9.3 cm-1
• Overtone and
combination
bands are
relatively intense
(only selected
bands shown in
the graph)
From S. Nidkorodov
Sample Near-IR Spectrum of H2O
• v1+v3 combination band
shown – a pure
vibrational transition.
• No obvious pattern in the
spectrum (this is very
typical for asymmetric
tops).
From S. Nidkorodov
12
Pathways for Loss of e- Excitation
From Wayne
• Photophysical
processes
– Lead to emission
of radiation
– Energy converted
to heat
– Read details in
book
• Photochemical
processes
– Dissociation,
ionization,
reaction,
isomerization
Photochemical processes
• Can produce new chemical species
• Photodissociation
– most important by far
– E.g. sole source of O3 in troposphere:
NO2(X2A1) + hv (290 < λ < 430 nm) →
NO(X2P) + O(3P)
• Others: intramolecular rearrangments,
photoisomerization, photodimerization, H-atom
abstraction, and photosensitized reactions
• Reminder: photochemistry drives the chemistry
of the atmosphere
13
Quantum Yields (φ)
• Relative efficiency of various photophysical and
photochemical processes:
φi =
• E.g.:
Number of excited molecules proceeding by process i
Total number of photons absorbed
NO3 + hv → NO3*
(3)
NO3 → NO2 + O (4a)
→ NO + O2 (4b)
→ NO3 + hv
(4c)
*
φ4a =
•
Number of NO 2 molecules formed
Total number of photons absorbed and so on
φi Are wavelength dependent, all important at different λ
Quantum Yields II
From F-P&P
14