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: 1 O3 + hv → O2 + O( D) 1 O( D) + 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) NO2 + hv → O + NO (source of O3 in the troposphere) 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: -1 D0(O2) = 495 kJ mol -1 D0(Cl2) = 243 kJ mol 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 • Wavelength, From F-P&P 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 c • Units used for photon energies and wavelengths: λ = – 1 eV = 8065.54 cm-1 = 96.4853 kJ/mol = 23.0605 kcal/mol = ν 11604.4 K E = hν -10 -6 – 1 Å = 0.1 nm = 10 m; micron = 10 m = 1000 nm 1 v = • Solve in class: Calculate the energy, frequency, and λ wavenumber of a green photon (λ = 530 nm). (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 v', J', … • Molecule can absorb radiation efficiently if: photon – The photon energy matches the energy E spacing between molecule’s quantum levels v", J", … – Optical transition between these quantum levels is allowed by “selection rules” – “Forbidden” transitions can occur but are weaker 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 E = hν v + 1 − hνx v + 1 + hνy v + 1 + ... vib ( 2) e ( 2) e ()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 -1 • Rigid rotor Erot = BJ (J +1) cm – No simultaneous vibration 2 h m1m2 2 B = 2 where I = R – Allowed energy levels: 8π I m1 + m2 • Nonrigid rotor 2 2 Erot = BJ (J +1) − DJ (J +1) + ... • 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 Possible different vibrational quanta rovibrational overlap with each other transition: v=0 → v=1 J=14 → J=15 HF molecular constants -1 9 Bv=0 = 20.557 cm (rotational constant) 9 ν = 4138.32 cm-1 (harmonic frequency) 9 νxe = 89.88 cm-1 (anharmonicity) 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 From F-P&P •O2 has triplet ground state, important exception – 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 • Some repulsive and some From F-P&P 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* Polyatomic Molecules From S. Nidkorodov 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: N = 3 ⇒ s = 3 C6H6: N = 12 ⇒ s = 30 C60: N = 60 ⇒ s = 174 2. Three independent axes of rotation, each characterized by its own rotational constant (A, B, C): Asymmetric tops A ≠ B ≠ C Prolate symmetric tops A < B = C Oblate symmetric tops A = B < C H2O molecule, meat grinder CH3F molecule; a pencil CH3 radical, planet Earth b b c a c a a c 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: -1 v1 = 1595 cm -1 v2 = 3652 cm -1 v3 = 3756 cm • 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: 2 NO2(X A1) + 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: Number of excited molecules proceeding by process i φ = i Total number of photons absorbed * • E.g.: NO3 + hv → NO3 (3) * NO3 → NO2 + O (4a) → NO + O2 (4b) → NO3 + hv (4c) Number of NO molecules formed φ = 2 4a 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.