CHEM 524 -- Course Outline (Part 13)

CHEM 524 -- Course Outline (Part 13)—2005 revision--minor

IX. Molecular Spectroscopy (Chap. 12 -- read)

A. Transition between states -- characterized by nuclear and electronic motion (source of energy regime)

Degrees of freedom—M-nucleii, n-electronsà(3m+3n), describe by state eqn.

Transition: DE = hn =E_i –E_j where i,j are real states

B. Types of motion - leads to differentiation of spectroscopy types

Translation not quantized—continuous distribution of energies

1. Rotation (motion of molecule as whole) -- sharp, low energy (m-wave)

--quantized angular momentum (conserved) Y_JM where J=0,1,2,3. . . , M=0, ±1, ±2. . ±J

E_J = BJ(J+1) [+ K²(A-B) ] B = (h/8p²c) (1/ I) I = S mr_i²

--bigger heavier molecules, lower B and DE_J

selection rules: DJ = ±1, 0, [DK = ±1, 0 ] [Raman, DJ = ±2, ±1, 0]

Thermally many levels populated: (2J+1)exp(-BJ(J+1)/kT)

-- not analytically useful as pure rotation transition—transitions too weak,

but impact all states—vapor phase see contributions

2. Vibration - internal motion (nucleii move to each other on a potential surface resulting from electron energy variation with nuclear postion) – slides introducing IR/Raman of proteins

-- measure absorption spectra in the infrared (or with Raman scattering,n_s=n₀±n_vib)

--states describe nuclear degrees of freedom: (3n-6) unless linear (3n-5)

a. Characteristic frequencies -- property of atoms/bonds –diatomic: n = (2p)^-1(k/m)¹^/2

k - curvature of potential surface - typically strong bond, bigger k

--k increase, frequency increase (eg. C=C ~1600 cm^-1, and C=C ~2200 cm^-1)

--mass increase, frequency decrease (eg. HCl ~2800 cm^-1, DCl ~2100 cm^-1)

b. Selection rules (harmonic source, violated when anharmonic)

Dn_i = ±1 , Dn_j = 0 i =/ j so DE_i = hn_i

fundamental transitions in 100-4000 cm^-1 range, lightest = highest (H₂)

3. Vapor -- rotation-vibration transitions combine (DJ = 0,±1), can get complex (NH₃)

condense phase --broaden vibrational bands (couple to matrix--libration, phonons)

4. Analytical -- Vibrational spectra useful for qualitative discrimination (examples, nitrobenzene, ethers, Raman-IR complementary, )

Quantitative: S/N and concentration can be limiting factors

Raman issue -- internal standard needed

C. Electronic Transitions

1. To bound state -- include. rot. and vib./ unbound poorly defined

vertical transition most intense (no nuclear change) [Franck-Condon]

2. Intensity depend on types (allowed or forbidden)

organic -- closed shell--in VUV (radical lower Energy)

-- p-system in UV, dominant utility--arenes, heteroaromatics, Azines

-- non-bonded electron pairs, heavy hetero-atoms (lower energy)

Transition metal complexes -- open shell

d-d -- vibronic allowed, weak but visible/characteristic

CT & d-p -- intense/higher energy

f-f & spin change -- very weak

D. Measurement: (Appendix E)

1. Beer-Lambert Law

(esu-cm)²

2. Einstein coefficient: absorption = emission (stimulated) ~ emission (spontaneous)

B_ij = 8p³D/3h²g_IB_ji = g_i/g_j B_ijoscillator strength: f_ij = 2.5x10^-34 B_ij/lm

3. Jablonski diagram -- follow the energy

Vib. Relax—energy from one vibrational level to another or to “heat”, i.e. general K.E. of surroundings (via collision)

IC—move energy to another electronic state without significant loss (DS=0),

ISC—move energy to triplet manifold from singlets (or vice versa) with little loss

Fluorescence –radiative relaxation of excited state (DS=0)

Phosphorescence—radiative relaxation of state with spin change (typical T₁àS₀)

Quantum Yield—ratio of photons out to photons in or rates of processes:

f = k_F/k_F+k_nr

Lifetimes and Quenching-- k_F = 1/t if fluorescence is only process, but if add quencher, lower quantum yield, shorten lifetime, t, because of competition with quenching

Links

Spectroscopy magazine, workbench columns

http://www.spectroscopymag.com/spectroscopy/article/articleList.jsp?categoryId=2942

Spectroscopy now has current happenings in various areas

http://www.spectroscopynow.com/coi/cda/list.cda?catId=2524&type=Link&sort=az&chId=7

Kaiser Optical Raman tutorial

http://www.kosi.com/raman/resources/tutorial/

Akron Organic Molecular spectroscopy unit:

http://ull.chemistry.uakron.edu/analytical/Mol_spec/

UIC’s organic course IR tutorial (Paul Robert Young), UC Boulder lab course and a UK course:

http://chipo.chem.uic.edu/web1/ocol/spec/IR1.htm

http://orgchem.colorado.edu/hndbksupport/irtutor/main.html