Molecular Spectroscopy

Types of transitions:

1) Electronic (UV-Vis-Near IR)1) Electronic (UV-Vis-Near IR)

2) Vibrational (IR)2) Vibrational (IR)

3) Rotational (microwave)3) Rotational (microwave)

Born – Oppenheimer Approximation

The wavefunction of a molecule is assumed to be the The wavefunction of a molecule is assumed to be the product of the electronic, vibrational and rotational wave product of the electronic, vibrational and rotational wave functions.functions.

The energy levels for each type of transition can be treated The energy levels for each type of transition can be treated independently.independently.

The total energy of the molecule is the sum of the The total energy of the molecule is the sum of the electronic, vibrational and rotational energy.electronic, vibrational and rotational energy.

Electronic Absorption Spectra

Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.Prentice Hall, Englewood Cliffs, NJ, 1991.

Atomic Orbitals (AO) Combine to give Molecular Orbitals (MO)

Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.Prentice Hall, Englewood Cliffs, NJ, 1991.

Bond Order = Bond Order = (# bonding e(# bonding e-- - # antibonding e - # antibonding e--) / 2) / 2

Total Spin Quantum # (S)Total Spin Quantum # (S)

S = S = ssii

Multiplicity = 2S + 1Multiplicity = 2S + 1

Are you getting the concept?

Calculate the bond order for COCalculate the bond order for COand predict its stability.and predict its stability.

Assuming that the same set ofAssuming that the same set ofmolecular orbitals applies, suggestmolecular orbitals applies, suggesta diatomic that would be unstable.a diatomic that would be unstable.

Selection RulesS = 0S = 0

i.e. ei.e. e-- cannot change spin during transition. cannot change spin during transition.

This rule can be broken with heavier nuclei.This rule can be broken with heavier nuclei.

There are other selection rules for diatomics but There are other selection rules for diatomics but S = 0 is the S = 0 is the only one valid for more complicated molecules.only one valid for more complicated molecules.

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Polyatomic Electronic Transitions

s orbitals – p orbitals –

non-bonding orbitals - n

Vibronic Transitions

Selection Rules:Selection Rules:

S = 0S = 0

vv = 0, = 0, 1, 1, 2, 2, 3, …3, …

J = J = 11

Frank – Condon PrincipleThe electronic transition is fast (10The electronic transition is fast (10-15-15 s) with respect to s) with respect to nuclear motions.nuclear motions.

Transitions where the position and momentum of the Transitions where the position and momentum of the nuclei don’t change are favored.nuclei don’t change are favored.

J. Michael Hollas, J. Michael Hollas, Modern SpectroscopyModern Spectroscopy, John Wiley & Sons, New York, 1992., John Wiley & Sons, New York, 1992.

Vibronic transitions where the wavefunctions line up are favored.

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

What do the intensity patterns tell us about the potential

diagrams?

J. Michael Hollas, J. Michael Hollas, Modern SpectroscopyModern Spectroscopy, John Wiley & Sons, New York, 1992., John Wiley & Sons, New York, 1992.

Progressions

J. Michael Hollas, J. Michael Hollas, Modern SpectroscopyModern Spectroscopy, John Wiley & Sons, New York, 1992., John Wiley & Sons, New York, 1992.

Series of bands with the Series of bands with the same same vv” (absorption) or ” (absorption) or vv’ ’ (emission).(emission).

Bands are separated by v’ Bands are separated by v’

Progressions

J. Michael Hollas, J. Michael Hollas, Modern SpectroscopyModern Spectroscopy, John Wiley & Sons, New York, 1992., John Wiley & Sons, New York, 1992.

Sequences

vv is constant. is constant.

Series of bands separated bySeries of bands separated by(v’-v”).(v’-v”).

V V ’’

V V ””More commonly observed in More commonly observed in emission experiments.emission experiments.

J. Michael Hollas, J. Michael Hollas, Modern SpectroscopyModern Spectroscopy, John , John Wiley & Sons, New York, 1992.Wiley & Sons, New York, 1992.

Shape of the electronic spectrum is determined by vibrational and rotational

structure.

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

UV-vis Absorption (Extinction) UV-vis Absorption (Extinction) SpectroscopySpectroscopy

Single-Beam or Double-Beam

Fixed or Dispersive

Common:Source – Tungsten Halogen Lamp (360-2000 nm)Sample – Liquid In CuvetteDispersion – Spectrograph w/ Diffraction GratingDetector – CCD

Beer’s Law: A = bc

Beer’s Law

Skoog, Hollar, Nieman, Skoog, Hollar, Nieman, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, Saunders , Saunders College Publishing, Philadelphia, 1998.College Publishing, Philadelphia, 1998.

Assumptions

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Apparent Deviations from Beer’s Law

Non-Zero InterceptNon-Zero Intercept

Improper blank Improper blank measurement or measurement or correction.correction.

Instrumental drift.Instrumental drift.

Skoog, Hollar, Nieman, Skoog, Hollar, Nieman, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, Saunders , Saunders College Publishing, Philadelphia, 1998.College Publishing, Philadelphia, 1998.

Absorbed/Emitted ColorsAbsorbed/Emitted Colors

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

ChromophoresOften transitions are localized in specific bonds or Often transitions are localized in specific bonds or functional groups within a molecule.functional groups within a molecule.

Group will have a characteristic Group will have a characteristic maxmax and and ..

Molecular structure or environment can affect Molecular structure or environment can affect maxmax and and ..

Shift to longer Shift to longer bathochromic (red) shift. bathochromic (red) shift.Shift to shorter Shift to shorter hypsochromic (blue) shift. hypsochromic (blue) shift.

Increase in Increase in hyperchromic effect. hyperchromic effect.Decrease in Decrease in hypochromic effect. hypochromic effect.

What effect does conjugation usually have?What effect does conjugation usually have?

hyperchromic effect / bathochromic shift

Characteristic Electronic TransitionsCharacteristic Electronic Transitions

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

L mol-1 cm-1

Characteristic Electronic TransitionsCharacteristic Electronic Transitions

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

L mol-1 cm-1

AuxophoreDoes not absorbDoes not absorb

Induces a bathochromic shift and hyperchromic effect when Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NHconjugated to a chromophore (e.g. -OH, -Br, -NH22).).

Solvent EffectsSolvent Effects

Hypsochromic shiftHypsochromic shift in n in n * transitions as solvent polarity * transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair.increases. Solvation stabilizes the nonbonding pair.

Bathochromic shiftBathochromic shift in in * transitions as solvent polarity * transitions as solvent polarity increases. Solvation stabilizes increases. Solvation stabilizes *, which is often more polar *, which is often more polar than than ..