SPECTROSCOPY

Light interacting with matter as an analytical tool

X-ray: core electron excitation

UV: valance electronic excitation

IR: molecular vibrations

Radio waves:Nuclear spin states(in a magnetic field)

Electronic Excitation by UV/Vis Spectroscopy :

 

Spectroscopic Techniques and Chemistry they Probe  

UV-vis UV-vis region bonding electrons  

Atomic Absorption UV-vis region atomic transitions (val. e-)

FT-IR IR/Microwave vibrations, rotations

Raman IR/UV vibrations  

FT-NMR Radio waves nuclear spin states

X-Ray Spectroscopy X-rays inner electrons, elemental

X-ray Crystallography X-rays 3-D structure  

  Spectroscopic Techniques and Common Uses  

UV-vis UV-vis regionQuantitative

analysis/Beer’s Law  

Atomic Absorption UV-vis regionQuantitative analysis

Beer’s Law

FT-IR IR/Microwave Functional Group Analysis

Raman IR/UVFunctional Group

Analysis/quant  

FT-NMR Radio waves Structure determination

X-Ray Spectroscopy X-rays Elemental Analysis

X-ray Crystallography X-rays 3-D structure Anaylysis  

Different Spectroscopies

• UV-vis – electronic states of valence e/d-orbital transitions for solvated transition metals

• Fluorescence – emission of UV/vis by certain molecules

• FT-IR – vibrational transitions of molecules• FT-NMR – nuclear spin transitions• X-Ray Spectroscopy – electronic transitions

of core electrons

Quantitative Spectroscopy

• Beer’s Law

Al1 = el1bc

e is molar absorptivity (unique for a given compound at l1)

b is path length

c concentration

Beer’s Law

• A = -logT = log(P0/P) = ebc

• T = Psolution/Psolvent = P/P0

• Works for monochromatic light• Compound x has a unique e at different

wavelengths

cuvette

sourceslit

detector

Characteristics of Beer’s Law Plots

• One wavelength

• Good plots have a range of absorbances from 0.010 to 1.000

• Absorbances over 1.000 are not that valid and should be avoided

• 2 orders of magnitude

Standard Practice

• Prepare standards of known concentration

• Measure absorbance at max• Plot A vs. concentration• Obtain slope • Use slope (and intercept) to determine

the concentration of the analyte in the unknown

Typical Beer’s Law Plot

y = 0.02x

0

0.20.4

0.6

0.81

1.2

0.0 20.0 40.0 60.0

concentration (uM)

A

UV-Vis Spectroscopy

• UV- organic molecules– Outer electron bonding transitions– conjugation

• Visible – metal/ligands in solution– d-orbital transitions

• Instrumentation

Characteristics of UV-Vis spectra of Organic Molecules

• Absorb mostly in UV unless highly conjugated

• Spectra are broad, usually to broad for qualitative identification purposes

• Excellent for quantitative Beer’s Law-type analyses

• The most common detector for an HPLC

Molecules have quantized energy levels:

ex. electronic energy levels. en

ergy

hv

ener

gy

}= hv

Q: Where do these quantized energy levels come from?A: The electronic configurations associated with bonding.

Each electronic energy level (configuration) has associated with it the many vibrational energy levels we examined with IR.

Broad spectra

• Overlapping vibrational and rotational peaks

• Solvent effects

Molecular Orbital Theory

• Fig 18-10

2s 2s

2p 2pn

C C

hv

C C

H

HH H

HH

max = 135 nm (a high energy transition)

Absorptions having max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.

Ethane

C C

hv

Example: ethylene absorbs at longer wavelengths:max = 165 nm = 10,000

= hv =hc/

hv

n

n

C O

n

The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden.”Example:

Acetone: nmax = 188 nm ; = 1860nmax = 279 nm ; = 15

C C

C C

C O

C OH

135 nm

165 nm

n183 nm weak

150 nmn188 nmn279 nm weak

A

180 nm

279 nm

C O

C C

HOMO

LUMO

Conjugated systems:

Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).

Note: Additional conjugation (double bonds) lowers the HOMO-LUMO energy gap:Example:

1,3 butadiene: max = 217 nm ; = 21,0001,3,5-hexatriene max = 258 nm ; = 35,000

O

O

O

Similar structures have similar UV spectra:

max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm

Lycopene:

max = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm

max(Actual) = 474.

Metal ion transitions

Degenerate D-orbitalsof naked Co

D-orbitalsof hydrated Co2+

Octahedral Configuration

E

Co2+

H2O

H2O

H2OH2O

H2O

H2O

Octahedral Geometry

Instrumentation

• Fixed wavelength instruments

• Scanning instruments

• Diode Array Instruments

Fixed Wavelength Instrument

• LED serve as source• Pseudo-monochromatic light source• No monochrometer necessary/ wavelength selection

occurs by turning on the appropriate LED• 4 LEDs to choose from

photodyode

sample

beam of light

LEDs

Scanning Instrument

cuvette

Tungsten Filament (vis)

slit

Photomultiplier tube

monochromator

Deuterium lampFilament (UV)

slit

Scanning Instrument

sources

• Tungten lamp (350-2500 nm)

• Deuterium (200-400 nm)

• Xenon Arc lamps (200-1000 nm)

Monochromator

• Braggs law, nl = d(sin i + sin r)

• Angular dispersion, dr/d = n / d(cos r)

• Resolution, R = /nN, resolution is extended by concave mirrors to refocus the divergent beam at the exit slit

Sample holder

• Visible; can be plastic or glass

• UV; you must use quartz

Single beam vs. double beam

• Source flicker

Diode array Instrument

cuvette

Tungsten Filament (vis)

slit

Diode array detector328 individual detectors

monochromator

Deuterium lampFilament (UV)

slit

mirror

Advantages/disadvantages• Scanning instrument

– High spectral resolution (63000), /– Long data acquisition time (several minutes)– Low throughput

• Diode array– Fast acquisition time (a couple of seconds),

compatible with on-line separations– High throughput (no slits)– Low resolution (2 nm)

HPLC-UV

Mobile phase

HPLC Pump

syringe

6-port valveSample

loop

HPLC column

UV detector

Solvent waste