Molecular Spectroscopy Visible and Ultraviolet Spectroscopy - UV/VIS Spectroscopy - UV/VIS...
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Transcript of Molecular Spectroscopy Visible and Ultraviolet Spectroscopy - UV/VIS Spectroscopy - UV/VIS...
Molecular SpectroscopyVisible and Ultraviolet Spectroscopy
- UV/VIS Spectroscopy- UV/VIS Spectrometer- Application for Quantitative Analysis
Ultraviolet: 190~400nm Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm
Internal Energy of Molecules
Etotal=Etrans+Eelec+Evib+Erot+Enucl
Eelec: electronic transitions (UV, X-ray)
Evib: vibrational transitions (Infrared)
Erot: rotational transitions (Microwave)
Enucl: nucleus spin (nuclear magnetic
resonance) or (MRI: magnetic resonance
imaging)
Electronic Spectroscopy
Ultraviolet (UV) and visible (VIS) spectroscopy This is the earliest method of molecular
spectroscopy. A phenomenon of interaction of molecules with
ultraviolet and visible lights. Absorption of photon results in electronic
transition of a molecule, and electrons are promoted from ground state to higher electronic states.
UV and Visible Spectroscopy
In structure determination : UV-VIS spectroscopy is used to detect the presence of chromophores like dienes, aromatics, polyenes, and conjugated ketones, etc.
Electronic transitions
There are three types of electronic transition
which can be considered; Transitions involving , , and n
electrons Transitions involving charge-transfer
electrons Transitions involving d and f electrons
Absorbing species containing , , and n electrons
Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy.
UV/VIS
Vacuum UV or Far UV (λ<190 nm )
Transitions
An electron in a bonding s orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to transitions are not seen in typical UV-VIS spectra (200 - 700 nm)
n Transitions
Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n transitions. These transitions usually need less energy than transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n peaks in the UV region is small.
n and Transitions
Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the excited state.
These transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the electrons.
Chromophore Excitation max, nm Solvent
C=C →* 171 hexane
C=On→*→*
290180
hexanehexane
N=On→*→*
275200
ethanolethanol
C-X X=Br, I
n→*n→*
205255
hexanehexane
Terms describing UV absorptions
1. Chromophores: functional groups that give
electronic transitions.
2. Bathochromic shift: shift to longer λ, also called red shift.
3. Hysochromic shift: shift to shorter λ, also called blue shift.
4. Hyperchromism: increase in ε of a band.
5. Hypochromism: decrease in ε of a band.
Selection RuleOrbital Spin States & Multiplicity
Singlet state (S):Most molecules have ground state with all electron spin paired and most excited state also have electron spin all paired, even though they may be one electron each lying in two different orbital. Such states have zero total spin and spin multiplicities of 1, are called singlet (S) states.
Total Spin Multiplicities
Multiplicity Triplet State
For some of the excited states, there are states with a pair of electrons having their spins parallel (in two orbitals), leading to total spin of 1 and multiplicities of 3.
Total Spin Multiplicities
Selection Rule
For triplet state: Under the influence of external field, there are three values (i.e. 3 energy states) of +1, 0, -1 times the angular momentum. Such states are called triplet states (T).
According to the selection rule, S→S, T→T, are allowed transitions, but S→T, T→S, are forbidden transitions.
Selection Rules of electronic transition
Electronic transitions may be classed as intense or weak according to the magnitude of εmax th
at corresponds to allowed or forbidden transition as governed by the following selection rules of electronic transition:
Spin selection rule: there should be no change in spin orientation or no spin inversion during these transitions. Thus, S→S, T→T, are allowed, but S→T, T→S, are forbidden. ( S=0 △ transition allowed)
→
Instrumentation
光源 分光器 樣品 偵測器 記錄器
Components of a SpectrophotometerLight Source
Deuterium Lamps- a truly continuous spectrum in the ultraviolet region is produced by electrical excitation of deuterium at low pressure. (160nm~375nm)
Tungsten Filament Lamps- the most common source of visible and near infrared radiation.
Components of a Spectrophotometer
Monochromator (分光器 /單光器 )
Used as a filter: the monochromator will select a narrow portion of the spectrum (the bandpass) of a given source
Used in analysis: the monochromator will sequentially select for the detector to record the different components (spectrum) of any source or sample emitting light.
MonochromatorCzerny-Turner design
Grating
Photomultiplier Detector
Principle of Photomultiplier Detector
The type is commonly used. The detector consists of a photoemissive catho
de coupled with a series of electron-multiplying dynode stages, and usually called a photomultiplier.
The primary electrons ejected from the photo-cathode are accelerated by an electric field so as to strike a small area on the first dynode.
Principle of Photomultiplier Detector
The impinging electrons strike with enough energy to eject two to five secondary electrons, which are accelerated to the second dynode to eject still more electrons.
A photomultiplier may have 9 to 16 stages, and overall gain of 106~109 electrons per incident photon.
Single and Double Beam Spectrometer
Single-Beam: There is only one light beam or optical path from the source through to the detector.
Double-Beam: The light from the source, after passing through the monochromator, is split into two separate beams-one for the sample and the other for the reference.
Quantitative AnalysisBeer’s Law
A=bc
: the molar absorptivity (L mol-1 cm-1)
b: the path length of the sample
c :the concentration of the compound in solution, expressed in mol L-1
Transmittance
I0 I
b
303.2
log)log(
)log(303.2)ln(
0
00
00
00
0
k
bcATI
I
I
Ikbc
I
I
dbkcI
dI
kcdbI
dI
I
IT
I
I
b
External Standard and the Calibration Curve
Fluorescence Spectroscopy
Excitation of Molecules
Excitation of molecules can be brought about by absorption of two bands of radiation, one centered about the wavelength 1(S0→S1) and the second around the shorter wavelength 2(S0→S2).
Vibrational Relaxation (VR)
The molecule can rapidly dissipate excess vibrational energy as heat by collision with solvent molecules.
Average lifetime of vibrational relaxation excited molecule is 10-12sec or less.
Fluorescence always involves a transition from the lowest vibrational level of an excited state to any one of the vibrational level of the ground state.
Internal Conversion (IC)
A molecule passes to a lower energy electronic state.
The molecule can pass from a low vibrational level of S2 to an equally energetic vibrational level of the first excited singlet S1.
The mechanism of internal conversion process S2→ S1 is not well understood.
Average lifetime of internal conversion process is 10-12sec or less.
Fluorescence
This process of emitting a photon for deexcitation of S1 to S0.
Average lifetime of fluorescence is 10-9~10-7sec.
Intersystem Crossing (ISC)
This process of non-radiative transfer from the singlet to the triplet state.
Intersystem crossing are most common in molecules that contain heavy atoms, such as iodine or bromine.
Interaction between the spin and orbital motions becomes large in the presence of such atoms.
Phosphorescence
From T1, the molecule can return to S0 by emission of photon.
A triplet-singlet transition is much less probability than a singlet-singlet conversion.
The excited triplet state with respect to emission ranges from 10-4 to 10sec.
Rate of Deactivation Processes
Absorption- 10-14~10-15sec Vibrational Relaxation and Internal Conversion - 10-12sec or less
Fluorescence- 10-9~10-7sec Phosphorescence- 10-3~10sec
Transition Types in Fluorescence
Fluorescence process seldom results from absorption of UV radiation of wavelengths lower than 250nm because such radiation is sufficiently energetic to cause deactivation of the excited states by pre-dissociation or dissociation
200nm radiation corresponds to about 140Kcal/mol; most molecules have at least some bonds that cane be ruptured.
Quantum Efficiency Comparisonfor →* and n→* transition The majority radiation of fluorescent compoun
ds is produced by a transition involving either the n→* or the →* excited state.
The molar absorptivity of a →* transition is ordinarily 100 to 1000 times greater than for an n→* process.
The inherent lifetime associated with a →* transition is shorter (10-7 to 10-9 sec compared with 10-5 to 10-7 sec for an n→* process) and kf is larger. The rate constant for intersystem crossing ki is smaller for →* excited states.
Fluorescence is more commonly associated with →* states than with n→* states because the deactivation processes that compete with fluorescence are less likely to occur.
Fluorescence and Structure
The most intense and most useful fluorescent behavior is found in compounds containing aromatic functional groups with low energy →* transition levels.
Compounds containing aliphatic and alicyclic carbonyl structures or highly conjugated double bond structure may also exhibit fluorescence.
H3C CH3
CH3
CH2OH
CH3 CH3 CH3
Organic Compounds for Fluorescence
Vitamin A has a blue fluorescence with an emission maximum at approximately 500nm in ethanol.
highly conjugated compound
Fluorescence Instrumentation
Source- xenon-arc lamp for higher wavelengths and mercury-arc lamp for shorter wavelengths
Lensing system- quartz lenses can be used in the ultraviolet wavelength range (200-380nm), and glass lenses can be used in the visible wavelength range (380nm-700nm)
Fluorescence Instrumentation
Emission wavelength selector: this system is generally placed at an angle of 90o with respect to the excitation axis to minimize interferences from transmitted and scattered exciting light.
Beam Splitter: A beam splitter is a mirror of partial reflectance. A proportion of light would be reflected from the beam splitter while the rest would be transmitted through it unaffected.