Spectroscopy copy is the study of the interaction between radiation and matter as of wavelength λ. It also can refer to interactions with particle ra response to an alternating field or varying frequency ν. f the response as a function of wavelength or more commonly frequenc red to as a spectrum . ctrometry is the measurement of these responses and an instrument wh forms such measurements is a spectrometer or spectrograph pplications :: Physical chemistry nalytical chemistry rganic and inorganic chemistry stronomy……..
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Spectroscopy Spectroscopy is the study of the interaction between radiation and matter as a function of wavelength λ. It also can refer to interactions with particle radiation or to a response to an alternating field or varying frequency ν. A plot of the response as a function of wavelength or more commonly frequencyis referred to as a spectrum.
Spectrometry is the measurement of these responses and an instrument which
performs such measurements is a spectrometer or spectrograph
Applications ::
Physical chemistry Analytical chemistry Organic and inorganic chemistry Astronomy……..
• Ultraviolet (UV)-visible spectroscopy corresponds to excitations of outer shell electron between the energy levels that correspond to the molecular orbital of the systems.
• Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. What functional groups are present in the compound.
• Mass spectrometry (MS) fragments the molecule and measures the masses. What is the size and formula of the compound.
• Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. What is the carbon-hydrogen framework of the compound.
• Fluorescence is the emission of photons from electronically excited states of atoms, molecules, and ions it is used to determine the excited state property of the molecule.
• MÖssbauer spectroscopy (or -ray spectroscopy) is concerned with transitions between energy levels within the nuclei of atom. It can be used to determine the valance state of metal in unknown compound.
• Raman effect may be defined as phenomenon due to which scattering of light has a slightly different frequency from that of incident light and there occurs a change in atomic oscillation within the molecule. It is helps in the elucidation of molecular structure, for locating functional groups or chemical bonds in the molecules.
Modern techniques for structure determination of unknown compounds include:
Types of Spectroscopy
Spectroscopy and the Electromagnetic Spectrum
The electromagnetic spectrum covers a continuous range of wavelengths and frequencies, from radio waves at the low-frequency end to gamma () rays at the high-frequency end
lower frequencylonger wavelength
higher frequencyshorter wavelength
Spectroscopy and the Electromagnetic Spectrum
Electromagnetic energy is transmitted in discrete amounts called quanta
Amount of energy, , corresponding to 1 quantum of energy (1 photon) of frequency () is expressed by the Planck equation
The energy of a photon varies directly with its frequency but inversely with its wavelength
-34
= =
= Planck's constant = 6.62 10 s
hch
h J
E N
N hcE
A
-4
A
is energy of Avogadro's number
of quanta , or, energy of one mole of photons
of wavelength
1.20 10 kJ/mol = =
(m)
It is known as one Einstein of energy
UV-Visible Spectroscopy
• Region of greatest interest to organic chemists from 2 x 10-7 m to ~8 x 10-7 meters
(i) Far (or vacuum) ultraviolet: The region from 10-200 nm can be studied in evacuatedsystems and is termed as “vacuum ultraviolet”. The atmospheric absorption below200 nm is a blessing to all, including the spectroscopists, since it prevents the hazardous (high energy) ultraviolet radiation in the sunlight from striking the earth’ssurface.(ii) Near or quartz ultraviolet : The region from 200-380 nmThe atmosphere is transparent in this region and quartz optics may be used to scan from 200-380 nm.(iii) Visible region: The region from 380-780 nmA tungsten filament lamp is generally used for the visible region of the spectrum.
In a typical experiment, the molecules or atoms start at lower energy and go to a higher energy level upon absorption of radiation of appropriate wavelength
After
EBefore
Absorptio
n
Ene
rgy
After
After
Before
Absorption can only occur when the energy of the radiation (calculated from the frequency or wavelength) matches the energy gap.
Ene
rgy
After
If there are several different upper levels (and there usually are) then several transitions will be observed.
UV/visible Spectroscopy
Chemical compounds are coloured because they absorb visible light.
In general, even organic compounds that are colourless will absorb UV light.
Where has the energy that was within the photons gone to ?
Absorption of visible light
In UV/visible spectroscopy the energy of the absorbed photon is used to drive the molecule into an excited electronic state.
In the excitation the energy of the whole molecule increases.
After
EBefore
Absorptio
n
Ene
rgy
This overall change is typically due to promotion of a single electron from a lower to higher energy orbital. The energy of the transition depends on the gap between the two orbitals.
In organic compounds which have only single bonds between the atoms the excitation energy is very high- lies in deep UV.
This excitation gives a dramatic decrease in bond order due to excitation from
a bonding to an anti-bonding orbital
With increasing conjugation, the decreasing energy gap is reflected by absorption at longer wavelengths.
The structures of many coloured compounds show they are very extensively conjugated.
HOOCCOOH
trans-Crocetin
16,17-DimethoxyViolanthrone
O
OMe OMe
ON
NN
N
O
O H
H
NH2
Xanthopterin
beta-Carotene
Substituents added to the compound may alter the energy of the orbitals which e- is excited from or to.
Auxochromes: substituents that alter the wavelength or intensity of the absorption due to the chromophore
ORANGE
O
O
NH2
PURPLE
O
O
NH2
OH
BLUE
O
O
NHCH3
NHCH3
O
O
OH
O
O
O
HO O-
Changes in chemical composition can give rise to pronounced colour changes since this can dramatically alter the energies of the orbitals involved in the transitions e.g. pH indicators.
-2H+
Phenolphthalein
pinkcolourless
O
O
OH
O
O
O
HO O-
N
N N
SO3-
CH3
CH3
N
N N+
SO3-
CH3
CH3
H
N
N N
SO3-
CH3
CH3
N
N N+
SO3-
CH3
CH3
H
Methyl orange
H+
red
orange-yellow
The basic laws of photochemistry:
1. The Grotthuss-Draper law: Only those radiation which are absorbed by the reacting system are effective in producing chemical change.
But the reverse of this law is not always true i.e. the system on absorbing light mayor may not result into a chemical reaction. In many cases, the absorbed light is converted into the kinetic energy of the absorbing molecules and thereby only heateffects are produced.
2. Laws of photochemical equivalence or Einstein law: Each light absorbing moleculein a photochemical reaction absorbs only one quantum of light which causes the activation.
E = NAhIt is conventionally known as one Einstein of energy.
The Quantitative Picture• Transmittance:
T = I/I0
B(path through sample)
I0
(power in)I
(power out)• Absorbance:
A = -log10 T = log10 I0/I
• The Beer-Lambert Law:
A = bcWhere the absorbance A has no units, since A = log10 I0 /I
is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample in cm
c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)
How Do UV spectrometers work?
Two photomultiplier inputs, differential voltage drives amplifier.
Matched quartz cuvettes
Sample in solution at ca. 10-5 M.
System protects PM tube from stray light
D2 lamp-UV
Tungsten lamp-Vis
Double Beam makes it a difference technique
Rotates, to achieve scan
Types of Electronic Transitions
Energy absorbed from UV radiation promotes an electron from highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)
* *
n * n *
• An electron in a bonding 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)
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.
• Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for 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.
n* and * Transitions(continue)
• Molar absorbtivities from n* transitions are relatively low because the electrons in the n-orbital are situated perpendicular to the plane of the bond and hence the probability of the jump of an electron is very low and range from 10 to100 L mol-1 cm-1 .
* transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1
cm-1 .
The corresponding absorption band in UV-Visible spectrum may be designated as follows:
• R band (group type, Radikalartig) originated from n - * transition. єmax < 100
Example:
Acetone
λ max 279 nm, єmax <100
Hypsochromic shift (blue shift) with an increase in solvent polarity
• K band (conjugation band, Konjugierte) form - * transition. High єmax ( > 104)
Example:
Dienes, Acetophenone
B band (Benzene band, Benzenoid bands) from the - * transition of Benzene. Broad band with fine structure between 230 – 270 nm. This band can be used to identify aromatic compound.
• E band (Ethylenic bands) also from - * transition of ethylenic band in benzene
E1 band and E2 bands of benzene occur near 180 and 200 nm respectively and their molar absorptivity varies between 2000 and 14000.
Important useful terms in UV-Vis spectroscopy• The absorbing groups in a molecule are called chromophores
• A molecule containing a chromophore is called a chromogen
• An auxochrome does not itself absorb radiation, but can enhance the absorption. If an auxochrome is attached to a C=C a bathochromic effect is observed and if to a double bond where n electron are available (C=O) a hypsochromic effect is observed.
• Bathochromic shift – red shift
• Hypsochromic shift – blue shift
• Hyperchromism – an increase in absorption
• Hypochromism – a decrease in absorption
• Isobestic Point: A point common to all curves produced in the spectra of a compound. e.g. phenol-phenolate conversion as a function of pH can demonstrate the presence of the two species in equm by the appearance of an isobestic point in the spectrum.
Solvent effect• The solvent in which the absorbing species is
dissolved also has an effect on the spectrum of the species.
• Peaks resulting from n* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital.
• The reverse (i.e. red shift) is seen for * transitions.
Choice of solvent:
The solvent should be of high purity, generally referred as “spectrograde”
A good solvent should be transparent over the desired range of wavelength. Usuallysolvents which do not contain conjugated system are most suitable for running the Uv-visible spectrum.
The fine structure of an absorption band depend on the polarity of the solvent. A non-polar solvent does not form hydrogen-bond with the solute and the spectrum of the Solute clearly approximate to its spectrum in the gaseous state. In a polar solvent hydrogen bonding forms a solute-solvent complex and the fine structure may disappear.
A solvent should be chosen so that it does not react chemically with the sample.
ON
CF3
O
Coumarin 153 (C-153)
Solvents for UV (showing high energy cutoffs)
Water 210
CH3CN 210
C6H12 210
Ether 210
EtOH 210
Hexane 210
MeOH 215
Dioxane 225
THF 220
CH2Cl2235
CHCl3 245
CCl4 265
benzene 280
Acetone 330
Various buffers for HPLC, check before using.
Woodward-Fieser rules for substituted dienes
(i) Parent acyclic diene 217 nm
(ii) Parent heteroannular diene 214 nm
(iii) Parent homoannular diene
Increment for substitution
253 nm
(iv) Alkyl group or ring residue (if the alkyl group is attached to two double bonds, count it twice)
+5 nm
(v) Exocyclic double bond (effect is two-fold if bond is exocyclic to two ring)
+5 nm
(vi) Double bond extending conjugation substituents on vinyl carbons (for each one)
+30 nm
(vii) Halogen (-Cl, -Br) + 5 nm
(viii) -OR, -O-(acyl) (-O-COR) + 6 nm, 0 nm
(ix) S-(alkyl) +30 nm
(x) -NRR- +60 nm
The effect of steric hindrance to coplanarity
Woodward rules give reliable result only for those compounds in which there is nostrain around the chromophore.
Longer the conjugation, higher will be the absorption maximum and larger will be the value of extinction coefficient. If in a structure the electron system is prevented from achieving coplanarity, there is a marked shift in the absorption maximum and max..
This departure is due to steric crowding which distorts the geometry of the chromophore. Hence the conjugation is reduced by reduction in the orbital overlap.
Study of charge transfer complex:
The formation of charge transfer complexes occurs between molecules which, whenmixed, allow transfer of electronic charge through space from an electron rich moleculeto an electron-deficient molecule.
The bond formation between the molecules occurs when filled orbitals (or non-bondingorbital) in the donor overlap with the depleted orbitals in the acceptor resulting in theproduction of new MO.
The transition between these newly formed orbitals are responsible for the new absorption band observed in the charge transfer complex.
The electron transfer from the donor to the acceptor is more complete in the excitedState than in the ground state and the wavelength of absorption is correlated with electron affinity of the acceptor and the ionization potential of the donor.
The Benesi-Hildebrand method is a mathematical approach used in the determination of the equilibrium constant K and stoichiometry of nonbonding interactions. This methodhas been typically used to study reaction equilibriums that form 1:1 guest-host complexes, where the guest and host are reactants, as shown below.
To observe one-to-one binding between a single host (H) and guest (G) using UV/Vis absorbance, the Benesi-Hildebrand method can be employed. The basis behind this method is that the acquired absorbance should be a mixture of the host, guest, and the host-guest complex.
With the assumption that the initial concentration of the guest (G0) is much largerthan the initial concentration of the host (H0), then the absorbance from H0 shouldbe negligible.
The absorbance can be collected before and following the formation of the HG complex. This change in absorbance (ΔA) is what is experimentally acquired, with A0 being the initial absorbance before the interaction of HG and A being the absorbance taken at any point of the reaction.
Using the Beer-Lambert law, the equation can be rewritten with the absorption coefficients and concentrations of each component
Due to the previous assumption that [G]0 >> [H]0, one can expect that [G] = [G]0. Δε represents the change in value between εHG and εG.
A binding isotherm can be described as "the theoretical change in the concentration of one component as a function of the concentration of another component at constant temperature." This can be described by the following equation:
By substituting the binding isotherm equation into the previous equation, the equilibrium constant Ka can now be correlated to the change in absorbance due to the formation of the HG complex.
Further modifications results in an equation where a double reciprocal plot can be made with 1/ΔA as a function of 1/[G]0. Δε can be derived from the intercept while Ka can be calculated from the slope.
Limitation:
Conjugation, Color, and the Chemistry of Vision
Colored organic compounds have extended conjugated systems
• “UV” absorptions extend into the visible region
-Carotene has max = 455 nm– When white light strikes -carotene wavelengths in
the blue region are absorbed while the yellow-orange colors are transmitted to our eyes
Conjugation, Color, and the Chemistry of Vision
-carotene is converted in the human body to 11-cis-retinal, an essential molecule for vision
Conjugation, Color, and the Chemistry of Vision
In the rod cells of the eye 11-cis-retinal is converted into rhodopsin, a light-sensitive substance
When light strikes rhodopsin, trans-rhodopsin (metarhodopsin II) is produced, which is accompanied by a change in geometry
The change in geometry causes a nerve impulse to be sent through the optic nerve to the brain, where vision is perceived
Metarhodopsin II is then recycled back into rhodopsin
Conjugation, Color, and the Chemistry of Vision
The cis-trans change in bond geometry accompanying vision
Solvent effect (cont)
• The reverse (i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift.
• This effect also influences n* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.