química orgânica i - w3.ualg.ptw3.ualg.pt/~abrigas/qoi13a_spec_uv.pdf · 1 afb qo i 2007/08 1...
TRANSCRIPT
1
AFB QO I 2007/08 1
Química Orgânica I
Ciências Farmacêuticas
Bioquímica
Química
AFB QO I 2007/08 2
Análise estrutural
2
AFB QO I 2007/08 3
Adaptado de:
� Organic Chemistry, 6th Edition; L. G. Wade, Jr.
� Organic Chemistry, William H. BrownWilliam H. Brown
AFB QO I 2007/08 4
methods determine the molecular structure
� Crystallography
� Mass Spectrometry
� Elemental Analysis
� Infrared Spectroscopy
� UV/VIS Spectroscopy
� Nuclear Magnetic Resonance
� Coupled methods: GC-MS; HPLC-NMR...
� (comparative methods for know structures)
3
AFB QO I 2007/08 5
methods� Crystallography: diffraction patterns relates to atomic
positions � Mass Spectrometry: molecular weight and fragmentation
pattern � Elemental Analysis: determination of elemental and isotopic
composition. � Infrared Spectroscopy: determine the presence or absence
of functional groups. � UV/VIS Spectroscopy: Spectrophotometry uses a
spectrophotometer to measure how much light is absorbed by the sample (Gives information about conjugated πelectron systems).
� Nuclear Magnetic Resonance (NMR): NMR spectroscopyworks by studying the magnetism of a nucleus by placing it in alignment with a magnetic field, and then using an electromagnetic field to disrupt this alignment.
AFB QO I 2007/08 6
E = hν and spectroscopy
4
AFB QO I 2007/08 7
AFB QO I 2007/08 8
Electromagnetic Radiation
� Common units used to express wavelength λλλλ
Angstrom (Å) 1 Å = 10-10 m
Relationto MeterUnit
1 mm = 10-3 m
1 nm = 10-9 m1 µµµµm = 10-6 m
Nanometer (nm)Micrometer (µµµµm)
Millimeter (mm)Meter (m) ----
5
AFB QO I 2007/08 9
Electromagnetic Radiation
�� Electromagnetic radiation:Electromagnetic radiation: light and other forms of radiant energy
�� Wavelength (Wavelength (λλ):): the distance between consecutive peaks on a wave
�� Frequency (Frequency (νν):): the number of full cycles of a wave that pass a given point in a second
�� Hertz (Hz):Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”)
AFB QO I 2007/08 10
�electronic
�vibrational
�rotational
�nuclear spin
UVUV--VisVis
infraredinfrared
microwavemicrowave
radiofrequencyradiofrequency
6
AFB QO I 2007/08 11
UV-vis spectroscopy
AFB QO I 2007/08 12
Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states.
∆∆∆∆E = hνννν
UV-vis
ν = c / λ
7
AFB QO I 2007/08 13
Why UV-vis?
� Many organic molecules have chromophores that absorb UV
� UV absorbance is about 1000 x easier to detect per mole than NMR
� Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology.
� Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors
AFB QO I 2007/08 14
…
� One of the best ways for identifying the presence of acidic or basic groups, due to big shifts in λ for a chromophore containing a phenol, carboxylic acid, etc.
λ
“bathochromic” shift“hypsochromic” shift
8
AFB QO I 2007/08 15
UV-vis - range
� Conjugated compounds can absorb light in the ultraviolet region of the spectrum
� The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is most useful in organic chemistry
AFB QO I 2007/08 16
UV-vis – range in detail
•σ → σ* and σ → π* transitions:
•high-energy, accessible in vacuum UV (λmax <150 nm).
•n → σ* and π → σ* transitions: non-bonding electrons
(lone pairs), wavelength (λmax) in the 150-250 nm region.
•n → π* and π → π* transitions: most common
transitions observed in organic molecular UV-Vis,
observed in compounds with lone pairs and multiple
bonds with λmax = 200-600 nm.
9
AFB QO I 2007/08 17
Energy
σ∗σ∗σ∗σ∗
ππππ
σσσσ
π∗π∗π∗π∗
n
σσσσ
σσσσ
ππππ
n
n
σσσσ∗∗∗∗
ππππ∗∗∗∗
ππππ∗∗∗∗
σσσσ∗∗∗∗
ππππ∗∗∗∗
alkanes
carbonyls
unsaturated cmpds.
O, N, S, halogens
carbonyls
Electronic transitions
AFB QO I 2007/08 18
Example: π → π* transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W):
HOMO πu bonding molecular orbitalLUMO πg antibonding molecular orbital
hν 170nm photon
calculated π → π* transitions
10
AFB QO I 2007/08 19
1,3-butadiene
� The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO)
AFB QO I 2007/08 20
Other electronic transitions
11
AFB QO I 2007/08 21
AFB QO I 2007/08 22
12
AFB QO I 2007/08 23
AFB QO I 2007/08 24
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
13
AFB QO I 2007/08 25
Transmittance vs Absorbance
� % Transmittance = (I / I0) * 100%
where:
I = intensity of transmitted light
I0 = intensity of incident light
� Absorbance = -log(I / I0)
AFB QO I 2007/08 26
Diode Array DetectorsDiode array
alternative puts
grating, array of
photosens.
Semiconductors after
the light goes through
the sample.
Advantage, speed,
sensitivity,
The Multiplex
advantage
Disadvantage,
resolution is 1 nm, vs
0.1 nm for normal
UV
Model from Agilent literature. Imagine
replacing “cell” with a microflow cell for
HPLC!
14
AFB QO I 2007/08 27
UV vis SpectrumA
bso
rban
ce
Wavelength, λλλλ, generally in nanometers (nm)
0.0400 800
1.0
200
UV Visible
λλλλmaxwith certain
extinction εεεε
Make solution of concentration low enough that A≤ 1
Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent.
Can extract the εεεε value if conc. (M) and b (cm) are known
UV bands are much broader than the photonic transition event. This is because vibration levels
are superimposed on UV.
AFB QO I 2007/08 28
Electronic Transitions and UV-visible Spectra in Molecules
15
AFB QO I 2007/08 29
Quantitative Use of UV Spectra
� Beers’ law:
A = εεεεcl� “A” absorbance
� “εεεε” is molar absorptivity (extinction coefficient
� “c” is concentration in mol/L
� “l” is path of light through sample in cm
AFB QO I 2007/08 30
meaning
� λmax: wavelength where UV absorbance for a compound is greatest
� Energy difference between HOMO and LUMO decreases as the extent of conjugation increases
16
AFB QO I 2007/08 31
Effect of Conjugation
� λmax increases as conjugation increases (lower energy)
� 1,3-butadiene: 217 nm
� 1,3,5-hexatriene: 258 nm
�Substituents on π system increase λmax
AFB QO I 2007/08 32
ΨΨΨΨ2222∗∗∗∗
ππππΨΨΨΨ1111ΨΨΨΨ1111
ΨΨΨΨ2222
ΨΨΨΨ3333∗∗∗∗
ΨΨΨΨ4444∗∗∗∗
∆E for the HOMO � LUMO transition is reduced
17
AFB QO I 2007/08 33
Energy
ethylene
butadiene
hexatriene
octatetraene
Lower energy =Longer wavelengths
AFB QO I 2007/08 34
18
AFB QO I 2007/08 35
AFB QO I 2007/08 36
Conjugation, Color and the Chemistry of Vision
� Visible region is about 400 to 800 nm� Extended systems of conjugation absorb in
visible region� β-Carotene, 11 double bonds in conjugation
� λmax = 455 nm
19
AFB QO I 2007/08 37
AFB QO I 2007/08 38
Conjugation, Color and the Chemistry of Vision
� β-Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:
20
AFB QO I 2007/08 39
Conjugation, Color and the Chemistry of Vision
� 11-cis-retinal is converted to rhodopsin in the rod cells of the retina.
� Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain
AFB QO I 2007/08 40
Transmission and Color
The human eye sees the complementary color to that which is
absorbed
21
AFB QO I 2007/08 41
Absorbance and Complementary Colors
AFB QO I 2007/08 42
Orange Common Food Uses
Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections
Lemon-yellow Common Food Uses
Custards, beverages, ice-cream, confections, preserves, cereals.
Orange-red Common Food Uses
Gelatins, puddings, dairy products, confections, beverages, condiments.
Royal BlueCommon Food Uses
Baked goods, cereals, snack foods, ice-cream, confections, cherries.
Bright BlueCommon Food Uses
Beverages, dairy products, powders, jellies, confections, condiments, icing.
The colors of M&M’s
22
AFB QO I 2007/08 43
NNO3S
SO3
N N
HO
• In the biological sciences these compounds are used as dyes to selectively stain different tissues or cell structures
• Biebrich Scarlet - Used with picric acid/aniline blue for staining collagen, recticulum, muscle, and plasma. Luna's method for erythrocytes & eosinophil granules. Guard's method for sex chromatin and nuclear chromatin.
Biological applications
AFB QO I 2007/08 44
NNO3S N
CH3
CH3
NHNO3S N
CH3
CH3
Yellow, pH > 4.4 Red, pH < 3.2
Methyl Orange
• In the chemical sciences these are the acid-base indicators used for the various pH ranges:
• Remember the effects of pH on aromatic substituents
pH paper
23
AFB QO I 2007/08 45
designing colours
AFB QO I 2007/08 46
UV spectra and structure
From Skoog and West et al. Ch 14
24
AFB QO I 2007/08 47
Empirical understandind of structural effects
R.B. Woodward, L.F. Fieser and others have predicted λmax for
π⇒π* in extended conjugation systems to within ca. 2-3 nm.
Homoannular, base 253 nm
heteroannular, base 214 nm
Acyclic, base 217 nm
Attached group increment, nm
Extend conjugation +30
Addn exocyclic DB +5
Alkyl +5
O-Acyl 0
S-alkyl +30
O-alkyl +6
NR2 +60
Cl, Br +5
AFB QO I 2007/08 48
Expected UV (Woodward)
O
Base value 217
2 x alkyl subst. 10
exo DB 5
total 232
Obs. 237
Base value 214
3 x alkyl subst. 30
exo DB 5
total 234
Obs. 235
Base value 215
2 ß alkyl subst. 24
total 239
Obs. 237
25
AFB QO I 2007/08 49
Distinguish Isomers!
HO2C
HO2C
Base value 214
4 x alkyl subst. 20
exo DB 5
total 239
Obs. 238
Base value 253
4 x alkyl subst. 20
total 273
Obs. 273
AFB QO I 2007/08 50
i. Bathochromic shift (red shift) – a shift to longer λ; lower energyii. Hypsochromic shift (blue shift) – shift to shorter λ; higher energyiii. Hyperchromic effect – an increase in intensityiv. Hypochromic effect – a decrease in intensity
200 nm 700 nm
εεεε
Hyp
oc
hro
mic
Hypsochromic
Hyp
erc
hro
mic
Bathochromic
Substituent effects
26
AFB QO I 2007/08 51
Quantitative analysis
Great for non-
aqueous titrations
Example here
gives detn of
endpoint for
bromcresol green
Binding studies
Form I to form II
Isosbestic points
Single clear point, can exclude
intermediate state, exclude light
scattering and Beer’s law applies
Binding of a lanthanide complex to
an oligonucleotide
AFB QO I 2007/08 52
Other Electronic Processes
� Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity
� Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity
� Singlet state: spins are paired, no net angular momentum (and no net magnetic field)
� Triplet state: spins are unpaired, net angular momentum (and net magnetic field)
27
AFB QO I 2007/08 53
� http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm#uv3b
� http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html