Slide 1

Spectroscopy of Organic Compounds Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry

Slide 2

The Identification of Organic Compounds

Slide 3

Methods in General Use

Slide 4

Spectroscopy The study of interaction of spectrum of light with a substance to be analysed, for its identification (i.e qualitative analysis) as well as determination of its amount (i.e quantitative analysis). The Absorption of Electromagnetic Radiation and the use of the Resulting Absorption Spectra to Study the Structure of Organic Molecules.

Slide 5

Structural Features we can address Spectroscopically Molecular weight Chemical Formula Functional groups Skeletal Connectivity, structural isomers Spatial-geometric arrangements, stereoisomerism, symmetry Presence and location of chromophores Chirality issues Some of these are more central than others. Sometimes we can stop when the answer is fit to purpose.

Slide 6

Techniques we will study in this Course NMR--looks at atoms by means of their nuclei. Connectivity pathways, spatial arrangements of atoms and 1:1 correspondence between signals and atoms Mass Spec--measures molecular weight, most fundamentally useful for unknowns. Controlable fragmentation can distinguish among rival possibilities IR -Vibrations characteristic of bonds, particulary for functional group identification. Excellent fingerprint UV--reports on conjugation and multiple bonds.

Slide 7

Limitations Can we ever achieve a fully secure structure? Harrisonin Heterocycles 1976, 5, 485. J. Nat. Prod. 1997, 60, 822

Slide 8

Limitations Can we ever achieve a fully secure structure? J. Nat. Prod. 2007, 70, 412. J. Am. Chem. Soc. 2008, 130, 804 (+)-Neopeltolide

Slide 9

Molecular Orbitals Molecular orbitals (MOs) are mathematical equations that describe the regions in a molecule where there is a high probability of finding electrons Molecular orbitals (MOs) are essentially combinations of atomic orbitals two types exist, bonding and antibonding orbitals Molecular orbitals (MOs) are built up (Aufbau principle) in the same way as atomic orbitals The following topic will help you to understand bonding

Slide 10

Sigma and pi bonds: Types of Molecular Orbitals There are three important types of molecular orbitals sigma pi n Each of these types of orbitals will be discussed further...

Slide 11

Sigma Bonds Pi Bonds 1s-2p 2p-2p symmetric to rotation about internuclear axis not symmetric Sigma and Pi Bonds END-TO-END OVERLAP SIDE-TO-SIDE OVERLAP

Slide 12

Pi ( ) Bonds In a multiple bond, the first bond is a sigma ( bond and the second and third bonds are pi ( bonds. Pi bonds are formed differently than sigma bonds. Non-Bonded Pairs : n.. : n

Slide 13

Visualizing MOs The hydrogen molecule Antibonding MO = region of diminished electron density Bonding MO = enhanced region of electron density

Slide 14

MOs for the 2p Electrons The two p x orbitals combine to form sigma bonding and antibonding MOs. The two p y orbitals and the two p z orbitals give pi bonding and antibonding MOs.

Slide 15

Molecular Orbital Diagram Bonding MOs s AOs = MOs p AOs = MOs Antibonding MOs = higher energy and MOs

Slide 16

MO diagram for He 2 + and He 2 Energy MO of He + * 1s 1s AO of He + 1s MO of He 2 AO of He 1s AO of He 1s * 1s 1s Energy He 2 + bond order = 1/2 He 2 bond order = 0 AO of He 1s He 2 does not exist!

Slide 17

The Electromagnetic Spectrum According to quantum mechanics, electromagnetic radiation has a dual and seemingly contradictory nature. Electromagnetic radiation can be described as a wave occurring simultaneously in electrical and magnetic fields. It can also be described as if it consisted of particles called quanta or photons.

Slide 18

The Electromagnetic Spectrum A wave is usually described in terms of its wavelength () or its frequency (). A simple wave is shown in Figure The distance between consecutive crests (or troughs) is the wavelength. The number of full cycles of the wave that pass a given point each second, as the wave moves through space, is called the frequency and is measured in cycles per second (cps), or hertz (Hz).

Slide 19

Photon a particle of light. Electromagnetic radiation ALL light. Visible AND invisible visible light, x-rays, gamma rays, radio waves, microwaves, ultraviolet rays, infrared.

Slide 20

All electromagnetic radiation travels through a vacuum at the same velocity. This velocity (), called the velocity of light, is 2.99792458 x 10 8 s -1 and relates to wavelength and frequency as c = The energy of a quantum of electromagnetic energy is directly related to its frequency: E= h Where h = Plancks constant, 6.63 x 10 -34 J s, = frequency (Hz) The higher the frequency () of radiation, the greater is its energy. Since =c/, the energy of electromagnetic radiation is inversely proportional to its wavelength: E= hc/ where c = velocity of light The shorter the wavelength () of radiation, the greater is its energy. The Electromagnetic Spectrum

Slide 21

Slide 22

Slide 23

Types of Spectroscopy Different regions of the electromagnetic spectrum are used to probe different aspects of molecular structure

Slide 24

Electromagnetic Spectrum When UV-visible spectra interacts with substance, absorption of light by substance causes the energy content of the molecules (or atoms) to increase. The total potential energy of a molecule generally is represented as the sum of its electronic, vibrational, and rotational energies: E total = E electronic + E vibrational + E rotational The amount of energy a molecule possesses in each form is not a continuum but a series of discrete levels or states. The differences in energy among the different states are in the order: E electronic > E vibrational > E rotational

Slide 25

Slide 26

Electromagnetic spectrum

Slide 27

UV-VIS spectrophotometer

Slide 28

UVVis Spectrophotometers A UVVis spectrophotometer measures the amount of light absorbed by a sample at each wavelength of the UV and visible regions of the electromagnetic spectrum. log(I 0 /I) = A, nm 200700 I0I0 I0I0 I0I0 I0I0 I

Slide 29

29 A.Instrumentation Two sources are required to scan the entire UV-VIS band: Deuterium lamp covers the UV 200-330 Tungsten lamp covers 330-700 As with the dispersive IR, the lamps illuminate the entire band of UV or visible light; the monochromator (grating or prism) gradually changes the small bands of radiation sent to the beam splitter The beam splitter sends a separate band to a cell containing the sample solution and a reference solution The detector measures the difference between the transmitted light through the sample (I) vs. the incident light (I 0 ) and sends this information to the recorder UV-Vis Spectroscopy Instrumentation and Spectra

Slide 30

30 B.Instrumentation Sample Handling Solvents must be transparent in the region to be observed; the wavelength where a solvent is no longer transparent is referred to as the cutoff Since spectra are only obtained up to 200 nm, solvents typically only need to lack conjugated systems or carbonyls Common solvents and cutoffs: acetonitrile 190 chloroform240 cyclohexane195 1,4-dioxane215 95% ethanol205 n-hexane201 methanol205 isooctane195 water190 UV-Vis Spectroscopy Instrumentation and Spectra

Slide 31

Additionally solvents must preserve the fine structure (where it is actually observed in UV!) where possible H-bonding further complicates the effect of vibrational and rotational energy levels on electronic transitions, dipole-dipole interacts less so The more non-polar the solvent, the better (this is not always possible) UV-Vis Spectroscopy Instrumentation and Spectra

Slide 32

C.The Spectrum 1.The x-axis of the spectrum is in wavelength; 200-350 nm for UV, 200-700 for UV-VIS determinations 2.Due to the lack of any fine structure, spectra are rarely shown in their raw form, rather, the peak maxima are simply reported as a numerical list of lamda max values or max max = 206 nm 252 317 376

Slide 33

The y-axis of the spectrum is in absorbance, A From the spectrometers point of view, absorbance is the inverse of transmittance: A = log 10 (I 0 /I) From an experimental point of view, three other considerations must be made: i.a longer path length, l through the sample will cause more UV light to be absorbed linear effect ii.the greater the concentration, c of the sample, the more UV light will be absorbed linear effect iii.some electronic transitions are more effective at the absorption of photon than others molar absorptivity, this may vary by orders of magnitude UV-Vis Spectroscopy Instrumentation and Spectra C.The Spectrum

Slide 34

Sample containers, usually called cells or cuvettes must have windows that are transparent in the spectral region of interest. There are few types of cuvettes: - quartz or fused silica - silicate glass - crystalline sodium chloride quartz or fused silica - required for UV and may be used in visible region silicate glass - cheaper compared to quartz. Used in UV crystalline sodium chloride - used in IR cuvette 34 UV-Vis Spectroscopy Sample Holder cuvette

Slide 35

REGIONSOURCESAMPLE HOLDER DETECTOR UltravioletDeuterium lampQuartz/fused silica Phototube, PM tube, diode array VisibleTungsten lampGlass/quartzPhototube, PM tube, diode array InfraredNernst glower (rare earth oxides or silicon carbide glowers) Salt crystals e.g. crystalline sodium chloride Thermocouples, bolometers Types of source, sample holder and detector for various EM region 35

Slide 36

Beers law e.g. 2,5-Dimethyl-2,4-hexadiene max (methanol) 242.5 nm ( = 13,100) 2014 by John Wiley & Sons, Inc. All rights reserved. Example: UV absorption spectrum of 2,5-dimethyl-2,4-hexadiene in methanol at a concentration of 5.95 x 10 -5 M in a 1.0 cm A=absorbance =molar absorptivity c=concentration =path length A= x c x A c x or =

Slide 37

Transmittance I0I0 I b

Slide 38

Path length / cm 00.20.40.60.81.0 %T 100502512.56.253.125 Absorbance 00.30.60.91.21.5

Slide 39

External Standard and the Calibration Curve

Slide 40

QUANTIZED MO ENERGEY LEVELS FOUND IN ORGANIC MOLECULES

Slide 41

UV/VIS Vacuum UV or Far UV (