Chapter 4 Ultraviolet and visible Absorption Spectroscopy

1 Chapter 4 Ultraviolet and visible Absorption Spectroscopy Properties of Electromagnetic Radiation Electromagnetic Radiation energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field result of the acceleration of a charged particle does not require a material medium and can travel through a vacuum Electromagnetic Radiation Electromagnetic Radiation vi = n li where vi => velocity n => frequency li => wavelength Electromagnetic Spectrum Type of Transition Wavelength Range Frequency Range (Hz) Type of Radiation nuclear <1 pm 1020-1024 gamma-rays inner electron 1 nm-1 pm 1017-1020 X-rays outer electron 400 nm-1 nm 1015-1017 Ultraviolet outer electron 750 nm-400 nm visible outer electron molecular vibrations m-750 nm 1x1014-4x1014 near-infrared molecular vibrations 25 m 1013-1014 infrared molecular rotations, electron spin flips* 1 mm-25 m 3x1011-1013 microwaves nuclear spin flips* >1 mm <3x1011 radio waves Electromagnetic Spectrum Interaction of EMR with Matter Jablonski diagram: Selection Rules The electron must be promoted without a change in its orientation.

2 S = 0 When s 0 transition is forbidden. It may occur with very low probability Some other from quantum mechanics Etotal (molecule) = Eelectronic + Evibrational + Erotational + Enuclear Absorption of Light Uv & Vis IR Microwave * Molecular and Atomic Absorption * Less extent Collisions between molecules lead to broadening of Absorption bands Types of Transitions Three types of transitions 1.

3 , , and n electrons 2. d & f electrons 3. charge transfer electrons Electronic transition in Formaldehyde Spectroscopy Nomenclature * vacuum UV Effect of Structure on lmax Cl < lmax Br < lmax I n * transitions occur at longer wavelengths is a function of 1. Cross sectional area of absorbing species ( ) 2. Transition probability (P) = 9X1019 P ( = 10-15 cm2 ( = about 105 for the average organic molecule Transition Multiplicity Consider two electrons paired in an orbital, and their possible transitions to an empty orbital.))

4 Ground state excited singlet excited triplet the ground state has all electrons in the lowest energyorbital organic compounds almost always have paired spins, thus their ground state is almost always a singlet singlet-triplet transitions are optically forbidden - light cannot both promote an electron to a new orbital and change its spin in an organic compound most Absorption spectra are due to singlet-singlet electronic transitions 21iiSsMS Electronic Transitions in Ethylene CCHHHH * * pzpzsp2sp2 Attention will be restricted to electrons involved with carbon-carbon bonding The two sp2 electrons form the -bond, while the two pz electrons form the -bond Absorption of a photon will promote one of the bonding electrons into an anti- bonding orbital, preserving electron spin The wavelength of absorbed light will follow Planck's Law, E = hc/l The transition energies are: * > *, * > * The * transitions are of most interest since they give us information about the conjugated double bond structure of a molecule * Transitions in Butadiene CCHHHCHCHH pzpzpzpz 1 2* 2 1* Each carbon atom has one electron in a pz-orbital The four pz electrons create two bonding -orbitals and two anti-bonding *-orbitals 2 2* Absorption is in the deep UV, it has an energy similar to that in ethylene The longest wavelength Absorption is due to the 1 1* transition intermediate wavelength Absorption is due to 2 1* and 1 2* transitions The long wavelength transition has an energy that decreases with the number of double bonds Rotational Broadening (cm-1)population (relative)

5 298 K Boltzmann's constant is cm-1 kT = ~200 cm-1 at room temperature the spacing of molecular rotational levels is a few tenths of reciprocal centimeters Thermal energy populates many rotational levels giving molecules an internal source of energy rotational energy available within a molecule can add to that of a photon, making a range of optical energy that can satisfy Planck's Law, E = hn + Erot the graph shows the energy of thermally populated rotational levels; the distribution has a width of ~700 cm-1 an electronic transition will be broadened by this width 500 nm transition will be 17 nm wide (491 - 508 nm) 400 nm transition will be 11 nm wide (394 - 405 nm) 300 nm transition will be 7 nm wide (296 - 303 nm) Vibronic Transitions 024605000100001500020000250003000035000i nternuclear distance (Angstroms)(a) (b) A simultaneous change in vibrational and electronic quantum numbers is called a vibronic transition if the inter-nuclear distances are not affected when the electron changes orbitals AND the transition is symmetry allowed, the spectrum will have a single peak and, no or very weak, vibronic bands If one or more vibrations have different equilibrium inter-nuclear coordinates, a vibronic sequence will appear in the spectrum - this is shown in (a) for a single vibrational mode (more than one can be affected)

6 If the electronic transition is symmetry forbidden, vibronic bands will appear for those vibrations that deform the molecule into a shape which has an allowed transition (b) symmetry allowed: = 103 - 105 M-1 cm-1 symmetry forbidden: 102 M-1 cm-1 State Diagrams and Absorption Spectra Absorption spectrum state diagram 5000(2 m)10000(1 m)15000(667 nm)20000(500 nm)25000(400 nm)30000(333 nm)E (cm-1)S0 v = 0v = 1S1* v = 01213S2* v = 0230-0transitioninfraredabsorptionsymmet ry forbidden 1 1* 1 2*with achange ininternucleardistancelmax0 cm-1( nm)long wavelengthabsorption bandground stateexcited singletstatesChromophores They are groups with one element of unsaturation (unsaturated linkages or groups) and cause coloring to the molecules when they are attached to a non-absorbing hydrocarbon chain Effect of Multichromophores on Absorption More chromophores in the same molecule cause bathochromic effect (Red shift: shift to longer wavelength) and hyperchromic effect (increase in intensity) Hypsochromic effect: Blue shift: shift to shorter wavelengths Hypochromic effect: decrease in intensity In the conjugated chromophores * electrons are delocalized over larger number of atoms causing a decrease in the energy of to * transitionsand an incrase in due to an increase in probability for transition Aromatic Hydrocabons They absorb at three bands: 260, 200 and 180 nm Policyclic aromatic (Naphthalene): exhibit regular shift towards longer wavelength (Red shift) Azo Compounds with the linkage N=N- show low intensity bands in the near Uv and Vis due to n to * transitions Azobenzenes absorb at about 445 nm the N=N- may be conjugated with the ring system.

7 UV Absorption spectra of benzene, naphthalene, and anthracene Auxochromes They are groups that do not confer color but increase the coloring power of a chromophore. They are functional groups that have non-bonded valence electrons and show no Absorption at l > 220 nm; they absorb in the far UV -OH and -NH2 groups cause a red shift Steric Effect Extended conjugation of orbitals requires coplanarity of the atoms involved in the - cloud delocalization for maximum resonance interaction Large bulki groups cause a perturbation of the coplanarity of the system . Thus lmax is usually shifted towards shorter and also decreases Linear Polyenes : 2 n name lmax (nm) max (M-1 cm-1) 1 ethylene 163 ? 2 butadiene 217 21,000 3 hexatriene 268 35,000 4 octatetraene 304 ? 5 decapentaene 328 120,000 CHCHn As the number of double bonds increases, the long wavelength Absorption shifts to higher values (called a red-shift) The molar absorptivity increases as the molecular orbital size increases To anticipate the spectrum, use the number of conjugated double bonds, CH2=CH-CH2-CH=CH2 has a spectrum closer to ethylene than butadiene.

8 Linear Fused Aromatics structure name lmax (nm) max (M-1 cm-1) benzene 255 220 naphthalene 315 320 anthracene 357 10,000 tetracene 471 10,000 As the number of fused rings increases, the long wavelength Absorption shifts to higher values The long wavelength transition is forbidden in benzene and naphthalene, but allowed in anthracene and tetracene To anticipate the spectrum use the number of conjugated double bonds, diphenylmethane has a spectrum that resembles toluene Linear Fused Aromatics Non-Linear Fused Aromatics structure name 0-0 band (nm) 0-0 (M-1 cm-1) 3,4-benzo phenanthrene 370 170 chrysene 360 800 pyrene 370 120 perylene 437 3,700 The 0-0 band appears at lower wavelengths than would be predicted by the number of fused rings (379 for anthracene and 479 for tetracene) The first three have band positions similar to anthracene and molar absorptivities similar to naphthalene Perylene has properties between anthracene and tetracene Non-Linear Fused Aromatics Linear Polyphenyls n name 0-0 band (nm) lmax(nm) max (M-1 cm-1) 1 benzene 264 255 220 2 biphenyl 288 248 1,600 3 p-terphenyl 320 276 3,300 4 p-quaterphenyl 340 294 4,000 n As the number of conjugated rings increases, the 0-0 band shifts to higher wavelengths The increase in wavelength is not as fast as the polyenes or linear aromatics because of the bond between the rings is wisted The spectrum is featureless because thermally induced oscillation about the twist angle adds width to the vibronic bands The molar absorptivity increases because the number of double bonds is increasing Linear Polyphenyls Alkyl Substituents name 0-0 band (nm) max (M-1 cm-1)

9 Benzene 264 220 toluene 269 290 ethylbenzene 269 260 propylbenzene 268 230 anthracene 377 9,800 9-methylanthracene 387 10,200 Alkyl substituents shift the 0-0 band of the parent aromatic a few nanometers to the red, and increase the molar absorptivity a small amount Multiple substituents will increase the shift by smaller increments The vibronic pattern in the spectrum will change because of the new vibrations Alkyl Substituents Substituents with Lone-Pairs of Electrons : 4 NHH When atoms with lone-pairs of electrons are attached to aromatic compounds they can effectively increase the size of the ring system An increase in the size of the ring system shifts the parent spectrum to the red Lone-pairs often break the symmetry of a molecule, converting a forbidden transition into a moderately allowed transition When a transition is made more allowed, there is an increase in the molar absorptivity When aromatic compounds with hydroxyl or amine substituents are dissolved in hydrogen bonding solvents, the Absorption bands become broad and vibronic structure is decreased or lost Halogen Substituents name 0-0 band (nm) max (M-1 cm-1)

10 Benzene 264 220 fluorobenzene 266 1,100 toluene 269 290 p-chlorotoluene 278 570 anthracene 377 9,800 9,10-dichloroanthracene 402 14,300 Halogen substituents shift the 0-0 band to the red The larger the halogen the larger the shift Halogens can break symmetry to make a transition more allowed Multiple substituents will increase the red shift Halogen Substituents Hydroxide and Amine Substituents name 0-0 band (nm) max (M-1 cm-1) benzene 264 220 hydroxybenzene (phenol) 278 2,400 aminobenzene (aniline) ~313 1,450 naphthalene 315 320 1-hydroxynaphthalene 325 5,400 2-hydroxynaphthalene 328 2,700 Hydroxide and amine substituents shift the 0-0 band to the red Both substituents create broad bands when the compound is dissolved in a hydrogen bonding solvent Both substituents can break symmetry to make a transition more allowed Molar absorptivities are in the range of 1,000 - 6,000 Hydroxyl and Amine Substituents cyclohexane cyclohexane ethanol ethanol methanol ethanol Ultraviolet Absorption spectra for 1,2,4,5-tetrazine (a.)