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)

2 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.

3 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

4 * Molecular and Atomic Absorption * Less extent Collisions between molecules lead to broadening of Absorption bands Types of Transitions Three types of transitions 1. , , 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

5 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.))

6 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.

7 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.

8 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)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.

9 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)

10 For a single vibrational mode (more than one can be affected) 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)