Presentation on theme: "X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field) Electronic."— Presentation transcript:
X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field) Electronic Excitation by UV/Vis Spectroscopy :
The wavelength and amount of light that a compound absorbs depends on its molecular structure and the concentration of the compound used. The concentration dependence follows Beer’s Law. A= bc = log I/I 0 Where A is absorbance is the molar absorbtivity with units of L mol -1 cm -1 b is the path length of the sample (typically in cm). c is the concentration of the compound in solution, expressed in mol L -1
- Single bonds are usually too high in excitation energy for most instruments (185 nm ) vacuum UV most compounds of atmosphere absorb in this range, so difficult to work with. - Types of electron transitions: i) , , n electrons Sigma ( ) – single bond electron Low energy bonding orbitalHigh energy anti-bonding orbital
Pi ( ) – double bond electron Low energy bonding orbitalHigh energy anti-bonding orbital Non-bonding electrons (n): don’t take part in any bonds, neutral energy level. Example: Formaldehyde
The n to * transition is at even longer wavelengths but is not as strong as to * transitions. It is said to be “forbidden.” Example: Acetone: n max = 188 nm ; = 1860 n max = 279 nm ; = 15 hv n n
135 nm 165 nm n 183 nmweak 150 nm n 188 nm n 279 nmweak A 188 nm 279 nm
Conjugated systems: Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). Additional conjugation (double bonds) lowers the HOMO-LUMO energy gap:
Similar structures have similar UV spectra: max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm
* transition in vacuum UV n * saturated compounds with non-bonding electrons ~ 150-250 nm ~ 100-3000 ( not strong) n *, * requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Vis ~ 200 - 700 nm n * : ~ 10-100 *: ~ 1000 – 10,000 The valence electrons are the only ones whose energies permit them to be excited by near UV/visible radiation. (bonding) (bonding) n (non-bonding) (anti-bonding) (anti-bonding) Four types of transitions * n * n * *
n * Transitions Still rather high in energy. between 150 and 250 nm. Not many molecules with n * transitions in UV/vis region max max H 2 O1671480 CH 3 OH184150 CH 3 Cl173200 CH 3 I258365 (CH 3 ) 2 S229140 (CH 3 ) 2 O1842520 CH 3 NH 2 215600 (CH 3 ) 3 N227900
n * and * Transitions Most UV/vis spectra involve these transitions. * are generally more intense than n *. max max type C 6 H 13 CH=CH 2 17713000 * C 5 H 11 C C–CH 3 17810000 * O CH 3 CCH 3 1861000 n * O CH 3 COH204 41 n * CH 3 NO 2 280 22 n * CH 3 N=NCH 3 339 5 n *
Ultraviolet Spectroscopy 200-400 nm photons excite electrons from a bonding orbital to a * antibonding orbital. Conjugated dienes have MO’s that are closer in energy. A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. =>
ChromophoreExampleSolvent max (nm) max Type of transition Alkenen-Heptane17713,000 ** Alkynen-Heptane178 196 225 10,000 2,000 160 *__*__ Carbonyln-Hexane 186 280 180 293 1,000 16 Large 12 n*n*n*n*n*n*n*n* CarboxylEthanol20441 n*n* AmidoWater21460 n*n* AzoEthanol3395 n*n* Nitro CH 3 NO 2 Isooctane28022 n*n* Nitroso C 4 H 9 NO Ethyl ether300 665 100 20 _n*_n* Nitrate C 2 H 5 ONO 2 Dioxane27012 n*n* Absorption Characteristics of Some Common Chromophores
16 Most organic spectra are complex superimposedelectronic and vibration transitions absorption bands usually broad detailed theoretical analysis not possible, effects of solvent & molecular details complicate comparison
- For Compounds with Multiple Chromophores: If isolated (more than one single bond apart) - are additive - constant CH 3 CH 2 CH 2 CH=CH 2 max = 184 max = ~10,000 CH 2 =CHCH 2 CH 2 CH=CH 2 max =185 max = ~20,000 If conjugated - shifts to higher ’s (red shift) 1,3 butadiene: max= 217 nm ; max= 21,000 1,3,5-hexatriene max= 258 nm ; max= 35,000
UV Spectral Nomenclature Red Shift (Bathochromic) – Peaks shift to longer wavelength. Blue Shift (Hypsochromic) – Peaks shift to shorter wavelength.
Solvent Effects - Intensity Solvents can induce significant changes in the intensity of peaks. Hyperchromic – Increase in absorption intensity. Hypochromic – Decrease in absorption intensity. Solvent max max Hexane2602000 Chloroform2634500 Ethanol2604000 Water2604000 Ethanol - HCl (1:1)2625200 Absorption characteristics of 2-methylpyridine
Solvent effects π -> π* transitions leads to more polar excited state that is more easily stabilized by polar solvent associations (H-bonds). The π* state is more polar and stabilized more in polar solvent relative to nonpolar one, thus in going from nonpolar to polar solvent there is a red shift or bathochromic shift (increase in λ max, decrease in ΔE).
Solvent effects For n -> π* transition, the n state is much more easily stabilized by polar solvent effects (H- bonds and association), so in going from nonpolar to polar solvent there is a blue shift or hypsochromic shift (decrease in λ max, increase in ΔE).
Auxochrome Substitutent groups which are not themselves optically active in this energy range, but which do interact with other chromophores to shift both intensity and wavelength. Derivative max max Pyridine2572750 2-CH 3 2623560 3-CH 3 2633110 4-CH 3 2552100 2-F2573350 2-Cl2633650 2-I272 400 2-OH23010000 Absorption Characteristics of Pyridine Derivatives
UVA and UVB UVA 320nm to 400nm (indirect interaction) both tans and burns the skin suppressing the immune system “immediate” sunburn reactive oxygen species UVB 290nm to 320nm (direct interaction) Skin cancer Aging Delayed sunburn
Tanning is based on the control of a complex series of natural chemical reactions. When exposed to ultraviolet radiation certain molecules in skin undergo rearrangement. This rearrangement leads to formation of Vitamin D from cholesterol, coloring of skin through the formation of melanins, and burning or cancer.
Tanning involves the formation of melanin polymers in our skin. Melanin monomers are already present in the outer layer of the skin, but in a reduced state. When oxidized upon exposure to UV, the melanin polymer forms and absorbs light in the visible and ultraviolet region.
MED = smallest dose (J/m 2 ) of UVB that produces a delayed sunburn SPF 34 should protect you from burning for thirty-four times the time of unprotected skin. Sun Protection Factor is defined as the ratio of delayed sunburn on protected skin to unprotected skin, where the protected skin is covered by 2mg/cm 2 of sunscreen. SPF is based on the physiological response in the wearer and not based on a direct comparison of the chemical properties or dosages of the compounds being used