13.20 Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated p electron systems 1

Transitions between electron energy states gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm DE = hn 2

X-axis is wavelength in nm (high energy at left, low energy at right) Conventions in UV-VIS X-axis is wavelength in nm (high energy at left, low energy at right) lmax is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially p electron system Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity (e) 3

UV Spectrum of cis,trans-1,3-cyclooctadiene 2000 Molar absorptivity (e) lmax 230 nm emax 2630 1000 200 220 240 260 280 Wavelength, nm 4

pÆp* Transition in cis,trans-1,3-cyclooctadiene LUMO y3* DE = hn y2 HOMO y1 Most stable p-electron configuration p-Electron configuration of excited state 5

pÆp* Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller DE), lmax shifts to longer wavelengths 5

Methyl groups on double bond cause lmax to shift to longer wavelengths Table 13.5 (p 525) Methyl groups on double bond cause lmax to shift to longer wavelengths H H H CH3 C C H CH3 H H lmax 170 nm lmax 188 nm 5

Table 13.5 (p 525) Extending conjugation has a larger effect on lmax; shift is again to longer wavelengths H H H H C C H C H H H H H lmax 170 nm lmax 217 nm 5

lmax 217 nm (conjugated diene) Table 13.5 (p 525) H C lmax 217 nm (conjugated diene) C H CH3 H3C lmax 263 nm conjugated triene plus two methyl groups 5

orange-red pigment in tomatoes Lycopene orange-red pigment in tomatoes lmax 505 nm 5