THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth UV/Vis (Electronic) Spectroscopy Electrons are raised from σ, π, n levels to n, π∗, σ∗ levels. All transitions are strictly quantized Δ E = hν σ* π* E
}
anti-bonding
n-π*
n π
non-bonding
}
π-π* n-σ* σ-σ*
bonding
σ
Spectral Range 800 - 400 nm
Visible (conjugated π-systems)
400-190 nm
UV (near)
190-100 nm
Vacuum UV
This technique can be used quantitatively; in a typical application the eluent of an HPLC chromatograph is detected by UV
Lambert–Beer Law A = ε x c x b = log I0/I I0/I
intensity of the incident/ transmitted light
ε
molar absorptivity or extinction coeffient (a characteristic property of substances) may be solvent dependent (hydrogen bonding solvents) general range of ε: 10 - 10
5
–1
c
concentration (mol l )
b
pathlength of the cell (usually1 cm; sometimes1 mm) 1
Organic Spectra
Electronic Spectroscopy
H. D. Roth
For quantitative analysis: Case 1: measure A, know ε and b calculate c; Case 2: measure A, know c and b calculate ε.
Some solvents (cut off, nm) cyclohexane
190
ethanol (95%)
198
hexane
187
methanol
198
CCl4
245
water
197
CHCl3
223
dioxane
215
CH2Cl2
215
isooctane
195
Beware of impurities (and sexist phrases): ("one man's signal is another's impurity") Chromophore a functional group that absorbs UV Bathochromic shift, a shift to longer wavelength (lower energies) Hypsochromic shift, a shift to shorter wavelength (higher energies) Auxochrome a group that causes a bathochronic shift (it shifts absorption to a more accessible region)
hypsochromic
shift
bathochromic
Spectra of systems with more than one chromophore are additive, unless the chromophores interact (charge transfer spectra, vide infra).
200
250
300
250
300
350
200
250
300
350
2
Organic Spectra
Electronic Spectroscopy
H. D. Roth
Electronic Transitions 1.
σ → σ* transitions are typical for alkanes; they require high energies, λmax
