INSTRUMENTAL ANALYSIS UPDATED FINAL PAPER
QUESTIONS AND SOLUTIONS GUARANTEE A+
✔✔where does cyclohexene absorb? - ✔✔-UV-Vis region because of the C=C, pi --> pi
star region
✔✔what happens when we extend conjugation? - ✔✔-lambda max shifts to longer
wavelength, and the molar absorptivity increases (~207 nm)
✔✔why can't 1,3-cylclohexadiene extend it's conjugation to benzene? - ✔✔because
benzene is aromatic, absorbs ~257 nm
✔✔molar absorptivity - ✔✔-The molar absorption coefficient is a measurement of how
strongly a substance absorbs light.
-The larger its value, the greater the absorption.
✔✔conjugation - ✔✔-When a compound is conjugated, the energy gap between the
ground state level and excited state level or the HOMO-LUMO gap decreases. Due to
this the wavelength of the light absorbed becomes longer.
-As conjugation in the compounds is extetnded, the gap further decreases and the light
absorbed is of higher wavelength.
✔✔assumptions to be careful of: - ✔✔1)molar absorptivity- not constant, the molar
absorptivity depends on instrumental conditions and the solvent conditions
2)the wavelength of lambda max- not a constant, solvent conditions must be known,
polar solvents result in a shift of the lambda max to longer wavelengths, nonpolar
solvents shift to short wavelengths
✔✔Transmittance of Beer's Law - ✔✔T=I/I0
-the instrument does this
✔✔Absorbance of Beer's Law - ✔✔-log(T)=log( I/I0)
✔✔Beer's Lambert Law - ✔✔log(I/I0)=Ebc
✔✔Beer's Lambert Law assumptions: - ✔✔1) incident radiation is monochromatic (1
wavelength)- (molar absorptivity is single valued)
2)uniform cross section- (b is single valued)
3) absorption of a substance (absorber) is independent of the presence of other
absorbers- ( abs total= abs1 + abs2 + abs3)
✔✔dispersive - ✔✔-dispose of light speed out into component
sources- line(individual wavelength)+ continuum source (many wavelengths)
, ✔✔Deuterium lamp (D2) - ✔✔-absorbs around 180-370 nm, vacuum UV; Quartz
absorbs @ shorter wavelengths; lifetime use is ~2000 hours
-continuum source
✔✔Tungsten-incandescent source - ✔✔-absorbs 350-2000 nm (IR)
-blackbody emitter, hot filament glows
-continuum source
✔✔Xe Arc - ✔✔-very high intensity, can be pulsed
-continuum source
✔✔Hallow cathode lamp - ✔✔-line source
-refer to graphs
✔✔LED - ✔✔-most often used
-refer to graphs
✔✔Mercury Vapor - ✔✔-refer to graphs
-detects aromaticity
✔✔wavelength to detect aromaticity - ✔✔~254 nm
✔✔constructive interference - ✔✔-in phase
- shift in 1 wavelength
-we see that color of light
✔✔destructive interference - ✔✔-out-of-phase
-shift in half a wavelength
-we don't see that color of light
✔✔wavelength selection devices - ✔✔-filters, polychromators, monochromators
✔✔absorption filters - ✔✔-dye impregnated glass on other material
-single absorption, Beer's Law works A=Ebc
-T=I/I0
-%T tends to be low
-used with fluorescence
✔✔cutoff filters - ✔✔-will pass above or below cutoff wavelength
-cuts off shorter or longer wavelengths
-refer to graphs
QUESTIONS AND SOLUTIONS GUARANTEE A+
✔✔where does cyclohexene absorb? - ✔✔-UV-Vis region because of the C=C, pi --> pi
star region
✔✔what happens when we extend conjugation? - ✔✔-lambda max shifts to longer
wavelength, and the molar absorptivity increases (~207 nm)
✔✔why can't 1,3-cylclohexadiene extend it's conjugation to benzene? - ✔✔because
benzene is aromatic, absorbs ~257 nm
✔✔molar absorptivity - ✔✔-The molar absorption coefficient is a measurement of how
strongly a substance absorbs light.
-The larger its value, the greater the absorption.
✔✔conjugation - ✔✔-When a compound is conjugated, the energy gap between the
ground state level and excited state level or the HOMO-LUMO gap decreases. Due to
this the wavelength of the light absorbed becomes longer.
-As conjugation in the compounds is extetnded, the gap further decreases and the light
absorbed is of higher wavelength.
✔✔assumptions to be careful of: - ✔✔1)molar absorptivity- not constant, the molar
absorptivity depends on instrumental conditions and the solvent conditions
2)the wavelength of lambda max- not a constant, solvent conditions must be known,
polar solvents result in a shift of the lambda max to longer wavelengths, nonpolar
solvents shift to short wavelengths
✔✔Transmittance of Beer's Law - ✔✔T=I/I0
-the instrument does this
✔✔Absorbance of Beer's Law - ✔✔-log(T)=log( I/I0)
✔✔Beer's Lambert Law - ✔✔log(I/I0)=Ebc
✔✔Beer's Lambert Law assumptions: - ✔✔1) incident radiation is monochromatic (1
wavelength)- (molar absorptivity is single valued)
2)uniform cross section- (b is single valued)
3) absorption of a substance (absorber) is independent of the presence of other
absorbers- ( abs total= abs1 + abs2 + abs3)
✔✔dispersive - ✔✔-dispose of light speed out into component
sources- line(individual wavelength)+ continuum source (many wavelengths)
, ✔✔Deuterium lamp (D2) - ✔✔-absorbs around 180-370 nm, vacuum UV; Quartz
absorbs @ shorter wavelengths; lifetime use is ~2000 hours
-continuum source
✔✔Tungsten-incandescent source - ✔✔-absorbs 350-2000 nm (IR)
-blackbody emitter, hot filament glows
-continuum source
✔✔Xe Arc - ✔✔-very high intensity, can be pulsed
-continuum source
✔✔Hallow cathode lamp - ✔✔-line source
-refer to graphs
✔✔LED - ✔✔-most often used
-refer to graphs
✔✔Mercury Vapor - ✔✔-refer to graphs
-detects aromaticity
✔✔wavelength to detect aromaticity - ✔✔~254 nm
✔✔constructive interference - ✔✔-in phase
- shift in 1 wavelength
-we see that color of light
✔✔destructive interference - ✔✔-out-of-phase
-shift in half a wavelength
-we don't see that color of light
✔✔wavelength selection devices - ✔✔-filters, polychromators, monochromators
✔✔absorption filters - ✔✔-dye impregnated glass on other material
-single absorption, Beer's Law works A=Ebc
-T=I/I0
-%T tends to be low
-used with fluorescence
✔✔cutoff filters - ✔✔-will pass above or below cutoff wavelength
-cuts off shorter or longer wavelengths
-refer to graphs
