separation techniques - CD spectroscopy Flashcards
(25 cards)
chiral definition
an object that is not superimposable upon its mirror image
enantiomer definition
a molecule that has a non superimposable mirror image
stereogenic centre definition
the atom at which the molecule is chiral/the chiral carbon
enantiomerically pure definition
a substance comprising a single enantiomer only
racemic mixture definition
a substance made up equal amounts of both enantiomers
how can different enantiomers be identified?
through their interactions with polarised light
what are the 2 types of polarised light?
linearly polarised light and circular polarised light
linearly polarised light definition
electron vector direction is constant, magnitude varies
circular polarised light definition
electric vector direction varies, magnitude is constant
what is the difference between normal light and plane polarised light?
in normal light, electric field vectors lie in all possible planes, whereas in plane polarised light, electric field vectors all lie in the same plane
optical activity definition
rotation of plane polarised light by a chiral substance
what happens when plane polarised light interacts with a chiral substance?
the light will be rotated, this rotation depends on how many molecules the light interacts with (it will be further rotated per molecule it interacts with)
polarimetry definition
measures angle of rotation (specific rotation) for a solution of known concentration or a pure compound in a cell of known length at known wavelengths of light
give the equation for specific rotation, for pure liquids and solutions
pure: [α]λ^T = α/(lρ)
solutions: [α]λ^T = α/(lc)
where [α]λ^T = specific rotation at certain temp+wavelength, α = measured rotation, l = path length (dm) ρ = density of liquid, c = concentration
what does [α]D^20 mean?
this is the specific rotation recorded at 20 C and at 589 nm (aka D line of Na, because sodium lines were used)
dextrorotation definition
clockwise rotation of plane polarised light
laevorotation definition
anticlockwise rotation of plane polarised light
what is CD spectroscopy?
circular dichromism spectroscopy measured the difference in absorbance of left handed circularly polarised light and right handed circularly polarised light, measured as a function of λ
- this difference can be detected when a chiral molecule contains one or more light absorbing groups (called chiral chromophores)
how is circular dichromism calculated?
CD = ΔA(λ) = A(λ)L-CPL - A(λ)R-CPL
where (λ)L-CPL/R-CPL = wavelengths of left/right handed circularly polarised light
how do enantiomers interact with L-CPL/R-CPL?
each enantiomer will interact with either type of light preferentially, the difference between its interactions with each type is what is measured
enantiomers interact oppositely, so they have mirror image CD spectra - this means that CD for a racemic mixture = 0
in molecules with many stereogenic centres, all must be opposite for CD to mirror/cancel out
what is the cotton effect?
the characteristic change in CD in the vicinity of UV-Vis absorption band
how is CD measured
ellipticity, θ
ellipticity, θ definition
a measure of the degree to which a polarised light wave deviates from a circular polarisation
also molar ellipticity [θ] = θ/molarity
what is the difference between ellipticity and molar ellipticity?
molar ellipticity is not dependent on concentration
