Chapter 6: UV-Vis Flashcards
What is the region for UV?
200 nm-400 nm
What is the region for visible light
400 nm-700 nm
HOMO
Highest Occupied Molecular Orbital
LUMO
Lowest Unoccupied Molecular Orbital
Chromophore
Molecule responsible for absorption
Oxochrome
Alters absorption, but does NOT absorb itself
Bathochromic Shift
Lambda max shifts to lower energy (Red Shift)
Hypsochromic
Lambda max shifts to higher energy (Blue Shift)
What is more useful?
a) sigma to sigma*
b) n to sigma*
c) n to pi*
d) pi to pi*
D) pi to pi*
They are the most favorable to UV-Vis due to conjugation.
What makes a better solvent: alkanes or alkenes?
Alkanes
Spin Selection Rule
Electronic transition can NOT change net spin (M=2S+1)
Frank-Condon Selection Rule
Symmetry of ground and excited states must overlap
LaPorte Rule
For centrosymmetric molecule transition between orbitals of same party (symmetry) are forbidden.
Ungerada
Changed by inversion
Gerada
Unchanged by inversion
Can you go from:
a) u to g
b) g to u
Both
a) u to g ONLY
What is A in Beer’s Law? And what is its units?
Absorbance, no units.
What is epsilon in Beer’s Law? And what is its units?
Molar Absorbtivity. M^-1*cm^-1
What is b in Beer’s Law? And what is its units?
Path length. cm.
What is c in Beer’s Law? And what is its units?
Concentration. M.
What are some assumptions of UV-Vis?
- Light is monochromatic (not true)
- Absorbing species art independently of each other, problematic if H-bonding intermolecular (solvent) attraction.
- There is no scattered light->minimize $$
Are absorbances addititive?
Yes.
What is advanced mode?
Determines epsilon at 1 nm intervals for all wavelengths in range from standards.
Limitations with UV-Vis
1) Problems at high concentrations
2) Deviations due to mismatched or dirty cells. (Avoid this by looking at where A=0, baseline, all readings go to the same baseline).
3) Need to keep A<1
4) Roll over can occur due to
-high concentration due to internal screening
-Detector
-Index of refraction could change
What is stray light?
Light that strikes the detector NOT at the nominal wavelength selected.
What can be used as the radiant source in UV-Vis?
-Deuterium lamps
-Tungsten lamps
-Xenon lamps
Double Beam
2 “cell holders” in sample compartment, one for “blank”, one for sample.
Essentially it “zeros on the fly” scans. It corrects for drift in light source.
Single Beam
Very simple and cheap. Need to change wavelength if scanning. (Remember T=P/P0).
Diode Array Polychromator
Removes exit slit and replaces with linear array of detectors.
Types of cuvettes
-Acrylic Plastic
-Glass
-Quartz
Sources of instrument noise
-Source
-Detector
-Signal processor/read out
What can you do to lower noise? (Hint: Think temperature)
Cool detectors.
