CH05 - Molecular Spectroscopy in UV-Vis Flashcards
Vacuum UV is absorbed by _______.
O2, water vapor and other gasses present in the air.
(Remember: CO2 causes hothouse effect by absorbing UV and vibrating)
Vacuum UV wavelength range is?
100Å-200nm (overlaps with X-ray)
UV wavelength range is?
200nm-400nm
Vis wavelength range is?
400nm-800nm
What type of bonds do sigma bonds form?
single (from superposition of s orbitals or from “axial” overlapping of p orbitals)
What type of bonds do pi bonds form?
double or triple (from superposition of p orbitals which are perpendicular to the inter-nuclear axis).
Are sigma bond absorption bands (sigma to sigma*) visible in UV\Vis?
No, absorption energies (frequencies) are higher than those of UV\Vis.
Are pi bond absorption bands (pi to pi*) visible in UV\Vis?
Yes
what kind of bonds do n electrons form?
non-bonding. Occurs in atoms right of C on the table (atoms with non-bonding pairs such as those on oxygen).
When an non-hybridized p orbital is full, it will form a _____ molecular orbital.
“n” (nonbonding) molecular orbital. Fully populated and non-hybridized p orbitals do not tend to participate in bond formation.
The energetic position of _______ molecular orbitals is the same as that of the original p orbitals.
“n” molecular orbitals
Which transitions between MOs absorb energy in UV\Vis?
π→π*
n→π*
n→σ*
What is the Franck–Condon principle?
During an electronic transition, a change from one vibrational energy level to another will be more likely to happen if the two vibrational wave functions overlap more significantly.
Simple molecules in the gas phase often show the _______ levels
superimposed on the electronic transitions. Those are absent from the _____ phase due to______.
Vibrational,
Liquid,
Interaction between solvent and solute molecules
Sample holders in UV Vis spectrum are made from ______ or ______.
Quartz and fused silica (glass). Glass absorbs IR but does not absorb UV\Vis.
A PMT detector has high _____ and high________ ratio.
Sensitivity, signal to noise
A semiconductor has a ________ valence band, an ________ conduction band at 0K and an energy gap________ than 2.5 eV.
Full, Empty, Smaller
An insulator has a________ valence band, an ________ conduction band at 0K and an energy gap________ than 2.5 eV.
Full, Empty, Greater
An conductor has an ________ valence band.
Half-empty (thus conducts electricity even at 0K)
_________ can be used as detectors for electromagnetic radiation.
Semiconductors (photons with sufficient energy can excite valence electrons to conduction band, thus inciting electric current).
How does a photo-diode detector work?
- PN junction is hed in reverse-bias (n to + and p to - of voltage source).
- A depletion zone is formed in the junction.
- A photon (with E>Egap) strikes the junction, forming hole-electron pairs in the depletion zone.
- Holes move toward “-“ terminal, electrons move toward “+” terminal, forming a current proportinal to the intensity of incident light.
When a diode array is used as a sensor, _____ wavelengths can be measured simultaneously.
Multiple
When a diode array is used as a sensor, the _____ is placed after the sample.
Dispersion device (prism or grating).
Describe optical diagram of a diode array spectrophotometer?
Source → Sample → Entrance Slit → dispersion device → diode array sensor
Advantages\disadvantages of photo-diode detector?
Advantage:
1. Fast - suitable for scanning fast reactions to determine
kinetics or measurements on flowing material (as in chromatography).
2. High SNR is achievable through multiple measurements - for applications involving low light levels.
Disadvantage:
1. Low resolution - suitable for molecular spectroscopy but not atomic spectroscopy.
What is a photomultiplier tube (PMT)?
Detector used in the UV/Vis spectral region that acts like a series of phototubes. The cathode and anode are biased by 400 - 2500 mV and separated by a series of intermediate dynodes, each progressively more positively biased, which provides an electron multiplication cascade.
What is a chromophore?
A group of atoms, with their associated electrons, in a molecule that absorbs light in certain wavelengths. (e.g. an Alkyne group)
What is an auxchrome?
Substituent groups with nonbonding electrons that are attached to the basic chromophore structure and change the position and/or intensity of the chromophore’s absorption band; typical examples include methyl, hydroxyl, alkoxyl, halogen, and amino groups.
What is a Batochromic Shift?
When wavelength of maximum absorption is shifted to a longer wavelength (“Red Shift”).
What is a Hypsochromic Shift?
When wavelength of maximum absorption is shifted to a shorter wavelength (“Blue Shift”).
What causes Batochromic\Hypsochromic shifts ?
Interaction of solute molecules with solvent.
What causes changes in absorption intensity (Hyperchromism\Hypochromism) for a given wavelength?
Changes in molecular structure (e.g. as a result of protonation\deprotonation of phenolphthalein)
In a Batochromic shift, dipole-dipole interactions and hydrogen bonds in a ____ solvent lowers energy of ____ orbitals more than that of π orbitals.
Polar, π*
For a given molecule, the π→π* bandgap in a ____ solvent is smaller than the π→π* bandgap in a ____ solvent.
Polar, Nonpolar
What electronic transition is affected by a Batochromic Shift?
π→π*
What electronic transition is affected by a Hypsochromic Shift?
n→π*
What causes a Hypsochromic Shift?
Compared to π* orbitals, “n” molecular orbitals are affected more strongly by interactions in polar solvent ⇒ n orbitals are lowered even more than π* ⇒ (n→π*) gap increases ⇒ wavelength shortens
____ transitions also undergo Hypsochromic Shifts, but are invisible since they are in the vacuum range.
n→σ*
When solvent forms hydrogen bonds with the molecules, the energy change in “n” orbitals ≈ ____
Energy of hydrogen bonds that were formed.
Increase in polarity of solvent ____ wavelength of n→π* transition.
Decreases (because interactions with polar solvent decreases energy of n orbital electrons, increasing n-pi gap. Hypsochromic shift)
In cyclical Ketones, positions of carbons are marked α,β,γ… ____
Along the conjugated diene chain (alpha is the nearest to carbonyl carbon, delta is the furthest)
What is an exocyclic double bond?
A double bond attached on one end to an atom which is shared between two rings.
λ_max in UV\Vis is different for two ____ of the same compound.
Isomers. Thus, UV-Vis absorption can be used to differentiate between isomers.
What is a Singlet State (singlet energy level)?
In quantum mechanics, a singlet state usually refers to a system in which all electron spins are paired.
What is a Triplet State (triplet energy level)?
A triplet state is a system with two unpaired electrons.
In Fluorescence, energy of photon is ____ and photon wavelength is ____ than that of the absorbed radiation.
Smaller, longer (this process of fluorescence is called “Stokes Fluorescence\Stokes Shift”
Describe the process of Fluorescence.
- UV radiation excites an electron from ground singlet state S0 to first excited singlet state S1.
- The electron relaxes in a radiationless process to the lowest vibrational state in S1. This is called an Internal Conversion (IC).
- From here, there are 2 options:
* The electron returns to ground state S0 while emitting a photon this process is called Fluorescence.
* The electron can relax to the ground state in a radiationless process as a result of intermolecular collision. This process is called “Quenching”.
Describe the process of Phosphorescence.
- UV radiation excites an electron from ground singlet state S0 to first excited singlet state S1.
- Inter System Conversion (ISC) to first excited triplet state T1 (radiation-less).
- From here, there are 2 options:
* The electron returns to ground state S0 while emitting a photon this process is called Phosphorescence.
* The electron can relax to the ground state in a radiationless process. This process is called “Quenching”.
In fluorescence, the lifetime of the electrons in the excited state is ____.
Very short (10-20ns)
Remember: Fluorescent paint shines only when lit by UV.
In phosphorescence, the lifetime of the electrons in the excited state is ____ in fluorescence.
Longer (ms to a few seconds)
As in “glow in the dark” materials.
Wavelengths emitted in ____ are longer than those emitted in ____ (choose fluorescence/phosphorescence also explain why).
Phosphorescence, Fluorescence
(Because energy of T1 state is lower than that of S1)
Reason for difference in lifetime of excited electrons in Fluorescence and Phosphorescence?
The process of phosphorescence involves a forbidden transition (S1->T1).
A rigid molecular structure increases ____.
F (intensity of fluorescence). Rigid molecules are less susceptible to quenching and radiation less relaxation.
Fluorescence occurs in molecules with ____ transitions.
π→π*
(aromatic compounds and polycyclic aromatic compounds)
