Electronic Spectroscopy & the Beer-Lambert Law Flashcards Preview

CHEM2302 - Molecular Spectroscopy > Electronic Spectroscopy & the Beer-Lambert Law > Flashcards

Flashcards in Electronic Spectroscopy & the Beer-Lambert Law Deck (48):

What equation is used to describe the population of excited rotational, vibrational or electronic states in an equilibrium system?

The Boltzmann Distribution


How is the Bolztmann equation related to energy level distribution?

The ratio of the populations of an excited state and the ground state is equal to the ratio of the degeneracy of those states multiplied by e to the negative power of the energy difference between the states divided by the Boltzmann constant times the absolute temperature.


What is the room temperature population of NMR energy levels?

Almost equally populated even at low temperatures, due to the small energy spacing between them.


What is the room temperature population of rotational energy levels?

Many rotational levels are populated at room temperature.


What is the room temperature population of vibrational energy levels?

Vast majority in ground state at room temperature, only populated at high temperature.


What is the room temperature population of electronic energy levels?

Not thermally populates at equilibrium. Must be excited.


What is the main advantage of emission spectroscopy over absorption?

Lack of background signal.


What measurement from spectroscopy gives the transmittance?

I/Io, often as a percentage.


What is absorbance?

Log10 (Io/I) OR Ecl


What are the dimensions of absorption and transmittance?

They are dimensionless.


What is the theoretical result of plotting absorbance against concentration?

A linear line through the origin.


In what way and circumstance do solutions deviate from the theoretical graph of absorbance against concentration?

The absorbance plateaus at high concentration.


What is the cause for deviation in the linear relationship between absorbance and concentration?

Refractive index changes with concentration.
The absorbance saturates, all molecules at excited.
Excitation may cause association/dissociation/reaction.
The molecules may fluoresce, contributing to the resulting intensity.


How is the total absorbance of a mixture of non-reacting solutes that obey the B-L law determined?

By the sum of the absorbances of each solute.


How can this be used to determine the concentration of the solutes?

By knowing their molar absorptivities and measuring the total absorbance at two different wavelengths to produce two simultaneous equations.


When trying to determine the relative concentrations of two solutes by spectroscopy, what two wavelengths should be used?

Ones at which there is great difference in the absorbance of each, one with high absorbance for one of the solutes and one where the second solute has the high absorbance.


What is an isobestic point?

A wavelength at which the absorption of a mixture does not change throughout a chemical reaction or other change, usually because the absorption of the reactant and product is the same for this wavelength.


What is the presence of an isobestic point an indication of?

True equilibrium between two species only, as it is far less likely that there will be a wavelength at which all three have the same extinction coefficient.


What categories can molecular orbitals be classed in?

Core and valence


What are valence molecular orbitals?

MOs that are involved in the bonding that changes when the molecule undergoes a chemical reaction.


What are core molecular orbitals?

The inner orbitals that are unchanged throughout physiochemical process, and hence are not involved in electronic spectroscopy.


How are molecular orbital models constructed?

Qualitatively or quantitatively, by Linear Combinatino of Atomic Orbitals (LCAO).


Describe sigma bonding molecular orbitals.

No nodal planes between nuclei, MOs overlap constructively along the axis between the atoms forming a 'single bond'.

Rarely involved in electronic spectra.


Describe sigma* bonding molecular orbitals.

Perpendicular nodal plane on the axis between the atomic centres. Destructive direct overlap created a very high energy orbital, corresponding to high frequency UV photons.


Describe pi bonding molecular orbitals.

A single nodal plane along the axis between the atomic centres. Constructive overlap forms abovve and below the axis producing a 'double bond' or delocalised aromatic bonding.


Describe pi* bonding molecular orbitals.

Two nodal planes, one along the axis between the atomic centres and one perpendicular to this at the midpoint. Electron density remains above and below each atom individually.
Higher energy than pi MOs.


What do the relative energies of pi and pi* MOs mean for spectroscopy?

The energy separation corresponds to the UV-vis (far smaller than sigma-sigma*),


How do localised and delocalised pi and pi* MOs compare in energy separation?

Delocalised pi-orbitals form new groups of orbitals with a HOMO and LUMO. The energy difference between these and normal pi - pi* is lower, so absorbs at longer wavelength.


What are the comparative electron excitation wavelengths between normal and delocalised pi-pi* transitions?

180 for normal, 260 for delocalised.


Describe n molecular orbitals.

There are non-bonding (thus neither bonding nor antibonding), with no restrictions on the number of nodal planes. They are valence orbitals but are not involved in intramolecular bonding - they are LONE PAIRS; the least tightly bound electrons in the molecule.


What impact on electronic spectra do n MOs have?

Excitation of these into empty pi* orbitals are responsible for most of the electronic transitions seen. They are much smaller energy transitions than pi-pi*.


How are chromophores basically classed?

As strong or weak, depending on whether their absorption is pi-pi* or n-pi* respectively.


What is the total spin state of a 'closed shell' molecule in its ground state?

0, all spins cancel as paired electrons take opposite spins.


What is the selection rule of radiative electronic transitions?

That electronic state transitions as a result of photon absorption/emission must occur with no change in spin multiplicity. Broken only by phosphorescence.


What are singlet excitations?

An electron excitation in which the electron has retained its original spin (thus total spin = 0).


What are triplet states?

Excites states in which part of the energy input is used to flip the spin state, causing a change in total molecular spin.


How can photonic excitation to a triplet state occur without breaking the selection rule?

Photon absorption to an excited singlet allows for non-radiative intersystem crossing relaxation to a triplet state.


How can triplet states become populated non-radiatively?

Electron bombardment
Collisions with other excited molecules
Chemical reactions


How do different relaxation pathways compete with one another?

The rate of each is very sensitive to circumstance and conditions, so whichever is significantly faster is likely to dominate.


How are the category of all ground state spin conserved electronic excitation levels named in a Jablonski diagram?

The singlet manifold.


How are the category of all ground state spin non-conserved electronic excitation levels named in a Jablonski diagram?

The triplet manifold.


What are axis of a Stern-Volmer plot?

1/If (y) against [Q] (x)


What is the intercept of a Stern-Volmer plot?

1/I abs


What can Stern-Volmer plots be used to derive?

The ratio of fluorescence in the absence of a quencher to in the presence of one.


What assumptions are made when calculating primary quantum yields?

The only way that the excited molecule is produced is through photonic absorption.
All excited molecules eventually relax.


What intrinsic fluorophore can be used in proteins, and for what?

Tryptophan, emission from surface residues are at lower energy than internal so monitoring the emission wavelength can be used to tack protein folding and unfolding.


What fluorophore is used in lipid membrane probing?

DPH acts as an extrinsic fluorophore when it is attached to lipids. It is only active when inside the membrane as water acts as a powerful quencher.


What fluorophore is often used with DNA?

DAPI, which base pairs with Thymine bases.