Electronic, Vibrational and Rotational Spectroscopy Flashcards
What 3 ways can EM radiation interact with matter?
Emission, Absorption and Scattering
What is a photon?
A specific amount (quantum) of energy
Low energy waves have a short wavelength. True or false?
False
Which region of the EM spectrum gives rise to a change in spin?
Radio Waves
Which region of the EM spectrum gives rise to a change in shape?
Infrared
Which region of the EM spectrum gives rise to a change in electronic distribution?
UV-vis and X-rays
Which region of the EM spectrum gives rise to a change in orientation?
Microwaves
What is meant by an amount of energy being Quantized?
It can only exist in discrete quantities
How is an absoption or emission line identified in a spectra?
The photon emitted or absorbed has a frequency which has an energy which corresponds to the energy gap between two quantized energy levels
What would a monochromator be used for?
To select a specific wavelength
What is detected in an absorption spectrometer?
Intensity of the photons passing through the sample
How do you calculate Transmittance (T)?
Intensity / Starting Intensity
How do you calculate Absorbance (A)?
log(1/T)
In an emission spectrometer, what is the purpose of the excitation source?
To electronically excite the sample to allow it to emit a photon when the sample returns to it’s relaxed state
If energy levels are close together, how will transitions be distributed?
In a Boltzmann Distribution
If energy levels are widely spread, how might this affect the spectrum produced?
The majority of the sample will be in the same energy state, so the spectrum may only show one band
How could you increase the number of molecules encountered by the photons in a spectrometer? Give 2 ways.
Increase the pathlength of the sample and increase the concentration of the sample
In the context of Rotational Spectroscopy, how is quantization applied?
Each molecule can rotate with a discrete amount of rotational energy
There are small differences in energy between the different rotational energy levels. True or False
True. Microwaves cause these transitions, so must be low energy.
E = BJ(J+1) is the equation which defines permitted rotational energy levels. What do B and J denote?
B: Rotational Constant
J: Rotational Quantum Number
E = BJ(J+1) is the equation which defines permitted rotational energy levels. What would the energy change be for levels 3=>4
E(3)= 3B(3+1) = 12B E(4)= 4B(4+1) = 20B E(3=>4)= 8B
How does the Moment of Inertia (I) relate to the Rotational Constant (B)?
B is proportional to 1/I
If a molecule is heavy and has long bonds, will it’s rotational energy gaps be large or small?
Small. The Rotational Constant will be lowered by the increased moment of inertia
What is the Gross Selection Rule for rotation?
There must be a change in dipole on rotation for an emission or absorption to occur
Can carbon monoxide display a rotational spectrum?
Yes. It has a permanent dipole, so it complies with the gross selection rule
What is the Specific Selection rule for Rotational Spectroscopy?
Changes in rotational energy levels can only increase or decrease by 1
Can a molecule start with a rotational energy level greater than 0?
Yes. Any molecule can start with a higher energy level. This gives rise to specific energy patterns in a spectrum