Lecture 1 - the basics Flashcards
What is spectroscopy?
The study of molecular structure and dynamics through the absorption, emission or scattering of light
Does does a basic experiment work?
- Light beam hits the sample and the intensity of this is measured by the detector.
- Peaks in the spectrum show up if the light interacts with the sample at a specific wavelength, where energy is absorbed to give the peaks.
- Different molecules give different peaks.
What is electromagnetic radiation?
- A propagating wave of electrical energy with an orthogonal (opp direction) of magnetic field.
- These are both oscillating with the same frequency
How does the electromagnetic spectrum work?
-As the wavelength (size of the waves) increases (gets longer) the energy is decreased.
-Example - microwaves are broad and low energy.
- Only the UV-vis section is coloured and can be seen by us - this ranges from 400nm - 700nm.
How is wavelength related to frequency? and what is frequency?
- The frequency is the number of waves produced or that pass by in a certain time (usually s) and is measured in Hz.
- Related by lamba x v = c
Where lamba = wavelengh (m)
v = freq (s-1)
c = speed of light, 2.98E8 (ms-1)
What is the difference between wavelength and wavenumber?
- Wavelength, lambda, is expressed in nm and is used in electric spectroscopy - like UV-vis
- Wavenumber, v hat, is expressed in cm-1 and is used in vibrational spectroscopy - like Raman and IR. V hat = 1/lambda
What is wave-particle duality theory?
- The amount of energy absorbed by a molecules is specific to the energy gap between levels in a specific molecule (DeltaE = E1 - E0 = hv) - only certain wl aloud which is predicted by each molecule.
- Explained using dual theory and classical cant explain well.
-Einstein produced the theory that light is made up of packets (photons) which can only be excited when they reach a threshold and can’t be excited until then, the are quantized as need to reach specific energies.
-Known as E = hv or E = hc/lambda
What is spectroscopy?
-Measurements of discrete amounts of energy which are passed between molecules and electromagnetic radiation when they interact - called energy exchanges and give structural info.
What are the different transition absorptions?
-Microwave region (low energy) - transitions between rotational states.
- IF region (higher energy) - transitions between vibronic states (and rotational)
- UV and visible region (highest energy) - transitions between electronic states (and vib and rot)
These energies can be separated based on the Born-Oppenheimer approximation.
What is the Born-Oppenheimer approximation?
-Assumes that the electron is so light compared to the nucleus so the nucleus is effectively stationary, this is because the mass of the nucleus»_space; electron. The electrons moved based on the nucleus position.
- The Hamiltonian is made up of kinetic and potential energy terms.
- The Schrodinger equation can be solved by firstly using the BO approx to separate the total wavefunction into electronic and nuclear wf’s.
- The electrons depends on the position of electrons (q) and nucleus (Q), whereas nucleus only depends on position of nucleus (Q).
- The total energy contributions can also be separated into Etotal = Ee and En.
-The nuclear wavefunction can be separated into rot and vib wf’s and so can the energy of En.
-Now the total wf = wf(Ee)wf(Ev)wf(Er)
and
Etot = Ee + Ev + Er.
-Now all contributions can be treated separately.
What energies are usually associated with each type of transition?
- As rotational spectrum is the lowest energy, the wavemumbers are also low.
-Rotational is around 0.5-50 cm-1 - Vibrational is around 500-4000cm-1
-Electronic is around >10000 cm-1
-Electronic has elec, rot and vib
-Vib has vib and rot
- Rot only has rot
What are the ad/dis of using each transition in spectroscopy?
-Rot - very low energy, only observed with gases at low temps.
-Vib - quick and reliable. Gives structural info on specific molecules.
-Elec - quic, reliable and cheap. Not molecular specific.
