Chemical bonding Lecture 7 Flashcards
What happens to an electronically excited molecule?
if the excited molecule can support bound vibrational quantum states. In that case, it can spontaneously emit a proton and return to one of the ro-vibrational levels in its ground electronic state.
If the excited molecule cannot support bound vibrational quantum states, it will quickly dissociate.
What is intersystem crossing?
it is a process that changes the molecule from a singlet state to a triplet state, this happens due to one of the electrons spin becoming flipped, making the electron in anti-bonding and bonding pi orbital parrel to each other
What is fluorescence? What is phosphorescence? Compare them.
Fluorescence - the process when the lowest vibrational quantum state in the singlet excited state emits a photon returning the molecule to its ground causing a transition to happen from the singlet excited state to the ground electronic state.
Phosphorescence - the process when the excited triplet state molecules emit a photon transitioning back to the ground electronic state
Fluorescence occurs much faster than phosphorescence since it is unlikely for an intersystem crossing to happen. Also, Fluorescence happens way faster than phosphorescence therefore the photon yield in phosphorescence is lower.
What makes the molecule ‘relax’?
if said molecules are in a solvent some of the vibrational energy could be transferred to the solvent molecules (hence it ‘relaxs’), this allows for a transition to the lowest vibrational quantum level which then causes fluorescence.
This relaxation can cause a delay in phosphorescence. (it competes with it)
This is a nonradiative process (no photon emitted)
what are vibrational and rotational spectroscopies used for?
Rotational - bond lengths and bond angles
vibrational - bond force constants and effective masses
What is the Beer-Lambert Law and what is its derivation?
Notessss
What is the laser-induced fluorescence experiment?
It is an experiment where the laser is shot at a sample to make it reach an excited state, where the sample can then spontaneously emit photons and return to its ground electronic state. The photons emitted are reflected into a detector and measured. From this experiment, we can find the energies of the ro-vibrational levels of the sample.
I think this is photoelectron spectroscopy but i am not too sure :)
How do we find the vibrational energy of diatomic molecules?
There are two representations of the vibrational energy of the diatomic molecule:
- the harmonic oscillator model - which uses a model of two masses attached by a spring, where Hooke’s law is constantly in play, the two masses are taken as the masses of the molecules and the spring is taken as the chemical bond, by using the reduced mass of the system and applying Hooke’s law we get an expression that shows the restoring force F(R) = -k (R-Re), taking the internal of this expression gives us the potential of the restoring force V(R) = -1/2k(R-RE)^2 which is parabola and when plotted gives us a very close approximation of the lowest vibrational energies
- The morse potential, which is the anharmonic potential function of diatomic molecules, gives a very good approximation for the vibrational energy levels, but not at short intermolecular distances. It is represented by: V(R) = De(1-exp[-a(R-Re)], De is the depth of the potential energy well, and a is the ‘width’, where a = (k/2De)^1/2.
It is important to note that as the v increases the difference of energies between the levels decreases until they reach zero (this is for Morse for harmonic the energy difference remains the same)
What is the expression for the frequency of the constant oscillation that occurs in the harmonic oscillator model?
V = 1/2pi (sqr(k/u))
u - reduced mass
FOR REPEATED OSCILLATIONS
How can we obtain the energies and wave functions of the harmonic oscillator model, and how can we represent them?
It is done by solving the one-dimensional Schrodinger equation, with the harmonic oscillator potential term added to the kinetic energy term.
Solving this is not required for me to know, but how to represent its graph and understand it is important.
Check notes for the graph as well as annotations and important information
What is the relation between the Force constant and the potential energy of the restoring force?
the greater the force constant the ‘stiffer’ the chemical bond, and the more energy is required to stretch the bond, therefore larger the potential
What are the vibrational transition rules? (For both infrared and ramen scatter)
Infrared -
1. The vibration of the molecule must cause a change in the electric dipole moment to be able to interact with E-field created by light.
2.strong abortions are only absorbed between vibrational states that differ by 1, in the vibrational quantum number change in v = +-1
Ramen Scatter:
1. the vibration of the molecule must cause a change in the molecule’s polarizability
2. strong abortions are only absorbed between vibrational states that differ by 1, in the vibrational quantum number change in v = +-1
why does rotational spectroscopy have only one strong transition observed but vibrational spectroscopy has many?
- in the harmonic oscillator all transition energies are equal
- the Boltzmann distribution makes that at room temp only the lowest vibrational level is significantly populated
How can we calculate the energy of the absorbed photon due to excitation for infrared radiation?
Energy = Ef - Ei
Energy = (Evib,f + Erot,f) - (Evib,i +Erot,i)
when change in v = 1 J can be +-1, and when the chaneg in j is 1 v can be =-1
CHECK WITH SOMEONE AND LOOK AT THE PAGE 1007!! PLEASE LOOK….sorry :)
What is the rotational spectroscopy for polyatomic molecules?
When we reach polyatomic molecules, describing the rotational spectroscopy can become very hard as they can have up to 3 moments of inertia.
For different types of molecules different rotations can be made, for example linear polyatomic molecule only has one moment of inertia and one unique axis, and symmetric molecules are easier to represent.
