Final Review
The final exam is on Tuesday May 7, at 12:30 in the regular Room. It will consist of two
parts, a multiple choice section which will cover the entire semester and the second part,
which will be over material we covered since the last exam. Circular Dichroism, NMR ,
Raman, IR etc. For the 1st part look over previous exams, quizzes, and exam reviews,
and the review below. If you have time, go through old homework and work homework
problems from the new material.
The absorption/emission spectroscopy that we talked about in class were:
IR absorption
Microwave absorption
UV-Vis Absorption
Circular Dichroism
MR
We talked about the selection rules associated with IR, must have a changing dipole
associated with the vibrational mode and for strictly harmonic oscillators v = +/- 1. How
many normal modes are present for a particular molecule, 3N-5 or 3N-6. What is the
fundamental transition, what is an overtone. The Morse potential allows one to include
the anharmonicity of the vibrations. It becomes important at high values of the
vibrational quantum number v. What is the expression for the vibrational energy when
anharmonicity is included? What about rotational spectra selection rules? J = + /-1 or 0.
The ideal model is the Rigid Rotor. Deviation from ideality comes from the centrifugal
distortion constant. This is only important at high J values. What does the expression for
the rotational energy like if this is included. If both the vibrational and rotational energy
levels are changing, then the spectrum has some extra structure on it. There can be a P,Q,
and R branch. What does each of these represent? What is an overtone, how about a
combination band?
Another type of interaction of light with matter is scattering. We talked about Rayleigh
Scattering, Raman Scattering (both Stokes and anti-Stokes Raman Scattering) and we
also talked about the selection rule that the polarizability of the molecule must change as
a result of the vibration for the vibrational mode to be Raman active. Both IR and Raman
Spectroscopy probe the vibrational modes of the molecule. How many vibrational
modes are there for a given molecule? It depends on if they are linear or nonlinear, why?
The birefringence in a molecule and chirality lead to optical rotation and circular
dichroism being useful as a probe for molecules with chiral structures. Thus molecules
can be characterized by polarized light. You should be able to explain how these
techniques can be used and what the difference is between the two. Is the circular
dichroism spectrum the same for the nucleoside or amino acid as it is for the
polynucleotide or the protein? What structural affects contribute.
Another topic is NMR which stands for ? This spectroscopy monitors changes in the
nuclear spin state. Nuclei can have both integral and half integral spin states. In the
presence of a magnetic field the spin states of a nucleus are no longer degenerate and the
energy separation between the states depends on the strength of the field and the
gyromagnetic ratio of the nucleus. That is E =  h/2 B0 = hL where L is the frequency
of the transition in the imposed magnetic field and is called the Larmor frequency. The
Larmor frequency is 500MHz for a proton in a field Bo = 11.7 T. Given a table of , you
should be able to give the Larmor frequency for other spin ½ nuclei as well. What are
some of the common spin ½ nuclei. If an NMR instrument is a 300MHz instrument,
what would the value of its magnetic field strength be. Why is NMR not considered to
be a very sensitive technique in the sense that it can measure concentration only at the %
level?
In NMR two important features that alter the transition frequency band are chemical
splitting and spin spin coupling. You should be able to explain each of these? Also
given a spectrum and a molecule you should be able to identify which peaks are
associated with a given NMR band. What is the spin-lattice relaxation time constant T1
and what is the other relaxation constant that is related to the bandwidth of the NMR
peaks. For the two dimensional NMR techniques, what is the difference between COSY
and NOESY. What is the Nuclear Overhauser Effect. What is it dependence on distance.
About how far away will neighbors make an effect?
What are Boltzmann statistics. How is the population of one energy related to the
population of another energy level. How are these related to the temperature. In both IR
and UV-Vis energy diagrams showing absorption and emission, we generally showed
absorptions that started from the ground state of the molecule. Why should they start
from the ground state, why not an excited state? We'll this is based on statistical
Thermodynamics and the Boltzmann distribution (see Chp 11). Basically the relative
populations of two states is give by n1/n0 = g exp(-(E1-E0)/kT). kT is about 200 cm-1 at
room temperature, and a typical IR transition occurs at 2350 cm-1 (assymmetric stretch in
CO2). Thus there is a small number in the excited state