Chem 7595
Sample Exam 1
For Study Use Only
(Covers lecture and reading materials from the start of the class up until the end of
NMR spectroscopy)
Name:
1.
(15 points)
A synthetic chemist colleague comes to you for help with an
unknown solid sample. The sample is small and he needs the sample returned.
Using the analytical methodology, help your colleague determine what this
sample is. When suggesting an analytical technology be sure to include why.
2.
(5 points) Calculate the total ionic strength () of a solution containing 0.1 M
NaCl and 0.2 M Mg(NO3)2 using the ionic strength equation:

 c z   c z
n
1
2
j 1
j
2
j
1
2
2
1 1

 c2 z 22  c3 z 32  
where c and z represent concentration and charge, respectively.
3. (10 points) What is the short-wavelength limit, in angstroms, to the continuum
radiation produced by a X-ray tube with a molybdenum target that is impacted by
electrons driven by a 50 kV potential?
hc
 h max
The Duane-Hunt equation is: eV0 
min
Note: 1 angstrom (Å) is 10-10 meters.
4. (10 points) Describe the process of Raman scattering. What type of radiation is used,
and what effect (e.g. energy levels) are measured? To achieve this measurement, what
major components are used in Raman instruments (drawing a block diagram might help).
5.a.
(5 points) What type of energy levels are accessed by nuclear magnetic resonance
(NMR) methods?
(10 points) We discussed several 2D (two-dimensional) NMR experiments in
class. Pick one and describe it here, focusing on the analytical information that it
would give you about an organic molecule. Drawing a simple picture of a
spectrum might help with your answer.
6.
(10 points) Sketch a simple block diagram of an atomic optical emission
spectrometer (OES) that can detect many different elements simultaneously, showing
only the major components. Show the path of emitted radiation from the source to the
detector.
What types of sources might this instrument use and which would you
recommend?
7. (10 points) Compare and contrast NMR methods based on (1) the nuclear Overhauser
effect and (2) the J-coupling.
8. (15 points) How would you evaluate the following three analytical techniques for the
problem of determining zinc in all oxidation states (i.e. total Zn) in boiler wastewater at
part-per-billion (ppb or g/L) levels? All three techniques are capable of detecting Zn
via various spectroscopic effects. The techniques are XRF (x-ray fluorescence), ICPOES (optical emission spectroscopy), and fluorometry using benzoin to complex Zn2+.
Please compare approximate detection limits, types of interferences, sampling issues, cost
and portability of the instrumentation, and any other information that you think is
relevant. Answering this question in a table form may help.
9. (10 points) Describe singlet and triplet excited electronic states, their approximate
lifetimes, and their utility in analytical chemistry.
Constant
Speed of light (vaccuum)
Planck’s constant
Electron charge
Symbol
c
h
e
Value
2.99792  108 m/s
6.62608  10-34 J s
1.602177  10-19 C
Exam 1 Answers:
1. The answers to this question will be considered individually based on their logic, and
the concepts taught in the first lecture of the semester.
2. The ionic strength is calculated as follows:


1
2
 c z   c z
n
1
2
j
j 1
(0.1)(1)

1
2
2
2
j
1
2
2
1 1

 c2 z 22  c3 z 32  
 (0.1)(1) 2  (0.2)( 2) 2  (0.4)(1) 2
(0.1)  (0.1)  (0.8)  (0.4)
  12 1.4

  0.7
3. Using the Duane-Hunt equation:
hc
eV0 
 min
and inserting the values of the physical constants, we have:
(6.62608  10 -34 ) (2.99792  10 8 )
19
 1.602177  10 (50000) 
min
Thus the wavelength for 50 kV (50000 V) is:
(6.62608  10 -34 ) (2.99792  10 8 )
min 
 0.24797 angstroms
1.602177  10 19 (50000)
(The actual element involved does not affect the result for continuum radiation).
4. (This is the complete description, the student only needs to touch on each of these in
some way to get this question right). Raman scattering occurs when monochromatic
light (UV-Vis-NIR) is scattered by a molecule, and the scattered light has been weakly
modulated by the characteristic frequencies of the molecule. The incident radiation
excites “virtual states” (distorted or polarized states) that persist for the short timescale of
the scattering process. Polarization changes are necessary to form the virtual state and
hence obtain the Raman effect. The Raman process involves inelastic scattering of a
photon when it is incident on the electrons in a molecule; when inelastically-scattered, the
photon loses some of its energy to the vibrational modes (energy levels) of the molecule
(Stokes process). It can then be experimentally detected as a lower-energy scattered
photon. The photon can also gain energy from the vibrations of the molecule (antiStokes process). Raman spectroscopy actually measures the difference between the
wavelengths of the incident radiation and the scattered radiation, which appears as a
vibrational spectrum. For block diagrams of Raman spectrometers (FT and CCD, etc...),
see the lecture or text. Remember to include the laser as this is the primary modern
source used in this technique!!!
5.a. nuclear spin energy levels
5.b. A number of 2D methods were discussed in the class and in the text. A few key
examples that could be presented include the COSY experiment, the NOESY experiment,
and HETCOR experiments such as HMQC and HMBC. To help, draw a diagram
showing the general layout for COSY and NOESY (with the diagonal) and for HMQC
and HMBC (showing the proton and heteronucleus spectra along the axes). See the
NMR lectures or the text for examples.
6. See the first OES slide sets (Optical Electronic Spectroscopy Lecture 1) or the text for
a block diagram of an emission spectrometer (e.g. slide 14, 30). The text has several
diagrams in Ch. 8-9, etc... Sources include those discussed in class, namely combustion
flames, ICP torches (probably the best choice for analytical work right now), spark
sources, lasers (LIBS), and others only discussed in the text. Any one of these will suit
as long as it is thoroughly justified – ICP will be the easiest to justify given its high
temperatures and other favorable properties, and will generally get the most credit in this
question.
7. NOE methods detect proximity between nuclear spins. For example, NOE methods
can be used to determine whether two protons on a molecule are within a few angstroms
of each other. (Extra credit if the student notes that NOE experiments generally cannot
rule out with certainty that two spins are NOT near each other – they can only really
prove that two spins are near each other). Presently, NOE methods are used more
frequently for stereochemical and conformational analysis than for structural analysis. Jcoupling based methods in NMR show whether two nuclei are bonded to one another,
from a direct covalent bond out to 3,4, 5 or more intervening bonds.
They can be
tailored to detect certain bonding arrangements, e.g. the HMQC experiment detects
protons directly attached to carbons (or nitrogens, etc...), while the HMBC experiment
detects protons indirectly attached by multiple bonds. Presently, J-based methods are
used more frequently for structural analysis than for stereochemical and conformational
analysis.
8. The answers to this question will be considered individually, but here are some
general notes. The question asks for you to consider all forms (oxidation states) of zinc.
Therefore, techniques that are sensitive to oxidation states do not have an inherent
advantage; they may in fact have a disadvantage because of the need to combine the
responses/signals of all oxidation states. The fluorescence method can be eliminated
because it only detects the +2 state. The ICP-OES and XRF methods will detect Zn in
all states, but ICP-OES is more sensitive that XRF (as noted in the XRF lecture and in the
textbook). Therefore, ICP-OES is the best answer. However, other answers will
receive some credit depending on how well they analyze the problem.
9. Excited electronic singlet states are produced by absorption of radiation (in the UVVis region), followed by direct excitation to a higher electronic energy level without
changing the spin of the electron. Triplet states are accessed by intersystem crossings
and involve a flip of the spin of the excited electron. Excited electronic singlet states can
decay to give rise to fluorescence phenomena, while excited triplet states give rise to
phosphorescence. Both techniques are analytically useful forms of luminescence
spectroscopy, as they allow for highly specific emission spectra to be recorded using
naturally fluorescent molecules, or if necessary probe molecules (fluorophores). (Note –
you can give an example here of the applications of these techniques from the book or
from the lecture materials. This may help the final score for this question, especially if
the example contains a clear description of the technique).