Chem 455 Quiz 3
10 April 2006
Name: ___________________
Question 1 (15 Points)
Infrared spectrometry, Raman spectrometry, and Nuclear Magnetic Resonance spectrometry are
use to evaluate aspects of molecular structure through analysis of electron behavior on the
examined molecules. Each method exploits a different route for examining the electron behavior
on a molecule. Please describe how Infrared spectrometry, Raman spectrometry, and Nuclear
Magnetic Resonance spectrometry probe electron properties on a molecule and describe the
electron properties examined.
IR spectrometry
Excitation of rotational and vibrational bands in a bond by photons in the infrared region
with a change in dipole moment on the molecule. The IR information includes rotational
and vibration frequencies of groups on the molecule as well as fingerprint that are
indicative of a specific molecule and functions
Raman spectrometry
Obtains similar information to IR spectrometry but relies upon variability of polarizibility
with bond length. The electron is excited to a higher atomic level with Visible light. The
scattering includes energy differences from the excitation wavelength based on the energy
of the excited rotational states.
NMR spectrometry
The nuclear spin is exploited in NMR. When the molecule is placed in a magnetic field,
different orientations are obtained with a spin ½ nucleus. The relaxation and behavior of
the nucleus is affected by the electrons and surrounding nuclei. In spectrometry the nuclei
are shielded by electrons and differences in the shield properties, stemming from
differences in the chemical environment, can be obtained.
Question 2 (15 Points)
a. Explain triplet and singlet states of electrons
Triplet has electrons aligned in the same spin direction, while singlet states the electrons are
aligned in opposite directions.
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b. Which state is related to phosphorescence?
The triplet state has paramagnetic properties and is related to phosphorescence.
c. Draw energy diagrams showing absorbance, fluorescence, and phosphorescence.
d. If absorbance of a molecule occurs at 250 nm, provide reasonable wavelength that
may be observed for fluorescence and phosphorescence.
The fluorescence wavelength would be greater than 250 nm, and the
phosphorescence greater than the fluorescence.
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Question 3 (10 Points)
a. What is quantum yield as related to fluorescence?
Quantum Yield is the ratio of molecules that luminance to total excited molecule with
transitions mainly n->p* or p ->p* and can be described
b. What can decrease the quantum yield for a molecule?
The fluorescence of a molecule can be decreased by increasing temperature, heavy atoms
in solvent, or dissolved O2 in solution. Higher concentration of molecules can lead to selfabsorption and a decrease in observed fluorescence.
Question 4 (10 Points)
Consider the bond C=O. The molecule in question was isotopically enriched with 13C. It is not
known if the C=O group was enriched. Calculate the vibrational frequency of C=O with normal
C and 13C to determine if IR spectrometry can be used to determine if the C=O group has the
enriched C. The atomic mass of 13C is 13.00335.

C isotope
normal
13C
C amu
12.0107
13.00335
O amu
15.9994
15.9994
1
k
k
 5.3E  12
2c 

mass C (kg)
1.99513E-26
2.16002E-26
mass O (kg)
2.65771E-26
2.65771E-26

1.13962E-26
1.19158E-26
cm-1
1569.985
1535.374
The difference in the wavenumber for C=O with normal and isotopically enriched C can be
differentiated by IR since the wavenumber difference is 35 cm-1.
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Question 5 (10 Points)
a. How does molecular symmetry influence IR spectroscopy?
The allowed bending and stretching frequencies are dictated by the symmetry of the
molecule. The point group can be used to determine the modes that generate bending and
stretching observable in IR. An example is the difference in the number of IR active modes
due to linearity or non-linearity of the molecule.
b. For the two molecules CH3-CH3 and CH3-CCl3 which has an IR active C-C stretching
frequency?
CH3-CCl3 has a change in dipole moment upon stretching of the C-C bond.
Question 6 (15 Points)
Label the components of the Raman spectrum, diagram, and equation below. You should
indicate which components are the same in the spectrum, diagram, and equation.
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m   0 E0 cos(2 ex t ) 
E0
E


rm ( ) cos(2 ( ex   )t )  0 rm ( ) cos(2 ( ex    )t )
2
r
2
r
Stokes
Rayleigh
Anti-Stokes
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m   0 E0 cos(2 ex t ) 
E0
E


rm ( ) cos(2 ( ex    )t )  0 rm ( ) cos(2 ( ex    )t )
2
r
2
r
 0 E0 cos(2 ex t ) is the Rayleigh component,
E0

rm ( ) cos(2 ( ex    )t ) is the Stokes, and
2
r
E0

rm ( ) cos(2 ( ex    ) is the anti-Stokes component
2
r
Question 7 (10 Points)
A molecule containing a proton is placed in at 5 T magnetic field at 25 ºC. What happens to the
N-1/2/N1/2 ratio if:
a. If the magnetic field is increased to 8 T
b. If the temperature is increased to 45 ºC
N 1/ 2
hB
 exp(
)
N1 / 2
2kT
From the equation an increase in the numerator of the exponential (B, magnetic field)
would decrease the ratio. An increase in temperature increases denominator and increases
ratio
Question 8 (15 Points)
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a. Provide the proton NMR spectrum for CH3CH2OH with a TMS standard. Include
splitting and use the following data
1.2 ppm (CH3), 3.6 ppm (CH2), and 5.25 ppm (OH).
Include TMS peak. CH3 split 3 times by CH2, CH2 split 4 times by CH3, then twice by OH (see
pattern above), OH split 3 times by CH2.
b. How would the peak at 1.2 ppm change if the compound was CCl2HCH2OH?
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The Cl deshields the proton so the peak would appear downfield closer to the alcohol. S Since
only 1 proton is on this group it would be 1/3 of the height of the CH3 peak and the CH2 would
appear as two peaks.
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