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exam_1_practice_B_w16.pdf-Chem 260/261 PRACTICE B First Exam
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exam 1 practice B w16.pdf-Chem 260/261 PRACTICE B ...
exam_1_practice_B_w16.pdf-Chem 260/261 PRACTICE B First Exam
exam 1 practice B w16.pdf-Chem 260/...
exam_1_practice_B_w16.pdf-Chem 260/261 PRACTICE B First Exam
Page 5
Name_________________________________
Chem 260/261
PRACTICE B First Exam 02/16/16
5
10.) Short Problem (15 points):
The Li atom can have its electrons in the 1s, 2s, and 2p orbitals.
Z
eff
(2s) is 1.26 while Z
eff
(2p) is 1.02. Calculate the energy difference between the 2s and 2p
orbitals in Li in Joules.
Δ
E =
J
Given your answer above, what wavelength of light would be required to excite an electron from
the 2s to 2p orbital. Give your answer in nm.
λ
=
nm


Page 6
Name_________________________________
Chem 260/261
PRACTICE B First Exam 02/16/16
6
11. Mastery Problem (30 pts):
In the space available solve 1 of the following 2 problems.
Indicate the question you want graded on the cover sheet.
Partial credit will be given for work
shown in the indicated area only.
You must show how the correct answer was obtained or no
credit will be given.
(11-A)
Cyanine dyes are used in industry and in biotechnology for labeling and analysis. They
are fluorescent and have many uses in biomedical imaging. Depending on the structure, they
cover the spectrum from IR to UV.
Cy3 and Cy5 are two of the most common cyanine dyes. They are usually synthesized with
reactive groups on either one or both of the nitrogen side chains so that they can be chemically
linked to nucleic acids or protein molecules.
One of these dyes absorbs at 550 nm and the other at 649 nm as shown in the spectrum below.
(I) (7 pts) Label the spectra indicating which dye corresponds to which spectrum. Explain your
choice. (Hint – the
π
electrons can be modeled using particle in a 1-D box.)


Page 7
Name_________________________________
Chem 260/261
PRACTICE B First Exam 02/16/16
7
(II) (8 pts) Using a particle in a 1-D box model estimate the length of Cy3 using the peak
absorption of the spectrum you’ve labeled above (550 nm or 649 nm).
Cy3 has 20
π
electrons.
Length (pm)
(III) (8 pts) The molar extinction coefficient of Cy5 at its maximum is 250,000 L cm
-1
mol
-1
. If a
1 cm sample cell has a transmittance of 25%, calculate the concentration of Cy5 in moles/L.
Concentration (Mol/L)


Page 8
Name_________________________________
Chem 260/261
PRACTICE B First Exam 02/16/16
8
(IV). (7 pts) In a paper published in Angewandte
Chemie 48, 299-303 (2009) Kundu et al.
demonstrated that hydrolyzing the cyanine dyes produced a species that could be used as a
sensor for reactive oxygen species in living cells. The hydrocyanines do not fluoresce in the
visible while the corresponding cyanines fluoresce strongly.
Reaction of hydrocyanine to form the cyanine
Fluorescence from hydro-Cy3 before and after
adding a reactive oxygen species.
Based on what you know about particle in a 1D box,
π
electrons, and spectroscopy, suggest a
reason why the hydro-Cy3 compound does not absorb 535 nm light or emit in the visible.


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