CHEMISTRY 110 - Dr



SECTION I: MS and NMR (100 points)

1) (15 points) Determine the structure for compound: C9H12O. NMR Solvent is acetone. Relative intensities (integral) of 1H NMR integrals and splitting patterns are indicated.

2) (15 points) Which of the following compounds would be expected to show spin-spin splitting in their 1H NMR spectra?

a) (3 points) [pic] or [pic]

b) (3 points) [pic] or [pic]

c) (3 points) [pic] or [pic]

d) (6 points) Draw a stick diagram indicating the peak splitting for each hydrogen in:

H3C (triplet) HCCl2 (triplet) H2C (doublet of quartets)

| | |

Both coupled to CH2 (N+1)

First, doublet due to coupling to HCCl2

Then quartet due to coupling to CH3

3) (20 points) Please answer the following general questions about NMR.

a. (4 points) Place the following nuclei in order of INCREASING NMR sensitivity: 1H, 12C, 13C, 14N, 15N (i.e., which NMR spectrum will have the highest signal-to-noise?).

12C,14N < 15N < 13C < 1H

b. (4 points) Which NMR spectrometer has the highest sensitivity: 300 MHz, 400 MHz, 500 Mhz or 600 Mhz?

600 MHz

c. (4 points) If the spin-spin relaxation time (T2) decreases from 100 msec to 35 msec, what happens to the NMR peak width?

Increases, v½ = 1/πT2

d. (4 points) List two methods to increase the resolution of an NMR spectrum.

Increase field strength (B0), zero-fill or apply a window function that emphasizes the end of the FID.

e. (4 points) What NMR observable is related to a dihedral angle? What NMR observable is related to a through-space distance?

J-coupling – dihedral angle

NOE – through space distance

4) (15 points) You are using MS to differentiate between two possible structures:

a. ( 5 points) What resolution would be needed to identify the correct structure?

[pic]

R =[(198.602 + 198.198)/2]/(198.602-198.198)

R=198.4/0.404 = 491.089

|Isotope |Symbol |Percent Abundance |

|1 |H |100 |

|2 |H |0.015 |

|12 |C |100 |

|13 |C |1.08 |

|14 |N |100 |

|15 |N |0.37 |

|16 |O |100 |

|17 |O |0.04 |

|18 |O |0.20 |

|32 |S |100 |

|33 |S |0.80 |

|34 |S |4.40 |

|35 |Cl |100 |

|37 |Cl |32.5 |

b. (5 points) If the observed M+2/M+ ratio is 4.4%, which is the correct structure (compound 1 or compound 2)?

Compound 1 M+2 Compound 2 M+2

C: 3 x (1.08/100)2 C: 2 x (1.08/100)2

H: 5 x (0.015/100)2 H: 5 x (0.015/100)2

Cl: 1 x 0.325 N: 2 x (0.37/100)2

O: 4 x (0.04/100)2 O: 2 x (0.04/100)2

0.325 S: 1 x 0.440

0.440

Compound 2

c. (5 points) Do the observed masses agree with the Nitrogen Rule? Explain

Yes, even mass (198) is consistent with an even number of Nitrogens. Compound 1 has zero and compound 2 has two nitrogens, both even numbers.

5) (20 points) Given the following mass spectra for D-glucose:

a. (5 points) Which spectra resulted from hard ionization? Why?

Spectrum A, since significant amount of fragmentation is present.

b. (5 points) Name a type of hard ionization

Electron-Impact source

c. (10 points) From the structure of D-glucose above, what is likely lost from the molecule to yield the molecular ion peak at 163?

Since the mass difference is 18, water is the likely fragment

6) (10 points) Briefly, describe Electrospray Ionization (ESI).

Liquid sample passed through needle at high potential (several KVs) creates fine droplets with charge. Evaporation of solvent, droplets become smaller, transfer of charge to molecule, become ions in ambient gas

SECTION II: Review (100 points)

7) (20 points) Please answer the following general questions about Spectroscopy.

a. (5 points) Place the following processes in order of INCREASING relaxation time: absorbance, fluorescence and phosphorescence.

absorbance < fluorescence < phosphorescence

b. (5 points) Place the following transitions in the order of INCREASING energy: electronic, rotational and vibrational.

rotational < vibrational < electronic

c. (5 points) According to Beer’s law, the amount of light absorbed is proportional to molar absorptivity (ε), which is a physical property of the molecule. Molar absorptivity depends on the transition probability and one other factor. What is that factor?

Cross-area of the molecule

d. (5 points) Black body radiation is a common process used in a light source. How do you create black body radiation?

Heating a filament or wire to a high temperature

8) (15 points) Please answer ONLY ONE of the two following questions:

a. Please describe how a photomultiplier tube works.

A current is generated proportional to the intensity of light that strikes the cathode coated with photoemissive material (Cs2O). A series of nine dynodes where each dynode is covered with a photoemissive material and a 90V potential is applied between each dynode. This greatly amplifies the signal. When light hits the cathode, an electron is emitted and attracted to dynode #1. This collision causes more electrons to be emitted and attracted towards dynode #2. Process continues until e- are collected at anode after amplification at 9 dynodes. One photon produces 106 – 107 electrons. Current is amplified and measured.

b. Please describe how Fourier Transfer IR (FTIR) works.

The light source is split by a central mirror into two beams of equal intensity. The beam goes to two other side mirrors, recombine and pass through sample. One side mirror is fixed and the other moves, so the two beams travel different distances before recombining. This results in constructive and destructive interference. Observe a plot of intensity versus distance (interferogram) which is converted to a spectrum by a Fourier transform. In this manner, FTIR collects absorbance for all frequencies at once.

9) (15 points) Please answer ONLY ONE of the two following questions:

a. Describe how source modulation works to remove spectral interference due to the flame in atomic absorption spectroscopy.

In order to measure the small signal change from the sample absorption against the large background of the flame, need to subtract out the flame to measure P/Po. There are two common solutions to the problem Use a chopper to alternate passing the light source (Hollow Cathode Lamp) through the sample or bypassing the sample. In this manner the observed intensity will alternate from just the flame or the sum of the flame and sample. Alternating subtracting the light intensity will result in only observing the signal from the sample

The same affect can be achieved by alternating the power of the hollow cathode lamp as a function of time (i.e., turning the lamp on and off).

b. Compare and contrast the Flame and Plasma (inductively coupled plasma-ICP) as an excitation source for atomic emission spectroscopy.

The flame is easy and cheap, but does not generate a high enough temperature for most elements besides alkali metals. Also, see flame temperature vary need to use an internal standard. AES is very dependent on temperature, because only observe emission from excited atoms where the population of excited atoms is a function of temperature.

ICP is more commonly and widely used for AES because of the much higher temperature obtained relative to a flame. Also, the temperature is more uniform resulting in better precision. ICP can rapidly analyze multiple elements and less interference.

10) (20 points) Please answer the following general questions about Chromatography.

a. (5 points) What are the two factors that determine a solute’s retention in gas chromatography?

Solute’s volatility and affinity with the stationary phase

b. (5 points) In slab electrophoresis, a biomolecule (protein, DNA, RNA) will migrate into a separate “zone” based on three physical characteristics of the biomolecule. Please identify one.

Either size, shape or charge.

c. (5 points) In liquid chromatography, solutes are pushed through columns by either gravity or pumps, how are they moved in capillary electrophoresis?

Electric field, potential or Electroosmotic flow

d. (5 points) Please name one source of peak broadening in liquid chromatography.

Either eddy diffusion, mobile phase mass transfer, stagnant mobile phase mass transfer, stationary phase mass transfer, longitudinal diffusion

11) (20 points) Please answer the following general questions about Electrochemistry.

a. (5 points) How does voltammetry differ from potentiometry and coulometry (i.e., what is measured, how it is measured and how the analyte is used)?

Voltammetry uses only a small amount of analyte while measuring current as a change in potential. Potentiometry measures potential near zero current and coulometry uses the entire sample at fix current or potential.

b. (5 points) For a typical voltammogram, why is there little to no current at the beginning of the curve?

Working electrode is not capable of reducing the analyte. Electrode potential is below the potential of the ½ reaction.

c. (5 points) There are two types of coulometry, the common amperostatic (constant current) and potentiostatic (constant potential). What is the major advantage of potentiostatic coulometry?

More specific, avoids redox of other species. Only redox at or below cell potential will take place.

d. (5 points) What is a common feature of membrane indicator electrodes (i.e., what is the general mechanism by which a membrane indicator measures a potential)?

Concentration gradient across membrane inner and outer surface., separation of charge generates a potential.

12) (15 points) Please answer ONLY ONE of the two following questions:

a. Describe one type of detector used in Liquid chromatography.

A number of possible detectors: Refractive Index Detector‚ Conductivity Detector‚ UV/Vis Absorbance Detector‚ Electrochemical Detector‚ Fluorescence Detector. Any description of how the detectors are used in LC are acceptable.

b. Provide a detailed description of sources of error in measuring a potential for an electrochemical cell (i.e., what could lead to deviations from the predicted potential from the Nernst equation?).

Any source of potential in the cell not derived from the redox reaction. This will include the liquid junction potential that occurs when two electrolytes of different ionic composition come into contact. This is caused by different rates of ion migration leads to separation of charge and a potential.

Any current (ion or electron movement) will result in a potential because of resistance. Ohm’s law E=IR. This includes mass transport of ions from the bulk solution to the electrode and non-faradic current – movement of ions to electrode surface as electrode is initially charged.

Also using concentrations instead of activity and side reactions that affectively change the activity of an ion will also distort the comparison between experimental and predicted cell potential

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1H NMR Spectra

3, s

2,d

2,d

3, t

2, q

s –singlet

d–doublet

t –triplet

q–quartet

[pic]

13C NMR Spectra

[pic]

[pic]

[pic]

[pic]

[pic]

[pic]

Compound 1

C6H11ClO5

Mass: 198.602

Compound 2

C4H10N2O5S

Mass:198.198

[pic]

[pic]

(B)

(A)

181

163

145

121

127

181

[pic]

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