XII. Electrochemistry
A.Electrochemical Sample Preparation
A solution for electrochemical measurements has at least three components: 1) the compound under investigation, 2) solvent and 3) supporting electrolyte. For cyclic voltammetry (CV), the concentration of the compound of interest varies between 0.001 M to 0.005 M depending on the electron transfer rate of the compound at the working electrode. The higher the concentration the better the signal to noise ratio of the CV. The concentration of the supporting electrolyte is normally 0.1 M; which is sufficient to decrease the cell resistance to a negligible quantity.
Electrochemical measurement is one of the most sensitive techniques and trace amounts of impurities may show up in the measurement. Therefore, sample preparation is very important in electrochemistry. Each one of the three components may be a source of impurities. The solvent and electrolyte, as well as the compound under study, should be reasonably pure.
The following sections will discuss some concerns about solvents, electrolytes and reference electrodes.
1.Solvent
The concerns about solvent in electrochemistry are: voltage limits or potential window, solubility of the compound under study and physical-chemical properties such as donor or solvating properties. Figure 12-1 shows the estimated potential ranges of aqueous and nonaqueous solutions.1 It should be realized that the voltage limits in Figure 12-1 can be reached only when both the solvent and electrolyte are extremely pure. The electrochemical experiment should be done under an inert atmosphere (N2 or Ar). Presence of trace amount of oxygen in all solutions and moisture in nonaqueous solutions may lead to new oxidation and/or reduction peaks in CV.
Table 12-1 gives the physical properties of some common solvents used in electrochemistry.2 Acetonitrile is one of the most frequently used solvents. Although somewhat difficult to purify and dry, acetonitrile is stable on storage after purification. A procedure to produce super dry acetronitrile is given in literature.3 For general purifications of solvents, one may refer to Section IX, "Solvent Purification", in this manual.
2.Supporting Electrolyte
a.The role of the supporting electrolyte.
The use of an indifferent or "inert" supporting electrolyte is indispensable in electrochemistry and affects the solvent medium in several ways: (a) it regulates cell resistance and mass transport by electrical migration; (b) it may control or "buffer" the level of hydrogen ion activity in solution; (c) it may associate with the electroactive solute, as in the complexing of metal ions by certain ligands; (d) it may form ion-pair or micellar aggregates with the electroactive species; (e) it largely determines the structure of the double layer and (f) it may impose positive or negative voltage limits because of its redox properties.
b.Choice of a supporting electrolyte.
In principle, any material which dissociates in a solvent into an ion pair can serve as a supporting electrolyte. For organic solvents, the commonly used electrolyts are tetralkylammonium salts such as [n-Bu4N][BF4] (TBABF4) and [n-Bu4N][PF6] (TBAP). The electrochemically pure electrolyts can be purchased from Aldrich. Since these salts are very hygroscopic, they need to be vacuum dried at 150°C for over 24 hours before being used or stored in the dry box.
3.Reference Electrodes
The commonly used reference electrodes are silver/silver chloride (Ag/AgCl) and calomel electrodes (Hg/Hg2Cl2). Figure 12-2 shows the potentials of these and other reference electrodes.1 These electrodes should always be kept in an aqueous solution containing KCl when not in use. Prolonged contact with organic solvents may destroy these reference electrodes.
A reference electrode based on the Ag/AgCl couple for use in non-aqueous media, where H2O as an impurity is an extreme precaution, can be easily prepared. First a silver wire is dipped into concentrated HNO3 (to ionize surface silver), then the wire is removed from the HNO3 solution and placed into concentrated HCl (to metathesise NO3- for Cl-) leaving surface bound AgCl. The Ag/AgCl wire is rinsed with H2O, acetone and dried. This reference electrode (quasi - Ag/AgCl reference) must be prepared each time before use and checked against a known reference such as a calomel electrode or the FeCp2/FeCp2+ couple to standardize it. Another non-aqueous reference electrode uses the FeCp2/FeCp2+ couple at platinum electrodes.4
4.Polishing Electrode
The commonly used working electrodes include Pt, Au, or glassy carbon disk electrodes. These can be purchased from BAS. It is important to polish the electrode surface before use. This can be accomplished by putting a few drops of water/1.0 micron alumnina slurry onto a polishing pad. The electrode is polished using a figure 8 motion to avoid grooving the electrode. In cases where the electrode has larger scratches, 1, 3, 5, or 15 micron diamond paste can be used, first working down incrementally until the 1 micron alumina can be used. All needed components can be obtained from BAS. A complete polishing kit carries the post no. MF-2060. Once the electrode has been polished, it should be rinsed with water, then sonicated in autonitrile to remove any residual particulate matter.
5.References
1.Bard, A. J. and Faulkner, L. R. "Electrochemical Methods, Fundamentals and Applications", John Wiley & Sons, New York, 1980.
2.Sawyer, D. T. and Roberts, J. L. "Experimental Electrochemistry for Chemists", John Wiley & Sons, New York, 1974.
3.Walter, M. and Ramaley L. Anal. Chem. 1973, 45, 165.
4.Koepp, H. M.; Wendt, H.; Strehlow, H. Z. Elektrochem. 1960, 64, 483.
B.Electrochemistry Equipment
1.Cyclic Voltammetry
Notes:
To run a C. V. in the dry box, simply connect the leads to the extension plugs on the side of the dry box. The cell is then set up in the usual way using the wires in the dry box. A non-aqueous reference electrode must be made by dipping a silver wire in HNO3 and then in HCl.
Procedure:
1. Check Set-up of Equipment.
a.Output from right of polentiostat attacked by BNC cable to Y input of chart recorder.
b.Connect BNC T-joint to left Ext.-IN on rear of potentiostat.
c.Connect one BNC cable from T-joint to signal output on universal programmer.
d.Connect another BNC cable from T-joing to X input of chart recorder. Connect the wires to reference (green wire), working electrode (red wire) and counter electrode (electrometer probe wire)(Figure 12-3).
2.Turn on:a. power of potentiostat/GA luanostat.
b. line of plot, paper hold.
c. power of programmer.
**** When not recording, the servo should always be at standby. ****
3.Define the zero point of plot as follows:
a.turn the servo "on"
b.push down the check buttons of X and Y
c.the place where the pen point is located is zero
d.the zero point can be changed by turning the zero switch of X and Y.
e.mark your zero point with a cross
4.Define the scan direction.
─ +
───────────┼──────────set X at +RT
set Y at -UP
+ ─
───────────┼──────────
set X at -RT
set Y at +UP
To verify the correct scan direction, connect a resistor to the potentiostat as shown in Figure 12-4 then scan according to Step 7. Watch the direction the pen moves and also watch how the potential changes on the potentiostat.
5.Define the initial and final potentials on the programmer. Put the operation mode of the potentiostat at "direct" and flip the cell selector switch to "Ext. Cell". The number shown on the screen is the open circuit potential. Set the initial potential of the programmer close to this number. Depending on the scan direction, the final potential should come close but not pass the initial potential.
6.Define the upperlimit and the lower limit on the programmer. The upper limit and lower limit are usually defined by the solvent being used. Usually they are ± 1.4 V.
7.Scanning: The lights of "one cycle" and "initial" on the programmer should be on. Turn on the servo of the X-Y recorder and switch the operation mode of the potentiostat from "direct" to "CONTR. E". Put the cell selector switch to "Ext. Cell" and put the pen down. Push the "activate" button on the programmer. After the scanning is finished, put the cell selector switch back to "off" and turn the servo off.
8.Finishing: Write down the following:
a. concentration of solution
b. solvent
c. supporting electrolyte
d. ranges of potential and current
e. current follower
f. initial potential and direction of scan
g. scan speed
h. working, reference and counter electrodes
i. x and y settings.
2.Rotating Disk Voltammetry
1.Connect the motor's 6 pronged lead into the outlet for the Pine Instrument ASR speed control. The speed control allows adjustment of the rotating speed.
2.Unscrew the top pulley at point 1, Figure 12-5. The working electrode can now be inserted into the hole at point 2. Now screw the pulley back down in order to secure the electrode.
3.The electrochemical cell can now be set up as described in the cyclic voltammetry section using the working electrode attached to the rotator the green working electrode lead should be clamped onto the black outlet at point 3). The final connection for the working electrode should now be completed. This is accomplished by flipping the silver colored screw forward at point 4. This will enable contact between the electrode and the lead via a carbon rod.
C.Coulometry
1.Cell Set-up
1.Two "L" shaped cells should be clamped together with a Nafion membrane between the two links (Figure 12-6).
2.The reference and counter electrode should be in one side of the cell while the working electrode should be in the other side (Figure 12-6).
3.A solution of the complex under study should be prepared as in the cyclic voltammetry section paying special attention to the quantity used. This solution should then be placed in the half of the cell which contains the working electrode.
4.A counter oxidant/reductant solution should be prepared (i.e. ferrocene/ferrocinium) and this solution should be placed with the reference and counter electrodes. Don't forget to add the supporting electrolyte.
2.Potentiostat and Y-Recorder Set-up
1.Turnoff the "ext. sig." inputs because the programmer will not be used.
2.The left side of the potentiostat will be used. (i.e. the "applied potential/current" section.)
3.One can use either section A or section B.
4.Set the potential that will be used.
5.Operating mode should be at CONTR E.
6.The Y-recorder should be connected to the Y lead which was originally attached to the y axis of the X-Y recorder. The X axis lead will not be used.
3.Run
1.Turn on Y-recorder. Be prepared to change the sensitivity.
2.Push the A (or B depending on which dials you set the applied potential), button and then switch to "ext. cell".
XIII. Infrared Spectroelectrochemistry
A. Infrared Sample Preparation
1.KBr Pellet
a.Dryness of KBr
Before used, the infrared grade KBr should be dried under vacuum at ca. 100 °C for one day.
b.Sample Preparation
For a solid compound, as much as the tip of a micro spatula with three or four times that amount of KBr is ground to a very fine powder. The powder is transferred to a KBr pellet press. The bolts of the KBr press are pressed to 30-40 newtons with a torque wrench. After several minutes, the bolts are loosened. A transparent IR sample is prepared. The useful IR range for KBr pellet is 4000 - 400 cm-1. After used, the KBr press and bolts should be cleaned first with H20 to remove any traces of KBr, then rinsed with acetone to remove any H20. Note: KBr is highly hygroscopic. It is recommended to prepare a sample in the glove bag even when the solid compound is air stable because H20 IR bands may obscure the IR bands you are looking for.
2.Nujol Mull
A solid compound is ground together with two or three drops of nujol (parafin oil). The liquid is applied to a NaCl plate with a rubber policeman. The other NaCl plate is placed on top of the first and pressed together to make sure no air is in between. The available IR range for nujol is 4000 - 400 cm-1 except in nujol band ranges 3000 - 2750 cm-1 and 1500 - 1350 cm-1 (Figure 13-1).
3.Solution
The IR cells with CaF2 window have the available IR range of 4000 - 1000 cm-1.
a.Air Stable Solution
IR cell is filled fully with a solution. If there are bubbles inside, one end of cell is raised for bubbles to come out. Teflon pulgs are put on to seal the cell.
b.Air Sensitive Solution
If the IR sample is prepared in the dry box, the procedure is the same as in a. However, when a sample is going to be prepared outside of the dry box the procedure is as follows: Two small rubber septa for 5 mm NMR tubes are placed on entrances. One entrance is connected to the N2 line by a needle while the other is connected to a bubbler via a needle. N2 gas is allowed to pass through the cell for several minutes. The needle of the N2 line is taken off and the solution is injected into the cell via a syringe.
c.Solution in the Thin Cell
If the stretching frequency you are looking for is in the stretching region of the solvent, a concentrated solution in a thin cell has to be prepared. The cell is so thin that the solution has to be injected in.
4.Fluorolube
Fluorolube is a saturated chlorofluorocarbon oil. The usable IR range for fluorolube is 5000 - 1430 cm-1 (Figure 13-2). IR samples are prepared as mulls in the same way as nujol mulls (section 2).
5.CIRCLE Cell
Cylindrical Internal Reflection. This method is suitable for routine analysis of aqueous solution. The reliable range is 3200 - 750 cm-1. Approximately 8 ml of solution is required for the macro CIRCLE while 2 ml of solution is required for the micro CIRCLE. Normally a concentration of 0.5% is needed.
6.Reflectance
This method is very useful when solvents have a strong absorbance, such as H20.
a.Reflection (see Figure 13-3). The cell looks like a syringe without the Luer lock tip. The cell is filled with 10 - 15 ml solution. The cylinder is then pushed down to make a thin layer solution.
b.Total Reflection (see Figure 13-4). This method is not commonly used.
XIV.IR Spectroscopy
For detailed instructions see 3600 IR DATA Station Self-Teaching Course.
A.Operation
a.Put sample in compartment.
b.Allow about 3 min for moisture to be purged from compartment.
c.Press SCAN key.
d.Type X
e.l.Type in number of scans.
2.Type in upper scan limit.
3.Type in lower scan limit.
4.Press RETURN key.
f.Type Y to start scanning.
- For solvent subtraction go to step O.
g.To clear screen of characters press CLEAR SCREEN key two times.
h.To display cursor press CURSOR key.
i.To move cursor push arrow keys.
j.To display grid on spectrum press GRID key.
- Most spectra are saved on floppy disk due to the high price of IR paper.
k.To save spectrum on floppy disk insert formatted* disk into disk drive 1.
1.Press SAVE key.
2.Type X.
3.Type in File ID; up to 5 characters. You must start File ID with a letter not a number.
* To format disk see Self-Teaching Course book pg. 1-4.
4.Type in identification; up to 60 characters long.
EXAMPLE:
SAVE X MKR37 IR2(C0)4 DMPM2 + H2
5.Press RETURN key.
l.To plot spectrum - type in SET SCALE ABSC 0.5 To set paper size.
l.Turn on Printer-switch on left hand back side of printer.
2.Press PLOT key.
3.Type X.
4.Type in upper limit of spectrum.
5.Type in lower limit of spectrum.
6.Type in 1 for solid line print out.
7.Type in B for borders on spectrum. In same field, type in C for printout of scan conditions.
8.Press RETURN key.
9.Type in Y.
m.To remove paper, press down on tear bar and tear paper off. Be careful - if you break the tear bar it costs $600 to be repaired.
n.Turn off printer and replace cover.
o.To subtract out solvent peaks load appropriate solution cell background disk into disk drive 1.
l.Press RETRIEVE key.
2.Type Y.
3.Type in solution background desired.
EXAMPLE:
RETRVE Y A18A
A=Cell A 18=DMSO A=Flat background
4.Press DIFF key.
5.Press RETURN key - spectrum is saved in memory Z in absorbance mode.
6.To convert memory Z into transmittance mode:
Press TAAT key.
Type Z.
Press RETURN key.
7.Press VIEW key.
8.Type Z.
B.Maintenance
About every 30 days check the screen on the rear of the electronics module. When the screen is dirty remove the screws and clean the screen as per the instructions on p. 4.5╧
About every 30 days check the CO2 and H20 content of the background spectrum. When the CO2 or H20 levels are unacceptable, replace the molecular sieves in the optical module as per the instructions on p. 4.5╧. At this time we only have one molecular sieve packet. The spare sieve packet should be placed in the IR while the two original sieve packets are regenerated. After regeneration the two original sieve packets should be placed back in the IR.
╧ 1700 series Infrared Fourier Transform Spectrometers Operator's Manual
(Green Book).
XV.Crystal Infraredspectroscopy
Not available at this time.
XVI.UV-Vis Sample Preparation
A.Solvents
The solvents used should be very pure and dry; they are usually available from the solvent stills or from the stockroom as spectro-grade. The UV cut-off (see Table) is the wavelength for which there is an absorbance > 1. Quantitative analysis should not be attempted below these wavelengths when working with various solvents.
TABLE Ultraviolet Cutoffs of Spectro-Grade Solvents
(lO-mm path vs. distilled water)
───────────────────────────────────────────────────────────────────────────
wavelength, wavelength,
Solvent nm Solvent nm
───────────────────────────────────────────────────────────────────────────
Acetic acid 260 Glycerol 207
Acetone 330Hexadecane 200
Acetonitrile 190Hexane210
Benzene 280Methanol210
l-Butanol2102-Methoxyethanol 210
2-Butanol260Methylcyclohexane 210
n-Butyl acetate254Methylethyl ketone330
Carbon disulfide380Methyl isobutyl ketone 335
Carbon tetrachloride 2652-Methyl-l-propanol230
l-Chlorobutane220N-Methylpyrrolidone285
Chloroform (stabilized Pentane210
with ethanol)245Pentyl acetate212
Cyclohexane 210l-Propanol 210
1,2-Dichloroethane 2262-Propanol 210
Dichloromethane235
1,2-Dimethoxyethane 240Pyridine330
N,N-Dimethylacetamide 268Tetrachloroethylene
N,N-Dimethylformamide 270 (stabilized with thymol) 290
Dimethysulfoxide 265Tetrahydrofuran 220
l,4-Dioxane215Toluene286
Diethyl ether2181,1,2-Trichloro-1,2,2-tri
Ethanol210 fluoroethane 231
2-Ethoxyethanol2102,2,4-Trimethylpentane 215
Ethyl acetate255o-Xylene290
Ethylene chloride228 Water 191
───────────────────────────────────────────────────────────────────────────
B.Solution Concentration
The concentration should be selected so that the most intense absorption peak or the absorption peak of interest has an absorbance of ≤ 1.0. This may be calculated using the Beer-Lambert Law if the molar extinction coefficient is known. Normally, solution concentrations fall in the range of 10-3 - 10-5 M. If the extinction coefficient is not known, the absorbance of at least three different solutions of known concentration may be plotted against concentration. The slope of the line will give the extinction coefficient.
Beer-Lambert Law
A = -log T = εbc
b ≡ cell path length in cm
ε ≡ molar extinction coefficient in M-1 cm-1
c ≡ solution concentration in M
C.Cells
There are two types of cells available in the lab, glass and quartz. The glass may be used down to wavelengths of about 310 nm and the quartz down to about 180 nm. The path length of each cell is near enough to 1.000 cm to be considered matched. The matching error of a pair of cells can be checcked by filling both with solvent and running a spectrum of one against the other. The rectangular curvettes with either teflon or glass caps may be used for air-stable solutions and reactions. For air sensitive solutions the freeze-pump-thaw cells should be used as sample cells with the curvettes used as reference cells.
D.Preparing Air-Sensitive Solutions
Freeze-pump-thaw cells are used when either the starting compounds or the photogenerated species are air-sensitive. Solutions of air-sensitive compounds may be prepared with degassed solvents and transferred to the cells in the dry box. For solutions in which the starting compound is air-stable but the excited state or photogenerated species are air-sensitive, the solutions may be prepared on the bench and then degassed using the freeze-pump-thaw technique. This is done using the high vacuum line. About 4ml of solution is placed into the freeze-pump-thaw cell (solution should not quite fill the test tube and should never extend up into the side arm). The stopcock is closed. The cell is attached to one of the flexible lines with Cajon fittings and clamped into place (Figure 14-1).
The Cajon tubing is opened to the line and evacuated (vacuum gauge should jump up to about 1 atm then slowly move down to about 0μm - if gauge does not go below 15μm check the connection for a leak). The solution vial is placed in a dewar of liquid nitrogen to freeze the solution (the solution is frozen when the liquid nitrogen ceases to boil violently). The stopcock on the cell is opened (again the vacuum gauge should jump to ~1 atm then slowly return to about 0-2 μm). When the cell has been evacuated the stopcock is open and the cell is removed from the liquid nitrogen. The solution is allowed to thaw; this may be hastened by rinsing the outside with a stream of acetone. The process is repeated two more times for a total of three freeze-pump-thaw cycles. When the sample has thawed, the stopcock to the vacuum is closed and the one to nitrogen opened. The cell is filled with nitrogen and disconnected from the line. When taking UV-vis spectra with freeze-pump-thaw cells, it is necessary to cover the sample compartment with a black felt cloth since the cells protrude out of the sample compartment.
E.Cleaning Cuvettes and Freeze-Pump-Thaw Cells
Do not touch the polished faces of the quartz cuvettes. Clean cuvettes with acetone and place in oven to dry (freeze-pump-thaw cells only) or air-dry. Clean polished faces with lens paper moistened in methanol when needed. When necessary the cuvettes may be cleaned with aqua regia or acidic peroxide for no more than 1 hour to remove metal plating from interior. Rinse well with deionized water followed by acetone. Follow the same procedures for the freeze-pump-thaw cells. Rinse stopcock plugs with acetone and air-dry. Inspect o-rings and replace when they are broken, badly shredded or fail to make a seal.
F.Solid Samples
Spectra of crystalline samples may be obtained by making a dilute nujol mull. The mull is then spread onto filter paper using a rubber policeman. The filter paper strips should be about 1 x 6 cm to fit in front of the cell holder. The spectra are then taken against an air reference. These spectra may not be used for quantitative analysis.
XVII.UV-VIS Spectrophotometer
A.Operation
a.Reference Manual: Uv-Vis 9420 Spectrophotometer User's Manual (located on shelf above the instrument).
b.Power-Up and Initialization.
1.Check that sample compartment is empty.
2.Turn on power switch (red switch to the rear of sample chamber.
3.Initialization display will appear on CRT - initialization procedure takes approximately 17 minutes.
c.Recording an Absorbance Spectrum.
1.At completion of initialization instrument will be in METHOD 1 - the spectrum mode. One may also get into METHOD 1 by pressing METHOD 1 ENTER.
2.Setting the Parameters.
a.Press DISPLAY LIST ENTER - experimental parameters will appear on CRT.
b.Setting experimental parameters:
1.ä Scale XX ENTER. Option of 0.5, 1, 2, 5, 10, 20 or 40 nm/cm scale on plotter - 40 nm/cm normal setting.
2.ä Set "Upper Limit" ENTER "Lower Limit" ENTER. Option of wavelength region from 900 nm to 185 nm.
3.SCAN SPEED XXX ENTER. Option of 4, 10, 20, 40, 100, 200 or 400 nm/min scanning speed - 200 nm/min normal setting.
4.%T.ABS X ENTER. Option of 1 for transmission mode or 2 for absorbance mode - absorbance mode is normal setting.
5.%T/Abs SCALE "Lower Limit" ENTER "Upper Limit" ENTER. Option of -500.0 to +500.0% for transmission mode and -5.0 to +5.0 for absorbance mode.
6.CHART SET X ENTER. Option of 1 for sequential plotting, 2 for overlap plotting or 3 for plotter off - off is normal setting.
3.Place the sample cell in the front cell holder and the reference cell in the rear cell holder.
4.Press MEASURE - spectrum will be taken and displayed on CRT.
Note:If Absorbance goes off scale you do not have to redo spectrum, you can change the scale in Method 11. The off scale data is stored in memory even though it is not displayed on the CRT.
5.Press METHOD 11 ENTER - Method 11 is the data processing mode.
6.Press MEMORY 1 ENTER - Spectrum from Method 1 are automatically stored in Memory 1: spectrum will appear on CRT.
7.If you want to change the absorbance scale enter the new scale as - % T/Abs Scale "Lower" ENTER "Upper" ENTER (lower limit is - 5 and upper limit is 5) then MEMORY 1 ENTER.
Note:You can not change the wavelength range, if you want a new range you must retake the spectrum in Method 1.
8.To print spectrum press CHART SET 1 ENTER then MEMORY 1 MEASURE.
9.To find peaks you can now move the cursor with the cursor -> or cursor <- keys, the wavelength and absorbance valve will be displayed on the CRT - to print this value press PRINT.
10.To print a parameter list press PRINT LIST.
11.Press CHART↑ to advance paper.
12.Record lamp time in log book.
Note:Lamps are expensive - do not leave the lamps on if you are not using the instrument.
d.Shut down
l.Remove cells from sample compartment.
2.If you will be taking more spectra and do not want to reinitialize each time you can turn off only the lamps by pressing METHOD 51 ENTER then 1 4 ENTER - this selects no light source. Later the lamps may be turned on by pressing 1 1 ENTER.
3.If you will not be using the instrument in the next few hours you can turn it off by turning off the power switch.
Note:All data stored in memories will remain with lamps off - data does not remain with power off.
e.For use of other modes and other applications see the users's manual
B.Maintenance
a.Changing Paper - see p. 1-9 to 1-11 in User's Manual
b.Desiccant Tubes - see p. 1-11 in User's Manual
c.Resources: User's Manual and Service Manual
d.IBM has sold their UV-VIS instrument division to Nicolet - all questions concerning parts and operations that cannot be found in the manuals should go to Nicolet:
Nicolet Inst. Corp.
255-1 Verona Rd.
Madison, WI 53771
1-800-356-8088
C.Magnetic Moment Determination by NMR
Measurement of magnetic moment by NMR spectroscopy has several advantages over the classic Gouy method. A considerably smaller sample is required, and in general measurements are made faster and more easily.
This technique is based on the principle that the position of a nuclear resonance depends, among other factors, up on the magnetic susceptibility of the medium surrounding that nucleus. Thus, the resonance peak of a solvent will be shifted if some paramagnetic substance is in solution. It is precisely this shift in resonance frequency that is our probe into the magnetic susceptibility of a dissolved compound. If solvent resonances are nonexistent or inconvenient to measure, some "indicator" compound is added to the solution and its frequency shift is measured.
Measurements are made using two coaxial tubes as shown in Figure 17-1. The inner tube, made of a sealed capillary, contains a known concentration of the paramagnetic substance in an appropriate solvent, plus some indicator compound if needed (See Table 17-1). The outer NMR tube contains only the solvent and indicator compound. By carefully adjusting the solvent level in the outer tube, the net weight of the inner tube is made just in excess of the buoyancy from the surrounding solvent. This slight negative buoyancy automatically centers the inner tube while spinning, so no spacer is needed.
When the NMR spectrum is scanned, the protons of the indicator compound come into resonance at different field strengths, as they are differently shielded due to the presence or absence of the paramagnetic substance. This frequency difference then, can be used to calculate magnetic moment.
For dilute solutions, the molar susceptibility of the dissolved paramagnetic substance can be well approximated by the expression:
where Xm is the molar susceptibility in cm3 mol-1; Δυ is the frequency separation of indicator peaks, in Hz; υo is the frequency (in Hz) at which the resonances are being studied (υo is 90 x 106 Hz for the PE - R32 instrument); m is the mass (in grams) of paramagnetic substance in 1 ml of solution; χo is the mass susceptibility of the solvent system (from Table 17-1); and M is the molecular weight of the compound being studied.
The "spin only" calculation for molar susceptibility is given by:
where N is Avogadro's number (6.022 x 1023 mol-1), β is the Bohr magneton, 9.2741 x 10-21 erg gauss-1; k is the Boltzmann constant, 1.3807 x 10-16 erg K-1; T is the absolute temperature; and μ is the magnetic moment in Bohr magnetons.
Rearrangement of equation (2) and determination of constants at T = 298K leads to an observed magnetic moment in Bohr magnetons of:
where χm is calculated from equation (1). This observed magnetic moment may be related to the number of unpaired electrons, n, by the equation:
Note that in the frequency shift determination we are interested only in the resonances of indicator (or solvent) protons, not in those of the compound under investigation. The mass susceptibilities of various solvent systems and suggested indicator compounds are compiled in Table 17-1. This correction essentially considers the diamagnetic contribution of the solvent (not considering the indicator compound) and is to be used for solvent systems with <8 - 10% indicator compound.
The NMR method of magnetic moment determination generally gives good results. When measurements are carefully made, peak separations down to 1 Hz may be measured, and magnetic moments at least as good as the Gouy method are obtainable.
D.Procedure for NMR Determination of Magnetic Moment
1.Choose appropriate solvent system (refer to Table 17-1) and prepare a known concentration of compound in a small volumetric flask, using deoxygenated solvents when possible (0.005 to 0.05 M solutions generally suffice).
2.With a syringe and a long, thin needle, fill a capillary tube 3/4 full of solution starting with needle at the bottom of the capillary to preclude air bubbles.
3.Seal the capillary with a burner. Place the capillary in an NMR tube and adjust solvent level to poise the capillary.
4.Run the NMR spectrum. Spectra are usually run unlocked and with the scale expanded.
5.Determine peak separation (in Hz) of inner and outer tube solvent lines. Calculate molar susceptibility of compound by:
where:Δυ = peak separation in Hz
m = grams compound per ml of solution
υo = spectrometer operating frequency (90 x 106 Hz for the PE-R32 instrument)
χo = solvent system susceptibilities from Table 17-1
M = molecular weight of compound
6.Calculate magnetic moment μ from:
If the temperature is too far removed from 298 K, recalculate the constant as given in the text.
7.Number of unpaired electrons n, is given by:
E.Sample Calculations - Magnetic Moment Determination
Sample: Cr(CNBu)6(PF6)2, M = 840.729
Solvent/Indicator: CH2Cl2/solvent peak
(χo = -0.549 x 10-6)
Sweep Range: 100 Hz
Sensitivity: 1/4
Peak Separation: 12.70 Hz
Concentration: 0.072 g/5.0 ml solution
(m = 0.0144 g/ml)
E.References and Notes
1.D.F. Evans, J. Chem. Soc. 2003(1959).
This is the original paper in which the method is developed and probably is the best paper here.
2.J. Lölinger and R. Scheffold, J. Chem. Ed., 49, 646(1972).
These authors do not make the magnetic correction for the solvent system, which will cause problems in compounds with low magnetic moments.
3.John L. Deutsch and Stephen M. Poling, J. Chem. Ed., 46, 167(1969).
This reference contains a good theory discussion.
4.Richard L. Carlin, J. Chem. Ed., 43, 521(1966).
This is a general reference on magnetic susceptibilities.
5.Paul B. Dorain, "Symmetry in Inorganic Chemistry", Addison-Wesley Publishing Co., Reading, MA, 1965: pp 100ff.
6.Alan Earnshaw, "Introduction to Magnetochemistry", Academic Press, London, 1968.
This text is a good general reference.