J. Loliger ond R. Scheffold
University of Fribourg CH-1700 Fribourg, Switzerland
Paramagnetic Moment Measurements by #MR A micro technique
The technique described was elaborated for studying the kinetics of ligand rearrangements of transition metal complexes (porphin and corrin type metal complexes) (I). The assembly shown utilizes a coaxial cell unit made of an ordinary nmr tube and a melting point capillary. This specific arrangement has often been used, e.g., in nmr spectroscopy of organic substances with an external standard and was found to be applicable in paramagnetic susceptibility measurements by nmr (@. The principles of the method and the theorctical background are well known. It was first described by Evans (5) in 1958 and has since then often been recapitulated, for example by Deutsch and Poling (4) in THIS JOURNAL in 1969. Method
The met,hod is based on the fact that the resonance condition (magnetic field, frequency) in a nmr experiment for a given nucleus depends, among other factors, also upon the volume susceptibility of the medium surrounding the nucleus. For such a susceptibility measurement a set of two coaxial nmr tubes is required (Fig. 1). Both of the tubes contain a solvent system consisting of a solvent plus a cert,ain percentage of an inert "indicator" compound (Table 1). The inner 'Only valid for dilute solutions, see Evans (8). 'For restrictions see Cotton and Wilkinson ( 6 )
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tube contains in addition a known amount of the substance of which the paramagnetic moment is to be d o termincd. Owing to the different volume susceptibility of the two solutions the nuclei (protons) of the "indicator" comnound in the two com~artmentsarc differently shieided. I t can be shown that the resulting shift diiference of the absorption signals of the "indicator" nuclei in the two compartments is related to the volume susceptibility K of the two solutions and therefore to the mass susceptibility X , and the magnetic moment p of the paramagnetic substance in the inner tube. The formula for a 60 Mcps nmr spectrometer, showing the relationship of the shift difference Au to the magnetic moment p, is given in eqa. (I).' The relationship of the number of unpaired electrons N ("spin only" values) to the magnetic moment p is given in eqn. (2)2 (5)
where p = magnetic moment in Bohr magnetons, c = concentration of solute in molc/ml (moles contained in 1 ml of solution!), a = constant: 2522.10-4 mol'/' "K-'/' ml-'/~ cps-'/2 (for 60 Mcps), T = absolute temperature OK, Au = frequency difference of the two signals of the "indicator" substances in cps, and N = number of unpaired electrons ("spin only").
Table 1.
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Choice of Solvent Systems
Solvent*
Indicrutor') MELTING POINT TUBE (SEALED)
The nmr spectrometer is locked on the signal of protons which are in bold-type (signal of solvent protons in the outer eomoartment). The crosssection of the inner tube is smaller than
SOLVENT AND "INDICATOR" SOLVENT, "INDICATOR" AND SUBSTANCE WITH UNKNOWN SUSCEPTIBILITY
~ i k a l of s Drotons whmh ar; in bold-type .. are used & "indicator" Zgnals. -
Table 2.
Advantages of the Micro Technique
Advantaees
Figure 1.
Comments
5 . lo-' males of paramagnetic material is suflicient for susceptibility measurernent,s 2.10-5 malelml. ~ - (Concentration: ~ ~ ~ ~ for a meltine ~ o i n tuhe t 25 ul of solutioi are requiredy. Long time stability Because the solvent signal from the proof spectrometer tons in the outer compartment is much more intense than the solvent signd from the protons in the melting point tube, the spectrometer locks t~utamaticdly on the more intense sienal. Easilv made from eom&erciallv available Availahilitv capillary or meltingpoint tube.
Cell assembly for micro technique.
Micro quantities
.
~
~~
A
SIGNAL OF "INDICATOR'~PROTONSIN MELTING POINT TUBE
41 I=
Equations (1) and (2) being independent of the type of nuclei are also valid for the protons of the solvent. The signal of the solvent nuclei, as well as the signal of the "indicator" nuclei, are split into two absorption signals with equal shift difference Av.
Figure 2. Example Warion T-601: Two resonance signals of 2% 1-butanol (Resonancesignals of water are not shown). Outer Compartment: 2% f-butanol in water. Inner Compartment: 2% t-butanol in voter as solvent for 8 2 mo of onhvdrom CuSO. in 1 0 ml of
Coaxial Cell Unit
solution. Result.: c = 0.514 X 10-'md/ml, 1.82 IBohr m~gneton4,N= 1.07.
In our laboratory the following arrangement for paramagnetic susceptibility measurements has been elaborated. It was checked with a variety of solvents and "indicator" pairs (examples see Table 1).
-
Av = 8 . 6 spr, T = 3 1 0 ° K - p
=
dependent on the magnetic moment of the substance and their concentration (Fig. 2). This assembly has many advantages over the one in which preeision-made coaxial nmr tubes are used. They are schematically presented in Table 2.
The inner part of the coaxial cell is a simple melting point tuhe oi approximately i.d. 1.0 mm and 0.d. 1.2 mm. It is filled by a pipetor asyringe that reaches the bottomof the tubeand is then sealed off (Fig. 1). The outer part is an "ordinary"nmr tube (5.0 mm0.d.).
Acknowledgrnenl
By carefully adjusting the solvent level of the outer compartment, the melting point tube is automatically centered, while the nmr tube is spun in the spectrometer--no spacer is needed. The unfilled space in the upper part of the melting point tube produces a buoyancy so that its weight is reduced. The net weight of the inner tube is made to be just in excess of the counterbalancing buoyancy by adding or taking away some of the solvent in the outer compartment. The fine adjustment of the y-resolution of the spectrometer and the neat centering of the melting point tube is readily verified by the sharpness and the wiggles of the more intense "indicator" proton nmr signal (signal of "indicator" protons in the outer compartment). The lineshape of the small signal (signal of "indicator" protons in the melting point tube) is more or less affected by the paramagnetic material, being
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SHIFT DIFFERENCE
We wish to thank Professor Dr. A. V. Zelewsky for drawing our attention to this method first employed by Evans (5). General References
sr~wooo,P. W.,
"Magnatochemirtry" (2nd ed.1. Interaoienoe Publ.. Ino. New York. 1956. E u s ~ s rJ. , W.. FEENEP. J., AND SUTCLIFF, H. H.."High Resolution Nuclear Magnetic Spectroscopy," Pergamon Preas, Oxiord 1967. Vol. 1, pg. 65.
Literature Cited S o n r r r o ~ oR., . LdrmEn. J.. Helv. Chim. A d o , t o bewbliahed. (21 B ~ n r u R. o o s e r . J. Cnm. E m c . . 49, 297 (19721. See hiso P m v o s ~ . L. R . . J ~ m n r s R. , V . , J. CHEN.EDUO. 4 5 , 6 7 5 (1968); they use melting point capillaries for mioro sample nmr. (3) EYANB, D . F.,Pmc. Chem.Soe., 115 (1958). EVANS, D . F . , J. Chem.Soc., 2003 (1959). J. L., POLIN.,S. M.,J. CHZM.EDUC., 46,167 (1969). (4) DBUTBC., (51 C o n o s , F. 8 . . WILK~NBON, G . , "Advanced Inorganic Chemistry." (2nd ed.1, IntentoiencePubl., Ino.,New York, 1966, pg. 633. (1)
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