M. T. Wans, R. E. Van Reel, and M. P. Eastman Universitv of Texos at El Poso El Paso, Texas 79912
ESR Studies of Hyperfine Interactions in DMSO-H,O Solutions
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A physical chemistry experiment
The nature of dimethyl sulfoxide (DMS0)-water solutions has been studied by a wide variety of techniqlles. These studies indicate a strong interaction between DMSO and water and the possible formation of-a 1:2 DMSO-Hz0 complex (1,Z). In the experiment proposed here simple esr measurements of the 14N hyperfine splitting of the ditertiary-hutyl nitroxide (DTBN) radical in DMSO-water solutions are used to confirm the strong interaction between DMSO and water and to suggest possible models for the DMSO-water system. This experiment can be carried out on any esr spectrometer that will handle liquid samples. No expensive or easily damaged accessories, such as a variable temperature controller, are required. The experiment is particularly appropriate for courses using Bettelheim's "Experimental Physical Chemistry'' (3), which investigates the DMSO-water system by a number of experimental methods. Background The DTBN radical (see Fig. 1) is representative of the recently discovered family of nitroxide radicals. These radicals are stable with respect to oxygen and to mild heating and are soluble and stable in a wide range of polar and nonpolar solvents. Because of their unusual properties nitroxide radicals have been utilized as "spin labels" in a wide variety of biological investigations (4). The Hamiltonian used to calculate the transition energies for DTBN in a magnetic field H can be represented by the expression (5) x = gOS,H aSJ, (1)
+
The first term in this expression arises from the interaction of the magnetic moment of the electron with the applied magnetic field. Here Sz is the operator for the comnouent of the electron soin aneular momentum alone the applied magnetic field, @ is the Bohr magneton, and g is a dimensionless factor which is equal to 2.006 for the DTBN radical (6). The second term in eqn. (1) arises from the interaction of the magnetic moment of the 14N nucleous with the magnetic moment of the unpaired electron.' In this term I, is the operator for the component of the nuclear spin angular mbmentum along the-applied magnetic field and a is the isotropic hyperfine coupling constant. This constant is a measure of the strength of the interaction between the magnetic moment of the electron and that of the '4N nucleous. For DTBN, S, the electron spin quantum number, is
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Port~onsnf this work were rupponed by a cram fnm the Perroleum Research Fund. adminlstrred by rhe ACS, and by 311 equipnwnt grant from the Ilnj\rrsity Research lntitutr o f the i . ' n ~ versity of Texas at El Paso. lBecause the unpaired electron in DTBN is highly localized on the N - 0 bond the small interaction of the electron with the t-butyl hydrogens is not considered in this experiment
equal to %, and I, the nuclear spin quantum number, is equal to 1. The allowed values of Ms, the components of S along the applied magnetic field, are &%, while the values of M, are 0, zt1. The application of the Hamiltonian in egn. (1) to the six spin states (Ms,M,) leads to the following expression for the energy levels. E ( M s , M d = M&BH + Q M J (2) In a typical esr experiment constant frequency electromagnetic radiation is applied to the sample in such a way that the magnetic field component of the radiation is perpendicular to the applied magnetic field. The applied field is then varied and resonance occurs, subject to the selection rules AMs = 1, AM1 = 0, when hu = g p H aMr (3)
+
Here u is the frequency of the applied electromagnetic radiation. Since MI has 21 + 1 possible values, 0 and *1, the esr spectrum of the DTBN radical consists of three equally spaced lines which occur at the resonant fields hulgp and hv/g@ a. Here the hyperfine splitting is expressed by a in magnetic field units. The DTBN radical displays the interesting property that the value of a is solvent dependent. For example a ~ , o= 17.2 G and ~ D M S O= 15.7 G. For a system consisting of both DMSO and HzO i t would he expected that a would assume a value intermediate between 17.2 G and 15.7 G. To be more specific, if the DTBN molecule moved through the solution randomly sampling the DMSO and water molecules, the value of a in the case of no strong interaction between the DMSO and water would be given by (5)
*
a =
XDMSO~DM +S XO H , O Q H ~
(4)
Here X represents the mole fraction of a component. based on the amounts of DMSO and water initially mixed. However, if strong interactions between DMSO and water were to take place the interaction between the solvent molecules and the radical in the mixed solvent systems would be considerably different from that expected on the basis of studies in the pure solvents. As a consequence,
0 Figure 1. The di-tert-butylnitroxideradical
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one would expect a t o deviate from "ideality" as expressed in eqn. (4). The ahove analysis assumes that the DTBN radical acts as a probe in a given solvent system and that a for DTBN is sensitive to the comoosition and structure of the surrounding solvent. This assumption is supported by the ohsewation that the value of a can he correlated with the Kosower Z factor for a wide variety of solvents (7). The expression of ideality (eqn. (4)) represents a result derived on the hasis of the modified Bloch equations (5,8). Here it is assumed that the DTBN probe is jumping from site to site in the solvent rapidly compared to the difference in resonance frequencies for DTBN in DMSO and in water. This is reasonahle to assume since the difference in resonance frequencies for DMSO and water is about l O + 7 sec-' while approximate calculations show that the frequency of jumps is on the order of 10+9 sec -1.
DTBN can be purchased from Eastman Organic Chemicals or synthesized by several methods (4). Although the cast of DTBN is quite high it should be realized that as many as 1000 ear samples can be prepared per gram of material. The first step in the experiment is to prepare the solutions of ~MSO and water. These solutions are deoxygenated by bubbling Nz through them. The removal of oxygen from the solution is desirable because oxygen broadens the esr lines and thus increases the amount of DTBN that must be added to produce an observable signal. After deoxygenation the appropriate amount of DTBN is added by means of a microsyringe. Concentrations of DTBN on the order of 10-3M have been found to be quite satisfactory for this experiment. Small aliquots of the deoxygenated solution are used to rinse and fill 1 mm capillary tubes for about 1 cm of their length. To avoid accidental breakage of the capillary tubes in the esr cavity it is a good idea to place the filled capillaries inside a 3 mm 0.d. Pyrex tube. The hyperfine splitting.between any two of the hyperfine lines can be measured. In general one should use the same two hyperfine lines for all measurements and measure all hyperfine splittings relative to those obtained in pure water. It is advisable to run the esr experiments at the lowest microwave powers eonsistent with a good signal to noise ratio. This precaution ensures minimum sample heating from microwave absorption.
Figure 2. a as a function of XDMso for Solutions 10-3M in DTBN at 24'C. The dotted line represents ideality.
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Results Figure 2 shows a plot of a versus X n ~ s o .It is clear that a deviates from ideality as represented by the dotted line. I n Figure 3 the deviations from ideality are plotted as a function of X n ~ s o .This figure shows that DMSO and water undergo strong interactions in mixed solvent systems with the strongest interaction occuring in S0.55. The results of the comvosition range 0.35 5 XDMSO a wide variety of otier studies (I,2J have indicated strong interactions between DMSO and water with the strongest interaction occurring in the composition range 0 . 3 . 5 X O M S 0.4. O ~ It should also he noted that the curve in Figure 3 shows three distinct regions, X o ~ s o 0.35, 0.35 X o ~ s o5 0.55, and XDMSO0.55. It seems reasonable to interpret these three regions as having differing solvent structures. Similar conclusions were reached by Wolford (2) after a review of the properties of the DMSO-water system. The ahove experiment can he exvanded to include other systems (dimethyl formamide andwater for example) and variable temperature studies. Models for the mixed solvent systems- which include DMSO-water complexes may he constructed and equations similar to eqn. (4) derived relatine a to the comnosition of the solution. While these models are interesting to construct i t should he realized that the available exnerimental evidence concernine DMSO-water systems i; insufficient to conclusively support any particular model.
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Journal of Chemical Education
