Anal. Chem. 1984, 56, 725-728
725
Spin-Lattice Relaxation Time and Chemical Shift for Structural Determination in Crude Oil and Petroleum Products by Carbon- 13 Nuclear Magnetic Resonance Spectrometry Tadeusz A. Holak,'* Dagfinn W. Aksnes,* and Michael Stocker2
Department of Chemistry, University of Bergen, N-5000 Bergen, Norway
13C NMR spln-lattice relaxation times of dominant resonances observed In the spectra of the oil samples were determined for solutions without and with a paramagnetic relaxation reagent, Cr(acac),, added. The average carbon chain length of the straight chain alkanes which are present In the petroleum samples can be estimated from the relaxation data, and It agrees with that determined from the NMR Intensity measurements. I n the aliphatic region of the l3C spectra, electron-nuclear spin-lattice reiaxatlon times obtained from solutlons containing Cr(acac), may asslst In the detection of the resonances originating from short alkyl chalns of alkyiaromatlc hydrocarbons.
13C NMR spectrometric elucidation of the chemical structures of petroleum (1-7) and coal liquids (8-14) gives direct information about the molecular carbon skeleton. Typically, conventional 13CNMR spectra are subdivided into broad chemical shift regions which are associated with broad chemical classifications. Usually, it is possible to identify spectral regions with aromatic and olefinic carbons, aliphatic carbons (among which long straight-chain alkyl peaks are often prominent), and some oxygen-bound carbon functionalities. Provided suitable precautions are taken (14-1 7), integration yields the relative abundance5 of distinctive 13C structural types. Due to the spectral congestion observed for fossil fuel materials, more detailed chemical shift classifications give rise to ambiguities of assignment and therefore to uncertainty of recognition of different structural types of carbon atoms. In general, a variety of NMR characteristics, such as chemical shifts, relative peak intensities, resonance multiplicity, resonance connectivities, and relaxation data, may be useful in determining structure via NMR spectrometry. For example, with the aid of the spin-lattice relaxation data the aromatic carbons may be unambiguously subdivided into protonated and nonprotonated carbons (10). Spin-lattice relaxation times ( Tl) have proved to be very useful for studying molecular dynamics of molecules in solution (18). Relaxation measurements have been used to properly assess the instrumental parameters for quantitative NMR analysis (19). Only a few investigations of relaxation times have been conducted, however, on systems composed of many different molecules. Recently Netzel et al. have measured Tlrelaxation times for light distillate and naphtha fractions of shale oil (5). This study will examine Ti's of crude oil (without and after weathering) and of two fractions of petroleum, which are representative of oil materials. Prospects for obtaining more detailed structural information than can be deduced from broad chemical shift classifications alone are discussed. Also, the relaxation times for the same petroleum solutions containing added Cr(acacI3 were measured. We have recently Present address: N.I.H. Research Resource for Multi-Nuclei NMR and Data Processing, Department of Chemistry, Syracuse University, Syracuse, NY 13210. Present address: Central Institute for Industrial Research, P.O. Box 350, Blindern, N-Oslo 3, Norway. 0003-2700/84/0356-0725$01.50/0
shown that in some model solutions of paraffinic and alkylbenzene hydrocarbons, the electron-nuclear relaxation times TIeof small side chain alkyl carbons of alkylbenzenes are shorter (ca. 30-40%) than those of the alkyl carbons of saturated paraffinic hydrocarbons thus making differentiating between alkylbenzene and saturated alkyl compounds possible (20). Shorter Tie's indicate preferential solvation of a given component toward C r ( a ~ a c (21). )~
EXPERIMENTAL SECTION The petroleum products examined were No. 2 fuel oil (FO) obtained from a local supplier, a vacuum residue of the Ekofisk North Sea oil (ER), and Arabian light crude petroleum (AL) and its two fractions obtained after weathering. The oil film was layered over water and weathered for 19 days. At the end of the weathering experiment the remaining oil (ALW) was decanted from a precipitated film of insoluble amber resin (ALR) (22). All viscosities were measured at 39 O C with Cannon-Manning semimicroviscometers. All spectra were obtained with Bruker CXP-100 and WM-250 FT spectrometers operating at 22.63 and 62.89 MHz for 13C, respectively. The 22.63-MHz measurements were made over a range of 3 kHz with 8 K transforms. Gated decoupling was used to suppress the nuclear Overhauser effect (NOE) with a 90' pulse and the delay time 10Tp The samples were dissolved in equivalent volumes of CC14or CDC13 (ALW, ALR). The spectra were run )~ were at a temperature of 39 & 2 OC. The C ~ ( a c a csolutions prepared by direct weighing into the NMR tube. The relaxation measurements were made without vacuum degassing of the samples, using the unmodified inversion-recoverypulse sequence for the paramagnetic solutions and the fast IRFT sequence (23) for the diamagnetic solutions. The 13Celectron-nuclear relaxation times, Tle,given in Table VI were obtained from the difference between the 13Crelaxation rates measured in the presence (Tlpa) and absence (Tldia,Table V) of the paramagnetic reagent, (Tie)-' = (TlWa)-' - (Ttia)-l.The calculated T,B values in Table VI are given to the third decimal place although the accuracy of the TlW and Tldiavalues is ca. 16-15%, depending upon the intensity of the peak. The oil samples may contain free radicals (and/or other paramagnetic species) which could augment normal diamagnetic relaxation. The contribution from the radical to the observed Tl's is minimal, however, as could be seen from Tables IV-VI. For the pure alkanes the observed T1values indicate efficient dipole-dipole nuclear relaxation (Table IV). Also the Tl's of the terminal carbons,for which the greatest reduction in Tlis expected because of the slowest relaxation, are similar for the pure compounds (Table IV) and for the oil samples of the comparable viscosities (Table V). It is concluded therefore that the oil liquids under investigation relax at rates similar to those of pure compounds. RESULTS AND DISCUSSION I3C NMR Spectra. The aliphatic region of the 13C NMR spectrum of the representative oil sample is shown in Figure 1. The resonance signals observed in the spectrum are due to straight chain, branched, cyclic, and alkylaromatic alkanes. It is evident from Figure 1 that straight-chain alkanes predominate. The five intense lines shown in the NMR spectrum, lines 18, 14, 7, 10, and 9, correspond to, respectively, C1, Cp, C3, Cd, and Cint (carbons beyond C4) of long straight-chain alkanes. The chemical shifts and relative intensities of the 0 1984 American Chemical Society
720
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 8
~
Table 11. Structural Parameters Obtained by the I3C NMR Methoda
ji,“, a
IO
61wmi
20
IO
Flgure 1. 13C NMR spectrum of the allphatlc region of the Arabian light crude petroleum (AL). -
Table I. 13C NMR Shifts for No. 2 Fuel Oil and Vacuum Residue of Ekofisk Oil, Aliphatic Region FO ER peak chemical re1 chemical re1 no. shifta intens shifta intens 1 39.26 15 39.40 6 2 37.29 15 37.19 10 3 36.95 14 37.00 20 4 36.54 8 36.44 13 5 34.25 13 34.40 8 6 32.61 23 32.70 19 7 31.77 93 31.64 38 8 29.87 19 29.82 36 9 29.55 205 29.61 220 10 29.21 96 29.36 40 11 28.32 15 28.31 8 12 27.78 21 27.02 8 13 26.96 14 24.30 10 14 22.48 109 22.51 46 15 19.95 12 20.01 2 16 19.46 21 19.52 19 17 18.96 4 13 19.01 18 13.81 87 14.01 40 rest 6.0-40.0 110 11.0-40.0 38 a Ppm (10.03) relative to Me,Si. dominant signals in the spectra of some samples are collected in Table I. The assignment of the major aliphatic structural units, except for a few relevant resonances, was not attempted here as the description of closely similar systems may be found in ref 5, 6, and 24-26. Quantitative Analysis. In our present study we focused our attention on the estimation of an average carbon-chain length of an average n-alkane present in the oil materials, using both the conventional method based on the determination of intensities of the appropriate resonance lines and on relaxation measurements. For the former case the approach of Netzel et al. has been applied (5). We have used slightly different experimental conditions and also mostly different solvents. The calibration curve for the determination of the average carbon-chain length of alkanes presented in ref 5 was therefore checked for three n-alkanes (ClotC12,and C16) and it was found to be identical with that of Netzel et al. (5), at least in the Cl0-CzO region. The method is based upon the reasonable assumption that the resonances due to branched alkanes contribute less than 10% of the C1, C4, and Cint resonances of the normal alkanes and therefore the average carbon chain length in the oil fractions can be estimated from a calibration of NMR area ratios [C, (n = 4, 5, 6, ...)/Cl]of known straight-chain alkanes. It can be seen from Table I1 that the shortest average chain length, Ni (ac), is found in the no. 2 fuel oil. Interestingly, “the solid” from the weathered Arabian crude light oil (sample ALR) has shorter Ni(6Jthan the ALW
sample q , CP Ni(S,) FO 1.17 13 ER 3.12 21 AL 1.12 18 ALW 0.98 18 ALR 0.95 17 See the notations in the text.
Ni(T1) Ni(T,, fr*) 14 14 24 19 18 26 21 39 18 32
Table 111. ’’C NMR Data for Aromatic Carbons relative intensities ___ nonaromaticity, sample protonateda protonated fa FO 150 75 0.20 ER 47 18 0.27 AL 44 18 0.30 ALW 45 23 0.29 ALR 40 10 0.37 a Chemical shifts for the protonated carbons 6, from 115.0 to 128.1-130.4 as determined from the difference in T,’s.
~
_
_
fraction. The aromaticity fa factors are given in Table 111. Aromaticity increases from the AL to ALR samples and generally it is regularly increasing from light to heavy fractions. Other average structural parameters, e.g., percentages of naphthenic carbons, linear and branching CH2carbons, etc. may be derived by using data from Tables I and I11 and a set of rules in ref 5 and 17. Relaxation Times. An inspection of the T1data in Table V for the C1, C2,C3, C4, and Cintcarbons (peaks 18,14,7,10, and 9, respectively) shows the variation in the Tl’s which results from segmental motion along the chain (27). In the interpretation of the 13C T1data, we will use a qualitative approach which should nevertheless provide the framework for the assessment of 13Crelaxation data for trends which reveal the general features of the molecular dynamics of the chain. In principle, it should be possible to obtain information about the average chain length (or average molecular weight) present in the complex mixture of the alkanes in the oil samples. In studies of neat n-alkanes, the relaxation times were found (28,29) (1)to decrease from the chain ends toward the center in each alkane, (2) to decrease progressively for a given carbon as the chain length increases, and (3) to decrease proportionally less a t the chain ends as the chain length increases. The same pattern is preserved for the Tl data of n-alkanes in ca. 40% solutions in CC14 (Table IV). It can be seen from Table IV that the ratio of the T{s of carbons C2/Cht increases linearly as the chain length of the hydrocarbon increases. We have used therefore this ratio parameter to estimate the average n-alkane length Nl(Tl) for the oil samples of similar concentrations and solvents (Table 11). By use of the ratio Tl(C2)/Tl(Ch,S instead of one T1value for a given carbon, the effect on the Tl’s of the different viscosities among the solutions is offset. In any case the effect of viscosities on the Tl’s is small because the viscosities are quite similar (Table 11),and for carbons for which fast internal motion contributes overwhelmingly to Teff, the pattern of Tl’s is weakly affected by the macroscopic viscosity or the rate of overall reorientation (30). This behavior is predicted theoretically (31). T1 values of carbon C1 were not used [i.e., Tl(C1)/Tl(CinJ] since it is known that for long chain alkanes the motion about terminal methyl is a factor of 2-6 faster than the motion of the whole molecule and is completely independent of solution macroscopic viscosity (29,30). I t is not surprising, therefore, that
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table IV. 13C Spin-Lattice Relaxation Times ( 6 ) for the Alkane Solutions' at 39.5 "C carbon n-alkane
viscosity, CP
1
2 5.91 5.59 5.33 4.82 5.56 4.33
3 5.00 4.80 4.65 3.67 4.59 3.31
4 4.7 0 4.08 3.81 2.72 3.49 2.65
5.81 0.7 5 5.74 0.79 5.65 0.83 C I* 5.57 1.07 C 16 6.24 0.80 c,, 5.13 1.15 c,, ' The solutions: 4.1:0.8:5.1 (v/v/v) of the alkane, indan, and CCI,, respectively. C IO Cll
-
727
TI(C,)I Ti ( Cin t 1.4 1.5
Tl(Cl)/ int Ti (Cint) 4.18 1.4 3.81 1.5 3.15 1.8 2.05 2.7 2.44 2.6 1.60 3.2 CDC1, solvent.
1.7
2.4 2.3 2.7
ered rigid) with average rotational rate ( T ~ ) - ~and , internal motions due to rotations about individual C-C bonds in the chain with rates (7J-l. The effective correlation time for a C-H vector is then given by
~
Table V . 13C TI Diamagnetic Relaxation Times (s) peak no. FO ER AL ALW ALR 4.22 4.00 4.17 3.52 18 5.14 2.83 2.93 2.96 14 3.81 2.17 2.11 2.31 2.64 7 3.66 1.70 1.43 1.89 1.44 1.14 10 2.72 0.89 1.12 0.62 1.16 9 1.96 1 2.23 1.5 1.6 2 1.12 0.85 0.6 0.49 3 1.74 0.36 1.17 0.7 0.56 1.10 0.79 0.62 4 2.0 5 3.39 1.6 2.00 2.0 6 2.26 1.12 0.96 1.51 8 1.24 1.04 0.74 1.20 11 2.11 0.8 0.71 1.13 12 3.98 1.43 1.9 1.14 13 2.1 0.27 1.06 0.88 0.76 15 3.43 1.4 2.0 1.54 16 2.54 1.65 1.39 1.18 17 4.21 0.97 1.96 1.52 1.17 Ar 3.29 2.19 1.11 1.22 1.46
where T~ is understood to contain all internal reorientational modes. The second contribution ( ~ 0 to ~ ~ ) ~ could f f be ~ obtained from the T,'s of the carbons close to the center of mass of the molecule for which ~ , ~ i=f lT ~ - ~It. should be remembered, however, that the Teff calculated for the inner carbons is subject to both internal and overall motions since in nalkanes with chains longer than Clo the resonances for the methylene carbons beyond C4 are not resolved. The dependence of T~ values upon the changes in shape and molecular weight can be predicted from the modified Stokes-Einstein equation (32) 47rf,77a3 TO=-=
3kT
0.74f,qMw
pkNoT
where a is the radius of the molecule in a medium of viscosity 11, k and T are the Boltzmann constant and absolute temperature, respectively, f, is a microviscosity coefficient that accounts for the heterogeneity in the solvent experienced by diffusing solute molecules, M, is the molecular weight, and Noand p are Avogardo's number and the density of the solute, respectively. Assuming the overall reorientation to be isotropic, the rotational correlation time T~ for the C-H pair is given by
whereas our Tl(Cl)/Tl(C,J ratios compare purely with those of the neat alkanes, our Tl(C2)/Tl(C,J ratios in solution have similar values to those which could be obtained by using the T1data of the neat alkanes, for which the viscosity increases by a factor of 6 on going from the Clo to the C18alkanes (28, 29). The average chain length for the studied samples is given in Table 11. Ni(Tl)compares favorably with the chain length obtained by the intensity method. Another approach based on a simplified model proposed by Lyerla et al. (28) resulted in the less satisfactory values of Ni. In this model the reorientation of a C-H vector is dependent upon an overall rotation of the molecule (consid-
(3) where rCHis the C-H internuclear distance (1.09 A), nH is the
-
-
Electron-Nuclear Relaxation Times ( 8 ) with Cr(acac), Table VI. 13C TIe peak no. AL ALW( 1) ALW( 2) ALW( 3 ) 18 0.890 5.92 1.44 1.047 14 0.863 5.47 1.58 1.125 7 0.885 4.94 1.50 1.162 10 0.775 3.62 1.59 1.479 9 0.894 3.92 1.94 1.233 1 1.028 14.40 2.06 2 2.89 1.350 3 0.875 18.35 1.58 1.464 4 0.654 4.00 2.33 1.095 5 0.960 0.99 0.778 6 0.786 1.92 1.495 8 0.860 5.93 1.50 1.266 11 0.709 6.12 2.006 12 1.095 1.91 1.60 1.093 13 0.665 3.38 1.51 1.363 15 0.566 1.91 1.25 1.037 16 0.677 8.01 1.67 1.073 1.88 0.964 17 0.671 5.61 Ar ' 0.414 0.54 0.420 Arb 0.453 0.63 0.512 Cr 4.0 1.1 4.3 5.3 ' Protonated carbons. Nonprotonated carbons. Molarity of Cr(acac), x l o 2 . ~
_
_
.
ALW(4) 0.724 0.750 0.740 0.666 0.801 0.370 0.592 0.748 0.567 0.408 0.749 0.782 0.997 0.459 0.748 0.649 0.64 1 0.507 0.340 0.384 7.5
ER
0.329 0.345 0.364 0.305 0.307
0.356
0.500 0.332 0.073 0.224 6.4
728
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
number of protons attached to each individual carbon, and
K is a constant equal to 3.60 X 1Olo As s - ~ . Equations 1, 2, and 3 predict a relationship between molecular weights and the T , value of the form
Mw = PT/Clrf,T+,
(4)
where C is a constant equal to 1.911 K mol cm-2 g-’ if the viscosity is in centipoise units. The unknown quantity in eq 4 is the microviscosity factor. The microviscosity theory of Gierier and Wirtz (33) suggests for neat liquids a microviscosity factor of 1/6. The 2H relaxation in neat liquids suggests a value of 1/12 (34). For large molecules, the classical diffusion theory should apply and f, = 1. For molecules between these limits, an intermediate value off, will be required. From eq 4 one can calculate “an effective microviscosity fador” f,* using the Tl data of the inner carbons (Table IV). This f,* factor should remove the discrepancies between calculated and experimental T1(Cint)which arise from the presence of the internal motion and the microviscosity effect. For the model alkane solutions this factor ranges from 0.1223 to 0.1422 with no apparent dependence upon the alkane present in the solution. The average value off,* = 0.1325 was therefore used in eq 4 to obtain the Mw’sof the oil samples. The solute densities were set equal to the densities of average alkanes as determined by the intensity Ni(6J method. The chain lehgth numbers derived from eq 4, Ni( Tl,fr*),are given in the last column of Table 11. It can be seen that although there is some parallelism between Ni(ai)’s and Ni(Tl,fr*)’s,the Ni(Tl,f,*) values do not yield a sufficiently consistent semiquantitative picture to be of practical usefulness. The T1data in Tables V and VI show the relative relationship between spin-lattice relaxation times with and without Cr(acac), added. It can be seen that in most samples the carbons, especially those in the inner part of the molecules, undergo efficient C-H dipole-dipole relaxation and addition of Cr(aca& to these solutions does not effectively quench nuclear Overhauser enhancements (NOE’s) at practical Cr(acac), concentrations (35). Gated decoupling must therefore be used even at the highest concentrations of Cr(acacI3. The data in Table VI also reveal the following trends. The peaks which are known to originate from the paraffinic hydrocarbons always have longer Tie's than those which could have contribution from naphthenic or alkylaromatic carbon atoms. The differences in Tie's are seen better for the higher Cr(acac), concentrations (i.e., AL, ALW(4), ER) since at the lower concentrations the calculated T1”s are subject to large errors. For the AL solution doped with Cr(aca&, the Tie's of the n-alkane carbons are ca. 0.89 s whereas the TT for peak 10 is 0.78 s (possible contribution from the CH2 group of ethylbenzene) and for the peaks 15-17 Tle = 0.68 s (possible contribution from methylnaphthalene etc.). A similar feature is observed for the ALW(4) solution. However, since the differences are approaching the accuracy of the Tie's for such samples (*E%, see also the Experimental Section) the lower Tie's for these signals may only suggest some contribution of the resonances of alkylaromatics to the given total resonance, and the Tie's may be used complementary with other techniques. In view of our model T,” studies (20)the contribution may correspond t o a local concentration of less than 20% of alkylaromatics compared to the given alkane participating in
the given resonance. For the Ekofisk oil residue, ER, it can be concluded that no such above-mentioned short-chain alkylaromatics are present in appreciable amounts. Substantially shorter Tie's for the aromatic carbon in all samples give evidence of preferential solvation of the aromatic components toward Cr(acac), (20). The dependence of 1/TIeupon the concentration of Cr(acac), is linear for all peaks thus showing that no strong specific interaction between Cr(acac), and the substrates exists. The slope for the aromatic carbons is a factor of about 2 greater than that for the aliphatic ones. Registry No. Cr(a~ac)~, 21679-31-2.
LITERATURE CITED (1) Shwlery. J. N.; Budde, W. L. Anal. Chem. 1976, 48, 1453. (2) Clutter, D. R.; Petrakis, L.; Stenger, R. L., Jr.; Jensen, R. K. Anal. Chem. 1972, 44, 1395. (3) Hajek, M.; Sklenar, V.; Sebor, G.; Lang, I.; Welsser, 0. Anal. Chem. 1978, 50. 773. , Knlght, S. A. Chem. Ind. 1967, 1920. I Netzel, D. A,; McKay, D. R.; Heppner, R. A,; Guffey, F. D.; Cooke, S. D.; Varle, D. L.; Llnn, D. E. Fuel 1981, 60,307. I Llndeman, L. P.; Adams, J. Q. Anal. Chem. 1971, 43, 1245. I Friedel, A.; Retcofsky, H. L. Chem. Ind. 1986, 455. 1 Retcofsky, H. L.; Schweighardt, F. K.; Hough, M. Anal. Chem. 1977, 49,585. Cantor, D. M. Anal. Chem. 1978, 50, 1185. Yoshlda, T.; Maekawa, Y.; Uchino, H.; Yokoyama, S. Anal. Chem. 1980, 52, 817. Pugmire, R. J.; Grant, D. M.; Zllm, K. W.; Anderson, L. L.; Oblad, A. G.; Wood, R. E. Fuel 1977, 56,295. Seshadrl, K. S.; Ruberto, R. G.; Jewell, D. M.; Malone, H. P. Fuel 1978, 57,549. Retcofsky, H. L.; Llnk, T. A. I n “Analytical Methods for Coal and Coal Products”; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 2, Chapter 24. Battle, K. D.; Ladner, W.R.; Martin, T. G.; Snape, C. E.; Wllllams, D. F. Fuel 1979, 58,413. Dorn, H. C.; Wooton, D. L. Anal. Chem. 1976, 48, 246. Glilet, S.; Delpuech, J.J.; Velentin, P.; Escalier, J.-C. Anal. Chem. 1980, 52,813. Glliet, S.; Rubinl, P.; Delpuech, J.J.; Escalier, J.4.; Vaientin, P. Fuel 1981, 60, 221, 226. Wright, D. W.; Axelson, D. E.; Levy, G. C. I n “Topics in Carbon-13 NMR Spectroscopy”; Levy, G. C., Ed.; Wlley-Interscience: New York, 1979; Chapter 2. Shoolerly, J. N. I n “Progress In NMR Spectroscopy”; Emsley, J. W., Feeney, J., Sutcllffe, L. H., Eds.; Pergamon: Oxford, 1977; pp 11, 79. Holak, T. A.; Aksnes, D. W. J. Magn. Reson. 1981, 45, 1. Levy, G. C.; Edlund, U.; Hoiioway, C. E. J. Magn. Reson. 1976, 24, 375. Tjessem, K., Department of Chemistry, Universlty of Bergen, N-5000 Bergen, Norway, personal communication. Canet, D.; Levy, G. C.; Peat, I. R. J. Magn. Reson. 1975, 18, 199. Deutsch, V. K.; Jamcke, J.; Zeigan, D. J . Prakf. Chem. 1976, 318, 177. Deutsch, V. K.;Jamcke, J.; Zeign, D. J. Pfakt. Chem. 1977, 319, 1. Carman, C. J.; Tarpley, A. R., Jr.; Goldsteln, J. H. Macromolecules 1973, 6 ,719. Reference 18, p 135. Lyerla, J. R., Jr.; McIntyre, H. M.; Torchla, D. A. Macromolecules 1974, 7, 11. Levine, Y. K.; Birdsall, N. J.; Lee, A. G.;Metcalfe, J. C.; Partington, P.; Roberts, G. C. K. J. Chem. Phys. 1974, 60, 2890. Levy, G. C.; Kamoroski, R. A.; Halstead, J. A. J. Am. Chem. SOC. 1974, 96,5456. Levine, Y. K.; Partlngton, P.; Roberts, G. C. K. Mol. Phys. 1973, 25, 497. Glllen, K. T.; Noggle, J. H. J. Chem. Phys. 1970, 53,801. Gierer, A,; Wlrtz, K. 2.Nafurforsch., TellA 1953, 8, 532. Glasel, J. A. J. Am. Chem. SOC. 1969, 91,4569. Levy, 0. C.; Edlund, U. J. Am. Chem. SOC. 1975, 97,4482.
RECEIVED for review October 28, 1983. Accepted December 23,1983. We thank the Royal Norwegian Council for Scientific and Industrial Research for support through a fellowship grant to T.A.H.
