522
Anal. Chem. 1982, 5 4 , 522-525
Determination of Chemical Functionality in Asphaltenes by High-Resolution Solid-state Carbon- 13 Nuclear Magnetic Resonance Spectrometry P. DuBols Murphy'* and B. C. Gerstein Ames Laboratoty-DOE
and Department of Chemistry, Iowa State University, Ames, Iowa 500 1 1
Victorla L. Weinberg and T. F. Yen Department of Chemical Engineering, University of Southern California, University Park, Los Angeles, California 90007
A solld-state 13C, 'H NMR study of two asphaltenes derlved from coal-llquld processes Is reported. The technlque of 13C dephaslng by the 'H dlpolar reservolr Is used to resolve both the aromatic and allphatlc NMR absorption bands Into tertlary plus secondary components and secondary/tertlary plus prlmarylquaternary allphatlc components. Integrated lntensltles of the varlous resolved absorptlon bands allow estlmations of the percentages of tertlary and quaternary aromatlc carbons, as well as those of the secondary/tertlary and prlmary/quaternary allphatlc carbons. First and second moments of the resolved absorptlon bands are belleved to be correlated wlth both the klnds and dlstrlbutlons of carbons In these asphaltenes. Studles of these moments suggest that the average number of polynuclear condensed rings In these asphaltenes Is much smaller than what Is thought to exist In a high-rank anthraclte coal. Furthermore, the allphatlc slde chalns attached to the polynuclear-rlng backbone may have large varlatlons In both lengths and branchlngs.
Solid-state NMR studies of whole coals and coal-derived materials such as asphaltenes are usually directed toward characterization of the chemical identities of the various constituents responsible for the observed properties (1-9). Elucidation of such structural information is particularly valuable in predicting the general chemistry of coal as well as in understanding the special pathways involved in coal conversion. Unfortunately, the variety of structural information that one seeks to identify by spectral fingerprinting usually leads to spectra in which much of the fine structure has been smeared into broad bands (1-3). In NMR spectra of these conglomerates there are two resolvable bands: the aromatic absorption and the aliphatic absorption (4-9). In the most favorable cases, the presence of certain structural units can be suggested by shoulders and other band contours; however, overinterpretation of such features can lead to unwarranted or tenuous assumptions. The purpose of the present work is to illustrate how the technique of 13C,'H dipolar dephasing (DD) (9) can be used to simplify the aromatic and aliphatic NMR absorptions of complicated systems such as asphaltenes. Useful information on the concentrations of quaternary and tertiary aromatic carbons and primary/quaternary and secondary/tertiary aliphatic carbons is obtained. The DD method has been previously used to measure the apparent ratio of quaternary to tertiary aromatic carbons in an anthracite coal (9). On the basis of this measurement, the average number of polycyclic condensed rings in anthracite was inferred. Present address: IBM Instruments, Inc., Orchard Park, Danbury, CT 06810. 0003-2700/82/0354-0522$01.25/0
EXPERIMENTAL SECTION Instrumental Section. 13C, 'H cross-polarization (CP) measurements (10) were made at 14.09 and 56.02 mHz, respectively, in a static field of 1.3 T and with a spectrometer designed in our laboratory (11). The spectrometer offset was ca. 1 kHz and the low-frequencyfilter bandwidth was 10 kHz for all measurements. All spectra were corrected for the slight frequencydependent attenuation from the low-frequency filter. Rotating-frame HI fields were 8 and 32 G for 'H and 13C,respectively, at the magic angle. The magic angle (54.7') was set by adjustment for minimum line width on an ammonium carbonate standard. Rotor speeds were measured at 2.7 kHz. The phase-alternated dipolar-dephasing cross-polarization pulse sequence has been described in detail elsewhere (9). A cross-polarization contact time of 1.5 ms was used. Magic-anglespinning (MAS)experiments required ca. 0.3 g of powdered samples. Approximately 30 000 signal averages with a period between accumulationsof 1.5 s were required. Proper zero-order phasing was determined with an adamantane standard. This zero-order phasing and the linear phasing required to compensate for the delay times TOand TDD were applied to the appropriate Fourier spectra (9). Chemical shift scales were calibrated with the methylene absorption of the spectrum of the adamantane standard recorded under identical conditions as those of the asphaltenes. All shift scales are drawn with respect to the 13Cof MelSi (tetramethylsilane) as the origin. The u convention,in which negative shifts are downfield and signify decreased shieldings, was used (10, 12-14). NMR shifts may be converted to the 6 convention by multiplying their sigma values by -1.0. Corrections for bulk susceptibilities were not made. Relative shifts are believed accurate to better than 3 ppm. All measurements were made at room temperature on freshly ground asphaltene powders. Preparation of Asphaltenes. The separation and preparation of the asphaltenes from coal liquefactionproducts of FMC-COED and CAT INC SRC have been previously described (16, 17). Elemental analyses (dmmf) of these pentane-insoluble, benzene-soluble coal derivatives yielded: for FMC-COED 81.1% C, 6.4% H, 8.2% 0, 1.7% N, 2.6% S; for CAT INC SRC 88.4% C, 5.6% H, 4.6% 0, 1.3% N, 0.1% S. Phenolic oxygen percentages,as inferred from trimethylsilation (IS), were 6.9% (FMC-COED)and 3.7% (CAT INC SRC). Average molecular weights, as inferred from VPO studies (IS),were 382 g (FMC-COED)and 486 g (CAT INC SRC). Average molecular diameters, as inferred from X-ray powder diffraction studies (IS), were 8.5 A (FMC-COED)and 9.4 A (CAT INC SRC). Apparent aromatic fractions, fa, which were measured in the present study with ISC,'H cross-polarization NMR, were 0.70 (FMC-COED)and 0.80 (CAT INC SRC). RESULTS AND DISCUSSION For the subsequent discussion, we explicitly define a primary carbon as one attached to three protons, a secondary carbon as one attached to two protons, a tertiary carbon as one attached to one proton and a quaternary carbon as one not attached to any protons. For aliphatic carbons, methyl groups are primary carbons, methylene groups are secondary carbons, and methine groups are tertiary carbons. For the 0 1982 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 I3C
AROMATIC REGION
ASPHALTENE
A S F W A LT E N E
FMC COED
F V C COED
123
13C-1H N M R TCP=1.5rnsec M A S = 2 7 kHz
81
i
6 6 ,
I
L
r 20-!o
20
40
60
a0
DIFFERENCE
-200
13
C
ALIPHATIC REGION
ASPHALTENE
FMC C O E D E
L 6.
8 \ \
20
40
60
EO
100
T~~USEC)
Figure 2. A plot of the relative area (Integrated Intensity) of the I3C aliphatic region (-80 to 15 ppm) of the FMC-COED asphattene vs. the The short decay represents contributions dipolar dephasing time, T,. from secondary/tertiary aliphatic carbons whlle the longer decay, primarylquaternary aliphatic carbons.
two possible aromatic types, an aromatic carbon attached to one proton will be referred to as a tertiary aromatic carbon and an aromatic carbon not attached to any protons will be referred to as a quaternary aromatic carbon. Detailed discussions of past applications of NMR to fuel research can be found in ref 1 to 3. The dipolar dephasing (DD) experiment has been described previously (9). Studies of model compounds with environments similar to what is thought to exist in coals and of an anthracite coal have shown that in such systems the abundant spins, or ‘H, appear to have sufficient homonuclear dipolar coupling to behave as one spin bath ( 9 , I O ) . Therefore, the rare spin, or 13C,magnetization as a function of the dephasing time TDD shows two decays: a short decay for those carbons (secondary and tertiary) which have protons as nearest neighbors and a longer decay for those carbons (quaternary) which do not have protons as nearest neighbors. Primary carbons or methyl groups, which show substantial motion at room temperatures, partially average the dipolar interaction and a much longer magnetic persistence is observed than what one might expect for a carbon with three nearest-neighbor protons. Figures 1 and 2 show the integrated intensities vs. dipolar dephasing time TDD for the aromatic region (-180 to -80 ppm) and aliphatic region (-80 to 15 ppm) of the asphaltene FMC-COED. The I3C NMR absorption spectrum of this asphaltene is shown as the top spectrum (labeled TDD = 0 ps) of Figure 3. We have found that short decays resulting from nearest neighbor dipolar interactions can be suitably approximated
TDD= 0 USEC
-
io0
T~D(USEC)
Figure 1. A plot of the relative area (integrated intensity) of the aromatic region (- 180 to -80 ppm) of the FMCCOED asphattene vs. The short decay represents contrithe dipolar dephasing time, T,. butions from tertiary aromatic carbons whlle the longer decay, quaternary aromatic carbons.
-
-120 -40 40 SHIFT (PPM,TMS)
Figure 3. The top spectrum (T, = 0 ps) shows the conventional I3C, IH cross polarization spectrum of the FMC-COED asphaltene. The middle spectrum (T, = 60 ps) shows the contribution of quaternary aromatic and primary/quaternaryaliphatic carbons to the top spectrum. The bottom spectrum (difference of top mlnus middle) shows tlie contrlbutions of tertiary aromatic and secondary/tertiary aliphatic carbons to the top spectrum.
by second-order or Gaussian decays: whereas longer decays, for which the dipolar interactions are much weaker, are well-fitted to first-order or Lorentzian decays (IO). The experimental data shown in Figures 1and 2 were resolved into two-component decays with Lorentzian-Lorentzian, Gaussian-Gaussian, and Gaussian-Lorentzian components. In till cases, the Gaussian-Lorentzian resolution gave the “best fit” based on the criterion of the standard deviation of errors. The two-componentGaussian-Lorentzian decay of a single absorption band, A(TDD), can be expressed as a function of the dipolar dephasing time, TDD A (TDD)= AOGexp[+.5(TDD/T’2G)2] + AOLexp[-TDD/T\LI (1) Where AOGand AOL are the initial areas (at TDD = 0) of the absorption band associated with the two decays and ThG and TiL are the decay constants for the two absorption-band decays, respectively. The total integrated intensity (area) of the absorption band with no dipolar dephasing (TDD = 0) is then A ( T D D = 0) = A O G A O L (2) It should be noted that at some time T bDwhich is greater than about 4 T 2 tho ~ short Gaussian component of the absorption band has more-or-lessvanished and only the longer Lorentzian component remains. The initial area (TDD = 0) of the absorption band, associated with the Lorentzian cornponent, AoL, can then be inferred by extrapolation of the residual area of this absorption band A ( T ~ Dback ) to TDD = 0 if T ~ isLaccurately known AOL= A(T’DD)exp(TbD/Tx,) (3) the initial area of the Gaussian component of this absorption band follows from solution of eq 2 as Aoa = A(TD, = 0) - A O L For both absorptions, the decay of the integrated intensities or areas, which is directly proportional to the magnetization, was resolved by least-squares analysis into two components: a short Gaussian component with T’ZG of approximately 17 and 12 ps for the aromatic and aliphatic bands, respectively, and a longer Lorentzian component with TiL of 200 and 80 ps for the aromatic and aliphatic bands, respectively. The convergence criterion for the least-squares analysis was that the residual sum-of-errors-squaredchanged by less than 0.001 for two consecutive iterations of the parameters. For the aromatic region, the short decay represents the loss of the magnetic information of those tertiary carbons which have one proton as nearest neighbor; whereas, the longer decay
+
524
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 ASPHALTENE
1 3 C 1 H NMR T =1.5 rnsec
CAT INC SRC
CP
M A S = 2 7 kHz
,, 8
Table I. A Summary of I3C NMR Spectral Parameters and Estimates of the Various Carbon Types of the Asphaltenes
doNr’ieId -
sample conditions I
U D
E
r/
2 -
-
CI,
-
TDD= 60 USEC
I r2%g===3c Lpi ,
0
.C X
rLZTRbPCLA
DIFFERENCE
-4z-?P=Gp
FMC-COED T D D= 0 T’DD= 0
difference
SHIFT (PPM.TMS)
The top spectrum (T, = 0 ps) shows the conventional 13C, ‘H cross polarization spectrum of the CAT INC SRC asphaltene. The middle spectrum ( T , = 60 ps) shows the contribution of quaternary aromatic and primary/quaternaryaliphatic carbons to the top spectrum. The bottom spectrum (difference of top mlnus mlddle) shows the contributions of tertiary aromatic and secondary/tertiary allphatlc carbons to the top spectrum. Flgure 4.
represents the more gradual loss of magnetization of those quaternary carbons which do not have any protons as nearest neighbors. For the aliphatic region, the short decay represents the loss of the magnetization of those secondary and tertiary carbons which have two or one proton as nearest neighbors. Apparently, primary carbons, owing to their substantial motion at room temperatures (9, 19))partially average the dipolar interaction and show a relatively long decay. Therefore, the longer aliphatic decay may be identified with quaternary aliphatic carbons which do not have nearest neighbor protons and/or primary carbons. There is good evidence based on the observed T’ZL of the longer decay that this long-lived magnetization is mainly due to primary carbons, i.e., methyl groups. Studies of model compounds have shown that the quaternary aliphatic carbons have TiLsimilar to quaternary aromatic carbons ( ~ 2 0 ps), 0 whereas primary carbons may have long or short TiL depending on their motion (9). Therefore, a T’2L of 80 ps for this long-lived magnetization suggests that this magnetization originates mostly from primary or methyl carbons. Supportive of such a conclusion is the FT IR spectra (18) which show bands assignable to methyl group stretching and bending modes. For a dephasing time, T b D = 60 ps, the short-decay absorptions have decayed below detectability, so that only the long-decay absorptions are detected. The 13CNMR spectrum of the tertiary aromatic carbons is generated from the aromatic band of the T D D = 0 parent spectrum which contains both the tertiary and quaternary information by subtracting out the extrapolated (described above) quaternary aromatic band. The aromatic region from -180 to -80 ppm associated with a T b D of 60 ps is first multiplied by the constant 1.35 [=exp(60/200), eq 31 based on the observed dephasing of this quaternary aromatic band. Similarly, the 13CNMR spectrum of the secondary and tertiary aliphatic carbons is generated from the aliphatic band of the T D D = 0 parent spectrum which contains the primary, secondary, tertiary, and quaternary aliphatic information by subtracting out the extrapolated primary/quaternary aliphatic band. The aliphatic region, associated with a T‘DD = 60 ps, from -80 to 15 ppm is first multiplied by the constant 2.12 [=exp(60/80), eq 31 based on the observed dephasing of the primary/quaternary aliphatic band. The results of these spectral manipulations are shown in Figure 3 for the asphaltene FMC-COED and in Figure 4 for the asphaltene CAT INC SRC. The decay constants inferred from the FMC-COED data were used to extrapolate the CAT INC SRC dephased spectrum. Appropriate spectral information is tabulated in Table I.
CAT INC SRC TDD= 0 T‘DD= 60
difference
carbon typesa
second first moment momente i. 1 0 % area + 3 ppm ppm2
Arom Aliph Q Arom P/Q Aliph TArom S/TAliph
68.8b 30.2b 40.lC 13.4d 29.7c 16.8d
-129.0 -27.2 -135.1 -18.5 -120.8 -34.2
141 136 133 20 35 110
Arom Aliph QArom P/Q Aliph T. Arom S/T Aliph
79.8b 20.2b 4O.gc 5.6d 38.gC 14.6d
-128.5 -28.4 -134.7 -15.9 -122.0 -33.2
137 132 128 21 60 86
a Arom = aromatic, Aliph = aliphatic, Q = quaternary, P = primary, S = secondary, T = tertiary. Relative uncertainty: + 5 parts in 100. Relative uncertainty: r 1 0 parts in 100. Relative uncertainty: f 20 parts in 100. e In ppm with respect to Me,Si, negative shifts are downfield ( u convention).
The top spectra of Figures 3 and 4 show the standard 13C, lH cross polarization spectra (15) ( T D D = 0 ps) of the FMCCOED and the CAT INC SRC asphaltenes, respectively. The middle spectra show the dipolar dephased spectra (T’DD = 60 ps). Both the aromatic and aliphatic regions of these spectra have been multiplied by the appropriate and different constants, described above, to extrapolate these spectra to their approximate intensities at T D D = 0 ps. The bottom spectra represent the difference spectra of the middle spectra subtracted from the top spectra. It should be noted that the aromatic region (-180 to -80 ppm) and the aliphatic region (=-80 to 15 ppm) of the T D D = 60 spectra represent the quaternary aromatic and primary/quaternary aliphatic information, respectively. Similarly, the aromatic and aliphatic regions of the difference spectra represent the tertiary aromatic and secondary/tertiary aliphatic information, respectively. Prominent shoulders occur in both asphaltene spectra on the low field side of the aromatic band at ca. -155 ppm. The T D D = 60 ps DD spectra clearly show this contribution to be from quaternary aromatic carbons. The location of the chemical shift (20) along with the known fact (18)that these asphaltenes contain from 4 to 7% phenolic groups is further evidence for the conclusion that these shoulders are due to phenolic carbons. Table I lists the first moments, or centers of mass, and second moments, related to the line widths, of the aromatic and aliphatic regions of the various spectra. A discussion of the significance of these moments and the procedures and precautions required to obtain accurate values has been previously given (8). First, it should be noted that the first and second moments of the 13Cabsorptions are very similar in each corresponding spectra for both asphaltenes. We believe this to be strong evidence that the carbon types are similar in both asphaltenes. Of course, the distributions of the various carbon types are somewhat different as is obvious from the integrated intensities. The first moments of the tertiary aromatic carbons are ca. 8 ppm upfield (more shielded) from the first moment of the tertiary aromatic carbons of benzene (-129 ppm); whereas, the first moments of the quaternary aromatic carbons are ca. 6 ppm downfield (less shielded) from that of benzene. The
ANALYTICAL CHEMI!STRY, VOL. 54, NO. 3, MARCH 1982
increased shieldings of the tertiary aromatic carbons with respect to that of benzene are consistent with such carbons being in polycyclic aromatic (PA) rings. In general, carbons in PA rings are less aromatic and thus expected to have increased shneldings when compared to the benzene carbons (8). The decreased shielding of the quaternary aromatic carbons with respect to the benzene carbons is consistent with such carbons being bridgehead carbons for the attachment of aliphatic chains andlor being phenols, i.e., attached to OH groups. Interestingly, for an anthracite coal the tertiary aromatic carbons which are located on the circumference of large polycyclic condensed rings have a first moment which is ca. 2 ppm more shielded than those of the asphaltenes (9). Furthermore, the quaternary carbons of this anthracite coal are mostly buried in the large polycyclic ring structures and have a first moment which is ca. 4 ppm more shielded than those observed for the quaternary carbons of these asphaltenes. Such observations suggest that the tertiary aromatic carbons are more “benzene-like”and that there are relatively few “buried” quaternary carbons in these asphaltenes as compared to what is found in an anthracite coal. The second moments of the quaternary aromatic carbons of the asphaltenes are ca. 1.5 times larger than the second moment of the quaternary aromatic carbons of the anthracite coal; whereas, the second moments of the tertiary aromatic carbons are somewhat smaller than the second moment of the tertiary aromatic carbons of the anthracite coal (9). Such observations favor a more uniform dispersion of the tertiary aromatic carbons and a less uniform dispersion of the quaternary aromatic carbons in these asphaltenes as compared to that found in the anthracite coal. In light of the above discussions, the average number of polycyclic condensed rings found in the asphaltenes must certainly be small and these rings probably have a high degree of substitution-a fact which is also suggested by the smaller X-ray diffraction diameters (18) of these asphaltenes as compared to that of the anthracite coal. The first moments of the secondary/tertiary aliphatic carbons are ca. 16 ppm deshielded with respect to the first moments of the primary/qpaternary aliphatic carbons. Such an observation is not inconsistent with the fact (20) that secondary and tertiary aliphatic carbons are less shielded than primary carbons. For reasons discussed above, we feel that the primarylquaternary aliphatic band is mainly contributed to by primary or methyl carbons. Further evidence that such a conclusion is valid can be reasoned from the relatively small values of the second moments of these bands (ca. 20 ppm). If a substantial amount of quaternary carbons which are noticeably deshielded from primary carbons were present, the second moments of these bands would be expected much larger. The second moments of the secondary/tertiary aliphatic carbons are relatively larger and seem to suggest a large distribution of such species. Thus, a large variety in both the lengths and branchings of the aliphatic side chains attached to the condensed ring structure seems likely. Table I also contains estimates of the percentages of the various carbon types, i.e., quaternary aromatic, tertiary aromatic, secondaryltertiary aliphatic, and primarylquaternary aliphatic. We feel that the relative uncertainty in the aromatic percentages is probably less than 10% (i.e., 4% in 40%). For example, the observed T;of the bridgehead aromatic carbons of PDTBB was measured as 192 fis (9) which is fairly close to that value of 200 ps observed for the decay of the quaternary
5i!5
aromatic carbons of the FMC-COED asphaltene. However, because of the much smaller overall contribution of the primary/quaternary aliphatic carbons, there may be a somewhat higher uncertainty in the values of these percentages. We estimate this uncertainty at 20% (i.e., 3% in 15%). Finally, both asphaltenes contain similar percentages of quaternary aromatic and secondaryltertiary aliphatic carbons. The FMC-COED contains a smaller percentage of tertiary aromatic carbons but a much larger percentage of primary aliphatic carbons than does the SRC CAT INC asphaltenle. In summary, heteronuclear 13C,lH dipolar dephasing NMR experiments have been used to extract more information froin complicated systems such as asphaltenes than is generally obtained via a simple 13C,lH cross polarization experiment alone. Estimates of the percentages of quaternary and tertiary aromatic carbons as well as of secondary/tertiary and primary/quaternary aliphatic carbons are made in these coalderived materiah. The values of the first and second momenits of the two resolvable components of the aromatic and aliphatic bands are shown to correlate quite well with the carbon types present and allow general speculation of some average structural properties of the molecules in these conglomerate systems. In particular, for the asphaltenes described in this work, the polycyclic condensed rings appear to have a condensation index of three or less, and the aliphatic side chains attached to these condensed rings may have large variations in both lengths and branchings.
LITERATURE CITED Davison, R. M. “MolecularStructure of Coal”, Report Number ICTIW TR08, IEA Coal Research, London, 1980. Berkowltz, N. “An Introductlon to Coal Technology”; Academic Press: New York, 1979. Retcofsky, H. L.; Llnk, T. A. “Analytical Methods for Coal and Coal Products”; Karr, C., Ed.; Academic Press: New York, 1978; Vol. 11, Chapter 24. Maciel, G. E.; Bartuska, V. J.; Mlknls, F. P. Fuel 1979, 58, 391. Zilm, K. W.; Puamlre, R. J.; Grant, D. M.; Wood, R. E.; Wiser, W. IH. Fuel 1979, 58,-11. Zllm, K. W.; Pugmire, R. J.; Larter, S. R.; Allan, J.; Grant, D. M., submltted for publication in Fuel. Gerstein. B. C.; Ryan, L. M.; Murphy, P. D. Prepr. Pap.-Am. Chenn. Soc., Div. FuelChem. 1979, 24, 90. Murphy, P. D.; Cassady, T. J.; Trahanovsky, W. S.; Gerstein, B. C:., submitted for publicatlon In Fuel. Murphy, P. D.; Gersteln, B. C., submitted for publication in Fuel. Mehrlng, M. “High Resolution NMR Spectroscopy In Solids”; SprlngetrVerlag: Berlln, 1976. Murphy, P. D. Ph.D. Thesis, Iowa State Universlty, Ames, IA, 1979. Wemmer, D. E.; Pines, A. J . Am. Chem. SOC.1981, 103, 34. Mennltt, P. Gary; Shatlock, Mlchael P.; Bartuska, Victor J.; Maclel, Gary E. J . Phys. Chem. 1981, 85, 2087. Murphy, P. D.; Stevens, W. C.; Cheung, T. T. P.; Gerstein, B. C.; Kurtz, D. M. J . Am. Chen,. SOC. 1981, 103, 4400. Murphy, P. D.; Gersteln, B. C. J . Am. Chem. SOC.1981, 103, 3282. Schwager, I.; Yen, T. F. Fuel, 1978, 57, 100. Yen, T. F. “Chemlstry and Structure of Coal-DerivedAsphaltenes and Preasphaltenes”; DOE Report No. EX-76-C-01-2031, 1980, 249 pp, available through NTIS. Weinberg, V. L.; Yen, T. F.; Gersteln, B. C.; Murphy, P. D. Prepr., Diit. Pet. Chem., Am. Chem. SOC.1981, 26, 810. Opella, S. J.; Frey, M. H.; Cross, T. A. J . Am. Chem. SOC. 197’8, 101, 5856. Maekawa, Y.; Yoshlda, T.; Yoshida, Y. Fuel 1979, 58, 864.
RECEIVED for review August 7,1981. Accepted November 23, 1981. The Ames Laboratory-DOE is operated for the U.3. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This research was supported by the Office of Basic Energy Sciences, Chemical Sciences Division. Partial support of D.O.E. Contract No. DEAC-2276ET-10626 and NS.F DAR Grant No. 800 8755 is also acknowledged.
