Energy & Fuels 1994,8, 244-248
244
Structural Characterization of Sulfur Compounds in Petroleum by S-Methylation and 13C NMR Spectroscopy Thomas K. Green,* Paul Whitley, Kanning Wu, William G. Lloyd, and Li Zhui Gan Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky 42101 Received June 11, 1993. Revised Manuscript Received September 20, 199P
A semiquantitative derivatization/NMR method for analysis of sulfur compounds in petroleum is described. The method relies on the conversion of sulfur compounds to their corresponding methylsulfonium salts following by 13CNMR analysis. The signals of the methyl carbons are then matched to those of the model sulfoniun salts. The initial results using a high sulfur crude petroleum are presented. The 13CNMR spectra of the S-methylated crude are consistent with a large proportion of thiophenic structures in this crude. Dibenzothiophene-type, benzonaphthothiophene-type,and naphthothiopene-type structures are tentatively identified in the crude. Quantitative l3C NMR analysis of the methylated crude shows that about 75-8096 of the sulfur functions in the crude are methylated to methylsulfonium salts. Introduction The speciation and quantification of organicsulfur forms in fossil fuels is an area of research which has received much attention in recent years.172 A general approach of many researchers has been derivatization of sulfur functions, particularly thiols, with various reagents containing NMR-sensitive nuclei. For example, 29SiNMR of trimethylsilyl derivatives of thiols has been used for characterization of thiols in coals and coals p r ~ d u c t s .More ~ recently, derivatization of thiols in petroleum with bis(tributyltin) oxide followed by ll%n NMR has been r e p ~ r t e d .Rose ~ and Francisco have reported an NMR method which involves derivatization of the thiols in petroleum residua to methyl thioethers using 13C-enriched methyl iodide in basea5 The derivatized thiols are identified by 13C NMR. They also applied this methylation chemistry to petroleum residua which had been subjected to alkali metal reduction, a reaction that is known to cleave C-S bonds in thiophenes and benzothiophenes.6 Finally, Verkade et al. have employed 31PNMR to analyze thiols in coal extracts and pyrolysis condensates that had been derivatized with chlorophospholanes.7 All of these derivatization/NMR methods are limited to thiols either native to the material being analyzed or formed by reduction of sulfides and/or thiophenes. None of these methods yield direct information about thiophenes and sulfides, the major sulfur constituents in crude petroleum.2 This paper describes a derivatization/NMR method which offers potential for identifying and quantifying both sulfidic and thiophenic forms of sulfur in crude petroleum. ~
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Abstract published in Advance ACS Abstracts, November 1,1993. (1) Stock, L. M.; Wolney, R.; Basdrichna, B. Energy Fuels 1989, 3, 651-61. (2) Orr, W. L.; Damste’, J. S. S. In Geochemistry of Sulfur in Fossil Fuels; ACS Symposium Series 429; Orr, W. L., White, C. M., Eds.; American Chemical Society: Washington, DC; Chapter 1, pp 2-29. (3) Howorth, 0. W.; Ratcliffe, G. S.; Burchill, P. Fuel 1990, 69, 297. (4) Rafii, E.; Ngassom, R. F.; Foon, R Lena, L.; Metzger, J. Fuel 1991, 70, 132. (5) Rose, K. D.; Francisco, M. A. Energy Fuels 1987, I, 233-239. (6) Rose, K. D.; Francisco, M. A. J. Am. Chem. SOC. 1988,110, 637. (7) Wroblewski, A. E.; Lensink, C.; Markuszewski, R.; Verkade, J. G. Energy Fuels 1988,2, 765-774. @
0S87-0624/94/250S-0244$04.50lQ
In this method, both sulfides and thiophenes are methylated with 13C-enrichedmethyl iodide in the presence of silver tetrafluoroborate to form methyl sulfonium salts. The methylated crude is then analyzed by 13C NMR spectroscopy. We demonstrate that the I3C chemical shift of the added methyl carbon is very sensitive to the nature of the sulfur atom to which it is bonded. A variety of sulfides and thiophenes can thus be distinguished by this technique. The method is applied to a high sulfur crude petroleum to illustrate the potential of this technique for identifying and quantifying thiophenes and sulfides in crude oils.
Experimental Section Methylation of Model Compounds. The procedure for methylation is similar to that given by Acheson and Harrison.* Approximately 1 mmol of the sulfur compound and 1.2 mmol of AgBF, were dissolved and stirred in 3.0 mL of dichloroethane (DCE)under argon. Methyl iodide (2.0mmol) was added via syringe to the stirred solution. A yellow precipitate (AgI) immediately formed in all cases. The reaction was allowed to stir overnight after which the solution was filtered to remove AgI. The AgI precipitate was washed with acetonitrile. The filtrate was then rotovaporized at 40-50 O C to remove solvent. If an oil formed, it was triturated with ether to give a solid. In some cases an oil formed which could not be precipitated as a crystalline solid,in which case the oil was dissolvedin acetonitrileds and analyzed directly by NMR. If a crystallinesolid formed, it was filtered and washed with water to remove excess AgBF4. The solid was then dissolved in acetonitrileand ether was added until the solution became slightly turbid. The solution was then cooled in dry ice/acetone to effect crystallization. The yields and melting pointa of the solids are given in Table 1. Methylation of Crude Petroleum. The high sulfur Arabian crudewas obtainedfrom Ashland Oil. The crude contained 84.7% C, 12.1% H,0.15% N, 2.9% S,and 0.15% 0 (by difference). Two basic procedures were used to effect methylation of the sulfur compounds in the crude, the primary difference being the order of addition of the reagents. Procedure A. Approximately 1 g of crude was used which contained 2.9% S or 0.91 mmol S. The crude and AgBFd (1.8 (8)Acheeon, R. M.; Harrison, D. R. J. Chem. SOC.C 1970, 1764.
0 1994 American Chemical Society
Energy &Fuels, Vol. 8, No. 1, 1994 245
Sulfur Compounds in Petroleum Table 1. NMR Data on Model Sulfonium Salts cation0
mn O C oil
% vield -
WNMR 21.0
lH NMR 2.7
-
22.7
2.7
110-11
82
21.7
2.5
224-25
80
22.2
2.8
119-21
94
25.3
3.1
248-50
61
25.9
2.7
-
26.1
3.2
oil
QC -.
oil 58-60
76
28.4
3.6
123-24
-
29.3
3.2
oil
-
28.3
3.2
68
31.7
3.2
-
31.2
3.3
69-70
oil
149-51
78
34.9
of the product dissolved. The insoluble portion remaining adhered to the test tube wall as an oil and the dark solution could be separated by removal with pipet. lH and 13C NMR spectra were obtained at 270 MHz using a JEOL CPF 270 FT-NMR. A pulse width of 2.8 ps and a pulse delay of 3.0 s were used for the 13C NMR spectra. The number of transients was usually 5000. Quantitatiue Analysis. Approximately 100mg of product and about 70 mg of anisole, used as an internal standard, were accurately weighed and dissolved in 2 mL of a 1:l (vol) mixture of chloroform-d and acetonitrile-da. About 30 mg of chromium acetylacetonate was added as relaxation agent. All components completely dissolved in the solvent mixture. 13C NMR spectra pulse were obtained under quantitative conditions using a 2.8-w~ width and a 15.0-spulse delay with gated decoupling to minimize NOE. The methyl peak of anisole, a t 55 ppm (relative to TMS), was integrated and compared to the integration value for the region of 18-38 ppm, the region of the spectrum where methyl sulfonium salts yield signals. The number of carbon atoms giving rise to these signals was calculated according to the following equation: mmol of carbon = g of product mg of anisole peak area (18-38 ppm) mg of product (1) 89.2 108 peak area (55pm) The factor 89.2 accounts for isotopic enrichment of the methyl iodide. Some peaks in the region 18-38 ppm are attributed to carbons native to the crude. In order to correct for these peaks, the same equation was applied to the spectrum obtained from the product using unenriched methyl iodide. The added methyl signals are not observed in the spectrum of this product, and thus the area of the peaks attributed to the native carbons can be calculated and subtracted. Thus the number of added methyl carbons alone can be calculated. mmol of CH, added g of product
mmol of carbon - mmol of carbon (2) g of I3CH3product g of "CH, product
3.3
l3
Results and Discussion
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mmol, 0.35 g) were dissolved in 3.0 mL of dry DCE in a conical vial fitted with a magnetic stirring bar and a septum. Both the crude and AgBF4 completelydissolved. Methyl iodide (1.8mmol, 0.35 g) was added via syringe to the stirred vial under nitrogen. An immediate yellow precipitate formed. The solution was stirred for 12 h. The AgI was then centrifuged and the supernatant was pipetted into a 25-mL round bottom flask. AgI was washed twice with excess DCE. The combined supernatants were rotovaporized at 50-60 "C under vacuum to remove solvent and excess methyl iodide. The residue was dried in a vacuum oven overnight at room temperature. Procedure B. This procedure differs from procedure A in that the crude and methyl iodide were first dissolved together in 1.5 mL DCE. A solution of AgBF4 in 1.5 mL of DCE was then added slowly via syringe over 15-30 min to the stirred solution. The relative amounts of reagents and crude were the same as in procedure A. After stirring for 12 h under nitrogen, the product was worked up in the same manner as procedure A. FT-NMR Analysis of Products. Qualitatiue Analysis. Approximately 100 mg of the product was added to 1 mL of acetonitrile-& in a test tube and shaken. A substantial amount
Model Compounds. Sulfur is a good nucleophile. As such, sulfides and thiophenes react with alkyl iodide to yield sulfonium s a l t ~ , as ~ Jshown ~ below. RSR + R'I
-
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(3)
The reaction is favored by polar solvents such as acetonitrile and acetone. Since iodide is also a good nucleophile, the reaction is reversible. However, if the reaction is carried out in the presence of silver tetrafluoroborate, silver iodide is precipitated, leaving the poor nucleophile tetrafluoroborate as the solute anion. Sulfides are methylated more readily than thiophenes in this reaction? Among the sulfides, dialkyl sulfides are methylated most readily and do not require the presence of the silver(1)ion. Diary1sulfides, being less nucleophilic due to delocalization of the lone pair of electrons on the sulfur, require the presence of silver(1) ion for high conversion. Thiophenes also require the presence of silver(I)ion for good conversion? Among the thiophenes, simple thiophene is most difficult to methylate and obtain in pure form. Acheson and Harrison were not able to isolate the tetrafluoroborate salt of thiophene but were able to ~~~
~~~~
(9) Lowe, P. A. In The Chemistry of Sulphonium Group; Stirling, C. J. M., Patai, S., Eds.; John Wiley: New York, 1981; Chapter 11. (10) Trost, B. M.; Melvin, L. S. Sulfur Ylids; Academic Press: New York, 1975.
Green et al.
246 Energy &Fuels, Vol. 8, No. I, 1994
isolate the hexafluorophosphonate salt in 10% yield.* The tetrafluoroborate salts of benzothiophene and dibenzothiophene were isolated in 73 and 93 % yields, respectively. Only limited 13C chemical shift information exists on methyl sulfonium salts.11J2Therefore, we prepared models for examination by NMR spectroscopy. The results are shown in Table 1, where the yields, melting points, and 13C and 1H chemical shifts of the added methyl groups (relative to TMS) are listed. Careful referencing to the center peak of the septet at 1.3 ppm of acetonitrile-d3 was made for determining the 13C chemical shifts. The yields of the salts ranged from 61 to 94%. The melting point ranges, where a crystalline solid could be obtained, were 1-2 "C, indicating high purity. The 'H NMR spectra also indicated that the salts were free of organic impurities, even in the cases where oils were obtained. The 1H and 13CNMR spectra were consistent with the structures in all cases. The 'H chemical shifts of the methyl hydrogens range from 2.5 to 3.6 ppm. Considering sulfidic derivatives, the chemical shift increases with the number of phenyl rings attached to the sulfur and ranges from 2.5 to 3.6 ppm. The 1H chemical shifts of the thiophenic derivatives vary little with structure, being about 3.2 ppm. The 13C chemical shifts of the methyl carbons range from 21.0 to 34.9 ppm. Among the sulfide derivatives (compounds 1-9), the chemical shift ranges from 21.0 to 29.3 ppm and generally increases with the number of aryl groups bonded to the sulfur. Exceptions to this trend include compounds 6 and 9. In the case of compound 6, the higher chemical shift is undoubtedly due to the increased ring strain of the 5-membered ring compared to the six-memberedring (compare compound 4). Compound 9 has two methyl groups attached which increases the chemical shift of the attached methyl carbon (compare compound 7). Among the thiophenic derivatives (compounds 10-14), the chemical shift ranges from 28.3 to 34.9 ppm, which increases with increased aryl substitution about the central thiophenium ring. Comparison of the chemical shifts of compounds 13 and 14 reveals the effect of an additional fused ring-a slight upfield shift. A comparison of compounds 11 and 12 reveals the same effect. While it clear that there is some overlap of the chemical shift range of sulfidic and thiophenic derivatives, the chemical shift information in Table 1indicates a potential for speciation of sulfur compounds in crude oils by this derivatization technique. In addition, although the yields of the salts are not quantitative, the yields are reasonably high, suggestingthat at least semiquantitative information may be achieved by this technique. Methylation of Crude Oil. Before discussing the results with the crude, we point out that a variety of other compound classes in fossil fuels (e.g., phenols, amines, etc.) can react with methyl iodide. The products of these reactions will also be observed by NMR. Typical 13C chemical shift ranges for 13CH3groups incorporated in crude oils are shown in Figure 1. Except for some N-CH3 signals, there are no other methyl signals which will interfere with the signals from the +S-CH3 carbons. Moreover, the high-sulfur crude which we employ here ~~~~
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(11)Barabrella,C.;Dembach,P.;Garbassi, A.TetrahedronLett. 1980, 21(21),2109-12. (12)Heldeweg, R.F.;Hogeveen, H. Tetrahedron Lett. 1974, (l), 7578.
80
70
60
50
40
30
20
10
0
Chemical Shift (ppm)
Figure 1. Typical 13C chemical shift ranges for incorporated into petroleum fractions.
13CH3 groups
has a very low nitrogen content (0.15%),reducing the likelihood of interfering N-CH3 signals. Sulfur Recovery. Sulfur analysis showed that the original crude contained 2.9% sulfur whereas the methylated product contained 4.0% sulfur. Thus the sulfur was enriched as a result of methylation of the crude and subsequent work-up. This enrichment was due to loss of a volatile fraction (31% ) of the crude during rotovaporization and vacuum drying of the product. However, the starting crude (1.00 g) contained 0.91 mmol sulfur and the product (0.69 g) contained 0.86 mmol sulfur. Thus, the sulfur was nearly quantitatively recovered in the methylated product. Qualitative NMR Analysis. For qualitative analysis, the solvent was acetonitrile-d3. There were two reasons for choosing this solvent for qualitative analysis. First, this solvent readily dissolves the model sulfonium salts formed in the methylation reaction. It should therefore dissolve the salts formed from the crude oil. Second, in order to make precise chemical shift comparisons between the models and the product, the same NMR solvent should be used, since it is known that solvent can affect chemical shift. We found that acetonitrile-ds dissolves a substantial amount, but not all, of the methylated product. lH NMR. The lH NMR spectra of the original crude and methylated crude both exhibited broad, intense peaks at 0.85 and 1.25 ppm, consistent with the occurrence of methyl and methylene hydrogens in the crude, respectively. These peaks were by far the most intense of any peaks. There is also evidence of aromatic hydrogen at 7.5 ppm. According to the model compound studies, the chemical shifts of +S-CH3 hydrogens are in the range 2.5-3.6 ppm (see Table 1). The signals in this range were very weak. These regions of the spectra are expanded and shown in Figure 2. The methylated crude clearly shows additional peaks not observed in the original crude, consistent with the formation of sulfonium salts. Particularly evident is the relatively intense, broad peak centered at 3.2 ppm. This peak is consistent with formation of either thiophenic or alkylaryl derivatives. I3C NMR. The aliphatic region of the 13C NMR spectrum of the crude dissolved in chloroform-d is shown in Figure 3 (spectrum A). The major peak is the spectrum is at 30 ppm. The spectrum of the product from S-methylationusing ordinary methyl iodide as methylating agent is shown above this spectrum (spectrum B). The solvent in this case is acetonitrile-d3. This spectrum is very similar to that of the original crude, with the same
Sulfur Compounds in Petroleum
Energy & Fuels, Vol. 8, No. 1, 1994 247
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Figure 3. l3C NMR spectra of Arabian Crude. spectrum A, original crude in CDCl,; spectrumB, S-methylatedcrude in CD3CN; spectrum C, S-methylatedcrude in CD3CN using 99% l3Cenriched methyl iodide. set of major peaks. Small differences in chemical shifts can be attributed to a solvent effect. There is no apparent evidence of added methyl groups in the spectrum of the methylated crude. There are two possible reasons for this result. First, the number of added methyl groups compared to the number of carbons present in the original crude is very small. If one assumes that one methyl group is added for every sulfur atom, this means that there are only 1.3methyl groups per 100carbon atoms in the original crude. Moreover, these methyl groups will be in a variety of chemical environments, with a broad distribution of chemicalshifts. These results point to the need for isotopic enrichment of the methyl iodide. The spectrum of the product using 99% 13C-enriched methyl iodide (procedure A, see Experimental Section) is also shown in Figure 3 (spectrum C). This spectrum is strikingly different. The difference is attributed to added methyl groups whose signals are enhanced due to isotopic enrichment. This spectrum is expanded in Figure 4 to illustrate more detail (shownas spectrum A). (The intense peak at 29.3 ppm is offscale.) Although complex, this spectrum was highly reproducible. Some of the peaks can be attributed to the original crude and are indicated with an asterisk. The majority of the peak area in the spectrum of the S-methylated crude is in the region of 28-36 ppm, exactly the chemical shift range expected if thiophenic compounds are the major sulfur types in the crude (see Table 1).Some preliminary assignments can be made. The region from 32.5 to 36.0 ppm is expanded and shown in Figure 5. There are six major peaks in this region with only one peak at 32.7 ppm attributed to the original crude. In order to make possible assignments for these peaks, we spiked the sample with authentic samples of S-methyldibenzothiophenium and S-methylbenzo[b]naphtho[ 1,2d]thiophenium
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Figure 5. l3C NMR spectrum of S-methylatedArabian Crude using 99 % W-enriched methyl iodide: spectrum A, unspiked sample; spectrum B, sample spiked with compounds 13 and 14 (see Table 1). tetrafluoroborate salts (compounds 13 and 14, Table 1, respectively). The spectrum of the spiked sample is shown above the unspiked sample. The peaks at 34.9 and 34.2 are seen to grow, indicating that these peaks correspond to these compound types. Thus, a tentative assignment can be made to these two peaks in the spectrum. In addition, the large peak at 31.2 ppm exactly matches the chemical shift of naphtho[2,3bl thiophenium tetrafluoroborate (compound 12), suggesting the presence of this compound type as well. Thiophenes have been identified in a large number of crude oils and asphaltenes6J3-17and are considered the most abundant types of sulfur compounds in many crudes. The specific types of thiophenes identified in this crude have been identified in other crudes by GC/MS techniques.18 (13) Rall,H.T.;Thompson,C.J.;Coleman,H.J.;Hopkins,R.L.Sulfur Compounds in Crude Oil; U.S. Bureau of Mines, Bulletin 659; US Government Printing Office: Washington, DC, 1972; 187 pp. (14) Ho T.Y.;Rogers, M. A,; Drusher, H. V.; Koons, C. B. Am. Assoc. Pet. Geol. Bull. 1974,58, 2338-2348. (15) Speight J. G.; Pancirov, R. J. Prepr.-Am Chem So., Diu. Pet. Chem. 1983,28(5), 1319. (16)Nicksic, S.W.; Jeffries-Harris, M. J. J . Inst. Pet. 1968, 54, 107. (17) Clerc, R. J.; ONeal, M. J. Anal. Chem. 1961,33, 380.
248 Energy & Fuels, Vol. 8, No. 1, 1994
Considering the sulfidic derivatives, the large peak at 29.3 ppm closely matches the +S-CH3 chemical shift of
dimethylphenylsulfonium tetrafluoroborate (compound 9). This type of sulfonium salt could arise from at least
three different sources: thiophenols, methylaryl sulfides, and diaryl disulfides. We have, in fact, isolated compound 9 in pure crystalline form from thiophenol, methylphenyl sulfide (thioanisole),and diphenyl disulfide. Both thiophe nols and diaryl disulfides are known constituents of crude oilsa2 Finally, the +S-CH3 chemical shift of dibenzylmethylsulfonium tetrafluoroborate (compound 3) at 21.7 ppm also matches one of the peaks observed in Figure 4, suggesting the presence of this type of salt. This result is consistent with the known existence of dialkyl sulfides in crude oils and a s p h a l t e n e ~ . l ~ J ~ J ~ ~ ~ ~ The methylated product corresponding to spectrum A in Figure 4was prepared by procedure A (see Experimental Section). In this procedure, the crude and AgBF4 were mixed prior to addition of methyl iodide. A possible limitation of this method is that Ag+ may incur partial complexation of sulfur before addition of methyl iodide, thus affecting reactivity of some sulfur species in the crude. In order to qualitatively assess what effect the order of addition of reagents might have on the spectrum, procedure B was also used. In this procedure, the crude and methyl iodide were dissolved in DCE and then a solution of AgBF4 in DCE was slowly added dropwise. Assuming rapid precipitation of AgI (which was observed), the concentration of Ag+ should be lower during the course of the reaction compared to procedure A. The 13C NMR spectrum of the product using procedure B is shown as spectrum B in Figure 4. This spectrum is identical to spectrum A except for the notable absence of major peaks at 29.3 and 28.6 ppm. The peak at 29.3 ppm corresponds to compound 9 in Table 1. As noted previously, this type of sulfonium salt could arise from at least three different sources: thiophenols, methylaryl sulfides, and diaryl disulfides. We do not know the source of this peak, but the absence of this peak in spectrum B suggests that Ag+ ion in high concentration may promote the formation of this type of sulfonium salt from one of these sources. Quantitative Analysis. To establish the number of methyl groups added to form sulfonium salts in the crude, the products from procedure B were analyzed by quantitative 13C NMR techniques. The solvent used was a 1:l (vol) mixture of chloroform-d and acetonitrile-d3. We found this solvent mixture to completely dissolve the methylated product whereas either solvent alone would not fully dissolve the product. An internal standard, anisole, and a relaxation agent, chromium acetylacetonate, were dissolved into the solution as well. The spectra obtained under quantitative conditions were qualitatively similar to those obtained under nonquantitative conditions. The number of added methyl groups per gram of methylated product was calculated from the integration resulh according to eqs 1 and 2 given in the Experimental Section. Only the region of 18-38 ppm was integrated, where signals from methyl sulfonium salts are expected. The results are given in Table 2. The number of added (18)SaetreR.;Somogvari, APrepr.-Am. Chem. Soc.,Diu.Pet. Chem. 1989,34 (2),268-274. (19)Yen, T.F.Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1972,17(4), F102. (20) Yen, T.F. Energy Sources 1974,1,447.
Green et al. Table 2. Quantitation of Methylated Crude mmol of sulfur/g of product 1.25 mmol of added CHdg of producta 0.99 (0.94) percent sulfur methylatedb 79% (75%) a Calculation according to eqs 1 and 2 in Experimental Section. Assumes one methyl added per sulfur.
methyl groups is less than the number of sulfur atoms in the product. This indicates that some sulfur species were not methylated by the procedure. An estimate of the percent conversion of the sulfur to sulfonium salts can be made if one assumes that one methyl group is added per sulfur atom. This assumption is valid for thiophenes and sulfides but not for thiols and disulfides. Given that thiophenes and sulfides are regarded as the major sulfur species in crude oils, this calculation should give a reasonable estimate of the conversion efficiency of the methylation reaction. As shown inTable 2, the conversion is 75-80% for the crude. Thus a large majority of the sulfur compounds are converted to methyl sulfonium salts by this technique. Finally, an estimate of the relative quantities of thiophenes and sulfides can be made by comparing the integration areas in the regions 28-38 ppm (thiophenes) and 18-28 ppm (sulfides). After correction for native carbons, this comparison gives 62 ?6 thiophenes and 38 9% sulfides. These numbers should be viewed with some caution since some sulfur species are not methylated by this reaction and there is some overlap of the chemical shift range of the thiophene and sulfide derivatives. Nevertheless, this preliminary estimate is reasonable and falls within the relative concentration range determined in other studies.21
Conclusions A semiquantitative derivatization/NMR method has been developed which offers potential for identifying sulfides and thiophenes in crude oils. An advantage of this technique is that nonvolatile sulfur species, if derivatized and solubilized, can be detected. Another advantage is that a variety of different sulfur compounds can be distinguished (e.g., different thiophene structures). A disadvantage is that some sulfur species may be left underivatized and therefore undetected. In addition, the highly volatile component of crude oils may be lost during work-up procedures, resulting in incomplete analysis of the sample, and in complications in calculating the semiquantitative estimates. -However, volatile sulfur species in crudes can be readily analyzed by other techniques such as GUMS. This derivatization/NMR technique should find its primary use in the analysis of nonvolatile sulfur species such as those found in petroleum residuum and asphalts. Finally, other nucleophiles other than sulfur can react with methyl iodide, such as amines, phenols, and perhaps some nucleophiliccarbons. Although an apparent disadvantage, this may be turned to an advantage. For example, we have found that pyridine is readily methylated by the CH&AgBF4 reagent and that the methyl 13C chemical shift of the methylpyridinium tetrafluoroborate lies far outside the chemical shift range of the methyl sulfonium salts. Thus, the technique may offer a means of analyzing nonvolatile nitrogen compounds as well. Acknowledgment. We thank Howard Moore of Ashland Oil for supplying the sample of petroleum. (21)Drushel, H.V.;Miller, J. F. Anal. Chem. 1956,27,495.
