Table V. Signal Variation (e) and Equilibrium Constant ( K ) of Isobutylbenzenetricarbonylchromium at Various Temperature (Conditions: 100 MHz, solvent, CDaCOCDs) T, "K 8 , ppm
K
308 0,202 2.62
278 0.223 2.96
253 0.239 3.26
228 0.253 3.57
213 0.266 3.89
Thus, the unavoidable conclusion, as recognized by Jackson et a!. (4) for the isopropylbenzene complex and in opposition to the conclusions of Barbieri and Taddei (7) and a fortiori to those of Jula and Seyferth (8), is that the modification of the spectra of some arene tricarbonylchromiums must be attributed to a conformational exchange I F? 11. This has been clearly observed here in the case of isobutylbenzene complex. CONCLUSION
The entropy effect AS E-O-isdifferent from the values found for ethyl- and isopropylbenzene complexes but similar to those given for toluenetricarbonylchromium ( 4 ) . This result means probably that the rotation of the isobutyl group is free, just as is the rotation of the methyl group in the toluene complex: it is possible that arrangements of the lateral chain and the larger distance between the ring and the tertiary carbon atom of the side chain minimize the interactions with the superimposed carbonyl group. The spectrum of the tert-butylbenzene complex shows no significant change in the same temperature range -60, +35 "C. This can be explained by a severe restriction of the rotation around the chromium-arene bond: the difference in the para proton chemical shifts measured in the free arene and in the complex at ambient temperature is 1.72 ppm (from chemical shifts in tert-butylbenzene measured at 220 MHz, ref. 4), very close to the value calculated at -273 OC for the difference in chemical shift of the meta proton in uncomplexed and complexed toluene rings (1.75 ppm) (from computed values of 6 relative to toluene, ref. 3). Therefore, the tert-butylbenzenetricarbonylchromium exists only in the conformation I1 at all investigated temperatures.
These preliminary results show that the association of physicochemical techniques can be very useful in the study of organometallic compounds: mass spectrometry and gas chromatography for identification and separation of the synthetized derivatives and nuclear magnetic resonance for the elucidation of their structure. It is remarkable that gas chromatography enables one to separate very complex mixtures and even to resolve closely related isomers. Kinetic studies and preparative scale separations by gas chromatography on various arenetricarbonylchromium derivatives are provided. The results of NMR experiments at variable temperature seem to solve the controversial problem of the eventual influence of the temperature on conformational exchanges. Further work in this field on complexes containing arenes substituted by groups with different electronic effects is in progress. ACKNOWLEDGMENT We thank Mrs. N. Sellier and S. Combrisson for recording mass and NMR spectra. RECEIVED February 1 , 1971. Accepted March 29, 1971.
Identification of New Steranes, Terpanes, and Branched Paraffins in Green River Shale by Combined Capillary Gas Chromatography and Mass Spectrometry E. J. Gallegos Chevron Research Company, Richmond, Calf. 94802 Combined gas chromatography-mass spectrometry, GC-MS, of the branched-cyclic hydrocarbon fraction of Green River shale is used in a provisional identification of the carbon skeletons of two perhydro-p-carotenes; five pentacyclic triterpanes, one of which is gammacerane; two tetracyclic trk-homoditerpanes; 11 tricyclic terpanes; seven tetracyclic steranes, 5-0- and 5-p-cholestane1 5-a- and 5-p-ergostane1 5-0- and 5-p-stigmastane, and 5-a-pregnane1 and finally nine branched paraffins. Twenty-two of these components are reported here for the first time. Presented is a detailed GC-MS analysis of the branched-cyclic hydrocarbon fraction from Green River shale. This analysis constitutes an extension of work already reported on this shale fraction by several other investigators. The analytical identification of 36 individual components in the saturate fraction of Green River shale further demonstrates the importance of combined GC-MS. The stereochemical importance of the ratio of intensities of the m / e 149 to m / e 151 fragment ions in the identification by mass spectra of 5-a- or 5-p-steranes is discussed.
MUCHWORK has been done recently on the isolation and identification of individual components in the branchedcyclic hydrocarbon extract from Green River shale. Cummins and Robinson (I) reported the uneven distribution of n-alkanes and the presence of large proportions of CX-, CIS-, Clg-(pristane), and Czo-(phytane) isoprenoid alkanes. Burlingame et al. (2) reported the analytical isolation and identification of CZ7-,CZ8-,and G9-steranes and a pentacyclic triterpane, C30Hs2.Eglington et al. (3) have identified cholestane C2?HdS;ergostane, C Z ~ H ~ sitostane, O; CZ~H~Z; and perhydro-@-carotene,c4OH7S. Robinson and Cummins isolated a crystalline solid from their .benzene extract of (1) J. J. Cummins and W. E. Robinson, J. Chem. Eng. Dafa, 9, 304 (1964). (2) A. L. Burlingame, P. Haug, T. Belsky, and M. Calvin, Proc. Nut. Acad. Sci., 54, 1406 (1965). (3) Sister Mary Murphy, T. J., A. McCormick, and G. Eglinton, Science, 157, 1040 (1967).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
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Green River shale which they, in collaboration with Whitehead (4), identified as gammacerane C30H52 m/e 412. Recently, Eglinton e f al. (5) have analytically isolated 5-pcholestane and 5-a-cholestane C27H48; a stigmastane isomer, CZ9Hs2;and gammacerane, C30H52; and, in addition, by gas chromatography and mass spectra they have provisionally identified two ambreanes, C30H56; ergostane, C28H50; and hopane, C30HS2;and indicate the presence of a pentacyclic nortriterpane, CZQH50; and a pentacyclic triterpane, C31H54. Anderson et al. (6) confirmed the presence of a stigmastane isomer, C ~ Q H ~ Z . Evidence is reported here from gas chromatography-mass spectrometry (GC-MS) data for provisional identification of the above and 22 additional compounds including other perhydrocarotenes, steranes, terpanes, and branched-paraffinic hydrocarbons not previously reported. Mass spectra are presented for 36 analytically isolated compounds in Green River shale. The relative intensities of m/e 151 to m/e 149 fragment ions in the mass spectra of steranes in shale are used to unambiguously distinguish between 5-CY and 5-/3 A/B ring juncture stereoisomers. EXPERIMENTAL The Green River shale taken from the vicinity of Grand Valley, Colo., was crushed to approximately 2-3 mm diameter particle size. The organic material was extracted with a 50 :50 benzene-methanol mixture at reflux temperatures. The predominantly branched-cyclic saturate fraction was isolated by making four passes through fully active alumina using cyclohexane as the eluant. The resulting viscous fraction showed no absorption in the region from 340-800 millimicrons. Precise mass measurements, mass spectra, and GC-MS of the sample were obtained on an AEI MS-9 mass spectrometer. A 150-foot, 0.02-inch i.d. capillary column coated with 7 x OV-17 was coupled directly to the source of the MS-9 without the use of an interface. The flow rate used was approximately 2 cc/minute. The G C trace was obtained from the total ion monitor of the MS-9. The mass spectra were all obtained at 20 eV in order to enhance parent peak intensities and to eliminate the ion cloud due to helium. The source temperature was held at 240 “C during the GS-MS runs. A temperature of 330 “C was maintained both at the injection port and column exit to the MS source. The chromatograph was programmed from 100 to 320 “C at 4 “C/ minute. The empirical formula of each peak in the mass spectrum of the whole fraction from m/e 69 to mJe 558 was determined by high resolution measurement. Mass measurement was accomplished both by “peak matching” and by scanning the spectrum and recoding on analog tape followed by digitization with subsequent data reduction cia computer (7). RESULTS Nuclear magnetic resonance (NMR), infrared (IR), ultraviolet (UV), and Raman spectra of the shale hydrocarbon extract are shown in Figure 1. (4) I. R. Hills, E. V. Whitehead, D. E. Anders, J. J. Cummins, and W. E. Robinson, Chem. Commun., 752 (1966). (5) W. Henderson, V. Wollrab, and G. Eglinton, Chem. Commur?., 710 (1968). (6) P. C. Anderson, P. M. Gardner, E. V. Whitehead, D. E. Anders. and W. E. Robinson. Geochim. Cosmochim. Acta, 38,
1304 (1’969). (7) A. L. Burlingame, “Topics in Organic Mass Spectrometry,” Wiley-Interscience, New York, N. Y . , 1970. 1152
0
Analysis of the NMR and UV spectra indicates a minor amount of aromatically mainly restricted to alkylbenzenes, indan, and tetralin-like components with perhaps some naphthalenes. IR and NMR suggest that multiring compounds and branched alkane-type molecules make up by far the major portion of components in this fraction. There are indications of minor amounts of olefinic compounds. See the Raman spectrum at 1650 cm-l. A low resolution mass spectrum of the branched-cyclic saturate fraction from Green River shale is shown in Figure 2. The most intense fragment ions in this mass range are m/e 191 and m/e 217. These are considered typical of many terpanes and steranes, respectively (2,4,6-10). GC-MS. A total ion monitor GC-MS trace of the branched cyclic hydrocarbon fraction from Green River shale is shown in Figure 3A. A comparable flame ionization detector trace using the same column is shown in Figure 3B. Mass spectra were taken at 15-second intervals, which is about the turnaround time of the magnet. Actual recording time of spectra from mass 560 to 100 is about four seconds. The GC peak number indicates the approximate position at which good single-compound mass spectra were recorded. A key relating the numbers shown on the GC-MS trace with the empirical formula, molecular weight, and relative amounts is given in Table I. Bar graph mass spectra of 36 individual components are presented in Figures 4-10. Background contributions to the mass spectra have been removed by comparison of mass spectra traces obtained both before and after the mass spectrum of interest, which in most cases coincides with a sharp peak in the total ion monitor trace. The best mass spectra of five complete GC-MS runs of this shale were used, and an average was taken of the best to obtain the line spectra presented. Column conditions were changed slightly over the five GC-MS runs so that small differences in relative retention times between components were observed. This proved to be an aid in obtaining better single-component mass spectra of a few of the less abundant compounds. An example of the quality of the original mass spectra is given in Figure 11. Note that this is for a component representing only 0.7% of the total. Negligible column bleed was observed. C4,,Hi8. Two perhydro-/3-carotene isomers with the empirical formula CaoH78were observed; both showed a large parent at m/e 558, a significant P - CH, at 543, a metastable 543’ 15, a diagpeak representing the transition 558+ nostic m/e 502 peak, and a base peak at m/e 125 in their mass spectra at 20-eV ionizing voltage. These mass spectra correspond closely to that reported by Eglinton et al. (3) for authentic perhydro-P-carotene. Presumably, C40Hi8 (I), see Figure 4, present in large amounts corresponds to the perhydro-P-carotene which was isolated and identified earlier by Eglinton et al. They prepared the authenic compound by hydrogenation of /3-carotene and subsequently -f
+
(8) W. Henderson, V. Wollrab, and G. Eglinton, “Identification of Steranes and Triterpanes from a Geological Source by Capillary GC-MS,” Advances in Organic Geochemistry, 1968, P. A. Schenck and I. Havenaar, Ed., Pergamon Press, Oxford, 1969. (9) H. Budzikiewicz, J. M. Wilson, and C. Djerassi, J. Amer. Chem. Soc., 85, 3688 (1963). (10) L. Tokes, G. Jones, and C. Djerassi, ibid., 90,5465 (1968).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
8
4
6
-- 0
2
PPm
I /
I.
I
MI
I
I
. I
~
I
I
Ultraviolet Spectrum
I
I
I + I I I - I . l
3OOo
m
I
I
1 - 1
I
A
1 1 1 u
I
I
I
I
I ~ I JI
t
I
1 1 4 - I
I
I ' I . + l I
i
1500
I
I
loo0
I
I
K)o
I
I = P ~ ~ * I
I
1
I l k 0
cm-' Figure 1. Branched-cyclic hydrocarbon fraction from Green River shale
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
1153
398 M o l e c u lar W e i g h t Figure 2. Low resolution mass spectrum of branched-cyclic hydrocarbon fraction from Green River shale
2
1
80
40 t, M i n u t e s
0
Figure 3A. Gas chromatography-mass spectrometry total ion monitor trace of the branched-cyclic hydrocarbon fraction from Green River shale
confirmed its presence in the branched cyclic fraction by coinjection. C40H78 (11) is a perhydro-/3-carotene isomer present in very small amounts and shows a somewhat longer retention time than C40H78 (I). The mass spectra, however, are apparently identical. There are indications from the five GC-MS runs made that there may be more than just two perhydrogenated carotene isomers. The similarity in mass spectra suggests that these isomers are a combination of three possibilities: cis-trans isomers, position isomers, and finally diastereoisomers. Two isomer envelopes of parent peaks in the mass range from about m/e 542 G 0 H e 2to m/e 550 C40Hi0 were resolved. This family of compounds has a slightly shorter retention time than the carotenes. There is a 2-mass unit difference between parents within an envelope. All show important P - CH3 fragment ions. A combined envelope of these 1154
peaks can be seen in the low resolution mass spectrum of Figure 2 next to the perhydro-/3-carotene peak at m/e 558. A metastable scan (11) of m/e 191 shows that precursors in the mass range from mje 542 to mje 550 do contribute to the m / e 191 fragment. This suggests a relationship of the C40 compounds to the terpanes. CalHsr. Two pentacyclic triterpanes of this empirical formula were observed. Both show large parents, important P - CH3 fragments, and base peaks at m / e 191. See G C peak 33, Figure 5 . Indications for the presence of the second isomer come from the rise and fall in intensity of the Pf, (P - CH3)+, and m / e 191 fragments in the mass spectra with elution time. The mass spectra are practically identical. They are resolved chromatographically. Peaks of the second isomer are obscured except for the most abundant ions. (11) J. H. Beynon, R. A. Saunders, and A. E. Williams, Nature, 204,67 (1964).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10,AUGUST 1971
Figure 3B. FID chromatography trace of the branched-cyclic hydrocarbon fraction from Green River shale
Table I. Key to Figure 3 GC
peak No. 1 2 3 4
5"
6" 7" 8" 9" 100 1l a 12" 13a 14a 1 9
a
Empirical formula C16H34 C18H38 C1'7H40 CmHtz CzlHt4 C20H36 Cn2H4s CzoHss C21H38 CnH38 CZlH36 CZ3HtZ C23H12 C24H44 C&s2
(1)
(11)
(1) (11) 5a
(1) (11)
Components Identified by GC-MS in Green River Shale GC Empirical Mole Re1 weight amount, % peak No. formula 226 254 268 282 296 276 310 276 290 290 288 318 318 332 352 316 316 346
1@ C23H40 (1) CzaHio (11) 17a 18" C25H46 (1) Those components not previously reported.
0.2 0.7 2.0 9.0 0.8 0.7 1.9 0.7 0.3 0.1
