Analytical Applications of Ion-Molecule Reactions Identification of C,H,, Isomers by Ion Cyclotron Resonance Spectrometry Michael L. Gross, Ping-Huang Lin, and Stanley J. Franklin Department of Chemisrry, University of Nebraska-Lincoln, Lincoln, Neb. 68508 Isomeric olefins and cyclic hydrocarbons are often difficult to analyze by conventional mass spectrometry. For this reason, the ion-molecule chemistry of the 1,3 butadiene radical cation and various C,Hl0 isomers was investigated with the hope of turning up differences which might be used in the complete identification of such similar compounds. The 1,3-butadiene radical cation produced at 10.9 eV reacts with 1-pentene, 3methyl-1-butene, 2-methyl-1-butene, and cis- and tranr2-pentene to produce CsHl,+, C7ti12' +,C7H11+,C6H10' -,and CsHQ+. Significant quantitative differences in the ionmolecule chemistry allow each isomer to be distinguished. The other isomeric pentene, 2-methyl-2butene undergoes only a charge exchange reaction with the butadiene radical cation. The final CsH, studied, cyclopentane, is totally inert toward the butadiene radical cation. The results indicate that ion-molecule reactions studied by ICR may prove to be an important complement to conventional mass spectrometry for such difficult analyses.
MASSSPECTROMETRY has proved to be a very important analytical tool for the identification and structure elucidation of complex molecules. A great wealth of information concerning the unimolecular modes of fragmentation of a molecular ion is available for samples as small as one nanogram. Usually these modes of fragmentation are sensitive to the original structure of the molecule. However, the mass spectra of isomeric olefins, cyclic hydrocarbons, and molecules containing these moieties demonstrate a major limitation of the technique. The spectra for a given set of isomers are often nearly identical, presumably because of the facile mobility of the double bond after ionization, and this results in either a common molecular ion or a common mixture of molecular ions. Examples include unsaturated fatty acids ( I ) , isomeric menthenes ( 2 ) , phenylpropenes ( 3 ) , I-phenylheptenes ( 4 ) , isomeric C7H12(3,C6HI0(6), and CjHlo (7) molecules. In the last three cases, it has been proposed that measurements of ionization and appearance potentials could serve to identify the isomers. These measurements are tedious, subject to error, and, of course, are not useful if the appearance potentials are unknown. The difficulty in making these measurements can be minimized by using the small laboratory computer, and we have recently documented the advantages of doing so ( 7 ) . Clearly, a more generally useful method, which still possesses the high sensitivity of mass spectrometry is desired. Such a method might involve ionization cia an ionmolecule reaction (chemical ionization). Here the ionization
process could involve formation of a collision complex possessing internal energy which is essentially equal to the exothermicity of the reaction provided the reacting ions possess low kinetic energies. Thus, a neutral molecule which requires identification may be converted to an ionic collisional complex possessing a very narrow distribution of internal energies. This complex may fragment more selectively than the molecular ion produced by electron impact ionization of the neutral molecule. The pioneering work of Field and Munson in chemical ionization (8) suggests that this approach to analysis may be worthwhile. Although most of the efforts in this area have not been directed at isomeric molecules, the fact that chemical ionization spectra are significantly different from electron impact spectra is an encouraging sign. One study of the chemical ionization of isomers involves various C7Hs compounds (toluene, cycloheptatriene, norbornadiene) (9). The electron impact mass spectra of these are identical, but the chemical ionization spectra are dramatically different. A convenient method of studying ion-molecule reactions at relatively low pressure and at low ion kinetic energies is ion cyclotron resonance spectrometry (ICR) (IO). The double resonance technique, which is unique to ICR, allows identification of the reaction pathways which lead to product ions. This is an important advantage in organic systems since ICR spectra may become quite complex. Bursey and coworkers (11) using ICR spectrometry have recently studied a gas-phase acetylation reaction as a chemical ionization technique. They found that the cross section for acetylation depends on the nature of atoms possessing unshared electron pairs, which are found in the neutral, and, thus, analytical information can be gained from these studies. These workers were not concerned with the identification of isomeric neutrals. Henis (12) has reported that isomeric C4Hs radical cations react differently with their respective neutrals whereas their unimolecular fragmentations are nearly identical. This result suggests that molecular ions may be selective chemical ionization agents. In this paper we report a study of a chemical ionization technique involving the 1,3,-butadiene molecular ion and various neutral CSHl0isomers. The work represents our initial attempt in determining whether ion-molecule reactions are useful in the analysis of molecules which are difficult to distinguish by conventional mass spectrometry. EXPERIMENTAL
(1) B. Hallgren, R . Ryhage, and E. Stenhagen, Acta. Chem., Scand., 13,845 (1959). ( 2 ) D. S. Weinberg and C. Djerassi:J. Org. Chem., 31,115 (1966). (3) Catalog of Mass Spectral Data, API Research Project 44, Carnegie Institute of Technology, Pittsburgh, Pa., Spectra 1210 and 1213. (4) A. F. Gerrard and C. Djerassi, J . Amer. Chem. SOC.,91, 6808 (1969). (5) R . E. Winters and J. H . Collins, ibid., 90,1235 (1968). (6) R. E. Winters and J. €I. Collins, Org. Mass Spectrom., 2, 299 (1969). (7) M. L. Gross and C. L. Wilkins, ANAL.CHEM.,43, 1624 (1971). 974
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
The 1,3-butadiene was obtained from Matheson Gas Products. The 2-methyl-2-butene was purchased from Aldrich and is reported to be >99% pure. The remainder of the (8) F. H . Field and M. S. B. Munson, J . Amer. Chem. SOC.,88, 2621 (1966). (9) F. H . Field, ibid., 89,5328 (1967). (10) J. D. Baldeschwieler and S. S. Woodgate, Accouizts. Chem. Res.,4,114(1971). (11) M. M. Eursey, T. E. Elwood, M. K. Hoffman, T. A. Lehman, ana J. M. Tesarek, ANAL,CHEM.,42, 1370 (1970). (12) J. M. S. Henis, ibid, 41(10), 22A(1969).
CjHlo isomers were Phillips research grade (>99.9 mole per cent purity). The ICR spectrometer was a Varian ICR-9 equipped with a dual inlet system. The standard flat cell (cross section 1.27 X 2.5 cm) was used for all measurements. The ionizing energy was 10.9 volts (nominal) and was set by use of a digital voltmeter. The emission current was 0.3 PA. The source and analyzer drift fields were 0.35 V/cm, and a trapping voltage of 0.60 V was employed. Single resonance spectra were obtained in the field modulation mode (amplitude = 20 G) with a marginal oscillator frequency of 153 kHz and a level which did not produce a change in the total ion current at resonance. All samples were prepared on a vacuum line and degassed thoroughly prior to use. The 1,3-butadiene was admitted to the cell from one of the inlets to a pressure of 2 X Torr (pressure read on the ion pump current monitor) and allowed to equilibrate for a few minutes. The C5H10 compound was then admitted via the second inlet. The partial pressure of CsHlowas initially determined by the ion pump current. At higher partial pressures, changes in the total ion current were used to determine the pressure since outgassing of the ion Torr. pump was observed at total pressures of 6-8 X Double resonance experiments were carried out in the pulsed mode with a level of 0.04-0.05 V/cm. Double resonance ejection studies are described later in the text. Conventional mass spectra were obtained with a double focusing Hitachi RMU-6D mass spectrometer with a source and inlet temperature of 200 “Cand an ionizing energy of 70 eV. RESULTS Mass Spectra. As was mentioned previously, isomeric olefins are difficult to identify by conventional mass spectrometry, and, therefore, should provide a test for the selectivity of ion-molecule reactions. We have chosen the C5Hloisomers as a trial case because it has been previously reported that these give identical mass spectral fragmentations (except for 1pentene and cyclopentane which yield mje 42 as the base peak rather than m/e 55) (7). The mass spectra of these compounds were remeasured at one sitting in order to check this point. We find good agreement between our spectra and the literature reports. No striking differences are observed in these data except for 1-pentene and cyclopentane as mentioned above. The spectra of cis- and tran~2-penteneare exactly
0
“‘I
3
?
0.6
Ii ZJ 0.5
0.4 03
0.2 01 0.0
0
3
2
I
PRESSURE OF 1,3 BUTADIENE
4 (X~~~TORR)
Figure 1. Relative abundances of product ions formed in the reaction of the 1,3-butadiene molecular ion with neutral butadiene Ionizing energy
=
10.9 eV
identical at 70 eV, and identification of these isomers is impossible even if authentic samples are available. Isomeric pentenes have been shown to undergo 1,2-hydrogen rearrangements upon ionization (13), and this fact partially explains the similarities in mass spectra. Ion-Molecule Chemistry. The ion-molecule reactions which occur in pure 1,3-butadiene were first examined at 10.9 eV (Figure 1). At this ionizing energy, the only primary ion is C4H6.+,the molecular ion. As can be seen, this radical cation disappears rather rapidly as the pressure is increased. For this reason, all subsequent studies were carried out at a partial pressure of 2 X 10-6 Torr to minimize these “internal” reactions. This approach differs dramatically from typical chemical ionization studies in which the reactant gas is present in a 100- to 1000-fold excess relative to the analytical sample (8). It is clear that the 1,3-butadiene radical cation would not serve as a good chemical ionization agent under these conditions since it would have largely disappeared producing a sig(13) B. J. Millard and D. F. Shaw, J. Ckem. SOC.B , 1966, 664.
7
2-VETHYL--BUTENE
I-PENTENE
0 28
3-METHYL-I-BUTENE
0 24
z OZ41
Yo201 W
4 020
rr
u
z
Z
3
m
2
4 016
w
> -
G
016
>
W
w
F 012
012
008
4 i!? 0 0 8
004
0 04
0 00 00 20 40 60 a0 100 I, PARTIAL PRESSURE OF CgHlo ( X I 0 6 TORR1
000
_I
cc
W
Lz
ooo
20 40 60 a0 100 PARTIAL PRESSUREOF C5 Hlc ( ~ 1 0 6 ~ 0 ~ ~ )
012-
20 40 6C 80 1083 I 0 PARTIAL PRESSURE OF C 5 HiO ( X I O ~ T O R R
00
Figure 2. Relative abundances of product ions formed in the reaction of the 1,3-butadiene molecular ion with C5H, isomers possessing a terminal double bond us. the partial pressure of C5H10 Relative abundance
=
+
(iJm8)/(ip/mn L: isl/msi), where subscripts s and p refer to secondary and primary ions, respectively i
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
975
O: TRANS-2-PENTENE
0 28
CIS-2-PENTENE
EO24
Figure 3. Relative abundances of product ions formed in the reaction of the 1,3butadiene molecular ion with neutral cisand trans-Zpentene cs. the partial pressure of C5HlO Relative abundance as defined for Figure 2. The lines, m/e 95*, are corrected for the reaction CjHlo 4-C4H6 CiHll+ CzHi -f
50201 016
+
I 20
60
40
80
100
120
PARTIAL PRESSURE OF C5HI0 ( X I 0 6 T O R R l
nificant number of secondary and even tertiary ions which would complicate the analyses. Radical cations of the nature of 1,3-butadiene must be generated a t partial pressures of the reactant gas equal t o or lower than that of the analyte rather than a t higher partial pressures. Also, these radical cations react via a collison complex presumably rather than by proton or hydride ion transfer. When mixtures of 1,3-butadiene and certain of the C5Hlo isomers are studied, product ions are found which increase in abundance as the partial pressure of CbH10 is made larger (Equation 1). This reaction scheme is only significant for five of seven C5H10 isomers here studied:
+ CH3 CiHiz.+ + CzH4 mje 96 CiHii+ + CzH5 mje 95 C6HlO” + C3H6 mje 82 + C3H7
t
C&Ii,+ mje 109
C4Ha.+
+ CsHio+[CgHis.+I*
=
(1)
rnje CsHgT 81 Le., 1-pentene, 3-methyl-l-butene, 2-methyl-l-butene, cis-2pentene, and trans-2-pentene. The results are plotted in Figures 2 and 3. The straight lines are consistent with a bimolecular process for product production. Two complications could affect the data. First, reactions of C5Hio+with its neutral precursor can be significant, especially at high partial pressures. We chose as low an ionizing energy as possible t o minimize this problem. In theory, these reactions can be eliminated since all the CjHlo isomers, except 2-methyl-2butene, have higher ionization potentials than C4H6(7) [IP = 9.07 eV (14-17)]. At 10.9 eV, both C&.+ and C5HlO‘+ are observed. Products from these “internal” reactions, however, occur at masses other than those in the mixture, and will not be considered further in this paper. Second, it is possible that the products in Equation 1 arise from reaction of C5Hl0.+with (14) K. Watanabe. T. Nakayama, and J. Mottl, J . Quatit. Spectrosc. Radiat. Transfer,2,369 (1962). (15) K . Watanabe. J . Cliem. Phys., 26,542 (1957). (16) B. Rrehm,Z. Naturforscli., 21a, 196 (1966). (17) M . I. Al-Joboury and D. W. Turner, J . Ckem. SOC.,1964, 4434. 976
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
neutral C4&. In this case, an increase in product abundance with increasing partial pressure of CjHlowould still be observed since C5H10.+ also increases with pressure. Pulsed double resonance studies show that all the products ions in Equation 1 decrease as the kinetic energy of C4H6. is increased, which indicates that the observed reactions are exothermic (18). In the cases of cis- and trans-2-pentenes, only, decreases in the product ion abundances are also found as the kinetic energy of C5Hi0. is increased. These double resonance studies were made a t the highest partial pressures of C5H10. Under this condition, the double resonance signal for irradiation of C5H10. ,is found to be ca. 70% of the signal for C4H6*-whereas the C5Hlo’+ ion is 4.2X more abundant than C4H6.+. However, t o ensure that reactions of C5Hlo’ + do not produce a large interference, double resonance ejection studies were initiated. Using a high amplitude rf signal (0.5 Vjcm) in the source region of the ICR cell, a n interfering precursor ion can be ejected if the frequency of the electric field is equal t o the cyclotron frequency of the interfering ion. Unfortunately, the product ion a t rnje 95 could not be held in the cell even at frequencies 20 kHz removed from the cyclotron frequencies of m/e 54 (C4Hs.+) or 70 (C5HlO’+), when the ionizing energy is 10.9 eV. At higher electron energies (15-20 eV), the situation is considerably improved; a small decrease in the normal abundance of rnje 95 is observed for high amplitude frequencies not equal t o a cyclotron frequency of a n ion present in the cell. We are uncertain as t o the origin of the problem a t the low total ion currents produced at 10.9 eV. Nevertheless, a correction scheme was devised at this ionizing energy. At smaller amplitudes of the ejection frequency of m/e 70, for example, m/e 95 does not disappear completely and is found t o decrease as mje 70 is irradiated. The decrease, Ai95, occurs because of two reinforcing factors: (1) partial ejection of C5H10 ’ + and (2) a n increase in the kinetic energy of the C5Hlo+ ions which are not ejected (Equation 2). +
The quantity, igi/(iio+ ig5),is a measure of the amount of mje 95 which is produced from CsHlo.+ and neutral C4Hs, and can be obtained if the fraction of CsHlo.+ ions which are ejected, f, is determined. This was done by monitoring the decrease in the (18) J. L. Reauchamp and S. E. Buttrill, Jr., J . Chem. Phys., 48, 1783 (1968).
total ion current during ejection. In addition, the decrease in igsdue to the increased kinetic energy of those CSH10. which are not ejected, (Ai9&, must be known. Since this quantity cannot be obtained absolutely, relative changes in (AigS)KE were measured at two rf amplitudes insufficient to eject a measurable quantity of C&O' + at high ionizing energy (18 ev). In this way, it is found that (Aig5)K~ was 1.25 X larger at an ejection amplitude for CsHlo.+ of 0.064 V/cm than at 0.04 V/cm. Measurement of Aig5 and f at both of these amplitudes allows Equation 2 to be solved for igs/(i70 igs) in a simultaneous manner. This approach assumes that changes in the rate constant as a function of ion kinetic energy are independent of ionizing energy. We have checked this point at lower ionizing energies using rf amplitudes less than 0.04 V/cm and found no significant differences. Thus, the factor i95/(i70 ig5) was determined for both cis- and trans2-pentene and was used to correct the observed abundances of m/e 95 (see Figure 3). The correction is rather minor, and we feel assured that the product ions, observed in the remaining cases where no double resonance signal is observed for C5H10. +, are produced nearly exclusively by reaction of C 4 H 6 . with neutral C5Hlo (Equation 1). Further confirmation of this result can be obtained by a study of the abundance of m/e 70 from cis- and trans-2-pentene as a function of the partial pressure of 1,3-butadiene. AddiTorr to tion of neutral C4H6 to a partial pressure of 2 X either the cis or trans isomers produces a 0.6 and 1.1 decrease, respectively, in the abundance of C5HlO' +. On the other hand, addition of cis- or trans-2-pentene to butadiene under the same conditions produces a decrease in C4H6.+ which is 37 and 15 X larger, respectively. Clearly, C5Hlo.+ is much less reactive with neutral C4H6 than is C4H6. + with neutral CSH10.
Table I.
+
m/e 81
82b 95
+
96b 109
Relative Slopes of the Least Squares Lines of Figures 2 and 3
3-Meth 1-Pen- yl-1-bu- 2-Methyl cis-2tene tene 1-butene Pentene 11.4 47.5 12.5 33.3 1.7
23.9
50.7 11.9 23.2 14.0
8.4 100.
41.1 10.8 23.4
+
trutis-2-
Pentene
13.0
7.4
27.9
29.3
70.3a 8.9 16.0
39.5a 11.2
13.6
+
Corrected for the reaction CjHio+ CJHs-,C7H1,+ C2H as indicated in the text. Corrected for a
+
+
DISCUSSION
Rather significa'nt and reproducible differences are found in the ion-molecule chemistry of neutral isomeric pentenes and the 1,3-butadiene molecular ion. For the sake of convenience in making comparisons, the relative slopes of the lines in Figures 2 and 3 are listed in Table I. A measurement of relative intensities at a fixed partial pressure of CSHIOshould yield identical values on the average. The three terminal olefins, 1-pentene, 3-methyl-l-butene, and 2-methyl-1-butene produce a collisional complex which preferentially loses C3H6 to form C6HlO. + ( m / e 82). This behavior is to be contrasted with the cis- and trans-2-pentenes for which the favored pathway is loss of C2H5to form C7H11+ (m/e 95). There are other notable differences which are useful in distinguishing the terminal olefins. On comparing 1-pentene and 3-methyl-1-butene, we find the latter isomer gives a much more pronounced loss of CH3 and C3H7 to yield m/e 109 and 81, respectively. The third terminal olefin, 2-methyl-1-butene, possesses a much larger cross section for production of m/e 82, 95, and 109. Here, m/e 95 and 109 are the second and third most abundant products at all pressures of C5Hl0, whereas for the other two terminal olefins, these products are of less importance. It is clear that a measurement of the relative abundance of the ionic products at one partial pressure of the various terminal olefins would yield information which could be used to unambiguously identify the isomer. In fact, analytical applications of this technique could be simplified by making measurements in this manner. The ion-molecule chemistry of cis- and trans-2-pentene is qualitatively similar. Both, for example, give favorable
production of m / e 95, 82, and 109. Certain quantitative differences are observed, however. The cross sections for formation of m/e 95 and 81 are 1.8 X greater in the cis isomer. These differences are not large, but are significant when one recalls that no differences are observed in conventional mass spectra of cis- and trans-2-pentene at 70 eV. Ion-molecule chemistry may well provide a method for distinguishing cis/ trans olefins. In addition, it appears that a double bond can be located with this technique since the preferential fragmentation of the complex is loss of three-carbon fragments for the terminal olefins and a two-carbon fragment for the 2-pentenes. More studies are in progress to test these possibilities. The sixth isomeric pentene, 2-methyl-2-butene, does not react with the 1,3-butadiene radical cation to produce any of the products of Equation 1. Instead, the only observable reaction in the pressure range investigated is charge exchange Equation 3. (3) Since the other pentenes have higher ionization potentials than 1,3-butadiene (7, 14-17), exothermic charge exchange can only occur for 2-methyl-2-butene. Two observations are consistent with Equation 3. First, pulsed double resonance studies show that CsHlo. + increases when C4H6' - i s irradiated. Typically, the rate constants for exothermic charge transfer become larger as the kinetic energy of the precursor ion is increased (18). Second, a large decrease is observed in the abundance of C4H6 + as neutral 2-methyl-2-butene is admitted to the cell containing 1,3-butadiene. The final CsHlo isomer investigated was cyclopentane. This compound was totally inert toward C4H6+. This result indicates the ease of distinguishing an olefin and its saturated cyclic isomer when using this technique. It is interesting to ask why ion-molecule reactions yield significant differences in the chemistry of isomeric pentenes but their mass spectral behavior is very similar. These differences are probably observed because the collisional complex, [CgH16. in Equation 1 possesses a unique structure for each CsHlo. As a result, the complex fragments selectively. There is a growing body of data which indicates that ion-molecule reactions observed in ICR spectrometry proceed through intermediate complexes which possess quite specific structures. The early studies of ion-molecule reactions of small hydrocarbon systems showed a great deal of randomization in the intermediate prior to decomposition. This randomization is complete in the intermediate complexes produced in the ion-molecule reactions of methane ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
977
( I 9 ) , acetylene (20), and ethene (21-23). However, in larger hydrocarbons such as allene and propyne (24), butenes through hexenes (25,26), and styrene (29, the reactions proceed with less randomization. The differences observed here seem to point to a unique complex in the reactions of the 1,3-butadiene radical cation and isomeric C5H1~’s.Labelling studies as well as studies with other isomeric compounds
are under way in order t o uncover the structure of the intermediate complexes and t o test the generality of the reactions observed here. It the mechanisms of such selective ionmolecule can be understood, it should be possible for the analyst to predict a priori the preferred decomposition pathways of a collisional complex. Further studies are necessary in order t o check if this possibility can be realized. ACKNOWLEDGMENT
(19) F. P. Abramson and J. H. Futrell, J . Chem. Phys., 45, 1925 (1 966). (20) J. H. Futrell and T. 0. Tiernan, J . Phys. Chem., 72,158 (1968). (21) T. 0.Tiernan and J. H. Futrell, ibid., p 3080. (22) M. T. Bowers, D. D. Elleman, and J. L. Beauchamp, ibid., p 3599. (23) J. J. Myher and A. G. Harrison, Can. J. Chem., 46, 101 (1968). (24) M . T. Bowers, D. D. Elleman, R. M. O’Malley, and K. R. Jennings, J . Phys. Chem., 74,2583 (1970). (25) F. P. Abramson and J. H. Futrell, ibid., 72, 1994 (1968). (26) J. M. S . Hems,J. Chem. Phys., 52,282(1970). (27) C. L. Wilkins and M. L. Gross, J . Amel.. Chem. SOC.,93, 895 (1971).
The senior author is indebted t o Professor F. W. McLafferty for his encouragement and inspiration.
RECEIVED for review October 4, 1971. Accepted January 10, 1972. This work was supported in part by the Petroleum Research Fund, administered by the American Chemical Society (Grant No. 1550-G), and by the University of Nebraska Research Council with summer fellowships to M L G and PHL. We also thank the National Science Foundation for providing funds for the purchase of the ICR spectrometer (Grant No. GU-2054).
Use of Electron Spin Resonance to Characterize the Vanadium(1V)-Sulfur Species in Petroleum F. E. Dickson, C. J. Kunesh,’ E. L. McGinnis, and Leonidas Petrakis Gulf Research & Deuelopment Company, Pittsburgh, Pa. 15230
Electron spin resonance spectrometry has been proposed as a potentially useful method of identifying vanadium(1V) environments in petroleum. Isotropic go values and hyperfine coupling constants have been examined for several square pyramidal vanadium(1V) environments, including several synthesized sulfur complexes, and the influence of both ligand and solvent has been studied. These data have been correlated with regard to go and A,, and the data confirm the usefulness of ESR in the characterization of vanadium(IV) environments. Data on fractions of petroleum residuals, separated chromatographically to concentrate polar and nonpolar materials, have revealed differences in the vanadium(1V) ESR parameters. An identification of the environments indicated by these parameters is suggested and represents a method whereby the nonporphyrin vanadium(1V) components in petroleum ultimately may be identified. THEEXISTENCE OF VANADIUM in petroleum and particularly in petroleum residuals is well documented. Although the concentration of vanadium is usually small, ranging from 0 t o 600 ppm, its deleterious effect on petroleum processing is disproportionately great. It is, therefore, of importance t o characterize t o as great a n extent as possible the chemical nature of the vanadium species present, thereby providing a possible lever in the alteration or removal of these materials. In spite of the advances in analytical technology over the past decade, electron spin resonance (ESR) still remains the single most important method available for the study of these materials of low vanadium content. Saraceno, Fanale, and Coggeshall ( I ) have shown that almost all the vanadium found 1 Present address, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 15213.
(1) A . J. Saraceno, D. T. Fanale, and N. D. Coggeshall, ANAL. CHEM., 33, 500 (1961).
978
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
in petroleum exists in the $4 oxidation state, fortuitously the state most amenable t o study by ESR. Further, the vanadium exists in the form of complexes with as much as 5 0 z in the form of porphyrins and the remaining 5 0 z as nonporphyrins (2). These latter substances are not well characterized, but are known t o be both volatile and quite stable. The ESR spectrum of the vanadium f 4 in natural asphaltenes is a typical axially anisotropic 16-line spectrum which resembles the spectrum of etioporphyrin(1) when dissolved in a viscous heavy oil ( I , 3). However, the ESR spectrum of asphaltenes recorded in various solvents and a t elevated temperatures reveals additional lines ( 4 ) . The exact species responsible for the extra resonances as well as the more prominent 16-line spectrum has not been determined. Yen et al. ( 2 ) and Boucher et al. (5) have recently taken a more rigorous approach to the analysis of ESR spectra of petroleum fractions and suggest that both the isotropic go value and the hyperfine coupling constant, A,, may possibly be used t o determine more explicitly the nature of the square planar complexes responsible for the characteristic spectrum. With the knowledge that petroleum residuals are rich in potentially coordinating heteroatoms such as N, 0, and S, we have examined the existing ESR data and have attempted to expand the available data, particularly as regards sulfur. Although much information has been published in recent years relative t o vanadyl square planar complexes containing nitro(2) T. F. Yen, L. J. Boucher, J. P. Dickie, E. C. Tynan, and G. B. Vaughan, J. Inst. Petrol., London, 55, 87 (1969). (3) D. E. O’Reilly, J . Chem. Phys., 29, 1188 (1958). (4) E. C . Tynan, andT. F. Yen, Fuel, 48, 191 (1969). (5) L. J. Boucher, E. C. Tynan, and T. F. Yen, “Electron Spin Resonance of Metal Complexes,” Plenum Press, New York, N.Y., 1969, p 111.
