Analytical Ion Cyclotron Resonance Spectrometry-Increased Discrimination between Stereoisomers by Ion-Molecule Reactions with 1,2-Dicyclopropylethanedione John D. Henion, Jar-lin Kao, Willard B. Nixon, and Maurice M. Bursey' Venable and Kenan Chemical Laboratories, The University of NoHh Carolina, Chapel Hill, N.C. 275 14
The relative rates of acylatlon by the molecular ion of the a-diketone 1,2-dlcyclopropylethanedlone do not agree with predictions of the most recent theory of colllsion rates of ions with molecules. Another parameter correlated with the accesslblllty of the carbonyl group of alkylcyciohexanones and decalones Is Important In descrlblng these rates. The a-decalones can be distinguished on this bask since, under our normal conditions for operation, the CISIsomer reacts too slowly for product to be detected. Most significantly, the new ion is more sensitive to the steric environment of the carbonyl group than Is CH3COCOCH3*+:reaction rates fail off more sharply than with biacetyl molecular Ion.
2) The limited mass range of the Varian ICR-9 spectrometer requires that R not be too large. 3) R+ must be so stable relative to RCO+ that the reactivity of RCOCOR is not altered in the mass spectrometer or ICR spectrometer. Examination of collected spectra of isopropyl and tert- butyl ketones quickly discounted these logical choices as bulky groups because their mass spectra are dominated by the R+ ions at mle 43 and mle 57, respectively ( 3 ) .On the other hand, the mass spectra of cyclopropyl ketones seemed to have an mle 69 ion (cC3H&O+) generally of much greater intensity than the m/e 41 peak (C3H5+), so that the reaction (Equation 2) corresponding to decarbonylation is less important.
C,H5CO* It was recently shown that an effect which could be interpreted as a steric effect is involved in the rates of reactions of several six-membered ring ketones with the molecular ion of biacetyl in the ion cyclotron resonance (ICR) spectrometer ( I ) . These compounds were cyclohexanone ( l ) , 2-methylcyclohexanone (2), 2-ethylcyclohexanone (3), 2-n-propylcyclohexanone (4), 2-n- butylcyclohexanone (5), cis-a-decalone (6),and trans-a-decalone (7). As the alkyl group in these substituted cyclohexanones becomes larger, the rate of the reaction (Equation 1) decreases in a way which cannot be accommodated by equating the rate of reaction to the rate of collision of the reactants calculated by any available theory. The reaction observed is in fact the only significant reaction which occurs upon collision of these two reagents at the pressures employed in our studies. The collapse of the colliding reagents to the acetylated product and an acetyl radical appears to be governed by at least one other parameter which qualitatively parallels the availability of the carbonyl group to an approaching ion. CH,COCOCH,'*
+
-
R?CO RzCO(COCH3)'
-
C,H5'
+
CO
4) The desired compound, 1,2-dicyclopropylethanedione, had been prepared previously, although the preparation requires several steps ( 4 ) . Accordingly, it was predicted that under suitable ionizing conditions the ICR spectrum of 1,2-dicyclopropylethanedione would be similar to that of biacetyl in the following ways. There would be only one major fragmentation (Equation 3),
C,H5COCOC3H,'+
--+
C,H,CO'
C3HSCO'
and only two ion-molecule reactions below sure, Equations 4 and 5. C,H,COCOC,H5"
+ C3H,COCOC,H,
(3)
Torr pres-
+
C3HSCOC(OH)C3H5' + C,H,COCOC3H,. C3H5COCOC,Hij.' + C,H,COCOC3H, -L C3H,COC(OCOC,H,)C3H,' + C3H5CO.
(4) (5)
Equation 4 is a proton transfer and Equation 5 is an acyl ion transfer to the neutral a-diketone molecule. The latter reaction would be expected to dominate the ICR spectrum One way to test the hypothesis that a steric parameter is a t the pressure conditions (30 to 40 pTorr) where we have involved is to carry out similar reactions with a reagent ion proposed analytical use of the ICR spectrometer. The collithat is bulkier itself and therefore more likely to interfere sion of the molecular ion with a neutral molecule, then, with bulky substituents in the ketones. The closest analogy would produce a reaction complex which could decompose to the reagent used for previous studies would be an a-dito products in two ways, Equations 4 and 5, of which one, ketone, RCOCOR, with larger R groups. The simplicity of the ICR spectrum of the original reagent, CH~COCOCHS, Equation 5 is much more important. Thus, the simplicity which was so desirable in the spectrum of biacetyl would be is another important reason for choosing this class of commaintained. As we shall see, even this simple spectrum pound ( 2 ) ;there are only a few peaks in the spectrum, and caused some interference in the study of one ketone. the reaction products of most compounds may be observed In fact, the spectrum of 1,2-di~yclopropylethanedione without interference. bore out these predictions. Figure 1 illustrates an ICR scan The choice of cyclopropyl as the R group was dictated by of the compound under conditions close to those we would the following considerations: use for analysis. Note particularly that there is no detecta1) the simple extension of the chain as in an n-alkyl ble product corresponding to Equation 4, which would have group may not lead to sufficient bulkiness; therefore R appeared at mle 139. must be branched. The ion-molecule reaction rate constant for the self-acylation of biacetyl, Equation 6, has been found to be 7.5 x Author t o whom correspondence should be addressed. 10-lo cm3/molecule-sec (5) and the rate constant
+
CH,CO* (1)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , A P R I L 1975
689
Figure 1. ICR spectrum of 1,2-dicyclopropylethanedione Ma+ has m/e 138; C3H5COf, m/e 69;C3H&OCO(C0C3H5)C3H5+, m/e 207.
For conditions, see Experimental
CH,COCOCH,**
-
+
CHSCOCOCH, CH,COCO(COCH,)CH,'
Figure 2. ICR spectrum of 15: 1 1,2dicyclopropylethanedione/cyclohexanone In addition to the peaks found in Figure 1, there are m/e 98, Mef of cyclohexanone; and m/e 167, acylated cyclohexanone. For conditions, see Experimental
+
CH,COo (6)
for Equation 5 has been found to be 7 ( f l ) X cm3/ molecule-sec, by methods described in the Experimental section. These two reactions are each about half as fast as theory predicts; Equation 5 is no more subject to effects within the collision complex than Equation 6. In this system, then, it is necessary to react an ion with a neutral whose carbonyl group is obviously blocked in order to see a steric effect, if indeed then. The reactions of dicyclopropylethanedione molecular ion with 1, 2, 3, 5, 6, 7, 8, and 9 were studied; the reaction of particular interest is the acylation
tone ion and the molecular ion of the ketone (which is not involved in any of the reactions) are observed. 3) There are no other precursors to the product ion. This was found to be so through double resonance experiments. Therefore, all of the intensity due to the product comes by the pathway to which it is implicitly ascribed in our treatment, Equation 7. With these restrictions, we may state that, at low conversion of reactant to product, the relative amounts of products formed in a competitive experiment will be proportional to the relative rates of the two reactions. Consider Equations 9 through 11. C,H5COCOC,H,.' C,H5COCOC,H5"
I
8
C,H,
C,H5COCOC,H5"
9
by the molecular ion, Equation 7 . To ensure the validity of the kinetic scheme used,
it was necessary to show that: 1) The product ion being studied does not decompose further. In the event, no ions with the molecular ion of dicyclopropylethanedione as precursor were found, other than those shown in Equations 5 and 7 . Nor were ions found with the acylated ketone, the product of Equation 7 , as precursor. The product ion, therefore, does not decompose, and its intensity may be taken as a measure of the amount formed. 2 ) The only product of the collision between the ion and the molecule is the acylated ketone, Equation 7 . It is conceivable that protonated products may be formed similarly to Equation 4 (Equation 8) C,H5COCOC,H5"
+ RzCO
+
R,COH'
+
C,H,COCOC,H,'
(8)
but, under the conditions employed, this alternative reaction channel for the colliding reactants was suppressed. At least 95% of the product of the reaction in every case was the acylated ion; Equation 7 was in most cases the only reaction observed. Figure 2 shows a typical ICR scan of a mixture of 1,2-dicyclopropylethanedioneand cyclohexanone. In addition to the reactant ions, only the acylated ke690
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4, APRIL 1975
+ A 5 C,H5COA' + C,H5C0. (9) kB +B C,H,COB' + C,H,CO' (10) kC +C C,H5COC' + C,H,CO* (11) -t
+
In any pair of these two neutrals compete to accept CsH&O+ from C ~ H ~ C O C O C ~ H SIn- +practice, . we have always let Equation 9 be the self-acylation of dicyclopropylethanedione, Equation 5 , and measured the rate ofacylation of one ketone relative to the self-acylation; that is, Equation 10 is always Equation 7 in practice. The relative rates of two ketones then were derived from comparison of the ratio kB/kA for one ketone and the ratio k c l k a for the other to obtain k$kc. Many experiments were performed to ensure a measure of precision in these ratios; pairs of compounds were compared on a t least three different occasions. The relative rates were thus found to be reproducible to at least f5%, in most cases f1%. As an internal check, cross comparisons were made for some sets of data; that is, the ratio kD/kB obtained by comparing D and B in one set of experiments was compared with the product of k c / k B and k d k c from data obtained previously. The ratios were the same within f1%. These relative rates offer hope of a more confident comparison of these closely related compounds than the measurement of absolute rates would, because of the larger error in absolute rates; the quoted errors in the absolute rates mentioned above are f15% ( 5 ) .The data are internally self-consistent.
EXPERIMENTAL Ketones 1 to 7 were used in a previous paper; their purity is discussed there ( I ) . Ketone 8 was prepared by a known procedure and purified by distillation, bp 171-172O (lit. ( 6 ) 170-171O); no impurities were detected by GLC or in its mass spectrum. Ketone 9 was a
commercial sample; no impurities were detected by GLC or in its mass spectrum. The reagent, 1,2-dicyclopropylethanedione,was prepared by a multistep literature procedure and purified by distillation ( 4 ) . To compare experimental results with theory, it was necessary to obtain values of a,the mean electric polarizability, and g ~ the , dipole moment, of each ketone. For ketones 1 to 7, these values have been reported (1). For the new compounds, the values of a were calculated according to the method of Le FBvre or derived from experimental data (7, 8); the dielectric constant was measured with a Weilheim Dipolmeter with a measuring frequency of 2 MHz, and the dipole moment was then obtained from the Higashi equation. The calculated and experimental results were compared and found to agree within f 2 % . The values of f i thus ~ obtained were compared with calculated (9) values; other calculated values were compared with literature data ( I O , I I ) and found to agree within 15%. The ICR spectra were obtained on a Varian ICR-9 spectrometer using the field modulation mode and a marginal oscillator frequency of 79.1 kHz. A standard flat cell was used with the following typical cell conditions: trap, 0.30 V; analyzer, split, -0.10 and +0.10 V; source, split, -0.25 and f0.25 V; a typical residence time for the precursor ion is 1 X sec. The ionizing energy was 15.1 eV. The emission current was operated between 10 and 50 nA. All samples were degassed by several freeze-pumpthaw cycles prior to use and were introduced through separate inlet ports. Typical pressures were in the range of 2 X Torr ketone and 3 X Torr biacetyl; calibration with a Datametrics Barocel capacitance manometer showed only a few percent correction from the ion pump reading, as has been our experience with organic molecules of this size. The calibrated data for methane gave a value for the rate constant of protonation of CH4 by CH4.+ within 8% of the accepted value. The double resonance and ion ejection experiments were performed as previously described (12, 13). All experiments were performed on different days, at least triplicate determinations being made. The mle 140 molecular ion of 2-propylcyclohexanone ( 3 ) is so close to that of 1,2-dicyclopropylethanedione,138, that the necessary preliminary double resonance experiments could not be done satisfactorily; this compound was therefore omitted from the studies, leaving a gap in the homologous series (see Table I). RESULTS AND DISCUSSION While the analytical utility (14, 15) of ICR spectrometry, like the analytical utility of mass spectrometry, may not have an immediate dependence upon the exact theory of the reaction rates involved, the present example comments strongly upon that theory, and so it is useful to discuss it. There is, as we indicated previously, only one channel for the decomposition of the collision complex between C ~ H ~ C O C O C ~ and H S -R2CO. ~ We therefore can compare the experimental rate (a rate of product formation) and the calculated rate (a rate of collision of reactants). Here we have calculated the rates of the various reactions by the ADO (average-dipole-orientation) theory, which assumes that the ion-molecule collision rate lies between the rate expected for a freely tumbling molecule approaching the ion and the rate expected when the dipole of the molecule is fixed in orientation toward the ion. The factor setting the average amount of dipole orientation has been derived from molecular parameters (16). The expression for the rate constant k has the form (Equation 12) k = 2 n e ( a / ~ ~ )+* /2 ~n e ~ p , ( 2 / n p k ~ ) 1 / 2 (12)
where a and are defined as above, e is the value of the ionic charge, g is the reduced mass, k is the Boltzmann constant, T is the absolute temperature, and C is the ADO fnctor explained above. In Equation 12, the first term is due to the polarizability contribution, and the second term is due to the effect of partially locking the orientation of the dipole. If C were 1, the dipole would be completely locked; in these examples, C has values near 0.20. Values of a, PD, and relative rate constants are given in Table I. The conclusions which may be drawn from these
Table I. Calculated a n d Experimental Relative R a t e Constants for Equation 7 k f hi
Compound (subst)
1 (H) 2 (2-CH3)
3 (2-CzH5) 5 (2-n-CdH9) 8 (2,2-di-CH3) g (4-C2H,)
6 (cis-a-decalone) 7 (trans-a-decalone)
a, A 3
uD, D
Calcd
10.73 12.81 14.65 18.33 14.65 14.65 17.40 17.55
3.08 2.91 2 .89 2.86 2.86 2.96 2.69 2.85
1.oo 0.99
1.oo
1.02 0.99
1.01 0.98 1.01
Exptl
1.oo 0.61 0.16 0.015 0.14 1.02 < 0.005 0.014
data are similar to those which were drawn from the data for the reaction between CH3COCOCH3.+ and the ketones (11. 1) There is clearly an effect upon reaction rate differing
from the theoretical rate, which measures collision frequencies. Thus, there is another parameter which indicates whether reaction occurs upon collision. The values of k l k l (calcd) indicate that the rates of collision are all rather similar; the values of k l k l (exptl) indicate that rates of reaction sometimes are small. 2) The decrease in the rate of reaction occurs in a fashion consonant with interpretation as a steric effect of the alkyl substituent. As the equatorial alkyl substituent adjacent to the carbonyl group grows in size (Compounds 1 to 5 ) , the relative rate of product formation decreases. This may be interpreted as an indication that reaction occurs a t the carbonyl group and that increasing the size of the alkyl substituent decreases the accessibility of this reactive site. A correspondence with solution chemistry of the carbonyl group of these compounds has been detailed before (1).It is particularly significant that 4-ethylcyclohexanone (9),in which the alkyl group cannot shield the carbonyl group, reacts a t the relative rate predicted by theory, while its isomer with the alkyl group more appropriately located for shielding, 3, is markedly less reactive. The compound with two methyl groups, 8, is significantly more retarded than the compound with only one. See Table I for verification. 3) Again, the decalones react at different rates; and again, the faster of the two is only slightly slower than the n-butyl derivative. The arguments for comparing the reactivity of the decalones with the n-butyl derivative have been presented before (1) and revolve around comparison of the trans -a-decalone, 7, with a butylcyclohexanone in which the alkyl group is bent back and tied to'another site in the ring. In the present case, the cis isomer, 6, is again the slower of the two, so slow that on the time scale set up by the standard conditions of these experiments, the acylation product of 6 was not detected. I t is satisfying that conditions have been attained in which one of a pair of isomers reacts detectably and in which the other isomer does not: this is an example of a stereoselective reaction whose direction might have been predicted from a knowledge of the solution chemistry of the isomers. It is not extremely satisfying because the intensity of the signal from the reacting isomer under these conditions is only a few times noise level. This glimpse of the possibility of using ICR spectrometry to distinguish isomers, nevertheless, has prompted us to pursue studies with the more readily available biacetyl to optimize conditions for distinguishing stereoisomers, and we will report these results later. 4) The key point which we wish to make using the new reagent, however, lies in the comparison of the relative ANALYTICAL CHEMISTRY, VOL. 47,
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rates in Table I with the relative rates of reactivity of many of the same compounds with biacetyl (I), namely: 1,l.W; 2, 0.70; 3, 0.25; 5 , 0.074; 6, 0.031; 7, 0.069. The ordering of compounds is the same with each ion: 1 > 2 > 3 > 5 > 7 > 6. Even more important, the decrease in the rate constant is greater for all compounds with C ~ H & O C O C ~ H Ythan + with CH3COCOCHr+. Most important, the decrease is greater for the larger ketones 5 , 6, and 7 with C3H5COCOC3H5-+ than for the smaller; for example, the relative S - ~ compared to 0.70 rate of 2 with C ~ H ~ C O C O C ~isH 0.61 for its relative rate with CH3COCOCH3.+, while the relative rate of 5 drops from 0.074 with CH3COCOCHr+ to 0.015 with C3H5COCOC3Hyf. The drop-off in relative rate on going to the bulkier ion is greater for the bulkier ketones. Thus, as predicted, 1,2-dicyclopropylethanedione reacts more selectively than biacetyl: it is more sensitive to the steric environment of the carbonyl group in these molecules. A sensitivity to steric effects, therefore, appears not only as a result of selected changes in the neutral molecule, but also as a result of selected changes in the ion as well. Such considerations should be kept in mind when designing specific reagents for analytical ion cyclotron resonance studies in the future.
ACKNOWLEDGMENT We thank Bill Burnsides for assistance in synthesizing the diketone.
LITERATURE CITED (1) M. M. Bursey, J. L. Kao, J. D. Henion, C. E. Parker, and T. I. S. Hwng, Anal. Chem., 46, 1709 (1974). (2) M. M. Bursey, T. A. Elwood, M. K. Hoffman, T. A. Lehman, and J. M. Tesarek, Anal. Chem., 42, 1370 (1970). (3) E. Stenhaaen. S. Abrahamsson. and F. W. McLaffertv. Ed., "Atlas of Mass SpeEtral Data," John Wiley and Sons, New York. N.Y., 1969. J. Kelder, J. A. J. Geenevasen. and H. Cerfontain, Svnth. Commun.. 2, 125 (1972). R. C. Dunbar. D. A. Chatfield, and M. M. Bursey, lnt. J. Mass Spectrom. /on Phys., 13, 195 (1974). R. E. Ireland and J. A. Marshall, J. Org. Chem.. 27, 1615 (1962). B. Kirshna and K. K. Srivastava. J. Chem. Phys., 27, 835 (1957); J. Chem. Educ., 32, 663 (1960). K. Higashi, Bull. lnst. Phys. Chem. Res. (Tokyo). 22, 805 (1943). P. A. Dobosh. CNINDO, Quantum Chemistry Program Exchange No. 141, Bloomington, Ind. A. L. McClellan, "Tables of Experimental Dipole Moments." W. H. Freeman and Company, San Francisco, Calif.. 1963, p 208. W. Huckel, Justus Liebigs' Ann. Chem., 441, 1 (1925). J. L. Beauchamp, L. R. Anders. and J. D. Baldeschwieler. J. Arner. Chem. SOC., 89, 4569 (1967). J. M. S. Henis. J. Amer. Chem. SOC., 90, 844 (1968). J. M. S. Henis, Anal. Chem., 41, (IO), 22A (1969). M. L. Gross, P.-H. Lin, and S. J. Franklin, Anal. Chem., 44, 974 (1972). T. Su and M. T. Bowers, J. Chem. Phys., 58, 3027 (1973).
RECEIVEDfor review August 8, 1974. Accepted December 2, 1974. This work was supported by the National Institute of General Medical Sciences (GM15994) and the Alfred P. Sloan Foundation. The ICR spectrometer was purchased through funds from Hercules, Inc., the Shell Companies Foundation, the North Carolina Board of Science and Technology (159), and the National Science Foundation (GU 2059).
Proposed Method for Mass Spectrometric Analysis for Ultra-Low Vapor Pressure Compounds Robert T. Mclver, Jr.,' Edward B. Ledford, Jr., and Judith S. Miller Department of Chemistry, University of California, Irvine, Calif. 92664
A trapped ion cyclotron resonance mass spectrometer with greatly improved mass range has been developed. The most novel feature of the instrument is its remarkable ability to trap gaseous ions. At a pressure of 4.7 X lo-' Torr, 80% trapping efflciency is observed after 3 seconds. This feature allows chemical ionization mass spectra to be obtained at very low pressures. A wide variety of positive and negative reagent ions can be generated by electron impact and allowed to react with the vapors of the sample to be analyzed. Calculations indicate that sample vapor pressures as Torr can be detected by this method. low as
One of the major problems presently confronting mass spectrometry is the analysis for low volatility compounds. This problem is especially acute in applications of mass spectrometry to studies of biological importance. In general, biological compounds exhibit high molecular weight, high polarity, and the ability to form strong hydrogen bonds. Such compounds have low vapor pressures and sublime slowly even a t elevated temperatures. The traditional approach to this problem has been to chemically derivatize l
692
Author to whom reprint requests should be addressed. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
samples in order to enhance volatility. However, chemical modification not only increases the molecular weight but also is time consuming, difficult, and uncertain, especially when only small samples are available for analysis. A number of new methods have been introduced recently for the analysis for low volatility compounds. Field desorption mass spectrometry has proved useful for a large number of polar compounds of low volatility ( I , 2). Rapid heating of a sample dispersed on a Teflon surface has been shown to greatly increase the rate of sample evaporation relative to competing surface decomposition reactions (3, 4 ) . Underivatized oligopeptides have been analyzed in a high pressure chemical ionization source by inserting a solid-sample probe directly in the plasma of reagent ions (5). The analytical potential of the ion cyclotron resonance technique has been discussed previously by several authors (6-11). But despite some initial success in developing specific reagent ions, the scope of the investigations was severely restricted by the limited mass range and low mass resolution of the early instruments. In this paper, we describe a trapped ion cyclotron resonance mass spectrometer which not only overcomes these problems but also provides
