(38) E. G. Smith and P. A. Baker, The Wiswesser Line-Formula Chemical Notation, 3rd ed., CIMI, Cherry Hill, N. J., 1975. (39) OCETH 13C NMR Data Collection, Laboratorium fur Organische Chernie, ETH, Zurich (40) J. T. Cierc, R. Knutti, H. Koenitzer, and J. Zupan, Fresenlus' 2 . Anal. Chem.. 283. 177 (1977). (41) Sadtler' Standard Spectra, Sadtler Research Laboratories, Philadelphia, Pa, (42) DMS, Documentation of Molecular Spectroscopy, Weinheim, Verlag Chemie, and Lond0n:Butterworth. (43) Aldermaston Mass Spectrometry Data Collection, Reading, U.K. (44) J. T. Cierc, private communication. (45) J. T. Clerc and J. Zupan, Proceedings of the IUPAC, International Symposium on Technique for the Retrieval of Chemical Information, London, Nov. 1976.
(46) D. S . Erley, Appl. Spectrosc., 2 5 , 200 (1971). (47) J. Zupan and D. Hadii, "Computers in Chemical Research Education and Technology", E. V. Ludena and F. Brito, Ed., Advances Studies Center IVIC, Caracas, Appartado 1827 (1977). (48) M. Penca, J. Zupan, and D. Hadii, Anal. Chim. Acta, in press. (49) . , T. L. Isenhour and P. C. Jurs. Anal. Chem.. 43 (101. 20A. (1971) and references cited therein.
RECEIVED for review May 9, 1977. Accepted June 29, 1977. Financial support of the Research Community of Slovenia is ~h~ work was supporkd in part also by the Pharmaceutical Factory KRKA.
Positional and Geometric Characterization of Olefinic Double Bonds by Fluorine Magnetic Resonance Spectrometry Michelle V. Buchanan, David F. Hiiienbrand, and James W. Taylor" Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Conversion of n-alkenes to hexafluoroacetone ketals has allowed the characterization of the double bond position and geometry using IgF NMR. Geometry can be determined by the chemical shift, with a 0.775 ppm separation between the cis and trans isomers. Double bond position can be determined by the value of bAB for the A3B3pattern, which decreases as the bond moves to the center of the molecule. A method by which AvABmay be estimated without the use of computer simulation is discussed. Application of these findings to the quantitative determination of geometric mixtures of linear alkenes is examined. Prior separation is required for analysis of positional mixtures but not for geometric mixtures.
T h e determination of both the geometry and the position of the double bond in large alkenes has proven to be a difficult problem because of the minor differences in physical properties of isomeric alkenes. One approach to characterizing these compounds is an analytical technique which provides a derivative wherein the differences between the isomers are maximized. For efficiency, the chemical preparation of the sample derivative needs to be simple, and the analysis should yield both the geometry and the position of the double bond without the use of standards. T h e derivatives which were first used to give information on both geometry and position of the double bond were cyclic esters. The first of these were cycloboronate esters (I) which were prepared from 1,2- and 1,3-diols (1-3). In another
I II m approach, a cyclic acetone ketal (11) was prepared by stereospecific conversion of the alkene to its diol using Os04, followed by condensation of the diol with acetone to form the corresponding cyclic ketal ( 4 , 5 ) . Mass spectral examination of the electron impact induced fragmentation gave ions which could be related to both the geometry and position of the 2146
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
original double bond. These isomers could be separated by gas-liquid chromatography to provide geometric identity between isomers of the same olefin, b u t positional isomers could not be distinguished if the original double bond were near the center of a long chain. Furthermore, this derivative was quite easily hydrolyzed so all handling procedures had to exclude moisture. The hexafluoroacetone (HFA) analogue of the acetone ketal (111) was synthesized to give more easily interpreted mass spectra because it would allow for the distinction between fragments originating in the ring as opposed to those from the hydrocarbon chain (6, 7). This derivative was successful in determining the bond position and was easily handled, but it could not be used to distinguish between geometric isomers without reliance on gas chromatographic retention times and standards. Because the HFA ketal has two sets of three equivalent fluorine atoms in the ring, it was thought that these fluorines might provide a sensitive probe into both the geometry and position of the double bond. The purpose of the present paper is to examine the use of HFA and "F NMR as a tool for the characterization of both the geometry and the position of double bonds in linear alkenes.
EXPERIMENTAL The 2,2-his(trifluoromethyl)-1,3-dioxolanes were prepared by first stereospecifically converting the olefin into the corresponding hromohydrin (IV). Then the bromohydrin was condensed with HFA to form the ketal (V). All olefins were used as received from
ET Y Chem Samples Co., Columbus, Ohio, and hexafluoroacetone was obtained from PCR, Inc., Gainesville, Florida. A detailed account of the experimental process for the preparation of the derivatives may be found in the literature (7). Preparative gas chromatography was performed using a Varian-Aerograph Model 705 gas chromatograph with a 0.95 mm 0.d. x 6.1 m aluminum column packed with 10% OV-1 on 60/80 mesh Gas Chrom Q. Normal operating conditions were: injector, 200 "C; flame ionization detector, 200 "C; detector split ratio, 1 O : l ;
-
-
~
Table I. Chemical Shifts of Hexafluoroacetone Derivatives vs. C, F, Octene Decene Dodecene Octene Nonene Decene Dodecene
,
"
cis-2 7912.6 7912.1 trans-2 7836.9 7836.7
cis-3 7913.6 7912.4 7915.1 trans-3 7843.8 7842.8 7842.7 7841.4
cis-4 791 6.4 7918.6
cis-5 7917.2
trans-4 7841.2
trans-5
7344.4
7845.4
,
Flgure 1. Proton-decoupled "F NMR spectrum of HFA derivatized cis-4-octene (AuB = 10.5 Hz) with computer simulated spectrum below
column, 150 "C; flow rates: air, 400 mL/min; hydrogen, 22 mL/min; helium (carrier), 200 mL/min. Injections of 50-100 gL were made and the heart cut material was collected a t room temperature. The proton-decoupled '9 NMR spectra were obtained at 94.176 MHz a t an ambient, probe temperature of 28 OC and deuterium lock using a Varian XL-100/15 NMR spectrcmeter with the Varian FT Accessory and Varian 620-L computer. The spectra were run with 50 pL of the compound and 10 pL of hexafluorobenzene (HFB) dissolved in 300 pL of acetone-& The chemical shifts of the compounds were referenced to the sharp singlet of HFB (136 Hz downfield from a hexafluorobenzene standard sealed in a capillary). HFB was used as the internal reference because it was found that the chemical shifts of the olefin derivatives could be quite sensitive to changes in concentration. When referenced to internal CFC13,this concentration effect is large (0.43 Hz upfield shift for each 1-pL increment of CFCll added, up to 30 pL), and would necessitate careful control of the concentrations of all species in solution to ensure reproducible chemical shifts. However, when the chemical shifts of the derivatives are referenced to internal HFB, the dependence of chemical shifts on concentration is negligible (0.05 Hz upfield shift for each l-kL increment of HFB added, up to 30 pL) compared to the 73 Hz separation between cis and trans isomers. Sample preparation and handling are also simplified using HFB. The problem of chemical shift changes due to concentration and solvent variation will be discussed in a subsequent paper (8). Quantitative determinations of mixtures of cis and trans olefins were made either by using an internal integral standard to calibrate the spectral integral, or by using standard additior. techniques. Ethyl trifluoroacetate (Aldrich Chemical Co.. Milwaukee, Wis.) was found to be a suitable internal integral standard because its chemical shift is close to, but does not interfere with, the derivatized olefin signals. Fourier transform NMR was used because the enhanced signal-to-noise ratio allowed accurate determination of the peak positions of the small side peaks of the spectra (Figure 1). The positions of these small peaks were greatly affected by small changes in Aum and JABwhereas the positions of the more intense central peaks changed very slightly. Thus, knowing the positions of the small peaks, it is possible to determine A u . and ~ J , k B more precisely. Confirmation of the values of A u . and ~ J,- was obtained by simulated spectra. These spectra were obtained on the University of Wisconsin Chemistry Department's IBM 7094 using Bothner-By and Castellano's LlzOCS3 self-iterative program (9). This program was easily modified to accept the nine equivalent coupling constants of the A3B3pattern in one parameter set. Only six parameters were allowed in one set in the original program. and The spectra were simulated by first inputting values of hAB JAB and allowing the computer to generate a table of frequencies and intensities of the lines expected in the spectrum. The calculated frequencies were then matched with the observed
Flgure 2. Proton-decoupled 19F NMR spectrum of a mixture of 90 p L of cis-4decene (7918.6 Hz) and 50 p L of trans-4decene (7844.4 Hz) with 50 pL of ethyl trifluoroacetate (8328.3 Hz) added as an integral standard
experimental frequencies and input to the computer. All calculated frequencies not observed in t.he experimental spectrum due to either low intensity or line overlap were not used. The computer then iteratively calculated, 'by a least-squares criterion, the chemical shifts and coupling constants which yielded the best fit. The best fit values for l v A B and J A B were used to plot the resulting spectra using a Bruker WH 270 (BNC 12 computer) and the ITHCAL six spin self-iterative program. The ITRCAL program had been modified to operate with 35! K of display by Jeff Luce of Nicoiet, Inc., Madison, Wis. For derivatives with values of A v p , B less than 5 Hz, the selfiterative LAO013 program was nct used because the smal! side peaks (Figure 1) were not visible. Without the small side peaks, not enough frequencies were available t o obtain unique values of A v A B and J A B in the simulations. In these cases, the ITRCAL program was used to generate a matrix of spectra with different values of Au,AB and J A B . The experimental spectra were then compared with these simulated spectra and fit using line width and intensities as criteria for a match.
RESULTS AND DISCUSSION The "F NMR of the HFA derivatives produce spectra with an A& pattern, such as displayed in. Figure 1. The 19Fspectra yield three parameters: chemical shift; J A B , t h e coupling constant; and AvAg, t h e difference in chemical shift between .the two sets of equivalent nuclei. T h e success of the analysis of the derivatized olefins using 19FNMR rests, therefore, on changes in these three spectral parameters with double bond position and geometry. T h e chemical shifts of the derivatives, taken as t h e center of t h e A3B3pattern, are shown in 'Table I. As can be seen from these data, t h e chemical shifts of t h e derivatives are constant with bond position but vary systematically with the original geometry about t h e double bond. T h e cis isomers exhibited an average chemical shift of 7915 Hz with t h e trans isomers showing 7842 Hz. These values are 84.04 and 83.27 ppm, respectively, from HFB, and represent a separation of 73 Hz at t h e 94.176 MHz l9F Larniour frequency. T h e separation of 73 Hz was found t o be sufficient for identifying which geometric cis or trans linear alkene was present as shown in Figure 2. T h e amounts of cis and trans ANALYTICAL CHEMISTRY, VOL. 49, NO. 1.4, DECEMBER 1977
0
2147
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Table 11. Quantitative hleasurement of Olefin Geometric Mixtures Integral intensity ratios Volumes of compounds Error, 7% added ( p L ) cisltrans Theoretical Experimental 0.20 0.21 5.0 10150 0.0 0.60 0.60 30150 0.80 0.80 0.0 40150 1.03 1.00 3.0 50150 1.23 2.5 1.20 60150 1.38 1.40 1.4 70150 1.60 1.58 80150 1.3 1.80 1.85 2.8 90150 2.06 2.00 100150 3.0
P 3
5 0 , 40.
30.
I04
10
isomers in a mixture were determined by integration, using ethyl trifluoroacetate to calibrate the spectral integral. As shown in Table 11, the amounts could be determined within *5% regardless of the particular linear olefin pairs employed. Computer simulation yielded both the A U ~values B and the coupling constants, JAB,,between the two sets of nuclei, A and B. These are shown in Table 111. T h e calculated values of the coupling constants for each series of geometric isomers are identical within experimental error (fO.l Hz) with the average value of J A B cis = 9.1 Hz and J A B trans = 8.4 Hz. Because these 4 J F F (four bond) couplings arise from through-space interactions (10, I I ) , they are sensitive to changes in distance between the two trifluoromethyl groups. T h e lower J A B value for the trans case suggests a possible increase in the angle between the two trifluoromethyl groups. This could occur if the steric interaction of the alkyl groups causes the ring to assume a slightly different conformation for t h e two geometric isomers. Except for the cis-2 case, within each geometric series, AvAB decreases as the double bond moves to the center of the molecule, as can be seen from the data in Table 111. This allows the original double bond position to be determined by the value of A v ~ B .Because of the unique symmetry when R1 = RP,both trans-5-decene and trans-4-octene yield values of AVAB= 0. As the alkyl groups become more dissimilar, the value of AvAB increases until it finally reaches a maximum at the trans-2 case. T h e AvAB cis values are larger overall than AuAB trans, since both alkyl groups are adjacent to one trifluoromethyl group in the cis case, making the environments of the two trifluoromethyl groups even more dissimilar. However, just as in the trans case, A v A B cis increases as the difference in the alkyl groups increases. The only exception is when the original molecule is cis-2-decene or cis-2-octene. B anomalously small. I n these cases, the values of A V ~ are T h e 1-octene derivative (7860.0 Hz vs. HFB), like the cis-2 olefin derivatives, has a small value of Avm. The data in Table I11 show that for the cis case, small A V A B values occur only when one alkyl chain has fewer t h a n two carbons. From molecular models, it may be seen that an alkyl group with two or more carbons can interact more strongly with its adjacent Table 111. Computer Simulated Values of A V A B and
Octene JAB
trans-2 trans-3 trans-4 trans-5 cis-2 cis-3
8.41
AVAB 7.44
JAB
30
40
50
60
70
80
*Y*B
Figure 3.
Spectral width as a function of AvABfor an
A3B3 pattern
trifluoromethyl group than can the smaller groups (R = H or CH3). It is possible that without this interaction, the two trifluoromethyl groups become more equivalent, yielding small AvAB values. Further, when R1 = H , the molecule is less symmetric than when R1= CHB, giving 1-octene a larger Aum value (4 Hz) than the cis-2 derivative. Computer simulations showed that as AvAB increased ( J A B remaining constant), the spectral width of the main portion of the spectrum, between points X and Y in Figure 1,increased as shown in Figure 3. All of the olefin derivatives in this study have AVABvalues between 0 and 15 Hz. In this region, a straight line relationship with spectral width may be assumed with a slope of 1.45. By dividing the spectral width in hertz between points X and Y by 1.45, the value of A V A B for the olefin derivatives may be estimated within a few tenths of a hertz (Table 111). Below AvAs = 1 Hz, the spectrum assumes the appearance of a singlet and the spectral width becomes complicated by line width. Above AVAB= 15 Hz, the slope becomes unity. Although any line position in the main part of the spectrum could have been monitored for a relationship with A v A B , points X and Y were chosen because these two features remain distinct as A V A B decreases to 1 Hz. T h e only overlap appears with trans-4decene and trans-5-decene, which cannot be distinguished using this estimation technique because both have Avm values less than 1Hz and thus, points X and Yare not distinct. However, if the linewidths of these two compounds are compared with that of the internal reference, HFB, they can be distinguished. Since both H F B and trans-5-decene are singlets, they have the same linewidth, whereas trans-4-decene is not a true singlet and has a linewidth broader t h a n that of HFB. In an analytical application, therefore, it is necessary only to determine the values of the points X and Y in hertz to determine the actual bond position. T h e geometry is determined by the location of the center of resonance. Thus the double bond position and geometry of an unknown linear olefin can be determined using the values of AvAB and the
JABa
Decene
Nonene AVAB'
20
AVAB AVABC
7.5
JAB 8.44
Dodecene
A ~ A B AVAB'
7.78
7.9
JAB
AVAB
AVAB'
1.7 8.4 1.5 1.6 1.7 8.4 0.2 >O b 8.4 0.0 Ob 8.4 9.1 0.2 0.2 9.1 14.0 9.01 13.85 13.6 9.14 14.17 13.9 14.22 9.11 9.6 9.14 10.53 9.09 cis-4 10.5 9.57 8.0 cis-5 7.96 9.16 See text for explanation. a Values for non-iterative simulations are given with fewer significant figures. See text. Values obtained from spectrum and Figure 3. 2148
8.4
1.9
8.4
0.0
1.9
...
8.4
1.8
1.9
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
chemical shifts from ”F NMR spectra of the hexafluoroacetone derivatized olefins. T h e proton decoupled spectra are obtained using the acetone-& solvent as a lock signal and are referenced to the singlet of internal hexafluorobenzene. Values of AvAB for the A3B3pattern may be obtained directly from the spectral width, eliminating the need for computer analysis and simulation of the spectra. Because of the employment of I9F,the NMR technique is sensitive only to the derivative of the olefin and the derivatives need not be purified as done in this study before being subjected to NMR analysis. Mixtures of geometric isomers and isomers of different bond positions could be determined by this technique coupled with GC/MS. T h e HFA derivatization technique has been shown to be useful for linear olefins and, more recently, we have also applied i t t o long chain (>C16)linear fatty acids. Work is currently in progress to expand the application of this analysis t o cyclic olefins and steroids.
LITERATURE CITED (1) C. J. W. Brooks and J. Watson, J. Chem. Soc., Chem. Commun., 952
(1967). (2) C. J. W. Brooks and I. Maclean. J . Chromatogr. Sci., 9, 18 (1971). (3) W. Blurn and W. J. Richter, Helv. Chim. Acta, 57, 1744 (1974). (4) R. E.Wolff, G. Woiff, and J. A. McCloskey, Tetrahedron,22, 3093 (1966). (5) J. A. McCloskey and M. J. McClelland. J . A m . Chem. Soc., 87, 5090 (1965). (6) B. M. Johnson and J. W. Taylor, Org Mass Spectrom., 7, 259 (1973). (7) B. M. Johnson and J. W. Taylor, Anal. Chem., 44, 1438 (1972). (8) M. V. Buchanan. D. F. Hilienbrand and J. W. Taylor, in preparation for submission to J . Magn. Reson. (9) A . A . Bothner-By and S. M. Castellano. in “Computer Programs for Chemistrv”. Vol. I. D. F. Detar. Ed., W. A. Beniamin, Reading, Mass., 1968, p p 10-53. (10) L. Petrakls and C H. Sederholm, J . Chem. Phys.. 35, 1243 (1961). (11) S. Ng and C. H. Sederholm, J . Chem. Phys., 40, 2090 (1964).
RECEIVED for review July 12,, 1977. Accepted September 15, 1977. This research has been supported by the National Science Foundation under grants MPS-74-23569 a n d MPS-75-21059.
Deconvolution and Background Subtraction by Least-Squares Fitting with Prefiltering of Spectra Peter J. Statham’ Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 947:?0
Deconvoiution of overlapped peaks in a spectrum Is complicated by the presence of a high background component. A method of background subtraction, whlch involves suppressing the background in both the specimen data and peak modeis wtth a digltai filter before proceeding to a conventlonal least-squares fit, Is analyzed. The dlmensions of a “top-hat’’ filter are found which give a suitable compromlse with regard to statistical accuracy and sensitivity to both background curvature and possible errors in the peak models. The major advantages over conventional techniques are that the shape of the background need not be known explicitly, there Is no need to find suitable points away from peaks for background scaling, and any background that is approximately linear over the range covered by a single peak will be effectively removed.
T h i s paper addresses itself primarily to the problem of finding peak areas in a digitized x-ray energy spectrum obtained with a solid-state detector, although the problem is essentially one of deconvolving overlapped peaks in the presence of a high background component. In the usual linear least-squares procedure, the sum of a number of functions is fitted to the specimen data to find the contribution from each peak. Each function must exactly model the peak, or series of peaks (for example, the Kcu and K(3 peaks for a given chemical element) it is meant to represent, and the procedure is only applicable when the specimen data can be represented by a linear sum of the model functions plus statistical “noise”. In the special case of x-ray spectra excited by electron bombardment of a flat polished specimen, the background Present address, Link Systems, Halifax Road, High Wycomhe, Bucks, England HP12 3SE.
shape can be predicted quite accurately ( I , 2) and the background can therefore be subtracted away provided that suitable points, away from peaks, can be found for scaling, or if the shape function is calculated at every point, the background can be included as an additional function in the fitting procedure. However, when electron-beam-excited specimens are not perfectly flat (for example, particles), or if we are dealing with x-ray fluorescence or y-ray spectra, the background shape is not so predictable and one has to exploit any features which distinguish background from peaks. In this respect, the most obvious feature is the slow variation of the background with energy compared with the faster variation of structure in the peaks. This suggests methods such as interpolating the background beneath the peaks using smooth analytical functions but this may be inaccurate when carried out over the large range required when several peaks overlap (3). T o get around this problem, coefficients for a polynomial function can be included as undetermined parameters in the least-squares analysis, thus removing the need for an explicit fit to available background points. However, the function used must accurately represent the background over the whole fitting range and while regions of high curvature, such as absorption steps, can be accommodated by including higher terms in t h e polynomial, this may make t h e technique unstable with regard to small errors in the peak model functions. An earlier study (3) discussed further methods of background correction where the background is implicitly taken into account in the peak deconvolution process. In the “iterative stripping” approach (3),peaks are removed in stages using a symmetric, zero-area weighting function a t each stage to estimate the area of each peak above local background. This method is quick and very easy to program but is limited to resolving peaks which are greater than 0.66 fwhm (full width half maximum) apart. T h e iterative process demands a ANALYTICAL CHEMISTRY, VOL. 49, NO. ‘14, DECEMBER 1977
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