Gas-Phase Infrared Spectroscopy for Determination of Double-Bond

Gas-phase Fourier transform infrared spectroscopyallows the determination of the ... 3011 cm™1 attributable to the cis =CH stretch vibration.1™3. ...
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Anal. Chem. 1995, 67, 1558-1567

Gas-Phase Infrared Spectroscopy for Determination of Double=BondConfiguration of Some Polyunsaturated Pheromones and Related Compounds Athula B. Attygalle,* Ale5 Svato5,t and Charles Wilcox

Baker Laboratory, Department of Chemistry, Come11 University, Ithaca, New York 14853 Simon Voennan

Research Institute for Plant Protection, P.O. Box 9060, 6700 GW Wageningen, The Netherlands

Gas-phaseFourier transform infrared spectroscopy allows the determination of the geometric configuration of disubstituted carbon-carbon double bonds present in linear polyunsaturated compounds on the basis of the characteristic absorptions observed in the 3036-3011 and 982-947 cm-I regions. Although the 3036-3011 cm-I band, attributable to a =CH stretch vibration, is conventionally considered to be unreliable for cis/trans determinations, data obtained from a large number of mono- and polyunsaturated acetates show that many useful deductions can be based on this absorption. Double bonds of the RCH=CHR' type separated by at least two methylene groups, present in polyunsaturated compounds, give rise to an absorption at 3013-3011 cm-' if the double bonds are not proximal to a functionalgroup or a terminal methyl group. Cis dienes whose double bonds are separated by only a single methylene group show the cis =CH stretch band at 3017 cm-I. For methylene-interrupted polyenes, this band shows a gradual hypsochromic shift as the unsaturation increases. Furthermore, a band at 1391 cm-' was recognized to represent the deformation vibration of the CH2 groups located between the double bonds. The intensity of this band is proportional to the number of skipped methylene groups. None of the isolated or methylene-interrupted dienes bearing only trans double bonds showed any significant absorptions above 3000 cm-'. However, spectra of trans-trans conjugated compounds show a =CH stretch band at 3016-3012 cm-' as a well-defined shoulder on the high-frequency side of the CH2 asymmetric stretch band, in addition to the wag absorption observed at 984982 an-],which is highly characteristic for trans-trans conjugated compounds. The spectra of cis-trans or trans-cis conjugated compounds are virtually identical and show a characteristic "fingerprint" consisting of two bands of similar intensity at 978-976 and 949-946 cm-l, in addition to the 4 - H stretch band observed at 3021-3017 cm-'. Interestingly, not only are the spectra of conjugated cis-cis compounds devoid of any signiscant absorption in the 982-947 cm-' region, but also the =CH stretch band is broad and appears as a poorly 1558 Analytical Chemistry, Vol. 67, No. 9,May 7, 7995

defined shoulder on the high-frequency edge of the CH2 asymmetric stretch band. Our recent studies on gas-phase Fourier transform infrared spectra of monounsaturated long-chain compounds led to many helpful generalizations for characterizing the configuration of double bonds of the RCH=CHR' type.' We have demonstrated that the presence or absence of the 3029-3011 and 968-964 cm-' absorption bands in gas-phase spectra allows unambiguous determination of cis and trans bonds in linear monounsaturated compounds such as insect pheromones and similar compounds. The spectra of most cis monoenes show an absorption at 30133011 cm-1 attributable to the cis =CH stretch However, for alkenes which are also acetates, this cis band shows a hypsochromic shift to 3029-3028 or 3018-3017 cm-' if a double bond is present respectively at the C-2 or C-3 carbon atoms. A similar shift toward higher frequency (3022-3021 cm-l) is also observed if the double bond is present at the penultimate carbon atom. In the present paper, we extend these generahations for monounsaturated compounds to polyunsaturated compounds. We now present the spectra of many polyunsaturated compounds to demonstrate the value of GC/FT-IR techniques in the confguration determination of geometric isomers of compounds bearing two or more double bonds. EXPERIMENTAL SECTION Reagents. (7E,S.Z)-7$Dodecadienyl acetate, (8E,lOE)-8,1@ dodecadienyl acetate, (82,1OE)-8,l@dodecadienyl acetate, (4E,7Z,102)-4,7,l@tridecatrienylacetate, (12E,14E)-12,14hexadecadienyl acetate, and all isomers of 9,124etradecadienyl acetate, 2,13octadecadienyl acetate, and 3,13-octadecadienylacetate were from the synthetic pheromone collection of the Research Institute for Plant Protection (Wageningen, The Netherlands)." The four isomers of 4,Ghexadecadienyl acetate and those of 4,11-hexadeca' Permanent address: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2,166 10 Prague 6, Czech Republic. (1) Attygalle, A B.; SvatoS, A; Wilcox, C.; Voerman, S. Anal. Chem. 1994,66, 1696-1703. (2) Doumenq, P.; Guiliano, M.; Mille, G. Int. j . Environ. Anal. Chem. 1989, 37, 235-244. (3) Doumenq, P.;Guiliano, M.; Mille, G. Analusis 1989,17, 39-49. (4) Voerman, S. Agric. Ecosyst. Environ. 1988,21, 31-41. 0003-2700/95/0367-1558$9.00/0 0 1995 American Chemical Society

dienyl acetate were prepared by acetylating the corresponding alcohols given to us by Prof. I. Tomida. 4,6,1@Hexadecatrienyl acetate isomers were from Dr. k Cork, and (4E,7Z)-4,7-tridecadienyl acetate and (72,112)-7,1l-hexadecadienyl acetate were from the Institute of Organic Chemistry (Czech Academy of Sciences, Czech Republic). (72,92)-7,SDodecadienyl acetate, (132)-13hexadecen-11-ynyl acetate, (13E)-13-hexadecen-ll-ynylacetate, (8Z,102)-8,l@dodecadienyl acetate, and the four isomers of 9,lltetradecadienyl acetate and llJ3-hexadecadienyl acetate were from Prof. H. J. Bestmann. (9E,l2@-9,12-0ctadecadienyl acetate and (92,122)-9,12-octadecadienylacetate were purchased from Sigma Chemical Co. (St. Louis, MO). (6Z,92,122)-6,9,12-Octadecatrienyl acetate, (92,122,152)-9,12,15ctadecatrienyl acetate, and (42,72,102,132,162,192)-4,7,10,13,16,19-docosahexaenyl acetate were made from the corresponding alcohols (Sigma). (62,9Z,122,152)-6,9,12,150ctadecatetraenylacetate was prepared from methyl (62,9Z,122,152)-6,9,12,15octadecatetraenoate(Sigma). (52,82,llZ,142)-5,8,11,14Eicosatetraenylacetate (arachidonyl acetate) was made from the corresponding acid (Sigma). (92,122)9,lZOctadecadienyliodide was made from linoleyl alcohol (Sigma). (92,122,152)-9,12,15[9,10,12,13,15,162H61 Octadecatrienyl acetate (1)and the corresponding compound (2) bearing only one olefhic deuterium atom at each carbon-carbon double bond were made from the respective alcohol^.^ Samples of (62,9Z)-alkadienes bearing 18, 20, and 22-27 carbon atoms were prepared by a CuCN/LiCl-cataIyzed coupling reaction of linolenyl bromide or iodide, prepared from methyl linolenate, with an appropriate Grignard reagent (ethyl- to nonWhen linolenyl ylmagnesium bromide) in THF at -20 bromide was used as the staring material, a small amount of (6Z,92)-6,9atadecadiene was also formed as a byproduct. (62,SZ)6,SNonadecadieneand (62,92)-6,9heneicosadienesamples were made from linolenyl ptoluenesutfonate by treating it with lithium dimethylcuprate and lithium di-n-pr~pylcuprate,~ respectively. The alkadienes synthesized exhibited the expected spectral data. The following data given for (62,92)-6,Sheneicosadieneare characteristic generally for the series. 'H NMR (CDC13, 400 MHz): 6 5.36 (m, 4H, CH=CH), 2.78 (t, J = 6.4 Hz, 2H, CHr8), 2.05 (m, 4H, CHr5,10), 1.4-1.2 (m, 24H, CHz), 0.89 (t,J= 7.4Hz, CHsl), 0.88 (t, J = 7.5 Hz, CH3-21). 13C NMR (CDCh, 100.6 MHz): 6 130.2 (=CH-6,10), 127.9 (=CH-7,9), 32.0 (CHr19), 31.5 (CHr3), 29.7 (CHr12), 29.7 (CHA), 29.6 (CHr18), 29.4 (CHr14,15,16,17), 29.3 (CHrl3), 27.4 (CHr5), 27.3 (CHrll), 25.6 (CHr8), 22.7 (CHr 2), 22.6 (CHr20), 14.2 (CHr21), 14.1 (CH3-1). ELMS m/z (relative intensity) 292 (22), 222 (l),194 (3), 180 (3), 166 (4), 152 (5), 138 (131, 124 (19), 110 (32), 96 (69), 95 (56), 82 (74), 81 (82), 68 (46), 67 (loo),55 (57), 43 (52), 71 (71). A sample of (32,6Z,92)-3,6,Sheneicosatrienewas prepared according to a published procedure.8 (32,6Z,92)-3,6,STricosatriene was available in our laboratory collection of chemicals. (3E,8Z)-3,8Tetradecadienylacetate! (3E,llz)-3,ll-tetradecadienyl acetate! (8Z,ll2)-8,11-tetradecadienylacetate? and (3E,8Z,llZ)3,8,11-tetradecatrienylacetatelowere synthesized in our laboratory. (5) SvatoS,A; Attygalle, A B.; Meinwald, J. Tetmhedron Lett. 1994,35,94979500. (6) Erdik, E. Tetrahedron 1 9 8 4 , 40, 641-657. (7) Jain, S. C.; Dussourd, D. E.; Conner, W. E.; Eisner, T.; Guerrero, A; Meinwald, J. /. Org. Chem. 1 9 8 3 , 48, 2266-2270. (8) Conner, W. E.; Eisner, T.; Vander Meer, R IC; Guerrero, A; Ghiringelli, D.; Meinwald, J. Behav. Ecol. Sociobiol. 1 9 8 0 , 7,55-63. (9) SvatoS. A; Attygalle, A B.; Meinwald, J., unpublished.

(42,7Z,lOZ)-4,7,l@Tetradecatrienyl acetate was synthesized according to the following procedure. 1,4Dibromobut-2-ynewas alkylated in THF at room temperature with l-(2H-tetrahydropyr a n y l o x y ) - b u t - ~ y n y ~ a ~bromide e s i ~ in the presence of CuBrS (CH3)2 (10 mol %).I1 The product, bromodiyne, was alkylated with 1-pentynylmagnesiumbromide under the above-mentioned conditions to form 1-(2H-tetrahydropyranyloxy)-4,7,l@tetradecatriyne. This unstable product was immediately hydroborated with dicyclohexylborane12to give (42,72,102)-l-(W-tetrahydropyranyloxy)4,7,1@tetradecatriene. The product was purified, deprotected by stirring with Dowex 50WX8 ion-exchange resin in methanol, and acetylated with AczO/pyridme to give the desired product (42,72,102)-4,7,l@tetradecatrienyl acetate [purity 96% (GC, DB-5 column)]. 'H NMR (C&, 400 MHz): 6 5.46 (m, 5H, CH=CH5,7,8,10,11),5.29 (dtt,J= 10.9, 6.7, 7.3, 1.5, 1.5 Hz, lH, =CH-4), 3.99 (t, 2H, t , J = 6.7 Hz, CHrl), 2.83 (dd,J= 5.7 Hz, 4H, CHr 6,9), 2.01 (m, 4H, (CHr3,12), 1.70 (s, 3H, CH3C=O), 1.50 (tt,J= 6.7, 6.7, 7.2, 7.4 Hz, CHr2), 1.35 (tq,]= 5.7, 7.7, 6.8, 7.4, 7.4 Hz, CHr13), 0.89 (t, J = 7.3, 7.4 Hz,CH3). NMR (CDCl3, 100.6 MHz): 6 171.1 (C=O), 130.2, 128.9, 129.6, 128.3, 127.8, 127.7 (CH=CH-4,5,7,8,10,11),63.9 (CHrl), 29.3 (CH23), 28.5 (CHrl2), 25.6 (CHr6,9), 23.6 (CHr2), 22.8 (CHr13), 21.0 (m&=O), 13.8 (CH3-14). ELMS: m/z (relative intensity) 250 (M+,4), 190 (2), 147 (6), 133 (13), 120 (14), 119 (14), 105 (20), 91 (60), 79 (loo), 67 (65), 61 (2), 55 (22), 43 (86). Instnunentation. Vapor-phase GC/FT-IR spectra (resolution, 8 cm-1) were recorded on a Hewlett-Packard (HP) 5890 gas chromatograph coupled to a HP 5965A IRD instrument equipped with a narrow-band (4000-750 cm-') infrared detector (mercury cadmium telluride) as described previously.' Purity and identity of samples were checked by GC/MS performed on a HP 5890 gas chromatograph coupled to a HP 5870B mass selective detector.' Ab Initio Spectral Calculations. Ab initio spectra were calculated with the Gaussian 92 for Windows package.13 RESULTS AND DISCUSSION In our previous study, we recognized a number of characteristic infrared absorptions that are useful in determining the configuration of linear monounsaturated compounds bearing RCH=CHR' type bonds.' In order to see how these generalizations can be extended to polyenes, we recorded gas-phase infrared spectra of a number of polyunsaturated acetates. Dienes Separated by Several Methylene Groups. It is evident from the data for dienes summarized in Table 1 that if the two double bonds are separated by two or more methylene groups, and none of the double bonds is proximal to a functional group or a terminal methyl group, a diene spectrum can be predicted reliably on the basis of the two correspondingmonoene spectra. In fact, the diene spectrum is essentially that constructed by superimposing the relevant portions of the spectra of the corresponding monoenes. For example, the spectra of (4E,llZ)and (42,11E)-4,1l-hexadecadienylacetates, which are essentially (10) Attygalle, A B.; Jham, G. N.; SvatoS, A; Frighetto, R. T. S.; Ferrara, F. A; Uchoa, M.; Vilela, E.; Meinwald, J., submitted. (11) Baker, R; O'Mahony, M. J.; Swain, C. J./. Chem. Res. (S) 1984,190-191, (12) Millar, J. G.; Underhill, E. W. Can. /. Chem. 1 9 8 6 , 64, 2427-2430. (13) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A; Replogle, E. S.; Gomperts, R; Andres, J. L.; Ragavachari, IC;Binkley, J. S.; Gonzalez, C.; Martin, R L;Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A Gaussian 92, Revision D.2; Gaussian Inc.: Pittsburgh, PA, 1992.

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Figure 1. Gas-phase infrared spectra of (42,112)-4,1l-hexadecadienyl acetate (A), (4€,112)-4,1l-hexadecadienyl acetate (B), (42,ll E)4,ll-hexadecadienylacetate (C), and (4€,11€)-4,ll-hexadecadienyl acetate (D) (resolution,8 cm-I).

indistinguishable from each other (Figure l),are congruent with those obtained by merging the spectra of Q - 4 - and (3-11- or (2)4 and (E)-11-hexadecenyl acetates.' Moreover, in the case of (42,11Z)-4,11-hexadecadienylacetate, the two cis =CH stretch (str) bands merge constructively to yield a relatively intense combined absorption at 3013 cm-I. Such constructive merging that results in a combined band of enhanced intensity is observed only if the frequenciesof the cis =CH bands of the corresponding monoenes are relatively close to each other. Since the =CH stretch bands in the spectra of (3-4-hexadecenylacetate and (2)11-hexadecenylacetate appear at 3013 and 3012 cm-', respectively, the combined cis band observed at 3013 cm-' in the spectrum of (42,11Z)-4,ll-hexadecadienylacetate is explicable, although the intensity of this band is not exactly twice that observed for monoene acetates. Nevertheless, the intensity of the 3013 cm-I band is suflicientiy large to show that it represents two cis bonds. The value of 0.475 obtained for the ratio Of A=c-Hcis str/&-o from the data of (42,112)-4,1l-hexadecadienylacetate is in fact 175% that observed for monoene acetates. In our previous investigation,' we established the ratio of A-C-H cis s t r / A ~for - ~monounsaturated acetates as 0.270 f 0.034 (n = 43). The cis out-of-plane deformation (de0 is known to occur near 700 ~ m - l ;however, ~.~ the significance of this absorption was not evaluated in this study since the narrow-band detector used could not record frequencies below 750 cm-'. Analytical Chemistry, Vol. 67,No. 9,May 1, 1995

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The relative intensity of the =CH stretch band (A-C-H&str/ Ac-0) observed for (4EJlZ)- and (4Z,llE)-4,11-hexadecadienyl acetates is 0.333 for both compounds, which is somewhat high compared to that of the monoenes. However, the values of 0.216 and 0.204 obtained respectively for the intensity ratios of Ac-o for (4E,llZ)- and (4Z,11E)-4,11-hexadecadienylacetate agree well with the ratio obtained for monoenes, 0.212 & 0.031 (n = 43).‘ Finally, the spectrum of (4E,llE)-4,1l-hexadecadienyl acetate shows a distinctive absorption at 967 cm-l, which represents the out-of-plane symmetric bending frequency of the protons on a trans double bond14 (trans wag) .15 The intensity of the observed absorption attests clearly that it represents two trans bonds. The ratio of A-c-H transwag/A~=~ is 0.397, which is 187%that obtained for monounsaturated acetates. This value agrees with the gasphase results of Ratnayake et a1.,I6who reported the intensity of the trans band of methyl (9E,12E)-9,12-octadecadienoateas 174% that of methyl (E)-Soctadecenoate (methyl elaidate). Similar results have been reported for solution spectra as well,I* although it was thought initially that the absorption caused by trans wag of methyl linolelaidate was twice that of methyl elaidate.” The 968-964 cm-I band has been utilized in numerous structural elucidations (see the condensed-phase IR review by O’Connor18 and the citations therein; although vibrational frequencies change with physical state and the values given for the condensed phase usually differ from those observed in the vapor phase, a correlation can always be foundlg) since its first recognition in 1947 by Rasmussen et alSzoOil chemists, in particular, employ this band for the determination of total trans unsaturation in fatsz1 In fact, the measurement of the intensity of this absorption band is the basis of quantitative determination of the content of isolated trans compounds in fats and oils.22 The spectra of 2,13-octadecadienylacetates further exemplify that the diene spectra can be analyzed on the basis of the corresponding monoene spectra (Figure 2). However, in this system, one of the carbon-carbon double bonds is located at C-2. From the spectra of hexadecenyl acetates,’ we established that 2 2 compounds show the cis =CH stretch absorption at an unusually high value of 3029-3028 cm-I. Interestingly, the spectrum of (22,13E)-2,13-octadecadienylacetate (Figure 2C) agrees well with the predictions and shows an absorption at 3027 cm-l, indicating the cis bond at C-2. On the other hand, the cis =C-H stretch absorption of (2E,132)-2,13-octadecadienylacetate is observed at 3011 cm-’, which agrees with the expected values for a cis bond located toward the middle of a carbon chain. In &-,j-J

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(14) Scholfield, C. R.; Jones, E. P.; Butterfield, R 0.; Dutton, H. J. Anal. Chem. 1963,35,1588-1591. (15) Colthup, N. B.; Daly, L. H.; Wiberly, S. E, In Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, CA, 1990. (16) Ratnayake. W. M. N.; Hollywood, R; O’Grady, E.; Beare-Rogers, J. L.J. Am. Oil Chem. SOC.1990,67, 804-810. (17) Jackson, J. E.; Paschke, R F.; Tolberg, W.; Boyd, H. M.; Wheeler, D. H. J. Am. Oil Chem. SOC.1952,29, 229-234. (18) O’Connor, R. T. J. Am. Oil Chem. SOC.1961,38, 648-659. (19) Doumenq, P.; Guiliano, M.; Bertrand, J. C.: Mille, G. Appl. Spectrosc. 1990, 44, 1355-1359. (20) Rasmussen, R. S.; Brattain. R. R; Zucco, P. S.J. Chem. Phys. 1947,15, 135-140. (21) International Union of Pure and Applied Chemistry, Commission on Oils, Fats and Derivatives. Standard Methods for the Analysis of Oils, Fats and Derivatives; Blackwell Scientific Publications: Oxford, 1987. (22) Oficial Methods and Recommended Practices of the American Oil Chemists’ Society; AOCS Official Method Cd 14-61, Americam Oil Chemists’ Society: Champaign, IL, 1989.

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Figure 2. Gas-phase infrared spectra of (2Z,132)-2,13-octadecadienyl acetate (A), (2€,134-2,13-octadecadienylacetate (B), (2Z,13€)2,13-octadecadienylacetate (C), and (2€,13€)-2,13-octadecadienyl acetate (D) (resolution,8 cm-I).

the case of (2Z,132)-2,13-octadecadienylacetate, the intensity of the cis =CH stretch absorption is unusually low (A3013cm-’/ A1760cm-~ = 0.365) for a compound bearing two cis bonds. A careful examination of this absorption band showed that it is, in fact, a complex peak which shows a clear bulge on the highfrequency side of the band. Since the =CH stretch frequencies of (2)-2-octadecenyl acetate and (2)-13-octadecenyl acetate are 3028 and 3012 cm-’, respectively, on the basis of the band merging principle we discussed before, the low intensity observed for the =CH stretch band of (2E,132)-octadecadienylacetate is not at all surprising. Moreover, from our previous results,’ we have established that the saturated, and most of the monounsaturated, acetates show a band, usually at 1370-1367 cm-l, which represents the symmetric CH3 deformations of the acetyl This band shifts specifically to 1375-1374 cm-l when a 2-2 double bond is present in the acetate molecule. This conclusion is corroborated by the observation of the 1374 cm-l band in the spectra of (22,132)- and (22,13E)-2,13-octadecadienylacetates (Figure 2 4 0 . Moreover, no other spectrum presented in Table 1shows such a high value for this band. Furthermore, the three-peak “fingerprint”centered (23) Katritzky, A. R.; Lagowski, J. M.; Beard. J. A. T. Spectrochim. Acta 1960, 16, 964-978.

around 1022 cm-' (this represents 0-CHrC asymmetric stretching, which usually appears at around 1040 cm-' for saturated and most unsaturated acetates'), recognized as characteristic for (I?,)2-alkenyl acetates, is seen in the spectra of (2E,13Z)- and (2E,13E)2,13-octadecadienyl acetates (Figure 2B,D). Of course, the 967 cm-I peak is much more intense @967cm-'/A1761 cm-1 = 0.474) in the spectrum of the latter, as expected for a compound bearing two isolated trms bonds. The band at 1025 cm-', with its characteristic shoulder on the lower frequency side, which was recognized as a diagnostic indicator of 2-2 unsaturation, is discernible in the spectrum of (2Z,132)-2,13-octadecadienylacetate (Figure 2A). However, in the spectrum of (22,13E)-2,13-0ctadecadienyl acetates (Figure 2C), this shoulder is obscured by the =CH wag absorption at 967 cm-'. Moreover, all saturated and monounsaturated acetates made from primary alcohols, except those bearing the double bond at C-2, show an absorption at 1233-1231 cm-l ascribable to the asymmetric stretching vibrations of the C-0 next to the carbonyl group.' Data from monoene acetates' show that this band appears characteristically at 1228 cm-' for compounds bearing a double bond at C-2. The diagnostic value of this band is exemplified by the fact that, of all the spectra reported in Table 1, only the four isomers of 2,13-octadecadienyl acetate (Figure 2) show a band at 1228 cm-l. The hypothesis that constructive merging that results in a combined band of relatively high intensity is observed only if the frequencies of the cis =CH bands of the corresponding monoenes are relatively close to each other is supported by the data of (32,132)-3,13-octadecadieny1acetate. The cis bands of (2)-3-and (2)-13-octadecadienyI acetates occur at 3017 and 3012 cm-', respectively.' The =CH stretch band of (32,132)-3,13-0ctadecadienyl acetate observed at 3014 cm-' shows a relative intensity of 0.441 (A3014 cm-1/A1761m-l), which is 163% that observed for monoene acetates. The spectra of the other three isomers of 3,13octadecadienyl acetate are also congruent with those predicted on the basis of bands in respective monoenes. For (I?,)-3 compounds, the 0-CHrC asymmetric stretching, usually o b served at 1040 cm-' for most acetates, is shifted to 1038-1037 cm-1. Interestingly, this absorption is observable not only in the spectra of (3EJ32)- and (3E,l3E)-3,15octadecadienylacetate but also in those of several other (E)-3 compounds, including a triene, listed in Table 1. The characteristic infrared absorptions observed in the spectra of other dienes bearing double bonds separated by two or more methylene groups (Table 1) can be explained on the aforementioned principles. For example, the spectra of (3E,82)-3,8 tetradecadienylacetate and (3E,llZj-3,11-tetradecadienyl acetate show, in addition to the cis =CH stretch and trans wag absorptions observed at 3012-3011 and 968-967 cm-l, respectively, another characteristic band at 1038-1037 cm-', which represents the 0-CHrC asymmetric stretching vibration of acetates bearing a trans bond at C-3.' Methylene-Interrupted Polyenes. Allcis dienes in which the double bonds are separated by only a single methylene group show the =CH stretch band characteristically at 3017-3016 cm-'. This is evident from the speciflc examples given in Table 1, such as (92,122)-9,12-octadecadienyl acetate, which shows the cis band at 3016 cm-I. This value is unaffected by the presence or absence of any remote functional groups since the spectrum of the hydrocarbon (62,92)-6,90ctadecadiene(Figure 3), and those of other (62,92)-6,9alkadienesbearing 19-27 carbon atoms given

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Wavenumber (em-')

Figure 3. Gas-phase infrared spectrum of (6Z,9Z)-6,9-octadecadiene (resolution, 8 cm-l).

in Table 2, show the =CH stretch absorption essentially at 3017 cm-'. Similar observations have been made for polyunsaturated fatty acid methyl esters bearing skipped cis bonds, which show this absorption at 3015 cm-l.I9 Furthermore, it is evident from methylene-interrupted diene spectra that the intensity of the =CH stretch band at 3017-3016 cm-', relative to those of the CHZbands at 2933-2932 or 29662964 cm-l, decreases gradually as the carbon chain length increases. In particular, the intensity ratio of 3017-3016 to 29662964 cm-' band can be used to estimate the number of double bonds present in a molecule if the corresponding value for the monoene with the same number of carbon atoms is known (see data in the last column of Table 2). For example, the values of this ratio for (2)-Sheneicosene, (62,92)-6,Sheneicosadiene,and (32,62,92,)-3,6,Sheneicosatriene are 0.258, 0.501, and 0.747, respectively. Such data allow the determination of the number of cis double bonds in a molecule. The decrease of this ratio as the carbon number increases is similar to the trend observed for the ratio of Aolefinic - C H ~ s v / A ~sp~ str 2 against the carbon number noted for l-alkene~?~ Moreover, the peak position of this band at 3017 cm-' is identical with that observed in the spectrum of (92,122)-9,12-0ctadecadienyliodide and those reported from gasphase spectra of other methylene-interrupted dienes such as methyl and ethyl linoleateaZ5Actually, the 3017 cm-' band is observed even when one of the methylene-interrupted double bonds has a trans configuration,as seen for the spectra of (4E,72)4,7-tridecadienyl acetate and (92,12E)-9,12-tetradecadienylacetate flable 1). A similar band, which occurs at 3018 cm-l, has been observed by Mossoba et aLZ6in GC/MI-FT-IR spectra of methyl (92,122)-9,12-0ctadecadienoateand its cis/trans and trans/cis isomers. In supercritical fluid chromatography(SFC)/FT-IR spectra, this band appears at 3017-3016 cm-1.27 For methylene-interrupted polyunsaturated acetates, the =CH stretch band shows a further small but gradual shift toward higher frequency as the degree of unsaturation increases. For example, the methylene-interrupted allcis octadecatrienyl and octadecatetraenyl, eicosatetraenyl, and docosahexaenyl acetates exhibit this band at 3018, 3019, and 3020 cm-', respectively (Table 1). A similar trend has been noted in the GC/MI-FT-IR spectra of cis unsaturated methyl The authors noted that MI-FT-IR spectra of cis monounsaturated methyl esters show the =CH stretch band at 3010-3009 cm-l, while those bearing two or three (24) Nyquist, R A The Interpretation of Vapor-Phase Infrared Spectra: Group Frequency Data; Sadtler Research Laboratories: Philadelphia, PA, 1984;Vol.

1. (25) Pouchert, C. J. The Aldrich Libraa of FT-IR Spectra, Vapor Phase, 1st ed.; Aldrich Chemical Co.: Milwaukee, WI, 1989 Vol. 3. (26) Mossoba, M. M.; McDonald, R E.; Chen, J.-Y. T.; Armstrong, D. J.; Page, S. W. J. Agric. Food Chem. 1990,38,86-92. (27) Calvey, E. M.; McDonald, R. E.; Page S. W.; Mossoba, M. M.; Taylor, L. T. J. Agric. Food Chem. 1991,39,542-548.

Analytical Chernistiy, Vol. 67,No. 9,May 1, 1995

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Table 2. Gas-Phase Infrared Data of Some Polyunsaturated Hydrocarbons (Resolution, 8 om-')

cis =CH str

band position,cm-I (percent transmission) CH2 asym + CHZ sym + CH3 asym str CH3 sym str CHZdef =CH-CHz-C=H

3017 (97.47) 3017 (99.44) 3017 (99.57) 3017 (98.71) 3017 (98.75) 3017 (99.43) 3017 (99.24) 3017 (99.57) 3017 (99.76) 3016 (99.82)

2933 (90.46) 2933 (97.59) 2933 (97.97) 2933 (93.47) 2933 (93.01) 2933 (96.58) 2933 (95.07) 2933 (97.02) 2933 (98.26) 2932 (98.71)

2866 (96.07) 2866 (99.05) 2865 (99.21) 2865 (97.45) 2865 (97.28) 2864 (98.68) 2864 (98.11) 2864 (98.87) 2864 (99.33) 2864 (99.51)

1460 (99.11) 1461 (99.78) 1462 (99.82) 1461 (99.46) 1460 (99.43) 1461 (99.74) 1460 (99.62) 1460 (99.82) 1460 (99.89) 1459 (99.93)

(32,62,92)-3,6,9heneicosatriene 3018 (97.43) (32,62,9Z) -3,6,9-trico satriene 3018 (97.75)

2932 (91.40) 2932 (90.87)

2864 (96.58) 2864 (96.39)

1459 (99.29) 1460 (99.25)

compound (62,92)6,9-octadecadiene (62,92)-6,8nonadecadiene

(62.92)-6,9-eicosadiene (62,92)-6,9-heneicosadiene (62,92)-6,9-docosadiene (62,92)-6,9-tricosadiene (62,92)-6,9-tetracosadiene (62,92)-6,9-pentacosadiene (62,92)-6,9-hexacosadiene (62,92)-6,9-heptacosadiene

0

&OK-3018

def

cm-l/

A2866-2864

1398 (99.50) 1398 (99.88) 1397 (99.92) 1397 (99.77) 1397 (99.76) 1396 (99.90)

cm-'

a a a

0.637 0.588 0.544 0.501 0.454 0.429 0.399 0.378 0.360 0.357

1395 (99.56) 1396 (99.60)

0.747 0.618

a

Very weak intensity.

methyl-interrupted cis groups exhibit the band at 3019-3018 cm-1.26,28 The gas-phase spectra of none of the methyleneinterrupted trans compounds show any significant bands above 3000 cm-l. However, the GC/MI-FTIR spectra of methyl (9E,12E)9J2-0ctadecadienoate show weak bands at 3035 and 3005 cm-', attributable to the trans =CH stretch.28Corresponding absorption bands are barely visible in the spectra of SFC/FT-IR27Therefore, the absence of any detectable absorptions above 3000 cm-' in the gas-phase spectra of the isolated or methylene-interrupted diunsaturated acetates bearing only trans double bonds is not surprising. A hypsochromic shift, similar to that observed for the cis =CH band in methylene-interrupted polyenes compared to that of monoenes, is also seen in the spectrum of (92,122,152)-9,12,15 [9,10,12,13,15,1&2H~]octadecatrienyl acetate (1)for the cis =CD stretch band (Figure 4B). We established previously' that the H2)60C0CH3

.

4000

4000

I

3000 Wavenumber (em-')

3000 Wavenumber (em-')

.

,

.

~

.

I

2000

I000

2000

1000

Figure 4. Gas-phase infrared spectra of (9Z,12Z,152)-9,12,15-[9,10,12,13,15,16-2H~]octadecatrienylacetate (1) (B) and the corresponding acetate bearing only one olefinic deuterium atom at each carbon-carbon double bond (2) (A) (resolution, 8 cm-').

wCC D

D D

D D

D

cis =CD stretch of a monoene bearing the double bond toward the middle of a carbon chain occurs at 2246-2244 cm-'. In this hexadeuterio compound, the cis =CD stretch band is observed at 2252 cm-'. Interestingly, the spectrum of the corresponding trideuterio compound (2), bearing only one olefinic deuterium atom at each carbon-carbon double bond, shows absorptions for both cis =CH and cis =CD stretching vibrations at 3023 and 2240 cm-l, respectively (Figure 4A).

4000

3000 2000 Wavenumber (em-')

Figure 5. Gas-phase infrared spectrum of (4Z,7Z,lOZ,13Z,16Z,19Z)4,7,10,13,16,19-docosahexaenyl acetate (resolution, 8 cm-').

symmetric CH3 deformation of acetate group at 1370-1368 cm-I (Table 1). In the spectra of methyleneintenupted polyunsaturated acetates bearing three or more double bonds, a new band appears at 1391 cm-' on the high-frequency edge of the 1370-1368 cm-l band. The intensity of this band increases as the number of double bonds increases. For example, in the spectrum of (42,72,102,132,162,192)-4,7,10,13,16,19-docosahexaeny1 acetate (Figure 5), the intensity of this band at 1391 cm-' is higher than that at 1369 cm-'. In fact, this band, which apparently represents the deformation vibrations of the diallylic CH2 groups located between the double bonds, is visible at 1398-1396 cm-I, even in most spectra of methylene-interrupted dienes (Table 2). Our assignment of this CH2 deformation vibration is supported by the fact that the 1391 cm-' band is absent in the spectra of

44=APJ(CH2)a0C0CH3 D

D

D

A band diagnostic of methylene-interrupted polyunsaturated compounds was recognized at 1391cm-l. The spectra of acetates, except those of 2 2 compounds,' usually show a band for the (28) Mossoba, M. M.; McDonald, R. E.; Armstrong, D. J.; Page, S. W. J ChrOmatOgr. S C ~1991,29, . 324-330.

1564 Analytical Chemistry, Vol. 67,No. 9,May 1, 1995

I000

deuterio compounds 1 and 2. Apparently, for compound 2 the problem of determining conligurations of double bonds in band is shifted and obscured by other absorptions. The band is compounds available only in nanogram quantities. In our previous also absent in the spectrum of hexadeuterio compound 1. In this study,' we showed that in vapor-phase spectra, the =CH stretch case, however, a new band is observed at 1175 cm-I (Figure 4B). band of trans compounds is shifted sufticiently toward lower Further support for the assignment of this band comes from ab frequency to be obscured by the CHZasymmetric stretch band at initio calculations at the 3-21G level,13 which predicts a CHZ 2940-2933 cm-' and as a consequence none of the 43 trans deformation band at 1487,1445,and 1421 cm-l for 1-butene, 1,4 monoalkenyl acetates investigated showed any significant absorp pentadiene, and (2E,5@-2,5heptadiene, respectively. Clearly, tions above 3000 cm-'. From the results discussed above, it is these values are not congruent with the respective literature equally clear that polyunsaturated compounds with one or more frequencies of 1460, 1424, and 1398 cm-1;29nevertheless, the isolated trans double bonds behave similarly. However, gas-phase tendency for this band to move to lower frequencies as the spectra of compounds bearing (Eflconjugated diene moieties substitution increases is evident. However, higher level calculado not conform to this generalization that spectra of trans tions on a more alkylated methylene-interrupted diene are compounds lack =CH absorptions above 3000 cm-'. Since required before we can conlirm that the calculated frequencyconjugation shifts the frequency of the =CH stretch band to indeed corresponds to the band we observed at 1391 cm-' for higher values, an absorption above 3000 cm-' is visible in the the long-chain compounds. spectra of (Efl-conjugated compounds. Particularly for ( E f l Since methylene-interrupted polyunsaturated compounds are compounds bearing a methyl group as one of the substituents ubiquitous in nature, this CHZ deformation band will be very (penultimate unsaturation), this band shifts sufticiently to higher helpful in the characterization of natural products available as frequency to show a well-resolved absorption at 3016 cm-', as mixtures and in minute amounts. Although it is known for shown by the spectra of (8E,10E)-8,lOdodecadienylacetate, condensed-phase spectra that the CHZdeformation band which (lOE,12E)-10,124etradecadienyl acetate, and (12E,14E3-12,14hexaappears near 1463 cm-' for alkanes is lowered to about 1440 cm-l decadienyl acetate (Table 1). However, if the (Eflconjugated when the group is next to a double bond,'5 and to 1434 cm-I when moiety is located toward the middle of the carbon chain, this shift it located between two double bonds,3O the band we describe here may be not large enough to yield a complete resolution from the for gas-phase spectra of skipped polyenes has not been recognized CHz asymmetric stretch band at 2940-2933 cm-'. For example, as a diagnostic absorption previously. the trans =CH stretch bands of (9E,llE)-9,1l-tetradecadienyl Our previous results demonstrated that compounds with acetate (Table l), and (11E,13E)-11,13-hexadecadienyl acetate internal double bonds exhibit no significant band for the C=C (Figure 6D) appear at 3012 and 3014 cm-', respectively, as a wellstretch, presumably because the stretching affords little or no defined shoulder on the high-frequency side of the CHZasymdipole moment change.' Only compounds with terminal unsatmetric stretch band. To the best our knowledge, the significance uration show a small but significant peak at 1641 cm-' for the of this 3016-3012 cm-' band for the recognition of ( E f l C=C stretch. The results obtained for polyenes are very similar. conjugated moieties has not been recognized. Apparently, the Of all the polyunsaturated compounds investigated (Table 1), only occasional occurrence of bands above 3000 cm-' for trans the highly unsaturated compounds showed at least a weak compounds, and the unavailability of a large data base of gasabsorption. For example, the spectrum of (4Z,7Z,lOZ,13Z,l62,192)phase spectra of unsaturated compounds, have prevented chemists 4,7,10,13,16,19docosahexaeny1 acetate (Figure 5) depicts a weak from making generalizations about the CH bands above 3000 band for the C=C stretch at 1653 cm-I. cm-l, which have been taken merely as indications of unsaturaConjugated Polyenes. The gas-phase IR spectra of conjution.15 Furthermore, the spectra of (E&conjugated compounds gated compounds are particularly interesting. The spectra show do not show the trans wag absorption at 970-967 cm-'. Instead, highly characteristic absorptionswhich are useful in determining a band which is highly characteristic for (E,E)-conjugated diene the configuration of conjugated carbon-carbon double bonds moieties appears at 983-982 cm-'. This band is evident in the unambiguously. Although Jackson et al.I7 recognized at a very spectra of the six examples of (Efl-conjugated compounds listed early stage that the infrared spectra obtained from solutions can in Table 1. Fortunately, the presence of this band at 983-982 distinguish between cis/trans and trans/trans conjugated isomers, cm-l not only ascertains the presence of an (EB-conjugated they exploited only the information derived from the 988-914 moiety but also alerts us to expect a band above 3000 cm-I which region. The most widely used methods for cis/trans coniiguration would otherwise be confused as a cis =CH band. The presence determination of conjugated compounds are 'H and I3C NMR of an absorption at 984 cm-I in the gas-phase spectrum of (8E,s p e c t r o s c ~ p y .However, ~ ~ ~ ~ ~ for NMR applications, microgram to lOE)-8,10dodecadienolhad been noted previously; however, it had milligram amounts of material in relatively pure form are required. been interpreted simply as a trans absorption rather than as a Thus, for samples available in only nanogram amounts and in diagnostic signal for an (Eflconjugated system.34At a very early impure form, NMR is not a practical choice. Insect pheromones stage, Jackson et recognized the significance of a band at frequently bear conjugated and chemists involved in 989-988 cm-I in solution spectra ((2%) as characteristic of ( E a structure elucidation of pheromones are often confronted with the conjugated bonds. For example, spectra of methyl (3E,5E)-3,5 (29) n2e National Institute of Standards and Technology (NIST) and Environmental nonadienoate and (3E,5E)-3,5tridecadienoaterecorded as CCh Protection Agency (EPA) Gas Phase Infrared Database; US. Department of Commerce: Gaithersburg, MD, 1992. solutions show this absorption at 990 ~ 1 1 1 - l . This ~ ~ band shifts (30) Colthup, N. B.ApPl. Spectrosc. 1980,34,1-6. further to 994 and 997 cm-l for compounds bearing three and (31) Ando, T.; Ohsawa, H. J. Chem. Ecol. 1993,19, 119-132. (32) Pfeffer, P. E.;Luddy, F. E.; Unruh, J.; Shoolery, J. N.]. Am. Oil Chem. SOC. 1977,54, 380-386. (33) Mayer, M. S.; McLaughlin, J. R Handbook of Insect Pheromones and S a Attractants; CRC Press: Boca Raton, FL, 1991.

(34) Leal, W.S.;Kuwahara, Y.; Matsuyama, S.; Suzuki, T.; Ozawa, T. J Braz. Chem. SOC.1992,3, 25-29. (35) Celmer, W. D.; Solomons, I. A. J. Am. Chem. SOC.1953,75, 3430-3435.

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bonds are expected to be present (relatively long GC retention times, and the presence of a strong molecular ion could indicate whether the two bonds are expected to be conjugated), the nonappearance of any bands in the 946-983 cm-1 region and the absence of any sharp bands in the 3036-3011 area in its GC/IR spectrum can therefore be taken as indirect evidence of a presence of a cis-cis conjugated system in the molecule. Only in the case 4000 3000 2000 I000 Wnvcnumber (cm-’) of (8Z,102)-8,l(ldodecadienylacetate was the cis =CH stretch absorption observed as a broad but well-resolved peak at 3024 cm-’ This compound, however, has a penultimate double bond which is known to contribute to the hypsochromic shift observed for this band.’ However, the intensity of the cis =CH stretch band in the spectrum of (8Z,lOZ)-8,l(ldodecadienylacetate is much lower than one would expect for a compound bearing two cis bonds 643024 cm-’/A1761 cm-l = 0.189). 4000 3000 2000 I000 Wnvcnumber (cm-’) The gas-phase spectra of (Ea or (Z,E)-conjugated dienes exhibit a highly characteristic fingerprint that shows two weak absorption of similar intensity at 979-977 and 948-947 cm-I, in addition to the =C-H stretch band at 3019-3017 The =C-H stretch absorption is shifted slightly to higher frequency, presumably because of conjugation. The fingerprint pattern and the =CH absorption are seen in the spectra of (llE,l32)-11,134000 300Q 2000 I000 hexadecadienyl acetate and (llZ,l3JZ)-11,13-hexadecadienyl aceWavenumber (cm-’) tate F i r e 6B,C), in several other spectra of cis/trans and trans/ cis conjugated compounds given in Table 1,and in those reported elsewhere for (7E,SZ)-7,4dodecadienylacetate and (92,1lE)-9,11tetradecadienyl a~etate.2~ In fact, Yamaoka et al. employed this reasoning to deduce the presence of a conjugated (E,Z)-moiety in a termite trail pheromone.4O For spectra recorded as solutions, two corresponding fingerprint bands appear at 982 and 948 4000 3000 2000 I000 cm-1.17735These two bands of equal intensity are generally Wnvenumber (em”) expected at 986 and 944 cm-I for condensed-phase spectra.15 Figure 6. Gas-phase infrared spectra of (llZ,132)-11,13-hexaUnfortunately, these bands have been used only seldomly in decadienyl acetate (A; the inset is an expansion of the 3080-2840 quantitative and qualitative analysis of cm-I region.),(1 1€,13Z)-11,13-hexadecadienyl acetate (B), (11Z,13€)In 11,13-hexadecadienyIacetate (C), and (1 1 €,13€)-11,13-hexadecaparticular, the significance of the =CH stretch band has been dienyl acetate (D) (resolution, 8 cm-’). generally overlooked. A hypsochromic shift, similar to that observed in the cis =CH four trans conjugated double bonds, re~pectively.’~*~~ Unfortustretch band of the above-mentioned conjugated compounds, is nately, the significance of these observations has not received seen in the spectrum of (132)-13-hexadecen-ll-ynylacetate (3). much recognition outside the realm of oil ~ h e m i s t s . ’ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ The =CH stretch band of (Z,Z)-conjugated compounds show an even further hypsochromic shift. However, the peak usually is disappointingly weak, consists of two or more poorly resolved bands, and appears as a broad and complex shoulder on the highfrequency side of the CH?asymmetric stretch band. For example, in the spectrum of (1lZ,13Z)-11,13-hexadecadienylacetate, this The =CH stretch band of this compound appears at 3029 cm-I peak appears as a broad band with three minima at 3036, 3008, (Figure 7A), in contrast to that of (Z)-13-hexadecenylacetate, in and 2970 cm-’ (see inset in Figure 6A). Similar results were which it occurs at 3012 cm-I. This observation confirms that obtained for (72,92)-7,9dodecadienylacetate and (92,1LZ)-9,11conjugation leads to a hypsochromic shift of the =CH stretch tetradecadienylacetate as well (Table 1). However, the absence band. In the spectrum of (13E)-13-hexadecen-ll-ynylacetate (4), of sharp bands in the region above 3000 cm-I in the spectra of the trans =CH wag absorption is observed at 959 cm-l as a @,Z)-conjugated compounds does not actually impede the idensomewhat broad band (Figure 7B). A similar shift has been noted tification of an unknown compound bearing such a moiety. GC and GC/MS analyses, which usually precede the GC/IR examination of an unknown, will inform us of the number of double bonds present in an unsaturated compound. Therefore, if two double (36) Colthup, N. B. Appl. Spectrosc. 1971,25, 368-371. (37) Szczepanska, H.; Chmielarz, B. Fette Seifen Anstrichm. 1984, 84, 273278. (38) Rheineck, A. E.; Zimmerman, D. D. Fette Seifen Anstrichm. 1970,72,So84.

1566 Analytical Chemistry, Vol. 67,No. 9,May 1, 7995

for solution spectra as well.35 This is intriguing since conjugation usually shifts this band to higher frequency, although electroneg-

A

4000

3000

2000

I000

Wavenumber (em-')

4000

3000

2000

1000

Wavenumber (cm-1)

Figure 7 . Gas-phase infrared spectra of (134-13-hexadecen-l l-

ynyl acetate (3)and (134-13-hexadecen-l l-ynyl acetate (4)(resolution, 8 cm-I).

ative groups tend to lower the frequency. Furthermore, a weak absorption is observed at 3024 cm-I in the spectrum of (13E)-13hexadecen-11-ynyl acetate. This seems to represent the trans =CH stretch band, which is resolved from the CH2 absorption as a result of conjugation. Independent support for the fact that conjugation leads to a hypsochromic shift of the =CH stretch band comes from ab initio calculations at the 3-21G levelI3for the simplified model systems, 1,ri-pentadieneand 1,Sbutadiene. The computations revealed that the general position of the vinyl frequencies in the methyleneintempted diene are on average 7 wavenumbers lower than that of the conjugated diene. Furthermore, calculations for geometric isomers of 2,ri-hexadiene and 3,soctadiene predicted that =CH stretch frequencies should appear around 3024, 3012, and 2985 cm-I for the (23-,(Ea-, and (E&-isomers of both systems, respectively. Evidently, the exact values differ from those of experimentaldata; however, the trends observed agree well with the recorded observations. The broad nature of the =CH stretch band of (23-conjugated compounds could be attributed to the uniquely low torsional energy barrier estimated for this isomer for the rotation around the sp2-sp2 single bond. Model calculations indicate that twisting around the central sp2-sp2 single bond could lead to a dispersion of the =CH frequencies. Furthermore, our ab intio calculations for cis-2-butenesuggest that the observed =C-H stretch band represents both in-phase and out-of-phase stretching modes with more contribution from the former. In the case of truns-2-butene, the inphase mode is forbidden;therefore, the observed band represents only the outof-phase stretch vibration. Although higher level calculations on a more complicated system are needed to extend these generalizations to the type of substances used in this study, it would be (39) Attygalle, A. B. Pure Appl. Chem. 1994,66, 2323-2326. (40) Yamaoka, R; Tokoro, M.; Hayashiya, K J. Chromatogr. 1987,399,259267. (41)Chiapault, J. R; Hawkins, J. M.]. Am. Oil Chem. SOC.1959,36,535-539.

rational to expect some similarities in the =C-H stretching modes of butene and those of higher olefinic systems. From the data presented in this paper, it is evident that the intensity of the cis =CH stretch absorption increases with the number of cis bonds present in a molecule, except for allcis conjugated compounds. However, this intensity is not as predictable as that of the trans =CH wag absorption, particularly when the individual wavenumbers of the merging bands are not similar. From the spectra of monounsaturated acetates, it was seen that the frequency range of the trans =CH wag absorption is relatively narrow (968-964 cm-I), whereas the cis =CH stretch absorption could appear from 3029 to 3011 cm-I. Although it might be not so dramatic as the intensity variations in the cis =CH stretch band of cis compounds, we could expect a similar variation even in that of the trans =CH wag of all-trans polyenes. It will not be surprising if the relative intensity of the trans =CH wag absorption of a compound such as (2E,12E)-2,12-tetradecadienylacetate (corresponding monoene bands appear at 969 and 964 cm-l, respectively) is somewhat lower than that of a diene which has corresponding monoene bands at identical frequencies. Finally, we should mention that although we have discussed only the spectra of polyunsaturated acetates and hydrocarbons in this paper, the generalizations we have made are not limited to these compounds. We are currently applying the band correlations discussed here to configuration determinations for polyunsaturated aldehydes, alcohols, and compounds bearing other functional groups with equal success. Furthermore, compared to NMR procedures which require relatively pure material in microgram to milligram amounts, the GC/FT-IR method is not only simple but also particularly advantageous because it is applicable to nanogram quantities of mixtures. ACKNOWLEDGMENT We are greatly indebted to Dr. J. Meinwald (Comell University) for critically reading the manuscript; Dr. Xiongwei Shi for participating in some of the synthetic work and Dr. k Cork (Overseas Development Natural Resources Institute, Kent, U.K), Dr. I. Tomida (Shinshu University, Japan), Dr. J. Vrkot (UOCHB, Czech Academy of Sciences, Czech Republic), and Dr. H. J. Bestmann (University of Erlangen, Germany) for sending us samples. We are grateful to the Hewlett-Packard Co. (Palo Alto, CA) for the donation of the GC/FT-IR equipment used in this study. Data given in this paper were presented in part by ABA at the 19th IUPAC Symposium on the Chemistry of Natural Products, Karachi, January 16-19, 1994. The partial support of this research by NIH Grant AI 12020 is acknowledged with pleasure. Received for review November 23, 1994. February 17, 1995.@

Accepted

AC941129S e Abstract

published in Advance ACS Abstracts, April 1, 1995.

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