Anal. Chem. 1994,66, 1696-1703
Gas-Phase Infrared Spectroscopy for Determination of Double Bond Configuration of Monounsaturated Compounds Athula B. Attygalle,’ Ale5 Svato5,t and Charles Wllcox Baker Laboratory, Department of Chemistry, Cornell Universiv, Ithaca, New York 14853 Slmon Voerman Research Institute for Plant Protection. P.0. Box 9060, 6700 GW Wageningen, The Netherlands
Gas-phase Fourier-transform infrared spectra allow unambiguous determination of the configuration of the double bonds of long-chain unsaturated compounds bearing RCH=CHR‘ type bonds. Although the infrared absorption at 970-967 cm-l has been used previously for the identification of trans bonds, the absorption at 3028-301 1cm-l is conventionally considered to be incapable of distinguishing cis and trans isomers. In this paper, we present a large number of gas-phase spectra of monounsaturated long-chain acetates which demonstrate that an absorption, highly characteristic for the cis configuration, occurs at 3013-3011 cm-l, while trans compounds fail to show any bands in this region. However, if a double bond is present at the C-2 or C-3 carbon atoms, thiscis = C H stretch absorption shows a hypsochromic shift to 3029-3028 and 3018-3017 cm-l, respectively. Similarly, if a cis double bond is present at the penultimatecarbon atom, this band appearsat 3022-3021 cm-l. All the spectra of trans alkenyl acetates showed the expected C-H wag absorption at 968-964 cm-l. In addition, the spectra of (E)-Z-alkenyl acetates show a unique three-peak “fingerprint” pattern which allows the identification of the position and configuration of this bond. Furthermore, by synthesizing and obtaining spectra of appropriate deuteriated compounds, we have proved that the 3013-3011 cm-l band is representative of the C-H stretching vibration of cis compounds of RCH=CHR’ type. Sex pheromones of moths (Lepidoptera) are usually mixtures of long chain aldehydes, acetates, and alcohols bearing one or more double bonds. Synthetic pheromone mixtures are now used for monitoring insect populations. An essential prerequisite for a successful synthetic attractant is the correct formulation. Very often the presence of minute amounts of a stereoisomer with the opposite geometric configuration inhibits drastically the attractivity of a synthetic pheromone. Therefore, the exact determination of the composition of natural and synthetic pheromone mixtures is vital for the successful application of pheromone technology to integrated pest management. However, this is not a trivial task, particularly for natural pheromones, since these compounds are often available only in nanogram quantities. Although excellent techniques are now available for determining the position of the double bonds, thenumber of methods usable for elucidating double-bond configurations are rather The most reliable method for configuration + Permanent address: Institute of Organic Chemistry and Biochemistry, Czech Academy of Science, 16610 Prague 6, Czech Republic.
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AnalflicalChemistry, Vol. 66,No. 70, May 15, 1994
determination is the comparison of gas chromatographic retention times with authentic material on stationary phases which allow reasonable resolution of cis and trans isomers. Infrared spectroscopy was one of the first techniques to be applied widely in natural product identification. The s i g nificance of the absorption at 980-965 cm-l, attributed to a trans C H wag, for the characterization of cis and trans alkenyl bonds, was recognized at a very early stage.6 For example, in lipid analysis, the absorption at 10.3 pm (970 cm-l) has been utilized to determine the amount of trans compounds in oil^,^-^ and the measurement of the intensity of this band constitutes the official basis for the determination of total trans unsaturation in fats.1° Although it was generally considered that cis alkenyl bonds do not give rise to major absorptions that lend themselves to characterization in usual infrared regions (ref 6, page 78; ref 7, page 43), an absorption at 3.3 pm (3030 cm-l) has been recognized as a diagnostic aid for cis bonds (ref 6, page 129). However, this band was not considered sufficiently distinct to be used in quantitative estimations (ref 7, page 79). Two recent papers on the recognition of cisltrans double bonds in unsaturated fatty acid methyl esters, in solutiong or gas phase8 utilized only the 967-cm-l absorption band. Apparently, infrared spectroscopy appears to be used less frequently in lipid analysis; a recent book on this subject does not mention any information about infrared applications.’ The neglect of the diagnostic value of the 3028-301 1-cm-l band in gas-phase spectra as characteristic for cis compounds of RCH-CHR’ type,l2*I3except for a couple of occasional examples,’”l6 may be due partly (1) Attygalle, A. B.; Jham, 0. N.; Meinwald, J. Anal. Chem. 1993, 65, 25282533. (2) Attygalle, A. B.; Morgan, E. D.Angew. Chem., Int. Ed. Engl. 1988,27,460478. (3) Hogge, L. R.; Millar, J. G. In Advances in Chromatography;Giddings, J. C., Grushka, E., Brown, P. R., Eds: Marcel Dekker: New York, 1987; Vol. 27, p 299. (4) Schmitz, B.; Klein, R. A. Chem. Phys. Lipids 1986, 39, 285-311. ( 5 ) Anderegg, R. J. Mass Spectrosc. Rev. 1988, 7, 395-424. (6) Christie, W. W. Lipid Analysis; Pergamon Press: Oxford, 1973. (7) Christie, W. W. Lipid Analysis; Pergamon Press: Oxford, 1982. (8) Ratnayake, W. M.N.; Hollywood, R.; O’Grady, E.; Beare-Rogers, J. L. J . Am. Oil Chem. SOC.1990.67, 804-810. (9) Toschi, T. G.; Capella, P.; Holt, C.; Christie, W. W. J . Sci. FoodAgric. 1993, 61, 261-266.
(10) Standard Methodsfor the Analysisof Oils, Fats andDerivatives;International Union of Pure and Applied Chemistry, Commission on Oils, Fats and Derivativw; Blackwell Scientific Publications: Oxford, 1987. (1 1) Hamilton, R. J.; Hamilton, S. Lipid Analysis; Oxford University Prws: Oxford, 1993. (12) Nyquist, R. A. The List of Vapor Phase Infrared Spectra; Sadtler Research Laboratories: Philadelphia, PA, 1984. (1 3) Herres, W . Capillary Gas Chromatography-Fourier Transform Infrared Spectroscopy, Theory and Applications: Huethig: Heidelberg, 1987.
0003-2700/94/036&1696$04.50/0
0 1994 American Chemical Society
to the fact that in condensed phased spectra, both cis and trans compounds are expected to show a medium absorption at 3020-2995 cm-l for the C-H stretch.17 Recent investigations by Mossoba et al. have shown that matrix-isolation Fourier transform infrared (GC/MI-FT-IR) spectra of unsaturated fatty acid methyl esters show bands at 30123010 cm-1, characteristic for isolated cis double bonds, and 3019-301 6 cm-l, for methylene interrupted all-cis-dienes and -trienes.18-20However, MI-FT-IR,18J9and supercritical fluid chromatography (SFC)/FT-IR,20 spectra also show two bands at 3035-3033 and 3005 cm-l characteristic for trans compounds. Thus, both these methods appear rather complicated for &/trans assignments of unsaturated, particularly polyunsaturated, compounds if only the above 3000-cm-' region is taken into consideration.
EXPERIMENTAL SECTION Reagents. (E)-g-Nonadecene and (2)-9-pentacosene were sent to us by Drs. E. D. Morgan and H. J. Bestmann,
respectively. The sample of (2)-5-tetradecen- 13-olide was a gift from Dr. A. C. Oehlschlager. All isomers of monounsaturated acetates were from the synthetic pheromone collection of the Research Institute for Plant Protection (Wageningen, The Netherlands).26 Samples of (2)-11-[l 1,12-2H2]tetradecen~land (E)-1 1[ 11,l 2-ZH2]tetradecenol were synthesized by the reduction of 1-(2H-tetrahydropyranyIoxy)-l l-tetradecyne. The trans isomer was prepared in 98% isomer purity (GC) by the reduction of the acetylenic precursor with LiAIZH4in diglyme at 140 OC ['H N M R 6 3.61 (2H, t, J = 7.2 Hz, -CH20-), 2.12-1.90 (4H, m, -CH2C=), 1.48-1.21 (16H, m, CH2), 1.65 ( l H , bs, OH), 0.91 (3H, t, J = 7.2 Hz, CH3)]. The In pheromone chemistry, infrared spectroscopy has been stereochemistry of the product was deduced by comparing its employed only for a handful of identifications.21 Even in the analytical data with those of the nondeuterated product few applications reported, deductions have been made mainly obtained by the LiAlH4 reduction of the acetylenic precursor on the basis of the presence or absence of an absorption around under similar condition^.^^ The cis isomer was prepared by 970 cm-l for a trans bond. For example, Murata et aLzz a modification of the procedure of Brown and Ahuja,Z8known observed an absorption at 3012 cm-' in the gas-phase IR to yield a pure cis product. The acetylenic compound was spectra of the compound (62,262)-pentatriacontadien-Zone reduced by P2-Ni, formed in situ from Ni(0Ac)zand NaB2H4 and its homologues. However, the cis geometry of the carbonin C2H502H, and deuterium, formed in situ from excess carbon double bonds was deduced by the absence of the trans NaBzH4 and CH302H. The stereoisomeric purity of the bands, not by the presence of the cis bands. Similarly, in the product was 95% (GC) ['H N M R 8 3.62 (2H, t, J = 7.0Hz), structure elucidation of cis-3,4-bis[(E)-l-butenyl]tetra2.10-1.90(4H,m,-CH2C=), 1.45-1.22(16H,m,CH2) 1.80 hydrofuran-2-01, the major component of the dorsal abdominal ( l H , bs, OH), 0.90 (3H, t, J = 7.2 Hz, CH3)]. glands of the spined citrus bug, Oliver et al.23deduced the Instrumentation. GC/FT-IR analyses were performed on trans configurations by the presence of a 968 cm-I GC/ a Hewlett-Packard (HP) 5890 gas chromatograph coupled to FT-IR band. However, this paper specifically mentions that a H P 5965A IRD instrument equipped with a liquid-nitrogena (2)-double bond might have escaped detection by IR; the cooled narrow-band (4000-750 cm-l) infrared detector authors were uncertain from the IR data whether the (mercury cadmium telluride). Helium was used as the carrier compound possessed two (E)-double bonds or one (E) and gas (80 kPa) and the effluent from the GC was mixed with one (2).From these examples, it is clear that most chemists the sweep gas helium (0.5 mL/min; transfer line temperature do not have the confidenceof using characteristic cis absorption 270 "C) and passed through the light pipe (120 mm; 1 mm in stereochemical elucidations. Furthermore, many textbooks i.d.; gold-plated internal surface; cell temperature, 280 "C), on spectroscopy fail to emphasize the significance of the cis with KBr windows, of the infrared instrument. Most spectra absorptions that appear in the C-H stretch region^.^^.^^ The were obtained at 8 cm-l resolution, except for a few samples purpose this paper is to establish convincingly the validity of for which resolution 4 cm-l was used. For GC/IR analyses a DB-5 (J & W Scientific; 0.25 pm film thickness; 25 m X using the 3028-301 1 cm-l absorption in the determination of 0.33 mm fused silica; 60 OC for 3 min and 10 OC/min to 250 cis bonds by gas-phase FT-IR spectroscopy. "C) or a Carbowax-20M capillary column (HP; 0.3 pm film thickness; 25 m X 0.31 mm fused silica; 60 OC for 2 min and (14) Leibrand, R. J. Basics of GCIIRD and GC/IRD/MR Hewlett-Packard Co.: Palo Alto, CA, 1993. 15 OC/min to 220 "C)was used. Samples (50-100 ng) were (15) Duncan, W. GC/IR/MS Analysis of C-18 Unsaturated Fatty Acid Esters; introduced by splitless injection (220 OC; purgedelay0.5 min). IRD Application Brief 89-3, Hewlett-Packard Co.: Palo Alto, CA, 1989. (16) Sebedio,J.L.;LeQuere,J.L.;Semon,E.;Morin,O.;Prevost,J.;Grandgirard, Interferograms were acquired at a rate of 0.3 s/scan and scans A. J . Am. Oil. Chem. SOC.1987,64, 1324-1333. across the width at half maximum of the chromatographic (17) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: San Diego, CA, 1990; pp 249-250. peak were coadded (about 8-10 scans). Reference spectra (18) Mossoba, M. M.; McDonald, R. E.; Chen, J.-Y. T.; Armstrong, D. J.; Page, were collected (four to seven scans) immediately before or S. W. J . Agric. Food Chem. 1990, 38, 86-92. (19) Mossoba, M. M.; McDonald, R. E.; Armstrong, D. J.; Page, S. W. J . Agric. after a GC peak. Food Chem. 1991, 39, 695-699. (20) Calvey, E. M.; McDonald, R. E.; Page, S. W.; Mossoba, M. M.; Taylor, L. Purity and identity of samples were checked by GC/MS T. J . Agric. Food Chem. 1991, 39, 542-548. performed on a H P 5890 gas chromatograph coupled to a H P (21) Heath,R. R.;Tumlinson,J. H.ln TechniquesinPheromoneResearch;Hummel, H. E., Miller, T. A., Eds.; Springer-Vcrlag: New York, 1984; p 303. 5870B mass selective detector, using a 25 m X 0.22 mm fused (22) Murata, Y.; Yeh, H. J . C.; Pannell, L. K.; Jones, T. H.;Fales, H. M.; Mason, silica column coated with DB-5 stationary phase (J &W R. T. J . Nat. Prod. 1991,54, 233-240. (23) Oliver,J.;Aldrich,J.R.;Lusby,W.R.; Waters,R.M.;James,D.G.Tetrahedron Scientific; 0.25 pm film thickness). For GC/MS analyses
Lett. 1992, 33, 891-894. (24) Silverstein, R. M.; Bassler, G.C.; Morill, T. C. Spectrometric Identification of Organic Compounds. 5th cd.;Wilcy: New York, 1991; p 163. (25) Pavia, D. L.; Lampman, G. M.; Kriz, G.S. Introduction to Spectroscopy; Saunders College Publishing: Philadelphia, PA, 1979.
~~
(26) Vocrman, S. Agric. Ecosyst. Enuiron. 1988, 21, 3 1 4 1 . (27) Rossi, R.; Carpita, A. Synthesis 1977, 561-562. (28) Brown, C. A.; Ahuja, V. K. 1. Org. Chem. 1986, 38, 2226-2230.
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Table 1. Gas-Phase Infrared SDedra of clsAlkenvl Acetates Ilerolutlon
= 8 cm-l)
band pimition, cm-1 (p¢ transm.ission)
compound name (2)-3-decenylacetate (d)-l-decenyl acetate (2)-5-decenylacetate (2)-6-decenylacetate (n-7-decenylacetate (2)-8-decenylacetate
3017 (98.46) 3013 (97.64) 3012 (97.81) 3012 (97.23) 3012 (97.00) 3022 (97.74)
2934 (93.86) 2936 (92.36) 2939 (93.11) 2940 (91.12) 2938 (90.16) 2935 (90.32)
2869 (97.60) 2869 (96.88) 2871 (97.07) 2873 (96.26) 2868 (96.02) 2866 (96.14)
1761 (93.36) 1761 (90.97) 1761 (91.66) 1761 (89.68) 1761 (88.99) 1761 (90.47)
1461 (99.15) 1460 (98.76) 1460 (98.82) 1461 (98.57) 1461 (98.45) 1458 (98.73)
1369 (98.19) 1370 (97.25) 1370 (97.39) 1370 (96.82) 1369 (96.59) 1369 (96.93)
1231 (88.75) 1232 (85.00) 1232 (85.94) 1232 (82.71) 1232 (81.87) 1232 (84.29)
1040 (97.43) 1043 (96.78) 1042 (97.34) 1045 (96.61) 1040 (96.54) 1040 (97.03)
(2)-2-dodecenylacetate (2)-3-dodecenyl acetate (2)-4-dodecenyl acetate (2)-5-dodecenyl acetate (&)-6-dodecenylacetate (2)-7-dodecenyl acetate (2)-8-dodecenylacetate (2)-9-dodecenylacetate (2)-10-dodecenyl acetate
3028 (98.41) 3017 (98.43) 3013 (98.59) 3012 (97.15) 3012 (97.69) 3012 (98.24) 3012 (97.62) 3012 (98.80) 3021 (98.70)
2933 (90.01) 2933 (91.41) 2934 (93.48) 2935 (88.45) 2936 (90.44) 2936 (92.62) 2936 (89.36) 2935 (94.35) 2934 (92.12)
2866 (96.19) 2866 (96.70) 2867 (97.47) 2868 (95.24) 2868 (96.06) 2868 (96.93) 2868 (95.65) 2866 (97.78) 2865 (96.95)
1760 (93.78) 1761 (93.59) 1761 (94.74) 1761 (90.17) 1761 (91.82) 1761 (93.82) 1761 (91.22) 1761 (95.54) 1761 (94.39)
1460 (98.93) 1461 (99.03) 1460 (99.20) 1460 (98.41) 1461 (98.68) 1460 (98.97) 1461 (98.54) 1460 (99.27) 1459 (99.16)
1374 (97.81) 1369 (98.24) 1369 (98.44) 1369 (96.92) 1369 (97.45) 1369 (98.09) 1369 (97.22) 1369 (98.63) 1369 (98.21)
1228 (88.73) 1231 (89.20) 1232 (91.13) 1232 (83.57) 1232 (86.18) 1232 (89.58) 1232 (85.54) 1232 (92.47) 1232 (90.56)
1025 (97.56) 1040 (97.20) 1042 (98.22) 1041 (96.83) 1044 (97.37) 1040 (98.06) 1040 (97.25) 1040 (98.59) 1039 (99.22)
(2)-2-tetradecenyl acetate (2)-3-tetradecenyl acetate (2)-4-tetradecenyl acetate (2)-5-tetradecenyl acetate (2)-6-tetradecenyl acetate (2)-7-tetradecenyl acetate (2)-8-tetradecenyl acetate (&)-g-tetradecenylacetate (2)-10-tetradecenyl acetate (2)-11-tetradecenyl acetate (2)-12-tetradecenyl acetate
3029 (95.92) 3018 (98.09) 3013 (92.88) 3012 (95.99) 3012 (99.05) 3012 (96.00) 3012 (98.88) 3012 (97.83) 3011 (96.83) 3012 (97.83) 3022 (97.21)
2934 (66.45) 2933 (87.28) 2934 (64.50) 2934 (80.00) 2935 (95.03) 2935 (80.90) 2935 (94.06) 2935 (88.30) 2935 (82.89) 2935 (87.42) 2934 (81.13)
2865 (84.41) 2865 (95.00) 2865 (84.54) 2866 (91.68) 2867 (98.02) 2867 (91.74) 2867 (97.60) 2866 (95.16) 2866 (92.79) 2865 (94.95) 2865 (92.12)
1760 (81.36) 1761 (92.71) 1761 (77.12) 1761 (87.16) 1761 (96.91) 1761 (87.56) 1761 (96.31) 1761 (92.71) 1761 (89.27) 1761 (92.53) 1761 (89.32)
1460 (95.64) 1461 (98.65) 1460 (95.31) 1460 (97.49) 1460 (99.44) 1460 (97.58) 1461 (99.32) 1461 (98.62) 1461 (97.86) 1461 (98.58) 1460 (98.04)
1375 (92.65) 1369 (97.89) 1369 (92.31) 1369 (95.78) 1369 (99.06) 1369 (95.97) 1369 (98.86) 1369 (97.70) 1369 (96.50) 1369 (97.64) 1369 (96.40)
1228 (70.54) 1231 (88.02) 1232 (64.69) 1232 (79.00) 1232 (94.73) 1232 (80.01) 1232 (93.79) 1232 (88.02) 1233 (82.83) 1233 (87.75) 1233 (82.91)
1025 (91.62) 1040 (97.20) 1042 (91.38) 1042 (95.74) 1044 (99.04) 1042 (96.02) 1040 (98.88) 1040 (97.70) 1044 (96.62) 1041 (97.61) 1041 (96.55)
(2)-3-hexadecenylacetate (2)-4-hexadecenylacetate (2)-5-hexadecenyl acetate (2)-6-hexadecenyl acetate (2)-7-hexadecenyl acetate (2)-8-hexadecenyl acetate (Z)-g-hexadecenyl acetate (2)-10-hexadecenylacetate (2)11-hexadecenyl acetate (2)-12-hexadecenyl acetate (2)-13-hexadecenylacetate
3017 (98.89) 3013 (98.71) 3012 (98.74) 3012 (98.71) 3011 (98.40) 3012 (98.94) 3011 (98.86) 3011 (98.76) 3012 (98.96) 3012 (98.58) 3012 (98.60)
2932 (89.79) 2933 (90.60) 2933 (91.43) 2933 (91.54) 2934 (89.98) 2934 (93.26) 2934 (92.57) 2934 (92.13) 2934 (92.92) 2934 (89.73) 2934 (89.89)
2864 (96.15) 2864 (96.42) 2865 (96.71) 2865 (96.70) 2865 (95.97) 2865 (97.29) 2865 (97.04) 2865 (96.83) 2865 (97.18) 2865 (95.96) 2864 (95.98)
1761 (95.31) 1761 (95.50) 1761 (95.73) 1761 (95.72) 1761 (94.84) 1761 (96.55) 1761 (96.21) 1761 (96.02) 1761 (96.46) 1761 (94.86) 1761 (95.18)
1461 (99.04) 1460 (99.05) 1460 (99.12) 1460 (99.12) 1460 (98.93) 1460 (99.28) 1460 (99.22) 1460 (99.19) 1460 (99.27) 1461 (98.91) 1460 (99.03)
1368 (98.64) 1369 (98.59) 1369 (98.65) 1369 (98.65) 1369 (98.40) 1369 (98.92) 1369 (98.82) 1369 (98.77) 1369 (98.91) 1369 (98.34) 1369 (98.52)
1231 (91.99) 1232 (92.40) 1232 (92.67) 1232 (92.63) 1232 (91.27) 1232 (94.16) 1232 (93.58) 1232 (93.27) 1232 (94.02) 1232 (91.36) 1232 (91.90)
1040 (98.22) 1042 (98.47) 1043 (98.68) 1044 (98.66) 1041 (98.46) 1040 (98.97) 1041 (98.83) 1040 (98.79) 1041 (98.91) 1042 (98.37) 1041 (98.56)
(2)-2-octadecenylacetate (2)-3-octadecenylacetate (Z)-g-octadecenyl acetate (2)-11-octadecenyl acetate (2)-13-octadecenvlacetate (2)-15-octadecen;l acetate
3028 (99.28) 3017 (99.71) 3011 (99.36) 3011 (98.82) 3012 (99.02) 3012 (99.60)
2932 (89.02) 2932 (96.45) 2933 (95.26) 2933 (91.12) 2933 (91.84) 2933 (96.33)
2864 (95.81) 2864 (98.71) 2865 (98.15) 2865 (96.49) 2864 (96.80) 2864 (98.58)
1760 (96.40) 1761 (98.69) 1761 (98.07) 1761 (96.28) 1761 (96.62) 1761 (98.61)
1460 (99.08) 1461 (99.73) 1461 (99.60) 1460 (99.13) 1460 (99.20) 1461 (99.73)
1374 (98.70) 1368 (99.64) 1369 (99.45) 1369 (98.82) 1369 (98.90) 1368 (99.59)
1228 (93.35) 1231 (97.72) 1232 (96.74) 1232 (93.71) 1232 (94.27) 1232 (97.61)
1025 (98.55) 1040 (99.53) 1040 (99.48) 1041 (98.86) 1041 (98.96) 1039 (99.62)
the oven temperature was held at 60 "C for 3 min and increased to 250 "C at 10 "C/min. Ab Initio Spectral Calculations. Ab initio spectra were calculated with the Gaussian 92 for Windows package.2g
RESULTS AND DISCUSSION In order to establish the characteristic infrared absorptions which are useful in the identification ofcisftrans configurations of pheromone-like unsaturated acetates bearing RCH=CHR' type bonds, we recorded the spectra of 13 isomers of decenyl acetates, 19 isomers of dodecenyl acetates, 23 isomers of tetradecenyl acetates, 22 isomers of hexadecenyl acetates, and 12 isomers of octadecenyl acetates (Tables 1-3). Once this large database of spectra was available, we were able to make many generalizations valuable in the characterization of the spectra of unknowns. (29) 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, K.; 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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In general, the spectra of all the cis compounds appeared similar (Table 1) and resembled those of saturated acetates. However, one characteristic absorption which appears above 3000 cm-l differentiated clearly the spectra of cis compounds from those of the saturated, and trans unsaturated, acetates (Table 2, Figure 1). For the majority of the cis compounds this characteristic absorption occurs between 3013 and 301 1 cm-1 (Table 1). However, for compounds bearing a cis double bond at the C-2 or C-3 position, or the penultimate carbon atom, the frequency of this absorption increases. For example, in the spectra of (2)-8-decenyl acetate, (2)10-dodecenyl acetate, and (2)-12-tetradecenyl acetate, all of which bear a cis double bond on the penultimate carbon atom, this =C-H stretch absorption appears characteristically at 3022-3021 cm-1 (Table 1). Similarly, in the spectraof (2)-2 compounds, as shown by the examples (2)-2-dodecenyl acetate, (2)-2tetradecenyl acetate, and (2)-2-octadecenyl acetate, it occurs at 3029-3028 cm-*. The magnitude of the hypsochromic shift observed for (2)-3-alkenyl acetates is somewhat less; for these compounds the =C-H stretch absorption appears at 30183017 cm-' (Table 1).
~~~~
~
~
Table 2. QacPhase Infrared Spectra d trsnaAlkenyl Acetates (Remlutlon = 8 cm-’)
band position, cm-l (percent transmission)
compound name (Eb3-decenyl acetate (E)-4-decenyl acetate (E)-5-decenyl acetate (E)-g-decenyl acetate (E)-7-decenyl acetate (E)-8-decenylacetate
2933 (91.98) 2934 (93.47) 2936 (92.98) 2937 (90.19) 2936 (91.83) 2934 (94.13)
2866 (97.08) 2865 (97.61) 2865 (97.32) 2868 (96.31) 2866 (96.76) 2866 (97.76)
1761 (92.21) 1761 (93.11) 1761 (92.20) 1761 (89.33) 1761 (91.47) 1761 (94.70)
1459 (99.00) 1452 (99.06) 1456 (98.91) 1457 (98.55) 1459 (98.80) 1457 (99.27)
1369 (97.75) 1369 (97.86) 1369 (97.54) 1369 (96.67) 1369 (97.36) 1369 (98.38)
1232 (87.02) 1232 (88.48) 1232 (86.99) 1232 (82.44) 1232 (85.89) 1232 (91.08)
1038 (97.09) 1043 (97.53) 1043 (97.50) 1043 (96.60) 1043 (97.23) 1040 (98.38)
967 (98.44) 967 (98.62) 968 (98.31) 967 (97.73) 966 (98.37) 965 (98.99)
(E)-a-dodecenylacetate (E)-3-dodecenyl acetate (E)-4-dodecenyl acetate (E)-5-dodecenyl acetate (E)-g-dodecenyl acetate (E)-7-dodecenyl acetate (E)-8-dodecenyl acetate (E)-9-dodecenyl acetate (E)-10-dodecenyl acetate
2933 (87.64) 2933 (91.38) 2933 (92.33) 2934 (93.10) 2934 (91.37) 2935 (90.22) 2935 (98.65) 2935 (90.37) 2934 (91.48)
2865 (95.42) 2865 (96.89) 2864 (97.14) 2865 (97.42) 2866 (96.72) 2866 (96.36) 2865 (99.52) 2865 (96.19) 2865 (96.75)
1762 (92.12) 1761 (93.97) 1761 (94.37) 1761 (94.61) 1761 (93.21) 1761 (92.38) 1760 (98.97) 1761 (92.82) 1761 (94.32)
1456 (98.64) 1459 (99.05) 1454 (99.11) 1456 (99.14) 1456 (98.91) 1458 (98.81) 1462 (99.86) 1460 (98.82) 1458 (99.08)
1368 (97.63) 1369 (98.20) 1369 (98.25) 1369 (98.30) 1369 (97.85) 1369 (97.62) 1368 (99.69) 1369 (97.75) 1369 (98.22)
1228 (86.15) 1231 (89.91) 1232 (90.55) 1232 (90.87) 1232 (88.62) 1232 (87.34) 1232 (98.26) 1232 (88.12) 1232 (90.49)
1022 (97.09) 1038 (97.71) 1042 (98.00) 1042 (98.31) 1043 (97.85) 1041 (97.57) 1044 (99.70) 1040 (97.70) 1040 (98.18)
968 (97.46) 968 (98.70) 967 (98.82) 968 (98.80) 967 (98.49) 967 (98.43) 967 (99.81) 966 (98.57) 965 (98.90)
(E)-2-tetradecenyl acetate (E)-3-tetradecenyl acetate (E)-4-tetradecenyl acetate (E)-5-tetradecenyl acetate (E)-g-tetradecenyl acetate (E)-7-tetradecenyl acetate (E)-8-tetradecenyl acetate (E)-g-tetradecenyl acetate (E)-10-tetradecenyl acetate (E)-11-tetradecenyl acetate (E)-la-tetradecenylacetate
2934 (90.75) 2033 (87.01) 2934 (79.03) 2934 (84.77) 2934 (83.79) 2934 (79.81) 2934 (84.63) 2934 (95.68) 2935 (82.99) 2934 (81.69) 2934 (79.32)
2864 (96.58) 2864 (95.11) 2864 (91.47) 2865 (94.07) 2865 (93.54) 2865 (91.85) 2865 (93.92) 2865 (98.39) 2865 (93.35) 2864 (92.35) 2865 (91.53)
1762 (95.48) 1761 (92.98) 1761 (87.70) 1761 (90.95) 1761 (90.09) 1761 (87.71) 1761 (90.82) 1761 (97.53) 1761 (90.04) 1761 (89.30) 1761 (88.77)
1458 (99.11) 1460 (98.71) 1456 (97.61) 1458 (98.29) 1458 (98.13) 1459 (97.66) 1459 (98.28) 1459 (99.52) 1459 (98.10) 1460 (97.89) 1459 (97.76)
1368 (98.63) 1369 (97.86) 1369 (95.87) 1369 (97.04) 1369 (96.76) 1369 (95.95) 1369 (97.00) 1369 (99.21) 1369 (96.75) 1369 (96.47) 1369 (96.23)
1228 (92.06) 1232 (88.52) 1232 (80.48) 1232 (85.19) 1232 (84.08) 1232 (80.39) 1232 (85.16) 1233 (95.86) 1233 (83.96) 1232 (82.90) 1232 (82.04)
1022 (98.38) 1038 (97.35) 1042 (95.40) 1042 (97.09) 1043 (96.82) 1042 (96.01) 1040 (97.18) 1040 (99.21) 1040 (96.77) 1041 (96.46) 1041 (96.27)
968 (98.56) 967 (98.50) 967 (97.21) 968 (97.86) 967 (97.64) 967 (97.22) 967 (98.00) 967 (99.46) 967 (97.91) 966 (97.83) 964 (97.69)
(E)-3-hexadecenyl acetate (E)-a-hexadecenyl acetate (E)-5-hexadecenyl acetate (E)-g-hexadecenyl acetate (E)-7-hexadecenyl acetate (E)-8-hexadecenyl acetate (E)-g-hexadecenyl acetate (E)-10-hexadecenyl acetate (E)-11-hexadecenyl acetate (E)-12-hexadecenyl acetate (E)-13-hexadecenyl acetate
2932 (88.80) 2932 (89.52) 2933 (90.94) 2933 (92.79) 2933 (89.90) 2933 (90.82) 2933 (92.72) 2933 (89.43) 2933 (88.52) 2933 (94.40) 2933 (90.29)
2863 (95.84) 2863 (96.06) 2864 (96.58) 2864 (97.33) 2864 (96.22) 2864 (96.53) 2864 (97.26) 2864 (95.98) 2864 (95.60) 2864 (97.95) 2864 (96.22)
1761 (95.11) 1761 (95.27) 1761 (95.76) 1761 (96.60) 1761 (95.19) 1761 (95.65) 1761 (96.60) 1761 (94.97) 1761 (94.58) 1761 (97.44) 1761 (95.59)
1460 (99.05) 1457 (99.04) 1458 (99.15) 1459 (99.35) 1458 (99.06) 1459 (99.12) 1459 (99.32) 1459 (98.97) 1459 (98.94) 1460 (99.50) 1460 (99.10)
1369 (98.56) 1369 (98.48) 1369 (98.64) 1369 (98.95) 1369 (98.47) 1369 (98.63) 1369 (98.94) 1369 (98.38) 1369 (98.30) 1369 (99.20) 1369 (98.61)
1231 (91.81) 1232 (92.02) 1232 (92.75) 1232 (94.21) 1232 (91.87) 1232 (92.66) 1232 (94.26) 1232 (91.57) 1232 (90.89) 1232 (95.65) 1232 (92.55)
1038 (98.25) 1042 (98.34) 1042 (98.69) 1043 (99.00) 1041 (98.53) 1041 (98.69) 1040 (98.97) 1041 (98.42) 1041 (98.33) 1042 (99.22) 1042 (98.65)
967 (99.05) 967 (98.99) 968 (99.05) 967 (99.26) 967 (98.95) 967 (99.04) 966 (99.28) 967 (98.94) 967 (99.93) 967 (99.51) 966 (98.20)
(E)-2-octadecenyl acetate (E)-3-octadecenyl acetate (E)-g-octadecenyl acetate (E)-11-octadecenyl acetate (E)-13-octadecenyl acetate (E)-lB-octadecenylacetate
2933 (93.53) 2932 (92.50) 2933 (98.24) 2933 (96.09) 2933 (97.53) 2933 (97.86)
2863 (97.63) 2863 (97.29) 2864 (99.34) 2864 (98.56) 2864 (99.07) 2863 (99.20)
1761 (97.85) 1761 (97.26) 1761 (99.31) 1761 (98.50) 1760 (99.06) 1761 (99.20)
1458 (99.50) 1460 (99.38) 1460 (99.84) 1458 (99.69) 1459 (99.80) 1460 (99.78)
1367 (99.35) 1368 (99.15) 1369 (99.76) 1369 (99.53) 1369 (99.72) 1369 (99.73)
1228 (96.06) 1231 (95.34) 1232 (99.82) 1232 (97.44) 1232 (98.42) 1232 (98.63)
1022 (99.27) 1038 (99.00) 1041 (99.76) 1041 (99.56) 1041 (99.73) 1042 (99.75)
968 (99.36) 967 (99.43) 966 (99.81) 967 (99.70) 968 (99.82) 965 (99.84)
Table 3. Gas-Phase Infrared Spectra of Three o-Unsaturated Acetates
compound name
band position, cm-1 (percent transmission)
9-decenyl acetate 3086 (98.96) 2935 (88.58) 2865 (95.79) 1761 (90.00) 1641 (99.38) 1459 (98.66) 1369 (96.91) 1232 (83.61) 1041 (96.75) 914 (98.32) 11-dodecenylacetate 3086 (99.09) 2934 (86.57) 2865 (94.97) 1761 (91.16) 1641 (99.44) 1459 (98.58) 1369 (97.22) 1232 (85.39) 1040 (97.12) 914 (98.48) 13-tetradecenylacetate 3085 (99.79) 2934 (95.46) 2864 (98.35) 1761 (97.77) 1641 (99.88) 1459 (99.62) 1369 (99.31) 1232 (96.32) 1041 (99.32) 914 (99.65)
Furthermore, there are additional features in the spectra of (Z)-2-alkenyl acetates that allow complete identification of the position and the configuration of the CH=CH bond in these compounds. From the spectra of the three (2)-2monoene acetates investigated, it was evident that, in addition to the C-H stretch absorptions of exceptionally high frequency (3029-3028 cm-I), the absorption at 1025 cm-l is also characteristic for a double bond located at C-2. For most of the monounsaturated acetates this IR band appears around 1040 cm-I; only the spectra of compounds bearing a (2)-2 or an (E)-2 bond show this absorption at 1025 or 1022 cm-l, respectively. Furthermore, for the (2)-2 compounds, the 1025cm-l band has a weak, but a very characteristic, shoulder on the lower frequency side (Figure 2). Another characteristic band diagnostic of the (2)-2 bond is the absorption at 1375-
1374 cm-I. For most of the compounds, including the (E)-2 and terminalunsaturated compounds (Table 3), this absorption occurs at 1369-1 368 cm-I. In contrast to the spectra of cis, or terminal unsaturated compounds which will be discussed later, none of the spectra obtained from trans compounds showed any absorption above 3000 cm-l. Since none of the spectra of trans compounds show this absorption, we emphasize that the 3028-301 1 cm-l absorption in gas phase FT-IR spectra is characteristic for cis alkenyl compounds. From the data available for condensed phase” and MI-FT-IR ~ p e c t r a , ~one * J ~would expect to see absorbances for trans =C-H stretch in the 3000-cm-l region. Since we did not observe any significant absorbances in this region for trans compounds, it appeared that either these bands are so weak that they are not visible at the sensitivities and Ana&ticalChemistry, Vol. 66, No. IO, May 15, 1994
1099
A I
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,
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.
,
,
,
...
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-
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I000
Wnvcnumbar (cm-1)
Wnvenimber (cni-l)
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r
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I006
Wavenumber (em-!)
Figure 1. Gas-phase infrared spectra of (Z)-5-tetradecenyl acetate (A), (€)-5-tetradecenylacetate(B), andtetradecylacetate (C)(resolution = 8 cm-l).
3000
2060
1 000
Wnveniiniber (em-’)
Figure 2. Gas-phase infrared spectra of (Z)-2dodecenyl acetate (A), (&2-tetradecenyl acetate (B), and (&2octadecenyl acetate (C) (resolution = 8 cm-’).
spectral resolutions of our experiments or they are shifted toward lower frequency and merge with the CH2 asymmetric stretch band. All the spectra of trans monounsaturated acetates appeared very similar and showed the well-established absorption at 968-964 cm-l for the trans C-H wag (Table 2). We observed that the spectra of (E)-2 compounds show a highly characteristic three-peak pattern centered around 1022cm-1asshown by those of (E)-2-dodecenyl acetate, (E)-Ztetradecenyl acetate, and (E)-2-octadecenyl acetate (Figure 3). Interestingly, this “fingerprint” pattern is evident in the vapor phase 1700
AnalyticalChemistry, Vol. 66, No. 10, M y 15, 1994
4000
3000
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Wavenumber (cm-I)
Figure 3. 3. Gas-phase Infrared spectra of (€)-2dodecenyl acetate (A), (€)-2-tetradecenyl acetate (B), and (€)-2-octadecenyl acetate (C) (resolution = 8 cm-1).
spectra reported for (E)-2-octenyl acetate and (E)-2-hexenyl acetate.30 Furthermore, the spectra of five examples of (E)-3 acetates examined show a characteristic band at 1038 cm-’ (Table 2). The frequency of this band is relatively low compared to those of all other monounsaturated acetates, excluding the 2-monounsaturated acetates. The spectra of long-chain unsaturated compounds bearing terminal double bonds show three absorptions that are useful for the characterization the end vinyl group (Table 3). In addition to the expected absorptions at 914 and 3085 cm-l, the spectra of 9-decenyl acetate, 11-dodecenyl acetate, and 13-tetradecenyl acetate show a small but significant peak at 1641 cm-l attributable to the C=C stretching vibration (Figure 4).31 None of the compounds with internal double bonds investigated in this study showed this band presumably because the stretching of the C=C bond of these compounds affords little or no dipole moment change due to the pseudosymmetry around the bond. The absorption frequencies of these three bands agree well with those reported for l - a l k e n e ~ . 1 2This ~ ~ ~is~ not ~ ~ surprising since the vinyl group is remote from the functional group in the w-unsaturated alkenyl acetates investigated here. In order to establish beyond doubt that the 3013-3011cm-l absorption is characteristic for cis compounds of RCH=CHR’ type, we synthesized cis and trans isomers of 11- [ 11,l 2-2H2]tetradecenol and their corresponding acetates and recorded the IR spectra of thesecompounds. For example, the spectrum of (Z)-11-tetradecenol (Figure 5A) and (Z)11-tetradecenyl acetate (Table l), shows the=C-H cis stretch absorption at 3011 cm-l. This band is clearly absent in the spectra of (Z)-11-[1 1,12-2H2Jtetradecen~l (Figure 5B) and (30) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, Vapor Phase, Edition 1; Aldrich Chemical Co.: Milwaukee, WI, 1989. ( 3 1 ) Spande, T. F.; Garraffo, H. M.; Daly, J. W.; Tokuyama, T.; Shimada, A. Tetrahedron 1992, 48, 1823-1836.
I
I
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:fe 0
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2000
1000
I 0clU
Wavenumber (cm-l)
100.0 99.0
r/---i-T
98.0 97.0 96.0 4000
c
4 3000
2000
I000
Wnvcnumber (cm-l)
Flgure 4. Gas-phase infrared spectra of Sdecenyl acetate (A), 11dodecenyl acetate (B), and 13-tetradecenyl acetate (C) (resolution = a cm-I).
Figure 6. Gasphase infrared spectra of ( e l 1-tetradecenol(A), (4 11-[l 1,12-*H2]tetradecenol(B), and ( & l l - [ l 1,12-*H2]tetradecenyl acetate (C) (resolution = 8 cm-'1.
fZ)-l I-lrtrrdccenol
I
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2000
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,
,
,
,
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I000
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,
,
,
, I000
,
Wavenumber (em-')
(Z)-ll-[l 1.12-zHzllelradcccnyI acetile
4000
3000
2000
I 000
Wnvenumbcr (em-')
Figure 5. Gas-phase Infrared spectra of ( e 1 1-tetradecenol(A), (2)11-[ 11,12-2H2]tetradecenol(e), and ( e 1 1-[l 1,12-2H2]tetradecenyl acetate (C)jresolution = a cm-'1.
(Z)-11-[l 1,12-2H2]tetradecenylacetate (Figure 5C), whilea new band is seen at 2246 and 2244 cm-l, respectively, for the cis =C-D stretching vibration. Most interestingly, an absorption is visible in the spectrum of (E)-1 1-[l 1,12-2H2]tetradecenol and (E)-1 1-[ 11,12-2H2]tetradecenyl acetate at 221 5 cm-l attributable to the =CD trans stretching vibration (Figure 6). Under the sensitivities and resolutions used in this investigation, a separate band in the 3000-cm-' region, for the =CH trans stretching vibration, was not observed for any of the trans compounds. It may be of interest to note here that the CD2 asymmetric and CD2 symmetric stretch absorp-
tions for methyl [7,7,8,8-2H4]palmitate are seen at 2193 and 2103 cm-l, respectively (Attygalle, unpublished data). From the data of monounsaturated acetates, it is apparent that the ratio of carbonyl absorbance (1761-1760 cm-l) to C-0 absorbance (1228-1233 cm-l) is nearly constant and independent of the chain length of the alcohol moiety and the position and configuration of the double bond ( X = 0.583 f 0.01 2, n = 86) (Figure 7). It is also apparent that the standard deviations shown in Figure 7 were relatively low for decenyl and hexadecenyl data sets. This is not surprising since these two data sets did not include values from 2-alkenyl isomers, for which the deviations of general band positions and intensities were slightly larger than those of the rest of the compounds (Tables 1 and 2). Since the intensities of the =CH stretch cis and =CH wag trans absorptions are diagnostic of the number of such bonds present in a molecule, H and [A+-H cis/ we estimated the ratios of [ A ~ - cis/A~+] A c a ] and found them to be 0.270 f 0.034 and 0.157 f 0.021 (n = 43), respectively. Similarly, the values of [A=c-H Ac=ol and [A=c-H trans/Aca] were 0.2 12 f 0.03 1 and 0.124 f 0.016 (n = 43), respectively. The intensity of the CH2 asymmetric stretch band relative to that of the carbonyl band increases with the increase in chain length (Figure 7, stippled bars) allowing unambiguous determination of the carbon chain length of the acetate, if necessary, by examining the gasphase IR spectrum of an unknown monounsaturated acetate. The cis and trans infrared bands are distinct also in unsaturated hydrocarbons. For example, the spectrum of (E)9-nonadecene shows a distinct absorption at 968 cm-' (Figure 8A) and no bands above 3000 cm-l, in contrast to that of (2)-9-pentacosene which shows the characteristic cis =C-H stretch band at 301 1 cm-l. The 3013-301 1 cm-l absorption is also evident in the spectra reported for (Z)-9-heneicosene Analytical Chemistry, Vol. 66, No. 10, May 15, 1994
1701
(A) Cis a c e t a t e s
(8) Tans a c e t a t e s
3.0
3.0 J
I
I
I
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6 8 4
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Figure 7. Mean absorbance ratios of some I R bands in alkenyl acetate spectra: (A) CISalkenyl acetates, open bars = &/Aco; hatched bars = A+H c r l A ~cross-hatched ; bars = A-H c$Ac-o; stlppied bars = Acnl/A-; (B) trans alkenyl acetates, open bars = A+.,/&; hatched ; bars = bars = A-4 I r . M I A ~checkered tranJAco;stippled bars = AcH,/&. Standard deviations are illustrated as vertlcal lines.
I
(E)-9-nonndccene
Figure 8. Gas-phase infrared spectrum of (Z)-5-tetradecen-l3sIide (resolution = 8 cm-l).
4000
3000
2000
I000
Wavenumber (em-')
Figure 8. Gas-phase infrared spectra of (4-9-nonadecene (A) and (+9-pentacosene (B) (resolution = 8 cm-').
and (Z)-9-tri~osene.~~ However, for cis alkenes such as ( Z ) 2-pentene30 and (Z)-2-nonadecene,32 this band appears not well resolved from the strong asymmetric CH2 stretch absorption. Unsaturated hydrocarbons are common in insect secretions. For example, pentadecene isomers have been characterized from the Dufour's gland secretion of ants belonging to the genus T e t r a m o r i ~ mThe . ~ ~infrared spectrum acquired by GC/FT-IR for the pentadecene peak obtained from an extract of 10 Dufour's glands of a species of Tetramorium showed an absorption at 3010 cm-', which indicated that the configuration of the double bond of the compound(s) represented by this chromatographic peak must be cis. (32) Sojik, L.; Ostrovsky, I.; Kubinec, R.; Kraus, G.; Kraus, A. J . Chromatogr.
The cis absorption is evident not only in open chain compounds but also in macrocyclic compounds bearing cis bonds. The infrared spectrum of (Z)-5-tetradecen-13-olide illustrated in Figure 9 shows clearly an absorption at 301 1 em-' which can be attributed to the presence of the cis bond at C-5. We have used the presence of the cis band in the identification of several natural products. For example, the IR spectrum of epilachnene, an azamacrolide isolated from a ladybird beetle, was very similar to that of (Z)-5-tetradecen13-olide and showed an absorption at 301 1 cm-I indicating a cis double bond in its structure.34 At resolutions of 4 to 8 cm-l, commonly obtained for GC/ FT-IR, the spectra of saturated hydrocarbons show only two strong bands in the C-H stretch region. The most intense peak represents the CH2 asymmetric stretch (2932 cm-I) while a band of medium intensity depicts the CH2 symmetric stretch (2864 cm-I). Sometimes a shoulder, which disappears as the carbon chain length increases above 18 atoms, is visible on the 2932-em-' band. This shoulder is attributable to the CH3 asymmetric stretch. According to most organic chemistry
1992, 609, 283-288.
(33) Billen, J. P. J.; Evershed, R. P.; Attygalle, A. B.; Morgan, E. D.; Ollett, D. G. J . Chem. Ecol. 1986, 12, 669-685.
1702 Analytical Chemistry, Vol. 66,No. 10, May 15, 1994
(34) Attyga1le.A. B.; McCormick, K. D.; B1ankespoor.C.L.; Eisncr,T.; Meinwald, J. Prof. Narl. Acad. Sci. V.S.A. 1993, 90, 5204-5208.
books, and publications on spectrometric identification of organic compounds, any C-H stretching bands observed above 3000 cm-l, if not due to aromatic, heteroaromatic, or acetylenic groups, can be considered as a clear sign of unsaturation. However, these generalizations based on condensed phase spectra cannot be extended directly to vapor phase spectra, as demonstrated by the fact that none of the trans unsaturated compounds in this study showed any significant band above 3000 cm-I. Generalizations which are valid for gas-phase spectra are not commonly available, except for the compilation by Nyquist.I2 However, this reference does not provide any generalizations about RCH=CHR' type compounds. In matrix-isolation Fourier transform infrared (MI-FTIR) spectra of polyunsaturated fatty acid methyl esters weak absorptions are observed in the 3035-3033-cm-l region, attributable to the =CH trans vibrations.18 From our data for gas-phase spectra, it is clear that no peaks are discernible above 3000 cm-l for any of the trans compounds examined. We believe that in gas-phase spectra the trans vinyl C H stretch band, although it should be present since allowed by symmetry, is displaced sufficiently to longer wavelengths so as to merge with the strong CHI asymmetric stretch absorptions. This supposition is substantiated by the fact that an absorption, attributable to the =CD trans stretching vibration, is visible at2215cm-I in thespectraof (E)-1 1-[l 1,12-2H2]tetradecen~l and (E)-1 1-[ 11,12-2H2]tetradeceny1acetate (Figure 6). Furthermore, no absorptions are present in the 3050-3000-cm-' region of the spectra of (Z)-11-[l 1,12-2H2]tetradecen~l and (Z)-11-[ 1 1,12-2H2]tetradecenyl acetate, instead a weak absorption is present at 2246-2244 cm-' for the =CD cis vibration (Figure 5). To further test the above hypothesis, we carried out ab initio calculations at the 6-31G** a procedure known to predict frequencies that agree reasonbly well with experimental values, for (Z)- and (E)-2-butene. The calculated spectra, after scaling the calculated harmonic frequencies in the standard way,35were in fact reasonably faithful reproductions of the observed gas-phasespectra.12 The calculatedvalue for the trans olefinic C H frequency was 19 wavenumbers below that of cis. Since the cis isomers show absorptions near 3012 cm-l, the unobserved trans band can be expected at 2993 cm-I, if we assume that the results from the calculation for 2-butene could be extrapolated to the high molecular weight cis and trans isomers investigated in this study. At 2993 cm-l the trans olefinic C H absorption would indeed lie at the highfrequency edge of the saturated CH absorptions and could well be unrecognizable. Indeed, for several trans isomers, the spectra show a shoulder at the edge the strong saturated CH2 absorption at 2935 cm-l that could well be the missing olefinic C H band. The support for using the calculated cis and trans olefinic frequency difference comes from a parallel calculation on the corresponding dideuterio analogs. From our experimental data, these lie at 2246 and 2215 cm-l, for cis and trans, respectively. The observed frequency difference of 3 1 wavenumbers is in excellent agreement with the calculated difference of 35 cm-I. (35) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; Defrees, D.J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. In?.J . Quontum Chem., Quorum Chem. Symp. 1981, IS, 269-218. (36) Attygalle, A. B.; SvatoS, A,; Voerman, S. Unpublished.
Both observed and calculated spectra reported here refer to the gas phase. The calculations say nothing about the conventional wisdom that solution spectra of both cis and trans alkenes have CH absorptions in the range 3040-3010 cm-l. Furthermore, it is not clear why the apparent hypsochromic solvent shift is greater for trans isomers. From the examples given, we have demonstrated that the presence or absence of 3028-301 1 and 968-964 cm-l absorptions in combination allow unambiguous determination of cis and trans bonds in long-chain unsaturated pheromones and similar compounds by gas-phase infrared spectra. In addition, this technique can ascertain unambiguously the presence of terminal (3086-3085 cm-I), (2)-2 (3028 and 1025 cm-l), or (E)-2 type (1022 and 968 cm-l) double bonds in long-chain compounds. The generalizations made here for monounsaturated compounds are in fact applicable to polyunsaturated compounds as well.36 Although condensed phase infrared spectroscopy was one of the first spectroscopic methods used in natural product characterization, its popularity waned as a result of recent advances in NMR technology. However, for samples available in only minute amounts, and in impure form, NMR spectroscopy is not the method of choice, since for routine NMR analysis microgram to milligram amounts of a relatively pure sample, and several hours of data acquisition time, are required. From the examples we have presented, it is evident that gasphase IR spectrometry offers a simple and straightforward procedure for the determination of the configurations of double bonds even when nanogram amounts of material are available. Since a gas chromatographic separation precedes the detection by the IR spectrometer, the samples need not be pure. Modern infrared instruments designed for gas-phase measurements, although not as sensitive as mass spectrometers, can record spectra of samples as minute as 5-50 ng. In our opinion, the potential of modern GC/FT-IR technique in natural product identification is not fully exploited. We hope this paper will stimulate chemists to take advantage of this technique for determining double bond configurations in the identification of pheromones and related natural products, as well as in the analysis of synthetic materials.
ACKNOWLEDGMENT We are greatly indebted to Dr. A. C. Oehlschlager (Simon Fraser University, Canada) for a sample of (Z)-5-tetradecen13-olide, Dr. E. D. Morgan (University of Keele, U. K.) for (E)-g-nonadecene and Tetramorium ants, and Dr. H. J. Bestmann (University of Erlangen, Germany) for (Z)-9pentacosene. We also thank Dr. J. Meinwald (Cornel1 University) for critically reading the manuscript. We are grateful to the Hewlett-Packard Company (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 A.B.A. at the annual meeting of the International Society of Chemical Ecology, Tampa/Clearwater, July 3 1-August 4, 1993. The partial support of this research by NIH Grant AI 12020 is acknowledged with pleasure. Received for review December 3, 1993.' 1994.
Accepted February 23, ~~
~~~~~
Abstract published in Aduance ACS Absrmcrs. April 1, 1994.
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