Characterization of vinyl-substituted, carbon-carbon double bonds by GC/FT-IR analysis.

Article abstract of DOI:10.1021/ac960890u

Vapor-phase infrared spectra allow the determination of the stereochemistry of carbon-carbon double bonds conjugated with a vinyl group. Cis and trans isomers of unsubstituted 1,3-alkadienes can be differentiated on the basis of the differences observed in the 900-1000 cm-1 region (spectra of cis isomers show two bands at 993 and 906 cm-1, while those of trans compounds show three absorptions at 998, 949, and 902 cm-1) and the 1590-1650 cm-1 region (the C=C stretch bands are observed at 1595 and 1642 cm-1 for cis compounds and at 1604 and 1650 cm-1 for trans compounds). Compounds bearing CH2=CHC(CH3)=CHCH2- and CH2=CHC(=CH2)-CH2- structural moieties, referred to as alpha- and beta-type compounds, are frequently encountered as natural products. For compounds bearing alpha-type groups, the cis/trans configuration of the trisubstituted double bond can be determined unambiguously. An absorption at 3095-3091 cm-1, for the =CH2 stretch vibration, is common to both of these groups; however, due to the presence of two =CH2 groups, the relative intensity of the band is much higher for beta-type compounds. For alpha-type compounds, a cis configuration at the C-3 carbon atom is characterized by a =CH2 wag absorption at 907-906 cm-1. For beta-type compounds and 3E-alpha-type compounds, this band appears at 899-897 cm-1. In addition, a wavy "fingerprint" pattern with two minima at 1632 (low intensity) and 1595-1594 cm-1 (high intensity) is characteristic for beta-type compounds. Our generalizations are based on spectra of cis and trans ocimene, myrcene, and dehydration products of many 3-methyl-1-alken-3-ols. Six isomers of farnesene can be characterized by GC/FT-IR. Furthermore, gas-phase IR allows the determination of the configuration of the trisubstituted double bond at C-3 in alpha-type farnesene congeners. For example, the homo- and bishomofarnesene isomers from Myrmica ants were shown to include a 3Z bond.

Full text of DOI:10.1021/ac960890u

Anal. Chem. 1997, 69, 1827-1836
Ch a ra c t e riza t io n o f Vin yl-S u b s t it u t e d ,
Ca rb o n -Ca rb o n Do u b le Bo n d s b y GC/FT-IR
An a lys is
†
Ale sˇ Sva to sˇ a nd Athula B. Attyga lle *
Baker Laboratory, Department of Chemistry, Cornell University, Ithaca, New York 14853
Vapor-phase infrared spectra allow the determination of
the stereochemistry of carbon-carbon double bonds
conjugated with a vinyl group. Cis and trans isomers of
unsubstituted 1 ,3 -alkadienes can be differentiated on the
basis of the differences observed in the 9 0 0 -1 0 0 0 cm-
1
region (spectra of cis isomers show two bands at 9 9 3 and
0 6 cm- , while those of trans compounds show three
1
9
-
1
absorptions at 9 9 8 , 9 4 9 , and 9 0 2 cm ) and the 1 5 9 0 -
1
6 5 0 cm- region (the CdC stretch bands are observed
1
-
1
at 1 5 9 5 and 1 6 4 2 cm for cis compounds and at 1 6 0 4
and 1 6 5 0 cm- for trans compounds). Compounds
to the C-3 atom.1 Although 1H NMR spectroscopy can be used
1
to characterize these carbon skeletons, relatively large sample
amounts (10-100 µg) in nearly pure form are required. Fre-
quently, natural product chemists are frustrated by the unavail-
ability of samples that large.
bearing CH
2 3 2 2 2
dCHC(CH )dCHCH - and CH dCHC(dCH )-
CH - structural moieties, referred to as r- and â-type
2
compounds, are frequently encountered as natural prod-
ucts. For compounds bearing r-type groups, the cis/ trans
configuration of the trisubstituted double bond can be
The farnesenes (4-9) are a group of common natural products
2-12
which bear the aforementioned R- and â-type carbon skeletons.
determined unambiguously. An absorption at 3 0 9 5 -
Although GC/ MS is the most frequently used technique for the
identification of farnesene isomers, characterization of all naturally
occurring isomers using mass spectral data alone is not straight-
forward. Particularly, the mass spectra of (E)-â- and (Z)-â-
farnesene are very similar. Consequently, many farnesenes
isomers recognized by GC/ MS have been reported without
complete stereochemistry.13 Moreover, many reports on the
identification of homo-, bishomo-, and trishomofarnesenes failed
to discuss the stereochemistry because of the absence of a
sensitive and reliable method for configuration determination of
small amounts of compounds found as complex mixtures.14-16
1
3
0 9 1 cm- , for the dCH
2
stretch vibration, is common
to both of these groups; however, due to the presence of
two dCH groups, the relative intensity of the band is
2
much higher for â-type compounds. For r-type com-
pounds, a cis configuration at the C-3 carbon atom is
characterized by a dCH
2
wag absorption at 9 0 7 -9 0 6
cm- . For â-type compounds and 3 E-r-type compounds,
1
-1
this band appears at 8 9 9 -8 9 7 cm . In addition, a wavy
fingerprint” pattern with two minima at 1 6 3 2 (low
“
-1
intensity) and 1 5 9 5 -1 5 9 4 cm (high intensity) is char-
acteristic for â-type compounds. Our generalizations are
based on spectra of cis and trans ocimene, myrcene, and
dehydration products of many 3 -methyl-1 -alken-3 -ols. Six
isomers of farnesene can be characterized by GC/ FT-IR.
Furthermore, gas-phase IR allows the determination of
the configuration of the trisubstituted double bond at C-3
in r-type farnesene congeners. For example, the homo-
and bishomofarnesene isomers from Myr m ica ants were
shown to include a 3 Z bond.
(
(
(
(
1) Connoly, J. D.; Hill, R. A. Dictionary of Terpenoids, Vol. 1, Mono- and
sesquiterpenoids; Chapman and Hall: London, 1991.
2) Bowers, W. S.; Nault, L. R.; Webb, R. E.; Dutky, S. R. Science 1 9 7 2 , 177,
1121-1122.
3) Nault, L. R.; Phelan, P. L. In Chemical Ecology of Insects; Bell, W. J., Carde,
R. T., Eds.; Chapman and Hall: London, 1984; pp 238-256.
4) Sakai, T.; Hirose, Y. Bull. Chem. Soc. Jpn. 1 9 6 9 , 42, 3615-3616.
(5) Detrain, C.; Pasteels, J. M.; Braekman, J. C.; Daloze, D. Experientia 1 9 8 1 ,
3, 345-346.
6) Attygalle, A. B.; Morgan, E. D. Chem. Soc. Rev. 1 9 8 4 , 245-278.
(7) Knight, D. W.; Rossiter, M.; Staddon, B. W. J. Chem. Ecol. 1 9 8 4 , 10, 641-
49.
8) Naya, Y.; Prestwitch, G. D. Tetrahedron Lett. 1 9 8 2 , 23, 3047-3050.
(9) Vander Meer, R. K.; Williams, F. D.; Lofgren, C. S. Tetrahedron Lett. 1 9 8 1 ,
651-1654.
10) Cavill, G. W. K.; Williams, P. J.; Whitefield, F. B. Tetrahedron Lett. 1 9 6 7 ,
201-2205.
(11) Fletcher, M. T.; Kitching, W. Chem. Rev. 1 9 9 5 , 95, 789-828.
4
(
6
(
Many natural products, particularly terpenes, bear vinyl groups
conjugated to carbon-carbon double bonds. A number of these
vinyl compounds are either disubstituted 1,3-butadienes with a
methyl group (designated as R-type, 1 , 2 ) or monosubstituted
1
(
2
(
(
(
12) Murray, K. E. Aust J. Chem. 1 9 6 9 , 22, 197-204.
13) Bergstr o¨ m, G.; Teng o¨ , J. Chem. Scr. 1 9 7 4 , 5, 28-38.
14) Ollett, D. G.; Morgan, E. D.; Attygalle, A. B.; Billen, J. P. J. Z. Naturforsch.
1 9 8 7 , 42c, 141-146.
1,3-butadienes (â-type, 3 ) with a large substituent group attached
†
Permanent address: Institute of Organic Chemistry and Biochemistry,
Academy of Sciences of the Czech Republic, Flemingovo n a´ m. 2, 166 10 Prague
, Czech Republic.
(15) Jackson, B. D.; Cammaerts, M.-C.; Morgan, E. D.; Attygalle, A. B. J. Chem.
6
Ecol. 1 9 9 0 , 16, 827-840.
S0003-2700(96)00890-6 CCC: $14.00 © 1997 American Chemical Society
Analytical Chemistry, Vol. 69, No. 10, May 15, 1997 1827

In the present study, we recorded vapor-phase infrared spectra
of many conjugated vinyl compounds. The generalizations made
from these spectra allowed not only the characterization of all
isomers of farnesene but also the determination of the configu-
ration of the trisubstituted double bond at the C-3 atom of R-type
homo- and bishomofarnesene isomers from Myrmica ants.
isomer were completely absent, the solvent was evaporated and
the residue dissolved in hexane. After purification on a Florisil
column, 7.9 mg of pure (3Z)-1,3-tridecadiene was obtained. 1H
NMR (400 MHz, CDCl ): δ 6.64 (1H, dddd, J ) 17.2, 11.2, 10.2,
3
1.5 Hz, dCH-2), 5.99 (1H, dd, J ) 10.3, 10.2 Hz, dCH-3), 5.45
(1H, dd, J ) 10.3, 7.8 Hz, dCH-4), 5.18 (1H, dd, J ) 17.1, 2.0 Hz,
dCH
7.8, 7.6 Hz, CH
(3H, t, J ) 6.7, 6.9 Hz, CH ). C NMR (100.3 MHz, CDCl ): δ
R
H
â
), 5.08 (1H, d, J ) 11.3 Hz, dCH
R â
H ), 2.17 (2H, dd, J )
EXPERIMENTAL SECTION
2
-5), 1.37 (2H, m, CH -6), 1.30-1.25 (12H, m), 0.87
2
13
Instrumentation. Vapor-phase GC/ FT-IR spectra (resolution,
3
3
8
cm-1) were recorded using a Hewlett-Packard (HP) 5890 gas
133.1 (dCH-2), 132.3 (dCH-3), 129.1 (dCH-4), 116.6 (dCH -1),
2
chromatograph coupled to a HP 5965A IRD instrument equipped
31.9 (CH -11), 29.6 (CH -8*), 29.5 (CH -9*), 29.3 (CH -10), 29.2
2
2
2
2
with a narrow-band (4000-750 cm-1) infrared detector (mercury-
(CH -6*), 29.2 (CH -7*), 27.7 (CH -5), 22.7 (CH -12), 14.1 (CH -
2
2
2
2
3
17
+
cadmium-telluride) as described previously. Purity and identity
of each sample were checked by GC/ MS performed on a HP 5890
gas chromatograph coupled to a HP 5870B mass selective detector
using a 25 m × 0.22 mm fused silica column coated with DB-5
stationary phase (J & W Scientific; 0.25 µm film thickness). For
GC/ MS analyses, the oven temperature was held at 60 °C for 3
min and increased to 250 °C at 10 °C/ min. NMR spectra were
recorded on a Unity-400 ( H NMR, 400 MHz; 13C NMR, 100.6
13). EI-MS: m/ z (relative intensity) 180 (22) M , 109 (10), 96
(25), 95(26), 82 (50), 81 (59), 79 (27), 69 (26), 68 (68), 67 (86), 57
(17), 55 (45), 54 (90), 43 (48), 41 (100). UV: λmax ) 227 nm.
(3 E)-1 ,3 -Tridecadiene (1 3 ). A mixture of 1,3-tridecadiene
stereoisomers (56 mg), hydroquinone (5 mg), and liquid sulfur
dioxide (condensed at -70 °C, ∼1.5 mL) was kept for 2 days at
19
-
20 °C. The unreacted diene was separated from the more polar
1
2
SO adduct (25 mg) by chromatography on Florisil using a
MHz; Varian) instrument for samples as CDCl
cal shifts, given in ppm, are expressed as δ values measured from
the residual CHCl signal (7.26 ppm) (tentative NMR assignments
are denoted by asterisks). Column chromatography was per-
formed on silica gel (Merck, H 60), and reactions were monitored
by TLC on Baker-flex silica gel IB2-F plates (J.T. Baker). UV
spectra were obtained using a diode array detector installed in a
HP 1090 instrument.
3
solutions. Chemi-
gradient of ether in hexane. The 3E isomer was isolated by
thermal decomposition of the sulfone. In fact, the sulfone
decomposes stereospecifically in the GC injector (280 °C),
releasing pure (3E)-1,3-tridecadiene.
3
Adduct. 1H NMR (400 MHz,
): δ 6.02 (2H, m, HCdCH), 3.74 (2H, q, J ) 16.1, 15.6, 15.1
CH ), 3.65 (1H, dd, J ) 7.3, 7.3 Hz, SO CH), 1.94 (1H, m,
-5a), 1.62 (1H, m, CH -5b), 1.48 (2H, m, CH
m, CH -7-12), 0.87 (3H, t, J ) 6.8, 6.8 Hz, CH
MHz, CDCl
(
3E)-1,3-Tridecadiene-SO
2
CDCl
3
Hz, SO
CH
2
2
2
2
2
2
-6), 1.4-1.2 (12H,
Chemicals. 1-Nonene (1 0 ), 2-nonanone, (()-nerolidol, (()-
13
2
3
). C NMR (100.3
(
E)-nerolidol, vinylmagnesium bromide, and allyltriphenylphos-
phonium bromide were purchased from Aldrich Chemical Co.
Milwaukee, WI), and (11Z)-15-methyl-1,11-hexadecadiene (1 1 )
3
): δ 130.4 (dCH-2*), 122.9 (dCH-3*), 64.5 (SO
CH ), 31.8 (CH -11), 29.4 (CH -7*), 29.4 (CH
-9*), 29.2 (CH -10*), 28.6 (CH -6), 27.0 (CH -5), 22.6
-12), 14.1 (CH -13).
3E)-1,3-Tridecadiene (1 3 ). 1H NMR (400 MHz, CDCl
2
CH-
-8*),
4
2
), 55.7 (SO
9.3 (CH
2
2
2
2
2
(
2
2
2
2
was a gift of Prof. H. J. Bestmann. All other chemicals were
purchased from commercial sources and used as obtained.
(
CH
2
3
(
3
): δ
(
3 Z)- and (3 E)-1 ,3 -Tridecadienes. Allyltriphenylphospho-
6
1
4
.31 (1H, ddd, J ) 17.1, 10.4, 10.3 Hz, dCH-2), 6.04 (1H, dd, J )
5.0, 10.4 Hz, dCH-3), 5.71 (1H, ddd, J ) 15.3, 7.5, 7.2 Hz, dCH-
nium bromide (0.70 g, 1.83 mmol) was dried at 100 °C and 0.1
mmHg for 3 h, suspended in dry THF (4 mL), and treated with a
sodium hexamethyldisilazide18 (3.2 mL, 1.90 mmol) solution in
toluene at room temperature for 0.3 h. The orange suspension
formed was stirred for 3.3 h, cooled in an ice bath, and treated
with distilled decanal (0.28 g, 1.8 mmol) in dry THF (0.4 mL).
The reaction mixture was stirred for 14 h and then quenched with
ice and HCl (2.5 mL of 35%). The product, a mixture of (3Z)- and
), 5.08 (1H, d, J ) 16.9 Hz, dCH
R
H
â
), 4.95 (1H, d, J ) 10.1 Hz,
-5), 1.30-1.25 (12H,
dCH
m), 0.87 (3H, t, J ) 6.7, 6.9 Hz, CH
CDCl ): δ 137.3 (dCH-2), 135.7 (dCH-3), 130.8 (dCH-4), 114.5
-1), 32.6 (CH -5), 31.9 (CH -11), 29.6 (CH -7*), 29.6 (CH
*), 29.5 (CH -9*), 29.3 (CH -10), 29.2 (CH -6*), 22.7 (CH -12),
4.1 (CH -13). EI-MS: m/ z (relative intensity) 180 (26) M , 109
R
H
â
), 2.07 (2H, dd, J ) 7.0, 7.0 Hz, CH
2
13
3
). C NMR (100.3 MHz,
3
(
dCH
2
2
2
2
2
-
8
1
2
2
2
2
+
3
(
3E)-1,3-tridecadienes (48:52 by 1H NMR), was extracted into
(
(
10), 96 (25), 95(26), 82 (50), 81 (58), 79 (26), 69 (24), 68 (68), 67
hexane and purified by silica gel column chromatography (0.25
g, 76% yield). An analytical sample was obtained by distillation
at 150-180 °C/ 20 mmHg.
86), 57 (17), 55 (45), 54 (90), 43 (48), 41 (100). UV: λmax ) 227
nm.
3
-Methyl-1 -decen-3 -ol. A solution of distilled 2-nonanone
282 mg, 2 mmol) in dry THF (5 mL) was treated with 1 M
vinylmagnesium bromide (Aldrich) in THF (2.2 mL) at 0 °C. After
.5 h, the reaction was quenched with saturated NH Cl solution,
(
3 Z)-1 ,3 -Tridecadiene (1 2 ). The mixture of (3Z)- and (3E)-
(
1
,3-tridecadienes (23 mg, 0.13 mmol) in CDCl (0.6 mL) was
3
treated with tetracyanoethylene (TCNE, 32 mg, 0.25 mmol) in an
NMR tube, and the progress of the reaction was monitored by
1H NMR spectroscopy.19,20 After 5 h, when signals for the 3E
0
4
and the mixture was extracted with ether. Flash chromatography
of the crude material afforded 167 mg (50%) of the desired alcohol.
1H NMR (400 MHz, CDCl
-CHd), 5.19 (1H, dd, J ) 1.2, 17.4 Hz, dCH
1.2, 10.7 Hz, dCH ), 1.40 (2H, m, CH C(OH)), 1.29 (1H, s,
OH), 1.27 (3H, s, C(CH )), 1.3-1.1 (8H, m, -CH -), 0.88 (3H, t, J
) 6.9 Hz, CH ).
-Methyl-1 -dodecen-3 -ol and 3 -methyl-1 -tridecen-3 -ol
): δ 5.90 (1H, dd, J ) 10.7, 17.4 Hz,
), 5.03 (1H, dd, J
3
(
(
(
(
(
16) Attygalle, A. B.; Morgan, E. D. J. Chem. Soc., Perkin Trans. 1 1 9 8 2 , 949-
51.
17) Attygalle, A. B.; Svato sˇ , A.; Wilcox, C.; Voerman, S. Anal. Chem. 1 9 9 4 , 66,
696-1703.
18) Bestmann, H. J.; Stransky, W.; Vostrowsky, O. Chem. Ber. 1976, 109, 1694-
700.
9
R â
H
)
R
H
â
2
1
3
2
1
3
19) Nesbitt, B. F.; Beevor, P. S.; Cole, R. A.; Lester, R.; Poppi, R. G. Tetrahedron
3
Lett. 1 9 7 3 , 4669-4670
20) Bestmann, H. J.; Attygalle, A. B.; Schwarz, J.; Garbe, W.; Vostrowsky, O.;
Tomida, I. Tetrahedron Lett. 1 9 8 9 , 30, 2911-2914.
were synthesized similarly, starting from 2-undecanone and
2-dodecanone, respectively.
1828 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

Dehydration of 3 -Methyl-1 -alken-3 -ols. A mixture of (3Z)-
-methyl-1,3-decadiene (1 4 ), (3E)-3-methyl-1,3-decadiene (1 5 ),
nerolidol (96.7% E isomer, 40 µL) as the starting material.
Natural Farnesenes. Five termite soldiers (Prorhinotermes
simplex), from a laboratory colony maintained in the Institute of
Organic Chemistry and Biochemistry, Czech Academy of Sci-
ences, were soaked in hexane (100 µL). About 20 gasters of
Myrmica sabuleti ants were crushed in hexane (200 µL) and
concentrated to 20 µL. Freshly peeled skins (4 g) from Granny
Smith apples (Wegmans Food, Ithaca, NY) in a glass distillation
apparatus were gently heated with a flame. The collected aqueous
condensate was extracted with hexane (0.5 mL), and the extract
was purified using a short column of Florisil (5 mm) by eluting
with a mixture of hexane/ ether (95:5 v/ v). Pea aphids (Acyrtho-
siphon pissum, 3.6 g) were extracted with hexane (5 mL), and the
extract was purified by a procedure similar to that used for the
apple sample.
3
and 3-heptyl-1,3-butadiene (1 6 ) was prepared from 3-methyl-1-
decen-3-ol, using POCl as the dehydrating agent (see the section
3
on preparation of farnesenes from nerolidol for experimental
details). Similarly, 3-methyl-1-dodecen-3-ol and 3-methyl-1-tri-
decen-3-ol were dehydrated by the same procedure.
4
-Methyl-1,3-undecadienes. Allyltriphenylphosphonium bro-
mide (460 mg, 1.20 mmol) was suspended in dry ether (9 mL)
and treated with n-butyllithium in hexane (0.75 mL, 1.2 mmol)
over a peroid of 10 min at 0 °C. The mixture was stirred for 55
min 0 °C, and 2-nonanone (140 mg, 1 mmol) in dry THF (3 mL)
was added. After the mixture was stirred for 10 h at room
temperature, ice and HCl (2 mL) were added. Products were
extracted with hexane and purified from the unreacted ketone
by silica gel column chromatography. This afforded a 35:65
mixture (15 mg, 9% yield) of (3E)- and (3Z)-4-methyl-1,3-undeca-
RESULTS AND DISCUSSION
Vapor-phase infrared spectra allow unambiguous recognition
of vinyl groups in volatile molecules by diagnostic bands observed
1
diene (ratios determined by GC and H NMR). Although the two-
isomer mixture was not separated, the NMR spectrum of the
mixture allowed the assignment of most signals corresponding
to each isomer.
2
at 3085-3083 (out-of-phase dCH stretch), 1641 (carbon-carbon
-
1
double bond), 993-992 (“trans” CH wag), and 914 cm (dCH
2
wag).2
1,22
In addition, spectra of low-molecular-weight vinylic
(
3E)-4-Methyl-1,3-undecadiene (1 7 ). GC: t
): δ 6.68 (1H, ddd, J ) 17.2, 10.7,
0.4 Hz, dCH-2), 5.84 (1H, db, J ) 10.7 Hz, dCH-3), 5.08 (1H,
), 4.97 (1H, dd, J ) 9.2, 1.8 Hz,
), 2.04 (2H, t, J ) 7.6 Hz, CH -5), 1.75 (3H, bs, CH -CHd),
.50-1.25 (10H, m, CH -6-10), 0.88 (3H, t, J ) 6.9 Hz, CH ).
EI-MS: m/ z (relative intensity) 166 (44), 137 (5), 123 (13), 109
R
) 14.15 min (DB-
-
1
). 1H NMR (400 MHz, CDCl
compounds show a band at 1828 cm which is an overtone of
the 914 cm- band. For example, the spectrum of 1-nonene (1 0 ,
Figure 1A) shows all the typical bands expected from a 1-alkene.
Our previous reports show that infrared bands for vinyl groups
can be recognized unambiguously even in the presence of
absorptions for other functional groups, although sometimes a
few of the vinyl bands may be obscured.17,23 Furthermore, we
have established vapor-phase infrared correlations for the recogni-
tion of cis/ trans configuration of CHdCH-type double bonds on
the basis of 3036-3011 and 982-947 cm- absorptions.17,24 From
the spectrum of (11Z)-15-methyl-1,11-hexadecadiene (11) depicted
in Figure 1B, it is evident that an isolated cis double bond can be
5
1
3
1
dd, J ) 16.8, 2.0 Hz, dCH
dCH
R â
H
R
H
â
2
3
1
2
3
(
(
(
(
5), 107 (5), 96 (15), 95 (90), 93 (90), 93 (15), 91 (13), 83 (14), 82
89), 81 (69), 80 (12), 79 (53), 78 (5), 77 (18), 69 (22), 68 (55), 67
100), 66 (10), 65 (12), 57 (8), 56 (7), 55 (42), 54 (9), 53 (32), 51
9), 43 (34), 42 (8), 41 (79).
1
(
3Z)-4-Methyl-1,3-undecadienes (18). GC: t
). 1H NMR (400 MHz, CDCl
): δ 6.66 (1H, ddd, J ) 17.1, 10.7,
0.4 Hz, dCH-2), 5.84 (1H, db, J ) 10.7 Hz, dCH-3), 5.06 (1H,
), 4.94 (1H, dd, J ) 9.5, 1.8 Hz,
), 2.15 (2H, t, J ) 7.6 Hz, CH -5), 1.77 (3H, bs, CH -CHd),
.50-1.25 (10H, m, CH -6-10), 0.88 (3H, t, J ) 6.9 Hz, CH ). EI-
MS: m/ z (relative intensity) 166 (49), 137 (50), 123 (12), 109 (12),
R
) 13.85 min (DB-
5
1
3
-1
recognized by the 3012 cm band, even in the presence of a 3085
cm- band for the vinylic dCH
1
stretch.
2
dd, J )16.8, 2.0 Hz, dCH
dCH
R â
H
1
,3 -Alkadienes. At a very early stage in the history of the
R
H
â
2
3
analytical use of infrared spectroscopy, Rasmussen and Brattain25
recognized that cis and trans 1,3-pentadiene isomers can be
differentiated by their vapor-phase infrared spectra. However,
general correlation data essential for the comprehensive inter-
pretation of vapor spectra of 1,3-alkadiene systems have not been
compiled. To investigate spectra of 1,3-alkadiene systems, we
1
2
3
9
7
5
6 (13), 95 (94), 93 (12), 83 (15), 82 (87), 81 (76), 80 (9), 79 (47),
7 (21), 69 (23), 68 (60), 67 (100), 66 (7), 65 (11), 57 (5), 56 (7),
5 (35), 54 (7), 53 (28), 51 (7), 43 (31), 42 (8), 41 (73).
4
-Methyl-1 ,3 -tridecadiene and 4 -Methyl-1 ,3 -tetradecadi-
synthesized a mixture of 3Z and 3E isomers of 1,3-tridecadiene.
ene. Using 2-undecanone and 2-dodecanone as starting materials,
cis and trans mixtures of 4-methyl-1,3-tridecadiene and 4-methyl-
Addition of tetracyanoethylene (TCNE)19,20 or SO
19
selectively to
2
the trans isomer followed by column chromatography afforded
pure samples of each isomer.
1,3-tetradecadiene were synthesized by procedures similar to that
described in the preceding section. Spectral data obtained were
congruent with the expected structures.
Mixture of Farnesene Isomers (4 -9 ). Nerolidol (a mixture
of E/ Z isomers (1:1), 40 µL, 0.158 mmol) was dissolved in dry
pyridine (1 mL) and cooled in an ice bath. Under an argon
The spectra of (3Z)-1,3-tridecadiene (1 2 ) and (3E)-1,3-trideca-
diene (1 3 ) (Figure 1C,D) show the vinyl stretch band at 3093-
-
1
3
092 cm , denoting that the conjugation of the vinylic group to
a carbon-carbon double bond shifts the 3085 cm- dCH
1
stretch
2
band to higher frequencies. A hypsochromic shift due to conjuga-
tion appears to be valid for 1,3-alkadienes in general, since the
atmosphere, POCl
3
(40 µL, 0.44 mmol) was added dropwise during
a period of 5 min. The resulting mixture was stirred on the bath
for 1 h and for 12 h at room temperature. The reaction mixture
was poured into ice with hexane (1 mL), and the organic layer
was separated. The aqueous layer was extracted twice with
hexane (0.5 mL). Combined organic layers were washed with
(21) Nyquist, R. A. The Interpretation of Vapor-Phase Infrared Spectra: Group
Frequency Data; Sadtler Research Laboratories: Philadelphia, PA, 1984; Vol.
1.
(
22) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, Vapor Phase, 1st ed.;
Aldrich Chemical Co.: Milwaukee, WI, 1989; Vol. 3.
(
(
23) Attygalle, A. B. Pure Appl. Chem. 1 9 9 4 , 66, 2323-2326.
24) Attygalle, A. B.; Svato sˇ , A.; Wilcox, C.; Voerman, S. Anal. Chem. 1 9 9 5 , 67,
4
saturated NaCl solution and dried over MgSO .
A mixture of (3Z,6E)-R-farnesene (7), (3E,6E)-R-farnesene (9),
and (6E)-â-farnesene (5 ) was prepared similarly, using (6E)-
1558-1567.
(25) Rasmussen, R. S.; Brattain, R. R. J. Chem. Phys. 1 9 4 7 , 15, 131-135.
Analytical Chemistry, Vol. 69, No. 10, May 15, 1997 1829

frequency. In addition, this interaction splits the band to a
complex pattern of two or more absorptions, with the lowest
frequency peak as the most intense.2
5,27-29
Similarly, in gas-phase
spectra, the conjugation of a vinyl group to a carbon-carbon
double bond leads to the appearance of two CdC stretch bands.
In the spectrum of (3Z)-1,3-tridecadiene, the two bands for in-
phase and out-of-phase stretch modes occur at 1642 and 1595
cm-1, respectively (Figure 1C). However, for (3E)-1,3-tridecadi-
ene, the bands are shifted to higher frequencies and occur at 1650
and 1604 cm- (Figure 1D). Moreover, the relative intensity of
the two bands is higher for the trans compounds, and the 1650
cm- band is slightly more intense than the 1604 cm-1 band.
Interestingly, the difference of the frequencies (∆ν) for the two
1
1
-
1
CdC bands is about 46-47 cm
for both isomers of 1,3-
tridecadiene.
Conjugation also affects the 993-992 (“trans” CH wag) and
-
1
9
(
14 cm (dCH
3Z)-1,3- tridecadiene, the dCH
while that for the “trans” CH wag mode appears to be unmodified
Figure 1C). On the other hand, the spectrum of the (3E)-1,3-
tridecadiene shows a characteristic three-peak pattern (Figure
2
wag) bands. For example, in the spectrum of
wag band is lowered to 906 cm-1,
2
(
-1
1
D). For isolated vinyl groups, the intensity of the 992 cm band
is lower than that of the 914 cm- band. However, in the trans
1
,3-diene spectrum, the intensity of the 998 cm- band is higher
1
1
than that at 902 cm-1. Furthermore, a weak absorption is noted
at 949 cm- in the spectrum of (3E)-1,3-tridecadiene. Previously,
we noted that, when a trans CHdCH bond and a cis CHdCH
1
bond are conjugated, the trans wag band splits into two absorp-
tions which appear at 978-976 and 949-946 cm-
1 24
.
For isolated
trans HCdCH groups, the in-phase wag band occurs near 965
cm-1. For cis moieties, the vibration in which the HCdCH bonds
wag in opposite directions is infrared inactive; however, its
frequency remains near 965 cm-
1 42,43
.
When cis and trans groups
are conjugated, they interact to give two bands near 977 and 948
cm-1, where both groups vibrate in-phase and out-of-phase with
each other.24 The nearly equal intensities result from the fact that
Fig u re 1 . Gas-phase infrared spectra of 1-nonene (A), (11Z)-15-
methyl-1,11-hexadecadiene (B), (3Z)-1,3-tridecadiene (C), and (3E)-
-
1
1
,3-tridecadiene (D) (resolution, 8 cm ).
the trans group is wagging for both vibrations, while the cis
waggings do not contribute to the intensities.42,43 For alkyl
substituted vinyl groups, the “trans in-phase” CH wag band
appears near 992 cm- (Figure 1A). When a vinyl group is
conjugated to a trans HCdCH group, they interact,42,43 and the
resulting bands appear near 998 and 949 cm-1, which involve both
vinyl and trans HCdCH group CH wags, vibrating in-phase and
spectra reported for 1,3-hexadiene and 1,3-octadiene also show
this band at 3090 and 3091 cm-1, respectively.26 In a previous
1
study,24 we noted similar hypsochromic shifts for the dCH stretch
bands corresponding to nonterminal-type conjugated CdC double
bonds.
In addition to the vinyl stretch band, spectra of cis and trans
,3-tridecadienes show an additional absorption above 3000 cm-1
out-of-phase with each other (Figure 1D). The in-phase band near
1
.
1
9
98 cm- is the more intense because all four “trans” hydrogen
For the cis compounds, this dCH stretch band appears at 3016
-
1
,
w
h
i
l
e
t
h
a
t
o
f
t
h
e
t
r
a
n
s
c
o
m
p
o
u
n
d
s
o
c
c
u
r
s
a
t
3
0
1
0
c
m
-
1
atoms move in the same direction. On the other hand, when there
c
m
.
-
1
is interaction between the vinyl 992 cm vibration and the cis
Although it is not common for trans compounds to show
absorptions above 3000 cm-1, apparently the effect of conjugation
shifts the trans dCH band sufficiently to higher frequencies to
965 cm-
1
infrared inactive vibration, only the vinyl contributes to
the conjugated group intensity observed at 993 cm- (Figure 1C).
The determination of the purity of cis or trans isomers of 1,3-
dienes by gas chromatography alone is not straightforward, since
both isomers coelute on most stationary phases such as DB-1,
DB-5, and Carbowax.19,30 The present data demonstrate that cis
and trans isomers of 1,3-alkadienes can be distinguished unam-
1
2
resolve the band from the more intense CH absorptions at 2934
cm-1, as we have noted previously.24
Although compounds with internal double bonds exhibit no
significant bands for CdC stretch modes, presumably because
the stretching affords little or no dipole moment change, the
spectra of vinyl compounds show a notable peak at 1642-1641
(
(
27) Blout, E. R.; Fields, M.; Karplus, R. J. Am. Chem. Soc. 1 9 4 8 , 70, 194-198.
28) Mitzner, B. M.; Theimer, E. T.; Freeman, S. K. Appl. Spectrosc. 1 9 6 5 , 19,
169-185
-
1
17,21,23
cm for the CdC stretch.
From generalizations for con-
densed-phase spectra, it is known that conjugation lowers the
(
29) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman & Hall:
(
26) The National Institute of Standards and Technology (NIST) and Environmental
Protection Agency (EPA) Gas Phase Infrared Database, version 1.0, 1992;
U.S. Department of Commerce, Gaithersburg, MD 20899.
London, 1975; p 45.
(30) T o´ th, M.; S o¨ zcs, G.; Van Nieukerken, E. J.; Phillip, P.; Schmidt, F.; Francke,
W. J. Chem. Ecol. 1 9 9 5 , 21, 13-27.
1830 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

Fig u re 2 . Gas-phase infrared spectra of (3Z)-3-methyl-1,3-deca-
diene (A), (3E)-3-methyl-1,3-decadiene (B), and 3-heptyl-1,3-buta-
diene (C) (resolution, 8 cm- ).
1
biguously by GC/ FT-IR. A composite infrared spectrum obtained
from a 60:40 mixture of trans and cis isomers of 1,3-tridecadiene,
in fact, showed the CdC bands at 1648 and 1603 cm-1. Thus,
the positions of these bands are useful for the determination of
isomeric purity of 1,3-alkadienes. In fact, monosubstituted 1,3-
dienes moieties are found in several natural products. For
example, a novel moth pheromone,30 the algal pheromone des-
marestene,31 and some alkaloids from poisonous frogs32 bear 1,3-
diene side chains. Although vapor-phase spectra of these frog
alkaloids have been recorded, no attempts have been made to
distinguish the cis and trans isomers on the basis of their CdC
or other absorptions.32
Substituted 1 ,3 -Alkadienes. To study the spectra of com-
pounds bearing 2-methyl-1,3-butadienyl (1 , 2 ) and 2-methylene-
3-butenyl moieties (3), the dehydration products of three alcohols,
3
-methyl-1-decen-3-ol, 3-methyl-1-dodecen-3-ol, and 3-methyl-1-
tridecen-3-ol, were prepared. The infrared spectra of the products
of 3-methyl-1-decen-3-ol, i.e., (3Z)-3-methyl-1,3-decadiene (1 4 ),
(
3E)-3-methyl-1,3-decadiene (15), and 3-heptyl-1,3-butadiene (16),
are depicted in Figure 2. These spectra revealed many significant
bands which allow unambiguous characterization of the 2-methyl-
1
,3-butadienyl and 2-methylene-3-butenyl moieties in volatile
compounds. Incidentally, GC/ MS is not helpful for this purpose,
since the mass spectra of each set of isomeric dienes are virtually
identical. To show how reliable structural predictions can be made
on the basis of very small differences in frequencies, data obtained
from the three sets of isomeric dienes are presented in Table 1.
As a rule, the precision and reproducibility of band positions in
vapor-phase infrared spectrometry are better than (1 cm-1.
(
(
31) M u¨ ller, D. G.; Gassmann, G.; Boland, W.; Marner, F.-J.; Jaenicke, L.
Naturwissenschaften 1 9 8 2 , 69, 290-291.
32) Spande, T. F.; Garraffo, H. M.; Daly, J. W.; Tokuyama, T.; Shimada, A.
Tetrahedron 1 9 9 2 , 48, 1823-1836.
Analytical Chemistry, Vol. 69, No. 10, May 15, 1997 1831

Fig u re 4 . Gas-phase infrared spectra of (Z)-â-farnesene (A) and
-
1
(
E)-â-farnesene (B) (resolution, 8 cm ).
cm- (Figure 3). The â-type compounds also show a profile with
1
-
1
two minima at 1632-1630, and 1595-1594 cm (Figure 2C);
however, the higher frequency band is much less pronounced.
This pattern is, in fact, visible even in the spectrum of myrcene
(
1
2 1 , Figure 3C). For myrcene, an additional band is observed at
660 cm-1; however, this represents the CdC stretch of the
Fig u re 3 . Gas-phase infrared spectra of (Z)-ocimene (A), (E)-
ocimene (B), and myrcene (C) (resolution, 8 cm ).
-
1
isolated trisubstituted double bond (Figure 3C). We found that
the frequency differences of the two CdC bands (∆ν ) ν - ν
is highly characteristic of the nature of the substituted 1,3-diene
1
2
)
2
The absorptions for out-of-phase dCH stretch vibrations of
dienes listed in Table 1 are shifted to higher frequencies, similar
moiety. From the data presented in Table 1, we conclude that
to those observed for linear 1,3-alkadienes. In fact, for (3Z)-3-
∆
ν values for cis-R-type (1 ), trans-R-type (2 ), and â-type (3 )
methyl-1,3-alkadienes, the dCH
2
stretch band occurs at 3095 cm-1,
-1
moieties are 47-51, 32-31, and 37 cm , respectively. These
values agree well with vapor-phase data obtained for (Z)-ocimene
while that for (3E)-3-methyl-1,3-alkadienes appears at 3094 cm-1.
Lowest values are always observed for â-type isomers, for which
1 9 , ∆ν ) 48 cm-1), (E)-ocimene (2 0 , ∆ν ) 31 cm-1), and
(
-
1
the band occurs at 3091 cm . Based on these observations, we
can generalize that the dCH stretch band occurs at a slightly
higher frequency for CH dCHC(CH )dCH- groups than for
CH dCHC(dCH )- groups. Moreover, the relative intensity of
the dCH stretch band is significantly higher for the â-type, since
these compounds bear two dCH groups, compared to the R-type
-1
myrcene (2 1 , ∆ν ) 35 cm ). Interestingly, similar frequency
differences can be observed in data reported from liquid-phase
spectra as well. From liquid-phase data reported by Ohloff et al.,33
2
2
3
2
2
we calculated ∆ν values for (Z)-ocimene, (E)-ocimene, and
-1
2
myrcene to be 48, 32, and 37 cm , respectively. Although
Murray12 and others35 have noted from liquid-phase data that the
2
with only one (Figure 2). Similar differences are evident in the
600 cm-1 absorption bands can be used to characterize the nature
1
spectra of isomeric monoterpene hydrocarbons (Z)-ocimene (19),
of this diene system, the validity of ∆ν values as a diagnostic
marker has not been recognized previously.
(
E)-ocimene (2 0 ), and myrcene (2 1 ) (Figure 3). For the
ocimenes, both of which bear a CH dCHC(CH )dCH- group, the
dCH
band is observed at 3094-3093 cm-1, whereas for myrcene
a relatively more intense dCH
band is seen at 3091 cm-1.
2
3
Farnesenes. All the isomers of farnesene bear an isopro-
pylidene moiety, an internal trisubstituted double bond of cis or
trans configuration, and a 1,3-diene system (4 -9 ). From vapor-
phase infrared spectra presented in Figures 4 and 5, it is clear
that band positions and intensities of each isomer are sufficiently
unique to allow unambiguous identification of the isomer. In this
way, three farnesenes obtained from a termite (P. simplex), an
apple (Granny Smith), and an aphid (A. pissum) were identified
unambiguously as the Z,E-R, E,E-R-, and E-â isomers, respectively
2
2
As observed for the linear 1,3-dienes, the band positions and
intensity profiles of the two CdC stretch bands in the 1600 cm-
region are diagnostically very useful. When the configuration at
the C-3 position is cis, the two bands are observed at 1643 and
1
1
596-1595 cm-1. These band positions are very similar to those
observed for unsubstituted cis 1,3-dienes; however, the relative
intensity of the higher frequency band is weaker. On the other
hand, the trans isomers show two much more well-defined, bands
at 1640 and 1608 cm-1, which, in fact, are different in both intensity
and position from those of linear trans 1,3-dienes. In the spectra
(
1
Table 2). In addition, generalizations we made for the substituted
,3-alkadienes can be applied to derive structural information for
each farnesene isomer.
The dCH
stretch band of farnesene spectra also shows a shift
to higher frequencies. Compared to that of R-farnesenes, the
2
-
1
of linear trans compounds, the 1650 cm band is stronger than
the 1604 cm- band (Figure 1D), whereas for compounds bearing
trans-2-methyl-1,3-butadienyl moieties, the 1640 band is weaker
than that at 1608 cm-1. In fact, the spectrum of (E)-ocimene (2 0 )
shows similar profiles for the respective bands at 1638 and 1607
1
(
(
33) Ohloff, G.; Seibl, J.; Kovats, E. Liebigs Ann. Chem. 1 9 6 4 , 83-101.
34) Brieger, G.; Nestrick, T. J.; McKenna, C. J. Org. Chem. 1 9 6 9 , 34, 3789-
3791.
(35) Anet, E. F. L. J. Aust. J. Chem. 1 9 7 0 , 23, 2101-2108.
1832 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

â-farnesene spectra, which show absorptions at 1632-1630, and
-1
1
595 cm , is indistinguishable for the two spectra since the diene
part of both molecules is identical. Moreover, this region is
virtually identical to that of myrcene. Similarly, the corresponding
regions are very similar in the spectra of the two 3E-isomers of
R-farnesene, which show absorptions at 1639/ 1638 and 1608/ 1607
-1
cm and resemble the corresponding regions of the (E)-ocimene
spectrum.
From data presented in Table 2, it is evident that, in farnesenes,
when the double bond at the C-3 position bears a Z configuration,
the dCH
wag band shifts characteristically to 907-906 cm-1,
2
while an E configuration, or a dCH
2
group at the C-3 position,
shifts it further to 899-897 cm- . Furthermore, in the spectra of
1
-
1
â-farnesenes, the intensities of the 899-898 cm absorption is
essentially double compared to those of R-fanesenes since the
2
â-farnesenes bear two dCH groups.
-
1
2
For 1-alkenes, the 914 cm band for the dCH wag vibration
is usually accompanied by a weak overtone at 1828 cm-1. The
analogous overtone band for the 907 cm- absorption of (Z,Z)-R-
1
farnesene and (Z,E)-R-farnesene is observed at 1814-1813 cm-
1
,
while the spectra of other four isomers show the overtone, arising
from the 898-896 cm- band, at 1798-1796 cm-1
1
.
Moreover, the gas chromatographic elution order of com-
pounds bearing the aforementioned three types structural moieties
on DB-1 and DB-5 stationary phases is consistent.35 The â-type
compounds always elute first, followed by the 3Z and 3E. The
elution pattern of myrcene, (Z)-ocimene, and (E)-ocimene follows
the same trend. Interestingly, the elution order of (6Z)-farnesene
and (6E)-farnesene follow the same behavior [6Z-â first, followed
by 3Z,6Z-R and 3E,6Z-R], indicating that, in these compounds,
retention behavior is determined essentially by the nature of the
conjugated diene moiety. In fact, the major differences in infrared
spectra are also governed by this diene moiety. Hence, it is not
surprising that Nishino and Bowers36 observed no basic differ-
ences among the spectra of four isomers of norfarnesene, which
lacks this conjugated diene moiety.
Fig u re 5 . Gas-phase infrared spectra of (Z,Z)-R-farnesene (A),
(
Z,E)-R-farnesene (B), (E,Z)-R-farnesene (C), and (E,E)-R-farnesene
-
1
(D) (resolution, 8 cm ).
Several farnesene isomers are known from ants and other
insects.37 Often, the structures are reported without complete
configurational information.13 This is particularly true for farne-
sene homologs. For example, the homofarnesene and bishomo-
farnesene isomers from Myrmica ants were reported as 7-ethyl-
intensity of this band in â-farnesenes is higher due to the presence
of two dCH groups in the latter (Table 2). As expected, the
frequency of the dCH band is consistently lower for â-farnesenes
3090 cm-1) than that for the R-farnesenes (3093-3091 cm-1).
Absorptions for the C-H stretches of other C-H bonds on
carbon-carbon double bonds can be expected between 3050 and
2
2
(
3
,11-dimethyldodeca-1,3,6,10-tetraene (2 2 ) and 7-ethyl-3,11-di-
methyltrideca-1,3,6,10-tetraene (2 3 ), respectively.16 We recorded
the spectra of these two farnesene homologs and the (Z,E)-R-
farnesene obtained from the ant M. sabuleti (Figure 6). Except
for the 2975-2973 cm- absorption, the three spectra appeared
very similar. The presence of 906, 1596-1594, and 3094-3093
-
1
2
990 cm . However, these bands are usually of lower intensity,
and in our spectra they all form a broad, unresolved hump on the
1
high-frequency edge of the band for the asymmetric C-H stretch
-
1
of the CH
farnesene isomers. Interestingly, the intensity of this CH
depends on whether the isomer bears a triply substituted 6Z or a
3
groups, which occur at 2975-2970 cm for all
-
1
cm bands in all three spectra disclosed that the configuration
3
band
at the C-3 must be cis in all three compounds. In addition, the
∆
ν values for the two CdC stretch bands were 47, 49, and 47
6
6
E double bond. In the spectra of three farnesene isomers with
Z configuration, compared to those bearing 6E configuration, this
-
1
cm for farnesene, bishomofarnesene, and bishomofarnesene,
respectively, confirming the stereochemistry of the double bond
at the C-3 atom as cis. (Unfortunately, the stereochemistry at
the C-6 position of the homofarnesene (2 2 ) and at C-6 and C-10
of the bishomofarnesene (2 3 ) could not be assigned with the
available data.) Interestingly, the 2742-2739 cm- band, which
is apparent in the spectra of compounds bearing an isopropylidene
moiety, is absent in the spectrum of the bishomofarnesene (2 3 ).
band is significantly stronger in intensity (Figures 4 and 5). The
other absorptions, such as the CH asymmetric stretch band
2930-2928 cm-1) and the combined CH
symmetric stretch and
symmetric stretch band (2871-2869 cm-1), are similar in
2
(
2
CH
3
1
intensities for all isomers.
As expected, the 1660-1590 cm- region of farnesene spectra
is complex. Nevertheless, valuable information about the nature
of the C-3 bond can be derived from these bands for CdC
stretching vibrations. For example, this region of (Z)-â and (E)-
1
(
36) Nishino, C.; Bowers, W. S. Tetrahedron 1 9 7 8 , 32, 2875-2877.
(37) Morgan, E. D.; Parry, K.; Tyler, R. C. Insect Biochem. 1 9 7 9 , 9, 117-121.
Analytical Chemistry, Vol. 69, No. 10, May 15, 1997 1833

1834 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

Fig u re 6 . Gas-phase infrared spectra of (Z,E)-R-farnesene (A), a
homofarnesene (B), and a bishomofarnesene (C), from Myrmica
-
1
sabuleti ants (resolution, 8 cm ).
Fig u re 7 . Gas-phase infrared spectra of (3E)-4-methyl-1,3-undeca-
-1
diene (A) and (3Z)-4-methyl-1,3-undecadiene (B) (resolution, 8 cm ).
The absorption bands present in the 1150-1080 cm-1 region
represents the -C(CH
are visible in this region in the spectrum of 3-heptyl-1,3-butadiene
1 6 , Figure 2C). In general, the intensity of bands in this region
3
)dCH- moieties. As expected, no bands
(
is weaker for â-farnesenes than for R-farnesenes. Similar observa-
tion were made with the ocimenes (1 9 , 2 0 ), for which the 1104-
-
1
-1
1
103 cm band is much stronger than that at 1106 cm for
myrcene (2 1 , Figure 3). This region is particularly useful to
distinguish between (E)- and (Z)-â-farnesenes. The (Z)-â-farne-
senes clearly shows two absorption minima at 1107 and 1080 cm-1,
while the (E) isomer show only one minimum at 1105 cm-
1
(
Figure 4).
Analytical Chemistry, Vol. 69, No. 10, May 15, 1997 1835

4
-Methyl-1 ,3 -undecadienes. Another structural moiety of
spectra of cis isomers of 4-methyl-1,3-tridecadiene and 4-methyl-
1,3-tetradecadiene (Table 3). The 1420 cm- band was insignifi-
cantly weak in the spectra of corresponding trans isomers.
In summary, we have demonstrated that GC/ FT-IR data allow
definitive deductions to be made about the stereochemistry of
carbon-carbon double bonds conjugated to a vinyl group. Since
such moieties are common in many natural compounds, we hope
that many chemists will realize the value of the GC/ FT-IR
technique, since NMR is not the method of choice for samples
available in minute amounts and in impure form.
1
considerable interest is the 4-methyl-1,3-dienyl group. For ex-
ample, the natural products (3E)-4,8-dimethyl-1,3,7-nonatriene and
3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene,38 which were later
shown to act as important chemical messengers in a tritrophic
level interaction,39,40 bear this moiety. The determination of the
configuration of the trisubstituted double bond in this moiety has
been considered a difficult analytical problem. To determine the
stereochemistry of the natural compounds, often both cis and
isomers had to be synthesized.38,40
(
To establish the gas-phase infrared correlations for the deter-
mination of the configuration of this group, we prepared three
Another GC/ IR technique that has been used recently is the
matrix isolation (MI) method.44 In this technique, the GC effluent
is mixed with argon and frozen into a solid matrix. Argon dilutes
minute amounts of analytes; therefore, in the condensed material,
analyte molecules are held almost individually. In this way, IR
band broadening due to intermolecular interactions between
analyte molecules and molecular rotations is reduced to a bare
minimum. As a result, the infrared bands are much narrower
and more intense than those observed in the vapor phase. For
the compounds discussed in this paper, more useful results could
have been obtained if the GC/ MI-IR method were applied.
However, MI is the most expensive GC/ IR procedure, since the
system has to operate under cryogenic temperatures and high-
vacuum conditions, and only a very few laboratories are fortunate
to have one. Since there are no commercial vendors currently, it
is unlikely that GC/ MI-IR will become a routine technique in the
near future.
4
-methyl-1,3-alkadienes, as a mixture of both geometric isomers,
from the reaction of allylidene ylide with 2-alkanones. For
example, a 35:65 mixture of 4-methyl-1,3-undecadiene isomers
(
1 7 , 1 8 ) was obtained when the ylide from allyltriphenylphos-
phonium bromide was allowed to react with 2-nonanone. The
geometry of each isomer in the mixture was determined from
their 1H NMR spectra, which show notable differences in the
chemical shifts of the methylene and methyl groups connected
2
to the sp carbon. Using the published values for (3E)- and (3Z)-
4
-methyl-1,3-nonadienes,41 the geometry of the less abundant
isomer 1 8 (eluted earlier on DB-5 and DB-1 GC phases) was
assigned to be 3Z. The chemical shifts values for the major isomer
1
7 corresponded to the published values for (3E)-4-methyl-1,3-
nonadiene.
From the spectra of cis and trans isomers of 4-methyl-1,3-
undecadiene (Figure 7), weak but diagnostic differences were
-
1
observed in the 1480-1360 cm region. The cis isomer showed
a band at 1418 cm located between the bands at 1460 (CH
and 1382 cm- (CH
ACKNOWLEDGMENT
-
1
2
def)
We are greatly indebted to Prof. J. Meinwald and Dr. N. B.
Colthup for offering many helpful suggestions and critically
reading the manuscript and Prof. H. J. Bestmann (University of
Erlangen, Germany) for sending us chemical 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. The
partial support of this research by NIH Grant GM 53830 is
acknowledged with pleasure.
1
3
sym, Figure 7B). In contrast, the spectrum
of the trans isomer showed this band only as a weak shoulder at
1
-1
1
420 cm- (Figure 7A). The significance of the 1418 cm band
was confirmed by the presence of a 1418-1416 cm- band in the
1
(
(
(
38) Maurer, B.; Hauser, A.; Froidevaux, J.-C. Tetrahedron Lett. 1 9 8 6 , 27, 2111-
112.
39) Turlings, T. C.; Tumlinson, J. H.; Lewis, J. W. Science 1 9 9 0 , 250, 1251-
253.
2
1
40) Dicke, M.; Van Beek, T. A.; Posthumas, M. A.; Ben Dom, N.; Van Bokhoven,
H.; De Groot, AE. J. Chem. Ecol. 1 9 9 0 , 16, 381-396.
41) Underiner, T. L.; Goering H. L. J. Org. Chem. 1 9 9 1 , 56, 2563-2572.
42) Colthup, N. B. Appl. Spectrosc. 1 9 7 1 , 25, 368-371.
Received for review September 5, 1996. Accepted March
(
(
(
X
3, 1997.
43) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman
Spectroscopy, 3rd ed.; Academic Press: San Diego, 1969; p 256.
44) Reedy, G. T.; Ettinger, D. G.; Schneider, J. F.; Bourne, S. Anal. Chem. 1 9 8 6 ,
AC960890U
(
X
5
7, 1602-1609.
Abstract published in Advance ACS Abstracts, April 15, 1997.
1836 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997