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The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
Professor W. J. Taylor stimulated to some degree our discussion on comparison of correlation and ordering. References and Notes (1) On leave from Ames Laboratory, Iowa State University, Ames, Iowa 50010. (2) F. Harary, "Graph Theory", Addison-Wesley, Reading, Mass., 1969. (3) G. R. Somayajulu and B. J. Zwolinski, J. Cbem. SOC.,Faraday Trans. 2, 70, 967 (1974), and other papers in this series. (4) A program Is available, see M RandiE, G. M. Brissey, R. B. Spencer, and C. L. Wilkins, "Computers in Chemistry", in press. (5) M. RandiE, A plenary lecture at the Bremen Conference on "Mathematical Structures in Chemistry", June, 1978 (to be published in MATCH). (6) J. E. Dubois, "Ordered Chromatic Graph and Limited Environment Concept", a chapter in "Chemical Applications of Graph Theory", A. T. Balaban, Ed., Academic Press, London, 1976. (7) M. Randie. Cbem. Pbvs. Lett.. 58. 180 11978). (8j M. RandiE,'J. Cbem. h f . Comput.'Sci., 18, 101 (1978); M. RandiE and C. L. Wilkins, ibid., 19, 23 (1979). (9) (a) J. R. Platt, J . Cbem. Pbys., 15, 419 (1947); J . Pbys. Cbem., 56, 328 (1952). (b) Although one can recognize graph theoretical content in a number of earlier (in particular see ref 18) works it appears that Platt (ref 9a) was first to specify the role of paths in a general expression for the nth C-C contribution pn to a property G by an additive expression: P,, = p o p , f n , f pZf,,, . . ., where p is the effect of a jth neighbor C-C bond on the C-C property and f,,, is the number of C-C bonds at a distance of jbonds along the skeleton from the nth C-C. (10) K. Altenburg, KolloidZ., 178, 112 (1961). (11) M. RandiC and C. L. Wilkins, J. Cbem. Inf. Comput. Sci., 19, 31 (1979). (12) D. H. Smith and R. E. Carhart, Tetrahedron, 32, 2513 (1976). (13) M. RandiE and C. L. Wilkins, submitted for publication. (14) M. RandiE and C. L. Wilkins, Cbem. Pbys. Lett., in press. (15) I t will be seen later that the comparison is valid for higher alkanes. (16) H. Wiener, J. Am. Chem. Soc., 69, 17, 2636 (1947); J. Phys. Cbem., 52, 425 1082 (1948). (17) W. J. Taylor, private communication. (18) W. J. Taylor, J. M. Pignocco, and F. D. Rossint, J. Res. NaN. Bur. Stand., 34, 413 (1945). Professor W. J. Taylor was kind to draw our attention to the relationship between path numbers pi and parameters z, and I,in private correspondence. (19) J. Karamata, Pub/. hatb. Univ. Belgrade, 1, 145 (1932).
+
+
Steinmetz et al. (20) For a brief outline and applications see: M. RandiE, Cbem. Pbys. Lett., 55, 547 (1978); Int. J . Quant. Cbem. (Symposium), 5, 245 (1978). (21) M. RandiE, J . Am. Cbem. Soc., 97, 6609 (1975). (22) L. B. Kier and H. H. Hall, "Molecular Connectivity in Chemistry and Drug Research", "Medicinal Chemistry", Vol. 14, Academic Press, New York, 1976. (23) L. Euler, Opera Omnia, Ser. 1 , 2, 241; see G. Polya, "Induction and Analogy in Mathematics", Princeton University Press, Princeton, N.J., 1954, p 91. (24) H. Hosoya, Bull. Cbem. SOC.Jpn., 44, 2332 (1971). (25) H. Hosoya, Bull. Cbem. SOC.Jpn., 45, 3415 (1972). (26) H. Hosoya, Tbeor. Cbim. Acta, 25, 215 (1972). (27) J. H. Van Vleck, Nobel Prize lecture, Science, 201, 113 (1978). (28) D. M. Grant and E. G. Paul, J . Am. Cbem. Soc., 86, 2984 (1964). (29) M. Randie, J . Cbem. Soc., Faraday Trans. 2 , submitted for publication. (30) The corresponding (pZrp3) coordinates for the mentioned compounds are: (13,6) for 2,2,4,44etramethylpentane; (7,6) for n-nonane; (8,8) for 4-methyloctane and 3-methyloctane; (9,lO) for 3-methyl-4ethylhexane; and (10,12) for 3,3-diethylheptane. (31) B. Everrii, "Cluster Analysis", Wiley, New York (A Halsted Press Book). (32) I f not otherwise stated, we compiled data from API Research Project 44. (33) E. A. Smolenskii, Russ. J. Pbys. Cbem., 38, 700 (1964); A. L. Seifer and E. A. Smolenki, ibid., 38, 1204 (1964). (34) M. Gordon and J. W. Kennedy, J. Cbem. Soc., Faraday Trans. 2, 68, 484 (1972). (35) E. Ruch, Tbeor. Cbim. Acta, 38, 167 (1975); E. Ruch and A. Schonhofer, ibid., 19, 225 (1970). (36) I. Gutman and M. RandiE, Cbem. Pbys. Lett., 47, 15 (1977). (37) G. H. Hardy, J. E. Littlewood, and G. Polya, "Inequalities", Cambridge University Press, London, 1934, p 45. (38) W. K. Chen, The Matrixand Tensor Quarterly, June, 1974, p 123 and Sep, 1974, p 1. (39) Such a reciprocal relationship is well known in graph theory and widely used. For example, by such consideration the famous coloring problem (The Four Color Conjecture) is transposed in coloring of vertices. For applications in chemistry, see ref 40. (40) A. T. Balaban and F. Harary, Tetrahedron,24, 2505 (1968); the deriied dual has been referred to as characteristic graph (strictly speaking it is not a dual in rigorous mathematical sense, as in chemical application one ignores the "outside" of the molecule, which is of no particular interest). 0. E. Polansky and D. H. Rouvray, MATCH, 2, 63, 91 (1976); A. T. Balaban, Tetrahedron, 27, 6155 (1971).
Conformational Analysis of Conjugated Polyenes by Nuclear Magnetic Resonance and Low Resolution Microwave Spectroscopy Wayne E. Steinmetz,*+' James E. Pollard, Jeffrey M. Blaney, Bruce K. Winker, In KI Mun, Fred J. Hickernell, and Stanley J. Hollenberg Seaver Chemistry Laboratory, Pomona College, Claremont, California 9 1711 (Received August 25, 1978) Publication costs assisted by Pomona College and the Petroleum Research Fund
The low resolution microwave spectra of trans,trans-3,6-heptadien-2-one and a series of conjugated acyclic polyene aldehydes were examined at room and dry ice temperatures. Proton-proton coupling constants were used to confirm the stereochemistry of the molecules examined. The evidence is consistent with the claim that the conjugated portion of the polyenes has a single planar conformation in which all the dihedral angles are s-trans. In the partially saturated molecules, the aliphatic side chain is oriented skew to the conjugated chain. Dihedral angles in the side chain are either s-trans or gauche. In some cases bands assignable to an excited vibrational state were observed. Introduction A system of conjugated double bonds is the key structural feature of many natural products, the most important example being the carotenoids.2 The demonstration that the carotenoid retinal is involved in the photochemistry of vision has stimulated a large number A 'H NMR study of all-trans- and of structural 11-cis-retinal in deuterioacetone indicated that a t room temperature the 11-cis isomer is present as an equilibrium 0022-3654/79/2083-1540$0 1.OO/O
mixture of the 12-s-cis and 12-s-trans conformer^.^ This conclusion has been supported by theoretical calculations4 and the measurement of electronic absorption spectra in a variety of solvents.6 Most structural studies to date including lH NMR work on all-trans-, g-cis-, and 13&retinal7 indicate that in an acyclic carbon chain the dihedral angle between two conjugated double bonds is almost always s-trans. The well-documented exceptions, e.g., 11-cis-retinal and perfluor0-1,3-butadiene,~ occur in 1979 American Chemical Society
Conformational Analysis of Conjugated Polyenes
molecules with significant steric repulsions. The present conformational study of a variety of polyenes was initiated to discover if the s-trans conformation is almost always the case. It is also hoped that the results of this and similar studies will permit the prediction of the conformation of other molecules. Low resolution microwave spectroscopy (LRMW) has been shown to be a rapid tool for semiquantitative, gas phage conformational analysis.g-12In this technique each near prolate, polar conformer gives a symmetric top spectrum which yields rotational constants and crude relative intensities.
Experimentail Section Unless otherwise noted, preparative and analytical gas phase chromatography (GPC) was run on a 4.7-m Chromosorb 102 column. Vapor pressure permitting, commercially procured samples were analyzed by GPC and were not found to require additional purification. The 'H NMR spectra of these materials served as a further check of their identity and purity. Preparative high-pressure liquid chromatography (LC) was run on a Waters liquid chromatograph with a 1.9-m 60 mesh Porasil-A column. The solvents wed, benzene and chloroform, were deoxygenated with a strealm of nitrogen prior to use. 'H NMR spectra were run on Varian A60 and HA100 spectrometers with internal TMS as a reference and CDC1, as a solvent. Unless otherwise noted, the probe temperature was ca. 35 'C. Complex second-order lH NMR spectra were simplified by the use of the lanthanide shift reagents Eu(fod), and P r ( f ~ d ) ~The . best results were obtained with the Eu(fod):+ An interpretable lH NMR spectrum was obtained with solutions 1 m in substrate and 0.3 m in shift reagent. The chemical shifts of heptadienone in the absence of shift reagent were obtained by an extrapolation to zero concentration of chemical shifts obtained from spectra with shift reagent. Microwave spectra were run at room and dry ice temperatures on a Hewlett-Packard Model 8460A microwave spectrometer in K band. The goal of the variable temperature studies was gross effects on the spectra. Temperature control was crude as the cell was not thermostatted. The cell temperatures, obtained by inserting a mercury thermiometer next to the Stark cell, do not reflect small spatial and temporal variations. Scan rates of 10 and 1 MHz/s with time constants of 0.3-3 s were used. Frequency measurements were based on the average of the peak maximurn in forward and reverse scans. Whenever possible, a sample pressure of ca. 80 mtorr and a Stark voltage of 18010 V were used. To minimize the problem of volatile impurities, all liquid samples were injected using the direct injection port and solid samples were introduced by total or near total vaporization of milligram samples. The compounds studied readily adsorbed on the cell so thorough pumping between samples was required to eliminate cross contamination. Only a-type R branch series were observed. The frequency of each band was divided by J -t. 1 and the results were averaged, yielding a LRMW value of B + C. All reported uncertainties are at the 95% confidence level. The L,RMW values were corrected to give an effective ground state rotational constant Bo + Co using the empirical correction of Farag and Bohn.13 In calculating rotational constants, the following bond lengths (A) and bond angles were used: (a) aliphatic chain parameters C-C(sp3-sp3), 1.533; C-C(sp3-sp2), 1.502; C-H, 1.094; LCCC, 112.4'; LHCH(methy1 top), 109.5'; LHCH(elsewhere), 106.1'; (b) carbonyl parameters: C=O, 1.219; LCCO, 123.3'; LHCO, 118.4'; (c) olefin parameters: C=C, 1.338; C-C(SP~-SP~), 11.466; C-H, 1.094; LCCH = 360' - 2
The Journal of Physical Chemistry, Vol. 83, No. 1 I , 7979
1541
--w126
12
Figure 1. Assumed CCC bond angles. Hydrogen atoms are not shown in the figure. The hydrogen bond angles are given in the text of the paper.
LCCX (X = 0 or C). The remaining assumed angles are indicated in Figure 1. Each s-trans dihedral angle was taken to be 180'. The value of the so-called skew dihedral angle for the orientation of the aliphatic side chain to the olefinic backbone was fixed at 120'. The bond lengths and bond angles were assumed to be invariant under rotational isomerism. Relaxation of the parameters has been documented14but the magnitude of these changes is small enough so that they have a small effect on the LRMW analysis.ll The partially unconjugated polyenes examined were obtained from commercial sources. trans-2-Hexenal, trans,trans-2,4-heptadienal, and trans,trans-2,4-octadienal were purchased from Aldrich Chemical Co.; trans-2methyl-2-pentenal, from Chemical Samples Co. Unless otherwise noted, other reagents and precursors of synthesized polyenes were obtained from Aldrich. During synthetic steps with air-sensitive materials, the apparatus was purged with nitrogen. The literature suggests that the materials prepared are easily oxidized. Our experience indicates that oxidation and polymerization is a long term (days to weeks) problem. Sensitivity to isomerization, however, varies considerably from compound to compound. Attempts to prepare pure cis-2-butenal consistently resulted in a cis/trans mixture16 whereas it was possible to To minimize prepare stable, pure trans-2,cis-4-hexadienal. these problems the products were stored in the dark in helium-purged sealed tubes at 0 'C. Linear Polyenes. all-trans-2,4-Hexadienal, 2,4,6-octatrienal, and 2,4,6&decatetraenal were prepared by the method of Kuhn and Grundmann.l6 A mixture of acetaldehyde and redistilled trans-2-butenal(2:1 molar ratio) with a trace of a piperidineacetic acid catalyst was refluxed 18 h in a round-bottom flask equipped with a dry ice cooled reflux condenser. The reaction mixture was filtered, combined with an equal volume of ether, and washed several times with 5% NaC1. After the ether layer was dried, the ether and unreacted starting material were removed on a rotary evaporator. Vacuum distillation through a Vigreux column yielded fractions at 33-34 "C, 0.25 torr (hexadienal); 40-60 'C, 0.42 torr (unknown aldehyde); 72-75 'C, 0.42 torr (octatrienal); and 95-100 'C, 0.15 torr (decatetraenal). The hexadienal was purified by GPC. Identity of the product was confirmed by comparison with the published IH NMR spectrum.17 The octatrienal and decatetraenal were purified by LC yielding samples >95% pure. The identity of the products was confirmed by lH NMR and mass spectrometry and by the UV absorption maxima of their solutions in benzene: octatrienal, 310 and 319 nm; decatetraenal, 330 and 350 nm.18
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The Journal of Physical Chemistry, Voi. 83,
No. 11, 1979
trans-2,cis-4-Hexadienal. 2-Butynal was prepared by a Sondheimer oxidationlg of an ether solution of 2-butyn-1-01 (Chemical Samples Co.) with an excess of freshly prepared activated Mn02. After a 2-h reaction period the oxides of manganese were filtered off and the product was recovered with 31% yield by distillation through a Vigreaux column, bp 105-1 06 "C. trans-Ethyl hex-%en4-ynoate was prepared from the butynal by a Wittig reaction.20 A benzene solution of the butynal was added dropwise with stirring to a benzene solution of triphenylcarbethoxymethylenephosphorane prepared by the method of Denny and Ross.21 After refluxing for 5 h and stirring overnight a t room temperature, the benzene was removed on a rotary evaporator. Pentane was added to the concentrate to enhance the precipitation of triphenylphosphine oxide which was filtered off. The pentane was stripped off on a rotary evaporator and the process was repeated until all the triphenylphosphine oxide was removed. The oxide was washed with cold ether. The ether washings and the reaction concentrate were vacuum distilled through a Vigreux column. The product was collected with 52% yield a t 80-81 "C, 8 torr (lit. 79.5-81.0 "C, 9 torr):22 IH NMR ti1, 1.20; ti2, 4.00; J3, 5.90; ti4, 6.48; 65, 1.93; J l z = 7.0 Hz; J 3 4 = 15.0 Hz; J 4 5 = 2.3 Hz. trans-Hexa-2-en-4-yn-1-01 was prepared by selective reduction of trans-ethyl hex-2-en-4-ynoate with LaA1H3(0C2H5). An ether solution of the reducing agent prepared by the method of Davidson et al.23was added dropwise to a chilled ether solution of the ester. After the addition was complete, the temperature was allowed to rise to room temperature, and the reaction mixture was worked up in the standard manner for hydride reduction^.^^ Removal of the solvent on a rotary evaporator left a crude product with 90% yield. IR and lH NMR spectra confirmed complete reduction of the ester and no detectable reduction of either the double or the triple bond: IH NMR 61, 4-57'; 62, 3.90; 63, 5.38; 64, 5.87; 65, 1.83; J 2 3 = 4.6 Hz; J 3 4 = 15.0 Hz; J46 = 1.6 Hz; IR (cm-') 3320(s), 2930(m), 2880(m), 2230(w), 1640(w), 1442(m), 1373(m), 1095(s), 1114(s), 956(s), 912(w). trans-2,cis-4-Hexadien-l-o1 was produced by a stereospecific, low pressure hydrogenation of an ether solution of the crude alkynol with a Lindlar catalyst.25 The reaction was terminated a t 9570 of the theoretical uptake of hydrogen. After filtration and removal of the ether on a rotary evaporator, vacuum distillation with no column yielded the product with 34% yield at 79-81 "C, 15 torr. The product was contaminated by ca. 30% with trans4-hexen-1-oi. The mixture was not separated but was converted to the aldehyde by a Sondheimer oxidation with MnOz. The oxidation was selective and only the hexadienol was oxidized. After removal of the oxides of manganese and the ether, the trans-2,&4-hexadienal was purified with a 1070 overall yield by GPC on a 2.8-m Carbowax 20M column. The 'H NMR spectrum of the purified product showed no trace of any trans,trans2,4-hexadienal. Examination of the IH NMR spectrum after the sample was stored for 2 years showed that the sample had not appreciably isomerized: lH NMR ?jl, 11.40; 6 2 , 6.15; 63, 7.55; 64, 6.05; 65,5.78; 66,1.93; J 1 2 = 7.9 Hz; J23 = 14.9 Hz; J 3 4 = 10.2 Hz; J 4 j = 11.6 Hz; J 5 6 = 5.4 Hz. trans,trans-3,5-Neptadien-Z-one. Following the procedure of Meerwein,26trans-2-butenal was added dropwise over a period of 2 h to a basic aqueous solution of acetone. After sitting 2 h at 10-12 "C, the m i x b e was neutralized with dilute sulfuric acid, acid with NaC1, and extracted with ether. After removal of the ether, the residue was vacuum distilled twice through a Vigreux column with a
Steinrnetz et al.
23% yield of product: IH NMR al, 2.17; a2, 6.0; &, 7.12; 64, 6.3; 65, 5.9; 66, 1.82;J 2 3 = 15.84 Hz; J 3 4 = 10.80 Hz; J 4 5 = 15.01 Hz; J 5 6 = 6.84 Hz; J 4 6 = 1.27 Hz; IR (cm-l) 3480(m), 2960(m), 1668(s), 1639(s), 1598(s), 1435(m), 1358(s), 1327(m), 1292(m), 1252(s), 1180(m), 1149(m), 996(s). all-trans-3-Methyl-2,4,6-octatrienal. The procedure of Weedon and which involves a Reformatsky reaction as the key step, was used. A mixture of alltrans-heptadienone and freshly distilled ethyl bromoacetate was carefully added dropwise to activated zinc28 covered with benzene while the temperature was controlled to maintain a gentle reflux. The hydroxyester formed was dehydrated by refluxing the mixture with p-toluenesulfonic acid for 7 h. The reaction was driven to completion by periodic removal of the water formed. The organic layer was washed with saturated sodium bicarbonate and dried over calcium sulfate. After removal of the benzene on a rotary evaporator, vacuum distillation of the residue through a Vigreux column gave the triene ester with 33 70 yield. Doubling of peaks in the lH NMR spectrum indicated that the formation of the double bond a t the 2 position produced a cis/trans mixture. The triene ester was converted to the aldehyde by a LiA1H4 reduction followed by a Sondheimer reduction as described above. The aldehyde was purified by LC with a partial separation of the 2-cis and 2-trans isomers. Although the 'H NMR spectrum of the freshly prepared material showed two sets of aldehyde doublets, the spectrum after storage showed a single doublet. In addition the LRMW spectrum showed a single series of bands with a rotational constant inconsistent with the cis isomer. trans,trans-3-Methyl-2,4-hexadienal.The methylhexadienal was prepared and purified in the same manner with trans-3-penten-2-one as the starting material. The trans stereochemistry of the starting material was confirmed by the proton-proton coupling constant between the olefinic protons, 16.0 Hz. This stereochemistry (at the 4 position) was maintained throughout the synthesis as confirmed by the proton-proton coupling constants. This is not unexpected as the mild synthetic conditions employed are known to maintain stereochemistry. As before the dehydration of the hydroxyester yielded a cisltrans mixture (at the 2 position) as indicated by the 'H NMR spectra at all succeeding stages of the synthesis. Similarly the product isomerized during storage to the all trans isomer: lH NMR 61, 9.70; 62, 5.7; 6 3 , 2.15; a4, 6.83; 65, 6.0; 66, 1.85; JJ12 = 7.8 Hz; J23 = 1.2 Hz; J 4 5 = 14.2 Hz; J56 = 6.5 Hz; J j g = 1.3 Hz. 3-Methyl-2-butenal. This aldehyde was prepared from l-bromo-3-methyl-2-butene (Chemicals Procurement Laboratory) via a Sommelet reaction.29 The bromoolefin was added to a solution of hexamethyltetraamine in dry chloroform with the production of a hexaaminium salt. An aqueous solution of the salt was added dropwise to the pot of a steam distillation apparatus. The product was recovered from the distillate by an ether extraction. After removal of the solvent, the product was purified by GPC: 'H NMR 61, 10.13; 62, 5.90; 63, 2.20; 64, 2.00; J12 = 8.0 Hz; J 2 3 = 1.3 Hz; J z 4 = 1.3 Hz; J 3 4 = 0.3 Hz. Results and Discussion The stereochemistry of the -CH=CH- groups was determined using proton-proton coupling constants. It is well known that a value of ca. 15 Hz indicates trans and 10-12 Hz indicates cis. The results shown in Table I support the claimed stereochemistry. The 'H NMR spectrum of methyl octatrienal was not completely assigned. However, the stereochemistry of the -CH=CH-
The Journal of Physical Chemistty, Vol. 83, No. 11, 1979
Conformational Analysis of Conjugated Polyenes
1543
TABLE I: Rotationad Constants and Proposed Conformations of Polyenes
BtC molecule
--
(LRMW)," MHz
B+C
Bo
MHz
K(ca1cd)
(calcd), MHz
t Go,
conformation*
trans- 2-butenal all-trans-2,4-hexadienal (VT)f trans-2,cis-4-hexadienal (VT) alZ-trans-2,4,6-octatrienal
4258.1(1.2)d 1604.4(0.2) 1838(4) 770.5(0.2)
4257.2 1604.3 1837 770.5
-0.992 -0.998 -0.986 -0.999
4277 1597 1840 765
T TT TT TTT
3-methyl-2-butenal (VT) all-trans-3-methyl-2,4.hexadienal all-trans-3,5-heptadien-2-one (VT)
3564( 20) 1537.7(0.7) 1349.4(0.1)
3554 1536.5 1348.8
-0.886 -0.970 -0.983
3678 1554 1372
T TT TT
742.3 2398 1620.4
-0.993 -0.945 -0,999
744 2418 1627
2021 1130.2
-0.969 -0.999
2067 1042
786.9
-0.999
776
908.6
-0.993
919
all-trans-3-methyl-2,4.6-octatrienal (VT) trans-2-methyl-2-pentenal (VT) trans-2-hexenal (VT) all-trans-2,4-heptadierial all-trans-2,4-octadien~ll(VT)
1338.3(0.2) 742.4( 0.2) 2401( 9 ) 1620.4(0.8) 1661(1) 2023r21 1130:2(0.2) 1107.3(0.2) 786.9(0.7) 776.3(0.9) 908.8( 0.2) 899.4( 1.0)
J," Hz 15.55e 15.34, 15.23g 14.9, 11.6 15.1, 1.4.7, 15.3l 14.2 15.84(0.28), 15.0 1( 0.1 5 ) ' ~ j
T T ~ TTT TS TST TST~ TSG TTS TTS~ TTST TTST~ TTSG TTSG~
15.0 14.9, 15.1' 15.3, 15.3'
The series of capital a The 95% confidence limits are given in parentheses. The same error bounds apply to Bo + Co. letters indicates the conformation at each dihedral angle in the molecule. T denotes s-trans; C, s-cis; G, gauche; and S, skew. Proton-proton coupling constants The conformation at the oxygenated end of the molecule is specified first, at the left. L. H. for the group -CH=CH-, in order starting at the oxygenated end of the molecule. The uncertainty is ca. 0.3 Hz. Scharpen, persjonal communication. e Reference 42. f VT denotes that the spectrum was run at, dry ice temperature as Vibrationally excited state. The constants were obwell as room temperature (normal conditions). g Reference 14. tained from a spectrum simplified by a paramagnetic shift reagent. The uncertainty is a 95% confidence limit obtained from a fit of all transitions with the Fortran program LAOCOON III. The program, written by A. A. Bothner-By and S. Castellano, was obtained from the Quantum Chemistry Program Exchange, Indiana University.
'
groups is given by the structure of the synthetic precursor, heptadienone , whose lH NMR spectrum was completely assigned with the assistance of shift reagents. The LRMW spectrum of the completely conjugated species examined (Tables I and 11)consists of a single series of bands. Some of thie molecules were studied at both dry ice and room temperatures but no appearance of new bands was noted. all-trans-2,4,6,8-Decatetraenal did not give a spectrum. This is not due to signal-to-noise problems because all the other unsubstituted polyenes in the series gave very iintense spectra. The calculated B + C is 425 MHz iwhich suggests that each band was cancelled by Stark components from adjacent bands. The data from the polyenes with a spectrum indicate that a single conformation is present although low concentrations (
