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Nuclear magnetic resonance studies of 1, 3-butadienes VII. Coupling constants involving the methyl protons of penta- and hexadienes
JOURNAL OF MAGNETJCRESONANCE2, 150-163 (1970)
Nuclear Magnetic ResonanceStudiesof 1,3-Butadienes VII. Coupling ConstantsInvolving the Methyl Protons of Penta- and Hexadienes* P.
ALBRIKTSEN~,
A. V. CUNLIFFE~, AND R. K. HARRIS
School of Chemical Sciences, University of East Anglia, Norwich, NOR 88C, England Received August 12, 1969; accepted October 28, 1969 The proton resonance spectra of the two isomers of pentadiene-1,3 and the three isomers of hexadiene-2,4 have been fully analysed using iterative computation together with the techniques of double resonance and subspectral breakdown. An appreciable magnitude is found for the long-range coupling constant involving methyl protons, including 7J = ca. 0.5 Hz. The data are used to discuss the mechanism of coupling in butadienes in terms of 0 and 7ccontributions. It is concluded that the A contribution to 4J is ca. -0.4 Hz and the n contribution to 5J is ca. $0.7 Hz. The spectrum of 2,5-dimethylhexadiene-2,4 is also discussed. INTRODUCTION
There has been considerable interest in the role of n electrons in spin-spin coupling for unsaturated (nonaromatic) compounds. It is generally accepted that the contact term is dominant in such situations and, therefore, that a rc contribution to coupling can only occur through (0, n) configuration interaction. Several theoretical papers have appeared on this topic (1-5). The butadiene system is of particular interest since there are two types of long-range coupling (through two and three bonds of the rc system) which might be expected to have important z contributions. Considerable information is available for coupling constants in butadienes (6-9), but most of the compounds studied have highly electronegative substituents, which render them unsuitable for testing theories of coupling. Consequently, we have studied a series of methyl-substituted butadienes that do not have this disadvantage. Moreover, the simplest theories relate the 7~contribution to coupling to ESR hyperfine splitting constants, and on this basis it has been suggested (10) that replacement of a dienic proton by a methyl group should result in a change of sign of J, together with a small increase in magnitude. Thus, measurement of coupling constants for methylated butadienes is useful for a discussion of 71 contributions to coupling. In this paper, we report work on cis- and truns-pentadiene-I,3 (I and II, respectively), on cis,cis-, cis,trans-, and truns,trans-hexadiene-2,4 (III, IV, and V, respectively), and, more qualitatively, on 2,5-dimethylhexadiene-2,4 (VI). There are no accurate data in the * Part VI is Ref. 36. t Present address: Kjemisk Institutt, Universitetet i Bergen, Norway. ‘, Present address: E.R.D.E., Waltham Abbey, Essex, England. 150
METHYL COUPLING
CONSTANTS IN PENTA- AND HEXADIENES
151
literature’ on such compounds-the paper by Koster and Danti (22) uses only first-order analysis of spectra. One reason for the lack of data is that the spectra are very complex, since they involve eight and ten protons for the pentadienes and hexadienes, respectively. The well-known iterative computer program (23) LAOCOON normally handles only systems of seven spins. Limited extension is possible, but only at a considerable cost in terms of computer time. However, the technique of subspectral analysis (14) makes it feasible to tackle spectra of such large spin systems. We have, therefore, obtained values-including the signs-of all the (H, H) coupling constants in the molecules studied. We shall classify these coupling constants as methyl-methyl, methyl-diene, and diene-diene, depending on whether they involve two, one, or no methyl protons. The notation of Bothner-By and Harris (7) will be used to denote the differing configurational relationships of the coupled protons. Thus, subscripts c, t, and s refer to coupling cis across a double bond, truns across a double bond, and across a single bond, respectively. A numerical prefix indicates the number of bonds through which coupling occurs. In addition, a single prime refers to a methyl-diene coupling, a double prime to methyl-methyl coupling, whereas the absence of any primes indicates a diene-diene coupling constant. Thus, the spectrum of cis-pentadiene depends on fifteen coupling constants, some of which are indicated below:
Occasionally, it is necessary to refer to protons involved in coupling constants more specifically; in such cases, the numbering scheme of Bothner-By and Harris (7) for the atoms in butadiene is used. The way in which this applies to the present compounds is indicated in Fig. 1. EXPERIMENTAL
The compounds were all commercially available from K & K laboratories. They were used without further purification, the NMR spectra being taken as evidence of purity. The materials (neat liquids) were introduced into 5-mm O.D. sample tubes; a small quantity of TMS and about 6% of benzene were added to serve as NMR reference and locking substances, respectively. The samples were degassed by the usual freezing and thawing procedure using a vacuum line, and the tubes were sealed under a vacuum during the freeze part of the cycle. The spectra were recorded on a Varian Associates HA-100 spectrometer operating at 100 MHz. The spectrometer was operated in the field-frequency lock mode with frequency sweep, for both single and double resonance experiments. All calculated 1 Since we have been involved in this work, we have learnt that Segre, Zetta, and Di Corato (II) have also analysed the spectra of the pentadienes, with results that are mostly in close agreement with our own. 11
152
ALBRIKTSEN, 5005
543.5 "I(
CUNLIFFE,
AND
6%01
6.259 p
HARRIS
@) 1.663
5945
@)
FIG. 1. Position notation and chemical shifts (ppm from the tetramethylsilane resonance) for methylated hutadienes. The shifts are accurate to 0.002 ppm (except for VI), but solution conditions varied slightly.
spectra were recorded at 50 Hz sweep width and calibrated every 5 Hz (in order to minimize the error in the sweep unit) using a Hewlett-Packard 5212 A frequency counter. The counter is accurate to 0.1 Hz for a IO-set count. The line positions were taken as an average of six spectra and are assumed to be accurate to better than 0.05 Hz. The spectra were recorded using a sweep time of 2500 set in order to minimize the ringing. The fine structure in the spectra made this necessary; the resolution was about 0.2 Hz. Computations were carried out using the Atlas computer at the Science Research Council’s Atlas Laboratory, Chilton, Didcot, England, or the IBM 360-50 computer at the University of Bergen. Graphical output was obtained using a Calcomp Plotter at Bergen. SPECTRAL
ANALYSIS
The spectrum of each compound
consists of two main regions, assigned to methyl protons (6 N 1.7) and diene protons (6 N 4.8 to 6.6). The separation between the regions is much greater than any of the coupling constants. The total spectra were extremely complex in all cases, due to the number of protons in the spin systems. Therefore, simplified spectra were obtained by decoupling the methyl protons from the remaining spins (1.5). The decoupled spectra were analysed (on the basis of ABCDE
spin
systems
for
the
pentadienes,
[All],
systems
for
the
symmetrical
hexadienes, and an ABCD system for cis,trans-hexadiene) using the iterative com-
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
153
puter program (13) LAOCOON II with some minor modifications. This procedure gave reasonably accurate values for the diene chemical shifts and diene-diene coupling constants-our final results showed that Bloch-Siegert effects (39) on this stage of the analysis were slight and mostly affected the diene chemical shifts to a similar extent. In order to obtain a complete analysis of the undecoupled spectra of the pentadienes, LAOCOON II, originally written to take seven spin-4 nuclei, was modified to deal with eight spin-i nuclei. The parameters obtained from the decoupled spectra, together with estimates of the remaining values, were used as a basis for iterative computation of the ABCDEX, spin systems. The final RMS error for cis-pentadiene on 217 fitted observed transitions was 0.050 when all 21 parameters were allowed to vary. The computed probable errors of the coupling constants are less than 0.01 Hz. The corresponding values for the tram isomer were 227 fitted transitions with an RMS error of 0.064. The spectra of the hexadienes are of the [ABX,], or ABCDX,Y, types; composite particle theory (14) shows that there are DDDDQQ, DDDDQD, and DDDDDD subspectra. We will discuss the [ABX,], case-the ABCDX,Y, system has similar features. The DDDDDD subspectrum resembles the complete spectrum for a spin system of type [ABX],. Use of the X approximation shows that for extreme values of the total spin component of the X nuclei of this subspectrum there is further breakdown into [ab], subspectra-these are based on the same coupling constants as the X-decoupled [ABX,], spectra apart from the fact that effective chemical shifts must be used as follows: Vb= VB+ S(JBX+ J,,,).
“0 = VA I!zB(J,.g,+ J/ix,),
Recognition of these subspectra enabled the coupling constants from the X-decoupled spectra to be confirmed and gave the additional parameters [J,,+J,,,[ and IJ,,+J,,,[. With this information it proved possible to iteratively fit the DDDDDD subspectra (calculated as [ABX],) using LAOCOON II. About 150 lines were fitted in each case, giving RMS errors of less than 0.1 Hz. Finally, the complete ten-spin spectra were calculated using the noniterative computer program (16) UEANMR 2, which incorporates magnetic equivalence factoring. A Lorentzian shape (with a common half-width) was given to each line and the calculated spectra were plotted graphically. The final result was a satisfactory fit to the observed spectra for most regions (see Fig. 2). Some regions still showed significant deviations (especially for V), but it was not felt to be worthwhile to refine the parameters further. Signs
of
Methyl-Proton
Coupling Constants
for
cis-Pentadiene
The spectrum of cis-pentadiene is approximately first-order, which makes it suitable for double resonance “tickling” experiments (15) to give relative signs of coupling constants. We have irradiated in the methyl region, illustrated in Fig. 3, while observing the resonances due to H,.. As can be seen, the methyl resonances consist of a doublet (I”J’i - 7.0 Hz) of doublets (j4J;j - 1.8 Hz). These are further split into 1 : 2 : 1 triplets (16J:,( - 16JL,1 - 0.6 Hz); finally, there are fine doublet splittings, scarcely resolved, due to I’&) - 0.2 Hz. Due to the small magnitude of ‘J: we were unable to selectively irradiate pairs of lines separated by this quantity,
HZ
r 640
fromTMS 1
i
65
I n
610
B
FIG. 2. Spectrum of protons H, and H, of cis,cis-hexadiene-2,4 at 100 MHz; A, observed; B, computed with the parameters of Fig. 1 and the tables; C, stick plot of the computed DDDDDD-subspectrum.
172 1
162 I HZ from TMS
FIG. 3. 100-MHz spectrum of the methyl protons of cis-pentadiene-I,3
(see text).
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
155
but it is possible to irradiate at (a) for example, frequency v’ of Fig. 3 with very low power and (b) for example, frequency v” of Fig. 3 with somewhat higher power. These experiments give the relative signs of (a) (j.l:, and “J& to 5J,t and 5JCt and (b) “J; to 3J,. It may be assumed that 3Jc, 5Jtl, and 5Jcr are positive; the tickling experiments then show that 4J: 6J&, and 6J:, are negative. The sign of 5JA was not obtained by this procedure. The diene protons of trans-pentadiene are strongly coupled and the computed fit to the spectrum was, therefore, sensitive to the relative signs of the coupling constants. The Spectrum of ciqtrans-Hexadiene-2,4
The loo-MHz spectrum is sufficiently close to first-order that good estimates of all the coupling constants may be obtained by inspection. The methyl groups each give rise to doublet patterns (due to vicinal coupling) with further fine structure, which clearly indicates the existence of a long-range methyl-methyl coupling constant of ca. 0.5 Hz. The high-frequency doublet band of one methyl group overlaps the low-frequency doublet band of the other (for a 10% CHCl, solution). Irradiation at this position selectively decouples the low-frequency lines of H3 and the highfrequency lines of H,, thus showing that the high-frequency methyl resonances arise from the group at position 4’ (this is also true for the neat liquid,‘as shown by the computer fitting of the spectrum). Some difficulty was encountered in decoupling both methyl groups simultaneously from the diene resonances. In particular, the optimum frequencies for decoupling from H, I and H4 were somewhat different, as might be expected. Iterative fitting of the diene transitions was performed using composite spectra from the different optimum decoupling frequencies. Little difficulty was experienced in the computation of the DDDDDD subspectrum of the full spin system using the data from the decoupled spectra plus first-order estimates of the parameters involving the methyl protons. Decoupling Experiments on cis,cis- and trans,trans-Hexadiene
The spectra of the diene protons with the methyl resonances decoupled are of [AB], type; the relative signs of 3J, (or 3J,) to “J, (or “JJ and of 3J, to ‘Jt, (or 5JJ are, therefore, easily obtained. The single resonance spectra of the methyl protons are complicated but symmetrical. In principle, decoupling the high-frequency diene bands (due to H, and H4) should reduce the methyl region to the X part of an [AX& spin system, with JAx = 3J’ - 7 Hz, J,,, = 6J& JAA = “JIt or 5JCC, and Jxx = 7JECor 7J;;. Such decoupling experiments were not entirely successful because of the relatively strong coupling between the A and B regions of the diene spectra, especially for the trans,trans isomer. The double resonance spectra of the methyl regions did, however, contain a pair of strong lines which may be assigned as the “N doublet” expected of the [AX,], spin system (17). The doublet splitting is INI = 13J’+6J’ Cf(. Moreover, the general appearance of the decoupled spectra were consistent with those expected for [AX312 spin systems with J,, small (16, 17). In particular, there is an additional fine structure which seems to lie almost entirely outside the N doublet (Fig. 4) thus indicating that 3J’ and 6JL, are of opposite sign and, therefore, that 6Ji, is negative. This is also suggested by the magnitude of N,
156
ALBRIKTSEN,
CUNLIFFE, AND HARRIS
FIG. 4. lOO-MHz spectrum of the methyl protons of cis,cis-hexadiene-2,4 HB decoupled (see text).
with protons Hz and
which is in each case ca. 0.7 Hz less than 3J’ as found for the pentadienes. Confirmation of the relative signs of the coupling constants is obtained from the iterative computer fitting of the DDDDDD subspectra. However, there is possibly some uncertainty in the sign of 7J” since this relies entirely on the computations. We have chosen a positive sign for 7Ja on theoretical grounds; it is possible, though unlikely, that a satisfactory fit could be obtained to the spectra using a negative 7J”. The Spectrum of 2,5-Dinzethylhexadiene-2,4 An attempt was made to obtain information for this molecule by decoupling experiments, with only partial success. Irradiation midway between the methyl chemical shifts changed the diene resonance from a broad to a sharp singlet; a value of 13Js(= 11.0 + 0.5 Hz was obtained from the decoupled 13C diene satellites (IJ,, = 149.6 * 1.5 Hz). This value indicates that the molecule is predominantly in the s-trans-planar conformation. Decoupling the diene protons reduced the methyl resonances to broad structureless single peaks with half-widths of ca. 0.8 Hz. These peaks were of an unusual shape, being appreciably broader near the base than would be expected for a Lorentzian line (the width at one-quarter height being ca. 2.8 Hz as compared with a value of about 1.4 Hz expected for a Lorentzian curve of the same half-width). This effect is presumably due to seven-bond methyl-methyl coupling. This explanation is substantiated by examination of the methyl 13C satellites under the same conditions. Both the decoupled and undecoupled satellites appeared to be appreciably broader than the corresponding main resonances, indicating nonzero ‘J:, and 7Jyf. More detailed spectral analysis was not possible, but the observations are consistent with the coupling constants expected by comparison with the other molecules studied.
(II)
--
a The b The c Ref. d Ref. ’ Ref. f Ref.
-
1 I .40 10.86 10.30 10.41 -
-
1.I4 2.5 2.08 -
10.30
1.89
“JS
COUPLIK
“JC
(IN
Hz)
11.05 10.80 10.17 11.6 10.02 10.88 -
10.24 (l’, 2) 10.86 (3, 4’) 10.22
CONSTANTS
AND
-
15.09
-0.79 (1, 3) -0.78 (2,4) -0.71 -0.84 -0.83 -
-0.85
‘JC
BUTADIENES
14.84 15.01 17.05 19.0 16.81
16.93 (1, 2) 15.06 (3, 4)
16.89
3Jt
FOR METHYLATED
1
-
-- I .27 -1.16 _-0.86
- 1.14 (2,4’) -0.81
---
COMPOUNDS~
‘Jt -0.81 (I’, 3)
RELAED
signs are chosen to be consistent throughout-some experimental evidence for signs is given in the text. magnitudes are in general expected to be accurate to i 0.05 Hz; probable errors listed by the computer programme 6. 21. 18. 19.
cis, cb-Hexadiene (III) cis, rrans-hexadiene (IV) trutzs, truns-Hexadiene (V) Butadiene’ Ethylened Propene” ci.s-Butene-2’ rruns-Butene-2 f
trans-Pentadiene
-~-__ cis-Pentadiene (I)
Compound
DIENE-DIENE
TABLE
-
0.76 0.60
0.61
0.81
“Jd
-
1.80 1.30
-
1.53
5Jtt
arc usually < 0.02 Hz.
-
0.68 0.69
0.74
5Jcc
b
5
5
a z s?
4 3 ?
2 % 9 z
2
F 2 n 0
c”
c)
34
s
158
ALBRIKTSEN, DISCUSSION
CUNLIFFE, AND HARRIS
OF COUPLING
CONSTANTS
Empirical Observations
The data for the diene-diene coupling constants (Table 1) in trans-pentadiene and trans,trans-hexadiene are close to the analogous values for butadiene (6), except for “J, for the CH,CH=CH group; it may be assumed that the trans-methyl groups have very little effect on the 71system or on the molecular geometry. Moreover, the values of 3J’ and “Ji (Table 2) are close to those in propene (18). Those coupling constants for ciqtrans-hexadiene which are not influenced by the cis-methyl group (see below) also fall into the same pattern. Thus, the only influence of trans-methyl groups seems to be to cause a drop of ca. 2.0 Hz for 3Ji from 17.05 Hz in butadiene to ca. 15.05 Hz. TABLE METHYL-DIENE
AND
Compound I II III
METHYL-METHYL BLITADIENES
2
COUPLING AND RELATED
CONSTANTS (IN COMPOUNDS~ b
Hz) FOR METHYLATED
3J’
7.10 6.69 7.13
-
-1.83
0.21
-
-0.54
-0.64"
-
-1.70
-
-
0.45
-
-0.76'
-0.74
-
-
0.47
-
-
-1.87
0.32
-
-0.70e
-
IV
6.77(4,4') 7.00 (1,l’)
-1.74
-1.85
0.10
0.58
-0.61
-
-0.76
0.64
V Propene c cis-Butene-2 d trans-Butene-2 d
6.84
-1.71
-
-
6.40
-1.75
-1.33
-
0.42 -
-
-0.84/ -
-
0.40 -
6.78
-
-1.79
-
-
-
-
-
-
6.54
-1.73
-
-
-
-
-
-
a The signs are chosen to be consistent throughout; some experimental evidence for signs is given in the text. b The magnitudes are in general expected to be accurate to & 0.05 Hz; probable errors listed by the computer programme are usually < 0.02 Hz. c Ref. 18. d Ref. 19. e Methyl group cis. r Methyl group trans.
This change parallels that observed for 3Jr from propene (18) (16.81 Hz) to transbutene-2 (19) (15.09 Hz)~ ; it is also in the same direction as the change caused in 3J, by a single methyl or vinyl group (6, IS) substituted into ethylene (“J, = 19.1 Hz) (21). The coupling constants for II and V show that these compounds are predominantly in the planar s-trans conformation. However, several of the coupling constants for cis-pentadiene and cis,cis-hexadiene show significant deviations from the butadiene (6) or propene (28) values. The 2 Data for the butenes are also given in Ref. 20; the values deviate considerably from those quoted here (19). We believe the data for the methylbutadienes (particularly the variations in “J, and “J; discussed below) support the butene results of Ref. 19.
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
159
data for cis,trans-hexadiene also show these deviations, which may be classified as follows : (i) “J, decreases (becomes more negative) by ca. 0.35 Hz. (ii) “J; decreases by ca. 0.5 Hz from the value in propene (this statement relies heavily on the propene 4J; value). (iii) 3J, increases by ca. 0.55 Hz for each &methyl group present. (iv) 3J’ increases by ca. 0.3 Hz. (v) 3~, increases by ca. 0.7 Hz. The anomaly here is, however, possibly much greater than this since the introduction of a single methyl or vinyl substituent into ethylene reduces “J, from 11.5 Hz (21) to 10.02 Hz (propene) (18) or 10.17 HZ (butadiene) (6). It can be seen that effects (i) and (ii) are essentially analogous, as are effects (iii) and (iv); the only difference is that (ii) and (iv) involve averages over the internal rotation of the methyl group, whereas (i) and (iii) are for relatively fixed orientations of the C-H bonds. There are parallel variations to (ii), (iv), and (v) in the series propene (18), trans-butene, and cis-butene (/9), so it seems that internal rotation about the sp2-sp2 bond in butadiene is not responsible for the anomalies (except, possibly, that in 3J,). The values of 5J and (jJ’, which are probably n-dominated (except 5Jtr -see next section), seem to be unaffected by cis-methyl groups, so it is unlikely that changes in the z system or in (C, C) bonds are involved. They also indicate that the compounds are predominantly in the planar s-trans conformation. It is clear that we are dealing with a steric interaction between a central C-H group of the diene chain and a c&methyl group three bonds away. Such an interaction is analogous to the one between the methyl groups in cis-butene (20) (hence the similarities between coupling constant variations in the methylbutadiene and butene series). A steric interaction of this type could lead to at least six distortions which would affect coupling constants : A. Bond length changes, already dismissed. B. Changes in s-electron density at the protons. This is unlikely since the vinyl group coupling constants in cis-pentadiene are unaffected. C. Variations in the potential barrier to internal rotation about the methyldiene bond. This has been discussed by Hecht and Victor (20), but cannot accout for effects (i) and (iii).3 D. A change in the dihedral angle for the stable conformation about the sp2-sp2 bond-as noted above; this would not account for the variations in the butene series. Moreover, it is inherently unlikely that the steric interaction would be strong enough to destroy conjugation to any appreciable extent. E. A change in populations of rotamers differing in dihedral angle about the sp2-sp2 bond. Again, this could not explain the butene data. Moreover, it would be expected that cis-methyl groups would force nonplanarity, leading to a lowering of 3Js, whereas an increase of 3J, is observed. However, recent data on temperature co~esponding
3 Hecht and Victor (20) suggest that the stable situation for cis-butene-2 is one in which the CH3 groups are twisted by ca. 14” (in opposite directions) from the position in which the double bond is eclipsed. In the case of butadiene, conjugation should prevent significant rotation about the central C-C bond. Thus, rotation of a c&methyl group by somewhat greater than 14” may be anticipated. The barrier to internal rotation of a cis-methyl group may be quite low (20).
160
ALBRIKTSEN,
CUNLIFFE, AND HARRIS
variations in “J, for butadiene and the pentadienes by Segre, Zetta, and Di Corato (II) suggest that butadiene and trans-pentadiene may have an appreciable (a few percent) population of a conformation that is not s-trans. F. Bond angle changes-specifically, it is expected that CCC angles may increase and CCH angles decrease. It seems clear that for cis-pentadiene the vinyl group angles (and, therefore, coupling constants) are unaffected, as are angles in the truns-CH,CH=CH group for cis,truns-hexadiene. Hecht and Victor (20) suggest that steric effects cause an increase of ca. 5” in the CCC angle for cis-butene. Thus distortions of the typeindicated in Fig. 5 are supposed for cis-pentadiene. The reduced CCH angles serve to explain the increases in 3.J, and 3J’ and probably that in “J, also. A corresponding double distortion for cis,cis-hexadiene would explain the doubled effect on 3J, for this compound. The effects on 4J, and 4J; are rather more difficult to envisage. It is possible that a distortion of type C is responsible for the variation of “J;, but a simultaneous explanation for the changes in “J, and “J; would be preferred-the distortion of Fig. 5 would ensure this, though the mechanism of such an effect remains obscure.
FIG.
5. Molecular
distortion
suggested for cis-pentadiene-1,3.
Some of the effects discussed above should also occur when there are bulky substituents other than methyl. The only well-documented case is that for chlorine substituents, which is probably complicated by the high electronegativity of chlorine. Effect (i) is observed for the 1,4-dichlorobutadiene (7), but effect (iii) is apparently reversed (truns-chlorine atoms appear to elevate 3J,). Moreover, the chloroethylenes do not show the anomalous variation of 3J, exhibited by the butenes-the value of 3J consistently decreases from ethylene (21) to vinyl chloride (22, 23) to cisdiihloroethylene (23). Substituent Efects on Chemical Shifts In view of uncertainties due to solvent and concentration effects, it is unwise to make quantitative conclusions from the data (Fig. 1). However, certain trends are apparent. A methyl group causes a high frequency shift [compared to butadiene (6)] of 0.26 to 0.48 ppm for the proton bonded to the same carbon atom; protons attached to the next carbon atom are shifted to low frequency by ca. 0.30 ppm by both cisand trans-methyl groups. These shifts are similar to those between ethylene (21) and propene (18) (+0.44, -0.33, and -0.41 ppm for gem, cis, and truns protons, respectively). Protons attached to carbon atoms of the distant double bond appear to be little affected (< 0.15 ppm) by methyl substitution, with one important exception :
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
161
A proton at C - 2 experiences a high frequency shift of ca. 0.3 ppm from a c&methyl group at C-4. Such a shift is shown by cis-pentadiene and also by cis,cis- and cis,trans-hexadiene. In the latter cases, it acts to virtually cancel the low-frequency shift of the nearer methyl group. These shifts may probably be attributed to steric effects (37,35) (Van der Waals shift), thus providing further evidence for a strong interaction between cis C-C bonds. The methyl protons themselves do not seem to show such a steric shift, possibly partly because a given proton is sterically affected for only one-third of the time and partly because there are complications from torsional variations.
G ad x Contributions to Coupling The long-ranged character of (H, H) coupling in unsaturated compounds has long been attributed to the effects of 71 electrons, through o-n configuration interaction. Karplus (3) developed a theory, following a suggestion by McConnell (I, 2), relating 7: contributions to coupling to ESR hyperfine splitting constants, which also involve G-Z configuration interaction. Cunliffe and Harris (4) extended the Karplus theory to conjugated compounds such as butadiene. Ditchfield and Murrell(5) have since sho\vn that the expressions developed by Karplus theory are incomplete and have uhed the double perturbation theory to solve the problem. Similar results have been obtained by Cunliffe, Grinter, and Harris (24) using a matrix diagonalization method. These authors have shown that appreciable 7~ contributions may occur for ‘.J and ‘J in butadienes. It :i of importance to test experimentally these theories by measuring the G and 71 contributions to coupling individually. The criteria for separating the two terms rest on the following qualitative conclusions about n coupling derived from the Karpliis (3) approach. (ii x-coupling contributions should be independent of configuration. For hutadiene this predicts “J: = 4J: and “J:, = 5Jz = ‘J:,. tii) Replacement of a hydrogen atom in ‘;: C(sp2)-H bond by a methyl group should change the sign of J” and slightly increase its magnitude. This test was first suggested by Hoffman (IO). 0;1 the basis of the first of these criteria, Bothner-By and Harris (8) suggested that in butacr’iene jJcc and “Jet are dominated by a 71contribution of ca. 0.65 Hz, whereas ‘J,, h:!s roughly equal contributions from n and cr effects. The CI term in 5J,t arises for the zig-zag conformation of bonds that is well-known (25) to convey an anomn!ously high 0 coupling. The approximate equality of “J, and 4Jt in butadiene suggests at first sight that these are also n-dominated. However, it appears unlikely that (T contributions are negligible over four bonds-it is possible that “J, and 4J, may contain equal 0 effects. ‘Th; compounds studied here allow tests to be made of x contributions using criterion (ii). It is found (Table 2) that 6J& N 6JX N 6J:,m -0.75 Hz. These findings fit remarkably well with the hypothesis of Bothner-By and Harris (8). The drop jn magnitude from 5J,, N + 1.3 Hz to ‘jJJ, - -0.75 Hz is particularly convincing. Introduction of the extra bond has evidently attenuated the anomalous c effect greatly. The values of 7J” for the hexadienes show the change in sign demanded by
162
ALBRIKTSEN,
CUNLIFFE,
AND
HARRIS
Hoffman’s replacement technique (IO), but the magnitudes are diminished for reasons which are not clear. However, it appears safe to conclude that for butadiene itself ‘JR is ca. +0.7 Hz. Table 2 also shows that ‘J’ , N 0.45 Hz, whereas “Jf N 0.2 Hz. We believe this difference is likely to be due to the steric effects discussed in the previous section, which are always present for 5JA. It may be noted that “J’; (1.18 Hz) < 5J: (I.60 Hz) for the butenes (19), showing a similar effect is present. On these grounds, we attribute the rr contribution to 4J in butadiene as ca. -0.4 Hz. This leaves a substantial CJ contribution, fortuitously equal for “J, and “J,, of ca. -0.45 Hz. Most known 4Jb coupling constants are positive, though small values have been indicated both experimentally (26, 27) and theoretically (28, 29). Values corresponding to the geometries occurring for the compounds studied here do not seem to be available, but large negative values are unlikely-the most negative values quoted have been for nonplanar situations. This is added evidence that the whole of the observed 4J and “J, coup lin g constants cannot be attributed to 0 contributions. The anomalous na;ure of the rough equality of “J, and “J, for butadiene is shown by the steric effects of &methyl groups, which increase “J, by ca. 0.35 Hz (a similar phenomenon occurs in the butene series, as pointed out above, causing “J, and “J, to be approximately equal only for cis-butene). The equality of “J, and “J, for butadiene is also destroyed (7) by electronegative substituents at C-2, whereas 5Jccand “Jet are virtually unaffected. This again leads one to suppose that a different coupling mechanism is involved and that the C-2 substituents affect 4Jz and “Jp substantially and to a different extent. It should be noted that if the drop in magnitude from 6J’ to 7J” is characteristic of methyl substitution (i.e., if it also occurs from 5Jz to 6J’), then the true magnitudes of 4Jn and ‘J” may be somewhat greater than 0.4 Hz and 0.7 Hz, respectively. The fact that the values suggested have 14J”l < l’J”I is in agreement with the simplest theories of n coupling (I, 2, 4). These use an average energy approximation and predict 4Jn = 0, 5J” > 0. Our suggested values of 4J” and 5Jn receive qualitative support from published data (30, 31, 32) on enynes and on diynes. Thus values for methylated enynes have been reported as follows: H H
Me0
CH, k==c
C ti
%Jt = +0.66
H H
C
H
C ti
\ H Hz
“Jc
= +O. 59 Hz
5J’
= +0.31
H Me0
H H yfi
F
7 CH,
“J;
= -0.58Hz
CHS “Jt’
= -0.6
Hz
Hz
The Hoffman hypothesis (IO) indicates 5J” N 0.6 HZ, 4J” (ethylenic) N 0.3 Hz [the value of 4J in the parent compound (33, 34) is dominated by a n contribution of the allylic type]. The values (35) of the coupling constants for diacetylene, methyldiacetylene, and dimethyldiacetylene are 5J = 2.2 Hz, 6J’ = 1.27 Hz, 7J” = 1.3 Hz, indicating a value of 5Ja - 0.6 Hz per z system (there are two equivalent n-coupling paths).
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
163
HEXADIENES
ACKNOWLEDGMENTS We thank Drs. Castellano and Bothner-By for a copy of the original LACOON computer program and Mr. J. Stokes for discussion and assistance with the computation. Dr. A. Segre kindly gave us details of results in advance of publication. One of us (A.V.C.) is grateful to the U.K. Science Research Council for a Research Studentship, and another of us (P. A.) is indebted to the Royal Norwegian Council for Scientific and Industrial Research for a Research Fellowship. REFERENCES 1. ’ ;: 1. 5. 6. 7. 8. 9. 10.
H. H. M. A. R. R. A. A. D. R.
M. MCCONNELL, J. Mol. Spectrosc. 1, 11 (1957). M. MCCONNELL, J. Chem. Phys. 30, 126 (1958). KARPLUS, J. Chem. Phys. 33, 1842 (1960). V. CUNLIFFE AND R. K. HARRIS, Mol. Phys. 13,269 (1967). DITCHFIELD AND J. N. MURRELL, Mol. Phys. 15, 533 (1968). T. HOBGOOD AND 3. H. GOLDSTEIN, J. Mol. Spectrosc. 12, 76 (1964). A. BOTHNER-BY AND R. K. HARRIS, J. Amer. Chem. Sot. 87, 3445 (1965). A. BOTHNER-BY AND R. K. HARRIS, J. Amer. Chem. Sot. 87, 3451 (1965). WENDISCH AND P. KURTZ, 2. Nuturforsch. 22b, 599 (1967). A. HOFFMAN, Mol. Phys. 1, 326 (1958); R. A. HOFFMAN AND S. GRONOWITZ,
ActalChem.
Stand. 13, 1477 (1959). II, 12. 13. 14. :5. 16. 17. 18. 19. 20.
-71. 22. 23.
A. D. S. P. R. R. R.
A. R. H. R. C. E.
L. SEGRE, L. ZETTA, AND A. DI CORATO, J. Mol. Spectrosc. 32, 296 (1969). F. KOSTER AND A. DANTI, J. Phys. Chem. 69,486 (1965). CASTELLANO AND A. A. BOTHNER-BY, J. Chem. Phys. 41, 3863 (1964). DIEHL, R. K. HARRIS, AND R. G. JONES, Prog. NMR Spectrosc. 3, 1 (1967). A. HOFFMAN AND S. FORSEN, Prog. NMR Spectrosc. 1, 15 (1966). K. HARRIS AND C. M. WOODMAN, Mol. Phys. 10, 437 (1966). K. HARRIS, Can. J. Chem. 42,2275 (1964). A. BOTHNER-BY AND C. NAAR-COLIN, J. Amer. Chem. Sot. 83, 231 (1961). K. HARRIS AND B. R. HOWES, J. Mol. Spectrosc. 28, 191 (1968). G. HECHT AND B. L. VICTOR, J. Amer. Chem. Sot. 90, 3333 (1968). M. LYNDEN-BELL AND N. SHEPPARD; Proc. Roy. Sot. Ser. A 269, 385 (1962). N. BANWELL AND N. SHEPPARD, Mol. Phys. 3, 351 (1960). B. WHIPPLE, W. E. STEWART, G. S. REDDY, AND J. H. GOLDSTEIN, J. Chem.
Phys. 34, 2136
(1961). 24. A. V. CUNLIFFE, R. GRINTER, AND R. K. HARRIS, to be published. 25. C. N. BANWELL AND N. SHEPPARD, Disc~css. Faraday Sot. 34, 115 (1962). 26. E. W. GARBISCH, Chem. Znd. 1715 (1964). 27. V. F. BYSTROV AND A. U. STEPANYANTS, J. Mol. Spectrosc. 21, 241 (1966). 28. M. BARFIELD, J. Chem. Phys. 41, 3825 (1964). 29. R. P. CAREY AND R. DITCHFIELD, Mol. Phys. 15, 515 (1968). 30. H. M. HUTTON AND T. SCHAEFER, Can. J. Chem. 45, 1165 (1967). 31. R-M. LEQUAN AND M-P. SIMONNIN, C.R. Acad. Sci. Ser. C 266, 825 (1968). 32. R-M. LEQUAN AND M-P. SIMONNIN, C.R. Acad. Sci. Ser. C 265, 125 (1967). 33. R. C. HIRST AND D. M. GRANT, J. Amer. Chem. Sot. 84,2009 (1962). 31. E. I. SNYDER, L. J. ALTMAN, AND J. D. ROBERTS, J. Amer. Chem. Sot. 84, 2004 35. E. I. SNYDER AND J. D. ROBERTS, J. Amer. Chem. Sot. 84, 1582 (1962). 36. A. A. BOTHNER-BY AND D. F. KOSTER, J. Amer. Chem. Sot. 90, 2351 (1968). 37. V. M. S. GIL AND W. A. GIBBONS, Mol. Phys. 8, 199 (1964). 38. W. A. GIBBONS AND V. M. S. GIL, Mol. Phys. 9, 167 (1965). 39. G. GOVIL AND D. H. WHIFFEN, Mol. Phys. 12 (1967).
(1962).
Nuclear Magnetic ResonanceStudiesof 1,3-Butadienes VII. Coupling ConstantsInvolving the Methyl Protons of Penta- and Hexadienes* P.
ALBRIKTSEN~,
A. V. CUNLIFFE~, AND R. K. HARRIS
School of Chemical Sciences, University of East Anglia, Norwich, NOR 88C, England Received August 12, 1969; accepted October 28, 1969 The proton resonance spectra of the two isomers of pentadiene-1,3 and the three isomers of hexadiene-2,4 have been fully analysed using iterative computation together with the techniques of double resonance and subspectral breakdown. An appreciable magnitude is found for the long-range coupling constant involving methyl protons, including 7J = ca. 0.5 Hz. The data are used to discuss the mechanism of coupling in butadienes in terms of 0 and 7ccontributions. It is concluded that the A contribution to 4J is ca. -0.4 Hz and the n contribution to 5J is ca. $0.7 Hz. The spectrum of 2,5-dimethylhexadiene-2,4 is also discussed. INTRODUCTION
There has been considerable interest in the role of n electrons in spin-spin coupling for unsaturated (nonaromatic) compounds. It is generally accepted that the contact term is dominant in such situations and, therefore, that a rc contribution to coupling can only occur through (0, n) configuration interaction. Several theoretical papers have appeared on this topic (1-5). The butadiene system is of particular interest since there are two types of long-range coupling (through two and three bonds of the rc system) which might be expected to have important z contributions. Considerable information is available for coupling constants in butadienes (6-9), but most of the compounds studied have highly electronegative substituents, which render them unsuitable for testing theories of coupling. Consequently, we have studied a series of methyl-substituted butadienes that do not have this disadvantage. Moreover, the simplest theories relate the 7~contribution to coupling to ESR hyperfine splitting constants, and on this basis it has been suggested (10) that replacement of a dienic proton by a methyl group should result in a change of sign of J, together with a small increase in magnitude. Thus, measurement of coupling constants for methylated butadienes is useful for a discussion of 71 contributions to coupling. In this paper, we report work on cis- and truns-pentadiene-I,3 (I and II, respectively), on cis,cis-, cis,trans-, and truns,trans-hexadiene-2,4 (III, IV, and V, respectively), and, more qualitatively, on 2,5-dimethylhexadiene-2,4 (VI). There are no accurate data in the * Part VI is Ref. 36. t Present address: Kjemisk Institutt, Universitetet i Bergen, Norway. ‘, Present address: E.R.D.E., Waltham Abbey, Essex, England. 150
METHYL COUPLING
CONSTANTS IN PENTA- AND HEXADIENES
151
literature’ on such compounds-the paper by Koster and Danti (22) uses only first-order analysis of spectra. One reason for the lack of data is that the spectra are very complex, since they involve eight and ten protons for the pentadienes and hexadienes, respectively. The well-known iterative computer program (23) LAOCOON normally handles only systems of seven spins. Limited extension is possible, but only at a considerable cost in terms of computer time. However, the technique of subspectral analysis (14) makes it feasible to tackle spectra of such large spin systems. We have, therefore, obtained values-including the signs-of all the (H, H) coupling constants in the molecules studied. We shall classify these coupling constants as methyl-methyl, methyl-diene, and diene-diene, depending on whether they involve two, one, or no methyl protons. The notation of Bothner-By and Harris (7) will be used to denote the differing configurational relationships of the coupled protons. Thus, subscripts c, t, and s refer to coupling cis across a double bond, truns across a double bond, and across a single bond, respectively. A numerical prefix indicates the number of bonds through which coupling occurs. In addition, a single prime refers to a methyl-diene coupling, a double prime to methyl-methyl coupling, whereas the absence of any primes indicates a diene-diene coupling constant. Thus, the spectrum of cis-pentadiene depends on fifteen coupling constants, some of which are indicated below:
Occasionally, it is necessary to refer to protons involved in coupling constants more specifically; in such cases, the numbering scheme of Bothner-By and Harris (7) for the atoms in butadiene is used. The way in which this applies to the present compounds is indicated in Fig. 1. EXPERIMENTAL
The compounds were all commercially available from K & K laboratories. They were used without further purification, the NMR spectra being taken as evidence of purity. The materials (neat liquids) were introduced into 5-mm O.D. sample tubes; a small quantity of TMS and about 6% of benzene were added to serve as NMR reference and locking substances, respectively. The samples were degassed by the usual freezing and thawing procedure using a vacuum line, and the tubes were sealed under a vacuum during the freeze part of the cycle. The spectra were recorded on a Varian Associates HA-100 spectrometer operating at 100 MHz. The spectrometer was operated in the field-frequency lock mode with frequency sweep, for both single and double resonance experiments. All calculated 1 Since we have been involved in this work, we have learnt that Segre, Zetta, and Di Corato (II) have also analysed the spectra of the pentadienes, with results that are mostly in close agreement with our own. 11
152
ALBRIKTSEN, 5005
543.5 "I(
CUNLIFFE,
AND
6%01
6.259 p
HARRIS
@) 1.663
5945
@)
FIG. 1. Position notation and chemical shifts (ppm from the tetramethylsilane resonance) for methylated hutadienes. The shifts are accurate to 0.002 ppm (except for VI), but solution conditions varied slightly.
spectra were recorded at 50 Hz sweep width and calibrated every 5 Hz (in order to minimize the error in the sweep unit) using a Hewlett-Packard 5212 A frequency counter. The counter is accurate to 0.1 Hz for a IO-set count. The line positions were taken as an average of six spectra and are assumed to be accurate to better than 0.05 Hz. The spectra were recorded using a sweep time of 2500 set in order to minimize the ringing. The fine structure in the spectra made this necessary; the resolution was about 0.2 Hz. Computations were carried out using the Atlas computer at the Science Research Council’s Atlas Laboratory, Chilton, Didcot, England, or the IBM 360-50 computer at the University of Bergen. Graphical output was obtained using a Calcomp Plotter at Bergen. SPECTRAL
ANALYSIS
The spectrum of each compound
consists of two main regions, assigned to methyl protons (6 N 1.7) and diene protons (6 N 4.8 to 6.6). The separation between the regions is much greater than any of the coupling constants. The total spectra were extremely complex in all cases, due to the number of protons in the spin systems. Therefore, simplified spectra were obtained by decoupling the methyl protons from the remaining spins (1.5). The decoupled spectra were analysed (on the basis of ABCDE
spin
systems
for
the
pentadienes,
[All],
systems
for
the
symmetrical
hexadienes, and an ABCD system for cis,trans-hexadiene) using the iterative com-
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
153
puter program (13) LAOCOON II with some minor modifications. This procedure gave reasonably accurate values for the diene chemical shifts and diene-diene coupling constants-our final results showed that Bloch-Siegert effects (39) on this stage of the analysis were slight and mostly affected the diene chemical shifts to a similar extent. In order to obtain a complete analysis of the undecoupled spectra of the pentadienes, LAOCOON II, originally written to take seven spin-4 nuclei, was modified to deal with eight spin-i nuclei. The parameters obtained from the decoupled spectra, together with estimates of the remaining values, were used as a basis for iterative computation of the ABCDEX, spin systems. The final RMS error for cis-pentadiene on 217 fitted observed transitions was 0.050 when all 21 parameters were allowed to vary. The computed probable errors of the coupling constants are less than 0.01 Hz. The corresponding values for the tram isomer were 227 fitted transitions with an RMS error of 0.064. The spectra of the hexadienes are of the [ABX,], or ABCDX,Y, types; composite particle theory (14) shows that there are DDDDQQ, DDDDQD, and DDDDDD subspectra. We will discuss the [ABX,], case-the ABCDX,Y, system has similar features. The DDDDDD subspectrum resembles the complete spectrum for a spin system of type [ABX],. Use of the X approximation shows that for extreme values of the total spin component of the X nuclei of this subspectrum there is further breakdown into [ab], subspectra-these are based on the same coupling constants as the X-decoupled [ABX,], spectra apart from the fact that effective chemical shifts must be used as follows: Vb= VB+ S(JBX+ J,,,).
“0 = VA I!zB(J,.g,+ J/ix,),
Recognition of these subspectra enabled the coupling constants from the X-decoupled spectra to be confirmed and gave the additional parameters [J,,+J,,,[ and IJ,,+J,,,[. With this information it proved possible to iteratively fit the DDDDDD subspectra (calculated as [ABX],) using LAOCOON II. About 150 lines were fitted in each case, giving RMS errors of less than 0.1 Hz. Finally, the complete ten-spin spectra were calculated using the noniterative computer program (16) UEANMR 2, which incorporates magnetic equivalence factoring. A Lorentzian shape (with a common half-width) was given to each line and the calculated spectra were plotted graphically. The final result was a satisfactory fit to the observed spectra for most regions (see Fig. 2). Some regions still showed significant deviations (especially for V), but it was not felt to be worthwhile to refine the parameters further. Signs
of
Methyl-Proton
Coupling Constants
for
cis-Pentadiene
The spectrum of cis-pentadiene is approximately first-order, which makes it suitable for double resonance “tickling” experiments (15) to give relative signs of coupling constants. We have irradiated in the methyl region, illustrated in Fig. 3, while observing the resonances due to H,.. As can be seen, the methyl resonances consist of a doublet (I”J’i - 7.0 Hz) of doublets (j4J;j - 1.8 Hz). These are further split into 1 : 2 : 1 triplets (16J:,( - 16JL,1 - 0.6 Hz); finally, there are fine doublet splittings, scarcely resolved, due to I’&) - 0.2 Hz. Due to the small magnitude of ‘J: we were unable to selectively irradiate pairs of lines separated by this quantity,
HZ
r 640
fromTMS 1
i
65
I n
610
B
FIG. 2. Spectrum of protons H, and H, of cis,cis-hexadiene-2,4 at 100 MHz; A, observed; B, computed with the parameters of Fig. 1 and the tables; C, stick plot of the computed DDDDDD-subspectrum.
172 1
162 I HZ from TMS
FIG. 3. 100-MHz spectrum of the methyl protons of cis-pentadiene-I,3
(see text).
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
155
but it is possible to irradiate at (a) for example, frequency v’ of Fig. 3 with very low power and (b) for example, frequency v” of Fig. 3 with somewhat higher power. These experiments give the relative signs of (a) (j.l:, and “J& to 5J,t and 5JCt and (b) “J; to 3J,. It may be assumed that 3Jc, 5Jtl, and 5Jcr are positive; the tickling experiments then show that 4J: 6J&, and 6J:, are negative. The sign of 5JA was not obtained by this procedure. The diene protons of trans-pentadiene are strongly coupled and the computed fit to the spectrum was, therefore, sensitive to the relative signs of the coupling constants. The Spectrum of ciqtrans-Hexadiene-2,4
The loo-MHz spectrum is sufficiently close to first-order that good estimates of all the coupling constants may be obtained by inspection. The methyl groups each give rise to doublet patterns (due to vicinal coupling) with further fine structure, which clearly indicates the existence of a long-range methyl-methyl coupling constant of ca. 0.5 Hz. The high-frequency doublet band of one methyl group overlaps the low-frequency doublet band of the other (for a 10% CHCl, solution). Irradiation at this position selectively decouples the low-frequency lines of H3 and the highfrequency lines of H,, thus showing that the high-frequency methyl resonances arise from the group at position 4’ (this is also true for the neat liquid,‘as shown by the computer fitting of the spectrum). Some difficulty was encountered in decoupling both methyl groups simultaneously from the diene resonances. In particular, the optimum frequencies for decoupling from H, I and H4 were somewhat different, as might be expected. Iterative fitting of the diene transitions was performed using composite spectra from the different optimum decoupling frequencies. Little difficulty was experienced in the computation of the DDDDDD subspectrum of the full spin system using the data from the decoupled spectra plus first-order estimates of the parameters involving the methyl protons. Decoupling Experiments on cis,cis- and trans,trans-Hexadiene
The spectra of the diene protons with the methyl resonances decoupled are of [AB], type; the relative signs of 3J, (or 3J,) to “J, (or “JJ and of 3J, to ‘Jt, (or 5JJ are, therefore, easily obtained. The single resonance spectra of the methyl protons are complicated but symmetrical. In principle, decoupling the high-frequency diene bands (due to H, and H4) should reduce the methyl region to the X part of an [AX& spin system, with JAx = 3J’ - 7 Hz, J,,, = 6J& JAA = “JIt or 5JCC, and Jxx = 7JECor 7J;;. Such decoupling experiments were not entirely successful because of the relatively strong coupling between the A and B regions of the diene spectra, especially for the trans,trans isomer. The double resonance spectra of the methyl regions did, however, contain a pair of strong lines which may be assigned as the “N doublet” expected of the [AX,], spin system (17). The doublet splitting is INI = 13J’+6J’ Cf(. Moreover, the general appearance of the decoupled spectra were consistent with those expected for [AX312 spin systems with J,, small (16, 17). In particular, there is an additional fine structure which seems to lie almost entirely outside the N doublet (Fig. 4) thus indicating that 3J’ and 6JL, are of opposite sign and, therefore, that 6Ji, is negative. This is also suggested by the magnitude of N,
156
ALBRIKTSEN,
CUNLIFFE, AND HARRIS
FIG. 4. lOO-MHz spectrum of the methyl protons of cis,cis-hexadiene-2,4 HB decoupled (see text).
with protons Hz and
which is in each case ca. 0.7 Hz less than 3J’ as found for the pentadienes. Confirmation of the relative signs of the coupling constants is obtained from the iterative computer fitting of the DDDDDD subspectra. However, there is possibly some uncertainty in the sign of 7J” since this relies entirely on the computations. We have chosen a positive sign for 7Ja on theoretical grounds; it is possible, though unlikely, that a satisfactory fit could be obtained to the spectra using a negative 7J”. The Spectrum of 2,5-Dinzethylhexadiene-2,4 An attempt was made to obtain information for this molecule by decoupling experiments, with only partial success. Irradiation midway between the methyl chemical shifts changed the diene resonance from a broad to a sharp singlet; a value of 13Js(= 11.0 + 0.5 Hz was obtained from the decoupled 13C diene satellites (IJ,, = 149.6 * 1.5 Hz). This value indicates that the molecule is predominantly in the s-trans-planar conformation. Decoupling the diene protons reduced the methyl resonances to broad structureless single peaks with half-widths of ca. 0.8 Hz. These peaks were of an unusual shape, being appreciably broader near the base than would be expected for a Lorentzian line (the width at one-quarter height being ca. 2.8 Hz as compared with a value of about 1.4 Hz expected for a Lorentzian curve of the same half-width). This effect is presumably due to seven-bond methyl-methyl coupling. This explanation is substantiated by examination of the methyl 13C satellites under the same conditions. Both the decoupled and undecoupled satellites appeared to be appreciably broader than the corresponding main resonances, indicating nonzero ‘J:, and 7Jyf. More detailed spectral analysis was not possible, but the observations are consistent with the coupling constants expected by comparison with the other molecules studied.
(II)
--
a The b The c Ref. d Ref. ’ Ref. f Ref.
-
1 I .40 10.86 10.30 10.41 -
-
1.I4 2.5 2.08 -
10.30
1.89
“JS
COUPLIK
“JC
(IN
Hz)
11.05 10.80 10.17 11.6 10.02 10.88 -
10.24 (l’, 2) 10.86 (3, 4’) 10.22
CONSTANTS
AND
-
15.09
-0.79 (1, 3) -0.78 (2,4) -0.71 -0.84 -0.83 -
-0.85
‘JC
BUTADIENES
14.84 15.01 17.05 19.0 16.81
16.93 (1, 2) 15.06 (3, 4)
16.89
3Jt
FOR METHYLATED
1
-
-- I .27 -1.16 _-0.86
- 1.14 (2,4’) -0.81
---
COMPOUNDS~
‘Jt -0.81 (I’, 3)
RELAED
signs are chosen to be consistent throughout-some experimental evidence for signs is given in the text. magnitudes are in general expected to be accurate to i 0.05 Hz; probable errors listed by the computer programme 6. 21. 18. 19.
cis, cb-Hexadiene (III) cis, rrans-hexadiene (IV) trutzs, truns-Hexadiene (V) Butadiene’ Ethylened Propene” ci.s-Butene-2’ rruns-Butene-2 f
trans-Pentadiene
-~-__ cis-Pentadiene (I)
Compound
DIENE-DIENE
TABLE
-
0.76 0.60
0.61
0.81
“Jd
-
1.80 1.30
-
1.53
5Jtt
arc usually < 0.02 Hz.
-
0.68 0.69
0.74
5Jcc
b
5
5
a z s?
4 3 ?
2 % 9 z
2
F 2 n 0
c”
c)
34
s
158
ALBRIKTSEN, DISCUSSION
CUNLIFFE, AND HARRIS
OF COUPLING
CONSTANTS
Empirical Observations
The data for the diene-diene coupling constants (Table 1) in trans-pentadiene and trans,trans-hexadiene are close to the analogous values for butadiene (6), except for “J, for the CH,CH=CH group; it may be assumed that the trans-methyl groups have very little effect on the 71system or on the molecular geometry. Moreover, the values of 3J’ and “Ji (Table 2) are close to those in propene (18). Those coupling constants for ciqtrans-hexadiene which are not influenced by the cis-methyl group (see below) also fall into the same pattern. Thus, the only influence of trans-methyl groups seems to be to cause a drop of ca. 2.0 Hz for 3Ji from 17.05 Hz in butadiene to ca. 15.05 Hz. TABLE METHYL-DIENE
AND
Compound I II III
METHYL-METHYL BLITADIENES
2
COUPLING AND RELATED
CONSTANTS (IN COMPOUNDS~ b
Hz) FOR METHYLATED
3J’
7.10 6.69 7.13
-
-1.83
0.21
-
-0.54
-0.64"
-
-1.70
-
-
0.45
-
-0.76'
-0.74
-
-
0.47
-
-
-1.87
0.32
-
-0.70e
-
IV
6.77(4,4') 7.00 (1,l’)
-1.74
-1.85
0.10
0.58
-0.61
-
-0.76
0.64
V Propene c cis-Butene-2 d trans-Butene-2 d
6.84
-1.71
-
-
6.40
-1.75
-1.33
-
0.42 -
-
-0.84/ -
-
0.40 -
6.78
-
-1.79
-
-
-
-
-
-
6.54
-1.73
-
-
-
-
-
-
a The signs are chosen to be consistent throughout; some experimental evidence for signs is given in the text. b The magnitudes are in general expected to be accurate to & 0.05 Hz; probable errors listed by the computer programme are usually < 0.02 Hz. c Ref. 18. d Ref. 19. e Methyl group cis. r Methyl group trans.
This change parallels that observed for 3Jr from propene (18) (16.81 Hz) to transbutene-2 (19) (15.09 Hz)~ ; it is also in the same direction as the change caused in 3J, by a single methyl or vinyl group (6, IS) substituted into ethylene (“J, = 19.1 Hz) (21). The coupling constants for II and V show that these compounds are predominantly in the planar s-trans conformation. However, several of the coupling constants for cis-pentadiene and cis,cis-hexadiene show significant deviations from the butadiene (6) or propene (28) values. The 2 Data for the butenes are also given in Ref. 20; the values deviate considerably from those quoted here (19). We believe the data for the methylbutadienes (particularly the variations in “J, and “J; discussed below) support the butene results of Ref. 19.
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
159
data for cis,trans-hexadiene also show these deviations, which may be classified as follows : (i) “J, decreases (becomes more negative) by ca. 0.35 Hz. (ii) “J; decreases by ca. 0.5 Hz from the value in propene (this statement relies heavily on the propene 4J; value). (iii) 3J, increases by ca. 0.55 Hz for each &methyl group present. (iv) 3J’ increases by ca. 0.3 Hz. (v) 3~, increases by ca. 0.7 Hz. The anomaly here is, however, possibly much greater than this since the introduction of a single methyl or vinyl substituent into ethylene reduces “J, from 11.5 Hz (21) to 10.02 Hz (propene) (18) or 10.17 HZ (butadiene) (6). It can be seen that effects (i) and (ii) are essentially analogous, as are effects (iii) and (iv); the only difference is that (ii) and (iv) involve averages over the internal rotation of the methyl group, whereas (i) and (iii) are for relatively fixed orientations of the C-H bonds. There are parallel variations to (ii), (iv), and (v) in the series propene (18), trans-butene, and cis-butene (/9), so it seems that internal rotation about the sp2-sp2 bond in butadiene is not responsible for the anomalies (except, possibly, that in 3J,). The values of 5J and (jJ’, which are probably n-dominated (except 5Jtr -see next section), seem to be unaffected by cis-methyl groups, so it is unlikely that changes in the z system or in (C, C) bonds are involved. They also indicate that the compounds are predominantly in the planar s-trans conformation. It is clear that we are dealing with a steric interaction between a central C-H group of the diene chain and a c&methyl group three bonds away. Such an interaction is analogous to the one between the methyl groups in cis-butene (20) (hence the similarities between coupling constant variations in the methylbutadiene and butene series). A steric interaction of this type could lead to at least six distortions which would affect coupling constants : A. Bond length changes, already dismissed. B. Changes in s-electron density at the protons. This is unlikely since the vinyl group coupling constants in cis-pentadiene are unaffected. C. Variations in the potential barrier to internal rotation about the methyldiene bond. This has been discussed by Hecht and Victor (20), but cannot accout for effects (i) and (iii).3 D. A change in the dihedral angle for the stable conformation about the sp2-sp2 bond-as noted above; this would not account for the variations in the butene series. Moreover, it is inherently unlikely that the steric interaction would be strong enough to destroy conjugation to any appreciable extent. E. A change in populations of rotamers differing in dihedral angle about the sp2-sp2 bond. Again, this could not explain the butene data. Moreover, it would be expected that cis-methyl groups would force nonplanarity, leading to a lowering of 3Js, whereas an increase of 3J, is observed. However, recent data on temperature co~esponding
3 Hecht and Victor (20) suggest that the stable situation for cis-butene-2 is one in which the CH3 groups are twisted by ca. 14” (in opposite directions) from the position in which the double bond is eclipsed. In the case of butadiene, conjugation should prevent significant rotation about the central C-C bond. Thus, rotation of a c&methyl group by somewhat greater than 14” may be anticipated. The barrier to internal rotation of a cis-methyl group may be quite low (20).
160
ALBRIKTSEN,
CUNLIFFE, AND HARRIS
variations in “J, for butadiene and the pentadienes by Segre, Zetta, and Di Corato (II) suggest that butadiene and trans-pentadiene may have an appreciable (a few percent) population of a conformation that is not s-trans. F. Bond angle changes-specifically, it is expected that CCC angles may increase and CCH angles decrease. It seems clear that for cis-pentadiene the vinyl group angles (and, therefore, coupling constants) are unaffected, as are angles in the truns-CH,CH=CH group for cis,truns-hexadiene. Hecht and Victor (20) suggest that steric effects cause an increase of ca. 5” in the CCC angle for cis-butene. Thus distortions of the typeindicated in Fig. 5 are supposed for cis-pentadiene. The reduced CCH angles serve to explain the increases in 3.J, and 3J’ and probably that in “J, also. A corresponding double distortion for cis,cis-hexadiene would explain the doubled effect on 3J, for this compound. The effects on 4J, and 4J; are rather more difficult to envisage. It is possible that a distortion of type C is responsible for the variation of “J;, but a simultaneous explanation for the changes in “J, and “J; would be preferred-the distortion of Fig. 5 would ensure this, though the mechanism of such an effect remains obscure.
FIG.
5. Molecular
distortion
suggested for cis-pentadiene-1,3.
Some of the effects discussed above should also occur when there are bulky substituents other than methyl. The only well-documented case is that for chlorine substituents, which is probably complicated by the high electronegativity of chlorine. Effect (i) is observed for the 1,4-dichlorobutadiene (7), but effect (iii) is apparently reversed (truns-chlorine atoms appear to elevate 3J,). Moreover, the chloroethylenes do not show the anomalous variation of 3J, exhibited by the butenes-the value of 3J consistently decreases from ethylene (21) to vinyl chloride (22, 23) to cisdiihloroethylene (23). Substituent Efects on Chemical Shifts In view of uncertainties due to solvent and concentration effects, it is unwise to make quantitative conclusions from the data (Fig. 1). However, certain trends are apparent. A methyl group causes a high frequency shift [compared to butadiene (6)] of 0.26 to 0.48 ppm for the proton bonded to the same carbon atom; protons attached to the next carbon atom are shifted to low frequency by ca. 0.30 ppm by both cisand trans-methyl groups. These shifts are similar to those between ethylene (21) and propene (18) (+0.44, -0.33, and -0.41 ppm for gem, cis, and truns protons, respectively). Protons attached to carbon atoms of the distant double bond appear to be little affected (< 0.15 ppm) by methyl substitution, with one important exception :
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
HEXADIENES
161
A proton at C - 2 experiences a high frequency shift of ca. 0.3 ppm from a c&methyl group at C-4. Such a shift is shown by cis-pentadiene and also by cis,cis- and cis,trans-hexadiene. In the latter cases, it acts to virtually cancel the low-frequency shift of the nearer methyl group. These shifts may probably be attributed to steric effects (37,35) (Van der Waals shift), thus providing further evidence for a strong interaction between cis C-C bonds. The methyl protons themselves do not seem to show such a steric shift, possibly partly because a given proton is sterically affected for only one-third of the time and partly because there are complications from torsional variations.
G ad x Contributions to Coupling The long-ranged character of (H, H) coupling in unsaturated compounds has long been attributed to the effects of 71 electrons, through o-n configuration interaction. Karplus (3) developed a theory, following a suggestion by McConnell (I, 2), relating 7: contributions to coupling to ESR hyperfine splitting constants, which also involve G-Z configuration interaction. Cunliffe and Harris (4) extended the Karplus theory to conjugated compounds such as butadiene. Ditchfield and Murrell(5) have since sho\vn that the expressions developed by Karplus theory are incomplete and have uhed the double perturbation theory to solve the problem. Similar results have been obtained by Cunliffe, Grinter, and Harris (24) using a matrix diagonalization method. These authors have shown that appreciable 7~ contributions may occur for ‘.J and ‘J in butadienes. It :i of importance to test experimentally these theories by measuring the G and 71 contributions to coupling individually. The criteria for separating the two terms rest on the following qualitative conclusions about n coupling derived from the Karpliis (3) approach. (ii x-coupling contributions should be independent of configuration. For hutadiene this predicts “J: = 4J: and “J:, = 5Jz = ‘J:,. tii) Replacement of a hydrogen atom in ‘;: C(sp2)-H bond by a methyl group should change the sign of J” and slightly increase its magnitude. This test was first suggested by Hoffman (IO). 0;1 the basis of the first of these criteria, Bothner-By and Harris (8) suggested that in butacr’iene jJcc and “Jet are dominated by a 71contribution of ca. 0.65 Hz, whereas ‘J,, h:!s roughly equal contributions from n and cr effects. The CI term in 5J,t arises for the zig-zag conformation of bonds that is well-known (25) to convey an anomn!ously high 0 coupling. The approximate equality of “J, and 4Jt in butadiene suggests at first sight that these are also n-dominated. However, it appears unlikely that (T contributions are negligible over four bonds-it is possible that “J, and 4J, may contain equal 0 effects. ‘Th; compounds studied here allow tests to be made of x contributions using criterion (ii). It is found (Table 2) that 6J& N 6JX N 6J:,m -0.75 Hz. These findings fit remarkably well with the hypothesis of Bothner-By and Harris (8). The drop jn magnitude from 5J,, N + 1.3 Hz to ‘jJJ, - -0.75 Hz is particularly convincing. Introduction of the extra bond has evidently attenuated the anomalous c effect greatly. The values of 7J” for the hexadienes show the change in sign demanded by
162
ALBRIKTSEN,
CUNLIFFE,
AND
HARRIS
Hoffman’s replacement technique (IO), but the magnitudes are diminished for reasons which are not clear. However, it appears safe to conclude that for butadiene itself ‘JR is ca. +0.7 Hz. Table 2 also shows that ‘J’ , N 0.45 Hz, whereas “Jf N 0.2 Hz. We believe this difference is likely to be due to the steric effects discussed in the previous section, which are always present for 5JA. It may be noted that “J’; (1.18 Hz) < 5J: (I.60 Hz) for the butenes (19), showing a similar effect is present. On these grounds, we attribute the rr contribution to 4J in butadiene as ca. -0.4 Hz. This leaves a substantial CJ contribution, fortuitously equal for “J, and “J,, of ca. -0.45 Hz. Most known 4Jb coupling constants are positive, though small values have been indicated both experimentally (26, 27) and theoretically (28, 29). Values corresponding to the geometries occurring for the compounds studied here do not seem to be available, but large negative values are unlikely-the most negative values quoted have been for nonplanar situations. This is added evidence that the whole of the observed 4J and “J, coup lin g constants cannot be attributed to 0 contributions. The anomalous na;ure of the rough equality of “J, and “J, for butadiene is shown by the steric effects of &methyl groups, which increase “J, by ca. 0.35 Hz (a similar phenomenon occurs in the butene series, as pointed out above, causing “J, and “J, to be approximately equal only for cis-butene). The equality of “J, and “J, for butadiene is also destroyed (7) by electronegative substituents at C-2, whereas 5Jccand “Jet are virtually unaffected. This again leads one to suppose that a different coupling mechanism is involved and that the C-2 substituents affect 4Jz and “Jp substantially and to a different extent. It should be noted that if the drop in magnitude from 6J’ to 7J” is characteristic of methyl substitution (i.e., if it also occurs from 5Jz to 6J’), then the true magnitudes of 4Jn and ‘J” may be somewhat greater than 0.4 Hz and 0.7 Hz, respectively. The fact that the values suggested have 14J”l < l’J”I is in agreement with the simplest theories of n coupling (I, 2, 4). These use an average energy approximation and predict 4Jn = 0, 5J” > 0. Our suggested values of 4J” and 5Jn receive qualitative support from published data (30, 31, 32) on enynes and on diynes. Thus values for methylated enynes have been reported as follows: H H
Me0
CH, k==c
C ti
%Jt = +0.66
H H
C
H
C ti
\ H Hz
“Jc
= +O. 59 Hz
5J’
= +0.31
H Me0
H H yfi
F
7 CH,
“J;
= -0.58Hz
CHS “Jt’
= -0.6
Hz
Hz
The Hoffman hypothesis (IO) indicates 5J” N 0.6 HZ, 4J” (ethylenic) N 0.3 Hz [the value of 4J in the parent compound (33, 34) is dominated by a n contribution of the allylic type]. The values (35) of the coupling constants for diacetylene, methyldiacetylene, and dimethyldiacetylene are 5J = 2.2 Hz, 6J’ = 1.27 Hz, 7J” = 1.3 Hz, indicating a value of 5Ja - 0.6 Hz per z system (there are two equivalent n-coupling paths).
METHYL
COUPLING
CONSTANTS
IN
PENTA-
AND
163
HEXADIENES
ACKNOWLEDGMENTS We thank Drs. Castellano and Bothner-By for a copy of the original LACOON computer program and Mr. J. Stokes for discussion and assistance with the computation. Dr. A. Segre kindly gave us details of results in advance of publication. One of us (A.V.C.) is grateful to the U.K. Science Research Council for a Research Studentship, and another of us (P. A.) is indebted to the Royal Norwegian Council for Scientific and Industrial Research for a Research Fellowship. REFERENCES 1. ’ ;: 1. 5. 6. 7. 8. 9. 10.
H. H. M. A. R. R. A. A. D. R.
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