- Email: [email protected]
NMR spectra of the BnAA′Bn′ type
JOUItNAL OF MOLECULAK HPECTHOSCOPY 28, 191-203 (196S)
NMR
Spectra
of the
Studies of But-2-ene
&AA’&
and Related
Type Compounds’
R. K. HARRIS AND B. R. HOWES* School of Chemical Sciences,
University
of East Anglia,
Norwich,
England
The spectra of cis- and trams- but-2-ene and 2-methyl propene have been analyzed. A discussion of the results places emphasis on the allylic and homoallylic coupling constants. The possibility of a “through-space” contribution to the methyl-methyl coupling constant in a-methyl propene is also discussed. The spectra of three trans.1,4-disubstituted but-2-enes have been analyzed; the rotational isomerism about the single bond joining trigonal to tetrahedral carbon is discussed. The possibility of two distinct long-range couplings between the CHzY groups is considered. The initial calculations for all the spectra were made on the basis of the X,AA’X,’ spin system. INTRODUCTION In IWIR studies of a series of related compounds it is important that accurate values for the coupling constants in the parent compound are known. It is with this in mind that the spectra of cis- and trans-but-2-ene and 2-methyl propene have been analyzed. Strictly speaking the spectra are of the BJA’B,’ type, but the chemical shift between the methyl proton resonances and those of the ethylenic protons is large compared to the coupling constants in all three cases, so initial discussion of the spectra on an X&4/X,’ basis is useful. In earlier papers of this series general expressions have been given for the X resonances for spin systems of t,he X,AA’X,’ type in which J xx’ is zero (5’) or small (3). Attempts were made to analyze the spectra of the but-2-enes using the perturbation theory (3) of Part II, but this approach proved to be unsatisfactory and the spin parameters were eventually obtained by making extensive use of computation, starting from informed guesses at the coupling constants. Initial information was obtained (a) by consideration (2) of the appearance of simple X,AA’X,’ spectra with JxxT = 0, (b) the observation of 13C satellites for the methyl resonances (1, L$),and (c) knowledge of the parameters for related compounds. In recording weak outer lines of the spectra of the parent molecules (without 13C) the tech1 The preceding paper in this series is R.ef. (1). 2 Present address: Institution of Electrical Engineers, St,evenage, Herts., England. 191
Southgate
House,
P. 0. Box 8,
192
HARRIS
AND
HOWES
niyue of using high radiofreyuency powers was employed (1). Our final comput’ations were carried out with the correct chemical shift differences, i.e., on a B,AA’B,’ basis. The signs of the (A, A’) and (B, R’) coupling constants relative to those for the (A, I?) parameters were not ohtained with any certainty (51, hut the suggested signs correspond to those reported for related compounds. Special emphasis in this work is placed on a discussion of the long-range coupling constants 4J and 5J in these molecules. Karplus (6) has predicted t)hat the pi-electron contribution to t*he (H, H’) coupling over three carbon at.oms (in t,he allylic system H-C-C=C-H’) and to the coupling over four carbon atoms (in the homoallylic system H-C-C=C-C-H’) is -1.7 and +2.0 cps, respectively. Further work (7) has predicted that t’he pi-electron contrihut’ion to 11 t,he met,hyl-methyl coupling in Z-methyl propene (in the system H-C -(‘-C---H’) is -2.0 cps. Thus “J in t’he but-Zene isomers is expected to he equal in magnitude but opposite in sign t’o 4J in g-methyl propene if t(he simple Karplus theory holds (6). E’urt’her predict’ions are that (a) differences in configuration about, the double bond should not affect t#he a-contributions to 4J and 5.1, and t,hat (b) n-coupling should depend in a cosine fashion on t,he dihedral angle ahout the H-C-C= bond, being zero when t#he sp3 proton is in t,he ethylenic plane. Many pieces of experimental dat,a hearing on these questions are no\v in the literature, hut) it is felt t’hat, the paramet,ers for the symmetrical compounds studied here are of special importance. Cnce coupling constants have been determined for the parent but-Z-cries a study of substit’uent effects becomes feasible. WP have therefore examined the PAIR spectra of three 1,4-disubstituted-trans-but-2-enes [(I), Y = Cl, Rr, (1x1 \vith a view to investigating the rot’ational isomerism (4) Y&C\
,H (2)
‘cd:
/’
“CH,Y (3)
c’f, (1)
ahout the single bond joining trigonal to tetrahedral carbon. Such studies have been undertaken in the past, for ally1 (g-12) and allylidene (11, 12) compounds. We wished to discover whet,her the internal rotation is affected by interaction between the two CHzY groups. Clearly t,his should occur most strongly for t’he cis isomers corresponding to (I), but it is necessary first of all to obtain inforrnation from t,he tmns isomers, which therefore act as model compounds. While t,his \vork was in progress Hecht, and Victor reported (IS) similar studies on ris and trans.1,4-dichlorobut-2-ene. Our parameters for the truns isomer do not. ent,irely agree with those given by Hecht and Victor (13).
&AA’&
NMR
SPECTRA:
193
BUTENES
In analyzing the spectra of the three compounds (I) we assumed initially that the XZAA’XZ’ spin system is appropriate. A similar procedure to that described for the but-2-enes was followed; the final computations were made on a B,AA’B,’ basis. However, it was recognized that in principle the spin system is actually BB’AA’B”B”, since the two protons of a methylene group may be magnetically nonequivalent (as in the simpler analogous case of BrHzC . CH,Cl). In other words, there may be two distinct long-range coupling constants. Computation was undertaken for the dichloro compound to see the effect on the spectrum of having two such couplings. EXPERIMENTAL
METHODS
The spectra were obtained at 100 Rlcps on a Varian HA100 instrument under HA conditions. The compounds cis- and trans-but-2-ene and 2-methyl propene are gases at room temperature and atmospheric pressure; they were introduced into the sample tubes under vacuum, and the tubes then were sealed. All three compounds were run as neat liquids under pressure. Trans-1,4-dichlorobut-2-ene was also examined as a neat liquid. Tram-l ,4-dibromo- and tram-l ,4-dicyanobut-2-ene, crystalline solids at room temperature, were run as 10% solutions in cyclohexane and methyl cyanide, respectively. Tetramethylsilane (TMS) was used as internal reference for all samples; spectra were recorded at ambient probe temperature (ea. 33°C). Chemical shifts are reported as ppm downfield from TMS (&scale). The computations were carried out using an I.C.T. Atlas computer with two programs: (a) a modified version of the iterative program (14) LAOCOON II, which at the time could not treat spin systems containing more than seven nuclei, and (b) the non-iterative program (3) UEA NMR II which uses composite particle factorization and can handle spin systems of eight or more nuclei if there is some magnetic equivalence. The latter program was fitted with a sub-routine (MRPLOT) which assigns a Lorentzian line shape to each transition and computes overall band shapes, producing data suitable for the operation of a BensonLehner graph plotter.
FIG. 1. The methyl
proton
resonances
of cis-but-2-ene
at 100 Mcps
191
HARRIS
AND
HOWEY
FIG. 2. The weak low-field outer lines of the met,hyl proton spectrum of cis-bnt-2.ene at 100 Mcps. (Upper) experimental spectrum using a relatively high radiofrequency power. (Lower) spectrum comput,ed with the parameters of Table I.
FIG. 3. The low-field elle at. 100 Mcps.
13C satellite
resonanres
SPECTRAL
of the methyl
proton
region
for lrans-bnt
-2.
ANALYSIS
(a) c’is- and tram-but-%enc. The procedure described in t,he int8roduction was followed, with emphasis on the X region of the spectra (Fig. 1 shows that of the cis isomer). The principal feature of this region is an intense doublet (2) of spacing 1N 1 = 1Jnx + J.,x, I. The remaining intensity is spread over a large number of lines, which are therefore individually weak. Since / JaA, ( > ( J.,, J.,xf 1 > 1Jxxf 1, the outer groups of lines are the most sensitive t’o J..,,, and must be observed if an accurate analysis is to be made. Figure 2 shows the lowfield outer lines for the cis isomer. The values of the long-range methyl methyl coupling constark were found in each case by observation of the ‘Y) satellites of the CH, region. The principal feature of each of t,hese satellites is a pair of 1: 3:3: 1 quartets derived from the N doublet of the spectrum due to The molecules containing only W. The quart.et spacing gives I Jx,l ] directly. spin q&em is stil1 tightly bound (since / .J,.t, / 2 / 2JCH - 3JcH I); t,he remairling int’ensity is spread over many lines and provides a broad background to the sharp doublet of yuartek The lowfield satellit,e for kans-but-2-ene is sho\vn in Icig. 3. In this case the innermost lines of the t,wo quartets overlap. The program IAOCOOX II was used to obt,ain accurate dat,a by iterat,ion to fit t.he specka. HoLyever, since this could only handle 7 spins, iteration was carried out on lines originating from the G-spin b3aa’b’ sub-spectrum only (Iii). The final iteration fol cis-but-2-cne gave a root-mean-square error of 0.07 cps for Wing 36 trarlsit’ions to 46 observed lines; a similar fit was obtained for the tram isomer. The spectra were then calculat,ed and plotted for the full system using t.he program UEA SSIK II.
&AA’&’
NMR
SPECTRA:
FIG. 4. The methyl proton resonances
FIG. 5. The low-field propene
W
satellite
resonances
BUTENES
of 2-methylpropene
of the methyl
proton
195
at 100 Mcps
region for 2.methyl-
at 100 Mcps.
(b) d-Methyl propene. The analysis of the spectrum of 2-methyl propene presented problems arising from deceptive simplicity. The deceptive simplicity is of the type described in (2) with / L/Jan, ( small, where L = Jax - JAXf . The methyl region is shown in Fig. 4. Observation of the W satellites of this region (Fig. 5), revealed a band with nine absorption maxima of approximate intensities 1:3:4: 5:6:5:4:3: 1. This pattern could be explained on a first-order basis3 as arising from overlapping of a double doublet of quartets with JAx m 3Jxx, and J Ax’ z 2Jxxl ; values for the methyl-methyl and methyl-ethylene couplings accurate to ~0.1 cps were obtained. The spectrum of the parent molecule (with3 Assumptions that W satellites are first order have recently been criticized (1, 16-18) in cases of symmetrical compounds. The assumptions appear to be justified in the present case; probably the difference in the cis and lrans vicinal (C,H) coupling constants is greater than the geminal (H, H) coupling constant (which is FZ 2 cps).
196
HARRIS
AND
HOWES
FIG. 6. The weak low-field outer lines of the CH, proton spectrum of 1,4-dichloro-/ranabut-2.ene: A. experimental trace; B. spectrum computed with a single magnitude (0.86 cps) for 5J, ; C. spectrum computed wit,h two values (0.6 and 1.2 cps) for 5Jt ; I). spectrum computed using the parameters of Ref. (13). The scale is in cps from the chemical shift of the CH, protons. out W) was not analyzed further, but its appearance is consistent with the parameters proposed (Table I). The geminnl coupling constant was not determined. (c) 1 ,d-Disubstituted-trans-but-d-enes. The method of analysis was essentially the same as that employed for the parent, compound, assuming there is 01~1~ one value for the long-range CHZ, CH, coupling constant for each molecule. However, it proved to be impossible to use the 1% satellites of the CH, region to provide a measure of this parameter. When approximate values for t,he other coupling constants in the molecules were known (from the simple theory of the X,AA’X’,’ system and from considerat,ion of the values for related compounds) the long-range coupling was found by comparison of the skucture of the weak outer lines of t.he CH, region with trial speckra computed with UEA NMR II and MRPLOT. The rms errors in t.he final B&A’& iteration with IAOCOOS II were of the order of 0.1 cps. Figure 6 shows the observed and computed lowfield outer CH2 lines for 1,4-dichloro-trans-but-Z-ene. Our parameters for this compound are not identical to t’hose listed in reference (13), particularly for J,, ,’ We believe this is because Hecht’ and Victor did not make use of the weak outer
&AA’&
NMR
SPECTRA: TABLE
CHEMICAL
197
BUTENES
I
SHIFTS (inppm downfield from TMS) AND COUPLING (in cps) FOR THE UNSUBSTITUTED BUTENEP
Compound”
&IT,
Cis-but-2-ene Trans-but-2-ene 2-Methylpropene”
SJJ,
'hII
~~___ 1.541 1.578 1.677
5.368 5.550 4.630
10.88 -
3Jt
3J,
15.09 -
4J,
6.78 6.54 -1.73 1. 35de’
CONSTANTS
4Jt
4J, ~~__
5J,
5Jt
-1.79 0.90de’
0.45d
1.18 -
1.60 -
a For the coupling constant notation see the text. b In principle it is not possible to distinguish between JAB and JAB’ for a &AA’&’ system. In practice, knowledge of related compounds makes distinction easy in the present cases. e 2J was not obtained. d Magnitudes e Assignment
f Whipple
only. uncertain-made
et al.
($1) report
have been done on an ABX,
by comparison
4J, = 4Jt = 1.25
basis,
wit’h related
cps,
and is therefore
TABLE CHEMICAL
SHIFTS
(in ppm
(in CpS) FOR Substituents
downfield
THREE
&HI
%X3
CN Cl
3.1-H 3.998
5.719 5.881
Br
4.446
6.217
a Values given by Hecht and Victor
from
but
15.60 15.09 17.08 15.10
analysis
appears
to
suspect.
II TMS)
1,4-I)ISUnSTITUTED
3Jf
compounds.
the spectral
AND COUPLING
CONSTANTS
trUnS-BUT-2-ENES
3JS 5.68 6.81 7.1” 7.50
4J, -1.77 -1.35 -1.5’ -1.05
&Jt 1.63 0.86 0.9s 0.50
(13).
lines in their analysis. Figure 6D shows that their value for JAAf causes misplacing of these lines. As mentioned in the introduction it may be seen by close inspection of the 1,4disubstituted-trans.but-2-ene molecules that there exists the possibility of two distinct long-range coupling constants between the two CHZY groups. Such a situation would be expected to affect the weak outer lines of the CH, region of the spectrum. The fact that a good fit was obtained by using the assumption that there was only one long-range coupling constant does not necessarily rule out the possibility of two such couplings. It was therefore decided to compute two spectsa (using UEA NMR II) for trans-1,4-dichlorobut-2-ene, one with an average coupling Jxx, equal to that given in Table II and the other with two coupling constants whose average came to Jxxr . The results of this computation for the low-field weak outer lines of trans-1,4-dichlorobut-2-ene are shown in Fig. 6C. 1:tis evident that only part of the spectrum is affected by the introduction of the two long-range couplings; an extra peak is seen, the intensity of which appears to
1%
HARRIS
AND HOWES
have originated from the intense central peak. From these computations it was concluded that if there are two distinct couplings then they differ by <0.3 cps. DISCUSSION
Tables I and II summarize the data obtained in the present study. The coupling const’ants are considered in general t,o be accurate to f0.1 cps. The notation used in referring t’o coupling constants is to indicate the number of formal chemical bonds through which coupling occurs by means of a superscript’ prefix. The orientation of the coupled protons is indicated by a subscript s&ix q, ( or 1 (geminal, cis or trans path across the double bond) or s (through a formal single bond). Thus in structure I .]I4 is denoted 3J, and JL’, is 4J, . Theoretical Considerations Of the values listed in Table I, the allylic and homoallylic coupling constSants are of most interest. The results for the allylic couplings in u's- and trans.but-2 ene are -1.79 and - 1.73 cps, respectively, i.e., equal within experimental error. This is somewhat surprising in view of the fact that in ally1 compounds (8 1,) 4TJ, and 4J, nearly always differ significantly (by 0.42 cps in the case of propene (8) i&elf), 4Jt being higher absolutely. However, other measurements (on met)hylsubst,itut#ed buta-1,3-dienes-see diagram below) in this laboratory4 indicat,c that 4J, is abnormally high when the methyl
=L/ I
I
-i\
14J,
14J+
-
1.4
1.75
‘--ii 1.55 1.65
group is cis to a C-C bond. It’ is in fact slightly higher than 4J, in the examples given. Theoretically, if contributions from u and K electrons are considered independently (6), it has been shown that the K terms are eyual for 4J, and 4J, (pro vided molecular geometry and electronic energy levels are the same in the case?: being compared). Moreover t’he r-contribution was predicted (6) to be -1.7 cps, close to the observed values. However the a-contributions to 4J, and 4*J, are expected (19) to differ significantly (by 0.3-0.4 cps, 4.J, being more positive). It may be that t’he explanation of the apparent anomaly discussed above lies in changes of geometry between cis- and t,ans-but-3-ene. An indicat’ion that’ this may be so is provided by the fact that there is a substantial difference in 4J, and 4cJ, for 2-methglpropene (Table I) whereas it has been reported that these coupling constants are usually equal (33) within experimental error for l-subAtuted isobut-1-enes, RCH: C,lIez . It is interesting to note that thereis a similar anomaly for allylic coupling involving protons at the 1 and 3 positions in 1 ,:l* .4. T. Cunliffe and R. K. Harris,
unpublished
work
&/IA’&,
NMR
SPECTRA:
199
BUTENES
butadienes @%)-the coupling constants through cis and trans paths are only equal wit’hin experimental error for the parent compound. In this case the protons concerned have a fixed geometry-always in the diene plane-whereas in the compounds studied here there are possible complications due to internal rotation of the methyl groups. Several values have been reported in the literature for cis and trans homoallylic coupling constants (21-25). These vary between 1.0 and 1.2 cps for the cis coupling and between 1.4 and 1.6 cps for the truns coupling. The values of 1.18 and 1.60 cps found for cis- and trans-but-2-ene show good agreement with those reported. It is evident that these couplings show remarkable consistency, there being very little change in magnitude on substitution, and the value of the trans coupling being consistently greater than the cis coupling by approximately 0.4 cps. It has been suggested (24) that the difference between the experimental values and the value of 2.0 cps predicted by Karplus (6) might be due to substitution of the olefinic protons. The results for these parent compounds show that this is not the case. It is interesting to speculate upon the origin of the difference in the cis and trctns homoallylic coupling.5 The “residual sigma contribution” (6) attributes any difference to coupling contributions associated with sigma electrons. If it is postulated that these are positive in sign, then this positive contribution would be expected to be larger (25) for the trans coupling (extended zig-zag configuration) and, therefore, the trans coupling would be expected to be greater than the cis coupling, which has always been found to be the case. On the other hand, the coupling due to sigma electrons is known to fall off fairly rapidly with the number of intervening bonds, and therefore the difference of 0.4 cps seems rather large. Alternatively the difference may be due to variations in triplet state energies (affecting the a-coupling) or in geometry between the isomers. The fact that the two possible 5J values for truns-1,4-dichlorobut-2-ene differ by < 0.3 cps is of interest. If only conformations analogous to those for the ally1 compounds (8) are considered, there are nine possible arrangements, which may be classed into four rotamers that are in principle distinguishable. If the two CH,Cl groups do not interact the number of separate species is reduced to three, having two, one or no Cl atoms in the ethylenic plane (degeneracies 1, 4, and 4, respectively). There are four possible values of 5J for fixed mutual orientations between the protons concerned. However the T-contributions to these coupling constants are equal for two orientations and zero for the other two (this occurs when at least one of the coupled protons is in the ethylenic plane (8)). If the two possible observable (average) values of 5J are expressed in terms of rotamer populations and fixed-orientation coupling constants, it can be seen that they are equal under the assumptions that (a) there is no interaction between the CHZY 6 A recent simplified M.O. roughly equal (ca. +1.7 cps).
treatment
(36) of coupling
still predicts
5J, and
6J~ to be
HARRIS
200
AND
HOWES
to coupling may be ignored. The observed groups, and (b) u-contributions qualky or near-equalit’y thus indicates that these assumptions are substantially valid. The methyl-methyl coupling in 2-methyl propene has a magnitude of 0.45 cps (sign unknown). Similar coupling constant)s reported in the literature lie mainl,y in the region of 0.5 cps (26, 87). The theory of Karplus (6) can be extended to include 2-methyl propene, i.e., to the prot’on-proton interaction through sigma bonds geminal to an unsaturated bond. In 2-methyl propene the two groups of protons may be considered to be coupled directly to the same T-electron which orier& the proton spins parallel to each other, thus resulting in a negative conpling. Since in 2-methyl propene [“along r” coupling (24)] and in but-%-ene [“across g” coupling (S4)] the same hyperfine constant plays a part, the CH, _ CH, coupling in 2-methyl propene should be equal to, and opposite in sign to, that in but-2-ene. It has, 011 the other hand, been suggested (34) that t,he presence (Jf a polarized double bond may be a necessary prerequisite for effective COUpling of t)his t,ype. The present measurements t’ogether with the value of 0.17 cps found4 for 2,4-dimethyl-2,4-pentadiene, sholv that t’his is not the case. There has been considerable discussion (24, $8) regarding t)he possibility of a “t,hrough,spnce” contribution (i.e., through direct, interaction het,ween the electron clouds of the nuclei concerned) to the CH3-CH3 coupling in acetone and other similar compounds. At this stage t’he evidence for “t,hrough-space” coupling between geminal groups is inconclusive. The results in this paper show that, the value of less in magnitude than the -2.0 ~~CII:!.CH3 iII Z-methyl propene is considerably cps predicted by Iiarplus (6), whereas the corresponding couplings in the but-Zenes are much closer in magnitude t’o the +2.0 cps predict#ed. Until furt,her evidence has been submitted it would seem wise at t)his point merely to say th;Lt, npxt from t#he negative pi coupling, there is also a positive sigma coupling between geminal met#hyl groups. TABLE <:OMPARISON
THE Vrcr.va~
OF
FOR
H Cr\; Cl Br a Ref. (8). h Ii. C. Hint, p Ref. (19). d Ref. (10).
III
ALLYLK:
CORRESPONDING
ALLYL
Ally1compounds
_________._
~
:?I,
4J,
6.40s 5.40h 6 56~” 7.26”
-1.75” -1.90” -1.41c” -1.2”
private
COUPLING
CONSTANTS
tlanS-l,1-I)ISUIBSTITUT~D BUT-~-ENES THE
Substituents
AND
communication
quoted
(it1rps)
AKD
COMPOUXDS
Hut-2.enes
~~
:‘J, 6.54 5.68 6.81 7.50
in Ref. (10).
4J,. -1.73 -1.77 -1.35 -1.05
-
BJA’B,’
NMR
SPECTRA:
201
BUTENES
Conformational Considerations In a series of papers (8-12) by Bot’hner-By et al. the coupling constants for ally1 compounds have been interpreted in conformational terms. Their population analysis was based on the three conformers (II), (III) and (IV).6 If it is assumed that the vicinal coupling constant,
I (II)
gauche rotamers ”
I (III)
cis rotamer
(IV)
J, , between gauche-oriented protons is the same for both rotamers, it can be shown that the observed (average) coupling, 3J,, is given by 3J, = $i(l
-
p)J1*O+ ;$(l
+ p)J60,
where p is the fractional population of the cis rotamer; JIB0 and J60 are the coupling constants between trans- and gauche-oriented protons, respectively (though it is probable that the gauche dihedral angle is not exactly SO”). Since in general Jlso > J60 the value of 3JJ,gives an indication of p. However, it must be emphasized that although the above relation has been used quantitatively the results are subject to a considerable error because of the relatively small range of values observed for 3J, and the uncertainties in J60 and J’*Ocaused by the effects of substituent electronegativity. On t’he other hand the general trends are clear: the population of the cis rotamer increases as the size of Y decreases, and as the electronegativity of Y increases. For the substituent’s used here p increases (8-12) in the order CN > H > Cl > Br. Our results for t,he substituted but-2-enes show that this order is preserved in this series also. Indeed the values of 3J, , given in Table III are very close in magnitude t’o those for the corresponding ally1 compounds. This is not surprising since very little steric interaction would be expected between the two CH,Y groups when bans oriented. However, the coupling constants in the but-2-ene series are between 0.2 and 0.3 cps greater than for the corresponding ally1 compounds. This difference could result from one of two effects: firstly, there may be a decrease in p, or secondly, the change may be a more direct effect of the substitution of the second CH,Y group. It is felt that substantial population changes are unlikely, but it should be noted that in going from propene to trans-but-2-ene, 3Js increases only by approximately 0.1 cps. Propenes substituted in the trans position by halogens have (29, SO) values of 3J, 0.4-0.6 cps higher than for propene itself. The allylic coupling con6The notation of Ref. (32) is used in the three structural formulas-the mutual orientation of C=C and C-Y is denot,ed. The distinction between the term cis rotamer and the term cis isomer (referring to a configurational isomer of a substituted but.ene-2) should be noted.
"02
HARRIS
AND
HOWE8
stants of corresponding members of the two series of compounds of Table III do not differ significantly (except possibly for the bromo-derivatives), t’hus arguing against substant’ial changes in p. Such differences as are observed would, on the other hand, correspond to slight decreases in p and are thus consistent with the variations in 3J, . One would expect far more steric interaction between the CHaIT rot,ors in t’he cis-but-2-enes. Hecht and Victor (13) have investigated this effect for the cis and buns-l ,4-dichlorobut-2-enes. They found that 3J, is ca. 0.8 cpn greater for the cis isomer (their value for the trans isomer is 0.3 cps higher than that reported here). This was taken to indicate a considerable change in p (a decrease, as expected on steric grounds). They also suggested that int’eraction between the CHZI’ groups might, cause some skewing of the rotors from t’he conformation analogous to (V). H.. 7 HJ\
H, ,H
c=c
I-i
&H \ H
(v) However, such skewing should not affect the coupling constant’s in t,he but-2-enes t)hemselves providing the tetrahedral angles at the methyl carbon atoms are re tained. This statement is based on the accept)ed (8,19) linear dependence of such coupling constants on cosine and cosine2 (or sine and sine2) functions of dihedral angles. We find that the vicinal coupling 3.J, in cis-but-2-ene is marginally greater (by ca. 0.2 cps) than in frans-but-2-ene. It is probable t’hat effects other than skewing, such as an actual repulsion between the methyl groups (result)ing in an increase in t’heir spatial separation) are operative. In the case of the l-halopropenes t,he value of 3J, for the cis isomer is (29, 50) invariably loruer (by O.lpO.F, cps) than for the frans isomer. The conformational conclusions drawn from the results of t’he vi&al couplings for the butene derivatives are also consistent with t#heresults of t)he long-range allylic couplings. It is pointed out by Bothner-By and Giinther (10) that a quantitative treatment based on the allylic couplings is unlikely to be valid since small changes occasioned by the different rotamer populations are easily obscured by the changes occasioned by the different electron-withdrawing abilities of the substituent groups. However, the variations in the allylic coupling constant in the series ally1 cyanide, chloride and bromide clearly show a trend in the ant,icipated direction; decreasing magnitude with increasing population of the qauch~ form. Parallel variat’ions are found for the butene series studied here. It is seen that the coupling between the bans CH21’ groups in the dicyano, dichloro and dibromo compounds decreases from 1.63 to 0.86 to 0.5 cps. Since the average pi coupling through the homoallylic system will be high for the cis orientation of each rotor and we have alreadv est,ablished from the vicinal cow
&AA’&’
NMR
SPECTRA:
BUTENES
203
plings that the populations of this orientation in these compounds follows the pattern CN > Cl > Br, then the trend in the values for the homoallylic couplings is in accordance with that expected. In fact this coupling constant would appear to be of greater use in discussions of conformation than 4J, . ACKNOWLEDGEMENTS The Science Research Council is thanked for an Advanced Course Studentship to ene of us (B.R.H.), and for use of the Atlas computer. We are grateful to Dr. A. V. Cunliffe for many fruitful discussions.
RECEIVED
April 9,1968 REFERENCES
1. E. G. FINER AND R. K. HARRIS, MoZ. Phys. 13, 65 (1967). 2. R. K. HARRIS, Can. J. Chem. 42, 2275 (1964). 5. R. K. HARRIS BND C. M. WOODMAN,MO~. Phys. 10,437 (1966). 4. E. G. FINER AND R. K. HARRIS, Mol. Phys. 12, 457 (1967). 5. R. M. LYNDEN-BELL, Mol. Phys. 6, 601 (1963). 6’. M. KARPLUS, J. Chem. Phys. 33, 1842 (1960). ‘Y. J. R. HOLMES AND D. KIVELSON, J. Am. Chem. Sot. 63, 2959 (1961). 8. A. A. BOTHNER-BY AND C. NAAR-COLIN, J. Am. Chem. Sot. 63, 231 (1961). 3. A. A. BOTHNER-BY,C.NAAR-COLINANDH.G~~NTHER,J. Am.Chem.Soc.t34,2748 (1962). 10. A. A. BOTHNER-BY AND H. GUENTHER,Discussions Faraday Sot. 34, 127 (1962). 11. A. A. BOTHNER-BY, S. CASTELLANO, ,YNDH. GUENTHER,J. Am. Chem. Sot., 87, 2439 (1965). 12. A. A. BOTHNER-BY, 8. CASTELLANO, S. EBERSOLE, AND H. GUNTHER, J. Am. Chem. Sot. 88, 2466 (1966). 19. H. G. HECHT AND R. L. T-ICTOR, J. Am. Chem. Sot. 89, 2532 (1967). 14.A. A. BOTHNER-BY AND S. CASTELL~NO, J. Chem. Phys. 41, 3863 (1964). I,?. P. DIEHL, R. K. HARRIS, AND R. G. JONES, Progr. NMR Spectry. 3, 1 (1967). 16. E. A. HILL AND J. D. ROBERTS, J. Am. Chem. Sot., 89, 2047 (1967). 17. R. K. HfZRRISAND C. M. WOODM.~N, J. Mol. Spectry. 26, 432 (1968). 18. G. GOVIL, J. Chem. Sot., A, 1967, 1416. 29. M. BARFIELD, J. Chem. Phys., 41, 3825 (1964). 20. A. T’. CUNLIFFE BND R. K. HARRIS, Mol. Phys. 13, 269 (1967). 21. R. It. FRASER, Can. J. Chem. 38, 549 (1960). 22. J.H. RICHARDS ‘ZNDW. H. BEXH, J. Org. Chem. 26, 623 (1961), and unpublished resulls quoted therein. 29. R. A. HOFFMAN AND S. GRONO~ITZ, Acta Chem. &and. 13, 1477 (1959). 84. W. H. DE JEU, R. DEEN, )\NDJ. SMIDT,Rec. Tree. Chim. 86, 33 (1967). 26. C. N. BAN~ELL .~NDN. SHEPP.IRD, Discussions Faraday sot. 34, 115 (1962). 26. N. \'a~ MEURS, Spectrochim. Acta 19, 1695 (1963). 27. R. F. C. BROWN, I.D. RAE, AND S. STERNHELL, Australian J. Chem. 18, 61 (1965). 28. M. ANTEUNIS AND D. TAVERNIER, Bull.sot.Chim. Beiges 76, 432 (1967). 29. R. A. BE.~UDETAND J. D. B~LDESCHWIELER, J.Mol. Spectry. 9, 30 (1962). SO. F. HRUSKA, D. W. MCBRIDE, AND T. SCHAEFER, Can. J. Chem. 46, 1081 (1967). Sf. E.B. WHIPPLE, J. H. GOLDSTEIN,AND L.MANDELL, J. Am. Chem. Sot. 82, 3010 (196nj, 52. R. D. MCLACHLAN AND R. A. NYQUIST, Spectrochim. Acla 24A, 103 (1967). SS. L. M. J.~CKMANAND R.. H. WILEY, J. Chem. Sot. 1960, 2881. 34. H. ROTTENDORF, S. STERNHELL, AND J. R. WILMSHURST, Australian J. Chem.18, 1759 (1965). $6. W. T. DIXON, J. Chem. Sot., B 1967, 1879.
NMR
Spectra
of the
Studies of But-2-ene
&AA’&
and Related
Type Compounds’
R. K. HARRIS AND B. R. HOWES* School of Chemical Sciences,
University
of East Anglia,
Norwich,
England
The spectra of cis- and trams- but-2-ene and 2-methyl propene have been analyzed. A discussion of the results places emphasis on the allylic and homoallylic coupling constants. The possibility of a “through-space” contribution to the methyl-methyl coupling constant in a-methyl propene is also discussed. The spectra of three trans.1,4-disubstituted but-2-enes have been analyzed; the rotational isomerism about the single bond joining trigonal to tetrahedral carbon is discussed. The possibility of two distinct long-range couplings between the CHzY groups is considered. The initial calculations for all the spectra were made on the basis of the X,AA’X,’ spin system. INTRODUCTION In IWIR studies of a series of related compounds it is important that accurate values for the coupling constants in the parent compound are known. It is with this in mind that the spectra of cis- and trans-but-2-ene and 2-methyl propene have been analyzed. Strictly speaking the spectra are of the BJA’B,’ type, but the chemical shift between the methyl proton resonances and those of the ethylenic protons is large compared to the coupling constants in all three cases, so initial discussion of the spectra on an X&4/X,’ basis is useful. In earlier papers of this series general expressions have been given for the X resonances for spin systems of t,he X,AA’X,’ type in which J xx’ is zero (5’) or small (3). Attempts were made to analyze the spectra of the but-2-enes using the perturbation theory (3) of Part II, but this approach proved to be unsatisfactory and the spin parameters were eventually obtained by making extensive use of computation, starting from informed guesses at the coupling constants. Initial information was obtained (a) by consideration (2) of the appearance of simple X,AA’X,’ spectra with JxxT = 0, (b) the observation of 13C satellites for the methyl resonances (1, L$),and (c) knowledge of the parameters for related compounds. In recording weak outer lines of the spectra of the parent molecules (without 13C) the tech1 The preceding paper in this series is R.ef. (1). 2 Present address: Institution of Electrical Engineers, St,evenage, Herts., England. 191
Southgate
House,
P. 0. Box 8,
192
HARRIS
AND
HOWES
niyue of using high radiofreyuency powers was employed (1). Our final comput’ations were carried out with the correct chemical shift differences, i.e., on a B,AA’B,’ basis. The signs of the (A, A’) and (B, R’) coupling constants relative to those for the (A, I?) parameters were not ohtained with any certainty (51, hut the suggested signs correspond to those reported for related compounds. Special emphasis in this work is placed on a discussion of the long-range coupling constants 4J and 5J in these molecules. Karplus (6) has predicted t)hat the pi-electron contribution to t*he (H, H’) coupling over three carbon at.oms (in t,he allylic system H-C-C=C-H’) and to the coupling over four carbon atoms (in the homoallylic system H-C-C=C-C-H’) is -1.7 and +2.0 cps, respectively. Further work (7) has predicted that t’he pi-electron contrihut’ion to 11 t,he met,hyl-methyl coupling in Z-methyl propene (in the system H-C -(‘-C---H’) is -2.0 cps. Thus “J in t’he but-Zene isomers is expected to he equal in magnitude but opposite in sign t’o 4J in g-methyl propene if t(he simple Karplus theory holds (6). E’urt’her predict’ions are that (a) differences in configuration about, the double bond should not affect t#he a-contributions to 4J and 5.1, and t,hat (b) n-coupling should depend in a cosine fashion on t,he dihedral angle ahout the H-C-C= bond, being zero when t#he sp3 proton is in t,he ethylenic plane. Many pieces of experimental dat,a hearing on these questions are no\v in the literature, hut) it is felt t’hat, the paramet,ers for the symmetrical compounds studied here are of special importance. Cnce coupling constants have been determined for the parent but-Z-cries a study of substit’uent effects becomes feasible. WP have therefore examined the PAIR spectra of three 1,4-disubstituted-trans-but-2-enes [(I), Y = Cl, Rr, (1x1 \vith a view to investigating the rot’ational isomerism (4) Y&C\
,H (2)
‘cd:
/’
“CH,Y (3)
c’f, (1)
ahout the single bond joining trigonal to tetrahedral carbon. Such studies have been undertaken in the past, for ally1 (g-12) and allylidene (11, 12) compounds. We wished to discover whet,her the internal rotation is affected by interaction between the two CHzY groups. Clearly t,his should occur most strongly for t’he cis isomers corresponding to (I), but it is necessary first of all to obtain inforrnation from t,he tmns isomers, which therefore act as model compounds. While t,his \vork was in progress Hecht, and Victor reported (IS) similar studies on ris and trans.1,4-dichlorobut-2-ene. Our parameters for the truns isomer do not. ent,irely agree with those given by Hecht and Victor (13).
&AA’&
NMR
SPECTRA:
193
BUTENES
In analyzing the spectra of the three compounds (I) we assumed initially that the XZAA’XZ’ spin system is appropriate. A similar procedure to that described for the but-2-enes was followed; the final computations were made on a B,AA’B,’ basis. However, it was recognized that in principle the spin system is actually BB’AA’B”B”, since the two protons of a methylene group may be magnetically nonequivalent (as in the simpler analogous case of BrHzC . CH,Cl). In other words, there may be two distinct long-range coupling constants. Computation was undertaken for the dichloro compound to see the effect on the spectrum of having two such couplings. EXPERIMENTAL
METHODS
The spectra were obtained at 100 Rlcps on a Varian HA100 instrument under HA conditions. The compounds cis- and trans-but-2-ene and 2-methyl propene are gases at room temperature and atmospheric pressure; they were introduced into the sample tubes under vacuum, and the tubes then were sealed. All three compounds were run as neat liquids under pressure. Trans-1,4-dichlorobut-2-ene was also examined as a neat liquid. Tram-l ,4-dibromo- and tram-l ,4-dicyanobut-2-ene, crystalline solids at room temperature, were run as 10% solutions in cyclohexane and methyl cyanide, respectively. Tetramethylsilane (TMS) was used as internal reference for all samples; spectra were recorded at ambient probe temperature (ea. 33°C). Chemical shifts are reported as ppm downfield from TMS (&scale). The computations were carried out using an I.C.T. Atlas computer with two programs: (a) a modified version of the iterative program (14) LAOCOON II, which at the time could not treat spin systems containing more than seven nuclei, and (b) the non-iterative program (3) UEA NMR II which uses composite particle factorization and can handle spin systems of eight or more nuclei if there is some magnetic equivalence. The latter program was fitted with a sub-routine (MRPLOT) which assigns a Lorentzian line shape to each transition and computes overall band shapes, producing data suitable for the operation of a BensonLehner graph plotter.
FIG. 1. The methyl
proton
resonances
of cis-but-2-ene
at 100 Mcps
191
HARRIS
AND
HOWEY
FIG. 2. The weak low-field outer lines of the met,hyl proton spectrum of cis-bnt-2.ene at 100 Mcps. (Upper) experimental spectrum using a relatively high radiofrequency power. (Lower) spectrum comput,ed with the parameters of Table I.
FIG. 3. The low-field elle at. 100 Mcps.
13C satellite
resonanres
SPECTRAL
of the methyl
proton
region
for lrans-bnt
-2.
ANALYSIS
(a) c’is- and tram-but-%enc. The procedure described in t,he int8roduction was followed, with emphasis on the X region of the spectra (Fig. 1 shows that of the cis isomer). The principal feature of this region is an intense doublet (2) of spacing 1N 1 = 1Jnx + J.,x, I. The remaining intensity is spread over a large number of lines, which are therefore individually weak. Since / JaA, ( > ( J.,, J.,xf 1 > 1Jxxf 1, the outer groups of lines are the most sensitive t’o J..,,, and must be observed if an accurate analysis is to be made. Figure 2 shows the lowfield outer lines for the cis isomer. The values of the long-range methyl methyl coupling constark were found in each case by observation of the ‘Y) satellites of the CH, region. The principal feature of each of t,hese satellites is a pair of 1: 3:3: 1 quartets derived from the N doublet of the spectrum due to The molecules containing only W. The quart.et spacing gives I Jx,l ] directly. spin q&em is stil1 tightly bound (since / .J,.t, / 2 / 2JCH - 3JcH I); t,he remairling int’ensity is spread over many lines and provides a broad background to the sharp doublet of yuartek The lowfield satellit,e for kans-but-2-ene is sho\vn in Icig. 3. In this case the innermost lines of the t,wo quartets overlap. The program IAOCOOX II was used to obt,ain accurate dat,a by iterat,ion to fit t.he specka. HoLyever, since this could only handle 7 spins, iteration was carried out on lines originating from the G-spin b3aa’b’ sub-spectrum only (Iii). The final iteration fol cis-but-2-cne gave a root-mean-square error of 0.07 cps for Wing 36 trarlsit’ions to 46 observed lines; a similar fit was obtained for the tram isomer. The spectra were then calculat,ed and plotted for the full system using t.he program UEA SSIK II.
&AA’&’
NMR
SPECTRA:
FIG. 4. The methyl proton resonances
FIG. 5. The low-field propene
W
satellite
resonances
BUTENES
of 2-methylpropene
of the methyl
proton
195
at 100 Mcps
region for 2.methyl-
at 100 Mcps.
(b) d-Methyl propene. The analysis of the spectrum of 2-methyl propene presented problems arising from deceptive simplicity. The deceptive simplicity is of the type described in (2) with / L/Jan, ( small, where L = Jax - JAXf . The methyl region is shown in Fig. 4. Observation of the W satellites of this region (Fig. 5), revealed a band with nine absorption maxima of approximate intensities 1:3:4: 5:6:5:4:3: 1. This pattern could be explained on a first-order basis3 as arising from overlapping of a double doublet of quartets with JAx m 3Jxx, and J Ax’ z 2Jxxl ; values for the methyl-methyl and methyl-ethylene couplings accurate to ~0.1 cps were obtained. The spectrum of the parent molecule (with3 Assumptions that W satellites are first order have recently been criticized (1, 16-18) in cases of symmetrical compounds. The assumptions appear to be justified in the present case; probably the difference in the cis and lrans vicinal (C,H) coupling constants is greater than the geminal (H, H) coupling constant (which is FZ 2 cps).
196
HARRIS
AND
HOWES
FIG. 6. The weak low-field outer lines of the CH, proton spectrum of 1,4-dichloro-/ranabut-2.ene: A. experimental trace; B. spectrum computed with a single magnitude (0.86 cps) for 5J, ; C. spectrum computed wit,h two values (0.6 and 1.2 cps) for 5Jt ; I). spectrum computed using the parameters of Ref. (13). The scale is in cps from the chemical shift of the CH, protons. out W) was not analyzed further, but its appearance is consistent with the parameters proposed (Table I). The geminnl coupling constant was not determined. (c) 1 ,d-Disubstituted-trans-but-d-enes. The method of analysis was essentially the same as that employed for the parent, compound, assuming there is 01~1~ one value for the long-range CHZ, CH, coupling constant for each molecule. However, it proved to be impossible to use the 1% satellites of the CH, region to provide a measure of this parameter. When approximate values for t,he other coupling constants in the molecules were known (from the simple theory of the X,AA’X’,’ system and from considerat,ion of the values for related compounds) the long-range coupling was found by comparison of the skucture of the weak outer lines of t.he CH, region with trial speckra computed with UEA NMR II and MRPLOT. The rms errors in t.he final B&A’& iteration with IAOCOOS II were of the order of 0.1 cps. Figure 6 shows the observed and computed lowfield outer CH2 lines for 1,4-dichloro-trans-but-Z-ene. Our parameters for this compound are not identical to t’hose listed in reference (13), particularly for J,, ,’ We believe this is because Hecht’ and Victor did not make use of the weak outer
&AA’&
NMR
SPECTRA: TABLE
CHEMICAL
197
BUTENES
I
SHIFTS (inppm downfield from TMS) AND COUPLING (in cps) FOR THE UNSUBSTITUTED BUTENEP
Compound”
&IT,
Cis-but-2-ene Trans-but-2-ene 2-Methylpropene”
SJJ,
'hII
~~___ 1.541 1.578 1.677
5.368 5.550 4.630
10.88 -
3Jt
3J,
15.09 -
4J,
6.78 6.54 -1.73 1. 35de’
CONSTANTS
4Jt
4J, ~~__
5J,
5Jt
-1.79 0.90de’
0.45d
1.18 -
1.60 -
a For the coupling constant notation see the text. b In principle it is not possible to distinguish between JAB and JAB’ for a &AA’&’ system. In practice, knowledge of related compounds makes distinction easy in the present cases. e 2J was not obtained. d Magnitudes e Assignment
f Whipple
only. uncertain-made
et al.
($1) report
have been done on an ABX,
by comparison
4J, = 4Jt = 1.25
basis,
wit’h related
cps,
and is therefore
TABLE CHEMICAL
SHIFTS
(in ppm
(in CpS) FOR Substituents
downfield
THREE
&HI
%X3
CN Cl
3.1-H 3.998
5.719 5.881
Br
4.446
6.217
a Values given by Hecht and Victor
from
but
15.60 15.09 17.08 15.10
analysis
appears
to
suspect.
II TMS)
1,4-I)ISUnSTITUTED
3Jf
compounds.
the spectral
AND COUPLING
CONSTANTS
trUnS-BUT-2-ENES
3JS 5.68 6.81 7.1” 7.50
4J, -1.77 -1.35 -1.5’ -1.05
&Jt 1.63 0.86 0.9s 0.50
(13).
lines in their analysis. Figure 6D shows that their value for JAAf causes misplacing of these lines. As mentioned in the introduction it may be seen by close inspection of the 1,4disubstituted-trans.but-2-ene molecules that there exists the possibility of two distinct long-range coupling constants between the two CHZY groups. Such a situation would be expected to affect the weak outer lines of the CH, region of the spectrum. The fact that a good fit was obtained by using the assumption that there was only one long-range coupling constant does not necessarily rule out the possibility of two such couplings. It was therefore decided to compute two spectsa (using UEA NMR II) for trans-1,4-dichlorobut-2-ene, one with an average coupling Jxx, equal to that given in Table II and the other with two coupling constants whose average came to Jxxr . The results of this computation for the low-field weak outer lines of trans-1,4-dichlorobut-2-ene are shown in Fig. 6C. 1:tis evident that only part of the spectrum is affected by the introduction of the two long-range couplings; an extra peak is seen, the intensity of which appears to
1%
HARRIS
AND HOWES
have originated from the intense central peak. From these computations it was concluded that if there are two distinct couplings then they differ by <0.3 cps. DISCUSSION
Tables I and II summarize the data obtained in the present study. The coupling const’ants are considered in general t,o be accurate to f0.1 cps. The notation used in referring t’o coupling constants is to indicate the number of formal chemical bonds through which coupling occurs by means of a superscript’ prefix. The orientation of the coupled protons is indicated by a subscript s&ix q, ( or 1 (geminal, cis or trans path across the double bond) or s (through a formal single bond). Thus in structure I .]I4 is denoted 3J, and JL’, is 4J, . Theoretical Considerations Of the values listed in Table I, the allylic and homoallylic coupling constSants are of most interest. The results for the allylic couplings in u's- and trans.but-2 ene are -1.79 and - 1.73 cps, respectively, i.e., equal within experimental error. This is somewhat surprising in view of the fact that in ally1 compounds (8 1,) 4TJ, and 4J, nearly always differ significantly (by 0.42 cps in the case of propene (8) i&elf), 4Jt being higher absolutely. However, other measurements (on met)hylsubst,itut#ed buta-1,3-dienes-see diagram below) in this laboratory4 indicat,c that 4J, is abnormally high when the methyl
=L/ I
I
-i\
14J,
14J+
-
1.4
1.75
‘--ii 1.55 1.65
group is cis to a C-C bond. It’ is in fact slightly higher than 4J, in the examples given. Theoretically, if contributions from u and K electrons are considered independently (6), it has been shown that the K terms are eyual for 4J, and 4J, (pro vided molecular geometry and electronic energy levels are the same in the case?: being compared). Moreover t’he r-contribution was predicted (6) to be -1.7 cps, close to the observed values. However the a-contributions to 4J, and 4*J, are expected (19) to differ significantly (by 0.3-0.4 cps, 4.J, being more positive). It may be that t’he explanation of the apparent anomaly discussed above lies in changes of geometry between cis- and t,ans-but-3-ene. An indicat’ion that’ this may be so is provided by the fact that there is a substantial difference in 4J, and 4cJ, for 2-methglpropene (Table I) whereas it has been reported that these coupling constants are usually equal (33) within experimental error for l-subAtuted isobut-1-enes, RCH: C,lIez . It is interesting to note that thereis a similar anomaly for allylic coupling involving protons at the 1 and 3 positions in 1 ,:l* .4. T. Cunliffe and R. K. Harris,
unpublished
work
&/IA’&,
NMR
SPECTRA:
199
BUTENES
butadienes @%)-the coupling constants through cis and trans paths are only equal wit’hin experimental error for the parent compound. In this case the protons concerned have a fixed geometry-always in the diene plane-whereas in the compounds studied here there are possible complications due to internal rotation of the methyl groups. Several values have been reported in the literature for cis and trans homoallylic coupling constants (21-25). These vary between 1.0 and 1.2 cps for the cis coupling and between 1.4 and 1.6 cps for the truns coupling. The values of 1.18 and 1.60 cps found for cis- and trans-but-2-ene show good agreement with those reported. It is evident that these couplings show remarkable consistency, there being very little change in magnitude on substitution, and the value of the trans coupling being consistently greater than the cis coupling by approximately 0.4 cps. It has been suggested (24) that the difference between the experimental values and the value of 2.0 cps predicted by Karplus (6) might be due to substitution of the olefinic protons. The results for these parent compounds show that this is not the case. It is interesting to speculate upon the origin of the difference in the cis and trctns homoallylic coupling.5 The “residual sigma contribution” (6) attributes any difference to coupling contributions associated with sigma electrons. If it is postulated that these are positive in sign, then this positive contribution would be expected to be larger (25) for the trans coupling (extended zig-zag configuration) and, therefore, the trans coupling would be expected to be greater than the cis coupling, which has always been found to be the case. On the other hand, the coupling due to sigma electrons is known to fall off fairly rapidly with the number of intervening bonds, and therefore the difference of 0.4 cps seems rather large. Alternatively the difference may be due to variations in triplet state energies (affecting the a-coupling) or in geometry between the isomers. The fact that the two possible 5J values for truns-1,4-dichlorobut-2-ene differ by < 0.3 cps is of interest. If only conformations analogous to those for the ally1 compounds (8) are considered, there are nine possible arrangements, which may be classed into four rotamers that are in principle distinguishable. If the two CH,Cl groups do not interact the number of separate species is reduced to three, having two, one or no Cl atoms in the ethylenic plane (degeneracies 1, 4, and 4, respectively). There are four possible values of 5J for fixed mutual orientations between the protons concerned. However the T-contributions to these coupling constants are equal for two orientations and zero for the other two (this occurs when at least one of the coupled protons is in the ethylenic plane (8)). If the two possible observable (average) values of 5J are expressed in terms of rotamer populations and fixed-orientation coupling constants, it can be seen that they are equal under the assumptions that (a) there is no interaction between the CHZY 6 A recent simplified M.O. roughly equal (ca. +1.7 cps).
treatment
(36) of coupling
still predicts
5J, and
6J~ to be
HARRIS
200
AND
HOWES
to coupling may be ignored. The observed groups, and (b) u-contributions qualky or near-equalit’y thus indicates that these assumptions are substantially valid. The methyl-methyl coupling in 2-methyl propene has a magnitude of 0.45 cps (sign unknown). Similar coupling constant)s reported in the literature lie mainl,y in the region of 0.5 cps (26, 87). The theory of Karplus (6) can be extended to include 2-methyl propene, i.e., to the prot’on-proton interaction through sigma bonds geminal to an unsaturated bond. In 2-methyl propene the two groups of protons may be considered to be coupled directly to the same T-electron which orier& the proton spins parallel to each other, thus resulting in a negative conpling. Since in 2-methyl propene [“along r” coupling (24)] and in but-%-ene [“across g” coupling (S4)] the same hyperfine constant plays a part, the CH, _ CH, coupling in 2-methyl propene should be equal to, and opposite in sign to, that in but-2-ene. It has, 011 the other hand, been suggested (34) that t,he presence (Jf a polarized double bond may be a necessary prerequisite for effective COUpling of t)his t,ype. The present measurements t’ogether with the value of 0.17 cps found4 for 2,4-dimethyl-2,4-pentadiene, sholv that t’his is not the case. There has been considerable discussion (24, $8) regarding t)he possibility of a “t,hrough,spnce” contribution (i.e., through direct, interaction het,ween the electron clouds of the nuclei concerned) to the CH3-CH3 coupling in acetone and other similar compounds. At this stage t’he evidence for “t,hrough-space” coupling between geminal groups is inconclusive. The results in this paper show that, the value of less in magnitude than the -2.0 ~~CII:!.CH3 iII Z-methyl propene is considerably cps predicted by Iiarplus (6), whereas the corresponding couplings in the but-Zenes are much closer in magnitude t’o the +2.0 cps predict#ed. Until furt,her evidence has been submitted it would seem wise at t)his point merely to say th;Lt, npxt from t#he negative pi coupling, there is also a positive sigma coupling between geminal met#hyl groups. TABLE <:OMPARISON
THE Vrcr.va~
OF
FOR
H Cr\; Cl Br a Ref. (8). h Ii. C. Hint, p Ref. (19). d Ref. (10).
III
ALLYLK:
CORRESPONDING
ALLYL
Ally1compounds
_________._
~
:?I,
4J,
6.40s 5.40h 6 56~” 7.26”
-1.75” -1.90” -1.41c” -1.2”
private
COUPLING
CONSTANTS
tlanS-l,1-I)ISUIBSTITUT~D BUT-~-ENES THE
Substituents
AND
communication
quoted
(it1rps)
AKD
COMPOUXDS
Hut-2.enes
~~
:‘J, 6.54 5.68 6.81 7.50
in Ref. (10).
4J,. -1.73 -1.77 -1.35 -1.05
-
BJA’B,’
NMR
SPECTRA:
201
BUTENES
Conformational Considerations In a series of papers (8-12) by Bot’hner-By et al. the coupling constants for ally1 compounds have been interpreted in conformational terms. Their population analysis was based on the three conformers (II), (III) and (IV).6 If it is assumed that the vicinal coupling constant,
I (II)
gauche rotamers ”
I (III)
cis rotamer
(IV)
J, , between gauche-oriented protons is the same for both rotamers, it can be shown that the observed (average) coupling, 3J,, is given by 3J, = $i(l
-
p)J1*O+ ;$(l
+ p)J60,
where p is the fractional population of the cis rotamer; JIB0 and J60 are the coupling constants between trans- and gauche-oriented protons, respectively (though it is probable that the gauche dihedral angle is not exactly SO”). Since in general Jlso > J60 the value of 3JJ,gives an indication of p. However, it must be emphasized that although the above relation has been used quantitatively the results are subject to a considerable error because of the relatively small range of values observed for 3J, and the uncertainties in J60 and J’*Ocaused by the effects of substituent electronegativity. On t’he other hand the general trends are clear: the population of the cis rotamer increases as the size of Y decreases, and as the electronegativity of Y increases. For the substituent’s used here p increases (8-12) in the order CN > H > Cl > Br. Our results for t,he substituted but-2-enes show that this order is preserved in this series also. Indeed the values of 3J, , given in Table III are very close in magnitude t’o those for the corresponding ally1 compounds. This is not surprising since very little steric interaction would be expected between the two CH,Y groups when bans oriented. However, the coupling constants in the but-2-ene series are between 0.2 and 0.3 cps greater than for the corresponding ally1 compounds. This difference could result from one of two effects: firstly, there may be a decrease in p, or secondly, the change may be a more direct effect of the substitution of the second CH,Y group. It is felt that substantial population changes are unlikely, but it should be noted that in going from propene to trans-but-2-ene, 3Js increases only by approximately 0.1 cps. Propenes substituted in the trans position by halogens have (29, SO) values of 3J, 0.4-0.6 cps higher than for propene itself. The allylic coupling con6The notation of Ref. (32) is used in the three structural formulas-the mutual orientation of C=C and C-Y is denot,ed. The distinction between the term cis rotamer and the term cis isomer (referring to a configurational isomer of a substituted but.ene-2) should be noted.
"02
HARRIS
AND
HOWE8
stants of corresponding members of the two series of compounds of Table III do not differ significantly (except possibly for the bromo-derivatives), t’hus arguing against substant’ial changes in p. Such differences as are observed would, on the other hand, correspond to slight decreases in p and are thus consistent with the variations in 3J, . One would expect far more steric interaction between the CHaIT rot,ors in t’he cis-but-2-enes. Hecht and Victor (13) have investigated this effect for the cis and buns-l ,4-dichlorobut-2-enes. They found that 3J, is ca. 0.8 cpn greater for the cis isomer (their value for the trans isomer is 0.3 cps higher than that reported here). This was taken to indicate a considerable change in p (a decrease, as expected on steric grounds). They also suggested that int’eraction between the CHZI’ groups might, cause some skewing of the rotors from t’he conformation analogous to (V). H.. 7 HJ\
H, ,H
c=c
I-i
&H \ H
(v) However, such skewing should not affect the coupling constant’s in t,he but-2-enes t)hemselves providing the tetrahedral angles at the methyl carbon atoms are re tained. This statement is based on the accept)ed (8,19) linear dependence of such coupling constants on cosine and cosine2 (or sine and sine2) functions of dihedral angles. We find that the vicinal coupling 3.J, in cis-but-2-ene is marginally greater (by ca. 0.2 cps) than in frans-but-2-ene. It is probable t’hat effects other than skewing, such as an actual repulsion between the methyl groups (result)ing in an increase in t’heir spatial separation) are operative. In the case of the l-halopropenes t,he value of 3J, for the cis isomer is (29, 50) invariably loruer (by O.lpO.F, cps) than for the frans isomer. The conformational conclusions drawn from the results of t’he vi&al couplings for the butene derivatives are also consistent with t#heresults of t)he long-range allylic couplings. It is pointed out by Bothner-By and Giinther (10) that a quantitative treatment based on the allylic couplings is unlikely to be valid since small changes occasioned by the different rotamer populations are easily obscured by the changes occasioned by the different electron-withdrawing abilities of the substituent groups. However, the variations in the allylic coupling constant in the series ally1 cyanide, chloride and bromide clearly show a trend in the ant,icipated direction; decreasing magnitude with increasing population of the qauch~ form. Parallel variat’ions are found for the butene series studied here. It is seen that the coupling between the bans CH21’ groups in the dicyano, dichloro and dibromo compounds decreases from 1.63 to 0.86 to 0.5 cps. Since the average pi coupling through the homoallylic system will be high for the cis orientation of each rotor and we have alreadv est,ablished from the vicinal cow
&AA’&’
NMR
SPECTRA:
BUTENES
203
plings that the populations of this orientation in these compounds follows the pattern CN > Cl > Br, then the trend in the values for the homoallylic couplings is in accordance with that expected. In fact this coupling constant would appear to be of greater use in discussions of conformation than 4J, . ACKNOWLEDGEMENTS The Science Research Council is thanked for an Advanced Course Studentship to ene of us (B.R.H.), and for use of the Atlas computer. We are grateful to Dr. A. V. Cunliffe for many fruitful discussions.
RECEIVED
April 9,1968 REFERENCES
1. E. G. FINER AND R. K. HARRIS, MoZ. Phys. 13, 65 (1967). 2. R. K. HARRIS, Can. J. Chem. 42, 2275 (1964). 5. R. K. HARRIS BND C. M. WOODMAN,MO~. Phys. 10,437 (1966). 4. E. G. FINER AND R. K. HARRIS, Mol. Phys. 12, 457 (1967). 5. R. M. LYNDEN-BELL, Mol. Phys. 6, 601 (1963). 6’. M. KARPLUS, J. Chem. Phys. 33, 1842 (1960). ‘Y. J. R. HOLMES AND D. KIVELSON, J. Am. Chem. Sot. 63, 2959 (1961). 8. A. A. BOTHNER-BY AND C. NAAR-COLIN, J. Am. Chem. Sot. 63, 231 (1961). 3. A. A. BOTHNER-BY,C.NAAR-COLINANDH.G~~NTHER,J. Am.Chem.Soc.t34,2748 (1962). 10. A. A. BOTHNER-BY AND H. GUENTHER,Discussions Faraday Sot. 34, 127 (1962). 11. A. A. BOTHNER-BY, S. CASTELLANO, ,YNDH. GUENTHER,J. Am. Chem. Sot., 87, 2439 (1965). 12. A. A. BOTHNER-BY, 8. CASTELLANO, S. EBERSOLE, AND H. GUNTHER, J. Am. Chem. Sot. 88, 2466 (1966). 19. H. G. HECHT AND R. L. T-ICTOR, J. Am. Chem. Sot. 89, 2532 (1967). 14.A. A. BOTHNER-BY AND S. CASTELL~NO, J. Chem. Phys. 41, 3863 (1964). I,?. P. DIEHL, R. K. HARRIS, AND R. G. JONES, Progr. NMR Spectry. 3, 1 (1967). 16. E. A. HILL AND J. D. ROBERTS, J. Am. Chem. Sot., 89, 2047 (1967). 17. R. K. HfZRRISAND C. M. WOODM.~N, J. Mol. Spectry. 26, 432 (1968). 18. G. GOVIL, J. Chem. Sot., A, 1967, 1416. 29. M. BARFIELD, J. Chem. Phys., 41, 3825 (1964). 20. A. T’. CUNLIFFE BND R. K. HARRIS, Mol. Phys. 13, 269 (1967). 21. R. It. FRASER, Can. J. Chem. 38, 549 (1960). 22. J.H. RICHARDS ‘ZNDW. H. BEXH, J. Org. Chem. 26, 623 (1961), and unpublished resulls quoted therein. 29. R. A. HOFFMAN AND S. GRONO~ITZ, Acta Chem. &and. 13, 1477 (1959). 84. W. H. DE JEU, R. DEEN, )\NDJ. SMIDT,Rec. Tree. Chim. 86, 33 (1967). 26. C. N. BAN~ELL .~NDN. SHEPP.IRD, Discussions Faraday sot. 34, 115 (1962). 26. N. \'a~ MEURS, Spectrochim. Acta 19, 1695 (1963). 27. R. F. C. BROWN, I.D. RAE, AND S. STERNHELL, Australian J. Chem. 18, 61 (1965). 28. M. ANTEUNIS AND D. TAVERNIER, Bull.sot.Chim. Beiges 76, 432 (1967). 29. R. A. BE.~UDETAND J. D. B~LDESCHWIELER, J.Mol. Spectry. 9, 30 (1962). SO. F. HRUSKA, D. W. MCBRIDE, AND T. SCHAEFER, Can. J. Chem. 46, 1081 (1967). Sf. E.B. WHIPPLE, J. H. GOLDSTEIN,AND L.MANDELL, J. Am. Chem. Sot. 82, 3010 (196nj, 52. R. D. MCLACHLAN AND R. A. NYQUIST, Spectrochim. Acla 24A, 103 (1967). SS. L. M. J.~CKMANAND R.. H. WILEY, J. Chem. Sot. 1960, 2881. 34. H. ROTTENDORF, S. STERNHELL, AND J. R. WILMSHURST, Australian J. Chem.18, 1759 (1965). $6. W. T. DIXON, J. Chem. Sot., B 1967, 1879.