Chemical shifts of stilbene derivatives

JOURNAL OF MOLECULAR SPECTROSCOPY 6, 85-91

Chemical

Shifts

of Stilbene

XIIKIO

Department

of Physics,

(1960)

Derivatives

KATATAMA

Duke University,

Dwhatn,

North Carolina

SND SHIZUO

I~TJJIWARA, .~ND

University The proton

magnetic

HIROSHI OWMU

SUZUKI,

of Electra-(‘ommunirations, resonance

TOICHI

KAGAI,

SIMAMURA

chemical

Tokyo,

Japarl

shifts of stilhene

derivatives

have

observed in carbon tetrachloride, cyclohexane, and henzene solutions. The observed shifts were compared with those calculated on the hasis of Pople’s model of circulating r-electrons, assuming various kinds of molecular structures.

been

1NTROI)UCTION

Stilbene and its derivatives exist in t,wo isomeric forms, the trans form and the cis form. Except for a few of the derivatives, the structure of these molecules is not known exactly. An extensive st,udy of ultraviolet, absorption specka of t,hese molecules was made by three of t#heauthors ( 1)) and the deviations of the structure from the planar form were est,imated by simple quantum mechanical calculations from the observed positions of t,he conjugation bands. Proton magnetic resonance was found to be useful in certain cases to investigate the steric effect and t,o distinguish between the cis-tram isomeric configurations. For example, the shifts of proton magnetic resonance of alkyl nitrites were observed by Piette et al (W), and the resonance spectra, which differ from each other, were obtained for the cis and the trans forms. The differences in the spectra mere interpreted as due to the formation of intramolecular hydrogen bonds in t,he cis form. Curtin et al. (3) observed the difference bet,ween chemical shifts of t,he cis and trans isomers of stilbene and those of some aromatic compounds and point,ed out, the usefulness of magnetic resonance to distinguish between the isomeric configurations. The general relat,ionship between the chemical shifts and t)he structure of the trans and cis forms was not established. EXPERIXIEKTAL

In t,he present article, proton magnetic resonance chemical shifts in stilbene derivatives have been observed and the general relationship bet,ween the chemical shifts and molecular struct,ures has been established. 85

KATAYAMA,

86

FUJIWARA,

SUZUKI,

SA(:AI

.4n’D SIMAMURA

Jlaterials measured are as follows: LY,cr’-dimethylstilbeue, 1 ,A’-dichloro-a ,cY’dimethylstilbene, 4,4’-dibromo-cr ,Lu’-dimethylstilhene, 4 ,A’-diiodo-a ,a’-dimethylstilbene, 4,4’-dimethoxy-a ,cr’-dimethylstilhene and a ,a’-diet,hylstilbene. These compounds exist in both the cis and the tray forms. All the compounds were prepared in our laboratories. Measurements were made at 27.0 Mc/sec by the high resolut,ion, slow sweep set-up constructed in our laboratory. It is of the same t,ype as the one described by Gutowsky et al. (4). Solutions of the materials in carbon tetrachloride were used with concentration of about 15%. The chemical shifts for these molecules are shown in Table I, in which the shifts of cr-protons are expressed in cps with reference to the shift of t,he aromatic protons. It is interesting to note that the shifts in Table I are larger for t)he trans form t.han for t,he cis form, except for that of stilbene. It may be ment)ioned that this result in t,he case of stilbene is in conformity with the measurements of Curt,in et al. (3). In order to determine the relative shifts of aromatic protons of the stilbene derivatives with reference to t,he shift of benzene, solutions in cyclohexane were used, and the shifts of aromatic protons were compared with those of benzene in cyclohexane. In solutions of cyclohexane, only cx,cr’-dimethylstilbene, cis-4 ,+I’dibromo-cu, a’-dimethylstilbene, cis- and trans-4,4’-dichloro-a ,cY’-dimethylstilbene and trans-cr-methylst,ilbene gave suitable concentratBion for the observation of the proton resonance signal. Table II gives bhe observed results of t’he shifts of aromatic protons referred to the shift of proton resonance of benzene in cyclohexane solutions. The shifts of the trans forms were observed at nearly the same magnet’ic field as that for benzene, while the shifts of the cis form were observed at a frequency several cps higher than that for benzene. The corrections to be made for volume susceptibility are only 3-3 cps in t’his case. It may, t’herefore, be safely said that the aromatic protons of the trans form are less shielded than those of the cis form The close contact. of two benzene rings in the cis form would TABLE

I

CHEMICAL SHIFTS OF (Y- AND CHI-P~o~o~~ REFERRED TO AROMATIC PROTONS IN THE SAME MOLECULE (27 Mc/secj Shift, cps Compound Tram Stilbene a ,a’-Dimethylstilbene 4,4’-Dichloro-ol,J-dimethylstilbene 4,4’-Dibromo-CL,&dimethylstilbene 4,4’-Diiodo-a,a’-dimethylstilbene

9 141 147 145 150” 148 (3,3’-HI 173 (CH,) 140 (CHx)

U,CZ’-Diethylstilbene

a Referred

to 2,2’-aromatic

protons.

CiS 18 128 125 132 132& 14” (3,3’-H) 163 (CH,) 131 (CHz)

nmr OF STILBENE

DERIVATIVES

TABLE

87

II

SHIFTS OF AROMATICPROTONSJN CYCLOHEXANESOLUTIONSREFERRED TO BENZENE (MEASUREDAT 27.0 Mc/sec) Shift, cps

Compound Trans-a,ol’-dimethylstilbene Cisa,a’-dimethylstilbene

0 +8

Trans.-l, 4’.dichloro-ol, a’-dimethylstilbene C&4,4’-dichloro-a,a’-dimethylstilbene Cis-4,4’-dibromo-cu,cu’-dimethylstilbene Trans-wmethylstilbene

+2 +6 $6 0

affect the magnetic

shielding of aromatic

culations in t.he following

protons, as will be shown by the cal-

sections. It is also evident from Tables I and II t,hat

protons of CHs groups of the tram form are more shielded than those of the cis form in stilbene derivatives.

The shifts could not be measured in the same solvent

for all the compounds because of solubility t’o be used for different

compounds,

limitations.

Different

and the shifts were obtained

solvent’s have by the two-

step method. SHIFT DUE TO T-ELECTRON

RING CURRENT

The results of the present experiment,s show that a close similarity tween the proton magnetic

resonance shifts shown in Table

absorption spectra of these compounds. Table III of the ultraviolet

spectra observed

exists be-

I and ultraviolet

shows the absorption maxima

by Suzuki, Nagai,

and Simamura.

Wave-

lengths of the absorption maxima of t,he cis isomers are longer than those of t’he corresponding maximum

tram isomers. Only in stilbene does the tram isomer have the than does t#he cis isomer. Table III also gives

at longer wavelength

the nuclear magnetic resonance shifts of t,he a-proton. TABLE

III

ABSORPTIONMAXIMA OF ULTRAVIOLETSPECTRAOF STILBENE DERIVATIVES ANU THE NUCLEAR MAGNETIC RESONANCESHIFTS (27 MC/see) OF THE ~-PROTONS Absorption maxima, A Shift, cps

Stilbene cY-Methylstilbene (Y,a’-Dimethylstilbene 4,4’-Dichloro-ol,a’-dimethylstilbene 4,4’-Dibromo-ol,a’-dimethylstilbene 4,4’-Diiodo-ol,a’-dimethylstilbene 4,4’-Dimethoxy-a,a’-dimethylstilbene ~,cx’-Diethylstilbene

Trans

Cis

2940 2720

2780 2670

-14 -

2435 2510 2500 2530 2490

2520 2580 2580 2630 2700

+5 +I8 +8 -

2375

2440

+8

88

IiATAYARL.4, FUJIW.4RA.

SUZUkiI, NAG.41 ANI) SI?rIA!dUI~:\

The nmlecuhr structure is known completely for ouly a few compounds. Cryatal structure analysis of trans-stilhene was reported by Robertson and Woodn-nrd (5). It) proved that two species of molecules are contained in the unit cell, but the structure of the two molecules is almost the same, i.e., the two benzene rings of the molecule are almost coplanar in hoth molecules. The crystal structure of trans-a ,a’-dimethylst,ilhene was analyzed by Katnyama (6 1. In this case the two benzene rings in the molecules are not coplanar. If we denote t’he angle between the plane of the ethylenic double bond and we may set 8i = 0? = 0 in that of each benzene ring as Bi and %?, respectively, t)he kzns-stilhene molecule. On the other hand, an analysis of cx,a’-dimethylstilhene shows that the angles of twist, & and t$ are nearly 70”, which is nearly in agreement8 wit’h the value calculated by a simple molecular orhital method. The molecular struct,ures of cis isomers have not heen analyzed in detail. However, the angle of twist may he calculated to he about 30” for the cis form of stilhene, using the experimental results of ultraviolet absorption spectra. This value of the angle fits well into the model constructed by using the appropriate van der Waal’s radii. In this comrection it may he mentioned that calculations of the proton magnetic resonance shifts of a-proton and aromatic protons, assuming the various molecular models, lead to very interesting results. In all these calculat’ions t,he molecular structures are assumed to he the same in solid or liquid phases. It has been well estahlished that the nuclear magnetic resonance frequency depends on the electronic environment of the nucleus and that the chemical shifts are due to magnetic shielding by the electron clouds. The distribution of electrons around a given nucleus changes from one compound to another depending upon the nature of t,he bonding to the rest of the molecule, and the change in electSron densit)y varies the effective magnetic field at, the nucleus. Aromatic molecules hav:e another contrihut’ion to the magnetic shielding caused by circulating currents of the ?r-electrons. For an estimate of this contribution in stilbene derivatives, the magnetic field at the positjions of the hydrogen nuclei caused by the ?r-electron circulating current, in benzene rings were calculated by assuming various values of the angle 0. The method of calculation is similar to the methods of Johnson and Rovey (‘?) and Waugh and E’essenden (8). The origin of the cylindrical coordinate system is taken to he the center of the hrnzene ring, and the p-axis is taken to he perpendicular to the plane of the benzene ring. The benzene ring is considered, for convenience of the calculation, to he a circle of radius a. The magnetic field H, at the position (p,z) may he expressed as follows : HZ

=

-!

2

a (1 + P)’ + 9

Ii =

(1

+;;.+2?

(1)

nmr OF STILBENE

89

DERIVATIVES

where p and z are expressed in units of a, and K(Ii) and E(k) are the elliptical integrals of t.he first and second kind. The circulating current, I around t.he henzene ring is then given as I=_

- ne?Ho &me ’

(2)

where Ho is the st,atic magnet,ic field and n is the number of s-electrons. The chemical shift. 6’, expressing in parts per million the difference between the positions of the resonance peaks for aromatic protons and for a close olefinic analog (in which no circulat,ing current exists but in which the bond hybridization is the same), is given by H,rr/H, , where Heff is t.he average projection of H, 011 the applied magnetic field. In carrying out the averaging, it must be recalled that both the current’ I and the projection of the resulting field on Ho nil1 diminish as the cosine of the angle between the plane of the loop and Ho . The results are as follows: 6’

x

lo-”

9

1

ATrrLU [( 1 + p)” +

= ___

x211’2[

K(L)

+ (; I

2

;)2-+zZ,

E(k)

I . (3)

The shifts, 6’, of Lu-protons in st.ilbene and its derivat,ives were calculated as a function of the angle of twist 0. Int’eraction of the distant, benzene ring with the a-proton was not considered as it is not significant. The results are shown in Fig. 1. The curve (a) represents 6’ of protons of cr-CHZ group in st.ilbene derivatives, and the curve (b) represents 6’ of a-proton stilbene. The curve (c) is obtained by calculating the magnetic field at the cent,er of a benzene ring caused by the r-electron currents on the other benzene ring. The shift. of the methyl groups was obtained at the position corresponding to the center of the circle described by the free rotation of the methyl group protons. It is well known that t,he a-electron cloud does not’ have its maximum density in t,he plane of the carbon atoms but it exists rather in two doughnut-like rings, one on each side. The spacing of these rings is not known, but it, is probably of the order of 1 A. Waugh and Fessenden (7) and Johnson and Bobey (8) have ,calculated the spacing by using a model consisting of two doughnuts, one above and one below the benzene ring, carrying the ring current. The curves (a) and (b) in Fig. 1 were calculated by using a-electron currents in the plane of the carbon atoms, for it is confirmed that, the shift) is not so sensitive to the spacing of the two doughnut-like ring currents. The curve (c) is, on the ot.her hand, sensitive to the spacing. The spacing of 1.2 A was used, which is the same value used by Johnson and Bovey (7). It has been assumed that t.he absolute values of OLand 02 are equal. The relative sign of angles of twist O1and es in the cis form could not he obtained by these calculations. As has been ment.ioned already, Tables I and II show that the a-prot,ons of the trans form of cr,c~‘-dimethylstilbene and its derivatives are more shielded

90

KATAYAhIA,

FUJIWARA,

SUZUKI,

30°

KAGAI

60’

AX’L)

SO’

8+ FIG. 1. The calculated

chemical

shifts 6’ as a function

of the angle of twist

e.

than the a-protons of the cis form of these compounds. This may be explained as due to the fact that the angles of t,wist 0 are smaller in the cis form than in the trans form, for the calculations based on the diamagnetic circulating currents of *-electrons have shown that the larger the angle of twist, the larger of the magnetic shielding. The conclusion that the angle 0 is larger in the trans form of a ,a’-dimethylstilbenze than in t’he cis form is in accordance with the results of ultraviolet absorpt’ion. On the other hand, the a-protons of trans stilbene are less shielded than those of cis stilbene. This may be explained as due to t,he fact t.hat the angle of twist 0 is 0” in the trans form while ~9in the cis form cannot become 0” because of the steric interference between two benzene rings. It may be recalled that the analysis of the ultraviolet absorpt,ion spectra shows that the angle of twist, of cisstilbene is about 30”. If we assume 13= 70” in the tram form and 0 = 30” in the cis form for (Y,cY’dimethylstilbene, the calculated difference of bhe shifts Al? is about 0.14. The observed difference of the shifts between the tram and t.he cis forms is 0.18, which is in rough agreement with the calculated value. Although the angle e of the cis form is not precisely determined experimentally, it is assumed larger than 30” from the analysis of t,he ultraviolet absorption spectra. If we assume a value for 0 larger than 30” for cis isomer, the calculated difference of the shifts should

nmr OF STILBENE

91

DERIVATIVES

become less than 0.14, and it is concluded that’ the effect of the diamagnetic circulating current of s-electrons is only a part of t’he shifts, although it would he the larger part. In the case of CY,~Y’-dimethylstilbene and its derivatives the aromatic protons of the 15s form are more shielded diamagnetically than t’hose of the trans form. The shifts of aromatic protons could be explained by the magnetic shielding at t,he center of the benzene ring produced by the circulating n-electron current,s in the other benzene ring. The curve (c) of Fig. 1 shows that for the cis isomer the planar form has resonance shifts at lower magnetic field and t,hat when 0 = 40” t,he shifts become equal to the resonance shift of benzene. This may be taken t’o indicate t’hat t’he angle 0 of the cis forms may be slightly more than 40”. The results are thus found to be in general agreement with the results of the ultraviolet, absorption spectra. However, we can conclude that the angle 8 in a! ,a’dimethylst,ilbene is larger in the tram form than in t,he cis form. For stilbene, quantitative agreement, between t’he observed and calculated shifts is not quite satisfact#ory. It is known that a-protons are much affected by the conjugation of the bonding electrons, which is dependent on the angle 8. The electron density on the a-proton varies with the angle 0. This may account for the lack of agreement between the observed and calculated results in the case of stilbene. ACBNOWLEDGMENT I am deeply indebted to Professor Walter Gordy for his interest Narasimha Rao for reading the mannscript. RECEIVED:

January

in this work and to Dr.

19, 1960. REFERENCES

1. H. SUZUKI, Bull. Chem. Sot. Japan 26, 145 (1952); Y. NAGAI AND 0. SIMAMAURA (unpublished). 2. L. H. PIETTE, J. D. RAY AND R. A. OGG, JR., J. Chem. Phys. 26,134l (1957). 8. D. Y. CURTIN, H. GRUEN, AND B. A. SHOULDERS, Chemistry & Industry 1205 (1958). 4. H. S. GUTOWSKY, L. H. MEYER, AND R. E. MCCLURE, Rev. Sci. Instr. 24, 644 (1953). 5. J. M. ROBERTSON AND I. WOODWARD, Proc. Roy. Sot. 162, 568 (1937). 6. M. KATAYAMA (unpublished). 7. C. E. JOHNSON, JR., AND F. A. BOVEY, J. Chem. Phys. 29, 1012 (1958). 8. J. S. WAUGH AND R. W. FESSENDEN, J. Am. Chem. Sot. 79,846 (1957).