The characteristic infrared absorption bands of cis-stilbene and its p,p'-disubstituted derivatives

Spectrocllimicn Actn,1968,Vol. 1!1,j,p. 1403to 1471.Prr_wmonPressLtd. Printedin SortllernIrelanll

The characteristic

infrared absorption bands of cis-stilbene its p,p’-disubstituted derivatives M. &I

Department

and

and H. KUNIMOTO

of Chemistry, Faculty of Science, The University Bunkyo-ku, Tokyo, Japan

of Tokyo

(Received 11 January 1963) Abstract-A characteristic absorption band is found for cis-stilbene and its p,p’-disubstituted derivatives in the 900-850 cm-l region. The position of the band moves to higher wave numbers when electron attracting substituents are introduced and to lower wave numbers with electron releasing substituents. The tendency may be reasonably explained from the orbital following concept. However, the isotope effect is very small when hydrogen atoms at c(- and a’-positions are replaced by deuteriums, the fact suggesting that the band at 900-850 cm-l is not purely originated from the CH out -of-plane deformation vibration.

ALTHOUGH the absorption due to the CH out-of-plane deformation vibrations of the ethylenic group in trans-stilbenes is clearly assigned and is used for the confirmation of the structure of the unknown stilbene derivatives, the assignment of the absorption due to the CH deformation of cis-ethylenic moiety has not been established. There is much confusion about the assignment and many investigators [l, 2, 31 have insisted that the band appearing as low as 700 cm-l should be assigned to the CH out-of-plane deformation of cis-olefins. There is only one paper pertaining to the absorption band of the ethylenic group of the cis-stilbenes which bear a substituent at 2-position. DETAR and CARPINO [4] found that it is possible to use the bands at 795-720 cm-’ and 920 cm-l for the diagnostic purpose of the cis-stilbenes, but no further proof has been obtained. It has also been recognized that the cis-compound shows a weak absorption at the position where the trans-compound absorbs strongly and the band has been attributed to the trans-compound, which exists as an impurity, by many authors [5, 6, 71. In the course of other studies, the present authors prepared various kinds of cis-stilbenes and measured their infrared spectra as liquid films or KBr disks. It is found that all the compounds show a characteristic band in the range of 900-850 cm-l.

[l] L. J. BELLAMY, The Infrared Spectra of Complex [2] [3] [4] [5] [6] 171

Molecules (2nd ed.) p. 34. Methuen, New York (1958). K. NAKANISHI, InfruredS~ectra--QuaZitatiwe Analyses and Exercises p. 30. Nankodo, Tokyo (1960). W. J. POTTS and R. A. XYQUIST, Spectrochim. Acta 9, 676 (1959). D. F. DETAR and L. A. CARPINO, J. Am. Chem. Sot. 79, 475 (1956). A. R. PHILPOTT and W. THAIN, Nature 166,1028 (1950). N. SHEPPARD and G. B. B. SUTHERLAND, Proc. Roy. Sot. A 196, 195 (1949). Infrared T. SHIMANOUCHI, Analysis of Infrared Spectra p. 56. Nankodo, Tokyo (1960); Spectra and Ramun Effects (1st ed.) p. 58. Kyoritsu Shuppan, Tokyo (1958). 1463

-

M. OKI and H. KUNIMOTO

1464

The origin of the absorption may naturally be considered to be the CH out-of-plane deformation of the ethylenic group. In this paper, further proof for this assignment will be presented and the assignment will be discussed in relation with the variation of hybridization. EXPERIMENTAL 1. Preparation

of materials

In any measurement of the absorption spectra of cis- and trans-isomers, the In this work, all the cis-stilbenes purity of the materials is particularly important. except p,p’-dinitro-cis-stilbene were prepared by partial hydrogenation of the corresponding tolan derivatives and were purified by fractional distillation in vacua and/or recrystallization from appropriate solvents. p,p’-Dinitro-cis-stilbene was prepared according to the method described for the preparation of similar compounds by RUGGLI et al. The details of the preparation of samples have been dealt elsewhere [S, 91. 2. Ultra-violet spectral measurements The measurement was carried out with a Beckman model DU spectrometer in order to check the purity of the new compounds, ethanolic solution being used. The ultraviolet spectra provide sufficient support for the scarce presence of the trans-compound if any. 3. Infrared spectral measurements The spectra were recorded as liquid film or KBr disks with a Hitachi model EPI 2 and a Nihonbunko model DS 301 spectrometer provided with NaCl optics. The spectral range from 650 t,o 4000 cm-l were covered with these instruments. The wavelength calibration was made by the use of the sharp peaks of indene, 1,2,4trichlorobenzene and polystyrene. RESULTS AND DISCUSSION

The results of the measurements are summarized in Tables l-4 and Figs. l-4. The absorption bands due to the CH out-of-plane deformation of benzene nucleus appears in the range of 860-800 cm-l in the cases of p,p’-disubstituted cis- and transstilbenes, whereas the band appears in the range of 760-750 cm-i in cis- and transstilbenes. This assignment is supported by the results of theoretical calculation and it is widely accepted that p-disubstituted benzenes show absorptions at 860-800 cm-l and monosubstituted benzenes at 770-730 cm-l [l]. Therefore, it should be comprehended that the absorption band appearing in the range 900-850 cm-l is characteristic of the &s-form. BELLAMY [lo] indicated that the wave numbers where the CH out-of-plane deformation absorption bands of 1,4-, 1,3- and 1,3,5_substituted benzenes appear are in linear relation with the sums of the Hammett’s sigma constants of the substituents, and that the bands are shifted by the substituents of large sigma values [8] H. KUNIMOTO,Nippon Kugaku Z&s&i. 84, 60 (1963). [9] H. KUNIMOTO,Nippon Kagaku Zasshi. 84, 65 (1963). [lo] L. J. BELLAMY,J. Chenz.Sot. 2818 (1955).

Characteristic Table

1. The infrared

absorption

p-R-C,H,CH=CHC,H,-K-p

980-950

trans

absorption

cm-’

958

(s.)

cis

936

(V.W.)

954

(V.W.)

tram9

953

(m.)

965

(m.)

900-850

cm-l

1465

bands of cis-stilbene

spectra of cis- and trans-stilbenes

CL3 R=H

infrared

860-800 cm-’

in the 1000-690 cm-l region 800-760 cm-l

760-690

cm-’

865 (IL)

-

751 (m.) 778 (VA)

-

-

761 (VA)

688 (v.9.)

781 (A.)

680 (V.W.) 703 (V.W.)

873 (a.)

830

(V.S.)

700 (V.8.)

R -OCH, 743 (w.)

873 (m.)

832 (m.)

772 (v.w.)

-

833 (s.)

-

-

827 (8.)

783 (III.)

722 (m.)

683 (v.w.) 701 (V.W.)

cis

917 (V.W.) 936 (v.w.)

tra?L3

920 (m.) 936 (m.)

cis

949 (V.W.) 964 (v.w.)

886 (m.)

tram

969 (v.s.)

-

825 (v.s.)

-

-

cis

952 (V.W.) 974 (V.W.)

887 (m.)

834 (V.S.)

774 (In.)

-

traras

948 (V.W.) 971 (VA)

-

828 (v.s.) 837 (s.)

-

-

cis

953 (V.W.) 968 (v.w.)

884 (m.)

854 (s.)

804 (m.)

trana

938 (m.) 968 (m.)

-

852 (w.)

779 (V.W.)

R =OCH,OCH,

R=CH,

-

833 (v.8.)

R =Br

R =OCOCH,

cis R =COOC,H,

895 (TV.)

681 (V.W.) 708 (v.w.) -

860 (w.)

774 (m.)

697 (v.w.) 709 (w.)

856 (w.) 977 (TV.)

-

862 (w.)

782 (m.)

-

trans

949 (w.)

-

858 (w.)

782 (m.)

703 (xv.)

cis

956 (v.w.) 971 (V.W.)

834 (VA) 856 (VS.)

764 (w.)

-

trans

951 (In.) 971 (In.)

-

831 (vs.)

-

cis

977 (V.W.)

897 (s.)

831 (w.) 858 (v.s.)

777 (w.)

695 (w.) 717 (In.)

trans

954 (w.) 972 (w.)

-

851 (v.s.) 858 (8.)

754 (In.)

692 (m.)

trans cis R =COOH

R=CN

K=XOz

895 (v.s.)

720 (V.W.)

to the lower wave numbers. That is, the more electron attracting substituent gives the band at the lower wave numbers for the out-of-plane deformation. He made also clear that this relationship was applicable for the compounds containing an unsaturated group conjugated with the aromatic ring, but not for the other type of compounds. The wave numbers, where the CH out-of-plane deformation vibration bands of p-disubstituted benzenes or the characteristic absorption bands of p,p,‘-disubstituted

M. OKI

1486

and H.

A

KUNIMOTO

mp

Fig. 1. The ultraviolet p,p’-dimethoxy-cis-stilbeno,

absorption spectra of p,p’-dimethoxy-trams_stilbene, p,p’-dimethoxymethoxy-cis-stilbene, and p,p’diacetoxy-cis-stilbene. --__-p,p’-dimethoxy-trans-stilbene - - p,p’-dimethoxy-cis-stilbene -p,p’-dimethoxymethoxy-cis-stilbene __ - p,p’-diacetoxy-cis-stilbene

Table

2. The absorptions due to the CH out-of-plane vibration of p-disubstituted benzenes

The absorptions due to the CH out-of-plane deformation vibration (cm-l)

p-R-C&H&H, C,H,CHO * R = OCH, R = OCH,OCH, trans CH,OC,H,CH=CHCH,-p* R = CH, R =Br R = OCOCH, R = COOH R = CN p-NCC,H,C-CC&H&N-p* R =NO, * These are spaSally substituents conjugating

deformation

748 819 818 842 794 807 842 843 814 839 850

selected as examples which contain the unsaturated with the benzene ring.

Characteristic

01220’

infrared

absorption

I

I

I

250

300

350

1, Fig. 2. The ult,raviolet

absorption

1467

bands of cis-stilbena

m/L

spectra

of 2.‘,~‘-dinitro-f,ans-stilbmc:

and p,p’-

dinitro-cis-stilbrne. p,p’-dinitro-truns-stilbene p,p’-tlinitro-cis-stilbene

-_-___

Table

3. The absorptions

p-H--C,H,CD

4XX,H,--H-p

R=H

of cis-a,a’-dideuterostilbencs 980-960

cm-’

966

(v.w.)

984

(V.W.)

-

R =OCH,

900 -831

cm-’

in the range of 1000-690 cm-’

860-800

cm-’

800-760

cmr’

760-960

cm-l

825 (s.)

741 (V.S.) 754 (VA)

694 (v.s.)

857 (v.B.)

832 (s.)

i80 (m.) 813 (In.)

700 (V.W.)

-

R =CH,

947

964 (v.w.)

856 (s.)

818 (s.) 824 (In.)

7G9 (m.) 808 (tn.)

R =Br

953 (v.w.)

859 (v.s.)

832 (VA)

769 (w.) 780 (n.)

(V.W.)

-

789 (w.) R =COOC,H, R =CN

959

(V.W.)

875 (In.)

857 (TV.)

776 (s.) 790 (w.)

705 (w.)

873 (VA)

836 (8.) 850 (s.)

773 (xv.)

713 (w.)

I

850

Wave number,

I

040

I

a30 cm-1

,

860

I

670

J

660

Fig. 3. The relationship between the aromatic CH out-ofplane deformation vibration of p,p’-disubstituted cis-stilbenes and the Hammett’s sigma constants [lo] of the substituents. Unbroken line, the corresponding p,p’-disubstituted cis-stilbenes. Dotted line, the corresponding p,p’-disubstituted cis-CL&dideuterostilbenes.

620

660

l

890 cm-f

860

Wave number,

670

900

Fig. 4. The relationship between the characteristic spectra of p,p’-disubstituted cis-stilbenes in the range of 900850 cm and the Hammett’s sigma constants [lo] of the substituente. the corresponding p,p’-disubstituted Unbroken line, cis-stilbenes. Dotted line, the corresponding p,p’-disubstituted cis-a,a’dideuterostilbenes.

850

Characteristic infrared absorption bands of cis-stilbene

1469

Table 4. Comparison of the absorption5 of the aromatic CH out-of-plane deformation vibrations and the characteristic absorption bands of cis- and trans-stilbenes in the range of 1000-690 cm-l P-R-CBH,CH=CHC,H,-IZ-p

980-850

cm-’

cis cis

cm-’

860-800

cm-l

865 (m.)

R=H wan.3

900-850

958 (8.) -

760-690

cm-’

751 (In.)

873 (s.)

800-760 cm-’

830 (VA)

761 (vs.) -

R =OCH,

trans cis

953

(m.)

965

(m.)

trans cis

920

trans

969

-

833 (s.)

-

873 (m.)

827 (s.)

-

R =OCH,OCH, (In.) 936 (In.) -

R =CH,

cis

(vs.) -

-

825 (vs.)

885 (m.)

834 (VA)

-

887 (In.)

837 (ES.) 854 (s.)

-

R ==Br

trans cis

(V.W.) 971 (vs.) 948

-

852 (a.)

-

884 (m.)

854 (s.)

-

-

852 (5.)

-

895 (w.)

860 (w.)

-

862 (IV.)

-

858 (w.) 856 (w.)

-

-

831 (vs.)

-

897 (8.)

858 (v.s.)

R=OCOCH,

tram

cis

938 (m.) 968 (IL) -

R =COOC,H,

tram

956 (xv.) 977 (TV.)

cis R=COOH

tra?u

cis

949 (w.) -

895 (VA)

R =CN

trans cis

951 (m.) 971 (m.)

R=NO,

trans

954 (w.) 972 (w.)

858 (~.a.)

-

stilbenes appear, are plotted against the Hammett’s sigma constants of the substituents using the results in Table 4. The relation is shown in Figs. 3 and 4. The results may be regarded that the good linear relationships are established in Figs. 3 and 4. It is apparent from the Figs. and Table 4 that the common characteristics of the substituents, which shift the CH out-of-plane deformation vibration bands of p-disubstituted benzenes and the characteristic absorption bands of p,p’-disubstituted cis-stilbenes to the higher wave numbers, are large Hammett’s sigma constant. That is, the substituents which have the tendency to deplete the electron densities of the aromatic ring and the central double bond of the stilbenes shift the wave numbers of the absorption due to the CH out-of-plane deformation vibration to the higher wave numbers. This suggests that the n-electron distributing above and below the plane of the benzene ring has an influence on the CH out-of-plane deformation vibration and that the n-electron of the double bond which is coplanar with that of aromatic ring has a great influence on the characteristic band. Such an

1470

M. OHI and H. KUXIIVIOTO

interaction was suggested by KROS et al. [ 121, who discussed the dependence of the CH out-of-plane deformation vibration in monosubstituted and p-disubstituted benzenes on n-electron densities from the theoretical grounds [IS] which point out that the hybridization of the molecule is transformed during the molecular deformaGon vibration. During the deformation vibration, generally, the bond-forming orbital belonging to the central atom has a tendency to follow the direction of the vibration of the outer atoms. This tendency can be best realized for the CH out-of-plane deformation vibrations in unsaturated systems because the carbon sp2 hybridized orbital, in following the movement of the hydrogen atom, enters the field of the x-electrons and, through overlapping with orbitals of n-electrons, is transformed into a hybrid containing the sp3 character to some extent. The more the carbon bonding orbital is able to follow the movement of the hydrogen atom by changing its hybridization, the more easily the vibrations will occur, hence relatively low vibration frequencies should be resulted. In benzenoid systems, this bond-orbital following tendency attains maximum in compounds having a maximum v-electron density associated with the ring. When the n-electron density is decreased by electron attracting substituents, there is less tendency for the carbon bonding orbital to follow the movement of the attached hydrogen atoms during deformation vibration. As a result, the effective force constant for the vibration will not be sensitively reduced in the case of the presence of an electron-withdrawing substituent, and relatively high CH out-ofplane deformation frequencies should arise. From the above discussion, it is easy to understand the linear relation between the position where the CH out-of-plane deformation absorption falls and the Hammett’s sigma constant in Fig. 3, because the Hammett’s sigma constant is a measure of the electron density. Furthermore, the linear relation between the wave number where the characteristic absorption of cis-stilbenes appears and the Hammett’s sigma constant may be interpreted similarly, provided t,hat the origin of these characteristic bands is the CH out-of-plane deformation of the ethylenic group which is coplanar with an aromatic ring. As to the 900-850 cm-l region, electron releasing substituents, such as methoxyl or methyl, show absorptions at the lower wave number. On the other hand, electron attracting substituents, such as nitro, show the bands at the higher wave number. These frequencies will be the result of a significant overlap of orbitals belonging to the a-bond between carbon and hydrogen and to the n-electron of the ethylenic bond during the course of the CH out-of-plane deformation vibration. During the vibration, the bond orbital of ethylenic carbon atom of cis-stilbenes, following the movement of the hydrogen atom of the ethylenic moiety, enters the field of v-electrons and is transformed into a hybrid containing some sp3 character through overlapping with orbitals of ethylenic n-electrons. When the para-position of benzene is subst’ituted by the electron attracting group, the n-electron density around the ethylenic carbon atoms is depleted through the benzene rings. As a result, the variation of hybridization will be hard to follow: the CH out-of-plane [12] R. D. KROS, V. A. FUSEL and M. MARGOSHES, J. Anz. Gem. Sot. 78, 1332 (1956). [13] J. W. LINNETTNI~ P. J. WHEATLY, Nature161,314, 971 (1948); Trans. Faraday Sot. 45,33, 39 (1949).

Characteristic

infrared

absorption

bands

of cis-stilbene

1471

deformation vibration and the characteristic absorption band is shifted to the higher wave numbers. The smaller the value of the Hammett’s constant of the group is, the richer the density of n-electron around the olefinic linkage will become, the contribution of sp3 configuration occurring more easily. The effective force constant is decreased as a result and the characteristic band appeared at the lower wave number. Inspection of the isotope effect of p,p’-disubstituted cis-X,X’-dideuterostilbenes in Fig. 3 and 4 reveals that. the CH out-of-plane deformation vibrations of p-disubstituted benzenes are hardly shifted to the lower wave numbers, but, on the other hand, the characteristic absorption bands of the cis-stilbenes are shifted to the lower numbers by 25 cm-l quite regularly. Though it is calculated theoretically that the isotope effect of deuterium is represented by YJY = l/l/2 (v is the position of the absorption due to the group containing hydrogen in wave number: vi is that due to the group containing deuterium), there are few cases in which the ratio of l/l/Z is realized for deuterium and hydrogen compounds, since the coupling of vibrations frequently occurs, the deviation from the value l/2/2 being marked when the product rule proposed by Teller-Redlich is not applicable. Thus the small shifts due to deuteration seems to suggest that the characteristic absorptions at 900-850 cm-l are not purely originated from the CH out-of-plane deformation vibration and some kinds of coupling must exist. Studies on the intensities and the nature of the aborptions at 900-850 cm-r are in progress. Acknowledgements-The helpful

suggcst,ions.

aut’hors wish to thank Dr. Y. URUSHIBARA

and Dr. T. SHIMANOUCHI

for