Physical Chemistry

L. M. SVERDLOV

CHARACTERISTIC FREQUENCIES OF OLEFINS

(Presented by Academician G. S. Landsberg, November 4, 1956)

The question of the characteristic frequencies of olefins was considered most thoroughly in the work of Hubo ((^{2})), in which, for each class of olefins, groups of characteristic frequencies are given, found by comparing observed spectra. It is obvious, however, that in order convincingly to confirm the selection of one or another group of frequencies as characteristic and to assign these frequencies to definite vibrations, a theoretical analysis is necessary (carried out according to the method of M. A. Elyashevich, B. I. Stepanov ((^{6})), and L. S. Mayants ((^{7}))).

In order to clarify the regularities of the vibrational spectra of complex olefins of the six possible types: (\mathrm{RHC{=}CH_2}), (\mathrm{R_1R_2C{=}CH_2}), cis- and trans-

Table 1

Characteristic frequencies of monoalkyl-substituted ethylene (\mathrm{RHC{=}CH_2})

[
\mathrm{H_2C\backslash \ C{=}C\ /\ H}
]
[
\mathrm{\ \ \ H/ \ \ \ \ \ \backslash H}
]

* No exact assignment.

(\mathrm{R_1CH{=}CHR_2}), (\mathrm{R_1R_2C{=}CHR_3}), (\mathrm{R_1R_2C{=}CR_3R_4}), we have carried out a complete theoretical calculation of the frequencies of normal vibrations of the simplest alkyl-substituted ethylenes: propylene, isobutylene ((^{8})), cis- and trans-butene-2 ((^{9})), trimethylethylene, and tetramethylethylene ((^{9})), since the spectrum of each of these molecules possesses its own characteristic features. As the calculation shows, each of the molecules considered has vibrations that are characteristic in form, in the broad sense ((^{7})), for a group of coordinates, correspond-

...corresponding to the structural element

$$
\begin{array}{c}
\begin{matrix} X & & Z \[-2pt] Y \end{matrix}
\,>!C{=}C!<\,
\begin{matrix} \[-18pt] \[-2pt] U \end{matrix}
\end{array}
$$

Since this structural group is included in compounds of the type

$$
\begin{array}{ccc}
\begin{matrix} R \ H \end{matrix}
!>!C{=}C!<!
\begin{matrix} H \ H \end{matrix}
&
\quad
\begin{matrix} R \ R \end{matrix}
!>!C{=}C!<!
\begin{matrix} H \ H \end{matrix}
&
\quad
\begin{matrix} R \ H \end{matrix}
!>!C{=}C!<!
\begin{matrix} R \ H \end{matrix}
\[18pt]
\begin{matrix} R \ H \end{matrix}
!>!C{=}C!<!
\begin{matrix} H \ R \end{matrix}
&
\quad
\begin{matrix} R \ R \end{matrix}
!>!C{=}C!<!
\begin{matrix} R \ H \end{matrix}
&
\quad
\begin{matrix} R \ R \end{matrix}
!>!C{=}C!<!
\begin{matrix} R \ R \end{matrix}
\end{array}
$$

then, consequently, the molecules of each of the indicated homologous series will, respectively, possess essentially the same characteristic frequencies as the molecules of propylene, isobutylene, cis-trans-butene-2, trimethylethylene, and tetramethylethylene. Thus, the groups of characteristic frequencies established by us are frequencies of stable characteristic vibrations for a series of compounds containing a definite chemical group. This conclusion is well supported by a large amount of experimental material on the combination and infrared spectra of the indicated homologous series (we collected data for more than 90 olefins).

Table 2

Characteristic frequencies of asymmetrically dialkyl-substituted ethylene (R_1R_2C{=}CH_2)

* Exact assignment is absent.

Tables 1–6 give the groups of characteristic frequencies established by us, with indication of the assignment (and with the approximate line intensity according to literature data ((^{1-4}))) for six homologous series, as well as data of other authors. In each table the structural element with which the entire group of characteristic frequencies is associated is indicated.

As follows from the tables, many frequencies have either been assigned incorrectly by other authors or have not been assigned at all to definite vibrations. In addition, in a number of cases Goubeau chose as characteristic such frequencies that apparently belong to the hydrocarbon radicals (R), and therefore cannot be regarded as characteristic frequencies of olefins, since

they are also observed in the spectra of all saturated hydrocarbons with a long carbon chain.

Let us next consider the effect of branching in the chain on the spectrum. In the case where there is branching in the chain far from the double bond, the characteristic nature of the vibrations noted in the tables should not be disturbed, since the structural groups with which these vibrations are associated are fully preserved. If, however, the branching occurs at a carbon atom adjacent to the double

Table 3

Characteristic frequencies of trans-alkenes RCH = CHR

[
\mathrm{H_2C}\backslash\mathrm{C{=}C}/\mathrm{H}, \qquad
\mathrm{H}/\mathrm{C{=}C}\backslash\mathrm{CH_2}
]

* No exact assignment.

Table 4

Characteristic frequencies of cis-alkenes RCH = CHR

[
\mathrm{H_2C}\backslash\mathrm{C{=}C}/\mathrm{CH_2}, \qquad
\mathrm{H}/\mathrm{C{=}C}\backslash\mathrm{H}
]

bond (for example, (\mathrm{R{-}CH{-}CH{=}CH_2})), then this must already affect the characteristic nature of certain vibrations, since in this case there is no longer that smallest group of coordinates for which these vibrations are characteristic. Both of these propositions are fully confirmed. Thus, for example, in the spectrum of the molecule 4-methyl-4-ethylhexene-1(¹) (with branching of the first type) all the characteristic frequencies noted in Table 1 are observed:

437, 627, 913, 995, 1180 (doublet), 1296, 1416, 1641, 3006, 3078 cm(^{-1}). On the other hand, in the case, for example, of the molecule 3,6-dimethylheptene-1 (with branching of the second type), the characteristic nature of the vibrations (\gamma(\mathrm{C{=}C{-}C})) and (\rho_{\mathrm{CCH}}) (which interact most strongly with the coordinates of the group (-\mathrm{CH_2{-}CH{=}}))

Table 5

Characteristic frequencies of trialkyl-substituted ethylenes
(\mathrm{R_1R_2C{=}CHR_3})

[
\begin{array}{c}
\mathrm{H_2C}\backslash \quad \quad / \mathrm{H}\[-2pt]
\mathrm{H_2C}/\ \mathrm{C{=}C}\backslash \mathrm{CH_2}
\end{array}
]

* No exact assignment is available.

Table 6

Characteristic frequencies of tetraalkyl-substituted ethylenes
(\mathrm{R_1R_2C{=}CR_3R_4})

* No exact assignment is available.

is already violated, whereas the characteristic nature of the remaining vibrations is still preserved: 914, 994, 1176, 1294, 1419, 1641, 2997, 3082 cm(^{-1}). Similar examples can also be given for olefins of the remaining five types.

Saratov Motor-Road Institute
named after V. M. Molotov

Received
19 IX 1956

CITED LITERATURE

1. P. A. Bazhulin, Kh. E. Sterin, Izv. AN SSSR, ser. fiz., 11, 456 (1947); S. V. Markova, P. A. Bazhulin, M. M. Sushchinskii, Optika i spektroskopiya, 1, 41 (1956).
2. J. Goubeau, Die Raman-Spectren von Olefinen. Beih. No. 56 zu der Zs. angew. Chem. (1948).
3. N. Sheppard, G. B. Sutherland, Proc. Roy. Soc., A196, 195 (1949); S. Sheppard, D. Simpson, Quart. Rev., 6, 1 (1952).
4. H. McMurry, V. Thornton, Anal. Chem., 24, 318 (1952).
5. P. F. Gruzdev, ZhFKh, 28, 507 (1954).
6. M. V. Vol’kenshtein, M. A. El’yashevich, B. I. Stepanov, Molecular Vibrations, Gostekhizdat, 1949.
7. L. S. Mayants, Tr. FIAN, 5, 65 (1950).
8. L. M. Sverdlov, O. N. Vinokhodova, DAN, 100, 34 (1955), L. M. Sverdlov, DAN, 106, 73 (1956).
9. L. M. Sverdlov, Optika i spektroskopiya, 1, 163 (1956).