NEWS OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV

Vol. 233
1974

RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

70. Dipole moments and polarity of the carbonyl group of acetyl derivatives of 9-alkylcarbazoles

S. S. ROGACHEVA, S. G. KLEPIKOVA, V. P. LOPATINSKII,
M. M. SUKHOROSLOVA, E. E. SIROTKINA

(Presented by the scientific-methodological seminar of the CTF)

The aim of the present work is to investigate the dipole moments of a homologous series of monoacetyl derivatives of 9-alkylcarbazoles and to study the electronic state of the C=O bond in this series.

The dipole moments were determined on the basis of the results of measurements of the concentration dependence of the dielectric constant and the specific volume of solutions of the substances in benzene, the concentration of which was varied from 1.0 to 3.0%. The calculation was carried out according to the formula [1]:

\[ \mu_0 = 0.01281 \cdot 10^{-18} \cdot [(P_{\infty}-R)T]^{\frac{1}{2}}, \]

where

\[ \mu_0 \]

is the dipole moment of the molecule;

\[ P_{\infty} \]

is the molar polarization at infinite dilution;

\[ R \]

is the molar refraction;

\[ T \]

is the absolute temperature.

The molecular refraction \(R\) is determined by the well-known Lorentz–Lorenz formula [5].

The error in determining the dipole moment is \(\pm 0.01D\). Tables 1 and 2 give the mean values from three measurements of all quantities needed for measuring the dipole moments.

For elucidating various features of molecular structure—the distribution of electron density, the character of intramolecular bonds, mutual influence, and reactivity—the dipole moments of individual bonds in the molecule are of particular interest.

For the experimental determination of the dipole moments of intramolecular bonds, the method proposed by Mecke [2] was chosen, consisting in measurements of the intensity of infrared vibrational spectra of molecules. It is known that the intensity of an absorption band corresponding to the transition \(v \to v'\) (\(v\) and \(v'\) are vibrational quantum numbers) is determined by the square of the corresponding pseudomoment \(M_{vv'}\), which can be expressed by the following formula:

\[ |\mu_{vv'}|^2 = \frac{3hc}{8\pi^2 \nu}\int \varepsilon_\nu d\nu = \frac{\left|\int f_v \mu_c f_{v'}^{*}\,dr\right|^2} {\int |f_v|^2\,dr \int |f_{v'}^{*}|^2\,dr}, \]

where \(\int \varepsilon_\nu d\nu\) is the area of the absorption band (measured with a planimeter); \(\varepsilon_\nu\) is the absorption coefficient;

\(\nu\) — frequency;
\(M_{\sigma\nu'}\) — pseudomoment;
\(f_{\nu}, f^{*}_{\nu'}\) — wave functions corresponding to the vibrational states under consideration;
\(r\) — interatomic state.

Table 1

To calculate \(M_c\), not only the wave functions \(f_{\nu}\) and \(f_{\nu'}\) must be known, but also the dependence of the bond moment on the interatomic distance, \(M_c=f(r)\), which is determined semiempirically. For the compounds studied, this dependence is not known; therefore only the pseudomoment of the \(C=O\) bond was measured.

All measurements were carried out in a benzene solution on an IKS-14 spectrophotometer (NaCl prism) at a temperature of \(20^\circ\text{C}\) (\(\pm 1^\circ\)). The concentration of the solutions was \(8 \cdot 10^{-3}\) mol/l, and the layer thickness was 0,1008 cm.

Since in the spectra of the compounds the bands of other bonds were partially superimposed on the \(C=O\) band, the method of separating the bands of Wilson and Wells [3] was used, with corrections for the wings according to Ramsay [4].

Table 2

The data for determining \(|M_{vv}|^2\) are given in Table 4. The error in measuring the pseudomoment of the \(C=O\) bond was \(\pm 13\%\).

Discussion of Results

The observed dependences of the dielectric constant and specific volume on concentration are linear in character (Tables 1 and 2). For all the substances investigated, with an increase in the concentration of the dissolved polar substance, the dielectric constant increases, since the number of polar particles in the volume increases.

Table 3

Table 4

From Table 1 it is evident that, for this homologous series, a decrease in the ratio \(\frac{\Delta \varepsilon}{\Delta \omega}\) is observed from the first member of the series to the subsequent ones. This is explained by the fact that the dielectric constant of the polar liquid increases not only with an increase in \(M_0\), but also with an increase in the number of molecules per unit volume.

Table 3 summarizes results that are in good agreement with the additivity rule and with Berthelot’s regularity [5].

The substances studied (Table 3) are polar, and as the linear length of the radical attached to the carbazole ring increases, the polarization and, consequently, the dipole moment \(M_0\) increase up to \(n\)-propyl; beginning with the \(n\)-propyl radical, however, the dipole moment changes hardly at all because of weakening of the induction effect of the radical.

This effect is also observed in other studies [8, 9, 10].

From Table 4 it is evident that the pseudomoment of the \(C=O\) bond increases in the series of acetyl derivatives of 9-alkylcarbazoles as the...

of the alkyl radical at nitrogen. Thus, the carbonyl group is sensitive to this kind of structural change in the molecule of 3-acetyl-9-alkylcarbazoles [11].

Conclusions

1. The dipole moments of acetyl derivatives of 9-alkylcarbazoles have been determined for the first time, and the polarity of the \(C=O\) bonds in them has been evaluated.

2. It has been established that, despite the great difference in the alkyl radicals, the dipole moments of the substances studied differ from one another only slightly, varying within the range \(3.70—3.86D\).

3. For the homologous series of monoacetyl-9-alkylcarbazoles, the Berthelot regularity is observed.

4. The pseudomoment of the \(C=O\) bond increases in the series of monoacetyl-9-alkylcarbazoles.

References

1. I. Halverstadt, W. Kumler, J. Am. Chem. Soc. 64, 2988, (1942).

2. V. N. Kondrat’ev. Structure of Atoms and Molecules. Fizmatgiz, Moscow, 1959.

3. E. B. Wilson, A. Wells, J. Chem. Phys. 14, 578, (1946).

4. D. A. Ramsay, J. Am. Chem. Soc. 74, 72, (1952).

5. V. K. Ioffe. Refractometric Methods of Chemistry. Goskhimizdat, Moscow, 1960.

6. P. Debye. Polar Molecules. ONTI, Moscow, 1931.

7. P. Debye, G. Sack. Theory of the Electrical Properties of Molecules. ONTI, Moscow, 1936.

8. L. L. Burshtein, G. P. Mikhailov. Zh. tekhn. fiz. 27, 694, 1957.

9. K. D. Nenitsescu. Organic Chemistry. Vol. I, Izdatinlit, Moscow, 1962.

10. S. T. Barsemyan. High-Molecular Compounds. 9, 749, (1967).

11. A. Gillam, E. Stern. Electronic Absorption Spectra of Organic Compounds. Izdatinlit, Moscow, 1957.