UDC 541.67+546.287

PHYSICAL CHEMISTRY

I. P. BIRYUKOV, M. G. VORONKOV, I. A. SAFIN

CORRELATION OF THE MEAN FREQUENCY OF NUCLEAR QUADRUPOLE RESONANCE WITH THE INDUCTION CONSTANT OF SUBSTITUENTS IN ORGANYLCHLOROSILANES

(Presented by Academician Ya. K. Syrkin, 12 IV 1965)

Data from the study of the spectra of nuclear quadrupole resonance (NQR) of organylchlorosilanes \((^{1-7})\) make it possible to conclude that the mean NQR frequency is an approximately additive quantity and can be used for a quantitative estimate of the influence of the polar (inductive) effect of substituents on the Si—Cl bond.

In the present work* we consider the dependence of the mean value of the NQR frequency at 77° K \((\nu_m^{77})\) of organylchlorosilanes of the type \(RR'R''SiCl^{35}\) (Table 1) on the polar (inductive) constants, associated with the central silicon atom, of any substituents \(R\), \(R'\), and \(R''\), \(\sigma_i^*\), of Taft \((^{8,9})\). The total induction effect of these substituents can be represented by the sum of the corresponding polar constants \(\sum_i \sigma_i^*\).

The correlation dependence between the quantities \(\nu_m^{77}\) and \(\sum_i \sigma_i^*\) approximately follows an additive law and can be represented by the linear equation:

\[ \nu_m^{77} = \nu_0^{77} + k \sum_i \sigma_i^*, \tag{1} \]

or

\[ \sum_i \sigma_i^* = k^{-1}(\nu_m^{77} - \nu_0^{77}) = \alpha \nu_m^{77} - \beta . \tag{2} \]

The results of calculating the parameters of equations (1) and (2) by the method of least squares for organylchlorosilanes belonging to the series \(RSiCl_3^{35}\), \(RR'SiCl_2^{35}\), \(RR'R''SiCl^{35}\), are given in Table 2.

* The linear dependence between the quantities \(\nu_m^{77}\) and \(\sigma_i^*\) was first reported in 1964 \((^4)\).

Table 1

Values of the mean NQR frequencies \(\nu_m^{77}\) of \(^{35}\mathrm{Cl}\) in organochlorosilanes in comparison with calculated values

Table 2

Parameters of equations (1) and (2)

For the majority of the organochlorosilanes we studied, the magnitude \(\sigma_{\mathrm{Cl}}^{*}\) proves to be considerably larger than the induction constants \(\sigma_R^{*}\) of the other substituents (except for \(\mathrm{CCl_3}\)**). Therefore it is quite natural that, in accordance with the number of \(\mathrm{Si{-}Cl}\) bonds in the molecule, the values of \(\nu_m^{77}\) are arranged on the correlation straight line in the form of three principal groups (the dependence

** Since \(\sigma_{\mathrm{CCl_3}}^{*} \simeq \sigma_{\mathrm{Cl}}^{*}\), the value of \(\nu_m^{77}\) for \(\mathrm{C_6H_5(CCl_3)SiCl_2}\) lies in the group of compounds with three chlorine atoms.

$\nu_m^{77}=f(\Sigma_i\sigma_i^*)$ is graphically presented in Fig. 1). The centers of these groups correspond to the following averaged values of $\nu_m^{77}$ (R, R′ R″ $\ne$ Cl): 19.0 (RSiCl$_3$); 17.9 (RR′SiCl$_2$) and 16.8 MHz (RR′R″SiCl), with an index of 1.1 MHz, and are separated from one another by $\simeq 2.9$ $(\sigma_{\mathrm{Cl}}^*)$ units of the $\sigma^*$ scale.

The average discrepancy between the values of $\nu_m^{77}$ calculated from equation (1) and the values found (Table 1) is $\pm 0.5$–0.6% ($\pm 0.53\%$ for RSiCl$_3$; 0.60% for RR′SiCl$_2$ and $\pm 0.63\%$ for RR′R″SiCl).

Earlier ($^4$) we noted that equation (2) can be used to calculate unknown values of $\sigma^*$ and to correlate known values of $\sigma^*$. Table 3 gives calculated values of $\sigma^*$ for several substituents bonded to the silicon atom in molecules of the RSiCl$_3$ series. The average discrepancy between the calculated and literature values of $\sigma^*$ is $\pm 0.15$*.

The data presented here indicate that the inductive effect of substituents (with the exception of the CH$_3$ group) bonded to the silicon atom generally remains the same as in the case when they are attached to a carbon atom.

The effect of $p_\pi-d_\pi$ interaction, which is so substantially reflected in the properties of silicon bonds with the most electronegative atoms (F, O, Cl, N) and with aryl and $\alpha$-alkenyl groups, is not noticeably manifested in the data obtained by the NQR method. This considerably distinguishes it from nuclear magnetic resonance, which is very sensitive to the phenomenon of $p_\pi-d_\pi$ interaction ($^{10}$). Consequently, comparison of the NQR and NMR data, which will be carried out elsewhere, proves very useful for solving certain questions concerning the nature of the chemical bond.

Table 3

Values of $\sigma_R^*$ of substituents R bonded to the central silicon atom in molecules of the RSiCl$_3$ series, calculated from $\nu_m^{77}$ by equation (2)

Institute of Organic Synthesis
Academy of Sciences of the Latvian SSR

Received
3 IV 1965

CITED LITERATURE

1. R. Livingston, J. Phys. Chem., 57, 496 (1953).
2. H. O. Hooper, P. J. Bray, J. Chem. Phys., 33, 334 (1960).
3. I. P. Biryukov, M. G. Voronkov, Izv. AN LatvSSR, ser. khim., No. 1, 115 (1965).
4. I. P. Biryukov, I. A. Safid, M. G. Voronkov, Conference on Magnetic Resonance, Krasnoyarsk, 1964.
5. M. G. Voronkov, I. P. Biryukov, Teor. eksp. khim., 1 (1965).
6. I. P. Biryukov, M. G. Voronkov et al., DAN, 162, No. 1 (1965).
7. I. P. Biryukov, M. G. Voronkov et al., DAN, 161, No. 6 (1965).
8. R. W. Taft, Chapter XIII in: Steric Effects in Organic Chemistry, Foreign Literature Publishing House, Moscow, p. 480.
9. V. A. Palm, Usp. khim., 30, 1069 (1961).
10. A. N. Egorochkin, Study of the Mutual Influence of Atoms in Molecules of Organoelement Compounds of Group IVB by the Proton Magnetic Resonance Method, Candidate’s dissertation, Novosibirsk, 1965.

* The rather substantial discrepancy in the value of $\sigma_{\mathrm{CH}_3}^*$ (−0.41), as we shall show subsequently, is regular (this discrepancy was not taken into account in deriving the average discrepancy).