SOME CONSIDERATIONS ON THE WAVE INTERACTION OF ATOMS

B. V. TRONOV

The Inductive Effect and the Conjugation Effect

Modern electronic chemistry recognizes two main types of mutual influence of atoms in molecules.

1. The effect of electrical interaction, or the inductive effect, is caused by the mutual attraction or repulsion of electric charges. Thus, if in the hydrocarbon radical of some carboxylic acid a hydrogen atom is replaced by a halogen atom, whose positive part (the nucleus with the electrons of the inner shells, which do not take part in chemical reactions) attracts electrons with great force, then this attraction is transmitted to all electron pairs of the molecule. As a result, the electron pair binding the hydroxyl hydrogen proves to be more neutralized than in the original acid, and the hydrogen is more readily split off in the form of a cation. This leads to an increase in the electrolytic dissociation constant, that is, to a strengthening of the acidic properties.

The inductive effect rapidly weakens with increasing distance between atoms, while retaining the direction of its action.

1. The effect that cannot be reduced to electrical forces and can be explained only with the aid of wave theory and quantum mechanics. Since this effect is manifested most clearly in molecules with conjugated bonds, it is usually called the conjugation effect. This type of interaction of atoms differs from the inductive effect in the following features:

a) the action of the conjugation effect has a periodic character. This explains a number of cases in which one and the same atom or atomic group, depending on its position relative to another atom (or group), either increases or decreases the reactivity of the latter. Thus the reactivity of a halogen directly bonded to a benzene ring, with respect to most reagents, is very low (for example, in chlorobenzene); in benzyl chloride, on the contrary, chlorine is distinguished by great activity. Farther on, the reactivity of chlorine is again lowered, though not as much as in chlorobenzene, and then a certain increase in activity is again observed. The difference in the properties of the links of the benzyl ring in all homologues and derivatives of benzene is also explained by the conjugation effect, as is manifested, for example, in the substitution rules;

b) in some systems of atoms the conjugation effect weakens little with increasing distance. Thus in acetaldehyde and in esters of acetic acid the hydrogens of the methyl group, under the influence of the carbonyl, become highly reactive. The methyl groups in these compounds readily enter, for example, into various condensation reactions. In propionic and butyric aldehydes, and also in the esters of the corresponding acids, the methyl hydrogens are inactive, i.e., the introduction into the chain of one or two methylene groups strongly weakens the loosening influence of the carbonyl. However, this influence is well transmitted [1] through one and even through two groups with a double bond (vinylogy) in crotonic and sorbic aldehydes, and in the esters of crotonic and sorbic acids.

The distance between the methyl and carbonyl groups in sorbic aldehyde and in the esters of sorbic acid is comparatively very large. Electrical interaction (the induction effect) at such a distance would be practically imperceptible. The question is rightly raised [2] of the possibility of transmitting the mutual influence of atoms through very long chains in high-molecular compounds. Of course, such transmission is possible only in the form of a conjugation effect, and it can be expected only when the chains are constructed in a corresponding way;

c) the conjugation effect depends to a large extent also on the spatial structure of molecules. It appears only in planar systems of atomic nuclei. Very characteristic in this respect is the change in the reactivity of chlorine in para-chloronitrobenzene under the influence of methyl groups arranged in different ways [3]. It is known that chlorine in the ortho and para positions relative to the nitro group is many times more active than the same halogen in chlorobenzene. Two methyls in the ortho position to the chlorine itself do not lower its reactivity, but methyls located next to the nitro group destroy its activating influence. This can be explained only by the fact that the oxygen atoms of the nitro group are displaced from the plane of the benzene ring.

Conjugation as a Wave Interaction

The enumerated features of the conjugation effect are undoubtedly connected with the wave nature of electrons and atomic nuclei. The conjugation effect has a periodic character, since periodicity is a general property of wave processes. The sharp dependence on the structure of molecules, and in particular on spatial structure, is explained by the fact that the waves forming electrons and atomic nuclei propagate in a definite way in the space occupied by the molecule, and the transmission through these waves of one or another interaction of atoms must require a quite definite configuration of the entire system participating in this interaction. Finally, it is precisely waves that can be combined in such a way that the interaction of atoms caused by them is transmitted over comparatively very large distances.

In atomic physics one can point to another example of the action of certain particles on one another, manifested at distances unusual for the given conditions.

Modern physics recognizes three types of fields.

1. The electromagnetic field, at present the best studied.

2. The gravitational field, with which special material particles, not directly observed, are associated—gravitons.

3. The nuclear field, which accounts for the very existence of complex (more complex than the proton) atomic nuclei. The forces acting inside the nucleus are very large, but the attraction of protons and neutrons caused by the nuclear field weakens extremely rapidly with increasing distance.

Let us consider from the standpoint of these ideas the absorption of neutrons by the nuclei of atoms of various elements [4]. It turns out that the effective cross sections of nuclei, determined from neutron absorption, may fail completely to correspond to the sizes of atomic nuclei. Thus, for uranium the capture cross section of slow neutrons is \(7.68 \cdot 10^{-24}\) sq. cm, whereas the smaller gadolinium nucleus has a capture cross section of \(46000 \cdot 10^{-24}\) sq. cm. From this we find that neutrons flying through uranium are stopped only at a distance a little greater than \(1.5 \cdot 10^{-12}\) cm from the center of the atomic nucleus, whereas gadolinium stops them at a distance of \(120 \cdot 10^{-12}\) cm from the center of the nucleus, i.e., almost 80 times farther away. What force stops these neutrons? This force cannot be gravitation, since the uranium nucleus is one and a half times heavier than the gadolinium nucleus

(atomic weights 238 and 157). Nor can the electric charge be at work here, since that of uranium is also almost one and a half times greater (atomic numbers 92 and 64). Finally, the forces of the nuclear field at such a considerable distance become negligibly small.

It remains to assume that in this case, too, some kind of wave interaction is manifested. Atomic nuclei and neutrons have a wave nature. They are nodes of certain waves, condensations in a material medium to which motion is inalienably inherent, and moreover wave motion. A distinctive interaction of waves, by combinations of which one or another particle is formed, may very well also lead to phenomena that cannot be explained from the standpoint of notions about three kinds of field. These include the conjugation effect considered by us in chemistry and the absorption of neutrons by atomic nuclei in atomic physics. Apparently, it is more correct to suppose that there is a single material medium that gives rise to electrons, protons, neutrons, photons, and so on, while the “fields” accepted by contemporary physics are connected only with definite motions in this medium.

The Universal Character of Wave Interaction

When, in electronic chemistry, the doctrine of two principal kinds of mutual influence of atoms—the induction effect and the conjugation effect—was being developed, some chemists were inclined to think that the latter effect could manifest itself only in molecules where there are conjugated systems of bonds in the old sense of this term, that is, alternating single and double bonds, and in any case systems with pi-electrons. There were debates about whether the “alternating effect” (according to the most characteristic feature of wave interaction—periodicity) exists in chains with only single bonds. The idea of an “alternating effect,” or alternating polarity, as a phenomenon very widespread in organic chemistry, had been advanced by some scientists in various forms already from the beginning of the 20th century. However, other authors denied alternation of charges in saturated chains [5].

Theoretically, from the fact that electrons and atomic nuclei, on the one hand, are particles with a definite electric charge, and, on the other hand, have a wave nature and wave properties, there inevitably follows the dual character of the mutual influence of atoms in molecules; and we do indeed know the effect of electrical interaction, or the induction effect, and the effect of conjugation, caused by wave interaction. But it is not only pi-electrons that have a wave nature, but all electrons in general. Consequently, in principle, wave interaction must manifest itself to a greater or lesser degree in all molecules, with all types of bonds. It is therefore understandable that chemists began to speak of “hyperconjugation,” then began to distinguish in greater detail several kinds of conjugation, such as pi-pi conjugation, i.e. conjugation in the old, narrow sense; sigma-pi conjugation, or conjugation of a single and a double bond; sigma-sigma conjugation, or conjugation of two single bonds. New types of conjugation have been especially thoroughly studied in the works of A. N. Nesmeyanov and his co-workers [6]. Conjugation of a single and a double bond explains, for example, the mutual influence of a methyl group and a double bond in the propylene molecule. The double bond adds hydrogen halides in a definite order (Markovnikov’s rule), and, on the other hand, under the influence of the double bond the hydrogens of the methyl group become very active.

Among the peculiar manifestations of conjugation belongs the above-noted periodicity in the influence of the benzene ring on the reactivity of chlorine, where the mutual influence of atoms—an influence undoubtedly

but of wave origin, is transmitted through several simple bonds.

Thus, the wave interaction of atoms, which causes the conjugation effect, is just as general a phenomenon as electrical interaction. The conjugation effect, like the induction effect, must operate in every molecule. The two kinds of mutual influence of atoms must always be superimposed on one another. Sometimes they reinforce one another, sometimes they weaken one another. Moreover, each of these effects may have either a static or a dynamic character. They may cause a definite change in the electron density in the molecule by themselves (polarization), or affect the molecule’s capacity to change its electron density under particular conditions (polarizability).

One of the most important tasks in the further development of the theory of chemical structure should be regarded as the further elucidation of the regularities governing the action of both kinds of mutual influence of atoms discussed here. We must be able not only to explain already known facts in this field, but also to predict new ones. For this purpose, a very large amount of work must still be carried out on the experimental study of organic compounds and their reactions, using both ordinary chemical methods and the new means of investigation available to modern physics.

Conclusions

1. Two basic mechanisms are known for the transmission of the mutual influence of atoms in molecules: the induction effect and the conjugation effect.

2. The induction effect is explained by the electrical interaction of atomic nuclei and electrons. The characteristics of the conjugation effect—namely, the periodicity of its action and its great dependence on the structure of the molecule—indicate that this effect is connected with the wave nature of electrons and atomic nuclei.

3. All electrons have a wave nature. Therefore, the conjugation effect must appear in all molecules, with all types of bonds. This has now been confirmed by numerous facts.

LITERATURE

1. Chichibabin A. E. Fundamental Principles of Organic Chemistry. 9th ed., vol. 1, p. 132, 1954.

2. Semenov N. N. “A New Stage in the Development of Chemical Science and Industry.” Pravda, No. 8 (14402), 1958.

3. The State of the Theory of Chemical Structure in Organic Chemistry. All-Union Conference, June 11–14, 1951, stenographic report, 1952.

4. Mury N. N. “Priroda” [Nature], 46, No. 12, pp. 21–26, 1957.

5. Remik A. Electronic Concepts in Organic Chemistry. 1950.

6. Reutov O. A. Theoretical Problems of Organic Chemistry, pp. 60–[[unclear: continuation]], 1956.