A SY3IPOSIUM ON MOLECULAR STRUCTURE1 INTRODUCTIOK TO T H E SYlIPOSIU~I HERRICK L. JOHNSTOK D e p a r t m e n t o j Chemistrv, Ohio S t a t e Uniaersity, Columbus, Ohio
Receiied Xocember BO, 2936
The topic of molecular structure is an appropriate one for the initial syniposium of this Division because of the important place held in the development of science by the molecular theory and by reasoning based on assumed molecular structures. The chemical advancements attained by the earlier applications of structural relationships are the inore impressive by reason of the fact that the existence of structure was known only by chemical inference until comparatively recent years. Necessarily, this permitted only very general, and naively simple, conceptions. Yet some remarkably accurate representations were attained,-for instance, the tetrahedral symmetry of carbon. For the accurate and relatively detailed representations of structure now possible for many molecules we are indebted to modern physics,-chiefly to quantum theory and the interpretation of molecular spectra. From these have come not only accurate molecular descriptions-moments of inertia, interatomic distances, bond angles, frequencies of vibration and rotation, force constants, magnetic moments, accurate energy relationsbut also new approaches t o problems of fundamental chemical interest, These include : the determination of highly precise values for reaction heats; the description of the primary processes in reactions initiated by This Symposium on Molecular Structure was held, as the first annual symposium of the Division of Physical and Inorganic Chemistry of the American Chemical Society, a t Princeton University, Princeton, Kern Jersey, on December 31, 1936 and January 1 and 2, 1937. The papers presented a t the Symposium vm-e classified for discussion into four groups: Part I, Spectra and Structure of Diatomic Rlolecules, and other topics-R. S. Nulliken, L e a d e r ; Part 11, Spectra and Structure of Polyatomic i\Iolecules-lT. A. Koyes, Jr., L e a d e r ; Part 111, Determinations of Structure by Methods Which are Non-spectroscopic-G. B. Kistiakowsky. L e a d e r ; P a r t IV, Some Chemical Applications of Knowledge of Structure-Saul Dushman, L e a d e r . These papers follow the introduction in order, in this issue and in the February issue. -4portion of the discussion of the papers will appear in the February issue. The arrangements for the symposium were under the direction of a committee consisting of H. L. Johnston, Chatrman, Farrington Daniels, Saul Dushman, W.31. Latimer, W.A. Noyes, J r . , H. S. Taylor, H . H Willard, and H. C. Urey.
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light; the elucidation of thermal activation processes and computation of reaction velocities; knowledge of mechanisms in the quenching of energyrich molecules; explanations of valence and the correlation of molecular configurations with the quantum states of component atoms. I n combination with statistical mechanics, studies of structure through spectra are yielding important thermodynamic data of chemical value. These include heat capacities, heat contents and entropies of gases, and the equilibrium constants of gaseous reactions. The accuracy which may be obtained from the utilization of experimental studies of spectra exceeds that which may be reached with even the best care by conventional methods. In addition, the temperature range over which the thermodynamic constants may be thus evaluated greatly exceeds that open to direct methods; the time of investigation is much reduced and the cost greatly lessened. Yet further chemical utility is exemplified in the discovery of new chemical molecules or the identification of rare intermediates. Examples of the first class are the following: the discovery of diatomic molecules in alkali metal vapors (25 per cent in sodium a t its boiling point); the discovery of isotopes (notably in silicon, oxygen, carbon, and nitrogen); and the discovery of ortho and para forms of homopolar molecules (notably hydrogen). The identification of free radicals, such as OH or CH, present as accompaniments to certain reactions and frequently existent in high temperature thermal decompositions, illustrates the second class. On the steric side, the conceptions obtained from physical evidence differ from the older ones based on chemical inference, principally in the substitution of dynamic structures for static ones. Bond stretching and bond bending vibrations characteristic of independent molecules in the gaseous state, and revealed by spectra, are found by independent lines of evidence to persist in the liquid and often in the solid state, with very little modification of frequency. Indeed there is very good reason to conclude that, where chemical bonds are preserved, vibration within these bonds persists even down to the absolute zero with energies normally little different from those a t room temperature. This conception of zero-point energy is proving important sometimes in distinguishing between reaction mechanisms by reaction velocity studies, also in making predictions regarding vapor pressures of isotopic molecules or of ortho and para forms of molecules. The rotation of molecules, resolved about axes mutually perpendicular, is revealed by the spectra and is found to persist in the liquid and, infrequently, in the solid state. In addition to rotation of the molecule as a unit, rotation of its component parts-such as the methyl group in certain aliphatic structures-is a part of the modern picture and is confirmed by lines of evidence outside of spectra. The motions to which we have been referring-vibration and rotation-are of exceedingly high
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frequency (high powers of ten per second). Consideration of the space effects of these several motions yields effective static models, differing sometimes in significant respects from those arrived a t by the older methods, and produces new conceptions of steric hindrance. For example, the presence of rotating groups or the amplitude of bending vibrations may effectively block off reactive portions of large molecules or influence physical behavior. These conceptions are of value in the consideration of anomalies in certain types of organic reactions. Steric data obtained by methods quite independent of the spectra confirm and supplement the accurate conclusions of that method and permit the study of molecules too complex for satisfactory spectroscopic study. The x-ray diffraction method has been successfully applied to determine the grouping of carbon atoms in some organic compounds, not only in crystal lattices but even in the liquid and gaseous states. The use of electron diffraction is an analogous method and possesses the advantage that higher intensities may be obtained in refracted beams from light atoms. There is even some promise that the method can be perfected to yield the spacing of hydrogen atoms. Distances determined by these methods and internuclear distances and bond angles obtained from the spectra are, of course, secured on dynamic systems and represent average values. The amplitudes of change from the averages amount to several per cent. Furthermore, there appears to be, in some cases, resonance between structures of approximately equal energy, which makes for greater stability. For determining reaction processes the nature and structures of what are termed excited states are apt to assume importance. Although the relative numbers of molecules present in such states is usually very small, they often constitute the paths by which reactions occur. Owing to the normally short lives of molecules in such excited states a considerable fraction of a chemical material may reach and pass through reactive states in unit time even though the equilibrium concentration is small. The spectra constitute the principal source of information on excited states, but studies by electron impact have been of importance in confirming the existence of quantum levels and in measuring their energies. The study of dipole moments, in the gaseous state xhere possible; the analysis of thermochemical data; the use of molecular beams; the evaluation of gas collision areas and the measurement of magnetic susceptibility are other methods-non-spectroscopic in character-which have served, on occasion, to distinguish between alternative structures or supply other desired information. Few of the approaches to problems of structure to which reference is made in this introduction were in use fifteen years ago. Except for diatomic molecules-and perhaps this exception should not be made-the develop-
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ment of these fields is only begun. Their continued development should be of considerable benefit to chemistry. The benefit will be greatest if chemists more generally realize the possibilities, and also the limitations and difficulties, in these physical methods of approach and if physicists who contribute to these fields recognize the manner in which chemist? utilize this material. I n securing papers for the symposium we have attempted to obtain manuscripts which relate to various aspects both of structure determination, in the broad sense, and of chemical utilization of such material. Unfortunately, we were not able to secure representation of two or three important branchei: in these fields. lye hope that these may be touched on in the discussion.
