GUEST AUTHOR Jerry Donohue
University of Southern California Los Angeles
I
Textbook Errors,
On Hydrogen Bonds
W h i l e the precise geometry of hydrogen bonds may appear, at first sight, to be a rather trivial pmperty on which to center attention, it was just such attention to detail which led, for example, t o the formulation of pmtein structures by Pauling and Corey and to the Watson-Crick hypothesis for the structure for DNA. It thus seems important to call attention to examples of errors which are a t present being propagated in textbooks.' A student can hardly be expected to develop a proper feeling for the geometry of hydrogen bonding if he is presented with impossible structures. One of these purports to give a partial arrangement of the water molecules in CuSOa.5H20. The determination of the structure of this compound, by X-ray diffraction2 found an elongated octahedral coordination for the copper atoms: four HzO arranged in a square about each C U + ~with , two more distant oxygen atoms of different sulfate ions completing the octahedron. The fifth water molecule bridges two sulfate groups of different C U ( H ~ O ) ~complexes. ++ Therefore, to formul a t e a s has frequently been done-the following complex,
where one water molecule forms two hydrogen bonds to the same sulfate ion, is not only contrary to the crystallographic results but also does violence to the geometrical requirement of a linear, or nearly linear 0 - H . .O system. It is obviously not possible to construct such a complex without introducing an unacceptably large deviation of the angle 0-H...O from 180' or without the requirement that the angle H-0-H deviate from a nearly tetrahedral value and to preserve a t the same time the characteristic 0...O distances of about 2.8 A. Another related error involves maleic acid for which the crystal structure known from X-ray diffra~tion.~ Suggestions of material suitable for this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as man" details as oossible. and oaticulsrlv geles 7, California. 'Since the purpose of this column is to prevent the spread snd continuation of errors rtnd not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent standard books to be presented. l B ~ ~C.vA,,~AND ~ LIPSON, ~ , H., Proe. Roy. Soc., A146, 570 119R4).
598
/
Journol o f Chemicol Education
47
This structure has been misinterpreted in one textbook as follows, and the error has now found its way into a t least one other textbook.
The above formulation in which the two hydroxyl gmups point a t each other brings the two hydrogen atoms impossibly close to each other and thus is ohviously incorrect. The original X-ray study implies the correct formulation: H
\
C=C
/H
This may be arrived a t by identification of the carbonoxygen bonds of 1.20 and 1.21 A as the two C=O bonds and those of 1.2T5 and 1.275 A as the two C-OH bonds (the hydrogen atoms were not located directly). One hydroxyl group is involved in intramolecular hydrogen bonding. -. the other in intermolecular, a result in complete agreement with expectation. Other examples of geometrically unacceptable hydrogen bonds may easily be found in textbooks, but these are quite often the result of exigencies of typography rather than an incomplete understanding of the geometry of the interactions. For example: H
H
H
does not represent the correct geometrical situation in ice, where each oxygen atom is tetrahedrally surrounded by four others. This and other examples fall into the same class as do many conventional figures used in texts and journals, such as
V for the regular hexagonal benzene ring. I n that instance, it is probably not necessary to emphasize the difference between the printed page and reality. A
word of caution in the case of hydrogen honds, on the other hand, appears to be appropriate. The question of what the student should be taught is readily answered. The concept of a hydrogen bond was first introduced4 in order to explain some of the anomalous properties of water, acetic acid, ammonium hydroxide, etc., in terms of a n iuteraction between the hydrogen atom of a n )N-H, \o-H, or F-H group
line joining the two heavier, electronegative atoms, and that it is much closer to one of these atoms than the other, corresponding to a covalent bond on the one hand and the electrostatic interaction on the other.5 It is probably desirable to emphasize these two conditions when first discussing hydrogen bonded structures. Comprehensive reviews are available6 in which the various properties of hydrogen honds are discussed.
with an electronegative atom such as fluorine, oxygen, or nitrogen. The properties of such "hydrogen bonds" have since been extensively investigated by numerous methods. This interaction is now considered to be essentially electrostatic in nature, and the conventional symbols N-H . .O, 0-H. . .O, and the like, imply two conditions which have, in fact, been amply verified by direct experiment. Direct experiment has shown that the hydrogen atom lies on, or close to, a
42, 1419 (1920).
,
'LATIMER, W. M., AND RODEBUSH, W. H., J. Am. Chem. Soc., 'So-called "symmetrical" hydrogen bands, such as F - - H - . F
.n . -
in KHFn, and "bifurcated" hydrogen bonds, such as N-B:
-
.n
in nitramide, represent exceptions to the usual behavior, and other examples of these have been only very rarely encountered. 'Pauling, L., "The Nature of the Chemical Bond," 3rd ed., Cornell University Press, 1960, chap. 12, together with thereviews cited on p. 449.
Volume 40, Number 7 7 , November 7 963
/
599
