July 26, 1951

Professor Eugene G. Rochow

Department of Chemistry

Harvard University

12 Oxford Street

Cambridge 38, Massachusetts

Dear Professor Rochow:

I am writing to ask you for some information about your ideas on the low temperature coefficient of viscosity of the silicone oils.

Recently someone has told me that in lectures to your students and in talks before local sections of the American Chemical Society you have stated that it has been shown that the explanation that I have offered in my textbook of the low temperature coefficient of viscosity is wrong, and have referred to the work of Roth and Harker on octamethylspiro(5,5)pentasiloxane.

So far as I am aware, there is no reliable evidence to show that my suggestion is wrong; and, on the other hand, I think there is good reason to doubt the suggestion made by Roth and Harker, that there is an abnormally easy librational motion of the methyl groups in the silicones, and that this librational motion is responsible, in some unexplained way, for the low values of the heat of vaporization and the temperature coefficient of viscosity of these oils.

You probably know that in their paper in Acta Crystallographica vol. 1, page 34 (1948), Roth and Harker merely suggest that "free rotation of CH, groups about siloxy bonds in the linear polymers is responsible for the small intermolecular forces characteristic of these compounds." They do not explain why free rotation of this sort should lead to small intermolecular forces, nor do I understand why it should. Moreover, although Roth and Harker make a quite definite statement that they have shown that there is an abnormal case of libration motion of the methyl groups, examination of their paper shows that it is quite likely that this conclusion is unjustified, and that instead it rests upon an error of judgment made by them in the selection of the value for the x-ray scattering factor of the methyl groups.

Prof. Rochow

Roth and Harker carried out their calculations for the methyl group of the x-ray scattering factor for the fluorine atom. This was a serious error on their part. It has been found that the scattering factor for the carbon atom, as given by theoretical calculations, serves well for the carbon atom in the methyl group, except that a small correction is needed at very small diffraction angles. The scattering factor for the fluorine atom is between 1.5 and two times as great as that for the carbon atom, and it is accordingly not surprising that Roth and Harker were unable to get satisfactory agreement between the observed and calculated intensities without making some additional correction. The correction that they introduced was that of using a temperature factor corresponding to an abnormally great libration of a methyl group. This temperature factor cuts down the scattering factor for fluorine to approximately the correct value for the carbon atom. In other words, the electron distribution for fluorine atom with a large temperature coefficient is just about correct for a methyl group with a normal temperature coefficient. Roth and Harker are not justified in concluding that the methyl group in their compound shows abnormal librational motion, relative to the siloxane nucleus of the molecule.

The foregoing statements would not, of course, be justified if Roth and Harker had been required to assign an abnormal motion to the methyl group only in the transverse directions, with a normal temperature coefficient in the directions along the bonds. However, they themselves state "Within the limits of the available data, the correction could probably be represented as a specific temperature factor applied to the CH3 groups, that is , exp(-BCH,(sin θ/λ)2), but this would seem incorrect since it implies abnormally large amplitudes for silicone methyl stretching vibrations." Thus an isotropic temperature coefficient correction for the fluorine scattering factors would, according to Roth and Harker, agree with their data. This isotropic temperature correction multiplied into the fluorine scattering factor gives, however, just the correct scattering factor for the methyl group. Roth and Harker point out that there is indication that the entire pentasiloxane molecule is undergoing significant libration within the crystal. This has been observed for other molecular crystals also.

I conclude by pointing out that Roth and Harker have not presented evidence to show that there is an abnormally great librational motion of the methyl groups relative to the siloxane nucleus in the pentasiloxane molecule, and that there is no sound reason to support their proposal that such an abnormal librational motion is responsible for the properties of the silicones. Also, so far as I know, there exists no evidence that contradicts the suggestion that I have made about the nature of silicone oils.

I hope that you will tell me what you think of this matter. I have looked in your book on the chemistry of silicones, without finding any discussion of the point. I have not yet seen the second edition of the book, however.

Sincerely yours,

Linus Pauling:W

Crystalline fibrin

Yesterday I decided that it might be possible that the chains in fibrin are parallel, and that the unit is only half as large as previously assumed: namely, with a = 4.75 A, b = 135 A, c = 10.03 A, β = 85.1º. on checking this possibility I found that the small unit accounts for all of the reflections that I had measured except one, a faint equatorial reflection with spacing 3.16 A. However, the possibility then suggested itself that this reflection be a β reflection. If it is assumed that it is due to copper Kβ – Rudall stated “probably trace of β” – then the spacing becomes 2.84 A, which is that of the strongest equatorial reflection.

On checking the other reflections I had found another very faint reflection that is to be interpreted as β, corresponding to the strongest non-equatorial spot, with spacing 2.81 A.