Philip A. Leighton

The Photoisomerization of the o‐Nitrobenzaldehydes I. Photochemical Results

Numerous disparate explanations have been advanced for the observed quantum yield in the photoisomerization of o‐nitrobenzaldehyde. The authors decided to gather further data on this reaction, varying such factors as the weight of the molecule and the physical state of the material. The results indicate that the reaction occurs intramolecularly, but that for o‐nitrobenzaldehyde itself the quantum efficiency is nearly one‐half in all cases. A detailed consideration of the mechanism is deferred to the second portion of this paper.

Homochromatic Spectrophotometry in the Extreme Ultraviolet

The d-log t curves of typical fast, medium, and slow emulsions when coated with oils fluorescing in the blue, near ultraviolet, and far ultraviolet respectively, have been studied in the region 900 to 5000 Angstroms by means of a vacuum spectrograph designed especially for photographic photometry, together with ordinary quartz spectrographs. The reciprocity and intermittency failures of the same emulsions, unoiled and when coated with the three oils, have been studied in the range 2300–5000A. The absorption and fluoresence spectra of the oils were also studied in the same range. It was found that the contrast obtained with oil coated plates is constant throughout that part of the spectrum in which the oil absorbs all of the incident light, and is equal to the contrast of the unoiled emulsion near the wave length of maximum intensity of fluorescence of the oil. As the oils studied absorb completely from their respective long wave limits down to at least 900A, the contrast obtained in the Schumann region was constant for a given oil-emulsion combination. This property greatly simplifies photographic photometry in the Schumann region, and enables one to assume the reciprocity law throughout that region when oil-emulsion combinations are used which have been found to obey the reciprocity law at the fluorescence maxima. Most of the secondary problems of photographic photometry are thus transferred from the difficult Schumann region to the near ultraviolet or visible where they can be more readily attacked. A device for varying intensities in the Schumann region is described, which is easily calibrated by means of the reciprocity law after the latter has been tested. A number of suggestions are made by which the use of fluorescent materials can be expected to simplify certain other problems of photometry.

Exchange Reactions with Deuterium II. The Photochemical Exchange Between Deuterium and Hydrogen Chloride

The exchange reaction between hydrogen chloride and deuterium, initiated photochemically, is found to be a chain reaction with quantum yields ranging from 2 to 19 under the experimental conditions employed. The rate determining step is the reaction Cl+D2→DCl+D. The proportions at the photostationary state are shifted from those at thermal equilibrium in a direction which indicates a somewhat stronger absorption by HCl than by DCl in the region 2000–2150A.

A Combined Recording Microphotometer, Densitometer and Comparator

An instrument which may be used as a registering microphotometer, a direct reading densitometer and a comparator has been constructed. It utilizes a radiometer as the light‐sensitive device. Records are taken on 4 by 10 inch photographic plates, direct readings on a 50 cm scale, and intervals are read by means of a 10 inch comparator screw.

The Photoisomerization of the o‐Nitrobenzaldehydes II. Mathematical Treatment

The probable course of the reaction, from the absorption of light to the formation of the isomer, is followed in roughly quantitative detail. In this manner, the low quantum yields, surprising at first sight for intramolecular processes, are explained by a consideration of the physical properties of the molecules.

Exchange Reactions with Deuterium I. Deuterium and Hydrogen Chloride

An apparatus for measuring exchange equilibria and rates of exchange of molecular deuterium with hydrogen containing compounds is described. The gas density balance is used to analyze the hydrogen‐deuterium mixtures. Results on the equilibria H2+2DCl = D2+2HCl, H2+DCl = HD+HCl are reported which check the theoretical values to within the limits of the experimental errors.