The inhibition of photochemical reactions by nitric oxide

Abstract

The effect of nitric oxide on the decomposition of certain organic compounds can only be explained by assuming that this substance efficiently breaks chains depending upon free radicles (Staveley and Hinshelwood 1936 a, b). The evidence being, however, indirect, it seemed desirable to determine the effect of small quantities of nitric oxide on a system in which the chain length was known with some certainty. In this way it might be possible to confirm the interpretation of the thermal results and to determine the efficiency with which nitric oxide would break the chains. A convenient example of such a system is provided by the photochemical decomposition of acetaldehyde, the quantum efficiency of which rises from about unity at 100° to several hundreds between 300° and 400° (Leermakers 1934a; Ackeroyd and Norrish 1936). Experiments have therefore been made on the inhibition of this photochemical reaction by nitric oxide. The investigation was extended to include propionic aldehyde, and then, since the results raised certain questions about he nature of catalytic action in general, the work was further extended to acetone, methyl formate, and methyl alcohol. Method The reactions were allowed to take place in a silica vessel with plane parallel ends of polished fused quartz, heated in an electric furnace also provided with windows of clear polished quartz. The temperature of the furnace was controlled by a Cooke and Swallow regulator and measured by a mercury thermometer. The reactions were followed by pressure measurements made with a quartz spiral gauge connected to the reaction vessel by a capillary tube. The volume of the spiral and capillary was small compared with that of the reaction vessel. For absolute measurements the gauge was used as a null instrument, but for measuring small changes of pressure with time, as required for the determination of the initial reaction rate, the passage of the image of the pointer over ten divisions of a micrometer eye-piece scale was observed directly, the instrument having previously been calibrated for each range of pressure. For the introduction of small quantities of nitric oxide a calibrated capillary pipette was included in the system. The source of light was a Hanovia quartz mercury vapour lamp. The beam passed through a spherical quartz flask containing a solution of nickel and cobalt sulphates, and then through a quartz cell containing potassium hydrogen phthalate solution. This system isolates the 3135 A line (Bowen 1935). The phthalate cell was refilled before each run.