Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

324 DETECTION AND ESTIMATION OF INTERMEDIATESthat stereospecific addition mav be expected from the singlet and non-specificaddition from the triplet.It seems that the interpretation of the results from these experiments is open toquestion. In the first place the retention of the stereospecificity, for singlet methyleneonly, requires both that the assumptions made about biradical formation fortriplet and direct addition for singlet are accurate, and that rotation about theC-C bond is rapid compared with ring closure for the biradical. Secondly, additionof singlet CH, in reaction (49) leads initially to an excited singlet adduct whichmay itself isomerise unless it is quenched, and the experiment must be performedunder conditions favourable to the quenching process. However, these same conditionsfavour the quenching of the singlet methylene itself, so that the proportionof singlet methylene detected may differ very appreciably from that produced.6. Miscellaneous physical techniques6.1 PRESSURE MEASUREMENTRecombination of atoms in a constant volume clearly leads to a decrease inpressure, and Smallw~od~~~ was able to follow the recombination of hydrogenatoms manometrically. A mixture of atomic and molecular hydrogen, produced inan electric discharge, was compressed suddenly, and the pressure changes measuredwith a recording diaphragm gauge. EliasZg2 has extended the technique to themeasurement of H, N and 0 atom concentrations in low pressure flow experiments.A section of the flow system may be isolated rapidly from the main flow,and the pressure changes within the isolated region followed with a sensitive transducer.It is claimed that [N] and [O] may be measured within 10% and [HIwithin 15 %. The value of the technique is that it is absolute, and it has been usedto confirm the validity of atom concentration determinations which employ thechemical titration reactions (Section 5).6.2 THERMAL .CONDUCTIVITY MEASUREMENTThermal conductivity measurements offer a further possible method for detectingunstable reaction intermediates. Such studies have been made by Senftlebenet al. to detect atomic hydrogen in the mercury sensitised photolysis of molecular