Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

8 TRAPPED RADICALS 335of NH, in Ar or N, matrices (14" K). In a CO matrix, the photolysis of NH,leads to the infrared absorption of HCO which further demonstrates the productionof atomic hydrogen in the photolytic process.Matrix isolation spectroscopy has been used also in numerous attempts to studythe species CH,. The first investigations of the photolysis of diazomethanein Ar and N, matrices (at 20" K) revealed new absorption features, which wereascribed to methylene and a CHzNz tautomer, and a later study using ultravioletand infrared absorption spectroscopy appeared to confirm these identifications.However, after publications7. *' of Herzberg's data on the gas-phase spectrumof CH,, it was appreciated that the solid-phase features could not resultfrom CH, absorption. A recent high resolution infrared in~estigation~'~ suggeststhat the unidentified absorption is due to methylene-imine (CH, = NH), formedtogether with HCN by the photolysis of CH,N, dimers. This result indicates theimportance of orientation in matrix photolysis, since CH, = NH is not formed inthe gas phase, nor is it formed by the matrix photolysis of diazirine. The absenceof absorption attributed to CH, is not to be taken as evidence that CH, is notproduced photochemically from CH,N,, but rather as evidence of the extremelyhigh reactivity of CH,. Indeed, Moore and Ph~entel~'~ find that CH, reacts withN, in the matrix: the photolysis of CHz15N14N in solid 14N2 leads to the appearanceof CH214N'4N, and the matrix photolysis of diazirine yields some diazomethane.On the other hand, CF, , formed in high concentration by the matrixphotolysis of CF,CO does not react318 with CO or CO,. The species CH,=NH(CD, =ND) is formed also by the photolysis of methyl azide (methyl d,-azide) inAr (4.2" K) or CO, (50" K) matrices319. Photoly~is~~~ in similar matrices of theaides FN,, ClN, and BrN, leads to compounds whose infrared spectra showthem to be NF, NCl and NBr: in the case of NBr there is considerable matrixdeactivation to the ground ,2- state. Cyanogen aide (NCN,) may be photolysedat wavelengths greater than 2800 A to form NCNin Ar, N,, CO or C02 matrices3".The radical was identified by its infrared absorption spectrum taken together withultraviolet absorption data. Isotope studies confirmed the identification, and itwas possible to estimate the force constants and thermodynamic properties of thespecies. If the photolysis is carried out in a CO matrix at wavelengths shorter than2800 A the infrared and ultraviolet absorption of CCO may be observed322.Finally mention should be made of an interesting and useful method, involvinga "rotating cryostat" developed by Thomas et aL3',, for the preparation of trappedhydrocarbon radicals. Beams of sodium atoms and halohydrocarbon moleculesare condensed in a vacuum of c torr on opposite sides of the exterior surfaceof a rapidly rotating stainless steel drum containing liquid nitrogen. Radicalsformed by the reactionNa+RX + NaX+RReferences pp. 336-342