The effect of deuteration on the quenching cross section (uQ2) has been determined for propane, cvclopropane, isobutane, and propylene. The results were as follows, with the uQ2 values, in .A2, in parentheses: C3Hs (1.2); CH3CDzCH3 (0.17); CD3CH2CD3 (1.0); C3D8 (0.09); cyclo-C3De (0.29); (CHJ3CH (4.8); (CH3)3CIl (0.44); C3H6 (29.8); and C3D6 (28.7). The deuterium isotope effect 1s most pronounced in the secondary C-H bonds of propane, and the tertiary C-H bond of isobutane. In the latter case, the isotope effect is estimated t o be 15-16. For propylene, perdeuteration has little, if any, effect on oQ2, indicating that C-H bonds are riot involved in quenching. Further implications of the results with respect t o the quenching process are discussed.In recent studies (1, 2) in this laboratory, on the relative initial rates of n-propyl and i-propyl radical formation in the reaction of Hg 6(3P1) atoms with propane and the deuterated propanes, unusually large deuterium isotope effects were encountered in the chemical quenching reactions. Furthermore, deuteration, especially of the secondary C-H positions in propane, has been found (3) to change markedly the relative rates of product formation in -reaction with Hg 6(3P1) atoms. In order to assess the overall effects of deuterium substitution in the mercury-photosensitized reactions of hydrocarbons, a knowledge of the quenching cross sections (aQ2) for the deuterated species is clearly necessary. In the present study, the effects of partial and total deuteration on cQ2 for a number of hydrocarbons of current interest in this laboratory have been determined. The light hydrocarbons (Phillips, research grade) and the deuterated hydrocarbons (hlerck Sharp and Dohme of Canada) used were all further refined by peak-isolation methods in gas-liquid chromatography. The separations were made with a Burrell, Model K-4 chromatograph on a silica gel column, with helium as the carrier gas. Traces of nitric oxide were removed from the nitrous oxide used, by slow distillation from a trap immersed in liquid oxygen. Further distillation of the nitrous oxide a t -131' (n-pentane slush) eliminated the remaining contaminants.For each deuterated hydrocarbon, the isotopic purity was determined by mass spectrometry. The results of the analyses follow, with the composition indicated in parentheses: cyclo-CsDs (92.8%; c~clo-CaDjH, 7.2%) ; (CH3)sCD (97.8%; (CH,)aCH, 2.2%) ; CHaCD2CH3 (98.2; C3Hs, 1.8%) ; C~D S (98.8%, C3D7H1 1.2%); CDaCHzCD3 (100%); C3D6 (94.9%; C~D B H , 5.1%). Since the general method employed has been previously described (4), only details specific for our adaptation of the procedure will be given. The mixt~lres of hq-drocarbon and nitrous oxide were exposed under static conditions in a cylindrical quartz cell, 5 cm in diameter, and 5 cm in length. The total pressure was maintained constant throughout a t 300 mm. Exposures were 5 minutes in duration, follou~ing a period of 3 hours for mercury vapor-liquid equilibration. The temperature for the runs was 25d~2'. In the c...