Simple Bimolecular Process Research Articles

The kinetics of elementary reactions involving the oxides of sulphur III. The chemiluminescent reaction between sulphur monoxide and ozone

The chemiluminescent reaction between sulphur monoxide (SO) and ozone has been studied in a fast flow system at pressures between 0·3 and 3·0 mmHg, These species undergo a rapid bimolecular reation (1) SO + O 3 = SO 2 + O 2 + 106 kcal/mole (1) to yield ground state products, where k 1 = 1·5 x 10 12 exp ( –2100/ RT ) cm 3 mole -1 s -1 . This reaction also yields electronically excited SO 2 molecules in the 1 B and 3 B 1 states. The 1 B SO 2 molecules are produced with up to 16 kcal/mole vibrational energy. Emission from the longer lived 3 B 1 state is vibrationally relaxed and provides no information about the initial energy distribution. Comparison with fluorescence studies shows that the 3 B 1 SO 2 molecules are produced mainly by collisional quenching of SO 2 molecules formed in the 1 B state. The formation of electronically excited SO 2 is also a simple bimolecular process, but it involves a higher energy barrier than formation of ground state SO 2 . Our measurements on the chemiluminescence, when combined with data on the quenching of the SO 2 fluorescence, yield the rate constants k 1a = 10 11 exp ( – 4200/ RT ) and k lb ≯ 3 x 10 10 exp ( –3900/ RT ) cm 3 mole -1 s -1 for the bimolecular reactions SO + O 3 = SO 2 ( 1 B ) + O 2 + 21 kcal/mole, (1 a ) SO + O 3 = SO 2 ( 3 B 1 ) + O 2 + 35 kcal/mole (1 b ) which form electronically excited SO 2 . No electronically excited O 2 appears to be formed. It is deduced that electronically excited SO 2 is produced by crossing to a separate potential surface at or near the transition state rather than by the formation of a highly vibrationally excited SO 2 molecule which crosses to the excited electronic state.

Photolysis of NO2in the presence of SO2at 3660 Å

Nitrogen dioxide was irradiated at 3660 Å in the presence of SO2. Quantum yields were measured as a function of SO2 pressure, and the specific rate constant for O+SO2→SO*3 was determined to be (1.1±0.1)× 109 l. mole–1 sec–1. The ratio of rate constants for the proposed reactions: SO*3→O+SO2 and SO*3+M→SO3+M, was estimated to be 0.077 mole l.–1 by the combination of the results of quantum yields with those of isotopic oxygen scrambling. A dark reaction was also observed which, if attributed to a simple bimolecular process, yields kD= 3.7 × 10–3 l. mole–1 sec–1.

Selective carboxymethylation of functional sulfhydryl groups at the active center of horse liver alcohol dehydrogenase

Glycerol-3-phosphatedehydrogenase (sn-glycerol-3-phosphate:NAD+ 2-oxidoreductase, EC 1.1.1.8) has been shown to be sensitive to inhibition by iodoacetate. The reaction of the enzyme with iodoacetate, which appears to be a simple bimolecular process, is accompanied by a corresponding loss of enzyme activity. In addition to changes in activity, the alkylation reaction was monitored by the incorporation of radioactivity from iodo[2-14C]acetate, by changes in amino acid composition, and by changes in the content of free sulfhydryl groups. It is concluded that there are two cysteine residues in the native dimeric enzyme which are essential for enzymic activity. The rate of inactivation was relatively insensitive to the presence of various compounds with the exception of NADH which markedly inhibited the reaction. Kinetic and binding studies showed that the binding of NADH prevents alkylation and, conversely, alkylation prevents NADH binding. From the pH dependence of the alkylation reaction, the pKa of the essential sulfhydryl groups was calculated to be 8.5 and it is suggested that the binding of coenzyme is independent of the state of ionization of these groups.