Nitrous oxide with excited molecules

The Reaction of Nitrous Oxide with Excited Molecules in the Radiolysis and Photolysis of Liquid Alkanes... 

Radiolysis. The photochemical experiments suggest that in the radiolysis a reaction of nitrous oxide with excited molecules would be expected in cyclohexane but should be less important in 2,2,4-trimethylpentane. The radiolysis results (Figure 3 and Table III) show that at nitrous concentrations less than 10 mM, where reactions of excited molecules are unimportant, G(N2) is the same for cyclohexane and 2,2,4-trimethylpentane solutions. At concentrations of nitrous oxide from 20 to 160 mM, G(No) from cyclohexane solutions is greater than G(N2) from 2,2,4-trimethylpentane solutions, and the excess yield increases with the concentration of nitrous oxide. [The nitrogen yields reported here for the concentration range 5-200 mM are in good agreement with those reported by Sherman (20)] Nitrous oxide reduces G(H2) from cyclohexane (16, 17, 18, 20, and Table III), but it has little effect on G(H2) and G(CH4) from 2,2,4-trimethylpentane. 

Hobroyd RA (1968) The reaction of nitrous oxide with excited molecules in the radiolysis and photolysis of liquid alkanes. In Gould RF (ed) Radiation chemistry II. Advances in Chemistry Series. American Chemical Society, Washington... 

Kinetic studies of the competitive reactions of other electron scavengers support this hypothesis (18, 20). In the radiolysis of solutions of nitrous oxide in alkanes, reactions with other intermediates must be considered. Radicals, hydrogen atoms, and positive ions can be eliminated (5, 20), but a reaction with excited molecules is possible. It has been reported... 

There are several possible explanations which need to be considered for the formation of N2 in the photolysis of cyclohexane-nitrous oxide solutions. These include direct absorption of vacuum ultraviolet light by nitrous oxide, photoionization of the solvent followed by electron attachment by nitrous oxide, and reaction of nitrous oxide with either excited cyclohexene or excited cyclohexane molecules. Of these possibilities only the last explanation—reaction of excited cyclohexane molecules with nitrous oxide—is important. 

On the other hand, it has been argued by Burton et al. (2, 3, 4) that the lifetime of singlet excited cyclohexane molecules is too short ( 10-13 sec.) to be observed. This conclusion is based largely on the behavior of scintillators containing fluorescent solutes dissolved in cyclohexane. It is important at this point to emphasize that at solute concentrations close to millimolar (which are usually employed in scintillator studies) excited cyclohexane molecules cannot be detected by nitrous oxide. Higher concentrations of both nitrous oxide and benzene (25) are required to observe energy transfer. This study confirms that in the radiolysis of cyclohexane, solutes at millimolar concentrations interact mainly with the electron and not with excited molecules. 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

For the diatomic molecules that were studied—nitrogen, oxygen, nitric oxide, and carbon monoxide—the concept of a Coulomb explosion appears to be relevant. The yield of atomic ions is high, 93% to 97%, and the ion kinetic energies of around 7 eV for +1 ions and about twice this value for -1-2 ions are consistent with the Coulomb repulsion model. For the polyatomic molecules the situation is different. The yield of atomic ions drops to 85% for carbon dioxide and to 74% for carbo i tetrafluoride. For excitation of a core to bound state resonance in nitrous oxide, involving the terminal nitrogen atom, the yield of atomie ions is only 63% (Murakami et al. 1986). These molecules do not simply explode following excitation of a core electron. 

The radiation induced cis-trans isomerization of 2-butene in benzene, benzene-dr, toluene, and pyridine has been studied. The triplet yield in benzene is 4.7 molecules per 100 e.v. benzene-dr, is similar. The yield in toluene is 6 molecules per 100 e.v. Nitrous oxide reduces the isomerization yield by about 40%. This indicates a G value of about 2 for excited states produced by the recombination of benzene ions with electrons. Xenon and krypton (up to 1M) increase the isomerization yield from 2.4 to 3.1 and 2.9 respectively, whereas argon has no effect. From the effect of xenon, which is attributed in part to enhanced intersystem crossing, it is estimated that the yield of singlet states which do not cross to the triplet in unperturbed benzene solutions is 1.3 molecules per 100 e.v. 

In the 1470-A. photolysis of cyclohexane-nitrous oxide solutions, nitrous oxide reacts with excited cyclohexane molecules to form nitrogen and oxygen atoms. The reaction of N20 with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N2) is the same for different alkanes at low 5 mM) N20 concentrations. At higher concentrations, G(N2) from the radiolysis of cyclohexane is greater than G(N2) from the radiolysis of 2,2,4-trimethylpentane solutions. The N2 yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N20. The yield of N2 in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 

In other alkanes the reaction of excited molecules with nitrous oxide is less important, and in some cases it may not occur at all. In n-hexane the N2 yields are about one-half what they are in cyclohexane. In other alkanes such as 2,2,4-trimethylpentane the yields of N2 were quite low. A small yield could be attributed to one of the other effects discussed, but if it is attributed to excited alkane molecules, then energy transfer to nitrous oxide is much less important in 2,2,4-trimethylpentane than in cyclohexane. 

Emission from electronically excited TiO molecules has been observed by Palmer and co-workers from the reaction of titanium tetrachloride or tetrabromide with potassium vapour in the presence of oxygen [277] and of nitrous oxide [278]. The potassium atoms presumably strip the halogen atoms from the titanium tetrahalide, and the titanium atoms then react with the oxygen or nitrous oxide producing electronically excited TiO molecules. 

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