Infrared Chemiluminescence Technique Research Articles

Detailed energy partitioning in the decomposition of chemically energized C 2H 3F

The HF product energy distribution in the reaction of F atoms with ·C 2H 3 radicals has been directly measured using a modification of the arrested relaxation infrared chemiluminescence technique in which the vinyl radicals are prepared and react to form the observed products in successive chemical reactions. The experimentally observed vibrational and rotational distributions are correctly reproduced by a calculation in which the reaction exoergicity is partitioned statistically among the products of the unimolecular decomposition of energized vinyl fluoride. Some properties of the transition state for this process are thereby elucidated. The statistical energy partitioning likely results from the relatively high (125 kcal mol −1) excitation of the vinyl fluoride intermediate.

Energy distribution among reaction products. XIV. F + CH 4, F + CH 3X(X  Cl, Br, I), F + CH nCl 4− n( n = 1−3)

The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction: ▪ For reaction (1) the detailed rate constants [ k(ν′ = 1) = 0.30; k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <⨍ ′ ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH 3Cl to CH 3Br to CH 3I, viz <⨍ ′ ν> = 0.50 (1a), <⨍ ′ ν> = 0.58 (1b), <⨍ ′ ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <⨍ ′ ν> = 0.50 (1a), <⨍ ′ ν> = 0.23 (2a), <⨍ ′ ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <⨍ ′ ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH 2Cl, CHCl 2, CCl 3, due to increase in the number of secondary encounters between HF and the departing radical.

ChemInform Abstract: INFRARED CHEMILUMINESCENCE STUDIES OF CHLORINE SUBSTITUTION REACTIONS WITH BROMINATED UNSATURATED HYDROCARBONS

The arrested relaxation infrared chemiluminescence technique has been used to investigate the chlorine atom substitution reactions of various olefinic compounds and bromobenzene. Analysis of the infrared emission spectra of the reaction products allows the determination of the product vibrational energy distribution. The observed vibrational energy distributions are compared to those which would be obtained if the products exhibited a statistical vibrational energy distribution. It was found that for the aromatic substrate the product vibrational energy distribution was statistical, while nonstatistical distributions were obtained for olefinic substrates in which Cl was substituted for Br. The implications of the results are discussed in the framework of the current theory of intramolecular energy transfer.

Infrared chemiluminescence studies of chlorine substitution reactions with brominated unsaturated hydrocarbons

The arrested relaxation infrared chemiluminescence technique has been used to investigate the chlorine atom substitution reactions of various olefinic compounds and bromobenzene. Analysis of the infrared emission spectra of the reaction products allows the determination of the product vibrational energy distribution. The observed vibrational energy distributions are compared to those which would be obtained if the products exhibited a statistical vibrational energy distribution. It was found that for the aromatic substrate the product vibrational energy distribution was statistical, while nonstatistical distributions were obtained for olefinic substrates in which Cl was substituted for Br. The implications of the results are discussed in the framework of the current theory of intramolecular energy transfer.

An experimental test of the Bernstein—Levine theory of branching ratios

The first test of the information-theoretic approach to branching ratios has been made for the reaction: F + HD å HF† ( V′, R′) + D å DF‡( V′, R′) + H. The vibrational ( V′) and rotational ( R′) product energy-distributions for both branches of this reaction have been obtained by the infrared chemiluminescence technique, and have been used in the calculation of an information-theoretic branching ratio, Γ HF/DF = 1.41 ± 0.18. This is in excellent agreement with the experimentally measured branching ratio of 1.45. However, results from classical trajectory calculations raise a question as to the significance of this agreement. Classical trajectory calculations (on various energy-surfaces) predict an increase in Γ with reagent J. The information-theoretic analysis applied to the product energy-distributions from these trajectory calculations leads to a qualitatively different result. As a possible alternative to the information-theoretic view, simple kinematic features are noted which could account for Γ > 1, as well as for the significant differences in product energy-distribution. On this alternative view, the two features are not indisolubly linked — the extent to which they appear in conjunction will depend on the nature of the energy surface.

ChemInform Abstract: INFRARED CHEMILUMINESCENCE INVESTIGATION OF THE ABSTRACTION REACTIONS BETWEEN FLUORINE ATOMS AND UNSATURATED COMPOUNDS

The abstraction reaction between fluorine atoms and various olefinic and aromatic substrates has been examined by the arrested relaxation infrared chemiluminescence technique. The relative populations of each rotational–vibrational quantum state of the HF produced in such reactions have been obtained. The fraction of reaction exothermicity entering HF vibration, as well as the relative total cross sections for the substitution and abstraction reactions have been estimated. The results indicate that, typically, about 40% of the exothermicity enters HF vibration, with the HF V=1 state the most likely to be populated, followed by V=2 and V=3.

Infrared chemiluminescence investigation of the abstraction reactions between fluorine atoms and unsaturated compounds

The abstraction reaction between fluorine atoms and various olefinic and aromatic substrates has been examined by the arrested relaxation infrared chemiluminescence technique. The relative populations of each rotational–vibrational quantum state of the HF produced in such reactions have been obtained. The fraction of reaction exothermicity entering HF vibration, as well as the relative total cross sections for the substitution and abstraction reactions have been estimated. The results indicate that, typically, about 40% of the exothermicity enters HF vibration, with the HF V=1 state the most likely to be populated, followed by V=2 and V=3.

Infrared chemiluminescence investigation of fluorine atom substitution reactions

The arrested relaxation infrared chemiluminescence technique has been used to investigate the fluorine atom substitution reactions of various olefinic and aromatic compounds. Analysis of the infrared emission spectra of the reaction products allows the determination of the product vibrational energy distribution. The observed vibrational energy distributions are compared to the distributions which would be obtained if the products exhibited statistical partitioning of the reaction energy. It was found that for all aromatic substrates studied and for olefinic substrates in which F was substituted for Cl or Br, the products exhibited statistical vibrational energy distributions, while nonstatistical distributions were obtained for olefinic substrates in which F was substituted for H or CH3. These results are discussed within a framework of exit channel effects on product energy distributions, and the effects of postcollision intramolecular relaxation.

Energy distribution among reaction products. H + NOCl, H + ICl

The infrared chemiluminescence technique has been used to obtain k(V′, R′, T′) (V′, R′, Tt́ are product vibrational, rotational and translational energies) for the reactions (i) H + ClNO → HCl + NO (energy-release E tot′ = 68.5 kcal mole −1) and (ii) H + CII → HCl + I ( E tot′ = 55.8 kcal mole −1). Reaction (i) exhibits inefficient conversion of energy-release into vibration in the new bond, characteristic of a light attacking atom reacting on a repulsive energy-surface. Reaction (ii) has a bimodal HCl product-energy-distribution suggesting that 18% of the reaction proceeds by direct attack at the Cl end of CII to yield low V′ and R′, and 82% by indirect reaction from the 1 end to give high V′ and R′.

Energy Distribution among Reaction Products. VII. H + F2

The reaction H+F2→ HF+F has been studied by means of the ``arrested relaxation'' variant of the infrared chemiluminescence technique. Detailed rate constants k (V′, R′, T′), where V′, R′, and T′ symbolize the vibrational, rotational, and translational energies in the reaction products, are reported in the form of a contour plot. The total detailed rate constants into specified vibrational quantum states for H+F2 (summed over the rotational levels of each v′ level) are k (v′ = 1) = 0.12, k (v′ = 2) = 0.13, k (v′ = 3) = 0.25, k (v′ = 4) = 0.35, k (v′ = 5) = 0.78, [k (v′ = 6) = 1.00], k (v′ = 7) = 0.40, k (v′ = 8) = 0.26, k (v′ = 9) &amp;lt; 0.16 (relative to the highest rate constant k(v̂′)=1.00). In common with other members of the H+X2 family of reactions (X is a halogen) the H+F2 reaction exhibits a comparatively low fractional conversion of the total available energy into internal excitation of the new molecule. The mean fractions entering vibration and rotation are f̄V′=0.53 and f̄R′=0.03. This behavior appears to be characteristic of reactions involving substantial ``repulsive'' energy release and a light attacking atom. Evidence is presented to show that H+F2 occupies a special place in the H+X2 family in that the repulsive energy release (energy released as the products separate) is restricted to the period when the F atoms are at close range. An approximate relation log Ea∝ ρ2f[a⊥/(1−a⊥)], relating the activation energy Ea to the X–X bond extension at the termination of energy release, ρ2f, and the attractive energy release a⊥, is derived and is tested for the family of reactions H+X2. There is only a very small increase in product rotational excitation with decreasing vibrational excitation. It follows that the translational energy of the products is markedly greater for successively lower v′ states.

Energy Distribution Among Reaction Products. V. H + X2 (X ≡ Cl, Br), D + Cl2

The ``arrested relaxation'' infrared chemiluminescence technique has been used to obtain the distribution of vibrational, rotational, and translational energies (V′, R′, and T′) in the products of the exothermic reactions (i) H + Cl2 → HCl + Cl, (ii) its isotopic analog D + Cl2 → DCl + Cl, and (iii) H + Br2 → HBr + Br. Detailed rate constants k(V′, R′, T′) are reported in the form of contour plots for these three reactions. The total detailed rate constants into specified vibrational quantum states (summed over the rotational levels of each v′ level) are: (i) for H + Cl2, k(v′ = 1) = 0.28, [k(v′ = 2) = 1.00], k(v′ = 3) = 0.92, k(v′ = 4) = 0.1, k(v′ = 5) = 0.05, k(v′ = 6) = 0.005 [the values for v′ = 5 and 6 are from P. D. Pacey and J. C. Polanyi, J. Appl. Opt. 10, 1725 (1971)]; (ii) for D + Cl2, [k(v′ = 1) ∼ 0.1], k(v′ = 2) ≈ 0.3, [k(v′ = 3) = 1.00], k(v′ = 4) = 0.9, k(v′ = 5) = 0.3, k(v′ = 6) = 0.06; (iii) for H + Br2, [k(v′ = 1) = 0.03], k(v′ = 2) = 0.18, [k(v′ = 3) = 1.00], k(v′ = 4) = 0.99, k(v′ = 5) = 0.2, k(v′ = 6) ≤ 0.002. All of these reactions exhibit a comparatively low fractional conversion of the total available energy into internal excitation of the new molecule. The mean fractions entering vibration plus rotation are (i) for H + Cl2, f̄V′ + f̄R′ ( = 0.39 + 0.07) = 0.46; (ii) for D + Cl2, f̄V′ + f̄R′ ( = 0.39 + 0.10) = 0.49; (iii) for H + Br2, f̄V′ + f̄R′ ( = 0.55 + 0.04) = 0.59. The relatively inefficient conversion of reaction energy into vibration is thought to arise from the combined effect of ``repulsive'' energy release and a light attacking atom. As in the case of other isotopic pairs of reactions there is a close parallelism in k(V′) though not in k(v′) between the members of the pair (H+Cl2 and D+Cl2 in the present case). The increased fractional conversion of the available energy into vibration for H+Br2 as compared with H+Cl2 is indicative of a less-repulsive potential-energy surface. This is in accord with expectation, based on the change in barrier height and consequent change in barrier location on the energy surface. These reactions exhibit only a very small increase in product rotational excitation with decreasing vibrational excitation. It follows that the translational energy of the products is markedly greater for successively lower v′ states.

Energy Distribution Among Reaction Products. IV. X+HY (X ≡ Cl, Br; Y ≡ Br, I), Cl+DI

The distribution of vibrational, rotational, and translational energies (symbolized by V′, R′, and T′) have been obtained for the products of the isotopic pair of reactions Cl + HI → HCl + I and Cl + DI → DCl + I. The experimental method was the ``arrested relaxation'' variant of the infrared chemiluminescence technique. Detailed rate constants k(V′, R′, T′) are reported in the form of contour plots for these reactions. The total detailed rate constants into specified vibrational quantum states for Cl+HI (summed over the rotational levels of each v′ level) are k(v′ = 1) = 0.22, k(v′ = 2) = 0.35, [k(v′ = 3) = 1.00], k(v′ = 4) = 0.74; and for Cl + DI, k(v′ = 1) ≈ 0.08, k(v′ = 2) = 0.14, k(v′ = 3) = 0.35, k(v′ = 4) = 0.73, [k(v′ = 5) = 1.00], k(v′ = 6) = 0.05 [relative to the highest rate constant k(v̂′) = 1.00, in each case]. Preliminary energy distributions are also reported for the products of two other reactions, Cl + HBr → HCl + Br and Br + HI → HBr + I, in the X + HY → HX + Y family (X and Y are halogen atoms). The members of this family of reactions channel a substantial fraction of the energy available to the products into vibrational and rotational excitation, and only a small fraction into relative translation. For the Cl+HI and also the Cl+DI reaction the fractions are f̄V′ + f̄R′ (= 0.71 + 0.13) = 0.84; in contrast to f̄T′ = 0.16. As a corollary there is a marked inverse correlation between vibrational and rotational excitation in the reaction products. Despite the fact that the detailed rate constants into specified product quantum states [k(v′, J′)] are markedly different for the isotopic pair of reactions, the fractional conversion of the available energy into vibration, rotation, and translation are in close agreement (to ∼ ± 1%). This close parallelism in f̄V′, f̄R′, f̄T′ is in accord with predictions from classical trajectory studies. It indicates the usefulness of such calculations even for the case of reactions which yield products with widely spaced (vibrational and rotational) quantum levels.

Initial vibrational energy level distributions determined by infra-red chemiluminescence

The reactions of atomic fluorine with HCl, HBr and HI have been studied by the infra-red chemiluminescence technique. Emission has been detected from all of the vibrational levels v⩽3 >(HCl), v⩽4 >(HBr) and v⩽6 >(HI) that may be expected to be directly populated. Population inversion is recorded in each case and relative primary rate constants are determined. The relative overall rate constants for the reaction of atomic fluorine with H2, CH4, HCl, HBr and HI have also been measured by a chemiluminescence method. The data are used to estimate the relative activation energies for the reactions.