Isotope effect studies chlorine

Although GC-C-IRMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study [614,677,678]. Measurement of chlorine stable isotope ratios in natural samples such as rocks and waters has become routine [626,679,680], but few measurements of chlorine isotopes in chlorinated aliphatic hydrocarbons have been reported [614]. A chlorine isotope effect was found in ferf-butyl chloride [681], demonstrating that 37Cl is more strongly bound to carbon than is 35Cl. Significant differences in the <5i7Cl values of some atmospheric chlorinated... 

Obviously, there is an isotope effect on the vibrational frequency v . For het-eroatomic molecules (e.g. HC1 and DC1), infrared spectroscopy permits the experimental observation of the molecular frequencies for two isotopomers. What does one learn from the experimental observation of the diatomic molecule frequencies of HC1 and DC1 To the extent that the theoretical consequences of the Born-Oppenheimer Approximation have been correctly developed here, one can deduce the diatomic molecule force constant f from either observation and the force constant will be independent of whether HC1 or DC1 was employed and, for that matter, which isotope of chlorine corresponded to the measurement as long as the masses of the relevant isotopes are known. Thus, from the point of view of isotope effects, the study of vibrational frequencies of isotopic isomers of diatomic molecules is a study involving the confirmation of the Born-Oppenheimer Approximation. 

For heavy atom isotope effects tunneling is relatively unimportant and the TST model suffices. As an example the dehalogenation of 1,2-dichloroethane (DCE) to 2-chloroethanol catalyzed by haloalkane dehalogenase DhlA is discussed below. This example has been chosen because the chlorine kinetic isotope effect for this reaction has been computed using three different schemes, and this system is among the most thoroughly studied examples of heavy atom isotope effects in enzymatic reactions. 

Urry (13) studied hydrogen abstraction from toluene a-d versus toluene by chlorine, bromine and CH COCV radicals and found that the attack on toluene a-d. is slower than that on toluene. The isotope effect was found to be in the range of 2.2 to 9.9. 

This study on the kinetic chlorine isotope effect in ethyl chloride50 was extended to secondary and tertiary alkyl halides pyrolyses51. The isotope effects on isopropyl chloride and terf-butyl chloride pyrolysis were found to be primary and exhibited a definite dependence on temperature. They increased with increasing methyl substitution on the central carbon atom. The pyrolysis results and model calculations implied that all alkyl chlorides involve the same type of activated complex. The C—Cl bond is not completely broken in the activated complex, yet the chlorine participation involves a combination of bending and stretching modes. 

Jones has studied the kinetic isotope effect in the chlorination of H2 and HT inducing the reaction with visible radiation and also with tritium p-particles. He found the same isotope effect with either method. The reactions and quantity of interest are... 

Corneil and Pimentel have studied, near room temperature, the hydrogen-chlorine explosion laser finding an isotope effect in the CI2-HD system for the reactions... 

The rates of reaction of hypophosphorous acid with iodine bromine ", chlorine ", iodine chlorides , iodate , selenious and tel-lurous acids, silver nitrate , cupric chloride and mercuric chloride" (all forming phosphorous acid or phosphites) have been measured, and the results of the earlier work summarized clearly" . All the data are consistent with the hypothesis that there is prior transformation to some reactive form (I). This form (I) does not discriminate very effectively between different oxidants and thus the oxidation steps are presumed to have rates close to the diffusion-controlled limit. The rates of formation of I deduced in these studies are close enough to the rates of deuterium and tritium exchange for the residual difference to represent an isotope effect. Mitchell wrote the formula H5PO3 for I. Others have supposed it to be a tautomer e.g. HPO(OH)2. Both the isotopic exchange results and the oxidation studies require that its formation and decomposition be subject to acid catalysis. For the general mechanism... 

The ring-closure mechanism of 2-chloroethanol has been studied on the basis of kinetic and equilibrium chlorine isotope effects. Epoxidation of the terminal double bond of farnesyl acetate has been achieved via the bromohydrin, obtained with NBS. A stereospecific method has been elaborated for the preparation of 1-alkynyloxiranes, starting from the monotosylate ester of acetylenic diols. 1-Alkynyloxiranes are also formed from a-hydroxy quaternary ammonium salts in alkaline medium (Eq. 57). ... 

In undeuterated dioxane, Hgq and coincidentally have the same chemical shift (at the field studied), so they cannot be differentiated at low temperatures. (See Sections 1-8 and 5-2.) In l,4-dioxane- 7 (an impurity in commercial l,4-dioxane- 5 g), both and Hgq exhibit isotope shifts to a lower frequency, but H x is shifted somewhat farther. As a result, the axial and equatorial protons give separate resonances at low temperatures, in contrast to the undeuterated material. Because of a chlorine isotope effect, chloroform is a poor substance for an internal lock or a resolution standard at fields above about 9.4 T. At high resolution, the chloroform proton resonance shows up as several closely spaced peaks, due to CH( 5c1)( C1)2, CH( C1)2( C1), CH( C1)3, and CH( C1)3. 

Thorough kinetics studies of the chlorination of aliphatic, alicyclic, and arylalkyl ketones with CBT were carried out by Indian workers (82PIA921). Kinetic measurements were performed using aqueous acetic acid and the addition of HC104 and NaCl. In the presence of mineral acid the reaction is first order in ketone and acid and zero order in CBT. A large kinetic isotopic effect was observed (for acetone kHlkD = 6.6). Addition of chloride ion causes some changes in the reaction order they become first order in CBT, 0.6 in ketone, and 0.2 in chloride ion. The rate constant for chlorination of substituted acetophenones correlate with a constants for substituents in the aryl ring (p is -0.57). On the basis of these data the mechanism in the absence and in the presence of chloride ion was developed. 

Isotope effect and relative rate studies also suggest an early TS for benzylic chlorination and bromination. The benzylic position is only moderately activated toward uncomplexed chlorine atoms. Relative to ethane, toluene reactivity is increased only by a factor of 3.3. The kinetic isotope effect observed for bromination and chlorination of toluene suggest little rehybridization at the TS. 

Possibly the measurement of the chlorine isotope effect for a series of suitably substituted substrates may help to decide whether or not greater stereospecificity is associated with decreased carbanion character of the transition state for elimination from alkenes. Of course, such studies must be accompanied by labelling experiments to exclude the possibility that alkynes arise by an apparent alpha-elimination (Section 5.1) involving formation of a carbanion at the alpha carbon, which subsequently eliminates alpha halogen simultaneously with migration of a substituent from the beta carbon. Results with alcoholic media, however, tend to indicate this alternative mechanism is unimportant if a normal beta-elimination is possible. 

In a recent review work (117) on the chemical and nano-structural characterization of NM/CeO catalysts, a detailed study of the H interaction with a Pt/CeO catalyst reduced at temperatures ranging fh)m 473 K to 773 K is reported. The experimental techniques used in this work were TPD-MS and Isotopic Transient Kinetics (ITK) of the H2/D2 exchange at 298 K. The catalyst sample was carefully selected in order to minimise the Pt and support sintering effects in the investigated range of reduction temperatures. Likewise, a chlorine-free metal precursor, [Pt(NH3)4](OH)2, was used in the preparation of the catalyst. 

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