Deuterium isotope studies

Although this process is unusual, a similar one has been suggested for a 9-substituted isoalloxazine derivative where C=N photoreactivity is also involved. Distinction between the above reaction and abstraction through a disfavored five-membered transition state (but from a far more reactive -CHgOH) has also been made on the basis of deuterium isotope studies. 

Application of pulsed ion gas-phase cyclotron resonance (ICR) spectroscopy to proton affinities of the derivatives 2-methoxypyridine and N- methylpyridin-2-one confirm previous deductions on the enthalpy of 2-hydroxypyridine-pyridin-2-one tautomerism (76JA6048), provided that the difference between the influences of O-methylation on 2-hydroxypyridine and A-me thy lat ion in pyridin-2-one are taken into account. These measurements have been further clarified and extended to other gas phase basicity measurements (79JA1361). A similar estimation of the gas phase basicities of 2- and 4-pyridinethiols and 2- and 4-pyridinethiones confirms that the thiol form is predominant in the gas phase (77TL1777), in line with previous studies involving mass spectrometric deuterium isotope studies (75BSB465). Photoelectron spectroscopy has also been employed in such studies (see Section 2.04.3.6 and Figure 21 for details) <77JCS(P2)1652>. 

The proposed transition states have been supported by deuterium isotope studies Evidence such as the decomposition behaviour in solution and the nature of the increases in the rates of decomposition along the series of chloro-formates methyl, ethyl, isopropyl, 2-butyl, /-butyl suggests that the transition states are somewhat polar °> . Lewis and Herndon found 2-methylbut-1-ene and 2-methylbut-2-ene as the olefinic products of the elimination reaction of neopentyl chloroformate, and the kinetic evidence supports a Wagner-Meerwein rearrangement in the gas-phase as in the case of neopentyl chloride pyrolysis (refs. 407, 408, 566). 

The reaction behaviour for the thermal decomposition of the chlorosulphites in solution parallels that of the chloroformates very closely. Thus, Boozer and Lewis " found that when 2-pentyl chlorosulphite is thermally decomposed in dioxane solution at 62 °C both 2-chloropentane and 2-pentene are formed. The relative amounts of the substitution and elimination reactions in this instance are 51 % and 49 % respectively. With allylic chlorosulphites, substitution is accompanied by rearrangement . Boozer and Lewis - have carried out configurational checks on the mechanism of thermal decomposition in solution and deuterium isotope studies have also been made . As with the chloroformates, the experimental evidence points to transition states of a polar nature . Gas phase decompositions of chlorosulphites have been studied by Rieche and Nau-mann and Geiseler et a/. - and radical-chain mechanisms have been proposed . 

Further experimental results (see also ref. 164) on the stereospecificity of [2 -1- 2] cycloadditions are available. Tetrafluoroethylene adds to both cis- and tra s-2-butene and to ethylene in a non-stereospecific manner, interpreted as evidence of biradical intermediates. Some stereospecificity is observed in allene-keten [2 + 2] cycloadditions and a one-step [,2, + J cycloaddition is indicated. Further deuterium-isotope studies indicate that the cyclodimerization of allenes by a [2 q- 2] addition is non-concerted, but their reactions in [3 + 2] and [4 + 2] cycloadditions are concerted. ... 

Additional significant experimental studies were also carried out by others. Arnett reported valuable calorimetric studies. Saunders showed the absence of the deuterium isotopic perturbation of eqnilibrium ex-... 

Hydrogen as it occurs in nature is predominantly composed of atoms in which the nucleus is a single proton. In addition, terrestrial hydrogen contains about 0.0156% of deuterium atoms in which the nucleus also contains a neutron, and this is the reason for its variable atomic weight (p. 17). Addition of a second neutron induces instability and tritium is radioactive, emitting low-energy particles with a half-life of 12.33 y. Some characteristic properties of these 3 atoms are given in Table 3.1, and their implications for stable isotope studies, radioactive tracer studies, and nmr spectroscopy are obvious. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... 

For an elegant study of the secondary deuterium isotope effects on a non-Schenck sensitized cis-trans isomerization, see Refs. 85-87. 

The measurement of deuterium isotope effects on chemical shifts is a useful tool for studies of tautomeric equilibrium (for details see the reviews 42 44). The deuterium isotope effect AX(D) is defined as the difference between chemical shifts in deuterated and non-deuterated sample [26]. 

T. Deuterium isotope effects on 13 C chemical shifts In the studies of proton transfer equilibrium of Schiff bases, the most informative are deuterium isotope effects measured for carbons bonded with proton donor groups (C-2 or C-7 for gossypol derivatives). For imines in which... 

Deuterium isotope effects in CDCI3 have been applied in studies of proton transfer equilibrium for optically active Schiff bases [24], [25] as well as their dirhodium tetracarboxylate adducts.11 The AC-2(D) values were in the range from +541 to —546 ppb for imines and from +524 to 255 ppb for their adducts. Exceptionally low values of deuterium isotope effects of - 20 —80 ppb were observed for the Schiff bases being derivatives of 3,5-dinitrosalicylaldehyde which exist in pure NH-form. This behaviour was explained by different electronic structures of the NH... 

This all seemed very reasonable at the time, but subsequent work was not consistent with it. A small but measurable amount of 180 exchange was reported for some amides in reasonably concentrated HC1 media,277,278 and for at least one amide the amount of exchange decreased with increasing acidity,277 which is the opposite of what would be expected with the Scheme 14 one-water-molecule mechanism taking over from the equation (74) three-water-molecule mechanism as the acidity increased. Also, the solvent deuterium isotope effect was found to be close to unity for at least one amide,278 a result that has since been confirmed,279 which is not what would be expected on the basis of either a three- or a one-water-molecule process.280 Because of this it was decided to reexamine the lactam hydrolysis data subsequent to the publication of the excess acidity analysis of the H NMR results for these,268 a new study appeared with rate constant data for four of these molecules in aqueous H2S04 media obtained by UV spectroscopy at several temperatures,281 and this was included too.282... 

The effect of deuterium isotope substitution on Tc has been studied for three members of the KL-(BEDT-TTF)2Ag(CF3)4(l,l,2-trihaloethane) family [12, 34-36]. In all cases, the Tc increased upon deuteration with the effect ranging from 0.21 to 0.36 K. These results are similar to the deuterium isotope effect observed in the p"-(BEDT-TTF)2SF5CH2CF2S03 [34, 37] and k-(BEDT-TTF)2Cu(NCS)2 [38] superconductors. 

A. Carbon-14 and Deuterium Isotope Effect Studies of the Diels-Alder... 

The intramolecular and intermolecular deuterium isotope effects in the cycloaddition of acrylonitrile to allene (equation 98) have been studied by Dolbier and Dai231,232. The intramolecular KIEs in the allene-acrylonitrile system were found to be 1.21 0.02 at 206°C and 1.14 0.02 at 225°C. A negligible intermolecular SKIE was found in the reaction of the mixture of tetradeuteriated and undeuteriated allene using a limited amount of acrylonitrile (ku/ku) = 1.04 0.05 at 190-210 °C for D0/D4 allene. An equilibrium deuterium IE of 0.92 0.01 was found at 280-287 5°C (15-45 h reaction time). 

Wells and coworkers53 have prepared a series of deuterated tetramethyltin compounds, which they used to study the long-range deuterium isotope effects on the proton chemical shifts of tetramethyltin. The various deuterated tetramethyltin compounds, with one to four trideuteromethyl groups on the tin atom, were prepared by a series of methyl group exchanges beginning with tri-trideuteromethyltin chloride and undeuterated tetramethyltin (Scheme 12). 

Wei and collaborators180 studied the long-range deuterium isotope effects on the proton chemical shifts of tetramethyllead and chlorotrimethylplumbane. The perdeuterated analogues of these organolead compounds were generated in situ by reaction of equimolar amounts of (CD3)3SnCl and (CH3)4Pb in methanol-d, in equation 118. 

At low hydroxide-ion concentrations, the rate of approach to equilibrium after a temperature jump decreases as the hydroxide-ion concentration increases. At higher concentrations the reaction becomes first order in hydroxide ion. The value of the kinetic solvent deuterium isotope effect on the reaction shows little variation over the range of hydroxide-ion concentrations studied as shown in Fig. 19. The ratio t-1(H20)/t 1(D20) at a particular concentration of OL (L = H or D) remains within the range 2.0 to 3.0 for OL" concentrations of 0.001 to 0.100 mol dm - 3 and provides little mechanistic information. Similar results were obtained in the original work (Perlmutter-Hayman and Shinar, 1978). 

A more successful strategy for developing sensitive and facile assays to monitor PLCBc activity involves converting the phosphorylated headgroup into a colorimetric agent via a series of enzyme coupled reactions. For example, phosphatidylcholine hydrolysis can be easily monitored in a rapid and sensitive manner by enzymatically converting the phosphorylcholine product into a red dye through the sequential action of alkaline phosphatase, choline oxidase, and peroxidase [33]. This assay, in which 10 nmol of phosphorylcholine can be readily detected, may be executed in a 96-well format and has been utilized in deuterium isotope and solvent viscosity studies [34] and to evaluate inhibitors of PLCBc [33] and site-directed mutants of PLCBc [35,36]. 

Kopple K, Meyerstein D, Meisel D (1980) Mechanism of the catalytic hydrogen production by gold sols. Hydrogen/deuterium isotope effect studies. J Phys Chem 84 870-875... 

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