Reactions of sulfur atoms. XIII. Experimental and calculated secondary hydrogen-deuterium kinteic isotope effect for the S(3P) + ethylene reaction

Abstract

ferred to the carbonyl group of these peptides. Instead, the pmr results suggest to these authors that the acid proton remains hydrogen bonded to the peptide carbonyl oxygen. As the concentration of acid increases, acid-polypeptide interaction and hydrogen binding without proton transfer increases. Intramolecular hydrogen bonding is, therefore, suppressed in favor of intermolecular hydrogen bonding between the acid and the polypeptide. This then is the driving force converting the polypeptide from the helical to the random coil form.The slight low-field shift and the small degree of line broadening of the I phenolic proton peak in the presence of poly-y-benzyl-L-glutamate and poly-L-methionine, respectively, encountered in the present studies also suggest that I hydrogen bonds to these polypeptides and that proton transfer does not occur. The pmr spectrum of poly-y-benzyl-L-glutamate was not altered in the presence of I. This indicates that either the low solubility or the moderate acid strength of I could not elicit peptide confirmational changes of the type previously reported for trifluoroacetic acid-polypeptide systems. 1, 2 3Hexachlorophene has now been shown to strongly hydrogen bond to polypeptides. The conclusions from the present investigations are useful in understanding the more complex interactions that occur between I and proteins.(23) J.Abstract: An experimental and theoretical study has been made of the secondary H/D kinetic isotope effect in the addition of S( 3P) atoms to ethylene. The values of the experimental kD/kH ratios were 1.14, 1.07, and 1.04 for the C2D4-C2Hl, CD2CHz-C2H4, and cis-CHDCHD-CzH4 reactant pairs, respectively. Calculation of the isotope effect was carried out within the framework of transition-state theory for two basic models of the transition state, a symmetrical ring and an asymmetrical ring-distorted structure. Either model can be made compatible with the experimental results by an appropriate and reasonable choice for the structural parameters of the transition state. Calculation of the isotope effect arising from each normal vibrational mode clearly shows that, contrary to current notions, the most important single factor inducing a secondary H/D isotope effect in addition reactions involving olefinic double bonds is not the out-of-plane bending motions of the CH bonds, but the gain in the isotopically sensitive vibrational degrees of freedom on going from reactant to activated complex. he addition of both lD2 excited and 3P ground-

Reactivities in Free-radical Polymerization

“…The rather small value of the secondary isotope effects observed when the alkene is substituted with deuterium at the reaction centre has been interpreted as supporting this picture of the reaction. Note, however, that detailed calculations of Strausz et al 23 do not support the simple interpretation of the magnitude of the secondary isotope effect given above. Note, however, that detailed calculations of Strausz et al 23 do not support the simple interpretation of the magnitude of the secondary isotope effect given above.…”

“…Styrene and vinyl acetate give linear (type 0) plots and many familiar monomers yield type 1 patterns, e.g. Bamford et al 53 developed relation (23) in which fc s is the rate coefficient for reaction (addition or transfer) of radical R # with substrate S, and evaluated p R (actually differences in p R ) from the experimental patterns. Monomers giving type 2 are less common, but include p-methoxystyrene, p-diethylaminestyrene and iV-vinylurethane.…”

Electrophilic Bromination of Specifically Deuterated Cyclohexenes: A Combined Experimental and Theoretical Investigation

“…The older 29 considers the change of some CH vibrational modes due to rehybridization: in a reaction that involves sp 2 to sp 3 rehybridization at the R-carbons an inverse secondary isotope effect should be observed. A more recent, alternative explanation of the secondary isotope effects was offered by Strausz et al 30 who attributed the effect to the creation of new, isotopically sensitive, vibrations in the transition state. The case of bromination of cyclohexene does not seem to fall into either category.…”

Octahedral metal carbonyls. 71. Kinetics and mechanism of benzene displacement from photogenerated (.eta.2-benzene)chromium pentacarbonyl

“…The slight difference between , and 2 may be Zhang et al attributable to secondary isotope effects. 33 Streitwieser et al have proposed that such an isotope effect might arise from the change in frequency of out-of-plane C-H bending modes for sp2-and sp3-hydridized carbons.34 Applying the Streitwieser concept to the interaction of Cr to an isolated double bond of benzene, which is accompanied by the sp2 to sp3 rehybridization of the two carbon atoms involved, one would expect a greater zero-point energy difference in ^-coordinated benzene than in free benzene, as is observed. However, since the interaction of benzene with [Cr-(CO)5] is relatively weak overall, secondary isotope effects might not be expected to be important in the transition state.…”