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The a-deuterium isotope effect has been examined for the solvolysis of a series of esters containing a fully deuterated methyl group. The possible sources of the effect have been divided into "thermodynamic" effects which appear to favor more rapid reaction of the protium compound and "zero point" effects where stiffening of out-of-plane vibrations may account for the direction of the observed isotope effects. It appears that the inverse isotope effect may be a measure of the spatial restrictions placed on the hydrogen atoms on the carbon atom in the activated complex.The substitution of deuterium for a-hydrogen in alkyl halides and substituted sulphonates has been shown to produce varying effects on reaction rates. Thus the solvolysis of methyl p-bromobenzenesulphonate in ~nethanol and the reaction with ~nethoxide ion are accelerated by a-deuteration ( 1 ) . Isopropyl bromide reacting with ethoxide ion is apparently unaffected by this substitution (2), and phenyl ethyl toluenesulphonate solvolyzes in formic acid a t a reduced rate when isotopically substituted a t the a-hydrogen (3). Johnson and Lewis systematized these changes, suggesting that they were the consequence of the sensitivity of the C-H bending vibrations to the presence of entering and leaving groups and noted a decrease in the isotope effect, kH/kD, with increasing necessity for nucleophilic attack from the rear ( 1 ) . Fro111 information available a t the time the); were unable to choose between several possible sources for the inverse isotope effect observed for nlethyl esters.Streitwieser and co-workers (4) have observed a reduction in the acetolysis rate for cyclopentyl p-toluenesulphonate followi~lg a-deuteration. They have attributed this to the change in vibrational frequencies involving the a-hydrogen, produced by the difference in hybridization between initial and transition states, together with the effect of the leaving group which was postulated to stiffen the out-of-plane vibrations of the hydrogen.Thus there appear to be three factors for consideration in assessing the zero-point contribution to the change in rate for nucleophilic displace~nents caused by a-deuterium substitution. These factors are: ( a ) the effect of the incoming group, ( b ) the effect of the leaving group, and (c) the change in hybridization.Factors ( a ) and (b) will be related, inasmuch as bulky entering or leaving groups will be expected to stiffen the out-of-plane C-H bending modes. This effect will be modified by distance, but clearly, stiffening will increase the disparity in the energy levels of deuterium and hydrogen compounds in the transition state, and will produce changes tending to reduce the free energy of activation for the deuteriu~ll compound, conlpared with that of the protiu~n compound. Reactions having the highest degree of nucleophilic interaction with large entering groups will presumably have the most stiffening.The change in hybridization will also affect the characteristic frequencies but presents a problem in that even a qualita...

The secondary p-deuterium isotope effects have been measured i n the water solvolytic reaction of allcyl halides and s~ilphonates for primary, secondary, and tertiary species. In every case the kinetic isotope effect was greater than unity ( k a l k o > 1). This isotope effect may be associated with varying degrees of hyperconjugation or altered non-bonding intramolecular forces. The esperinlents make it difficult to decide which effect is 1110st important.As an extension of our morlc (1, 2, 3) on cleuterium kinetic isotope effects we have measured the Itinetic consequences of P-deuteration for a series of compounds solvolyzing in water, RX + HzO 4 ROH + H+ + X-. [I]The kinetic effect proclucecl by substituting deuterium for P-hydrogen in molecules undergoing solvolytic substitution has been studied by Shiner (4, 5 , B), Lewis et al. (7, 8 , 9), Streitwieser et al. ( l o ) , ancl others (11, 12, 13). This paper presents a survey for a series of alkyl halicles ancl sulphonates arlcl the results of these measure~nents have been collected in Table I. DISCUSSION I t will be notecl that in all cases the deuterated compounds react more slo\vly than their protium analogues, the magnitucle of the total isotope effect increasing both absolutely and relatively as the transition state becomes more ionic. T h u s the effect of deuteration for a given alkyl group is greater in the s~~l p h o n a t e s than for the halides and increases with increasing a-meth? lation. Since some of the colnpou~lds have been extensively deuteratecl, it lvill be necessar) to consider as sources of the observed isotope effect not only those factors involved in the activation process contributing to changes in the zero-point energ?-such as changes in hyperconjugatio~~ and non-bonclecl interaction b u t also the consequence ol mass increase. This fdctor has been discussed bj. us in an earlier paper a s a " t h e r m o d~ nanlic" effect involving changes in translational and rotational properties \vhich affect the thermoc1~-nnmic properties of the ~noleculc independently of the zero-point energy shifts. '' l'hermodynamic" FactorsA l t h o~~g h a cletailecl evaluation of these elements is not yet possible, the relative values of hulkD evident in Table I provicle a basis for a semiquantitative discussion of the importance of possible co~ltributors to the tleuteriuln isotope effect.'dTanz~script receized

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Factor analysis has been used to resolve the overlapping XPS spectra obtained in the x-ray-induced transformation of Pt(en)2(OH)2Cl2 to Pt(en)2Cl2. This is achieved without prior knowledge of the data set nor the establishment of any initial curve fitting criteria.