systematically from a tetrahedral reactant-like transition state for nl - 1 to trigonal bipyramidal for symmetric models (nl N 0.5) to a tetrahedral product-like model for n1 - 0. All other variables were taken equal to reactant (or product) values selected from standard sources.14 A zero or imaginary reaction coordinate frequency vL* was obtained by introducing an inter- action constant F12 equal to or greater than (FlF2)’/’, respectively. l5 The calculated carbon-14 and chlorine-37 isotope effects for reaction 1 are shown in Figure 1 for Y equals 0 (I), C1 (11), and S (111). As can clearly be seen, the carbon isotope effect goes through a maximum as nl de- creases. The results support the qualitative sugges- tions that an SN~ reaction can be viewed as an “RCH2 group” transfer from X to Y, with mechanistic impli- cations similar to those of hydrogen-transfer reactions. Compared to the symmetric chlorine exchange reac- tion, where the maximum isotope effect occurs for nl = 0.5 (Figure 1, curve 11), one can see that the tran- sition state corresponding to the maximum isotope effect becomes more reactant-like, nl > 0.5 (curve I) as the forming bond (C-0) becomes stronger than the rupturing C-CI bond, or more product-like, n1 < 0.5 (curve 111), as the forming bond (C-S) becomes weaker than the rupturing C-C1 bond. This trend is consis- tent with the Hammond postulate. l6 The chlorine isotope effect, on the other hand, in- creases continuously as one goes from a reactant-like to a product-like transition state (i.e., as nl decreases). The chlorine isotope effect for a forming 0-C bond (curve I) is caiculated to be somewhat larger than for a forming S-C bond (curve 111), in contrast to the recent experimental results of Grimsrud and Taylor. l7 How- ever, the calculated isotope effects for these two cases are not significantly different, and solvation of the leaving group and/or nucleophile may appreciably affect the chlorine isotope effect but would not be ex- pected to alter the trends in the carbon isotope effect. The experimentalg isotope effects for solvolysis (curve I, Y = 0) are: k/k* - 1.08 for carbon-I4 and 1.008 for chlorine-37. The results in Table I1 show Table II. Effect of Reaction Coordinate Frequency on Calculated Kinetic Isotope Effects at 30’ for Reaction 1 of Text“ VL * k a/k l4 ka6/k87 0 1.05911 1.01289 139i 1.07310 1.01087 204i 1 .08159 1 .OO926 2643 1.08686 1.00825 a Cutoff model 2 (see Table I) was used with n, = 0.6 and nz = 0.4. that a more realistic reaction coordinate frequency (vL * imaginary rather than zero) yields calculated carbon- 14 and chlorine-37 isotope effects in better (14) G. Herzberg, “Molecular Spectra and Molecular Structure 11. Infrared and Raman Spectra of Polyatomic Molecules,” Van Nostrand, Princeton, N. J., 1945; G. W. Wheland, “Advanced Organic Chem- istry,” 3rd ed, Wiley, New York, N. Y., 1960; E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, “Molecular Vibrations,” McGraw-Hill, New York, N. Y., 1955. (15) H. S. Johnston, W. A. Bonner, and D. J. Wilson, J. Chem. Pfiys., 26, 1002 (1957); M. J. Stern and M. Wolfsberg, ibid., 45,2618 (1966). (16) G. Hammond, J. Amer. Chem. Soc., 77,334 (1955). (17) E. P. Grimsrud and J. W. Taylor, ibid., 92,739 (1970). Y I .08 I OXYGEN II CHLORINE m SULFUR 1365 ) I-n, = n2 Figure 1. Calculated carbon-14 (upper curves) and chlorine-37 (lower curves) isotope effects as a function of bond order for re- action 1 using cutoff model 1. agreement with experiment. Details of the calcula- tions, including cutoff procedures and comparisons with available experimental data, will be published subsequently. To our knowledge, no heavy-atom kinetic isotope effect results have been reported which illustrate the bell-shaped behavior shown in Figure 1. Experiments are being carried out in this laboratory to test this con- cept. (18) (a) NDEA Fellow, 1965-1968; Phillips Petroleum Fellow, 1968- 1969; (b) NDEA Fellow, 1969-1971; NSF Trainee, 1971-1972; (c) NSF Trainee, 1971-1972. L. B. Sims,* Arthur Fry, L. T. Netherton J. C. Wilson,18a K. D. Reppond,18b S. W. Crook18o Department of Chemistry, University of Arkansas Fayetteuille, Arkansas 72701 Received October 9, 1971 Synthesis and Chemistry of a-Lactones’ Sir: Synthesis of a-lactones poses a long-standing chal- lenge. a-Lactones have been invoked as intermediates in such diverse transformations as nucleophilic dis- placements, free-radical processes, thermal elimina- tion~,~ and photochemical reaction^.^ A variety of in- (1) Photochemical Transformations. XLIV. Cyclic Peroxides. VII. (2) W. A. Cowdrey, E. D. Hughes, and C. K. Ingold, J. Cfiem. Soc.. 1208 (1937); S. Winstein and H. J. Lucas, J. Amer. Cfiem. Soc., 61, 1576 (1939); E. Grunwald and S. Winstein, ibid., 70, 841 (1948); F. G. Bordwell and A. C. Knipe, J. Org. Cfiem., 35,2956 (1970). (3) C. Walling and E. S. Savas, J. Amer. Chem. Soc., 82, 1738 (1960); P. D. Bartlett and L. B. Gortler, ibid., 85, 1864 (1963); L. B. Gortler and M. D. Saltzman, J. Org. Cfiem., 31, 3821 (1966); J. E. Lemer and R. G. Zepp,J. Amer. Cfiem. Soc., 92,3713 (1970). (4) D. G. H. Ballard and B. J. Tighe, J. Cfiem. Soc. B, 702 (1967); B. J. Tighe, Cfiem. Znd. (London), 1837 (1969). (5) W. Adam and R. Rucktaschel, J. Amer. Cfiem. Soc., 93, 557 (1971). Communications to the Editor