Energy surface carbonyl addition

Figure 8.2 Energy surface for addition of nucleophile Nuc to a carbonyl with concerted proton transfer from an acid HA. The lowest-energy path is indicated by the heavy line from point A to point B. Points C and D are the high-energy intermediates of the two possible stepwise paths. The circled point is the transition state.

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

The carbonyl sulfide calculations were not as straightforward as the 2D nitrous oxide work. One issue is that the transition involves three transition dipoles to various states, whereas in nitrous oxide the absorption is dominated by a single transition. The calculation for OCS only considered a single transition. In addition the quality of the potential energy surfaces was not as high, and the 2D approximation not as good, for OCS relative to NNO. An illustration of this is that the predicted OCS spectrum has a maximum at 214 nm while the experimental spectrum has a maximum at 223 nm. In comparison the difference in peak location for NNO was only 3 nm. 

Figure A.4 shows the usefulness of the reaction cube as a data structure. Additions to carbonyls often occur between different charge types, and frequently three-dimensional energy surfaces are used to clarify the various equilibria. We have seen two faces of this cube before as individual energy surfaces. The bottom faee of the cube is Figure 7.16, polarized multiple bond addition/elimination mechanisms in basic media. The back face of the cube is Figure 7.17, polarized multiple bond addition/elimination mechanisms in acidic media.

The reactions of P-donor nucleophiles with the metal carbonyl cluster Rh4COi2 have been studied over a considerable time period.It is widely accepted that the reaction is associative. This latest investigation is aimed at quantifying the effects of the electronic and steric properties of the nucleophiles upon the kinetic parameters. A rapid substitution reaction step using an excess of the nucleophile was monitored by SF spectrophotometry. Second-order rate constants were obtained from the variation of the pseudo-first-order rate constants with nucleophile concentration. Contributions to these constants from the properties steric effect, TT-activity, and, in addition, an aryl effect of the nucleophiles were assessed in a multi-parameter equation. The outcome is a successful understanding of the relative reactivities of many P-donors toward the rhodium cluster. The data were also represented by a three-dimensional potential energy surface. 

The introduction of an >-substituent (CN, Cl, or OH) into a primary n-alkyl chloride considerably enhances the rate of 5 n2 chloride exchange in the gas phase. Reactivity trends suggest that the acceleration is due primarily to through-space solvation of the transition state, especially charge-dipole interactions. Potential-energy surfaces are discussed. In further work by the same group, the translational energy dependence of the rate constants of several gas-phase 5 n2 and carbonyl addition-elimination reactions has been measured by FT-ICR spectroscopy. The results were interpreted by RRKM calculations. 

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