Sn2 reactions gas phase

Figure 13-3. Reaction path for the gas-phase SN2 reaction of Cl + CH3C1.

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

Statistical rate theories have been used to calculate rate constants for gas-phase Sn2 reactions.1,7 For a SN2 reaction like Cl" + CH3Clb, which has a central barrier higher than the reactant asymptotic limit (see Figure 1), transition state theory (TST) assumes that the crossing of the central barrier is rate-limiting. Thus, the TST expression for the SN2 rate constant is simply,... 

Deng, L., V. Branchadell, and T. Ziegler. 1994. Potential Energy Surface of the Gas-Phase Sn2 Reactions X + CH2X = XCH3 + X- (X = F, Cl, Br, I) A Comparative Study by Density Functional Theory and ab initio Methods. 

Hu, W.-P. and Truhlar, D. G. Modeling transition state solvation at the single-molecule level test of correlated ab initio predictions against experiment for the gas-phase SN2 reaction of microhydrated fluoride with methyl chloride, J.Am.Chem.Soc., 116 (1994), 7797-7800... 

Figure 9. A qualitative double-minimum potential energy surface for gas phase Sn2 reactions.

Introduction 198 Experimental techniques 200 Ion cyclotron resonance spectrometry 201 Flowing afterglow 203 High pressure mass spectrometry 204 General features of gas-phase ion-molecule reactions 204 Gas-phase SN2 reactions involving negative ions 206 Thermochemical considerations 206 General aspects of gas-phase SN2 reactions 207 Stereochemistry 209... 

Nucleophilicity and leaving group ability 211 Effect of solvation on the gas-phase reaction 212 Mechanism of the gas-phase SN2 reaction 213 Potential energy surfaces for gas-phase SN2 reactions 214 Recent theoretical developments 218 Some examples of gas-phase SN2 reactions involving positive ions 220 Nucleophilic displacement reactions by negative ions in carbonyl systems 222 General features 222... 

An elegant and clever experiment along these lines (Lieder and Brauman, 1974) confirms that the gas phase SN2 reaction proceeds by a predominant backside attack. Analysis of neutral products in the icr experiment shows that for reaction (27), backside attack, and consequently inversion at the reaction centre, occurs to the extent of 92 + 6%. For reaction (28) the same type of experiment indicates that inversion of configuration amounts to 87 + 7% of the reaction. 

It is clear from the raw data that gas-phase SN2 reactions are not amenable to treatment by linear-free energy relationships of the type observed in solution (Pearson et al., 1968). 

Gas-phase SN2 reactions have been shown to span a range of efficiencies of at least three orders of magnitude. This range may be considerably larger but... 

It is early yet to decide whether this approach is the most appropriate to account for gas-phase SN2 reactions. However it does provide a very useful approach to understanding and predicting the outcome of these reactions. 

Further insight into gas-phase SN2 reactions can be gained from recent calculations by Wolfe et al. (1981a,b). For the series of reactions (46), optimization of the transition-state geometry using a split Gaussian representation for the atomic orbitals reveals that the carbon—fluorine bond lengths vary... 

Some examples of gas-phase SN2 reactions involving positive ions... 

As mentioned on p. 99, gas-phase reactions are under charge control and, therefore, almost by definition, FO theory is inappropriate for their study. Such a conclusion would be precipitous. Note to begin with that only the anion behaves in an unusual manner, the comportment of its partner being normal. An FO study of gas-phase SN2 reactions (X- + RY —> XR + Y ) is therefore perfectly possible. We can also study the competition of electrophilic sites. On the other hand, FO theory will give questionable conclusions for the regioselectivity of anions (e.g. O-alkylation versus C-alkylation of enolates). For these problems, a more thorough study, requiring in particular transition states determination, is necessary. 

Standard basis sets are not suitable for calculating anions. The reason is that the exponents tf (which give the size of AOs) are optimized for neutral molecules. With these exponents, the electrons of anions are still compelled to move in the same volumes, so their repulsion is unduly exaggerated. A small basis set is not flexible enough to alleviate this effect. In particular, the 3-21G basis set gives unreliable activation energies and geometries for ionic reactions. Take a gas-phase SN2 reaction. It is... 

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