Structural Effects in the Gas Phase

Having considered how solvents can strongly affect the properties of molecules in solution, let us consider some of the special features that arise in the gas phase where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, since current theoretical calculations do not usually treat solvent effects, experimental data from the gas phase are important for comparison with theoretical results. Frequently quite different trends, particularly in substituent effects, are seen when systems in the gas phase are compared to similar systems in solution. 

CHAPTER 4 STUDY AND DESCRIPTION OF ORGANIC REACTION MECHANISMS 

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for a proton. Table 4.9 compares Af/g s with AHaq for a series of phenol ionizations. 

Discussion of the techniques for gas phase equilibrium measurements can be found in Ion Cyclotron Resonance Spectrometry, T. A. Lehman and M. M. Bursey, Wiley-Interscience, New York, 1976 and in Gas Phase Ion Chemistry, Vols. 1 and 2, M. T. Bowers (ed.), Academic Press, New York, 1979. 

Another area of gas phase substituent effects which has attracted considerable interest is the acidity of the simple alcohols methanol, ethanol, isopropyl alcohol, and t-butyl alcohol relative to one another and to water. In the gas phase the order is t-BuOH EtOH MeOH H20. This is different from results in solution. Table 4.10 gives pK values for water and DMSO. 

A comparison of phenol acidity in dimethyl sulfoxide versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. While the effect of substituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for dimethyl sulfoxide is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and dimethyl sulfoxide.  

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Electric birefringence (EB) or the Kerr effect is widely used in molecular optics as a method for the investigation of the molecular structure of low molecular weight substances. The study of the Kerr effect in the gas phase or in solutions in combination with other methods, such as refraction, light scattering, dielectric measurements etc. permits to ascertain the spatial arrai ement of atoms in the molecule and thus to calculate the main values of the polarizability tensor of the molecule and to obtain information about the value and direction of its dipole moment ... 

The loss or gain of an electron by a molecule is typically accompanied by structural changes, which in turn also create a barrier to electron transfer. We are familiar with this effect in the gas phase when the transferred electron enters an antibonding orbital. Section 5.1.3.1. In solution, the barrier arising from intramolecular reorganization is known as an inner-sphere effect because it occurs within the solvation sphere. The barrier arising from solvent reorganization is then... 

The reason for this enliancement is intuitively obvious once the two reactants have met, they temporarily are trapped in a connnon solvent shell and fomi a short-lived so-called encounter complex. During the lifetime of the encounter complex they can undergo multiple collisions, which give them a much bigger chance to react before they separate again, than in the gas phase. So this effect is due to the microscopic solvent structure in the vicinity of the reactant pair. Its description in the framework of equilibrium statistical mechanics requires the specification of an appropriate interaction potential. 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

However, the Baker-Nathan effect has now been shown not to be caused by hyperconjugation, but by differential solvation. This was demonstrated by the finding that in certain instances where the Baker-Nathan effect was found to apply in solution, the order was completely reversed in the gas phase. ° Since the molecular structures are unchanged in going from the gas phase into solution, it is evident that the Baker-Nathan order in these cases is not caused by a structural feature (hyperconjugation) but by the solvent. That is, each alkyl group is solvated to a different extent. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

It came as a surprise, however, that Aug prefers a trigonal planar D3h structure in the gas phase [16] and is not octahedral as one might assume, suggesting that gold clusters do not follow the usual pattern of typical Lennard-Jones, Morse or Gupta systems, which all favor a maximum number of close atom-atom contacts. The preferred planarity of small gold cluster compounds is due to relativistic effects [17]. 

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