The absolute influence of stepwise solvation on nucleophilicity is explored with flowing afterglow measurements of rate constants in the gas phase at room temperature for nucleophilic displacement reactions of CH3C1 and CH3Br with the solvated homoconjugate anions A-*(AH),, with AH = H20, D20, CH30H, C2H50H, CH3COCH3, HCOOH, and CH3COOH. The specific rates are followed as a function of stepwise solvation for additions of up to three molecules of solvent (from n = 0-3). Comparisons are made with the kinetic behavior of the analogous reactions proceeding in solution. The observed trends in specific rate with solvation are discussed in terms of overall energetics and models for the intermediate potential energy profile.Nucleophilic displacement (SN2) reactions between anions and methyl halides are known to be extremely sensitive to the medium in which they proceed. For example, solution measurements have shown that nucleophilic displacement can be as much as a million times faster in dipolar aprotic than in protic solvents.' These same reactions have now also been executed in the gas phase where they have been found to proceed in the total absence of solvent with specific rates as much as 20 orders of magnitude higher than those in solution.2 The gas-phase experiments have provided, for the first time, a direct measure of the absolute influence of bulk solvent on the kinetics and efficiencies of these reactions.In addition to the solvent-free reactions, solvated nulceophilic displacement reactions of the type in (1) can also be executed in the gas phase, and their kinetics can be followed as a function A-4, + CH3B -B-6, + [ ( n -m)S + CH,A] (1) of stepwise solvation of the nucleophilic as indicated in Figure 1. For example, reactions of the type in (2) were executed in the gas phase in our early gas-phase studies of solvated nucleophilic Cl-.(ROH), + [ ( n -m)ROH + ROCH3] (2) displacement for R = CH,, C2H5, (CH3),CH, and (CH3)3C and for values of n equal to 0 and A significant decrease in reactivity was observed for all alkyl groups with the addition of just the one molecule of solvent to the nucleophile. More recently we have reported in brief a quantitative study for reactions of the type in (3) between hydrated hydroxide ions and methyl bromide OH-.(H,O), + CH3Br -Br-.(H,O), + [ ( n -m)H20 + C H 3 0 H ] (3)for additions of up to three molecules of water.4 Here we report these results in more detail as part of an extended study of the influence of solvation on nucleophilic displacement in the gas phase.With our previous measurements of intrinsic reactivities as a benchmark,, we have followed as a function of stepwise solvation the kinetics of reactions of homoconjugate anions A-.(AH),, with both CH$l and CH3Br. The measurements were carried out at room temperature with additions of up to three solvent molecules to the nucleophile. While such amounts of solvation scarcely correspond to the solvent environment found in solution, they do begin to bridge the gap between the totally unsolvated gas phase and solution. A...
Flowing afterglow measurements are reported which reveal the influence of stepwise solvation on the nucleophilicity of F− and Cl− in the gas phase at room temperature. The specific rates of nucleophilic displacement reactions with CH3Cl and CH3Br are followed for additions of up to three molecules of solvent for F− solvated with D2O, CH3OH, and C2H5OH and for Cl− solvated with CH3OH, C2H5OH, CH3COCH3, HCOOH, and CH3COOH. The observed precipitous response of the specific rate to solvation is attributed to intermediate features of plausible reaction energy profiles.
A new series of hypervalent radicals has been generated by neutralizing a fast beam of NH4(NH,),+ or CH,NH,(NH,),+ ions in the electron transfer reactions NH4(NH3),' + K (or Na) -NH4(NH3),* + K+ (or Na') (for n = 0-2) and CH,NH3(NH3),+ + K -CH,NH3(NH3),* + K+ (for n = 0-1). Mass spectra of the reaction products have been obtained by the technique of charge stripping with NO2 as collisional ionization gas. New results on the stabilities of CH,NH3, CH3N2H6, N3HI0, and their deuterated forms are presented. The effects of internal excitation on the decomposition pathways of the radicals are discussed. These species constitute a novel class of radicals with ionization energies estimated to be in the range 3 to 4.5 eV.We have previously described a technique' for generating a variety of metastable and hypervalent molecules by neutralizing a fast beam of positively charged ions with an appropriate electron donor. In a recent study2 we reported observations that the ammonium radical (NH,), which is slightly unstable thermodynamically, can form stabilized molecular clusters in combination with NH3. In the present study the technique is extended to investigate the stabilities of other members of this series of hypervalent molecules. Results of the work indicate that these molecules constitute a new class of free radicals with ionization energies in the range 3-4.5 V. Experimental SectionThe apparatus and the experimental procedures were described in detail previ~usly.~*~ The primary ions were produced by chemical ionization of NH, and CH3NH2/NH3 mixtures in a high-pressure ion source. At high pressures cluster ions are produced efficiently by the three-body reactions4 NH4+(NH3), + 2NH3 -NH4+(NH3),+, + NH3 (n = 0-2) (1) and CH3(NH3),' + 2NH3 -CH3(NH3),+1+ + NH3 (n = 1-2) (2) All of the deuterated ions were produced by the analogous reactions employing deuterated or partially deuterated reagents. The ions were then accelerated through 6 keV mass resolved in a magnetic sector and focussed into a neutralization chamber containing a few mtorr of K or Na metal vapor. The chamber was 0.5 in. thick and has a knife edge entrance slit of 0.025 X 0.200 in. and an exit slit of 0.025 X 0.250 in. The resulting neutral species were permitted to continue undeflected toward the channeltron electron multiplier located 40 cm from the center of the neutralization chamber while the remaining ions were removed from the beam by means of electrostatic deflector plates. After traversing the electrostatic field, a fraction of the neutral beam along the beam axis was re-ionized by charge stripping. For this purpose, a collisional ionization chamber, 4.25 in. long with 0.020 X 0.280 in. entrance and exit slits, was positioned between the detector and the neutralization chamber. This second chamber was pressurized with NO2 gas at approximately 70 mtorr. The collisionally re-ionized beam (a -lo-'* cm2) was deflected electrostatically off axis and detected by translating the detector normal to the beam axis. Since an ion is deflected a distance Ax propo...
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