The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH(2) and MePH(2)BH(3) differ by about 118 kJ mol(-1) (see picture).The gas-phase acidity of a series of phosphines and their corresponding phosphineborane derivatives was measured by FT-ICR techniques. BH(3) attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol(-1)). This acidity-enhancing effect of BH(3) is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH(3) association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH(3) group is present. Our study also shows that deprotonation of ClCH(2)PH(2) and ClCH(2)PH(2)BH(3) is followed by chloride departure. For the latter compound deprotonation at the BH(3) group is found to be more favorable than PH(2) deprotonation, and the subsequent loss of Cl(-) is kinetically favored with respect to loss of Cl(-) in a typical S(N)2 process. Hence, ClCH(2)PH(2)BH(3) is the only phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.

This work employs Fourier transform ion cyclotron resonance (FT-ICR) and the Gaussian quantum chemistry composite methods W1 and G2 to experimentally and computationally analyze gas-phase basicities (GB) for a series of weak bases in the basicity region around and below water. The study aims to clarify the long-standing discrepancy between reported GB values for weak bases obtained via high-pressure mass spectrometry (HPMS) and ICR; the ICR scale is observed to be more than 2 times contracted compared to the HPMS scale. The computational results of this work support published HPMS data. This agreement improves with increasing sophistication of the computational method and is excellent at the W1 level. Several equilibria were also re-examined experimentally using FT-ICR. In the experiments with some polyfluorinated weak bases (hexafluoro-2-propanol and nonafluoro-2-methyl-2-propanol), it was found that two protonation processes compete in the gas phase: protonation on oxygen and protonation on fluorine. In these species, protonation on fluorine proceeds faster and is statistically favored over protonation on oxygen but leads to cations that are thermodynamically less stable than oxygen-protonated cations. The process may also lead to the irreversible loss of HF. The rearrangement of fluorine-protonated cations to oxygen-protonated cations is very slow and is further suppressed by the process of HF abstraction. These results at least partially explain the discrepancy between published HPMS data and earlier FT-ICR findings and call for the utmost care in using FT-ICR for gas-phase basicity measurements of heavily fluorinated compounds. The narrower dynamic range of ICR necessitates the measurement of several problematic bases and produces some differences between the ICR results in the present work and the published HPMS data; the wider dynamic range allows HPMS to overcome these difficulties in connecting the ladder.

We have carried out a study of the energetics, structural, and physical properties of o-, m-, and p-hydroxybenzophenone neutral molecules, C(13)H(10)O(2), and their corresponding anions. In particular, the standard enthalpies of formation in the gas phase at 298.15 K for all of these species were determined. A reliable experimental estimation of the enthalpy associated with intramolecular hydrogen bonding in chelated species was experimentally obtained. The gas-phase acidities (GA) of benzophenones, substituted phenols, and several aliphatic alcohols are compared with the corresponding aqueous acidities (pK(a)), covering a range of 278 kJ.mol(-1) in GA and 11.4 in pK(a). A computational study of the various species shed light on structural effects and further confirmed the self-consistency of the experimental results.

In our Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry [1] studies on the gas-phase reactivity of Group 14 derivatives, we initially examined the behavior of tetramethylsilane, Si(CH 3 ) 4 (TMS). Using mild electron ionization (nominal energies between 10 and 12 eV) and pressures in the range 10 À8 -10 À5 mbar, the main ion observed is the trimethylsilyl cation, Si(CH 3 ) 3 + , formed through the fast decomposition of the radical cation Si(CH 3 ) 4 + C. A striking feature of the spectrum is the presence at low pressures of a weak signal ( % 1 %) at m/z 161 that fades away a few seconds after the ionization process. It corresponds to the most abundant isotopomer of the ion Si 2 (CH 3 ) 7 + (I + ). The stoichiometry of this species suggests a somewhat unusual electronic structure, which prompted us to carry out a more detailed study.Some important databases [2] do not report the existence of I + , although this ion was reported in 1974 by Klevan and Munson [3] as the product of reaction (1), which takes place under high-pressure conditions (0.3 Torr).Using high-pressure mass spectrometry, Stone and coworkers carried out a careful study of this reaction, as well as of similar processes involving Ge and Sn derivatives.[4] They also determined D r H m (1) and D r S m (1) for reaction (1) as (À22.3 AE 0.4) kcal mol À1 and (À35.2 AE 0.9) cal mol À1 K À1 , respectively. These values lead to a value for D r G m (1) of (À11.8 AE 0.7) kcal mol À1 . Herein, we report our observations that under the lowest pressures indicated above, kinetic excitation of Si(CH 3 ) 3 + by means of on-resonance irradiation or slightly off-resonance irradiation (SORI) under multiple-collision conditions [5] increases the yield of I + . In the presence of a cooling gas and without ion excitation, the abundance of this ion increases rather slowly. As expected from the value of D r G m (1), the equilibrium abundance observed under our working conditions is small.[6] Figure 1 shows a spectrum of ion I + .Collision-induced decomposition (CID) experiments on I + using argon as the target gas and energies at the center of mass below 2 eV show that Si(CH 3 ) 3 + is the only ion formed. This observation also indicates a "fragile" bond between Si(CH 3 ) 3 + and TMS, in agreement with the results reported in reference [3].Similar experiments using perdeuterated tetramethylsilane, Si(CD 3 ) 4 (TMSD), yield the ion Si 2 (CD 3 ) 7 + (I D + ), and CID experiments conducted as indicated for I + lead to the formation of Si(CD 3 ) 3 + as the sole ionic product. The study of mixtures of TMS and TMSD was generally conducted using 1:1 mixtures and nominal pressures of the individual reagents of about 1-2 10 À7 mbar, with argon being added up to pressures of 8 10 À7 -1.