Rebeca Herrero

The ever-surprising chemistry of boron: enhanced acidity of phosphine.boranes.

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.

Gas-Phase Basicities Around and Below Water Revisited

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.

Energetics and Structural Properties, in the Gas Phase, of trans-Hydroxycinnamic Acids

We have studied the energetics and structural properties of trans-cinnamic acid (CA), o-, m-, and p-coumaric acids (2-, 3-, and 4-hydroxycinnamic acids), caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), iso-ferulic acid (3-hydroxy-4-methoxycinnamic acid), and sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid). The experimental values of Δ(f)H(m)°(g), determined (in kJ·mol(-1)) for CA (-229.8 ± 1.9), p-coumaric acid (-408.0 ± 4.4), caffeic acid (-580.0 ± 5.9), and ferulic acid (-566.4 ± 5.7), allowed us to derive Δ(f)H(m)°(g) of o-coumaric acid (-405.6 ± 4.4), m-coumaric acid (-406.4 ± 4.4), iso-ferulic acid (-565.2 ± 5.7), and sinapic acid (-698.8 ± 4.1). From these values and by use of isodesmic/homodesmotic reactions, we studied the energetic effects of π-donor substituents (-OH and -OCH(3)) in cinnamic acid derivatives and in the respective benzene analogues. Our results indicate that the interaction between -OCH(3) and/or -OH groups in hydroxycinnamic acids takes place without significant influence of the propenoic fragment.

Neutral, ion gas-phase energetics and structural properties of hydroxybenzophenones.

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.

Energetic and structural study of bisphenols.

We have studied thermochemical, thermophysical and structural properties of bisphenols A, E, F, and AP. In particular, the standard enthalpies of sublimation and the standard enthalpies of formation in the gas phase at 298.15 K for all these species were experimentally determined. A computational study, through M05-2X density functional theory, of the various species shed light on structural effects and further confirmed, by means of the isodesmic reaction scheme, the excellent consistency of the experimental results. Our results reflect also the fact that energetic substituent effects are transferable from diphenylalkanes to bisphenols.

Dissociation Dynamics and Thermochemistry of Tin Species, (CH3)4Sn and (CH3)6Sn2, by Threshold Photoelectron−Photoion Coincidence Spectroscopy

The dissociative photoionization of tetramethyltin (Me₄Sn) and hexamethylditin (Me₆Sn₂) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me₄Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E₀ = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me₃Sn(•) loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me₄Sn and Me₆Sn₂ are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol⁻¹: Δ(f)H(m)(o)(Me₃Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me₅Sn₂(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me₃Sn(•),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me₂Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(MeSn(+),g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol⁻¹: BDE(Me₃Sn-Me) = 284.1 ± 9.9; BDE(Me₃Sn-SnMe₃) = 252.6 ± 14.8.

Hydrogen-Bonding Interactions of (CF3)3CH and (CF3)3C− in the Gas Phase. An Experimental (FT-ICR) and Computational Study

Hydrogen-bonding interactions involving 2-(trifluoromethyl)-1,1,1,3,3,3-hexafluoropropane (1H) and 1(-) have been quantitatively studied by means of Fourier transform ion cyclotron resonance spectrometry. The existence of the species (1HCl)(-) and (1H1)(-) was demonstrated, and their thermodynamic stabilities were determined experimentally and computationally. In addition, some of their structural features were analyzed.