Jean-François Gal

Dinuclear Copper Intermediates in Copper(I)-Catalyzed Azide–Alkyne Cycloaddition Directly Observed by Electrospray Ionization Mass Spectrometry†

The mechanism of the CuAAC reaction has been investigated by electrospray ionization mass spectrometry (ESI-MS) using a combination of the neutral reactant approach and the ion-tagging strategy. Under these conditions, for the first time, putative dinuclear copper intermediates were fished out and characterized by ESI(+)-MS/MS. New insight into the CuAAC reaction mechanisms is provided and a catalytic cycle is proposed.

Linear solvation energy relationships. Part 32. A co-ordinate covalency parameter, ξ, which, in combination with the hydrogen bond acceptor basicity parameter, β, permits correlation of many properties of neutral oxygen and nitrogen bases (including aqueous pKa)

Family-dependent (FD) basicity properties are defined as those which have a linear relationship with the hydrogen bond acceptor (HBA) basicity parameter, β, only when families of bases having similar HBA sites are considered separately. Family-independent (FI) properties are those which have a linear relationship with β when all bases are considered together. FD properties can be correlated and meaningfully related to FI properties if, in addition to the β parameter, an empirical co-ordinate covalency parameter, ξ, is used in equations of the form, XYZ=XYZ0+bβ+eξ. Values of ξ are –0.20 for PO bases, 0.00 for CO and SO bases, 0.20 for single-bonded oxygen bases, 0.60 for pyridine bases, and 1.00 for sp3-hybridized amine bases. By means of the above equation proton transfer basicities (pKa) are for the first time related to hydrogen bond basicities in a correlation involving all the above types of bonding sites.

Thermogravimetric calibration of permeation tubes used for the preparation of gas standards for air pollution analysis

Sources of VOC (Volatile Organic Compounds) reference-materials at ppm and ppb levels are needed for calibration of air monitoring instruments. The permeation-tube technique is considered effective for the preparation of low concentration standards of high accuracy and stability. In this work, purpose-built PTFE permeation tubes, containing benzene, toluene, ethylbenzene, o-xylene or m-xylene (BTEX) were accurately and rapidly calibrated. Using the sensitive thermo-balance of a thermogravimetric apparatus, very low permeation rates were determined by the continuous monitoring of the tube weight loss as a function of time. Permeation rates in the range from 25 to 350 ng min(-1) were determined with precision. Thermogravimetry appears to be a rapid method for the measurement of weight loss at constant temperature, allowing rapid characterization and recalibration of permeation tubes. A detailed study on toluene, chosen as a typical case, showed that there are variations of the permeation rate in the long term. The temperature dependence of the permeation coefficient was also explored and permeation rates were shown to display an Arrhenius behavior in the temperature range 304-324 K. Thermodynamic parameters influencing the permeation were discussed.

Comparison of brönsted acidities of neutral NH-acids in gas phase, dimethyl sulfoxide and water

Abstract The Bronsted acidities of several neutral NH-acids (substituted diphenylamine, substituted anilines and imides) were measured in the gas phase (pulsed FT ICR spectrometry), dimethyl sulfoxide and aqueous solution. Comparison of the Bronsted acidities of neutral NH-acids in the gas phase, dimethyl sulfoxide and water was also carried out. It was shown that substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into dimethyl sulfoxide and water. The strongest solvent-induced attenuation of the substituent effects is characteristic of the meta-substituted anilines whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by 2.83 times and with transfer into water by 4.13 times. At the same time, the reaction series of para and/or ortho-π-acceptor substituted anilines, amides, imides and substituted diphenylamines are less sensitive to a change in gas phase for DMSO or water. In the special case of para-acceptor substituted anilines it was demonstrated that the specific solvation induced an increase in the acidity of the para- and/or ortho-acceptor substituted anilines as compared with the behavior of the corresponding meta-substituted anilines by amounts up to 10 pKa units.

Lithium-cation and proton affinities of sulfoxides and sulfones: A fourier transform ion cyclotron resonance study

The kinetic method was used for the quantitative determination of lithium-cation affinities by Fourier transform ion cyclotron resonance. This method was applied to a series of XYSO and XYSO2 compounds. Proton basicities of the SO and SO2 compounds were also determined. When comparison is made between Li+ basicities and proton basicities, a linear regression encompassing XYSO and XYSO2 families suggests that Li+ may be bonded in a similar way to the SO and SO2 moieties, that is, to only one oxygen on the latter. PM3 calculations support this hypothesis.

Experimental and theoretical study of carbon suboxide C3O2, protonated carbon suboxide C3HO2+ and C3HO2· radical in the gas phase

Abstract The proton affinity (PA) of carbon suboxide OCCCO has been determined by Fourier transform ion cyclotron resonance as 791 kJ mol−1. Ab initio calculations at the MP4(SDTQ)/6–31G∗∗//6–31G∗∗ + ZPE(6–31G∗∗) level of theory give a proton affinity for C3O2 of 789 kJ mol−1. The gas-phase reactivity of ions a [OCCHCO]+ and ions b [OCCCOH]+, generated from various precursors has been studied by mass spectrometry techniques (metastable ion kinetic energy (MIKE), collision induced decomposition (CID) and neutralization — reionization (NR) mass spectra). From the experimental and theoretical results it follows that the gas-phase protonation of carbon suboxide yields ion a. At high internal energies these ions show competing losses of H′ and CO. The radical [OCCHCO]·, the neutral counterpart of protonated carbon suboxide, has a lifetime of at least 1 μs.

The heats of formation, gas-phase acidities, and related thermochemical properties of the third-row hydrides GeH4, AsH3, SeH2 and HBr from G2 ab initio calculations

Abstract Ab initio molecular orbital calculations at the G2 level have been used to calculate several thermochemical properties of the third-row hydrides GeH 4 , AsH 3 , SeH 2 and HBr. The heats of formation at 0 K and 298 K were calculated for these hydrides, for the anions GeH 3 − , AsH 2 − , SeH − and Br − , and for the corresponding free radicals GeH 3 . , AsH 2 . , SeH . and Br . . Also obtained in the present study have been the gas-phase acidities and bond dissociation enthalpies of the above hydrides, and the electron affinities of the free radicals. Our results generally agree well with recent experimental values. Comparisons of our G2 acidities, bond dissociation enthalpies and electron affinities with literature G2 results for the first- and second-row analogues of the above species show that in each case the trends across the three rows are similar, with the second and third rows exhibiting the closest behavior. The electron affinities of the second- and third-row radicals are nearly identical, resulting in the trends for the acidities and bond dissociation enthalpies being very similar to one another.

The Gas-Phase Acidity of HCP, CH3CP, HCAs, and CH3CAs: An Unexpected Enhanced Acidity of the Methyl Group

The gas-phase acidities of methylidynephosphine, HCtbond;P, ethylidynephosphine, CH(3)Ctbond;P, and ethylidynearsine, CH(3)Ctbond;As, have been measured by means of Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry and calculated at the CCSD(T)/6-311+G(3df,2p)//QCISD/ 6-311+G(df,p) level of theory. An analysis of these results shows that, in contrast to the well-known fact that HCtbond;N is a stronger acid than CH(3)Ctbond;N, CH(3)Ctbond;P and CH(3)Ctbond;As are more acidic than HCtbond;P and HCtbond;As, respectively. The most important consequence of this unexpected effect is that while HCtbond;P and HCtbond;As are found to be weaker acids than HCtbond;N, the opposite trend is found for the corresponding methyl derivatives, the acidity of which increases as CH(3)Ctbond;N<CH(3)Ctbond;P<CH(3)Ctbond;As. Also the effects of deprotonation on the structures and the vibrational frequencies of HCtbond;X and CH(3)Ctbond;X (X=N, P, As) compounds are qualitatively similar, but quantitatively very different for nitrogen- as compared with phosphorus- and arsenic-containing compounds. A rationalization of these differences in terms of the bonding differences is presented.

Mass spectrometric characterization of metal triflates and triflimides (Lewis superacid catalysts) by electrospray ionization and tandem mass spectrometry

Trifluoromethylsulfonate (triflate) and bis(trifluoromethylsulfonyl)imide (triflimide) salts, well-known Lewis acid catalysts, present some difficulty in their characterization. By using nitromethane as the solvent, useful electrospray mass spectra in positive and negative ion mode were obtained for salts of metals in oxidation states +2 and +3. In positive mode, addition of a strong Lewis base (triphenylphosphine oxide, TPPO), capable of displacing a triflate (TfO(-)) or a triflimide (Tf(2)N(-)) anion, is necessary for obtaining useful spectra. Under these conditions of solvent and added ligand, the most abundant ions were [M(2+)(A(-))(TPPO)(2)](+) or [M(3+)(A(-))(2)(TPPO)(2)](+) with A(-) = TfO(-) or Tf(2)N(-). The MS/MS spectra of these diagnostic ions provide additional analytical information. The breakdown curves, in the form of % dissociated as a function of the ion activation energy, offer a mean for investigating the bonding in these ions.

Fourier transform-ion cyclotron resonance study of the gas-phase acidities of germane and methylgermane; bond dissociation energy of germane

An accurate gas-phase acidity for germane (enthalpy scale, equivalent to the proton affinity of GeH3−), ΔHacido(GeH4) = 1502.0 ± 5.1 kJ mol−1, is obtained by constructing a consistent acidity ladder between GeH4, and H2S by using Fourier transform-ion cyclotron resonance spectrometry, and 0 and 298.15 K values for the first bond dissociation energy of GeH4 are proposed: D0o(H3Ge-H) = 352 ± 9 kJ mol−1; Do(H3Ge-H) = 358 ± 9 kJ mol−1, respectively. These results are compared with experimental and theoretical data reported in the literature. Methylgermane was found to be a weaker acid than germane by approximately 35 kJ mol−1: ΔHacido = 1536.6 kJ mol−1.