Pierre-Charles Maria

Gas-phase structural (internal) effects in strong organic nitrogen bases

Gas-phase basicities (GB) of strong organic bases containing the imino group were re-examined in the light of the re-evaluated GB values for the reference bases given in a recent compilation of Hunter and Lias. Structural (internal) effects which influence the basicity are discussed and general relations for the GB prediction are proposed for simple alkyl amidines and guanidines. These relations were used for estimation of cyclization and intramolecular H-bonding effects. Copyright

Gas-phase lithium-cation basicities of some benzene derivatives: An experimental and theoretical study

Abstract The gas-phase lithium-cation basicities of a series of monosubstituted benzene derivatives, namely C 6 H 5 X (X=H, Me, CHCH 2 , OH, OMe, SH, Cl, Br) have been measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of the corresponding complexes and their relative stabilities were investigated with B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) density functional theory calculations. In all cases, the π-complexes are favored with respect to those in which the metal monocation interacts with the substituent. These latter kind of complexes, which are entropically favored with respect to the π-complexes, are found to be chelated species, in which Li + bridges the heteroatom of the substituent and the ipso carbon atom. The Li + basicity of the benzene derivatives investigated reflects the electron-donor ability of the aromatic moiety as a function of the substituent. Consistently, there is a linear correlation between the Li + basicity and the frequency of the vertical displacement of Li + with respect to the aromatic ring.

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 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.

On the Use of the Kinetic Method for the Determination of Proton Affinities by Fourier‐transform Ion Cyclotron Resonance Mass Spectrometry

The use in Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry of the collision-induced dissociation of proton-bound dimers, the kinetic method or Cookss method, is tested. This method is compared with the proton-transfer equilibrium method. Good agreement between the two methods is observed. Advantages and limitations of the FTICR kinetic method are briefly discussed.

Computational Study of Cesium Cation Interactions with Neutral and Anionic Compounds Related to Soil Organic Matter

The gas-phase cesium cation affinities (CsCAs) and basicities (CsCBs) for 56 simple neutral compounds (mostly aromatic molecules) and 41 anions (carboxylates and phenolates) were calculated using density functional theory (DFT), in the context of the interaction of Cs(+) with soil organic matter (SOM). The B3LYP/def2-TZVP method gives in general CsCAs and CsCBs in a good agreement with experimental data. The strong deviations in case of NO(3)(-) and CsSO(4)(-) anions need further experimental investigations as the high-level CCSD(T) calculations support B3LYP results. Different cesium cation complexation patterns between Cs(+) and the neutral and anionic systems are discussed. As expected, the strongest CsCAs are observed for anions. The corresponding quantities are approximately by 4-5 times higher than for the neutral counterparts, being in the range 90-118 kcal/mol. The weakest cesium cation bonding is observed in the case of unsubstituted aromatic systems (11-15 kcal/mol).