Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase.

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Nitriles with High Gas-Phase Basicity-Part II Transmission of the Push-Pull Effect through Methylenecyclopropene and Cyclopropenimine Scaffolds Intercalated between Different Electron Donor(s) and the Cyano N-Protonation Site.

This work extends our earlier quantum chemical studies on the gas-phase basicity of very strong N-bases to two series of nitriles containing the methylenecyclopropene and cyclopropenimine scaffolds with dissymmetrical substitution by one or two electron-donating substituents such as Me, NR2, N=C (NR2)2, and N=P (NR2)3, the last three being strong donors. For a proper prediction of their gas-phase base properties, all potential isomeric phenomena and reasonable potential protonation sites are considered to avoid possible inconsistencies when evaluating the energetic parameters and associated protonation or deprotonation equilibria B + H+ = BH+. More than 250 new isomeric structures for neutral and protonated forms are analyzed. The stable structures are selected and the favored ones identified. The microscopic (kinetic) gas-phase basicity parameters (PA and GB) corresponding to N sites (cyano and imino in the cyclopropenimine or in the substituents) in each isomer are calculated. The macroscopic (thermodynamic) PAs and GBs, referring to the isomeric mixtures of favored isomers, are also estimated. The total (pushing) substituent effects are analyzed for monosubstituted and disubstituted derivatives containing two identical or two different substituents. Electron delocalization is examined in the two π-π conjugated transmitters, the methylenecyclopropene and cyclopropenimine scaffolds. The aromatic character of the three-membered ring is also discussed.

Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity.

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species were the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results essentially showed that the enthalpy of BF3 adduct formation in DCM solution was clearly influenced by specific interactions, with DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allowed us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain for identified reasons could be complemented by the computed values.

On the Lewis Basicity of Phosphoramides: A Critical Examination of Their Donor Number through Comparison of Enthalpies of Adduct Formation with SbCl5 and BF3.

The Lewis basicity of a series of phosphoryl compounds was examined using DFT and ab initio methods, including solvation effects. The enthalpies of adduct formation with two archetypal Lewis acids, antimony pentachloride and boron trifluoride, used to define the donor number DN and the BF3 affinity (BF3 A) respectively, were examined. The BF3 adducts allow the use of the high-accuracy G4 approach, whereas for SbCl5 adducts, three different DFT formalisms, including empirical dispersion corrections, were used because the G4 formalism is not available for third-row elements. For a comparison with experimental data, solvation effects were taken into account by using the polarizable continuum model. The experimental BF3 affinities were well reproduced by G4 calculations when including PCM solvation. Conversely, comparisons of our calculated values and experimental results reported in the literature show that SbCl5 enthalpies for phosphoramides are in error. In particular the DN for HMPA should be revised.

Enhanced Basicity of Push-Pull Nitrogen Bases in the Gas Phase.

Nitrogen bases containing one or more pushing amino-group(s) directly linked to a pulling cyano, imino, or phosphoimino group, as well as those in which the pushing and pulling moieties are separated by a conjugated spacer (C═X)n, where X is CH or N, display an exceptionally strong basicity. The n-π conjugation between the pushing and pulling groups in such systems lowers the basicity of the pushing amino-group(s) and increases the basicity of the pulling cyano, imino, or phosphoimino group. In the gas phase, most of the so-called push-pull nitrogen bases exhibit a very high basicity. This paper presents an analysis of the exceptional gas-phase basicity, mostly in terms of experimental data, in relation with structure and conjugation of various subfamilies of push-pull nitrogen bases: nitriles, azoles, azines, amidines, guanidines, vinamidines, biguanides, and phosphazenes. The strong basicity of biomolecules containing a push-pull nitrogen substructure, such as bioamines, amino acids, and peptides containing push-pull side chains, nucleobases, and their nucleosides and nucleotides, is also analyzed. Progress and perspectives of experimental determinations of GBs and PAs of highly basic compounds, termed as "superbases", are presented and benchmarked on the basis of theoretical calculations on existing or hypothetical molecules.

Exceptionally High Proton and Lithium Cation Gas-Phase Basicity of the Anti-Diabetic Drug Metformin.

Substituted biguanides are known for their biological effect, and a few of them are used as drugs, the most prominent example being metformin (1,1-dimethylbiguanide, IUPAC name: N,N-dimethylimidodicarbonimidic diamide). Because of the presence of hydrogen atoms at the amino groups, biguanides exhibit a multiple tautomerism. This aspect of their structures was examined in detail for unsubstituted biguanide and metformin in the gas phase. At the density functional theory (DFT) level {essentially B3LYP/6-311+G(d,p)}, the most stable structures correspond to the conjugated, push-pull, system (NR2)(NH2)C═N-C(═NH)NH2 (R = H, CH3), further stabilized by an internal hydrogen bond. The structural and energetic aspects of protonation and lithium cation adduct formation of biguanide and metformin was examined at the same level of theory. The gas-phase protonation energetics reveal that the more stable tautomer is protonated at the terminal imino C═NH site, still with an internal hydrogen bond maintaining the structure of the neutral system. The calculated proton affinity and gas-phase basicity of the two molecules reach the domain of superbasicity. By contrast, the lithium cation prefers to bind the less stable, not fully conjugated, tautomer (NR2)C(═NH)-NH-C(═NH)NH2 of biguanides, in which the two C═NH groups are separated by NH. This less stable form of biguanides binds Li+ as a bidentate ligand, in agreement with what was reported in the literature for other metal cations in the solid phase. The quantitative assessment of resonance in biguanide, in metformin and in their protonated forms, using the HOMED and HOMA indices, reveals an increase in electron delocalization upon protonation. On the contrary, the most stable lithium cation adducts are less conjugated than the stable neutral biguanides, because the metal cation is better coordinated by the not-fully conjugated bidentate tautomer.

Can Nitriles Be Stronger Bases Than Proton Sponges in the Gas Phase? A Computational Analysis.

DFT calculations have been performed for a series of push-pull nitriles [(R2N)n(X═Y)iC≡N, where i = 0, 1, or 2, n = 1, 2, or 3, R2N = H2N, Me2N, or C4H8N, X = CH, N, or P, Y = CH or N]. The possible protonation N-sites (N-cyano, N-imino, and N-amino) have been examined and their proton affinities (PA) estimated. For all compounds in the series, even for those containing the guanidino, phosphazeno, and diphosphazeno pushing groups, the N-cyano atom is the favored site of protonation. The n-π conjugation strongly decreases the PA value of the pushing amino group in favor of the pulling cyano one. Nitriles with the phosphazeno groups [(R2N)3P═N-P(R2N)2═N and (R2N)3P═N] exhibit the strongest basicity in the series. Some of them (with PA > 1000 kJ mol(-1)) are stronger bases than DMAN, the so-called "proton sponge". Nitriles bearing the guanidino group [(R2N)2C═N] are less basic than those with the phosphazeno group [(R2N)3P═N] but more basic than those with the formamidino group (R2N-CH═N) containing the same substituent R. The N-imino atoms, present in the transmitter group (X═N, X = CH, N, or P), display PA values lower than those of the N-cyano site by more than 30 kJ mol(-1). When proceeding from the unsubstituted derivatives (R = H) to the methylated ones (R = Me), the Me groups at the N-amino atom increase the PA value of the N-cyano site for Me2N-X═Y-C≡N (X, Y = CH or N) by ca. 30-60 kJ mol(-1). For the guanidino and phosphazeno derivatives containing two and three amino groups, respectively, this effect is not additive. The four Me groups for (Me2N)2C═N-C≡N and the six Me groups for (Me2N)3P═N-C≡N increase the PA(N-cyano) values by only 30-50 kJ mol(-1). The C≡N bond lengths of the neutral forms are well correlated with the PA(N-cyano) values.

Evolution of Chemical Research in Nice, Côte d'Azur: From Early Laboratories to the 'Institut de Chimie de Nice'.

The 'Institut de Chimie de Nice' (ICN), founded in 2012, celebrates its 10th anniversary in 2022. Today, the ICN is part of the University Côte d'Azur (UCA), one out of nine excellence universities in France. ICN is also affiliated to the CNRS. We use the institute's anniversary to reflect on the origins and the successful evolution of research in chemical sciences in Nice, France. We outline research topics and their development towards modern chemistry in Nice that are characterized by innovation and territorial anchoring. At present, four research axes, namely aroma and perfume chemistry, medicinal chemistry, radiochemistry, and material chemistry structure the institute. ICN has created five start-up companies and includes a technological platform. The ICN is central part of the university and contributes to the advancement in chemical sciences as evidenced by both fundamental research and active contributions to local partnerships.

Purine tautomeric preferences and bond-length alternation in relation with protonation-deprotonation and alkali metal cationization.

Quantum chemical calculations were carried out for deprotonated (P-) and protonated purine (PH+) and for adducts with one alkali metal cation (P-M+ and PM+, where M+ is Li+ or Na+) in the gas phase {B3LYP/6-311+G(d,p)}, a model of perfectly apolar environment, and for selected structures in aqueous solution {PCM(water)//B3LYP/6-311+G(d,p)}, a reference polar medium for biological studies. All potential isomers of purine derivatives were considered, the favored structures indicated, and the preferred sites for protonation/deprotonation and cationization reactions determined. Proton and metal cation basicities of purine in the gas phase were discussed and compared with those of imidazole and pyrimidine. Bond-length alternations in the P, PH+, P-M+, and PM+ forms were quantitatively measured using the harmonic oscillator model of electron delocalization (HOMED) indices and compared with those for P. Variations of the HOMED values when proceeding from the purine structural building blocks, pyrimidine and imidazole, to the bicyclic purine system were also examined. Generally, the isolated NH isomers exhibit a strongly delocalized π-system (HOMED > 0.8). Deprotonation slightly increases the HOMED values, whereas protonation and cationization change the HOMED indices in different way. For bidentate M+-adducts, the HOMED values are larger than 0.9 like for the largely delocalized P-. The HOMED values correlate well in a comprehensive relationship with the relative Gibbs energies (ΔG) calculated for individual isomers whatever the purine form is, neutral, protonated, or cationized. When PCM-DFT model was utilized for P-, PH+, PM+, and P-M+ (M+ = Li+) both electron delocalization and relative stability are different from those for the molecules in vacuo. The solvation effects cause a slight increase in HOMEDs, whereas the ΔEs decrease, but in different ways. Hence, contribution of particular isomers in the isomeric mixtures of PH+, PM+, and P-M+ also varies. HOMED variations for the favored neutral, deprotonated, protonated, and lithiated forms of purine in the gas phase and aqueous solution.

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).

ICP-MS determination of trace elements in aerosols using a dynamic reaction cell: first results in PM10 comparing road and aerial traffic in Nice area (France).

An analytical methodology was developed for the determination of 21 trace elements in suspended particulate matter (PM) using a microwave digestion procedure associated with an inductively coupled plasma mass spectrometry (ICP-MS). The dynamic reaction cell (DRC) of the instrument was carefully optimized to eliminate polyatomic species causing spectral interferences for three specified elements (Cr, Fe, Mn). With this method, the detection limits based on the analysis of seven quartz fibre filters (QFF) considering a one-week sample (250 m3) varied between 0.2 and 650 pg m(-3) for trace elements and between 2.1 and 5.6 ng m(-3) for major elements (Fe, Ti, Zn). The recovery of the analytes was tested with NIST SRM 1648 urban dust within 10% of the certified values only for 3-4 mg of material. The first results were discussed for a field campaign which was carried out simultaneously in the heaviest traffic road tunnel of the centre of Nice and near the landing-taking-off runways in the international airport of Nice Côte d'Azur. The behaviour of some combustion tracers was especially studied.

SPME sampling of BTEX before GC/MS analysis: examples of outdoor and indoor air quality measurements in public and private sites.

This article presents the results of an exploratory application of the Solid Phase MicroExtraction (SPME) technique to the analysis of BTEX (benzene, toluene, ethylbenzene and xylenes) at the microg/m3 level in outdoor and indoor air. The salient features of the method validation are reported. As shown by the various examples of field sampling described, SPME technique appears as a method of choice for fast qualitative analysis and quantitative determination of Volatile Organic Compounds (VOC). The small dimensions of the SPME sampling system and the short sampling time let envisage its utilisation for the rapid diagnostic of outdoor and indoor air quality.

A quadrupole ion trap and Fourier transform ion cyclotron resonance concerted study of the kinetics of an ion/molecule association reaction: a chemometric approach.

The effects of different experimental parameters on rate constant measurements performed by mass spectrometry were investigated with a two-level fractional factorial design. This chemometric technique allows a study of the effects of selected factors and of their interactions on the response of an experiment by performing a limited number of analyses. The selected factors were: sample pressure, energy of the ionising electrons, reaction time and ionisation time. In this work, two mass spectrometric techniques were compared: Fourier transform ion cyclotron resonance (FT-ICR) and quadrupole ion trap (QIT) mass spectrometries. Experimental results were obtained from a study of a reaction system consisting of the condensation between triethylphosphite and its fragment ion (CH(3)CH(2)O)(2)P(+). Apparent bimolecular rate constants are clearly larger when determined by QIT than by FT-ICR, because of collisional stabilisation of the adduct ion by helium buffer gas introduced in the QIT spectrometer. However, the QIT rate constant extrapolated to zero helium pressure is almost identical to the FT-ICR value; this supports the conclusion regarding the buffer gas effect. Minor effects evidenced by the chemometric method were attributed to the sample pressure and to the reaction time.

Is allylphosphine a carbon or a phosphorus base in the gas phase?

The gas-phase basicity of allylphosphine (2-propenylphosphine) was measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry techniques. A complete survey of the allylphosphine-H(+) potential energy surface, carried out through the use of high-level G2(MP2) and B3LYP/6-311+G(3df,2p) calculations, allows us to conclude that, under low-pressure, low-energy ICR conditions, the interaction between the protonated reference base, B(ref)H(+), and allylphosphine leads to a complex in which B(ref)H(+) attaches to the phosphorus atom of allylphosphine, where the electrostatic potential is strongly attractive. Hence, in the first step only the phosphorus protonated species should be formed. Its isomerization to yield the C(beta)-protonated form, which is the global minimum of the potential energy surface, implies a very high activation barrier that cannot be overtaken under normal experimental ICR conditions. Therefore, the main conclusion of our study is that allylphosphine behaves as a phosphorus base in the gas phase, even though the C(beta)-protonated structure is the most stable protonated species. We have also shown that both C(beta)- and C(gamma)-protonation triggers a cyclization of the system. An analysis of the bonding of the different protonated species as compared with that of the neutral system is presented.

Change of the favored routes of EI MS fragmentation when proceeding from N1, N1-dimethyl-N2-arylformamidines to 1,1,3,3-tetraalkyl-2-arylguanidines: substituent effects.

Although series of N(1), N(1)-dimethyl-N(2)-arylformamidines and of 1,1,3,3-tetraalkyl-2-arylguanidines are structurally analogous and similar electron-ionization mass spectral fragmentation may be expected, they display important differences in the favored routes of fragmentation and consequently in substituent effects on ion abundances. In the case of formamidines, the cyclization-elimination process (initiated by nucleophilic attack of the N-amino atom on the 2-position of the phenyl ring) and formation of the cyclic benzimidazolium [M-H](+) ions dominates, whereas the loss of the NR(2) group is more favored for guanidines. In order to gain information on the most probable structures of the principal fragments, quantum-chemical calculations were performed on a selected set. A good linear relation between log{I[M-H](+)I [M](+*)} and sigma(R)(+) constants of substituent at para position in the phenyl ring occurs solely for formamidines (r = 0.989). In the case of guanidines, this relation is not significant (r = 0.659). A good linear relation is found between log{I[M-NMe(2)](+)/I [M](+*)} and sigma(p)(+) constants (r = 0.993).

A study of the cesium cation bonding to carboxylate anions by the kinetic method and quantum chemical calculations.

Collision-induced dissociation (CID) of the Cs(+) heterodimer adducts of the nitrate anion (NO(3)(-)) and a variety of substituted benzoates (XBenz(-)) [(XBenz(-))(Cs(+))(NO(3)(-))](-) produces essentially nitrate and benzoate ions. A plot of the natural logarithm of their intensity ratio, ln[I (NO(3)(-))/I(XBenz(-))], versus the calculated cesium cation affinity (DFT B3LYP) of the substituted benzoate ions (equivalent to the enthalpy of heterolytic dissociation of the salt) is reasonably linear. This suggests that the kinetic method can be used as a source of data on the intrinsic interaction between the anionic and the cationic moieties in a salt.

Interaction of the cesium cation with mono-, di-, and tricarboxylic acids in the gas phase. A Cs+ affinity scale for cesium carboxylates ion pairs.

Humic substances (HS), including humic and fulvic acids, play a significant role in the fate of metals in soils. The interaction of metal cations with HS occurs predominantly through the ionized (anionic) acidic functions. In the context of the effect of HS on transport of radioactive cesium isotopes in soils, a study of the interaction between the cesium cation and model carboxylic acids was undertaken. Structure and energetics of the adducts formed between Cs+ and cesium carboxylate salts [Cs+RCOO-] were studied by the kinetic method and density functional theory (DFT). Clusters generated by electrospray ionization mass spectrometry from mixtures of a cesium salt (nitrate, iodide, trifluoroacetate) and carboxylic acids were quantitatively studied by CID. By combining the results of the kinetic method and the energetic data from DFT calculations, a scale of cesium cation affinity, CsCA, was built for 33 cesium carboxylates representing the first scale of cation affinity of molecular salts. The structural effects on the CsCA values are discussed.

Complexes between lithium cation and diphenylalkanes in the gas phase: the pincer effect.

The gas-phase lithium cation basicities (LCB values, Gibbs free energies of binding) of alpha,omega-diphenylalkanes Ph-(CH(2))(n)-Ph (n=2, 3, or 7) and 1,1-diphenylethane Ph-CH(Me)-Ph were investigated by means of Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry. Their structures, and those of the corresponding Li(+) complexes were optimized at the B3LYP/6-31G(d) level and their relative stabilities calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level. Whereas the most stable conformers of the free diphenylalkanes were found to adopt a completely stretched aliphatic chain connecting the two benzene rings, the most stable Li(+) complexes correspond to conformers in which the alkali metal cation interacts simultaneously with both benzene rings through the folding of the aliphatic chain ("pincer effect"). This chelation brings about a significant enhancement of the Li(+) binding enthalpies (LBE values), which were calculated to be approximately 75 kJ mol(-1) higher than those evaluated for conventional (singly coordinated) pi complexes in which the metal cation interacts with only one of the benzene rings. The increase of the corresponding lithium cation basicities, however, (Gibbs free energies of Li(+) binding, LCB values) was calculated to be smaller by approximately 15 kJ mol(-1) as the pincer effect is entropically disfavored. The good agreement between the calculated LCB values, assuming a statistical distribution of the different conformers present in the gas phase, and the experimental LCB values measured by means of FTICR mass spectrometry are considered indirect evidence of the existence of the pincer effect.

Bonding energetics in clusters formed by cesium salts: a study by collision-induced dissociation and density functional theory.

In relation to the interaction between (137)Cs and soil organic matter, electrospray mass spectrometry experiments and density functional theory (DFT) calculations were carried out on the dissociation of positively charged adducts formed by cesium nitrate and cesium organic salts attached to a cesium cation [Cs(CsNO(3))(CsA)](+) (A = benzoate, salicylate, hydrogen phthalate, hydrogen maleate, hydrogen fumarate, hydrogen oxalate, and hydrogen malonate ion). These mixed clusters were generated by electrospray from methanol solutions containing cesium nitrate and an organic acid. Collision-induced dissociation of [Cs(CsNO(3))(CsA)](+) in a quadrupole ion trap gave [Cs(CsNO(3))](+) and [Cs(CsA)](+) as major product ions. Loss of HNO(3) was observed, and also CO(2) loss in the case of A = hydrogen malonate. Branching ratios for the dissociation into [Cs(CsNO(3))](+) and [Cs(CsA)](+) were treated by the Cooks' kinetic method to obtain a quantitative order of bonding energetics (enthalpies and Gibbs free energies) between Cs(+) and the molecular salt (ion pair) CsA, and were correlated with the corresponding values calculated using DFT. The kinetic method leads to relative scales of Cs(+) affinities and basicities that are consistent with the DFT-calculated values. This study brings new data on the strong interaction between the cesium cation and molecular salts CsA.

Ab initio and experimental thermodynamic and kinetic study of proton-assisted bond activation in gaseous hydrocarbons: deconvolution of reaction efficiencies in the case of adamantane.

1) Protonation at all possible sites of adamantane (C(10)H(16)) was studied at the MP2/6-311++G(3df,2p)//MP2/6-311++G(d,p) level. This provided values of the changes in the thermodynamic state functions for these processes. Whenever direct comparison was possible, the agreement with experimental data was very good. 2) By the same means, the reaction paths linking the various species obtained in these reactions were analyzed. 3) Fourier transform ion cyclotron resonance (FT-ICR) spectroscopy was used to determine the rate constants for proton transfer from 16 protonated reference bases to adamantane in the gas phase. Also, the rate constants for the formation of ionic products in these reactions were determined. 4) The experimental reaction rates were successfully predicted and refined on the basis of a simple mechanistic model based on the reaction profiles indicated above. 5) Our results hint at the potential usefulness of this approach for mechanistic studies.