Modeling intrinsic basicities and acidities

Rational Design of Cyclopentadiene-Based Super- and Hyperacids Based on Aromaticity

The design and synthesis of neutral organic superacids have been of interest recently due to their vast applications in chemistry and material sciences, for example, olefinic polymerization, isolation of highly reactive short-lived cations, etc. Cyclopentadiene behaves as a mild organic acid, producing a stable conjugate base by gaining aromaticity and conjugation after deprotonation. To stabilize conjugate bases of organic acids to show superacidities and hyperacidities, we considered aromatic phenyl substituents with cyclopentadiene (mono-, di-, and triphenyl-substituted cyclopentadiene and their cyano derivatives). The MP2, DFT (B3LYP, M06-2X), and CBS-QB3 methods were used to calculate the gas-phase proton affinities of the parent cyclopentadiene, and the DFT methods were chosen for the substituted cyclopentadiene as they yield an experimental proton affinity of cyclopentadiene of 253.66 kcal/mol (Expt. 253.6 ± 1.3 kcal/mol). The stable trisubstituted cyclopentadiene derivative shows gas-phase enthalpies of deprotonation (ΔHacid) of 245 and 239 kcal/mol with DFT B3LYP and M06-2X methods, respectively, with values in the range of hyperacidity. Some of the tautomers of cyclopentadiene derivatives show hyperacidity, with proton affinity values of 205-240 kcal/mol. Triphenyl-substituted cyclopentadiene behaves as a moderate acid but transforms into a superacid after replacing the phenyl group with nitrobenzene, which is a stronger acid than H2SO4 (ΔHacid = 298 kcal/mol). The nucleus-independent chemical shift (NICS) shows the extent of conjugation in the derivatives of cyclopentadienyl anions after deprotonation, which affects the stabilities of conjugate bases, i.e., the acidities of the protonated species. The calculated harmonic oscillator model of aromaticity and NICS indices reveal that the stability of conjugate bases as well as their acidities increase with increasing aromaticity of cyclopentadiene rings after deprotonation in all molecules.

Predicting a New Δ-Proton Sponge-Base of 4,12-Dihydrogen-4,8,12-triazatriangulene through Proton Affinity, Aromatic Stabilization Energy, and Aromatic Magnetism

Herein, we report designing a new Δ (delta-shaped) proton sponge base of 4,12-dihydrogen-4,8,12-triazatriangulene (compound 1) and calculating its proton affinity (PA), aromatic stabilization, natural bond orbital (NBO), electron density ρ(r), Laplacian of electron density ∇2 ρ(r), (2D-3D) multidimensional off-nucleus magnetic shielding (σzz (r) and σiso (r)), and scanning nucleus-independent chemical shift (NICSzz and NICS). Density functional theory (DFT) at B3LYP/6-311+G(d,p), ωB97XD/6-311+G(d,p), and PW91/def2TZVP were used to compute the magnetic shielding variables. In addition, relevant bases like pyridine, quinoline, and acridine were also studied and compared. The protonation of compound 1 yields a highly symmetric carbocation of three Hückel benzenic rings. Comparing our findings of the studied molecules showed that compound 1 precedes others in PA, aromatic isomerization stabilization energy, and basicity. Therefore, the basicity may be enhanced when a conjugate acid gains higher aromatic features than its unprotonated base. Both multidimensional σzz (r) and σiso (r) off-nucleus magnetic shieldings outperformed electron-based techniques and can visually monitor changes in aromaticity that occur by protonation. The B3LYP/6-311+G(d,p), ωB97XD/6-311+G(d,p), and PW91/def2TZVP levels showed no significant differences in detailing isochemical shielding surfaces.

A DFT study to design super- and hyperacids with 1-(cyclopenta-2,4-dien-1-yl)-4-nitrobenzene and 3-(cyclopenta-2,4-dien-1-ylmethylene)-6-methylenecyclohexa-1,4-diene molecules

DFT calculations reveal that poly-cyano-substituted 1-(cyclopenta-2,4-dien-1-yl)-4-nitrobenzene and 3-(cyclopenta-2,4-dien-1-ylmethylene)-6-methylenecyclohexa-1,4-diene can function as super- to hyper-acids.

Predicting collision-induced dissociation mass spectra: understanding the role of the mobile proton in small molecule fragmentation

RATIONALE

Intramolecular proton migration has been reported to be required for fragmentation by collision-induced dissociation (CID). If the collision energy is required to provide energy for proton movement to a ‘dissociative’ site, it may be possible to predict the optimal collision energy for fragmentation using quantum computational chemistry software. A greater understanding of the mechanism(s) of proton migration is necessary.

METHODS

The product ion spectra of seven compounds were obtained at collision energies stepped in the range from 5 to 50 eV, with precursor ions being generated in positive ion mode by both atmospheric pressure chemical ionization (APCI) and electrospray ionisation (ESI) (using an ESCi ionisation source with or without corona discharge, respectively). The products ions observed at each collision energy were assessed in terms of structure to ascertain if they were formed as a result of protonation at the initial ionisation site or if the proton had migrated to a dissociative site.

RESULTS

Proton migration was shown to be independent of collision energy, stability of the protonated molecule and the distance that the proton moved. Therefore, proton migration is not a barrier to fragmentation as the proton appears to be fully mobile at 5 eV. As proton migration is independent of collision energy for these compounds, whereas fragmentation is energy dependent, protonation at the dissociative site alone is not sufficient to cause bond cleavage.

CONCLUSIONS

The role of collision energy in bond cleavage may be to increase the vibrational energy of the bond and/or increase the rate of bond cleavage such that it occurs within the residence time of the ion within the collision cell rather than to supply the energy for proton migration. Therefore, quantum chemistry alone cannot predict the collision energies appropriate for fragmentation on the basis of modelling proton movements.

Understanding collision-induced dissociation of dofetilide: a case study in the application of density functional theory as an aid to mass spectral interpretation

Fragmentation of molecules under collision-induced dissociation (CID) conditions is not well-understood. This may make interpretation of MSMS spectra difficult and limit the effectiveness of software tools intended to aid mass spectral interpretation. Density Functional Theory (DFT) has been successfully applied to explain the thermodynamics of fragmentation in the gas phase by the modelling the effect that protonation has on the bond lengths (and hence bond strengths). In this study, dofetilide and four methylated analogues were used to investigate further the potential for using DFT to understand and predict the CID fragmentation routes. The products ions present in the CID spectra of all five compounds were consistent with charge-directed fragmentation, with protonation adjacent to the cleavage site being required to initiate fragmentation. Protonation at the dissociative site may have occurred either directly or via proton migration. A correlation was observed between protonation-induced bond lengthening and the bonds which were observed to break in the CID spectra. This correlation was quantitative in that the bonds calculated to elongate to the greatest extent gave rise to the most abundant of the major product ions. Thus such quantum calculations may offer the potential for contributing to a predictive tool for aiding the accuracy and speed mass spectral interpretation by generating numerical data in the form of bond length increases to act as descriptors flagging potential bond cleavages.

Predicting collision-induced dissociation spectra: semi-empirical calculations as a rapid and effective tool in software-aided mass spectral interpretation

RATIONALE

Fifteen molecules were modelled using quantum chemistry, prior to interpreting their collision-induced dissociation (CID) product ion spectra, in a 'blind trial' to establish if calculated protonation-induced bond elongation could be used to predict which bonds cleaved during CID. Bond elongation has the potential to be used as a descriptor predicting bond cleavage.

METHODS

The 15 molecules were modelled with respect to protonation-induced bond length changes using Density Functional Theory (DFT). Significant bond elongations were highlighted to flag potential bond cleavages. CID product ion spectra, obtained using positive ion electrospray ionisation (Waters Synapt G1), were interpreted to establish if observed bond cleavages correlated with calculated bond elongations. Calculations were also undertaken using AM1 (Austin Model 1) to see if this rapid approach gave similar results to the computationally demanding DFT.

RESULTS

The AM1-calculated bond elongations were found to be similar to those generated by DFT. All the polarised bonds observed to cleave (n = 82) had been calculated to elongate significantly. Protonation, possibly via proton migration, on the most electronegative atom in the bond appeared to initiate cleavage, leading to a 100% success rate in predicting the bonds that broke as a result of protonation on a heteroatom. Cleavage of carbon-carbon bonds was not predicted.

CONCLUSIONS

Cleavage of the polarised bonds appears to result from protonation on the more electronegative atom of the bond, inducing conformational changes leading to bond weakening. AM1-calculated bond length changes act as a descriptor for predicting bond cleavage. However, the impetus for cleavage of the unpolarised bonds may be product ion stability rather than bond weakening.

Gas-phase dissociation of 1,4-naphthoquinone derivative anions by electrospray ionization tandem mass spectrometry

Gas-phase dissociation pathways of deprotonated 1,4-naphthoquinone (NQ) derivatives have been investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The major decomposition routes have been elucidated on the basis of quantum chemical calculations at the B3LYP/6-31 + G(d,p) level. Deprotonation sites have been indicated by analysis of natural charges and gas-phase acidity. NQ anions underwent an interesting reaction under collision-induced dissociation conditions, which resulted in the radical elimination of the lateral chain, in contrast with the even-electron rule. Possible pathways have been suggested, and their mechanisms have been elucidated on the basis of Gibbs energy and enthalpy values for the anions previously described at each pathway.

Gas-phase basicities of polyfunctional molecules. Part 1: Theory and methods

The experimental and theoretical methods of determination of gas-phase basicities, proton affinities and protonation entropies are presented in a tutorial form. Particularities and limitations of these methods when applied to polyfunctional molecules are emphasized. Structural effects during the protonation process in the gas-phase and their consequences on the corresponding thermochemistry are reviewed and classified. The role of the nature of the basic site (protonation on non-bonded electron pairs or on pi-electron systems) and of substituent effects (electrostatic and resonance) are first examined. Then, linear correlations observed between gas-phase basicities and ionization energies or substituent constants are recalled. Hydrogen bonding plays a special part in proton transfer reactions and in the protonation characteristics of polyfunctional molecules. A survey of the main properties of intermolecular and intramolecular hydrogen bonding in both neutral and protonated species is proposed. Consequences on the protonation thermochemistry, particularly of polyfunctional molecules are discussed. Finally, chemical reactions which may potentially occur inside protonated clusters during the measurement of gas-phase basicities or inside a protonated polyfunctional molecule is examined. Examples of bond dissociations with hydride or alkyl migrations, proton transport catalysis, tautomerization, cyclization, ring opening and nucleophilic substitution are presented to illustrate the potentially complex chemistry that may accompany the protonation of polyfunctional molecules.

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

An efficient but reasonably accurate B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) computational procedure showed that pentasubstituted cyclopentadienes such as (CN)5C5H, (NO2)5C5H, and (NC)5C5H containing strongly electron-withdrawing groups are neutral organic superacids of unprecedented strength. The boldface denotes the atom attached to the cyclopentadiene framework. All of them exhibit prototropic tautomerism by forming somewhat more stable structures with C=NH, NO2H, and N=CH exocyclic fragments, respectively. The acidity (DeltaH(acid)) of these is lower, but only to a rather small extent. The DeltaH(acid) enthalpies of these last three tautomers are estimated to be 271, 276, and 282 kcal mol(-1), respectively. Hence, the most stable tautomers of (CN)5C5H and (NC)5C5H represent a legitimate target for synthetic chemists. On the other hand, (NO2)5C5H is less suitable for practical applications, because of its high energy density. The origin of the highly pronounced acidity of these compounds was analyzed by using the recently developed triadic formula. It is found that very high Koopmans' ionization energy (IE)n(Koop) of conjugate bases exerts a decisive influence on acidity. It follows as a corollary that the overwhelming effect leading to very high acidity is due to the properties of the final state. An alternative picture is offered by homodesmotic reactions, wherein the cyclic systems are compared with their linear counterparts. It is found that the acidity of cyclopentadiene (CP) is a consequence of aromatic stabilization in the CP- anion. The extreme acidity of pentacyanocyclopentadiene (CN)5C5H is due to aromatization of the five-membered ring and a strong anionic resonance effect in the resultant conjugate base. The neutral organic superacids predicted by the present calculations may help to bridge the gap between existing very strong acids and bases.

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.

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics, structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.