Gas-phase acidities: a comparison of density functional, MP2, MP4, F4, G2(MP2, SVP), G2(MP2) and G2 procedures

Ab initio investigations on the gas phase basicity and nonlinear optical properties of FLinOH species (n = 2–5)

The superalkali hydroxide (FLi₅OH) possesses alkalide characteristics which is responsible for its remarkable mean hyperpolarizability i.e. NLO properties.

An empirical model for gas phase acidity and basicity estimation

Gas phase acidity and basicity estimation models have been developed for acidic and basic functional groups of amino acid side-chains and also for a number of small organic molecules. The acidic functional groups include aliphatic and aromatic alcohol, and aliphatic and aromatic carboxylic acid, and the basic functional groups include aliphatic, aromatic and hetero-aromatic amines, and also pyridino-, pyrazolo- and imidazolo-groupings. The models are described in terms of a linear combination of descriptors that highly influence reactivity at the reaction centres of the functional groups. In order to describe the chemical environments of the deprotonating and protonating sites, atomic descriptors such as the effective atomic electronegativity and effective atomic polarizability of the atoms in the reaction field and the electrostatic potentials at the reaction sites have been introduced. The coefficient of determination (r(2)) of each model is above 0.8, apart from the imidazole model. The models are readily applicable, ranging from simple organic molecules to proteins.

Accurate benchmark calculations on the gas-phase basicities of small molecules

Accurate benchmark calculations of gas-phase basicities of small molecules are presented and compared with available experimental results. The optimized geometries and thermochemical analyses were obtained from MP2/aug-cc-pVTZ calculations. Two different ab initio electron-correlated methods MP2 and CCSD(T) were employed for subsequent gas-phase basicity calculations, and the single-point energies were extrapolated to the complete basis set (CBS) limit. The overall accuracy for different ab initio methods is compared, and the accuracy in descending order is CCSD(T)_CBS > CCSD(T)/aug-cc-pVDZ > (MP2/aug-cc-pVQZ approximately MP2_CBS) > HF/aug-cc-pVQZ. The best root-mean-squared-error obtained was 1.0 kcal mol(-1) at the CCSD(T)_CBS//MP2/aug-cc-pVTZ level for a test set of 41 molecules. Clearly, accurate calculations for the electron correlation energy are important for the theoretical prediction of molecular gas-phase basicities. However, conformational effects were also found to be relevant in several instances when more complicated molecules were examined.

Computation of pKaValues of Substituted Aniline Radical Cations in Dimethylsulfoxide Solution

A newly developed computation strategy was used to calculate the absolute pKa values of 18 substituted aniline radical cations in dimethylsulfoxide (DMSO) solution with the error origin elucidated and deviation minimized. The B3LYP/6-311++G(2df,2p) method was applied and was found to be capable of reproducing the gas-phase proton-transfer free energies of substituted anilines with a precision of 0.83 kcal/mol. The IEF-PCM solvation model with gas-phase optimized structures was adopted in calculating the pKa values of the substituted neutral anilines in DMSO, regenerating the experimental results within a standard deviation of 0.4 pKa unit. When the IEF-PCM solvation model was applied to calculate the standard redox potentials of anilide anions, it showed that the computed values agreed well with experiment, but the redox potentials of substituted anilines were systematically overestimated by 0.304 eV. The cause of this deviation was found to be related to the inaccuracy of the calculated solvation free energies of aniline radical cations. By adjusting the size of the cavity in the IEF-PCM method, we derived a reliable procedure that can reproduce the experimental pKa values of aniline radical cations within 1.2 pKa units to those from experiment.

An Investigation of Chlorophenol Proton Affinities and Their Influence on the Biological Activity of Microorganisms

The proton affinities of 15 chlorophenols are calculated by ab initio methods. Straight correlation between proton affinities and changes in the electronic structure is observed. The proton affinities decrease linearly with the electronic density gain on the chlorine atoms, as the liberation of the proton increases. To confirm the importance of the proton affinities on the toxicity of chlorophenols, calorimetric responses of these molecules and related ones where the acid proton is changed to a methyl group (anisol and its chlorinated derivatives) were used to verify their effects on Chromobacterium violaceum. The results confirmed that the chlorophenols are more toxic than the respective chloroanisols and suggest that high proton affinities are associated with low toxic activity. The toxicity of the chlorophenols can be associated with the respiratory mechanism in some microorganisms.

Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO – a density functional study

It is shown by using the density functional theory (DFT) calculations that Rees tricyclic[10]annulene 1 and its benzo-annelated derivative 2, substituted by CN groups at all peripheral sp2 carbon positions, represent hyperstrong neutral superacids in the gas-phase and DMSO.

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.

Gas phase dissociation energies of saturated AHn*+ radical cations and AHn neutrals (A = Li-F, Na-Cl): dehydrogenation, deprotonation, and formation of AHn-2*+ - H2 complexes

The dissociation energies corresponding to the two possible A-H cleavages of A (A = Li-F and Na-Cl) radical cations (loss of a H(+) and loss of a H(.)) have been computed at the CCSD(T)/ 6-311++G(3df,2pd) level of theory and compared to those of their neutral precursors. Removing an electron from AH(n)() decreases dramatically its deprotonation energy, especially for the A molecules (C and ), which become one of the most acidic species of the row, their acid character being only exceeded by FH(.+) and ClH(.+), respectively. However, dehydrogenation energies only decrease for the systems on the left side of the row (up to C and SiH(4)(.+)) for which the electron is removed from a A-H bonding orbital. Nevertheless, the loss of hydrogen is the more favorable cleavage in all cases except FH(.+). Ionization of SiH(4) leads to a Jahn-Teller distorted structure that corresponds to a Si - H(2) complex. Other - eta(2)H(2) complexes in the doublet spin state have also been found to be stable for A = Be, Mg, Al, and P, the hydrogen molecule complexes being more stable than their corresponding radical cations, for Be, Mg, and Al.

Mass spectrometry for on-line monitoring of perfluoro compounds using Li+ ion attachment techniques

Ion attachment mass spectrometry is being developed for continuous measurement of perfluoro compounds (PFCs) found in the atmosphere as a result of semiconductor manufacturing processes. Studies were made on 5 greenhouse gases, CF4, CHF3, C2F6, SF6, and c-C4F8, to develop improved methods for PFC analysis, particularly at levels found in the atmosphere (the parts-per-billion concentration range). The results demonstrate the feasibility of performing real-time measurements of the trace amounts of PFCs encountered in process facilities by generating adduct ions from Li+ ion attachment. The identification and detection of c-C4F8 is described as an example of the utility of this new method.

Computational chemistry: a useful (sometimes mandatory) tool in mass spectrometry studies

In this review, we present a brief summary of the theoretical methods most frequently used in gas-phase ion chemistry. In subsequent sections, the performance of these methods is analyzed, paying attention to the reliability of geometries, vibrational frequencies, energies, and entropies. The possible pathologies of the different methods, in the form of instabilities of the wave function or spin contamination problems, are discussed. Several examples are presented to illustrate the usefulness of ab initio or density functional theory (DFT) methods to predict the existence of elusive molecules and/or to characterize non-conventional structures, and to rationalize the charge redistributions normally associated with ion-molecule interactions and which result in bond-weakening or bond-reinforcement effects. Finally, the role of non-classical structures in ion-molecule interactions is also illustrated with different examples.

Alkali-metal ion/molecule association reactions and their applications to mass spectrometry

This review will be concerned primarily with thermal alkali-metal ion association reactions of the general type: A+ + M + N reversible (A + M)+ + N. Where A denotes a positively charged alkali metal ion, M is neutral species and N works as a third body. As indicated, most association reactions are reversible, and the A-M bond derives primarily from electrostatic forces whose energy (affinity) is typically 50 kcal/mol or less, often much less. The review includes reaction mechanism, instrumentation, and application to mass spectrometry, together with reaction rates and alkali ion affinities of the classified compounds.

Theoretical calculations of glycine and alanine gas-phase acidities

The gas-phase acidities of glycine and alanine were determined by using a variety of high level theoretical methods to establish which of these would give the best results with accessible computational efforts. MP2, MP4, QCISD, G2 ab initio procedures, hybrid Becke3-LYP (B3LYP) and gradient corrected Becke-Perdew (BP) and Perdew-Wang and Perdew (PWP) nonlocal density functionals were used for the calculations. A maximum deviation of approximately 13 and 18 kJ/mol from experimental data was observed for the computed delta Hacid and delta Gacid values, respectively. The best result was obtained at G2 level, but comparable reliability was reached when the considerably less time consuming B3LYP, BP, and PWP density functional approaches were employed.