Peeter Burk

Critical test of performance of B3LYP functional for prediction of gas-phase acidities and basicities

Abstract The gas-phase acidities and basicities for 49 acids and 32 bases, calculated using B3LYP hybrid DFT method and 6-31G ∗ , 6-31+G ∗ , 6-311+G ∗∗ , and 6-311+G(3df,3pd) basis sets are compared with corresponding experimental values. The best results were obtained with 6-311+G(3df,3pd) basis set; the average absolute errors were below 2.5 kcal/mol both for basicities and acidities. Good results for both acidities and basicities (the average absolute errors were ⩽3 kcal/mol) were also obtained using the 6-311+G ∗∗ basis set and even with a moderate 6-31+G ∗ basis set (mean absolute errors were

IEF-PCM Calculations of Absolute pKa for Substituted Phenols in Dimethyl Sulfoxide and Acetonitrile Solutions

Absolute (nonrelative) pKa calculations for substituted phenols were carried out in nonaqueous media, demonstrating the predictive power of the integral equation formalism PCM method with a mean unsigned error of 0.6 pKa units for DMSO and 0.7 pKa units for MeCN at the B3LYP/6-31+G** level of theory combined with the scaled B3LYP/6-311+G** gas-phase data. At the same time, the correlation between the calculated and experimental pKa values yielded the value of the linear regression slope very close to unity for both DMSO and MeCN. Computation of pKa of neutral acids in nonaqueous solutions with a reasonable precision obviously depends on carefully tuned cavities, optimized for nonaqueous solutions. The ability of continuum solvation model to compensate charge escape from the cavity, which is prominent in the case of anions, is also required. And finally, good quality gas-phase data is essential to achieve required pKa precision.

SUPERACIDITY OF NEUTRAL BRONSTED ACIDS IN GAS PHASE

Quantum chemical calculations of potentially superacidic neutral Brönsted acids were carried out using the PM3 method. It was shown that the PM3 method can be used to predict the gas phase acidities of acidic compounds only if empirical corrections are made. A strong acidifying effect is predicted for a new family of compounds in which an sp2 oxygen is substituted by an (DOUBLE BOND) NSO2CF3 group. So, for example, such replacement is expected to result in acid strengthening by 47.5 kcal/mol in the case of CH3CHO and by 22.7 kcal/mol in the case of CF3SO2OH. The acidities of such compounds are predicted to be increased further (nonadditively) by stepwise replacements of (DOUBLE BOND) O by (DOUBLE BOND) NSO2CF3. The geometries of known superacidic systems were reproduced quite well by PM3 method. The geometries of several superacidic systems were analyzed.

Dihydrogen Generation from Amine/Boranes: Synthesis, FT-ICR, and Computational Studies†

A Fourier transform ion cyclotron resonance spectrometry (FT-ICR) study of the gas-phase protonation of ammonia-borane and sixteen amine/boranes R(1)R(2)R(3)N-BH(3) (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H(2)) are R(1)R(2)R(3)N-BH(4)(+) and R(1)R(2)R(3)N-BH(2)(+), respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6-311+G(d,p)] level) of the thermodynamic stabilities of the R(1)R(2)R(3)N-BH(3) adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R(1)R(2)R(3)N-BH(4)(+) ions is characterized by a three-center, two-electron bond between B and a loosely bound H(2) molecule. Unexpected features of some R(1)R(2)R(3)N-BH(4)(+) ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.

Critical Test of Some Computational Chemistry Methods for Prediction of Gas-Phase Acidities and Basicities.

Gas-phase acidities and basicities were calculated for 64 neutral bases (covering the scale from 139.9 kcal/mol to 251.9 kcal/mol) and 53 neutral acids (covering the scale from 299.5 kcal/mol to 411.7 kcal/mol). The following methods were used: AM1, PM3, PM6, PDDG, G2, G2MP2, G3, G3MP2, G4, G4MP2, CBS-QB3, B1B95, B2PLYP, B2PLYPD, B3LYP, B3PW91, B97D, B98, BLYP, BMK, BP86, CAM-B3LYP, HSEh1PBE, M06, M062X, M06HF, M06L, mPW2PLYP, mPW2PLYPD, O3LYP, OLYP, PBE1PBE, PBEPBE, tHCTHhyb, TPSSh, VSXC, X3LYP. The addition of the Grimmes empirical dispersion correction (D) to B2PLYP and mPW2PLYP was evaluated, and it was found that adding this correction gave more-accurate results when considering acidities. Calculations with B3LYP, B97D, BLYP, B2PLYPD, and PBE1PBE methods were carried out with five basis sets (6-311G**, 6-311+G**, TZVP, cc-pVTZ, and aug-cc-pVTZ) to evaluate the effect of basis sets on the accuracy of calculations. It was found that the best basis sets when considering accuracy of results and needed time were 6-311+G** and TZVP. Among semiempirical methods AM1 had the best ability to reproduce experimental acidities and basicities (the mean absolute error (mae) was 7.3 kcal/mol). Among DFT methods the best method considering accuracy, robustness, and computation time was PBE1PBE/6-311+G** (mae = 2.7 kcal/mol). Four Gaussian-type methods (G2, G2MP2, G4, and G4MP2) gave similar results to each other (mae = 2.3 kcal/mol). Gaussian-type methods are quite accurate, but their downside is the relatively long computational time.

Calculation of the properties of acid sites of the zeolite ZSM-5 using ONIOM method

Abstract The performance of the ONIOM method was tested in reproducing the properties of the acid sites of the zeolite ZSM-5. The acid sites in three crystallographic locations (Al3–O19(H)–Si6, Al6–O18(H)–Si9 and Al7–O17(H)–Si4) have been modelled by three combinations of two-layer ONIOM method with varied size (two and eight T atoms) and calculation method (ab initio and DFT) used for the chemically important (active) part. The best correspondence with experimental data was achieved in case of Al6–O18(H)–Si9 site calculated with ONIOM(B3LYP/6-311+G∗∗:MNDO) method using eight T atoms in model system for the following properties: stretching vibrational frequencies, 1H NMR chemical shifts of the bridged hydroxyl groups, vibrational frequency shifts and changes in 1H NMR chemical shifts of the hydroxyl groups upon adsorption of CO.

Critical test of PM3 calculated gas-phase acidities

SummaryGas-phase acidities have been calculated for 175 compounds using the PM3 semiempirical molecular orbital model. With some exceptions, PM3 seems to be a useful tool for the investigation of gas-phase acidities. The main problems encountered involve two rather different classes of acids: one which generates small anions (e.g., halide ions, hydride ion, etc.), in which the charge is localized on one atom, and, a second, represented by anions that contain bulky electron acceptor substituents characterized by an extensive negative charge delocalization. In some cases (anilines, amides, alcohols, and phenols) the average error in predicted gas-phase acidity can be significantly reduced by employing an empirically derived correction.Comparison with AM1 results shows that both methods are of roughly equal quality with the exception of hypervalent molecules where PM3 is better (averaged unsigned errors are 11.8 and 17.0 kcal/mol for PM3 and AM1, respectively).

Gas-Phase Basicities Around and Below Water Revisited

This work employs Fourier transform ion cyclotron resonance (FT-ICR) and the Gaussian quantum chemistry composite methods W1 and G2 to experimentally and computationally analyze gas-phase basicities (GB) for a series of weak bases in the basicity region around and below water. The study aims to clarify the long-standing discrepancy between reported GB values for weak bases obtained via high-pressure mass spectrometry (HPMS) and ICR; the ICR scale is observed to be more than 2 times contracted compared to the HPMS scale. The computational results of this work support published HPMS data. This agreement improves with increasing sophistication of the computational method and is excellent at the W1 level. Several equilibria were also re-examined experimentally using FT-ICR. In the experiments with some polyfluorinated weak bases (hexafluoro-2-propanol and nonafluoro-2-methyl-2-propanol), it was found that two protonation processes compete in the gas phase: protonation on oxygen and protonation on fluorine. In these species, protonation on fluorine proceeds faster and is statistically favored over protonation on oxygen but leads to cations that are thermodynamically less stable than oxygen-protonated cations. The process may also lead to the irreversible loss of HF. The rearrangement of fluorine-protonated cations to oxygen-protonated cations is very slow and is further suppressed by the process of HF abstraction. These results at least partially explain the discrepancy between published HPMS data and earlier FT-ICR findings and call for the utmost care in using FT-ICR for gas-phase basicity measurements of heavily fluorinated compounds. The narrower dynamic range of ICR necessitates the measurement of several problematic bases and produces some differences between the ICR results in the present work and the published HPMS data; the wider dynamic range allows HPMS to overcome these difficulties in connecting the ladder.

Quantum chemical calculations of geometries and gas-phase deprotonation energies of linear polyyne chains

The molecular geometries of polyyne chains H(CC)nH with their deprotonated forms (anions) have been optimized using ab initio LCAO-SCF molecular orbital (MO) method and density functional theory at different basis set levels. The polyynes possess a series of alternating single and triple bonds. On the theoretical side the persistence of bond alternation and the effect of chain lengthening on the individual bond length in linear conjugated polyyne chains has been investigated. The common conclusion has been drawn that the bond alternation will persist and that bond length variation will be small. The triple bond length increases progressively toward the asymptotic limits as the value of n increases progressively. If the split-valence basis set was employed, the total charges obtained using the Mulliken population analysis yielded unrealistic values. Using natural bond orbital (NBO) analysis or Baders analysis, the net charges of the individual atoms converge very rapidly to their asymptotic limits, and the central atoms have almost zero charges in contrast to the Mulliken population analysis results. The reliability of deprotonation energies of neutral polyynes and their monoanionic derivatives calculated from the differences in molecular energy of the parent chains and the corresponding anions E(H(CC)n−)–E(H(CC)nH) and E(−(CC)n−)–E(H(CC)n−) was tested for different basis sets. The increase of the number of CC bonds in the chain decreases these differences asymptotically. The studied compounds are the best available building blocks in bimetallic compounds with useful properties in molecular electronics and nonlinear optics.

Why are carboxylic acids stronger acids than alcohols? The electrostatic theory of Siggel–Thomas revisited

Abstract The electrostatic explanation of Siggel and Thomas for the acidity differences between series of acids was investigated critically. Acidities and electrostatic potentials at the protons of XOH, XNH 2 , and XCH 3 derivatives (X=H, CH 3 , HCO, NO 2 , and F) were calculated at the Becke3LYP/6-311+G ∗∗ level. Final state (anion) relaxation energies were obtained as proposed by Siggel and Thomas. Examination of initial and final state contributions revealed deficiencies in the Siggel–Thomas method as the final-state (relaxation) energies still retain contributions from the electronic structure of the initial state. Hence, this relaxation energy is not reliable as a measure of resonance stabilisation in anion. The application of Siggel–Thomas approach in two opposite directions—dissociation of neutral acid and protonation of an anion—leads to contradictory conclusions about whether the acidity difference between methanol and formic acid is determined by the neutral acid or anion. Hence, this scheme cannot be used to determine the initial state (neutral acid) and final state (anion) contributions to acidity as proposed by Siggel and Thomas.