Acid-base equilibria in nonpolar media. 3. Expanding the spectrophotometric acidity scale in heptane

The UV-vis spectrophotometric ion-pair acidity scale in heptane has been significantly expanded: it includes now 21 bulky CH and NH indicator acids and spans for about 10 pKip units. The phosphazene base t-Bu-P4 was used for deprotonating. The correlations between acidities in heptane versus gas-phase acidities or acidities in DMSO or 1,2-dimethoxyethane have been made for some compounds. It was demonstrated that the substituent effects on the acidity of the studied CH acids are attenuated ca. 1.24 times when the gas phase is substituted for the nonpolar solvent, heptane. In its turn, for the series of NH acids, the latter is found to be a somewhat more differentiating solvent than DMSO.     

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.     

Gas‐phase acidity of bis[(perfluoroalkyl)sulfonyl]imides. Effects of the perfluoroalkyl group on the acidity

The gas‐phase acidity (GA) values were determined for a number of perfluoroalkyl‐substituted sulfonylimides by measuring proton‐transfer equilibria using a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The GA scale below 286.5 kcal mol^?1 for (CF₃SO₂)₂NH was extended and partially revised. The GA value of (C₄F₉SO₂)₂NH which is currently the strongest acid was revised from 284.1 to 278.6 kcal mol^?1. The effect of fluorine atoms on the acidity of perfluoroalkyl‐substituted sulfonylimides was described with the following model where N_(α), N_(β), N_(γ), and N_(δ) are the numbers of fluorine atoms at α, β, γ, and δ position in R_fSO₂ (R_f = perfluoroalkyl group), respectively. This correlation indicates that the electron‐withdrawing ability of the R_fSO₂ group can be described in terms of the number of fluorine atoms in the perfluoroalkyl group corrected by taking into account their positions. Copyright © 2014 John Wiley & Sons, Ltd.     

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

The molecular geometries of polyyne chains H(CC)_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 Bader's 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(CC)_n⁻)–E(H(CC)_nH) and E(⁻(CC)_n⁻)–E(H(CC)_n⁻) was tested for different basis sets. The increase of the number of CC 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. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 73–85, 2001     

A comprehensive self-consistent spectrophotometric acidity scale of neutral Brønsted acids in acetonitrile

For the first time, the self-consistent spectrophotometric acidity scale of neutral Br?nsted acids in acetonitrile (AN) spanning 24 orders of magnitude of acidities is reported. The scale ranges from pK(a) 3.7 to 28.1 in AN. The scale includes 93 acids that are interconnected by 203 relative acidity measurements (DeltapK(a) measurements) and contains compounds with gradually changing acidities, including representatives from all of the conventional families of OH (alcohols, phenols, carboxylic acids, sulfonic acids), NH (anilines, diphenylamines, disulfonimides), and CH acids (fluorenes, diphenylacetonitriles, phenylmalononitriles). The CH acids were particularly useful in constructing the scale because they do not undergo homo- or heteroconjugation processes and their acidities are rather insensitive to traces of water in the medium. The scale has been fully cross-validated: the relative acidity of any two acids on the scale can be found by combining at least two independent sets of DeltapK(a) measurements. The consistency standard deviation of the scale is 0.03 pK(a) units. Comparison of acidities in many different media has been carried out, and the structure-acidity relations are discussed. The large variety of the acids on the scale, its wide span, and the quality of the data make the scale a useful tool for further acidity studies in acetonitrile.     

Theoretical study of structure and basicity of some alkali metal oxides,hydroxides, and amides

Ab initio (TZV *, SBK *, and 3–21G * or 6–31G * basis sets) calculations were performed to predict the geometries and gas-phase proton affinities of Li₂O, LiOH, LiNH₂, Na₂O, NaOH, NaNH₂, K₂O, KOH, and KNH₂. © 1994 John Wiley & Sons, Inc.