In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.



Volume 54 No. 2 June 2005


Special issue on physical organic chemistry

Preface; 59

Peeter Burk

The pervasive influence of hyperconjugation: the intrinsic (gas phase) acidities of adamantane, adamantanol, and some of their fluorinated derivatives; 60–69

José-Luis M. Abboud, Masaaki Mishima, and Takaaki Sonoda

Abstract. In this work the results of a computational study at the B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) level of the title compounds, the conjugated bases derived therefrom, and some ancillary compounds are reported. The most significant conclusions drawn are (i) that 1-H perfluoroadamantane and perfluoro-1-adamantanol are expected to be endowed with an intrinsic gas phase acidity close to 310 kcal mol–1, similar to that of strong conventional acids such as trifluoroacetic or methane sulphonic acids, and (ii) hyperconjugative effects on the structures of these species and their corresponding anions are seen to be quite significant although their detailed dissection remains a topic for future studies.

Key words: gas phase acidity, 1-H-perfluoroadamantane, perfluoro-1-adamantanol, DFT calcula­tions, hyperconjugation.

Computational study of cesium cation–humic substance interactions. A neutral analogue ligand molecules approach; 70–84

Peeter Burk, Sven Tamp, Jaana Tammiku‑Taul, Pierre‑Charles Maria, and Jean‑François Gal

Abstract. Interactions between the cesium cation (Cs+) and humic substances were studied by the calculations of complexes between Cs+ and small ligand molecules mimicking different moieties in humic substances. The cesium cation gas-phase affinities (CsCAs) and basicities (CsCBs) of 52 model compounds (mainly aromatic) were calculated using the density functional method. The most common binding patterns of Cs+ are discussed. The CsCAs are mainly governed by ion-induced dipole p-type interaction (CsCA around 11 kcal/mol) in unsubstituted fused aromatic systems. The cesium cation tends to form a more localized type of bond in substituted aromatic compounds via the lone pairs of electronegative atoms (O, N) of substituents resulting in enhanced CsCA (around 20 kcal/mol). If the model compound is able to form chelate-type complexes with the Cs cation, CsCA may be even higher (20–28 kcal/mol).

Key words: cesium cation basicity, cesium cation affinity, humic substances, DFT calculations.

Comparative semiempirical, ab initio, and DFT study of interactions between polypyrrole pentamer dication and some anions; 85–93

Peeter Burk, Imbi Rauk, and Tarmo Tamm

Abstract. Complexes between pyrrole pentameter dication and chloride, bromide, and perchlorate anions were studied using AM1, PM3, HF/3-21+G(*), and B3LYP/6-31+G** methods. The resulting complexation energies were in agreement with the trends in the ionic radiuses of the anions. Semiempirical methods were found to underestimate the complexation energy of the perchlorate anion and in the case of PM3 also the bromide anion. HF/3-21+G(*) also gave a too narrow range of complexation energies, in addition nonplanarity of the dication was not observed. The DFT B3LYP/6-31+G** method, while being most consistent, yielded some complexation energies with slightly higher BSSE than expected.

Key words: polypyrrole, anions, DFT, ab initio, AM1, PM3.

Self-consistent acidity and basicity scales in nonaqueous solvents; 94–115

Ivo Leito, Ivari Kaljurand, Toomas Rodima, Agnes Kütt, Aino Pihl, Eva-Ingrid Rõõm, Lilli Sooväli, Vahur Mäemets, Viljar Pihl, and Ilmar A. Koppel

Abstract. A method for creating self-consistent acidity and basicity scales in nonaqueous solvents is presented and the current progress in applying such scales to different solvents is reviewed. The method is based on UV–Vis spectrophotometric titration of a mixture of two acids (or two bases) with a non-absorbing base (acid). From the titration data the relative acidity (relative basicity) of the two compounds – the difference of their  values  – is obtained. When a large set of compounds that have different acid (base) strengths is available and  measurements are carried out in such a way that each compound is measured against several other compounds, a self-consistent acidity (basicity) scale can be built. The relative acidities (basicities) of the compounds included in the scale can be determined at very high accuracy: many of the error sources, most importantly moisture and impurities in the solvent and in the compounds, influence both compounds in a similar way and at least partially cancel out. Using this method, self-consistent acidity scales have been created in heptane and acetonitrile and self-consistent basicity scales have been developed in acetonitrile and THF. These scales are discussed and future plans are outlined.

Key words: acidity scales, basicity scales,  values, dissociation constants, nonaqueous solvents.