Chemistry June 2005 Vol 54/2 []

CONTENTS & ABSTRACTS

InEnglish. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Chemistry



Volume 54 No. 2June 2005



Special issue on physical organic chemistry

Preface;59

Peeter Burk

Thepervasive influence of hyperconjugation: the intrinsic (gas phase) acidities ofadamantane, 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 theB3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) level of the title compounds, theconjugated bases derived therefrom, and some ancillary compounds are reported.The most significant conclusions drawn are (i) that 1-Hperfluoroadamantane and perfluoro-1-adamantanol are expected to be endowed withan intrinsic gas phase acidity close to 310 kcal mol–1, similar tothat of strong conventional acids such as trifluoroacetic or methane sulphonicacids, and (ii) hyperconjugative effects on the structures of thesespecies and their corresponding anions are seen to be quite significantalthough 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 neutralanalogue ligand molecules approach; 70–84

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

Abstract. Interactions between the cesium cation (Cs+) andhumic 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 52model compounds (mainly aromatic) were calculatedusing the density functional method. The most common binding patterns of Cs+are discussed. The CsCAs are mainly governedby ion-induced dipole p-typeinteraction (CsCA around 11 kcal/mol) in unsubstituted fused aromaticsystems. The cesium cation tends to form a more localized type of bond insubstituted 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-typecomplexes with the Cs cation, CsCA may be even higher (20–28 kcal/mol).

Key words: cesium cation basicity, cesium cation affinity, humicsubstances, DFT calculations.

Comparativesemiempirical, ab initio, and DFT study of interactions between polypyrrolepentamer 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 inagreement with the trends in the ionic radiuses of the anions. Semiempiricalmethods were found to underestimate the complexation energy of the perchlorateanion 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 nonplanarityof the dication was not observed. The DFT B3LYP/6-31+G** method, while beingmost consistent, yielded some complexation energies with slightly higher BSSEthan 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, AinoPihl, Eva-Ingrid Rõõm, Lilli Sooväli, Vahur Mäemets, Viljar Pihl, and Ilmar A.Koppel

Abstract. A method for creating self-consistent acidity and basicityscales in nonaqueous solvents is presented and the current progress in applyingsuch scales to different solvents is reviewed. The method is based on UV–Visspectrophotometric titration of a mixture of two acids (or two bases) with anon-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 differentacid (base) strengths is available and  measurements are carried out in such a way that each compound ismeasured against several other compounds, a self-consistent acidity (basicity)scale can be built. The relative acidities (basicities) of the compoundsincluded in the scale can be determined at very high accuracy: many of theerror sources, most importantly moisture and impurities in the solvent and inthe compounds, influence both compounds in a similar way and at least partiallycancel out. Using this method, self-consistent acidity scales have been createdin heptane and acetonitrile and self-consistent basicity scales have beendeveloped in acetonitrile and THF. These scales are discussed and future plansare outlined.

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