The choice of basis set in ab initio calculations has been the subject of numerous theoretical studies. Early SCF calculations utilized mainly spht-va-lence basis sets such as 3-21G and 4-31G. The importance of inclusion of d polarization functions on sulfur atoms has been stressed by several authors. For instance, Suleimenov and Ha found that the omission of d polarization functions leads to a substantially lower barrier for the internal rotation ( 16 kj mol for the central bond of H2S4) and produces an unreahstically large S-S bond length for the most stable rotamer [4]. In general, the use of [Pg.2]

Since two mechanisms are possible for the competition between association and reaction, detailed ab initio calculations of the potential surface are even more necessary in theoretical determinations of the rates of association channels. More experimental work is also needed it is possible that as a larger number of competitive systems is studied, our understanding of the competition will increase. Critical systems for interstellar modeling include the association/reactive channels for C+ and bare carbon clusters, as well as for hydrocarbon ions and H2. [Pg.28]

Seitz and East selected five isomeric protonated octane isomers (CgH "1"), all featuring C—H—C or C—H—H 3c-2e bonds for their theoretical studies (ab initio calculations at the MP2/6-3 lG(d) level of theory).847 In most cases, dissociation into ion-molecule complexes was found to be again barrierless. Proton affinities of C—C and C—H bonds are in the range 154—187 kcal mol 1 and 139-150kcalmol-1, respectively. [Pg.222]

Tandem m s, and theoretical calculations have been used to study the gas-phase aldose-ketose isomerization. Results obtained with (l->3)-linked disaccharides supported a mechanism involving hydride transfer rather than a 1,3-hydrogen shift. Related theoretical studies (ab initio and semiempirical MO methods) of aldose-ketose isomerization in aqueous solution suggested that the favoured pathways in the absence and presence of metal ions are proton transfer via an enediol and hydride transfer, respectively. A strong influence of the acetonitrile mole fraction on the [Pg.13]

Tautomerism of 2,1,3-benzothiadiazinone 2,2,-dioxide 63 and its N-monosubsti-tuted derivatives has been studied experimentally by 3H and 13C NMR and theoretically by ab initio calculations. In the gas phase, the oxo form 63b was found to be more stable (AE — 4-6 kcal/mol) than the hydroxy form 63a however, due to the higher dipole moment of the latter this energy difference is greatly decreased in solution. Whereas both tautomers 63a and 63b were observed in non-polar solvents (THF) and protic solvents (methanol), only hydroxy tautomer 63a was detected in polar aprotic solvents (DMSO-<76) and in the solid state. The tautomerism is also present in the N-monosubstituted derivatives of 63, which exist primarily as oxo tautomers (99T12405). [Pg.31]

H2S2 (hydrogenpersulfide), the smallest member of the polysulfane series [15], has been studied extensively by molecular spectroscopy and theoretical calculations [16] (and references therein). By now, accurate knowledge of its structure, torsional potential and vibrational modes has been established. Ab initio calculations readily reproduce these properties [17]. The value of the SSH angle in hydrogen disulfide was a subject of controversies for some time. However, recent experiments led to a different value which is in favour of the ab initio calculated value [17]. [Pg.4]

The well-known molecule (2R,3R)-(-l-)-tartaric acid [2] has been studied both as its dg and isotopomer. As small molecules are less demanding concerning computation time, the molecule could be studied at three theoretical levels ab initio calculations using the 6-31G, 6-31G " and DZP basis sets were performed and compared with the experimental [Pg.792]

Whereas oxaziridine and diaziridine were partial subjects of comprehensive theoretical studies on cyclic compounds (73MI50800), diazirine and some of its simple derivatives were the special target of quantum chemical investigations. Since diazirine, the lowest molecular weight heterocycle, has only five atoms and is of high symmetry, there was a chance for ab initio calculations, which followed some semiempirical studies. [Pg.197]

These monodimensional compounds, rich in it electrons, have been the object of several experimental[35] and theoretical work[20,35,36]. Perry et al.[35], using a powder SHG technique, have studied diaryl polyynes and have shown that some of them exhibit second order hyperpolarizabilities of very high magnitude. On another hand, Jameson and Fowler[36] carried out ab initio calculations in order to study basis sets effects on the electrical properties of acetylene and diacetylene. Furthermore, Chopra et al [20], then Maroulis and Thakkar [37] have been interested in the influence of the lengthening of the [Pg.303]

Evidently, several aspects of this exciting area are difficult to study with experimental techniques. The different species are short-lived, reactive, and exist only under rather extreme conditions. These are conditions under which theoretical studies can contribute a lot to our understanding. Theoretical work has indeed been reported on smaller clusters with n-2 -10(42-50) as well as on some of the larger ones(25-41). The present work reports ab initio calculations for a number of large carbon clusters of relevance to the chemical problems addressed above. [Pg.36]

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