J.J.C. Teixeira-Dias

Weak hydrogen bonding. Part 2. The hydrogen bonding nature of short C–H ⋯π contacts: crystallographic, spectroscopic and quantum mechanical studies of some terminal alkynes

In a number of crystalline terminal alkynes, the alkyne C–H groups form short intermolecular contacts with other alkyne or with aromatic moieties. A particularly short C–H ··· phenyl contact is found in (±)-3-phenylbut-1-yn-3-ol, with an H ··· midpoint separation of only 2.51 A. Crystallographic, spectroscopic and computational evidence is given to show that these contacts represent hydrogen-bond-type interactions. Quantum chemical calculations indicate hydrogen bond energies in the range 1–2.2 kcal mol–1. Interconnected contacts such as CC–H ··· CC–H ··· CC–H exhibit a pronounced cooperativity effect, with energy increases of ∼0.5 kcal mol–1 compared with isolated contacts. Based on these results, the role of the terminal alkyne group in cooperative hydrogen bond networks is described as topologically paralleling that of hydroxy groups.

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Mesophase formation in lead(II) decanoate

Abstract Structures of the thermotropic mesophases of lead(II) decanoate are reassigned following optical and X-ray diffraction studies. These results, and those of D.S.C., Raman and 207Pb N.M.R. spectroscopy, indicate formation of a lower temperature mesophase involving mainly increased lateral disorder, and a higher temperature Lα (smectic A) phase resulting from chain disordering and decreased lead-carboxylate interaction. Comparison of experimental thermodynamic data for the phase transitions with theoretical data in the literature indicates that the entropy change for the lower to higher mesophase transition is dominated by the increase in chain disorder.

The dependence of exchange energy on orbital overlap

Several methods of estimating exchange energies have been tested for the interaction of two hydrogen atoms in 2s, 2p or hybrid valence states. The simplest relationship previously used, X = KS 2/R, does not give an accurate picture of the dependence on internuclear distance. A two-term expression X = S 2(AR -1 + BR -2) is considerably better but fails for the case of exchange between orthogonal orbitals (S = 0). An alternative expression in which C 0 - C is the penetration part of the two-electron coulomb integral, is just as accurate, contains no more constants and holds equally well for orthogonal and non-orthogonal orbitals. C 0 - C is a better measure of the net overlap of two orbitals than is the overlap integral S, because the latter is zero for orbitals which are orthogonal but nevertheless overlap. We have found no method of deducing the constants K, A or B other than by calculating the exchange energy exactly at one or two values of R.

The calculation of electric polarizabilities of hydrocarbons with particular attention to the bond-additive property

By adding contracted hydrogen 2p orbitals to the basis of an SCF LCAO MO calculation good values are obtained for the polarizabilities of the saturated hydrocarbons. CNDO parameters are obtained for these orbitals by comparison of empirical and non-empirical calculations. The calculations fail to support the bond-additive model for the components of the polarizability tensor but the average polarizability is found to be bond-additive. These results are in agreement with experiment. The failure of the additivity of components is due both to the flow of electrons between bonds and to the large internal fields induced in the molecule.

Molecular orbital calculations of substituent effects on directly bonded 13C-H coupling constants

The effect of substituents on 13C-H coupling constants is discussed, within the framework of the Pople-Santry MO theory of nuclear spin coupling, by making a theoretical analysis of the contributions due to differences of coulomb and resonance integrals as well as due to the presence of electron lone-pairs. Earlier treatments based on correlations of J CH with s characters are shown to be less justified mainly because the description of CH bonds in terms of localized MOs is not valid, and no physical meaning can be rigorously attributed to concepts such as rehybridization.

s-cis and s-trans Conformers of formic, thioformic and dithioformic acids. An ab initio study

Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6–31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the ‘charge’-‘charge flux’-‘overlap’(CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis(thiol) > s-trans(thiol) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms.

A molecular mechanics force field for conformational analysis of simple acyl chlorides, carboxylic acids and esters

Abstract A molecular mechanics force field for conformational and vibrational studies on acyl chlorides, carboxylic acids and esters is developed. The resulting force field can also be applied to alkanes, alcohols and ethers. Chlorine atoms and alkyl groups are considered as α-substituents.

Thermotropic Polymorphism in Liquid-Crystalline Lead(II) Alkanoates

Abstract Lead(II) carboxylates with even chain lengths from octanoate to octadecanoate have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dilatometry, together with Raman, infrared, and 207Pb N.M.R. spectroscopy. The reassignment of the high temperature mesophase as smectic A (Lx) for the octanoate to dodecanoate is confirmed. This phase is most stable for the decanoate. The intermediate temperature mesophase is a highly ordered three dimensional lamellar structure based on ionic layers. The alkyl chains in this phase are partially disordered (melted) through the introduction of gauche conformations and show decreased lateral interactions; however, they remain largely in an extended conformation due to the constraints imposed by the ionic network. Melting of the alkyl chains occurs therefore in more than one step. Re-cooling to the crystalline phase appears to result in a different polymorphic crystalline form (albeit still lamellar) from the...

Solid state inclusion compound of S-ibuprofen in β-cyclodextrin: structure and characterisationElectronic supplementary information (ESI) available: crystal and data collection parameters and relevant O⋯O contacts (divided in six different groups) for βCD∶S-Ibu. See: http://www.rsc.org/suppdata/nj/b2/b207272f/

A crystalline inclusion complex was isolated from the reaction of β-cyclodextrin (βCD) with aqueous S-(+)-ibuprofen (S-Ibu). The existence of a true inclusion complex in the solid state was confirmed by a combination of powder X-ray diffraction (XRD), single crystal X-ray diffraction, thermogravimetric analysis (TGA), FTIR and 13C CP MAS NMR spectroscopies. The inclusion compound crystallises in the non-centrosymmetric monoclinic space group C2 with a 2∶1 host∶guest stoichiometry. The crystal structure consists of a head-to-head dimer of βCD molecules stacked along the crystallographic c axis, thus forming a slightly tilted channel-type structure.