Prediction of excess enthalpies for ternary aqueous solutions of nonelectrolytes

The excess enthalpiesH _xy ^E of ternary aqueous solutions of nonelectrolytes are used to test the possibility of making predictions for ternary solutions from the properties of the binary solutions only. Two methods are proposed: one is based on the empirical rule (h _xx ·h _yy )^1/2=h _xy . Another leads to the numerical prediction of the overallH _xy ^E . Both are successful for most of the pairs of solutes for whichH _x ^E ,H _y ^E >0.     

Ab initio pseudopotential study of the fluxional behavior in tetrahydroborate complexes. Many-body contributions to the energy barriers of NaBH4, AlH2BH4, and GaH2BH4

The relative stabilities of the bidentate and tridentate configurations of the complex hydrides NaBH₄, AlH₂BH₄, and GaH₂BH₄ have been computed both at the Hartree-Fock and post-Hartree-Fock levels using the ab initio pseudopotential method. For both compounds correlation effects favor the configurations with the highest coordination of the metal. The changes with respect to HF results are not very large, so that second-order perturbative computation of correlation energy is sufficient to give accurate results.     

Structure validation of natural products by quantum-mechanical GIAO calculations of 13C NMR chemical shifts

Geometry optimization and GIAO (gauge including atomic orbitals) (13)C NMR chemical shift calculations at Hartree-Fock level, using the 6-31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental (13)C NMR chemical shifts for fourteen low-polar natural products, containing 10-20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of (13)C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical (13)C chemical shifts of both correct and incorrect structures matched the experimental data.     

Energies,structures, and electronic properties of molecules in solution with the C-PCM solvation model

The conductor-like solvation model, as developed in the framework of the polarizable continuum model (PCM), has been reformulated and newly implemented in order to compute energies, geometric structures, harmonic frequencies, and electronic properties in solution for any chemical system that can be studied in vacuo. Particular attention is devoted to large systems requiring suitable iterative algorithms to compute the solvation charges: the fast multipole method (FMM) has been extensively used to ensure a linear scaling of the computational times with the size of the solute. A number of test applications are presented to evaluate the performances of the method.     

Conformational behaviour of non-fused biheterocycles. Part XV. Isomeric phenylisoxazoles

The conformational behaviour of isomeric phenylisoxazoles has been investigated at the STO-3G level optimizing the most significant geometrical parameters. The energy minima of the two isomers having an heteroatom in an ortho position correspond to planar structures, whereas the 4-isomer has a twisted equilibrium conformation. This confirms that H? H non-bonded repulsions are stronger than (lone-pair)-H ones. The similar conformational behaviour of corresponding phenylfurans and phenylisoxazoles shows that the second heteroatom has a negligible effect and that the conformational behaviour of non-fused biheterocycles is dictated by the nature of the ortho substituents.     

DFT calculations of the electric field gradient at the tin nucleus as a support of structural interpretation by 119Sn Mössbauer spectroscopy

DFT calculations, using an all-electron basis set and with full geometry optimization, were performed on 34 Sn(II) and Sn(IV) compounds of known structure and (119)Sn M?ssbauer parameters, to obtain the theoretical values of the electric field gradient components, V(xx), V(yy), and V(zz), at the tin nucleus. These were used to determine the quantity V = V(zz)[1+ 1/3((V(xx) - V(yy))/((V(zz))(2)](1/2), for each investigated compound, which is related to the quadrupole splitting (DeltaE) parameter according to DeltaE = 1/2eQV, where e is the electronic charge and Q is the quadrupole moment of the tin nucleus. The linear fitting of the correlation plot of the experimental DeltaE, versus the corresponding calculated V values, produced a slope that is equal to 0.93 +/- 0.03 and a correlation coefficient R = 0.982. The value of Q obtained, 15.2 +/- 4.4 fm(2), is in agreement with that previously experimentally determined or calculated by analogous procedures. The calculation method is able to establish the sign of the electric field gradient component V(zz), in agreement with the sign of DeltaE determined experimentally by M?ssbauer-Zeeman spectroscopy. The calculated structural parameters are in good agreement with the corresponding experimental data, determined by X-ray crystallography in the solid state, with average structural deviations of about 3 % for bond lengths and angles in the tin environment. Calculated values of DeltaE were obtained from the calibration fitting constant and from the values of V. By comparing experimental and calculated DeltaE parameters, the structure assignment of configurational isomers was successful in two test cases, in agreement with the experimental X-ray crystallographic structures. These results indicate that the method can be used as a tool to support the routine structure interpretation of tin compounds by (119)Sn M?ssbauer spectroscopy.     

Conformational analysis of polypropylene chains bound to model catalytic sites

The models of catalytic sites for isospecific Ziegler-Natta polymerization proposed previously (edges and reliefs on lateral surfaces of violet TiCl₃) are compared in order to verify their validity in the presence of long alkyl groups and of analyzing the conformational constraints imposed by the TiCl₃ surface on the growing chain. Our results indicate that sites on edges of lateral surfaces parallel or nearly parallel to the c axis may be not suitable for the coordination of long alkyl chains; the coordination of long alkyl chains at less hindered sites, such as edges of inclined lateral surfaces or reliefs, do not involve any increase of repulsive interactions for an isobutyl group. Further, these latter sites allow the optimum spiralization of the chain at a very early stage of growth, and their isospecific behaviour is not affected by the presence of configuration defects in the growing chain.     

Theseus: A new software package for the handling and analysis of thermal denaturation data of biological macromolecules

A new software package (THESEUS) has been assembled for the analysis of the DSC data, Concerning the thermal denaturation of biological macromolecules. The system is useful to obtain accurate physico-chemical information, bypassing the casual and systematic errors, very common in these experiments. It can also be used for handling data from other instruments and methodologies giving thermodynamic, spectroscopic or other kind of data as a function of temperature. Because many of the researches in this field are of exploratory nature and continuously new unfolding mechanisms are described or hypothesized in the current literature, we have written and assembled this powerful and flexible program of general applicability, in order to put the operator in a position to control each step of the calculation procedure and use his own experience for choosing the better way to solve unexpected problems.     

Enthalpies and entropies of fusion of some substituted dipeptides

Enthalpies and temperatures of fusion or transition for four substituted dipeptides (Nacetylamides of glycyl-L-alanine (NAGAA),L-alanyl-L-alanine (NAA₂A),L-prolyl-glycine (NAPGA) andL-leucyl-L-proline monohydrate (NALPA·H₂O)) were determined by differential scanning calorimetry and the entropies of fusion derived. The results obtained have been compared with those of the corresponding substituted aminoacids and some of their racemic crystalline mixtures. The enthalpies and entropies of fusion of some substituted aminoacids have been redetermined. The results are discussed in comparison with crystal structural data, which has been reported in the literature or determined recently by some of the authors. Rationalization of the fusion parameters was attempted mainly on the basis of the number of intramolecular hydrogen bonds and the packing densities in the crystals.This work was carried out with the financial support from the Italian C.N.R. (Rome) and from the Ministry of University and Scientific and Technological Research.