Gustavo L.C. Moura

Mesoionic rings as efficient asymmetric bridges for the design of compounds with large optical nonlinearities

Abstract Through semi-empirical AM1 calculations of second-order nonlinear optical properties of molecules containing mesoionic rings, we propose that these should be considered as promising substitutes for the polyene bridges normally present in compounds displaying large hyperpolarizabilities, β. In order to substantiate our reasoning, we computed the AM1 static hyperpolarizabilities, β(0), of twenty molecules whose experimental values have already been determined. Our results indicate that the AM1 β(0) are indeed able to yield qualitative inferences about experimentally obtained values. Finally, we present a number of theoretically designed molecular structures which should display high hyperpolarizability values.

MESOIONIC 2-N-CYCLOALKYLAMINO-5-ALKYL-1,3-DITHIOLIUM-4-THIOLATES

Abstract Seven 2-(N-cycloalkylamino-1,3-dithiocarbamoyl)-carboxylic acids and seven mesoionic 2-N-cycloalkylamino-5-alkyl-1,3-dithiolium-4-thiolates have been conveniently synthesized. They were characterized by elemental analysis, I.R., U.V., mass and 1H NMR spectrometry, plus 13C NMR spectrometry in some cases. Important questions concerning the structure of the mesoionic compounds, relevant to mesoionic compounds in general, are addressed. In particular, we refer to the degree of separation of regions of positive and negative charge, bond orders, electron and charge delocalization and aromaticity. In this discussion we cite some of our X-ray diffraction and theoretical studies. We conclude that there are regions of positive and negative charge in which there is delocalization of electrons and charge with bond orders between 1 and 2. However, the shared regions are separated by what are essentially single bonds. Thus, they, and mesoionic compounds in general, should not be considered as formally aromati...

Molecular modeling of the octacoordinated tetracarbonato-Nd(III), [Nd(CO3)4]5−, complex and its nonacoordinated fluoro- and aquo-adducts

Abstract Theoretical investigation on the structures of the octacoordinated [Nd(CO3)4]5- and the nonacoordinated [Nd(CO3)4.OH2]5- complexes, using the SPARKLE parameters of the lanthanides within MOPAC, revealed that they possessed dodecahedral and square antiprismatic structures respectively with an average Nd-O distance of 0.249 nm. These structures and the Nd-O distances agreed well with those experimentally found in the crystal structures. Replacing the water molecule with a fluoride ion or a mondentatecarbonato ligand resulted in a nonacoordinated distorted square antiprismatic structures where the trans-carbonato groups were twisted. The corresponding decacoordinated structures with two fluoride ions or a bidentatecarbonato group, [Nd(CO3)4·F2]7- and [Nd(CO3)5]7-, were also investigated. In both cases considerable twisting of the transcarbonato groups was observed.

Insights into the spontaneity of hydrogen bond formation between formic acid and phthalimide derivatives.

We evaluated a group of phthalimide derivatives, which comprise a convenient test set for the study of the multiple factors involved in the energetics of hydrogen bond formation. Accordingly, we carried out quantum chemical calculations on the hydrogen bonded complexes formed between a sample of phthalimide derivatives with formic acid with the intent of identifying the most important electronic and structural factors related to how their strength and spontaneity vary across the series. The geometries of all species considered were fully optimized at DFT B3LYP/6-31++G(d,p), RM1, RM1-DH2, and RM1-D3H4 level, followed by frequency calculations to determine their Gibbs free energies of hydrogen bond formation using Gaussian 2009 and MOPAC 2012. Our results indicate that the phthalimide derivatives that form hydrogen bond complexes most favorably, have in their structures only one C=O group and at least one NH group. On the other hand, the phthalimide derivatives predicted to form hydrogen bonds least favorably, possess in their structures two carbonyl groups, C=O, and no NH group. The ability to donate electrons and simultaneously receive one acidic hydrogen is the most important property related to the spontaneity of hydrogen bond formation. We further chose two cyclic compounds, phthalimide and isoindolin-1-one, in which to study the main changes in molecular, structural and spectroscopic properties as related to the formation of hydrogen bonds. Thus, the greatest ability of the isoindolin-1-one compound in forming hydrogen bonds is evidenced by the larger effect on the structural, vibrational, and chemical shifts properties associated with the O–H group. In summary, the electron-donating ability of the hydrogen bond acceptor emerged as the most important property differentiating the spontaneity of hydrogen bond formation in this group of complexes.

Hydrogen-bonded complexes of α,β-unsaturated carboxylic esters with phenols: a molecular mechanics study

Abstract The hydrogen-bonded complexes between phenol derivatives and methyl acrylate, methyl trans -crotonate and methyl trans -cinnamate were studied using the interactive molecular modelling program PCMODEL which uses the features of Allingers MMX force field, including pi-valence electron self-consistent field (Pi-VESCF) calculations. The results successfully reproduce experimental Δ H values associated with the formation of the hydrogen-bonded complexes and account for the experimentally observed dependences of Δ H on the electron releasing/withdrawing abilities of the substituents in the phenol aromatic ring or in the ester. In addition, the conformational change in the ester moiety of the hydrogen-bonded complexes gives rise to energetically similar hydrogen bonds, in agreement with previous infrared spectroscopic results. In general, the results obtained confirm or expand the previous spectroscopic or quantum mechanical results of M. Dulce G. Faria (J. Mol. Struct., 263 (1991) 87; Vib. Spectrosc., 2 (1991) 43, 107), and lead to the possibility of successfully applying the MMX/Pi-VESCF method to the study of hydrogen-bonded complexes involving other biochemically important molecules.

Quantum molecular mechanics—a noniterative procedure for the fast Ab Initio calculation of closed shell systems

In this article, we advance the foundations of a strategy to develop a molecular mechanics method based not on classical mechanics and force fields but entirely on quantum mechanics and localized electron‐pair orbitals, which we call quantum molecular mechanics (QMM). Accordingly, we introduce a new manner of calculating Hartree–Fock ab initio wavefunctions of closed shell systems based on variationally preoptimized nonorthogonal electron pair orbitals constructed by linear combinations of basis functions centered on the atoms. QMM is noniterative and requires only one extremely fast inversion of a single sparse matrix to arrive to the one‐particle density matrix, to the electron density, and consequently, to the ab initio electrostatic potential around the molecular system, or cluster of molecules. Although QMM neglects the smaller polarization effects due to intermolecular interactions, it fully takes into consideration polarization effects due to the much stronger intramolecular geometry distortions. For the case of methane, we show that QMM was able to reproduce satisfactorily the energetics and polarization effects of all distortions of the molecule along the nine normal modes of vibration, well beyond the harmonic region. We present the first practical applications of the QMM method by examining, in detail, the cases of clusters of helium atoms, hydrogen molecules, methane molecules, as well as one molecule of HeH+ surrounded by several methane molecules. We finally advance and discuss the potentialities of an exact formula to compute the QMM total energy, in which only two center integrals are involved, provided that the fully optimized electron‐pair orbitals are known.

Comparison between the theoretical models and experimental structures of some octacoordinated Ln(III)-bis-dipyridyl-bis-dichloroacetato-diaquo complexes and their phenanthroline analogues

Abstract The title complexes were modeled using the semiempirical MOPAC and the newly developed SPARKLE paramerters of the lanthanides. The calculated bond distances and angles agreed well with those found from crystal structure measurements. This technique allows us to screen a large number of molecules and get structural information within a very short time.