RESUMO

We present time-dependent density functional theory (TDDFT) calculations of the electronic optical rotation (ORP) for seven oxirane and two aziridine derivatives in the gas phase and in solution and compare the results with the available experimental values. For seven of the studied molecules it is the first time that their optical rotation was studied theoretically and we have therefore investigated the influence of several settings in the TDDFT calculations on the results. This includes the choice of the one-electron basis set, the exchange-correlation functional or the particular polarizable continuum model (PCM). We can confirm that polarized quadruple zeta basis sets augmented with diffuse functions are necessary for converged results and find that the aug-pc-3 basis set is a viable alternative to the frequently employed aug-cc-pVQZ basis set. Based on our study, we cannot recommend the generalized gradient functional KT3 for calculations of the ORP in these compounds, whereas the hybrid functional PBE0 gives results quite similar to the long-range correct CAM-B3LYP functional. Finally, we observe large differences in the solvent effects predicted by the integral equation formalism of PCM and the SMD variant of PCM. For the majority of solute/solvent combinations in this study, we find that the SMD model in combination with the PBE0 functional and the aug-pc-3 basis set gives the best agreement with the experimental values.

RESUMO

Stable dimers aspartate-aspartate have been studied in aqueous and gas phase through theoretical simulations. The polarizable continuum model (PCM) has been applied to simulate the effect of the hydration on monomers and complexes. The quantum theory of atoms in molecules (QTAIM) and the interacting quantum atoms (IQA) scheme has been used to inquire into if, in the aqueous phase, individual hydrogen bonds have attractive electrostatic components. In all cases a spontaneous formation of the complexes in the aqueous phase are observed, while in the gas phase a considerable energy barrier must be overcome (between 100.8 to 263.2 kJ mol-1 ). The intermolecular distance at which this barrier is indicates when the hydrogen-bond interactions begin to take importance between the dimers and the corresponding molecular recognition among them. The IQA analysis shows that in aqueous phase, the hydrogen bonds N-H⋅⋅⋅O are mainly electrostatic in nature with a certain covalent character which increases linearly with the decrease of internuclear distances H⋅⋅⋅O. The H⋅⋅⋅H interactions observed are stabilizing and they are mainly quantum in nature.

Assuntos

RESUMO

High-level quantum chemical calculations are performed to investigate C=Se⋅⋅⋅Se=C interactions. Bounded structures are found with binding energies between -4 and -7 kJ mol-1 . An energy decomposition analysis shows that dispersion is the more attractive term, and in all cases save one, the electrostatic interaction is attractive despite each selenium atom having a positive σ-hole at the extension of the C=Se bond. The topological analysis of the molecular electrostatic potential and L(r)=-∇2 ρ(r) function, and natural bond orbital analysis reveal that these particular Se⋅⋅⋅Se contacts can be considered to be quadruple Lewis acid-base interactions.

RESUMO

The experimental (1) H nuclear magnetic resonance (NMR) spectrum of 1H-pyrazole was recorded in thermotropic nematic liquid crystal N-(p-ethoxybenzylidene)-p-butylaniline (EBBA) within the temperature range of 299-308 K. Two of three observable dipolar DHH -couplings appeared to be equal at each temperature because of fast prototropic tautomerism. Analysis of the Saupe orientational order parameters using fixed geometry determined by computations and experimental dipolar couplings results in a situation in which the molecular orientation relative to the magnetic field (and the liquid crystal director) can be described exceptionally by a single parameter. Copyright © 2016 John Wiley & Sons, Ltd.

RESUMO

The nature of F-BrX-R interactions (with X = F, Cl, Br, I and R = -H, -F) has been investigated through theoretical calculation of molecular potential electrostatic (MEP), molecular polarizability, atoms in molecules (AIM) analysis and energetic decomposition analysis (EDA). A detailed analysis of the MEPs reveals that considering only the static electrostatic interactions is not sufficient to explain the nature of these interactions. The molecular polarizabilities of X-R molecules suggest that the deformation capacity of the electronic cloud of the lone pairs of the X atom plays an important role in the stability of these complexes. The topological analysis of the L(r) = -»∇(2)ρ(r) function and the detailed analysis of the atomic quadrupole moments reveal that the BrX interactions are electrostatic in nature. The electron acceptor Br atom causes a polarization of the electronic cloud (electronic induction) on the valence shell of the X atom. Finally, the electrostatic forces and charge transfer play an important role not only in the stabilization of the complex, but also in the determination of the molecular geometry of equilibrium. The dispersive and polarization forces do not influence the equilibrium molecular geometry.

RESUMO

In this paper a theoretical study has been carried out to investigate the nature of the unusual halogen-halogen contacts in the complexes R-X···X-R (with R = -H, -Cl, -F and X = Cl, Br, I). AIM, NBO, and MEP analyses have been used to characterize X···X interactions. Formation of the unusual X···X interactions leads to a significant increase of electron charge density in the bonding region between the two halogen atoms. The geometry and stability of these complexes is mainly due to electrostatic interactions lump(X1) → hole(X2) and lump(X2) → hole(X1) [or equivalently [VS,min(X1) → VS,max(X2) and VS,min(X2) → VS,max(X1)] and the charge transfers LP(X1) → σ*(R-X2) and LP(X2) → σ*(R-X1). In other words, these findings suggest that the electrostatic interactions and the charge transfer play a substantial role in determining the optimal geometry of these complexes, as in conventional halogen bonds, even though the dispersion term is the most important attractive term for all the complexes studied here, save one.

Assuntos

RESUMO

A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug'-cc-pVDZ level of theory. Two types of A-X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C-X distances and small binding energies (8-54 kJ·mol(-1)). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C-X distances and large binding energies (148-200 kJ·mol(-1)), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A(-) and (+)[X-CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C-X bond is also identified as a significant stabilizing interaction in type-II complexes.

Assuntos

RESUMO

We applied a methodology capable of resolving the optical rotatory power into atomic contributions. The individual atomic contributions to the optical rotatory power and molecular chirality of the methylhydroperoxide are obtained via a canonical transformation of the Hamiltonian by which the electric dipolar moment operator is transformed to the acceleration gauge formalism and the magnetic dipolar moment operator to the torque formalism. The gross atomic isotropic contributions have been evaluated for the carbon, the nonequivalent oxygen, and the nonequivalent hydrogen atoms of methylhydroperoxide, employing a very large Gaussian basis set which is close to the Hartree-Fock limit.

Assuntos

RESUMO

A theoretical study of linear and cyclic clusters of (HCN)n and (HNC)n (up to n = 10) has been carried out by means of DFT and MP2 ab initio methods. The transition states linking the cyclic clusters show high energetic barriers that prevent the spontaneous transformation of the high-energy clusters, (HNC)n, into the low-energy ones, (HCN)n. The effect of the protonation/deprotonation of the linear clusters has also been explored. The results show that (HNC)n clusters with n values larger than six are thermodynamically more stable as charged systems than as neutral ones. The geometrical results have been analyzed using a Steiner-Limbach plot. The electron density and its Laplacian at the bond critical points correlate with the corresponding bond distances by means of two exponential functions, one for the open shell and another for the closed shell cases.

Assuntos

RESUMO

The cooperativity effects on both the electronic energy and NMR indirect nuclear spin-spin coupling constants J of the linear complexes (HCN)n and (HNC)n (n = 1-6) are discussed. The geometries of the complexes were optimized at the MP2 level by using the cc-pVTZ basis sets. The spin-spin coupling constants were calculated at the level of the second-order polarization propagator approximation with use of the local dense basis set scheme based on the cc-pVTZ-J basis sets. We find strong correlations in the patterns of different properties such as interaction energy, hydrogen bond distances, and spin-spin coupling constants for both series of compounds. The intramolecular spin-spin couplings are with two exceptions dominated by the Fermi contact (FC) mechanism, while the FC term is the only nonvanishing contribution for the intermolecular couplings. The latter do not follow the Dirac vector model and are important only between nearest neighbors.

Assuntos