Spectroscopic properties of vitamin E models in solution

Understanding Nonlinear Optical Phenomena in N-Pyrimidinyl Stilbazolium Crystals via a Self-Consistent Electrostatic Embedding - DFT Approach

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been used to investigate the nonlinear optical (NLO) properties of phenolic N-pyrimidinyl stilbazolium cationic chromophore in its corresponding noncentrosymmetric crystals. Such a cationic chromophore, the OPR (4-(4-hydroxystyryl)-1-(pyrimidin-2-yl)pyridinium), consists of a strong electron donor, the 4-hydroxyphenyl group, and a strong electron acceptor, the N-pyrimidinylpyridinium group based on two electron-withdrawing groups. The in-crystal NLO properties were determined by applying a supermolecule approach in combination with an iterative electrostatic scheme, in which the surrounding molecules of a unit cell are represented by point charges. With CAM-B3LYP, our absolute estimates for the largest diagonal component of the second-order nonlinear susceptibility tensor of OPR-based crystals range from 64.00 to 80.34 pm/V in the static regime and from 162.09 to 175.52 pm/V at 1907 nm. These values are significant when compared to those of benchmark stilbazolium-based crystals. Furthermore, the third-order susceptibility, which is related to the nonlinear optical process of the intensity-dependent refractive index, is also significant compared to the results for other organic crystals, such as chalcone derivatives. With TD-CAM-B3LYP, the two-state model effectively explains the similarity in the first hyperpolarizability values in the crystalline phase. This similarity arises from the combination of the oscillator strength and the charge transfer of the crucial transition. Therefore, phenolic organic salt crystals show great promise for various nonlinear optical applications.

Theoretical investigation on the linear and nonlinear optical properties of DAPSH crystal

The linear polarizability, first and second hyperpolarizabilities of the asymmetric unit of DAPSH crystal are studied and compared with available experimental results. The polarization effects are included using an iterative polarization procedure, which ensures the convergence of the dipole moment of DAPSH embedded within a polarization field generated by the surrounding asymmetric units whose atomic sites are considered as point charges. We estimate macroscopic susceptibilities from the results of the polarized asymmetric units in the unit cell, considering the significant contribution of the electrostatic interactions in crystal packing. The results show that the influence of the polarization effects leads to a marked decrease of the first hyperpolarizability, compared with the respective isolated counterpart, which improves the concordance with the experiment. There is a minor influence of polarization effects on the second hyperpolarizability but our estimated result for the third-order susceptibility, related to the NLO process of the intensity dependent refractive index, is significant as compared with the results for other organic crystals, such as chalcone-derivatives. In addition, supermolecule calculations are conducted for explicit dimers in presence of the electrostatic embedding to illustrate the role played by the electrostatic interactions in the hyperpolarizabilities of the DAPSH crystal.

Anharmonicity and Spectra-Structure Correlations in MIR and NIR Spectra of Crystalline Menadione (Vitamin K3)

Mid-infrared (MIR) and near-infrared (NIR) spectra of crystalline menadione (vitamin K3) were measured and analyzed with aid of quantum chemical calculations. The calculations were carried out using the harmonic approach for the periodic model of crystal lattice and the anharmonic DVPT2 calculations applied for the single molecule model. The theoretical spectra accurately reconstructed the experimental ones permitting for reliable assignment of the MIR and NIR bands. For the first time, a detailed analysis of the NIR spectrum of a molecular system based on a naphthoquinone moiety was performed to elucidate the relationship between the chemical structure of menadione and the origin of the overtones and combination bands. In addition, the importance of these bands during interpretation of the MIR spectrum was demonstrated. The overtones and combination bands contribute to 46.4% of the total intensity of menadione in the range of 3600-2600 cm-1. Evidently, these bands play a key role in shaping of the C-H stretching region of MIR spectrum. We have shown also that the spectral regions without fundamentals may provide valuable structural information. For example, the theoretical calculations reliably reconstructed numerous overtones and combination bands in the 4000-3600 and 2800-1800 cm-1 ranges. These results, provide a comprehensive origin of the fundamentals, overtones and combination bands in the NIR and MIR spectra of menadione, and the relationship of these spectral features with the molecular structure.

Updating atomic charge parameters of aliphatic amino acids: a quest to improve the performance of molecular modeling via sequential molecular dynamics and DFT-GIAO-NMR calculations

In this work, we observe the behavior of the dipole moment, atomic charges, solute-solvent interactions and NMR spectroscopy of aliphatic amino acids in a water solution via the computational simulations of classical molecular dynamics and DFT quantum calculations. Our results indicate that the convergence of the atomic charge of the solute, from an iterative process, together with the dipole moment of the amino acid, alters the lifetime of hydrogen bonds present in the first solvation shell, resulting in the modification of its structure and dynamics. Using GIAO-DFT-NMR calculations, we assessed the impact of these structural solute-solvent modifications on the magnetic shielding constants of the solute carbon atoms. In this sense, we evaluate the importance of an update in parameters that describe atomic charges present in the CHARMM36 force field.

Molecular dynamic simulations, GIAO-NMR and TD-DFT spectroscopy analyze for zwitterionic isoleucine (ILE)N , 1 ≤ N ≤ 6, in water solution

In this article, we investigate the effects of the isoleucine (ILE)_N amino acid chain growth, N = 1.0.6, the ILE conformational effect as well as the solvent presence on the electrical and magnetic spectroscopic properties when these compounds are in aqueous solution. Computational molecular dynamics simulations were performed to include the solvent medium and generate uncorrelated configurations involving solute-solvent structures. The charge point model for solvent was used to obtain the results for quantum mechanical calculation, in special DFT calculations, for (ILE)_N structures. Our results for the magnetic shielding constant obtained via GIAO-DFT-NMR calculations show that there is evidence of a magnetic behavior that characterizes the number of peptide bonds and, therefore, how the N isoleucine polypeptide chain is composed. TD-DFT results also show an absorption band shift to larger wavelengths indicating a dependence on N growth.

15N NMR Shifts of Eumelanin Building Blocks in Water: A Combined Quantum Mechanics/Statistical Mechanics Approach

Theoretical results for the magnetic shielding of protonated and unprotonated nitrogens of eumelanin building blocks including monomers, dimers, and tetramers in gas phase and water are presented. The magnetic property in water was determined by carrying out Monte Carlo statistical mechanics sampling combined with quantum mechanics calculations based on the gauge-including atomic orbitals approach. The results show that the environment polarization can have a marked effect on nitrogen magnetic shieldings, especially for the unprotonated nitrogens. Large contrasts of the oligomerization effect on magnetic shielding show a clear distinction between eumelanin building blocks in solution, which could be detected in nuclear magnetic resonance experiments. Calculations for a π-stacked structure defined by the dimer of a tetrameric building block indicate that unprotonated N atoms are significantly deshielded upon π stacking, whereas protonated N atoms are slightly shielded. The results stress the interest of NMR experiments for a better understanding of the eumelanin complex structure.

Spectroscopic properties and solute–solvent structural analyses for ANR polypeptides in water solution: a sequential Monte Carlo/quantum mechanics (S-MC/QM) theoretical study

We present a theoretical study of the solvent influence on A_NR polypeptides, with N = 1, 2, 3, 4 and 5, in the gas phase and in water solution using the sequential MC/QM methodology combined with GIAO-DFT calculations.

Solvent effects on the electrical and magnetic spectroscopic properties of azo-enaminone derivatives in methanol and in water

We have investigated the solvent effects on the nonlinear and spectroscopic optical properties of some azo-enaminone molecules in the gas phase and in methanol and water solutions.