Ultraviolet light spectroscopic characterization of ibuprofen acid aggregation in deionized water

This work provides a description of the aggregation equilibria of ibuprofen acid in deionized water at temperatures between 20 and 40 °C in the 0.1-20.1 ppm concentration range. For this goal, we have made use of UV-Visible spectroscopy. A calculation algorithm was developed to obtain the aggregate orders and thermodynamic parameters from the experimental absorbance values. Monomeric ibuprofen acid was found to be absent in water solutions. In addition to the dimer, two aggregates formed by 32 and 128 monomeric units were found to co-exist in solution at the highest concentration tested. A critical micelle concentration of 7.8 ppm was estimated for this system. The appearance of the first aggregate occurs when the pH drops below the pKa value, which was determined to be 4.62. At higher ibuprofen concentrations, a sudden jump in the electrical conductivity coincides with the onset of formation of the second aggregate. A varied menu of alternatives is offered with respect to the calibration curve of ibuprofen in water, though the linear calibration of ibuprofen concentration with absorbance might be reasonably performed at 224 nm. Finally, the dissolution rate of the commercial ibuprofen used in this work was found to obey the Noyes-Whitney first order equation.

Theoretical study of cis-trans isomer of 2-hydroxy-5-methyl-2'-nitroazobenzene: DFT insight

CONTEXT

The synthesis of azobenzene materials is an important aspect of the research in the field of photo-switch materials. It is currently thought that azobenzene molecules exist in the cis and trans form of molecular structure configuration. However, the reaction process allowing for reversible energy switches from trans to cis form is still challenging. Therefore, it is crucial to understand the molecular properties of the azobenzene compounds in order to provide reference for future synthesis and application. Affirmation supporting this perspective has been substantially derived from theoretical results in the isomerization process and whether these molecular structures may affect the electronic properties entirely needs to be confirmed. In this study, I give my effort to understand the molecular structure properties of the cis and trans form of azobenzene molecule from 2-hydroxy-5-methyl-2'-nitroazobenzene (HMNA). Their chemistry phenomena are investigated using the density functional theory (DFT) method. This study shows that the trans-HMNA has a molecular size of 9.0 Å and the cis-HMNA has a molecular size of 6.6 Å. The trans-HMNA exhibits an electronic transition of π → π* type driven by an azo bond, whereas the cis-HMNA exhibits an electronic transition of n → π* type with respect to the non-bonding electrons of oxygen and nitrogen atoms. Therefore, the HMNA mechanism pathway from trans to cis form is feasible to undergo at the inversion pathway in the ground state.

METHODS

All DFT calculations were performed using the Gaussian Software Packages (Gaussian 09 Revision-A.02 and GaussView 5.0.8). Gaussum 3.0 software was selected to visualize the molecular orbital levels in the density of states diagram. The optimized molecular geometrical parameter was calculated using B3LYP/cc-pVTZ level in the gas phase. TD-DFT with M06-2X/cc-pVTZ level was used as a method for the precise interpretation of excited states in molecular systems.

On the magnetic circular dichroism of benzene. A density-functional study

Spectroscopy of magnetic circular dichroism (MCD) provides enhanced information on molecular structure and a more reliable assignment of spectral bands than absorption alone. Theoretical modeling can significantly enhance the information obtained from experimental spectra. In the present study, the time dependent density functional theory is employed to model the lowest-energy benzene transitions, in particular to investigate the role of the Rydberg states and vibrational interference in spectral intensities. The effect of solvent is explored on model benzene-methane clusters. For the lowest-energy excitation, the vibrational sub-structure of absorption and MCD spectra is modeled within the harmonic approximation, providing a very good agreement with the experiment. The simulations demonstrate that the Rydberg states have a much stronger effect on the MCD intensities than on the absorption, and a very diffuse basis set must be used to obtain reliable results. The modeling also indicates that the Rydberg-like states and associated transitions may persist in solutions. Continuum-like solvent models are thus not suitable for their modeling; solvent-solute clusters appear to be more appropriate, providing they are large enough.

On the structure of prilocaine in aqueous and amphiphilic solutions

The solvation of prilocaine has been investigated in pure water and in amphiphilic solutions using a combination of neutron diffraction and simulations.

Non-covalent interaction of benzene with methanol and diethyl ether solid surfaces

We have investigated the interactions involved at the interface of binary, layered ices (benzene on methanol and on diethyl ether) by means of laboratory experiments and ab initio calculations on model clusters.

Efficient electron-promoted desorption of benzene from water ice surfaces

We study the desorption of benzene from solid water surfaces during irradiation of ultrathin solid films with low energy electrons.

Ground and excited states of naphthalene–water (naphtha–W6) clusters: a computational study

An MP2 and DFT study of the structures of naphthalene–water hexamer clusters has been performed for both the prism and cage forms of the cluster.