Barrier For Molecules Research Articles

A Review: Microneedle Drug Delivery

The various common techniques for transdermal medication delivery include hypodermic needles, topical lotions, and transdermal patches. Because the stratum corneum layer of the skin acts as a barrier for molecules, the action of most therapeutic medicines is restricted, and only a few molecules are able to penetrate and reach the location of action. A novel type of delivery technology known as micro needles aids in improving medication distribution through this channel and addressing the many issues associated with traditional formulations. Because of the problems associated with oral medication delivery methods, transdermal drug administration utilising micro needles is gaining popularity. The possibilities and uses of micro needles are discussed in this review. Micro needles of many sorts can be manufactured, including solid, dissolving, hydrogel, coated, and hollow micro needles. The fabrication process chosen is determined on the kind and material of the micro needle. This technique is now being used in a variety of sectors, including oligonucleotide distribution, vaccine administration, insulin delivery, and even cosmetics.

Turn: Weak Interactions and Rotational Barriers in Molecules—Insights from Substituted Butynes

The nature of the bonding and a definite preference for an eclipsed geometry in several substituted but-2-ynes, including certain novel derivatives are uncovered and examined. In particular, we consider the molecular species R3C-C≡C-CR3 (where R= H, F, Cl, Br, I, and CN), their R3C-B≡N-CR3 analogues, and a few novel exo-bridge systems with intramolecular hydrogen bonds running parallel to the C-C≡C-C chain. In some cases, the potential energy surfaces are remarkably flat-so flat, in fact, that free rotation is predicted for those molecules at very low temperatures. A systematic investigation of the bonding in the halogenated butynes demonstrates that the eclipsed conformation actually becomes more stable relative to the staggered form as R becomes larger and less electron-withdrawing. The rotational barriers (the differences in energy between the eclipsed and staggered geometries) are magnified significantly, however, in a special case where selected R groups at the ends of the R3C-C≡C-CR'3 molecule form hydrogen bonds parallel to the C-C≡C-C core. In those systems, the hydrogen bonds serve as a weak locking mechanism that favors the eclipsed conformation. A comparison of HF and uncorrected DFT methods versus the MP2(full), CCSD(T), and other dispersion-corrected methods confirms that correlation accounts to a significant extent for barriers in substituted butyne compounds. In the hydrogen-bonded systems, the barriers are comparable to and larger in some cases than the barriers observed for the more extensively studied ethane molecule.

Ab initio and DFT study to understand the physics behind the conformational barriers of isobutyl cyanide molecule

Origin of the conformational barrier of isobutyl cyanide (IBCN) molecule has been investigated. Ab initio and density functional theory calculations predict the existence of two minima on the potential energy surface corresponding to trans-gauche (TG) and gauche-gauche (GG) rotameric forms and one enantiomeric gauche-trans (GT) form of the IBCN molecule. The barrier heights between TG and GG, TG and GT are estimated as 5.7 and 3.78 kcal/mol respectively. Origin of conformational barrier of molecule has been studied by partial relaxations and with the aid of nuclear virial and natural bond orbital analysis technique. The relaxations of C1–C2 bond and C1–C2–C3 and C1–C2–C4 bond angles are estimated to play a pivotal role for conformational barrier of the molecule.

Cytokines and adhesion molecules in mesothelial cell pathophysiology

The mesothelium is a simple squamous epithelium of mesodermic origin lining peritoneal, pleural and pericardial cavities. The mesothelial lining provides a selective barrier for molecules and cells trafficking between the blood and serous cavities. Serous cavities can be involved by several pathological conditions such as infectious, immune and neoplastic processes that are frequently associated with the formation of effusions and leukocyte recruitment. Mesothelial cells are both a source and a target of proinflammatory mediators, chemokines and growth factors. They actively participate in the regulation of immunity, inflammation, and healing, by expressing in a regulated way cytokines and adhesion molecules.

Molecular adsorption on unrelaxed and relaxed ionic steps. Application to Ar, CO, CO2, and NH3 adsorbed on MgO(001)

The adsorption energy and the equilibrium valley of various molecules adsorbed on a step of MgO(001) substrate are determined on the basis of interatomic potentials. The step acts as a trapping site for all molecules adsorbed on the lower terrace and as a barrier for molecules on the upper terrace. The height of the potential barrier increases or decreases when compared to the diffusion barrier on a perfect surface, depending on the electric nature of the molecule. While nonpolar admolecules experience a potential barrier at the upper step edge, this is a characteristic of the strong ionic substrate surface field to drastically reduce the potential barrier for polar molecules. The relaxation and the rumpling of the step ions change the adsorption energy by about 10%, resulting in an increase for a rare-gas adatom and in a decrease for all polar species, but they have little influence on the molecular orientation.

ChemInform Abstract: THEORETICAL ANALYSIS OF EQUILIBRIUM GEOMETRIES AND BARRIERS OF ROTATION IN MOLECULES H2X2, WITH X = O, S, SE, AND TE

A model analysis is presented of equilibrium geometries and rotational barriers in molecules of composition H2X2, with X=O, S, Se, and Te, applying exchange perturbation theory. The basis of the model is an assumed analogy with the phenomenon of ‘‘superexchange’’ (indirect exchange) in solids with paramagnetic cations. A diamagnetic center in the superexchange model is formed by two spin‐paired electrons in an ‘‘effective’’ 1s Slater orbital determined from the valence‐shell diamagnetic susceptibility of the X atom. The model is conceptually related to the so‐called ‘‘through‐space’’ (i.e., direct) and ‘‘through‐bond’’ (i.e., indirect) interactions, of recent interest in organic chemistry. A survey of the literature is given concerning alternative interpretations of equilibrium geometries and barriers (cis and trans), in particular with respect to the possible influence of lone‐pair orbitals. The results of the model are found to be comparable to those of large‐scale ab initio calculations.

Theoretical analysis of equilibrium geometries and barriers of rotation in molecules H2X2, with X=O, S, Se, and Te

A model analysis is presented of equilibrium geometries and rotational barriers in molecules of composition H2X2, with X=O, S, Se, and Te, applying exchange perturbation theory. The basis of the model is an assumed analogy with the phenomenon of ‘‘superexchange’’ (indirect exchange) in solids with paramagnetic cations. A diamagnetic center in the superexchange model is formed by two spin-paired electrons in an ‘‘effective’’ 1s Slater orbital determined from the valence-shell diamagnetic susceptibility of the X atom. The model is conceptually related to the so-called ‘‘through-space’’ (i.e., direct) and ‘‘through-bond’’ (i.e., indirect) interactions, of recent interest in organic chemistry. A survey of the literature is given concerning alternative interpretations of equilibrium geometries and barriers (cis and trans), in particular with respect to the possible influence of lone-pair orbitals. The results of the model are found to be comparable to those of large-scale ab initio calculations.

Improved bond-orbital calculations of rotation barriers in molecules containing lone pairs of electrons

Improved bond-orbital calculations at the ab initio level using a STO–3G basis and assuming rigid rotation and experimental geometries have been performed for the eclipsed and staggered forms of CH3NH2 and CH3OH, for 11 conformations of N2H4 and 12 of NH2 OH. The one-configuration wavefunction of strictly localized non-orthogonal bond-orbitals is improved in second-order perturbation theory by admitting CI with non-orthogonal configurations arising from single excitations to antibonding orbitals. The results obtained using bond orbitals variationally optimized with respect to hybridization and polarity parameters show that in the cases of lower local symmetry (N2H4 and NH2OH) second-order delocalization is essential for a correct description of the dependence of molecular energy on the dihedral angle. All conformational parameters obtained in second order are in good agreement with the corresponding MO–SCF quantities in the same basis.

Analysis of rotational barriers in molecules on the basis of exchange perturbation theory

Exchange perturbation theory of the Rayleigh-Schrödinger type in first and second orders is applied to a calculation of barriers of internal rotation about single bonds in molecules. The barrier is assumed to arise as a result of exchange interactions between nonbonded atoms; the electrons of these atoms are replaced by one effective electron per atom, with a Gaussian charge distribution. Spins of effective electrons on different atoms are taken to be parallel. The method is applied to a large variety of molecules; calculated results for the barriers are in substantial agreement with experiment. A decomposition of the interaction energy for different conformers and for the barrier itself is given in terms of two- and many-atom exchange interactions.

The Barrier to Internal Rotation in Ethane

The internal rotation barrier in ethane appears susceptible to "explanation" at a qualitative, intuitively useful, reasonably correct level. At this level, localized and delocalized MO's each produce a description. For a rigid rotation, the delocalized description is basically the "orbital control" type familiar from Woodward-Hoffmann rules (30), Walsh's rules (31), and the formation of diatomic molecules (32). This description indicates that long-range eclipsed H---H antibonding is responsible for the barrier. The equivalent localized-bond description invokes destructive interference or repulsion between eclipsed C-H bonds. The extension of this kind of description to other molecules requires care. Subsequent to rigid rotation, the relaxation of ethane into its optimum eclipsed geometry produces almost no energy change but produces an unknown degree of energy "redistribution." I have emphasized that the theoretical distinction between "orbital control" and "steric interaction" is not precise and that a clarification in terminology may be desirable in this connection. The discussion of barriers given here applies only to a limited class of molecules, exemplified by ethane. No doubt, additional factors enter into a proper description of barriers in molecules having lower symmetry or more polar bonds, or both (33). Finally, I should point out that approaches other than those in categories 1 to 3 have been made in efforts to rationalize or predict barriers to internal rotation (34). The Hellmann-Feynman theorem has been applied to find the torque on ethane at conformations between staggered and eclipsed (35). The integral Hellmann-Feynman theorem has served as a basis for discussing barriers in terms of transition densities (36) and has led to an electrostatic model for barriers which has given some remarkably successful barrier predictions (37). These approaches are mathematically valid, and it is perfectly legitimate to try to extract physical explanations from them also. Thus, several valid explanations for the barrier to internal rotation in ethane are possible. However, I feel that the explanations proposed above in terms of delocalized or localized orbitals are preferable at present inasmuch as they are couched in terms and concepts currently in the mainstream of chemical thinking.