Interference of H-bonding and substituent effects in nitro- and hydroxy-substituted salicylaldehydes

Two intramolecular interactions, i.e., (1) hydrogen bond and (2) substituent effect, were analyzed and compared. For this purpose, the geometry of 4- and 5-X-substituted salicylaldehyde derivatives (X = NO2, H or OH) was optimized by means of B3LYP/6-311 + G(d,p) and MP2/aug-cc-pVDZ methods. The results obtained allowed us to show that substituents (NO2 or OH) in the para or meta position with respect to either OH or CHO in H-bonded systems interact more strongly than in the case of di-substituted species: 4- and 3-nitrophenol or 4- and 3-hydroxybenzaldehyde by ∼31%. The substituent effect due to the intramolecular charge transfer from the para-counter substituent (NO2) to the proton-donating group (OH) is ∼35% greater than for the interaction of para-OH with the proton-accepting group (CHO). The total energy of H-bonding for salicylaldehyde, and its derivatives, is composed of two contributions: ∼80% from the energy of H-bond formation and ∼20% from the energy associated with reorganization of the electron structure of the systems in question

Application of the LFER in Organic Electrochemistry: Interpretation of the Hammett Reaction Constant (e) for Processes of Reversible Electroreduction in Aprotic Solvents

The paper is an overview of the application of the concept of linear free energy relationships (LFER), most notably through the Hammett equation, to investigations of reversible organic electrochemical reactions in aprotic solvents. In sequence the following topics are covered: the effect of solvent properties on the equilibrium, ion pairing effects, the direct application of Hammett\u27s equation on polarographic data, entropic and enthalpic contributions to substituent effects, and the quantum chemical interpretatation of the reaction rate constant in aprotic media. The application of polarographic techniques is advocated as more sensitive and easier to interpret than the techniques of homogeneous kinetics

On relation between substituent effect and aromaticity in monocyclic systems

Aromaticity/aromatic and substituent/substituent effects belong to the most commonly used terms in organic chemistry and related fields. They are used for more than a century, and so far are the subject of thousands publications a year. The quantitative description of the aromaticity of planar π-electron cyclic molecules is based on four criteria: (i) they are more stable than their acyclic unsaturated analogues, (ii) bonds have intermediate lengths between those for the single and double ones, (iii) external magnetic field induces π-electron ring current, and (iv) aromatic systems prefer reactions in which the π-electron structure is preserved. conserved. Quantitative characteristics based on these criteria, named as aromaticity indices, allow to relate aromaticity to the substituent effect. This latter can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used. In the first case, a descriptor named SESE (substituent effect stabilization energy) is obtained, while in the second case – cSAR (charge of the substituent active region), which is the sum of the charge of the ipso carbon atom and the charge of the substituent. The application of these substituent effect descriptors to a set of π-electron systems, such as: benzene, quinones, cyclopenta- and cyclohepta-dienes, as well as some azoles, allowed to draw the following conclusions: (i) The less aromatic the system, the stronger the substituent influences the π-electron system. Highly aromatic systems are resistant to the substituent effect, in line with the organic chemistry experience that aromatic compounds dislike reactions leading to changes in the π-electron structure of the ring. (ii) Intramolecular charge transfer (resonance effect) is privileged in cases where the number of bonds between the electron-attracting and electron-donating atoms is even. These effects are much weaker when this number is odd. Classically, it may be related to traditional para vs meta substituent effects in benzene derivatives. We should note that in electron-accepting groups, such as CN or NO2 (and others), electron-accepting atoms are second counting from Cipso. (iii) In all cases, when the substituent changes number of π-electrons in the ring in the direction of 4N+2, its aromaticity increases, for example electron-donating substituents in exocyclic substituted pentafulvene, or a halogen atom in complexes with heptafulvene

Effect of Alkali Metal Cations on Length and Strength of Hydrogen Bonds in DNA Base Pairs

For many years, non-covalently bonded complexes of nucleobases have attracted considerable interest. However, there is a lack of information about the nature of hydrogen bonding between nucleobases when the bonding is affected by metal coordination to one of the nucleobases, and how the individual hydrogen bonds and aromaticity of nucleobases respond to the presence of the metal cation. Here we report a DFT computational study of nucleobase pairs interacting with alkali metal cations. The metal cations contribute to the stabilization of the base pairs to varying degrees depending on their position. The energy decomposition analysis revealed that the nature of bonding between nucleobases does not change much upon metal coordination. The effect of the cations on individual hydrogen bonds were described by changes in VDD charges on frontier atoms, H-bond length, bond energy from NBO analysis, and the delocalization index from QTAIM calculations. The aromaticity changes were determined by a HOMA index

Aromaticity Induced by Electric Field: The Case of Polycalicenes

Local and global π-electron delocalization occurring in planar poly-1,7-[<i>N</i>]­calicenes is investigated with use of 10 aromaticity measures based on different physical properties. Systematic change of aromatic character is observed along chains of connected calicene units. Multidimensionality of the aromaticity phenomenon is studied with use of principal component analysis (PCA). The structural characteristics are compared with the properties of the isolated calicene molecule exposed to external electric fields of various intensities. Interrelations between the value of electric field applied and physical properties of the calicene molecule are discussed in the context of calicene unit affected by its surroundings in polycalicene chains. The patterns of global π-electron delocalization are described in graph theory terminology, and interconnections between local and global aromaticity in these systems are established