Chalcogen bonds tuned by an N–H···π or C–H···π interaction: investigation of substituent,cooperativity and solvent effects

In the present work, ab-initio calculations are performed to investigate cooperativity effects between chalcogen bond and H···π interactions in XHY···NCH···C₆H₆ and XHY···CNH···C₆H₆ complexes, where X = F, Cl, Br, CN, NC, and Y = S, Se. The nature of these interactions and the mechanism of cooperativity are studied by means of quantum theory of atoms in molecules, noncovalent interaction index, many-body analysis of interaction energy and electron density shift analysis. For each ternary complex, the shortening of the Y···N(C) distance is more pronounced than that of the H···π. The cooperative energies of these complexes are all negative which demonstrate a positive cooperativity between the Y···N(C) and H···π interactions. The many-body analysis of interaction energy reveals that the two-body energy term has the largest contribution to the total interaction energies of ternary complexes. A good linear correlation is established between the three-body energy and cooperative energy values in the ternary systems. The cooperative energies of XHY···CNH···C₆H₆ complexes indicate a larger sensitivity on the polarity of solvent than XHY···NCH···C₆H₆ ones.     

Quantum computational,spectroscopic and molecular docking studies on 6-amino-3-bromo-2-methylpyridine

In order to confirm the vibrational assignments, the density functional hypothesis has been used for 6-amino-3-bromo-2-methylpyridine (6A3B2MP). The entire energy distribution is used to materialize the several vibrational modes of 6A3B2MP (TED). The DFT/B3LYP approach is used to examine the molecular optimum limits and electronic characteristics of 6A3B2MP. Investigations have been made into the molecular orbital, Fukui function analysis, natural bond orbital (NBO), and molecular electrostatic potential (MEP) characteristics of 6A3B2MP. Theoretical studies have focused on the UV–vis spectra with different solvents. According to the outcomes of the molecular docking technique, the 6A3B2MP is docked with two target proteins, which is crucial for the emergence of cancer. Therefore, this research paves the door for the development of medicines that are specially formulated.     

Metal-enhanced Singlet Oxygen Generation: A Consequence of Plasmon Enhanced Triplet Yields

In this Rapid Communication, we report the first observation of Metal-Enhanced singlet oxygen generation (ME¹O₂). Rose Bengal in close proximity to Silver Island Films (SiFs) can generate more singlet oxygen, a three-fold increase observed, as compared to an identical glass control sample but containing no silver. The enhanced absorption of the photo-sensitizer, due to coupling to silver surface plasmons, facilitates enhanced singlet oxygen generation. The singlet oxygen yield can potentially be adjusted by modifying the choice of MEF (Metal-Enhanced Fluorescence) & MEP (Metal Enhance Phosphorescence) parameters, such as distance dependence for plasmon coupling and wavelength emission of the coupling fluorophore. This is a most helpful observation in understanding the interactions between plasmons and lumophores, and this approach may well be of significance for singlet oxygen based clinical therapy.     

Entropy production and thermal conductivity of a dilute gas

It is known that the thermal conductivity of a dilute gas can be derived by using kinetic theory. We present here a new derivation by starting with two known entropy production principles: the steepest entropy ascent (SEA) principle and the maximum entropy production (MEP) principle. A remarkable feature of the new derivation is that it does not require the specification of the existence of the temperature gradient. The known result is reproduced in a similar form.     

Carbene–aerogen bonds: an ab initio study

Through the use of ab initio calculations, the possibility of formation of σ-hole interaction between ZO₃ (Z = Ar, Kr and Xe) and carbene species is investigated. Since singlet carbenes show a negative electrostatic potential on their divalent carbon atom, they can favourably interact with the positive electrostatic potential generated by the σ-hole of Z atom of ZO₃. The characteristic of this interaction, termed as ‘carbene–aerogen’ bond, is analysed in terms of geometric, interaction energies and electronic features. The energy decomposition analysis indicates that for all complexes analysed here, the electrostatic energy is more negative than the polarisation or dispersion energy term. According to the electron density analysis, some partial covalent character can be ascribed to Xe???C interactions. In addition, the carbene–aerogen bond exhibits cooperative effects with the H???O hydrogen-bonding interaction in ternary complexes where both interactions coexist. For a given carbene, the amount of these cooperative effects increases with the size of the Z atom. The results obtained in this work may be helpful for the extension and future application of σ-hole intermolecular interactions as well as coordination chemistry.     

A Computational Study of Chair–Twist Energy Differences and the Chair–Chair Conformational Interconversion in Tetrahydro-2H-thiopyran (Thiacyclohexane,Thiane)

Ab initio molecular orbital theory with the LANL2DZ, 3-21G, 6-31G(d), 6-31+G(d), 6-31+G(d,p), 6-311+G(d,p),6-31G(2d), 6-31G(3d), and 6-311G(d,p) basis sets and density functional theory (B3P86, B3LYP, B3PW91) have been used to calculate the structures, relative energies, enthalpies, entropies, and free energies of the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran (tetrahydrothiopyran, thiacyclohexane, thiane, pentamethylene sulfide). All levels of theory calculated similar energy values and the effect of basis sets on the calculated energies was small. The chair conformer of tetrahydro-2H-thiopyran was 5.27 kcal/mol more stable than the 1,4-twist conformer, which was slightly more stable (0.81 kcal/mol) than the 2,5-twist conformer. The chair–1,4-twist and chair–2,5-twist free energy differences ( G°_{c – t}) were 5.44 and 5.71 kcal/mol, respectively. Intrinsic reaction coordinate [IRC, minimum-energy path (MEP)] calculations connected the transition state between the chair and the 2,5-twist conformers. This transition state is 9.73 kcal/mol higher in energy than the chair conformer and the energy differences between the chair and the 1,4-boat and 2,5-boat transition states were 8.07 and 6.38 kcal/mol, respectively. Stereoelectronic hyperconjugative interactions were observed in the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran. The stereoelectronic hyperconjugative effects in the chair conformer of tetrahydro-2H-thiopyran have been compared to those in the respective chair conformers of tetrahydro-2H-pyran, tetrahydro-2H-selenane, and tetrahydro-2H-tellurane.     

A new scaling procedure to correct semiempirical MEP and MEP-derived properties

Summary The MNDO, AM1, PM3, and ab initio 6–31G^* and 6–31+G^* MEPs for 21 neutral and 12 charged molecules were computed in layers ranging from 1.2 to 2.0 times the van der Waals radii of atoms. Semiempirical and ab initio MEPs for each layer and two groups of layers were compared to gain insight into the relationships between semiempirical and ab initio MEPs. A detailed statistical study allowed us to obtain a new set of scaling coefficients able to correct the semiempirical MEPs to provide better representations of the ab initio values. The corrected semiempirical MEPs were used to obtain electrostatic charges, whose quality was tested by the comparison between semiempirical Coulombic MEPs and ab initio quantum mechanical MEPs.     

A theoretical study of potential energy surface and some chemo-physical descriptors of Aspirin,Coupling the rotation of the ester and carboxyl groups

In this paper, the effect of the simultaneous rotation of the carboxyl (COOH) and ester (R'COOR) functional groups on the potential energy surface (PES) of aspirin is studied. Relative energies are reported at the HF/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels of theory. To understand the activity and selected electrophilic attack sites, electric dipole moments, atomic charges, HOMO-LUMO energy gap, natural bond orbital (NBO), and molecular electrostatic potential (MEP) analyses, as well as the main structural parameters of the identified conformers, are studied at the same theoretical level. Finally, an NBO analysis is used to demonstrate charge transfer between lone pairs and localized bonds.     

Assessing the MALDI-TOF MS sample preparation procedure to analyze the influence of thermo-oxidative ageing and thermo-mechanical degradation on poly (Lactide)

Multiple processing by means of successive injection cycles was used to simulate the thermo-mechanical degradation effects on the oligomeric distribution of PLA under mechanical recycling. Likewise, an accelerated thermal ageing over PLA glass transition was performed in order to simulate its service life. MALDI-TOF MS was used for the analysis and the sample preparation procedure was assessed by means of a statistical Design of Experiments (DoE). The quality effects in use for the analysis were signal-to-noise ratio and Resolution. Different matrixes, analyte/matrix proportions and the use of NaTFA as cationization agent were considered. A deep inspection of the statistical results provided a better understanding of the influence of the different factors, individually or in combination, to the signal. The application of DoE for the improvement of the MALDI measurement of PLA stated that the best combination of factors (levels) was the following: matrix (s-DHB), proportion analyte/matrix (1/5 V/V), and no use of cationization agent. Degradation primarily affected the initially predominant cyclic [LA_C]_n and linear H[LA_L]_nOH species, where LA stands for a PLA repeating unit. Intramolecular and intermolecular transesterifications as well as hydrolytic and homolytic reactions took place during the formation and disappearance of oligomeric species. In both degradation mechanisms induced by thermal ageing and thermo-mechanical degradation, the formation of H[LA_L]_nOCH₃ by intermolecular transesterifications was highlighted.     

α,α-Difluorophosphonohydroxamic Acid Derivatives among the Best Antibacterial Fosmidomycin Analogues

Three α,α-difluorophosphonate derivatives of fosmidomycin were synthesized from diethyl 1,1-difluorobut-3-enylphosphonate and were evaluated on Escherichia coli. Two of them are among the best 1-deoxy-d-xylulose 5-phosphate reductoisomerase inhibitors, with IC₅₀ in the nM range, much better than fosmidomycin, the reference compound. They also showed an enhanced antimicrobial activity against E. coli on Petri dishes in comparison with the corresponding phosphates and the non-fluorinated phosphonate.