Relaxed energetic maps of kappa-carrabiose: a DFT study

Nonlinear optical properties DFT calculations of polyacethylene and copolymers models substituted with aldimines chromophores as side chains

Aldimine derivatives chromophores grafted on polyacetylene oligomers were first designed to investigate the nonlinear optical (NLO) response of the resulting materials using CAMB3LYP method. The effects of different factors such as the chain length separating the substitutions as well as the configurations and the orientations of these latter, were examined and discussed. In a second part of this paper, NLO responses, in particular the static hyperpolarizabilities of polyvinyl oligomers substituted by aldimine chromophores via carboxylic groups and by pyrrolidone groups were calculated for different configurations. The stability and the hydrogen bonding in each oligomer were also discussed.

Theoretical investigation on the improper hydrogen bond in κ-carrabiose⋯Y (Y = HF, HCl, HBr, NH3, H2O, and H2S) complexes

The nature of H-bonds in κ-carrabiose⋯Y (Y = HF, HCl, HBr, NH₃, H₂O, and H₂S) complexes was studied. For this aim, the structure of isolated κ-carrabiose was optimized using three global hybrids functional: B3LYP, PBE0, and M06-2X combined with 6-311G** basis set. Subsequently, the κ-carrabiose in the presence of HF, HCl, HBr, NH₃, H₂O, and H₂S was optimized using the CBS-4 M method. NBO analyses were then carried out at the MP2/6-311G** level of theory. A particular interest was focused on C₍₁₈₎-H₍₃₄₎⋯Y bond. The results reveal that the C₍₁₈₎-H₍₃₄₎⋯Y bond is an improper H-bond since a significant contraction of C₍₁₈₎-H₍₃₄₎ was observed during the complexation leading to a significant blueshifted stretching frequency. The NBO analyses have shown that the formation of the improper H-bonds C₍₁₈₎-H₍₃₄₎⋯Y (Y = F, Cl, Br, N, O, and S) is principally due to the increase of the s-character of the hybrid orbital in carbon atom (rehybridization) in κ-carrabiose⋯Y complexes. Regarding the polarization, it was proved that more the H-bond center (carbon in C₍₁₈₎-H₍₃₄₎⋯Y) becomes less positive, the hydrogen more positive, and Y more negative; more the contraction of the C₍₁₈₎-H₍₃₄₎ bond is important. It was also confirmed for intramolecular H-bonds in κ-carrabiose⋯Y complexes that the rehybridization is responsible for H-bonds nature either proper or improper.

Testing the CP-correction procedure with different DFT methods on H-bonding complexes of κ-carrabiose with water molecules

Interaction of water molecules with κ-carrabiose disaccharide, within three H-bonding complexes, was investigated. Particular interest was focused on the way with which the BSSE correction has to be performed. Two strategies were used, either performing BSSE correction during or after optimization. For this aim, several DFT-functionals (hybrid GGA and hybrid meta-GGA) and 6-31 + G* basis set were considered. The results demonstrated the uselessness of including of BSSE-CP correction during optimization for all complexes. From a structural point of view, a proper H-bonding description was obtained using the PBE0 functional for all complexes. The basis set effect on the BSSE using B3LYP functional was also investigated. The reliability of B3LYP/6-31 + G** and B3LYP/6-31++G** models for the complexes involving one or two water molecules was reported while the use of B3LYP/6-311 + G** or B3LYP/6-311++G** levels was shown to be more appropriate for larger complexes equivalent to that involving three water molecules. CP-corrected interaction energies were demonstrated to be closer to CBS-4 M interaction energies than the uncorrected ones. Graphical abstract Functional and basis set effects on BSSE.

Density functional conformational study of 2-O-sulfated 3,6 anhydro-α-D-galactose and of neo-κ- and ι-carrabiose molecules in gas phase and water

We examined the conformational preferences of the 2-O-sulfated-3,6-α-D-anhydrogalactose (compound I) and two 1,3 linked disaccharides constituting-κ or ι-carrageenans using density functional and ab initio methods in gas phase and aqueous solution. Systematic modifications of two torsion angles leading to 324 and 144 starting geometries for the compound I and each disaccharide were used to generate adiabatic maps using B3LYP/6-31G(d). The lower energy conformers were then fully optimized using B3LYP, B3PW91 and MP2 with several basis sets. Overall, we discuss the impact of full relaxation on the energy and structure of the dominant conformations, present the performance comparison with previous molecular mechanics calculations if available, and determine whether our results are impacted, when polarization and diffuse functions are added to the 6-31G(d) basis set, or when the MP2 level of theory is used.