ωB97XD/6-311++G(d,p) calculations were carried out to investigate the hydrogen-bonding interactions between adrenaline (Ad) and water. Six Ad-H(2)O complexes possessing various types of hydrogen bonds (H-bonds) were characterized in terms of their geometries, energies, vibrational frequencies, and electron-density topology. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses were performed to elucidate the nature of the hydrogen-bonding interactions in these complexes. The intramolecular H-bond between the amino and carboxyl oxygen atom of Ad was retained in most of the complexes, and cooperativity between the intra- and intermolecular H-bonds was present in some of the complexes. H-bonds in which hydroxyls of Ad/water acted as proton donors were stronger than other H-bonds. Both hydrogen-bonding interactions and structural deformation play important roles in the relative stabilities of the complexes. The intramolecular H-bond was broken during the formation of the most stable complex, which indicates that Ad tends to break the intramolecular H-bond and form two new intermolecular H-bonds with the first water molecule.

Hydrogen-bonding interactions in adrenaline–water complexes: DFT and QTAIM studies of structures, properties, and topologies

The tryptophan-water (Trp-H2O) complexes formed by hydrogen bonding interactions were investigated at the ωB97XD/6–311+ +G(d,p) level. Five Trp-H2O complexes possessing various types of hydrogen bonds (H-bonds) were characterized by geometries, energies, vibrational frequencies. The nature of the H-bonds were characterized by the natural bond orbital (NBO) and the quantum theory of atoms in molecule (QTAIM) analyses as well. The intramolecular H-bond formed between the amino and carboxyl oxygen atom of tryptophan was retained in most of the complexes, and the cooperativity between the intra and intermolecular H-bonds exist in some complexes. The intramolecular H-bond and some intermolecular H-bonds are strong and have partial covalent character. The H-bonds formed between carboxyl and oxygen/nitrogen atoms are stronger than other H-bonds. The H-bonds involving methylene of tryptophan as H-donor are weak H-bonds. For all complexes, ΔE
 ele and ΔE
 ex makes major contributions to the total interaction energy (ΔE
 MP2), while ΔE
 disp is the smallest component of the interaction energy. Both hydrogen bonding interaction and structural deformation play important roles in the relative stabilities of the complexes. Regardless of strong H-bonds, the stabilities of some complexes are weakened by the serious structural deformations.

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Density functional theory, natural bond orbital and quantum theory of atoms in molecule analyses on the hydrogen bonding interactions in tryptophan-water complexes

The directionality of inter- and intramolecular OHO hydrogen bonds has been compared. For intramolecular bridges it is determined by an orbital formed in the proton transfer process. For intermolecular bonds, the hydrogen-bonded proton is attached to two lone pairs of the acceptor and the OHO angle is not fixed but can change in a broad range. Depending on the OHO angle, the interaction changes continuously from electrostatic interaction to strong OHO hydrogen bond.

Directionality of inter- and intramolecular OHO hydrogen bonds: DFT study followed by AIM and NBO analysis.

The effect of an external electric field on water clusters of the (H 2 O) n type, with [1 ⩽  n  ⩽ 15], in the ground state was analyzed at the B3LYP/cc-pVTZ level of theory. The calculations showed that an external electric field changes the number of hydrogen bonds, reduces the cluster sizes and increases the strength of the inter-cluster hydrogen bonds. The particular symmetry of the cluster and the null dipole moment in these specific configurations suggest that their stability can be associated with a perfect alignment of the water molecules, maximizing attractive electrostatic interactions caused by changes in the charge distribution of the clusters.

Electrical field effects on dipole moment, structure and energetic of (H2O)n (2 ⩽ n ⩽ 15) cluster

The (CH3OH)n (n = 2–8) clusters formed via hydrogen bond (H-bonds) interactions have been studied systemically by density functional theory (DFT). The relevant geometries, energies, and IR characteristics of the intermolecular OH···O H-bonds have been investigated. The quantum theory of atoms in molecule (QTAIM) and natural bond orbital (NBO) analysis have also been applied to understand the nature of the hydrogen bonding interactions in clusters. The results show that both the strength of H-bonds and the deformation are important factors for the stability of (CH3OH)n clusters. The weakest H-bond was found in the dimer. The strengths of H-bonds in clusters increase from n = 2 to 8, moreover, the strengths of H-bonds in (CH3OH)n (n = 4–8) clusters are remarkably stronger than those in (CH3OH)n (n = 2, 3) clusters. The small differences of the strengths of H-bonds among (CH3OH)n (n = 6–8) clusters indicate that a partial covalent character is attributed to the H-bonds in these clusters. The linear relationships between the electron density of BCP (ρb) and the H···O bond length of H-bonds as well as the second-perturbation energies E(2) have also been investigated and used to study the nature of H-bonds, respectively.

Combined DFT with NBO and QTAIM studies on the hydrogen bonds in (CH3OH)n (n = 2–8) clusters

Investigation of intermolecular hydrogen bond interactions in crystalline L-cysteine by DFT calculations of the oxygen-17, nitrogen-14, and hydrogen-2 EFG tensors and AIM analysis.

Roya A. Kiyani

Papers

Investigation of intermolecular hydrogen bond interactions in crystalline L-cysteine by DFT calculations of the oxygen-17, nitrogen-14, and hydrogen-2 EFG tensors and AIM analysis.