14N and 17O electric field gradient tensors in benzamide clusters: Theoretical evidence for cooperative and electronic delocalization effects in N–H⋯O hydrogen bonding

Systematic study of the substitution effect on the tetrel bond between 1,4-diazabicyclo[2.2.2]octane and TH3X

A tetrel bond was characterized in the complexes of 1,4-diazabicyclo[2.2.2]octane (DABCO) with TH3X (T = C, Si, Ge; X= -Me, -H, -OH, -NH2, -F, -Cl, -Br, -I, -CN, -NO2). DABCO engages in a weak tetrel bond with CH3X but a stronger one with SiH3X and GeH3X. SiH3X is favorable to bind with DABCO relative to GeH3X, inconsistent with the magnitude of the σ-hole on the tetrel atom. The methyl group in the tetrel donor weakens the tetrel bond but an enhancing effect is found for the other substituents, particularly -NO2. The substitution effect is also related to the nature of the tetrel atom. The halogen substitution from F to I has a weakening effect in the CH3X complex but an enhancing effect in the SiH3X complex and a negligible effect in the GeH3X complex. The above abnormal results found in these complexes can be partly attributed to the charge transfer from the lone pair on the nitrogen atom of DABCO into the anti-bonding orbital σ*(T-X) of TH3X. The stability of both SiH3X and GeH3X complexes is primarily controlled by electrostatic interactions and polarization.

A theoretical study on the characteristics of the intermolecular interactions in the active site of human androsterone sulphotransferase: DFT calculations of NQR and NMR parameters and QTAIM analysis

A theoretical study at the level of density functional theory (DFT) was performed to characterize noncovalent intermolecular interactions, especially hydrogen bond interactions, in the active site of enzyme human androsterone sulphotransferase (SULT2A1/ADT). Geometry optimization, interaction energy, (2)H, (14)N, and (17)O electric field gradient (EFG) tensors, (1)H, (13)C, (17)O, and (15)N chemical shielding (CS) tensors, Natural Bonding Orbital (NBO) analysis, and quantum theory of atoms in molecules (QTAIM) analysis of this active site were investigated. It was found that androsterone (ADT) is able to form hydrogen bonds with residues Ser80, Ile82, and His99 of the active site. The interaction energy calculations and NBO analysis revealed that the ADT molecule forms the strongest hydrogen bond with Ser80. Results revealed that ADT interacts with the other residues through electrostatic and Van der Waals interactions. Results showed that these hydrogen bonds influence on the calculated (2)H, (14)N, and (17)O quadrupole coupling constants (QCCs), as well as (1)H, (13)C, (17)O, and (15)N CS tensors. The magnitude of the QCC and CS changes at each nucleus depends directly on its amount of contribution to the hydrogen bond interaction.

Cooperative and substitution effects in enhancing the strength of fluorine bonds by anion−π interactions

In this work, the cooperative effects between anion−π and fluorine bond interactions are studied by ab initio calculations at the MP2/6-311++G** level. Cooperative effects are observed in complexes in which anion−π and fluorine bond interactions coexist. For each complex, the shortening of the binding distance in the fluorine bond is more prominent than that in the anion−π bond. Favorable cooperativity energies are found with values that range between –0.51 and –0.76 kcal/mol. The atoms in molecules and molecular electrostatic potential analyses are carried out for these complexes to understand the nature of anion−π and fluorine bond interactions and the origin of the cooperativity.

A DFT INVESTIGATION ON BASIS SET SIZE AND HYDROGEN-BONDING EFFECTS ON 17O AND 2H NQR PARAMETERS

A systematic theoretical study on various maleic acid (MA) clusters has been carried out employing density functional theory (DFT) methods. The performance of two different functionals namely B3LYP and M06 in the prediction of geometries, 17 O and 2 H nuclei quadrupole coupling constant (CQ) values of the MA clusters has been assessed comparing the results to those experimental data. For DFT calculations, several basis sets have been used, including the recently developed Jensen's polarization-consistent basis set families, pcJ-n and pcS-n (n = 0,1,2,3). Calculations at the basis set limit indicate that the value of CQ(2 H ) in monomer MA, changes by 0.01–0.04 kHz for each of the final two basis set increments, and seems reasonable to conclusion that the pcJ-3 result is within a few kHz of the basis set limit. Convergence with respect to basis set size was found to be very good, and the pcJ-1 and pcS-1 basis sets provided a good compromise between the basis set limit and computational expense. In most cases, the differences between B3LYP and M06 results for a given basis set are in a range of 1–2%. On the other hand, no systematic changes in the CQ(17 O ) or CQ(2 H ) were found for basis sets larger than double-ζ. Thus, the usual assumption that double-ζ basis set (pcJ-1 and pcS-1) results in the acceptable CQ values, seems to be valid in the case of 17 O and 2 H nuclei.

Density functional theory investigation of cocaine water complexes

Twenty cocaine-water complexes were studied using density functional theory (DFT) B3LYP/6-311++G** level to understand their geometries, energies, vibrational frequencies, charge transfer and topological parameters. Among the 20 complexes, 12 are neutral and eight are protonated in the cocaine-water complexes. Based on the interaction energy, the protonated complexes are more stable than the neutral complexes. In both complexes, the most stable structure involves the hydrogen bond with water at nitrogen atom in the tropane ring and C=O groups in methyl ester. Carbonyl groups in benzoyl and methyl ester is the most reactive site in both forms and it is responsible for the stability order. The calculated topological results show that the interactions involved in the hydrogen bond are electrostatic dominant. Natural bond orbital (NBO) analysis confirms the presence of hydrogen bond and it supports the stability order. Atoms in molecules (AIM) and NBO analysis confirms the C-H···O hydrogen bonds formed between the cocaine-water complexes are blue shifted in nature.

Characterization of cooperative effects in linear alpha-glycylglycine clusters

The aspects of N-H...O=CNH, N-H...O=CO and C-H...O=CNH interactions are analyzed by applying ab initio and DFT methods as well as Bader theory. We investigated geometry, binding energies, (17)O, (15)N chemical shift tensors, and Atoms in Molecules (AIM) properties of alpha-glycylglycine (alpha-glygly) clusters, via MP2, B3LYP and PW91(XC) methods. Dimer stabilization energies and equilibrium geometries are studied in various levels of theory. MP2 and DFT calculations reveal that for alpha-glygly clusters, stability of N-H...O and C-H...O hydrogen bonds are enhanced significantly as a result of cooperativity effects. Furthermore, a covalent nature is also detected for some hydrogen bondings. The n-dependent trend of (17)O and (15)N chemical shift tensors was reasonably correlated with cooperative effects in hydrogen-bond interactions. Regarding the various N-H...O=CNH, N-H...O=CO and C-H...O=CNH hydrogen bondings, capability of the alpha-glygly clusters for electron localization at the N-H...O and C-H...O bond critical points, depends on the cluster size. This leads to cooperative changes in the hydrogen-bond length and strength as well as (17)O and (15)N chemical shift tensors.