X·F (X=C, N, O and S) Noncovalent Weak Intermolecular Interactions

A number of X···F (X=C, N, O and S) noncovalent weak intermolecular interaction systems of CH3-F···XO2 (X=C, N, O and S) has been investigated at B3LYP/6-311++G(d, p) computational level. A topological analysis of the electron density for the X···F (X=C, N, O and S) noncovalent weak bonds was performed using Baders theory of atom-in-molecules (AIM). The interaction content of the F···X in H3CF···CO2 complex would mainly represent more π property than others. The interaction energies data without (ΔE) and with (ΔEcp) BSSE correction showed that the stability of the four complexes of the H3CF···DB2 system increases in the order of H3CF···O3 < H3CF···NO2 < H3CF···CO2 < H3CF···SO2.

Theoretical Study on the GeH4•••Y (Y=He, Ne, Ar and Kr) Systems

MP2/aug-cc-pvtz level was used to optimize geometries of the complexes between GeH4 and Y(Y=He, Ne, Ar and Kr). The structures and electronic properties of the blue-shift hydrogen bonds complexes GeH4…Y(Y=Ar, Kr) were investigated. The calculated interaction energies with basis set super-position error (BSSE) correction revealed that the relative stabilities of the complexes in the order: GeH4…He ˂ GeH4…Ne ˂ GeH4…Ar ≈ GeH4…Kr. The calculated results showed that the interactions between GeH4 and Y（Y=He, Ne）belong to van der Waals force, and those between GeH4 and Y（Y=Ar, Kr）belong to weak hydrogen bond. NBO (natural bond orbital theory) and electron behavior analysis showed that GeH4…Y(Y= Ar, Kr) hydrogen bond is with a non-electrostatic property.

ESTIMATION ON THE INTRAMOLECULAR 8- AND 12-MEMBERED RING N–H…O=C HYDROGEN BONDING ENERGIES IN β-PEPTIDES

Computation of accurate hydrogen bonding energies in peptides is of great importance in understanding the conformational stabilities of peptides. In this paper, the intramolecular 8- and 12-membered ring N – H … O = C hydrogen bonding energies in β-peptide structures were evaluated. The optimal structures of the β-peptide conformers were obtained using MP2/6-31G(d) method. The MP2/6-311++G(d,p) calculations were then carried out to evaluate the single-point energies. The results show that the intramolecular 8-membered ring N – H … O = C hydrogen bonding energies in the five β-dipeptide structures β-di, β-di-R1, β-di-R2, β-di-R3, and β-di-R4 are -5.50, -5.40, -7.28, -4.94, and -6.84 kcal/mol with BSSE correction, respectively; the intramolecular 12-membered ring N – H … O = C hydrogen bonding energies in the nine β-tripeptide structures β-tri, β-tri-R1, β-tri-R2, β-tri-R3, β-tri-R4, β-tri-R1', β-tri-R2', β-tri-R3' and β-tri-R4' are -10.23, -10.32, -9.53, -10.30, -10.32, -10.55, -10.09, -10.51, and -9.60 kcal/mol with BSSE correction, respectively. Our calculation results further indicate that for the intramolecular 8-membered ring hydrogen bondings, the structures where the orientation of the side chain methyl group is "a–a" have stronger intramolecular hydrogen bondings than those where the orientation of the side chain methyl group is "e–e", while for the intramolecular 12-membered ring hydrogen bondings, the structures where the orientation of the side chain methyl group is "e–e" have stronger intramolecular hydrogen bondings than those where the orientation of the side chain methyl group is "a–a". The method is also applied to estimate the individual intermolecular hydrogen bonding energies in the dimers of amino-acetaldehyde, 2-amino-acetamide, 2-oxo-acetamide, and oxalamide, each dimer having two identical intermolecular hydrogen bonds. According to our method, the individual intermolecular hydrogen bonding energies in the four dimers are calculated to be -1.71, -1.50, -4.67, and -3.22 kcal/mol at the MP2/6-311++G(d,p) level, which are in good agreement with the values of -1.84, -1.72, -4.93, and -3.26 kcal/mol predicted by the supermolecular method.