Theoretical Studies On The Dihydrogen-bonded Complexes Containing Alkali Metal Or Alkali-Earth Metal Hydrides

Dihydrogen bonding is an unconventional hydrogen bonding. It has strength and directionality comparable with those found in classical hydrogen bonding. Consequently, it can influence structure, reactivity and selectivity in solution and solid state, finding thus potential utilities in catalysis, crystal engineering, and materials chemistry. The theoretical studies were performed on the structures, interactions and properties of some representative systems containing dihydrogen bond and other interactions in this thesis. The results obtained on new structures and intermolecular interactions may be valuable for improving our understanding of the nature of intermolecular interaction, and enriching our knowledge of dihydrogen bonds and other weak intermolecular interactions.There are five main aspects included in this thesis:(1) The dihydrogen-bonded complexes of ethylene and its chlorine derivatives with magnesium hydride have been systematically investigated at the MP2/6-311++G(d,p) level. The studied complexes are divided into three groups based on the optimized structures. The most stable complexes with interaction energies between 3.4 and 5.9 kcal/mol present circular structures enclosed by CH…H and HMg…Cl bonds. The other linear structures with interaction energies between 0.5 and 2.0 kcal/mol are stabilized by only CH…H dihydrogen bond. All investigated complexes exhibit slight elongation of C–H bond accompanied by a red shift of C–H stretching frequency. With increasing chlorine atoms on ethylene, the geometries, frequencies, interaction energies, and the electron density in the C–Hσ* antibonding orbital of the complexes all increase or decrease gradually. The nature of the electrostatic interaction in this type of dihydrogen bond has also been unveiled by means of the atoms in molecules (AIM) and natural bond orbital (NBO) analysis. The effect of ring structures on the dihydrogen bonding systems has been considered by comparing the geometric data and AIM parameters. Moreover, the calculated direction of net charge transfer of ring structure complexes is contrary to the previous found in dihydrogen bonded systems.(2) The C–H…H dihydrogen-bonded complexes of methane, ethylene, acetylene, and their derivatives with magnesium hydride have been systematically investigated at the MP2/aug-cc-PVTZ level. The results confirm that the strength of dihydrogen bonding increases in the following order of proton donors: C(sp³)–H < C(sp²)–H< C(sp)–H and chlorine substituents enhance the C–H…H interaction. For the majority complexes with cyclic type of structure, the bond length variations and red-shifts of Mg-H proton-accepting bond are more sensitive than the corresponding value of C-H proton-donating bond. The nature of the electrostatic interaction in these C–H…H dihydrogen bond has also been unveiled by means of the atoms in molecules (AIM) analysis. The natural bond orbital (NBO) analysis suggests that the charge transfer in cyclic complexes have dual-channel character. The direction of net charge transfer of cyclic complexes is contrary to the previous found in dihydrogen bonded systems.(3) The dihydrogen-bonded complexes of ethylene and its chlorine derivatives with sodium hydride have been systematically investigated at the MP2/6-311++G(d,p) level. The studied complexes are divided into three groups (including Linear, Five- and Six-membered cyclic structures) based on the optimized structures. The structural, energetic and topological parameters are presented and analyzed in terms of their possible correlation with the interaction energies and the intermolecular H…H distances. The nature of the electrostatic interaction in this type of dihydrogen bond has also been unveiled by means of the atoms in molecules (AIM) and natural bond orbital (NBO) analysis. The effect of ring structure on the dihydrogen bonding systems has been considered by comparing to corresponding linear structure. NBO analysis suggests that the EDT in cyclic structures have dual-channel character.(4) The properties of the dihydrogen-bonded (DHB) complexes MH…HC2Cl3 (M=Li, Na, K) were calculated by the MP2/6-311++G(d,p) method. The studied complexes are divided into three groups (including Linear, Five- and Six-membered cyclic structures) based on the optimized structures. For the same optimized structure, if the acceptors listed in order of increasing proton-accepting ability are LiH < NaH < KH. It is undoubted that such observations are related with the gradually decreasing of electronegativity for Li, Na and K. The nature of the electrostatic interaction in this type of dihydrogen bond has also been unveiled by means of the atoms in molecules (AIM) and natural bond orbital (NBO) analysis. The effect of ring structure on the dihydrogen bonding systems has been considered by comparing to corresponding linear structure.(5) The B3LYP/6-31++G(d,p) and B3LYP/6-311++G(3df,2p) calculations are carried out to investigate the structures and properties of dihydrogen-bonded CaH₂…HY (Y = C₂H, C₂Cl, C₂BeH,CN, and NC) complexes. Our calculations revealed two possible structures for CaH₂ in CaH₂…HY complexes: linear (I) and bent (II). The bond lengths, interaction energies, and strength for H…H interactions obtained by both B3LYP/6-31++G(d,p) and B3LYP/6-311++G(3df, 2p) methods are quite close to each other. The inverse ratio correlation indicates interaction energy decrease with the increase of the electron density at Ca–H bond critical point. The AIM results point out that for all of Ca–H…H–Y interactions considered here the Laplacian of the electron density at H…H bond critical point are positive indicating the electrostatic nature in these Ca–H…H–Y dihydrogen bond systems.