| The types of non-covalent interactions(NCIs)are getting more and more abundant with the development of theoretical and experimental studies,leading to a bright and deep understanding of the nature of non-covalent interactions.The cooperative of various types of NCIs plays fundamental but essential roles in the three-dimensional structure of proteins and nucleic acids,self-assembly of supramolecular materials,organic synthesis and molecular recognition.As one of the gas-phase spectroscopy experiments,highresolution Fourier transform microwave(FTMW)spectroscopy combined with pulsed supersonic-jet technique is a reliable approach to study the structural,energetic and internal dynamic information present in bare molecules or weak-bonded molecular clusters,avoiding the interference of condensed environments on molecular systems.FTMW technique is associated with the measurements of the quantized rotational transitions energies of the gas-phase molecules,which is closely related to the molecular mass distributions in space,thus can obtain accurate structural information of targeted systems.The combination of FTMW with high-level theoretical calculations can obtain the energy,internal dynamics information,ect.,providing favourable conditions for the effective analysis of the nature of NCIs.Typically,σ-/π-hole regions in molecules refers to the phenomenon of electron deficiency in a particular region.The switch of NCI is not only related to the electronegativity of the molecule itself,but also to the characteristics of partner molecules.At present,however,there are the following deficiencies in the σ-/π-hole non-covalent interactions:a)the interaction strength after the σ-/π-hole region forms van der Waals forces with the noble gas molecules;b)whether the typical molecule containing the σhole region has other interaction sites;c)σ-hole and π-hole regions compete when participating in non-covalent interactions;d)the influence of molecules containing σhole on the π electronic properties of heterocyclic aromatic molecules;e:)The effect of σ-hole type hydrogen bonds on the conformational behavior of water before and after forming complexes with chiral molecules.To solve these problems,several typical σ-/πhole molecules were selected,utilizing FTMW technique combined with theoretical approaches,to study the structures,energies and internal dynamics of their complexes formed with rare gases,CO2,heterocyclic aromatic and water molecules,further reveal the nature of noncovalent interactions.The details are as follows:(1)There is a σ-hole region around the H atom in pentafluoroethane(CF3CF2H)and a π-hole above the double-bonded π system in chlorotrifluoroethylene(CF2=CFCl).In the van der Waals complexes of CF3CF2H-Ar and CF2=CFCl-Ar,only one conformer was observed by rotational spectroscopy in each complex:The Ar atom locates above theσ-hole or π-hole region.The van der Waals interaction energies of the two systems are estimated to be about 2～3 kJ mol-1 by pseudo-diatomic approximation theory.However,the interaction formed in the σ-hole region is slightly weaker than the one formed in theπ-hole region.(2)In chlorotrifluoromethane(CF3Cl),the σ-hole region on the Cl atom has comparable electrophilicity to that of a proton donor,and is usually used as one of the typical model molecules for studying halogen bonds,binding to various electron donors of different strengths with the formation of C-Cl…B halogen bonds.CO2 molecule containing carbonyl group(C=O)can either form hydrogen or halogen bonds through the interaction of O atoms with stronger protons or halogen bond donors,or form n→π*interactions with less electrophilic molecules through the π-hole on the C=O.We carried out the rotational spectroscopic study on CF3Cl-CO2 complex,which reveals that CF3Cl and CO2 are not bound together through C-Cl…O halogen bond,but rather the lone-pairs(LPs)ring(“equatorial belt")of the Cl atom is activated by the π-hole of CO2 to form the n→π*interaction in the most stable isomer.However,the π*characteristics in molecules are not always dominated by substituents,the stronger electronegative atoms in partner molecules can also induce π*characteristics.As one of the heterocyclic aromatic molecules,furan exhibits either ether-like or diene-like features with different kind of partner molecules.Our rotational study of the CF3Cl-furan complex reveals that,in the most stable isomer,the aromatic π*feature of furan is shown by the Cl LPs in CF3Cl,with the formation of n→π*Aro,in the less stable isomer,CF3Cl interacts with furan through the Cl σ-hole with one of the double bond in furan to form a Cl…π-C=C-halogen bond.The results of the two molecular systems of CF3Cl further emphasise the influence of partner molecules on the overall non-covalent interactions.(3)Solvent molecules may have an effect on the structure and favourable conformation of the chiral molecules.Proton H on water molecules can also be treated asσ-hole donor,involved in the formation of intermolecular hydrogen bondings.In this thesis,the rotational spectra of bare 1-phenylethanol and 3-hydroxy-2-butanone,as well as their monohydrates were studied.For the monomers,the accurate structures were obtained by measuring the specta of 13C isotopic species.In the 1-phenylethanol-H2O complex,one conformer was observed with analyzing the intermolecular hydrogen bonds.Comparing the 1-phenylethanol structure before and after forming complex with water,it is concluded that the water molecule has less influence on the conformation.In the study of 3-hydroxy-2-butanone-H2O complex,it was found that the addition of water molecule destoryed the original intramolecular hydrogen bond,and formed new intermolecular hydrogen bonds.Such kind of addition led to a greater impact on the conformation of the 3-hydroxy-2-butanone monomer.These results indicate that water molecule have little effect on the conformation of chiral molecules with low flexibility,but have a greater influence on molecules with high flexibility. |
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