| Intermolecular interaction plays a crucial role in structures and properties of materials,mechanisms and trends in the chemical reaction process,and attracts the interest of both experimental and theoretical chemists.Plenty of theories and effective approaches have been developed by theoretical chemists.Nowadays,the EDA method,which will be introduced in Chapter 2,is developing rapidly and wildly adopted in the investigation of intermolecular interaction.In Chapter 3,the internal rotation barrier of the XH3-YH3(X,Y=C,Si)molecules、is discussed by means of EDA method.Moreover,the barriers of the internal rotation in ethane,methylsilane,and disilane are investigated.The rotation barriers and the inter-conversion of energy potential surfaces of three geomeical variation processes from the three geometrical variation processes are decomposed into the electrostatic,exchange-repulsion,polarization,correlation and geometrical relaxation terms.EDA results for the six paths are similar,showing that the process is approximately unrelated to each other.Three conclusions were drawn according to our research.Firstly,the origin of the rotation barrier is free from the order of the processes in the various paths.Secondly,the disilane rotation barrier is similar to ethane and methylsilane,which mainly arises from the rigid rotation process.The exchange-repulsion(Pauli repulsion)governs the rigid rotation process,showing its dominant role for the barrier.The electrostatic and polarization terms play the secondary roles in the rigid rotation process and the whole rotation.Thirdly,the radicals5 geometrical variation and the σ bond expansion,which can be regarded as the geometrical relaxation,hardly contribute to the barrier but greatly influence over the total EDA terms.In Chapter 4,the physical origins of the C-O single bond rotations in glycine are explored theoretically.The rotation barriers are decomposed into the electrostatic,exchange-repulsion,polarization,correlation and geometrical relaxation terms.In general,the C-O single bond rotations are controlled by Pauli repulsion and polarization interactions.It is confirmed that there is only one intramolecular hydrogen bond in the second stable conformer but not in others by the AIM analysis.The C-O single bond rotations without obvious inter-group interaction between NH2 and COOH are dominated by Pauli repulsion and polarization interactions.On the other hand,polarization interaction is the most important energy component in GLY3 and GLY5,due to the intramolecular hydrogen bond formed between two sides.It is concluded that the physical origin of the C-O rotation in glycine can be different to that in formic acid,which is regarded as a prototype of C-O rotation.The importance of the intramolecular interaction for the rotation is confirmed.In Chapter 5,the nature of blue-shift hydrogen bond is discussed.As a multi-reference wavefunction method,the wavefunction of a system in valence bond(VB)theory could be described as a series of VB structures.This theory combines the explicit physical picture with the intuitive chemical sense.Both further understanding and clearer theoretical model of intermolecular interactions can be discovered by using the EDA method based on VB wavefunction.Two consistent conclusions were drown by means of both the LMO-EDA method as well as the EDA based on VB wavefunction.Firstly,no crucial difference between the red-and blue-shifting H bond was found.Our results show that the frozen energy terms play a dominant role in majority HB systems.The induce term and the charge transfer term in VB calculation,tend to decrease with the C-H vibration frequency.It should be charge transfer effects that contribute little in all HB systems by means of both the LMO-EDA and VB studies.The bond order changes combined with VB structures changes in VBSCF calculation and the charge distribution also support this results.Consequently,charge transfer could not be attribute to the difference between the red-shift HB and the blue-shift HB.Secondly,the significance of polarization was found by means of VB method,since a decrease of weight of(F3C-H…Y)VB structure and a increase of the(F3C-…H+…Y)structure were found using VB method,due to the polarization of the Y group. |
