Theoretical Study On The Structures, Stabilities And Bonding Of Dimer Radical Cation Systems

In this paper Ab initio calculations have been carried out to investigate the structures, stabilities and bonding of the dimer radical cation systems (1) (AH3-H2O)+(A=N,P, As) and (2) (XH3-YH3)+ (X,Y=N, P, As). We performed the Natural Bond Orbital (NBO) analysis to study the nature of interactions in the systems. On the basis of Natural Population Analysis (NPA), we reported Wiberg bond indexes (WBIs), natural charges and natural spin density populations, respectively. The "atoms in molecules" (AIM) theory was also applied to find the bond critical points (BCPs) and to analyze the nature of bonding in terms of electron densities (pBCP). Laplacians (▽2pBCP), the total electron energy density H (H= G+V) and its components (the local kinetic energy density G and the local potential energy density V) at BCPs. Based on the results of NBO and AIM analysis, we obtained the binding energies of different structures. On the basis of the above research, we studied the systems (3)(XH2P∴NH2X)+(X=H, F, Cl, Br, NH2, OH, CH3) to discuss the effect of different substituent groups on the 2c-3e hemi-bond interaction. The main contents are the following:1. Structures, stabilities, binding energies and bonding nature of the dimer radical cation systems (AH3-H2O)+(A= N, P, As) have been studied using B3LYP, MP2 and CCSD methods. The local minima of (NH3-H2O)+ include a hemi-bonded structure (H3N+···OH2)(A-1), a proton-transferred hydrogen-bonded structure (H3NH+···H)(A-3), and a non-proton transfer hydrogen-bonded structure (H2N-H+···OH2) (A-2) which is the most stable conformer. Both the P and As complexes have four local minima. In addition to the three structures found for the nitrogen analogue, a second proton-transferred hydrogen-bonded structure (H2A···H-OH2+) was found. For the P-complexes the hemi-bonded structure (H3P+···H2) is the most table complex, while for the As-compounds the hemi-bonded structure (H3As+···OH2) and hydrogen-bonded structure (H2As···H-OH2+) are the most table ones. The NBO and AIM analysis showed that the most stable structures of each system are partially covalent interactions in nature.2. Structures, bonding and relative stabilities of the dimer radical cations (XH3-YH3)+(X,Y=N, P, As) have been studied theoretically. Two kinds of structures (hemi-bonded and proton-transferred isomers) are obtained for each system. For (NH3-N/P/AsH3)+ the stable conformer is the proton-transferred structure; for (PH3-PH3)+and (AsH3-AsH3)+the stable structure is the hemi-bonded one; for (PH3-AsH3)+three proton-transferred and one hemi-bonded isomers were found with the stability order:(H-PH3+···AsH2) Ⅰ> (H3PH+···AsH2) Ⅱ> (H3P···AsH3+)> (H3AsH+···PH2)Ⅲ. The hemi-bonds have large interaction energies 25.2-35.1 kcal/mol and are partially covalent in nature, while the proton-transferred structures have moderate interaction energies 6.5～22.2 kcal/mol.3. Ab initio MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVTZ calculations have been carried out in a systematic investigation of (XH2P···NH3)+and (XH2P···NH2X)+complexes for X=H, F, Cl, Br, NH2, OH and CH3, as well as selected complexes (XH2P···H2X’)+with different substituents X bonded to P and N. In general, the electron-withdrawing substituents of XH2P+ and the electron-donating groups of NH2X’can strengthen the P-N interaction in hemi-bonded complexes. The interaction distances for the complexes (XH2P···H2X)+ are range from 2.029-A for (FH2P···NH2CH3)+ to 2.412A for (H2NH2P···NH2F)+, the binding energies are range from 17 to 54 kcal/mol, and the Wiberg bond indexes are in range of 0.14～0.39. The NBO analysis shown that for a spin there is a strong charge transfer from the N lone pair into the P-A σ* antibond; for β spin the N-P bonds are described as a single electron σP-N bonds except in (H2NH2P···NH2F)+. According to topological analysis, the P-N bonds in majority of hemi-bonded complexes are partly covalent in nature, but the P-N bonds in (FH2P···NH2CH3)+ and (FH2P···NH3)+ complexes is a typical covalent bond in nature. These two hemi-bonded complexes can be regarded as a single molecular. Binding energies for all the complexes correlate with intermolecular N-P distances as well as with bonding parameters obtained from AIM analyses.