Theoretical Studies On Mechanisms Of Several Important Chemical Reactions And Solvent Effects

In the present paper,ab initio quantum mechanics method was employed to investigate several important reactions,and also the solvent effects of some of these reactions have been studied.The density functional theory RB3LYP/6-311++G(d,p)was employed to study the mechanism in the gas phase,and the solvent effect of the reactions has been studied using the conductor-like polarizable continuum model.The reaction mechanism of SiH₃ with NO on singlet potential energy surface was studied in the third chapter.The results show that only four products are less than reactants in energy, P2(HSiN+H₂O),P3(HNSi+H₂O),P5(H₂SiONH)and P6(HSiONH₂).From the energy point view, Only initial adducts IM01 is easily obtained in the usual temperature and pressure,many reactions are quite difficult,even can't take place because of higher energy barrier.The energy of almost all intermediates are lower than the reactants.For example,IM08 and IM09 are -480.99 and -420.38 kJ.mo1^-1respectively.Some of these intermediates are in fact final products.By theoretical computation we get the paths of the products of P2(HSiN+H₂O),P3(HNSi+H₂O),also predicted the paths of another four products:PI(HSiNO+H₂),P4(HSiNO+H₂),P5(H₂SiONH)and P6 (HSiONH₂).In addition,the paths to P2(HSiN+H₂O)and P3(HNSi+H₂O)found by other researchers are only R(SiH₃+NO)→IM01(SiH₃NO)→IM03(SiH₂NHO)→IM04(SiH₂NOH)→P2(HsiN+H₂O),R(SiH₃+NO)→IM01(SiH₃NO)→IM03(SiH₂NHO)→IM04(SiH₂NOH)→IM05(SiHNHOH)→P3(HNSi+H₂O).Because there is a transition state between P2(HSiN+H₂O) and P3(HNSi+H₂O),we found there are more paths to these products,all of which are feasible.In the paper we got other products and the paths,which are to be confirmed by experiment.In the fourth chapter,we studied the reaction mechanism of nitrosobenzene with formaldehyde in gas phase and solvents on a singlet potential energy surface.Two reaction paths,the concerted and the stepwise mechanisms were found,both of which produce the experimental product: N-phenylhydroxamic acid C₆H₅NOHCHO.According to E_T(30)andε,we selected six solvents water,ethanol,acetonitrile,dichloromethane,THF and cyclohexane.The solvent effects of the reaction have been studied using the conductor-like polarizable continuum model(CPCM)at the level of theory CPCM/RB3LYP/6-311++G(d,p).The calculated results are as follows:(1)two reaction paths,the concerted and the stepwise mechanisms were found,and the stepwise mechanism is the most favorable one in the gas phase;(2)the solvation effect reduces the activation energy,but the reaction of nitrosobenzene with formaldehyde is not sensitive to the solvent polarities,which is different from the reaction in acid as catalyzer;(3)the theoretical result confirmed the view proposed by Pilepi(?)that N is more nucleophilic than O in nitroso compound;(4) the theoretical result also confirmed that the reaction is difficult in any solvent without catalyzer in room temperature,since the lowest activation energy of the stepwise mechanisms in water is 144.36 kJ·mol^-1.In the fifth chapter,we studied the reaction mechanism of nitrosobenzene, 2-methyl-2-nitrosopropane with formaldehyde in gas phase and solvents.According to E_T(30)andε,we choiced three solvents ethanol,acetonitrile,water.The solvent effect of the reaction has been studied using the conductor-like polarizable continuum model(CPCM)at the level of theory CPCM/RB3LYP/6-311++G(d,p).The calculated results are as follows:(1)two reaction paths of nitrosobenzene,2-methyl- 2-nitrosopropane with formaldehyde in gas phase,the concerted and the stepwise mechanisms were found,and the stepwise mechanism is the most favorable one in the gas phase;(2)the solvation effect reduces the activation energy,but the reaction of nitrosobenzene, 2-methyl-2-nitrosopropane with formaldehyde is not sensitive to the solvent polarities,which is different from the reaction in acid as catalyzer;(3)the theoretical result confirmed the view proposed by Pilepi(?)that N is more nucleophilic than O in nitroso compound;(4)the theoretical result also confirmed that the reaction is quite difficult in any solvent without catalyzer in room temperature,since the lowest activation energy of the stepwise mechanisms of nitrosobenzene and 2-methyl-2- nitrosopropane with formaldehyde in water is 144.36 and 124.10 kJ·mol^-1respectively; (5)from the thermodynamics view,our work confirmed the view proposed by W.Lijinsky that the acyclic nitroso compound is more active than the phenyl nitroso compound.