Theoretical Study and NBO Analysis of the Decomposition Kinetics of Oxetane, 2-Methyloxetane and 2,2-Dimethyloxetane  

作　　者：Arezoo Tahan Abolfazl Shiroudi 

机构地区：[1]Semnan Branch,Islamic Azad University [2]Chemistry Department,Shahrood Branch,Islamic Azad University

出　　处：《Chinese Journal of Structural Chemistry》2011年第5期697-705,共9页结构化学（英文）

摘　　要：A theoretical study of the thermal decomposition kinetics of oxetane （1）, 2-methyloxetane （2）, and 2,2-dimethyloxetane （3） has been carried out at the B3LYP/6-311＋G＊＊, B3PW91/6-311＋G＊＊, and MPW1PW91/6-311＋G＊＊ levels of theory. The MPW1PW91/6-311＋G＊＊ method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1～3 yields formaldehyde and the corresponding substituted olefin. Based on the optimized ground state geometries using MPW1PW91/6-311＋G＊＊ method, the natural bond orbital （NBO） analysis of donor-acceptor （bond-antibond） interactions revealed that the stabilization energies associated with the electronic delocalization from σC3-C4 bonding to σ＊O1-C2 antibonding orbitals decrease from compounds 1 to 3. The σC3-C4→σO1-C2 resonance energies for compounds 1～3 are 2.63, 2.59 and 2.45 kcal mol-1, respectively. Further, the results showed that the energy gaps between σC3-C4 bonding and σ＊O1-C2 antibonding orbitals decrease from compounds 1 to 3. Also, the decomposition process in these compounds are controlled by σ→σ＊ resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Csp3 atom （which is attached to oxygen atom）. NBO analysis shows that the occupancies of σCsp3-O bonds decrease for compounds 1～3 as 3〈2〈1, and those of σCsp3-O bonds increase in the opposite order （3 〉 2 〉 1）. This fact illustrates a comparatively easier thermal decomposition of the sCsp3-O bond in compound 3 compared to compound 2, and in compound 2 compared to compound 1. NBO results indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.A theoretical study of the thermal decomposition kinetics of oxetane （1）, 2-methyloxetane （2）, and 2,2-dimethyloxetane （3） has been carried out at the B3LYP/6-311＋G＊＊, B3PW91/6-311＋G＊＊, and MPW1PW91/6-311＋G＊＊ levels of theory. The MPW1PW91/6-311＋G＊＊ method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1～3 yields formaldehyde and the corresponding substituted olefin. Based on the optimized ground state geometries using MPW1PW91/6-311＋G＊＊ method, the natural bond orbital （NBO） analysis of donor-acceptor （bond-antibond） interactions revealed that the stabilization energies associated with the electronic delocalization from σC3-C4 bonding to σ＊O1-C2 antibonding orbitals decrease from compounds 1 to 3. The σC3-C4→σO1-C2 resonance energies for compounds 1～3 are 2.63, 2.59 and 2.45 kcal mol-1, respectively. Further, the results showed that the energy gaps between σC3-C4 bonding and σ＊O1-C2 antibonding orbitals decrease from compounds 1 to 3. Also, the decomposition process in these compounds are controlled by σ→σ＊ resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Csp3 atom （which is attached to oxygen atom）. NBO analysis shows that the occupancies of σCsp3-O bonds decrease for compounds 1～3 as 3〈2〈1, and those of σCsp3-O bonds increase in the opposite order （3 〉 2 〉 1）. This fact illustrates a comparatively easier thermal decomposition of the sCsp3-O bond in compound 3 compared to compound 2, and in compound 2 compared to compound 1. NBO results indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

关 键 词：unimolecular ab initio molecular modeling decomposition DFT NBO 

分 类 号：O643.1[理学—物理化学]