α-丙氨酸分子在扶手椅型SWBNNT(9,9)内的手性转变机制

The chiral shift mechanism ofα-alanine inside single-walled armchair SWBNNT(9,9)

采用组合的量子化学ONIOM(our own n-layered integrated molecule orbit and molecule mechanics)(B3LYP/6-31++G(d,p):UFF)方法,研究了限域在SWBNNT(9,9)内α-丙氨酸的分子结构和手性转变通道.为得到高水平的能量,在ONIOM(B3LYP/6-311++G(3df,3pd):UFF)水平,计算了各个包结物的单点能.分子结构分析表明:与单体α-丙氨酸相比,受限在SWBNNT(9,9)内时,骨架碳氮原子间的键长不同程度地缩短,骨架碳原子的键角及骨架碳氮原子的二面角略有增大.反应路径研究发现:α-丙氨酸分子在SWBNNT(9,9)内的手性转变有两条同单体情况大致相同的反应通道,不存在单体情况的含有羰基H和甲基H协同转移过程的反应通道.手性转变反应过程的势能面计算发现:与单体α-丙氨酸手性转变反应过程的主要能垒相比较,在纸外面的氢从手性碳直接到羰基氧的过渡态产生的能垒,从326.5kJ·mol-1降到319.7kJ·mol-1;氢首先在羧基内转移,而后手性碳的氢在纸面外转移到羰基,这两个过程的能垒从198.0kJ·mol-1和320.3kJ·mol-1降到135.5kJ·mol-1和302.7kJ·mol-1.结果表明:限域在SWBNNT(9,9)内的α-丙氨酸,其手性转变过程中不同的氢转移反应能垒被不同程度地降低.

Methods of combined quantum chemistry ONIOM（Our own N-layered Integrated molecule Orbit and molecule Mechanics）are widely used to study the chemical reaction system associated with nanotubes.This work will study the system which is divided into two layers.The inner layer isα-alanine,using the B3 LYP method based ondensity functional theory,and basis set selection 6-31＋＋G（d,p）.The outer layer,a single arm boron nitride nanotubes SWBNNT（9,9）is processed by using molecular mechanics UFF（universal force field）force field,and then the molecular structure and the hiral transition reaction mechanism ofα-alanine confined in SWBNNT（9,9）are studied.Besides,the stable point and transition state of chiral shift reaction process have been optimized.By analyzing the imaginary frequency vibration mode of transition states and intrinsic reaction coordinate（IRC）calculation for the transition states,the reliability of the transition state has been determined.In order to obtain relatively high levels of energy of the system,we depict potential energy surface of the relatively exact chirality process of transformation and calculate a single point energy of each Inclusion on the ONIOM（B3LYP/6-311＋＋G（3df,3pd）：UFF）level.Then we draw the potential energy surface of the reaction process to correct the total system energy of zero-point vibrational energy.The analysis to molecular structure shows that compared with the monomerα-alanine,the bond lengths of skeleton C-N are shorten in different degree whenα-alanine is confined in SWBNNT（9,9）.At the same time,the bond angles of skeleton carbons and the dihedral angles of skeleton C-N increase slightly.The study of reaction channel of chiral shift shows that there are two roughly same reaction with channels of chiral shift ofα-alanine in SWBNNT（9,9）,and only the reaction channel of collaborative transfer hydrogen in carbonyl and methyl without monomer.The calculation of potential energy surface in chiral shift reaction shows that compared with the monomerα-alanine chiral transformation,whenα-alanine is limited in SWBNNT（9,9）,the energy barrier of outboard hydrogen which transfers from chiral carbon directly to oxygen in carbonyl reduces from 326.5kJ·mol-1 to 319.7kJ·mol-1.The energy barriers of hydrogen transfer inside the carboxyl and hydrogen in chiral carbon transfer from outboard to carbonyl respectively decrease from 198.0kJ·mol-1 and 320.3kJ·mol-1 to 135.5kJ·mol-1 and 302.7kJ·mol-1.The results show that the energy barriers of different hydrogen transfer decrease in different degree,whenα-alanine is limited in SWBNNT（9,9）.