α-丙氨酸分子在扶手椅型SWCNT(9,9)内的手性转变机制

The chiral shift mechanism ofα-alanine molecule in SWCNT(9,9) of armchair

采用组合的量子化学ONIOM(B3LYP/6-31++G(d,p):UFF)方法,研究了限域在SWCNT(9,9)内α-丙氨酸的分子结构和手性转变机制.为得到相对高水平的体系能量和反应势能面,在ONIOM(B3LYP/6-311++G(3df,3pd):UFF)水平上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SWCNT(9,9)碳纳米管内时,键长均略缩短,骨架原子的二面角基本不变.反应路径研究发现,α-丙氨酸分子在SWCNT(9,9)内的手性转变路径有2条与单体情况大致相同.不存在单体情况的有羰基H和甲基H协同转移过程的反应通道.对手性转变反应过程势能面的计算发现,S型α-丙氨酸在SWCNT(9,9)内向R型转变与单体α-丙氨酸手性转变反应过程的主要能垒相比较,路径1的最高能垒同样是由在纸外面的氢从手性碳直接到羰基氧的过渡态产生的,能垒基本不变;路径2是氢先在羧基内转移,此过程的能垒由194.5降到137.6kJ·mol-1.而后纸外面的氢从手性碳转移到羰基,此过程的能垒由317.1降到302.9kJ·mol-1.研究结果表明,SWCNT(9,9)对α-丙氨酸的限域影响使手性转变反应过程的某些能垒降低,改变或部分改变了反应路径.

The molecular structure and chiral shift mechanism of α-alanine in SWCNT （9,9） are studied in this research, which combines the method of quantum chemistry ONIOM （B3LYP/6-31 ＋ ＋ G （d, p） .. UFF）. To obtain a high level of energy, each single-point energy of inclusion complexes is calculated at ONIOM （B3LYP/6-311 ＋ ＋ G （3 df, 3 pd） .. UFF） level. The analysis of molecular structure shows that compared with the monomer α-alanine, the bond lengths of the a-alanine limited in SWCNT （9,9） within the carbon nanotubes are slightly shorter, while the dihedral angle between backbone atoms remains unchanged. In the research of the reaction path, we find that there are two chiral transition paths of α-alanine molecule in SWCNT （9,9） which are roughly the same as the single situation. There.is no single case in the reaction channel of cooperative transfer process of H in carbonyl and methyl. By calculating the potential energy surfaces of chiral shift reaction, we find that the maximum energy barrier of path 1 in the process that S-type α-alanine in SWCNT （9,9） transfers to R-type compared to the monomer chiral shift reaction process of a-alanine, is also generated from the transition state that the hydrogen out of the paper transforms from the chiral carbon directly into the carbonyl oxygen, whose energy barrier is substantially unchanged. Path 2 is the proces that hydrogen first transfers into the carboxyl group, whose energy barrier drops from 194. 5 to 137.6 kJ mo1-1. Then, the hydrogen out of the paper transfers from the chiral earbon to carbonyl group, its ener- gy barrier drops from 317.1 to 302.9 kJ ~ mo1-1. The results in this research show that the influence of SWCNT （9,9） to a-alanine decreases the certain energy barrier of the chiral transition, which changes or partly changes its response paths.