三自旋体系核自旋单重态的制备与单重态二维谱的实现

核自旋单重态是一种特殊的自旋状态，其寿命远长于相应自旋的横向和纵向弛豫时间，能够被用于研究分子的慢扩散、慢运动、特征信号选择等过程.目前单重态的研究主要集中于孤立的两自旋体系.而本文以N-乙酰基天冬氨酸（NAA）分子中由亚甲基和次甲基的三个氢原子核构成的三自旋体系为研究对象，将亚甲基中的两个氢核制备成单重态.利用优化控制和数值计算方法，分别设计了包含和不包含次甲基氢核耦合的单重态制备脉冲，结果发现，不考虑次甲基氢耦合设计的优化脉冲，其在实际三自旋体系中的单重态制备效率会显著下降.另外，我们以单重态为起点，实现了针对次甲基和亚甲基的信号选择COSY谱和NOESY谱，结果表明基于单重态的二维谱能够有效避免谱峰重叠现象，提高谱图分辨率，并有助于提高分子结构解析的准确性.

Preparing Nuclear Spin Singlet State in a Three-spin System and Its Application in 2D Spectrum

Nuclear spin singlet state is a special spin state,whose main characteristic is that its lift-time can be much longer than the corresponding longitudinal/transverse relaxation time.It can be used to study molecular slow diffusion,slow motion,special signal selection or other molecular motion.In the literature,singlet states are mainly studied in an isolated two-spin system.Here we discuss the nuclear spin singlet state preparation in a three-spin system.The system consisting of three protons in the molecule N-acetyl aspartic acid(NAA)was used as an example.Specifically,we used optimal control theory and numerical calculation method to design the pulse sequence and to transfer the two spins in methylene group into singlet state.The shaped pulses including and not including the proton in the methyne group were designed respectively.Our simulation results indicate that to ensure a high efficiency for the singlet state preparation,the coupling of the proton in the methyne group should be included in the pulse calculation.Furthermore,the singlet state can be combined with two-dimensional pulse sequences such as COSY and NOESY.The experimental results show that some correlation peaks could be selectively observed in the two-dimensional spectrum based on singlet state.It will be useful for the spectral peak assignment especially in the case of serious spectral overlap.