有机课件ch8 2nmr1核磁共振信号强弱.pdfVIP

NMR Spectroscopy ©20112 Wei Dong Nanjing University of Science Technology s Nuclear Ma ic Resonance Nuclei of isotopes with an odd number of protons, an odd number of neutrons, or both, exhibit mechanical spin phenomena which are associated with angular momentum. This angular momentum is characterized by a nuclear spin quantum number, I such that, I = 1/2 n, where n is an integer. When I = 0, nuclei do not posses spin angular momentum and are NMR-silent; 12C, 16O are examples of this. Nuclei with I = 1/2 posses spin angular momentum and can be studied in the NMR; these lude 1H, 13C, 19F, 31P, etc. 核磁 信号的强弱 13 1 15 19 31 有机化合物的基本元素 C、 H、 N、 F、 P等 都有核磁 信号，且自旋量子数均为1/2，核磁 信号相对简单，已广泛用于有机化合物的结构测定。 然而，核磁 信号的强弱是与被测磁性核的天 1 然丰度和旋磁比的立方成正比的，如 H的天然丰度为 19 31 99.985%， F和 P的丰度均为100%，因此，它们的 13 信号较强，容易测定，而 C的天然丰度只有1.1%， 15 17 很有用的 N和 O核的丰度也在1%以下，它们的 信号都很弱，必须在 变换核磁 波谱仪上经 过多次扫描才能得到有用的信息。 Nuclear Spin • A nucleus with an odd atomic number or an odd mass number has a nuclear spin. • The spinning charged nucleus generates a ma ic field. = External Ma ic Field When ced in an external field, spinning protons act like bar ma s. Th i f l i ff li ma Two Energy States The ma ic fields of the spinning nuclei will align either with the external