NMR中自旋耦合与裂分

• Generally, spin-spin splitting of single absorption peaks into multiplets (doublets, triplets, quartets, etc.) arises from different numbers of neighboring protons. • Conversely, the splitting in the resonance signals gives us exact information about the number of neighboring protons. This is a very important point in NMR spectroscopy in regard to structural analysis. This phenomenon, signal splitting, is called spin-spin splitting.
• In looking at these factors, we have neglected the point of how the alignment (parallel or antiparallel) of the magnetic moment of proton Hb in the external magnetic field can affect the chemical shift of the neighboring proton Ha. • Different alignment of the magnetic moment of the proton Hb, will cause fine splitting in the signal of proton Ha. • As a consequence, the number of signals in the spectrum will increase and the appearance of the spectrum will be complex. At the same time, we will obtain more information from the spectrum for interpretation.
自旋偶合与自旋裂分
spin coupling and spin splitting
• We have seen that differences in the chemical shift of protons are caused by the small local magnetic fields of the electrons that surround the various nuclei. • Chemical shift is also affected by the magnetic anisotropy of different functional groups.
• If the neighboring proton Hb is aligned parallel to the external field, the total effective magnetic field at the proton Ha is slightly larger than it would be otherwise. • Conversely, if proton Hb is aligned antiparallel to the external magnetic field, the effective field at proton Ha is slightly smaller than it would be otherwise. Thus, the applied field that is needed to bring proton Ha into resonance is slightly increased. • In summary, proton Ha will be under the influence of an increased (H1) and decreased (H2) magnetic field depending on how the neighboring proton Hb is aligned. These magnetic fields are shown below:
Htotal is the magnetic field (including the effect of the neighboring groups) influencing proton Ha and H^y is the magnetic moment of Hb
• Ha proton resonance will occur at two different values of the applied field, and proton Ha will have two different resonance frequencies. Therefore, the signal of proton Ha will split into a doublet. This phenomenon is called spin-spin coupling. We can also say that Ha couples with proton Hb and resonates as a doublet.
• The methylenic protons located between two carbonyl groups are equivalent and possess no neighboring protons. • While their resonance signal appears as a sharp singlet at 3.2 ppm, the other methylenic protons next to the methyl group resonate as a quartet. • Analysis of these structures indicates that ethyl acetate has two different methyl groups. One of them resonates as a singlet, the other one as a triplet. In the case of diethyl malonate, there are two different methylenic protons, which resonate as a singlet and a quartet, respectively. These observations lead us to the following conclusion: a methyl group (-CH3) can resonate as a singlet, doublet or triplet depending on the molecular structure. This splitting mode is also valid for methylene (-CH2-) and methine protons (-CH-).
ຫໍສະໝຸດ Baidu
EXPLANATION OF SPIN-SPIN SPLITTING • ethyl acetate diethyl malonate
• As can be seen from the formula, ethyl acetate contains two different methyl groups, the chemical shifts of which, as expected, are different. One methyl group is attached directly to the carbonyl group, and the other to the methylene (-CH2-) group. However, the appearances of the signals belonging to these methyl groups are completely different. • While the methyl group that is bonded to the carbonyl group resonates as a singlet at around 2.00 ppm, the other methyl group resonates at 1.3 ppm as a triplet (three peaks). The methyl group that is bonded to the carbonyl carbon does not have any neighboring protons. • However, the other methyl group has two neighboring protons. We can conclude from this observation that the neighboring protons (OCH2-) are responsible for the splitting in the signal of the methyl group. Furthermore, methylenic protons resonate at 4.1 ppm as a quartet (four peaks). It is evident that the methyl protons are responsible for the splitting in the resonance signal of the -CH2protons.
How this splitting arises?
• 4-methoxy-3-buten-2-one
• There are two kinds of methyl groups, each of which resonates at different ppm values. • Inspection of the olefinic protons shows their nonequivalency. However, these signals, in contrast to the other methyl resonances, are split into doublets. • Let us first analyze the Ha proton. If this proton is irradiated with electromagnetic radiation of the proper frequency, energy absorption occurs, and the lower energy state (parallel alignment) flips to the higher energy state (antiparallel alignment), which is called resonance. • Now we address the question of how proton Hb behaves in the magnetic field while proton Ha resonates. Nucleus Hb possesses a magnetic moment (µa). According to the Boltzmann distribution law, proton Hb aligns either with or against the external magnetic field.