第二章 核磁共振与化学位移

第一章 核磁共振波谱分析法 第二节 核磁共振与化学位移 nuclear magnetic resonance spectroscopy nuclear magnetic resonance and chemical shift • Protons resonate at different frequencies according to their chemical environments and locations. • Even in a homogeneous magnetic field, the strengths of the local magnetic fields that influence the protons differ slightly from the external magnetic field. • Generally speaking, all protons of a given compound located in 'different' chemical environments are under the influence of different magnetic fields. • Conversely, protons located in the same chemical environment are under the influence of the same magnetic field. • According to the basic NMR equation (eq. 13), if the protons are under the influence of different magnetic fields, they will naturally also have different resonance frequencies. • This fact will help us to recognize different protons in NMR spectroscopy. • Now we can raise the question of why the magnetic fields around the protons are different from the applied external magnetic field. • When we were discussing resonance, we were concerned only with the external magnetic field, always ignoring the effect of the electrons surrounding the protons. • Electrons under the influence of an external magnetic field generate their own magnetic fields, either increasing or decreasing the influence of the external magnetic field. • Electrons are charged particles. Therefore, the applied external magnetic field induces circulations in the electron cloud surrounding the nucleus causing the electrons to generate their own magnetic fields. • In accordance with Lenz's law, the induced magnetic field (the secondary magnetic field), opposed to the external magnetic field, is produced. • Since the induced magnetic field opposes the external magnetic field, the strength of the external magnetic field around the nucleus is then reduced. The reduction of the external magnetic field around a nucleus is called shielding. • In summary, when a compound is placed in a homogeneous magnetic field, the protons will be under the influence of different magnetic fields because of tiny magnetic fields induced when opposing the external magnetic field. • In certain cases, the proton may be under the influence of an increased magnetic field. Consequently, to fulfill the resonance condition, the strength of the external magnetic field has to be reduced. This is called deshielding. • For example, assume that a proton resonates in the presence of a magnetic field of 14,000 G. We bring this proton in an external magnetic field of 14,000 G. Now, the electrons around this proton will generate a secondary magnetic field. • Let us assume that the strength of this induced magnetic field is 50 G. The proton is no longer under the influence of a field of 14,000 G. Since the induced magnetic field opposes the external magnetic field, the proton is then under the influence of a total magnetic field of 13,950 G. • This value is not sufficient to bring the proton into resonance, which would require the strength of the external magnetic field to be increased to 14,050 Hz. • The electrons are said to shield the proton. • All protons will have different shielding, since the electron density around the protons and the chemical environments are different. Consequently, the effective magnetic field around the protons will vary from one proton to another. In accordance with the NMR equation, the protons will have different resonance frequencies. Shielding and deshielding effects cause the absorption of the protons to shift from the position at which a bare proton would absorb. • These shifts, the resonance of different protons at different frequencies, are called chemical shifts. • If two protons are located in the same chemical environment, they will be under the influence of the same magnetic field and then their resonance signals will overlap. • After the discussion of the differential shielding of individual protons in a given magnetic field, we have to define the position of a resonance signal in an NMR spectrum. Chemical shifts are measured with reference to the absorptions of protons of reference compounds. • The most generally used reference compound is tetramethylsilane (TMS). In practice, a small amount of tetramethylsilane is usually added to the sample so that a standard reference absorption line is produced. The distances between the signals of the sample and the reference signal are given in hertz (Hz). There are many reasons why tetramethylsilane is used as a reference. • 1. The TMS signal, which is at the right-hand side of the spectrum, is clearly distinguished from most other resonances. Since the carbon atom is more electronegative than silicium, the methyl groups bonded directly to the silicium atom are shielded more, and therefore resonate at the high field (on the right-hand side of the spectrum). There are also some compounds whose protons resonate on the right side of the TMS signal. These signals mostly arise from the protons located in the strong shielding area of the aromatic compounds. • 2. TMS is a cheap and readily available compound. • 3. TMS is chemically inert. There is no reaction between TMS and the sample. • 4. TMS has a low boiling point (27 °C). It can be easily removed from the sample by evaporation after the spectrum is recorded. • 5. The 12 protons of TMS produce a sharp signal. Even at lower TMS concentrations, the reference signal can be easily recognized. • The distance between the TMS signal and the sample signal is called the chemical shift of the corresponding proton. The resonance frequency of a proton depends on the strength of the magnetic field. Since many different kinds of NMR spectrometers operating at many different magnetic field strengths (1.4-21 T) are available, chemical shifts given in Hz will vary from one instrument to another. Finally, the reported chemical shifts of a given compound will vary between different research groups working with different sized NMR spectrometers. This system would lead to chaos. Therefore, it is necessary to develop a scale to express chemical shifts in a form that is independent of the strength of the external magnetic field. • defined the chemical shift as the distance between the reference signal and the sample signal: • This difference varies with the size of the operating NMR spectrometer. The frequency difference (chemical shift) A z ^will increase with the increased magnetic field strength. To remove the effect of the magnetic field on chemical shift, we divide the chemical shift (in Hz) by the frequency of the spectrometer (in Hz) and multiply it by 106: • Since the chemical shifts are always very small (typically < 6000 Hz) compared with the total field strength (commonly the equivalent of 100, 400 and 600 million Hz), the factor 106 is introduced in order to simplify the numerical values, and thus the 6-values are always given in parts per million (ppm). • Now the chemical shifts of NMR absorptions expressed in ppm or 8 units are constant, regardless of the operating frequency of the spectrometer. 常见结构单元化学位移范围 0123456789101112131415 化学位移 H C C H C O H C O OH H3C O ~3.7 H3C N ~3.0 H3C C ~2.1 O H3C C ~1.8 C H3C C ~0.9 δ(ppm)