《仪器——课件HNMR》由会员分享，可在线阅读，更多相关《仪器——课件HNMR（117页珍藏版）》请在金锄头文库上搜索。
1、Proton Nuclear Magnetic Resonance Spectrometry (1H NMR),Typical 1H NMR Spectra,sixtet,quintet,Parameters: chemical shift area of peaks coupling constant,Introduction,Magnetic Properties of Nuclei,nuclear magnetic moment spin quantum number I,Even mass:,# protons & # Neutrons Both Even : I=0 (4He, 12
2、C ) No spin,# Protons & # Neutrons Both Odd : I=1, 2, . (Integer),Odd mass:,# protons odd & # Neutrons Even : I=1/2, 3/2, . (Half-integer),# protons even & # Neutrons odd : I=1/2, 3/2, . (Half-integer),1H and 13C both have spin = 1/2 1H is 99% at natural abundance 13C is 1.1% at natural abundance,Th
3、e nuclei that are most useful to organic chemists are:,+,The distribution of nuclear spins is random in the absence of an external magnetic field.,+,An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field.,H0,Directional Quantization,If a nucleu
4、s with Magnetic Moment m is placed in a magnetic field B0. The direction of spinning nuclei is quantized and described with m （magnetic quantum number）,m is the magnetic or directional quantum number it can take values: m = I , I-1 , . , -I,There are then 2I + 1 orientations in the field 1H, 13C 2 d
5、irections,+,There is a slight excess of nuclear magnetic moments aligned parallel to the applied field.,H0,Energy Level and Resonant Transitions,For spin I = the energy gap between the 2 levels is:,n = g B0 /2p,m = + ,m = - ,B0,DE = g B0 = h n (g magnetogyric ratio）,The frequency of absorbed electro
6、magnetic radiation is proportional to the energy difference between two nuclear spin states which is proportional to the applied magnetic field,Some Important Relationships in NMR,DE = g B0 = h n,n = g B0 /2p,Methods to Realize NMR,Continuous-Wave (CW) NMR Spectrometer: The sample is held in a stron
7、g magnetic field, and the frequency of the source is slowly scanned (in some instruments, the source frequency is held constant, and the field is scanned). Fourier Transform (FT) NMR instrument: All frequencies in a spectrum are irradiated simultaneously with a radio frequency pulse,The frequency of
8、 absorbed electromagnetic radiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T: 1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1) 13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1),Some Import
9、ant Relationships in NMR,在核磁共振中， DE很小，自发辐射的几率几乎为零。因此若要能在一定的时间间隔内持续检测到核磁共振信号，必须有某种过程存在，使高能级的原子核能够回到低能级，以保持低能级分布居数始终略大于高能级分布居数，这个过程就称为弛豫。,Relaxation （弛豫）,The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment. This is why NMR is s
10、uch a useful tool for structure determination.,Chemical Shift,Shielding,= g B0 (1-)/2p ：shielding constant B0 ,Chemical shift (, ppm) measured relative to TMS,Upfield Increased shielding,Downfield Decreased shielding,(CH3)4Si (TMS),B0,Chemical shift is a measure of the degree to which a nucleus in a
11、 molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts.,Chemical Shift,Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard.,Chemical Shift,In 1H, 13C: ref. is TMS (tetrameth
12、ylsilane),d is dimensionless (independent from the applied field) n changes with the applied field,Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard.,Chemical Shift,In 1H, 13C: ref. is TMS (tetramethylsilane),d is dimensionless (independent from the ap
13、plied field) n changes with the applied field,(1)Its protons are more shielded than most all organic protons (2) It is structurally symmetrical and gives a single, intense, sharp absorption. (3) It is chemically inert, volatile and soluble in most organic solvents.,Reasons to Choose TMS as a Standar
14、d Compound,Effects of Field Strength on Signal Resolution,Dispersion or separation of signals is increased with increasing Magnetic field strength. The chemical shift in ppm is constant, But the value in Hz is increased.,Factors Influencing Chemical Shift,Electronegativity of Neighboring Groups,Elec
15、tronegative substituents decrease the shielding of methyl groups,Electronegative substituents decrease shielding,O2NCH2CH2CH3,d 1.0,d 4.3,d 2.0,CHCl3 7.3 CH2Cl2 5.3 CH3Cl 3.1,Effect is cumulative,CH3 more shielded than CH2 ; CH2 more shielded than CH,Methyl, Methylene, and Methine,Protons attached t
16、o sp2 hybridized carbon are less shielded than those attached to sp3 hybridized carbon,CH3CH3, 7.3, 5.3, 0.9,But protons attached to sp hybridized carbon are more shielded than those attached to sp2 hybridized carbon,But protons attached to sp hybridized carbon are more shielded than those attached to sp2 hybridized carbon,Prot
