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Requirements

Nuclear Magnetic Resonance. Requirements Molecules must contain nuclei that have a nuclear angular momentum (or nuclear spin) quantum number I >0 which results in ( 2 I + 1 ) magnetic quantum numbers (states) m = + I , I -1, I -2, …, - I +1,- I .

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Requirements

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  1. Nuclear Magnetic Resonance Requirements Molecules must contain nuclei that have a nuclear angular momentum (or nuclear spin) quantum number I>0 which results in (2I + 1) magnetic quantum numbers (states) m = + I, I -1, I -2, …, - I +1,- I. All nuclei with odd mass, odd atomic number or both are NMR active: I = ½for 11H,136C, 199F, 3115P and I = 1for 21H, 147N For NMR measurements magnetic nuclei must be in a magnetic field as the eigen states of m are degenerated in an non-magnetic environment. hn = DE = g(h/2p)Bo DE is the energy difference between the generated states and depends on the applied external magnetic field Bo and the nucleus specific gyro-magnetic ratio g. The energy differences, however, are very small for all nuclei and electromagnetic radiation of radio frequency is sufficient (5-100 MHz at a field of 2.3 T).

  2. Sample Preparation and Measurement The resonance frequency no for protons in 1H-NMR are at 60 MHz in a 1.41 T magnetic field (100 MHz at 2.34 T, 300 MHz at 7.05 T, and 500 MHz at 11.7 T. The sample is dissolved in a solvent that does not contain the magnetic nucleus of interest and this is why deuterated solvents are used in 1H-NMR: e.g.C6D6, CDCl3,or CCl4. An NMR tube of 5 mm diameter is typically filled with 2 mg of compound dissolved in 0.5 mL of solvent (1H-NMR) and spun in the magnetic field to average out field inhomogeneities.

  3. Chemical Shift NMR would be useless if all protons in a sample had the same resonance frequency. Fortunately, the resonance frequency is slightly varied by the chemical environment of each proton. The frequency difference n of a nucleus is measured relative to a standard (tetramethylsilane (TMS) for 1H- and 13C-NMR) and have been named chemical shift d. d is given in ppm. d(X) = 106Dn/n with d(TMS) = 0 Example: The protons of a methyl group have a resonance frequency of 126 Hz lower than TMS at an observation frequency of 60 MHz. dH(CH3) = 106 (126/60*106) = 2.10 ppm

  4. NMR Scale The advantage of using the dimensionless ppm unit is that the scale is independent of the external magnetic field. Example for 1H-NMR with TMS as reference and 300 MHz resonance frequency. 3000 Hz 0 Hz 0 ppm 10 ppm Field Strength Effect 60 MHz 300 MHz

  5. NMR Scales (1H-NMR) 0 Hz 0 ppm 3000 Hz 10 ppm 300 MHz, 7.05 T (downfield) higher frequency-less shielded (upfield) lower frequency-more shielded 0 Hz 0 ppm 6000 Hz 10 ppm 600 MHz, 14.1 T

  6. Chemical Shift and Molecular Structure The resonance position of a given nucleus is determined by its shielding constant s. s is made up of four terms, the diamagnetic shielding sdia, the paramagnetic shielding spara, shielding due to neighbouring groups sintra, and shielding due to intermolecular effects sintra. s can possess positive and negative values. neff = (g/2p)Bo(1-s) Shielding and deshielding effects are produced by local magnetic fields generated by circulating electron densities. Thus, changes in the local electron density will influence the chemical shift. A positive value of d is left of TMS, which as a high electron density around its protons, and means the nuclei are deshielded relative to TMS. For example, we can deduced from the increasing ppm values in 1H-NMR given below that C is more electronegative than H: R3CH>R2CH2>RCH3>CH4 1.6 1.2 0.8

  7. Chemical Shift of 1H DE = hn0 = gB0h/2p, n0 = gB0/2p, with Blocal = Bo(1-s) s is the electronic shielding of a specific proton and its resonance frequency with shielding is termed chemical shift d; n0 = gB0(1-s)/2p An increase in d means a 1H nucleus is deshielded relative to TMS and vice versa; As d depends on electron density around nuclei, the electronegativity of atoms close to the nuclei will affect the chemical shifts; d for H in H3CX with X = F, HO, H2N, H, Me3Si, or Li are 4.26, 3.38, 2.47, 0.23, 0.0, and -0.4 (very solvent depending), respectively;

  8. The electron density and, consequently, d are not only influenced by the inductive effect but also by resonance; Hybridization of the carbon to which a proton is attached also influences the electron density around the proton; As the proportion of s-character increases form sp3 to sp hybridization, bonding electron move closer to the carbon and the proton becomes more deshielded;

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