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NMR spectra of some simple molecules Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2) N S H0 Ho Because the protons have a magnetic field associated with them, the field changes as across the nmr tube. Diffusion tends to offset this field gradient Chemical Shifts Heff = The magnetic field felt at the proton Heff = Hext + Hlocal +Hp; Heff : magnetic field felt by the nuclei Hext : external magnetic field Hlocal: local field induced by the external field Hlocal: Electrons in a chemical bond are considered to be in motion and are charged. This induces a local magnetic field which can shield (oppose) or deshield (enhance) the magnetic field experienced by the nucleus. Since the precessional frequency of the nucleus is governed by Heff, changes in this field as a result of local fields caused by bonding electrons, the resonance frequency of magnetically and chemically non-equivalent nuclei differ resulting in slightly different values of . This is the origin of the chemical shift. The local magnetic field is induced by the external field and is directly proportional to the external field Hlocal : the effect of the external magnetic field on the bonding electrons depends on electron density and molecular structure. Hlocal is directly proportional to Hext Remember H is a vector. This property has both magnitude and direction δ = ppm; ppm = part/million Intensity Increasing frequency 0 10 9 8 7 6 5 4 3 2 1 0 ppm TMS Typical chemical shifts for protons: 0 –10 ppm In a 300 MHz instrument, differences in range about 3000 Hz (3000 Hz shifts relative to a total of 300*106 cycles /sec) In a 600 MHz instrument, differences in range about 6000 tau = 10-δ Intensity aromatic CH -CH= CH2 CH3 0 10 9 8 7 6 5 4 3 2 1 0 ppm Typical chemical shifts for protons: 0 –10 ppm Intensity Typical range of chemical shifts for 13 C 0 2 0 0 1 6 0 1 2 0 8 0 4 0 0 p p m TMS >C=C< CR4 >C=O aromatic Intensity CHR3 R2CH2 CH3 0 2 0 0 1 6 0 1 2 0 8 0 p p m Typical chemical shifts for 13C: 0 to 220 ppm 4 0 0 Common terms used in NMR (terms originating from use of CW instruments) Shielded: the induced local field opposes the external field Upfield shift: shift toward lower frequency; higher magnetic field, lower energy Deshielded: the induced local field field augments the external field Downfield shift: shift toward higher frequency; lower magnetic field higher energy Intensity increasing frequency, increasing energy increasing magnetic field 0 10 9 8 7 6 5 ppm 4 3 2 1 0 Sigma bonds electron cloud Field due to circulating e- Hexternal field nucleus Field felt by the nucleus Heff = Hext - Hlocal For resonance either Hext must be increased or decreased relative to the situation where Hlocal = 0 π bonds in acetylenes Hext Hlocal H H O shielding cone π bonds in alkenes Hlocal and aldehydes Hext deshielding region π bonds in aromatic compounds Hext Hlocal H Field felt by the nucleus Heff = Hext + Hlocal For resonance either Hext must be decreased or increased relative to the situation where Hlocal = 0 Hext H -3.0 H 0.3 H CH2 H H H H 9.3 An Example of A Simple Spectrum Area: 9:1:2 Information from NMR 1. chemical shift 2. area 3. multiplicity Other Factors Influencing Hlocal Hlocal is influenced by all local fields; the field effect of the bonding electrons results in the chemical shift, a relatively small perturbation Hlocal is induced by the external field and depends on its magnitude What about the field effects due to the local protons Hp? Suppose we have two identical protons attached to the same carbon. What are the possible spin states of this system and how do they effect the local magnetic field? Nomenclature used to describe spin-spin coupling First Order Spectra: Chemical shift difference ∆ > 10 J AX ; A2X; A3X; AMX; A3MX; A3M2X; … J is a measure of the effective magnetic field of neighboring protons. The effect is generally considered to be transmitted through chemical bonds and not through space Non-first Order Spectra: Chemical shift ∆ < 10 J AB ; A2B; A3B; ABC; A3CB; A3B2X; A3B2C … A2 Case, J = 0 H-C-C-C-C-H All transitions are at υA Energy or H Remember: Ne/Ng = e-H/RT 1 A2 Case H-C-H For positive J +J/4 A +J/4 -3J/4 A +J/4 J =0 No H – H interaction: J = 0 H – H interaction A2 Case For negative J -J/4 A +3J/4 -J/4 A -J/4 No H – H interaction H – H interaction J =0 AX; X > A J=0 A Relative ordering of energy levels without AX interactions X X Energy A A X Both opposed to magnetic field AX; X > A J>0 J=0 X +J/2 +J/4 A + J/2 -J/4 Relative ordering of energy levels with AX interactions X -J/2 -J/4 A X A – J/2 +J/4 Both opposed to magnetic field For positive J In the absence of coupling, ie J = 0 Intensity In the presence of coupling, ie J ≠ 0 J X 0 10 9 8 7 A 6 5 ppm 4 3 2 J 1 0 AX; X > A -J/4 A – J/2 +J/4 X -J/2 Relative ordering of energy levels with AX interactions A X Both opposed to magnetic field X +J/2 +J/4 A + J/2 For negative J -J/4 Intensity X A J J 0 10 9 8 7 6 5 ppm 4 3 2 1 0 A2X X > A No AX interaction, JAA ≠ 0 A2 X A2X X > A A +J/2 X+J/2 0 A -J/2 No AX interaction X X X-J/2 A -J/2 0 A+J/2 A2 X For positive JAX X A2X X > A A +J/2 0 A+J/2 A -J/2 AX interaction Note that the A transitions are twice as intense A -J/2 0 A+J/2 A2 X A+J/2 A-J/2 A-J/2 J=0 For positive J No A2X coupling A2X coupling X A The 2nS +1 Rule The number of lines observed for a particular nucleus as a result of n “identical” neighbors is 2nS + 1 where S is the spin of the neighboring nucleus. For most nucleus, S = ½, the relationship simplifies to n+1 lines “identical” in this context refers to nuclei that have the same or very similar coupling constants to the nucleus being observed. number of “identical neighbors” multiplicity of nucleus observed 1 2 (1:1) 2 3 (1:2:1) 3 4 (1:3:3:1) 4 5 (1:4:6:4:1) 5 6 (1:5:10:10:5:1) Examples of First Order Spectra H OH C CH3 CH3 CH3CH2OH What information do you get out of a 1H NMR spectrum? Chemical Shift? An indication of the type of proton and its environment Multiplicity? An indication of the number of nearest neighbors and their proximity Area? A measure of the relative number of hydrogen nuclei in the molecule The compound has a IR frequency of 1720 cm-1 and a molecular formula of C4H8O. What is its structure? O CH3 C CH2 CH3 3 3 2 CH3 CH3 CH2 O O CH2 O C C C O CH2 CH3 O CH2 CH3 H CH3 CH3 CH2 O O CH2 O C C CH2 CH3 C geminal 2J vicinal 3J 4J 5J Magnitude of the Vicinal Coupling Constant J Karplus Equation 3J CHCH H H = 10 cos2(φ) where φ is the dihedral angle Summary of the Field Dependence of and J is the local field that is induced by the magnitude of the external field, Ho. is therefore chemical shift dependent. J is dependent on the magnetic moment of the proton and is therefore independent of the external field, Ho. Effect of Magnetic field strength on 1H NMR Spectra Raccoon H5 H3 H2 60 MHz, 600 Mz H4 H1 H1= H2 = H3 1.0 J12 = -10; J13 = -10; J23 = -10 H4 = H5 = 1.5 J14 = 7; J 15 = 7; J4,5 = -12 Effect of Magnetic field strength on 1H NMR Spectra H1 Raccoon CN 60 MHz, 600 Mz H2 H3 H1= 8.0 J12 = 8; J13 = 17; J23 = -6 H2 = 8.6 J H3 = 8.9