NMR spectra of some simple molecules Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)
N S H 0
H o 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 H eff = The magnetic field felt at the proton H eff = H ext + H local + H p ; H eff : magnetic field felt by the nuclei H ext : external magnetic field H local : local field induced by the external field H local : 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 H eff, 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
H local : the effect of the external magnetic field on the bonding electrons depends on electron density and molecular structure. H local is directly proportional to H ext Remember H is a vector. This property has both magnitude and direction
δ = ppm; ppm = part/million Increasing frequency Intensity 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*10 6 cycles /sec) In a 600 MHz instrument, differences in range about 6000
tau = 10-δ
aromatic Intensity -CH= CH CH 2 CH 3 0 10 9 8 7 6 5 4 3 2 1 0 ppm Typical chemical shifts for protons: 0 10 ppm
Typical range of chemical shifts for 13 C Intensity 0 2 0 0 1 6 0 1 2 0 8 0 4 0 0 ppm TMS
>C=O aromatic >C=C< CR 4 CHR3 Intensity R 2 CH 2 CH 3 0 2 0 0 1 6 0 1 2 0 8 0 4 0 0 p p m Typical chemical shifts for 13 C: 0 to 220 ppm
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
increasing frequency, increasing energy Intensity increasing magnetic field 0 10 9 8 7 6 5 4 3 2 1 0 ppm
Sigma bonds electron cloud Field due to circulating e - nucleus H external field Field felt by the nucleus H eff = H ext -H local For resonance either H ext must be increased or decreased relative to the situation where H local = 0
π bonds in acetylenes H ext H local H H
O shielding cone π bonds in alkenes and aldehydes H local deshielding region H ext
π bonds in aromatic compounds H local H ext H Field felt by the nucleus H eff = H ext + H local For resonance either H ext must be decreased or increased relative to the situation where H local = 0
H ext H H -3.0 H 9.3 0.3 H H H H CH 2
An Example of A Simple Spectrum Area: 9:1:2 Information from NMR 1. chemical shift 2. area 3. multiplicity
Other Factors Influencing H local H local is influenced by all local fields; the field effect of the bonding electrons results in the chemical shift, a relatively small perturbation H local is induced by the external field and depends on its magnitude What about the field effects due to the local protons H p? 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 ; A 2 X; A 3 X; AMX; A 3 MX; A 3 M 2 X; 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 ; A 2 B; A 3 B; ABC; A 3 CB; A 3 B 2 X; A 3 B 2 C
A 2 Case, J = 0 H-C-C-C-C-H All transitions are at υ A Energy or H Remember: N e /N g = e - H/RT 1
A 2 Case H-C-H For positive J +J/4 A A +J/4-3J/4 +J/4 J = 0 No H H interaction: J = 0 H H interaction
A 2 Case For negative J -J/4 A +3J/4 -J/4 No H H interaction A -J/4 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 +J/4 X +J/2 A + J/2 -J/4 Relative ordering of energy levels with AX interactions X -J/2 -J/4 A X Both opposed to magnetic field For positive J A J/2 +J/4
In the absence of coupling, ie J = 0 In the presence of coupling, ie J 0 Intensity 0 X J A J 10 9 8 7 6 5 4 3 2 1 0 ppm
AX; X > A -J/4 A J/2 +J/4 Relative ordering of energy levels with AX interactions X -J/2 X +J/2 +J/4 A X Both opposed to magnetic field A + J/2 For negative J -J/4
Intensity X A J J 0 10 9 8 7 6 5 4 3 2 1 0 ppm
A 2 X X > A No AX interaction, J AA 0 A 2 X
A 2 X X > A A +J/2 0 X +J/2 A -J/2 No AX interaction X X -J/2 X X A -J/2 0 A +J/2 A 2 X For positive J AX
A 2 X X > A A +J/2 A -J/2 0 A +J/2 A +J/2 AX interaction Note that the A transitions are twice as intense A 2 X A -J/2 0 A +J/2 A -J/2 A -J/2 J = 0 For positive J
No A 2 X coupling A 2 X 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 C OH CH 3 CH 3
CH 3 CH 2 OH
What information do you get out of a 1 H 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 C 4 H 8 O. What is its structure? O CH 3 C CH 2 CH 3 3 3 2
CH 3 O CH 2 O CH 3 CH 2 O C C C O CH 2 CH 3 H CH 3 O CH 2 O CH 3 CH 2 O C C C O CH 2 CH 3 CH 2 CH 3
geminal 2 J vicinal 3 J 4 J 5 J
Magnitude of the Vicinal Coupling Constant J Karplus Equation H H 3 J CHCH = 10 cos 2 (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, H o. is therefore chemical shift dependent. J is dependent on the magnetic moment of the proton and is therefore independent of the external field, H o.
Effect of Magnetic field strength on 1 H NMR Spectra Raccoon H 5 H3 H2 60 MHz, 600 Mz H 4 H 1 H 1 = H 2 = H 3 1.0 J 12 = -10; J 13 = -10; J 23 = -10 H 4 = H 5 = 1.5 J 14 = 7; J 15 = 7; J 4,5 = -12
Effect of Magnetic field strength on 1 H NMR Spectra Raccoon H 1 CN 60 MHz, 600 Mz H 2 H 3 H 1 = 8.0 J 12 = 8; J 13 = 17; J 23 = -6 H 2 = 8.6 J H 3 = 8.9