1. Using NMR Spectra to Analyze Molecular Structure 10-4 The position of an NMR absorption of a nucleus is called its chemical shift. Chemical shifts depend upon the electron density around a nucleus and are thus controlled by the structural environment of the nucleus. The NMR chemical shifts provide important clues for determining the molecular structure of a chemical compound.

2. The position of an NMR signal depends on the electronic environment of the nucleus. In the high-resolution 1 H NMR spectrum of chloro(methoxy)methane above, two separate resonance absorptions of hydrogen are observed. These absorptions reflect the differing electronic environments of the two types of hydrogen nuclei present. Electrons in the bonds connecting the hydrogen atoms to the molecule affect the NMR absorptions.

3. Bound hydrogens are connected to a molecule by orbitals whose electron density varies: Bond polarity Hybridization of the attached atom Presence of electron withdrawing/donating groups The electrons in these orbitals are affected by the external magnetic field, H 0, in such a way as to generate a small local magnetic field, h local, opposing H 0. The total magnetic field seen by the hydrogen nucleus is the sum of these two fields and is thus reduced. The hydrogen nucleus is said to be shielded from H 0 by its electron cloud.

4. The degree of shielding of a nucleus depends upon its surrounding electron density. Adding electrons increases shielding. Removing electrons causes deshielding. Shielding causes a displacement of an NMR peak to the right in the spectrum (shifted upfield). Deshielding causes a displacement to the left (shifted downfield).

5. Chemically equivalent hydrogens in a molecule all have identical electronic environments and therefore show NMR peaks at the same position. In the NMR spectrum of 2,2-dimethyl-a-propanol, there are three different peaks due to absorptions by: Nine equivalent methyl hydrogens on the butyl group (most shielded); One hydrogen on the OH; Two equivalent methylene hydrogens.

6. The chemical shift describes the position of an NMR peak. Rather than reporting the exact frequency of each resonance in an NMR spectrum, we measure frequencies relative to an internal standard, tetramethylsilane, (CH 3 ) 4 Si. To remove the effect of differing applied magnetic fields using different spectrophotomers, the frequencies relative to tetramethylsilane are divided by the frequency of the spectrometer. This yields the chemical shift (δ), a field-independent number measured in ppm.

7. For (CH 3 ) 4 Si, δ is defined as 0.00. The spectrum above would be reported as: 1 H NMR (300 MHz, CDCl 3 ) δ = 0.89, 1.80, 3.26 ppm

8. Functional groups cause characteristic chemical shifts. Each type of hydrogen in a molecule has a chemical shift which depends upon its chemical environment.

9. The absorptions of alkane hydrogens occur at relatively high field. Hydrogens close to an electron withdrawing group (halogen or oxygen) are shifted to relatively lower field (deshielding). The more electronegative the atom, the more the deshielded methyl hydrogens are relative to methane.

10. Multiple substituents exert a cumulative effect.

11. The deshielding influence of electron withdrawing groups diminishes rapidly with distance.

12. Hydroxy, mercapto, and amino hydrogens absorb over a range of frequencies. The absorption peak of the proton attached to the heteroatom may be relatively broad. This variability of chemical shift is due to hydrogen bonding and depends upon: Temperature; Concentration; Presence of H-bonding species such as water (moisture). When line broadening is observed, it usually indicates the presence of OH, SH, or NH 2 (NHR) groups.

13. Tests for Chemical Equivalence 10-5 In general, chemically equivalent protons have the same chemical shift. To identify chemically equivalent nuclei, we often have to resort to symmetry operations to decide on the expected NMR spectrum for a compound.

14. Tests for Chemical Equivalence 10-5 Molecular symmetry helps establish chemical equivalence. Rotational symmetry results in equivalent protons when the group of protons is rapidly rotating, as in a methyl group.

15. Conformational interconversion may result in equivalence on the NMR time scale. In the case of the rapid rotation of the methyl group in chloroethane, or the rapid conformation flip in cyclohexane, the observed chemical shifts are the averages of the values that would be observed without the rapid rotation or flip.

16. In the case of cyclohexane, the single line in the NMR spectra at δ = 1.36 ppm at room temperature becomes two lines at a temperature of -90 o C, one at δ = 1.12 ppm for the six axial hydrogens and one at δ = 1.60 for the six equatorial hydrogens. At this temperature, the conformational flip of the benzene is slower than the NMR time scale. In general, the lifetime of a molecule in an equilibrium must be on the order of one second to allow its resolution by NMR.