Nuclear Magnetic Resonance (NMR) Spectroscopy

1. Nuclear Magnetic Resonance (NMR) Spectroscopy Part 1 Carbon 13 NMR

2. Theory of NMR • The positively charged nuclei of certain elements (e.g., 13C and 1H) behave as tiny magnets. • In the presence of a strong external magnetic field (Bo), these nuclear magnets align either with ( ) the applied field or opposed to ( ) the applied field. • The latter (opposed) is slightly higher in energy than aligned with the field. DE is very small

3. Theory of NMR • The small energy difference between the two alignments of magnetic spin corresponds to the energy of radio waves according to Einstein’s equation E=hn. • Application of just the right radiofrequency (n) causes the nucleus to “flip” to the higher energy spin state • Not all nuclei require the same amount of energy for the quantized spin ‘flip’ to take place. • The exact amount of energy required depends on the chemical identity (H, C, or other element) and the chemical environment of the particular nucleus.

4. Theory of NMR • Our department’s NMR spectrometer (in Dobo 245) has a superconducting magnet with a field strength of 9.4 Tesla. On this instrument, 1H nuclei absorb (resonate) near a radiofrequency of 400 MHz; 13C nuclei absorb around 100 MHz. • Nuclei are surrounded by electrons. The strong applied magnetic field (Bo) induces the electrons to circulate around the nucleus (left hand rule). (9.4 T)

5. Theory of NMR • The induced circulation of electrons sets up a secondary (induced) magnetic field (Bi) that opposes the applied field (Bo) at the nucleus (right hand rule). • We say that nuclei are shielded from the full applied magnetic field by the surrounding electrons because the secondary field diminishes the field at the nuclei.

6. Theory of NMR • The electron density surrounding a given nucleus depends on the electronegativity of the attached atoms. • The more electronegative the attached atoms, the less the electron density around the nucleus in question. • We say that that nucleus is less shielded, or is deshielded by the electronegative atoms. • Deshielding effects are generally additive. That is, two highly electronegative atoms (2 Cl atoms, for example) would cause more deshielding than only 1 Cl atom. C and H are deshielded C and H are more deshielded

7. Chemical Shift • We call the relative position of absorption in the NMR spectrum (which is related to the amount of deshielding) the chemical shift. It is a unitless number (actually a ratio, in which the units cancel), but we assign ‘units’ of ppm or d (Greek letter delta) units. • For 1H, the usual scale of NMR spectra is 0 to 10 (or 12) ppm (or d). • The usual 13C scale goes from 0 to about 220 ppm. • The zero point is defined as the position of absorption of a standard, tetramethylsilane (TMS): • This standard has only one type of C and only one type of H.

8. Chemical Shifts

9. Chemical Shifts • Both 1H and 13C Chemical shifts are related to three major factors: • The hybridization (of carbon) • Presence of electronegative atoms or electron attracting groups • The degree of substitution (1º, 2º or 3º). These latter effects are most important in 13C NMR, and in that context are usually called ‘steric’ effects. • First we’ll focus on Carbon NMR spectra (they are simpler)

10. CMR Spectra • Each unique C in a structure gives a single peak in the spectrum; there is rarely any overlap. • The carbon spectrum spans over 200 ppm; chemical shifts only 0.001 ppm apart can be distinguished; this allows for over 2x105 possible chemical shifts for carbon. • The intensity (size) of each peak is NOT directly related to the number of that type of carbon. Other factors contribute to the size of a peak: • Peaks from carbon atoms that have attached hydrogen atoms are bigger than those that don’t have hydrogens attached. • Carbon chemical shifts are usually reported as downfield from the carbon signal of tetramethylsilane (TMS).

11. 13C Chemical Shifts

12. Predicting 13C Spectra • Problem 13.6 Predict the number of carbon resonance lines in the 13C spectra of the following (= # unique Cs): 4 lines plane of symmetry

13. Predicting 13C Spectra • Predicte the number of carbon resonance lines in the 13C spectra of the major product of the following reaction: 7 lines 5 lines plane of symmetry

14. Predicting 13C Spectra

19. C6H12O2