NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY … or NMR for short

13 C – nmr 13 C ~ 1.1% of carbons 1)number of signals: how many different types of carbons 2)splitting: number of hydrogens on the carbon 3)chemical shift: hybridization of carbon sp, sp 2, sp 3 4)chemical shift: environment

Nuclear Magnetic Resonance (nmr) -the nuclei of some atoms spin: 1 H, 13 C, 19 F, … -the nuclei of many atoms do not spin: 2 H, 12 C, 16 O, … -moving charged particles generate a magnetic field (  ) -when placed between the poles of a powerful magnet, spinning nuclei will align with or against the applied field creating an energy difference. Using a fixed radio frequency, the magnetic field is changed until the ΔE = E EM. When the energies match, the nuclei can change spin states (resonate) and give off a magnetic signal. ΔE

magnetic field = 14,092 gauss for 1 H v = 60,000,000 Hz (60 MHz) nmr spectrum intensity chemical shift (ppm) magnetic field 

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=h. Application of just the right radiofrequency (  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.

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

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  (Greek letter delta) units. For 1 H, the usual scale of NMR spectra is 0 to 10 (or 12) ppm (or  ). The usual 13 C 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.

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 ppm apart can be distinguished; this allows for over 2x10 5 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).

13 C CHEMICAL SHIFTS

PREDICTING 13 C SPECTRA Problem 13.6 Predict the number of carbon resonance lines in the 13 C spectra of the following (= # unique Cs): 4 lines plane of symmetry

PREDICTING 13 C SPECTRA Predicte the number of carbon resonance lines in the 13 C spectra of the major product of the following reaction: 7 lines 5 lines plane of symmetry

PREDICTING 13 C SPECTRA Predicte the number of carbon resonance lines in the 13 C spectra of the major product of the following reaction: 7 lines 5 lines plane of symmetry

2-bromobutane a c d b CH 3 CH 2 CHCH 3 Br 13 C-nmr