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

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

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.

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)

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.

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 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.

Chemical Shifts

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)

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).

13C Chemical Shifts

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

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

Predicting 13C Spectra

C6H12O2