Nuclear Magnetic Resonance - Organic Chemistry - Lecture Slides, Slides of Organic Chemistry

Download Nuclear Magnetic Resonance - Organic Chemistry - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Nuclear Magnetic Resonance (NMR) A structure determination tool docsity.com Spectroscopy (General) • Spectroscopy is the study of the interaction between energy and matter • Different energy types are used in different types of spectroscopy Low energy- stimulates vibrations Moderate energy- stimulates electronic transitions Lower energy- stimulates nuclear spins Figure 9.2 from text docsity.com NMR Background III • The alignments do not have the same energy, and radio frequency energy can be used to probe the energy difference • Different magnet strengths, however, change the energy difference • Therefore a normalized scale (ppm) is used Figure 9.9 from text docsity.com NMR Spectral Interpretation • 4 features of an NMR spectrum provide information about structure – Number of peaks – Position of peaks (chemical shift) – Relative peak areas – Peak splitting docsity.com NMR Spectra (Example) 1H peaks occur in range from 0-12ppm 13C peaks occur in range from 0-200ppm This spectra is a 1H spectra with: 3 peaks @ 1.0 ppm: split into triplet 1.9 ppm: split into multiplet 3.4 ppm: split into triplet Relative areas not provided although 1.0 ppm peak > 3.4 ppm peak docsity.com Chemical Shifts to Know • Terminal alkane: ~1ppm • Mid-alkane: slightly > 1ppm • Near electronegative atom: depends on atom but ~ 3.5ppm • Alkene (vinyl) protons: ~5ppm • Aromatic protons: ~7ppm Relate protons in other environments to the most relevant of these! docsity.com H3C C CH3 CH3 CH3 Relative Peak Areas • Relative (but not absolute) peak areas relate to the number of nuclei contributing to the signal HC HC C H CH CH C CH3 Integrated areas will have 5:3 ratio You only know all nuclei are the same docsity.com Peak Splitting • Since spinning nuclei generate magnetic fields, neighboring nuclei of different spins result in different energy absorbances • Splittinga = (nb+1)(nc+1)(nd+1)… docsity.com Problem 1 • What features do you expect to see in the 1H NMR spectrum for 2-bromobutane? docsity.com Problem 2 • What structure do you think gave the following spectra? (The compound gives a broad absorbance @ 3245 cm-1 in its IR spectra) Relative peak areas are shown above each peak. (2) (1) (3) docsity.com Problem 3 • Two structures with formulas C3H8O gave the spectra on the following slides (IR, 1H NMR, 13C NMR) • Determine their structures, their IUPAC names, and their structural relationship docsity.com No peaks at >100ppm Structure Structure A B T T T T T 80 60 40 20 a 100 80 60 40 20 o pam docsity.com Problem 4 • A structure with MW=74 gives the 1H NMR spectrum to the right (no peaks above 6ppm). IR shows no absorbance above 3000 cm-1 • Draw and name the structure (2) (3) docsity.com