Chapter 10 NMR Theory

1. Chapter 10 NMR Theory • Identifying Molecules • Physical and Chemical Tests • Known compounds are easy to identify by melting point, TLC, etc… • Literature values for chemical properties already exist • New compounds must be identified without this knowledge • Elemental Analysis: C 63.24% H 14.12% O 22.64 • Chemical Tests: ROH will react with Na; ROR would not • We need more informative tests • Spectroscopy = analyzing molecules based on how they absorb radiation • Electromagnetic Radiation Spectrum

2. Molecules absorb “quanta” of light energy only when the light is of the correct energy—all or nothing • DE = hn h = 6.626 x 10-27 erg.s n = frequency • Absorption causes some electronic or mechanical motion called an Excitation to occur • Different types of excitations require different energies • IR light causes excitation of vibrations • UV light excites e- from filled to unfilled orbitals • X-Rays excite e- from inner to outer shells • Radio Waves cause alignment of nuclei with magnetic fields 6) A Spectrometer records the absorption as a visual plot of what energy of light and how much of that light is absorbed

3. Proton Nuclear Magnetic Resonance Spectroscopy (radio waves) • Nuclear Spin 1) Hydrogen nuclei = 1H = proton have a nuclear spin, often represented as 2) Because of its charge (+), the spinning creates a magnetic field 3) We can analyze the proton as a tiny bar magnet 4) Without a magnetic field, a collection of protons have all possible directions of spin 5) When we expose this collection to a magnetic field (Ho), the protons align with and against the magnetic field • a spin state is aligned with the field and has lower energy • b spin state is aligned against the field and has higher energy

4. Spectroscopy requires at least 2 energy states so that absorption can occur • We must put the sample in a magnetic field to do NMR • Resonance = when the frequency of the radio waves is just right to cause absorption and excitation from a to b states. • Relaxation = loss of energy by the proton to return to a state (heat)

5. DE between a and b depends on how strong the magnetic field is. • For Ho = 21,150 G, DE = 90 MHz (90,000,000 Hz) • For Ho = 42,300 G, DE = 180 MHz • For Ho = 70,500 G, DE = 300 MHz • DE (a-b) = hn = 9 x 10-6 kcal/mol = very small amount of energy • Out of a million protons, only a few more will be a than b B. Other Nuclei and Resolution 1) 14N, 35Cl, 31P, and 13C all have nuclear spins and are NMR active • Different Nuclei absorb at different energies • High Resolution NMR looks at differences within a single nuclei type

6. NMR Instrumentation • Continuous Wave Instruments • Scan through the frequency range and are slow • Usually old, low field (90 MHz) instruments • Fourier Transform Instruments • Pulse at all frequencies at once and are fast • Requires computing to do math “Fourier Transform” for real spectra • Usually high field (200-900 MHz) supercooled electromagnets (4 K uses liquid He, surrounded by 77 K liquid N2) • Proton Chemical Shift and Structure Determination • Position of an 1H NMR peak • Electron density around the proton in question, determines how much of the magnetic field Ho is “felt” by that proton • The frequency of radio wave needed to excite that proton (thus where that proton appears on the spectrum) depends on the Ho field “felt” • Molecular Structure determines the electron density • Work backwards: spectrum to e- density to molecular structure

7. Electron Density effects on protons • Any bound 1H is surrounded by orbitals and atoms • Electron donating and electron withdrawing ability (polarity) of neighbor atoms control how much electron density each 1H is surrounded by • Electrons move to oppose Ho, creating ho = local magnetic field • Consequently, the overall magnetic field felt at each proton is reduced or shielded due to the electrons of the molecule • Adding electron density increases the shielding (need higher Ho for resonance to occur) • Removing electron density decreases shielding = deshielding (need a lower Ho for resonance to occur) • 1H NMR spectra have higher field moving to the right Downfield Upfield

8. Every chemically different proton of a molecule has a different amount of shielding leading to a unique peak on the spectrum • 2,2-dimethyl-1-propanol 1H NMR Spectrum

9. Chemical Shift • The frequency of radio waves needed for resonance of a proton varies with the size of the magnetic field (can’t compare spectra from different instruments directly) • 90 MHz magnet would give proton resonance of f • 180 MHz magnet would give 2f as the resonance frequency • Frequency is directly proportional to the size of the magnetic field • Pick a standard reference: Si(CH3)4 = tetramethylsilane (TMS) • 12 equivalent protons for a strong signal • Resonance frequency is upfield (to the right) of almost any other proton type, so it doesn’t interfere in the spectrum • Use a unit that compares how far other proton resonances are from TMS • Double the magnet size, double distance from TMS • Now, spectra from different size magnets are directly comparable • Chemical Shift (d) • Unit ppm = parts per million (Hz/MHz) • TMS assign as 0.00 ppm

10. For 2,2-dimethyl-1-propanol peaks are at 78Hz, 258Hz, 287Hz (90MHz) • Functional Groups and Chemical Shift • Each functional group has a characteristic amount of shielding or deshielding due to the atom types that make it up • This leads to a specific chemical shift for protons near or on that group • Tables 10-2 and 10-3 in your book • Alkanes or alkyl groups: lots of shielding, upfield resonances • Any electronegative atoms: deshielding, downfield resonances • The closer a –CH2- group is to an electronegative atom, the larger the shift • Effect of electronegative atoms is cumulative • ROH, RNH2, RSH chemical shifts are variable. They depend on how much hydrogen bonding occurs (solvent, temperature, concentration)