Chemistry 125 Lecture 59 March 22 2010 NMR

Chemistry 125: Lecture 59 March 22, 2010 NMR Spectroscopy Chemical Shift and Spin-Spin Coupling This For copyright notice see final page of this file

NMR: locating protons within molecules using uniform field ?

Fractional difference in applied field 0. 00000248 ! Listen at fixed frequency. Tune H to “hear” precession. 2. 48 ppm Requires very high uniformity of field to avoid “MRI” The “Chemical” Shift HO-CH 2 -CH 3 http: //www. wooster. edu/chemistry/is/brubaker/nmr H Oscilliscope Trace (1951)

Dr. Lauterbur became interested in possible biological applications of nuclear magnetic resonance after reading a paper in 1971 by Raymond V. Damadian, who described how some cancerous tissues responded differently to the magnetic fields than normal tissue. Until then, most scientists placed the samples in a uniform magnetic field, and the radio signals emanated from the entire sample. Dr. Lauterbur realized that if a non-uniform magnetic field were used, then the radio signals would come from just one slice of the sample, allowing a two-dimensional image to be created. i. e. one particular frequency The nuclear magnetic resonance machine at SUNY was shared among the chemistry professors, and the other professors needed to perform their measurements in a uniform magnetic field. Dr. Lauterbur had to conduct his work at night, returning the machine to its original settings each morning.

Some of the Magnetic Resonance Spectrometers in Yale's have put classical structure proof Chemistry Department by chemical transformation (and even IR!) out of business. One Yale “natural products” organic professor, whose research used chemical transformations to determine molecular structures, abandoned organic chemistry to take up fundamental research on quantum theory (and later became a professional studio photographer).

500 MHz

500 MHz

600 MHz

600 MHz

800 MHz * 3 ~8 = 512 times as sensitive as 100 MHz (not to mention the chemical shift advantage discussed below) * 1) Boltzmann factor 2) Energy quantum 3) Electronics sensitivity

EPR (Electron Paramagnetic Resonance) (for Free Radicals with SOMOs) e magnet is 660 x H+!

EPR (Electron Paramagnetic Resonance) 9 GHz ~3000 Gauss (0. 3 Tesla)

New 900 MHz (21 Tesla) NMR spectrometers NHFML - Florida State University Varian Associates NHFML now has a pulsed field NMR at 45 Tesla (there is no charge for use, but you have to have a great experiment

Which peak is which set of protons? 1 2 Area (integral) 3 HO-CH 2 -CH 3 http: //www. wooster. edu/chemistry/is/brubaker/nmr number of protons, because they are so similar (not like IR) Oscilliscope Trace (1951)

O O O ? O O O 3: 1 1) O 3 2) H 2 O 2 ? Structural proof by chemical HO-C C-OH degradation 1955 Advertisement O O cis-caronic acid (venerable) C H 2. 9 1 H C http: //www. wooster. edu/chemistry/is/brubaker/nmr

Higher Resolution Shows Splitting Advantage of “similarity” of protons (unlike IR where various modes have very different changes in dipole moment, and thus very different signal strengths) 1959 http: //www. wooster. edu/chemistry/is/brubaker/nmr

Ethyl Acetate averages field inhomogeneities 1959

A 90° pulse makes spinning nuclei (1 H, 13 C) “broadcast” a frequency that tells their LOCAL magnetic field.

Components of Effective Magnetic Field. Bmolecular (diamagnetic) Applied Field: Inhomogeneous ~ 30, 000 G for MRI CAT scan. (4 G/cm for humans, 50 G/cm for small animals) Bapplied Homogeneous for Chemical NMR Spectroscopy Beffective (spin sample) Molecular Field: Net electron orbiting - “Chemical Shift” (Range ~12 ppm for 1 H, ~ 200 ppm for 13 C) Nearby magnetic nuclei - “Spin-Spin Splitting” (In solution JHH 0 -30 Hz ; JCH 0 -250 Hz)

Chemical Shift and Shielding high electron density Note: Electron orbiting to give B is driven by B; so B B. Bmolecular (diamagnetic) CH 3 C Bapplied C-H ? TMS ! ? ? ? Beffective deshielded downfield shielded Cf. Table 15. 4 p. 720 low e- density high chemical shift high frequency upfield high e- density low chemical shift low frequency

1/r 3 Bapplied Suppose molecule undergoes rotational averaging. PPM Electrons Orbiting Other Nuclei average around over sphere circle ZERO! Ignore Diamagnetism them! from Orbiting Electrons

Electrons Orbiting Other Nuclei Bapplied average over sphere NOT ZERO! Diamagnetic “Anisotropy” (depends on direction) Unless orbiting depends on molecular orientation

Diamagnetic Anisotropy Benzene “Ring Current” B 0 can only drive circulation about a path to which it is perpendicular. 15. 30 If the ring rotates so that it is longer perpendicular to B 0, the ring current stops. no

Aromaticity: PMR Chemical Shift Criterion ? 14 electrons (4 3 + 2) TMS DIAMAGNETIC electrons ANISOTROPY 10 (distorted) HCCl 3 8 H TMS DIAMAGNETIC ANISOTROPY! 2 H -4. 23

Diamagnetic Anisotropy Acetylene “Ring Current” 15. 32 The H nuclei of acetylene lie on the orbital axis when there is ring current. (B 0 diminshed; Warning! signal shifts upfield). This handy picture of diamagnetic The H nucleianisotropy of benzene due to ring lie outside the current orbital path may well when therebe is nonsense! ring current. (B 0 augmented; (Prof. Wiberg showed it to signal shifts downfield). be nonsense for 13 C. )

Spin-Spin Splitting

Three peaks from four different sets of 15. 36. jpg molecules in the sample. ~1: 2: 1 Triplet

15. 37. jpg ~1: 3: 3: 1 Quartet

Isotropic JH-H is mediated by bonding electrons (through-space part is anisotropic, averaged to zero by tumbling)

End of Lecture 59 March 22, 2010 Copyright © J. M. Mc. Bride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-Non. Commercial-Share. Alike 3. 0). Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol . Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. Mc. Bride, Chem 125. License: Creative Commons BY-NC-SA 3. 0