Chemistry 125: Lecture 60 March 24, 2010 NMR Spectroscopy Isotropic J and Dynamics This For copyright notice see final page of this file

ZERO! average over sphere Electrons Orbiting Other Nuclei B applied

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

15.44.jpg Not spatial proximity! 3.07 Å 1.85 Å 2.38 Å Might overlap be greater for anti C-H bonds ?? HOMO-3 When the “up” electron of this MO is on Nucleus A only its “down” electron is available to be on Nucleus B In tumbling molecules, nuclear spins communicate not through space, but through paired electrons on the nuclei. Through-space interaction of dipoles averages to zero on tumbling.

15.44.jpg good p  -p  good s-s bad p  -p  bad s-s 2 bad s-p  good s-p  ; good p  -s s-p  > s-s and p   p  (lecture 13 frame 2) Better Overlap! ++  ++

H No “handle” if same chem shift (see Frame 11 below) 2-13 Hz, depends on conformation (overlap) 13 Hz2 Hz H gauche ~7 Hz 11 Hz (approximate way to measure torsional angle!)

J C-H 13 Amplify 30x 126 Hz Coupling electron must be on the C 13 nucleus. 99% of sample is C 12 (instrumental artifact)

Hybridization and J coupling sp 3 sp 2 sp

Units for  and J ppmHz

Energy Scale Same Frequency (Hz) Scale (relative to TMS) Same ppm (  ) Scale 0 Hz300 Hz

Two unsymmetrical Doublets  a b

Lose the “handle” as  becomes smaller than J   By increasing  (Hz) big magnet reduces asymmetry and gives “ideal” pattern. Note:  is measured in Hz (not ppm) between the weighted average positions of the lines in the two doublets

Slope up toward splitting partner quartet * * * * doublets of * * * * Cf. Fig , p. 738

Dynamics (the NMR Time Scale / Decoupling) Sec pp Sec. 15.6e p. 739

ROH chemical shift? CH 3 CH 2 OH spin-spin splitting? average of many H-bond structures (dependent on concentration & temperature) average for exchange among many molecules (H + / solvent) Three OH peaks from three different kinds of molecules (with different neighboring CH 2 proton spins) Fig p

Form A Why doesn’t IR show OH averaging? 110 x Hz102 x Hz Difference ~ Hz (This particular sample is a solid, but no averaging is observed in solution spectra either.)

How long does it take to measure frequency precisely? 1 second 20 Hz 22 Hz 21 Hz But a 1 sec pulse samples full range of phases. 0.5 sec (1/  ) is long enough to sample the full range, favorable and unfavorable. Match with a short pulse of 20 Hz light is nearly as good. No net interaction with light. 1 sec light pulse distinguishes 20 Hz from 21 Hz. light field

What if protons are “exchanging” faster than 1/  ? 1 second 20 Hz 22 Hz 21 Hz 22 Hz 20 Hz 22 Hz20 Hz Very good match with the 21Hz average frequency  single, sharp peak.

When do Peaks Average? When atoms don’t stay put long enough to tell the difference in frequency. e.g. If two peaks differ by 100 Hz, you must count for ~0.01 sec to tell them apart. These IR peaks differ by Hz. Exchange of position is not that fast.

One average chemical shift (no splitting observed) equatorial axial 15.58

Compared to What? The NMR Time Scale p Hz Coalescence at ~30 flips per second axial equatorial d 11 to avoid complications from spin-spin splitting.

a Probably no splitting (broad singlet) because of rapid OH proton exchange among different molecules 1:1 doublet from single H on neighboring carbon (J about 7 Hz) Almost anyplace (  1-6) depending on concentration and temperature (averaged H-bonding) Slightly deshielded by two oxygen atoms on neighboring carbon  1-2 (a real analogue is at  1.3) b c Deshielded by two oxygen atoms on the same carbon  4-5 (a real analogue is at  4.7) 1:3:3:1 quartet from three H atoms on neighboring carbon (J about 7 Hz)

d 1:1 doublet from single H on neighboring C (J about 7 Hz) Seven line multiplet from 6 H atoms on neighboring carbons (J about 7 Hz) Slightly deshielded by oxygen atom on neighboring carbon  1 (a real analogue is at  1.1) Deshielded by oxygen atom on the same carbon (but less than c)  4 (a real analogue is at  3.8) e f Slightly deshielded by oxygen atom on neighboring carbon.  1 (a real analogue is at  1.1) 1:1 doublet from single H on neighboring carbon (J about 7 Hz) different from d (diastereotopic)

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