Diamagnetic Anisotropy, Spin-Spin Coupling Chemistry 125: Lecture 60 March 23, 2011 NMR Spectroscopy Chemical Shift and Diamagnetic Anisotropy, Spin-Spin Coupling This For copyright notice see final page of this file

Components of Effective Magnetic Field. Bapplied Bmolecular (diamagnetic) Beffective Applied Field Molecular Field: Net electron orbiting - “Chemical Shift” (Range ~12 ppm for 1H, ~ 200 ppm for 13C) Nearby magnetic nuclei - “Spin-Spin Splitting” (In solution JHH 0-30 Hz ; JCH 0-250 Hz)

Diamagnetic Anisotropy The Chemical Shift: Electron Orbiting and Diamagnetic Anisotropy

Chemical Shift and Shielding high electron density Chemical Shift and Shielding Note: Electron orbiting to give B is driven by B; so B  B. d+ d- Beffective Bmolecular (diamagnetic) Bapplied TMS R-OH (depends on conc, T) CH X X = O, Hal, N RC CH O RC OH O RC H O H C C Alkyl R-H H 1 2 3 4 5 6 7 8 9 10 11 d (ppm) ! ??? CH3C C-H deshielded shielded downfield upfield low e- density high e- density high chemical shift low chemical shift high frequency low frequency

Electrons Orbiting Other Nuclei  1/r3 Bapplied  1/r3 Suppose the studied nucleus is fixed relative to the other nucleus by bond(s). net from average around circle net from average over sphere PPM Suppose molecule in fluid undergoes rotational averaging. ZERO! Ignore electrons on other atoms! Diamagnetism from Orbiting Electrons

Diamagnetic “Anisotropy” Electrons Orbiting Other Nuclei Bapplied NOT net from average over sphere reinforces Bapplied suppose less orbiting for this molecular orientation ZERO! Unless orbiting depends on molecular orientation Diamagnetic “Anisotropy” (depends on orientation)

Diamagnetic Anisotropy Benzene “Ring Current” B0 can only drive circulation about a path to which it is perpendicular. B0 Net deshielding of aromatic protons; shifted downfield If the ring rotates so that it is no longer perpendicular to B0, the ring current stops.

Aromaticity: PMR Chemical Shift Criterion 14  electrons (43) + 2 ? 1 2 3 4 5 6 7 8 8 H 2 H TMS TMS Vogel, Aromaticity, 1967, p. 119 DIAMAGNETIC ANISOTROPY DIAMAGNETIC ANISOTROPY! 10  electrons (distorted – less overlap & ring current) HCCl3  -4.23 9 -1 -2 -3 -4 d (ppm) Boekelheide (1969)

Aromaticity: PMR Chemical Shift Criterion “Anti-Aromatic” Dianion Metallic K adds 2  electrons to give 16 (4n) -2 CH3 signals shift downfield by 26 ppm despite addition of “shielding” electrons. 14  electrons (43) + 2 9 1 2 3 4 5 6 7 8 HCCl3 -1 -2 -3 -4 TMS Vogel, Aromaticity, 1967, p. 119 DIAMAGNETIC ANISOTROPY DIAMAGNETIC ANISOTROPY! THF solvent Shrink Scale  -4.23 4 6 8 10 12 14 16 18 20 22 2 -2 -4 d (ppm) Boekelheide (1969)

Diamagnetic Anisotropy Acetylene “Ring Current” H The H nuclei of benzene lie beside the orbital path when there is ring current. (B0 at H reinforced; signal shifts downfield). H Warning! This handy picture of diamagnetic anisotropy due to ring current may well be nonsense! (Prof. Wiberg showed it / /to be nonsense for 13C.) H The H nuclei of acetylene lie above the orbiting path when there is ring current. (B0 at H diminshed; signal shifts upfield).

Spin-Spin Splitting

Chem 220 NMR Problem 1 (of 40) CH3C OCH2CH3 O C. H Triplet (1:2:1) Four (22) sets of molecules that differ in spins of adjacent H nuclei “Spin Isomers” so similar in energy that equilibrium keeps them equally abundant d (ppm) 1 2 3 4 5 6 7 8

C. O H CH3C OCH2CH3 J in Hz vs. Chemical Shift in NMR Problem 1 (of 40) Influence of CH2 on CH3 must be the same as that of CH3 on CH2 CH3C OCH2CH3 O C. H and independent of Bo Triplet (1:2:1) Eight (23) sets of molecules that differ in spins of adjacent H nuclei J in Hz Quartet (1:3:3:1) 7.3 7.3 1: 1 7.3 7.3 7.3 1 2 2: 1 3 3: 4: 1 6 4 binomial coefficients d (ppm) 1 2 3 4 5 6 7 8 vs. Chemical Shift in (Orbiting driven by Bo)

HO-CH2-CH3 ? ? 5.1 Hz 7.2 Hz 124 Hz 13CH3 ? Subtle Asymmetry d Dd 0.018 ppm 5.1 Hz 7.2 Hz × 400 MHz J = 7.2 Hz DMSO-d5 Subtle Asymmetry HO-CH2-CH3 1.070 1.052 d Doublet of Quartets 7.2 5.1 124 Hz D is a weaker magnet than H. 13CH3 1:4:6:4:1 Quintet? D can be oriented 3 ways in Bo. 7.2 CD3SCD2H O 1.8 Hz 1.1% of C 1:2:3:2:1 Quintet ? ? H2O d (ppm) ?

What determines the Strength of Spin-Spin Splitting?

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

Not spatial proximity! Might overlap be greater for anti C-H bonds ?? In tumbling molecules, nuclear spins communicate not through space, but through paired electrons on the nuclei. When the “up” electron of this MO is on Nucleus A HOMO-3 J depends on the s-orbital content of molecular orbitals. only its “down” electron is available to be on Nucleus B J = 6-8 Hz J = 1-3 Hz J = 0-1 Hz Might overlap be greater for anti C-H bonds ?? 3.07 Å 1.85 Å 2.38 Å J = 0-3 Hz J = 12-18 Hz J = 6-12 Hz Not spatial proximity! Through-space interaction of dipoles averages to zero on tumbling.

 Better Overlap! Which gives better overlap? Examine the overlap of the components. + + Backside overlap is counterintuitive.  Better Overlap! good p-p bad p-p 2 bad s-p + good s-p; good p-s + s-p > s-s or p-p (See Lecture 12) good s-s bad s-s

C Overlap s-ps s-s p-ps Overlap Integral 1.0 0.8 0.6 0.4 0.2 0.0 1.2 1.3 1.4 1.5 Å C s-ps Interesting that s-p is greater than EITHER s-s or p-p s-s p-ps C Overlap

(approximate way to measure a rigid torsional angle!) 10-20 No “handle” for rf if same chem shift (see Frame 26 below) invisible H H 11 Hz 13 Hz 2 Hz gauche ~7 Hz 2-13 Hz, depends on conformation (overlap) (approximate way to measure a rigid torsional angle!)

End of Lecture 60 March 23, 2011 Copyright © J. M. McBride 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-NonCommercial-ShareAlike 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. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0