Chemistry 125: Lecture 58 March 4, 2011 Normal Modes: Mixing and Independence Practical IR NMR Spectroscopy Precession This For copyright notice see final page of this file

Butane C 4 H 10 3 x (4 + 10) = 42 degrees of freedom - 3 (translation) - 3 (rotation) = 36 vibrations C 4 : 3 stretch, 2 bend, 1 twist 10 C-H : 10 stretch, 20 bend or twist Mixed (according to frequency-match / coupling) into 36 normal modes.

C 8 Straight Chain Hydrocarbons Octane C 8 H 18 C-H stretch C-CH 3 umbrella + C-C stretch CH 2 rock CH 2 wag CH 2 scissors 26 atoms  72 normal modes (not all IR active) C-H stretch “Breathing” gives no net dipole change - no IR peak Half of C 4 H 10 ’s ten C-H stretch normal modes have no “handle” E(t) helps push 8 H in and out E(t) helps push 4 Hs up and down Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.

C 8 Straight Chain Hydrocarbons Octane C 8 H 18 4-Octyne1-Octyne Why not in 4-octyne? C C H sp CCH CC (symmetric compound has no handle) Functional Group Identification Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.

C 8 Straight Chain Hydrocarbons Octanetrans-4-Octenecis-4-Octene 2-Methyl-2-Hexene C C dipole change (weak) Functional Group Identification

Twist reduces  overlapFolding preserves  overlap 967 cm cm -1 hardereasier 828 cm -1 IR Active out-of-plane C-H bending (paired H atoms move in the same direction) Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.

The Jewel in the Crown of Infrared Spectroscopy C O C O

1681 CH 3 C O NH 2 CH 3 -C=O(X) strong & independent 1727 CH 3 C O H 1715 CH 3 C O CH 3 CH 3 C O OCH CH 3 C O Cl n N  * C=O n O  * C-Cl n O  * C-OMe :: : C=O weakened by resonance C=O strengthened by resonance

1683?1618 CH 3 C O CH=CH CH 3 C O NH 2 CH 3 -C=O(X) strong & independent 1727 CH 3 C O H 1715 CH 3 C O CH 3 CH 3 C O OCH CH 3 C O Cl n N  * C=O n O  * C-Cl n O  * C-OMe + - C=O weakened by resonance C=O strengthened by resonance C=O C=C coupling out-of-phase mostly C=O Double ! in-phase mostly C=C But strong peak is at higher frequency than ketone C=O C=C ? ? ?

Sankaran & Lee, J. Phys. Chem. A 2002, 106, Difference (new spectrum - old) 1718 & 1623 grow 1696 shrinks hours irradiation at 308 nm IR Spectrum of Methyl Vinyl Ketone in Ar at 13K anti periplanar syn periplanar “Mostly C=C” is very weak because small C=O vibration fights its dipole change “Mostly C=C” is strong because small C=O vibration helps it Calculated Positions for s-trans Calculated Positions for (a different species) C=OC=C ? ? combination of two lower frequency transitions?

Exaggerated Amplitude Actual Amplitude C=O C=C Coupling in MVK In-Phase Normal Mode (1618 cm -1 ) C=C stretch C=O stretch = 9

Cf. Frames of Lecture 8 (Erwin-Goldilocks on Vibrational Amplitude) Exaggerated Amplitude Actual Amplitude C=O C=C Coupling in MVK Out-of-Phase Normal Mode (1720 cm -1 ) C=C stretch C=O stretch = -1/6

IR in the “Real” World of a Polymorphic Multibillion-Dollar Pharmaceutical *different crystal forms * crystal packing “isomers” (different solubility, bioavailability)

Form A "Fingerprint" C-H stretch Form A NH 2 + ? !

Form B "Fingerprint" C-H stretch Form B NH 2 +

Form C "Fingerprint" C-H Form C NH 2 + (truncated)

Form C Paroxetine Hydrochloride Form A Form B Patent Dispute: Can one detect 5% of protected B (675) in the presence of 95% of unprotected A (665) ?

Spectroscopy for Structure and Dynamics Electronic (Visible/UV) Vibrational (Infrared) NMR (Radio) Do precession problems on 125 webpage. Magnetism Precession

young chemist Michael Faraday (discovered benzene In illuminating gas) with the kind permission of Alfred Bader

Faraday 1831 Discovers Magnetic Induction and Invents Fields Magnetism from electric current Electricity from changing magnetism Thirty years later Maxwell built these into a comprehensive theory of light and electromagnetism.

“many simple things can be deduced mathematically more rapidly than they can be really understood in a fundamental or simple sense… the precession of a top looks like some kind of a miracle involving right angles and circles, and twists and right-hand screws. What we should do is to understand it in a more physical way. Precession Feynman, I, 20-6

90° Phase Lag  “falls around” FORCE from string and gravity via spokes Right RIM POSITION back topbottomfront Left VELOCITY Where on the rim is the rightward velocity maximum? Where it has been pushed to the right for the longest time. above center force to right below center force to left accumulating rightward velocity

Radio Frequency Precession of “Spin = 1/2” Nuclei in Magnetic Field of ~23.5 kGauss MHz MHz x 3 x = cm -1 cm -1 x 2.9 = °K H1H1 F 19 P 31 C 13 O 17 1% 99.98% 6% 66,000 MHz e - CT Public Radio MHz = cm -1 = 0.01 °K equilibrium Up:Down ratio e E/T = e 0.01/300 = WCBS 0.88 excess of 3 in 200,000 ! two quantized angles for S = 1/2 We know these nuclei spin, because they are magnetic, and they precess when an applied field tries to twist them.

90° RF Pulse and the “Rotating Frame” Applied Magnetic Field Precessing proton gives rise to constant vertical field Will rotating horizontal field generate 100 MHz RF signal? No, because there are many precessing protons with all possible phases. Consider a “rotating frame” in which the observer orbits at 100 MHz - protons seem to stand still as if no applied field. (just long enough to rotate all nuclear spin axes by 90°). Fast precession (~100 MHz) Slow precession (~0.1 MHz) Horizontal fields cancel. Subsequent precession generates 100 MHz RF signal in lab frame. 100 MHz RF in lab frame Until “relaxation” reestablishes equilibrium. and rotating horizontal field. Pulse a very weak magnetic field fixed in this rotating frame

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

End of Lecture 58 March 4, 2011 Copyright © J. M. McBride 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).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