Chemistry 125: Lecture 58March 4, 2011

Normal Modes:Mixing and Independence

Practical IR NMR Spectroscopy

Precession This

For copyright notice see final page of this file

Butane C4H10

3 x (4 + 10) = 42 degrees of freedom

- 3 (translation) - 3 (rotation) = 36 vibrations

C4 : 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.

C8 Straight Chain Hydrocarbons

OctaneC8H18

C-H stretch

C-CH3 umbrella+ C-C stretch

CH2

rockCH2 wag

CH2 scissors

26 atoms 72 normal modes (not all IR active)

C-H stretch

“Breathing” gives no net dipole change - no IR peak

Half ofC4H10’s tenC-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.

C8 Straight Chain Hydrocarbons

OctaneC8H18

4-Octyne 1-Octyne

3315630

2120

Why not in 4-octyne?

CC Hsp

CC H

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.

C8 Straight Chain Hydrocarbons

Octane trans-4-Octene cis-4-Octene

2-Methyl-2-Hexene

828

710

967

1655

CC

dipolechange(weak)

Functional Group

Identification

Twist reduces overlap Folding preserves overlap

967 cm-1 710 cm-1

harder easier

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 The Jewel in the Crown of Infrared SpectroscopyInfrared SpectroscopyC

O

C

O

1681

CH3CO

NH2

CH3-C=O(X) strong & independent

1727

CH3CO

H

1715

CH3CO

CH3

CH3CO

OCH3

17461806

CH3CO

Cl+

-+

-

+

-

nN *C=OnO *C-Cl nO *C-OMe

: :

:

C=O weakened by resonance

C=O strengthened by resonance

1683?1618

CH3CO

CH=CH2

1681

CH3CO

NH2

CH3-C=O(X) strong & independent

1727

CH3CO

H

1715

CH3CO

CH3

CH3CO

OCH3

17461806

CH3CO

ClnN *C=OnO *C-Cl nO *C-OMe

+

-

C=O weakened by resonance

C=O strengthened by resonance

C=O C=C coupling

out-of-phasemostlyC=O

Double !

in-phasemostlyC=CBut strong

peak is at higher

frequency than ketone

C=O C=C

1720

1683?1618

1618

1720 ?

?

Sankaran & Lee, J. Phys. Chem. A 2002, 106, 1190-1195

1718

1696

1623

Difference(new spectrum - old)

1718 &1623 grow1696 shrinks

1718

1696

1623

2.5 hours irradiationat 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

CalculatedPositionsfor s-trans

Calculated Positions

for

(a different species)

C=O C=C

?

?

combination of two lower frequency

transitions?

ExaggeratedAmplitudeActual

Amplitude

C=O C=C Coupling in MVKIn-Phase Normal Mode (1618 cm-1)

C=C stretchC=O stretch

= 9

Cf. Frames 25-26 ofLecture 8

(Erwin-Goldilocks on

Vibrational Amplitude)

ExaggeratedAmplitudeActual

Amplitude

C=O C=C Coupling in MVKOut-of-Phase Normal Mode (1720 cm-1)

C=C stretchC=O stretch

= -1/6

IR in the “Real” Worldof a

PolymorphicMultibillion-Dollar

Pharmaceutical*different crystal forms

*

crystal packing “isomers”(different solubility, bioavailability)

Form AN O O

O

H

H+

F

Cl -

HO

HO

H

H"Fingerprint"

C-H stretch

Form A

N O O

O

H

H+

F

Cl -

NH2+

?

!

Form B

"Fingerprint"C-H stretch

Form B

N O O

O

H

H+

F

Cl -

NH2+

Form C

"Fingerprint"C-H

Form C

N O O

O

H

H+

F

Cl -

NH2+

(truncated)

Form C

Paroxetine Hydrochloride

Form A

1598

674

1183

1240

1562

665

592

1194

1604

1221

1248

Form B

1574

675

12301182

1598 1240 1574 1230 1562 1194 665 592

Patent Dispute: Can one detect

5% of protected B (675) in the presence of 95% of unprotected A (665)?

Spectroscopy forStructure and Dynamics

Electronic (Visible/UV)

Vibrational (Infrared)

NMR (Radio)

Do precession problems on 125 webpage.

MagnetismPrecession

young chemistMichael Faraday(discovered benzeneIn illuminating gas)

with

the

kind

per

mis

sion

of

Alf

red

Bad

er

Faraday 1831Discovers Magnetic Induction

and Invents Fields

Magnetism fromelectric current

http://micro.magnet.fsu.edu/electromag/java/faraday/

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”

FORCEfrom string and gravity via spokes

Right

RIM POSITION

back backtop bottomfront

LeftVELOCITY

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 10-5 = cm-1

cm-1 x 2.9 = °K

H1

F19

P31

C13

O17

1%

99.98%

6%

66,000 MHz e-

CT Public Radio 90.5

100 MHz = 0.003 cm-1 = 0.01 °K

equilibrium Up:Down ratio (@RT):

eE/T = e0.01/300 = 1.00003WCBS 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 riseto 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 generates100 MHz RF signal in lab frame.

100 MHz RFin lab frame

Until “relaxation”reestablishes equilibrium.

and rotating horizontal field.

Pulse a very weak magnetic field fixed in this rotating frame

A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency

that reports theirlocal magnetic field.

End of Lecture 58March 4, 2011

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