Vibrational spectra
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Vibrational Spectra. Molecules are not Static Vibration of bonds occurs in the liquid, solid and gaseous phase Vibrating  Energy  Frequency (and the appropriate frequencies for molecular vibrations are in the Infrared region of the electromagnetic spectrum

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Vibrational Spectra

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Vibrational spectra

Vibrational Spectra

Molecules are not Static

Vibration of bonds occurs in the liquid, solid and gaseous phase

Vibrating  Energy  Frequency (and the appropriate frequencies for molecular vibrations are in the Infrared region of the electromagnetic spectrum

Vibrations form therefore, a fundamental basis for spectroscopy in chemistry--the bonds are what makes the chemistry work in structure and function

For Organic Chemistry the most important uses of these vibrations is for analysis of:

•functional groups

•structural identity, “fingerprinting”

The IR spectra in this evenings talk are from the SDBS data base.


Tonight s lecture

Tonight’s lecture

Infrared Spectroscopy for Structure Determination

A little theory

Some notes on sampling

Defer discussion of instrumentation

Defer discussion of solids analysis

Lots of examples, working through trends in related structure classes

How to interpret, use data.


Some perspective

Some Perspective

IR’s are not as thoroughly interpretable as NMR, Mass Spec

Lacks the quantitative character on atom-atom basis that NMR has. (all the chromophores are not equal)

Not used as much for identification as NMR and MS have become more accessible

Still very useful for confirmation of structure cf. Reference spectra.

Diagnostic for functional groups that may be silent or ambiguous in the NMR

Quite sensitive, and can measure in all sorts of strange matrix e.g on surfaces, extremely useful for solid state characterization.


Infrared vs raman

Infrared vs. Raman

These two spectroscopies measure the same thing, vibrations, in different ways.

IR is a absorption measurement, while Raman measures scattered light from a laser source, that in being scattered, is superimposed with the vibrational structure of the molecule.

The selection rules are different--IR bands are active if in the act of the vibration, the dipole moment of the molecule changes. Raman band are active if the polarizability of the molecule changes.

Often these two are complementary to each other

Molecules of high symmetry frequently will not show IR activity


Ir measurement

IR Measurement

Neat film, or melt between two sodium chloride plates

Solid solution in KBr, ground together and pressed into a transparent pellet

Solution with appropriate blank region of solvent. Solution IR can be used to minimize broadening from self-association, H-bonding.

Salt plates of CsCl2 for lower frequency window transparency (down to 200 cm-1)

Mulling (grinding with mineral oil as dispersion), spread on salt plate

Many different reflectance techniques, light must pass into the sample and reflect out.


Vibration levels are quantized like everything else

Vibration levels are quantized, like everything else

Like a harmonic oscillator

With L-H, L moves most easily

Can couple to other springs but heavy atoms can block or minimize this effect

From Skoog and West


What kind of vibrations are these

What Kind of vibrations are These?

Bonds can…….

These can number into the hundreds.

Some are symmetrical, some antisymmetrical and many are coupled across the molecule

Can be calculated. One easy approximation is:

Bend

Stretch

k is the “force constant”, like the Hookes Law restoring force for a spring. Known and tabulated for different vibrations

Wag (rock)

The “reduced mass” where m1, m2 are the masses on either side of vibration


Regions of frequencies

Regions of Frequencies

Spectral RegionFrequency(Hz) Wavenumber(cm-1) Wavelength (,m)

After Table 16-1 of Skoog and West, et al. (Chapter 16)


What kinds of bonds absorb in which regions

What kinds of Bonds Absorb in which Regions?

Bending is easier than stretching--happens at lower energy (lower wavenumber)

Bond Order is reflected in ordering--triple>double>single (energy)

with single bonds easier than double

easier than triple

Heavier atoms move slower than

lighter ones

The k in the frequency equation is in mDyne/Å of displacement

Single bond str 3-6 mD/Å

Double bond str. 10-12 mD/Å

Triple Bond 15-18 mD/Å


The fundamentals

The Fundamentals

  • These oscillating electric dipoles match in frequency the incoming e-field oscillations of IR light.

  • All the simple possibilities. For n atoms in a molecule;

    • Linear: 3n – 5 modes

    • Non-linear: 3n – 6 modes

    • Example for a methylene,given n=3

antisymmetric

symmetric

R

R

H

H

stretching

R

H

R

H

R

R

H

H

in-plane

bending

R

H

R

H

scissoring

rocking

While useful, this oversimplifies, since molecular orbital picture requires that atoms can’t vibrate without affecting the rest of the molecule.

R

R

H

H

out-of-plane

R

H

R

H

bending

wagging

twisting


Calculated ir bands for ch 2 in formaldehyde

Calculated IR bands for CH2 in formaldehyde

Formaldehyde spectrum from: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2

Results generated using B3LYP//6-31G(d) in Gaussian 03W.


Looking at a spectrum

Looking at a Spectrum

Divide the spectrum in to two regions:

4000 cm-1 1600 cm-1most of the stretching bands, specific functional groups (specific atom pairs). This is the “functional group” region.

1600 cm-1  400 cm-1Many band of mixed origin. Some prominent bands are reliable. This is the “fingerprint” region. Use for comparison with literature spectra.

Wavenumber is cm-1=104/()


Some ir media are better than others

Nujol (mineral oil)

CHCl3

CDCl3

Some IR Media are better than others

Nujol, for example wipes out the hydrocarbon region for transparency..

Note that some older IR spectra while they are linear in frequency, the wavelength scale compresses the higher we go, Affects the appearance of spectra, not the line positions.


Reaction monitoring

Reaction Monitoring

Gas phase IR

Pivaldehyde + methylamine

Note loss of C=O

Thank to Drs. Dalton and Mascavage


A functional group chart

A Functional Group Chart

2000

800

group

4000

3600

3200

2800

1200

1600

2400

Cl

I

Br


The hydrogen stretching region

The hydrogen stretching region

Tip--

Draw a line straight up from 3000 cm-1. Intensity on left is Csp2-H, to the right is Csp3-H

Arom- H

3000cm-1


Amines

Amines

3500, 3300 cm-1 doublet, frequently (without, with H-bonding effect) NH stretch

1600 cm-1 NH2 scissoring - broad

700-900 cm-1 NH2 wagging- broad, strong

1080 cm-1 C–N str. --weak for alkyl

1300 cm-1 Ar–N str. strong

R-NH2

R–NH–R

3400 cm-1 singlet str.

Weak C–N 1125 cm-1

No good IR bands, adj CH2 will shift to 2800 cm-1. A tert amine salt NH strong at 2500 cm-1

R–NR–R


Amine salts

Amine Salts

R-NH3+ like methyl, but broad

NH str. centered at 3000, br.

deformation 1520-1570 br.

R2NH2+ 3000 br, spikes at 2200,2500 cm-1

NH2 scissor at 1600 cm-1, broad


Alcohols

phenethanol

1-phenylethanol

Alcohols

C–O–H stretch 3600 cm-1 in dilute solution

Typically H-bonding and at lower frequency ~3400 cm-1

C–O stretch in same region as C–C but much more intense

Position is sensitive to subs. pattern

RCH2–OH1050 cm-1

R2CH–OH1110 cm-1

R3C–OH1175 cm-1

2-piperidinylethanol


Bands that should appear together

Bands that should appear together

SERVE AS CROSS CHECK

e.g. see a triple bond? Check for C–H str.

see C=O, check for OH, C–O

I will point a few of these out as we go...


Alkynes

3,5-dimethyl-1-hexyn-3-ol

6-methyl5-hepten-1-yne

phenylacetylene

benzonitrile

Phenyl-1-butyne

Alkynes

Also wk overtones at 1820,1790 cm-1


Functional groups can be nmr silent or ambiguous ir can play a key role in identification

Functional Groups can be “NMR Silent” or ambiguous. IR can play a key role in Identification

1800, 1750 cm-1 (cyclic, has more intense 1750, acyclic, more instense 1800

C-O-C vs 1180-1220cm-1

1800 cm-1 doublet, Fermi resonance

Poor resonance for 2p-3p, but strong inductive effect


More nmr silent groups nitro groups

More NMR silent groups--Nitro groups

Analogous to Carboxylate ion. Strong bands

Aromatic nitro

1520, 1350 cm-1

Aliphatic nitro

1550, 1370 cm-1


Double bonds

Double Bonds

1640 cm-1 is double bond stretch

not seen for symmetrical molecules

lower freq by conjugation, more intense

=C–X lowers to 1590 cm-1

Ca. 890-910 cm-1 and 985cm-1 are o.o.p bendings for terminal =CH2

Cis vs Trans?

Disubstituted –HC=CH-

960-970 cm-1trans o.o.p bend

675-730 cm-1 cis o.o.p. bend

Medium intensity


Methylenes

Methylenes

1460 cm-1 CH2 scissoring

725 cm-1 characteristic rocking for 4 or more CH2’s in a row (non-cyclic)


The carbonyl stretch is our friend

The Carbonyl Stretch is our friend…

Carbonyl stretch changes its position for variation in specific structure

THIS BAND IS ALWAYS STRONG!!!

Good rules to remember…

C=O conjugated to double bond goes lower in frequency

With electronegative substituent (O, Cl) goes to higher frequency

C=O in strained ring, goes to higher frequency

C=O…(H hydrogen bonds lower the frequency)


Ketones sensitive to strain

O

O

O

Ketones--sensitive to strain

1715 cm-1

1750 cm-1

1780 cm-1

Ca. 30 cm-1 higher for every C atom removed

-diketones, str-str for open chain, IR inactive; in ring, 1720,1740

-haloketones--can see second band from rotamer populations (1720, 1745)


Ketones sensitive to conjugation

Ketones--Sensitive to conjugation

1660-1700 cm-1 rotational isomers cause doubling. S-trans 1674, S-cis 1699

1580-1640 cm-1 for enol

1715 cm-1 for the keto bond

1650-1700 cm-1

Along with br. OH str.


Aldehydes

Ethyl-2-butenal

cyclohexylcarboxaldehyde

Aldehydes

Doublet straddles 2800 cm-1 (Fermi resonance)

Fundamental at ca. 2800

Bending at 1400, gives overtone at 2800


Carboxylic acids

Hept-3-enoic acid

Phenylbutyric acid

Cyclohexanecarboxylic acid

Carboxylic Acids

1715 cm-1

br OH stretch

Good example of the broadening from H-bonding

Also C—O 1280 cm-1, often a doublet

O—H o.o.p bend br 920 cm-1

Salts have1600,1350 cm-1 broad!


Esters and lactones

Butyl acetate

Esters and Lactones

1735 cm-1

1300-1100 intense, often doublet

Cyclohexyl acetate


Effects of conjugation

Effects of conjugation

Lowers to 1715 cm-1

Raises to 1770 cm-1

Similar, to 1715 cm-1

:

Weakens DB character

Strengthens DB character (inductive over resonance)


Lactones similar effects

Lactones, similar effects

1735 cm-1

1770 cm-1

1715 cm-1

1765 cm-1


Amides

N-benzylbenzamide

phenylacetamide

phenylacetanilide

amides

NH str 3300 cm-1

C=O 1650 cm-1

NH bend 1640 cm-1

Moves to 1550 for R-C(=O)-NHR’

L-a-aspartyl-L-phenylalanine 1-methyl ester


Methyl groups

CH3 “umbrella”

Ca. 1375 cm-1

“t-butyl split”

“isopropyl split”

isopropylcyclohexane

t-butylcyclohexane

Moves lower by 20 cm-1

methylcyclopentane

Methyl Groups

But…

OCH3, NCH3 do not give this band


Benzene rings substitution patterns

Complements the out-of plane bendings, related to the number of adjacent H…

750,700

750

5

750

From Crewes, Rodriguez and Jaspars, ch 8

4

750

850,780,

700

Out-of-plane bending combinations, quite small, but in a normally clean region of IR. Reliable even with nitro or carboxyl substitution

820

3

780

2

820

1

850

Unreliable with NO2, CO2H subs

Benzene rings--substitution patterns

Ring pucker at ca 700 cm-1is IR active for mono, 1,3-di-, 1,3,5-tri-, 1,2,3-tri- subs rings.


Putting the s in silent sulfur containing groups

Putting the S in Silent; Sulfur containing groups

wk SH at 2580

1360, 1180 cm-1

1050 cm-1

Extremely wk at 590-700 cm-1

1390, 1200 cm-1

1320, 1140 cm-1

Induction strengthens D.B. resonance not significant

1340, 1160 cm-1


Silence is deafening the ir guys think silence is golden

Silence is Deafening(the IR guys think “silence is golden”

R-N=C=O strong at 2250-2290 cm-1

R-N=C=S strong 2000-2190 cm-1

R-N=N=N strong 2100-2200 cm-1

R-N=C=N-R strong 2150 cm-1

R-CN 2250 cm-1

R-N+C:- 2130-80 cm-1

R-C=C=C-R’ (allenes) 1900-2000 cm-1 with very strong 850 wag if terminal


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