Vibrational Spectroscopy

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Vibrational SpectroscopyA rough definition of spectroscopy is “the study of the interaction of matter with energy (radiation in the electromagnetic spectrum).” A molecular vibration is a periodic distortion of a molecule from its equilibrium geometry. The energy required for a molecule to vibrate is quantized (not continuous) and is generally in the infrared region of the electromagnetic spectrum.

re = equilibrium distance between A and B

For a diatomic molecule (A-B), the bond between the two atoms can be approximated by a spring that restores the distance between A and B to its equilibrium value. The bond can be assigned a force constant, k (in Nm-1; the stronger the bond, the larger k) and the relationship between the frequency of the vibration, , is given by the relationship:

DAB = energy required to dissociate into A and B atoms

or, more typically

where , c is the speed of light, is the frequency in “wave numbers” (cm-1) and is the reduced mass (in amu) of A and B given by the equation:

m mA B

Infrared RadiationPortion of the electromagnetic spectrumbetween visible light and microwavesfull range for IR is 10000-400 cm-1

of importance here is 4000-400 cm-1 (wavenumbers)or 2.2-25 m (wavelength)

Note: cm-1 is proportional to Energy cm-1 = 104/m

this energy is absorbed by molecules and converted to molecular vibration

IR Absorption

IR absorptions are characteristic of entire moleculeor essentially a molecular fingerprint

vibration spectrum appear as bands molecular vibration is not a single energy as also depends onmolecular rotation

band intensities expressed as either transmission (T)or absorption (A)

A = log10(1/T)

Molecular Vibrations

Stretching is a rhythmical movement along a bond

Bending is a vibration that may consist of a changein bond angle (twisting, rocking and torsional vib.)

Vibrations that result in change of dipole momentgive rise to IR absorptionsalternating electric field produced by changing dipolecouples the molecular vibration to the oscillating electric field of the radiation

Vibrations for H2O and CO2

3650 cm-1 3756 cm-1 1596 cm-1

Symmetrical asymmetrical scissoring stretch stretch

(inactive in IR)for CO2

1340 cm-1 2350 cm-1 666 cm-1

Bending for CH2

Asymmetric symmetric in-plane out-of-planestretch stretch bend bend2926 cm-1 2853 cm-1 1465 cm-1 1350-1150 cm-1

+ -Out-of planebend or twist1350-1150 cm-1

In plane bend or rocking 1350-1150 cm-1

Assignments of BandsFor a stretching frequency interruption based on

Hooke’s Law: Frequency = 1/2c[(k/(MxMy/Mx+My)]1/2

where f = force constant of bond and M is mass

f is about 5 x105 dyne/cm for single bond 2x that for double bond and 3x that for triple bond

C-H stretch:calc: 3040 cm-1 actual CH3: 2960-2850 cm-1

Note: for C-D: stretch is 21/2 x that of C-H

InstrumentationRequirements: source of IR radiation, sample, detector

IR Spectrophotometer

Sample HandlingIR spectra can be obtained for gases, liquids and solids

Liquids: may be neat or in solutionNeat: between to NaCl plates (0.01 mm film)(NaCl does not absorb until 600 cm-1)thick samples absorb too strongly: poor spectrumSolution: cells are 0.1-1 mm thick(0.1-1 mL in volume)requires second cell of pure solvent to correct for absorptions of solvent

Solids: usually as a mull (supension) in nujol oil (free of IR absorptions 4000-250 cm-1

or dispersed in KCl pellet

Spectral Interpretation

Precise and complete interpretation is NOT possiblethus must use IR in conjunction with other techniques

butFunctional group region: 4000-1300 cm-1:

eg: OH, NH, C=O, S-H, CC Many functional groups exhibit characteristic bands

Fingerprint regions: 1300-650 cm-1absorptions here are usually complexsome interpretation is possible similar compounds give similar spectra but fingerprint is unique

Organic Functional Groups

An Organic ExampleCN stretch 2226

Aromatic C-H bands

Nuclear Magnetic Resonance

Sample in a magnetic field absorbs radio frequencyradiation

absorption depends on certain nuclei in molecule

initially we deal with 1H (proton) NMR

inspection of NMR provides much more structural datathan MS or IR

Magnetic NucleiNuclei with odd mass, odd atomic number or both have quantized spin angular momentum eg 1

1H, 21H, 13

6C, 147N, 31

spin quantum number, I = 0, 1/2, 1, 3/2 …..

For 11H,13

6C,3115P: I = 1/2

For 21H, 14

7N I = 1 (nonspherical charge distribution: electric quadrupole)

number of states in magnetic field 2I+1

In a Magnetic Field

E=(h/2)Bo

Bo is related to strengthof magnetic field

h is Planck’s Constant

E is in the radio frequency range

Absorbance of RFIn magnetic field spinning nucleus precesses about applied magnetic field(Larmor Frequency)

when same frequency RFis appliedelectric field of radiation and electric field of precessing nucleus coupleE is transferred and spin changes -Resonance

Relaxation

How is this energy dissipated?

T1 spin-lattice or longitudinal relaxation processtransfer of E from excited protons to surrounding protons

T2 spin-spin transverse relaxationtransfer of E among precessing protons, result is line broadening

Instrumentation

Magnetic field, radio frequency generator

Instrument

1945-46 at Stanford

Professor Bloch

Nobel Prize 1952

Sample

Typically if want to observe 1H NMR need to avoidsolvent with protons

used deuterated solvent or solvent with no protonsfor example: C6D6, CDCl3 or CCl4

sample is held in a 5mm tube typically 2 mg in 0.5 mL)

sample is spun in the magnetic field to average out field inhomogeneities

Magnets1953: 1.41 Tessla or 60 MHz for proton resonanceNow: 200-500 MHz magnets are common

as high as 900 MHz in some NMR research Labs

magnetic fields are large:

in the case of 500 MHz magnetic 5Gauss lines forma a15 ft sphere about the magnets

Modern Instrument

Chemical ShiftElectron density in a magnetic field circulates generatinga magnetic field in opposition to the applied fieldthus shielding the nucleus….

Since electron density for each type of proton environment is different get different resonanceabsorption of RF

eff = (/2Bo(1-) is the shielding constant

reference position relative to the standard TMStetramethylsilane

CH3H3C

NMR ScaleSet TMS to zero Hz (300 MHz magnet)

if we use this scale must specify the strength of magnet

as frequency of resonance will change with field

better to use dimensionless units: (ppm)

freq/applied field x 106 =

0 Hz3000 Hz

0 ppm10 ppm

NMR Scales

0 ppm10 ppm

300 MHz

0 ppm10 ppm

6000 Hz 0 Hz

0 Hz3000 Hz

600 MHz

Field Strength Effect

60 MHz

300 MHz

Chemical Shifts

As the shift depends somewhat on electron density electronegativity may be a guide for chemical shifts

electron density around protons of TMS is high

positive increases to left of TMS

increase means deshielded relative to TMSsince C is more electronegative than C expect:

R3CH>R2CH2>RCH3>CH4

1.6 1.2 0.8

NMR Scales

0 ppm10 ppm

300 MHz

0 ppm10 ppm

6000 Hz 0 Hz

0 Hz3000 Hz

600 MHz

Higher frequency-less shielded

Lower frequency-more shielded

Acetylenebased on electronegativity expect higher chemical shift than ethylene Apparent anomaly H-CC chemical shift is 1.8 ppm

WHY?linear molecule: if aligned with magnetic fieldthen -electrons can circulate at right angles to field and generate magnetic field in opposition to applied field thus: protons experience diminished field and thus resonance at lower frequencythan expected

1.7-1.8 ppm

AldehydesDeshielded position of aldehyde protonobserved at 9.97 ppm (acetaldehyde)

Benzene

Ring current effect deshields aromatic protons 7.0-8.0 ppm (depending on substitution)

[18]Annulene

Outside protons are deshielded 9.3 ppm

protons on inside shielded -3.0 ppm

Acetophenone

All protons are deshielded due tp ring currents

Ortho-protons are further deshielded due to carbonyl

meta, para 7.40 ppm

ortho 7.85 ppm

Ring current effect infer planarity and aromaticity

General Regions of Chemical Shifts

10 9 8 7 6 5 4 3 2 1 0 ppm

aldehydicAromatic

alkenedisubstituted aliphatic

monosubstituted aliphaticalkyne

Integration: Benzyl Acetate

Integration 5:2:3

At high resolution see multiplet

Spin-spin Coupling

Chemically inequivalent protons: field of one proton affects the other normally only see up to 3-bond coupling

Spin-spin Coupling

J is the coupling constant

Each proton has a unique absorption but effected by magnetic field of other proton

Coupling

C-H sees CH2 protons CH2 sees C-H proton

(+1, 0, -1) (+1/2, -1/2)

Ethylbenzene

Typical ethyl patternA2B3

quartet

triplet

Pascal’s Triangles

Isopropylbenzene

Ethanol in CDCl3

Rapid exchange of OH: do not see coupling

CH3CH2OH

Ethanol in DMSO

CH3CH2OH

No exchange

Doublet of Quartets

CH3CH2OH

Can see: J(CH2-OH) and J(CH3-CH2)

N-methylcarbamate

14N has I =1, if exchange is rapid no couplingintermediate or slow --broad NH;

H-C-N-H Coupling

In trifluoroacetic acid, amine is protonatedsee methylene coupling to N-H protons

Fluoroacetone, CH3COCH2F

19F has I = 1/2

Other Magnetic Heteroatoms2H (Deuterium): I = 1; simplifies proton spectrum as H-D coupling is smallX-CH2-CH2-CH2-COY X-CH2-CH2-CD2-COYtriplet, quintet, triplet triplet, slightly broad triplet

31P: I = 1/2 (100% natural abundance)large coupling constants P-H 200-700 Hz

29Si: I = 1/2 (4.7% Natural abundance)Si-CH 6 Hz; low intensity (satellites)

13C: I = 1/2 (1.1% Natural abundance)not seen unless enriched with 13C

Chemical Shift Equivalence

Nuclei are chemical shift equivalent if they are interchangeable through a symmetry operation or by a rapid process.

Rotation about a simple axis (Cn)Reflection through a plane of symmetry ()Inversion through a center of symmetry (i)

Rotation and Reflection

C2 axis of rotation Environments are indistinguishable

CO2HH3C

Reflection through a plane; protons are mirror images of each other (enantiotopes)

Enantiotopes and Diastereotopes

Enantiotopic by i

CO2HH3C

Methylenes are diastereotopicnot equivalent couple to each other

Chiral moelcule

Diastereotopic protons(achiral molecule)

CO2HHO

Plane makes H1’s and H2’sequivalent

no plane through CH2’s thus the protons are diastereotopic

Diastereotopic protons can not be placed in same chemical environment

Rapid Exchange

Equilibrium at low T

At high T see an average spectrum

13C NMR Spectroscopy

12C not magnetically active but 13C has I = 1/2 Natural abundance is 1.1%

sensitivity is 1/5700 of 1H this problem is overcome with Fourier Transform (FT) NMR instrumentation (1970’s)

use broadband decoupling of protons so see no coupling and get NOE enhancement in 13C signal intensity

13C{1H} NMR

13C samples usually run in CDCl3 and chemical shifts are reported relative to TMS

300 MHz for 1H NMR == 75.5 MHz for 13CNMR

10 mg in 0.4 mL of solvent in 5 mm tube

13C NMR of diethylphthalate

Proton coupled

13C{1H} NMR of diethylphthalate

Proton decoupled

13C{1H} NMR of diethylphthalate

Proton decoupled10-s delay

Peak Intensityin 13C NMR the relaxation times vary over a wide range so peak areas do not integrate for the correct number of nuclei

long delays could work but the time required is prohibitive

NOE response is not uniform for all C atom environments

C atoms without protons attached give low intensity

Deuterium Substitution

Substitution of D for H results in decreased intensity

deuterium has I = 1 so 13C is split into 3 lines ratio 1:1:1

possible spin states for D are -1, 0 +1

thus CDCl3 exhibits a 1:1:1 triplet in 13C NMR

Chemical Shifts

Carbon chemical shifts parallel (generally) proton shifts but with a much broader range

eg. Two substituents on a benzene ring

para: three carbon peaks ortho: three peaksmeta: four peaks R

t-butyl alcohol

2,2,4-trimethyl-1,3-pentanediol

Alkenes, Alkynes and Aromatics

Alkenes: sp2 carbons seen in range 110-150 ppm

Alkynes: sp carbons seen in range 65-95 ppm

Aromatic: benzene 128.5 ppm substituted +/-35ppm

substituted carbons decreased peak heightlonger T1 and diminished NOE

Carbon based Functional Groups

Ketones: R2CO 203.8 ppm(acetone)

Aldehydes: RHCO 199.3 ppm (Acetylaldehyde)

Carboxylic acids: RCO2H 150-185 ppm

Nitriles: RCN 150-185 ppm

Oximes: R2CN(OH) 145-165 ppm

Example

H3C CH2

11.5011.00

18.759.75

13C-1H Coupling

Coupling is less important than in 1H NMR

since routinely decoupled.

One-bond C-H coupling: 110-320 Hztwo bond: -5 to 60 Hzthree bond: about same as two bond for sp3 C but for aromatics three bond is often bigger than two bondin Benzene: 3JC-H = 7.4 Hz, 2JC-H = 1.0 Hz

Example Spectra 1: C5H10O

Singlet: 211.8 ppmdoublet

quartetH3C

CH CH3

Example Spectra 2: C4H10O

doublet

quartettriplet

Example Spectra 3: C11H14O2

doublet

quartettriplet

singlet

Other Nuclei for NMR

Nuclei Spin Nat. Abund.2H (1) 0.0156Li (1) 7.4215N (1/2) 0.3719F (1/2) 10023Na (3/2) 10029Si (1/2) 4.731P (1/2) 100

19F NMR Spectrum of fluoracetone

19F NMR: Fluoracetophenone

29Si NMR Spectrum of TMS

29Si NMR:triethylsilane

29Si NMR:1,1,3,4-tetramethyldisiloxane

31P NMR Spectrum of H3PO4

31P NMR Spectrum

(Solvent)NH3C

H3C CH3

(Solvent)

H3C CH3

31P NMR

31P NMR PPh2

DiastereomersPPh2

PRR'R''

Vibrational Spectroscopy

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