Vibrational Spectroscopy A 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. equilibrium distance between A and B r e 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: D AB r AB 0 D AB = energy required to dissociate into A and B k 2 c k 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 m m m A B A B
Transcript
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
re
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
rAB0
DAB = energy required to dissociate into A and B atoms
k
2
ck
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 m
m mA B
A 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
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
15P
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
Si
CH3
CH3H3C
H3C
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
Hb
Ha
Hx
CN
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
H
H H
H
HH
H
H
H
H
H H
H
H
H
H
HH
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
-1/2
+1/2
+1/2
-1/2
OR
RO
H H
Spin-spin Coupling
J is the coupling constant
OR
RO
H H
Each proton has a unique absorption but effected by magnetic field of other proton
Coupling
OR
H
H H
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
NH
O
O
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
J2J4
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
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
H H
ClCl
C2 axis of rotation Environments are indistinguishable
H H
FCl
H H
CO2HH3C
Reflection through a plane; protons are mirror images of each other (enantiotopes)
Enantiotopes and Diastereotopes
H
H3C
Cl
H
H
Cl
CH3
H
Enantiotopic by i
H H
CO2HH3C
HO H
Methylenes are diastereotopicnot equivalent couple to each other
Chiral moelcule
Diastereotopic protons(achiral molecule)
H2 H1
CO2HHO
H
H2H1
HO2C
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
R
RR
R
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
HO
N
H3C CH2
CH3
OHN
H3C CH2
CH3
11.5011.00
29.00
159.2
18.759.75
21.50
158.7
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
