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 The IR spectra in this evenings talk are from the SDBS data
Transcript
Vibrational SpectraMolecules 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
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 PerspectiveIR’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. RamanThese 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
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
From Skoog and West
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
What Kind of vibrations are These?Bonds can…….
Stretch
Bend
Wag (rock)
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:
μν k12103.5 −×=
21
21
mm
mm
+=μ The “reduced mass” where m1, m2
are the masses on either side of vibration
k is the “force constant”, like the Hookes Law restoring force for a spring. Known and tabulated for different vibrations
Regions of Frequencies
After Table 16-1 of Skoog and West, et al. (Chapter 16)
Near -to visible- IR (NIR)
Combination bands
3.8 x 1014 to 1.2
x 1014
12800 to 4000 0.78 to 2.5
Mid Infrared
Fundmental bands for organic molecules
1.2 x 1014 to 6.0
x 1012
4000 to 200 2.5 to 50
Far IR
Inorganics organometallics
6.0 x 1012 to 3.0
x 1011
200 to 10 50 to 1000
Spectral Region Frequency(Hz) Wavenumber(cm-1) Wavelength (,μm)
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
R
R
H
H
R
R
H
H
R
R
H
H
R
R
H
H
R
R
H
H
R
R
H
H
antisymmetric symmetric
rockingscissoring
in-planebending
stretching
out-of-plane bending
wagging twisting
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
While useful, this oversimplifies, since molecular orbital picture requires that atoms can’t vibrate without affecting the rest of the molecule.
Calculated IR bands for CH2 in formaldehyde
Formaldehyde spectrum from: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2Results generated using B3LYP//6-31G(d) in Gaussian 03W.
Looking at a SpectrumDivide the spectrum in to two regions:
4000 cm-1 1600 cm-1 most of the stretching bands, specific functional groups (specific atom pairs). This is the “functional group” region.
1600 cm-1 400 cm-1 Many 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/(μ)
Some IR Media are better than
others
CHCl3
CDCl3
Nujol (mineral oil)
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
Gas phase IR
Pivaldehyde + methylamine
Note loss of C=O
Thank to Drs. Dalton and Mascavage
A Functional Group Chart
O-H str
NH str
COO-H
=C-H str
Csp3-H
C-H
-(C=O)-H
CN
CC
C=O
-C=N
-C=C
phenyl
C-O
C-N
F C-X
4000 3600 3200 2800 2400 2000 1600 1200 800 group
IBrCl
The hydrogen stretching region
3000cm-1
Tip--
Draw a line straight up from 3000 cm-1. Intensity on left is Csp2-H, to the right is Csp3-H
