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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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:
•structural identity, “fingerprinting”
The IR spectra in this evenings talk are from the SDBS data base.
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.
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.
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
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.
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
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:
k is the “force constant”, like the Hookes Law restoring force for a spring. Known and tabulated for different vibrations
The “reduced mass” where m1, m2 are the masses on either side of vibration
Spectral Region Frequency(Hz) Wavenumber(cm-1) Wavelength (,m)
After Table 16-1 of Skoog and West, et al. (Chapter 16)
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
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
While useful, this oversimplifies, since molecular orbital picture requires that atoms can’t vibrate without affecting the rest of the molecule.
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.
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/()
CDCl3Some 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.
Gas phase IR
Pivaldehyde + methylamine
Note loss of C=O
Thank to Drs. Dalton and Mascavage
Draw a line straight up from 3000 cm-1. Intensity on left is Csp2-H, to the right is Csp3-H
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
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-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
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...
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
Analogous to Carboxylate ion. Strong bands
1520, 1350 cm-1
1550, 1370 cm-1
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?
960-970 cm-1 trans o.o.p bend
675-730 cm-1 cis o.o.p. bend
1460 cm-1 CH2 scissoring
725 cm-1 characteristic rocking for 4 or more CH2’s in a row (non-cyclic)
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)
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
Along with br. OH str.
Lowers to 1715 cm-1
Raises to 1770 cm-1
Similar, to 1715 cm-1
Weakens DB character
Strengthens DB character (inductive over resonance)
Complements the out-of plane bendings, related to the number of adjacent H…
From Crewes, Rodriguez and Jaspars, ch 8
Out-of-plane bending combinations, quite small, but in a normally clean region of IR. Reliable even with nitro or carboxyl substitution
Unreliable with NO2, CO2H subsBenzene 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.
wk SH at 2580
1360, 1180 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
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-CN 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