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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.

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Presentation on theme: "Vibrational Spectra Molecules are not Static Vibration of bonds occurs in the liquid, solid and gaseous phase Vibrating  Energy  Frequency (and the appropriate."— Presentation transcript:

1 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.

2 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.

3 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.

4 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

5 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 CsCl 2 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.

6 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

7 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: The “reduced mass” where m 1, m 2 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

8 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 to 1.2 x to to 2.5 Mid Infrared Fundmental bands for organic molecules 1.2 x to 6.0 x to to 50 Far IR Inorganics organometallics 6.0 x to 3.0 x to 1050 to 1000 Spectral RegionFrequency(Hz) Wavenumber(cm -1 ) Wavelength (,  m)

9 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 mD/Å Triple Bond mD/Å

10 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 antisymmetricsymmetric rockingscissoring in-plane bending 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.

11 Calculated IR bands for CH 2 in formaldehyde Formaldehyde spectrum from: Results generated using B3LYP//6-31G(d) in Gaussian 03W.

12 Looking at a Spectrum Divide 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 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 =10 4 / (  )

13 Some IR Media are better than others CHCl 3 CDCl 3 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.

14 Reaction Monitoring Gas phase IR Pivaldehyde + methylamine Note loss of C=O Thank to Drs. Dalton and Mascavage

15 A Functional Group Chart O-H str NH str COO-H =C-H str C sp3 -H  C-H -(C=O)-H CNCN CCCC C=O -C=N -C=C phenyl C-O C-N F C-X group I Br Cl

16 The hydrogen stretching region 3000cm -1 Tip-- Draw a line straight up from 3000 cm-1. Intensity on left is C sp2 -H, to the right is C sp3 -H Arom- H

17 Amines 3500, 3300 cm -1 doublet, frequently (without, with H-bonding effect) NH stretch 1600 cm -1 NH 2 scissoring - broad cm -1 NH 2 wagging- broad, strong 1080 cm -1 C–N str. --weak for alkyl 1300 cm -1 Ar–N str. strong R-NH 2 R–NH–R 3400 cm -1 singlet str. Weak C–N 1125 cm -1 R–NR–R No good IR bands, adj CH 2 will shift to 2800 cm -1. A tert amine salt NH strong at 2500 cm -1

18 Amine Salts R-NH 3 + like methyl, but broad NH str. centered at 3000, br. deformation br. R2NH br, spikes at 2200,2500 cm -1 NH2 scissor at 1600 cm -1, broad

19 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 RCH 2 –OH1050 cm -1 R 2 CH–OH1110 cm -1 R 3 C–OH1175 cm -1 phenethanol 1-phenylethanol 2-piperidinylethanol

20 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...

21 Alkynes 6-methyl5-hepten-1-yne phenylacetylene 3,5- dimethyl-1- hexyn-3-ol benzonitrile Phenyl-1-butyne Also wk overtones at 1820,1790 cm -1

22 Functional Groups can be “NMR Silent” or ambiguous. IR can play a key role in Identification 1800 cm -1 doublet, Fermi resonance Poor resonance for 2p-3p, but strong inductive effect 1800, 1750 cm -1 (cyclic, has more intense 1750, acyclic, more instense 1800 C-O-C vs cm -1

23 More NMR silent groups-- Nitro groups Analogous to Carboxylate ion. Strong bands Aromatic nitro 1520, 1350 cm -1 Aliphatic nitro 1550, 1370 cm -1

24 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 cm -1 and 985cm -1 are o.o.p bendings for terminal =CH2 Cis vs Trans? Disubstituted –HC=CH cm -1 trans o.o.p bend cm -1 cis o.o.p. bend Medium intensity

25 Methylenes 1460 cm -1 CH 2 scissoring 725 cm -1 characteristic rocking for 4 or more CH 2 ’s in a row (non-cyclic)

26 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)

27 Ketones--sensitive to strain O O O 1715 cm cm 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)

28 Ketones--Sensitive to conjugation cm cm -1 rotational isomers cause doubling. S-trans 1674, S-cis cm -1 for enol 1715 cm -1 for the keto bond Along with br. OH str.

29 Aldehydes Ethyl-2-butenal cyclohexylcarboxaldehyde Doublet straddles 2800 cm -1 (Fermi resonance) Fundamental at ca Bending at 1400, gives overtone at 2800

30 Carboxylic Acids Phenylbutyric acid Cyclohexanecarboxylic acid Hept-3-enoic acid 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! 1715 cm -1 br OH stretch Good example of the broadening from H- bonding

31 Esters and Lactones 1735 cm intense, often doublet Butyl acetate Cyclohexyl acetate

32 Effects of conjugation Lowers to 1715 cm -1 Similar, to 1715 cm -1 Raises to 1770 cm -1 : Weakens DB character Strengthens DB character (inductive over resonance)

33 Lactones, similar effects 1735 cm cm cm cm -1

34 amides phenylacetamide N-benzylbenzamide phenylacetanilide L-  -aspartyl-L- phenylalanine 1-methyl ester NH str 3300 cm -1 C=O 1650 cm -1 NH bend 1640 cm -1 Moves to 1550 for R-C(=O)-NHR’

35 Methyl Groups isopropylcyclohexane t-butylcyclohexane methylcyclopentane “isopropyl split” “ t-butyl split” But… OCH 3, NCH 3 do not give this band Moves lower by 20 cm -1 CH 3 “umbrella” Ca cm -1

36 Benzene rings--substitution patterns 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 Ring pucker at ca 700 cm -1 is IR active for mono, 1,3-di-, 1,3,5-tri-, 1,2,3-tri- subs rings. 850,780, , Unreliable with NO 2, CO 2 H subs Complements the out-of plane bendings, related to the number of adjacent H…

37 Putting the S in Silent; Sulfur containing groups 1320, 1140 cm cm , 1160 cm , 1180 cm -1 wk SH at , 1200 cm -1 Extremely wk at cm -1 Induction strengthens D.B. resonance not significant

38 Silence is Deafening (the IR guys think “silence is golden” R-N=C=O strong at cm -1 R-N=C=S strong cm -1 R-N=N=N strong cm -1 R-N=C=N-R strong 2150 cm -1 R-C  N 2250 cm -1 R-N +  C: cm -1 R-C=C=C-R’ (allenes) cm -1 with very strong 850 wag if terminal


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