Infrared Spectroscopy

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1 x-rays ultraviolet (UV) visible Infrared (I) microwaves radiowaves near I middle I far I λ (cm) 8 x x x x 10-2 µ ν (cm -1 ) 13,000 4, ν (cm -1 1 ) = λ (cm) µ = micron (older unit) E = 1-10 kcal/mol bond vibrations O C H C H stretching bending ν (cm -1 ) = 1 λ (µm) x 10,000 stretching & - changes in bond lengths bending - changes in bond angles

2 Only vibrations that produce a change in dipole moment are observed in the I ' δ + δ - C C C C ' C O ' ~1640 cm -1 ~2100 cm -1 ~1730 cm -1 ALWAYS a dipole so always seen in spectrum if present H 3 C CH 3 C C H 3 C CH 3 H 3 C C C CH 3 I "invisible" if molecule is symmetric weak band if similar groups

3 Fundamental Stretching & Bending Vibrations O C H C H stretching bending

4 Hooke s Law Can approximate position (wavenumber) of spectral band using Hooke s Law m 1 m 2 K µ and µ = m 1 m 2 m 1 + m 2 K so: m 1 m 2 m 1 + m 2 m = mass of atom K = force constant of bond c = velocity of light (3x10 10 cm/sec)

5 Hooke s Law m 1 m 2 m 1 + m 2 Can thus derive some qualitative relationships: à stronger bonds absorb at higher frequencies (higher K = higher n ) C C C C C C ν (cm -1 ) ~2159 ~1650 ~1200 Bond Strength (kcal/mol) à as mass of atom increases, frequency decreases C H C C C O C Br C I ν (cm -1 ) increasing atomic mass

6 Other Vibrations Spectra include a number of other peaks. In addition to fundamental stretching and bending absorptions. Overtones: lower intensity vibration at an integral times the fundamental frequency ν overtone = n x ν fundamental Combination bands: the sum of two interacting vibrational frequencies, but only certain combinations are allowed ν combination = ν + ν 2 Difference bands: similar to combination bands, but the difference of two interacting vibrations ν difference = ν - ν 2 Fermi resonance: when a fundamental absorption couples with an overtone or combination band, most often observed for C=O

7 Overtones 1st overtone usually at twice normal ν à typically weak C O ~1720 cm -1 1st overtone ~3440 cm -1 near OH, 2 NH, terminal alkyne

8 Fermi esonance Overtone falls close to a fundamental band à intensity is greatly enhanced usually results in doubling of that band

9 The Infrared Spectrometer Dispersive Infrared Spectrophotometer measures one frequency of light at a time scan speed is relatively slow

10 The Infrared Spectrometer Fourier Transform Infrared Spectrophotometer (FT-I) measures all frequencies of light at a time each scan requires less time; lower resolution than dispersive I collect lots of scans & average à higher resolution overall faster acquisition, higher resolution than dispersive instrument overall

11 Sample Handling solution cell sample can be gas, liquid or solid samples can be taken in solution - subtract out solvent by taking background/reference spectrum liquids/oils samples often taken neat samples typically applied to polished NaCl plate (I inactive) adventitious water can be problematic à lead to false signals - false ID of OH, CO 2 H, 2 NH solids more problematic - solids can be deposited as thin film on NaCl plate dissolve, drop on plate, then evaporate some solvent solvents: CCl 4, CHCl 3 & CH 2 Cl 2 - sample can be mixed with Nujol (petroleum oil, high boiling) obscures aliphatic region (less useful) - sample can pressed into KBr pellet may take some practice - AT: apply solid (or liquid) directly to spectrometer salt plate KBr pellet

12 The I Spectrum (w) (m) (s) functional group region fingerprint region

13 Troubleshooting neat: wet sample; sloping to high energy water bands solid film: film too thick (or solvent evaporated between salt plates)

14 Troubleshooting mull: too much sample applied KBr or solid film: too little sample applied

15 General Absorbance anges in the I X=C=Y (C, O, N, S) N 3 stretching vibrations fingerprint region bending vibrations

16 Correlation Chart (Adapted from Pavia Table 2.3) complete table can be found on class web site etc.

17 The I Spectrum (w) (m) (s) functional group region fingerprint region (s) (m) (w) strong medium weak

18 Alkanes Simple spectra with few peaks C-H stretch most prominent (around 3000 cm -1 ) - sp 3 C-H stretch between cm -1 saturated, unstrained hydrocarbons typically to the right of 3000 cm -1 C-H stretch on vinyl or aromatic carbon to left of 3000 cm -1 - CH 2 /CH 3 bending vibrations between cm -1 CH 2 bend around 1465 cm -1 CH 3 bend around 1375 cm -1 - long chain (four or more CH 2 groups in long chain may see a band at 720 cm -1 weak, often obscured in more complex molecules C-C stretch not interpretively useful

19 Alkanes decane cyclohexane

20 Alkanes (bromomethyl)cyclopropane Br cyclopropane C-H ca cm -1

21 Alkenes Spectra more complex than alkanes =C-H stretch to left of 3000 cm -1 - sp 2 C-H stretch between cm -1 =C-H out of plane bending vibrations between cm -1 - can sometimes be used to determine degree of alkene substitution - may be obscured in complex systems C=C stretch occurs between cm -1 - intensity varies with dipole - conjugation shifts absorbance to lower frequencies; increases intensity

22 Alkenes 1-hexene

23 Alkenes cis-2-pentene H H H 3 C CH 3 C=C sp 2 C-H

24 Alkenes trans-2-pentene C=C (very weak) H 3 C H H CH 3 sp 2 C-H

25 Alkenes Conjugation Effects Conjugation moves C=C stretch to lower frequencies (~15-20 cm -1 ) O 1643 cm cm cm cm cm -1 Conjugation increases amount of s-character; weakens bond, lowers force constant May see multiple absorbances if alkene is conjugated to another double bond

26 Alkenes trans-1,3-pentadiene H 3 C H H H CH 2 sp 2 C-H C=C

27 Alkenes C=C stretch: effect of ring strain in endocyclic alkenes 1650 cm cm cm cm cm -1 decreasing absorption frequency 1646 cm -1 ~1611 cm -1

28 Alkenes C=C stretch: substituent effects in endocyclic alkenes 1656 cm cm cm cm cm cm cm cm cm cm cm cm -1 increasing absorption frequency

29 Alkenes C=C stretch: effect of ring strain in exocyclic alkenes 1940 cm cm cm cm cm cm -1 increasing absorption frequency more s character more p character

30 Alkenes C=C stretch in acyclic alkenes * * vinyl vinylidene H ~1645 cm -1 H ~1655 cm -1 H H H cis H H ~1660 cm -1 * trans H H ~ cm -1 trisubstituted tetrasubstituted H ~ cm -1 ~ cm -1 vinyl ether O ~ cm -1 (usually strong) * =C-H bending absorbance may supplement assignment

31 Alkenes Out of Plane Bending and Alkene Substitution! approx frequency 990, 910 cm cm cm cm cm -1 none BEWAE! Fingerprint egion

32 Alkynes C-H stretch usually near 3300 cm -1 - sp C-H stretch between cm -1 C C stretch vibrations between cm -1 - conjugation shifts absorbance to lower frequencies; - absorbances for symmetrically substituted alkynes may be weak or absent

33 Alkynes 1-hexyne

34 Alkynes 4-octyne

35 Alkynes allyl 2-butynoate sp 2 C-H O O C=C C C C=O

36 Aromatic Ings =C-H stretch to left of 3000 cm -1 - sp 2 C-H stretch between cm -1 =C-H out of plane bending vibrations between cm -1 - can often be used to determine ring substitution pattern C=C stretch between cm -1 - two to three peaks; often occur in pairs at 1660 and 1475 cm -1 Overtone/Combination bands appear between cm -1 - can sometimes be used to assign ring substitution pattern

37 Aromatic ings toluene

38 Aromatic ings Out of Plane Bending mono: ortho: meta: para cm -1 (strong) cm -1 (strong)* cm -1 (strong) cm -1 (strong) 780 cm -1 (strong) cm -1 (moderate)* cm -1 (strong) * may be absent

39 Aromatic ings overtones

40 Aromatic ings toluene cm -1 (strong) cm -1 (strong)*

41 Aromatic ings o-diethylbenzene cm -1 (strong)

42 Aromatic ings m-diethylbenzene cm -1 (strong) 780 cm -1 (strong) cm -1 (moderate)*

43 Aromatic ings p-diethylbenzene cm -1 (strong)

44 Alcohols & Phenols O-H stretch occurs between cm -1 - position and shape vary depending on amount of hydrogen bonding free O-H: sharp peak between cm -1 H-bonded O-H: broad peak between cm -1 C-O-H bending vibrations between cm -1 - not diagnostic C-O stretch between cm -1 - can sometimes provide information about alcohol structure

45 Alcohols & Phenols 2-butanol p-cresol

46 Alcohols & Phenols OH Stretch: Hydrogen Bonding vs. Free OH hydrogen bonded OH free and hydrogen bonded OH (dilute solution) free and hydrogen bonded OH (very dilute solution)

47 Alcohols & Phenols cyclohexanol OH neat 3331 cm -1 C-O-H bend OH solution in CCl cm -1

48 Alcohols & Phenols OH 3322 cm -1 methyl salicylate OCH 3 O O H neat 3190 cm -1 OCH 3 O O H solution in CCl cm -1

49 Alcohols & Phenols C-O stretch and structure phenol 3 alcohol 2 alcohol 1 alcohol C-O stretch 1220 cm cm cm cm -1 decreasing frequency OH OH OH OH 1237 cm cm cm cm -1

50 Alcohols & Phenols effect of unsaturation on C-O stretch 3 alcohol 2 alcohol 1 alcohol C-O stretch 1150 cm cm cm -1 secondary alcohols OH OH OH 1070 cm -1 (Δ 70 cm -1 ) 1070 cm -1 (Δ 70 cm -1 ) 1060 cm -1 (Δ 60 cm -1 ) primary alcohols OH OH 1017 cm -1 (Δ 33 cm -1 ) 1030 cm -1 (Δ 20 cm -1 )

51 Alcohols & Phenols free OH stretch and structure phenol 3 alcohol 2 alcohol 1 alcohol free O-H stretch 3610 cm cm cm cm -1 increasing frequency OH OH OH OH 3614 cm cm cm cm -1

52 Ethers Difficult to identify by I C-O stretch most prominent feature ( cm -1 ) - aliphatic ethers show one strong band (~1120 cm -1 ) - phenyl/vinyl alkyl ethers show two strong bands (~1250 and 1140 cm -1 ) Many other FG show C-O stretch - may distinguish ether from alkane - can distinguish from alcohol (no OH stretch) - can distinguish from carbonyl derivative (no C=O stretch)

53 Ethers dibutyl ether 1023 cm -1 decane

54 O O Ethers ethoxybenzene O C-O-C 1250 cm cm -1 ethyl vinyl ether O C-O-C 1207 cm cm -1

55 Ethers ethoxybenzene O deformation bands 1,3-dioxolane O O

56 Amines N-H stretch occurs between cm -1 - signal varies with amine structure 1 amines show two bands 2 amines have one band 3 amines have no absorbance in this region N-H bend vibrations between cm -1-1 amines: amines: ~1500 cm -1 not diagnostic C-N stretch between cm -1 H N H H N N 1 2 3

57 Amines cyclohexylamine NH 2 N-H stretch N-H bend

58 Amines dibutylamine H N

59 Amines N-methylaniline

60 Amines tributylamine

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