Organic Spectra Infra Red Spectroscopy H. D. Roth. THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth. Infra Red Spectroscopy

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1 rganic Spectra Infra Red Spectroscopy. D. Roth TERY and INTERPRETATIN of RGANI SPETRA. D. Roth Infra Red Spectroscopy Infrared spectroscopy (IR) is an analytical technique concerned with molecular vibrations and rotations; we will be concerned mostly with vibrations such as stretching of bonds or bending of groups. The nature of the bond or group determines the vibrational frequency. To observe an IR band, a change in dipole moment must accompany the motion. Therefore polar covalent bonds (=,, l) have strong IR bands, whereas symmetrically substituted bonds or functions do not absorb at all. R R The dipole moment of a two-atomic entity or along a bond connecting two groups can be formulated as µ = ( δ δ ) d The charge of the electron is esu and atomic distances fall into the range of 10 8 cm (Å); typical dipole moments fall into the order of esu. For convenience this quantity is defined as 1 D (debye). Applications of IR Spectry 1. Identify groups by characteristic frequencies. 2. Positive identification by comparison with known spectra (Aldrich IR Library). 3. Fourier transform (FT)-IR very sensitive, used for trace analysis G-FT-IR even more sensitive. 4. Quantitative Analysis - Beer's Law cannot be used easily. In special cases calibration curves can be set up using known concentrations. alibrations checked with polystyrene film (strong bands at 1603, 1946 cm-1) 1

2 rganic Spectra Infra Red Spectroscopy. D. Roth Four types of samples are used routinely in IR spectroscopy Films Solutions Mulls Pellets (Nal cell) l4, S2, 3l (why not 2?) fine suspension in Nujol pressed from mixture with finely ground KBr What kind of vibrations give rise to IR transitions (and can be observed)? 1. Stretching "A B" (self-explanatory; stretching frequencies are most useful for structure identification) Molecules with two identical atoms (, l) or groups (3, N) on the same (or N) may have two coordinated stretching frequencies, i.e., symmetrical and antisymmetrical, illustrated below for a 2 group: Symmetrical Antisymmetrical 2. Bending, a deformation of one or more bond angles; for example a methyl group has a symmetrical bending mode, increasing/decreasing all three R angles equally, and an unsymmetrical mode where one R angle is changed in the opposite sense. R Symmetrical R Unsymmetrical 2

3 rganic Spectra Infra Red Spectroscopy. D. Roth 3. Additional types of molecular motions, giving rise to IR bands, include Scissoring Wagging Twisting Rocking In these drawings the vibrations are illustrated for a 2 group; analogous vibrations occur also for groups such as (3)2, l2, F2, etc. Number of vibrations to be expected Non-linear molecules of N atoms have 3 N 6 "normal vibrational modes 2 3 atoms 3 modes l 4 atoms 6 modes N3 4 5 atoms 9 modes ethanol 9 atoms 21 modes acetone 10 atoms 24 modes 2-chlorobutane 14 atoms 36 modes pentose 20 atoms 54 modes pinene 26 atoms 72 modes terpenes bviously, in complex molecules there will be many similar and overlapping bands, reducing the overall number of bands, and rendering certain regions of the IR spectrum less useful. For example bands in the alkyl region, although different from alkenyl and alkyne bands (vide infra), does not lend itself to specific assignments. 3

4 rganic Spectra Infra Red Spectroscopy. D. Roth Identification of functional groups Spectral Range cm 1 haracteristic frequencies of individual groups cm 1 "Fingerprint" region; allows comparison with known spectra cm 1 Differently substituted aromatics; various bending frequencies verview cm 1,, N cm 1, N cm 1 =, =, =N, N= cm 1,, N For example, we can follow an esterification, R R' R R' 2 (would you use an Nal cell?) or the reaction of acetic anhydride with an alcohol (3 )2 R 3 R 3 by observing characteristic,, and frequencies. haracteristic frequencies Alkenes Free (sharp) cm 1 bonded (broad) cm 1 R bend cm 1 stretch cm 1 The characteristic frequencies for various types of alkenes are not very different. 1 and 13 NMR will be of much greater value for the proper assignment of alkenes. 4

5 rganic Spectra Infra Red Spectroscopy. D. Roth R-=2 R2=2 cis-r-=-r trans-r-=-r R2=R2 Alkynes: A) Internal R R' 1645 cm cm cm cm cm cm 1 R R symmetrical ompare R N cm 1 This type of carbon has a very characteristic 13 frequency B) Terminal R cm cm 1 ompare R= cm 1 or R cm 1 frequencies reflect bond strengths R R R 2 sp sp2 sp3 strongest weakest most acidic least acidic cm cm cm 1 R N sp N sp 2 N sp 3 weakest intermediate strongest base base 5

6 rganic Spectra Infra Red Spectroscopy. D. Roth The carbonyl (>=) stretching frequency ( cm 1) is used to illustrate subtle substituent effects on the frequency; substituents change frequencies in a predictable manner. Three different effects illustrated: a) Inductive effects b) Resonance c) Strain effects Example A: inductive effects F 3 F 3 F cm 1 The electron withdrawing effect of the F3 or F functions cause changes in the dipole moment relative to 3 or alkyl substituted ketones. F 3 F 3 Example B: effect of resonance X N X = N Example : effects of ring strain 1691 F 1700 cm 1 F cm kcal mol-1 Stretching frequency increases with increasing strain energy 6

7 rganic Spectra Infra Red Spectroscopy. D. Roth Aldehydes R = stretch α,β-unsaturated aldehydes and stretch actually the first overtone of the carbonyl bending frequency (~1390). [Also unistakeable 1 (>9.5 ppm) and 13 chemical shift (>200 ppm).] Esters Lactones (cyclic esters) δ,ε... γ β cm 1 (cf., cyclic ketones) Acids = cm cm 1 This frequency only in very dilute solution, dimers, etc cm 1 antisym stretch arboxylate ions R- (2 bands) cm 1 sym stretch cm 1 antisym stretch Anhydrides (2 bands) cm 1 sym stretch Acid chlorides cm 1 Aroyl chlorides have a weak second band at cm 1 Amides cm 1 The amide = frequency is lowest among carbonyls because of resonance N 7

8 rganic Spectra Infra Red Spectroscopy. D. Roth armonic scillator Evib = h c ϖ (vq 1/2) ν vib = 1 2π K µ 1/2 ( ) µ = m A x m B m A m B Evib ω vq νvib K vibrational energy harmonic wavenumber (cm-1) related to vibrational frequency and to the potential energy function vibrational quantum number (v = 0,1,2,3...n) energy levels are evenly spaced. vibrational frequency force constant (millidynes/å) µ reduced mass (a measure of the total mass that is vibrating). Note: do not mistake ν (vibrational frequency, νvib) for vq (vibrational quantum number) and vice versa 8

9 rganic Spectra Infra Red Spectroscopy. D. Roth The role of isotopic substitution on IR stretching frequencies The effect is largest for the lightest atom; we compare the vibrational frequencies of D vs. or D vs. bonds vs D Stretching Frequency ν vib-d (µ = = - ) 1/2 ν vib (µ -D ) 1/2 µ-d = 12 x = ; (µ -D)1/2 = µ- = 12 x = ; (µ-)1/2 = ν-d ν- = = Accordingly, the typical frequency (3,000 cm 1) is reduced to 2200 cm 1 upon D-substitution. vs D Stretching Frequency ν-d ν- µ-d = µ- = = (µ -)1/2 (µ-d)1/2 = x = x = ν- = 3650 cm-1 ν -D = 2657 cm -1 Δν = 1000 cm -1 9

10 rganic Spectra Infra Red Spectroscopy. D. Roth Isotope substitution in bonds between two heavy atoms results in less dramatic changes. An example involving two "heavy" elements: ν13-14n ν12-14n = µ13-n = (µ13-n)1/2 = µ12-n = (µ12-n)1/2 = ν12-n = 2100 cm-1 ν13-n = 2056 cm -1 Δν = 44 cm -1 The olor of Water Water has the three vibrational frequencies: ν cm -1 a stretching mode ν cm -1 a bending mode ν cm -1 a combination mode The fourth overtone of ν3 would occur at a wavenumber of ~15,000 cm 1, i.e., a wavelength of ~665nm. Although very weak, this overtone can be observed, if the "cell" is longer than the usual thin film (IR) or 10 cm (UV/VIS). Is this the reason why algae are green? 10

11 rganic Spectra Infra Red Spectroscopy. D. Roth FD FR TUGT What would the color of water be on a planet where D, and not, is the predominant isotope of hydrogen? More on overtones ptical fibers for telecommunications used to be manufactured from the thermal reaction of Sil4 with molecular oxygen. Sil4 2 > Si2 2 l2 Sil4 is hygroscopic and reacts with water by hydrolysis Sil4 2 > Sil3 l Even minor impurities would seriously affect the performance of the optical fibers, since the fourth overtone of the Si stretching frequency, at a wavenumber of ~15,000 cm-1, corresponding to a wavelength of ~665nm, would absorb the light of the diode lasers used for the transmission of optical data. Although the overtone is very weak, it becomes prohibitive for cables that are many miles long, i.e., for a "cell" many kilometers long. 11

12 rganic Spectra Infra Red Spectroscopy. D. Roth Specific EXAMPLES 1) 3085 cm 1 = 1600 cm 1 = 2) 3350 cm 1 3) 1700 cm 1 = 4) 1690 cm 1 = 5) 1720 cm 1 = R 3480 cm 1 6) N 3395 cm cm 1 = 7) 1768 cm 1 = 8) 2250 cm 1 N 12