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CHM 504 Infra-red Spectroscopy Electromagnetic Spectrum Objectives of IR spectroscopy • To identify the functional groups present in molecules • To identify compounds by comparison with spectra of known compounds in a database. Molecular vibrations Natural frequency of vibration depends on •Mass of each atom; light atoms (e.g., H) vibrate faster. •Strength of bond; strong bonds vibrate faster. Modes of vibration • Molecules with more than two atoms can vibrate in several different ways. Each way is called a MODE. • Every mode of vibration has a natural frequency. • Each mode of vibration involves distortions of – Bond length : STRETCHING – Bond angle : BENDING Vibrations of H2O Symmetric stretching Asymmetric stretching Bending Interaction of matter with infra-red energy • Molecules can absorb energy in the form of infra-red radiation. • The radiation being absorbed must have the same frequency as one of the modes of vibration of the molecule. • When a molecule absorbs one photon of IR radiation of appropriate frequency, the corresponding mode of vibration increases its amplitude. • The frequency of vibration does not change. Vibrations and infra-red radiation Infra-red radiation • Amplitude of vibration increases • Frequency of vibration unchanged Energy of IR radiation • IR radiation is a form of electromagnetic radiation. • Every photon of electromagnetic radiation has a quantum of energy, given by the equation E = hn where h is Planck’s constant, 6.626 10-34 J s. • The frequency n is related to the wavelength l by the equation ln = c where c is the speed of light. Measurement of IR radiation • Can be measured as either frequency (cycles per second, Hz, s-1) or wavelength (micrometers, mm). • For historic reasons, usually measured as wavenumber (cycles per centimeter, cm-1). • Wavenumber is a kind of frequency; the greater the wavenumber, the higher the energy. • Symbol for wavenumber is n. Wavenumber and wavelength 4 10 1 n (cm ) l (mm) Absorption of IR radiation due to vibrations occur in the range 10 000 - 100 cm-1 (1 - 100 mm) Range of real interest in IR spectroscopy: 4000 - 400 cm-1 (2.5 - 25 mm) Infra-red Spectrum • Graph • Horizontal axis: – Wavelength (2.5 - 25 mm) OR – Wavenumber (4000 - 400 cm-1) – usually. • Vertical axis: – Absorbance (A) OR – Transmittance (T) – usually (0 - 100%). IR spectrum of 3-hydroxyacetophenone Some definitions I Transmittance: T 100 % I0 Absorbance: I0 A log I 100 A log T Spectral bands or peaks • A complex molecule has many modes of vibration. • Each mode has a characteristic frequency. • The molecule can absorb IR radiation of each characteristic frequency. • Each such absorption appears as a band or peak in the IR spectrum. Identification of molecules • Each molecule has a unique set of modes of vibration. • Therefore each molecule has a unique spectrum. • Unknown molecule can be identified by comparing spectrum with spectra of known molecules. Exact one-to-one matching of peaks sufficient to identify molecule. • Computer required to search database of known spectra. Qualitative information from IR spectra • Many modes of vibration principally involve specific bonds or functional groups. • Peaks corresponding to those vibrations reveal the presence of those functional groups. • Each peak has 3 characteristics that can provide information: – Wavenumber – Intensity (strong, medium, or weak) – Shape (sharp, normal, or broad) • Not every peak provides useful information. Shapes of peaks sharp normal broad Analysis of IR spectrum • Spectrum can be divided into 4 regions: – – – – Region 1: 4000 - 2550 cm-1 Region 2: 2500 - 2000 cm-1 Region 3: 1900 - 1400 cm-1 Region 4: 1400 - 400 cm-1 • Different types of vibrations, corresponding to different functional groups, are found in different regions. 4000 - 2550 cm-1 • C–H, N–H, O–H, and (rarely) S–H stretching vibrations. • Can be distinguished based on wavenumber, intensity, and shape. • Very important in the identification of – – – – Alcohols / phenols Carboxylic acids Primary / secondary amines Amides – Terminal alkynes (R–CC–H) 2500 - 2000 cm-1 • Triple bond stretching (R–CC & R–CN) • Stretching in cumulative pairs of double bonds (X=C=Y, where X and Y could be C, N, or O). • In most spectra, this area is blank. 1900 - 1400 cm-1 • C=C, C=O, and C=N stretching peaks. • Stretching of bonds that are intermediate between single and double (1300 - 1600 cm-1). For example, • Often the most important part of the spectrum Functional groups visible in 1900 1400 cm-1 region • Aldehydes and ketones • Other C=O containing functional groups, including – – – – Carboxylic acids Acid chlorides and anhydrides Esters Amides • C=C double bonds • Aromatic rings 1400 - 400 cm-1 • Stretching of single bonds to atoms other than H, e.g., C–C, C–O, C–N, C–Cl, etc. • C–H bending peaks. • Usually lots of peaks. Most cannot be identified or interpreted. • Region of spectrum that tends to be unique for a given compound. • Sometimes referred to as the fingerprint region. Identification of functional groups • Based on presence of key peaks in spectrum. • Wavenumber, shape, and intensity of peak should be considered. • Some functional groups cannot be easily identified using IR, e.g., those containing only single bonds other than O-H, N-H. – Alkyl halides – Ethers – Tertiary amines Alkanes (alkyl groups) • C–H stretching: 2850 - 2960 cm-1 • C–H bending: 1150 - 1390, 1450 - 1465 cm-1 • Many overlapping bands • Presence of these peaks is not informative, since most compounds contain alkyl groups. Alkenes Three types of diagnostic peaks. 1. C=C stretching (1640 – 1670 cm-1, w to m) 2. C–H stretching (3000 – 3100 cm-1) 3. C–H bending IR spectrum of 1-hexene Alkynes 1. CC stretching. R–CC–H 2140 - 2100 cm-1 (medium) R–CC–R’ 2260 - 2190 cm-1 (weak or absent) 2. C–H stretching. 3330 - 3270 cm-1 (strong, sharp) 3. C–H bending. A terminal alkyne (R–CCH) is easily recognised; an internal alkyne is very difficult to spot using IR. IR spectrum of 1-hexyne Aromatic hydrocarbons 1. C=C stretching: two sets of peaks. (i) 1600 - 1585 cm-1 (weak - medium) (ii) 1500 - 1400 cm-1 (weak - medium) • Each set typically contains two peaks. • The second peak in each set may be weak, absent, or appear as a “shoulder.” 2. C–H stretching. 3100 - 3000 cm-1. (indistinguishable from alkene C-H stretch) 3. C–H bending IR spectrum of toluene Alcohols and phenols 1. O–H stretching. broad) 3550 - 3200 cm-1 (strong, Broad because of H – bonding. In dilute solutions, sharp peak at ~3600 cm-1. 2. C–O stretching. 1260 - 1000 cm-1 (strong) (Cannot distinguish between alcohols and phenols based on IR) IR spectrum of (CH3)2CHCH2OH The carbonyl group Identifying a carbonyl-containing functional group • C=O stretching peak – Very strong peak - often the strongest in the spectrum • Consider other characteristic peaks – O–H stretching of carboxylic acid – C–H stretching of aldehyde – N–H stretching and bending of amide – C–O stretching of ester • Absent such peaks, probably a ketone Aldehydes • C=O stretching. 1680 – 1740 cm-1 • C–H stretching. 2830 - 2690 cm-1 – 1-2 peaks, relatively weak – No other peaks appear in this range Ketones • C=O stretching : 1670 – 1750 cm-1 • Can be distinguished from aldehydes by absence of C–H peaks at 2830 - 2690 cm-1. IR spectrum of CH2CH2CH2CHO IR spectrum of CH3C(O)CH2CH2CH3 Carboxylic acids • C=O stretching. 1680 – 1720 cm-1 • O–H stretching: very broad, distinctive peak, not very strong, stretching from ~3300 to ~2500 cm-1 (centered at ~3050 cm-1); C–H stretching peaks are usually superimposed. IR spectrum of CH3(CH2)4CO2H Esters • C=O stretching. 1715 – 1770 cm-1. • C–O stretching: 1250 - 1140 cm-1 – A very strong peak. – Absent in ketones; can be used to distinguish between esters and ketones. – However: its presence does not guarantee an ester! Could be a ketone with a C–O single bond elsewhere. IR spectrum of CH3C(O)OCH2CH3 Amides • C=O stretching: 1680 – 1630 cm-1. • N–H stretching: 3400 – 3180 cm-1. – Primary amides: 2 peaks – Secondary amides: 1 peak – Tertiary amides: missing IR spectrum of CH3CONH2 Primary and secondary amines N–H stretching: 3400 - 3250 cm-1 – Primary (RNH2) : 2 peaks – Secondary (R2NH) : 1 peak – Relatively weak and sharp. Much weaker than amide N–H stretching. – Easy to distinguish from strong, broad OH stretching. – May be obscured if OH, NH2 in same molecule. IR spectrum of CH3(CH2)3NH2