Química Orgânica I Ciências Farmacêuticas Bioquímica Química AFB QO I 2007/08 1 IR spectroscopy AFB QO I 2007/08 2 1
Adaptado de: Organic Chemistry, 6th Edition; L. G. Wade, Jr. Organic Chemistry, William H. Brown AFB QO I 2007/08 3 Infrared Spectroscopy IR spectrum of 3-methyl-2-butanone AFB QO I 2007/08 4 2
Infrared Spectroscopy scale The vibrational IR extends from 2.5 x 10-6 m (2.5 µm) to 2.5 x 10-5 m (25 µm) the frequency of IR radiation is commonly expressed in wavenumbers wavenumber (ν) - : the number of waves per centimeter, with units cm -1 (read reciprocal centimeters) expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 400 cm -1 ν = 10-2 m cm -1 2.5 x 10-6 m = 4000 cm -1 10-2 m cm -1 ν = = 400 cm -1 2.5 x 10-5 m AFB QO I 2007/08 5 Molecular vibrations Covalent bonds vibrate at only certain allowable frequencies. http://www.askthenerd.com/ocol/spec/ir/irf.htm AFB QO I 2007/08 6 3
Stretching Frequencies Frequency decreases with increasing atomic mass. Frequency increases with increasing bond energy. AFB QO I 2007/08 7 Vibrational Modes Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes. AFB QO I 2007/08 8 4
Fingerprint of Molecule Whole-molecule vibrations and bending vibrations are also quantized. No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm -1. Complex vibrations: 600-1400 cm -1, called the fingerprint region. AFB QO I 2007/08 9 IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will absorb weakly or not at all. AFB QO I 2007/08 10 5
An Infrared Spectrometer AFB QO I 2007/08 11 FT-IR Spectrometer Has better sensitivity. Less energy is needed from source. Completes a scan in 1-2 seconds. Takes several scans and averages them. Has a laser beam that keeps the instrument accurately calibrated. AFB QO I 2007/08 12 6
IR spectra interpretation AFB QO I 2007/08 13 Molecular vibrations Fundamental stretching and bending vibrations for a methylene group AFB QO I 2007/08 14 7
Some metylene vibrations http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html AFB QO I 2007/08 15 Molecular Vibrations Consider two covalently bonded atoms as two vibrating masses connected by a spring the total energy is proportional to the frequency of vibration the frequency of a stretching vibration is given by an equation derived from Hooke s law for a vibrating spring ν = 4.12 K = a force constant, which is a measure of the bonds strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 10 5 dynes/cm µ = reduced mass of the two atoms, (m 1 m 2 )/(m 1 + m 2 ), where m is the mass of the atoms in grams AFB QO I 2007/08 16 K µ 8
Molecular Vibrations ν = 4.12 K µ From this equation, we see that the position of a stretching vibration is proportional to the strength of the vibrating bond is inversely proportional the masses of the atoms connected by the bond The intensity of absorption depends primarily on the polarity of the vibrating bond AFB QO I 2007/08 17 Carbon-Carbon Bond Stretching Stronger bonds absorb at higher frequencies: C-C 1200 cm -1 C=C 1660 cm -1 C C <2200 cm -1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C 1640-1680 cm -1 conjugated C=C 1620-1640 cm -1 aromatic C=C approx. 1600 cm -1 AFB QO I 2007/08 18 9
Carbon-Hydrogen Stretching Bonds with more s character absorb at a higher frequency. sp 3 C-H, just below 3000 cm -1 (to the right) sp 2 C-H, just above 3000 cm -1 (to the left) sp C-H, at 3300 cm -1 AFB QO I 2007/08 19 Correlation Tables Characteristic IR absorptions for the types of bonds and functional groups we deal with most often Bond Stretching Frequency (cm -1 ) Intensity O-H 3200-3650 weak to strong N-H 3100-3550 medium C-H 2700-3300 w eak to medium C=C 1600-1680 w eak to medium C=O 1630-1820 strong C-O 1000-1250 strong AFB QO I 2007/08 20 10
Hydrocarbon Alk ane C-H CH 3 C-C Alk ene C-H C=C Alkyne C-H C C Arene C-H C=C C-H Vibration Intens ity Stretching 2850-3000 Medium Bending 1450-1475 Mediu m Bending 1375 and 1450 Weak to medium (N ot useful for interpretation - too many b ands Stretching Stretching Frequen cy (cm -1 ) 3000-3100 1600-1680 Weak to medium Weak to medium Stretching 3300 Mediu m to stron g Stretching 2100-2250 Weak Stretching 3030 Weak to medium Stretching 1450-1600 Mediu m Bending 690-900 Strong AFB QO I 2007/08 21 Alkanes IR spectrum of decane AFB QO I 2007/08 22 11
Alkenes IR spectrum of cyclohexene AFB QO I 2007/08 23 Alkynes IR spectrum of 1-octyne AFB QO I 2007/08 24 12
Alkynes IR Spectra AFB QO I 2007/08 25 Aromatics IR spectrum of toluene AFB QO I 2007/08 26 13
Alcohols Bond O-H (free) O-H (H bonded) C-O Frequency, cm -1 Inten sity 3600-3650 Weak 3200-3500 Medium, broad 1000-1250 Medium AFB QO I 2007/08 27 Alcohols IR spectrum of 1-hexanol AFB QO I 2007/08 28 14
Ethers IR spectrum of dibutyl ether AFB QO I 2007/08 29 Ethers IR spectrum of anisole AFB QO I 2007/08 30 15
Amines IR spectrum of 1-butanamine (Fig 12.11) AFB QO I 2007/08 31 CHO, CO, COOH Carbonyl Group O RCR' O RCH Ketones C=O Aldehydes C=O C-H Vibration Frequen cy (cm -1 ) Intensity Stretchin g 1630-1820 Strong Stretching 1630-1820 Strong Stretching 2720 Weak O RCOH Carboxylic acids C=O Stretching 1700-1725 Strong O H Stretching 2500-3300 Strong (broad) AFB QO I 2007/08 32 16
Aldehydes and Ketones IR spectrum of menthone AFB QO I 2007/08 33 Carbonyl groups The position of C=O stretching vibration is sensitive to its molecular environment as ring size decreases and angle strain increases, absorption shifts to a higher frequency O O O O 1715 cm -1 1745 cm -1 1780 cm -1 1850 cm -1 conjugation shifts the C=O absorption to lower frequency O O O H 1717 cm -1 1690 cm -1 1700 cm -1 AFB QO I 2007/08 34 17
acid derivatives O RCNH 2 O RCOR' O O RCOCR RC N Amides C=O N H Stretching 3200, 3400 M edium (1 amides have two N-H stretches) (2 amides have one N-H stretch) Carboxylic esters C=O Stretching 1735-1800 Strong sp 2 C O Stretching 1200-1250 Strong sp 3 C O Acid anhydrides C=O Stretching 1740-1760 and Strong 1800-1850 C O Stretching 900-1300 Strong Nitriles C N Stretching 1630-1680 Strong Stretching 1000-1100 Strong AFB QO I 2007/08 35 Stretching 2200-2250 M edium Carboxylic acids IR spectrum of pentanoic acid AFB QO I 2007/08 36 18
O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, 2500-3500 cm -1, due to strong hydrogen bonding. AFB QO I 2007/08 37 Esters IR of ethyl butanoate AFB QO I 2007/08 38 19
Carbon - Nitrogen Stretching C - N absorbs around 1200 cm -1. C = N absorbs around 1660 cm -1 and is much stronger than the C = C absorption in the same region. C N absorbs strongly just above 2200 cm -1. The alkyne C C signal is much weaker and is just below 2200 cm -1. AFB QO I 2007/08 39 A Nitrile IR Spectrum AFB QO I 2007/08 40 20
Summary of IR Absorptions AFB QO I 2007/08 41 Strengths and Limitations IR alone cannot determine a structure. Some signals may be ambiguous. The functional group is usually indicated. The absence of a signal is definite proof that the functional group is absent. Correspondence with a known sample s IR spectrum confirms the identity of the compound. AFB QO I 2007/08 42 21
Raman spectroscopy http://www.chemsoc.org/exemplarchem/e ntries/2004/birmingham_jones/raman.html #Raman http://www.kosi.com/raman/resources/tec hnotes/1101.pdf IR bands arise from a change in the dipole moment of a molecule, Raman bands arise from a change in the polarizability AFB QO I 2007/08 43 Theoretical Background Molecule Absorption (i.e. IR spectroscopy) Electromagnetic wave Scattering (i.e. Raman Spectroscopy) Diffraction (i.e. x-ray diffraction) AFB QO I 2007/08 44 22
Normal Raman Instrumentation AFB QO I 2007/08 45 IR and Raman Comparison of IR and Raman for a CO 2 AFB QO I 2007/08 46 A. Fadini and F.M. Schnepel, Vibrational Spectroscopy 23