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FALL 2011

CHEM 430. IR SPECTROSCOPY. FALL 2011. Long Lecture Play. Dr. Justik. Introduction. The method provides a rapid and simple method for observing the functional group species present in an organic molecule

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FALL 2011

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  1. CHEM 430 IR SPECTROSCOPY FALL 2011 Long Lecture Play Dr. Justik

  2. Introduction • The method provides a rapid and simple method for observing the functional group species present in an organic molecule • The spectrum is a plot of the percentage of IR radiation that passes through the sample (% transmission) versus some function of the wavelength of the radiation related to covalent bonding CHEM 430 – NMR Spectroscopy

  3. Introduction Instrumentation. Modern IR spectrometers are based on the Michelson interferometer • Fourier transform infrared ( FT– IR) spectrometers: The absorption spectrum is obtained by means of Fourier transformation of an interferogram. • Dispersive infrared spectrometers: Earlier instruments based on monochromators that disperse the radiation from an IR source into its component wavelengths - spectrum is obtained by measuring the amount of radiation absorbed by a sample as the wavelength is varied. • Raman spectroscopy provides information complementary to that obtained from IR spectroscopy. CHEM 430 – NMR Spectroscopy

  4. Vibrations of Molecules The IR Spectroscopic Process. • The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration • When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms • For a simple diatomic molecule, this model is easy to visualize: CHEM 430 – NMR Spectroscopy

  5. H H H H C C C C C C H H H H H H C C C C H H Vibrations of Molecules The IR Spectroscopic Process. • There are two types of bond vibration: • Stretch – Vibration or oscillation along the line of the bond • Bend– Vibration or oscillation not along the line of the bond asymmetric symmetric scissor rock twist wag in plane out of plane 5

  6. Vibrations of Molecules The IR Spectroscopic Process. • Each stretching and bending vibration occurs with a characteristic frequency • Typically, this frequency is on the order of 1.2 x 1014 Hz (120 trillion oscillations per sec. for the H2 vibration at ~4100 cm-1) • The corresponding wavelengths are on the order of 2500-15,000 nm or 2.5 – 15 microns (mm) • When a molecule is bombarded with electromagnetic radiation (photons) that match the frequency of one of these vibrations (IR radiation), it is absorbed and the bonds begin to stretch and bend more strongly (emission and absorption) • When this photon is absorbed the amplitude of the vibration is increased NOT the frequency CHEM 430 – NMR Spectroscopy

  7. Vibrations of Molecules The IR Spectroscopic Process. • The result of the spectroscopic process is a spectrum of the various stretches and bends of the covalent bonds in an organic molecule CHEM 430 – NMR Spectroscopy 7

  8. Vibrations of Molecules The IR Spectroscopic Process. • The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1 (Remember E = ħnor E = ħc/l) • This unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed – chemists like this, physicists hate it High frequencies and highwavenumbers equate higherenergy is quicker to understand than Short wavelengths equate higher energy CHEM 430 – NMR Spectroscopy 8

  9. Vibrations of Molecules The IR Spectroscopic Process. • This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequency • IR spectra are observed for what is called the mid-infrared: 400-4000 cm-1 • The peaks are Gaussian distributions of the average energy of a transition CHEM 430 – NMR Spectroscopy 9

  10. Vibrations of Molecules The IR Spectroscopic Process. • So how does the IR detect different bonds? • The potential energy stretching or bending vibrations of covalent bonds follow the model of the classic harmonic oscillator (Hooke’s Law) Remember: E = ½ ky2 where: y is spring displacement k is spring constant Potential Energy (E) Interatomic Distance (y) CHEM 430 – NMR Spectroscopy 10

  11. 154 pm 10 pm 10 pm 4o Vibrations of Molecules The IR Spectroscopic Process. Aside: Physically here are the movements we are discussing: • Stretching vibration: a typical C-C bond with a bond length of 154 pm, the displacement is averages 10 pm: • Bending vibration: For C-C-C bond angle a change of 4° is typical, which corresponds to an average displacement of 10 pm. CHEM 430 – NMR Spectroscopy 11

  12. Vibrations of Molecules The IR Spectroscopic Process. • The energy levels for these vibrations are quantized as we are considering quantum mechanical particles • Only discrete vibrational energy levels exist: • Note there is no energy level below n = 0, at any temperature above absolute zero there is always the first vibrational energy level rotational transitions – (in microwave region) Potential Energy (E) Vibrational transitions, n Interatomic Distance (r) CHEM 430 – NMR Spectroscopy 12

  13. Vibrations of Molecules The IR Spectroscopic Process. • However, the application of the classical vibrational model fails apart for two reasons: • As two nuclei approach one another through bond vibration, potential energy increases to infinity, as two positive centers begin to repel one another • At higher vibrational energy levels, the amplitude of displacement becomes so great, that the overlapping orbitals of the two atoms involved in the bond, no longer interact and the bond dissociates • We say that the model is really one of an aharmonic oscillator, for which the simple harmonic oscillator model works well for low energy levels CHEM 430 – NMR Spectroscopy 13

  14. Vibrations of Molecules The IR Spectroscopic Process. • Here is the derivation of Hooke’s Law we will apply for IR theory: • Vibrational frequency given by: n : frequency K: force constant – bond strength m: reduced mass = m1m2/(m1+m2) • Reduced massis used, as each atom in the covalent bond oscillates about the center of the two masses CHEM 430 – NMR Spectroscopy 14

  15. Vibrations of Molecules The IR Spectroscopic Process. • What does this mean for the different covalent bonds in a molecule? Let’s consider reduced mass, m, first: • The C-H and C-C single bonds differ by only 16 kcal/mole: 99 kcal · mol-1 vs. 83 kcal · mol-1 (similar K) • Due to the reduced mass term, these two bonds of similar strength show up in very different regions of the IR spectrum: C─C 1200 cm-1m = (12 x 12)/(12 + 12) = 6 (0.41) C─H 3000 cm-1m = (1 x 12)/(1 + 12) = 0.92 (0.95) • A smaller atom therefore gives rise to a higher wavenumber (and n and E) CHEM 430 – NMR Spectroscopy 15

  16. Vibrations of Molecules The IR Spectroscopic Process. • What does this mean for the different covalent bonds in a molecule? When greater masses are added, the trend is similar (K’s here are different) C─I 500 cm-1 C─Br 600 cm-1 C─Cl 750 cm-1 C─O 1100 cm-1 C─C 1200 cm-1 C─H 3000 cm-1 A smaller atom therefore gives rise to a higher wavenumber (and  n and E) and a larger atom gives rise to lower wavenumbers (and  n and E) CHEM 430 – NMR Spectroscopy 16

  17. Vibrations of Molecules The IR Spectroscopic Process. • What does this mean for the different covalent bonds in a molecule? Let’s consider bond strength, K: • A C≡C bond is stronger than a C=C bond is stronger than a C-C bond wavenumber, cm-1DHf • From IR spectroscopy we find: C≡C ~2100 200 C=C ~1650 146 C—C ~1200 83 Note the good correlation with the heats of formation for each bond! Stronger bonds give higher wavenumbers (and  higher n and E) CHEM 430 – NMR Spectroscopy 17

  18. Vibrations of Molecules The IR Spectroscopic Process. • The y-axis of the IR spectrum is in units of transmittance, T, which is the ratio of the amount of IR radiation transmitted by the sample (I) to the intensity of the incident beam (I0); % Transmittance is T x 100 T = I / I0 %T = (I / I0) X 100 • IR is different than other spectroscopic methods which plot the y-axis as units of absorbance (A). A = log(1/T) • As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional vs. concentration of other functionalities, it can be used vs. itself if standardized!!! CHEM 430 – NMR Spectroscopy 18

  19. Vibrations of Molecules The IR Spectroscopic Process. • The intensity of an IR band is affected by two primary factors: • Whether the vibration is one of stretching or bending • Electronegativity difference of the atoms involved in the bond: • For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. • The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending CHEM 430 – NMR Spectroscopy 19

  20. Vibrations of Molecules The IR Spectroscopic Process. • It is important to make note of peak intensities to show the effect of these factors: • Strong (s) – peak is tall, transmittance is low • Medium (m) – peak is mid-height • Weak (w) – peak is short, transmittance is high • * Broad (br) – if the Gaussian distribution is abnormally broad (* this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded CHEM 430 – NMR Spectroscopy 20

  21. II. Infrared Group Analysis • A. General • The primary use of the IR spectrometer is to detect functional groups • Because the IR looks at the interaction of the EM spectrum with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds • Since most “types” of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum • Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks) • There are 4 principle regions: 4000 cm-1 2700 cm-1 2000 cm-1 1600 cm-1 400 cm-1

  22. We will pick up next time with peak intensities, width of bands and some simple symmetry rules, as well as instrument design Monday we should finally get to functional groups where we will apply in depth the general topics we have discussed in the introductory material No Problem set for today! But take this time to review some organic: - bond strengths – both inter and intra-molecular - bond distances for more organic-y bonds - hybridization models - Periodic table and properties – you should know the position and EN’s of H, B, C, N, O, F, Si, P, S, Cl, Br and I

  23. IR Spectroscopy • I. Introduction • The IR Spectrum • The intensity of an IR band is affected by two primary factors: • Whether the vibration is one of stretching or bending • Electronegativity difference of the atoms involved in the bond: • For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak • The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment • Typically, stretching will change dipole moment more than bending • It is important to make note of peak intensities to show the effect of these factors: • Strong (s) – peak is tall, transmittance is low • Medium (m) – peak is mid-height • Weak (w) – peak is short, transmittance is high • * Broad (br) – if the Gaussian distribution is abnormally broad • (*this is more for describing a bond that spans many energies) • Exact transmittance values are rarely recorded

  24. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • We have learned: • That IR radiation can “couple” with the vibration of covalent bonds, where that particular vibration causes a change in dipole moment • The IR spectrometer irradiates a sample with a continuum of IR radiation; those photons that can couple with the vibrating bond elevate it to the next higher vibrational energy level (increase in A) • When the bond relaxes back to the n0 state, a photon of the same n is emitted and detected by the spectrometer; the spectrometer “reports” this information as a spectral band centered at the n of the coupling • The position of the spectral band is dependent on bond strength and atomic size • The intensity of the peak results from the efficiency of the coupling; e.g. vibrations that have a large change in dipole moment create a larger electrical field with which a photon can couple more efficiently

  25. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Remember, most interesting molecules are not diatomic, and mechanical or electronic factors in the rest of the structure may effect an IR band • From a molecular point of view (discounting phase, temperature or other experimental effects) there are 10 factors that contribute to the position, intensity and appearance of IR bands • Symmetry • Mechanical Coupling • Fermi Resonance • Hydrogen Bonding • Ring Strain • Electronic Effects • Constitutional Isomerism • Stereoisomerism • Conformational Isomerism • Tautomerism (Dynamic Isomerism)

  26. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Symmetry H2O • For a particular vibration to be IR active there must be a change in dipole moment during the course of the particular vibration • For example, the carbonyl vibration causes a large shift in dipole moment, and therefore an intense band on the IR spectrum • For a symmetrical acetylene, it is clear that there is no permanent dipole at any point in the vibration of the CC bond. No IR band appears on the spectrum

  27. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Symmetry H2O • Most organic molecules are fortunately asymmetric, and bands are observed for most molecular vibration • The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes • Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

  28. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Symmetry H2O • Symmetry also effects the strength of a particular band • The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes • Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

  29. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Mechanical Coupling • In a multi-atomic molecule, no vibration occurs without affecting the adjoining bonds • This induces mixing and redistribution of energy states, yielding new energy levels, one being higher and one lower in frequency • Coupling parts must be approximate in E for maximum interaction to occur (i.e. C-C and C-N are similar, C-C and H-N are not) • No interaction is observed if coupling parts are separated by more than two bonds • Coupling requires that the vibration be of the same symmetry

  30. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Mechanical Coupling • For example, the calculated and observed n for most C=C bonds is around 1650 cm-1 • Butadiene (where the two C=C systems are separated by a dissimilar C-C bond) the bands are observed at 1640 cm-1 (slight reduction due to resonance, which we will discuss later) • In allene however, mechanical coupling of the two C=C systems gives two IR bands – at 1960 and 1070 cm-1 due to mechanical coupling • For purposes of this course, when we discuss the group frequencies, we will point out when this occurs

  31. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Fermi Resonance • A Fermi Resonance is a special case of mechanical coupling • It is often called an “accidental degeneracy” • In understanding this, for many IR bands, there are “overtones” of the fundamental (the n’s you are taught) at twice the wavenumber • In a good IR spectrum of a ketone (2-hexanone, here) you will see a C=O stretch at 1715 cm-1 and a small peak at 3430 cm-1 for the overtone overtone fundamental

  32. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Fermi Resonance • Ordinarily, most overtones are so weak as not to be observed • But, if the overtone of a particular vibration coincides with the band from another vibration, they can couple and cause a shift in group frequency and introduce extra bands • If you first looked at the IR (working “cold”) of benzoyl chloride, you may deduce that there were two dissimilar C=O bonds in the molecule

  33. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Fermi Resonance • In this spectrum, the out of plane bend of the aromatic C-H bonds occurs at 865 cm-1; the overtone of this band coincides with the fundamental of C=O at 1730 cm-1 • The band is “split” by Fermi resonance (1760 and 1720 cm-1)

  34. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Fermi Resonance • Again, we will cover instances of this in the discussion of group frequencies, but this occurs often in IR of organics • Most observed: • Aldehydes – the overtone of the C-H deformation mode at 1400 cm-1 is always in Fermi resonance with the stretch of the same band at 2800 cm-1 • The N-H stretching mode of –(C=O)-NH- in polyamides (peptides for the biologists and biochemists) appears as two bands at 3300 and 3205 cm-1 as this is in Fermi resonance with the N-H deformation at 1550 cm-1

  35. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Hydrogen Bonding • One of the most common effects in chemistry, and can change the shape and position of IR bands • Internal (intramolecular) H-bonding with carbonyl compounds can serve to lower the absorption frequency 1680 cm-1 1724 cm-1

  36. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Hydrogen Bonding • Inter-molecular H-bonding serves to broaden IR bands due to the continuum of bond strengths that result from autoprotolysis • Compare the two IR spectra of 1-propanol; the first is an IR of a neat liquid sample, the second is in the gas phase – note the shift and broadening of the –O-H stretching band Gas phase Neat liquid

  37. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Hydrogen Bonding • Some compound, in addition to intermolecular effects for the monomeric species can form dimers and oligomers which are also observed in neat liquid samples • Carboxylic acids are the best illustrative example – the broadened O-H stretching band will be observed for the monomer, dimer and oligomer

  38. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Ring Strain • Certain functional group frequencies can be shifted if one of the atoms hybridization is affected by the constraints of bond angle in ring systems • Consider the C=O band for the following cycloalkanones: • 1815 1775 1750 1715 1705 cm-1 • We will discuss the specific cases for these shifts during our coverage of group frequencies

  39. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Electronic Effects - Inductive • The presence of a halogen on the a-carbon of a ketone (or electron w/d groups) raises the observed frequency for the p-bond • Due to electron w/d the carbon becomes more electron deficient and the p-bond compensates by tightening

  40. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Electronic Effects - Resonance • One of the most often observed effects • Contribution of one of the less “good” resonance forms of an unsaturated system causes some loss of p-bond strenght which is seen as a drop in observed frequency

  41. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Electronic Effects - Resonance • In extended conjugated systems, some resonance contributors are “out-of-sync” and do not resonate with a group • Example:

  42. IR Spectroscopy • I. Introduction • The IR Spectrum – Factors that affect group frequencies • Electronic Effects - Sterics • Consider this example: • In this case the presence of the methyl group “misaligns” the conjugated system, and resonance cannot occur as efficiently • The effects of induction, resonance and sterics are very case-specific and can yield a great deal of information about the electronic structure of a molecule

  43. IR Spectroscopy • Group Frequencies and Analysis • Introduction • When approaching any IR spectrum be sure to use the larger-to-smaller region approach- do not immediately focus on any one single peak (even –OH or C=O) • From the Hooke’s Law derivation we are using we find that the IR can be conveniently be divided into four major regions: 4000 cm-1 2700 cm-1 2000 cm-1 1600 cm-1 400 cm-1

  44. IR Spectroscopy • Group Frequencies and Analysis • Introduction • If supporting information is available – molecular formula, chemical inferences – (i.e. this was the product of an oxidation reaction), assume this information is correct and the analysis of the IR should support it (later in your careers you can doubt information given to you) • If a molecular formula is available, do an HDI! • Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it. • The most common mistakes in spectral analysis are those of “jumping the gun” to a conclusion (usually based on some small, insignificant peak) or taking a random haphazard approach to the spectrum (gee, here is an IR, oh, let’s start looking for phosphorus this time) • Be methodical, develop a scheme and stick to it!

  45. IR Spectroscopy • Group Frequencies and Analysis • Before we begin – Each functional group will be described as follows: • Group • General – What is most recognizable? What makes it different from similar groups? • Group Frequencies (cm-1): Scale on bottom summarizes band positions and strengths Strong - Medium - Weak -

  46. IR Spectroscopy • Group Frequencies and Analysis • The Hydrocarbons • Alkanes • General – due to the small electronegativity difference between C and H, hydrocarbon bands are of medium intensity at best and give simple spectra • Group Frequencies (cm-1):

  47. IR Spectroscopy • Group Frequencies and Analysis • The Hydrocarbons • Alkanes – Dodecane – C12H26

  48. IR Spectroscopy • Group Frequencies and Analysis • The Hydrocarbons • Alkanes – Cyclopentane – C5H10

  49. IR Spectroscopy • Group Frequencies and Analysis • The Hydrocarbons • Alkanes • Additional – If the 1400-1350 region is free of interference, the presence of certain alkyl groups can be discerned: H C C H H C C H H H C C H H Methylene Methyl Scissor 1465 Bendasymm 1450 Bendsymm 1375 usually overlap gem-dimethyl 1380 1370 t -butyl 1390 1370

  50. IR Spectroscopy • Group Frequencies and Analysis • The Hydrocarbons • Alkanes • Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane: vs.

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