Chapter 16: Structure Determination 2 - Infrared & UV-Vis Spectroscopy PDF

CHAPTER 16 Structure Determination 2: Infrared Spectroscopy and Ultraviolet–Visible Spectroscopy  Index of Hydrogen Deficiency (Degree of Unsaturation) Number of multiple bonds and/or rings that are present for a molecular formula (CnHmNxOyXz etc) 1) Deter...

CHAPTER 16 Structure Determination 2: Infrared Spectroscopy and Ultraviolet–Visible Spectroscopy  Index of Hydrogen Deficiency (Degree of Unsaturation) Number of multiple bonds and/or rings that are present for a molecular formula (CnHmNxOyXz etc) 1) Determine the hypothetical number of hydrogens for an acyclic, saturated analogue (CnH2n+2) 2) Make corrections for other atoms: – For each N or P, add one H to the calculated formula – For each O or S, make no changes in numbers of H’s in calculated formula – For each F, Cl, Br, or I, subtract one H from the calculated formula 3) Subtract true number of hydrogens from calculated hypothetical number – difference in hydrogens divided by 2 is the unsaturation (number of rings and/or pi bonds) Example: 10/28/2024 C7H17N vs C7H5N 2 The Electromagnetic Spectrum 16.1 OVERVIEW OF INFRARED SPECTROSCOPY Electromagnetic radiation has characteristics of both waves and particles. Wavelength and frequency are inversely related: As one increases, the other must decrease. Relationship between Wavelength and Frequency Behaving as a particle, electromagnetic radiation exists as photons. Each photon possesses a characteristic energy that depends only on its frequency (or wavelength). The energy of a given photon is directly proportional to its frequency: As one increases, so does the other. For IR we use wavenumbers n = 1/l(cm) Why use wavenumbers? Directly proportional to energy E = hn = hc/l = hcnE a n The IR Spectrum Classifying Absorptions A strong absorption is one whose percent transmittance is near zero (near the bottom of the spectrum). A weak absorption is one whose percent transmittance is near 100% (near the top of the spectrum). 16.2 GENERAL THEORY OF INFRARED SPECTROSCOPY Absorption of an Infrared Photon Only IR photons of certain frequencies can be absorbed by a molecule. 16.4 THE BALL-AND-SPRING MODEL FOR EXPLAINING INFRARED PEAK LOCATIONS For a ball-and-spring model, vibrational frequency increases as the masses decrease (a to b). as the stiffness of the C H C O C Br E spring increases (b to c). smaller mass larger mass E C C C C C C stronger bond weaker bond Bond Vibrational Energy and Spectroscopy The frequency at which a peak appears in an IR spectrum is typically the same as the frequency of the molecular vibration responsible for the absorption of the photon. 16.3 LOCATION OF PEAKS IN AN INFRARED SPECTRUM Each type of vibration is typically found within a characteristic range of frequencies. Characteristic Absorption Frequencies Type of Bond Compound class Frequency Range in centimeters to the Appearance Type of Compound Frequency Appearance negative Type of 1Compou Frequency Appearance Bond Class Range Bond nd Class Range O single bond H Alcohol and phenol (centimeters 3200 to 3600 (centimeter Broad, strong superscript s Carboxylic acid negative 1) 2500 to 3200 superscript Broad, strong negative 1) N single bond H Amine Oxygen- Alcohol 3200-3600 Broad, 3300 to 3500 Carbon- Nitrile 2210-2260 Medium Medium hydrogen and phenol strong nitrogen single Amide 3350 tripleto 3500 bond Medium bond C single bond H Alkane Oxygen- Carboxylic 2500-3000 Broad, 2800 to 3000 Carbon- Alkyne ~2200 2100-2260 Variable Variable hydrogen acid strong carbon single Alkene 3000 tripleto 3100 bond ~2100 Weak bond Alkyne Nitrogen- Amine 3300-3500 Medium Approximately Carbon- 3300 Ketones 1680-1750 Strong Strong hydrogen oxygen or single Aldehyde 2720 and 2820 double 1715 aldehyd Strong 1725 bond bond es C triple bondNitrogen- N NitrileAmide 3350-3500 Medium 2210 to 2260 Carbon- Esters 1735-1745 1730-1750 Medium Strong hydrogen oxygen C triple bondsingle C Alkyne 2100 to 2260 double Variable 3000 hydrogen aldehydes oxygen ic acid single Esters 1730 to double 1750 Strong bond bond Carbon-Carboxylic Alkene acid 3000-3100 Weak 1710 to Carbon- 1780 Amide 1650 1630-1690 Strong Strong hydrogen oxygen single Amide 1630 to double 1690 Strong bond bond C double bond C Alkene Carbon- Alkyne Approximately Strong 1620 to Carbon- 1680 Alkene 1620-1680 Variable Variable hydrogen 3300 carbon single Aromatic 1450 to double 1550 Variable 1050-1300 bond bond C single bond O Alcohol, Carbon- ester,2720 and Aldehyde Strong 1050 to Carbon- 1150 Aromatic 1450-1550 Medium Variable ether hydrogen 2820 carbon single double bond bond SOLVED PROBLEM: Use Table 16-1 to estimate the frequencies where IR absorption peaks would appear for each of the stretching vibrations indicated. Major Regions of the IR Spectrum Higher Lower Higher n Lower n Higher E Lower E 16.5 INTENSITIES OF PEAKS IN AN INFRARED SPECTRUM A stretching mode of vibration involving a highly polar bond (such as C=O or O—H) tends to have a strong IR absorption. A stretching mode of vibration involving a nonpolar bond (such as C=C) tends to have a: weak or nonexistent IR absorption if the two portions connected by the bond are similar. medium intensity IR absorption if the two portions connected by the bond are dissimilar. Intensity in Stretching in Hept-1-ene and Hept-3-ene 16.6 SOME IMPORTANT INFRARED STRETCHES When you first encounter an IR spectrum of an unknown molecule, it may seem daunting. Even a simple compound can have dozens of peaks in its IR spectrum. Where do we begin? Look at the peaks above 1400 cm−1 (outside the fingerprint region). Learn to recognize peaks that are easy to spot and provide unambiguous information.  IR Spectra of Hydrocarbons hexane note C-H stretches and bends CH3 H3C C-H bend sp3 C-H stretch 10/28/2024 27  Cyclohexane C-H bend sp3 C-H stretch 10/28/2024 28  IR Spectra of Hydrocarbons 1-Hexene note position of sp2 C-H stretches sp2 sp3 C-H C-H C-H Bend C=C stretch H3C C-H oop bend C-H stretch 10/28/2024 29  Intensity in Stretching No change in dipole – no C=C signal 10/28/2024 30  IR Spectra of Hydrocarbons Toluene note position of sp2 C-H stretches of aromatic ring what is that absorbance at 1606 cm -1 ? CH3 C-H Stretch Aromatic sp2 and sp3 C=C stretch C-H oop bend 10/28/2024 31 The C—H Stretch Intensity  C=C Stretching Frequencies C-H Stretch sp2 and sp3 Bending vibrations for monosubstituted Ring 700 and 750 Bending vibration for monosubstituted Ring ~830 10/28/2024 33 The O—H Stretch The Cause of Broad OH Peaks The N—H Stretch Bands representing N—H stretches share many similarities with those of O—H stretches, including being generally easy to identify. N—H stretches appear between 3300 and 3500 cm−1. Hydrogen bonding involving N—H bonds usually causes these bands to be rather broad, although typically not as broad as those for an analogous O— H stretch. Identifying N—H Stretches N—H Stretching Vibrations in a Primary Amine or Amide Two N—H stretching bands appear for a primary amine or amide because an NH2 group has two different modes of vibration involving the two N—H bonds: symmetric stretching and asymmetric stretching.  Aromatic C=C stretch C-H stretch N-H stretch (2) H3C NH2 C-H “oop” bending Aromatic C=C stretch H N CH3 N-H stretch (1) C-H stretch C-H “oop” bending 10/28/2024 39 The Carbonyl (C=O) Stretch For each of the following compound classes, the C=O stretch typically appears in the corresponding range: Ester 1730–17501735 cm−1cm-1 Aldehyde 1720–17401725 cm−1cm-1 Ketone 1710–1730 cm−11715 cm-1 Know these base values Amide 1630–1690 cm−11650 cm-1 Carboxylic Acid 1710 cm-1  Propanamide N-H stretch C-H stretch (2) C=O stretch ??? O C N-H bend CH3 CH2 NH2 10/28/2024 41  Ethyl Butanoate C-O stretch C-H stretch O C=O stretch C CH3 CH2 CH2 O CH2 CH3 10/28/2024 42  Nonanal Aldehyde C-H stretch (2) C=O stretch C-H stretch O CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 C H 10/28/2024 43 Infrared Spectrum of Heptanal 10/28/2024 45  O O H3C H CH3 N H 3C N H CH3 10/28/2024 46 C=O and Conjugation When a C=O group is conjugated to a C=C or C≡C bond, the frequency of the C=O absorption is typically lowered by 20–40 cm−1.  4-Methyl-3-penten-2-one C-H stretch O C=O stretch CH3 C CH3 C C C=C stretch CH3 H 10/28/2024 48 Alkyne (C≡C) and Nitrile (C≡N) Stretches C≡C and C≡N bonds both appear as sharp bands at roughly the same position, with C≡C between 2100 and 2260 cm−1 and C≡N between 2210 and 2260 cm−1. One way to distinguish between the two functional groups is to search for an alkyne C—H peak at ~3300 cm−1. The presence of one, which appears as a sharp peak, is consistent with a terminal alkyne (C≡C—H). The absence of one may still leave us the choice between a C≡N or an internal alkyne (R—C≡C—R). RC≡CR stretching bands typically have weak absorption intensities, while RC≡N absorptions have moderate intensities.  IR Spectra of Hydrocarbons 1-Hexyne note position of sp C-H stretch at 3208 cm-1 what is that absorbance at 2128 cm-1 ? C≡C stretch C-H Bend sp H3C C C C-H H stretch Terminal Alkyne – need to see both the C≡C and sp C-H stretches 10/28/2024 52  IR Spectra of Hydrocarbons H3C C C H CH3 H3C C C CH3 10/28/2024 53  sp C-H C≡N stretch stretch absent sp3 C-H stretch H3C C N 10/28/2024 54 The C—H Stretch Alkyne C—H, ~3300 cm−1 (moderate intensity) Alkene C—H, 3000–3100 cm−1 (variable intensity) Aromatic C—H, 3000–3100 cm−1 (variable intensity) Alkane C—H, 2800–3000 cm−1 (variable intensity) Aldehyde C—H, two peaks, ~2720 and ~2820 cm−1 (moderate intensity) 16.7 STRATEGIES FOR SUCCESS: STRUCTURE ELUCIDATION USING INFRARED SPECTROSCOPY Structure Elucidation Using IR Spectroscopy  HO OH OH O O 10/28/2024 58  HO OH OH O O 10/28/2024 59  C8H 8O 1691 10/28/2024 60  C8H 8O 1691 O CH3 10/28/2024 61  C6H10O 10/28/2024 62  O 10/28/2024 63 16.8 A DEEPER LOOK: INFRARED BENDING VIBRATIONS Type of Bond Number of Frequency Range Appearance Bands in centimeters to the negative 1 R single bond C H double 2 910 and 990 (two Strong bond C H 2 peaks) R 2 C double bond C H 2 1 890 Strong R C H double bond C H R cis 1 660 to 730 Strong R C H double bond C H R 1 970 Strong trans R 2 C double bond C H R 1 815 Moderate Aromatic monosubstituted 2 700 and 750 Strong Aromatic ortho 1 750 Strong Aromatic meta 2 780 and 880 Strong Aromatic para 1 830 Strong Determining Substitution Patterns from Bending Vibrations 16.9 AN OVERVIEW OF ULTRAVIOLET–VISIBLE SPECTROSCOPY 16.10 ULTRAVIOLET–VISIBLE SPECTRA AND MOLECULAR STRUCTURE: CONJUGATION AND LONE PAIRS Notice that there is a peak (λmax) in the spectrum, centered at 217 nm. Effects of Structure on λmax Compound Lambda Compound Lambda Compound Lambda maximum maximum maximum (nanomete (nanomete (nanomete rs) rs) rs) Alkanes Less than Empty Cell Empty Cell Empty Cell Empty Cell and 150 cycloalkan es Ethene, 161 Buta-1,3- 217 Cyclohexa- 256 with two diene, 1,3-diene, carbons showing a showing a bonded by zigzag hexagonal a double four-carbon six-carbon bond. Each chain with ring with carbon double double bonded to bonds bonds two between between hydrogen carbons 1 carbons 1 atoms. and 2, and and 2, and carbons 3 carbons 3 and 4. and 4. Hex-1-ene, 177 cis-Penta- 223 Hexa-1,3,5- 274 showing a 1,3-diene, triene, zigzag six- showing a showing a carbon zigzag five- zigzag six- chain with carbon carbon a double chain with chain with bond a double double between bond bonds carbons 1 between between and 2. carbons 1 carbons 1 and 2, and and 2, a cis carbons 3 double and 4, and bond carbons 5 between and 6. carbons 3 and 4. Penta-1,4- 178 trans- 223.5 Methanal 280 diene, Penta-1,3- or showing a diene, formaldehy zigzag five- showing a de, carbon zigzag five- showing a chain with carbon central double chain with carbon bonds a double atom between bond bonded to carbons 1 between an oxygen and 2, and carbons 1 atom carbons 4 and 2, and above it by and 5. a trans a double double bond, and bond to a between hydrogen carbons 3 atom on and 4. either side by single bonds. Cyclohexen 182 2- 224 Octa- 290 e, showing Methylbuta 1,3,5,7- a -1,3-diene tetraene, hexagonal or showing a six-carbon isoprene, zigzag ring with a showing a eight- double zigzag carbon bond four-carbon chain with between chain with double carbons 1 double bonds and 2. bonds between between carbons 1 carbons 1 and 2, and 2, and carbons 3 carbons 3 and 4, and 4, carbons 5 where and 6, and carbon 2 is carbons 7 bonded to and 8. a methyl group. Hex-1-yne, 185 Cyclopenta 239 Propenal or 340 showing a diene, acrolein, zigzag six- showing a showing a carbon pentagonal three- chain with five-carbon carbon a triple ring with chain with bond double a double between bonds bond carbons 1 between between and 2. carbons 1 carbons 2 and 2, and and 3, carbons 3 where and 4. carbon 1 is a part of an aldehyde group. Empty Cell Empty Cell Empty Cell Empty Cell Beta- 455 carotene, which shows two similar units, one of which has been flipped and fused with the other. Each unit shows a six-carbon ring where carbon 6 is bonded to a chain of nine carbon atoms numbered from 7 to 15. Double bonds exist between carbons 5 and 6, carbons 7 and 8, carbons 9 and 10, carbons 11 and 12, and carbons 13 and 14. Carbon 1 is bonded to two methyl groups, and carbon atoms 5, 9, and 13 are each bonded to a methyl group. Carbon 15 in the first unit is bonded to carbon 15 in the second unit 16.11 A DEEPER LOOK: MOLECULAR ORBITAL THEORY AND ULTRAVIOLET–VISIBLE SPECTROSCOPY The absorption of a UV– vis photon causes an electron transition from a lower-energy MO to a higher-energy MO. The longest-wavelength λmax corresponds to the transition of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), called the HOMO–LUMO transition. Conjugation and HOMO–LUMO Transitions Nonbonding Electrons and HOMO–LUMO Transitions The transition that occurs in formaldehyde upon absorption of a UV–vis photon is not a π ⟶ π* transition. Instead, it is from the nonbonding orbital to the π* orbital, which is called an n ⟶ π* transition.  HO HO HO OH O + NaOH O O O O O O O UV–Vis Absorption and Color Liquid formaldehyde is colorless. lmax = 280 nm β-carotene, the compound responsible for the color of carrots, is orange. lmax = 455 nm Complementary Colors of Visible Light Formaldehyde absorbs no visible photons. All wavelengths of visible light pass through. When a compound absorbs a particular visible photon, that wavelength is removed from white light and we only see what remains. β-carotene absorbs blue light from white light and so we see its complementary color: orange. Beer–Lambert Law The magnitude of absorbance is governed by the Beer– Lambert law: A = εlC This law states that absorbance, A, is directly proportional to three different variables: 1. Concentration, C, of the species responsible for absorbing light 2. Length, l, of the sample through which the light travels 3. Molar absorptivity, ε, an experimentally derived quantity that is characteristic of a given species at a given wavelength of radiation (reflects the probability that light of a given wavelength will be absorbed when it encounters light-absorbing molecules) UV–Vis Spectroscopy in Quantitative Measurements UV–vis is most often used to quantify the amount of one or more compounds. Follows the Beer–Lambert law For kinetics, measure change of absorbance vs. time