Spectroscopy CHAPTER ELEVEN

Spectroscopy CHAPTER ELEVEN
Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Electromagnetic Radiation
Electromagnetic radiation: Light and other forms of radiant energy. Wavelength (): The distance between consecutive identical points on a wave. Frequency (): The number of full cycles of a wave that pass a point in a second. Reported in hertz. Hertz (Hz): The unit in which radiation frequency is reported; s-1 (read “per second”). Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Electromagnetic Radiation

Molecular Spectroscopy
Molecular spectroscopy: The study of which frequencies of electromagnetic radiation are absorbed or emitted by substances and the correlation between these frequencies and specific types of molecular structure. We study two types of molecular spectroscopy. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Figure 11.1 Absorption of energy in the form of electromagnetic radiation excites an atom or a molecule in energy state E1 to energy state E2. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Infrared Spectroscopy
The vibrational IR extends from 2.5 x 10-6 m to 2.5 x 10-5 m. The frequency of IR radiation is commonly expressed in wavenumbers. Wavenumber (n): The number of waves per centimeter, cm-1 (read reciprocal centimeters). Expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm-1. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Infrared Spectroscopy
Figure 11.2 Infrared spectrum of aspirin. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.
Molecular Vibrations Atoms joined by covalent bonds undergo continual vibrations relative to each other. The energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed. The energies associated with transitions between vibrational energy levels for most covalent bonds are from 2 to 10 kcal/mol (8.4 to 42 kJ/mol). These energies correspond to frequencies in the infrared region between 4000 to 400 cm-1. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Molecular Vibrations For a bond to absorb IR radiation: The frequency of radiation must match the frequency of a bond vibration. The bond undergoing vibration must be polar. The greater the polarity, the stronger the absorption. Covalent bonds which do not meet these criteria are said to be IR inactive. The multiple bonds in these nonpolar molecules are not IR active. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Alkynes Terminal alkynes show C-H stretching at cm-1. Alkynes show triple bond stretching between 2100 and 2260 cm-1 Figure 11.6 Infrared spectrum of 1-octyne. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Alcohols The position and intensity of an O–H absorption depends on the extent of hydrogen bonding. O–H stretching occurs between 3200–3500 cm-1. C–O stretching occurs between 1050 and 1250 cm-1. Figure Infrared spectrum of 1-pentanol Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Ethers The C–O stretching of ethers is similar to that of alcohols and is observed in the region1070 to 1150 cm-1. Figure 11.8 Infrared spectrum of diethyl ether. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Amines N–H stretching vibrations appear in the region 3100 to 3500 cm-1. 1° amines have two absorptions in this region. 2° amines have one. 3° amines have none. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Carboxylic Acids The carboxyl group, —COOH, has two characteristic peaks. C=O stretching between 1700 and 1725 cm-1. O–H stretching between 2400 and 3400 cm-1. Figure Infrared spectrum of butanoic acid. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Esters Esters show strong C=O stretching absorption between 1735 and 1800 cm-1. In addition, they display strong C–O stretching from 1000 to 1250 cm-1. Figure Infrared spectrum of ethyl butanoate. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Amides The C=O stretching of amides occurs between 1630–1680 cm-1. 1° and 2° amides show N–H stretching from 3200 to cm-1. 3° amides do not show N–H absorption. Figure Infrared spectrum of N,N-diethyldodecanamide (a tertiary amide) Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Index of Hydrogen Deficiency
Index of hydrogen deficiency (IHD): The sum of the number of rings and pi bonds in a molecule. To determine IHD, compare the number of H in an unknown compound with the number in a reference hydrocarbon that has the same number of carbons and has no rings or pi bonds. The molecular formula of the reference hydrocarbon is CnH2n+2 Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Index of Hydrogen Deficiency
For each atom of a Group 7 element (F, Cl, Br, I), add one H. No correction is necessary for the addition of atoms of Group 6 elements (O, S) to the reference hydrocarbon. For each atom of a Group 5 element (N, P), subtract one hydrogen. Problem: Calculate the IHD of isopentyl acetate C7H14O2. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Nuclear magnetic resonance (NMR) spectroscopy: A spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogen atoms using 1H-NMR spectroscopy. carbon atoms using 13C-NMR spectroscopy. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Origins of NMR An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2. A spinning charge creates an associated magnetic field. In effect, an electron behaves as if it were a tiny bar magnet and has what is called a magnetic moment. Within a collection of 1H and 13C atoms, nuclear spins are completely random in orientation. When nuclei with a spin quantum number 1/2 are placed in an applied magnetic field, a small majority of nuclear spins become aligned with the applied field in the lower energy state. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Origins of NMR Figure For 1H and 13C, only two orientations are allowed, with the applied magnetic field and against the applied magnetic field. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Origins of NMR In an applied field strength of 7.05 tesla (T), which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1H is approximately J ( cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz). 13C is approximately J ( cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz). Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Nuclear Magnetic Resonance
Resonance: The absorption of electromagnetic radiation by a spinning nucleus and the resulting “flip” of its spin from a lower to a higher energy state. Figure H and 13C nuclei in the absence and in the presence of an applied magnetic field. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

If we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H; the same is true of 13C nuclei. But hydrogens in organic molecules are not isolated from other atoms; they are surrounded by electrons, which are caused to circulate by the presence of an applied magnetic field. The circulation of electrons creates local magnetic fields that oppose the applied magnetic field, thereby shielding hydrogen nuclei from the external field. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small. For example, the difference in resonance frequencies for hydrogens in CH3Cl compared to hydrogens in CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

It is customary to measure the resonance frequency (signal) of individual nuclei relative to the resonance frequency (signal) of a reference compound. The reference compound now universally accepted is tetramethylsilane (TMS). For a 1H-NMR spectrum, signals are reported by their shift from the twelve H signal in TMS. For a 13C-NMR spectrum, signals are reported by their shift from the four C signal in TMS. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Equivalent Hydrogens Equivalent hydrogens: Hydrogens that have the same chemical environment. Example: Determine the number of sets of equivalent hydrogens in each compound, and the number of hydrogens in each set. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Equivalent Hydrogens A molecule with 1 set of equivalent hydrogens gives one 1H-NMR signal. Each of these four symmetrical molecules has one set of equivalent hydrogens and gives one signal in its 1H-NMR spectrum. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Signal Areas Relative areas of signals are proportional to the number of H giving rise to each signal. Modern NMR spectrometers electronically integrate and record the relative area of each signal. Figure H-NMR spectrum of tert-butyl acetate. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Signal Splitting; the (n + 1) rule
Peak: The units into which an NMR signal is split; doublet, triplet, quartet, etc. Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens. (n + 1) rule: The 1H-NMR signal of a hydrogen or set of equivalent hydrogens is split into (n + 1) peaks by a nonequivalent set of n equivalent neighboring hydrogens. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Interpreting NMR Spectra
Esters Hydrogens on the a-carbon to the carbonyl group of an ester are slightly deshielded and give 1H signals at  Hydrogens bonded to the carbon of the ester oxygen are more strongly deshielded and give 1H signals at  Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Interpreting NMR Spectra

Solving NMR Problems Example Prenol is a colorless liquid with molecular formula C5H10O. It has a fruity odor and is commonly used in perfumes. Propose a structural formula for prenol. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Solving NMR Problems Example Compound B is a colorless liquid with the molecular formula C7H14O. Propose a structural formula for Compound B. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Spectroscopy CHAPTER ELEVEN

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