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Chapter 12
Infrared Spectroscopy and Mass Spectrometry

12-01

Title	The Electromagnetic Spectrum
Caption	Figure 12-1 The electromagnetic spectrum is the range of all possible frequencies, from zero to infinity. In practice, the spectrum ranges from very low radio frequencies to very high energy gamma rays.
Notes	Different wavelengths have different energies and the energies have distinct molecular effects. X-rays cause ionization of molecules because of its high energy, while microwave will affect rotational motion of the molecule. Infrared rays will cause molecular vibrations. The higher the frequency the shorter the wavelength.
Keywords	wavelength, x-rays, microwave rays, molecular vibrations, rotational motion

12-01-01UN

Title	Molecular Vibrations
Caption	If the bond is stretched, a restoring force pulls the two atoms together toward their equilibrium bond length. If the bond is compressed, the restoring force pushes the two atoms apart. If the bond is stretched or compressed and then released, the atoms vibrate.
Notes	Molecular vibrations depend on the masses of the atoms. Heavy atoms vibrate slowly so they will have a lower frequency that lighter atoms. The frequency of a vibration decreases with increasing atomic weight. Frequency also increases with bond energy so a C=C double bond will have a higher frequency than a C-C single bond.
Keywords	molecular vibrations, frequency

12-01-02T01

Title	Table for Bond Stretching Frequencies
Caption	Table 12-1 Bond Stretching Frequencies
Notes	The stretching frequency decreases with increasing atomic weight. Between similar atoms, the frequency increases with increasing bond energy.
Keywords	stretching frequency

12-02

Title	Stretching and Bending of Bonds
Caption	Figure 12-2 A nonlinear molecule with n atoms has 3n - 6 fundamental vibrational modes. Water has 3(3) - 6 = 3 modes. Two of these are stretching modes, and one is a bending mode.
Notes	Stretching can be symmetric when both O-H bonds are stretched at the same time. In an anti-symmetric stretch one O-H bond stretches while the other O-H bond is compressed. Bending, also known as scissoring, occurs when the H-O-H angle decreases and increases giving the effect of a pair of scissors.
Keywords	stretching, bending, symmetric, antisymmetric

12-04

Title	Diagram of an Infrared Spectrometer
Caption	Figure 12-4 Block diagram of an infrared spectrometer.
Notes	An infrared spectrometer measures the frequency of IR light that a compound absorbs.
Keywords	spectrometer

12-06b

Title	IR Spectrum of n-Octane
Caption	Figure 12-6b Infrared spectrum of n-octane. Notice that the frquencies shown in an routing IR spectrum range from about 600 cm-1 to about 4000 cm-1.
Notes	Because the atoms involved in stretching and bending will determine the frequency, IR is mostly used to identify the presence of functional groups in a molecule. An alkane will show stretching and bending frequencies for C-H and C-C only. The C-H stretching is a broad band between 2800 and 3000 cm-1, a band present in virtually all organic compounds. In this example, the importance lies in what is not seen, i.e., the lack of bands indicates the presence on no other functional group.
Keywords	stretching, bending, functional groups

12-07

Title	IR Spectra for n-Hexane and 1-Hexene
Caption	Figure 12-7 Comparison of the IR Spectra for n-hexane and 1-hexene. The most important absorptions in the 1-hexene are the C=C stretch at 1642 cm-1, and the unsaturated stretch at 3080 cm-1.
Notes	Notice that the bands of the alkane are present in the alkene; the C-H stretch between 2800 - 3000 cm-1 and the C-H bending.
Keywords	absorption, stretch, bending

12-07-01UN

Title	C-H Stretching Frequencies
Caption	Alkanes, alkenes, and alkynes also have characteriztic C-H stretching frequencies. Carbon-hydrogen bonds involving sp3 hybrid carbons generally absorb at frequencies just below (to the right) 3000 cm-1, while those involving sp2 carbons absorb just above (to the left) 3000 cm-1.
Notes	A greater percent of s character in the hybrid orbitals will make the C-C bond stronger. An sp3 hybridized carbon has a 25% s character, an sp2 has around 33% s character, and an sp carbons has 50% s character. The C-H bond of an sp3 carbon will be slightly weaker than the C-H of an sp2 or an sp carbon.
Keywords	frequencies, stretch, s character

12-08a,b

Title	IR Spectra of 1-Octyne and 4-Octyne
Caption	Figure 12-8 (a) The IR spectrum of 1-octyne shows characteristics absorptions at 3313 cm-1 and at 2119 cm-1. (b) Neither of this absorptions are seen in the spectrum of 1-octyne.
Notes	Terminal alkynes have a characteristic C-H (alkynyl) stretch at around 3300 cm-1 and a C-C triple bond stretch at around 2100 - 2200 cm-1. Internal alkynes cannot exhibit the acetylenic C-H stretch. The very small dipole moment of the disubstituted triple bond limits the stretching and makes the C-C triple band bond not visible.
Keywords	terminal alkyne, internal alkyne, acetylenic hydrogen, alkynyl, dipole

12-09

Title	IR Spectrum of Alcohols
Caption	Figure 12-9 The IR spectrum of 1-butanol shows a broad, intense O-H stretching absorption centered around 3300 cm-1. The broad shape is due to the diverse nature of the hydrogen bonding interactions of alcohol molecules.
Notes	The alcohol O-H absorbs around 3300 cm-1 and it usually is a broad, strong band. This band is due to different hydrogen bonding arrangements taking place. There is a C-O stretch band centered near 1050 cm-1. Although a stretching band around this region can be due to stretches other than C-O, the lack of this band around 1000 - 1200 cm-1 strongly suggests the lack of a C-O bond.
Keywords	hydrogen bonding

12-10

Title	IR Spectrum of Amines
Caption	Figure 12-10 The IR spectrum of dipropylamine shows a broad N-H stretching absorption centered around 3300 cm-1. Notive the spike in this broad absorption.
Notes	The hydrogen bonds that form between nitrogen and hydrogen are weaker than those with oxygen and hydrogen. Amines, like alcohols, will have a broad band centered around 3300 cm-1, but not as strong. There could be spikes superimposed on the broad peak depending on the number of hydrogens that the nitrogen has; a secondary amine will have one spike, a primary amine will have two spikes. Tertiary amines will not show spikes because there is no N-H bond.
Keywords	dipropylamine, spikes, broad band, hydrogen bonding

12-10-01UN1-3

Title	Ketones, Aldehydes, and Acids
Caption	The C=O stretching vibrations of simple ketones, aldehydes and carboxylic acids occur at frequencies around 1710 cm-1. These frequencies are higher than those for C=C double bonds because the C=O double bond is stronger and stiffer.
Notes	The C=O stretch is strong and unmistakable. Depending on what else is attached to the carbonyl, there are other bands to look for to differentiate between aldehydes, ketones, and acids.
Keywords	ketones, aldehydes, acids

12-11a,b

Title	IR of Carbonyl Compounds
Caption	Figure 12-11 (a) 2-heptanone and (b) butyraldehyde. Both show intense carbonyl absorptions near 1710 cm-1.
Notes	The spectrum of 2-heptanone shows a strong absorption at 1718 cm-1. The aldehyde has the C=O stretch at 1720 cm-1 but it also has tow different stretch bands for the aldehyde C-H bond at 2720 and 2820 cm-1.
Keywords	2-heptanone, butyraldehyde

12-12

Title	IR of Carboxylic Acids
Caption	Figure 12-12 IR spectrum of hexanoic acid.
Notes	Carboxylic acids show a broad O-H absorption from about 2500 to 3500 cm-1. This broad absorption gives the entire C-H stretching region a broad appearance. The C=O double bond stretch will be sharp and intense at 1711 cm-1. Both peaks need to be present to identify the compound as a carboxylic acid.
Keywords	carboxylic acid, broad peak

12-12-05UN

Title	Frequencies of Conjugated Carbonyl Compounds
Caption	The delocalization of the pi electrons reduces the electron density of the carbonyl double bond, weakening it and lowering the stretching frequency from 1710 cm-1 to about 1685 cm-1 for conjugated ketones, aldehydes and acids.
Notes	The C=C double bond stretch may not be apparent in the IR spectrum because it is so much weaker than the C=O absorption. The presence of the C=C double bond is inferred from its effect on the C=O frequency and the presence of unsaturated =C-H absorptions above 3000 cm-1.
Keywords	delocalization, conjugation, conjugated ketone

12-14

Title	IR of Nitriles
Caption	Figure 12-14 Nitrile triple bond stretching absorptions are at slightly higher frequencies than those of alkyne triple bonds.
Notes	A carbon nitrogen triple bond has an intense and sharp absorption centered at around 2200 to 2300 cm-1. Nitrile bonds are more polar than carbon-carbon triple bonds, so nitriles produce stronger absorptions than alkynes.
Keywords	nitrile, alkyne

12-14-04UN

Title	Summary of IR Stretching Frequencies
Caption	The IR spectrum has distinct regions. The left of the spectrum shows the C-H, O-H, and N-H stretches. The triple bonds absorb around 2200 cm-1 followed by the double bonds to the right at around 1700 cm-1. The region below 1400 cm-1 is called the fingerprint region.
Notes	It is very difficult to use the fingerprint region of the IR spectrum to identify an unknown because of its complexity. However, it can be used to confirm the identity of an unknown by matching peak by peak this region.
Keywords	fingerprint region

12-15

Title	Diagram of a Mass Spectrometer
Caption	Figure 12-15 Diagram of a mass spectrometer. A beam of electrons causes molecules to ionize and fragment. The mixture of ions is accelerated and passes through a magnetic field, where the paths of lighter ions are bent more than those of heavier atoms. By varying the magnetic field, the spectrometer plots the abundance of ions of each mass.
Notes	The exact radius of curvature of an ion's path depends on its mass-to-charge-ration, symbolized by m/z. In this expression, m is the mass of the ion (in amu) and z is its charge. The vast majority of ions have a +1 charge, so we consider their path to be curved by an amount that depends only on their mass.
Keywords	spectrometer, ionization, fragmentation, magnetic field,

12-16

Title	Mass Spectrum for 2,4-Dimethylpentane
Caption	Figure 12-16 Mass spectrum of 2,4-dimethylpentane.
Notes	In the spectrum, the tallest peak is called the base peak and it is assigned an abundance of 100%. The % abundance of all other peaks are given relative to the base peak. The molecular ion (M+) corresponds to the mass of the original molecule.
Keywords	base peak, molecular ion

12-17

Title	Gas Chromatography - Mass Spectrometry (GC-MS)
Caption	Figure 12-17 Block diagram of a gas chromatogram-mass spectrometer. The gas chromatograph column separates the mixture into its components. The quadrupole mass spectrometer scans mass spectra of the components as they leave the column.
Notes	As the sample passes through the column the most volatile components move through the column faster than the more volatile components. The separated components leave the column at different times, passing through a tranfer line into the ion source of the mass spectrometer, where the molecules are ionized and allowed to fragment.
Keywords	gas chromatograph, mass spectrometer, column

12-17-02UN

Title	Isotopic Effect of Chlorine
Caption	Mass spectra of 2-chloropropane. Most heavier elements do not consist of a single isotope but contain heavier isotopes in varying amounts. The heavier isotopes give rise to small peaks at higher mass numbers than the M+ molecular ion peak.
Notes	The height of the M+, M+1, and M+2 peaks will depend on the isotopic composition of the element in question. Chlorine is a mixture of 75.5% 35Cl and 24.5% 37Cl. The molecular ion peak M+ has 35Cl be 3 times higher than the M+2 peak that has 37Cl.
Keywords	isotopic composition

12-17-03UN

Title	Isotopic Effect of Bromine
Caption	Mass spectra of 1-bromopropane. Notice that the M+ and the M+2 peaks are about the same height.
Notes	Bromine is a mixture of 50.5% 79Br and 49.5% 81Br. The molecular ion peak M+ has 79Br be as tall as the M+2 peak that has 81Br.
Keywords	isotopic abundance

12-19

Title	Mass Spectrum of 2-Methylpentane
Caption	Figure 12-19Mass spectrum of 2-methylpentane. The base peak corresponds to the loss of a propyl radical to give an isopropyl cation.
Notes	Fragmentation of a branched alkane commonly occurs at a branch carbon atom to give the most highly substituted cation and radical.
Keywords	2-methylpentane, cation, radical, bae peak, fragmentation

12-20

Title	Mass Spectrum of 2-Hexene
Caption	Figure 12-20The radical cation of 2-hexene cleaves at an allylic bond to gie a resonance-stabilized methallyl cation, m/z = 55.
Notes	Whenever possible, fragmentation will produce resonance-stabilized species such as methallyl cations, allylic cations, and acylium cations.
Keywords	methallyl, allylic bond, resonance-stabilized

12-21

Title	Mass Spectrum of 3-Methyl-1-Butanol
Caption	Figure 12-21The mass spectrum of 3-methyl-1-butanol. The strong peak at m/z 70 is actually the M-18 peak, corresponding to the loss of water. The molecular ion is not visible because it loses water easily.
Notes	Whenever possible, fragmentation will produce resonance-stabilized species such as methallyl cations, allylic cations, and acylium cations.
Keywords	allyl, allylic bond, resonance-stabilized, base peak

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