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__ can be measured using liquid, solid, or gaseous samples that are placed in the beam of infrared light. A drop of liquid can be placed as a think film between two salt plats made of NaCl or KBr, which are transparent to __ at most important frequencies. A solid can be ground with KBr and pressed into a disk that is placed in the light beam.
- infrared spectra
- infrared light
Alternatively, a solid sample can be ground into a pasty mull with paraffin oil. As with a liquid, the mull is placed between two salt plates. Solids can be dissolved in common solvents that do not have absorptions in the areas of interest. Gases are placed in a longer cell with polished salt windows. These __ often contain mirrors that reflect the beam through the cell several times for __.
- gas cells
- stronger absorption
An __ measures the frequencies of infrared light absorbed by a compound. In a simple infrared spectrometer, two beams of light are used. The __ passes through the sample cell, while the __ passes through a reference cell that contains only the solvent. A __ alternately allows light from each of the two beams to enter the monnochromator.
- infrared spectrometer
- sample beam
- reference beam
- rotating mirror
The __ uses prisms or diffractioin gratings to allow only one frequency of light to enter the detector at a time. It scans the range of __ as a pen that moves along the corresponding frequencies on the x axis of the chart paper. __ appear toward the left of the chart paper.
- infrared frequencies
- higher frequencies (shorter wavelengths)
The detector signal is proportional to the __ in the sample and reference beams, with the reference beam compensating for any absorption by air or by the solvent. The detector signal controls movement of the pen along the y axis, with __ (no absorption) at the top of the paper, and __ (absorption of all the light) at the bottom.
- difference in the intensity of light
- 100% transmittance
- 0% transmittance
__ are called that because they disperse light into all the different frequencies and measures them individually. __ require expensive prisms and diffraction gratings, and they must be manually aligned and calibrated on a regular basis. Since only one frequency is observed at a time, __ require strong IR sources, and they require 2 to 10 minutes to scan through a complete spectrum. __ are being replaced by __ for most uses.
- dispersive instruments x3
- dispersive infrared spectrometers
- Fourier transform infrared spectrometers
A __ uses an __ to measure an IR spectrum. The infrared light goes from the glowing source to a beamsplitter, usually made of polished KBr, placed at a 45 degree angle. Part of the beam passes through the __ and part is reflected at a right angle. The reflected beam strikes a stationary mirror, while the transmitted beam strikes a mirror that moves at a constant speed.
- Fourier transform infrared spectrometer (FT-IR)
The beams returm from the mirrors to recombine at a beamsplitter. The beam from the moving mirror has traveled a different distance than the beam from the fixed mirror, and the two beams combine to create an interference pattern called an __. This __, which simultaneously contains all frequencies, passes through the sample compartment to reach the detector.
The __ contains all the info contained in the spectrum shown in the lower half. The __ is said to be in the __, corresponding to the energy seen by the detector as the mirror moves through the signal. A standard computer algorithm called a __ converts the __ to the __ spectrum that allows us to see the strength of absorption as a function of the frequency (or wavelength).
- interferogram x2
- time domain
- Fourier transform
- time domain
- frequency domain
The __ has several major advantages over the dispersive instrument: (What is it?)
less energy is needed from the __, and less time is needed for a scan. Several scans can be completed in a few seconds and averaged to improve the signal. __ and __ are also improved because a __ is used alongside the IR beam to control the speed of the moving mirror and to time the collection of data points. The laser beam is a precise frequency reference that keeps the spectrometer accurately calibrated
- FT-IT spectrometer
- its sensitivity is better because it measures all frequencies simultaneously rather than scanning through the individual frequencies
- resolution and accuracy
- laser beam
Since most organic compounds contain at least some saturated C-H bonds and some CH2 and CH3 groups, all these bands are common. In fact without an authentic spectrum for comparison we could not what?
look at this spectrum and conclude that the compound is octane
Hydrocarbons contain only __ and __ bonds. An __ does nto provide enough info to identify a structure conclusively, but the absorptions fo the carbon-carbon and carbon-hydrogen bonds can indicate the presence of __ and __ bonds.
- infrared spectrum
- double and triple
Stronger bonds generally absorb at __ because of their greater stiffness.
What are the carbon-carbon bond stretching frequencies?
__ are not very reliable. We use the __ primarily to confirm the identity of an unknown compound by comparison with an authentic spectrum.
- C-C 1200 cm^-1
- C=C 1660 cm^-1
- C***C < 2200 cm^-1
- C-C single bond absorptions (and most other absorptions in the fingerprint region)
- fingerprint region
The absorptions of __ are useful for structure deterrmination. Most __ produce observable stretching absorptions in the region of 1600 to 1680 cm^-1. The specific frequency of the __ stretching vibration depends on whether there is another __ nearby. When two __ are one bond apart they are said to be __.
- C=C double bonds
- unsymmetrically subbed double bonds
- double bonds
__ are slightly more stable than isolated __ because there is a small amount of __ between them. This overlap between the pi bonds leaves a little leess electron density in the double bonds themselves. As a result, they are a little less stiff and vibrate a little more slowly than an __, which absorb around 1640-1680 cm^-1, while __ absorb around 1620 to 1640 cm^-1.
- conjgated double bonds
- isolated double bonds
- pi bonding
- isolated double bond
- conjugated double bonds
The effect of __ is even more pronounced in __ which have three __ in a six-membered ring. __ are more lie 1.5 bonds than true __, and their reduced __ results in less stiff bonds with lower stretching frequencies, around 1600 cm^-1.
- aromatic compounds
- three conjugated double bonds
- aromatic C=C bonds
- double bonds
- pi bonding
__ in alkynes are stronger and stiffer than __ or __, and they absorb infrared light at __. Most __ have stretching frequencies between 2100 and 2200 cm^-1. __ usually give sharp C***C stretching signals of moderate intensity. The C***C stretching absorption of an __ may be weak or absent, however, due to the symmetry of the __ with a very small or zero dipole moment.
- carbon-carbon triple bonds
- single or double bonds
- higher frequencies
- alkyne C****C triple bonds
- Terminal alkynes
- internal alkyne
- disubbed triple bond
Alkanes, alkenes, and alkynes also have characteristic __. Carbon-hydrogen bonds involving __ ybrid carbon atoms generally absorb at frequences just __ 3000 cm^-1. Those involving __ hybrid carbons absorb just __ 3000cm^-1. We explain this difference bythe amount of __ in the carbon orbital used to form the bond. The __ is closer to the nucleus than the __, and stronger, stiffer bonds result from orbitals with more ++. Even if an alkene's C=C absorption is weak or absent, the unsaturated C-H stretch above 3000 cm^-1 reveals the presence of the double bond.
- C-H stretching frequencies
- s character
- s orbital
An __ is 1/4 s character, and an __ is 1/3 s character. We expect the bond using the __ orbital to be slightly stronger, with a higher vibration frequency. The C-H bond of a __ is formed using an__ wth about one-half s character. This bond is stiffer than a C-H bond using an sp3 or sp2 hybrid carbon, and it absorbs at a higher frequency: about 3300 cm^-1.
- sp3 orbital
- sp2 orbital x2
- terminal alkyne
- sp hybrid orbital
Even if the __ stretching absorption is weak or absent, the __ just above 3000 cm^-1 suggests the presence of an __.
The O-H bonds of alcohols and the N-H bonds of amines are __ and __. The vibration frequences of each therefore occur at __ than those of most C-H bonds (except for alkynyl ***C-H bonds)
- unsaturated =C-H stretching absorption
- alkene double bond
- strong and stiff
O-H and N-H stretching frequencies
- alcohol O-H 3300 cm^-1 broad
- acid O-H 3000 cm^-1, broad
- amine N-H 3300 cm^-1, broad with spikes
Alcohol OH bonds absorb over a wide range of frequencies, centered around 3300 cm^-1. Alcohol molecules are involved in __, with different molecules having different instantaneous arrangemnets. The O-H stretching frequencies reflect this diersity of __ arrangements resulting in very broad absorptions.
Like alcohols __ give OH absorptions that are broadened by __. The broad acid OH absorption is usually centered around 3000 cm^-1, however (compared with 3300 cm^-1 for an alcohol) because of the stronger __ between acid molecules.
- carboxylic acids
- hydrogen bonding
- hydrogen bonding
COmpounds with C-O bonds (__ and __, for example) generally show strong absorptions n teh range of 1000-1200 cm^-1; however, there are other __ that also absorb in this region. Therefore, a strong peak between 1000 and 1200 cm^-1 does not necessarily imply a C-O bond, but the absence of an __in this region suggests the absence of a__ For simple __, this unreliable C-O absorption is usually the only clue that the compound might be an ether.
- alcohols and ethers
- functional groups
- C-O bond
Amine N-H bonds also have stretching frequencies int eh 3300 cm^-1 region or even slightly higher. Like alcohols, amines participate in __ that can broaden the N-H absorptions. With amines, however, the absorption is somewhat __, and there may be one or more __ superimposed on the broad N-H stretching absorpiton: often one N-H spike for the __ and two N-H spikes for the __
- hydrogen bonding
- sharp spikes
- signle N-H bond of a secondary amine (R2NH)
- symmetric and antisymmetric stretch of the two N-H bods in a primary amine.
These sharp spikes, combined with the presence of nitrogen in the molecular formula, help to do what? __ have no NH bonds, and they do not give rise to NH stretching absorptions in the IR spectrum.
- distinguish amines from alcohols
- Tertiary amines