Ch. 12 - Infrared Spectroscopy Flashcards
Gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves, and radio waves are all types of ______?
Electromagnetic radiation
Define wavelength
The distance between consecutive peaks on a wave
Define frequency
The number of full cycles of a wave that pass a given point in a second; it is given the symbol v (Greek nu) and reported in hertz (Hz), which has the units s-1.
What unit is frequency measured in?
Hertz
True or False:
wavelengths and frequency are inversely proportional
True
Define: Molecular Spectroscopy
Define: Infrared (IR) spectroscopy
A spectroscopic technique in which a compound is irradiated with infrared radiation, absorption of which causes covalent bonds to change from a lower vibration state to a higher one. Infrared spectroscopy is particularly valuable for determining the kinds of functional groups present in a molecule.
At what wavelength range is Infrared spectroscopy dependent?
2.5 x 10-6 to 2.5 x 10-5 m
2.5 x 10-6 to 2.5 x 10-5 Is also known as?
Vibrational infrared region
Define: wavenumbers
The frequency of electromagnetic radiation expressed as the number of waves per centimeter, with units cm-1 (read: reciprocal centimeters).
In frequencies, what is the vibrational infrared region?
4000 to 400 cm-1
Why are NaCl and KBr often used in sample preparation for spectroscopy?
As ionic solids, they have no covalent bonds to absorb infrared radiation.
True or false:
In general, the greater the bond dipole, the greater change in dipole moment caused by a vibration.
True
Define: Infrared active
Any molecular vibration that leads to a substantial change in dipole moment is observed in an IR spectrum
Are these molecules infrared active or inactive?
Infrared inactive. This is because the carbon-carbon double and triple bonds of symmetrically substituted alkanes and alkanes do not absorb infrared radiation because vibration does not result in substantial bond dipole change.
What is the formula for allowed fundamental vibrations of a nonlinear molecule containing n atoms?
3n - 6
What is the formula for allowed fundamental vibrations of a linear molecule containing n atoms?
3n - 5
Define: Raman spectroscopy
What is the equation, derived from Hooke’s law for a vibrating spring, for the frequency of a stretching vibration? And what does each variable mean?
True or False:
Frequencies calculated using the equation for the frequency of a stretching vibration is generally precise enough to determine molecular structure
False
True or false:
The stronger the bond is and the lighter the atoms are, the higher the frequency of the stretching vibration will be.
True.
Hooke’s law depends on both the strength of the vibrating bond and the masses of the atoms connected by the bond.
Without doing a calculation, which of this pair will occur at the higher frequency?
Although O has a larger mass, the carbon-oxygen bond is much stronger than the carbon-carbon bond.
Without doing the calculation, which of this pair will occur at the higher frequency?
The triple bond is much stronger than the double bond, causing it to occur at a higher frequency regardless of the carbon-oxygen double bond.
Without doing the calculation, which of this pair will occur at the higher frequency?
Assuming the single bonds are similar in force constants, the hydrogen being so small in mass would result in a higher frequency when put into the equation.
What is the stretching frequency of an O–H bond? Is it classified as weak, strong, or medium?
3200 - 3650 cm-1
Weak to strong
What is stretching frequency of an N–H bond? Intensity?
3100 - 3550
Medium
What is the stretching frequency of a C–H bond? Intensity?
2700 - 3300
Weak to Medium
What is the stretching frequency of a Carbon-Carbon double bond? Intensity?
1600 - 1680
Weak to Medium
What is the stretching frequency of a Carbon-Oxygen double bond? Intensity?
1630 - 1820
Strong
What is the stretching frequency of a C–O? Intensity?
1000 - 1250
Strong
What is the fingerprint region?
True or False:
Infrared spectra of alkanes are usually complex with many peaks.
False.
Infrared spectra of alkanes are usually simple with few peaks.
What are the 4 major infrared absorptions of alkanes?
Include:
- hydrocarbon
- vibration type
- frequency
- intensity
What are the 2 major infrared absorptions of alkenes?
Include:
- hydrocarbon
- vibration type
- frequency
- intensity
What are the 2 major infrared absorptions of alkynes?
Include:
- hydrocarbon
- vibration type
- frequency
- intensity
What are the 3 major infrared absorptions of arenes?
Include:
- hydrocarbon
- vibration type
- frequency
- intensity
What are the 3 major infrared absorptions of alcohols?
include:
- bond
- frequency
- intensity
How many bands can be seen in primary amines and secondary amines?
Primary amines show 2 bands due to one being caused by symmetric stretching and the other by asymmetric stretching.
Secondary amines show 1 band of absorption in the same region as primary.
Tertiary amines show no bands as there are no N-H bonds
What frequency do aldehydes and ketones fall under in infrared spectroscopy?
Between 1630 and 1820 cm-1
True or False:
As ring size decreases and angle strain increases, the Carbon-Oxygen double bond absorption shifts to a higher frequency
True
What occurs when a carbonyl group has the presence of an adjacent carbon-carbon double bond or benzene ring?
The Carbon-Oxygen double bond shifts to a lower frequency.
If you were shown an IR spectrum showing a peak at 1700-1725 cm-1 and another at 2500-3300 cm-1, what is likely the molecule you are observing?
It is likely a Carboxylic Acid.
*Carboxylic acids show two peaks. One from the carbonyl at 1700-1725 cm-1 and another due to the O-H bond at 2500-3300 cm-1
Describe the systematic approach to solving IR problems
