p.1
History of IR Spectroscopy
Who discovered infrared (IR) radiation and in what year?
Sir William Herschel discovered infrared (IR) radiation in 1800.
p.1
History of IR Spectroscopy
What did Coblentz contribute to the field of IR spectroscopy?
Coblentz laid the groundwork for IR spectroscopy with a systematic study of organic and inorganic absorption spectra.
p.1
History of IR Spectroscopy
What was a significant challenge in early IR spectroscopy measurements?
It could take 4 hours to record the full spectrum since each point had to be measured separately.
p.1
Molecular Vibrations and IR Activity
What realization came from the early work in IR spectroscopy?
Each compound has its own unique IR absorption pattern, and certain functional groups absorb at similar wavelengths in different molecules.
p.1
Interpretation of IR Spectra
What does the IR absorption spectrum provide for a molecule?
The IR absorption spectrum provides a 'fingerprint' of a molecule with covalent bonds, which can be used to identify the molecule.
p.1
Interpretation of IR Spectra
What is a primary use of IR spectroscopy?
Qualitative identification of organic and inorganic compounds is a primary use of IR spectroscopy.
p.1
Absorption Considerations in IR Spectroscopy
How can IR spectroscopy be used in relation to functional groups?
The spectrum provides a quick way to check for the presence of a given functional group, such as a carbonyl group, in a molecule.
p.29
Interpretation of IR Spectra
What is the standard representation of infrared spectra?
Infrared spectra are typically represented as percent of transmittance versus wavenumber.
p.7
Interpretation of IR Spectra
What is the reputation of infrared spectroscopy from a quantitative point of view?
Infrared spectroscopy has a very well gained reputation for its power, flexibility, and reliability.
p.28
Molecular Vibrations and IR Activity
Why is IR spectroscopy considered a useful tool?
IR spectroscopy is useful for molecular characterization and quantification because no two unique molecules produce the same IR spectrum.
p.3
Infrared Regions and Their Applications
What is infrared spectroscopy used for?
Infrared spectroscopy is a powerful technique for the determination and identification of molecular structure and has various quantitative applications within analytical chemistry.
p.21
Types of Molecular Vibrations
What does Figure 7 compare regarding bonds?
It compares the wavelength, stretching frequency, and wavenumber of bonds with different strengths (single, double, and triple bonds).
p.29
Interpretation of IR Spectra
How are wavenumbers plotted in an IR spectrum?
Wavenumbers are plotted in decreasing order from left to right.
p.4
Electromagnetic Spectrum Overview
How is frequency defined in the context of electromagnetic waves?
Frequency (ν) is the number of wave cycles that pass through a point in one second, measured in Hertz (Hz).
p.26
Absorption Considerations in IR Spectroscopy
What is commonly used to measure the amount of infrared radiation absorbed by a sample in infrared spectroscopy?
Transmittance is commonly used in infrared spectroscopy.
p.25
Dipole Moments and IR Absorption
What is the effect of symmetric C≡C stretching on acetylene's dipole moment?
The symmetric C≡C stretching does not alter acetylene’s dipole moment and is not infrared active.
p.2
Electromagnetic Spectrum Overview
What marks the upper energy end of the IR region?
Visible radiation (red light) marks the upper energy end or minimum wavelength end of the IR region.
p.7
Interpretation of IR Spectra
What type of information can be achieved with Infrared spectroscopy?
Infrared spectroscopy can provide information about the type of atoms within the molecule, the type of bonds between atoms, and the molecular structure.
p.28
Absorption Considerations in IR Spectroscopy
What happens to infrared (IR) radiation when it interacts with a sample?
Some wavelengths of IR radiation are absorbed by the sample, while others are transmitted through it.
p.3
Principles of Infrared Radiation
What is spectroscopy?
Spectroscopy can be defined as the interaction between matter and light.
p.19
Calculation of Molecular Vibrations
What is the force constant for a single C-H bond?
The force constant for a single bond is 5 x 10^5 dyne/cm.
p.24
Dipole Moments and IR Absorption
What must a molecule do to be considered infrared active?
A molecule must undergo a change in its dipole moment when vibrating.
p.12
Dipole Moments and IR Absorption
What determines whether a molecule with more than two atoms has a permanent dipole moment?
It depends on the geometry of the molecule.
p.28
Interpretation of IR Spectra
What does the IR spectrum represent?
The IR spectrum represents the IR absorption and transmission of a molecule.
p.19
Calculation of Molecular Vibrations
Why do calculated values for C-H bonds differ from observed values?
The calculation does not take into account the environment of the C-H group within the molecule.
p.4
Electromagnetic Spectrum Overview
What is wavelength and how is it measured?
Wavelength (λ) is the length of one complete wave cycle, measured in centimeters (cm).
p.15
Types of Molecular Vibrations
What are the frequencies for the asymmetric stretching and bending vibrations of CO2?
The asymmetric stretching frequency is 2350 cm−1 and the bending vibration occurs at 666 cm−1.
p.14
Types of Molecular Vibrations
What are the in-plane bending modes mentioned in the text?
Scissoring and rocking are the in-plane bending modes where the H atoms remain in the same plane as the C atom.
p.27
Absorption Considerations in IR Spectroscopy
What is ε in the context of absorbance?
In the context of absorbance, ε is the molar absorptivity, also referred to as the extinction coefficient.
p.14
Types of Molecular Vibrations
What are the principal modes of vibration between carbon and hydrogen in an alkane?
The principal modes are symmetrical stretching, asymmetrical stretching, scissoring, rocking, wagging, and twisting.
p.1
Principles of Infrared Radiation
What type of spectroscopy is primarily used by analytical and organic chemists?
IR spectroscopy and spectrometry, primarily absorption spectroscopy, is used by analytical and organic chemists.
p.22
Molecular Vibrations and IR Activity
What happens to the reduced mass (μ) in the context of molecular vibrations?
The reduced mass (μ) decreases, which can affect the vibrational frequency of the molecule.
p.5
Electromagnetic Spectrum Overview
What is the relationship between energy and wavelength?
Energy is inversely proportional to wavelength; short wavelengths have high energy and vice versa.
p.12
Dipole Moments and IR Absorption
Why does carbon dioxide have no net dipole moment despite having two C=O bond dipoles?
Because the molecule is linear, the bond dipoles cancel each other out.
p.16
Absorption Considerations in IR Spectroscopy
What must equal the frequency of the incident radiation for a molecule to absorb IR radiation?
The natural frequency of vibration of the molecule must equal the frequency of the incident radiation.
p.15
Types of Molecular Vibrations
What are the three types of vibrations that can occur in a CO2 molecule?
Symmetric stretch, asymmetric stretch, and bending vibration.
p.4
Electromagnetic Spectrum Overview
What is the electromagnetic spectrum?
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation, which can be considered as waves or particles traveling at the speed of light.
p.15
Dipole Moments and IR Absorption
Which vibrational mode of CO2 does not change the dipole moment?
The symmetric stretching vibration.
p.29
Principles of Infrared Radiation
How is the wavenumber usually expressed?
The wavenumber is usually expressed as cm - 1.
p.20
Molecular Vibrations and IR Activity
What is the relationship between bond strength and vibrational frequency for single, double, and triple bonds?
The vibrational frequency increases from single to double to triple bond due to increased bond strength.
p.27
Absorption Considerations in IR Spectroscopy
How can transmittance be calculated?
Transmittance can be calculated using the formula: T = I/I0, where I0 is the intensity of the incident radiation and I is the intensity of the radiation leaving the sample.
p.16
Molecular Vibrations and IR Activity
What modifies the energy difference between vibrational energy levels?
The energy difference between the vibrational energy levels is modified by coupling to rotational energy levels and coupling between vibrations.
p.4
Electromagnetic Spectrum Overview
How is the energy of a photon related to wavelength and frequency?
The energy of a photon (E in Joules) is directly proportional to frequency; therefore, high energy radiation will have a high frequency.
p.17
Dipole Moments and IR Absorption
What happens to the IR spectrum of alkenes and alkynes when the bond is symmetrically substituted?
No band will be seen in the IR spectrum.
p.17
Dipole Moments and IR Absorption
What happens to the dipole moment when two equal masses in a diatomic molecule vibrate?
No change in the dipole moment will occur as the molecule vibrates.
p.23
Types of Molecular Vibrations
What does Figure 8 compare regarding bonds with different reduced masses?
It compares wavelength, stretching frequency, and wavenumber.
p.29
Infrared Regions and Their Applications
In which region are most infrared analyses carried out?
Most infrared analyses are carried out in the mid-infrared region (400 to 4000 cm - 1).
p.20
Molecular Vibrations and IR Activity
What happens to the vibrational frequency of a bond as its strength increases?
The vibrational frequency of a bond increases as the strength of the bond increases.
p.16
Absorption Considerations in IR Spectroscopy
What equation must the frequency of the radiation satisfy for IR absorption?
The frequency of the radiation must satisfy ΔE = hν, where ΔE is the energy difference between the vibrational states involved.
p.12
Dipole Moments and IR Absorption
What is important for predicting the IR absorption behavior of molecules with more than two atoms?
Any change in the dipole moment on vibration is important, not just the net dipole moment of the molecule.
p.19
Calculation of Molecular Vibrations
What is the force constant for a double C-H bond?
The force constant for a double bond is 10 x 10^5 dyne/cm.
p.30
Calculation of Molecular Vibrations
What is the wavenumber corresponding to infrared radiation of wavelength 2,500 nm?
The wavenumber is calculated as 1 / (2.5 x 10^-4 cm) = 4,000 cm^-1.
p.26
Absorption Considerations in IR Spectroscopy
What does Figure 10 illustrate in the context of infrared radiation?
Figure 10 illustrates a beam of infrared radiation of power I0 directed at a sample solution, showing the radiation leaving the sample with radiant power I.
p.4
Electromagnetic Spectrum Overview
What is the speed of light in cm/sec?
The speed of light is 3 x 10^10 cm/sec.
p.2
Principles of Infrared Radiation
What is the energy comparison between IR radiation, visible radiation, and radio waves?
IR radiation is of lower energy than visible radiation but of higher energy than radio waves.
p.25
Dipole Moments and IR Absorption
Do homonuclear diatomic molecules exhibit infrared active modes?
No, homonuclear diatomic molecules such as Cl2, H2, N2, etc., will exhibit no infrared active modes as there is no change in their dipole moment during vibration.
p.13
Types of Molecular Vibrations
What are the two types of stretching modes for a group of three or more atoms?
The two types are symmetrical stretching and asymmetrical stretching.
p.6
Interpretation of IR Spectra
Why is infrared spectroscopy considered a useful method for structural analysis?
Infrared spectroscopy is considered useful for structural analysis because it can provide different types of structural information if the molecule under investigation is infrared active.
p.24
Types of Molecular Vibrations
What are the simplest modes of vibration that are infrared active?
The simplest modes of vibration that are infrared active are stretching and bending modes.
p.29
Electromagnetic Spectrum Overview
What is the range of the infrared region in terms of wavenumbers?
The infrared region spans from 33 to 12820 cm - 1.
p.10
Dipole Moments and IR Absorption
What indicates a polar bond in a molecule?
A polar bond is indicated by charge separation, resulting in a partial positive charge on one atom and a partial negative charge on the other.
p.16
Dipole Moments and IR Absorption
What is required for a vibration to result in IR absorption?
The vibration must cause a change in the dipole moment of the molecule.
p.3
Dipole Moments and IR Absorption
How do atoms interact with electromagnetic radiation?
Atoms can absorb energy from electromagnetic radiation, which alters the state of the atoms within the molecule.
p.2
Infrared Regions and Their Applications
What are the wavelengths of IR radiation of interest to chemists studying organic molecules?
The wavelengths fall between 2 and 20 μm.
p.27
Absorption Considerations in IR Spectroscopy
What does the Beer-Lambert law state?
The Beer-Lambert law states that absorbance (A) is equal to εlc, where ε is the molar absorptivity, l is the path length, and c is the concentration.
p.4
Electromagnetic Spectrum Overview
What is Planck's constant?
Planck's constant (h) is 6.6 x 10^-34 Joules - sec.
p.25
Molecular Vibrations and IR Activity
What happens when either of the H atoms in acetylene is substituted?
Substitution of either of the H atoms produces an asymmetric alkyne bond which will be IR active.
p.2
Infrared Regions and Their Applications
What is the maximum wavelength end of the IR region defined as?
The maximum wavelength end is defined somewhat arbitrarily; some texts consider the far-IR to extend to 1000 μm.
p.13
Types of Molecular Vibrations
Describe symmetrical stretching in a CH2 group.
In symmetrical stretching, both H atoms move away from the C atom.
p.7
Interpretation of IR Spectra
Is infrared spectroscopy sufficient to determine the complete structure of a molecule?
More often than not, infrared spectroscopy is insufficient to determine the complete structure, and additional techniques such as NMR and mass spectroscopy are used.
p.10
Dipole Moments and IR Absorption
What happens to electron density when two atoms with different electronegativities form a bond?
The electron density in the bond is not equally distributed, shifting towards the more electronegative atom.
p.19
Calculation of Molecular Vibrations
What is the observed stretching frequency range for C-H bonds?
The observed values for C-H bonds are in the region 3320 - 2700 cm -1.
p.10
Dipole Moments and IR Absorption
In the molecule hydrogen fluoride (HF), which atom has a partial negative charge?
The fluorine atom (F) has a partial negative charge.
p.12
Dipole Moments and IR Absorption
How does the geometry of water affect its dipole moment?
Water has a bent geometry, causing the vector sum of the bond dipoles to not cancel, resulting in a permanent net dipole moment.
p.20
Molecular Vibrations and IR Activity
How does reduced mass (μ) affect the vibrational frequency of a bond?
The vibrational frequency of a bond increases as the reduced mass (μ) decreases.
p.10
Dipole Moments and IR Absorption
How is the dipole in a bond represented?
The dipole is indicated by a crossed arrow placed with the point on the more negative end of the bond.
p.16
Dipole Moments and IR Absorption
How is the amount of radiation absorbed related to the dipole moment during vibration?
The amount of radiation absorbed is proportional to the square of the rate of change of the dipole during the vibration.
p.27
Absorption Considerations in IR Spectroscopy
What is the relationship between absorbance and transmittance?
Absorbance (A) is related to transmittance (T%) by the equation A = 2 - log(T%).
p.14
Types of Molecular Vibrations
What are the out-of-plane bending modes described?
Wagging and twisting are the out-of-plane bending modes where the H atoms move out of the plane containing the C atom.
p.14
Types of Molecular Vibrations
What are bends also referred to as in the context of molecular vibrations?
Bends are also called deformations, and the term antisymmetric is used in place of asymmetric in various texts.
p.17
Molecular Vibrations and IR Activity
How can a bond be treated to understand molecular vibrations?
A bond can be treated as a simple harmonic oscillator composed of two masses (atoms) joined by a spring.
p.13
Types of Molecular Vibrations
What is stretching in molecular vibrations?
Stretching involves a change in bond lengths resulting in a change in interatomic distance.
p.1
Absorption Considerations in IR Spectroscopy
Can IR absorption be used for quantitative measurements?
Yes, IR absorption can also be used to provide quantitative measurements of compounds.
p.30
Calculation of Molecular Vibrations
What is the formula to calculate the wavenumber from wavelength?
The wavenumber (in cm^-1) can be calculated using the formula: wavenumber = 1 / wavelength (in cm).
p.15
Dipole Moments and IR Absorption
Why is the symmetric stretching vibration of CO2 considered IR inactive?
Because it does not change the dipole moment of the molecule.
p.3
Molecular Vibrations and IR Activity
What changes occur in molecular vibrations due to absorbed energy?
The absorbed energy usually manifests in alterations to the frequency and amplitude of molecular vibrations, which can be measured and plotted to produce an infrared spectrum.
p.19
Calculation of Molecular Vibrations
What is the force constant for a triple C-H bond?
The force constant for a triple bond is 15 x 10^5 dyne/cm.
p.14
Types of Molecular Vibrations
What does the + sign in the circle indicate regarding molecular movement?
The + sign indicates movement above the plane of the page toward the reader.
p.2
Infrared Regions and Their Applications
How is the entire IR region divided?
The IR region can be divided into NIR (0.75 to 2.5 μm), mid-IR (2.5 to 20 μm), and far-IR (20 to 200 μm).
p.13
Types of Molecular Vibrations
What are the common types of molecular vibrations excited by IR radiation?
The common types are stretching vibrations and bending vibrations.
p.13
Types of Molecular Vibrations
What does bending in molecular vibrations entail?
Bending involves a change in bond angle or a change in the position of a group of atoms with respect to the rest of the molecule.
p.6
Molecular Vibrations and IR Activity
What type of vibrations are associated with the Far-Infrared region?
The Far-Infrared region is associated with vibrations of molecules containing heavy atoms, molecular skeleton vibrations, and crystal lattice vibrations.
p.30
Calculation of Molecular Vibrations
What is the wavelength in cm for 2,500 nm?
The wavelength in cm is 2,500 nm × 1 x 10^-7 cm/nm = 2.5 x 10^-4 cm.
p.10
Dipole Moments and IR Absorption
What type of molecule is hydrogen fluoride (HF) and what is its dipole moment?
HF is a linear diatomic molecule with one polar bond, making it polar and giving it a dipole moment.
p.4
Electromagnetic Spectrum Overview
What is the relationship between frequency and wavelength?
Frequency and wavelength are inversely related; as one increases, the other decreases.
p.15
Dipole Moments and IR Absorption
Which vibrational modes of CO2 are IR active?
The asymmetric stretching and bending vibrations.
p.2
Electromagnetic Spectrum Overview
How do the wavelengths of IR radiation compare to visible radiation?
IR wavelengths are longer than those in the red end of the visible region, which ends at about 0.75 μm.
p.14
Types of Molecular Vibrations
What does the - sign in the circle indicate regarding molecular movement?
The - sign indicates movement below the plane of the page away from the reader.
p.17
Dipole Moments and IR Absorption
What occurs in the IR spectrum when the bond in alkenes or alkynes is asymmetrically substituted?
A stretching frequency corresponding to the alkene or alkyne bond will be present.
p.13
Types of Molecular Vibrations
What occurs during asymmetrical stretching in a CH2 group?
In asymmetrical stretching, one H atom moves away from the C atom while the other moves toward it.
p.6
Infrared Regions and Their Applications
What are the three well-defined infrared regions in infrared spectroscopy?
The three well-defined infrared regions are Far-Infrared (400 - 33 cm-1), Mid-Infrared (4000 - 400 cm-1), and Near Infrared (12820 - 4000 cm-1).
p.6
Infrared Regions and Their Applications
What is the primary use of the Mid-Infrared region?
The Mid-Infrared region is useful for organic analysis.
p.6
Infrared Regions and Their Applications
What is the significance of the Near Infrared region in analysis?
The Near Infrared region is significant for overtones and is very useful for quantitative analysis.
