13.2
IR Spectroscopy: Molecular Vibration Overview
When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamenta…
IR-active molecules exhibit two fundamental modes of vibration that result in IR absorption.
Stretching vibrations occur along the bond line and can change the bond length. Stretches can be symmetric or asymmetric.
Bending vibrations change the bond angle and are subdivided into in-plane or out-of-plane modes. Scissoring and rocking represent the former type, whereas wagging and twisting represent the latter.
The total number of fundamental vibrations allowed for a nonlinear molecule like ethanol containing n number of atoms is 3n − 6; however, only 3n − 5 fundamental vibrations are allowed for a linear molecule such as HCl.
A bond's stretching and bending vibrations occur at a characteristic fundamental absorption frequency and involve the excitation of bonds from the ground state to the lowest energy excited state.
Integral multiples of fundamental absorption frequencies are termed overtones.
The fusion of two IR active vibrational frequencies produces a combination band.
The coupled vibration generated by the interaction of the fundamental band with the overtone or combination band produces Fermi resonance.
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Q1: What are the two main types of molecular vibrations in IR spectroscopy?
IR-active molecules exhibit stretching and bending vibrations. Stretching vibrations occur along the bond line and change bond length, while bending vibrations change bond angles without occurring along the bond line. Both types result in characteristic IR absorption when bonds transition from ground state to the lowest energy excited state.
Q2: How do symmetric and asymmetric stretching vibrations differ?
In symmetric stretching, two bonds elongate and contract together simultaneously. Asymmetric stretching involves one bond moving in one direction while another bond moves in the opposite direction. These different motions produce distinct peak splitting in IR spectra through IR spectrum peak splitting symmetric vs asymmetric vibrations.
Q3: What are the types of bending vibrations in IR spectroscopy?
Bending vibrations are classified as in-plane or out-of-plane modes. In-plane bending includes scissoring (symmetric) and rocking (asymmetric) motions. Out-of-plane bending comprises twisting (symmetric) and wagging (asymmetric) motions. These distinct bending modes contribute unique absorption frequencies to IR spectra and enable molecular identification.
Q4: How many fundamental vibrations are allowed for nonlinear and linear molecules?
Nonlinear molecules like ethanol containing n atoms allow 3n − 6 fundamental vibrations. Linear molecules such as HCl allow only 3n − 5 fundamental vibrations. This difference arises because linear molecules have one fewer rotational degree of freedom, reducing the number of independent vibrational modes available.
Q5: What is the difference between overtones and combination bands in IR spectroscopy?
Overtones are integral multiples of fundamental absorption frequencies generated by any physical vibration in a molecule. Combination bands result from the fusion of two IR-active vibrational frequencies. Both phenomena appear in IR spectra and provide additional information about molecular structure beyond fundamental absorption peaks.
Q6: What is Fermi resonance and how does it affect IR spectra?
Fermi resonance is a coupled vibration created by the interaction of a fundamental band with either an overtone or combination band. This interaction produces spectral features that differ from simple superposition of individual bands, affecting peak positions and intensities in IR spectra.
Q7: Why do stretching and bending vibrations occur at characteristic frequencies?
Stretching and bending vibrations occur at characteristic fundamental absorption frequencies because bonds have specific energy requirements to transition from ground state to the lowest energy excited state. These frequencies depend on bond strength, atomic masses, and molecular geometry, making them unique identifiers for different functional groups.