13.2
Spettroscopia IR: Vibrazione molecolare - Panoramica
Quando la radiazione infrarossa (IR) attraversa una molecola legata covalentemente, i legami passano da livelli vibrazionali inferiori a superiori. I…
Le molecole IR-attive mostrano due modalità fondamentali di vibrazione che provocano l'assorbimento IR.
Le vibrazioni di stiramento si verificano lungo la linea di legame e possono modificare la lunghezza del legame. Gli allungamenti possono essere simmetrici o asimmetrici.
Le vibrazioni di flessione modificano l'angolo di legame e sono suddivise in modalità nel piano o fuori dal piano. Le forbici e il dondolio rappresentano il primo tipo, mentre lo scodinzolamento e la torsione rappresentano il secondo.
Il numero totale di vibrazioni fondamentali consentite per una molecola non lineare come l'etanolo contenente n numero di atomi è 3n − 6; tuttavia, solo 3n − 5 vibrazioni fondamentali sono consentite per una molecola lineare come l'HCl.
Le vibrazioni di allungamento e flessione di un legame si verificano a una caratteristica frequenza di assorbimento fondamentale e comportano l'eccitazione dei legami dallo stato fondamentale allo stato eccitato a più bassa energia.
I multipli integrali delle frequenze di assorbimento fondamentali sono chiamati armonici.
La fusione di due frequenze vibrazionali attive IR produce una banda combinata.
La vibrazione accoppiata generata dall'interazione della banda fondamentale con l'armonico o la banda combinata produce la risonanza di Fermi.
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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.