Question

Suggest which types of transitions \(\left(\sigma^{*}+\sigma\right.\) \(\pi^{*}+\pi, \sigma^{*} \prec n\) or \(\pi^{*}+n\) ) give rise to significant features in the electronic spectra of (a) pentane (13.15) (b) pent-l-ene (13.16), (c) octa- 2,4,6 -triene (13.17) and \((d)\) ethanol (13.18)

Short answer

(a) \(\sigma^* + \sigma\); (b) \(\pi^* + \pi\); (c) \(\pi^* + \pi\); (d) \(\sigma^* \prec n\).

Step-by-step solution

Understand the Transitions

Molecular electronic transitions can broadly be categorized into: - **\(\sigma^* + \sigma\):** Excitation involving \(\sigma\) bonds often seen in saturated molecules.- **\(\pi^* + \pi\):** Involves \(\pi\) electrons, common in unsaturated compounds with multiple bonds.- **\(\pi^* + n\):** Non-bonding to antibonding \(\pi\), seen in molecules with lone pair electrons.- **\(\sigma^* \prec n\):** Lone pair to \(\sigma^*\), also involving lone pair electrons but in saturated systems.

Analyze Pentane

Pentane is a fully saturated molecule with only \(\sigma\) bonds and no \(\pi\) systems or lone pair electrons significant for electronic transitions. Thus, the most likely transition is \(\sigma^* + \sigma\).

Analyze Pent-1-ene

Pent-1-ene has both \(\sigma\) and \(\pi\) bonds due to its one double bond. The electronic transitions would involve \(\pi \rightarrow \pi^*\) due to this unsaturation, leading to the main electronic absorption feature.

Analyze Octa-2,4,6-triene

Octa-2,4,6-triene has multiple \(\pi\) bonds due to the conjugated system. The primary significant transition in the spectrum would be \(\pi^* + \pi\) transitions, contributing to a strong absorption due to the extended conjugation.

Analyze Ethanol

Ethanol has \(\sigma\) bonds and an -OH group providing lone pair electrons. Likely transitions include \(\sigma^* \prec n\), as the lone pair on oxygen can be involved in such a transition.

Key concepts

Molecular Spectroscopy

Molecular spectroscopy is a powerful technique used to study the electronic transitions in molecules. These transitions occur when electrons absorb energy and move from a lower energy level to a higher one, often detected in the UV-Vis spectrum. In this field, understanding the nature of chemical bonds and electron pairs is crucial, as they influence which transitions are allowed or prominent.

• **Electronic transitions:** When energy is absorbed, electrons jump between molecular orbitals.
• **Influence of molecular structure:** The type and arrangement of bonds affect the energy levels and the absorbance spectrum.

In basic terms, the way molecules absorb light relates to their electronic structure. These insights help identify and analyze organic compounds based on their spectral features.

Sigma Bonds

Sigma () bonds are the strongest type of covalent chemical bond. Formed by the direct overlap of atomic orbitals, sigma bonds primarily exist in saturated organic molecules like alkanes. In molecular spectroscopy, transitions involving sigma bonds are typically high in energy and therefore less common in direct observation via UV-Vis.

• **Characteristics of sigma bonds:**
  • Strong covalent bond, formed via head-to-head overlap.
  • Stability and existence in single-bonded carbon chains.
• **Role in electronic transitions:**
  • Sigma to sigma () transitions require higher energy, usually occurring in the vacuum UV region.

Saturated hydrocarbons like pentane offer a classic example where  bonds play a vital role, yet the transitions are often not easily detected due to their high energy requirements.

Pi Bonds

Pi () bonds are less strong than sigma bonds and arise from the side-to-side overlap of p orbitals. They typically exist in unsaturated molecules such as alkenes and alkynes. In spectroscopy, pi bonds are significant because they generally absorb at lower energies than sigma bonds, making  to ")) u transition relatively easy to detect.

• **Formation:**
  • Formed from the overlap of parallel p orbitals.
  • Occurs in double and triple bonds where additional bonds beyond the sigma bond form a pi bond.
• **Spectral significance:**
  • Pi to pi () transitions contribute to high absorbance in compounds like pent-1-ene or conjugated systems such as octa-2,4,6-triene.
  • These are often central to understanding and designing materials that interact with light in specific ways.

The presence of pi bonds in a molecule dramatically influences its spectroscopic behavior, facilitating the study of electronic transitions.

Lone Pair Electrons

Lone pair electrons refer to pairs of valence electrons not involved in bonding. They are found in atoms such as oxygen, nitrogen, and halogens. Lone pairs play a crucial role in molecular electronic transitions, particularly in transitions involving non-bonding to antibonding orbitals.

• **Nature of lone pairs:**
  • Unbonded electron pairs residing in outer shells.
  • They can occupy non-bonding molecular orbitals.
• **Influence in spectroscopy:**
  • Transitions like ")) n to ")+ ) n are observed in molecules with lone pairs, such as in ethanol.
  • The absorption features due to these transitions often appear in the UV region.

Understanding lone pair electrons and their electronic transitions is essential for the analysis and interpretation of molecular spectroscopy, providing insights into the detailed electronic structure of molecules.