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Related Concept Videos

Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control01:23

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

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Updated: Jun 4, 2026

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems
09:57

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems

Published on: February 10, 2020

Ab initio based surface-hopping dynamics study on ultrafast internal conversion in cyclopropanone.

Ganglong Cui1, Weihai Fang

  • 1College of Chemistry, Beijing Normal University, Beijing, PR China.

The Journal of Physical Chemistry. A
|February 17, 2011
PubMed
Summary
This summary is machine-generated.

Cyclopropanone

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Area of Science:

  • Photochemistry
  • Theoretical Chemistry
  • Chemical Physics

Background:

  • Cyclopropanone exhibits fluorescence quenching below 365 nm.
  • This phenomenon is attributed to rapid S(1) → S(0) internal conversion.
  • Conical intersections are key to ultrafast internal conversion processes.

Purpose of the Study:

  • To investigate the vibrational-mode-dependent S(1) → S(0) internal conversion in cyclopropanone.
  • To elucidate the role of conical intersections in this process.
  • To estimate the S(1) lifetime and internal conversion timescale.

Main Methods:

  • Utilized ab initio based surface hopping dynamics.
  • Determined a new conical intersection between S(1) and S(0) states using state-averaged CASSCF/cc-pVDZ calculations.
  • Performed nonadiabatic dynamics simulations to analyze the conversion pathways.

Main Results:

  • Identified a critical conical intersection facilitating ultrafast S(1) → S(0) internal conversion.
  • Found that internal conversion is most efficient with initial kinetic energy in C═O vibrational modes, particularly C-O stretching and O-C-C-C out-of-plane torsion.
  • Estimated S(1) lifetime and internal conversion timescale, consistent with experimental observations.

Conclusions:

  • The ultrafast S(1) → S(0) internal conversion via a newly identified conical intersection is responsible for the fluorescence disappearance in cyclopropanone.
  • Vibrational modes associated with the C═O group significantly influence the efficiency of internal conversion.
  • Theoretical dynamics simulations provide strong evidence supporting the proposed mechanism.