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Thermochemical studies of pyrazolide.
Adam J Gianola1, Takatoshi Ichino, Shuji Kato
1JILA, University of Colorado and National Institute of Standards and Technology, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440, USA.
The Journal of Physical Chemistry. A
|July 11, 2006
Summary
This study measured the photoelectron spectrum of 1-pyrazolide anion, determining the electron affinity of the 1-pyrazolyl radical and gas-phase acidity of pyrazole. It also characterized the 5-pyrazolide anion, providing insights into pyrazole
Area of Science:
- Physical Chemistry
- Quantum Chemistry
- Spectroscopy
Background:
- Pyrazole is a fundamental heterocyclic aromatic organic compound.
- Understanding the electronic properties and stability of pyrazole derivatives is crucial in various chemical applications.
- Previous studies have explored pyrazole's reactivity, but detailed electronic characterization of its anionic and radical forms is limited.
Purpose of the Study:
- To experimentally determine the electron affinity (EA) of the 1-pyrazolyl radical.
- To measure the gas-phase acidity of pyrazole.
- To investigate the electronic structure and vibrational properties of 1-pyrazolide and 5-pyrazolide anions and their corresponding radicals.
Main Methods:
- Photoelectron spectroscopy of 1-pyrazolide anion generated via hydroxide (HO-) deprotonation of pyrazole in a flowing afterglow ion source.
- Gas-phase acidity measurements using a flowing afterglow-selected ion flow tube.
- Computational analysis including Franck-Condon (FC) simulations and B3LYP/6-311++G(d,p) electronic structure calculations.
Main Results:
- The electron affinity (EA) of the 1-pyrazolyl radical was determined to be 2.938 ± 0.005 eV.
- Gas-phase acidity values for pyrazole were measured: ΔacidG(298) = 346.4 ± 0.3 kcal mol⁻¹ and ΔacidH(298) = 353.6 ± 0.4 kcal mol⁻¹.
- The EA of the 5-pyrazolyl radical was found to be 2.104 ± 0.005 eV, with significant vibrational progression indicating geometric differences between the anion and radical.
Conclusions:
- The study provides key thermochemical data for pyrazole, including its N-H bond dissociation energy (BDE) derived from EA and acidity measurements.
- The photoelectron spectrum reveals near-degeneracy of low-lying states in 1-pyrazolyl and suggests significant nonadiabatic effects.
- Comparison with isoelectronic systems like pyrrole and imidazole offers broader insights into the electronic properties of five-membered heterocycles.