Thermochemical Studies of Pyrazolide^†

The 351.1 nm photoelectron spectrum of 1-pyrazolide anion has been measured. The 1-pyrazolide ion is produced by hydroxide (HO^-) deprotonation of pyrazole in a flowing afterglow ion source. The electron affinity (EA) of the 1-pyrazolyl radical has been determined to be 2.938 ± 0.005 eV. The angular dependence of the photoelectrons indicates near-degeneracy of low-lying states of 1-pyrazolyl. The vibronic feature of the spectrum suggests significant nonadiabatic effects in these electronic states. The gas phase acidity of pyrazole has been determined using a flowing afterglow-selected ion flow tube; Δ_acidG₂₉₈ = 346.4 ± 0.3 kcal mol^-1 and Δ_acidH₂₉₈ = 353.6 ± 0.4 kcal mol^-1. The N−H bond dissociation energy (BDE) of pyrazole is derived to be D₀(pyrazole, N−H) = 106.4 ± 0.4 kcal mol^-1 from the EA and the acidity using a thermochemical cycle. In addition to 1-pyrazolide, the photoelectron spectrum demonstrates that HO^- deprotonates pyrazole at the C5 position to generate a minor amount of 5-pyrazolide anion. The photoelectron spectrum of 5-pyrazolide has been successfully reproduced by a Franck−Condon (FC) simulation based on the optimized geometries and the normal modes obtained from B3LYP/6-311++G(d,p) electronic structure calculations. The EA of the 5-pyrazolyl radical is 2.104 ± 0.005 eV. The spectrum exhibits an extensive vibrational progression for an in-plane CCN bending mode, which indicates a substantial difference in the CCN angle between the electronic ground states of 5-pyrazolide and 5-pyrazolyl. Fundamental vibrational frequencies of 890 ± 15, 1110 ± 35, and 1345 ± 30 cm^-1 have been assigned for the in-plane CCN bending mode and two in-plane bond-stretching modes, respectively, of X̃ ²A‘ 5-pyrazolyl. The physical properties of the pyrazole system are compared to the isoelectronic systems, pyrrole and imidazole.