High-resolution energy-selected study of the reaction NH3+→NH2++H: Accurate thermochemistry for the NH2/NH2+ and NH3/NH3+ systems

Abstract

Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH3+ to form NH2++H near its threshold. The breakdown curves for NH2+ and NH3+ thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH2+ from NH3. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D0=4.6017±0.0025 eV) for NH3, allows the determination of the D0(NH2+–H) and IE(NH2), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (ΔHf0°) for NH3 and H and the AE(NH2+), we obtain the ΔHf0°(NH2+)=302.60±0.08 kcal/mol. The PFI-PE spectrum for NH3 exhibits a step-like feature at the 0 K AE(NH2+), indicating that the dissociation of excited NH3 in high-n (n⩾100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ⩽10−7 s. This step confirms the AE(NH2+) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation [Dixon et al., J. Chem. Phys. 115, 2576 (2001)]. The comparison between theoretical predictions and the best experimental results for the NH2/NH2+ and NH3/NH3+ systems indicates that the accuracy of the computational scheme used is ⩽0.4 kcal/mol.