Threshold photoelectron spectroscopy and dissociative photoionization of NCN and NCN3: The NCN enthalpy of formation

Kurzfassung

The NCN radical is a key intermediate in prompt-NO formation. For flame simulations that involve detailed NCN radical chemistry such as the prompt-NO switch reaction NCN + H, it is vital to have both accurate kinetic and thermodynamic data on NCN available. This study employed synchrotron-based imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on NCN3 and NCN to determine a consistent and accurate enthalpy of formation for NCN. The obtained value of DELTAHf 0K(NCN) = 450.5 ± 3.5 kJ/mol (DeltaHf 298 K(NCN) = 451.0 ± 3.5 kJ/mol) provides experimental evidence for the recently recommended, similarly low theoretical values. The determination is based on thermodynamic cycles with input from the first reported threshold photoelectron spectrum (TPES) of the NCN radical, indicating an adiabatic ionization energy of IE(NCN) = 12.608 ± 0.027 eV, complemented by the appearance energy of NCN+ from dissociative ionization of NCN3, AE(NCN+) = 12.060 ± 0.015 eV. We also report the adiabatic ionization energy of NCN3, IE(NCN3) = 10.995 ± 0.010 eV along with Franck–Condon spectral modeling. Novelty and significance statement: The enthalpy of formation of the biradical NCN has been a subject of debate until recently. Over the years, theoretical calculations, kinetic models and experiments have yielded very different values, ranging from 445 - 473 kJ/mol. However, most recent high-level theoretical calculations seem to converge to a consensus value of about 451 kJ/mol, but an independent experimental verification of this value is still lacking to clarify the discrepancy between two previously reported spectroscopic measurements of 452 kJ/mol and 466 kJ/mol. We took advantage of the synchrotron-based imaging photoelectron photoion coincidence spectroscopy setup at the Swiss Light Source to record the first threshold photoelectron spectrum of NCN, where NCN radicals were generated by a flash pyrolysis of NCN3. By combining the derived adiabatic ionization energy with the appearance energy of NCN+ from the dissociative photoionization of NCN3, we were able to verify the lower enthalpy of formation value.