Thermochemistry of ReN and IrN: Bond dissociation energies of ReN+ and IrN+ and ionization energies of ReN and IrN

The bond dissociation energies (BDEs) of ReN+ and IrN+ have been measured using a newly constructed cryogenically cooled ion photodissociation spectrometer to be D0(Re+-N) = 40 615(100) cm-1 [5.036(12) eV] and D0(Ir+-N) = 37 825(75) cm-1 [4.690(9) eV]. The ionization energy (IE) of IrN was measured to be IE(IrN) = 75 490(300) cm-1 [9.360(37) eV] using resonant 2-photon ionization (R2PI) spectroscopy. The values of D0(Ir+-N) and IE(IrN), along with previous measurements of D0(IrN) and IE(Ir), were used to complete the thermochemical cycle D0(IrN) + IE(Ir) - IE(IrN) - D0(Ir+-N) = 0, obtaining 264 ± 311 cm-1 after propagating errors by quadrature. Because IE(IrN) is by far the least precise value in the cycle, a more precise value is obtained using the other measurements in the thermochemical cycle, giving IE(IrN) = 75 754(83) cm-1 or 9.392(10) eV. The corresponding thermochemical cycle for ReN initially did not agree, prompting a remeasurement that gave IE(ReN) = 68 276(40) cm-1 [8.465(5) eV]. The previous D0(ReN) measurement was also reassessed, giving D0(ReN) = 45 575(75) cm-1 [5.651(15) eV]. The corresponding cycle for ReN now gives -134 ± 131 cm-1. Predissociation-based measurements of BDEs necessarily provide upper limits only; there is always the possibility of a barrier to dissociation that could cause the measured values to be too high. The present results demonstrate that for molecules such as ReN and IrN, which have a high density of states at the ground separated atom limit, predissociation-based methods provide a highly accurate measurement of the BDE.