Heats of Formation of C₆H₅^•, C₆H₅⁺, and C₆H₅NO by Threshold Photoelectron Photoion Coincidence and Active Thermochemical Tables Analysis

Abstract

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol(-1), respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C-H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol(-1). Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.

Active Thermochemical Tables: dissociation energies of several homonuclear first-row diatomics and related thermochemical values

“…In the very early stages of the development of the ATcT TN, when the network was much smaller, the resulting ATcT enthalpy of formation of N atom was strongly dominated by the same spectroscopic determination by Büttenbender and Herzberg [59]. Consequently, this determination was marked as a 'weak link' in the TN, and steps were undertaken to enhance the TN section surrounding N. Table 2) coincides with the previously reported [10] interim value. The CODATA [8] value is more than an order of magnitude less accurate and is 0.24 kJ/mol higher; perhaps a not entirely unrelated detail is that the bond dissociation energy explicitly quoted by CODATA should have produced an uncertainty for the enthalpy of formation of nitrogen atom of ±0.3 0 kJ/mol, but in the final analysis, the CODATA Task Group has decided for some reason to increase it to ±0.4 0 kJ/mol.…”

“…4 Other than gradually gaining in accuracy as the TN was expanded over time, these ATcT values have not changed significantly since the previously reported versions [1,3,4,[9][10][11]. The CODATA value [8], subsequently adopted in the popular tables of Gurvich et al [12,13] and in the JANAF Tables 5 [14,15], was already quite accurate, D f H°2 98 (H) = 217.998 ± 0.006 kJ/mol, and thus in this particular case, the improvement in accuracy by almost three orders of magnitude is probably of no practical thermochemical consequence.…”

“…Table 2 also lists the current ATcT enthalpy of formation of carbon atom, D f H°0(C) = 711.395 ± 0.054 kJ/mol (716.880 kJ/mol at 298.15 K). The latter value has changed very slightly since the previously reported [10] interim ATcT value.…”

“…The ATcT TN approach provides a number of significant advantages over the traditional sequential approach, described in more detail elsewhere [1,2,9,10]. Because the results are obtained by simultaneously satisfying all relevant determinations present in an internally consistent TN, the ATcT values are not only more accurate, 1 but also more robust 2 than those obtained from a sequential approach.…”

Accurate ab initio ro-vibronic spectroscopy of the $\tilde X^2 \Pi$X̃2Π CCN radical using explicitly correlated methods

“…This is in good agreement with the previous harmonic results of Martin et al 16 Intensities of Renner-Teller subbands are shown in Table VII The enthalpy of formation at 0 K, ΔH f (0K), for X 2 Π 1/2 CCN was computed from the final zero-point energy corrected atomization energy (290.9 kcal/mol as calculated from the ΣD e value shown in Table I of 296.13 kcal/mol) together with spin-orbit corrections for both the molecule (+0.06 kcal/mol) and atoms (-0.14 kcal/mol), i.e., ΣD 0 = 290.8 kcal/mol. This is about 2 kcal/mol larger than the previously computed value by Martin et al 16 Using reference ΔH f (0K) values for the atoms 73 (170.024 kcal/mol for C and 112.469 kcal/mol for N), the resulting 0 K enthalpy of formation, 161.7 kcal/mol, is expected to be accurate to within about 0.5 kcal/mol, with the majority of this uncertainty arising from the higher order correlation contributions to the atomization energy.…”