Accurate ab initio predictions of ionization energies and heats of formation for the 2-propyl, phenyl, and benzyl radicals.
The ionization energies (IEs) for the 2-propyl (2-C(3)H(7)), phenyl (C(6)H(5)), and benzyl (C(6)H(5)CH(2)) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction have been also made in these calculations. Although a precise IE value for the 2-C(3)H(7) radical has not been directly determined before due to the poor Franck-Condon factor for the photoionization transition at the ionization threshold, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. The comparison between the predicted value through the focal-point analysis and the highly precise experimental value for the IE(C(6)H(5)CH(2)) determined in the previous pulsed field ionization photoelectron (PFI-PE) study shows that the CCSD(T)/CBS method is capable of providing an accurate IE prediction for C(6)H(5)CH(2), achieving an error limit of 35 meV. The benchmarking of the CCSD(T)/CBS IE(C(6)H(5)CH(2)) prediction suggests that the CCSD(T)/CBS IE(C(6)H(5)) prediction obtained here has a similar accuracy of 35 meV. Taking into account this error limit for the CCSD(T)/CBS prediction and the experimental uncertainty, the CCSD(T)/CBS IE(C(6)H(5)) value is also consistent with the IE(C(6)H(5)) reported in the previous HeI photoelectron measurement. Furthermore, the present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can be used to provide reliable IE predictions for C(3)-C(7) hydrocarbon radicals with an uncertainty of +/-35 meV. Employing the atomization scheme, we have also computed the 0 K (298 K) heats of formation in kJ/mol at the CCSD(T)/CBS level for 2-C(3)H(7)/2-C(3)H(7) (+) ,C(6)H(5)/C(6)H(5) (+), and C(6)H(5)CH(2)/C(6)H(5)CH(2) (+) to be 105.2/822.7 (90.0/806.4), 351.4/1148.5 (340.4/1138.8), and 226.2/929.0 (210.3/912.7), respectively. Comparing these values with the available experimental values, we find that the discrepancies for the 0 and 298 K heats of formation values are < or =2.6 kJ/mol for 2-C(3)H(7)/2-C(3)H(7) (+),< or =4.1 kJ/mol for C(6)H(5)/C(6)H(5) (+), and < or =3.2 kJ//mol for C(6)H(5)CH(2)C(6)H(5)CH(2) (+).
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Department of Chemistry, University of California, Davis, California 95616, USA.
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