Thermochemistry of icosahedral closo-dicarboranes: a composite ab initio quantum-chemical perspective

Abstract

We obtained accurate thermochemical properties for the ortho-, meta-, and para-dicarborane isomers (C2B10H12) by means of explicitly correlated high-level thermochemical procedures. The thermochemical properties include heats of formation, isomerization energies, C–H and B–H bond dissociation energies (BDEs), and ionization potentials. Of these only the ionization potentials are known experimentally. Our best theoretical ionization potentials, obtained by means of the ab initio W1–F12 thermochemical protocol, was 241.50 kcal mol–1 (para-dicarborane), 238.45 kcal mol–1 (meta-dicarborane), and 236.54 kcal mol–1 (ortho-dicarborane). These values agree with the experimental values adopted by the National Institute of Standards and Technology (NIST) thermochemical tables to within overlapping uncertainties. However, they suggest that the experimental values may represent significant underestimations. For all isomers, the C–H BDEs are systematically higher than the B–H BDEs because of the relative stability of the boron-centred radicals. The C–H BDEs for the three isomers cluster within a narrow energetic interval, namely between 110.8 kcal mol–1 (para-dicarborane) and 111.7 kcal mol–1 (meta-dicarborane). The B–H BDEs cluster within a larger interval ranging between 105.8 and 108.1 kcal mol–1 (both obtained for ortho-dicarborane). We used our benchmark W1–F12 data to assess the performance of a number of lower cost composite ab initio methods. We found that the Gaussian-3 procedures (G3(MP2)B3 and G3B3) result in excellent performance with overall root-mean-square deviations (RMSDs) of 0.3–0.4 kcal mol–1 for the isomerization, ionization, and bond dissociation energies. However, the Gaussian-4 procedures (G4, G4(MP2), and G4(MP2)-6X) showed relatively poor performance with overall RMSDs of 1.3–3.7 kcal mol–1.

Fullerenes Pose a Strain on Hybrid Density Functional Theory

“…In G4­(MP2) theory, the CCSD­(T) energy is approximated using the following formula. Here, HF/{T,Q} indicates extrapolation of the HF energy from truncated versions of the aug-cc-pVTZ and aug-cc-pVQZ basis sets . All ab initio calculations involved in the G4­(MP2) procedure were calculated using Molpro 2016. , G4­(MP2) theory has been found to reproduce isomerization energies in conjugated, aromatic, and cage systems within chemical accuracy. − All the C 40 isomers were confirmed to have singlet ground states at the MN15/Def2-TZVPP level of theory. The adiabatic singlet–triplet separation energies are presented in Table S1 of the Supporting Information.…”

“…The G4­(MP2) composite procedure approximates the CCSD­(T) energy in conjunction with a triple-ζ-quality basis set (CCSD­(T)/TZ) via an MP2-based basis-set additivity scheme . G4­(MP2) theory has been found to consistently obtain structural isomerization energies with chemical (or better) accuracy, arbitrarily defined as 1 kcal mol –1 = 4.2 kJ mol –1 ). − Of particular relevance to the present work is the excellent performance of G4­(MP2) for isomerization energies in polycyclic aromatic hydrocarbons, conjugated → nonconjugated isomerizations in dienes, C 8 H 8 structural isomerizations, and isomerization energies in carborane cages . In addition, here we evaluate the performance of G4­(MP2) theory for the above C 20 and C 24 CCSD­(T)/CBS isomerization energies of Manna and Martin.…”

Performance of DFT for C60 Isomerization Energies: A Noticeable Exception to Jacob’s Ladder

“…16,17,25,26 For example, for isomerization energies of ortho, meta, and para carboranes (C2B10H12) the deviations between G4(MP2) and W1-F12 amount to less than 1 kJ mol -1 . 27 The DFT exchange-correlation functionals considered in the present study (ordered by their rung on Jacob's Ladder) 7 are given in Table 1. Empirical D3 dispersion corrections 28,29,30 are included in some cases using the finite Becke-Johnson 31 and zero-damping potentials (denoted by the suffix -D3BJ and -D3, respectively).…”

“…It should be pointed out that ideally, we should be using a higher-level composite ab initio method such as W1–F12, which approximates the CCSD­(T) energy closer to the basis set limit for calculating the C 60 isomerization energies; , however, these calculations proved beyond our computational resources. Nevertheless, the G4­(MP2) isomerization energies should be of sufficient accuracy for the purpose of benchmarking DFT and MP2-based procedures due to a large degree of systematic error cancelation between reactants and products. ,,, For example, for isomerization energies of ortho, meta, and para carboranes (C 2 B 10 H 12 ) the deviations between G4­(MP2) and W1–F12 amount to less than 1 kJ mol –1 …”

Thermodynamic properties of formation estimated to biodiesel esters using Gaussian quantum chemistry software and group contribution method of Constantinou and Gani

“…Gaussian features a user-friendly computing environment and robust methods for a variety of calculations. There are several applications, such as characterization of chemical bonds [27][28][29], study of molecular vibrational dynamics [27], reactivity [29], potential redox [30], orbital optimization [31] and calculation of chemical properties [15,29,32,33].…”