Bond Dissociation Energies in Second-Row Compounds

Abstract

Heats of formation at 0 and 298 K are predicted for PF3, PF5, PF3O, SF2, SF4, SF6, SF2O, SF2O2, and SF4O as well as a number of radicals derived from these stable compounds on the basis of coupled cluster theory [CCSD(T)] calculations extrapolated to the complete basis set limit. In order to achieve near chemical accuracy (+/-1 kcal/mol), additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and vibrational zero-point energies. The calculated values substantially reduce the error limits for these species. A detailed comparison of adiabatic and diabatic bond dissociation energies (BDEs) is made and used to explain trends in the BDEs. Because the adiabatic BDEs of polyatomic molecules represent not only the energy required for breaking a specific bond but also contain any reorganization energies of the bonds in the resulting products, these BDEs can be quite different for each step in the stepwise loss of ligands in binary compounds. For example, the adiabatic BDE for the removal of one fluorine ligand from the very stable closed-shell SF6 molecule to give the unstable SF5 radical is 2.8 times the BDE needed for the removal of one fluorine ligand from the unstable SF5 radical to give the stable closed-shell SF4 molecule. Similarly, the BDE for the removal of one fluorine ligand from the stable closed-shell PF3O molecule to give the unstable PF2O radical is higher than the BDE needed to remove the oxygen atom to give the stable closed-shell PF3 molecule. The same principles govern the BDEs of the phosphorus fluorides and the sulfur oxofluorides. In polyatomic molecules, care must be exercised not to equate BDEs with the bond strengths of given bonds. The measurement of the bond strength or stiffness of a given bond represented by its force constant involves only a small displacement of the atoms near equilibrium and, therefore, does not involve any reorganization energies, i.e., it may be more appropriate to correlate with the diabatic product states.

Structures, relative energies, and ligand dissociation energies of iridium carbonyl phosphine clusters

“…From 1b to 1f, the energy differences in both eq PH 3 and ax PH 3 LDEs are less than 3 kcal/mol. In most cases, the strength of the bonds is reflected by bond lengths and LDEs, but this may not always be true [86]. The carbonyl cis to the ax (eq) PH 3 is replaced by PH 3 to form 1e from 1b and 1c.…”

Theoretical Investigation of Carbon–Sulfur Triple Bonds

“…Similar problems have been discussed for the ABDEs of alkenes [47][48][49] and secondrow compounds. [50] In the case of CH 2 CH 2 and CF 2 CF 2 , the ADBE of ethylene is much larger as compared with tetrafluoroethylene. However, when diabatic dissociation is considered, the DBDEs are similar because CF 2 CF 2 dissociates to 3 CF 2 instead of 1 CF 2 as in the adiabatic process.…”

“…To do so, we first estimated the SÀO bond energy as the average of the values determined for SOH [52] and HSOH, [28] which are 76.0 and 69.6 kcal mol . This stems from the fact that it represents more closely the bonding in HCSOH, which, as indicated by Grant et al, [50] is the most important aspect to take into account when dealing with DBDEs. If the latter value is adopted as the reference value, the DBDE of HCSOH is weaker than those of CS, SCS, and SCO, but stronger than that of CH 2 S. Therefore, none of the three approaches used to compute the DBDE of HCSOH supports the presence of a CS bond.…”

Bond Dissociation Energies of Organophosphorus Compounds: an Assessment of Contemporary Ab Initio Procedures

“…17 Most studies have focussed on sulfur-containing compounds, [18][19][20] however some also tackled this issue for phosphorus-containing species. 9,21,22 It was overall found that the use of high-exponent d functions is very important in the determination of accurate energetics, whereas the impact on structures, i.e., bond lengths and angles, is slight. The present paper uses a large basis set, without however the explicit inclusion of tight d functions, focussing on the assessment of the electronic structure method.…”