In this study, the thermodynamics and barrier heights associated with the fragmentation reactions of a set of fifteen 1,4,2-oxaselenazoles into isoselenocyanates (molecules with promising anticancer activity) and carbonyl derivatives, have been studied using the high-level CBS-QB3 quantum chemical protocol. Of the systems studied, attachment of a CF₃-substituent at the C5-position affords the system with the largest gas-phase free energy barrier (190.1 kJ mol^–1), whilst substitution at the C5-position with two –NMe₂ substituents affords a heterocycle with the lowest free energy barrier (67.8 kJ mol^–1). The presence of solvent (acetonitrile) was shown to reduce the free energy barriers in all cases, with the two systems mentioned above having condensed-phase free energy barriers of 180.8 and 42.0 kJ mol^–1, respectively.

Knowledge of the energies required to induce homolytic cleavage of the C–Br bonds of brominated organic molecules, a process that affords carbon-centered radicals and Br•, is of fundamental importance. Although some data pertaining to the strength of C–Br bonds can already be found in the literature, the chemical diversity of the species for which bond dissociation energies (BDEs) are available is somewhat limited. In this data article, we report a comprehensive set of homolytic C–Br BDEs, obtained using the G4 thermochemical protocol, for brominated organic molecules with wide structural diversity. The species in this set have C–Br BDEs (at 298 K) that differ by as much as 188.3 kJ mol^–1, with α-bromoalanine having the lowest BDE (214.1 kJ mol^–1) and 1-bromobut-1-yne having the largest (402.4 kJ mol^–1). Of particular relevance to biological systems are the BDEs of 8-bromoguanine (345.3 kJ mol^–1), 8-bromoadenine (345.6 kJ mol^–1), 5-bromocytosine (348.8 kJ mol^–1) and 5-bromouracil (350.3 kJ mol^–1).

Numerous reactions between sulfenyl chlorides and organic molecules proceed by way of radical reactions, in which homolytic cleavage of the S-Cl bond is a key step. Owing to the lack of data concerning the quantitative effect of substituents in governing the strength of S-Cl bonds toward homolytic cleavage, the present article reports an extensive dataset of gas-phase S-Cl bond dissociation enthalpies (BDEs) obtained using the high-level G4 thermochemical protocol. The BDEs of the species in this set range from 207.2 (ONSCl) to 279.7 kJ mol⁻¹ (tBuSCl), with HSCl having a BDE of 267.6 kJ mol⁻¹. In addition, the gas-phase S-Cl BDEs of six sulfonyl chlorides have also been reported, and these range from 267.2 kJ mol^–1 (CF₃SO₂Cl) to 283.0 kJ mo1^-1 (FSO₂Cl).

Although the relative energies of the various tautomers of cytosine and 5-chlorocytosine have been studied in detail, little is known about the relative energies of the N-chlorinated isomers of cytosine. N-chlorination of cytosine is known to occur when cytosine is exposed to hypochlorous acid, a potent oxidant that is formed during the inflammatory process, and is presumed to have consequences concerning, for example, the structural integrity of DNA and RNA. In this study, the high-level G4(MP2) thermochemical protocol has been employed in studying the relative gas-phase free energies of thirty isomers that may be formed upon N-chlorination of cytosine. Using free energy of solvation corrections obtained by way of continuum models (CPCM and SMD), an approximation of the effect of aqueous solvation on the relative energies of the isomers has also been taken into account. In both the gas- and aqueous-phases, the lowest energy isomer is an exocyclic N-chloroimino derivative.

This study reports the gas-phase homolytic P–H BDEs of a set of 30 phosphine-type oxides (i.e., R¹R²P(=O)H) obtained using the W1w thermochemical protocol. We note that the P–H BDEs (at 298 K) of the species in this dataset differ by as much as 157.2 kJ mol^–1, with (H₂B)₂P(=O)H having the lowest BDE (249.3 kJ mol^–1) and F₂P(=O)H having the highest (406.5 kJ mol^–1). Furthermore, using the full set of 30 all-electron, non-relativistic, vibrationless bottom-of-the-well W1w P–H BDEs as reference values, we have identified several well-performing DFT methods that could be applied to the computation of the P–H BDEs of phosphine-type oxides. The best-performing DFTs (in conjunction with the A'VTZ basis set) were shown to be MN12-SX (MAD = 1.7 kJ mol^–1) and MN12-L (MAD = 2.7 kJ mol^–1).

In this article, the performance of a wide range of conventional and double-hybrid DFT methods (in conjunction with Dunning basis sets of double-, triple- and quadruple-zeta quality), as well as a number of Gaussian-n thermochemical protocols are assessed for their ability to compute accurate homolytic N–F bond dissociation energies (BDEs). Their performance is evaluated against a previously reported set of 31 highly accurate gas-phase N–F BDEs obtained using the benchmark-quality W2w thermochemical protocol (See: R.J. O'Reilly, A. Karton, L. Radom, J. Phys. Chem. A 2011, 115, 5496.). Out of all of the DFT/basis set combinations investigated, ωB97 and M06-2X (in conjunction with the aug'-cc-pVDZ basis set) offer the lowest mean absolute deviations (MADs =2.4 and 2.7 kJ mol^–1, respectively). Of the Gaussian-n procedures, G3X offers the best performance (MAD =1.4 kJ mol^–1), whilst the significantly more economical G3X(MP2)-RAD method also offers excellent performance (MAD =1.8 kJ mol^–1).

We herein report a dataset of 28 homolytic C–Cl bond dissociation energies (BDEs) (to be known as the CCl28 dataset), obtained using the benchmark-quality W1w thermochemical protocol. This set contains chlorinated organic molecules that consist of either sp3-, sp2- or sp-hybridized C–Cl bonds. The species in this set have C–Cl BDEs (at 298 K) that differ by as much as 168.4 kJ mol^–1, with allyl chloride having the lowest BDE (291.7 kJ mol^–1) and 1-chloroprop-1-yne having the largest (460.1 kJ mol^–1). Given the benchmark quality of the CCl28 dataset, it may serve as a useful reference for assessing the performance of more approximate quantum chemical methods, such as density functional theory (DFT) and double-hybrid DFT methods.

The gas-phase homolytic S–F bond dissociation energies (BDEs) of 21 sulfenyl-type fluorides (RSF) have been obtained using the W1w thermochemical protocol. The BDEs (at 298K) for the species in this set range from 316.2 (HCCSF) to 368.1 (H₂CCHSF) kJ mol^–1. We additionally report fluorine-transfer energies (FTEs), corresponding to the energetics of fluorine transfer from RSF to H₂S. At 298K, the FTEs range from –10.7 (H₂AlSF) to 90.7 (MeHNSF) kJ mol^–1. We have also assessed the performance of a wide range of density functional theory (DFT) and double-hybrid DFT methods (in conjunction with the A'VQZ basis set) for the calculation of these quantities. For the calculation of S–F BDEs, the M06-2X procedure offers the best performance, with a mean absolute deviation (MAD) of 1.6kJ mol^–1, whilst for the FTEs, B2K-PLYP and DSD-PBEP86 offer the best performance with MADs of 0.5kJ mol^–1.