The explanation of the hypothetical HAlCl(4) acid instability is provided on the basis of theoretical considerations supported by ab initio calculations. The equilibrium structures of LiAlCl(4), NaAlCl(4), and KAlCl(4) salts were examined and compared to that of their corresponding parent acid. The process of formation of the representative NaAlCl(4) salt was analyzed, and the interaction energy between NaCl and AlCl(3) was estimated to be ca. 55 kcal/mol while that between HCl and AlCl(3) (when the HAlCl(4) species is formed) was calculated to be smaller by an order of magnitude (ca. 8 kcal/mol). The hypothetical HAlCl(4) acid was identified as an HCl...AlCl(3) adduct (with the hydrogen chloride tethered weakly to the quasi-planar aluminum chloride molecule). The electron affinity of the neutral AlCl(4) superhalogen molecule was found to be the factor determining the ability to form a stable compound of MAlCl(4) type.
The calculations performed at the OVGF/6-311++G(3df,3pd)//MP2/6-311++G(d,p) level for the representative NaX(2)(-) and AlX(4)(-) anions matching the MX(k+1)(-) superhalogen formula and utilizing 9-electron systems (i.e., consisting of various possible combinations of atoms containing nine electrons when brought together) revealed that the OH, Li(2)H(3), and NH(2) groups might be considered as alternative ligands X due to their thermodynamic stability and large values of electron binding energy (approaching or even exceeding 6 eV in some cases). All aluminum-containing AlX(4)(-) anions (excluding Al(HBLi)(4)(-)) were predicted to be thermodynamically stable, whereas the NaX(2)(-) anions for X = CH(3), HBLi, CLi, BeB, and H(2)BeLi were found to be susceptible to the fragmentations leading to Na(-) loss. Among the MX(k+1)(-) (M = Na, Al; X = Li(2)H(3), OH, H(2)BeLi, BeB, NH(2), HBLi, CH(3), Be(2)H, CLi) anions utilizing systems containing 9 electrons (and thus isoelectronic with the F atom) the largest vertical electron detachment energy of 6.38 eV was obtained for Al(OH)(4)(-).
In this contribution we have investigated experimentally and theoretically the interaction of low energy electrons with gas phase thiothymine (a sulphur containing analogue of thymine). We observe that the presence of the sulphur atom within thiothymine strongly controls the fragmentation dynamics. With the exception of the (M - H)(-) anion formation, the most favorable reaction channels are associated with a loss of sulphur containing negative fragments (i.e., the formation of S(-), SCN(-) and (M - S)(-)) suggesting that these resonances are localized at the C=S group. Hence the present results demonstrate that certain reactions can be controlled by substitution of the sulphur atom at specific molecular sites within nucleobases. Our study thus represents a starting point for a physicochemical understanding of the action of sulphur-containing antimetabolites when used in chemoradiotherapy.
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