A Gas-Phase Basicity Scale for Selenocarbonyl Compounds Based on High-Levelab Initioand Density Functional Theory Calculations

Abstract

A basicity scale for selenocarbonyl derivatives which covers a wide range of values (60 kcal/mol) has been established through the use of high-level ab initi o and DFT calculations. In our theoretical survey we have included selenoformaldehyde and the corresponding BH2, CH3, NH2, F, and Cl mono- and disubstituted derivatives, as well as carbonyl selenide, thiocarbonyl selenide, and selenoketene. With the only exception of selenoketene, which is a carbon base, all selenocarbonyl compounds investigated behave as selenium bases in the gas phase. Selenocarbonyl derivatives are predicted to be equally or slightly more basic than the thiocarbonyl analogues and, therefore, more basic than the corresponding carbonyl compounds. We have also shown, by means of G2-type calculations, that substituent effects on the relative stability of the neutral and the protonated forms of selenocarbonyl series are also rather similar to those estimated, at the same level of theory, for the thiocarbonyl series. For the neutrals these substituent effects are always stabilizing. Protonated species are strongly stabilized by σ- and π-electron donors, while they are destabilized by σ-withdrawing substituents. This explain the enhanced basicity of the methyl and amino derivatives and the low intrinsic basicity of the halogen derivatives. For the thiocarbonyl series the G2 calculated proton affinities are in very good agreement with the experimental values, which allow us to be confident in our estimates regarding the proton affinities of the selenocarbonyl derivatives investigated. The B3LYP/6-311+G(3df,2p) estimated proton affinities are slightly higher than the G2 values. The keto−enol isomerization of the methyl, hydroxy, and amino monosubstituted derivatives has been also studied.

Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

“…Also, the protonation of thiocarbonyl derivatives, which takes place on the sulfur atom, is accompanied by a slight lengthening of the CS bond, reflecting the slightly higher electronegativity of sulfur as compared with that of carbon (Abboud et al, 1993). A similar result was found for selenocarbonyl bases (González, Mó, & Yáñez, 1999).…”

Why Are Selenouracils as Basic as but Stronger Acids than Uracil in the Gas Phase?

“…It has been well established, from both theoretical and experimental viewpoints, that thiocarbonyl derivatives are more basic than carbonyl compounds 54. A basicity scale for selenocarbonyls, based on high‐level ab initio calculations,55 also showed that these compounds exhibit a basicity similar to that of their thiocarbonyl analogues and therefore also larger than that of carbonyls. However, this seems not to be the case as far as the protonation of 24SU is concerned.…”

Acidity enhancement of unsaturated bases of group 15 by association with borane and beryllium dihydride. Unexpected boron and beryllium Brønsted acids

“…38,39 Such a model is based on the use of fully optimized geometries obtained using the B3LYP hybrid functional and a 6-31+G(d,p) basis set expansion. Final energies are then evaluated in single point calculations using the same functional but a larger 6-311+G(3df,2p) basis set.…”

“…In our survey we have employed a theoretical model which has been proven to be rather reliable for the description of protonation and deprotonation processes, and, at the same time, is not very time consuming. 38,39 Such a model is based on the use of fully optimized geometries obtained using the B3LYP hybrid functional and a 6-31+G(d,p) basis set expansion. Final energies are then evaluated in single point calculations using the same functional but a larger 6-311+G(3df,2p) basis set.…”