Abstract:ABSTRACT:The effect of the basicity of methyl-amines on hydrogen bonding HB with HCOOH is examined in both gas and solution phases. In the gas phase, the strength Ž . of HB may be related to the proton affinity PA difference between the carboxylate anion Ž y . Ž . and the methyl-amine, ⌬PA s PA HCOO y PA NR . The changes in the driving 3 potential ⌬PA are explained on the basis of electronic substituent effects. The electronic substituent effects are rationalized in terms of local reactivity indices such as th… Show more
“…Hence, PA, the Fukui function, and the local softness seem to be suitable for use as global and local quantities appearing in eq 4, respectively. These relationships have been shown to correctly assess the substituent electronic effects on both gas-phase basicity of alkylamines and acidity of alcohols and thioalcohols. ,, Additionally, an interpretation framed on the local hard and soft acids and bases (HSAB) principle has been given in agreement to previous results reported by Li and Evans 2 Nucleophilic Condensed-to-Site Fukui Functions Values (First Entry) and the Net Charges from the Mülliken Population Analysis (Second Entry) for the Most Important Atoms in Each System for Compounds 1−8 3 Nucleophilic Condensed-to-Site Fukui Functions Values (First Entry) and the Net Charges from the Mülliken Population Analysis (Second Entry) for the Most Important Atoms in Each System for Compounds 9−13 …”
Site selectivity and reactivity in some strong organic super bases RN=C(NR(2))R, containing several possible sites for protonation, have been analyzed qualitatively in terms of global and local chemical reactivity descriptors defined in the context of conceptual density functional theory. On the basis of the condensed-to-site Fukui function values, the protonation site is predicted to be located at the imino nitrogen. Linear relationships between global energy-dependent quantities and the variation of the Fukui function (or the local softness) at the protonation site have been also found. These linear relationships can be interpreted within a local HSAB rule framework, as observed for instance for the gas-phase basicity of alkylamines and gas-phase acidity of alcohols and thioalcohols. This work extends the range of applicability of reactivity models developed for the treatment of substituent effects based on empirical energy-density Hammett-like relationships.
“…Hence, PA, the Fukui function, and the local softness seem to be suitable for use as global and local quantities appearing in eq 4, respectively. These relationships have been shown to correctly assess the substituent electronic effects on both gas-phase basicity of alkylamines and acidity of alcohols and thioalcohols. ,, Additionally, an interpretation framed on the local hard and soft acids and bases (HSAB) principle has been given in agreement to previous results reported by Li and Evans 2 Nucleophilic Condensed-to-Site Fukui Functions Values (First Entry) and the Net Charges from the Mülliken Population Analysis (Second Entry) for the Most Important Atoms in Each System for Compounds 1−8 3 Nucleophilic Condensed-to-Site Fukui Functions Values (First Entry) and the Net Charges from the Mülliken Population Analysis (Second Entry) for the Most Important Atoms in Each System for Compounds 9−13 …”
Site selectivity and reactivity in some strong organic super bases RN=C(NR(2))R, containing several possible sites for protonation, have been analyzed qualitatively in terms of global and local chemical reactivity descriptors defined in the context of conceptual density functional theory. On the basis of the condensed-to-site Fukui function values, the protonation site is predicted to be located at the imino nitrogen. Linear relationships between global energy-dependent quantities and the variation of the Fukui function (or the local softness) at the protonation site have been also found. These linear relationships can be interpreted within a local HSAB rule framework, as observed for instance for the gas-phase basicity of alkylamines and gas-phase acidity of alcohols and thioalcohols. This work extends the range of applicability of reactivity models developed for the treatment of substituent effects based on empirical energy-density Hammett-like relationships.
“…Ever since their inception, the condensed Fukui functions (which we have referred to as Fukui indices for the sake of brevity) and local softness indices have received overwhelming attention − as local reactivity descriptors. A number of methods and algorithms have been proposed − to evaluate .…”
Condensed Fukui function values of a number of closed-shell molecules have been calculated from Stockholder charges which were obtained using ab initio HF and DFT/B3LYP methods. The global softness parameters needed to evaluate local softness have been calculated using both Koopmans' approximation and the energy difference method. The calculated reactivity indices (condensed Fukui function and atomic softness) were used to predict the sites of electrophilic and nucleophilic attack in the molecules under investigation. In all cases, the atoms with the maximal value of condensed Fukui function and local softness are predicted to be the preferred sites of electrophilic or nucleophilic addition. The predictions thus made are in agreement with experiment and independent of the theoretical models used. The performance of the relative electrophilicity and relative nucleophilicity indices also was tested, but they were found not to have any special advantage as local reactivity descriptors.
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