Inductive and Mesomeric Effects of the [60]Fulleropyrrolidine Fragment and [60]Fullerene Sphere: A Quantitative Evaluation Based on Theory and Experiments

Abstract

Inductive and mesomeric effects of the [60]fulleropyrrolidine (Pyr=C(60)) and the [60]fullerene (C(60)) molecular frameworks have been investigated by linear free energy relationship analyses. The electronic effects of these moieties have been studied by expressly designing [60] fulleropyrrolidine derivative 1, which has a C-H bond alpha to both a carbonyl group (CH(alpha)) and a C(60) cage. The extent of polarization of such a bond was then used to probe electron-withdrawing perturbation induced by the fullerene sphere. Thermodynamic measurements based on both theoretical and experimental approaches allowed the acidity of ketone 1 to be measured; this resulted in about a 1 x 10(6)-fold increase in acidity with respect to that of the structurally correlated acetophenone. Experimental and theoretical kinetic determinations were used to determine the stereolability of ketone 1, which was, in fact, chiral due to the stereogenic carbon alpha to the carbonyl group. Also, these results confirmed the very strong tendency for the C(60) sphere to promote breaking of the C-H(alpha) bond. This means that the Pyr=C(60) and C(60) fragments are able to express electron-withdrawing effects stronger than those of halogen atoms, but less effective than those of carbonyl derivatives (e.g., ketones, carboxylic acids or acyl derivatives). Finally, the sigma(p), sigma(I) and sigma(-)(R) descriptors of inductive and/or mesomeric effects were successfully estimated for both Pyr=C(60) and C(60) fragments thanks to the integrated use of an original procedure based on semi-empirical calculations and Hammett/Taft correlations involving mono-or dual-parameter equations

Effect of Ring Size on the Tautomerization and Ionization Reaction of Cyclic 2-Nitroalkanones: An Experimental and Theoretical Study

“…Nevertheless, a rationalization of the p K a values of a series of related compounds can be successfully attempted in terms of factors, such as bond polarization and/or relative stability of the protonated and deprotonated species. Both factors can be conveniently expressed by molecular descriptors, attainable at different levels of theory, which, in turn, can be used in a linear free energy relationship (LFER) analysis to estimate p K a values. ,,− These descriptors have been previously calculated at both low level of theory (empirical, AM1, HF/STO-3G, HF/3-21G, HF/3-21G(d)) ,,,− ,,− and Hartree–Fock (HF) or DFT methods with larger basis sets. Although, as a general rule, the highest possible computational methods have to be adopted, there are several examples where the use of HF or DFT methods with larger basis sets does not necessarily lead to significant improvements in correlations with the experimental data. − ,, It has also been demonstrated that the atomic charge densities on the α-hydrogens, δ(H α ), can be used to monitor even small variations of acidity as a function of conformational and/or configurational changes, when these changes influence the degree of polarization of the bonds involved in the dissociation process. ,,, Thus δ(H α ) values have been calculated and used to predict the p K a of the investigated ketones as previously reported. , As a first step, we have determined the most representative conformers of the keto form of the experimentally investigated 2-nitrocycloalkanones both at the semiempirical AM1 and B3LYP/6-31G(d,p) levels of theory.…”

“…Although, as a general rule, the highest possible computational methods have to be adopted, there are several examples where the use of HF or DFT methods with larger basis sets does not necessarily lead to significant improvements in correlations with the experimental data. [26][27][28][29]44,46 It has also been demonstrated that the atomic charge densities on the α-hydrogens, δ(H α ), can be used to monitor even small variations of acidity as a function of conformational and/or configurational changes, when these changes influence the degree of polarization of the bonds involved in the dissociation process. 15,16,22,23 Thus δ(H α ) values have been calculated and used to predict the pK a of the investigated ketones as previously reported.…”

Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus‐Centered Three‐Bladed Propellers: Tris‐aryl Phosphane Oxides

“…M.p. 303-304 8C; 1 H NMR (300 MHz, CDCl 3 ): d = 9.05 (d, 3 JA C H T U N G T R E N N U N G (H,H) = 9.66 Hz, 3 H), 8.74 (br s, 3 H), 8.13 (d, 3 27 (11), 575 (17), 449 (28), 189 (29); UV (c = 1.77 10 À5 in CHCl 3 ): l max (e) = 244 (31 192), 287 (23 932), 347 (13 672).…”

“…Attention was preliminarily focused on the atomic charges located on phosphorus (d P ) and oxygen (d O ) atoms of the P=O group, as well as on the length of the P=O bond d P=O (Table S1 of Supporting Information). The choice of the first two descriptors was suggested by the knowledge that charges localized on acidic hydrogen atoms or on atoms carrying acidic hydrogen atoms, assessed by quantum mechanical calculations at either semi-empirical [11][12][13][14][15][16][17][18][19] or ab initio level, [11,[20][21][22][23][24][25][26] are effective descriptors of acidity that can sensitively reflect even small conformational [16,17] and configurational [16][17][18]20] changes in the structure of the substrate. Interestingly, several examples [11,[22][23][24][25][26] showed that improvements in the level of theory of the quantum mechanical methods used to assess the molecular descriptors (e.g., HF or DFT instead of semiempirical methods, as well as larger basis sets within HF or DFT approaches) do not necessarily lead to significant improve- ment in the quality of the linear free-energy relationship (LFER) correlations.…”

“…The choice of the first two descriptors was suggested by the knowledge that charges localized on acidic hydrogen atoms or on atoms carrying acidic hydrogen atoms, assessed by quantum mechanical calculations at either semi-empirical [11][12][13][14][15][16][17][18][19] or ab initio level, [11,[20][21][22][23][24][25][26] are effective descriptors of acidity that can sensitively reflect even small conformational [16,17] and configurational [16][17][18]20] changes in the structure of the substrate. Interestingly, several examples [11,[22][23][24][25][26] showed that improvements in the level of theory of the quantum mechanical methods used to assess the molecular descriptors (e.g., HF or DFT instead of semiempirical methods, as well as larger basis sets within HF or DFT approaches) do not necessarily lead to significant improve- ment in the quality of the linear free-energy relationship (LFER) correlations. In other words, unfavorable LFERs obtained through descriptors calculated at low levels of theory do not afford definitive indication about their inadequacy, while, on the contrary, favorable findings are suggestive of their good sensitivity.…”