An MNDO study of substituent effects in carbocations: the unimportance of inductive effects

1986 ◽  
Vol 64 (3) ◽  
pp. 532-538 ◽  
Author(s):  
Amin Mohamed Aissani ◽  
James Clayton Baum ◽  
Richard Francis Langler ◽  
Jack Leon Ginsburg
Keyword(s):  
Gas Phase ◽  
Substituent Effects ◽  
Simple Groups ◽  
Special Importance ◽  
Isodesmic Reactions ◽  
Polarization Effects ◽  
Inductive Effects ◽  
Mndo Calculations ◽  
A Site

Stabilization effects for both saturated and unsaturated substituents were examined using MNDO calculations. (By "saturated substituents" we mean to imply that the substituent is attached to C+ by sp3 carbon, whether or not the substituent has a site of unsaturation at some point remote from the carbocationic center.) Relative gas phase stabilities of regioisomeric carbocations have been calculated. For simple groups, directly bound to [Formula: see text], π donation, σ donation, and polarization effects are found to be important. Saturated-substituent stabilization effects are examined by means of reaction enthalpies for isodesmic reactions between the appropriate neutral molecules and [Formula: see text]. Traditional analyses of substituent effects based solely on inductive effects for saturated groups have little or no significance. Inductive effects have no special importance in determining saturated substituent stabilization effects on carbocations.

Computational study of substituent effects on gas-phase stabilities of Meisenheimer complexes

2015 ◽  
Vol 93 (12) ◽  
pp. 1327-1334 ◽  
Author(s):  
Kazuhide Nakata ◽  
Mizue Fujio ◽  
Hans-Ullrich Siehl ◽  
Yuho Tsuno
Keyword(s):  
Gas Phase ◽  
Narrow Range ◽  
Computational Study ◽  
Substituent Effects ◽  
Orbital Interaction ◽  
Conjugation System ◽  
Isodesmic Reactions ◽  
Meisenheimer Complexes ◽  
Stabilization Energies ◽  
Respective Species

The total stabilization energies (TSEs) and anion stabilization energies (ASEs) of ring-substituted (X-) Meisenheimer complexes featuring two NO2 groups in the ring were determined using appropriate isodesmic reactions. The structures and energies of respective species were calculated at the B3LYP/6–311+G(2d,p) level of theory. Ten series of substituent effects were examined by varying substituent Y, which is connected to the sp3 carbon of the ring. The substituent effects were successfully analyzed using an extended Yukawa–Tsuno equation, [Formula: see text]. The r− values for the TSEs were identical to those for the ASEs, whereas the s values for the TSEs were significantly different from those for the ASEs. This shows that the effect of neutral species contributes to the s values of the TSEs. The r− and s values for the ASEs of all Meisenheimer complexes were distributed in a narrow range because substituent Y was insulated from the π-conjugation system. The r− values were large and the s values were small. This shows that the r− and s values were independent of each other and that the extended three-term Yukawa–Tsuno equation was intrinsic for substituent-effect analyses of anions. Although the variation was not substantial, the change in the r− values was clearly explained by the orbital interaction between substituent Y and the π-conjugation system. The r− values exhibited a good correlation with the bond lengths between the ring and the 4-NO2 group among all Meisenheimer complexes and benzylic anions. These facts provide a physical meaning: the r− value is a parameter that reveals the degree of the additional π interactions between the electron-withdrawing substituents and the π-conjugation systems of the ring.

A MNDO study of planar thioacyl-substituted carbocations

1992 ◽  
Vol 70 (1) ◽  
pp. 111-115 ◽  
Author(s):  
Jack Leon Ginsburg ◽  
Richard Francis Langler ◽  
Raj Kumar Raheja ◽  
Laura Precedo
Keyword(s):  
Gas Phase ◽  
Substituent Effects ◽  
Mndo Method ◽  
Saturation Effect ◽  
Semiempirical Calculations ◽  
Isodesmic Reactions ◽  
Electron Population ◽  
Resonance Saturation ◽  
Carbonium Ions

Relative gas phase stabilities of pairs of planar regioisomeric thioacyl-substituted carbocations have been calculated using the MNDO method. These systems are stabilized by good π donors. The role of S π-electron population as a gauge of substituent effects is examined. Similar results were obtained using isodesmic reactions to gauge substituent effects. Results of this and earlier studies are consistent with the resonance saturation effect. Keywords: semiempirical calculations, carbonium ions.

scholarly journals Computational study of the substituent effects on the gas-phase stabilities of phenylboranylmethyl anions

2017 ◽  
Vol 89 (11) ◽  
pp. 1685-1694
Author(s):  
Kazuhide Nakata ◽  
Mizue Fujio
Keyword(s):  
Gas Phase ◽  
Benzene Ring ◽  
Computational Study ◽  
Resonance Effect ◽  
Substituent Effects ◽  
Nbo Analysis ◽  
Side Chain ◽  
Isodesmic Reactions ◽  
Effect Analysis ◽  
Orbital Interactions

AbstractThe relative gas-phase stabilities of ring-substituted phenylboranylmethyl anions were computationally determined using isodesmic reactions. The energies of species included in the reactions were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were analyzed by the extended Yukawa-Tsuno equation, and unexpectedly substantial r− (0.59) and s (0.65) values were found for the fully-optimized planar anion. The substantial through-resonance effect quantified by the r− value was observed, although it is not possible to draw a canonical form in which the negative charge is delocalized on the benzene ring. Substituent effects were also analyzed for the anions in which the dihedral angle (φ) between the side chain plane and the benzene ring was fixed. The r− value decreased significantly by changing the φ from 0° to 90°, while the s value changed little. NBO analyses revealed that the r− value is proportional to the sum of the π–π* and σ–π* orbital interactions between the side chain and the benzene ring. This fact shows that the through-resonance effect quantified by the r− value is present at all φ, and therefore, the anion cannot become an ideal σ0-reference system. The constant saturation effect quantified by the s value can be explained by the constant charge distributed to the benzene ring. The combination of substituent-effect analysis and NBO analysis successfully revealed the nature of the anion.

Theoretical insight into the substituent effects on the antioxidant properties of 8-hydroxyquinoline derivatives in gas phase and in polar solvents

2018 ◽  
Vol 96 (5) ◽  
pp. 453-458
Author(s):  
Anes El-Hadj Saïd ◽  
Sidi Mohamed Mekelleche ◽  
Taki-Eddine Ahmed Ardjani
Keyword(s):  
Electron Transfer ◽  
Gas Phase ◽  
Antioxidant Properties ◽  
Substituent Effects ◽  
Hydrogen Atom Transfer ◽  
Polar Solvents ◽  
Antioxidant Power ◽  
Theoretical Insight ◽  
Insight Into

The objective of this work is to perform a theoretical analysis of the antioxidant properties of a series of 8-hydroxyquinolines (8-HQs) to rationalize the available experimental results and to design new potent 8-HQ derivatives. The study was carried out in gas phase and in methanol at the DFT/B3LYP/ 6-311++G(d,p) computational level. The formation of stable ArO• radicals is discussed on the basis of different mechanisms, namely, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and single proton loss electron transfer (SPLET). The obtained results show that the HAT mechanism is, thermodynamically, more favoured in gas phase, whereas the SPLET pathway is more favoured in polar solvents. The calculated thermochemical descriptors allow classification of the antioxidant power of the studied compounds.

Sign in / Sign up

Export Citation Format

Share Document