Molecular orbital treatment of substituent effects. I. Structures of some carbon acids and their conjugate bases

1983 ◽  
Vol 61 (1) ◽  
pp. 45-49 ◽  
Author(s):  
Kenneth E. Edgecombe ◽  
Russell J. Boyd
Keyword(s):  
Molecular Orbital ◽  
Substituent Effects ◽  
Stabilization Energy ◽  
Basis Set ◽  
Carbon Acids ◽  
Resonance Stabilization ◽  
Stabilization Energies ◽  
Conjugate Bases ◽  
Equilibrium Geometries

The STO-3G optimized structures of three series of carbon acids, CHnX4−n, where n = 1, 2, or 3 and X = F, CN, or NO2 and their corresponding conjugate bases are compared with the limited number of experimentally determined structures. The 6-31G* equilibrium geometries of [Formula: see text] are included as a check on the reliability of the STO-3G structures of the anions. Although the effects of successive substitutions on the STO-3G structures are generally systematic, a few apparent anomalies are readily explained in terms of resonance stabilization. Calculations at the 3-21G level, on the STO-3G optimized structures, indicate that the stabilization energy associated with increasing the number of orbitals in the basis set is approximately additive. The stabilization energies are 0.865 ± 0.007 au per fluorine substituent, 0.661 ± 0.008 au per cyano substituent, and 1.561 ± 0.027 au per nitro substituent.

Substitutent Effects on the Geometrical Properties of 1-Phenylallyl Alcohol

2005 ◽  
Vol 60 (4) ◽  
pp. 265-270
Author(s):  
Salim Y. Hanna ◽  
Salim M. Khalil ◽  
Moafaq Y. Shandala
Keyword(s):  
Electron Density ◽  
Molecular Orbital ◽  
Substituent Effects ◽  
Heats Of Formation ◽  
Geometrical Parameters ◽  
Molecular Orbital Calculations ◽  
Electron Densities ◽  
Geometrical Properties ◽  
Stabilization Energies

Abstract Optimized geometrical parameters, electron densities, heats of formation and stabilization energies have been obtained on X-substituted phenylallyl alcohols, where X is H, OCH3, NH2, CN, F and CH3 at ortho, meta, and para positions, using MINDO-Forces SCF-molecular orbital calculations. The substituent effects on the geometrical parameters and the electron density are discussed.

1H nuclear magnetic resonance and molecular orbital studies of the conformations of 3-fluoroanisole

1989 ◽  
Vol 67 (6) ◽  
pp. 1027-1031 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Substituent Effects ◽  
Spin Coupling ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Minimal Basis ◽  
Parent Molecule ◽  
Nuclear Magnetic

The proximate spin–spin coupling constant between the methyl protons and the ring protons, 5J(H,OCH3), is extracted from a full analysis of the 1H and 19F nuclear magnetic resonance spectra of 3-fluoroanisole in CS2 and acetone-d6 solutions. The values of 5J(H,OCH3) imply that the less polar cis conformer is slightly more stable at 300 K than the more polar trans conformer in both solvents, in agreement with geometry-optimized STO-3G MO computations for the free molecule. The latter also find a higher barrier to internal rotation of the methoxy group for 3-fluoroanisole than for the parent molecule. The present results are compared with other measurements of the conformer ratio for the vapor and for solutions. The STO-3G and 6-31G structures of the cis and trans conformers are compared. The C—F bond length is computed more reliably with the minimal basis set, as is the COC bond angle. The internal angles of the benzene moiety are, of course, found more accurately with the 6-31G basis. The computations indicate additivity of the substituent effects on the internal angle, as found experimentally for a variety of benzene derivatives. Keywords: 1H NMR of fluoroanisole, conformations of fluoroanisole, molecular orbital calculations for fluoroanisole.

The aromatic character and resonance stabilization energies of substituted cyclopentadienyl and indenyl cations

2005 ◽  
Vol 83 (9) ◽  
pp. 1287-1298 ◽  
Author(s):  
J A Pincock ◽  
A WH Speed
Keyword(s):  
Triplet State ◽  
Bond Length ◽  
Positive Charge ◽  
Singlet State ◽  
Substituent Effects ◽  
Bond Alternation ◽  
Carbon Carbon Bond ◽  
Resonance Stabilization ◽  
Stabilization Energies ◽  
Electron Donating Groups

Calculations (B3LYP/6-31G(d)) have been used to assess the aromaticity of 5-X substituted indenyl (4) and cyclopentadienyl (5) cations with X = O–, NH2, OCH3, CH3, F, H, CN, and N2+. Two criteria were used, the aromatic stabilization energy (ASE), as determined by isodesmic reactions, and bond alternation, as determined from the Julg index (A) on the basis of carbon–carbon bond lengths. Substituent effects on the singlet state of the cyclopentadienyl cations resulted in significant decreases in antiaromatic character for electron-donating groups as indicated by larger A values (A = –0.25 for X = H and +0.26 for X = NH2). These decreases paralleled increases in the C-2—C-3 bond length and good linear correlations were obtained between A vs. the C-2—C-3 bond length and A vs. the ASE. These effects were rationalized by the stabilization by the electron-donating groups of the positive charge at C-5 generated as a consequence of a Jahn–Teller distortion leading to a lowest energy singlet state with a HOMO of a2 symmetry. In contrast, the lowest energy triplet state for each of the substituted cyclopentadienyl cations has little bond alternation (A > 0.9) and, by this criterion, is not significantly antiaromatic. The triplet state is more stable than the singlet state for the unsubstituted case and those with electron-withdrawing groups (ΔEST = –11.3 and –9.3 kcal/mol for X = H and CN, respectively) (1 cal = 4.184 J), but less stable for electron-donating groups (ΔEST = +15.0 kcal/mol for X = NH2). For the indenyl cations 4, the ASE values were almost independent of the substituent and the A values only decreased slightly for electron-donating groups. The A values also indicated that the indenyl cations could be divided into two moieties, an X-substituted pentadienyl cation with considerable delocalization and little bond alternation, and a 2,3-butadiene one with considerable bond alternation. This separation also placed the major portion of the positive charge on the pentadienyl part. The lack of symmetry in the substituted indenyl cations rationalizes the selective reactivity of the 5-methoxy-substituted cation at C-1. Finally, the resonance stabilization energies (RSE) of the substituted cations gave a linear correlation with the RSEs of 4-substituted benzylic cations.Key words: indenyl cations, cyclopentadienyl cations, substituent effects, stabilization energies.

scholarly journals Huckel theory with overlap for the three-electron bond

1978 ◽  
Vol 31 (1) ◽  
pp. 199 ◽  
Author(s):  
RD Harcourt
Keyword(s):  
Molecular Orbital ◽  
Stabilization Energy ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
Constructive Interference ◽  
Hückel Theory ◽  
Resonance Stabilization ◽  
Hückel Molecular Orbital Theory

For a three-electron bond between two equivalent atoms, Huckel molecular orbital theory with overlap generates a resonance stabilization energy which is formally identical to that obtained from molecular orbital theory when electronic repulsions are included and the Mulliken approximation (ab ≈ ½S(aa+bb)) is invoked. The latter energy is the constructive interference energy.

The Stabilization Energy of the GLU-LYS Salt Bridge in the Protein/Water Environment: Correlated Quantum Chemical ab initio, DFT and Empirical Potential Studies

2008 ◽  
Vol 73 (6-7) ◽  
pp. 921-936 ◽  
Author(s):  
Jan Řezáč ◽  
Karel Berka ◽  
Dominik Horinek ◽  
Pavel Hobza ◽  
Jiří Vondrášek
Keyword(s):  
Quantum Chemical ◽  
Ion Pairs ◽  
Water Environment ◽  
Salt Bridge ◽  
Ion Pair ◽  
Stabilization Energy ◽  
Basis Set ◽  
Salt Bridges ◽  
Empirical Potential ◽  
Stabilization Energies

The stabilization energies of Glu-Lys salt bridges are calculated at the CCSD(T) complete basis set limit to provide a reasonable description of the strength of the ion-pair bond in the gas phase. The effect of the environment (protein with ε = 4 and water with ε = 80) on the stabilization energy was introduced via a modification of the quantum chemical DFT energy, for which the COSMO methodology was adopted. The other (standard) approach was based on incorporating a dielectric constant into the Coulomb electrostatic term of the Amber empirical potential function and utilizing the generalized Born model implemented in the Amber program. The environment affects the stabilization energy of the salt bridge dramatically: The protein reduces the energy to less than one half of the original value, whereas water sometimes changes stabilization to destabilization. Both theoretical procedures, based on completely different theoretical backgrounds, yield very similar results, which strongly support their validity. An ion pair is converted to an ion-neutral pair when its pH is changed. This transformation is connected with a strong reduction of the stabilization energy regardless of the environment. The substantial differences in the stabilization energies of ion pairs and ion-neutral pairs contradict the negligible changes of free energy detected experimentally. Evidently, the contribution of formation and hydration entropy is significant and compensates for the large stabilization energies.

Molecular orbital treatment of substituent effects. II. Structures of some carboxylic Acids and their conjugate bases

1984 ◽  
Vol 62 (12) ◽  
pp. 2881-2886 ◽  
Author(s):  
Kenneth E. Edgecombe ◽  
Russell J. Boyd
Keyword(s):  
Carboxylic Acids ◽  
Molecular Orbital ◽  
Substituent Effects ◽  
Conjugate Bases

An ab initio Molecular Orbital Study of Bonding in Fluorocarbonium Ions

1971 ◽  
Vol 49 (22) ◽  
pp. 3708-3713 ◽  
Author(s):  
N. C. Baird ◽  
R. K. Datta
Keyword(s):  
Ab Initio ◽  
Molecular Orbital ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Minimal Basis ◽  
Fluorine Substitution ◽  
Abstraction Reaction ◽  
Carbonium Ions ◽  
Hydride Abstraction ◽  
Resonance Stabilization

Ab initio molecular orbital calculations are reported for the series of carbonium ions (CH3)+, (FCH2)+, and (F2CH)+ and for their neutral molecule counterparts CH4, CH3F, and CH2F2. The energies and wavefunctions for the carbonium ions have been calculated both with and without including the carbon 2pπ orbital in the minimal basis set in order to unravel the inductive destabilization and resonance stabilization due to fluorine substitution. The increase in bonding energy with multiple fluorine substitution is less than linear, due primarily to nonadditivity in the dative carbon–fluorine π bonding. The "saturation" effect noted previously for the hydride abstraction reaction enthalpies is shown to be due primarily to stability effects in the neutral molecules themselves rather than to energetic effects of the carbonium ions.

The acidities of 4-arylsulfonylmethylpyridines and N-methyl and N-benzyl 4-arylsulfonylmethylpyridinium cations in aqueous solution

1992 ◽  
Vol 70 (10) ◽  
pp. 2635-2644 ◽  
Author(s):  
Stefan Wodzinski ◽  
John W. Bunting
Keyword(s):  
Aqueous Solution ◽  
Substituent Effects ◽  
Linear Free Energy Relationship ◽  
Acidity Function ◽  
Linear Free Energy ◽  
Phenyl Sulfone ◽  
Carbon Acids ◽  
Pyridinium Cations ◽  
Conjugate Bases ◽  
Insight Into

The pKa values for the deprotonation of a series of 4-(X-phenylsulfonylmethyl)pyridines (6) (pKa = 19.89 (X = H); ρ = 3.0) were determined in aqueous dimethyl sulfoxide solutions at 25 °C using the H0q acidity function. The pKa values were also measured for the corresponding series of N-mefhyl 4-(X-phenylsulfonylmethyl)pyridinium cations (2) (pKa = 11.27 (X = H); ρ = 1.45) and also for a series of N-(X-benzyl) 4-phenylsulfonylmethylpyridinium cations (7) (pKa = 10.70 (X = H); ρ = 0.65) in aqueous solution (ionic strength 0.1 at 25 °C). Comparison of the substituent effects upon the pKa values of the sulfonyl-activated carbon acids 2, 6, and 7 with the substituent effects upon the pKa values of the corresponding three series of ketones gives insight into the electron-density distribution in the carbanionic conjugate bases of sulfone and ketone carbon acids. Extrapolation of a linear free energy relationship between the pKa values of neutral sulfones and ketones allows the estimation of pKa = 28.7 for the deprotonation of methyl phenyl sulfone in aqueous solution.

Substituent effects in some autoxidation and hydrogen abstraction reactions in terms of their field (F) and resonance (R) components inclusive of unique positional weighting factors

1980 ◽  
Vol 58 (24) ◽  
pp. 2754-2760 ◽  
Author(s):  
Lalit N. Patnaik ◽  
Nigamananda Mallick ◽  
M. K. Rout ◽  
Swoyam P. Rout
Keyword(s):  
Transition State ◽  
Hydrogen Abstraction ◽  
Substituent Effects ◽  
Reference Compound ◽  
General Validity ◽  
Correlation Studies ◽  
Weighting Factors ◽  
Benzyl Radical ◽  
Resonance Stabilization ◽  
Stabilization Energies

Substituent effects in some autoxidation and hydrogen abstraction reactions have been studied in terms of the field (Fk) and resonance (Rk) components proposed by Swain and Lupton and the unique positional weighting factors fj and rj incorporated by Williams and Norrington with the aid of the following equation:[Formula: see text]where Pi's are the rate parameters, Pi0 being that for a standard reference compound. The correlations are found to be quite satisfactory, showing the general validity of the above equation for the types of reactions considered. From the information obtained from the present correlation studies, an attempt has been made to resolve the conflict with regard to the nature of the transition state involved in the hydrogen abstraction reactions – that is, whether or not there is charge separation in the transition state. The results of the present studies seem to suggest that both the differences in the resonance stabilization energies of the ring-substituted benzyl radical and the contribution of polar structures to the transition state are to be invoked to explain the observed data.

A Comprehensive Computational Study on OCH+-Rg (Rg = He, Ne, Ar, Kr, Xe) Complexes

2003 ◽  
Vol 68 (3) ◽  
pp. 489-508 ◽  
Author(s):  
Yinghong Sheng ◽  
Jerzy Leszczynski
Keyword(s):  
Computational Study ◽  
Relativistic Effects ◽  
Dft Method ◽  
Basis Set ◽  
Rare Gas ◽  
Geometrical Parameters ◽  
Minor Effect ◽  
Dissociation Energies ◽  
Equilibrium Geometries

The equilibrium geometries, harmonic vibrational frenquencies, and the dissociation energies of the OCH+-Rg (Rg = He, Ne, Ar, Kr, and Xe) complexes were calculated at the DFT, MP2, MP4, CCSD, and CCSD(T) levels of theory. In the lighter OCH+-Rg (Rg = He, Ne, Ar) rare gas complexes, the DFT and MP4 methods tend to produce longer Rg-H+ distance than the CCSD(T) level value, and the CCSD-calculated Rg-H+ bond lengths are slightly shorter. DFT method is not reliable to study weak interaction in the OCH+-He and OCH+-Ne complexes. A qualitative result can be obtained for OCH+-Ar complex by using the DFT method; however, a higher-level method using a larger basis set is required for the quantitative predictions. For heavier atom (Kr, Xe)-containing complexes, only the CCSD method predicted longer Rg-H+ distance than that obtained at the CCSD(T) level. The DFT method can be applied to obtain the semiquantitative results. The relativistic effects are expected to have minor effect on the geometrical parameters, the H+-C stretching mode, and the dissociation energy. However, the dissociation energies are sensitive to the quality of the basis set. The nature of interaction between the OCH+ ion and Rg atoms was also analyzed in terms of the interaction energy components.

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