Valence-Bond and Hückel Molecular Orbital Diradicals—Alternant versus Nonalternant Effects

2003 ◽  
Vol 56 (12) ◽  
pp. 1225 ◽  
Author(s):  
Jerry Ray Dias
Keyword(s):  
Molecular Orbital ◽  
Valence Bond ◽  
Polar Species ◽  
Alternant Hydrocarbon

Two distinct classes of diradicals are examined and compared. Valence-bond diradicals are topologically enforced, whereas Hückel molecular orbital diradicals can undergo skeletal distortions and transform to a nonradical form and tend to gain or lose electrons to form stable polyions. Alternant hydrocarbon diradicals are nonpolar species and nonalternant hydrocarbon diradicals tend to be polar species and less prevalent.

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

2019 ◽  
Author(s):  
Ishita Bhattacharjee ◽  
Debashree Ghosh ◽  
Ankan Paul
Keyword(s):  
Excited State ◽  
Potential Energy ◽  
Molecular Orbital ◽  
High Spin ◽  
Potential Energy Surfaces ◽  
Valence Bond ◽  
Deep Minimum ◽  
State Potential ◽  
Energy Surfaces ◽  
Mo Theory

The question of quadruple bonding in C<sub>2</sub> has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C<sub>2</sub>, N<sub>2</sub> and Be<sub>2</sub> and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the<sup> 2S+1</sup>Σ<sub>g/u</sub> (with 2S+1=1,3,5,7,9) states of C<sub>2</sub> and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N<sub>2</sub> and HC≡CH, the presence of a deep minimum in the <sup>7</sup>Σ state of C<sub>2</sub> and CN<sup>+</sup> suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.

A variational method for calculating magnetic shielding constants in molecules

1957 ◽  
Vol 243 (1233) ◽  
pp. 264-273 ◽  
Keyword(s):  
Variational Method ◽  
Molecular Orbital ◽  
Diamagnetic Susceptibility ◽  
Valence Bond ◽  
Wave Functions ◽  
Magnetic Shielding ◽  
Magnetic Nucleus ◽  
Shielding Constants ◽  
Good Agreement

The general variational method is applied to the problem of calculating magnetic shielding constants in molecules. Using approximate variation functions together with simple molecular-orbital and valence-bond wave functions calculations have been made for the molecules hydrogen, methane, ethylene and acetylene. An approximation using the calculated diamagnetic susceptibility is used for electrons which are not localized near the magnetic nucleus considered. The results are in good agreement with experiment and in particular it is shown that the shielding constant for acetylene should lie between those for methane and ethylene.

The correspondence between the resonance and molecular orbital theories

1952 ◽  
Vol 214 (1119) ◽  
pp. 482-493 ◽  
Keyword(s):  
Organic Chemistry ◽  
Molecular Orbital ◽  
Valence Bond ◽  
Quantum Mechanical ◽  
Orbital Method ◽  
Molecular Orbital Method ◽  
Resonance Theory ◽  
Bond Method ◽  
Additional Assumptions

The resonance theory of organic chemistry is critically examined from a theoretical view­point. It is stressed that this theory is not rigorously founded on the quantum-mechanical valence bond method, but involves additional assumptions which cannot be defended. The practical success of the resonance theory must therefore be explained in some other way. It is here shown that a remarkable correspondence exists between the resonance theory and the molecular orbital method; and it is suggested that the resonance theory owes its success more to this correspondence than to the validity of its own premises.

scholarly journals Valence Bond and Molecular Orbital: Two Powerful Theories that Nicely Complement One Another

2021 ◽  
Author(s):  
John Morrison Galbraith ◽  
Sason Shaik ◽  
David Danovich ◽  
Benoît Braïda ◽  
Wei Wu ◽  
...  
Keyword(s):  
Molecular Orbital ◽  
Valence Bond

Comparison of Valence‐Bond and Molecular‐Orbital Results for Some Ion‐Radicals

1967 ◽  
Vol 46 (3) ◽  
pp. 870-875 ◽  
Author(s):  
Thomas H. Brown ◽  
Martin Karplus
Keyword(s):  
Molecular Orbital ◽  
Valence Bond
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