Pauli exclusion principle, hydrogen bonds

According to the Pauli Exclusion Principle, two electrons in the same orbital must have opposite spins. Thus, the two electrons of triplet (spin-unpaired) methylene must occupy different orbitals. In triplet methylene, sp-hybridized carbon forms one bond to each of two hydrogens. Each of the two unpaired electrons occupies a p orbital. In singlet (spin-paired) methylene the two electrons can occupy the same orbital because they have opposite spins. Including the two C-H bonds, there are a total of three occupied orbitals. We predict sp2 hybridization and planar geometry for singlet methylene. 

This expression is the VB wavefunction for the bond in molecular hydrogen. For technical reasons related to the Pauli exclusion principle (see the following Justification), this wavefunction can exist only if the two electrons it describes have opposite spins. Bonds do not form because electrons tend to pair their spins bonds are allowed to form when the electrons pair their spins. 

The opposite extreme to a shared interaction occurs when two closed-shell systems interact, as found in ionic, hydrogen-bonded, van der Waals, and repulsive interactions. In such closed-shell interactions, the requirement of the Pauli exclusion principle leads to the removal of electron density from the region of contact, the interatomic surface. All of the curvatures of Pb are relatively small in magnitude but the positive curvature of Pb along the bond path is dominant and V pb > 0. Since the electron density contracts away from the surface, the interaction is characterized by a relatively low value of pb and the electron density is concentrated. separately in each of... 

Its usefulness in the analysis of bonding in EDA complexes and hydrogen-bonded species notwithstanding, the above scheme has come under criticism on the grounds of its reliance on fictitious wavefunctions that violate the Pauli exclusion principle. Numerous formalisms that do not suffer from this serious conceptual flaw have been put forward. " Neither those foimalisms nor the constrained space orbital variation (CSOV) method, which has been employed in a... 

The possibilities of the exchange of the hydrogen electron witli either of the helium electrons is restricted in the first place by the Pauli principle. The exchange of electrons 1 and 3 results in the appearance of two electrons with parallel spins existing in one orbital which is not permitted by the exclusion principle. There remains in consequence only the possibility of the exchange of electrons 2 and 3, which because they have parallel spins, will, as has been shown in the last chapter, lead not to bond formation but to mutual repulsion. Thus no molecule HeH is formed. 

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