Molecular orbital hydrogen bonding

In each of the B-H-B bridges, only two electrons bond the three atoms together by having the orbitals on the boron atoms simultaneously overlap the hydrogen Is orbital. A bond of this type is known as a two-electron three-center bond. In terms of molecular orbitals, the bonding can be described as the combination of two boron orbitals and one hydrogen orbital to produce three molecular orbitals, of which only the one of lowest energy is populated ... 

As we know from Section 2.1, electronegativity is the power of an atom in a molecule to attract electrons to itself. When two atoms have different degrees of electronegativity, the bond between them will have partial ionic character. If electrons are available to occupy the resulting molecular orbitals, the bond will have some covalent character. Hydrogen bonds are formed when the electronegativity of A relative to H in an A—H covalent bond... 

According to MO theory, the overlap of the Is orbitals of two hydrogen atoms leads to the formation of two molecular orbitals one bonding molecular orbital and one antibonding molecular orbital. Abonding molecular orbital has lower energy and greater... 

Figure 2.7 The close approach of two hydrogen atoms, each with an electron in a Is orbital, leads to the formation of two molecular orbitals, a bonding iris molecular orbital and an antibonding (J 1s orbital. In the H2 molecule, both electrons occupy the bonding orbital, and a strong bond with energy liibond results...

As in the case of molecular hydrogen, combining two atomic orbitals produces two molecular orbitals—one bonding and one antibonding. To generalize, combining n atomic orbitals produces n molecular orbitals. 

An Elementary Molecular Orbital Model Bond Energies and Pauli Repulsions in Homonuclear Diatomics The Hydrogen Molecular Ion H2 (N=l)... 

The carbon atom has a share in eight electrons (Ne structure) whilst each hydrogen atom has a share in two electrons (He structure). This is a gross simplification of covalent bonding, since the actual electrons are present in molecular orbitals which occupy the whole space around the five atoms of the molecule. 

We shall examine the simplest possible molecular orbital problem, calculation of the bond energy and bond length of the hydrogen molecule ion Hj. Although of no practical significance, is of theoretical importance because the complete quantum mechanical calculation of its bond energy can be canied out by both exact and approximate methods. This pemiits comparison of the exact quantum mechanical solution with the solution obtained by various approximate techniques so that a judgment can be made as to the efficacy of the approximate methods. Exact quantum mechanical calculations cannot be carried out on more complicated molecular systems, hence the importance of the one exact molecular solution we do have. We wish to have a three-way comparison i) exact theoretical, ii) experimental, and iii) approximate theoretical. 

Valence bond and molecular orbital theory both incorporate the wave description of an atom s electrons into this picture of H2 but m somewhat different ways Both assume that electron waves behave like more familiar waves such as sound and light waves One important property of waves is called interference m physics Constructive interference occurs when two waves combine so as to reinforce each other (m phase) destructive interference occurs when they oppose each other (out of phase) (Figure 2 2) Recall from Section 1 1 that electron waves m atoms are characterized by their wave function which is the same as an orbital For an electron m the most stable state of a hydrogen atom for example this state is defined by the Is wave function and is often called the Is orbital The valence bond model bases the connection between two atoms on the overlap between half filled orbifals of fhe fwo afoms The molecular orbital model assembles a sef of molecular orbifals by combining fhe afomic orbifals of all of fhe atoms m fhe molecule... 

We will use the valence bond approach extensively m our discussion of organic molecules and expand on it shortly First though let s introduce the molecular orbital method to see how it uses the Is orbitals of two hydrogen atoms to generate the orbitals of an H2 molecule... 

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