Bonding in polyatomic molecules

In Chapter 2, we considered three approaches to the bonding in diatomic molecules  

A polyatomic species contains three or more atoms. 

2 Valence bond theory hybridization of atomic orbitals 

The word hybridization means mixing and when used in the context of atomic orbitals, it describes a way of deriving spatially directed orbitals which may be used within VB theory. Like all bonding theories, orbital hybridization is a model, and should not be taken to be a real phenomenon. 

Hybrid orbitals may be formed by mixing the characters of atomic orbitals that are close in energy. The character of a hybrid orbital depends on the atomic orbitals involved and their percentage contributions. The labels given to hybrid orbitals reflect the contributing atomic orbitals, e.g. an sp hybrid possesses equal amounts of s and p orbital character. 

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We now turn to the bonding in polyatomic molecules, particularly the hydrocarbons and other common species of organic chemistry. The formulation of such important concepts as... 

The scheme described above, reconforted by the post-HF calculations [57] where the coordinate representing the distance between the nuclei in the diatomic molecule (or any bond in polyatomic molecules), lead to the pervading picture of a diatom connected adiabatically with two non-interacting atoms at infinite distance. From a compuational point of view, this picture is quite useful and widely employed. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

As in the case of a covalent bond in polyatomic molecules, a distinction should be made between the bond energy related to the bond distance and the experimentally observed dissociation energy of a hydrogen bond which includes the energy changes in the polarized systems. 

There are several qualitative approaches to bonding in polyatomic molecules, but we shall discuss here the most widely used and currently popular approach. This approach involves setting up appropriate atomic orbitals for the atoms and considering that each bond arises from the attractive electrical forces of two or more nuclei for a pair of electrons in overlapping atomic orbitals, with each orbital on a different atom. The geometry of the bonds is assumed to be determined by the geometry of the orbitals and by the repulsive forces between the electrons. In the course of showing how this approach... 

Valence-bond theory, as described so far, cannot account for bonding in polyatomic molecules like methane, CH4, nor for their bond angles. For example, if we tried to apply the theory to methane, we would note that... 

Use molecular orbital theory to describe bonding in polyatomic molecules such as the benzene molecule, Section 3.12. 

The vibrational frequency depends on the reduced mass and the force constant. Often the individual bonds in polyatomic molecules generate vibrational frequencies which depend only slightly on the rest of the molecule. For example, carbon-oxygen double bonds are found in a wide variety of organic molecules (such as acetone, (CH3)2C=0). The C=0 stretch is excited at v = 1750 cm-1 in virtually any such molecule, and is often used to confirm the existence of a C=0 group in an unknown... 

Many chemists combine the LCAO and VB methods to describe bonding in polyatomic molecules. They use the VB model to describe the localized a bonds that provide connectivity for the molecule structure and use the LCAO method to describe the de-localized tt bonds that distribute electrons over the entire structure. 

The contents of Part 1 is based on such premises. Using mostly 2x2 Hiickel secular equations, Chapter 2 introduces a model of bonding in homonuclear and heteronuclear diatomics, multiple and delocalized bonds in hydrocarbons, and the stereochemistry of chemical bonds in polyatomic molecules in a word, a model of the strong first-order interactions originating in the chemical bond. Hybridization effects and their importance in determining shape and charge distribution in first-row hydrides (CH4, HF, H20 and NH3) are examined in some detail in Section 2.7. 

Qicun Shi research is still underway to combine FSS with Gaussian basis functions to treat larger molecular systems and also to investigate the effect of external fields on the molecular stability and whether or not one can use this approach to selectively break chemical bonds in polyatomic molecules. 

In triangular H3 each hydrogen atom furnishes one AO and 2/3 of an electron, the number of neighbors being two for any hydrogen so, the simplest delocalized bond is a a bond. Actually, a bonds in polyatomic molecules are only localized if they involve e.g. spn hybrids ( = 1,2,3), i.e. if the AO basis functions can be transformed 35,36,37) jn such a way that any of the resulting hybrid AO s can overlap in only one direction. 

Analyze the hybrid orbitals used in bonding in polyatomic molecules and ions... 

Measuring the strength of covalent bonds in polyatomic molecules is more complicated. For example, measurements show that the energy needed to break the first O H bond in H2O is different from that needed to break the second O H bond ... 

TABLE 9.4 Some Bond Dissociation Energies of Diatomic Molecules and Average Bond Energies for Bonds in Polyatomic Molecules... 

Bond dissociation energies for diatomic molecules (in color) have more significant figures tiian bond energies for bonds in polyatomic molecules because die bond dissociation energies of diatomic molecules are directly measurable quantities and not averaged over many compounds. 

The concept of atomic orbital overlap should apply also to polyatomic molecules. However, a satisfactory bonding scheme must account for molecular geometry. We will discuss three examples of VB treatment of bonding in polyatomic molecules. 

Covalent bonds in polyatomic molecules and ions are formed by the overlap of hybrid orbitals, or of hybrid orbitals with unhybridized ones. Therefore, the hybridization bonding scheme is still within the framework of valence bond theory electrons in a molecule are assumed to occupy hybrid orbitals of the individual atoms. 

Chapter 4 Valence bond theory multiple bonding in polyatomic molecules 105... 

In the previous section, we emphasized that hybridization of some or all of the valence atomic orbitals of the central atom in an XY species provided a scheme for describing the X—Y f7-bonding. In, for example, the formation of sp, sp and sp d hybrid orbitals, some p or d atomic orbitals remain unhybridized and, if appropriate, may participate in the formation of TT-bonds. In this section we use the examples of C2H4, HCN and BF3 to illustrate how multiple bonds in polyatomic molecules are treated within VB theory. Before considering the bonding in any molecule, the ground state electronic configurations of the atoms involved should be noted. 

Despite its successes, the application of valence bond theory to the bonding in polyatomic molecules leads to conceptual difficulties. The method dictates that bonds are localized and, as a consequence, sets of resonance structures and bonding pictures involving hybridization schemes become rather tedious to establish, even for relatively small molecules (e.g. see Figure 4.10c). We therefore turn our attention to molecular orbital (MO) theory. 

J. Barrett (1991) Understanding Inorganic Chemistry The Underlying Physical Principles, Ellis Horwood (Simon Schuster), New York - Chapters 2 and 4 give a readable introduction to group theory and bonding in polyatomic molecules. 

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