Ethane orbital overlap

When rotation occurs about a bond there are two sources of strain energy. The first arises from the nonbonded interactions between the atoms attached to the two atoms of the bond (1,4-interactions) and these interactions are automatically included in most molecular mechanics models. The second source arises from reorganization of the electron density about the bonded atoms, which alters the degree of orbital overlap. The values for the force constants can be determined if a frequency for rotation about a bond in a model compound can be determined. For instance, the bond rotation frequencies of ethane and ethylamine have been determined by microwave spectroscopy. From the temperature dependence of the frequencies, the barriers to rotation have been determined as 12.1 and 8.28 kJ mol-1, respectively1243. The contribution to this barrier that arises from the nonbonded 1,4-interactions is then calculated using the potential functions to be employed in the force field. 

Because of the cylindrical symmetry of cr bonds (Section 3.8), orbital overlap in the C-C single bond of ethane is exactly the same regardless of the geometric relationships among other atoms attached to the carbons (Figure 4.1). The different arrangements of atoms that result from rotation about a single bond are called conformations, and a specific conformation is called a conformer (conformational isomer). Unlike constitutional isomers, which have different connection. of atoms, different conformers have... 

FIGURE 1.23 Orbital overlap description of the sp -sp CT bond between the two carbon atoms of ethane. 

In a molecule such as ethane H C-CH, each carbon atom has four sp hybrids. Between the two carbon nuclei, there is a localized molecular orbital built up from two hybrids, one for each carbon atom, and pointing at one another. The alignment of the hybrid axes gives the most stable system. Indeed the orbital overlap is maximum and therefore the interaction between atomic orbitals is the strongest The corresponding energy is the highest in absolute... 

Where J is the J, 3-bond coupling constant, cp is the dihedral XCCY dihedral angle, and A, B, and C are empirically derived substituent-dependent parameters. The effects of orbital overlap on coupling constants become obvious by examining the geometric dependence of vicinal H-H coupling in ethane (Figure 12.67). 

As pointed out earlier (Chapter 3), the carbon-carbon double bonds in 1,3-butadiene (CH2=CH-CH=CH2) are about the same length (134pm) as normal carbon-carbon double bonds, for example, 134pm in ethene (ethylene, CH2=CH2), but the carbon-carbon single bond between carbon atoms C2 and C3 is shorter (148 pm) than a normal carbon-carbon single bond, for example, 154 pm in ethane (CH3CH3) and this is attributed to both (a) orbital overlap in the lowest occupied MO and (b) the fact that the C2-C3 a bond may be thought of as made up by... 

The two areas of orbital overlap in the n bond (above and below the plane of the molecule) cause ethene to be a rigid molecule there is no rotation around the carbon—carbon bond. This is in contrast to ethane, and other non-cyclic alkanes, and leads to the existence of cis-trans (geometric) isomerism in alkenes (Chapter 20). 

Chemical bonds are characterized by their length and energy. In this section we are concerned with the structural features that change the normal values of these properties. Any structural feature that shortens the distance between 2 atoms increases the energy needed to dissociate the atoms. This happens because the closer the atoms are, the more their orbitals overlap. The generalization is substantiated by the observed distances and energies of the bonds between the 2 carbon atoms in ethane, ethylene, and acetylene ... 

The orbital overlap shown in (a) leads to the lower potential energy of the staggered conformation of ethane. 

Hence we have two molecular orbitals, one along the line of centres, the other as two sausage-like clouds, called the n orbital or n bond (and the two electrons in it, the n electrons). The double bond is shorter than a single C—C bond because of the double overlap but the n electron cloud is easily attacked by other atoms, hence the reactivity of ethene compared with methane or ethane. 

FIGURE 2 10 The C—C ct bond in ethane pictured as an overlap of a half filled sp orbital of one carbon with a half filled sp hybrid orbital of the other... 

Here, the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

---