Sigma orbitals

For examples of the use of these terms see CYCLOADDITION and SIGMATROPIC REARRANGEMENT. See also ANTI PI (jl) ORBITAL SIGMA (o) ORBITAL. 

Define the following terms bonding molecular orbital, antibonding molecular orbital, pi molecular orbital, sigma molecular orbital. 

Why do we hybridize atomic orbitals to explain the bonding in covalent compounds What type of bonds form from hybrid orbitals, sigma or pi Explain. 

An orbital pointing toward an adjacent orbital is called a sigma bond the Greek letter a is analogous to s). Thus in methane the four tetrahedral orbitals of the carbon atom are a orbitals, and the four C—H bonds are cr bonds. 

The tendency for maximum orbital overlap disposes some atoms, as, for example, the N in NH3 or the O in H2O, to use an s-p-type hybrid AO rather than an unhybridized p orbital. Sigma bonds formed from s-p-type hybrid orbitals are stronger than those formed from p orbitals. 

Note the similarity with the letters p, d, and / in the atom case. Both the ground state orbital and the first excited-state orbital of hJ are CT(sigma) orbitals. Sigma orbitals of diatomic molecules contain the factor o and have orbital regions that are cylindrically symmetric about the bond axis. The other orbitals can be chosen to include either the real functions or the complex functions, and have nodal planes containing the internuclear axis. ... 

Which ligands offer the best combination of donor (nug) and acceptor (tz ug) orbitals Sigma-bonding strength of donor lone pairs is expected to be (inversely) correlated with electronegativity, with,... 

MMVB is a hybrid force field, which uses MM to treat the unreactive molecular framework, combined with a valence bond (VB) approach to treat the reactive part. The MM part uses the MM2 force field [58], which is well adapted for organic molecules. The VB part uses a parametrized Heisenberg spin Hamiltonian, which can be illustrated by considering a two orbital, two electron description of a sigma bond described by the VB determinants... 

The H3 and H4 systems were discussed above. Another type of sigma bonds involves a p orbital lying along the reaction coordinate, as, for example, in reaction (15) (Fig. 8). 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

UNSUBSTITUTED BUTADIENE. Butadiene anchors were presented in Figures 1(3) and 13. The basic tetrahedral character of the conical intersection (as for H4) is expected to be maintained, when considering the re-pairing of four electrons. Flowever, the situation is more complicated (and the photochemistiy much richer), since here p electrons are involved rather than s electrons as in H4. It is therefore necessary to consider the consequences of the p-orbital rotation, en route to a new sigma bond. 

A bond m which the orbitals overlap along a line connecting the atoms (the inter nuclear axis) is called a sigma (a) bond The electron distribution m a ct bond is cylm drically symmetric were we to slice through a ct bond perpendicular to the mternuclear axis Its cross section would appear as a circle Another way to see the shape of the elec tron distribution is to view the molecule end on... 

CT and the antibondmg orbital ct ( sigma star ) The bonding orbital is characterized by a region of high electron probability between the two atoms while the antibondmg orbital has a nodal surface between them... 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

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