Sigma notation

The CT (sigma) notation indicates orbitals that are symmetric to rotation about the line connecting the nuclei ... 

The electron density in both molecular orbitals is symmetrical about the axis between the two nuclei. This means that both of these are sigma orbitals. In MO notation, the Is bonding orbital is designated as eru. The antibonding orbital is given the symbol An asterisk designates an antibonding orbital... 

An (i+j+. ..) cycloaddition is a reaction in which two or more molecules (or parts of the same molecule) provide units of i, j,. .. linearly connected atoms. These units become joined at their respective termini by new sigma bonds so as to form a cycle containing ( +/+. ..) atoms. In this notation, (a) a Diels-... 

Some texts will draw a circle in a six-membered ring to symbolize an aromatic. A semicircle with a plus may be also used to symbolize a sigma-complex. Don t use the circle notation when doing mechanisms, because it is so easy to lose track of electrons. 

In Eq. (5.17), we have introduced the standard notation for a sum, a capital Greek sigma (E). The letter k is called the summation index. Its initial value is given under the capital sigma and its final value is given above it. Unless otherwise stated, k is incremented by unity for each term. There are n terms indicated in this sum. 

We again use the standard notation of a capital Greek sigma ( ) for a sum, introduced in Eq. (5.17). The quantity pi is equal to Ni/N and is the fraction of the members of the entire list that are equal to x,. If we were to sample the entire list by choosing a member at random, the probability that this member would equal x. 

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