Through-bond axis

Nitroparaffias (or nitroaLkanes) are derivatives of the alkanes ia which one hydrogen or more is replaced by the electronegative nitro group, which is attached to carbon through nitrogen. The nitroparaffins are isomeric with alkyl nitrites, RONO, which are esters of nitrous acid. The nitro group ia a nitroparaffin has been shown to be symmetrical about the R—N bond axis, and may be represented as a resonance hybrid ... 

As Figure 10-19 shows, bonds that form from the side-by-side overlap of atomic p orbitals have different electron density profdes than a bonds. A p orbital has zero electron density—a node—in a plane passing through the nucleus, so bonds that form from side-by-side overlap have no electron density directly on the bond axis. High electron density exists between the bonded atoms, but it is concentrated above and below the bond axis. A bond of this type is called a pi ( r) bond, and a bonding orbital that describes a ttbond is a tt orbital. 

Figure 10-30 shows the constmction of the 2 -based molecular orbitals. One pair of MOs forms from the p orbitals that point toward each other along the bond axis. By convention, we label this as the z-axis. This end-on overlap gives Cp and

Severai common poi /atomic oxoanions, inciuding suifate, perchiorate, and phosphate, have inner atoms from the third row of the periodic tabie. In these anions, vaience d orbitais are avaiiable to participate in bonding. Figure 10-47 shows how a n orbitai can form through side-by-side overiap of a d orbital on one atom with a.7, p orbital on another atom. As with other itt bonds, electron density is concentrated above and below the bond axis. 

As predicted by elementary hybrid bonding theory, the multiple bonds of the chemist s Lewis-structure diagram are usually found to correspond to two distinct types of NBOs (1) sigma-type, having exact or approximate cylindrical symmetry about the bond axis (as discussed in Sections 3.2.5-3.2.7), and (2) pi-type, having a nodal mirror plane passing through the nuclei 44... 

This molecule has two different sets of symmetry axes. There is a C2 axis passing through the carbon atom at right angles to the bond direction and there is an oo fold axix (C ) passing through the bond axis itself. The names symmetric stretch and anti symmetric stretch are self evident. The symmetric stretch produces no dipole which remains zero. So this vibration is not infra red active. 

Two fiu-ther types of centre on the axis passing through the nuclei are introduced -the bond-axis functions, designated (ba), lie midway between the (ac) and (be) centres having the coordinates... 

The possibilities of double or triple bond formation are most easily discussed in terms of molecular orbitals. The principal type of multiple bond consists of two parts first, a pair of electrons in an orbital symmetrical about the axis of the bond and second, one or more pairs of electrons in orbitals which are not symmetrical about the axis. The first pair of electrons form a bond which differs in no way from the ordinary single, or passed through the axis. They may be regarded as formed by the interaction of two p orbitals, one on each atom, with axes parallel to each other and perpendicular to the axis of the bond. We shall refer to orbitals of this type as rr orbitals. 

One mode of interconversion of proton donor and acceptor roles in HF--HF to FH—FH passes through a transition state with C2 cyclic character. The energy barrier to this interconversion is estimated as approximately 1 kcal/mol. A similar barrier is encountered in the transition path which moves hydrogens from one side of the H-bond axis to the other. The transition state consists of a fully linear HF--HF configuration of all four atoms. The two transition states have interfluorine distances that differ by some 0.25 A. 

The rotation operation (C ) (also called proper rotation) is rotation through 360°/n about a rotation axis. We use counterclockwise rotation as a positive rotation. An example of a molecule having a threefold (C3) axis is CHCI3. The rotation axis is coincident with the C—H bond axis, and the rotation angle is 360°/3 = 120°. Two C3 operations may be performed consecutively to give a new rotation of 240°. The resulting... 

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