Question

Below are four frontier molecular orbitals for N_2F_2. Designate the C_2h symmetry representation of each. Like colors represent the same sign on the wavefunction. Draw an arrow between occupied (HOMO) and unoccupied(LUMO) pair(s) that are connected by a dipole-allowed absorption.(*Note the coordinate system of each wavefunction.)

Answer 1

N₂F₂, is a halide of nitrogen, and contains a double bond in its structure, between the two nitrogen atoms. It can adapt two different geometrical forms, cis and Trans, each with distinct symmetry.

The cis-form has symmetry, C2v, while the trans-form, with the two fluorine atoms on opposite sides of each other, is C2h. For the cis-form, the HOMO and LUMO have the p orbitals perpendicular to the plane of the molecule are characteristic of sp2-hybridized atoms, and this is why the F-N-N angle is close to 120 ⁰. The bond order is close to 2 for the N=N bond.

Trans-N₂F₂ is slightly different, with an angle close to sp3-hybridized atoms 109.5⁰ . Its molecular orbitals are slightly different. The LUMO shows the same behavior as that of cis-N₂F₂, but the HOMO is slightly different, looking more like sp3-hybridized orbitals. The lone pairs on the nitrogen atoms in trans-N₂F₂ are visible in the HOMO, and as they jot out in the plane of the molecule, that is a feature of sp2-hybridized orbitals. Atoms carrying lone pairs being sp3-hybridized tend to arrange the lone pairs in tetrahedral arrangements relative to bonds formed with other atoms. Sp2-hybridized adopt a trigonal planar arrangement for both lone pairs and bonds to other atoms.  

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