Odd-electron molecules

The transition metals, lanthanides and actinides provide many examples of compounds with unpaired electrons because of partial core character of the nd and nf subshells for these groups of elements, as discussed above. 

---

Unlike nitrogen monoxide, nitrogen dioxide has properties more typical of an odd electron molecule. It is a coloured (brown), reactive gas which dimerises to the diamagnetic colourless gas dinitrogen tetroxide, N2O4. in which the odd electron is paired. The structure of dinitrogen tetroxide can be represented as a resonance hybrid of ... 

Nitric oxide is the simplest thermally stable odd-electron molecule known and, accordingly, its electronic structure and reaction chemistry have been very extensively studied. The compound is an intermediate in the production of nitric acid and is prepared industrially by the catalytic oxidation of ammonia (p. 466). On the laboratory scale it can be synthesized from aqueous solution by the mild reduction of acidified nitrites with iodide or ferrocyanide or by the disproportionation of nitrous acid in the presence of dilute sulfuric acid ... 

Example shows that no energy barrier exists for the combination of two NO2 molecules to form N2 O4. The activation energy for this reaction is zero because NO2 is an odd-electron molecule with a lone electron readily... 

The case of interaction between an even-electron molecule A and an odd-electron molecule B can be discussed in a similar manner. Eq. (3.20) is modified to be... 

As has been mentioned in Chap. 5, the singly occupied MO in odd-electron molecules and radicals plays the role of HO or LU or both MO s according to the orbital energy relationship and the orbital overlapping situation. The importance of SO distribution is easily understood... 

Only NH3, S03 and C102 have central atoms that obey the octet rule. N02 and C102 are odd-electron molecules. 

The nitrogen fluoride series has an unexpected maximum at NF2 but the sulfur fluorides show alternations in D SFn—F) values with minima at odd-electron molecules. If rehybridization energies at ZXSF4—F) and ZHSF2—F) stages are superimposed on the regular... 

Only the thiazyl trifluoride can be said to have a single S—F bond within the error limits. The stronger bond in the monofluoride accords with a bond order of 2.4 for S—N estimated from a force constant-distance relationship. The odd-electron molecule NSF2 has the weakest bond. 

When a mixture of the odd-electron molecules NO and N02 is cooled to — 20°C, the single electrons pair and the two radicals combine to form the dark blue liquid dinitrogen trioxide, N203 (1 and Fig. 15.6). This compound, in which the oxidation number of the nitrogen is +3, is unstable as a gas. It is the anhydride of nitrous acid, HN02, and forms that acid when it dissolves in water ... 

Effect of O2 and NO. These are radical inhibitors and scavenge the CH radicals, removing the source of C2H6, CH4 and the minor products. 02 has two unpaired electrons and NO is an odd electron molecule. This means that these unpaired electrons can pair up with the unpaired electron on radicals to produce a single bond. This removes the radical characteristics, which are dependent on the presence of at least one unpaired electron. The yields will then drop to zero. This is observed and confirms the central role of CH in the mechanism. 

Only CIO2 is known. Chlorine dioxide is an odd-electron molecule. Theoretical calculations suggest that the odd electron is delocalized throughout the molecule, and this probably accounts for the fact that there is no evidence of dimerization in solution, or even in the liquid or solid phase. Its important Lewis structures are shown below ... 

They are both odd-electron molecules. N02 dimerises reversibly at low temperatures to make N204, but... 

Since the LE model is based on pairs of electrons, it does not handle odd-electron cases in a natural way, although Lewis structures are sometimes written for these species. To treat odd-electron molecules accurately, we need a more sophisticated model. 

Experimentally, nitric oxide is indeed found to be paramagnetic. Note that this odd-electron molecule is described very naturally by the MO model. In contrast, the LE model, in the simple form used in this text, does not readily describe such molecules. 

Nitrogen dioxide (NO2), which is also an odd-electron molecule, has a V-shaped structure. The reddish brown, paramagnetic NO2 molecule readily dimerizes to form dinitrogen tetroxide,... 

---