Dinitrogen dissociation energy

Dinitrogen has a dissociation energy of 941 kj/mol (225 kcal/mol) and an ionisation potential of 15.6 eV. Both values indicate that it is difficult to either cleave or oxidize N2. For reduction, electrons must be added to the lowest unoccupied molecular orbital of N2 at —7 eV. This occurs only in the presence of highly electropositive metals such as lithium. However, lithium also reacts with water. Thus, such highly energetic interactions ate unlikely to occur in the aqueous environment of the natural enzymic system. Even so, highly reducing systems have achieved some success in N2 reduction even in aqueous solvents. 

The new dinitrogen complex [Ni(CO)3N2] can be generated in a pressure cell by UV photolysis of tetracarbonylnickel in liquid krypton, doped with N2 at 114K. The decomposition of this complex was followed over the temperature range 122-127 K and a value of the Ni—N2 bond dissociation energy estimated at lOkcal moN1.2474... 

In cocondensation experiments of nickel atoms in various CO/N2 matrices, mixed carbonyl-dinitrogen species Ni(CO)x(N2)4 x (jc = 1-3) were observed.243 The Ni(CO)3(N2)244 245 species was also generated by the UV photolysis of Ni(CO)4 in N2-doped liquid Kr, and the decay kinetics were studied.246 The Ni—N2 dissociation energy was estimated to be about 42kJmor1. 

Dissociation energies must be considerably greater than 1 7 if the entity R1-R2 is to be sufficiently stable to be recognized as a molecule. In practice there are wide variations in Z)(Ri - R2) from molecule to molecule. At the lower end of the scale we have )(N02 -NO2) in dinitrogen tetroxide as low as 12-9 kcal mole in accordance with the well-known ease of dissociation of this compound. At the other end we have the molecules carbon monoxide and nitrogen where D is probably more than 200 kcal mole"h Appreciable dissociation of CO and IS2 can only take place at temperatures usually found only in certain Hames and detonations. 

From our knowledge on structure and reactivity of the dinitrogen molecule, e. g., the very high dissociation energy of the triple bond (940 kJ mol ), it is obvious that the major problem is not the formation of the diazenido complex (1), but the consecutive steps (2)-(4). In particular, it is unlikely that diazene (HN=NH) is an intermediate, as it is thermodynamically very unfavorable. Chemical research on nitrogen fixation concentrated, therefore, in recent years on reduction mechanisms of dinitrogen complexes (see review of Shilov, 1992). 

Fig. 20.7. First row bond distances and bond energies in prototype moiecules with N-N and N-O single and N=0 double bonds. Second row bond distances and NO bond energies in the three gaseous mononitrogen oxides. Third row bond distances and N-N bond dissociation energies of three gaseous dinitrogen oxides.

Nernst pointed out in 1904 that in a dissociating gas, energy transfer could take place as follows molecules absorb energy at the hot surface and are dis.sociated subsequent recombination at the cold surface releases this energy as thermal energy. He showed that this would result in a great increase in the coefficient of thermal conductivity with temperature as a gas became dissociated, and that this could be seen to occur in dinitrogen tetroxide He considered that the measured coefficient of thermal conductivity, could be wTitten... 

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