Abstract
The nitro group is active in metabolic systems and can be found as an integral part of a number of useful curative drugs and many toxic substances. The basis for much of this activity is not fully understood. It is not necessarily caused directly by through-bond electronic effects but may also be due to direct H-bonding to nitro or to indirect interference by the nitro group with existing H-bonding. An unusual effect of a nitro substituent on kinetic results from urethane addition/elimination reactions (Scheme 1) has been ascribed to some form of self-association, which was neither specified nor quantified. To investigate self-association phenomena caused by a nitro group, a bond energy/bond order formula for N–O bonds has been developed and then used to interpret relative amounts of covalent and ionic contributions to total N–O bond energy. Calculated bond energies were then used to obtain enthalpies of formation for H-bonds to nitro groups in crystals and in solution. Similar results from solution data reveal that direct H-bonding to nitro is much weaker than in crystals, unless intramolecular H-bonding can occur. The results revealed that the 'self-association' effects observed for nitro substituents in urethanes (Scheme 1) were not caused by nitro participating directly in intermolecular bonding to NH of another urethane but by an indirect intramolecular action of the nitro group on pre-existing normal NH– O amide/amide type H-bonding.
Highlights
Unusual behaviour of the nitro group can be seen in results from kinetic experiments on the elimination/addition reaction of substituted arylurethanes (Scheme 1)
Coulson discussed the dilemmas involved in treating molecules as collections of almost independent bonds in relation to quantum methods, which consider molecular orbitals as encompassing all atoms to varying extents at any one instant.3a For isolated bonds in heteronuclear diatomic molecules, Coulson demonstrated that a total wave function (Ψ) should comprise a covalent part (Ψcovalent) and an ionic part (Ψionic) to allow for ionic contributions to total bond energy
The two terms are related by the expression, Ψ = Ψcovalent + λΨionic, which allows for the temporal probability that two electrons in a bond may both be near one of the atoms making up the bond
Summary
Unusual behaviour of the nitro group can be seen in results from kinetic experiments on the elimination/addition reaction of substituted arylurethanes (Scheme 1). E.21a These experimental and calculated magnitudes for the charges on oxygen and nitrogen in a standard NO2 group are almost identical, giving confidence in the accuracy and meaning of the terms Etotal, Ecovalent and Eionic, obtained from the N–O bond energy/bond order formula.
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