Bond energy/bond order relationships for N-O linkages and a quantitative measure of ionicity: the role of nitro groups in hydrogen-bonding

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.

Pauling’s Conceptions of Hybridization and Resonance in Modern Quantum Chemistry

“…The correlations reveal slight S-shaped curvatures, or more specifically, near-perfect linear correlations around each integer (single dominant NLS) or half-integer (two-state bond-shift [85]) bond order with connecting curvatures to accommodate the slightly different slopes of different bond types, but their essential linearity is suggested by the robust |χ| 2 coefficients. Such correlations strongly support the useful predictive associations of NRT bond orders with experimentally measurable quantities, consistent with well-known empirical relationships connecting a variety of bond properties, including bond lengths [86][87][88], bond energies [89][90][91][92], IR vibration frequencies [93,94], and NMR spin-spin coupling constants [95]. Variations in the NRT weights for the four resonance structures across the IRC, as Variations in the NRT weights for the four resonance structures across the IRC, as well as concomitant changes in natural bond orders, are entirely consistent with the electronpushing, curly-arrow representation that the bench chemist would use to depict the reaction mechanism (Figure 8).…”

“…The correlations reveal slight S-shaped curvatures, or more specifically, near-perfect linear correlations around each integer (single dominant NLS) or half-integer (two-state bond-shift [ 85 ]) bond order with connecting curvatures to accommodate the slightly different slopes of different bond types, but their essential linearity is suggested by the robust |χ| 2 coefficients. Such correlations strongly support the useful predictive associations of NRT bond orders with experimentally measurable quantities, consistent with well-known empirical relationships connecting a variety of bond properties, including bond lengths [ 86 , 87 , 88 ], bond energies [ 89 , 90 , 91 , 92 ], IR vibration frequencies [ 93 , 94 ], and NMR spin-spin coupling constants [ 95 ].…”

“…The correlations reveal slight S-shaped curvat near-perfect linear correlations around each integer (single dom (two-state bond-shift [85]) bond order with connecting curva slightly different slopes of different bond types, but their esse by the robust |χ| 2 coefficients. Such correlations strongly sup associations of NRT bond orders with experimentally measur with well-known empirical relationships connecting a variety ing bond lengths [86][87][88], bond energies [89][90][91][92], IR vibratio NMR spin-spin coupling constants [95]. Figure 7 shows the correlation of natural bond order with bond length for geometries across the IRC.…”

Nε-Acryloyllysine Piperazides as Irreversible Inhibitors of Transglutaminase 2: Synthesis, Structure–Activity Relationships, and Pharmacokinetic Profiling

“…This results in an energy gain of 3.1 kJ/mol (0.74 kcal/mol) due to the presence of the nitro group compared to H. A similar value (3.31 kJ/mol, 0.79 kcal/mol) is obtained by the use of the K i instead of the k inact / K I values. Typical energy gains of H-bonds are in between 4 to 12 kJ/mol and for ionic interactions up to 20 kJ/mol but strongly depend in both cases on desolvation effects accompanied by the effect of enthalpy–entropy compensation. − In view of a possible H-bond, the nitro group generally represents a weak H-bond acceptor (p K a (−NO 2 H + ) = −11; ref ). However, the potential of aromatic nitro groups is higher than that of aliphatic ones .…”

Resonance Theory Reboot

“…Note that NBO/NRT algorithms only require input of the 1-RDM Γ true (1) (and AO descriptors for the basis in which Γ true (1) is represented), but no information about molecular geometry, symmetry, Kohn–Sham MOs, orbital energies, or other numerical descriptors that might be correlated with experimental properties. Thus, NRT bond orders provide suitable blind predictors for expected correlations with bond lengths, bond frequencies (Badger’s rule), bond energies, and the like.…”