Some years ago, we extended this model,32 by introducing polarizabilities on all atoms,and considered allelectrostatic interactions between monopoles and induced dipoles atall three atomic centers. Where Debye used fully ionized atoms, weused a charge transfer q. Our model explains thebond angle in terms of the ratio of thepolarizabilities of the freeatoms αA and αB (for a list of atomicpolarizabilities we refer to Table S19 ofthe Supporting Information). By observing the change of optimum bondangle with the change of the polarizability ratio, two bond angletrends were identified: (1) the bond angle decreases for more polarizablecentral atoms, and (2) the angle increases for more polarizable outeratoms. This leads to the prediction of bond angle trends with justthe information available from the period table of elements. Atomicpolarizability within a group increases for atoms from higher periods(rows). Similarly, if the ratio of polarizabilities is changed by,for example, electronic excitation, trends can be explained. Whenthis model is applied to the CO2 molecule, the rationalizationof its linear ground state geometry is that there is too little polarizableelectron density around the carbon nucleus to initiate bending throughthe induction of a dipole at the carbon atom. In the lowest singletexcited states of CO2, there is substantial charge transferfrom oxygen to carbon: about 0.25 electrons. These states33 have a bond angle of about 120°. When themodel is applied to the H2O molecule, the density of electronsaround the oxygen atom that are not involved in bonding and are notcore electrons is sufficiently polarizable for a dipole to be inducedto stabilize a nonlinear geometry.
The 1H nuclear magnetic resonance (NMR) spectrum shows signals for α-(δ 8.5), γ-(δ7.5) and β-protons (δ7). By contrast, the proton signal for benzene is found at δ7.27. The larger chemical shifts of the α- and γ-protons in comparison to benzene result from the lower electron density in the α- and γ-positions, which can be derived from the resonance structures. The situation is rather similar for the 13C NMR spectra of pyridine and benzene: pyridine shows a triplet at δ(α-C) = 150 ppm, δ(β-C) = 124 ppm and δ(γ-C) = 136 ppm, whereas benzene has a single line at 129 ppm. All shifts are quoted for the solvent-free substances. Pyridine is conventionally detected by the gas chromatography and mass spectrometry methods.
Pyridine supports a series of radical reactions, which is used in its dimerization to bipyridines. Radical dimerization of pyridine with elemental sodium or Raney nickel selectively yields 4,4'-bipyridine, or 2,2'-bipyridine, which are important precursor reagents in the chemical industry. One of the name reactions involving free radicals is the Minisci reaction. It can produce 2-tert-butylpyridine upon reacting pyridine with pivalic acid, silver nitrate and ammonium in sulfuric acid with a yield of 97%. 2b1af7f3a8