Tautomeric equilibria in relation to pi-electron delocalization.

The coordination chemistry of 2-pyridone and its derivatives

Rotationally resolved fluorescence excitation spectra of two vibronic bands in the S1←S0 electronic transition of 2‐hydroxypyridine (2HP), and of the corresponding bands in the hydroxy‐deuterated molecule, have been obtained. A comparison of the rotational constants of the two molecules shows that the two bands both originate in the zero‐point vibrational level of the planar keto tautomer of 2HP, 2‐pyridone (2PY), and terminate in different zero‐point levels of 2PY that have different out‐of‐plane equilibrium geometries at nitrogen. Additionally, all four bands exhibit ‘‘anomalous’’ rotational line intensities that are shown to result from an in‐plane inertial axis reorientation which occurs on absorption of the photon. Likely atomic displacements that are responsible for this ‘‘rotational’’ Duschinsky effect, which may have significant dynamical consequences in 2PY and other molecules, are discussed.

Inertial axis reorientation in the S1←S0 electronic transition of 2-pyridone. A rotational Duschinsky effect. Structural and dynamical consequences

Theoretical considerations of the molecular hyperpolarizabilities (β) of donor−acceptor substituted aromatic molecules show that the transfer of a proton from the electron-donor group of an acid to the electron acceptor group of a base could increase β for both resulting ionic species relative to the corresponding neutral molecules. To take advantage of this, the cocrystallization of 4-nitrophenol with various organic bases was studied. Cocrystallization of 4-nitrophenol with 4-(dimethylamino)pyridine gives yellow crystals that display a strong second harmonic signal by the Kurtz and Perry powder test. The material crystallizes in the orthorhombic space group P212121 and is composed of 4-(dimethylamino)pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP2). Theoretical calculations of the hyperpolarizabilities predict that the values of β are greater for the 4-(dimethylamino)pyridinium and 4-nitrophenolate ions than for the corresponding neutral molecules. A second noncentrosymmetric material has been grown ...

Nonlinearity enhancement by solid-state proton transfer : A new strategy for the design of nonlinear optical materials

Publisher Summary This chapter discusses aromaticity of heterocycles. At a time when the validity of the concept of aromaticity is questioned, increasingly, it seems appropriate to review various definitions in common use and to survey their applications in heterocyclic chemistry. From a qualitative viewpoint the characteristics of an aromatic compound are easy to define. A compound is said to be aromatic if it is cyclic and unsaturated with an enhanced stability over simple olefinic compounds. The characteristic aromatic reactivity of the type described is clearly a matter of degree and is difficult to assess quantitatively. From a simple consideration of pyridine, which reacts less readily with electrophiles but more readily with nucleophiles at the ring carbon atoms than does benzene, it soon becomes apparent that attempts to discuss even relative aromatic character using kinetic criteria is fraught with difficulties; this is apart from the fundamental objection that chemical reactivity is not solely a function of ground-state stability.

Aromaticity of Heterocycles

https://journals.iucr.org/q/issues/1953/07/00/a00954/a00954.pdf

B.R. Penfold

Papers

The electron distribution in crystalline α-pyridone