The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]- and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

Polycrystalline all–trans retinal (ATR) has been excited at 647.1 nm under pressure up to 9 GPa at room temperature and the resulting Raman spectrum has been measured. The large majority of Raman bands in the 700–1700 cm−1 range have positive pressure coefficients, i.e. bonds stiffen upon compression. The Raman spectrum is recovered after the compression–decompression cycle indicating that the molecular unit does not undergo an irreversible transformation under our experimental conditions. Model density functional (DF) calculations with the B3-LYP exchange-correlation functional and the 6-31G* basis set have been performed on 1-cis,3-trans,5-trans,7-trans,9-trans,1,5,9-trimethyldecapentaenal (TMDPL) mimicking the polyenic moiety of ATR. By shortening the single and double CC bonds of TMDPL up to 0.0015 Å it was possible to fit the frequency dependence on pressure of the most intense polyenic bands with single and double bond compressibilities ≈0.00292 and 0.00117 Å/GPa, respectively. The trans → cis isomerization in the crystal state under pressure and in the presence of the 647.1 nm excitation light is suggested by the Raman spectrum in the fingerprint region. A simplified model is proposed for the occurrence of this process in the crystal phase.

The purpose of this paper is to review Dalton’s contributions to science in various fields of research in relation to the first intimation of the chemical atomic theory. Early “germs” of his physical ideas may be found in the initial meteorological studies where water vapour is viewed as an “elastic fluid sui generis” diffused in the atmosphere and not as a species chemically combined with the other atmospheric gases. The next object of Dalton’s attention was atmosphere itself. He discarded affinity between atmospheric gases as a possible cause of homogeneity and, making recourse to Newtonian Principles, considered the repulsive forces among particles. Experiments on the “nitrous air test” and on the diffusion and solubility of gases were instrumental to arrive at the chemical atomic theory. The slow, laborious, and persevering work of Dalton to get the first table of atomic weights is a fascinating piece of science which may be fully appreciated by referring to his laboratory notebooks.

The thermal denaturation of proteins is considered as a process by means of which chemical equilibrium can be introduced to undergraduate students of Chemistry related curricula. In this approach chemical potential μ, Gibbs energy G, degree of advancement ξ and Le Châtelier principle are integrated with chemical equilibrium. With reference to α-Chymotrypsinogen A as a test case, the process is discussed in terms of a simplified two-state model. The activity is addressed to physical chemistry students in combination with computer-aided work mainly involving storage/manipulation of large data sets and plot preparation.

The purpose of this paper is to review the basic steps in the development of the atomic–molecular theory with the support of the original documentation. The most important contributions are examined, from the first intimation of the theory by Dalton to the Avogadro hypothesis, intended to reconcile empirical relations observed by Gay-Lussac in gaseous reactions with the atomistic ideas. The genesis and the structure of Dalton’s theory are described, and attention is focused on the rules of the greatest simplicity by means of which the relative weights and sizes of elementary and compounds atoms were determined. The first striking achievements are outlined. A peculiar trait of the Daltonian principles is represented by the rejection of a simple relation between specific gravities of elastic fluids and weights of the ultimate particles. On the contrary, the fixed (and small) ratios of reacting volumes in gaseous reactions, a result discovered by Gay-Lussac, suggest that particles combine according to simple numbers. The equal volume/equal number of particles hypothesis was a substitute better than Dalton’s rules for the determination of the atomic weights and led to the prediction of elementary polyatomic molecules.

Symmetry plays a fundamental role in molecular spectroscopy