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.

Symmetry plays a fundamental role in molecular spectroscopy [...]

In this paper we report on the Raman scattering of antimalarial artesunate, as a neutral species and as an anion in sodium salt, and on the surface-enhanced Raman scattering (SERS) of the anion adsorbed on silver nanoparticles. The vibrational assignment of the strongest Raman peaks of artesunate has been made with reference to the Raman data on artemisinin and by identifying peaks due to modes largely localized on the succinic side chain of artesunate. The structure of artesunate, as well as that of the anion, has been optimized by means of ab-initio calculations at the DFT/B3LYP/cc-pVDZ calculation level and found in good agreement with the experimental molecular geometry. The conformational minima of the flexible side chain both in the anion and in the anion complexed with Ag+ have been characterized by ab-initio methods at the same level of accuracy and eight pairs of conformers have been predicted. From these data the vibrational frequencies and Raman intensities have been obtained. This has allowed to assign the SERS spectrum of chemisorbed artesunate to conformers with nearly all-trans side chain. For these conformers the artemisinin core lies as far as approximate to 9 A from the silver ion and the active SERS modes are localized on the succinic fragment. Minor contributions to the SERS intensity are due to the di-silver complex and a more sterically hindered mono complex. It is a major conclusion of this study that the interaction of artesunate with the silver surface is exclusively through the side chain, leaving untouched the peroxide group responsible of the antimalarial activity. (C) 2020 Elsevier B.V. All rights reserved.

The purpose of this article is to present a short review of Sadi Carnot work on heat engines and on the role his adherence to the caloric theory may have had. The essential points developed in the Réflexions are reviewed as forerunners of the science of thermodynamics. The antecedents that may have inspired the brilliant scientific insights of Carnot are reviewed together with the reception of the Carnot principles in the engineering and in the scientific community until the formulation of the two principles of modern thermodynamics.

The insertion of azobenzene moiety in complex molecular protein or peptide systems can lead to molecular switches to be used to determine kinetics of folding/unfolding properties of secondary structures, such as α-helix, β-turn, or β-hairpin. In fact, in azobenzene, absorption of light induces a reversible trans ↔ cis isomerization, which in turns generates a strain or a structure relaxation in the chain that causes peptide folding/unfolding. In particular azobenzene may permit reversible conformational control of hairpin formation. In the present work a synthetic photochromic azobenzene amino acid derivative was incorporated as a turn element to modify the synthetic peptide [Pro7,Asn8,Thr10]CSF114 previously designed to fold as a type I β-turn structure in biomimetic HFA/water solution. In particular, the P-N-H fragment at positions 7–9, involved in a β-hairpin, was replaced by an azobenzene amino acid derivative (synthesized ad hoc) to investigate if the electronic properties of the novel peptidomimetic analog could induce variations in the isomerization process. The absorption spectra of the azopeptidomimetic analog of the type I β-turn structure and of the azobenzene amino acid as control were measured as a function of the irradiation time exciting into the respective first ππ* and nπ* transition bands. Isomerization of the azopeptidomimetic results strongly favored by exciting into the ππ* transition. Moreover, conformational changes induced by the cis↔ trans azopeptidomimetic switch were investigated by NMR in different solvents.

: Treatment of epoxides with bis(trimethylsilyl)-selenide under strictly controlled conditions allows to isolate b -hydroxy selenols which evidence an unexpected stability, taking into account their known propensity to afford diselenides. Also thiiranes and aziridines lead to functionalized selenols bearing a thiol and a N -Ts- or N -Boc-protected amino moiety on b -position. These selenols were stable enough to react with different electrophiles. Ab-initio DF calculations on two suitable model systems, n -propyl selenol and b hydroxy derivative, allow to ascribe the observed low tendency to oxidation to noncovalent interactions between the selenol moiety and the (cid:2) OH group.

1,6-Methano[10]annulene solutions in cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs, exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow one to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results in the critical species for methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6-methano[10]annulene.

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 chemosensing properties of the polyaza-macrocycle 1-(6,7)-acridine-3,6,9,12-tetraaza-tridecaphane have been investigated by means of emission fluorescence spectroscopy, considering halide ions as substrates. As in the case of the free ligand, the fluorescence emission of the complexes is due to the acridinium species which are formed after photoinduced proton transfer reaction. The complexation constants have been obtained for the bi- and tri-protonated ligands in deoxygenated aqueous solutions. Two different emission behaviours have been observed varying the anion. Fluoride and chloride give rise to fluorescence enhancement whereas bromide and iodide strongly quench the emission. The macrocycle shows an unusual higher selectivity towards the chloride anion rather than fluoride. The fluorescence emission has been modelled considering a modified Stern-Volmer equation, taking into account the quenching effects of the largest anions, which can be considered negligible for fluoride and chloride anions. Ab initio calculations allow us to interpret the fluorescence emission of the complexes in terms of activation energy related to the proton transfer reaction responsible for the emission process.

Gold nanoshells, with a silica core and different core and shell dimensions, have been extensively investigated. Optical far-field properties, namely extinction and absorption, have been separately determined by means of spectrophotometry and photoacoustic spectroscopy, respectively, in the 440-900 nm range. The enhancement factor for surface-enhanced Raman scattering, which is related to near-field effects, has been measured from 568 to 920 nm. The absorption contribution to extinction decreases as the overall diameter increases. Moreover, absorption and scattering display different spectral distributions, the latter being red shifted. The Surface Enhanced Raman Scattering enhancement profile, measured using thiobenzoic acid as a Raman probe, is further shifted to the red. The latter result suggests that the enhancement is dominated by the presence of hot spots, which are possibly related to the surface roughness of gold nanoshell particles.

The wavelength dependence of the laser-induced photoacoustic signal amplitude has been measured for water dispersions of 10, 61, and 93nm diameter gold nanospheres. The whole region of the localized surface plasmon resonance has been covered. This “photoacoustic excitation profile” can be overlayed with the extinction spectrum between 450nm and 600nm in the case of the smallest nanoparticles. At variance, the larger-sized nanoparticles display a progressive deviation from the extinction spectrum at longer wavelength, where the photoacoustic signal becomes relatively smaller. Considering that photoacoustics is intrinsically insensitive to light scattering, at least for optically thin samples, the results are in agreement with previous theoretical work predicting (i) an increasing contribution of scattering to extinction when the nanoparticle size increases and (ii) a larger scattering component at longer wavelengths. Therefore, the method has a general validity and can be applied to selectively determine light absorption by plasmonic systems.

Synthesis and characterization of a new macrocyclic compound, composed by a triethylentetraamine chain linking the 4 and 5 positions of an acridine moiety, are reported. The molecule, devised as a fluorescent chemosensor for anions, has revealed an intriguing pH-dependent spectroscopic behavior, whose features are the specific object of this article. Ligand protonation in aqueous solution has been analyzed by means of potentiometric, (1)H NMR, UV-vis, and fluorescence emission measurements. The molecule binds up to four protons in the pH range 2-11. Protonation takes place on the aliphatic tetraamine chain, while the acridine nitrogen does not participate to proton binding even at very low pH. Differently from acridine, the UV-vis spectra are almost unaffected by the pH. On the opposite, the emission spectra are strongly pH-dependent. In fact, at low pH values, the spectra show a blue-shifted emission, resembling that of unprotonated acridine, while at slightly acidic and alkaline pH the fluorescence features a red-shifted band similar to that of acridinium cation. This unusual behavior occurs in the mono-, bi-, and triprotonated forms of the compound and is interpreted as due to an excited state proton transfer from an aliphatic ammonium group adjacent to the acridine moiety to the acridine nitrogen. In the fully protonated state, this process is prevented owing to unfavorable molecular arrangements mainly determined by electrostatic repulsions. This interpretation is supported by quantum mechanical calculations as well as molecular dynamics simulations.

The transient evolution of protoporphyrin IX (PPIX) and hemin following the Soret band excitation was measured in the 410–600 nm spectral region with sub-picosecond time resolution. In PPIX the relaxation pathway was characterized in the femto- and picosecond time scale by two processes with time constants of 350 fs and ∼6 ps, describing the evolution of the system through internal Q_y → Q_x conversion and vibrational relaxation and cooling in the Q_x state. The lifetime of the Q_x state was found to be 10.4 ns by time resolved fluorescence measurements. In hemin, the ground state is completely recovered in tens of picoseconds through pathways involving CT and (d,d) states. The experimentally observed vibrational dynamics is mainly due to “hot” ground state transitions.

CO recombination kinetics has been investigated in the type II truncated hemoglobin from Thermobifida fusca (Tf-trHb) over more than 10 time decades (from 1 ps to ∼100 ms) by combining femtosecond transient absorption, nanosecond laser flash photolysis and optoacoustic spectroscopy. Photolysis is followed by a rapid geminate recombination with a time constant of ∼2 ns representing almost 60% of the overall reaction. An additional, small amplitude geminate recombination was identified at ∼100 ns. Finally, CO pressure dependent measurements brought out the presence of two transient species in the second order rebinding phase, with time constants ranging from ∼3 to ∼100 ms. The available experimental evidence suggests that the two transients are due to the presence of two conformations which do not interconvert within the time frame of the experiment. Computational studies revealed that the plasticity of protein structure is able to define a branched pathway connecting the ligand binding site and the solvent. This allowed to build a kinetic model capable of describing the complete time course of the CO rebinding kinetics to Tf-trHb.

Nanostructured Ag platforms have been obtained by simple chemical procedure and characterized by AFM (atomic force microscopy) measurements, for use in biosensing by means of SERS (surface-enhanced Raman scattering) spectroscopy. The SERS efficiency of these substrates has been verified by microRaman measurements on small RNA chains with different nucleobase content, showing sensitivity near attomole level. It is our opinion that these metal substrates may be widely used as appropriate sensors for detecting biomolecules in many applications concerning medical diagnostics, pharmacological research and nanomaterials technology.

Four nanostructured Ag substrates have been fabricated with different surface morphologies and tested with surface-enhanced Raman scattering (SERS) experiments by adsorption of adenine. Their SERS efficiency has been compared and related to the surface roughness resulting from atomic force microscopy measurements. Chemical etching of silver by thiourea/Fe(III)nitrate produces homogeneously roughened plates, exhibiting the largest three-dimensional surface and the best SERS enhancement. They mostly exhibit surface protrusions with sizes around 200 nm, thus matching the best condition for obtaining SERS enhancement by laser excitation at 785 nm. This is quite important in the case of biomolecules, whose samples often present strong fluorescence bands, which usually are not observed with red-shifted exciting lines. Moreover, these Ag platforms, owing to their uniform nanostructured surfaces, are suitable for obtaining reproducible results from microRaman investigation. In conclusion, the present nanofabrication of Ag surfaces allows obtaining SERS-active substrates, which combine high reproducibility and sensitivity and can be successfully employed in the molecular recognition of different organic ligands or biomolecules like nucleic acids and proteins.

The electronic and vibrational properties of polyPCDA have been examined by means of ab initio calculations on oligomers performed as a function of the number of monomeric units and of the torsional angle τ between them. The vertical excitation energies S0→S1 and the double and triple CC stretching frequencies of blue, red and bluish-green forms of the polyPCDA have been correlated to conformational structures with different torsional angles.

Surface-enhanced Raman scattering (SERS) of adenine-containing microRNA chains has been obtained by adsorption on roughened silver substrates. The spectral features of all of these samples appear dominated by the bands of adenine. By comparison with the SERS spectra of adenine and adenosine obtained on the same substrates, along with DFT calculations on the interaction sites of adenine and adenosine with silver, inferences are discussed about the structural arrangement of the microRNA chains with respect to the metal surface. This approach gives suitable guidelines in order to investigate the adsorption of complex biomolecules on metal substrates.

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.

The photochemical rearrangement of azoxybenzene to 2-hydroxyazobenzene has been investigated and the Raman spectrum of the photoproduct has been measured with micro-Raman instrumentation. The reaction mechanism and the spectral data are discussed by means of density functional calculations.