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a detailed electrochemical and photophysical comparative study of three parallel series of phenyl, thienyl and furyl substituted ethylenes has been carried out, implemented by the computational calculation of selected terms. Relationships have been highlighted between molecular structure (number and type of aromatic rings) and important functional properties (in particular, electronic features and oligomerization ability). Interestingly, some of the studied heteroaryl-ethylenes show emission in the solid state displaying an aggregation-induced emission behavior.

Polymorphism in the highly flexible organic Schiff-base macrocycle ligand 3,6,9,17,20,23-hexa-azapentacyclo(23.3.1.111,15.02,6.016,20)triaconta-1(29),9,11,13,15(30),23,25,27-octaene (DIEN, C24H30N6) has been studied by single-crystal X-ray diffraction and both solid-state and gas-phase density functional theory (DFT) calculations. In the literature, only solvated structures of the title compound are known. Two new polymorphs and a new solvated form of DIEN, all obtained from the same solvent with different crystallization conditions, are presented for the first time. They all have P\bar 1 symmetry, with the macrocycle positioned on inversion centres. The two unsolvated polymorphic forms differ in the number of molecules in the asymmetric unitZ′, density and cohesive energy. Theoretical results confirm that the most stable form is (II°), withZ′ = 1.5. Two distinct molecular conformations have been found, named `endo' or `exo' according to the orientation of the imine N atoms, which can be directed towards the interior or the exterior of the macrocycle. Theendoarrangement is ubiquitous in the solid state and is shared by two independent molecules which constitute an invariant supramolecular synthon in all the known crystal forms of DIEN. It is also the most stable arrangement in the gas phase. Theexoform, on the other hand, appears only in phase (II°), which contains both the conformers. Similarities and differences among the occurring packing motifs, as well as solvent effects, are discussed with the aid of Hirshfeld surface fingerprint plots and correlated to the results of the energy analysis. A possible interconversion path in the gas phase between theendoand theexoconformers has been found by DFT calculations; it consists of a two-step mechanism with activation energies of the order of 30–40 kJ mol−1. These findings have been related to the empirical evidence that the most stable phase (II°) is also the last appearing one, in accordance with Ostwald's rule.

The development of novel second-order nonlinear optical (NLO) organic materials in the solid state is an important and growing research field, as it is attractive for laser industry, electro-optic devices, data safety issues, and so on. In recent decades, some experimental electron charge density studies have been performed on solid organic NLO-phores, aimed at extracting from the ground-state charge density the information on the hyperpolarizability tensor [Koritsanszky & Coppens,Chem.Rev.101 (2001), 1583]. However, other groups [Whitten et al., J. Chem. Phys. 125 (2006), 174505, 1] claimed that the one-particle charge density does not contain sufficient information to determine the NLO properties. Even though it is impossible to obtain reliable quantitative approximations of the beta tensor elements directly from the charge density, it is also true that the former are influenced by the molecular structure and intermolecular interactions. In this work, we look for possible indirect correlations between topological properties of the charge density and non-linear optical properties in a suitable, simple test-case. We focus on N-methyl pyridinium tetrazolate (PYRTE) [Beverina et al., Chem. Comm. 2011, 47, 292], a high optical gap chromophore exhibiting high thermal, chemical and photochemical stabilities, whose NLO are observable only in solution due to the centrosymmetric crystal structure. First, we investigate how the crystal field affects molecular dipole moment, and therefore optical properties, of the solid. As expected, the in-crystal molecular dipole moment undergoes a significant enhancement, determining an important change in the second-order tensorial parameters of PYRTE. We then compare the predicted hyperpolarizability of the title compound with that of suitable chemical derivatives. Possible correlations among non-local integral topological descriptors, such as the Source Function, and the computed second order hyperpolarizabilities are tentatively established. Eventually, we look for possible PYRTE derivatives that, while maintaining good predicted NLO properties, are also potentially able to crystallize in non-centrosymmetric space groups. To this end, the CLP model [Gavezzotti, The Coulomb-London-Pauli (CLP) model of intermolecular interaction: Description and User's Manual v. 3.6.2013; Milano, 2013.] is applied to rank in silico generated structures from a thermodynamic viewpoint.

A new constituent, aloeresin H (1), was isolated from Cape aloe, a bittering and laxative agent. Its structure was elucidated by degradation experiments combined with 1D and 2D NMR data. Aloeresin H represents the first C,C-diglucoside discovered in commercial samples of the drug and its polyketide origin can be interpreted in terms of two-chain condensation. Possible conformations of the virtual aglycone were obtained as energy minima by quantum mechanical calculations and were found to be consistent with particular NOE associations observed in the original glucoside.

3-Diethylamino-4-(4-methoxyphenyl)-1,1-dioxo- 4H-1?6,2-thiazete-4-carbonitrile (DTC) is a synthetic compound that exhibits a significant similarity with ?-sultamic drugs. Its core chemical moiety is an uncommon four-membered thiazete-1,1- dioxide heterocycle. Former crystallographic investigations carried out at room temperature on different DTC polymorphs and a chemically related compound showed a very unusual structural feature: within the conjugated -N-C=N-SO2- system, the formally single N-C bond is, on average, 0.018 Å shorter than the formally double N=C bond. In this work, the charge density distribution of DTC has been explored by both single-crystal X-ray diffraction at T = 100(2) K and quantum mechanical simulations, with the aim of gaining insights into the subtle interplay between structure, electron delocalization, and crystal field polarization effects. To this end, both local and nonlocal topological descriptors provided by the Quantum Theory of Atoms in Molecules have been employed. Topological and structural changes of crystalline and in vacuo DTC have been related to the smaller or larger importance of resonance forms in the -N-C=N-SO2- moiety. A rationale for the mentioned C-N/C=N bond length inversion is found in terms of the large DTC dipole moment enhancement occurring in the crystal, which stabilizes highly polar resonant forms that exploit more favorable electrostatic interactions with neighboring molecules. In turn, this causes a significant electronic rearrangement within the molecule that results in an unusual and counterintuitive bond length alternation pattern. Possible implications from the viewpoint of the accurate in silico modeling of crystal structures are discussed

Four-membered ring thiosulfonium ions may be obtained quantitatively and under mild conditions by anionotropic rearrangement of C-(tert-butyl)-substituted thiiranium ion precursors. Thus, t-4-(tert-butyl)-r-1,2,2,c-3-tetramethylthietanium tetrafluoroborate or hexachloroantimonate (2a or 2b, resp.) were formed from thiiranium ion 1. The thietanium salts 2a and 2b were characterized by X-ray crystal-structure analysis. Their cation geometry was also optimized by ab initio calculations at the RHF/6-31G*//RHF/6-31G* level, as were those of its stereoisomer 3 and of the unsubstituted S-methylthietanium ion 5. Comparison of 2, 3, and 5 with 4 – the only other thietanium ion studied by XRD, where the C-atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton – indicates that, in these systems, relief from substituent overcrowding is easily achieved by a folding of the four-membered ring along the line connecting the two opposite C-atoms. The corresponding ring-deformation normal mode has a calculated frequency as low as 109 cm−1 in ion 5, to be compared with a frequency of 138 cm−1 in methylcyclobutane. For thietanium ion 2, the frequencies of the two normal modes involving such ring deformation have calculated values of 61 and 85 cm−1.

Some ZSM-5 zeolite samples with an SiO 2 Al 2 O 3 ratio of 10.3 have been analysed by temperature-programmed desorption (TPD) of ammonia to investigate how the rate-determining step of the desorption-diffusion process depends on some experimental parameters and on the physicochemical characteristics of the zeolite. By comparison with previous results on a ZSM-5 sample with SiO 2 Al 2 O 3 = 33.6 , a shift of the probable rate-determining step from intracrystalline diffusion of the desorbed gaseous base to desorption of the latter without appreciable readsorption has been noticed. On the other hand, both the change of the temperature ramp from 10 to 2 K/min and the substitution of Na+ for H+ by ion exchange shift the rate-determining step to desorption with free readsorption of the desorbed base.

The Source Function (SF) [1,2] enables one to view chemical bonding and other chemical paradigms under a new perspective and using only information from the electron density observable, rho, and its derivatives. Being completely independent from the tools used to get rho, the SF represents a very useful descriptor, able in many cases to bridge the gap between the rich information one gains from an ab-initio wavefunction of an ideal system and that, quite often more limited, but referred to a real system, obtained from an experimental rho derived from X-ray diffraction data. The potential uses of the SF are, however, yet not fully explored. In this lecture we discuss our recent work where the question of whether the SF is or is not capable to reveal electron delocalization has been carefully addressed [3,4]. Question arose because of a recent claim [5] according to which "pi-electron delocalization in the benzene ring is not manifest in the SF when the reference point (rp) - the point at which the atomic sources for its density are calculated - is taken at the C-C bond critical point (bcp)". Reasoning behind this statement was the null contribution from pi-molecular orbitals (MOs) to rho in their nodal plane. In all inspected cases and regardless of the theoretical or experimental derivation of rho, the answer to the question referred to above seems instead to be convincingly positive. Such a SF ability to reveal electron delocalization is independent from a sigma and pi separation of rho, since the SF tool was applied to the total rho. This observation is important in view of the possibility to recover electron delocalization effects using both rho's derived experimentally (hence without sigma and pi separation being allowed) and rho's where the departure from symmetry inhibits a proper separation of sigma and pi contributions. Using a MO approach, the sigma and pi contributions to the SF values can also be revealed and quantified. [1]Bader, R.F.W., Gatti, C. Chem. Phys. Lett. 1998, 287, 233-238. [2] Gatti, C., Cargnoni, F., Bertini, L. J Comput Chem 2003, 24, 422-436. [3]Gatti, C. Struct. Bond. 2011, 1 $DOI: 10.1007/430_2010_31 2. [4]Monza, E., Gatti, C., Lo Presti, L., Ortoleva, E. J. Phys. Chem. A 2011, 115, 12864-12878. [5]Farrugia, L.J., P. Macchi, J. Phys. Chem. 2009, A113, 10058-10067.

The analysis of t.p.d.a. spectra from ZSM5 is reported, assuming various chemical or physical steps as rate-determining. After saturation, quite long isothermal desorption times (up to 50 h) were needed, before starting the temperature ramp, to obtain reproducible data. Intracrystalline diffusion was found to be the probable rate-determining step with apparent activation energy of ca. 13 kcal/mol. A comparison of the present results with the ones obtained with HNaY zeolites provided information on the most probable hindering phenomena characterizing the overall diffusive process.

The conversion of a di-tert-butyl-methylthiiranium ion into thietanium ion, that is reported in the literature as taking place spontaneously at 25 degrees C in a CD(2)Cl(2) solution, has been discovered to occur quantitatively at room temperature (RT) also in the crystalline state. The ring enlargement reaction is accompanied, in the solid phase, by a modest deterioration of the quality of the sample under investigation, and all three specimens here studied by in situ crystallography maintained their single-crystal nature up to 100% conversion. The rearrangement reaction implies the breaking of a C-S bond and the formation of a new bond of the same type, together with the migration of a methyl group. The extent of the corresponding atomic displacements has been measured by comparing the initial and final crystal structures. Several intermediate stages of the process have been investigated and characterized by the site occupancy factor of the episulfonium ion. The RT temporal evolution of this factor and that of the unit-cell volume indicate multi-step kinetics, with processes of simple molecular reorientation or displacement before and after the main, central stage, where the conversion reaction takes place. The overall kinetics is well described by an Avrami-Erofeev equation, with exponent m = 1.75(3) and rate coefficient k = 10.4(3) x 10(-8) s(-1) at 25 degrees C. Ab initio calculations in the gas phase predict a three-step mechanism resulting in a slightly spontaneous reaction with an overall decrease of entropy.

Different polymorphs have different intensive physical properties and it is still impossible to predict from scratch if a change in the crystallization conditions will result in different crystal structures or not. In this contribution, possible correlations are highlighted among charge density features, molecular conformation and interaction energetics in the two known polymorphic forms of (DTC)[1,2], an isothiazole β-sultamic derivative. A tentative rationale is provided for the relative stability of the two forms on the basis of their different self-recognition patterns. Both polymorphs crystallize in the same P21/n space group and show very different non-covalent networks of weak C-H–X (X = N,O,π) interactions due to the dissimilar conformation of the asymmetric units (ASU). Accurate multi-temperature (100 K ≤ T ≤ 298 K) single-crystal X-Ray diffraction experiments were carried out and the evolution of crystal packing and self-recognition energetics were monitored through periodic quantum-mechanical calculations at fixed geometries. Preliminary results show that dispersive/repulsive and electrostatic non-covalent interactions dominate the crystal packing in both polymorphs. At T=100 K the form A have a tighter packing, as it shows a greater propensity in being involved in H bonds than B (see the Hirshfeld surface fingerprint plots[3] of forms A -left- and B -right- here reported). This reflects in greater density, whereas the estimated DFT cohesive energies of the two forms are similar. DTC has enough molecular flexibility to access various favourable arrangements during the nucleation, as the interconversion between the A and B conformers in the gas phase takes place with a very small activation energy. The possible role of the solvent in favouring either of the two observed conformations is discussed.

Closed-shell SCF MO calculations and geometry optimization were performed with the STO-3G, 3-21G and 6-31 G* basis sets for ten molecules containing the NCl group: ClNC, ClNO, ClNO2, ClNCO, ClNNN, Cl3N, Cl2NH, ClNH2, ClNCH2 and ClNCH(NH2). The hydrogenation energies were evaluated and qualitatively correlated with the RNCl bond angle and the Cl gross atomic population. For the molecules considered here, the NCl bond is rather weak and its stability - larger for longer than for shorter NCl distances - increases as the RNCl angle narrows. There appears to be no significant double-bond character for the shortest NCl bonds.

Extended Huckel calculations were performed for adsorption and dissociation of N2 on different planes of iron and rhenium single crystals. The supermolecular model is used throughout. The results suggest the order of reactivity 111 > 100 > 110 for the iron crystals, and 0001 1 0 11 2 0 01 1 0 for rhenium Different least-energy reaction paths for N2 dissociation are suggested for the two metals.

An analysis of TPD data relating to the desorption of ammonia from two different samples of partially decationated Y-zeolite is reported, assuming various chemical or physical steps to be rate-determining. The process is shown to be controlled by intracrystalline surface diffusion, with values of the apparent activation energy E a = 30.8 and 27.1 kcal/mol, respectively, for the two zeolite samples. These values include the retardation effect associated with the hindering interaction of the diffusing ammonia molecules with the acid sites of the zeolite. Values of the apparent effective diffusion coefficient ranging from ca. 3 10 −18 to ca. 4 10 −14 cm 2 /s have been calculated for the 423–573 K temperature range.

2,3-Diamino-1,3-butadienes ( 1 ) reacted with β-nitrostyiene or nitroethene to afford a mixture ofisomeric 1,5-diaminoo-5-methyl-4-nitro-cyclopentenes ( 3 and 4 ). The configuration of the main isomers ( 3 ) was established by X-ray diffraction analysis which also revealed the great steric crowding of this molecule. The conformation of the cyclopentene ring in 3 in the solid state is also discussed. The enamines ( 3 and 4 ) were hydrolyzed to the corresponding aminoketones ( 5 and 6 ) which, on reduction, gave the same diaminoketone ( 7 ).

Fox six bridged diimino [14] annulenes with the two bridging N atoms connected by aliphatic chains (molecules 1–6 ), isodesmic alkyl separation enthalpies have been calculated in the MINDO/3 approximation, taking syn -1,6 : 8,13- bis (methylimino) [14] annulene as the reference compound. Calculated geometries for the annulenes are in satisfactory agreement with X-ray diffraction results, already known for molecules, 1 , 2 and 3 , and presented here for the first time for molecules 4 , 5 and 6 . Conformational features and their main trends as n (the number of methylene fragments in the aliphatic chain) increases from 1 to 6, are discussed both on the basis of the experimental geometries and in terms of the semiempirical results for the isodesmic reactions. The corresponding Δ H r values show a well-defined correlation both with the dihedral angles at the anthracene skeleton and the degree of pyramidalization of the nitrogen bridging atoms. The greater “softness” of the latter with respect to the “stiffness” of methine groups, which can replace the N atoms in the corresponding isoelectronic annulenes, has been quantitatively evaluated by ab initio 6-31G * calculations on four simple molecules (NH 3 , CH 4 ,NH 2 CH 2 NH 2 , and CH 3 CH 2  CH 3 ).

Current interest on the chemisorption of aromatics on metal surfaces has both theoretical and applied aspects. No determination of adsorption energies

A quantitative description of conjugative interactions between the phenyl rings and the butenyne fragment in phenyl-substituted vinylacetylenes is presented in terms of the properties of the electronic charge distribution at the critical points of the CC bonds. The structure of one of the two molecules investigated has been determined by X-ray single-crystal analysis as part of this study, while the other was apparently the only one available from the literature for phenyl-substituted vinylacetylenes. The study includes a topological analysis of the unsubstituted parent compound, whose CC bond ellipticities show unchanged values when calculated for three different models of the molecular geometry.

The chemisorption of acetylene on various faces of bcc tungsten, modelled by clusters of various sizes, has been studied by the Extended Huckel method. Relative binding energies on the various sites have been obtained, and the bonding modes of acetylene are discussed, also in comparison with experiment. It is concluded that, whenever possible, acetylene is di-σ bonded to the surface, and sp2hybridization is considered likely in all cases.

Absorption FTIR spectra have been recorded for solutions and solid phases of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. The crystal structures of the two compounds have been established by XRD and compared with those of other bis(dialkylamino) analogues already studied by the same technique. Ab initio calculations for the squaraine led to a 6–31G** geometry substantially similar to the experimental one, indicating that crystal packing only marginally perturbs the equilibrium configuration of the isolated molecule. In contrast, the geometry of the 3-cyclobutene-1,2-dione derivative shows a degree of planarity considerably greater in the crystal than in the free molecule at the MP2/6–31G* minimum-energy nuclear configuration. The squaraine is predicted to be more stable, at all basis set levels here adopted, than its isomer. Differences and similarities between experimental IR frequencies and their ab initio estimates are presented.

The results of an X-ray diffraction study of 13,13-difluoro-1,7-methano[12]annulene at room temperature and at 125 K are presented and the crystal, molecular, and electronic structure is discussed. Environmental effects on the structure are discussed by an analysis of the influence of temperature, and by comparison with the geometry of a previously studied bridged [12]annulene.

The application of extended-Huckel calculations to comparative predictions of the chemisorption of acetylene and a number of organic fragments on various metals is tested. The reactivity of the adsorbates to chemisorption on different metals is found to be similar. Ni, Rh and Pt catalyse the rearrangement of adsorbed acetylene to vinylidene, but Ni is found to catalyse the reverse reaction also. However, Ni is the best catalyst for carbon–carbon bond-breaking in acetylene. Adsorbate–adsrobate interactions (of the van der Waals type) force the acetylene molecule to tilt out of the plane parallel to the surface on Ni(100). These interactions are scarcely significant on the (111) face of these three metals. Some results for chemisorption on Co(111) and Ir(111) are presented.