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A method for structure solution in thin films that combines grazing incidence X-ray diffraction data analysis and crystal structure prediction was presented in a recent work (Braun et al. Cryst. Growth Des.2019, DOI: 10.1021/acs.cgd.9b00857). Applied to phenytoin form II, which is only detected in films, the approach gave a very reasonable, but not fully confirmed, candidate structure with Z = 4 and Z′ = 2. In the present work, we demonstrate how, by calculating and measuring the crystal Raman spectrum in the low wavenumber energy region with the aim of validating the candidate structure, this can be further refined. In fact, we find it to correspond to a saddle point of the energy landscape of the system, from which a minimum of lower symmetry may be reached. With the new structure, with Z = 4 and Z′ = 2, we finally obtain an excellent agreement between experimental and calculated Raman spectra., Experimental low wavenumber Raman spectroscopy validates the presence of a non-centrosymmetric packing arrangement that is energetically very close to a centrosymmetric saddle point structure.

Films of the chemical precursor (t)Boc-quinacridone obtained by the spin-coating and bar-assisted meniscus shearing methods were subjected to thermal deprotection to recover the organic semiconductor quinacridone in its crystalline form. We found that the final crystal structure of the semiconductor on the Si/SiO2 substrate is in fact determined by the chemical precursor starting structure, which is in turn induced by the deposition method. Indeed, the samples prepared by spin coating display the precursor structure known from the literature, which transforms into the beta-quinacridone phase. The shearing technique instead yields highly homogeneous films composed of a novel (t)Boc-quinacridone polymorph, which acts as a trigger for the subsequent formation of a pure, well oriented alpha-quinacridone phase. Although this crystalline form is the least stable of the many quinacridone polymorphs, here it turns out to be selectively induced and stabilized. Finally, the organic field effect transistor charge mobility of the alpha-quinacridone films was measured., T. S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). This work was supported by the MIUR-PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes towards a bio-inspired materials design” (HARVEST) protocol 201795SBA3. This work was funded by DGI (Spain) with the project GENESIS PID2019-111682RB-I00, the Generalitat de Catalunya (2017-SGR-918), and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S). The authors thank Dr Raphael Pfattner (ICMAB-CSIC) for fruitful discussions.

We propose the use of solid solutions for the modulation of the kinetic in [2+2] photocycloaddition. Solid solutions with general formula [1H]BrxCl1‒x (1 = 4-amino-cinnamic acid; 0 < x < 1) were prepared via mechanochemical methods, structurally characterized, and found to undergo a single-crystal-to-single-crystal [2+2] photodimerization. The effect of the solid solutions composition on the kinetics was successfully investigated via a combination of FTIR and Principal Component Analysis (PCA), and the results were compared with those obtained for the parent materials [1H]Cl and [1H]Br. All systems follow the same nearly first-order law, and the overall reaction kinetics is influenced by a subtle interplay of structural and electronic factors as well as other processes regulating the formation and lifetime of the reacting excimer.

The 3:2 Charge Transfer (CT) co-crystal (Perylene)3(TCNQF1)2 is grown by the Physical Vapor Transport (PVT) method, and characterized structurally and spectroscopically. Infrared analysis of the charge sensitive modes reveals a low degree of charge transfer (less than 0.1) between donor and acceptor molecules. The crystal is isostructural to the other 3:2 CT crystals formed by Perylene with TCNQF2 and TCNQF4, whereas such stoichiometry and packing is not known for the CT crystals with non-fluorinated TCNQ. The analysis of the isostructural family of 3:2 Perylene–TCNQFx (x = 1,2,4) co-crystal put in evidence the role of weak F…HC bonding in stabilizing this type of structure, The work has been supported by the local research funds of Bologna and Parma Universities.

We show that the development of highly accurate density functional theory calculations coupled to low-frequency Raman spectroscopy constitutes a valid method for polymorph characterization alternative/complementary to X-ray. The method is applied here to the temperature-induced, first-order phase transition of coronene, known for a long time, but has remained structurally uncharacterized due to crystal breaking during the process. The astonishing fidelity of the Raman calculated spectra to the experiments allows us to unambiguously identify the low-temperature phase with the β-coronene polymorph, recently reported as new and obtained in the presence of a magnetic field. We also suggest that additional measurements are needed to confirm that a magnetic field can actually drive the growth of a β-polymorph surviving indefinitely at ambient temperature.

The photodimerization of anthracene derivatives constitutes a model system for intermolecular [4 + 4] cycloadditions. In this paper we deal with the elusive 9-anthracene–carboxylic case and study the crystal state of the head-to-tail dimer, obtained by the reaction of the monomer in various solvents both in its unary and solvated forms, by X-ray diffraction and confocal Raman microscopy in the lattice phonon region. A number of solvates have been identified, and their structures have been solved and here presented. The 9-anthracene–carboxylic acid dimer appears to be an exemplary case of a molecular crystal easily prone to host solvent molecules in the interstices of the framework generated by homomolecular hydrogen bonds. Alternatively, the hydrogen bonds between solvent molecules and the carboxylic group may establish supramolecular structures of closely packed architectures. Raman microscopy has also allowed us to investigate the short-lived dimer, which is produced in the crystal-to-crystal photoreaction triggered by the irradiation of the monomer single crystal.

Halogenation of conjugated molecules represents a powerful approach to tune the electronic structure of molecular thin films through inductive effects and long-range intermolecular electrostatic interactions. The mixing of halogenated molecules with their pristine counterparts has recently proven successful in altering the blend’s energy levels to adjust the open-circuit voltage of organic solar cells by the mixing ratio. Here, we show that the prevailing rationale for this effect is not equally valid for different molecular orientations. We provide a comprehensive experimental and theoretical analysis of the prototypical blend formed by pentacene and perfluoropentacene to relate structure with electronic properties. We find a mixed-stack structural motif in standing and lying orientations depending on the substrate nature. In the standing orientation, the ionization potential lies in between the values of the pure components, in line with the established picture of averaged molecular quadrupole moments. For the lying orientation, however, we experimentally observe an ionization potential lower than both pristine values, which seems at odds with this simple rationale. Electrostatic simulations based on the knowledge of the atomistic structure of the films capture the complex experimental scenario for both orientations. In particular, the ultralow ionization potential of films formed by lying molecules is identified as a signature of the monolayer structure, where quadrupolar interactions are responsible for a difference of ca. 0.4 eV in the highest occupied molecular orbital energy as compared to thicker films with the same molecular orientation.

Recently, structural studies in the thin-film phase of 6,6′-dibromoindigo, the main component of the Tyrian purple dye, revealed the presence of surface-induced structures. This finding is important, as the system is a good candidate for applications in organic electronics, and knowledge of the film structure is needed to explain the physical properties of the conducting layer. In this work, combining the results of low-frequency Raman spectroscopy, X-ray diffraction, and accurate density-functional theory (DFT) calculations of the vibrational properties of the proposed surface-induced structures, we suggest that a newly detected 6,6′-dibromoindigo bulk crystal form is structurally closely related to the surface-induced modifications. Thus, 6,6′-dibromoindigo behavior shows similarities and differences at the same time, with respect to other investigated systems., We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C—SCSCDFT). T.S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). The authors thank Prof. Roland Resel for the fruitful discussions. This work has been funded by the PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes toward a bio-inspired materials design” (HARVEST) protocol 201795SBA3.

A careful analysis of the vibrational spectra of two isostructural charge transfer systems with potential applications in optoelectronics, (perylene)3‐(TCNQF4)2, and (perylene)3‐(TCNQF2)2, highlights the presence of three strong infrared bands, polarized along the (perylene‐TCNQFx‐perylene) trimeric units present in the crystal. These bands are recognized as due to the Holstein, or electron–molecular vibration (e–mv) interaction. Use of appropriate modeling, based on the analysis of the frequency shifts due to the e–mv coupling, allows to extract the relevant Holstein coupling constants for the HOMO of the perylene molecule. The so‐obtained experimental values agree very well with density functional calculations, thus validating the results. The perylene relaxation energy, or small polaron binding energy, is about 70 meV, of the same order of magnitude as that of pentacene., The authors thank Prof. Enrico Da Como for useful discussions in the early stages of this work. The work was supported by the local research funds of Bologna and Parma Universities.

Singlet exciton fission is a spin-allowed process in organic semiconductors by which one absorbed photon generates two triplet excitons. Theory predicts that singlet fission is mediated by intermolecular charge-transfer states in solid-state materials with appropriate singlet-triplet energy spacing, but direct evidence for the involvement of such states in the process has not been provided yet. Here, we report on the observation of subpicosecond singlet fission in mixed films of pentacene and perfluoropentacene. By combining transient spectroscopy measurements to nonadiabatic quantum-dynamics simulations, we show that direct excitation in the charge-transfer absorption band of the mixed films leads to the formation of triplet excitons, unambiguously proving that they act as intermediate states in the fission process.

We report on the solution growth of the two known structures of Perylene-F2TCNQ charge transfer complexes. The transformation accompanied by a marked morphological change from needle 1:1 to platelet 3:2 crystal structure is observed in the mother liquor. Lattice phonon Raman spectroscopy is used for an easy structure identification of the different morphologies before and after the process. X-ray and lattice phonons spectra of reference samples obtained by physical vapor transport is used to identify the two complexes. A fully spectroscopic analysis of the intramolecular Raman modes is presented to estimate the degree of ionicity, which is found to agree with the value previously reported.

International audience; Thin films of the natural semiconductor indigo have been grown on Si/SiO2 substrates by physical vapour deposition under ultra-high vacuum conditions, monitoring the influence of the deposition rate on structure, morphology and thickness. This was done to allow well-defined films to be made in a controlled way, suitable for the manufacture of organic devices and the investigation of charge transport phenomena. The films have been characterized by X-ray diffraction, atomic force microscopy, microRaman spectroscopy and transmission electron microscopy, finding that the growth at the solid interface of polymorph B, the only phase present in the films, is governed by the formation of strong multiple hydrogen-bonds between neighbouring molecules, which play a predominant role in the interface organization of the molecular assemblies.

The classic photodimerization reaction of 9-cyanoanthracene in the solid state has been studied by means of Raman microscopy, analyzing the time evolution both of the molecular and of the lattice phonon spectra from the reactant to the product. This has allowed us to verify that the formation of the dimer crystal structure always takes place with some delay with respect to the onset of the chemical reaction, as already observed in another crystal to crystal photoreaction.1 This evidence points to a phenomenon of reconstructive phase transition.2 The kinetic analysis of the spectroscopic data confirms the previous hypothesis of an autocatalytic mechanism for the process.

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics.

Raman microscopy in the lattice phonon region coupled with X-ray diffraction have been used to study the polymorphism in crystals and microcrystals of the organic semiconductor 9,10-diphenylanthracene (DPA) obtained by various methods. While solution grown specimens all display the well-known monoclinic structure widely reported in the literature, by varying the growth conditions two more polymorphs have been obtained, either from the melt or by sublimation. By injecting water as a nonsolvent in a DPA solution, one of the two new polymorphs was predominantly obtained in the shape of microribbons. Lattice energy calculations allow us to assess the relative thermodynamic stability of the polymorphs and verify that the energies of the different phases are very sensitive to the details of the molecular geometry adopted in the solid state. The mobility channels of DPA polymorphs are shortly investigated.

We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-penty1-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (similar to 20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.

We report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab-initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4-tetraphenyl-1,3-butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (Ci) or 2-fold (C2) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of Ci and C2 conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra.

This work assesses the reliability of different van der Waals (vdW) methods to describe lattice vibrations of molecular crystals in the framework of density functional theory (DFT). To accomplish this task, calculated and experimental lattice phonon Raman spectra of a pool of organic molecular crystals are compared. We show that the many-body dispersion (MBD@rsSCS) van der Waals method of Ambrosetti et al. and the pairwise method of Grimme et al. (D3-BJ) outperform the other tested approaches (i.e., the D2 method of Grimme, the TS method of Tkatchenko and Scheffler, and the nonlocal functional vdW-DF-optPBE of Klimeš et al.). For the worse-performing approaches the results could not even be fixed by the introduction of scaling parameters, as commonly used for high-energy intramolecular vibrations. Interestingly, when using the experimentally determined unit cell parameters, DFT calculations using the PBE functional without corrections for long-range vdW interactions provide spectra of similar accuracy as the MBD@rsSCS and D3-BJ simulations.

We revisit the polymorphism of paracetamol by means of a micro-Raman technique, which has proved to be a powerful tool for structure recognition. Distinct lattice phonon spectra clearly identified the pure phases. Confocality enabled us to detect phase mixing between form II and either I or III on a micrometric scale in the same crystallite. Following the most recent findings on surface-mediated structures, we also investigated spin-coated films grown on glass, gold, and polystyrene substrates, confirming the selectivity of these surfaces for the metastable form III, which shows an unprecedented stability over a time span of several months. A mechanism of its transformation to phase II, via a partially ordered intermediate state, is suggested by polarized Raman measurements.

Within the reinvestigation of Ni–Sb carbonyl cluster chemistry, we report here the synthesis and characterization of the new [Ni19Sb4(CO)26]4– cluster and the synthesis, structure, magnetic characterization and electrical resistivity of the viologen salts of the previously known [Ni13Sb2(CO)24]n– (n = 2, 3) anionic species. The crystal structures of [NEt4]4[Ni19Sb4(CO)26], [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 and [EtV]3[Ni13Sb2(CO)24]·1.5THF (EtV = 1,1′-diethyl-4,4′-bipyridilium cation, DMF = N,N-dimethylformamide, THF = tetrahydrofuran) are described. Notably, the unit cell of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 involves a mixture of two [Ni13Sb2(CO)24]3– trianions and one [Ni13Sb2(CO)24]2– dianion, as it also contains eight [EtV]+· radical monocations, which are assembled in infinite stacks. In contrast, the unit cell of the [EtV]3[Ni13Sb2(CO)24]·1.5THF salt contains four [Ni13Sb2(CO)24]3– trianions along with twelve [EtV]+· radical monocations, four of which are arranged into two pairs of isolated dimers, whereas the other two sets of four form two infinite stacks that extend over the whole crystal. The charges of the miscellaneous ions have been assigned on the basis of electroneutrality and spectroscopic evidence. More specifically, the infrared spectra of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14, both in the solid state and in solution, clearly indicate the presence of a 2:1 mixture of [Ni13Sb2(CO)24]3– and [Ni13Sb2(CO)24]2– anions. Resistivity measurements performed on pellets of powdered samples indicate that the [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 salt substantially behaves as an insulator. A study of the magnetic behaviour of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 evidences pairing among the electrons of the EtV+· molecules, in agreement with DFT calculations, and the odd-electron clusters behave as paramagnetic centres of spin S = 1.

We present a Raman investigation of polymorphism in 1,1,4,4-tetraphenyl-butadiene (TPB), a well-known blue luminescent molecule, which retains its emissive properties in the solid state. The use of low-wavenumber (10–150 cm−1) Raman microscopy, very sensitive to the crystal packing, allows us to single and pick up four different polymorphs. X-ray analysis shows that the TPB molecules assume different conformations in the various polymorphs, a fact revealed also by the Raman spectra in the region of intra-molecular vibrations. Lattice dynamics calculations yield a preliminary assignment of the low-wavenumber Raman spectra and provide information of the relative stability of the phases. Copyright © 2013 John Wiley & Sons, Ltd.

Confocal Raman microscopy in the lattice phonon region has been used to study the polymorphism of the organic semiconductor 5,6,11,12-tetraphenyl-tetracene (rubrene). Following literature guidelines, crystals of rubrene have been prepared using a number of solution growth and vapour deposition methods, obtaining samples of different morphologies which could be related to the various polymorphs of this compound. The technique has enabled us an easy and non invasive identification of the three known polymorphs and of their phase homogeneity with a lateral spatial resolution below 1 μm.

We present a working example of phase recognition by lattice phonon Raman spectroscopy, a technique which has proved to be a powerful tool for identifying new polymorphs. The spectra show that a newly resolved triclinic structure of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF) is one of its four polymorphic modifications, labeled the δ-phase, which was previously identified only from spectroscopic data. The now available XRD data allow for the computation of the minimum potential energy structure. Calculated density and energy of the triclinic polymorph are found to be very close to the previously identified β polymorph. These similarities account for the often encountered co-existence of the β and δ crystal phases.

A multiscale investigation of N,N-bis(n-octyl)-x:y, dicyanoperylene-3,4:9,10- bis(dicarboximide), PDI8-CN2, shows the same molecular arrangement in the bulk and in thin films sublimated on SiO2/Si wafers. Non-conventional powder diffraction methods and theoretical calculations concur to provide a coherent picture of the crystalline structure. X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses of films of different thickness depos- ited at different substrate temperatures indicate the existence of two temper- ature-dependent deposition regimes: a low-temperature (room temperature) regime and a high-temperature (80­120 °C) one, each characterized by dif- ferent growth mechanisms. These mechanisms eventually result in different morphological and structural features of the films, which appear to be highly correlated with the trend of the electrical parameters that are measured in PDI8-CN2-based field-effect transistors.

Absorption, photoluminescence, and Raman scattering measurements have been performed on a fourfold alkoxy-substituted phthalocyanine in solution and in the discotic liquid crystal phase. As already known for this class of compounds, the features of the electronic spectra in the condensed state are determined by the spatial closeness and arrangement of the systems of the phthalocyanine cores, with the occurrence of collectively excited states and excimer-like emission. Quantitative information about the alignment of the molecules which compose the liquid crystal phase has been obtained from the analysis of polarized Raman spectra of homeotropically aligned samples.

The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed., Following on from the success of the previous crystal structure prediction blind tests (CSP1999, CSP2001, CSP2004 and CSP2007), a fifth such collaborative project (CSP2010) was organized at the Cambridge Crystallographic Data Centre. A range of methodologies was used by the participating groups in order to evaluate the ability of the current computational methods to predict the crystal structures of the six organic molecules chosen as targets for this blind test. The first four targets, two rigid molecules, one semi-flexible molecule and a 1:1 salt, matched the criteria for the targets from CSP2007, while the last two targets belonged to two new challenging categories – a larger, much more flexible molecule and a hydrate with more than one polymorph. Each group submitted three predictions for each target it attempted. There was at least one successful prediction for each target, and two groups were able to successfully predict the structure of the large flexible molecule as their first place submission. The results show that while not as many groups successfully predicted the structures of the three smallest molecules as in CSP2007, there is now evidence that methodologies such as dispersion-corrected density functional theory (DFT-D) are able to reliably do so. The results also highlight the many challenges posed by more complex systems and show that there are still issues to be overcome.

We report a combined spectroscopic and structural study of polymorphism of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) blue luminescent crystals. We have identified two polymorphs, which can be grown by crystallization of the vapor. The first, α-TPB, has the monoclinic noncentrosymmetric structure (P2 1, two molecules per unit cell), whose lattice parameters have already been reported in the literature. The second, β-TPB, is a new polymorph, monoclinic centrosymmetric (P21/c, four molecules per unit cell). The previously reported triclinic structure, γ-TPB, has not been identified in samples grown by the physical vapor transport method. The effects of the different molecular configuration and packing of α- and β-TPB on Raman scattering, IR, and UV-vis optical properties are discussed.

Car-Parrinello molecular dynamics (CPMD) calculations are presented for a Na (+)(Phe) complex in aqueous solution and for various stable Na (+)(Phe) complexes and Na (+)(H 2O) n clusters in the gas phase (with up to six water molecules). The CPMD results are compared to available experimental and ab initio reference data, to DFT results obtained with various combinations of density functionals and basis sets, and to previous classical mechanics MD simulations. The agreement with the reference data in the gas phase validates the CPMD method, showing that it is a valid approach for studying these systems and that it describes correctly the competing Na (+)-Phe and Na (+)-H 2O interactions. Analysis of MD trajectories reveals that the Na (+)(Phe) complex in aqueous solution maintains a stable configuration in which the Na (+) cation hovers above the phenyl ring, at an average distance of 3.85 A from the ring center, while remaining strongly bound to one of the carboxylic oxygens of Phe. Constrained MD simulations indicate that the free energy barrier opposing dissociation of the complex exceeds 5.5 kcal/mol. We thus confirm that "cation- pi" interactions between alcali cations and the pi ring, combined with other kinds of interactions, may allow aromatic amino acids to overcome the competition with water in binding a cation.

Using sexithiophene as a benchmark compound, we present a very effective strategy for searching the potential energy minima of a crystalline material, described in terms of rigid molecules with Coulombic and atom-atom interactions. The strategy involves uniform sampling of the many-body energy hypersurface, mechanical identification of all constraints deriving from the crystallographic symmetry, and a "sight-resight" method, originally introduced in wildlife ecology, for assessing the completeness of the search. Thousands of distinct potential energy minima, with a surprising variety of structural arrangements, are identified for sexithiophene. Despite the large number of competing minima, the system presents a small number of deep minima, with very different structures and not particularly congested in energy or density. The two deepest minima correspond to the structures of the two known experimental polymorphs, which are satisfactorily described.

International audience; For molecules with C-H bonds, a local mode description of the bonds is often adopted to interpret the C-H stretching overtone spectra, although a representation in terms of normal mode is also possible. Equivalence between the two alternative representations demands simple numerical equations between the normal mode anharmonicity constants xrs and the quartic anharmonic coupling constants Krstu , the so-called x-K relations. The relations appropriate to low symmetry molecules with two non-equivalent C-H bonds (ν1 , ν2), such as XHC=CHY, are derived in this paper. The consequences of these relations are illustrated with a detailed analysis of the C-H stretching overtone region, where the naive application of the relations obtained for more symmetric molecules could lead to a different interpretation of the overtones 2ν1 , 2ν2 and of the combination band ν1 + ν2 . The relations are tested with theoretical calculations for difluoro- and chlorofluoro-substituted ethenes, propadiene, and butatrienes.

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.

Raman phonon spectra of 9,10-dimethylanthracene crystals have been measured as a function of pressure in the range 0–10 GPa for both lattice and intramolecular modes. Quasi-harmonic lattice dynamics calculations have been used to analyze the effects of the vibrational degrees of freedom on the pressure dependence of the crystallographic structure and phonon wave numbers. We find that the coupling between lattice and intramolecular vibrations contributes significantly to the pressure dependence of the phonon spectra, which is accurately reproduced by the calculations. Vibrational effects on the structure, instead, are less significant.

Raman phonon spectra of tetrathiafulvalene (TTF) crystals in their monoclinic (α-TTF) and triclinic (β-TTF) polymorphic modifications have been recorded as a function of temperature in the 30 to 130 cm−1 range. Along with other model potentials, the set of parameters already adopted for the study of ionic and neutral forms of bis-ethylen-dithio-tetrathiafulvalene (BEDT-TTF) has been used to compute, within a quasi-harmonic lattice dynamics framework, the crystallographic structures and the phonon frequencies of the two polymorphs as a function of temperature with good results. Introduction of the internal vibrational degrees of freedom of the TTF molecules in the crystals improves the computational approach and proves to be a valuable tool to judge the importance of the coupling between lattice phonons and intramolecular vibrations.

The crystal structure and lattice phonons of the $(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}$ superconducting $\ensuremath{\beta}$ phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the quasiharmonic lattice dynamics (QHLD) method. The empirical atom-atom potential is that successfully employed for neutral BEDT-TTF and for nonsuperconducting $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}.$ The rigid molecule approximation has to be relaxed by allowing mixing between lattice and low-frequency intramolecular vibrations. Such a mixing is found to be essential to account for the specific heat measurements, and also yields good agreement with the observed Raman and infrared frequencies. The crystal structure and its temperature and pressure dependence are also properly reproduced, though the effect of the mixing is less important in this case. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The charge transfer integrals are evaluated by the extended H\"uckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function ${\ensuremath{\alpha}}^{2}(\ensuremath{\omega})F(\ensuremath{\omega}),$ which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling constant $\ensuremath{\lambda}$ is found to be $\ensuremath{\sim}0.4.$

An effective one-dimensional Hamiltonian has been determined for the NH 2 inversion motion in aniline. The anharmonic potential is represented by a quartic polynomial, fitted to the experimental vibrational transitions in the electronic states S 0 and S 1 . In the S 0 state the barrier height is about 580 cm −1 and the equilibrium angle between the NH 2 and the ring planes is 44°, while in the S 1 state the potential well is extremely flat without a well-defined equilibrium angle. These results are compared with those obtained by ab initio methods. The long-standing discrepancy between the rotational and vibrational results is partially clarified.

Using a model potential function we have performed a molecular dynamics simulation of several static and dynamical properties of glycerol in the crystal, glass and liquid phases. Comparison with available experimental data shows an excellent agreeent and proves the validity of the potential model used. For the calculation of the molar specific heat of the liquid and of the glass we have developed a theoretical approach which takes into account the contributions of the conformational structure energy and of the vibrational energy computed using the Bose–Einstein statistics.

We present a quasi harmonic lattice dynamics calculation for the crystal structure and the phonon frequencies of orthorhombic and monoclinic benzene as a function of pressure and temperature. We employ a site-site potential model including Coulombic interactions, which were found necessary to obtain the correct low pressure phase. The calculations agree with the measurements in both phases and reproduce very well the experimental phase diagram. When applicable, quasi harmonic lattice dynamics is an accurate and extremely efficient method, which promises to outperform molecular dynamics and Monte Carlo simulations.

We choose naphthalene as a model system and perform a quasi-harmonic lattice dynamics calculation. The entropic and zero-point effects due to the lattice phonons are investigated and some commonly used potential models are tested. We find that quite reliable predictions of the crystallographic structure and the phonon frequencies of both temperature and pressure are feasible.

The photochemical reaction of 9,10-dinitroanthracene (DNO(2)A) to anthraquinone (AQ) + 2NO has been studied by means of lattice phonon Raman spectroscopy in the spectral region 10-150 cm(-1). In fact, crystal-to-crystal transformations are best revealed by following changes in the lattice modes, as even small modifications in the crystal structure lead to dramatic changes in symmetry and selection rules of vibrational modes. While analysis of the lattice modes allowed for the study of the physical changes, the chemical transformation was monitored by measuring the intramolecular Raman-active modes of both reactant and product. On the basis of the experimental data it has been possible, at a microscopic level, to infer crucial information on the reaction mechanism by simultaneously detecting molecular (vibrational modes) and crystal structure (lattice phonons) modifications during the reaction. At a macroscopic level we have detected an intriguing relationship between incident photons and mechanical strain, which manifests itself as a striking bending and unfolding of the specimens under irradiation. To clarify the mechanisms underlying the relationship between incoming light and molecular environment, we have extended the study to high pressure up to 2 GPa. It has been found that above 1 GPa the photoreaction becomes inhibited. The solid-state transformation has also been theoretically modeled, thus identifying the reaction pathway along which the DNO(2)A crystal lattice deforms to finally become the crystal lattice of the AQ product.

We have used confocal micro Raman spectroscopy, atomic force microscopy (AFM), and x-ray diffraction (XRD) to investigate pentacene films obtained by vacuum deposition on SiO${}_{2}$ substrates. These methods allow us to follow the evolution of lattice structure, vibrational dynamics, and crystal morphology during the growth from monolayer, to TF, and, finally, to bulk crystal. The Raman measurements, supported by the AFM and XRD data, indicate that the film morphology depends on the deposition rate. High deposition rates yield two-dimensional nucleation and quasi-layer-by-layer growth of the T-F form only. Low rates yield three-dimensional nucleation and growth, with phase mixing occurring in sufficiently thick films, where the T-F form is accompanied by the ``high-temperature'' bulk phase. Our general findings are consistent with those of previous work. However, the Raman measurements, supported by lattice dynamics calculations, provide additional insight into the nature of the TFs, showing that their characteristic spectra originate from a loss of dynamical correlation between adjacent layers.

We have performed a molecular dynamics calculation of the vibrational spectrum of glassy silica. The simulated spectra compare favourably with the neutron scattering experiments, in both frequency and momentum space. A detailed analysis of one-body and two-body correlation functions yields an assignment of the spectrum which exhibits a high degree of internal consistency and agrees with the current understanding of the experimental data.

The paper reports a complete analysis of the phonon structure of crystalline picene, a recently announced organic semiconductor. Both lattice and intramolecular vibrations are investigated. An exhaustive assignment of lattice phonons is obtained through polarized Raman spectra assisted by lattice dynamics calculations based on a well tested atom–atom potential model. Raman, infrared spectra and density functional (DFT) calculations are used for the characterization of intramolecular modes. Coupling between low-frequency molecular vibrations and lattice phonons is accounted for. Molecule-to-molecule transfer integrals, as well as the Peierls and Holstein (non-local and local) coupling constants, are evaluated through the semiempirical method INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization).

The computational protocol we have developed for the calculation of local (Holstein) and non-local (Peierls) carrier-phonon coupling in molecular organic semiconductors is applied to both the low temperature and high temperature bulk crystalline phases of pentacene. The electronic structure is calculated by the semimpirical INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization) method. In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intra-molecular modes with inter-molecular (lattice) phonons. A clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency intra-molecular phonons, which modulate the on-site energy (Holstein coupling). The results of calculation agree well with the values extracted from experiment. The comparison with similar calculations made for rubrene allows us to discuss the implications for the current models of mobility.

The thermochemical constants for the oxidation of tyrosine and tryptophan through proton coupled electron transfer in aqueous solution have been computed applying a recently developed density functional theory (DFT) based molecular dynamics method for reversible elimination of protons and electrons. This method enables us to estimate the solvation free energy of a proton (H+ ) in a periodic model system from the free energy for the deprotonation of an aqueous hydronium ion (H3 O+ ). Using the computed solvation free energy of H+ as reference, the deprotonation and oxidation free energies of an aqueous species can be converted to pK a and normal hydrogen electrode (NHE) potentials. This conversion requires certain thermochemical corrections which were first presented in a similar study of the oxidation of hydrobenzoquinone [J. Cheng, M. Sulpizi, and M. Sprik, J. Chem. Phys. 131, 154504 (2009)]. Taking a different view of the thermodynamic status of the hydronium ion, these thermochemical corrections are revised in the present work. The key difference with the previous scheme is that the hydronium is now treated as an intermediate in the transfer of the proton from solution to the gas-phase. The accuracy of the method is assessed by a detailed comparison of the computed pK a , NHE potentials and dehydrogenation free energies to experiment. As a further application of the technique, we have analyzed the role of the solvent in the oxidation of tyrosine by the tryptophan radical. The free energy change computed for this hydrogen atom transfer reaction is very similar to the gas-phase value, in agreement with experiment. The molecular dynamics results however, show that the minimal solvent effect on the reaction free energy is accompanied by a significant reorganization of the solvent. © 2011 American Institute of Physics.

The nature of the interaction of a series of α-oligothiophenes (1T up to 6T) with a Si(001) surface was studied by means of density functional theory (DFT) structural optimizations. For 1T, we foun...