Amparo Navarro

Measurement and ab initio modeling of the inelastic neutron scattering of solid melamine: Evidence of the anisotropy in the external modes spectrum

Abstract The inelastic neutron scattering spectrum of melamine has been measured and a normal coordinates analysis has been performed in order to interpret the vibrational dynamics. This study reveals the anisotropy in the external mode spectrum and its important role in the internal modes region. Thus, the Debye–Waller factor has taken a value for the out-of-plane vibrations four times greater than that for the in-plane vibrations. A molecular force field refinement has been carried out in independent symmetry coordinates (D 3h ) in order to confirm the vibrational assignments. The final force field is free of redundancies and therefore the corresponding force constants are unambiguous.

Density functional theory study of the optical and electronic properties of oligomers based on phenyl-ethynyl units linked to triazole, thiadiazole, and oxadiazole rings to be used in molecular electronics

In the present work, we have studied from a theoretical perspective the geometry and electronic properties of the series of related compounds 2,5-bis(phenylethynyl)-1,3,4-thiadiazole, 2,5-bis(phenylethynyl)-1,3,4-oxadiazole, and 2,5-bis(phenylethynyl)-1,2,4-triazole as candidates for electron-conducting polymers and compounds with desirable (opto)electronic properties. The effect of the ethynyl group (-C[Triple Bond]C-) on the structure and electronic properties was also studied. The influence of planarity on electrical conductivity has been studied by a natural-bond-orbital analysis. The (opto)electronic properties and conducting capability were investigated through the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, excitation energy, bond length alternation, LUMO energy, electron affinities, and intramolecular reorganization energy. Finally, the evolution of some properties such as optical bandgap and electron affinity with the increase of the number of repeat units in the oligomer chain has been checked.

Theoretical study of the effect of ethynyl group on the structure and electrical properties of phenyl-thiadiazole systems as precursors of electron-conducting materials

2,5-Bis(phenylethynyl)-1,3,4-thiadiazole (PhEtTh) and 2,5-diphenyl-1,3,4-thiadiazole (PhTh) are expected to be building blocks for polymer materials that could be employed to conduct electricity due to their narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. In this work, a theoretical, comparative study about the effect of the ethynyl group on the planarity and electrical conductivity of this kind of systems has been carried out. Thus, several ab initio (Hartree-Fock, Moller-Plesset) and DFT (B3LYP, B3PW91, M05, M05-2X) methods and basis sets (6-31G(*), 6-31G+G(**), 6-311G(**), cc-pVDZ, cc-pVTZ) have been tested. As a result, PhEtTh showed better properties for its use as electric conducting material relative to PhTh due to its smaller HOMO-LUMO gap, as well as its enhanced trend to retain the planarity provided the reduction in steric hindrances that the ethynyl group (-C[triple bond]C-) permits. Solvent effects were also modeled for ethanol and chloroform under the conductor-like polarizable continuum model approximation. Finally, electronic transitions in gas and solution phases were predicted by using TDDFT approximation in order to compare the theoretical lambda(max) with the experimental values reported in literature for both compounds.

Vibrational analysis of the inelastic neutron scattering spectrum of s-triazine and trichloro-s-triazine

Abstract The inelastics neutron scattering (INS) spectra of s-triazine and trichloro-s-triazine at 5 K provide the first observation of the IR and Raman inactive modes, v 4 and v 5 . A full assingment is made by fitting a harmonic force field to the INS spectra profile. In the case of trichloro-s-triazine we demonstrate that this approach can be used successfully for a molecule without H atoms.

The vibrational analysis of styrene, revisited

Abstract In this report we present a new proposal of vibrational analysis for the styrene molecule on the basis of an a priori scaled force field and a comparison between calculated infrared intensities and experimental absorbances. This has been done in order to clarify some discrepancies appearing in previous assignments for ν27,ν31,ν32,ν34 and ν38 modes. As experimental data we have used new IR and Raman spectra recorded at room temperature. The force field has been built up using as scale factors the arithmetic mean of those obtained for 3-fluoro, 4-fluoro, 3-chloro and 4-chlorostyrene. The root-mean-square deviation (rms) between experimental and calculated wave numbers turns out to be 6.7 cm−1, which demonstrates the reliability of the methodology used.

Vibrational analysis of the inelastic neutron scattering spectrum of pyridine

Abstract A vibrational analysis of the pyridine molecule has been performed by combining inelastic neutron scattering (INS) data and quantum mechanical calculations at the RHF, MP2 and B3LYP levels, with the 6-31G** and 6-311G** basis sets. Firstly, in order to test which level of theory is the best in reproducing the INS profile, this was calculated from the atomic displacement matrix of the pyridine molecule. DFT proved to be the most suitable option reproducing the above-mentioned spectrum, for both wave numbers and intensities. Secondly, in order to approximate the calculated spectrum to the observed one, the initial force constants matrix calculated at the B3LYP/6-311G** level was symmetrized using a set of independent symmetry coordinates (C2v). All the diagonal and some off-diagonal force constants were fitted until the difference between observed and calculated spectra was minimized. Good agreement between both calculated and experimental INS spectra supports the validity of our ‘empirical’ (or effective) force field.

Influence of the alkyl and alkoxy side chains on the electronic structure and charge-transport properties of polythiophene derivatives.

Density Functional Theory has been used to study the structural, electronic and charge-transport properties of two regio-regular head-to-tail polythiophene derivatives, i.e. poly(3-hexyl-thiophene), P3HT, and poly(3-oxyhexyl-thiophene), P3OHT. The effect of substituents on the electronic structure was analyzed by means of bandwidth, bandgap, effective mass, total and partial densities of states and crystal orbital overlap populations. Electronic couplings were estimated from band diagrams as the splitting of the valence band. The neutral and cationic states of isolated oligomers were optimized using the supercell approximation. The hole-transfer rates and mobilities were evaluated according to Marcuss theory. Results provide a compelling illustration of the effect of side chains on the crystal packing, electronic structure and charge-transport properties. Thus, the hole mobility calculated for the alkyl derivative was 0.15 cm(2) V(-1) s(-1) (experimental mobility is 0.10 cm(2) V(-1) s(-1)), while the alkoxy derivative has a theoretical mobility of 0.49 cm(2) V(-1) s(-1). The obtained results hopefully could motivate experimentalists to try out P3OHT for an improved charge carrier mobility.

The Role of Linear Alkyl and Alkoxy Side Chains in the Modulation of the Structure and Electrical Properties of Bithiophene:a Theoretical Study

In the present work, we have studied from a theoretical perspective the geometry and electronic properties of 2,2′-bithiophene (BT) and its derivatives 3,4′-alkyl-2,2′-bithiophene (3,4′ABT) and 3,4′-alkoxy-2,2′-bithiophene (3,4′OABT). Properties such as planarity, bond lengths, electron density, highest occupied molecular orbital → lowest unoccupied molecular orbital (HOMO → LUMO) excitation energy and π-delocalization energy, which are related to the electrical conductivity, were calculated and analyzed as a function of both the nature and length of the substituent. The oxidation process was also studied for the single-polaronic state. The ionization potential and the intramolecular reorganization energy were calculated following the semiclassical Marcus treatment. As a conclusion, the introduction of alkoxy chains in 3,4′-positions improves the electrical properties with respect to the bithiophene molecule and the corresponding alkyl derivatives.

Refined, scaled and canonical force fields for the cis- and trans-3-fluorostyrene conformers. An interplay between theoretical calculations, IR/Raman and INS data

We have performed a force field calculation for 3-fluorostyrene following the refinement and the scaling methodologies, starting in both cases from the force constants matrix calculated at the B3LYP/6-311++G** level. To do this, the structures of the cis- and trans- conformers were first obtained at different levels of theory. While HF and MP2 predict non-planar structures, only DFT calculations yield a planar one for both conformers, in agreement with experimental data. The final rms deviations for the calculated wavenumbers are 0.46 cm−1 and 0.23 cm−1, in the case of the refinement method, and 5.01 cm−1 in the case of the scaling method, for the cis- and trans- conformers, respectively. The refined force constants obtained in terms of independent symmetry coordinates have been expressed in terms of simple valence internal coordinates in the unambiguous canonical form for comparison. The refined values for the C–C stretching force constants suggest that the vinyl C–C bond is similar in strength to ethene while the bond connecting the vinyl moiety to the benzene ring is similar in strength to those of the benzene C–C bonds. As to the scaling procedure, the values obtained for each force constant match the characteristic value for a C–C single bond. All these results agree well with those from an analysis of the barriers to internal rotation and also geometric data.

Theoretical estimation of the optical bandgap in a series of poly(aryl-ethynylene)s: A DFT study

Aimed to optimize the ratio accuracy/computational cost, in this work we study the performance of three different theoretical methodologies in the calculation of the optical bandgap for a test set made of a number of poly(aryl-ethynylene)s related polymers. Infinite, ideal polymer chains were first optimized by means of periodic calculations. Different length oligomers were afterward generated by direct replication of the corresponding periodic structure and their optical bandgaps were calculated by means of different time dependent-density functional theory (TD-DFT) methodologies. These results were fitted to an exponential function for each oligomer family in order to get a theoretical estimation of the optical bandgap for each polymer to be compared to the experimental reported values. The best result was obtained for TD-M06-2X yielding an average deviation of 3.4% with respect to the experimental values.