Mauricio Alcolea Palafox

Scaling factors for the prediction of vibrational spectra. I. Benzene molecule

The performance of semiempirical, ab initio, and density functional methods in calculating and describing the vibrational frequencies of benzene was determined. Different levels were used. The modes were characterized by the magnitude and direction of the displacement vector. The error in the calculated frequencies was reduced using two procedures to obtain the scaled frequencies. Scaling equations were determined for each theoretical method. Specific scale factors were calculated to reduce the error in the ring modes of benzene derivatives.

Quantum chemical predictions of the vibrational spectra of polyatomic molecules. The uracil molecule and two derivatives

This work describes the different scaling procedures used to correct the quantum-chemical theoretical predictions of the IR and Raman vibrational wavenumbers. Examples of each case are shown, with special attention to the uracil molecule and some derivatives. The results obtained with different semiempirical and ab initio methods, and basis sets, are compared and discussed. A comprehensive compendium of the main scale factors and scaling equations available to obtain the scaled wavenumbers is also shown.

The hydration effect on the uracil frequencies: an experimental and quantum chemical study

This work describes the performance of different quantum chemical theoretical methods in calculating the vibrational frequencies of uracil and some derivatives, and the effect of hydration on the uracil frequencies. The Raman spectra of polycristalline uracil with different water contents are discussed. To correct the deficiency of the theoretical quantum chemical methods, several procedures are described in detail. Two of them are new. For these new procedures, scaling factors and scaling equations were determined at different levels. With them, a significant reduction in the error of the predicted frequencies was obtained over the one-factor scaling standard procedure. A comparison of the cost/effective method and procedure of scaling was carried out on uracil molecule. Scale factors transferred from uracil to related molecules provided an a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments.

Density functional theory calculations and vibrational spectral analysis of 3,5-(dinitrobenzoic acid)

The FT-IR and Raman spectra of 3,5-dinitrobenzoic acid (DNBA) have been recorded and analyzed. The equilibrium geometry, various bonding and harmonic vibrational wavenumbers have been calculated with the help of density functional theory (DFT) method. Most of the vibrational modes are observed in the expected range. Mulliken population analysis shows the interactions C-N-O⋯H-C and C-O⋯H-C. The most possible interaction is explained using natural bond orbital (NBO) analysis. The strengthening and polarization of the CO bond increases due to the degree of conjugation. HOMO-LUMO energy and the thermodynamic properties are also evaluated.

Femtosecond laser reshaping yields gold nanorods with ultranarrow surface plasmon resonances

Laser-shaping nanoparticles For many applications of the plasmon resonances of metal nanoparticles, it is necessary to have narrow resonance lines. However, most methods for synthesizing nanoparticles create a distribution of sizes and shapes that broaden the resonance lines. González-Rubio et al. annealed gold nanorods dispersed in an aqueous solution of a surfactant with carefully tuned ultrafast (femtosecond) laser pulses. This approach reshaped the nanoparticles to create a near-uniform distribution with resonance lines nearly as sharp as for a single nanorod. Science, this issue p. 640 Heating nanoparticles with femtosecond laser pulses can reshape them and sharpen their plasmon resonances. The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.

On the Connection between the Complexation and Aggregation Thermodynamics of Oxyethylene Nonionic Surfactants

Density and sound velocity data for aqueous solutions containing nonionic surfactants of the homologue series of polyoxyethylene(n) nonyl phenyl ethers (NPEn, n = 5 and 40) were analyzed in the absence and presence of beta-cyclodextrin (beta-CD) at 298 K. Thus, the critical micelle concentration of the surfactants and their apparent and partial molar volumes and compressibilities were measured. From a pseudophase separation model, the partial molar volumes and compressibilities of both pure surfactants in the micelle state and those of NPE40 in the monomer phase have been determined directly. For the ternary systems, increases of the molar volumes and compressibilities of NPE5 and NPE40 at infinite dilution and shifts of the cmc were observed compared to the binary systems. Luminescent measurements of the complexation process between NPE40 and beta-CD showed 1:1 + 1:2 (NPEn/2 beta-CD) stoichiometries for the complexes, with thermodynamic equilibrium constants that were in good agreement with previous results for NPE5 in the presence of beta-CD. This resemblance allowed us to use these results to indirectly determine the molar partial properties of NPE5 in the monomer state and understand the changes in the thermodynamic properties of NPE5 due to aggregation. From the aggregation and complexation data, a folding of the surfactants at the monomer state, in which the hydrophobic moieties of NPEn are surrounded by the EO chain, has been found. The oxyethylene group contributions at the monomer and micelle state of the NPEn homologue series have been estimated. The values of the transfer thermodynamic properties of both surfactants and beta-CD at infinite dilution conditions have been discussed in terms of a new extended model, in which the balance between the released water from the cavities of two beta-CDs and the different hydrophobic moieties of the surfactant that enter the macrocycle was considered.

Vibrational spectral investigations and density functional theory study of 4-Formylbenzoic acid.

An accurate assignment of the vibrational spectra of 4-Formylbenzoic acid in solid state was carried out. For this purpose density functional calculations (DFT) were performed to clarify wavenumber assignments of the experimentally observed bands for the first time. A scaling of the wavenumbers was carried out to improve the calculated values. Good reproduction of the experimental wavenumbers is obtained and the % error is very small in the majority of cases. The NBO analysis was carried out, and it reveals hyper conjugative interaction, ICT and stabilization of molecules.

Interpretation of the vibrational spectra of chloramide, H2NCl, on the basis of post-Hartree-Fock force field

Abstract The ab initio RHF and MP2 force fields were calculated for chloramide with the use of the 6-31G∗, 6-31G∗∗ and 6-311G∗∗ basis sets. The ab initio force fields obtained were scaled by fitting to the available experimental vibrational frequencies for the H2NCl, H2N37Cl and HDNCl species. This makes it possible to extend the set of transferable scale factors we recommended earlier. Minimal differences were observed between the scaled MP2 and RHF force fields, although the inclusion of electron correlation improved the ab initio data and showed better agreement with the observed frequencies obtained by scaling. The effect produced by empirical anharmonicity corrections to the observed NH(ND) stretching frequencies on the results of the fitting was tested. The experimentally observed satellite band at 1032 cm−1 in the infrared spectrum of the parent substance was identified, and it was attributed to the presence of HNCl2 in the sample, in contrast to previous assignments to vibrations of H2NCl or any of its isotopomers.

A reinvestigation of the molecular structure of dimethyl-N-nitramine by gas electron diffraction, ab initio calculations of the molecular geometry and the force field and vibrational spectra

Abstract The molecular structure of dimethyl- N -nitramine was reinvestigated with gas-phase electron diffraction (ED) and ab initio calculations. Ab initio calculations using different basis sets and HF, MP2 and DFT all predict a molecule with C s symmetry and a pyramidal amine N bond configuration. The vibrational spectra were interpreted from the scaling of the harmonic force field, and vibrational amplitudes required for the ED analysis were calculated from this scaled force field. The following values ( r g bond lengths in A and ∠ α angles in degrees with errors equal to three standard deviations) were found for the main parameters: r (N–O)=1.232(3), r (N–N)=1.387(3), r (N–C)=1.466(3), r (C–H) ave =1.114(9), ∠CCN=116.1(6), ∠CNC=122.4(27), ∠ONO=127.6(12), ∠NCH ave =109.9(18). The sum of the bond angles around the amine N atom is 354.6(28)°. The geometrical parameters obtained from the ED analysis are in agreement with the ab initio calculations except that a more pyramidal amine N bond configuration is predicted by ab initio.

Substituent Effect of Chiraldiphenyl Salen Metal (M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II)) Complexes for New Conceptual DSSC Dyes

The authors have designed and synthesized new chiral salen-type metal (M = Fe, Co, Ni, Cu, Zn) complexes (1-5) for new conceptual dyes (co-sensitizer or colorful multi-dyes) of DSSCs (dye-sensitized solar cells). The authors measured substituent effects on their absorption spectra and redox properties, and compared them with TD-DFT (time-dependent density functional theory) calculations. Electron withdrawing groups resulted in red-shift of ultraviolet-visible (UV-Vis) spectra. For the first time, the authors also proposed and confirmed the importance of substituent effects on their electric transition dipole moments, calculated by TD-DFT for designing dyes. Chemisorption for TiO2 of the complex by carboxyl groups was confirmed by XPS measurement. In view of electronic properties, all compounds have the possibility to be dyes of DSSCs.