Piotr Borowski

A theoretical study of the low-lying excited states of ozone

Abstract A theoretical study has been performed on the five lowest excited states of the ozone molecule using multiconfigurational second-order perturbation theory (CASPT2). The predicted order of states is: 3A2 (T0 = 1.15 eV), 3B2 (T0 = 1.33 eV, 3B1 (T0 = 1.33 eV), 1A2 (T0 = 1.44 eV) and 1B1 (T0 = 1.88 eV). Corresponding experimental data are: 1.18, 1.30, 1.45, 1.58, and 2.05 eV, respectively. Equilibrium geometries, harmonic frequencies of symmetric vibrations, and vertical excitation energies are also reported. The dissociation limit De for the ground state of ozone is found to be 1.08 eV, in agreement with the experimental value (1.13 eV). The calculations make use of a modified Fock operator in the CASPT2 theory. Relative energies of states with a different number of open shells are substantially improved. The modified CASPT2 method was checked by calculating spectroscopic constants of the oxygen molecule.

Quantum cluster equilibrium theory treatment of hydrogen-bonded liquids: water, methanol and ethanol

The quantum cluster equilibrium (QCE) theory was used in order to predict the composition of the hydrogen bonded liquids: water, methanol and ethanol. The calculations were based on high accuracy theoretical data obtained at the DFT/B3LYP/6-311 G(d,p) level of theory. All investigated liquids are predicted to be composed of big clusters: hexamers in the case of water, tetramers, pentamers, hexamers and heptamers in the case of methanol and pentamers in the case of ethanol. The content of big clusters in a liquid phase as predicted by QCE is overestimated. We have found two confirmations of this. First of all, the behaviour of the liquid water isobar clearly demonstrates that there should be a substantial amount of small clusters in order to obtain the correct temperature dependence of the molar volume. Indeed, the theoretical molar volume close to the boiling point is by about 0.6cm3 lower than the experimental one. The molar volume is too low due to the overestimated population of big clusters resulting in too high a liquid density. Second, the temperature dependence of the chemical shift of the hydroxyl protons in liquid methanol and ethanol, obtained as the population weighted average of the chemical shift of individual clusters, is shifted down field as compared to experiment by as much as 2ppm. This is because big clusters with strongly deshielded hydroxyl protons contribute too much to the weighted average. Possible shortcomings of the QCE approach are discussed.

The development and characterisation of novel hybrid sol–gel-derived films for optical pH sensing

This paper focuses on the development and detailed characterisation of a novel hybrid sol–gel material derived from 3-glycidoxypropyltrimethoxysilane (GPTMS) and ethyltriethoxysilane (ETEOS). The material has been doped with a fluorescent pH-sensitive dye, modified 8-hydroxy-1,3,6-pyrene trisulfonic acid (HPTS) and its application as a high-performance optical ratiometric pH sensor has been demonstrated. The optimum sol–gel-based material composition has been characterised using atomic force microscopy (AFM), scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM-EDX), thermal gravimetric-infrared (TGA-IR) analysis, Fourier transform infrared (FT-IR) and Raman spectroscopy, solid state 29Si and 13C nuclear magnetic resonance (NMR). AFM and SEM images demonstrated a high degree of homogeneity of the pH films. FT-IR, Raman and NMR results have shown that hydrolysis and condensation reactions were almost completed and extensive crosslinking occurred in the final material. They also revealed that an opening of the epoxy ring in GPTMS took place which was key to optimum pH response. The pH sensor produced using the hybrid material compares very well with the current state-of-the-art and exhibits very good reversibility, reproducibility, stability, short response time and no leaching. The dynamic range extends from pH 5.0 to 8.0 which is compatible with bioprocessing, environmental and clinical monitoring applications.

Theoretical determination of the 1H NMR spectrum of ethanol

GIAO calculations of the 1H NMR chemical shifts for ethanol at the SCF and DFT levels of theory are presented. The importance of molecular geometry and basis set is discussed. Vibrational correction to the hydroxyl proton chemical shift is also considered in calculations for the monomer of ethanol. The final theoretical results for the monomer obtained at the optimized DFT/B3LYP/6-311G(d,p) geometry with the 6-311G++ (d,p) basis set for NMR are in very good agreement with gas phase experimental data. For the liquid phase ethanol the hydrogen bonding effects are taken into account by performing calculations on various clusters of ethanol. It is shown that inaccuracy due to molecular geometry and basis set in the monomer of ethanol is magnified significantly in calculations for its clusters. In this context the structure of liquid ethanol as predicted recently by quantum cluster equilibrium (QCE) theory is discussed.

An effective scaling frequency factor method for scaling of harmonic vibrational frequencies: Application to 1,2,4-triazole derivatives.

Scaling of harmonic frequencies of a molecule is one of the methods of improving the agreement between the calculated from a quadratic force field and experimental vibrational spectrum. An application of the recently proposed effective scaling frequency factor (ESFF) method to the complicated 1,2,4-triazole derivatives is presented. The calculations are based on the DFT/B3LYP/6-311G** quadratic force fields. It is shown that the ESFF method is capable of providing the high-quality spectra with regard to the scaled frequencies, comparable to these obtained with the well-established scaled quantum mechanical (SQM) force field method. Using the recommended scaling factors for the 11-parameter calculations, the RMS value obtained for a set of 293 vibrational modes of four compounds is only 8.7 and 8.5cm(-1), for SQM and ESFF, respectively, provided the hydrogen bonded C=O bond was excluded from the general non-hydrogen XX stretch group, and the scaling factor attributed to this bond was optimized. The new, 9-parameter set of scaling factors provides SQM- and ESFF-scaled frequencies that are of comparable quality to those of the 11-parameter calculations. In addition, it provides (on average) more reliable band splittings in the middle region of the spectrum, and the order of the scaled frequencies corresponds to that of the experimental bands. The straightforward application of the ESFF method to estimate the value of the scaled frequency is also presented.

Theoretical analysis of solvent effects on nitrogen NMR chemical shifts in oxazoles and oxadiazoles

Using quantum chemistry methods we have evaluated the solvent effects on the (14)N NMR chemical shifts in five oxa- and oxadiazoles dissolved in twelve solvents. These solvents differ in their polarity with the dielectric constants varying from 2 to 80. Moreover, three of them have a hydrogen-bond donor character. All possible hydrogen-bonding in the water solution with the oxygen and nitrogen (hydrogen-acceptor) centers in oxazoles (2) and oxadiazoles (3) have been considered in our studies. It has been shown that both the pure solvent and hydrogen-bonding effects are significant and result in (14)N magnetic shielding increase. In water solutions the pure solvent effect is larger than the hydrogen-bonding effect. In addition, the solvent effect has been analyzed in terms of its direct and indirect contributions. It should be emphasized that our theoretical results for (14)N chemical shifts in oxa- and oxadiazoles remain in a very good agreement with the accurate experimental data.

Theoretical determination of the infrared spectra of amorphous polymers.

The simple procedure of calculating the infrared spectra of polymers is presented. It is based on selecting the relevant, medium-size representative fragments of a polymer, for which the vibrational frequencies are computed within the harmonic approximation, in conjunction with the multiparameter scaling techniques. Scaling is necessary to predict the reliable fundamentals, which, along with the calculated intensities and properly chosen band widths, reproduce the observed band shapes with high accuracy. Applications to the three polymers: poly(methyl methacrylate), poly(vinyl acetate), and poly(isopropenyl acetate) are presented. The simulated spectra are in good agreement with the experiment. The assignment of bands is reported. The obtained results indicate strong delocalization of the vibrational modes within polymers, which is in accord with the most recent experimental finding [Macromolecules2008, 41, 2494-2501]. Good agreement between the observed and the calculated spectra of deuterated PMMA confirms the correctness of our approach. The preliminary results obtained for the highly irregular macromolecular compound (vinyl-functionalized silica) are also shown.

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.

Adsorption of L-Histidine onto Functionalized Mesoporous SBA-15 Organosilicas

Adsorption of L-histidine (LH) over mesoporous SBA-15 silicas functionalized by amine-, thiol- and cyano-functional groups has been studied. These organosilicas have been synthesized by co-condensation of proper monomers in the presence of a template (Pluronic P-123), and characterized by elemental analysis, nitrogen adsorption, X-ray diffraction and transmission electron microscopy. The highest LH static adsorption capacity was observed for the cyano-functionalized sample [approximately 70 mg g−1 (450 mol g−1)]; by contrast, pristine SBA-15 had a lowest adsorption capacity of approximately 25 mg g−1 (170 mol g−1).

Solvent effects on the nitrogen NMR chemical shifts in 1-methylazoles – a theoretical study

We have investigated solvent effect on the nitrogen chemical shifts in a series of 1-methylazoles. The detailed results for 1-methylazoles – systems containing one (1-methylpyrrole), two (diazoles), three (triazoles) and four (tetrazoles) nitrogen atoms in the heteroaromatic ring – have been presented. We have examined twenty six popular DFT functionals to calculate the nitrogen magnetic shielding constants in the gas phase and 12 different solvents within the conductor-like screening model (COSMO) and the explicit solvation model (ESM), as well as their combination (ESM + COSMO) in the case of water solutions. The vibrational corrections for the analyzed systems have been also reported. Additionally, the solvent effect on the nitrogen chemical shifts has been analyzed in terms of its direct and indirect contributions. Our theoretical vibrationally corrected results properly reproduce the experimental data. In the calculations of N NMR chemical shifts, the best results were achieved with the B97-2 functional with the mean absolute error as small as 3 ppm for a range exceeding 270 ppm in the tested azole systems.