Extending the molecular size in accurate quantum-chemical calculations: the equilibrium structure and spectroscopic properties of uracil

The equilibrium structure of uracil has been investigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level in conjunction with a triple-zeta basis set have been carried out. Extrapolation to the basis set limit, performed employing the second-order M?ller-Plesset perturbation theory, and inclusion of core-correlation and diffuse-function corrections have also been considered. Based on the available rotational constants for various isotopic species together with corresponding computed vibrational corrections, the semi-experimental equilibrium structure of uracil has been determined for the first time. Theoretical and semi-experimental structures have been found in remarkably good agreement, thus pointing out the limitations of previous experimental determinations. Molecular and spectroscopic properties of uracil have then been studied by means of the composite computational approach introduced for the molecular structure evaluation. Among the results achieved, we mention the revision of the dipole moment. On the whole, it has been proved that the computational procedure presented is able to provide parameters with the proper accuracy to support experimental investigations of large molecules of biological interest.     

A generalization of Bartlett’s decomposition

Bartlett’s decomposition provides the distributional properties of the elements of the Cholesky factor of A=G^TG where the elements of G are i.i.d. standard Gaussian random variables. In this paper the most general case where the elements of G have a joint multivariate Gaussian density is considered.     

Noncovalent interactions in the gas phase: the anisole-phenol complex

The present paper reports on an integrated spectroscopic study of the anisole-phenol complex in a molecular beam environment. Combining REMPI and HR-LIF spectroscopy experimental data with density functional computations (TD-M05-2X/M05-2X//N07D) and first principle spectra simulations, it was possible to locate the band origin of the S(1) ← S(0) electronic transition and determine the equilibrium structure of the complex, both in the S(0) and S(1) electronic states. Experimental and computational evidence indicates that the observed band origin is due to an electronic transition localized on the phenol frame, while it was not possible to localize experimentally another band origin due to the electronic transition localized on the anisole molecule. The observed structure of the complex is stabilized by a hydrogen bond between the phenol, acting as a proton donor, and the anisole molecule, acting as an acceptor through the lone pairs of the oxygen atom. A secondary interaction involving the hydrogen atoms of the anisole methyl group and the π electron system of the phenol molecule stabilizes the complex in a nonplanar configuration. Additional insights about the landscapes of the potential energy surfaces governing the ground and first excited electronic states of the anisole-phenol complex, with the issuing implications on the system photodynamic, can be extracted from the combined experimental and computational studies.     

Effective method for the computation of optical spectra of large molecules at finite temperature including the Duschinsky and Herzberg-Teller effect: the Qx band of porphyrin as a case study

The authors extend their recent method for the computation of vibrationally resolved optical spectra of large molecules, including both the Duschinsky rotation and the effect of finite temperature in the framework of the Franck-Condon (FC) approximation, to deal with the more general case of the Herzberg-Teller (HT) model, where also the linear dependence of the transition dipole moment on the nuclear coordinates is taken into account. This generalization allows us to investigate weak and vibronically allowed transitions by far extending the range of application of the method. The calculation of the spectra of sizable molecules is computationally demanding because of the huge number of final vibrational states that must be taken into account, and the inclusion of HT terms further increases the computational burden. The method presented here automatically selects the relevant vibronic contributions to the spectrum, independent of their frequency, and it is able to provide fully converged spectra with a modest computational requirement. The effectiveness of the method is illustrated by computing the HT absorption and fluorescence Q(x) spectra of free-base porphyrin both at T=0 K and at room temperature, performing for the first time an exact treatment of vibrations in harmonic approximation. Q(x) spectra are compared to experiments and FC/HT interferences are analyzed in detail.     

Ab initio prediction of the emission color in phosphorescent iridium(III) complexes for OLEDs

We performed fully first principles quantum mechanical calculations of the ground and excited state geometries and harmonic vibrational frequencies of two prototype cationic Ir(III) complexes showing high emission quantum efficiencies. Thanks to recent theoretical advances, we have been able for the first time to simulate their vibrationally resolved emission spectra. Our results, in good agreement with the experiment, allow us to calculate the CIE coordinates and therefore the emission color of this important class of emitters for OLEDs and LECs.     

Implementation and validation of a multi-purpose virtual spectrometer for large systems in complex environments

Despite impressive advances of computational spectroscopy, a robust and user-friendly multi-frequency virtual spectrometer is not yet available. This contribution summarises ongoing efforts in our research group toward the implementation and validation of such a tool with special reference to the building blocks of biomolecules in their natural environment. Our integrated computational tool allows the computation of several kinds of spectra, including vibrational (e.g. IR, VCD), electronic (e.g. absorption, emission, ECD) as well as magnetic resonance (e.g. ESR, NMR) for both closed- and open-shell systems in vacuo and in condensed phases, and includes facilities for drawing, comparing, and modifying all the computed spectra. A number of test cases involving a combination of different spectroscopic ranges will be discussed in order to point out strengths, limitations, and ongoing developments of our research plan.     

Study of Late Roman and Byzantine glass by the combined use of analytical techniques

In the present work, we report on a detailed characterization of Late Roman and Byzantine (from 2nd to 8th century A.D.) glass specimen found in Catania (Sicily, Southern Italy) by the employment of two complementary techniques, namely scanning electron microscopy coupled with energy-dispersive spectrometry (SEM–EDS) and Fourier transform infrared (FT-IR) absorbance spectroscopy. The glass fragments come from archaeological excavations in the courtyard of St. Agata la Vetere's church in Catania. In particular, compositional data were obtained by scanning SEM–EDS measurements and the investigation of the degradation on the surface of the glass specimen was made combining the SEM–EDS observations with those performed through FT-IR absorbance. The whole set of results contributed to define the provenance of the specimens. Furthermore, the data furnished new evidences of the technical development of glass production in Late Roman and Byzantine period in Catania.     

Order parameters of alpha,omega-diphenylpolyenes in a nematic liquid crystal from an integrated computational and 13C NMR spectroscopic approach

The orientational order parameters and conformational behavior of five relatively large rodlike molecules, biphenyl, trans-stilbene, 1,3-diphenyl-butadiene, 1,3,5-diphenyl-hexatriene, and 1,3,5,7-diphenyl-octatetraene, dissolved in the thermotropic liquid crystal ZLI-1167, have been studied using an integrated approach combining (13)C NMR measurements and quantum mechanical computations of carbon chemical shift tensors. Besides biphenyl, the phenyl moiety of all structures has been found to have a high rotational mobility in the temperature range of the present experiments. The rank-two order parameter in the nematic phase is found to increase steadily from the shortest to the longest term of the series at any temperature within the nematic range. The molecular biaxiality order parameter is found to be small and essentially constant with temperature, giving further support to the common assumption of effective uniaxiality for these probes.