Showing papers by "Cristina Puzzarini published in 2013"
TL;DR: The structures, relative stabilities, and infrared spectra of the six low-energy conformers of glycine have been characterized using a state-of-the-art quantum-mechanical approach allowing the bond distances, conformational enthalpies and vibrational frequencies to be determined well within the chemical accuracy.
Abstract: The structures, relative stabilities, and infrared spectra of the six low-energy conformers of glycine have been characterized using a state-of-the-art quantum-mechanical approach allowing the bond distances, conformational enthalpies and vibrational frequencies to be determined well within the chemical accuracy. Transition state structures governing interconversion among the different energy minima have also been characterized. In detail, the gas-phase thermodynamic properties (at 15 K and 410 K) of the glycine conformers considered have been obtained with a 1 kJ mol−1 accuracy, and it has been shown that the employment of DFT geometries usually reduces such accuracy by at most 0.1 kJ mol−1. Regarding molecular structures, the use of two different composite schemes allowed us to further confirm the suitability of a rather cost-effective approach and provide geometrical parameters with an overall accuracy better than 0.002 A for distances and 1 degree for angles. Thanks to a hybrid CC/DFT approach, the infrared spectra of all conformers considered and of several deuterated isotopologues have been reproduced (when experimental data were available) or predicted with an accuracy of 10 cm−1. Finally, the joint thermodynamic and spectroscopic investigation allowed us to shed some light on the possible observation of elusive conformers. On the whole, the high accuracy of the computational results allows us to draw a fully consistent interpretation of the available experimental data and to obtain a more complete characterization of the potential energy surface of glycine.
113 citations
TL;DR: The structure and vibrational spectra of a marginally stable conformer of glycine (usually referred to as VIp or ttc) recently detected in low-temperature matrices have been characterized by a state-of-the-art computational approach allowing an overall quality well within the chemical accuracy.
Abstract: The structure and vibrational spectra of a marginally stable conformer of glycine (usually referred to as VIp or ttc) recently detected in low-temperature matrices have been characterized by a state-of-the-art computational approach allowing an overall quality for bond distances, rotational constants, conformational enthalpies and vibrational frequencies well within the chemical accuracy. The high accuracy of the computational results allows us to draw a fully consistent interpretation of the available experimental data and to obtain a more complete characterization of an elusive glycine conformer.
81 citations
TL;DR: This work simulates the IR spectra of eight halogenated molecules, using two common hybrid and double-hybrid density functionals in conjunction with both double- and triple-ζ quality basis sets (SNSD and cc-pVTZ) as well as employing the coupled-cluster theory with basis sets of at least triple- ζ quality.
Abstract: The recent implementation of the computation of infrared (IR) intensities beyond the double-harmonic approximation [J Bloino and V Barone, J Chem Phys 136, 124108 (2012)]101063/13695210 paved the route to routine calculations of infrared spectra for a wide set of molecular systems Halogenated organic compounds represent an interesting class of molecules, from both an atmospheric and computational point of view, due to the peculiar chemical features related to the halogen atoms In this work, we simulate the IR spectra of eight halogenated molecules (CH2F2, CHBrF2, CH2DBr, CF3Br, CH2CHF, CF2CFCl, cis-CHFCHBr, cis-CHFCHI), using two common hybrid and double-hybrid density functionals in conjunction with both double- and triple-ζ quality basis sets (SNSD and cc-pVTZ) as well as employing the coupled-cluster theory with basis sets of at least triple-ζ quality Finally, we compare our results with available experimental spectra, with the aim of checking the accuracy and the performances of the computat
72 citations
TL;DR: The computational composite scheme purposely set up for accurately describing the electronic structure and spectroscopic properties of small biomolecules has been applied to the first study of the rotational spectrum of 2-thiouracil.
Abstract: The computational composite scheme purposely set up for accurately describing the electronic structure and spectroscopic properties of small biomolecules has been applied to the first study of the rotational spectrum of 2-thiouracil. The experimental investigation was made possible thanks to the combination of the laser ablation technique with Fourier transform microwave spectrometers. The joint experimental–computational study allowed us to determine the accurate molecular structure and spectroscopic properties of the title molecule, but more importantly, it demonstrates a reliable approach for the accurate investigation of isolated small biomolecules.
72 citations
TL;DR: The integrated theoretical model proposed is based on accurate post-Hartree-Fock computations of energies, structures, properties, and harmonic force fields coupled to DFT corrections for the proper inclusion of vibrational effects at an anharmonic level (as provided by general second-order perturbative approach).
Abstract: A state-of-the-art computational strategy for the evaluation of accurate molecular structures as well as thermodynamic and spectroscopic properties along with the direct simulation of infrared (IR) and Raman spectra is established, validated (on the basis of the experimental data available for the Ip glycine conformer) and then used to provide a reliable and accurate characterization of the elusive IVn/gtt and IIIp/tct glycine conformers. The integrated theoretical model proposed is based on accurate post-Hartree–Fock computations (involving composite schemes) of energies, structures, properties, and harmonic force fields coupled to DFT corrections for the proper inclusion of vibrational effects at an anharmonic level (as provided by general second-order perturbative approach). It is shown that the approach presented here allows the evaluation of structural, thermodynamic, and spectroscopic properties with an overall accuracy of about, or better than, 0.001 A, 20 MHz, 1 kJ·mol–1, and 10 cm–1 for bond dist...
71 citations
TL;DR: The role of theory in guiding and supporting experiment is detailed through a few examples, and the interplay of experiment and theory is discussed in terms of the information of physical and chemical interest that can be derived.
Abstract: Rotational spectroscopy is known to be a technique that is widely used to infer information on molecular structure and dynamics. In the last few decades, its role in the field of atmospheric and astrophysical investigations has rapidly grown. However, several are the challenging aspects in rotational spectroscopy, since the detection and analysis of spectra as well as interpretation of obtained results are not at all straightforward. Quantum chemistry has reached such an accuracy that can be used to disentangle these challenging situations by guiding the experimental investigation, assisting in the determination of the spectroscopic parameters, and extracting information of chemical interest. This perspective provides an overview of the theoretical background and computational requirements needed for the accurate evaluation of the spectroscopic parameters of relevance to rotational spectroscopy. The role of theory in guiding and supporting experiment is detailed through a few examples, and the interplay of experiment and theory is discussed in terms of the information of physical and chemical interest that can be derived.
56 citations
TL;DR: It is demonstrated that an accuracy of 1 kHz, or even better (i.e., an accuracy better than 1 part in 10(9)), and a frequency resolution of 50 kHz can be routinely obtained in the THz region, which allows sub-Doppler resolution to be exploited in this frequency region.
Abstract: We report the first thorough investigation of the Lamb-dip effect in the THz region, which in turn allows sub-Doppler resolution to be exploited in this frequency region. It is demonstrated that an accuracy of 1 kHz, or even better (i.e., an accuracy better than 1 part in 109), and a frequency resolution of 50 kHz (i.e., a resolution better than 5 parts in 108) can be routinely obtained in our laboratory. It has also shown that Lamb-dip spectra can be recorded using either a Fabry–Perot interferometric cell or a free-space cell. Hydrogen sulfide (H2S), sulfur dioxide (SO2), deuterated water (D2O), and methyl fluoride (CH3F) have been selected as examples for demonstrating the accuracy and resolution reachable, thus providing the most accurate frequency values in the 1.0–1.2 THz frequency range for these molecules. Measurements for SO2 have also been employed in a global fit, thus improving its spectroscopic parameters for the vibrational ground state.
44 citations
TL;DR: The vibrational analysis of the gas-phase infrared spectra of chlorofluoromethane provided the basic data to model the atmospheric behavior of this greenhouse gas and was used to validate the spectroscopic predictions yielded by the HDCPT2 approach.
Abstract: The vibrational analysis of the gas-phase infrared spectra of chlorofluoromethane (CH2ClF, HCFC-31) was carried out in the range 200–6200 cm−1. The assignment of the absorption features in terms of fundamental, overtone, combination, and hot bands was performed on the medium-resolution (up to 0.2 cm−1) Fourier transform infrared spectra. From the absorption cross section spectra accurate values of the integrated band intensities were derived and the global warming potential of this compound was estimated, thus obtaining values of 323, 83, and 42 on a 20-, 100-, and 500-year horizon, respectively. The set of spectroscopic parameters here presented provides the basic data to model the atmospheric behavior of this greenhouse gas. In addition, the obtained vibrational properties were used to benchmark the predictions of state-of-the-art quantum-chemical computational strategies. Extrapolated complete basis set limit values for the equilibrium geometry and harmonic force field were obtained at the coupled-cluster singles and doubles level of theory augmented by a perturbative treatment of triple excitations, CCSD(T), in conjunction with a hierarchical series of correlation-consistent basis sets (cc-pVnZ, with n = T, Q, and 5), taking also into account the core-valence correlation effects and the corrections due to diffuse (aug) functions. To obtain the cubic and quartic semi-diagonal force constants, calculations employing second-order Moller-Plesset perturbation (MP2) theory, the double-hybrid density functional B2PLYP as well as CCSD(T) were performed. For all anharmonic force fields the performances of two different perturbative approaches in computing the vibrational energy levels (i.e., the generalized second order vibrational treatment, GVPT2, and the recently proposed hybrid degeneracy corrected model, HDCPT2) were evaluated and the obtained results allowed us to validate the spectroscopic predictions yielded by the HDCPT2 approach. The predictions of the deperturbed second-order perturbation approach, DVPT2, applied to the computation of infrared intensities beyond the double-harmonic approximation were compared to the accurate experimental values here determined. Anharmonic DFT and MP2 corrections to CCSD(T) intensities led to a very good agreement with the absorption cross section measurements over the whole spectral range here analysed.
35 citations
TL;DR: This contribution presents a comprehensive study of the molecular structure and spectroscopic properties of the phenyl radical with the aim of validating an accurate computational protocol able to deal with conjugated open-shell species and succeeds in obtaining reliable and accurate results.
Abstract: The coupled-cluster singles doubles model with perturbative treatment of triples (CCSD(T)) coupled with extrapolation to the complete basis-set limit and additive approaches represent the “golden standard” for the structural and spectroscopic characterization of building blocks of biomolecules and nanosystems. However, when open-shell systems are considered, additional problems related to both specific computational difficulties and the need of obtaining spin-dependent properties appear. In this contribution, we present a comprehensive study of the molecular structure and spectroscopic (IR, Raman, EPR) properties of the phenyl radical with the aim of validating an accurate computational protocol able to deal with conjugated open-shell species. We succeeded in obtaining reliable and accurate results, thus confirming and, partly, extending the available experimental data. The main issue to be pointed out is the need of going beyond the CCSD(T) level by including a full treatment of triple excitations in order to fulfil the accuracy requirements. On the other hand, the reliability of density functional theory in properly treating open-shell systems has been further confirmed.
31 citations
TL;DR: In this article, the authors identify an important class of ion-molecule reactions proceeding via carbocation collision complexes, and its implications in the chemistry of Titan's thermosphere and ionosphere.
31 citations
TL;DR: In this paper, a complete database of self-broadening coefficients for 1635 transitions in a wide quantum number range ( 0 ≤ K ″ a ≤ 16, 2 ≤ J ″ ≤ 68 ) is compiled, presented and made available, based on the ATC (Anderson-Tsao-Curnutte) approximation.
Abstract: Sulfur dioxide (SO2) is a molecule of proved atmospheric relevance, the main sources being anthropogenic, which is one of the main causes of acid rains. Besides, it is also of interest in astrophysics, as it is present in the atmosphere of Venus and in star forming regions. For these reasons SO2 is one of the target molecules in all of the most important spectroscopic databases which collect the spectroscopic line-by-line parameters for atmospheric remote sensing, astrophysics soundings, and climate changing investigations. Although over the years the spectroscopic properties of this molecule have been widely studied, and line-by-line listings of line positions and intensities have been compiled, at present an analogous systematic and complete database of broadening coefficients is still lacking. The aim of this work is to fill in this vacancy, starting from self-broadening coefficients, by coupling experimental measurements to theoretical calculations. The laboratory experiments are carried out for 12 pure rotational transitions of the vibrational ground state (and 2 of vibrational excited states) and for 25 ro-vibrational lines of the ν1 band, lying in the 9 µm atmospheric window. Theoretical calculations of broadening coefficients are performed employing a semiclassical formalism based on the ATC (Anderson–Tsao–Curnutte) approximation. From the interplay between theory and experiment the vibrational and quantum number dependence of the collisional cross-sections is first assessed and studied and then a complete database of self-broadening coefficients for 1635 transitions in a wide quantum number range ( 0 ≤ K ″ a ≤ 16 , 2 ≤ J ″ ≤ 68 ) is compiled, presented and made available.
TL;DR: Using the Lamb-dip technique, the hyperfine structure in the rotational spectra of H2(33)S and (33)SO2 has been resolved and the corresponding parameters--that is, the sulfur quadrupole-coupling and spin-rotation tensors--were determined.
Abstract: Using the Lamb-dip technique, the hyperfine structure in the rotational spectra of H233S and 33SO2 has been resolved and the corresponding parameters—that is, the sulfur quadrupole-coupling and spin–rotation tensors—were determined. The experimental parameters are in good agreement with results from high-level coupled-cluster calculations, provided that up to quadruple excitations are considered in the cluster operator, sufficiently large basis sets are used, and vibrational corrections are accounted for. The 33S spin-rotation tensor for H2S has been used to establish a new sulfur nuclear magnetic shielding scale, combining the paramagnetic part of the shielding as obtained from the spin–rotation tensor with a calculated value for the diamagnetic part as well as computed vibrational and temperature corrections. The value of 716(5) ppm obtained in this way for the sulfur shielding of H2S is in good agreement with results from high-accuracy quantum-chemical calculations but leads to a shielding scale that is about 28 ppm lower than the one suggested previously in the literature, based on the 33S spin-rotation constant of OCS.
01 Jan 2013
TL;DR: For the bromine atom and the hydrogen bromide molecule, this paper reported results for the electric field gradient at the Bromine nucleus based on quantum-chemical calculations, which were obtained by using coupled-cluster methods for the treatment of electron correlation.
Abstract: For the bromine atom and the hydrogen bromide molecule, we report results for the electric-field gradient at the bromine nucleus based on quantum-chemical calculations. Highly accurate values are obtained by using coupled-cluster methods for the treatment of electron correlation, by minimising remaining basis-set effects through the use of large atomic-orbital sets, and by taking into account relativistic effects. For hydrogen bromide, zero-point vibrational corrections are considered as well. The obtained results for the bromine electric-field gradients are used to derive values for the 79Br quadrupole moment: 308.1 and 309.3 mb based on data for the bromine atom and hydrogen bromide, respectively. As these values deviate from those reported in recent compilations of nuclear quadrupole moments (see, for example, P. Pyykko, Mol. Phys. 106, 1965 (2008)), we suggest to replace the currently accepted value of 313(3) mb for 79Br by the mean value from the present investigation, i.e., 308.7(20) mb. The remaini...
TL;DR: In this paper, the Born-Oppenheimer breakdown corrections to rotational constants in the case of diatomic molecules are extended to open-shell species, and the deviation from the Born−Oppenheim equilibrium rotational constant is obtained by considering three contributions: the adiabatic correction to the equilibrium bond distance, the electronic contribution to the moment of inertia requiring the computation of the rotational g-tensor, and so-called Dunham correction.
Abstract: The quantum-chemical protocol for computing Born–Oppenheimer breakdown corrections to rotational constants in the case of diatomic molecules is extended to open-shell species. The deviation from the Born–Oppenheimer equilibrium rotational constant is obtained by considering three contributions: the adiabatic correction to the equilibrium bond distance, the electronic contribution to the moment of inertia requiring the computation of the rotational g-tensor, and the so-called Dunham correction. Values for the Born–Oppenheimer breakdown parameters of CN, CO+, and BO in their 2Σ+ electronic ground states are reported based on coupled-cluster calculations of the involved quantities and compared to available experimental data.
01 Jun 2013
TL;DR: An overview of the theoretical background and computational requirements needed for the accurate evaluation of the spectroscopic parameters of relevance to rotational spectroscopy is provided.
Abstract: Rotational spectroscopy is known to be a technique that is widely used to infer information on molecular structure and dynamics. In the last few decades, its role in the field of atmospheric and astrophysical investigations has rapidly grown. However, several are the challenging aspects in rotational spectroscopy, since the detection and analysis of spectra as well as interpretation of obtained results are not at all straightforward. Quantum chemistry has reached such an accuracy that can be used to disentangle these challenging situations by guiding the experimental investigation, assisting in the determination of the spectroscopic parameters, and extracting information of chemical interest. This perspective provides an overview of the theoretical background and computational requirements needed for the accurate evaluation of the spectroscopic parameters of relevance to rotational spectroscopy. The role of theory in guiding and supporting experiment is detailed through a few examples, and the interplay of experiment and theory is discussed in terms of the information of physical and chemical interest that can be derived.
01 Jan 2013
TL;DR: An introductory overview of computational aspects of computational astrochemistry and the role played by molecular spectroscopy in the field of molecular astrophysics can be found in this paper, where the authors provide an introductory overview.
Abstract: Astrochemistry and molecular astrophysics are interdisciplinary fields involving chemistry, physics, and astronomy and aiming at the understanding of the chemical evolution of the universe Both fields span astronomical observations, modeling, and theoretical and/or experimental laboratory-based investigations This contribution provides an introductory overview, focusing on the computational aspects of astrochemistry (computational astrochemistry) and on the role played by molecular spectroscopy in the field of molecular astrophysics