Estudo Geralhttps://estudogeral.sib.uc.ptThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 08 Dec 2021 18:41:18 GMT2021-12-08T18:41:18Z5031Dissociation of ground and nσ* states of CF3Cl using multireference configuration interaction with singles and doubles and with multireference average quadratic coupled cluster extensivity correctionshttp://hdl.handle.net/10316/17930Title: Dissociation of ground and nσ* states of CF3Cl using multireference configuration interaction with singles and doubles and with multireference average quadratic coupled cluster extensivity corrections
Authors: Lucena Jr, Juracy R.; Ventura, Elizete; Monte, Silmar A. do; Araújo, Regiane C. M. U.; Ramos, Mozart N.; Fausto, Rui
Abstract: Extended complete active space self-consistent field (CASSCF), multireference configuration interaction with singles and doubles (MR-CISD), and multireference average quadratic coupled cluster (MR-AQCC) calculations have been performed on the ground (S0) and first excited (nσ*,S1) states of the CF3Cl molecule. Full geometry optimizations have been carried out for S0 as well as “relaxed” potential energy calculations for both states, along the C–Cl bond distance. Vertical excitation energies (ΔEvertical), dissociation energies (ΔEdiss), dissociation enthalpies (ΔHdiss), and the oscillator strength (f) have also been computed. Basis set effects, basis set superposition error (BSSE), and spin-orbit and size-extensivity corrections have also been considered. The general agreement between theoretical and available experimental results is very good. The best results for the equilibrium geometrical parameters of S0 (at MR-AQCC/aug-cc-pVTZ+d level) are 1.762 and 1.323 Å, for the C–Cl and C–F bond distances, respectively, while the corresponding experimental values are 1.751 and 1.328 Å. The ∠ClCF and ∠FCF bond angles are in excellent agreement with the corresponding experimental values (110.3° and 108.6°). The best calculated values for ΔEvertical, ΔHdiss, and f are 7.63 eV [at the MR-AQCC/aug-cc-pV(T+d)Z level], 3.59 eV[MR-AQCC/aug-cc-pV(T+d)Z level+spin-orbit and BSSE corrections], and 2.74×10−3 (MR-CISD/cc-pVTZ), in comparison with the corresponding experimental values of 7.7±0.1 eV, 3.68 eV, and 3.12×10−3±2.50×10−4. The results concerning the potential energy curves for S0 and S1 show a tendency toward the nonoccurrence of crossing between these two states (in the intermediate region along the C–Cl coordinate), as the basis set size increases. Such tendency is accompanied by a decreasing well depth for the S1 state. Dynamic electronic correlation (especially at the MR-AQCC level) is also an important factor toward an absence of crossing along the C–Cl coordinate. Further investigations of a possible crossing using gradient driven techniques (at CASSCF and MR-CISD levels) seem to confirm its absence.
Tue, 30 Oct 2007 00:00:00 GMThttp://hdl.handle.net/10316/179302007-10-30T00:00:00ZA comparative multi‐reference configuration interaction study of the low‐lying states of two thione isomers of thiophenolhttp://hdl.handle.net/10316/91160Title: A comparative multi‐reference configuration interaction study of the low‐lying states of two thione isomers of thiophenol
Authors: Lima, Filipe Belarmino de; Pereira Rodrigues, Gessenildo; Lucena Júnior, Juracy Regis; Ventura, Elizete; Rui Fausto; Reva, Igor; Monte, Silmar Andrade do
Abstract: Multi-reference configuration interaction, MR-CI (including extensivity
corrections, named +Q) calculations have been performed on S0 to S3 states of cyclohexa-2,4-
diene-1-thione (thione 24) and cyclohexa-2,5-diene-1-thione (thione 25), which are thione
isomers of thiophenol. Several types of uncontracted MR-CIS and MR-CISD wavefunctions
have been employed, comprising MR-CI expansions as large as ~ 365 x 106 configuration
state functions. The nature of the studied excited states has been characterized. Vertical
excitation energies (E) and oscillator strengths (f) have been computed. The most intense
transitions (S0→S2 for 24 and S0→S3 for 25) do not change with the wavefunction, although a
variation as large as ~ 1 eV has been obtained for the S3 state of 24, at the highest (MR-CI+Q)
level. On the other hand, E changes at most ~ 0.56 eV for 25, as the wavefunction changes,
at the same level. The S1 state of both thiones has nπ* character and is in the visible region.
For 24 S2 and S3 are ππ* and nπ* states, respectively, while for 25 the reverse order has been
obtained. S2 and S3 are in the range from ~ 3.5 to 5.2 eV, again at the highest level. It is the
first time that the excited states of the title molecules are studied. The computed results agree
with the experimental onsets of photoreactions of thiones 24 and 25 found by Reva et. al.
(Phys. Chem. Chem. Phys. 2015, 17, 4888).
Thu, 11 Jun 2020 00:00:00 GMThttp://hdl.handle.net/10316/911602020-06-11T00:00:00ZMatrix Isolation FTIR Spectroscopic and Theoretical Study of 3,3-Dichloro-1,1,1-Trifluoropropane (HCFC-243)http://hdl.handle.net/10316/10423Title: Matrix Isolation FTIR Spectroscopic and Theoretical Study of 3,3-Dichloro-1,1,1-Trifluoropropane (HCFC-243)
Authors: Lucena Jr., J. R.; Sharma, A.; Reva, I. D.; Araújo, R. M. C. U.; Ventura, E.; Monte, S. A. do; Braga, C. F.; Ramos, M. N.; Fausto, R.
Abstract: The molecular structure and infrared spectrum of the atmospheric pollutant 3,3-dichloro-1,1,1-trifluoropropane (HCFC-243) were characterized experimentally and theoretically. The theoretical calculations show the existence of two conformers, with the gauche (G) and trans (T) orientation around the HCCC dihedral angle. Conformer G was calculated to be more stable than form T by more than 10 kJ mol−1. In consonance with the large predicted relative energy of conformer T, only the G form was identified spectroscopically in cryogenic argon (10 K) and xenon (20 K) matrices prepared from room-temperature equilibrium vapor of the compound. The observed infrared spectra of the matrix-isolated HCFC-243 were interpreted with the aid of high-level density functional theory calculations and normal coordinate analysis. For experimental identification of the weakest IR absorption bands, the spectrum of HCFC-243 in the neat solid state at 145 K was obtained. This spectrum also confirmed the sole presence of the G conformer in the sample. Natural bond orbital and atomic charge analyses were carried out for the two conformers to shed light on the most important intramolecular interactions in the two conformers, in particular those responsible for their relative stability.
Thu, 20 Nov 2008 00:00:00 GMThttp://hdl.handle.net/10316/104232008-11-20T00:00:00Z