Browsing by Author "Coelho, Daniela"
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- Conformational and structural analysis of 2-allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide as probed by matrix-isolation spectroscopy and quantum chemical calculationsPublication . Gómez-Zavaglia, A.; Kaczor, A.; Coelho, Daniela; Cristiano, Maria Lurdes Santos; Fausto, R.2-Allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (ABIOD) has been studied by matrix-isolation infrared spectroscopy and quantum chemical calculations. A conformational search on the B3LYP/6-311++G(3df,3pd) potential energy surface of the molecule demonstrated the existence of three conformers, Sk, Sk0 and C, with similar energies, differing in the orientation of the allyl group. The calculations predicted the Sk form as the most stable in the gaseous phase, whereas the Sk0 and C conformers have calculated relative energies of ca. 0.6 and 0.8–3.0 kJ mol 1, respectively (depending on the level of theory). In agreement with the relatively large (>6 kJ mol 1) calculated barriers for conformational interconversion, the three conformers could be efficiently trapped in an argon matrix at 10 K, the experimental infrared spectrum of the as-deposited matrix fitting well the simulated spectrum built from the calculated spectra for individual conformers scaled by their predicted populations at the temperature of the vapour of the compound prior to matrix deposition. Upon annealing the matrix at 24 K, however, both Sk and Sk0 conformers were found to convert to the more polar C conformer, indicating that this latter form becomes the most stable ABIOD conformer in the argon matrix.
- Estudos de reactividade em derivados de compostos heterocíclicosPublication . Coelho, Daniela; Cristiano, Maria Lurdes SantosOs álcoois alílicos são bastante abundantes na natureza, estando presentes em vários sistemas biológicos. São ainda intermediários importantes na síntese de vários tipos de compostos com, um vasto leque de aplicações. Neste projecto de investigação, procedeu-se à modificação química do álcool alílico natural nerol, através da sua conversão em éter de tetrazol e de benzisotiazol, dois sistemas heteroaromáticos. O principal objectivo incidiu na elucidação mecanística das isomerizações térmicas dos correspondentes éteres. Este álcool inclui na sua estrutura duas insaturações, o que faz com que os correspondentes éteres possuam um sistema alilvinílico que possibilita uma isomerização do tipo “Claisen” e um sistema 1,5- dieno que torna possível uma isomerização térmica do tipo “Cope”. No decorrer deste trabalho de investigação, foi ainda feita uma análise detalhada dos padrões de fragmentação de uma série de éteres alílicos e benzílicos de benzisotiazol por espectrometria de massa. Estudos precedentes da fotoquímica de 4-aliltetrazolonas e o conhecimento da aplicação de derivados alílicos de benzisotiazol no controlo de pragas nas plantações de arroz no Japão, China e Coreia do Sul, estiveram na base de uma investigação da fotoquímica de derivados alílicos de 1,2-benzisotiazol-1,1-dióxido.
- Experimental and theoretical studies of the gas-phase reactions of O(1D) with H2O and D2O at low temperaturePublication . Hickson, Kevin; Bhowmick, Somnath; Suleimanov, Yury; Brandão, João; Coelho, DanielaHere we report the results of an experimental and theoretical study of the gas-phase reactions between O($^1$D) and H$_2$O and O($^1$D) and D$_2$O at room temperature and below. On the experimental side, the kinetics of these reactions have been investigated over the 50-127 K range using a continuous flow Laval nozzle apparatus, coupled with pulsed laser photolysis and pulsed laser induced fluorescence for the production and detection of O($^1$D) atoms respectively. Experiments were also performed at 296 K in the absence of a Laval nozzle. On the theoretical side, the existing full-dimensional ground X$^1$A potential energy surface for the H$_2$O$_2$ system involved in this process has been reinvestigated and enhanced to provide a better description of the barrierless H-atom abstraction pathway. Based on this enhanced potential energy surface, quasiclassical trajectory calculations and ring polymer molecular dynamics simulations have been performed to obtain low temperature rate constants. The measured and calculated rate constants display similar behaviour above 100 K, showing little or no variation as a function of temperature. Below 100 K, the experimental rate constants increase dramatically, in contrast to the essentially temperature independent theoretical values. The possible origins of the divergence between experiment and theory at low temperatures are discussed.
- Quasiclassical study of a termolecular reaction: a more detailed description of the HO2 collisional stabilization processPublication . Mogo, César; Brandão, João; Wang, Wenli; Coelho, Daniela; Rio, CarolinaWe present detailed studies of the collisional stabilization of the HO*2 radical in a mixture of hydrogen atoms and oxygen molecules using molecular dynamic studies and accurate potential energy surfaces. Following previous work on this process's global temperature and pressure dependencies, we analyze each collider's role and estimate specific rate constants and their temperature dependence.
- Quasiclassical study of a termolecular reaction: Application to the HO2 collisional stabilization processPublication . Mogo, César; Brandão, João; Wang, Wenli; Coelho, Daniela; Rio, CarolinaWe present a multiprocess reaction dynamics program to study the termination reaction H + O-2 + M -> HO2 + M, one main uncertainty source in hydrogen combustion studies. We simulate the behavior of a mixture of hydrogen atoms and oxygen molecules at different conditions of temperature and pressure, using classical mechanics and accurate Potential Energy Surfaces. In this simulation we treat all the reaction channels, including the stabilization step, in the same dynamical procedure. The contribution of the collisional effects on all the reactions is also accounted for. Following the fate of the excited HO2* radical, we analyze the collisional stabilization and estimate the kinetic parameters of the involved reactions. A Lindemann-Hinshelwood type reaction scheme is shown to be able to describe the kinetics of this reaction as a function of pressure and temperature. This way, we propose a procedure to directly study termolecular reactions.
- Sigmatropic rearrangements in 5-allyloxytetrazolesPublication . Frija, L.; Reva, I. D.; Ismael, Amin; Coelho, Daniela; Fausto, R.; Cristiano, Maria Lurdes SantosMechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3¢]-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3¢]- and [3,3¢]-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3¢]-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products.