Victor M.F. Morais

Energetics of Coumarin and Chromone

Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for coumarin and chromone were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T = 298.15 K, were then derived: coumarin, -(163.4 +/- 3.3) kJ x mol(-1), and chromone, -(126.1 +/- 2.5) kJ x mol(-1). The temperatures of fusion, T(fusion), and fusion enthalpies, at T = T(fusion), were also reported. Additionally, theoretical calculations were done using different methods: DFT/B3LYP, MCCM (MC-UT/3 and MC-QCISD/3), and also the more accurate G3MP2 method. Good agreement between experimental and theoretical data was achieved. Some correlations between structure and energy were also made, and the aromaticity of the compounds was evaluated by the nucleus independent chemical shifts (NICS).

Experimental and computational thermochemistry of three nitrogen-containing heterocycles: 2-benzimidazolinone, 2-benzoxazolinone and 3-indazolinone

The standard molar enthalpies of combustion, sublimation, and formation of three nitrogen-containing heterocycles, namely, 2-benzimidazolinone, 2-benzoxazolinone and 3-indazolinone were determined calorimetrically. The standard (p° = 0.1 MPa) molar enthalpies of formation in the gas phase were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and from the standard molar enthalpies of sublimation at T = 298.15 K, measured by Calvet microcalorimetry. Møller–Plesset calculations at the MP2 level and density functional calculations with the B3LYP functional and extended basis sets were also performed to determine the energetically preferred tautomeric form of the molecules. The results were qualitatively independent of the calculational level, where in general the DFT calculations were in better agreement with experiment than those from MP2. The gas and solid phase enthalpic differences between imines and amides, wherein –CH=N- is contrasted with –CONH-, have been studied and roughly constant values have been found.

Semi-empirical valence bond potential energy surfaces for homonuelear alkali trimers

Semi-empirical valence bond surfaces for the homonuclear alkali trimers (M 3) have been obtained by evaluating the Coulomb and exchange integrals in the London equation, respectively, as half the sum and difference of new reliable analytical potential energy curves for the 1Σ g + and 3Σ u + states of the dimers (M 2). Calculations were also carried out using an effective 3Σ u + universal diatomic function which was chosen to reproduce the experimentally known binding energy of Li3. The ground state equilibrium structures of those trimers are predicted to be an obtuse triangle (2 B 2 symmetry) bound with respect to M 2(1Σ g + )+M(2 S) and ordered Li3>Na3>K3>Rb3>Cs3 according to their stabilities. The surfaces are found to be extremely flat with two saddle points, an acute isosceles triangle (2 A 1 symmetry) and a linear symmetric structure (2Σ u + ) lying above but very close to the minimum. The force constants of the 2Σ u + , 2 A 1 and 2 B 2 structures, as well as the fundamental frequencies and zero poin...

Saccharin: a combined experimental and computational thermochemical investigation of a sweetener and sulfonamide

The standard molar enthalpies of combustion, sublimation, and formation in the crystalline and gaseous phase at a temperature of 298.15 K have been experimentally determined for saccharin and for benzenesulfonamide. These compounds were also studied theoretically using density functional theory, the B3LYP functional and extended basis sets.

Are isatin and isatoic anhydride antiaromatic and aromatic respectively? A combined experimental and theoretical investigation

This paper reports the results of our thermochemical/calorimetric determination of the enthalpies of combustion, phase change, and formation of isatin, isatoic anhydride, and N-methylisatin. The density functional calculations accompanied by vibrational and thermal corrections were also performed for these compounds and N-methylisatoic anhydride. Through a combination of theoretical calculations and associated isodesmic reactions, we have deduced that isatin has some antiaromatic character and isatoic anhydride enjoys some aromatic stabilization.

Thermochemical and theoretical study of some methyldiazines

The standard (p0 = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.

THERMOCHEMICAL AND THEORETICAL STUDIES OF 4-METHYLBIPHENYL, 4,4'-DIMETHYLBIPHENYL, 4,4'-DIMETHYL-2,2'-BIPYRIDINE

The standard (p°=0.1 MPa) molar enthalpies of formation for 4-methylbiphenyl, 4,4′-dimethylbiphenyl and 4,4′-dimethyl-2,2′-bipyridine were derived from the standard molar enthalpies of combustion, in oxygen, at T=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15 K, were measured by Calvet microcalorimetry. Theoretical calculations at the ab initio restricted Hartree–Fock (RHF) level with second-order Moller–Plesset (MP2) perturbation theory correlation corrections and density functional theory (DFT) have been carried out for all these molecules in order to access their stabilitites relative to the non-substituted ones. The theoretical results are in general good agreement with the experimental results.

Study of energetics and structure of 1,2,3-benzotriazin-4(3H)-one and its 1H and enol tautomers.

This paper reports an experimental and computational study on the energetics of 1,2,3-benzotriazin-4(3H)-one. The standard (p° = 0.1 MPa) molar enthalpy of formation of solid 1,2,3-benzotriazin-4(3H)-one, at T = 298.15 K, was derived from its standard massic energy of combustion measured by static bomb combustion calorimetry in oxygen. The Calvet high-temperature vacuum sublimation technique was used to measure the respective standard molar enthalpy of sublimation at T = 298.15 K. From these two experimentally determined thermodynamic parameters, we have calculated the standard molar enthalpy of formation of 1,2,3-benzotriazin-4(3H)-one in the gas phase at T = 298.15 K, (200.9 ± 3.8) kJ·mol(-1). Interrelations between structure and energy for 1,2,3-benzotriazin-4(3H)-one, the tautomer 1,2,3-benzotriazin-4(1H)-one, and the enol tautomer 1,2,3-benzotriazin-4-ol were discussed based on density functional theory (DFT) calculations with the B3LYP hybrid functional and the 6-311++G(d,p) basis set. The gas-phase enthalpy of formation of 1,2,3-benzotriazin-4(3H)-one was estimated from quantum chemical calculations using the G3(MP2)//B3LYP composite method. Nucleus-independent chemical shifts (NICS) were also calculated with the purpose of analyzing the aromaticity of the benzenic and heterocyclic rings of the title molecule and others related tautomerically to it.

Thermochemical and theoretical studies of some benzodiazines

The standard molar enthalpies of formation of quinoxaline, quinazoline and phthalazine at 298.15 K in the gaseous state have been determined from the standard molar enthalpies of combustion of the crystalline solids and the respective enthalpies of sublimation: quinoxaline, 240.3 ± 3.3 kJ mol–1; quinazoline, 243.1 ± 2.7 kJ mol–1; phthalazine, 329.9 ± 3.3 kJ mol–1. Ab initio full geometry optimizations at the 3-21G and 6-31G* levels were also performed for these molecules. MP2/RHF/3-21G//3-21G energies were calculated for all isomers, and used to estimate their isodesmic resonance energies.

Thermochemical study of the ethylpyridine and ethylpyrazine isomers

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four liquids: 2-ethylpyridine, 4-ethylpyridine, ethylpyrazine and 2,3-diethylpyrazine were measured by static bomb calorimetry in an oxygen atmosphere. The values of the standard molar enthalpies of vaporization, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K: 2-ethylpyridine (79.4 +/- 2.6) kJ mol(-1); 4-ethylpyridine (81.0 +/- 3.4) kJ mol(-1); ethylpyrazine (146.9 +/- 2.8) kJ mol(-1); and 2,3-diethylpyrazine (80.2 +/- 2.9) kJ mol(-1). The most stable geometries of all ethylpyridine and ethylpyrazine isomers were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G* and 6-311G**. These calculations were then used to obtain estimates of the enthalpies of formation of all isomers, including those not experimentally studied, through the use of isodesmic reactions. A discussion of the relationship between structure and energetics of the isomers is also presented.