Rafael Notario

Basicity of N-H- and N-Methyl-1,2,3-triazoles in the Gas Phase, in Solution, and in the Solid State − An Experimental and Theoretical Study

The gas-phase and aqueous basicities of six 1,2,3-triazoles have been determined, the former by FT-ICR and the latter by spectrophotometry and 1H NMR. The gas-phase experiments agree very well with the Gibbs free energies calculated at the B3LYP/6-31G* level. In contrast, only semiquantitative ascertainments are possible when basicities in the gas phase and in solution are compared. It is possible, with the aid of calculations, to obtain a complete picture of the complex equilibria involved in C-substituted N-H-1,2,3-triazoles. The crystal structures of 4(5)-phenyl-1,2,3-triazole (4) and 4(5)-nitro-1,2,3-triazole (15) have been determined. In the gas phase, 2H tautomers b always predominate, while in aqueous solution, both 1H and 2H tautomers − a and b − are present. Finally, in the solid state, 1 exists as a 1:1 mixture of 1a and 1b, while 4 is in the 4b tautomeric form and 15 is a 1H tautomer 15a. These conclusions − a in the gas phase, a + b in solution, and equal probabilities of finding either a or b in the crystal − are probably general for all 1,2,3-triazoles.

Organic Thermochemistry at High ab Initio Levels. 1. A G2(MP2) and G2 Study of Cyclic Saturated and Unsaturated Hydrocarbons (Including Aromatics)

With the purpose of exploring the reliability of the enthalpies of formation calculated using G2 methods, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G2(MP2) and G2 levels through both atomization reactions and bond separation isodesmic reactions, and comparison with experimental values has been made. A linear relationship between the differences between experimental and calculated (from atomization reactions) heats of formation and the number of formal double bonds is obtained.

Structure-Energy Relationship in Barbituric Acid: A Calorimetric, Computational, and Crystallographic Study

This paper reports the value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K for barbituric acid. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and transference (transpiration) method in a saturated N2 stream and a gas-phase enthalpy of formation value of -(534.3 +/- 1.7) kJ x mol(-1) was determined at T = 298.15 K. G3-calculated enthalpies of formation are in very good agreement with the experimental value. The behavior of the sample as a function of the temperature was studied by differential scanning calorimetry, and a new polymorph of barbituric acid at high temperature was found. In the solid state, two anhydrous forms are known displaying two out of the six hydrogen-bonding patterns observed in the alkyl/alkenyl derivatives retrieved from the Cambridge Crystallographic Database. The stability of these motifs has been analyzed by theoretical calculations. X-ray powder diffraction technique was used to establish to which polymorphic form corresponds to the commercial sample used in this study and to characterize the new form at high temperature.

Experimental and computational thermochemical study of α-alanine (DL) and β-alanine.

This paper reports an experimental and theoretical study of the gas phase standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of α-alanine (DL) and β-alanine. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline α-alanine (DL) and β-alanine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g), and H2O(l), measured by static-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of both amino acids were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation at T = 298.15 K was derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpy of formation of α-alanine (DL) and β-alanine in the gaseous phase, Δ(f)H(m)°(g), as −426.3 ± 2.9 and −421.2 ± 1.9 kJ·mol(−1), respectively. Standard ab initio molecular orbital calculations at the G3 level were performed. Enthalpies of formation, using atomization reactions, were calculated and compared with experimental data. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out.

Comparison of brönsted acidities of neutral NH-acids in gas phase, dimethyl sulfoxide and water

Abstract The Bronsted acidities of several neutral NH-acids (substituted diphenylamine, substituted anilines and imides) were measured in the gas phase (pulsed FT ICR spectrometry), dimethyl sulfoxide and aqueous solution. Comparison of the Bronsted acidities of neutral NH-acids in the gas phase, dimethyl sulfoxide and water was also carried out. It was shown that substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into dimethyl sulfoxide and water. The strongest solvent-induced attenuation of the substituent effects is characteristic of the meta-substituted anilines whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by 2.83 times and with transfer into water by 4.13 times. At the same time, the reaction series of para and/or ortho-π-acceptor substituted anilines, amides, imides and substituted diphenylamines are less sensitive to a change in gas phase for DMSO or water. In the special case of para-acceptor substituted anilines it was demonstrated that the specific solvation induced an increase in the acidity of the para- and/or ortho-acceptor substituted anilines as compared with the behavior of the corresponding meta-substituted anilines by amounts up to 10 pKa units.

Experimental and theoretical study of the structures and enthalpies of formation of 3H-1,3-benzoxazole-2-thione, 3H-1,3-benzothiazole-2-thione, and their tautomers.

This paper reports an experimental and theoretical study of the structures and standard (p(o) = 0.1 MPa) molar enthalpies of formation of 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione. The enthalpies of combustion and sublimation were measured by rotary bomb combustion calorimetry and the Knudsen effusion technique, and gas-phase enthalpies of formation values at T = 298.15 K of (42.0 +/- 2.7) and (205.5 +/- 3.8) kJ x mol(-1) for 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione, respectively, were determined. G3-calculated enthalpies of formation are in excellent agreement with the experimental values. The present work discusses the question of tautomerism explicitly for both compounds and compares the energetics of all the related species. A comparison of the theoretical results with the structural data is also reported.

Experimental and Computational Thermochemical Study and Solid-Phase Structure of 5,5-Dimethylbarbituric Acid

This paper reports an experimental and computational thermochemical study on 5,5-dimethylbarbituric acid and the solid-phase structure of the compound. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(706.4 +/- 2.2) kJ x mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB(2) stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 +/- 0.5) kJ x mol(-1). From these results a value of -(590.6 +/- 2.3) kJ x mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in reasonable agreement with the experimental value. 5,5-Dimethylbarbituric acid was characterized by single crystal X-ray diffraction analysis. In the crystal structure, N-H...O=C hydrogen bonds lead to the formation of ribbons connected further by weak C-H...O=C hydrogen bonds into a three-dimensional network. The molecular and supramolecular structures observed in the solid state were also investigated in the gas phase by DFT calculations.

Gas-Phase Basicities Around and Below Water Revisited

This work employs Fourier transform ion cyclotron resonance (FT-ICR) and the Gaussian quantum chemistry composite methods W1 and G2 to experimentally and computationally analyze gas-phase basicities (GB) for a series of weak bases in the basicity region around and below water. The study aims to clarify the long-standing discrepancy between reported GB values for weak bases obtained via high-pressure mass spectrometry (HPMS) and ICR; the ICR scale is observed to be more than 2 times contracted compared to the HPMS scale. The computational results of this work support published HPMS data. This agreement improves with increasing sophistication of the computational method and is excellent at the W1 level. Several equilibria were also re-examined experimentally using FT-ICR. In the experiments with some polyfluorinated weak bases (hexafluoro-2-propanol and nonafluoro-2-methyl-2-propanol), it was found that two protonation processes compete in the gas phase: protonation on oxygen and protonation on fluorine. In these species, protonation on fluorine proceeds faster and is statistically favored over protonation on oxygen but leads to cations that are thermodynamically less stable than oxygen-protonated cations. The process may also lead to the irreversible loss of HF. The rearrangement of fluorine-protonated cations to oxygen-protonated cations is very slow and is further suppressed by the process of HF abstraction. These results at least partially explain the discrepancy between published HPMS data and earlier FT-ICR findings and call for the utmost care in using FT-ICR for gas-phase basicity measurements of heavily fluorinated compounds. The narrower dynamic range of ICR necessitates the measurement of several problematic bases and produces some differences between the ICR results in the present work and the published HPMS data; the wider dynamic range allows HPMS to overcome these difficulties in connecting the ladder.

Experimental and theoretical study of the structures and enthalpies of formation of the synthetic reagents 1,3-thiazolidine-2-thione and 1,3-oxazolidine-2-thione.

This paper reports an experimental and a theoretical study of the structures and standard (p(o) = 0.1 MPa) molar enthalpies of formation of the synthetic reagents 1,3-thiazolidine-2-thione [CAS 96-53-7] and 1,3-oxazolidine-2-thione [CAS 5840-81-3]. The enthalpies of combustion and sublimation were measured by rotary bomb combustion calorimetry, and the Knudsen effusion technique and gas-phase enthalpies of formation values at T = 298.15 K of (97.1 +/- 4.0) and -(74.4 +/- 4.6) kJ.mol(-1) for 1,3-thiazolidine-2-thione and 1,3-oxazolidine-2-thione, respectively, were determined. G3-calculated enthalpies of formation are in reasonable agreement with the experimental values. In the solid state, 1,3-thiazolidine-2-thione exists in two polymorphic forms (monoclinic and triclinic) and 1,3-oxazolidine-2-thione exits in the triclinic form. The isostructural nature of these compounds and comparison of their molecular and crystal structures have been analyzed. The experimental X-ray powder diffractograms have been compared with the calculated patterns from their structures for identification of the polymorphic samples used in this study. A comparison of our results with literature thermochemical and structural data for related compounds is also reported.

Empirical treatment of solvent-solute interactions: medium effects on the electronic absorption spectrum of ?-carotene

Solvent effects on the wavenumber of the maximum of the longest wavelength electronic absorption band of all-trans-β-carotene were determined in 34 solvents. Together with results from previous studies, a data set for 51 solvents, mostly non-hydrogen bond donors, was constructed. This information was analyzed in terms of reaction field models and also showed its value for correlation purposes when used either alone or in combination with standard empirical solvent polarity–polarizability scales.