Structure of 1-(1-adamantyl)pyrazoles : Crystal geometry and carbon-13 nmr spectroscopy

Crystal structures of 1-(1-adamantyl)pyrazole, 1a, and 1-(1-adamantyl-3-ol)-4-nitropyrazole, 2a, have been solved by X-ray analysis. The space groups and cell parameters are P2₁, a, 7.4021(3), b, 10.7529(5),c, 6.9651(2)Å, β, 90.206(3)° for 1a with Z = 2 and P2/n, a, 31.1172(14), b, 6.8506(1), c, 12.0313(3)Å, β 94.873(3)° for 2a with Z = 8. Refinements were carried out down to R values of 0.043 (R_w, = 0.046) and 0.079 (R_w = 0.061) for the 951 (2σ(I)) and 2461 (3σ(I)) observed reflections respectively. The conformation about the bond between the heterocycle and the carbocycle is discussed on theoretical grounds INDO calculations): the adamantane behaves as a free rotor. The stcricinteractions of the adamantyl residue with the methyl substituents in 2- and 5-position of pyrazole are apparent in the C-13 chemical shifts.     

Crystal and molecular structure of three biologically active nitroindazoles

3-Bromo-1-methyl-7-nitro-1H-indazole (1), 3-bromo-2-methyl-7-nitro-2H-indazole (2) and 3,7-dinitro-1(2)H-indazole (3) have been synthesized and characterized by X-ray diffraction, ¹³C and ¹⁵N NMR spectroscopy in solution and in solid-state. The dihedral angles obtained in the crystal structures are in good agreement with the molecular parameters calculated using DFT B3LYP calculations employing the 6-311++G(d,p) basis set. Compounds 1 and 2 present intermolecular halogen bonds between the bromine and the oxygen atoms of the nitro group and in compound 3 inter- and intramolecular hydrogen bonding exists.     

Why Does Pivalaldehyde (Trimethylacetaldehyde) Unexpectedly Seem More Basic Than 1‐Adamantanecarbaldehyde in the Gas Phase? FT‐ICR and High‐Level Ab Initio Studies

Fourier transform ion cyclotron resonance spectroscopy (FT ICR) techniques, including collision-induced dissociation (CID) methodology, were applied to the study of the gas-phase protonation of pivalaldehyde (1) and 1-adamantanecarbaldehyde (2). A new synthetic method for 2 was developed. The experiments, together with a thorough computational study involving ab initio and density functional theory (DFT) calculations of high level, conclusively show that upon monoprotonation in the gas phase, compound 1 yields monoprotonated methyl isopropyl ketone 3. The mechanism of this gas-phase acid-catalyzed isomerization is different from that reported by Olah and Suryah Prakash for the reaction in solution. In the latter case, isomerization takes place through the diprotonation of 1.     

Conservative methods for the Toda lattice equations

We are concerned with the numerical integration of the Todalattice equations by using different conservative methods. Numericalexperiments suggest that the global error for isospectral schemesdecreases exponentially with time but it is almost constantfor either symplectic or more general integrators. We providea theoretical explanation for these experimental findings.     

Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol⁻¹. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol⁻¹. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol⁻¹. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes.