Gary Nichols

Sublimation enthalpies at 298.15 K using correlation gas chromatography and differential scanning calorimetry measurements

The sublimation enthalpies of 17 hydrocarbons are obtained by combining the technique of correlation gas chromatography (CGC), to evaluate vaporization enthalpies at 298.15 K, and differential scanning calorimetry (DSC) to measure fusion enthalpies. Vaporization enthalpies at 298.15 K obtained by CGC are compared to values measured directly from vapor pressure measurements at temperatures above the melting point by adjusting the experimental vaporization enthalpy for the effects of temperature. Vaporization enthalpies obtained by these two methods agree within3877 J mol ˇ1 . Fusion enthalpies are similarly adjusted for temperature. Sublimation enthalpies, obtained by combining temperature adjusted fusion, and vaporization enthalpies agree within2580 J mol ˇ1 . The sublimation enthalpies of azulene and 1,8-cyclotetradecadiyne are also measured by head-space analysis resulting in values of 76880 and 94348 J mol ˇ1 at 298.15 K, respectively. # 1998 Elsevier Science B.V.

The estimation of melting points and fusion enthalpies using experimental solubilities, estimated total phase change entropies, and mobile order and disorder theory.

Melting points and fusion enthalpies are predicted for a series of 81 compounds by combining experimental solubilities in a variety of solvents and analyzed according to the theory of mobile order and disorder (MOD) and using the total phase change entropy estimated by a group additivity method. The error associated in predicting melting points is dependent on the magnitude of the temperature predicted. An error of +/- 12 K (+/- 1 sigma) was obtained for compounds melting between ambient temperature and 350 K (24 entries). This error increased to +/- 23 K when the temperature range was expanded to 400 K (46 entries) and +/- 39 K for the temperature range 298-555 K (79 entries). Fusion enthalpies were predicted within +/- 2sigma of the experimental values (+/- 6.4 kJ mol(-1)) for 79 entries. The uncertainty in the fusion enthalpy did not appear dependent on the magnitude of the melting point. Two outliers, adamantane and camphor, have significant phase transitions that occur below room temperature. Estimates of melting temperature and fusion enthalpy for these compounds were characterized by significantly larger errors.

Structural characterization of crystalline inclusion complexes formed from 1,3,5-triaroylbenzene derivatives—a new family of inclusion hosts

A series of crystalline inclusion complexes formed from substituted 1,3,5-triaroylbenzene hosts and small molecule guests has been structurally characterized by X-ray crystallography. The new inclusion hosts examined do not possess functional groups capable of participating in strong non-covalent interactions, thus C–H⋯O hydrogen bonding appears to significantly influence the crystal packing observed in most of the solid state structures. The thermochemical properties of the inclusion complexes were examined using differential scanning calorimetry.

A Protocol for Deriving Values for Δfus H m(298.15 K) and Δvap H m(298,15 K). Applications in Obtaining Δsub H m (298.15 K)

An indirect method for obtaining sublimation enthalpies is described. The method consists of combining experimental or estimated fusion enthalpies adjusted to 298.15 K with either vaporization enthalpies measured at elevated temperatures and adjusted for temperature or obtained directly at 298.15 K by correlation gas chromatography. Some relationships used to adjust phase change enthalpies with temperature are discussed and their use is demonstrated on a series of compounds. These equations are used to adjust the fusion enthalpies of hydrocarbons to 298.15 K. The corresponding fusion entropies at 298.15 K are parameterized using group additivity relationships and the resulting group values are used to estimate fusion entropies and enthalpies at 298.15 K. The techniques discussed are applied to the evaluation of the sublimation enthalpies of perylene and coronene.

Theoretical study of the 4-pyridylmethyl and the 2- and 5-pyrimidyl-methyl cations and radicals

Geometry optimized SCF–MO calculations in the INDO approximation have been performed on the 4-pyridyl-methyl (1), 2-pyrimidylmethyl (3), and 5-pyrimidylmethyl (5) radicals and their corresponding cations (2), (4), and (6). The spin distribution in (1), (3), and (5) and the charge distribution in (2), (4), and (6) provide evidence that mesomeric effects in these systems are similar to those in the benzyl radical and cation. The optimized geometries all have a quinonoid distortion which is more pronounced in the cation. The π-bond orders and the ring–CH2·(or ring-CH2+) rotational barriers were obtained. The results are discussed in terms of resonance hybrid structures. The ionization potentials of the radicals were obtained and found to be in the order (3) > (1) > (5). The calculated ionization potential of (1) was 8.71 V in good agreement with the experimental value of 8.40 V obtained by electron impact measurements.