Enthalpies of Sublimation of M(η5-C5H5)2Cl2 (M = Ti, Zr, Hf, V, Nb, W) Complexes

Diogo, Hermı́nio P.; Piedade, Manuel E. Minas da; Gonçalves, Jorge M.; Monte, Manuel J.S.; Silva, Manuel A.V. Ribeiro da

doi:10.1002/1099-0682(20011)2001:1<257::aid-ejic257>3.0.co;2-2

Cited by 18 publications

(11 citation statements)

References 20 publications

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“…The Knudsen effusion method was also used to obtain the enthalpy of sublimation of piracetam (Form II) from vapor pressure against temperature measurements. The apparatus and operating procedure were previously described 34–37. The bottom of the vacuum chamber consisted of a brass block in which up to three effusion cells can be inserted.…”

Section: Methodsmentioning

confidence: 99%

Thermodynamic Characterization of Three Polymorphic Forms of Piracetam

Picciochi¹,

Diogo²,

Piedade

2011

Journal of Pharmaceutical Sciences

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Section: Methodsmentioning

confidence: 99%

Thermodynamic Characterization of Three Polymorphic Forms of Piracetam

Picciochi¹,

Diogo²,

Piedade

2011

Journal of Pharmaceutical Sciences

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“…Enthalpy of Sublimation Measurements. The enthalpy of sublimation of nicotinic acid was determined from vaporpressure measurements by the Knudsen effusion method [39][40][41] and by Calvet microcalorimetry. 42,43 The Knudsen effusion setup was a modified version of that previously described.…”

Section: Methodsmentioning

confidence: 99%

“…42,43 The Knudsen effusion setup was a modified version of that previously described. [39][40][41] The main change consisted in the adaptation of a new cylindrical brass block, which can accommodate up to three bronze cells with different holes, to the bottom of the vacuum chamber. The block was surrounded by a tubular furnace whose temperature was controlled to better than (0.1 K by a Eurotherm 902P thermostatic unit driven by a K type thermocouple embedded in the block.…”

Section: Methodsmentioning

confidence: 99%

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Energetics and Structure of Nicotinic Acid (Niacin)

et al. 2010

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The standard molar enthalpies of formation and sublimation of crystalline (monoclinic, space group P2(1)/c) nicotinic acid (NA), at 298.15 K, were determined as Delta(f)H(m)(o)(NA, cr) = -344.7 +/- 1.2 kJ x mol(-1) and Delta(sub)H(m)(o)(NA) = 112.1 +/- 0.5 kJ x mol(-1) by using combustion calorimetry, drop-sublimation Calvet microcalorimetry, and the Knudsen effusion method. The experimental determinations were all based on a sample of NIST Standard Reference Material 2151, which was characterized in terms of chemical purity, phase purity, and morphology. From the above results, Delta(f)H(m)(o)(NA, g) = -232.6 +/- 1.3 kJ x mol(-1) could be derived. On the basis of this value and on published experimental data, the enthalpy of the isodesmic reaction nicotinic acid(g) + benzene(g) --> benzoic acid(g) + pyridine(g) was calculated as -3.6 +/- 2.7 kJ x mol(-1) and compared with the corresponding predictions by the B3LYP/cc-pVTZ (-3.6 kJ x mol(-1)), B3LYP/aug-cc-pVTZ (-3.7 kJ x mol(-1)), B3LYP/6-311++G(d,p) (-4.2 kJ x mol(-1)), G3MP2 (-4.3 kJ x mol(-1)), and CBS-QB3 (-4.0 kJ x mol(-1)) quantum chemistry models. The excellent agreement between the experimental and theoretical results supports the reliability of the Delta(f)H(m)(o) (NA, cr), Delta(sub)H(m)(o)(NA), and Delta(f)H(m)(o)(NA, g) recommended in this work. These data can therefore be used as benchmarks for discussing the energetics of nicotinic acid in the gaseous and crystalline states and, in particular, to evaluate differences imparted to solid forms by the production and processing methods. Such differences are perhaps at the root of the significant inconsistencies found between the published enthalpies of sublimation of this important active pharmaceutical ingredient and thermochemical standard. The molecular packing in the crystalline phase studied in this work was also discussed and its influence on the molecular structure of nicotinic acid was analyzed by comparing bond distances and angles published for the solid state with those predicted by the B3LYP/cc-pVTZ method. No advantage in terms of accuracy of the structural predictions was found by the use of the larger aug-cc-pVTZ or 6-311++G(d,p) basis sets.

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“…Enthalpy of Sublimation Measurements. The enthalpies of sublimation of E - and Z -heptacyclene were determined by using the Knudsen effusion apparatus and operating procedure previously described . The temperature of the tubular furnace surrounding the brass block containing the effusion cells (up to three cells with effusion holes of different dimensions can be inserted in the block) was controlled to better than ±0.1 K by a Eurotherm 902P thermostatic unit and a K-type thermocouple placed in contact with the inner wall of the furnace.…”

Section: Methodsmentioning

confidence: 99%

Energetics of the Thermal Dimerization of Acenaphthylene to Heptacyclene

et al. 2006

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The energetics of the thermal dimerization of acenaphthylene to give Z- or E-heptacyclene was investigated. The standard molar enthalpy of the formation of monoclinic Z- and E-heptacyclene isomers at 298.15 K was determined as Delta(f)H(m)o (E-C24H16, cr) = 269.3 +/- 5.6 kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, cr) = 317.7 +/- 5.6 kJ x mol(-1), respectively, by microcombustion calorimetry. The corresponding enthalpies of sublimation, Delta(sub)H(m)o (E-C24H16) = (149.0 +/- 3.1) kJ x mol(-1) and Delta(sub)H(m)o (Z-C24H16) = (128.5 +/- 2.3) kJ x mol(-1) were also obtained by Knudsen effusion and Calvet-drop microcalorimetry methods, leading to Delta(f)H(m)o (E-C24H16, g) = (418.3 +/- 6.4) kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, g) = (446.2 +/- 6.1) kJ x mol(-1), respectively. These results, in conjunction with the reported enthalpies of formation of solid and gaseous acenaphthylene, and the entropies of acenaphthylene and both hepatcyclene isomers obtained by the B3LYP/6-31G(d,p) method led to the conclusion that at 298.15 K the thermal dimerization of acenaphthylene is considerably exothermic and exergonic in the solid and gaseous states (although more favorable when the E isomer is the product), suggesting that the nonobservation of the reaction under these conditions is of kinetic nature. A full determination of the molecular and crystal structure of the E dimer by X-ray diffraction is reported for the first time. Finally, molecular dynamics computer simulations on acenaphthylene and the heptacyclene solids were carried out and the results discussed in light of the corresponding structural and Delta(sub)H(m)o data experimentally obtained.

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Enthalpies of Sublimation of M(η5-C5H5)2Cl2 (M = Ti, Zr, Hf, V, Nb, W) Complexes

Abstract: The standard molar enthalpies of sublimation of M(η 5 -C 5 H 5 )Cl 2 (M = Ti, V, Zr, Nb, Hf, and W) compounds, at 298.15 K, were determined by the Knudsen effusion method to be: 124.4±2.9 kJmol −1 (M = Ti), 108.5±4.6 kJmol −1 (M =

Cited by 18 publications

References 20 publications

Thermodynamic Characterization of Three Polymorphic Forms of Piracetam

Thermodynamic Characterization of Three Polymorphic Forms of Piracetam

Energetics and Structure of Nicotinic Acid (Niacin)

Energetics of the Thermal Dimerization of Acenaphthylene to Heptacyclene

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