Heat Of Formation Values Research Articles

Molecular mechanics study on conformation of perylene-quinonoid photosensitizers

Using molecular mechanics method, values of the heat of formation (HF) of different conformations of perylenequinonoid photosensitizer, hypocrellin A (HA) and hypocrellin B (HB) were calculated and the variances of HF after phenolic protons’ dissociation were calculated as well. The following was found: (i) The HF values of four conformational isomers of HA and HB are similar to each other, so the four isomers can transform to each other at room temperature. (ii) There exists the difference between the ability of dissociation of phenolic protons of HA and that of HB, the former is higher than the latter. (iii) There exist two intramolecular hydrogen bonds in HA and HB. The bond energy is approximately 8 kJ/mol and the energy of conformation I is lower than that of conformation II The bond energy of HA is lower than that of HB. (iv) There exists a low energy spot when phenolic hydroxyl bond twists 180 from the psition where hydrogen bond is formed, which suggests that this kind of conformation probably exists. (v) The phenolic hydrogen of HBMC forms hydrogen bond with the neighbouring nitrogen, which is its structure basis of possessing photosensitizing activity.

Sodium Fire Code SFIRE1C for Pool Fire Characteristics

Sodium pool fire code, SOFIRE II, written for the constant value of stoichiometric combustion ratio and heat of reaction is used to compute the buildup of pressure and temperature in a containment. In the SOFIRE II model, for the formation of a mixture of Na2O and Na2O2 in the sodium pool, the input stoichiometric combustion ratio and heat of formation values need to be varied to corresponding values admissible for the mixture. In the present work, the SOFIRE II one-cell model is revised and the present version SFIRE1C (Sodium FIRE 1 Cell model) accounts for the formation of Na2O in an early stage of the fire and shifts to the formation of Na2O2 at a later stage. Thus SFIRE1C computes in a more realistic manner the reaction products which are formed in the pool. The model for sodium oxide aerosol release is also modified in this version, by incorporating a more appropriate aerosol release rate equation. The calculated values using the SFIRE1C one-cell model are compared with sodium pool fire experimental results.

Sodium Fire Code SFIRE1C for Pool Fire Characteristics.

Sodium pool fire code, SOFIRE II, written for the constant value of stoichiometric combustion ratio and heat of reaction is used to compute the buildup of pressure and temperature in a containment. In the SOFIRE II model, for the formation of a mixture of Na2O and Na2O2 in the sodium pool, the input stoichiometric combustion ratio and heat of formation values need to be varied to corresponding values admissible for the mixture. In the present work, the SOFIRE II one-cell model is revised and the present version SFIRE1C (Sodium FIRE 1 Cell model) accounts for the formation of Na2O in an early stage of the fire and shifts to the formation of Na2O2 at a later stage. Thus SFIRE1C computes in a more realistic manner the reaction products which are formed in the pool. The model for sodium oxide aerosol release is also modified in this version, by incorporating a more appropriate aerosol release rate equation. The calculated values using the SFIRE1C one-cell model are compared with sodium pool fire experimental ...

Fast Flow Kinetic Studies of the Reaction CH2OH + HCl ⇋ CH3OH + Cl. The Heat of Formation of Hydroxymethyl

AbstractThe kinetics of the chlorination equilibrium of methanol, CH2OH + HCl ⇋ CH3OH + Cl (1, – 1), have been studied using the fast flow technique with Laser Magnetic Resonance and Electron Paramagnetic Resonance detection. The rate constants of the forward and reverse reactions were found to be k1 = (2.3±0.9) ·1011 exp [‐(20.9±2.9) kJ·mol−1/RT] cm3/mol·s in the temperature range 500 K≤T≤812 K and k‐1 = (3.7±0.4)±1013 cm3/mol·s at room temperature. These kinetic results were utilized in third law and second law procedures to obtain the value of the heat of formation of the hydroxymethyl radical. The entropy of CH2OH, S0f,298(CH2OH) = (254±4) J/mol·K, and the activation energy of reaction (‐1), E‐1 = (0±4) kJ/mol, which are required for the calculations, were selected by critical assessment of the available literature data. The average of the third law and second law determinations provided SfH0298(CH2OH) = ‐(9±6) kJ/mol. This value is higher by 17 kJ/mol than the heat of formation value in common use and implies a stronger C – H bond energy in methanol, D298(H–CH2OH) = 410 kJ/mol. These new thermochemical quantities are in excellent agreement with the very recent recommendations of Seetula and Gutman [J. Phys. Chem. 96, 5401 (1992)] and have important implications for modeling studies of complex chemical systems such as the combustion of methanol.

Classical conformational analysis of strained organic molecules. I. [l,m,n]Propellanes with l,m,n equal to 2, 3, and 4

AbstractClassical conformational analysis was applied to the study of the variation of heats of formation (HOF) and geometrical parameters of small‐ring propellanes. As expected, the calculated HOF is the smallest for [4,4,4] propellane I and the largest for [2,2,2]propellane X. In spite of the different trends exhibited by the strain energy components, the calculated HOF values may be reproduced within a simple additivity scheme. The calculated bridgehead–bridgehead bond length is the longest for [2,2,2]propellane (1.655 Å), lowers with increasing molecular size, and is equal to 1.558 Å for [4,4,4]propellane. In agreement with experimental findings the latter value is significantly larger than the standard value of 1.54 Å typical for unstrained hydrocarbons. Other calculated geometrical parameters as well as HOF values cannot be compared with experimental and/or theoretical results since there is a lack of reliable experimental data. Surprisingly, [2,2,2]propellane was found to be more flexible than the less strained bicyclo[2,2,2]octane.

A thermochemical approach to the gas-phase reduction of metal sulphates

The reduction of different metal sulphates by hydrogen occurs at different temperatures and can lead to a variety of products. It has been shown that the starting temperatures for the reduction of several sulphates are directly related to the magnitude of their (exothermic) heats of formation per equivalent. The thermochemical data also provide a basis for the interpretation of the relative thermal stabilities (under nitrogen atmosphere) of several metal sulphates. A consideration of the appropriate heat of formation values can also be useful in understanding the different reduction products obtained for different metal sulphates. Finally, the reduction behaviour of some binary metal sulphates is also consistent with the thermochemical data.

An investigation of arsenic and antimony additives in unseeded and potassium-seeded [formula omitted] flames

Studies have been completed of the reactions involved with arsenic and antimony additives in H 2 O 2 flames and H 2 O 2 flames seeded with potassium. The flame was atmospheric and a molecular beam of the frozen species was introduced into the mass spectrometer through a four-stage differential pumping system operating from 1 to 10 −11 atm. Arsenic compounds found in the flame included AsO(g), HAsO 3(g) and H 3AsO 4(g). Heat of formation values, ΔH f298 , were calculated as −145.4±5 kcal/mole for HAsO 3(g) and −259.7±5 kcal/mole for H 3AsO 4(g). Antimony compounds included Sb(g), SbO(g), HSbO 3(g) and H 3SbO 4(g), yielding ΔH f298 values of 18.7±5 kcal/mole for SbO(g), −137.4±5 kcal/mole for HSbO 3(g), and −262.9±5 kcal/mole for H 3SbO 4(g). The formation of the potassium compounds KH 2AsO 4(g) and KH 2SbO 4(g) was also established. Heats of formation values, ΔH f298, obtained for the K-substituted compounds were −299.5±5 and −303.7±5 kcal/mole for KH 2AsO 4(g) and KH 2SbO 4(g), respectively.

The true potential energy curves, Morse r-centroids and Franck-Condon factors for the bands of the E2Σ+- X2Σ+ system of CN

The true potential energy curves for the bands of E 2 Σ +- X 2 Σ + system of the astrophysically important CN molecule have been calculated by RKRV and authors new methods and the agreement between the methods is found to be excellent. The ground state dissociation energy for the CN molecule calculated from the three-parameter Lippincott function is found to be in excellent agreement with the heat of formation value. The r-centroids and Franck-Condon factors for the bands of this system are also calculated and presented.