Flammability limits of olefinic and saturated fluoro-compounds.

Flammability limits were measured for a number of olefinic and saturated fluoro-compounds in a 12l spherical glass vessel. The obtained data together with the ones of previous studies have been analyzed based on the F-number scheme of flammability limits. The flammability limits of these compounds have been found to be explained very well by the present scheme of interpretation. The flammability limits are dependent upon distribution of F atoms in a molecule as well as upon F-substitution rate itself. It has been found that -O-CF(3) group in a molecule conspicuously decreases the flammability of the compound, while -C-CF(3) group does not much. For olefinic compounds, distribution of F atoms around double bonds markedly diminishes the flammability of the molecule.

A study on flammability limits of fuel mixtures.

Flammability limit measurements were made for various binary and ternary mixtures prepared from nine different compounds. The compounds treated are methane, propane, ethylene, propylene, methyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. The observed values of lower flammability limits of mixtures were found to be in good agreement to the calculated values by Le Chatelier's formula. As for the upper limits, however, some are close to the calculated values but some are not. It has been found that the deviations of the observed values of upper flammability limits from the calculated ones are mostly to lower concentrations. Modification of Le Chatelier's formula was made to better fit to the observed values of upper flammability limits. This procedure reduced the average difference between the observed and calculated values of upper flammability limits to one-third of the initial value.

Burning velocity measurements of nitrogen-containing compounds.

Burning velocity measurements of nitrogen-containing compounds, i.e., ammonia (NH3), methylamine (CH3NH2), ethylamine (C2H5NH2), and propylamine (C3H7NH2), were carried out to assess the flammability of potential natural refrigerants. The spherical-vessel (SV) method was used to measure the burning velocity over a wide range of sample and air concentrations. In addition, flame propagation was directly observed by the schlieren photography method, which showed that the spherical flame model was applicable to flames with a burning velocity higher than approximately 5 cm s(-1). For CH3NH2, the nozzle burner method was also used to confirm the validity of the results obtained by closed vessel methods. We obtained maximum burning velocities (Su0,max) of 7.2, 24.7, 26.9, and 28.3 cm s(-1) for NH3, CH3NH2, C2H5NH2, and C3H7NH2, respectively. It was noted that the burning velocities of NH3 and CH3NH2 were as high as those of the typical hydrofluorocarbon refrigerants difluoromethane (HFC-32, Su0,max=6.7 cm s(-1)) and 1,1-difluoroethane (HFC-152a, Su0,max=23.6 cm s(-1)), respectively. The burning velocities were compared with those of the parent alkanes, and it was found that introducing an NH2 group into hydrocarbon molecules decreases their burning velocity.

Flammability limits of isobutane and its mixtures with various gases.

Flammability limits of isobutane and five kinds of binary mixtures of isobutane were measured by the ASHRAE method. Propane, nitrogen, carbon dioxide, chloroform, and HFC-125 (1,1,1,2,2-pentafluoroethane) were used as the counter part gases in the mixtures. The observed data were analyzed using the equations based on Le Chatelier's formula. The flammability limits of mixtures with propane were well explained by the original Le Chatelier's formula. The flammability limits of mixtures with nitrogen and the ones with carbon dioxide were adequately analyzed by the extended Le Chatelier's formula. It was found that the extended Le Chatelier's formula is also applicable to the flammability limits of mixtures with chloroform and HFC-125.

Analysis of the intermolecular interactions between CH3OCH3, CF3OCH3, CF3OCF3, and CH2F2, CHF3.

The intermolecular interaction energy curves of CH(3)OCH(3)-CH(2)F(2), CF(3)OCH(3)-CH(2)F(2), CF(3)OCF(3)-CH(2)F(2), CH(3)OCH(3)-CHF(3), CF(3)OCH(3)-CHF(3), and CF(3)OCF(3)-CHF(3) complexes were calculated by the MP2 level ab initio molecular orbital method using the 6-311G** basis set augmented with diffuse polarization functions. We investigate the fluorine substitution effects of both methane and dimethyl ether on intermolecular interactions. In addition, orientation dependence of intermolecular interaction energies is also studied with utilizing eight types of orientations. Our analyses demonstrate that partial fluorinations of methane make electrostatic interaction dominant, and consequently enhance attractive interaction at several specific orientations. On the contrary, fluorine substitutions of dimethyl ether substantially decrease the electrostatic interaction between ether and CH(2)F(2) or CHF(3); thus, there is no such characteristic interaction between the C-H of fluorinated methane and ether oxygen of CF(3)OCF(3) as conventional hydrogen bonding, due to reduced polarity of fluorinated ether. The combination of different pairs of the electrostatic interaction is therefore responsible for the intermolecular interaction differences among the complexes investigated herein and also their orientations.

Ab initio investigation on the reaction path and rate for the gas-phase reaction of HO + H2O <--> H2O + OH.

This article describes an ab initio investigation on the potential surfaces for one of the simplest hydrogen atom abstraction reactions, that is, HO + H2O <--> H2O + OH. In accord with the findings in the previously reported theoretical investigations, two types of the hydrogen-bonding complexes [HOH--OH] and [H2O--HO] were located on the potential energy surface. The water molecule acts as a hydrogen donor in the [HOH--OH] complex, while the OH radical acts as a hydrogen donor in the [H2O--HO] complex. The energy evaluations at the MP2(FC) basis set limit, as well as those through the CBS-APNO procedure, have provided estimates for enthalpies of association for these complexes at 298 K as -2.1 approximately -2.3 and -4.1 approximately -4.3 kcal/mol, respectively. The IRC calculations have suggested that the [H2O--HO] complex should be located along the reaction coordinate for the hydrogen abstraction. Our best estimate for the classical barrier height for the hydrogen abstraction is 7.8 kcal/mol, which was obtained from the CBS-APNO energy evaluations. After fitting the CBS-APNO potential energy curve to a symmetrical Eckart function, the rate constants were calculated by using the transition state theory including the tunneling correction. Our estimates for the Arrhenius parameters in the temperature region from 300 to 420 K show quite reasonable agreement with the experimentally derived values.

Ab initio study on the structures of fluorinated formates and hydrogen abstraction reaction with OH radical.

The conformational potential energy surfaces for mono- and difluoromethyl formate have been determined by using a modified G2(MP2) level of calculations. The structures and vibrational frequencies for the conformers of mono- and difluoromethyl formate have been reported. The hydrogen abstraction reaction channels between these two formates and OH radicals have been studied at the same level of theory. Using the standard transition state theory and taking into account the effect of tunneling across the reaction barrier, we have estimated the rate constant for hydrogen abstraction by OH radical. The effect of successive fluorine substitution for methyl hydrogen on the conformational stability and on the hydrogen abstraction rate has been analyzed.

Analysis of the intermolecular interaction between CH(3)OCH(3), CF(3)OCH(3), CF(3)OCF(3), and CH(4): high level ab initio calculations.

The intermolecular interaction energies of the CH(3)OCH(3)-CH(4), CF(3)OCH(3)-CH(4), and CF(3)OCF(3)-CH(4) systems were calculated by ab initio molecular orbital method with the electron correlation correction at the second order Møller-Plesset perturbation (MP2) method. The interaction energies of 10 orientations of complexes were calculated for each system. The largest interaction energies calculated for the three systems are -1.06, -0.70, and -0.80 kcal/mol, respectively. The inclusion of electron correlation increases the attraction significantly. It gains the attraction -1.47, -1.19, and -1.27 kcal/mol, respectively. The dispersion interaction is found to be the major source of the attraction in these systems. In the CH(3)OCH(3)-CH(4) system, the electrostatic interaction (-0.34 kcal/mol) increases the attraction substantially, while the electrostatic energies in the other systems are not large. Fluorine substitution of the ether decreases the electrostatic interaction, and therefore, decreases the attraction. In addition the orientation dependence of the interaction energy is decreased by the substitution.

RF number as a new index for assessing combustion hazard of flammable gases.

A new index called RF number has been proposed for assessing the combustion hazard of all sorts of flammable gases and their mixtures. RF number represents the total expectancy of combustion hazard in terms of flammability limits and heat of combustion for each known and unknown compounds. The advantage of RF number over others such as R-index and F-number for classification of combustion hazard has been highlighted.