Search

The adsorption of nitrogen (78 K) and neopentane (253 to 273 K) on two different samples of synthetic calcium carbonate has been used to characterize the state of their surface, with respect to microporosity. The need for well defined standards is revealed by the study of natural Ca/Mg carbonates.

Using the technique of molecular modulation spectrometry, we have measured directly the rate constants of several reactions involved in the oxidation of methyl radicals at room temperature: k1 is in the fall-off pressure regime at our experimental pressures (20–760 torr) where the order lies between second and third and we obtain an estimate for the second-orderlimit of (1.2 ± 0.6) × 10−12 cm3/molec · sec, together with third-order rate constants of (3.1 ± 0.8) × 10−31 cm6/molec2 · sec with N2 as third body and (1.5 ± 0.8) × 10−30 with neopentane; we cannot differentiate between k2a and k2c and we conclude k2a + (k2c) = (3.05 ± 0.8) × 10−13 cm3/molec · sec and k2b = (1.6 ± 0.4) × 10−13 cm3/molec · sec; k3 = (6.0 ± 1.0) × 10−11 cm3/molec · sec.

The gas‐phase Auger line shapes of the linear alkanes C1 through C6 and of neopentane are presented and analyzed. The general shape of the spectra are characteristic of carbon in a tetrahedral environment with the major feature in all cases occurring at ∼249 eV. The relatively large spectral changes found between methane and ethane results from the direct interaction of the terminal methyl groups in ethane, and the spectra of the higher alkanes are shown to be a composite of contributions from terminal methyl and interior methylene group carbon atoms. Theoretical analysis based on a one‐electron approximation is shown to be capable of making a molecular orbital assignment by comparing calculated vertical transitions to features in the Auger spectra of ethane and propane, and, in the case of ethane, of differentiating between the 2Eg and 2A1g assignment of the ground state of (C2H6)+. A one‐electron based molecular orbital treatment, however, is shown to partially break down in propane and neopentane. Anal...

The formation of benzene and cyclohexane from n-hexane above 300°C on films of iridium and of iridium—gold is believed to come about by ring enlargement of methylcyclopentane intermediate as well as by the direct 1,6-cyclization route previously proposed. Evidence for the former is two-fold. First, cyclization of both n-pentane and 3-methylpentane (showing feasibility of 1,5-cyclization) and skeletal rearrangement of both isobutane and n-pentane (showing feasibility of ring enlargement), in all cases in the presence of hydrogen, take place catalytically on films of iridium (above 250°C) and of iridium—gold (above 300°C). Secondly, methylcyclopentane has been detected among the products of n-hexane conversion on supported Ir and Ir—Au catalysts in a flow reactor. Study of the exchange reactions between methane and deuterium and between neopentane and deuterium lead to the conclusion that 1,1-diadsorbed alkane entities can form reversibly on iridium at relevant temperatures. This suggests that a carbene-type intermediate is possible for the 1,5-cyclization.

High and low frequency Raman scattering experiments have been performed in the whole temperature range of plastic neopentane (140–257 K). High frequency spectra are well accounted for by Larsson’s isotropic two‐step stochastic model when the vibrational contribution is not neglected. The discussion shows that the results are the best for the spectral part where the isotropic aspect of the model is less important. The low frequency spectra are explained by a transient dielectric anisotropy of the molecules induced by their deformation as they overcome a high potential barrier during reorientation movements. The analysis of both spectra provide consistent results for the temperature dependence of the fraction of molecules performing large amplitude rotations.

Physical adsorption of the title compounds on rhombic sulfur of 0.4 to 0.5 m2/g is reported. The isotherms are of type II for N2, Ar and C5H12, of type III for SO2 and CO2, and linear for SF6. There is no hysteresis. The method of Ross & Olivier shows that the surface is relatively heterogenous (γ 17). Isosteric heats of adsorption and c values of the B.E.T. equation are also reported.

Gas-liquid critical pressures of mixtures of octamethylcyclotetrasiloxane with neopentane, 2,3-dimethylbutane, cyclopentane and tetramethylsilane, critical temperatures of octamethylcyclotetra-siloxane + tetramethylsilane mixtures and the vapour pressure of tetramethylsilane near the critical region are reported. The critical temperature and pressure data for these and other mixtures are compared with values calculated from the van der Waals one fluid theory assuming different combining rules and equations of state. The limitations of the criticality condition solutions are discussed briefly.

Electron impact spectra of methane, ethane, and neopentane have been obtained at scattering angles of 0° and 90° and at impact energies from ?30 to 250 eV. The data are consistent with the lowest excitation in all of these systems to involve promotion of an electron to a 3s Rydberg‐like orbital. Differences between 0° and 90° onsets are attributed to large‐angle intensity enhancements of transitions to the 3s Rydberg triplets. At 90° all of the spectra exhibit very similar intensity redistributions with strong enhancement of transitions in the 12 and 15 eV region as compared to lower‐lying transitions. Assignments of the spectra and possible origins of the angular dependence are discussed.

Measurements of the photoconductivity of liquid n‐pentane, n‐hexane, cyclopentane, cyclohexane, neopentane, neohexane, and tetramethylsilane induced by far UV radiation (7⩽hν⩽10 eV) have been performed. We have studied the influence of the incident photon energy, of the applied electric field strength, and of the concentration of two electron scavengers (carbon tetrachloride and perfluoromethylcyclohexane). Our results show that in going from the gas phase to the liquid phase, the ionization energy threshold values of these low dielectric constant liquids are lowered by about 1.6 eV. The introduction of an electron scavenger reduces drastically the solvent photocurrent. Our results are consistent with the quenching, by the scavenger, of a solvent expanded excited state, precursor of the geminate ion pair, and not with an epithermal electron scavenging as in high energy photon irradiation.

The structures and reactions of radical cations of prototype alkanes such as ethane, propane, isobutane, neopentane, etc. produced in SF6 and in other halocarbon matrices irradiated at 4 K have been extensively studied by ESR. The geometrical structures as well as the unpaired electron orbitals of these cations are unequivocally characterized based on the simple symmetry considerations as well as the INDO MO calculations. The ESR hyperfine coupling constants determined for these cations as well as their deuterated homologs give conclusive evidence for the interpretations. The unpaired electron in linear alkane cations is delocalized over the in‐plane C–H and C–C σ bonds forming delocalized σ radicals, whereas that in highly branched alkane cations is more confined to one of the C–C bonds forming localized σ radicals. The H1s spin density, giving a large hyperfine coupling in the linear alkane cations, is largely contributed from a direct participation of the C–H bond in the unpaired electron orbital, wher...

Mixtures of neopentane and hydrogen were pyrolyzed in a flow system at 826–968 K and 27–400 mm Hg. Measurements of the yields of CH4 and C2H6 at various conditions enabled calculation of the rate c...

The overtone spectra of ethane in the gas phase at room temperature and neopentane in the liquid phase at 0 °C are measured and analyzed in the region of the pure local mode overtones corresponding to ΔvCH=3, 4, and 5. Overtone spectra are also measured and analyzed for 2, 2, 3, 3‐tetramethylbutane in hexachlorobutadiene solution and for hexamethylbenzene in carbon tetrachloride solution, both at room temperature in the region of ΔvCH=3 and 4. For ethane, neopentane, and tetramethylbutane, the dominant contribution to the overtone band corresponds to a pure CH3 local mode overtone. Combination peaks between different CH3 local oscillators are characterized. Combination peaks are also identified involving a CH3 local oscillator and a low frequency normal mode, with the transitions originating from both the ground vibrational state and a state containing one quantum of the low frequency mode. A computer decomposition of the overtone bands is carried out and a satisfactory fit is achieved in terms of Lorentz...

The early stages (0.01–1%) of the pyrolysis of neopentane have been studied in a wall-less reactor between 945 and 1016 K in excess argon at 600 Torr (1 Torr = 133.3 N m–2). The radical chain mechanism deduced from conventional studies is shown to give a good account of the kinetics of formation of methane and ethane; the rate constants for chain initiation neo-C5H12→ t-C4H.9+ CH.3(1), and chain propagation by methyl radicals CH.3+ neo-C5H12→ CH4+ C5H.11(4), are obtained. The values of the latter agree with the results of conventional studies but the former are approximately ten times smaller than previous estimates. The implications of these rate constants for the thermodynamic values of the t-butyl radical are discussed. By using ΔfH(t-C4H.9) to obtain the activation energy, the Arrhenius equation for the homogeneous gas-phase rate constant k1 deduced from the mean experimental value at 983 K, log10(k1/s–1)=–2.56 ± 0.02, is k1/s–1= 1.38 × 1015 exp (–40051/T).

The experimental data for the reactions of oxygen atoms with methane, ethane, and neopentane at temperatures below ca. 600 K have been reexamined. In the case of CH4 and C2H6 reactions, detailed computer models have been assembled to test the assumptions regarding stoichiometries that were made in the original studies in order to derive elementary rate coefficients from the experimentally observed reaction rates. It was found in both cases that the measurements are especially sensitive to secondary reactions not taken into account and impurities in the reagent alkane. Because the original reports did not include sufficient experimental details, it is not now possible to correct their results quantitatively. However, it appears, qualitatively, that the values for the O + CH4 and O + C2H6 rate coefficients were overestimated by factors of approximately 2 to 3 in the 250–400 K temperature range, with the error increasing as T decreases. Although the experimental results for the O + neopentane reaction are not as sensitive to the same kinds of complications, a comparison of the low-temperature measurements with those for the O + ethane reaction suggests that the previously recommended rate coefficients, based on the data of Herron and Huie, are probably also too high by a factor of 2 to 3.

The last five years have seen an increasing interest in clathrate ices as a result of the discovery of extensive deposits of natural gas hydrates in permafrost regions. Twenty-six new clathrate hydrates have been identified, mainly by NMR, including a tetragonal hydrate of dimethyl ether. N-butane and neopentane have been found to be enclathrated in natural gas hydrates, the former as a gauche conformer. As a result of their high symmetries, encaged neopentane, CF4, SF6, and SeF6 exhibit a Resing apparent-phase-change effect in the temperature range of NMR line narrowing. There is increasing evidence that reorientational jumps of water molecules are more frequent than translational jumps in clathrate ices. This is certainly so for ethylene oxide-d4 and tetrahydrofuran-d8 hydrates for which two regions of proton line narrowing and two T 1ρ minima have been observed. The reorientational motions of most guest molecules in structure II hydrates only become isotropic on a time scale long enough to permit the cage configurations to be averaged to 4 3m symmetry by reorientation of the water molecules. The orientations of the water molecules remain disordered to the lowest temperatures.

The x‐ray diffraction pattern of liquid tertiary butyl alcohol (CMe3OH) near the melting point has been measured. The data are analyzed to yield the molecular structure. The distinct structure function Hd(k) derived from the data is compared with that obtained previously for liquid neopentane (CMe4). The replacement of one methyl group in CMe4 by a hydroxyl group to produce CMe3OH results in significant changes in Hd(k). The data show clearly that hydrogen‐bonded hydroxyl groups occur in liquid CMe3OH with an O⋅⋅⋅O distance of 2.74 A, and that each hydroxyl group has two nearest neighbors at this distance.

The pyrolysis of 2,2,3,3-tetramethylbutane (TMB) has been studied using the wall-less reactor. Under homogeneous conditions up to 670°C, the activation energy is 43.2 kcal/mol and log A is 10.70. The products are 2-methylpropene, hydrogen, propene, 2-methyl-2-butene, ethane, ethene, methane, neopentane, and 2,3-dimethyl-2-butene in order of abundance. Addition of toluene, cyclopentadiene, or nitrous oxide to the system increases both the activation energy and the A factor; whereas oxygen decreases both the activation energy and the A factor. Incorporation of stainless-steel rods to provide surface has only a modest effect upon the rate. In view of this evidence, it appears that the homogeneous pyrolysis of TMB at higher pressures is a chain reaction under the described conditions.

The x‐ray diffraction pattern of liquid neopentane has been measured at eight different temperatures between the melting point (−17°C) and the critical point (161°C). In the data analysis, the methyl groups were treated as single scattering centers, and the molecular structure derived from the large‐angle part of the x‐ray data was found to be the same as that found for the low density vapor. Subtraction of the molecular structure yields a set of distinct structure functions descriptive of intermolecular atom pair correlations. It is shown that a realistic model for the orientational correlations between molecules in the liquid must reproduce the experimental structure functions beyond the first maximum. The radial distribution functions derived from the data indicate well defined Me⋅⋅⋅Me contacts between neighboring molecules at low temperature (high density).

Neopentane was pyrolyzed in a flow reactor system at 793–953 K and 20–400 mm Hg. The rate constant for the initiation reaction,calculated from the observed C2H6 yield, was 1017.7±0.3 exp (−356 ± 6 kJ mol−1/RT)s−1, in good agreement with earlier determinations in other temperature ranges. The rate constant of the reaction,calculated from the observed CH4 and C2H6 yields, was 1010.5 ± 0.1 exp (−67 ± 2 kJ mol−1/RT) 1 mol−1 s−1, four to ten times faster than predicted on the basis of earlier work at 404–608 K. From 404–953 K, the Arrhenius plot for this reaction is strongly curved.