Ralph E. Weston

H3 Activated Complex and the Rate of Reaction of Hydrogen Atoms with Hydrogen Molecules

The method recently proposed by Sato for determining potential energy surfaces is compared with the semiempirical method of Eyring and co‐workers. The two methods are found to be equally empirical. The potential energy surface for the H3 complex has been constructed with the Sato method to give an activation energy which agrees with the experimental value. The H3 complex is found to be linear and symmetrical, with a bond length of 0.93 A and vibrational frequencies of 2108, 877, and 1918i cm—1. Anharmonicity constants for the real frequencies make a negligible contribution to the zero‐point energy. A large contribution to the H+H2 reaction from tunneling through the potential barrier is predicted, contrary to experimental data. However, at 1000°K, the calculated pre‐exponential factor and ratios of rate constants for isotopic species are in reasonably good agreement with experimental values.

Infrared Spectrum and Force Constants of the Nitrite Ion

The infrared spectra of the following compounds have been investigated: microcrystalline NaNO2, KNO2, and AgNO2; and aqueous solutions of NaNO2 and KNO2. The spectra of N15‐labeled compounds have also been obtained. The spectrum of the high‐temperature modification of NaNO2, which exists above 160°, has been observed. The fundamental frequencies for NO2— (in crystalline NaNO2) are: ν1(a1)=1328±2 cm−1 (N14),1303±2 (N15);ν2(a1)=828.2±0.4 (N14),823.6±0.4 (N15);ν3(b1)=1261±3 (N14). These are used to calculate the following force constants, in units of millidynes/A: fd=7.69±0.10,fα/d2=1.75±0.04,fdd=1.94±0.13,fdα/d=0.505±0.113.

Deuterium Isotope Effect in the Reaction of Hydrogen Molecules with Chlorine Atoms and the Potential Energy of the H2Cl Transition Complex

The relative rates of reaction of H2 and HD with chlorine atoms have been measured over the temperature range of 243–350°K. In this temperature interval, the ratio of the second‐order rate constants, R=kH2/ΣkHD, is equal to (1.24∓0.03) exp (490±6/RT). A search was made for HD after an unequilibrated mixture of H2 and D2 was half converted to hydrogen chloride by photochemical reaction with Cl2. From the failure to detect 0.02% HD in the unreacted hydrogen, a lower limit is set for the ratio of the rate constants of the reactions H+Cl2→ lim k16HCl+Cl and H+HCl→ lim k17H2+Cl.It is shown that the pre‐exponential factors in R for the HD experiments and in the analogous experiments on HT are in quantitative agreement with theoretical calculations for either linear or triangular transition states, subject to the sole restriction that k(HD+Cl→HCl+D) is approximately equal to k(HD+Cl→DCl+H). An intercomparison is made between the experimental difference in activation energies between H2, HD, HT, and D2 for reacti...

Deuterium‐Isotope Effect in the Chlorine Exchange between Hydrogen Chloride and Chlorine Atoms. A Study of Models for the Tunnel Effect

The rates of the exchange reactions of chlorine atoms with hydrogen chloride and with deuterium chloride were measured as a function of temperature. Chlorine atoms were produced by photodecomposition of chlorine gas and their relative concentration was monitored by their reaction with deuterium: Cl+D2→DCl+D. The temperature dependence of the logarithm of the ratio of rates was found to be nonlinear in the temperature range of 30° to 150°C. Various attempts to calculate the isotope effect assuming different types of tunneling are discussed.

Determination of Hydrogen‐Atom Concentration by Lyman‐α Photometry. I. Oscillator Strength of the Hydrogen‐Atom 2P32,12←2S12 Transition. II. Kinetics of the Reaction of Hydrogen Atoms with Acetylene and Ethylene

A photometric method of determining hydrogen‐atom concentration in the gas phase has been developed. It consists of a hydrogen—neon lamp emitting Lyman‐α radiation at 1216 A, and a nitric oxide‐filled ion chamber which serves as the detector. A fast‐flow system with a microwave discharge in an H2–He mixture was used as the source of hydrogen atoms. This arrangement permits the determination of H‐atom concentrations of 1011 atoms/cc. A photometric calibration curve was obtained by using titration with NO2 as an absolute measure of H‐atom concentration.The oscillator strength of the Lyman‐α transition was determined with this apparatus. Depending on the assumed profile of the emission line and the extent of self‐reversal in the source, oscillator strengths of 0.4 to 1.1 (compared with the theoretical value of 0.8324) were obtained.Rates of reaction of H atoms with ethylene and acetylene were determined at various total pressures and concentrations of reactants. Both reactions are first order with respect to...

THE STRUCTURE OF PHOSPHINE

Several low J microwave transitions of PH2D and PHD2 have been observed in the K band. The effect of centrifugal distortion on the rotational levels has been taken into account approximately, with the result that errors in the computed structure due to this cause now appear to be negligible in comparison with those due to vibration‐rotation interaction. One obtains for the P–H distance 1.4177 and 1.4116A in the two cases; for the H–P–H angle, 93°21.6′ and 93°15.4′. The dipole moment has been measured for both species with agreement within experimental error.

Counting vibrational quanta with a diode laser probe: Bending and stretching excitation in CO2 caused by collisions with hot atoms from excimer laser photolysis

A high resolution (10−3 cm−1) cw diode laser probe technique has been developed and used to determine the number of CO2 vibrational quanta of each kind (ν3 antisymmetric stretch, ν2 bend, and ν1 symmetric stretch) produced as a result of collisions between translationally hot hydrogen atoms and CO2 molecules. The experimental method relies on probes of the CO2 vibrational transitions mnlp → mnl( p+1) all of which ‘‘ride’’ the large oscillator strength of the fundamental 0000 → 0001 antisymmetric stretching transition. Transitions with different values of m, n, l, and p are easily separated due to the narrow spectral characteristics of the diode laser and the small anharmonicities associated with different vibrational quantum numbers. The probability for excitation of a CO2 ν3 quantum by collisions with hot hydrogen atoms produced by 193 nm excimer laser photolysis of H2S is about 1% per gas kinetic collision. Bending (ν2) quanta are produced about 5–6 times more efficiently than (ν3) antisymmetric stretch...

Fluorescence excitation spectra, lifetimes, and quenching of S2(B 3Σu−) in the wavelength region 280–315 nm

Fluorescence lifetimes have been determined for S2 vapor excited to the B 3Σu− state at wavelengths in the range 280–315 nm, with a spectral resolution of ∼0.5 cm−1. For states with v′⩽8, excitation in the bandhead region leads to fluorescence lifetimes of ∼30–45 ns. In addition, low‐intensity peaks with much longer lifetimes are observed throughout the absorption spectrum. The decreased intensity and increased lifetime are attributed to perturbations by the B′ 3πu state. For the v′ = 9 state, lifetimes generally decrease as the excitation wavelength moves away from the bandhead and at some wavelengths a double exponential decay was observed. Although the v′ = 10 state is known to predissociate, weak fluorescence peaks with very short lifetimes (<3 ns) were observed. The addition of Ar as a quenching gas leads to complex fluorescence decay curves with two or three components for both v′ = 3 and v′ = 9 states. Lifetimes and quenching rate constants obtained in this work agree with those measured previously...

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Abstract Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm −1 per atom. Subsequent collisions of these “hot” atoms with CO 2 and N 2 O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.

Some rotations like it hot: selective energy partitioning in the state resolved dynamics of collisions between CO2 and highly vibrationally excited pyrazine

Abstract The collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been studied with high resolution diode laser spectroscopy. The vibrationally hot pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions in the 0000 and 0001 vibrational levels of CO2 produced by collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. In addition, the CO2 translational recoil velocity was measured for a number of rotational levels in each vibrational state. The results of these experiments reveal that very little rotational and translational excitation accompanies the energy transfer from hot pyrazine to excited vibrational levels of CO2. In contrast, rotational excitation of the CO2 ground state due to collisions with highly excited pyrazine is significant and is accompanied by a substantial enhancement in the CO2 translational energy. These results are consistent with a picture in which vibration-vibration (V → V) energy transfer processes, leading to vibrational excitation of the bath, are dominated by long range attractive forces, and vibration-translation/rotation (V → T/R) energy transfer, which leaves the bath vibrations unexcited, is dominated by short range repulsive forces.