Walter G. Rothschild

Motional characteristics of large molecules from their Raman and infrared band contours: Vibrational dephasing

A simple application of Kubo’s line shape theory to the domain of vibrational frequencies is presented, compared with the experiment, and used for predictions. In vibrational modes where vibrational dephasing processes are predominant in determining the shape of the condensed phase band contour, the formalism shows that the vibrational correlation obeys a fast modulation mechanism (’’motional narrowing’’), e.g., in quinoline, tetravinyl tin, and isopropyl alcohol. However, even smaller molecules such as chloroform and methyl iodide show similar characteristics and only in exceptional cases (the uncoupled O–D stretch of D2O in H2O) is the modulation slow. This behavior is a consequence of the short modulation times (order of fractions of a picosecond) in the liquid which determine the phase loss of the vibrational amplitude after a brief period of a quasistatic distribution of molecular environments (’’rigid lattice’’) —times which can be approximately identified with the inverse average collision frequenc...

Dynamic behavior of three-way catalysts

Three-way catalysts (TWCs) are employed in the exhaust treatment for gasoline-fueled vehicles to achieve simultaneous control of CO, hydrocarbons (HCs) and NO emissions. A typical TWC contains precious metals (Pt, Pd, or Rh) and metal oxides which are beneficial for stability of the catalyst support (..gamma..-alumina) or enhancement of the catalyst activity under dynamic operating conditions. When the TWC is used in vehicles with closed-loop control of the air/fuel (A/F) ratio near the stoichiometric point for the control of the exhaust emissions, its performance has been found, in general, to be better than predicted from steady-state catalyst efficiency. This improved performance relates to several characteristic processes occurring under dynamic operating conditions, including oxygen storage, water-gas shift (WGS) reaction, and HC retention on the catalyst. The authors note here salient observations from a study of the dynamic behavior of a monolith TWC with the ultimate goal to obtain quantitative descriptions of the transient reactions involved. 17 references.

Dynamics of molecular reorientational motion and vibrational relaxation in liquids. Chloroform

Vibrational and rotational (dipole and second−order tensor) correlation functions were obtained by Fourier inversion of infrared and Raman vibrational band contours of the three ∥ and one ⊥ fundamental of liquid CHCl3, CDCl3, and isotopically pure CH35Cl3. All correlation functions are nonexponential at short times and approximately exponential for long times. The symmetry axis of the molecule reorients by ’’free’’ jumps of about 1/3 rad, turning through a root−mean−square angle of 1 radian within 2psec by about 13 orientational jumps. Computer simulations show that J diffusion is too fast beyond 1 psec and that M diffusion fits the data up to 4 psec (τJ = 0.12 psec); thereafter, M diffusion is too slow. The Raman rotational correlation time is approximately equal to the NMR quadrupolar correlation time; the infrared rotational correlation time is only 0.75 of a corresponding dielectric relaxation time. Vibrational relaxation in the symmetric near−infrared carbon−hydrogen stretch is of the same order of i...

Molecular Motion in Liquids: On the Prevalence of Large‐Size Rotational and Translational Diffusion Steps

It is shown experimentally (Fourier inversion of vibrational band contours) that the molecular orientational motion in some representative common liquids [methylene chloride (CH2Cl2), methyl iodide (CH3I), chloroform (CHCl3), cyclohexane (C6H12)] appears to consist of angular jumps of the order of 20° to 60°. During a jump, which is completed in times of the order of 0.4 × 10−12 sec, the molecules are observed to rotate as if they were in their vapor phase, regardless of whether or not the orientational motion involves the tilting of a permanent dipole moment or of the size of the molecule. The infrared data are compared with results from nuclear magnetic relaxation studies. It is seen that the rotational motion in liquids, as it is observed by these techniques, is not sensitive to molecular association, weak hydrogen bonding, etc. On the basis of the extended Torrey model of intermolecular spin–lattice relaxation, it appears that molecular translational diffusion in a large variety of liquids, some of th...

Rotational Diffusion Tensor of Liquid Methylene Chloride from Its Infrared Spectrum

The rotational motion of the three inertial axes of liquid methylene chloride (CH2Cl2, point group C2υ) has been studied by computing the autocorrelation functions through the Fourier inversion of the observed band contours. Three fundamentals [symmetry species a1 (283 cm−1), b1 (895 cm−1), and b2 (1265.5 cm−1)], as well as three summation bands [symmetry species b1 (2308 cm−1, 2414 cm−1) and b2 (2688 cm−1)], one overtone [symmetry species a1 (2525 cm−1)], and one difference band [symmetry species b2 (452 cm−1)], were investigated. The experimental results indicate that the liquid‐phase molecules do not obey Debye‐type diffusion but, on the contrary, are able to make orientational jumps of 27° – 38° around their inertial axes. The decay of the coherence of the rotational motion takes between 0.6 × 10−12 and 1.1 × 10−12 sec. Further‐more, the anisotropy of the rotational motion in the liquid is observed to be about the same as that of freely rotating CH2Cl2 molecules. The results imply that rotational moti...

Molecular Motion in Liquids: Rotational and Vibrational Relaxation in Highly Polar and Strongly Associated Systems

The characteristics of the rotational‐vibrational relaxation of acetonitrile, benzonitrile, 2‐propanol, and p‐dioxane were observed by determining the correlation functions of several infrared and Raman rotation‐vibration bands of these liquids. Benzonitrile and 2‐propanol were also investigated as dilute solutions in CCl4 and in CS2, whereas p‐dioxane was also observed in solution with a variety of electron‐donating, electron‐accepting (among them chloroform, water), and nonpolar molecules. The results show that the intermolecular forces need about 0.1 × 10−12–0.2 ×−12 sec to cause an observable effect on the molecular motion. Regardless of the polarity or association of the liquids or solutions investigated, rotational relaxation is the predominant mode of decay for times less than 0.5 × 10−12 sec after the onset of observation: The C≡ N stretch of benzonitrile and the investigated modes of p‐dioxane are, probably, exceptions. It is also shown that the vibrational correlation functions do not decay pure...

Infrared spectra of chemisorbed CO on Rh/γ–Al2O3: Site distributions and molecular mobility

Chemisorption of CO on a 0.92 wt. % Rh/γ‐Al2O3 catalyst gave three species of adsorbed CO molecules: Species RhCO, at about 2000 cm−1; species Rh(CO)2, showing a doublet with peak frequencies at 2100 and 2030 cm−1; and a bridged species Rh2CO, leading to a very broad band peaked between 1850 and 1900 cm−1. The thermostability of these adsorbed species is in the sequence Rh2CO≳RhCO≳Rh(CO)2. Only the doublet band of Rh(CO)2 and the very broad band of the bridged species Rh2CO were found on well dispersed Rh metal in its δ phase. The band contours of the RhCO and Rh(CO)2 species were Fourier transformed into the time domain. The result showed that the configuration RhCO was spread over an approximately four times wider distribution of adsorption sites (surface–adsorbed CO interactions) than that of the Rh(CO)2 species. Furthermore, it was shown that the CO molecules adsorbed as RhCO species are ’’floppy,’’ undergoing some type of fast (probably librational) motion with a characteristic time of 0.1×10−12 sec....

The Effect of Fuel and Operating Variables on Hydrocarbon Species Distributions in the Exhaust from a Multicylinder Engine

Abstract Measurements of the concentrations of individual exhaust hydrocarbon species have been made as a function of engine operating variables (φ, rpm, EGR, spark timing, and coolant temperature) in a 2·3-liter four-cylinder engine. Three fuels were used in these experiments: propane, isooctane (2,2,4-trimethylpentane), and an unleaded gasoline (indolene clear). The results show that a change in operating variable can change not only the total hydrocarbon concentration but also the distribution of species in the exhaust. All three fuels show similar trends when an operating variable is changed. Fuel-air equivalence ratio is a critical parameter in controlling exhaust hydrocarbon emissions. Beginning near stoichiometric, the total hydrocarbon concentration and the percentage contributions of methane and acetylene to the exhaust increase as the mixture becomes richer. These species contribute less than 2 percent to the total hydrocarbon emissions at <0.95. Their contribution rises to 15–25 percent at φ...

Rotational Motion in Solution: Hydrogen Halides in Cyclohexane

The dipole correlation function and the average kinetic rotational energy of hydrogen fluoride, hydrogen chloride, and deuterium chloride have been computed from the published far‐infrared absorption bands [P. Datta and G. M. Barrow, J. Chem. Phys. 43, 2137 (1965)]. The computations show that the dissolved hydrogen halide molecules undergo fast, large‐angle rotational diffusional jumps: for instance, the dissolved HF molecules which are in their most populous state have regained their original orientation after a time interval of 0.2 × 10−12 sec following the disturbance of their equilibrium state. The coherence of their rotational motion has completely decayed after about 0.4 × 10−12 sec. A comparison with the dipole correlation function of the classical ensembles of freely rotating molecules shows that the intermolecular forces between HF and cyclohexane become observable only after a time interval of 0.08 × 10−12 sec; the corresponding value for dissolved DCl is 0.14 × 10−12 sec. These time intervals s...

On the Existence of Conformers of Cyclobutyl Monohalides. III. Assignments of the Fundamentals of Bromocyclobutane and Chlorocyclobutane

The infrared spectra of the liquid and the vapor and the Raman spectra of the liquid of bromocyclobutane and chlorocyclobutane are reported. The 30 fundamentals of each halide are assigned and an approximate picture of the motions of those atoms, which are mainly involved in the fundamental modes, is given. There is one‐to‐one correlation between the fundamentals of bromocyclobutane with the fundamentals of chlorocyclobutane. This signifies that the amount of interaction between different modes is generally of the same order of magnitude in both halides, in contrast to the corresponding cyclopropyl halides. There is little symmetry correlation between the fundamentals of the cyclobutyl halides and cyclobutane; the out‐of‐plane ring‐bending (ring‐puckering) mode is an exception. In the Appendix, computer calculations of A‐ and C‐type band envelopes are presented and compared with observations; the computations include vibration—rotation interaction.