Walter G. Rothschild

Bio: Walter G. Rothschild is an academic researcher from Ford Motor Company. The author has contributed to research in topics: Raman spectroscopy & Motional narrowing. The author has an hindex of 23, co-authored 54 publications receiving 1648 citations.

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

Abstract: 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 frequency. In order to compare theory and experiment quantitatively, the model requires that the vibrational second spectral moment must be measured in addition to the vibrational correlation function. Comparisons with vibrational memory functions, obtained from the correlation functions via the modified Langevin equation, corroborate the usefulness of the model.

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

Abstract: 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

Abstract: 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 them strongly associated, occurs by a jump of the order of a molecular diameter after a trapping time of the order of 10−11–10−10 sec. As far as can be predicted, liquids which obey a classical diffusion model of very small diffusion steps would be the exception rather than the rule. The possibility is discussed that a dimer is the diffusing entity in acetic acid.

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

Abstract: 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...

Catalytic Properties of Ceria and CeO2-Containing Materials

Abstract: Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Vibrational dynamics of liquids and solids investigated by picosecond light pulses

Abstract: With well-defined coherent light pulses of several ${10}^{\ensuremath{-}12}$ sec duration we are in a position to investigate a variety of ultrafast vibrational processes in liquids and solids. Several new experimental techniques have been devised to study directly the dynamics of different vibrational modes and molecules in the electronic ground state. A first light pulse excites the vibrational system via stimulated Raman scattering or by resonant infrared absorption. A second interrogating pulse allows one to determine the instantaneous state of the excited system. Using a coherent probing technique one can measure the dephasing time of homogeneously broadened vibrational transitions and a collective beating of multiple isotope levels. In addition, one can investigate inhomogeneously broadened vibrational modes and observe the dephasing time of a small molecular subgroup. Different information is obtained when the coherent anti-Stokes Raman scattering of the probe pulse is measured. The population lifetime of known vibrational modes can be investigated and evidence for inter- and intra-molecular interactions is obtained. In a third probing technique, the vibrationally excited jolecules are promoted to the first electronic state by a second pulse and the fluorescence is measured. In this way it is possible to see the very rapid change of population of the primary excited vibrational mode. The article gives a detailed theoretical treatment of different excitation and probing processes. Several experimental techniques successfully applied in the authors investigations are outlined and a variety of results is presented and discussed. New information, not available from other experimental methods, is obtained.

Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: Implications for oxygen-storage properties

Abstract: Steady-state, water-gas-shift kinetics were measured on model, ceria-supported, Pd, Pt, and Rh catalysts and compared to rates obtained on alumina-supported catalysts. When ceria was calcined at low temperatures prior to addition of the precious metal, the specific rates were found to be identical for each of the metals, with an activation energy of 11 ± 1 kcal/mol and reaction orders of zero and one for CO and H 2 O respectively. For comparison, specific rates on Rh/alumina were at least two orders of magnitude lower. However, ceria structure strongly affected the results. When ceria was calcined to high temperatures to increase crystallite size, prior to the addition of Pd, specific rates were a factor of 50 lower at 515 K and the activation energy was found to be much higher, 21 ± 1 kcal/mol. By comparison with results from an earlier study of CO oxidation [17], we propose that water-gas shift on ceria-supported metals occurs primarily through a bifunctional mechanism in which CO adsorbed on the precious metal is oxidized by ceria, which in turn is oxidized by water. Deactivation of the catalyst following growth in the ceria crystallite size is due to the decreased reducibility of large ceria crystallites. The implications of these results for automotive, emission-control catalysts is discussed.

Stochastic Liouville, Langevin, Fokker–Planck, and Master Equation Approaches to Quantum Dissipative Systems

Abstract: Half century has past since the pioneering works of Anderson and Kubo on the stochastic theory of spectral line shape were published in J. Phys. Soc. Jpn. 9 (1954) 316 and 935, respectively. In this review, we give an overview and extension of the stochastic Liouville equation focusing on its theoretical background and applications to help further the development of their works. With the aid of path integral formalism, we derive the stochastic Liouville equation for density matrices of a system. We then cast the equation into the hierarchy of equations which can be solved analytically or computationally in a nonperturbative manner including the effect of a colored noise. We elucidate the applications of the stochastic theory from the unified theoretical basis to analyze the dynamics of a system as probed by experiments. We illustrate this as a review of several experimental examples including NMR, dielectric relaxation, Mossbauer spectroscopy, neutron scattering, and linear and nonlinear laser spectroscop...

Highly sensitive fluorescence probes for nitric oxide based on boron dipyrromethene chromophore-rational design of potentially useful bioimaging fluorescence probe.

Abstract: Boron dipyrromethene (BODIPY) is known to have a high quantum yield (phi) of fluorescence in aqueous solution but has not been utilized much for biological applications, compared to fluorescein. We developed 8-(3,4-diaminophenyl)-2,6-bis(2-carboxyethyl)-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (DAMBO-P(H)), based on the BODIPY chromophore, as a highly sensitive fluorescence probe for nitric oxide (NO). DAMBO-P(H) had a low phi value of 0.002, whereas its triazole derivative (DAMBO-P(H)-T), the product of the reaction of DAMBO-P(H) with NO, fluoresced strongly (phi = 0.74). The change of the fluorescence intensity was found to be controlled by an intramolecular photoinduced electron transfer (PeT) mechanism. The strategy for development of DAMBO-P(H) was as follows: (1) in order to design a highly sensitive probe of NO, the reactivity of o-phenylenediamine derivatives as NO-reactive moieties was examined using 4,5-diaminofluorescein (DAF-2, a widely used NO fluorescence probe), (2) in order to avoid pH-dependency of the fluorescence intensity, the PeT process was controlled by modulating the spectroscopic and electrochemical properties of BODIPY chromophores according to the Rehm-Weller equation based on measurement of excitation energies of chromophores, ground-state reduction potentials of PeT acceptors (BODIPYs), and calculation of the HOMO energy level of the PeT donor (o-phenylenediamine moiety) at the B3LYP/6-31G level, (3) in order to avoid quenching of fluorescence by stacking of the probes and to obtain probes suitable for biological applications, hydrophilic functional groups were introduced. This strategy should be applicable for the rational design of other novel and potentially useful bioimaging fluorescence probes.