R. W. Vaughan

Infrared spectra of chemisorbed CO on Rh

The infrared spectrum of CO chemisorbed on alumina‐supported Rh atoms has been investigated. In agreement with previous work, three types of adsorbed species have been clearly distinguished on the basis of their C–O stretching frequencies. Species I, assigned as Rh(CO)2, is formed only with Rh atoms which are isolated from each other. Species II, assigned as Rh–CO, and III, assigned as Rh2CO, are formed on Rh clusters having two or more Rh atoms. CO‐species II and III undergo interactions with neighbor CO species causing an increase in wave number as coverage increases. Based on infrared intensity measurements for species I, the OC–Rh–CO angle is ∼90°. Chemisorbed 13CO yields the expected infrared spectrum on Rh, and rapid isotopic exchange between 13CO(ads) and 12CO(g) is observed which cannot be explained by the observed rate of desorption of CO from the supported Rh surface.

Correlations between proton chemical shift tensors, deuterium quadrupole couplings, and bond distances for hydrogen bonds in solids

Correlations between parameters characterizing the proton chemical shift tensors for O–H⋅⋅⋅O bonded protons in solids and O⋅⋅⋅O hydrogen bond distances and deuterium quadrupole coupling constants (e2qQ/h) are presented. The isotropic value of the chemical shift correlates very well with both RO⋅⋅⋅O and e2qQ/h, whereas the correlations involving the chemical shift anisotropy show much more scatter. A quantitative agreement is found with theoretical calculations performed by Ditchfield.

Heteronuclear dipolar modulated chemical shift spectra for geometrical information in polycrystalline solids

A double resonance multiple pulse nuclear magnetic resonance experiment is proposed and demonstrated which allows the use of heteronuclear dipolar interactions to determine geometrical, orientational, and motional information in polycrystalline solids. 13C–1H double resonance experiments are performed in polycrystalline benzene and calcium formate to demonstrate the capabilities of the technique in its present form.

A 13C NMR study of the adsorbed states of CO on Rh dispersed on Al2O3

The results of nuclear magnetic resonance (NMR) spectroscopy have been analyzed with respect to previous infrared studies of CO adsorbed on Rh dispersed on Al2O3 to quantify the site distribution and to describe the adsorbed state. The 13C NMR spectra account for all the 13CO adsorbed on a 2.2% Rh on Al2O3 substrate. Although the spectra from the different adsorbed states of CO overlap, the line shapes may be separated into two components based on differences in the 13C spin–lattice relaxation times. These two components have been assigned to the 13CO dicarbonyl formed on single Rh atoms and to 13CO adsorbed on Rh rafts. The component attributed to the CO adsorbed on the raft sites is further separated into linear and bridged CO state contributions based on chemical shift information, yielding a quantitative distribution of the three adsorbed states of CO on Rh. The 13CO distribution is used to estimate the molar integrated intensities of the infrared spectrum of 13CO on Rh at high coverage and to determi...

Proton nmr studies of annealed plasma-deposited amorphous Si:H films

Abstract Proton magnetic resonance data are presented for plasma-deposited amorphous Si:H as a function of annealing temperature up to 650°C. The data indicate that hydrogen diffuses internally before major evolution occurs, that transfer of hydrogen occurs from a heavily clustered phase to a dilute phase coincident with evolution and that evolution occurs initially from the heavily clustered phase. Comparison with infrared data indicates that the heavily clustered phase can be either SiH x ( x = 2,3) or SiH.

13C chemical shift tensor in K2Pt(CN)4Br0.3 · 3H2O

Single crystal rotation spectra have been obtained for the 13C resonance in K2Pt(CN)4Br0.3 · 3H2O and the 13C chemical shift tensor determined. The principal values are (relative to the carboxyl carbon in CH3COOH): σ11=261±10 ppm (parallel to C–N bond), σ22 = −48±10 ppm (perpendicular to C–N bond and Pt–Pt chain), and σ33=−10±5 ppm (parallel to Pt–Pt direction). Since the 14N quadrupole interaction is of similar size to the 14N Zeeman interaction in this material, large effects of the 14N quadrupole interaction are observed on the 13C spectra.

Double resonance interferometry: Relaxation times for dipolar forbidden transitions and off‐resonance effects in an AX spin system

Experimental and theoretical results of a nuclear magnetic double resonance interferometric study of a model AX spin system are presented. Measurements of the characteristic relaxation times of off-diagonal density matrix elements corresponding to magnetic-dipole-forbidden transitions are presented, and the use of such relaxation time constants to obtain information including cross-correlations of the fluctuating fields at the A and X nuclear sites is discussed. (Author)

13C NMR chemical shift tensors of metal carbonyls

The principal components of the 13C NMR chemical shift tensors of metal carbonyls containing between one and six metal atoms were determined from their powder patterns. The tensors of terminally bound CO groups are highly anisotropic (380±60 ppm) and nearly axially symmetric. The tensors of bridging CO groups are much less anisotropic, due to significant asymmetry in the electron orbitals about the C–O internuclear axis. The tensors vary only slightly for different transition metals. There is no intramolecular rearrangement of the metal carbonyls in the solid state at frequencies greater than 10 kHz, except in Fe3(CO)12.

A nuclear magnetic resonance investigation of high surface area silica-aluminas

Abstract A combination of conventional and multiple pulse nuclear magnetic resonance (NMR) techniques has been used to examine the hydroxyl protons on a series of high surface area silica-aluminas whose composition varied from 0–100% SiO 2 . Protons remaining on these materials after being calcined at 500 °C are found to exist as immobile and widely separated hydroxyl groups at room temperature. The concentrations of both SiOH and AlOH groups are reported as a function of adsorbent composition. With the high silica content samples, 100-75% SiO 2 , no AlOH groups were detectable, while on the 50% and lower SiO 2 content samples most surface protons were in AlOH groups; thus, there is indication of a major change in local structure between 75 and 50% silica content. The isotropic part of the chemical shift tensor for hydroxyls on pure SiO 2 is −4.2 ppm relative to TMS and the anisotropy extracted from the powder pattern assuming an axially symmetric tensor is +6.9 ppm, substantially smaller than those found for hydroxyl groups in the solid state.

Catalytic Decomposition of Formaldehyde on Single Rhodium Atoms.

The interaction of H2CO with alumina‐supported Rh has been studied by transmission infrared spectroscopy. It has been shown that isolated Rh atoms catalyze the decomposition of H2CO to produce carbonyl hydride species HxRh–CO (x=1,2). Species of the type Rh(HCO) and Rh(HCOH) are not observed to form in detectable amounts. Hydride bonding to rhodium carbonyl species is associated with electron donation to Rh causing a decrease in ?CO due to electron donation into 2π*‐CO orbitals. Hydride displacement by CO may occur on exposure to CO(g). The conversion of the species H–Rh(CO)2 to H–Rh(CO) has been followed spectroscopically during desorption.