John D. Ewbank

Electron-diffraction study of hydrogen isotope effects in cyclohexane

Abstract The electron-diffraction data for cyclohexane and perdeuterated cyclohexane were analyzed and the results were compared. It was found that r g (C-C) = 1.535 A (±0.002) for both compounds; r g (C-H) = 1.116 A (±0.004) and r g (C-D) = 1.109 A (±0.003). Observed hydrogen isotope effects in mean amplitudes agree very well with calculated ones. The C-C bond distance of cyclohexane is in good agreement with some of the previous studies, which is of importance in view of a recent scaling controversy involving this parameter.

Ab initio studies of structural features not easily amenable to experiment: Part 49. Conformational analysis and molecular structures of ethylenediamine and aminoethanol

Abstract The structures of eleven conformations of aminoethanol and of ten conformations of ethylenediamine were determined by ab initio gradient geometry optimization on the 4–21G level. The calculations show that many energetically different conformers exist for the gauche and trans forms (NCCO or NCCN torsions) of both systems. The results indicate that structural effects on NH bond distances associated with aliphatic N⋯N or NH⋯O type hydrogen bonding are less noticeable than those obtained previously for other XH⋯Y interactions. In the same way as CH and CC bonds in aliphatic compounds studied previously, NH bonds in the same NH2-group anti-periplanar to CH are found here to be slightly longer (∼0.001 A) than NH bonds antiperiplanar to CC. The most stable 4–21G conformations of ethylenediamine (NCCN ≅ 58° and 62°) are identical with the main conformer identified by others in the gas electron diffraction data of this compound (NCCN = 64° ± 4), but the calculations afford a more detailed description of the NH2 arrangements (NC torsions) than that obtained by the gas phase experimental data. Structural effects of electron lone-pair orientation are discussed. A small but potentially significant discrepancy exists between the electron diffraction rg average CC bond distance (1.545 A ± 0.008) and the calculated rg value (1.528 A ± 0.003; 4–21G average value empirically corrected as described previously). Experimental and calculated average rg CN bond distances are 1.469 A ± 0.004 and 1.463 A ± 0.006, respectively. Some aspects of the CC bond discrepancy are discussed in detail.

Molecular orbital constrained gas electron diffraction studies: Part I. Internal rotation in 3-chlorobenzaldehyde

Abstract The geometries of the trans and cis forms of 3-chlorobenzaldehyde (I and II, respectively) were optimized using standard single determinant MO theory with the STO—3G basis. The calculated energy difference between I and II was 90 cal mole −1 . Mean amplitudes of vibration were calculated for the system by vibrational analysis. Computed molecular properties — amplitude differences and ab initio differences between some geometrical parameters — were combined with the gas electron diffraction data for the compound to constrain least-squares data analysis. The model which was in best agreement with experiment represented a conformational equilibrium with a composition of 64(16)% trans and 36% cis for the vapor of 3-chlorobenzaldehyde at 120°C. This result contradicts a previous vapor phase far infrared investigation of the compound and the torsional potential derived from it. Potentially misleading factors may, therefore, generally have to be taken into account whenever torsional potentials are derived from vibrational data. Beyond the scope of the structural problem considered, the paper demonstrates the advantages of molecular orbital constrained electron diffraction (MOCED) studies.

Ab initio studies of structural features not easily amenable to experiment: Part 39. Conformational analysis of glycine and alanine

Abstract The NCCO torsional potential energies of glycine and alanine were determined by 4-21G ab initio calculations with geometry gradient optimization at each point. The potential energy of glycine is characterized by minima at 0 and 180°, and by a barrier at approximately 75°. The 180° local minimum is part of an energy plateau extending from approximately 150 to 210°. The alanine NCCO potential is characterized by minima at 0 and 150°, and by asymmetric barriers in the 60–90° and 270–300° regions. The 150° minimum is part of an asymmetrically stepped energy plateau extending from approximately 120 to 240°. It is shown that the qualitative features of the alanine potential can be simulated from the CCCO and NCCO torsions of propanoic acid and glycine. Some quantitative differences between the simulated and the actual potentials may point to cooperative effects in the total system. A similar, qualitative prediction is made for the NCCO potential of α-aminoisobutyric acid from glycine and 2-methyl propanoic acid.

Real‐time data acquisition for gas electron diffraction

An instrument for gas electron diffraction (GED) studies is described which eliminates the photographic intermediary and sector device which are currently used almost universally in molecular structure determinations using GED. Specifically, the scattered electrons are detected by a fluorescent screen which is optically coupled to a custom multichannel analyzer (EG&G model 1412 photodiode array detector and model 1218 controller, combined with a MINC‐DEC LSI 11/23 computer). Rapid operation not only eliminates photographic film development and densitometry, but also allows for repeated measurements to be easily made. Dynamic data acquisition provides for virtually steady‐state conditions in the experimental environment, minimizing systematic errors and providing a means for instrument control. The design has been completed to the extent that small angle scattering data are now readily recorded in this novel way. The data range accessible in the current configuration (maximum s values of approximately 20–2...

The stroboscopic gas electron diffraction method for investigation of time-resolved structural kinetics in photoexcitation processes

Abstract The emergence of a novel tool of structural chemistry is reviewed; pulsed-electron beam (stroboscopic) gas electron diffraction (GED) synchronous with photoexcitation. About 10 years ago, the first stroboscopic electron diffraction experiments of irradiated gaseous species were performed at Moscow State University, yielding qualitative evidence that intensity changes upon irradiation can be detected in this way. More recently, development of prototype on-line GED data recording techniques at the University of Arkansas allowed for the first successful observations, with quantitatively model-fitted GED signals, of photochemical reactions, i.e. the 193 nm photodissociations of carbon disulfide and of chlorine-substituted ethenes. In addition to summarizing some of the current structural work, the paper describes the characteristic aspects of pulsed-beam GED, the requisite on-line data recording, and non-conventional data analysis techniques capable of interpreting GED signals from non-equilibrium ensembles in arbitrary vibrational states.

Instrumentation for gas electron diffraction employing a pulsed electron beam synchronous with photoexcitation

A novel instrument is described capable of recording gas electron diffraction (GED) patterns of excited molecular states or transient species with pulsed electron beams. The system incorporates (1) a pulsed optical beam for electronic excitation of materials under study, (2) a synchronously pulsed source of 30–50 keV electrons in a space‐charge‐limited beam, (3) necessary vacuum environment and sample‐handling capabilities, and (4) detection and signal processing equipment using an on‐line procedure developed at the University of Arkansas. Data obtained for several test gases demonstrate successful operation of the instrument. The 193 nm laser photofragmentation of carbon disulfide, CS2, is described in detail. In agreement with a recent time‐of‐flight mass spectrometric study of the same process, carbon monosulfide was observed as the reaction product. This study is the first quantitatively successful joint exercise of on‐line multichannel GED data recording and a stroboscopic electron source. The method...

Reinvestigation of the microwave spectrum of cyanocyclobutane: assignment of the axial conformer

Abstract The microwave spectrum of the axial conformer of cyanocyclobutane has been assigned on the basis of its ab initio structure. From dipole moment and relative intensity measurements it has been possible to determine the relative energy with respect to the previously assigned equatorial conformer: E (axial) − E (equatorial) = 258 ± 50 cm −1 .

INSTRUMENTATION FOR TIME-RESOLVED ELECTRON DIFFRACTION SPANNING THE TIME DOMAIN FROM MICROSECONDS TO PICOSECONDS

Recent instrumental improvements which successfully extend the time resolution of pulsed beam electron diffraction to the picosecond regime are described. Based on modifications of an existing nanosecond apparatus, a new sample inlet system, electron pulse generation laser, and amplified detector have been incorporated into the design such that significant improvements in both the signal level and ultimate time resolution are achieved; an upper estimate of the electron pulse width is ∼20 ps. Enhancements are such that, for operation in the nanosecond time domain, an entire diffraction pattern over a useful range of scattering angles may be collected from a gas-phase sample in a single electron pulse.

Structural and vibrational kinetics of photoexcitation processes using time resolved electron diffraction

Abstract This paper describes the historical background and current status of structural and vibrational kinetic studies of photoexcitation processes using pulsed beam gas electron diffraction. The development of time resolved electron diffraction (TRED) required essential changes in the traditional experimental and theoretical procedures of gas electron diffraction. On the experimental side, over the last decade or so, research at the University of Arkansas has led to construction of a prototype on-line data recording system that, combined with a laser-driven pulsed photocathode, enables time-resolved investigations spanning the time domain from microseconds to picoseconds. On the theoretical side, new techniques allow one to model TRED data in terms of a molecules potential energy surface or, alternatively, to apply stochastic procedures to solve the inverse problem, i.e. to determine the PES from the diffraction data. The importance of considering appropriate vibrational properties is demonstrated. Diverse aspects of TRED modeling of dissociation and pre-dissociation processes are described, including formalisms based on wave packet dynamics, spatial anisotropy, chaotic nuclear dynamics, Wigner distribution functions, tomographic reconstruction, and coherently excited molecular ensembles.