State-resolved thermalization of singlet and mixed singlet-triplet states of CH2

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH(2) (~a (1)A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH(2) is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH(2) population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times.     

Photoinduced Rydberg ionization spectroscopy of the B state of benzonitrile cation

Photoinduced Rydberg ionization (PIRI) spectra of the second excited electronic state of benzonitrile cation were recorded via the origin and 6a1 and 6b1 vibrational levels of the cation ground electronic state. This B<--X transition was verified to be a forbidden 2B2<--2B1 transition with an origin at 17,225 cm-1 above the ground ionic state. By the use of vibronic coupling calculations, as well as symmetry analysis and comparison of the PIRI spectra via different ground vibrational levels, a nearly complete assignment of the vibrational structure was made, and the vibrational frequencies of the B 2B2 state of benzonitrile cation were obtained based on the assignments. Comparisons of the experimental spectra with simulations from the vibronic structure calculations are also used to validate the theoretical procedures used in the simulations.     

The calculation of vibrational intensities in forbidden electronic transitions

A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained.     

Magnetic properties of smooth terminating dipole bands in Te

Three strongly coupled sequences have been established in ^110,112Te up to high spins. They are interpreted in terms of deformed structures built on proton 1-particle–1-hole excitations that reach termination at I∼40?$I\sim 40?$. This is the first observation of smooth terminating dipole structures in this mass region. Lifetime measurements have allowed the extraction of experimental B(M1;I→I−1)$B(\mathrm{M}1;I\to I-1)$ and B(E2;I→I−2)$B(\mathrm{E}2;I\to I-2)$ reduced transition rates for one of the dipole bands in ¹¹⁰Te. The results support the deformed interpretation.     

Observation of the state of CH2 by optical–optical double resonance

We report the observation of levels in the state of CH₂ via optical–optical double resonance spectroscopy. Direct transitions between the lowest singlet state and the state are allowed by symmetry, but weak because they correspond to a two electron excitation in the single configuration approximation to the electronic wavefunction. The observed transitions involve sequential single photon absorptions at visible and near infrared wavelengths using state intermediate levels. Recent ab initio results (S.N. Yurchenko et al., J. Mol. Spectrosc. 208 (2001), 136) predicted the positions of some of the levels which are confirmed by the present results. The new spectra provide accurate energies for rotational levels in the , l = 0 level of the state.     

Solid-state (55)Mn NMR spectroscopy of bis(μ-oxo)dimanganese(IV) [Mn(2)O(2)(salpn)(2)], a model for the oxygen evolving complex in photosystem II

We have examined the antiferromagneticly coupled bis(μ-oxo)dimanganese(IV) complex [Mn(2)O(2)(salpn)(2)] (1) with (55)Mn solid-state NMR at cryogenic temperatures and first-principle theory. The extracted values of the (55)Mn quadrupole coupling constant, C(Q), and its asymmetry parameter, η(Q), for 1 are 24.7 MHz and 0.43, respectively. Further, there was a large anisotropic contribution to the shielding of each Mn(4+), i.e. a Δσ of 3375 ppm. Utilizing broken symmetry density functional theory, the predicted values of the electric field gradient (EFG) or equivalently the C(Q) and η(Q) at ZORA, PBE QZ4P all electron level of theory are 23.4 MHz and 0.68, respectively, in good agreement with experimental observations.     

Kinetic investigations of product inhibition in the amino alcohol-catalyzed asymmetric alkylation of benzaldehyde with diethylzinc

In situ reaction rate measurements help to define the role of product inhibition in the asymmetric alkylation of benzaldehyde with diethylzinc using (-)-MIB as a chiral reagent. Reaction calorimetry and kinetic modeling demonstrated that the rate behavior over consecutive reactions may only be rationalized when reversible binding of the product alkoxide is taken into consideration. These results may have implications for the conversion dependence of product enantioselctivity in reactions using enantioimpure catalysts.     

A note on adaptive-wall wind tunnels

The scheme proposed by Ferri/Baronti and by Sears, in which the walls of a wind tunnel are iteratively adjusted in accordance with certain flow-perturbation measurements, is reviewed briefly.The iterative process is then simulated, within the scope of the theory of small-perturbation subsonic flows, for three cases of sinusoidal bodies in two-dimensional and circular tunnels. One of these cases is that of a lifting sinusoidal wing of small span tested in a circular tunnel.It is shown that convergence occurs in all cases if the relaxation parameterk, which is the fraction of the measured discrepancy distribution employed after each iteration, is chosen anywhere within a rather wide range of positive values. It is suggested that the results of this study cast light on the adaptive-wall scheme in general.

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Zusammenfassung Es wird ein kurzer Überblick über das von Ferri/Baronti und von Sears vorgeschlagene Prinzip der Verwendung eines Unterschallwindkanals mit anpaßbarer Geometrie gegeben. Die Veränderung der Geometrie erfolgt durch iterative Verstellung der Windkanalwände gemäß bestimmter Messungen der Störströmung. Anschließend wird der iterative Prozeß für drei sinusförmige Körper in zweidimensionalen und kreisförmigen Windkanälen mathematisch simuliert, wobei die Theorie kleiner Störungen für die Unterschallströmung zugrunde gelegt wird. Einer dieser Fälle ist ein sinusförmiger Tragflügel kleiner Spannweite mit Auftrieb, der in einem Windkanal mit kreisförmigem Querschnitt untersucht wird. Es wird gezeigt, daß die Konvergenz immer dann gewährleistet ist, wenn der Relaxationsparameterk innerhalb eines relativ großen Bereiches positiver Werte gewählt wird. Dieser Parameter ist ein Bruchteil der Abweichung, die sich nach jeder Iteration gegenüber der gemessenen Verteilung einstellt. Es ist anzunehmen, daß die Ergebnisse dieser Untersuchung ganz allgemein zum Verständnis des Prinzips der Windkanäle mit anpaßbaren Wänden beitragen.

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Dedicated to Prof. Dr. Nikolaus Rott, with warmest regards and congratulations, on the occasion of his 60th birthday.

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This investigation was carried out under Contract No. AFOSR 76-2954.     

A comparison of the performance of calorimeters

Isothermal calorimetry is finding extensive application in a number of research areas. This popularity is reflected in the number of commercially available instruments which are capable of yielding a variety of thermodynamic and kinetic parameters. Whilst there has been much discussion of ways in which to validate any values returned from these instruments very little has been done quantitatively to compare the relative performances of different instruments. This paper highlights the use of a test and reference reaction quantitatively to compare the performance of three instruments (Thermometric TAM, THT μRC and a Setaram HSDSC III); the specifications of these instruments provide a range from high-sensitivity, long equilibration time to lower-sensitivity, short equilibration time. The comparison is made through a statistical analysis of values returned for the rate constant, enthalpy of reaction and activation energy for the base catalysed hydrolysis of methyl paraben. The statistical analysis from the data set discussed here indicates that there is no significant difference between the returned thermodynamic and kinetic parameters from the TAM and μRC. The analysis revealed however that the HSDSC returns values for the rate constant which are significantly different from both the TAM and μRC, although it is noted that this instrument was not specifically designed to operate in a step-isothermal mode and that it was possible to apply a correction to the data. In all cases the enthalpy data returned from all instruments were statistically similar although the μRC and HSDSC returned values which were, for the rate constant and activation energy, less precise than those obtained from the TAM. As well as highlighting the importance of using test and reference reactions, this study also shows that proper instrument selection is an important factor when designing a calorimetric experimental series.