Heteroatom derivatives of indene: V. Vibrational spectra of benzimidazole

Infrared and Raman measurements for benzimidazole are presented and discussed, including its argon-matrix infrared spectrum. To assist in the assignment, benzimidazole's harmonic force fields for the 321G* and 631G* levels were scaled by scaled factors derived by fitting the respective computed force fields of other indene derivatives to previously reported experimental vibrational frequencies. Comparison to the best set of experimental wavenumbers, usually taken from the matrix, shows mean 321G* and 631G* deviations of 7.0 and 5.8 cm⁻¹ for the planar modes, and 14.0 and 6.8 cm⁻¹ for the nonplanar modes, respectively, with much of the error residing in imino-hydrogen group modes. Standard entropies are derived with the matrix wavenumbers and the methods of statistical mechanics. An attempt to determine standard entropies by calorimetric methods was unsuccessful. The triple-point temperature T_tp and enthalpy of fusion Δ¹_crH_m only are reported.     

Non-maxwellian velocity distributions and molecular beam intensities in sonic nozzle expansions

The thermal conduction model is used to calculate the second moment of the kinetic energy distributions in both the parallel and perpendicular degrees of freedom of sonic nozzle expansions. Comparisons with the first moment affords a measure of the departure from maxwellian distributions in these coordinates. Agreement between the computed parallel velocity distribution and measurements described in the literature is good. A Monte Carlo simulation is then used to get further information on the perpendicular velocity distribution at large distances and, in particular, on the intensity of molecular beams which can be skimmed from these expansions.The sonic nozzle expansion constitutes a potentially useful medium for the study of molecular relaxation phenomena. Our understanding of the evolution of the parallel temperature is now virtually complete. The present study has also shown how Monte Carlo calculations can supplement the thermal conduction model to get information on the less tractable problem of the perpendicular velocity distribution. The methodology can be readily extended to polyatomic media [5].We have focused on the central portion of the perpendicular velocity distribution, as this is the regime of greatest interest. It is disappointing to f     

Systematics of evaporation

Beginning with rather basic principles, general relations are obtained for evaporative rate constants. These are established both as a function of energy and of temperature. In parallel with this, expressions are developed for the kinetic energy distribution of the separating species. Explicit evaluation of the rate constants in the case of “chemical” evaporation from an entity containingn monomeric units yields as a typical result $$k(T)(s^{ - 1} ) = 3 \cdot 10^{13} n^{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}} \exp [6/n^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}} ]\exp ( - \Delta E_a (n)/k_B T)$$ Experimental evidence in support of this relation is cited. Applications to thermionic emission are also noted.     

Simulation of Pake doublet with classical spins and correspondence between the quantum and classical approaches

Problems of interacting quantum magnetic moments become exponentially complex withincreasing number of particles. As a result, classical equations are often used to modelspin systems. In this paper we show that a classical spins based approach can be used todescribe the phenomena essentially quantum in nature such as of the Pake doublet.     

Chemical identification of icosahedral structure for cobalt and nickel clusters

Reactions of bare and hydrogenated cobalt and nickel clusters with ammonia and with water are used to determine cluster geometrical structure. Saturation measurements determine the total number of ammonia binding sites on cluster surfaces. A pattern of minima in the number of such sites is found to correlate with the sequence of closed shells and subshells expected for icosahedral packing in the 50- to 120-atom size range (50- to 200-atom range for hydrogenated clusters). In many cases there are 12 sites at the minima, the number that would be expected for preferred ammonia binding sites on closed (sub)shells of icosahedral clusters. The equilibrium adsorption of a single water molecule provides a sensitive measure of changes in cluster-water binding energy. A pattern of binding energy maxima is found, once again correlating with icosahedral structure, but for clusters having one metal atom more than the closed (sub)shells. In general, hydrogenation enhances the patterns of minima and maxima. These observations are explained in terms of the expected nature of ammonia and water binding to icosahedral clusters.     

Kinetic energy release distribution in the fragmentation of energy-selected iodopropane ions

The technique of threshold photoelectron-photoion coincidence (PEPICO) has been employed to determine the average kinetic energy release and the kinetic energy release distribution (KERD) for the iodine loss from 1- and 2-iodopropane ions as a function of the ion internal energy. The KERDs at all precursor-ion energies investigated (0–3 eV excess energy) have the shape of statistically expected distributions, 1-iodopropane ions which dissociate with an apparent 0.16 eV reverse activation barrier, are shown to isomerize at low energies prior to dissociation, to produce subsequently the 2-propyl C₃H₇^? structure. At high energies they may form a different C₃H₇^? isomer. The experimentally observed average kinetic energy releases are approximately a factor of 2 greater than expected statistically suggesting that not all vibrational modes participate in the energy disposal. The secondary dissociation of the C₃H₇^? isomers to C₃H₃ which is inhibited by a reverse activation barrier of = 0.4 eV indicates that the 1- and 2-iodopropane ions dissociate 75% and 60% respectively, to form the excited 1(²P_1/2) atoms.     

Kinetic methods for quantifying magic

Two methods are described for obtaining activation energies for evaporation from isolated clusters. They involve measurements of (1) kinetic energy distributions during evaporation and (2) metastable decay probabilities. The two methods supplement each other and can be applied to data garnered in the same apparatus. The methods are illustrated with applications to data in the literature on copper and carbon clusters. An even/odd alternation in evaporation energies from the copper clusters is consistent with a similar alternation in electron affinities, but not with elementary theory. Thermodynamic properties of carbon clusters are also deduced and discussed.