Abstract : The goal of this program is to theoretically evaluate compounds comprised of first row-atoms that might serve as the basis of new propulsion schemes. This theoretical work is based on the premise that species which show promise as high energy density materials may exhibit novel bonding mechanisms which distinguish them from conventional stable molecules. Two, possibly overlapping, categories of species have been considered: (1) electron deficient compounds, which are certain compounds of B and Be not having sufficient valence electrons to distribute two per chemical bond, and (2) mixed metal clusters of the form Li(n)B(m)H(k). A comprehensive study of the electron deficient compound BH4 as well as results for the mixed metal clusters Li3Be and Li3B is presented. In addition, results for the excited states and correlation diagram of BH2 obtained in support of the experimental program at Phillips Laboratory are detailed. Preliminary Isp estimates, used in program planning as well as calculations exploring the 'superalkali-superhalogen' concept that was judged not to be promising are summarized.

Calculations designed to characterize the transition state and determine the barrier height for rearrangement of nitromethane to methyl nitrite are reported. Structures of CH3NO2, CH3ONO, dissociation products, CH3 + NO2, and CH3O + NO, and the transition state for nitro–nitrite rearrangement have been optimized at the MCSCF/4-31G level. The geometry of the transition state may be approximately described as separated CH3 and NO2 species with extremely long C—N and C—O bond lengths, 3.396 and 3.654 Å, respectively. Energies have been obtained by large-scale multireference single- and double-excitation CI calculations (6-31G* basis). The transition state is calculated to lie 56.7 kcal/mol above nitromethane (with zero-point energy). A C—N bond dissociation energy of 51.7 kcal/mol is obtained. Results are compared with the infrared multiphoton dissociation experiment of Wodtke, Hintsa, and Lee. Keywords: nitromethane, abinitio calculations, transition state, rearrangement, dissociation.

Calculations of cross sections for mixing of Xe(5p{sup 5}nl)(nl=6s,6p,5d) by thermal collisions with He and Ar are reported. Ab initio structure calculations have been performed using the COLUMBUS codes, which include spin orbit effects and relativistic effects in the heavy atom core. A model Hamiltonian approach has been developed that allows the necessary nonadiabatic coupling matrix elements to be determined. The dynamics equations are solved using the coupled channel method, and calculated mixing rates are compared with recent experiments. The mixing rates depend strongly on the particular transition and on the collision partner (He or Ar). The calculations provide an explanation for most of the state-to-state rates in terms of the features of the potential curves.

The authors have recently reported calculations for low-lying excited state potential curves of XeAr, and they have used these potential curves for coupled channel scattering calculations of inelastic scattering of Xe. A typical process is Xe + Ar {yields} Xe + Ar. Dissociative recombination (DR) corresponds to different arrangements of the same particles. DR may be written ArXe{sup +}+e{yields}Xe+Ar. Because the same Hamiltonian describes inelastic scattering and DR, much of the authors earlier work can be extended to provide the potential curves and coupling matrix elements necessary to treat DR. Part of the task is to determine the energies of very highly excited states of Xe Ar; this can be done by using Multichannel Quantum Defect Theory (MQDT) to extrapolate the authors earlier results. Preliminary results will be reported.

An attempt to locate stable excited electronic states of CH5+ found only states that are dissociative in character. In view of these results, the hypothesis of stabilization via a stable electronic state suggested for CH3+ + H-2, radiative association needs to be reconsidered.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical study of nitro-nitrite rearrangement of nitramideRoberta P. Saxon and Megumu YoshimineCite this: J. Phys. Chem. 1989, 93, 8, 3130–3135Publication Date (Print):April 1, 1989Publication History Published online1 May 2002Published inissue 1 April 1989https://pubs.acs.org/doi/10.1021/j100345a050https://doi.org/10.1021/j100345a050research-articleACS PublicationsRequest reuse permissionsArticle Views194Altmetric-Citations50LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts

Absolute two-photon absorption cross sections have been measured for the np $^{3}P_{2}$,1,0\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{2}$ transitions in atomic oxygen (n=4 and 5) using the technique of two-photon--excited fluorescence. For the n=4 transition at 200.6 nm the integrated cross section is ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2) =(4.8\ifmmode\pm\else\textpm\fi{}2.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}36}$ ${\mathrm{cm}}^{4}$. The corresponding number for the n=5 transition at 192.5 nm is (0.${7}_{\ensuremath{-}0.5}^{+0.7}$)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}36}$ ${\mathrm{cm}}^{4}$. These numbers are in moderately good agreement with ab initio calculations presented previously [Phys. Rev. A 34, 199 (1986)] for n=4 and in this paper for n=5. Calculations are also reported for the 4f $^{3}F_{4}$,3,2\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{\mathrm{J}\mathcal{'}\mathcal{'}}$ transition at 194.2 nm. Atoms in the np $^{3}P$ excited states produce positive ions and electrons upon collisions with ${\mathrm{O}}_{2}$. In each case, Penning ionization and associative ionization are possible, and cannot be distinguished.Total rate constants for collisional ionization by ${\mathrm{O}}_{2}$ at room temperature are (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$for n=4 and (4\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. Einstein A coefficients are calculated for all allowed fluorescence transitions originating from the 4p $^{3}P$ and 5p $^{3}P$ electronic states. For each state, the sum of the calculated Einstein A coefficients agrees well with the experimental zero-pressure decay rate. Rate constants for collisional removal of population from these states by the background gas (which is mostly ${\mathrm{O}}_{2}$) are 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=4 and 7.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. The photoionization cross section for atoms in the 4p $^{3}P$electronic state is (3\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$. For atoms in the 5p $^{3}P$ state the photoionization cross section is less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$.

Two-photon absorption cross sections for transitions from the O${(\mathrm{}}^{3}$P) ground electronic state to the 3p ${}^{3}$P, 4p ${}^{3}$P, and the autoionizing ${\mathrm{O}}^{+}$(${\mathrm{}}^{2}$D)3p' and ${\mathrm{O}}^{+}$(${\mathrm{}}^{2}$P)3p'' states have been calculated by explicit evaluation of the perturbation-theory summation using matrix elements calculated from configuration-interaction wave functions and sum rules to correct for truncation. The role of different contributions in the summation has been analyzed. For the 3p ${}^{3}$P state, a cross section of ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}J) of (1.319\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$ has been obtained, which is in excellent agreement with the experimental value for the quantity ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2)${G}^{(2)}$ =(2.66\ifmmode\pm\else\textpm\fi{}0.80)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$, given in the companion paper, if the photon statistical factor, ${G}^{(2)}$, is evaluated in the chaotic field limit (${G}^{(2)}$=2).

: Summary: Photon Impact: Atomic Photoionization - Experiment and Theory, Molecular Photoionization - Experiment and Theory, Photodetachment, Single Photon Dissociation, Multiphoton Processes in Atoms and Molecules; Electron-Atom Collisions: Elastic Processes, Alignment and Orientation, Resonances, General Threshold and Autoionization, (e,2e) and Post Collision Interactions, Spin-Dependent Processes; Electron-Molecule Collisions: Electronically Elastic Processes, Electronic Excitation and Ionization, Dissociation; Electron-Ion Collisions: Elastic, Inelastic, Ionizing, and Recombination; Positron-Atom, Positron-Molecule Collisions; Exotic Collisions; Atom-Atom Collisions: Ionization, Associative and Chemi-Ionization; Ion-Atom Collisions: Resonant Transfer and Excitation, Direct Excitation and Ionization, Autoionization, Electron Detachment, Quasimolecular Collisions, Alignment and Orientation, Production and Ionization of Multiply Charged Ions, Charge Transfer of Multiply Charged Ions, Capture into Continuum and Convoy Electrons, Radiative and Non-Radiative Capture at High Energies, Inner Shell Vacancy Decay; Ion(Atom)-Molecule Collisions: Rotational-Vibrational Excitation; Molecular Charge Transfer, Ionization and Detachment, Reactive Scattering, Three-Body Processes; Ion-Ion Collisions; Rydberg States and Reactions; Field Assisted Collisions; Clusters; Experimental Techniques.

In the early days of quantum chemistry research, major emphasis was placed on method development and calculations were performed on physical systems more to evaluate calculational methods than to gain new insight into these systems. The field has now matured to the point where present-day methods are capable of providing reliable information for systems of experimental interest, including those where large numbers of excited states are involved. Predicting the cross sections and products of photodissociation requires knowledge of large numbers of excited state potential curves or surfaces, particularly repulsive potential curves that are not easily studied spectroscopically. Also, theoretical study of collisions between excited species requires information about excited state potential curves. Calculations of excited state potential curves discussed in this paper support the assertion that current quantum chemistry techniques are now providing physical insight for systems of interest.

The radicals and ions observed in comets result from photodissociation and photoionization of molecules. According to current models, a comet is composed chiefly of a large, solid nucelus of frozen gases (parent molecules) such as H2O, HCN, and NH3. It is believed comets were formed at the same time and in the same region of space as the major planets and that their chemical composition is the same as that of the early solar system. As the comet nears the Sun, the surface heats up, liberating the frozen gases as well as dust particles. Solar radiation photodissociates the parent molecules into fragments that are observed by resonance fluorescence. Both polyatomic molecules, present in the interstellar medium, and cometary radicals were observed. Using laboratory photo-dissociation data and computer models, astronomers are attempting to identify the parent molecules that account for all observed radicals and ions.