High level electronic structure predictions of thermochemical properties and molecular structure are capable of accuracy rivaling the very best experimental measurements as a result of rapid advances in hardware, software, and methodology. Despite the progress, real world limitations require practical approaches designed for handling general chemical systems that rely on composite strategies in which a single, intractable calculation is replaced by a series of smaller calculations. As typically implemented, these approaches produce a final, or “best,” estimate that is constructed from one major component, fine-tuned by multiple corrections that are assumed to be additive. Though individually much smaller than the original, unmanageable computational problem, these corrections are nonetheless extremely costly. This study presents a survey of the widely varying magnitude of the most important components contributing to the atomization energies and structures of 106 small molecules. It combines large Gaussia...

A survey of factors contributing to accurate theoretical predictions of atomization energies and molecular structures.

This new calibration atlas is based on frequency rather than wavelength calibration techniques for absolute references. Since a limited number of absolute frequency measurements is possible, additional data from alternate methodology are used for difference frequency measurements within each band investigated by the frequency measurements techniques. Data from these complementary techniques include the best Fourier transform measurements available. Included in the text relating to the atlas are a description of the heterodyne frequency measurement techniques and details of the analysis, including the Hamiltonians and least-squares-fitting and calculation. Also included are other relevant considerations such as intensities and lincshape parameters. A 390-entry bibliography which contains all data sources used and a subsequent section on errors conclude the text portion. The primary calibration molecules are the linear triatomics, carbonyl sulfide and nitrous oxide, which cover portions of the infrared spectrum ranging from 488 to 3120 cm-1. Some gaps in the coverage afforded by OCS and N2O are partially covered by NO, CO, and CS2. An additional region from 4000 to 4400 cm-1 is also included. The tabular portion of the atlas is too lengthy to include in an archival journal. Furthermore, different users have different requirements for such an atlas. In an effort to satisfy most users, we have made two different options available. The first is NIST Special Publication 821, which has a spectral map/facing table format. The spectral maps (as well as the facing tables) are calculated from molecular constants derived for the work. A complete list of all of the molecular transitions that went into making the maps is too long (perhaps by a factor of 4 or 5) to include in the facing tables. The second option for those not interested in maps (or perhaps to supplement Special Publication 821) is the complete list (tables-only) which is available in computerized format as NIST Standard Reference Database #39, Wavelength Calibration Tables.

Wavenumber calibration tables from heterodyne frequency measurements

The atomic characteristics, which govern changes in bonding properties due to relativistic effects in heavy atoms, are identified from a scattering theoretic standpoint. It is shown that within an all-electron calculation scalar relativistic corrections to valence orbitals relevant to atomic bonding properties can be made via a local pseudopotential for all elements. The present approach reproduces molecular geometries and vibrational frequencies excellently for a test set of relatively simple molecules, where good experimental data are available. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 423–433, 1998

A scattering theoretic approach to scalar relativistic corrections on bonding

The fundamental role of Eckart vectors and Eckart frames is demonstrated in the theort of the vibration--rotation Hamiltonian for a polyatomic molecule. (AIP)

Eckart vectors, Eckart frames, and polyatomic molecules

Multiconfigurational second-order perturbation theory: A test of geometries and binding energies

The infrared-active combination bands of SF/sub 6/ gas were remeasured and the band origins estimated. These data were combined with the Raman frequencies of Bosworth et al., and with the spectrum of SF/sub 6/ in cryogenic liquid oxygen solutions reported by Bertsev et al., to obtain estimates for the 21 anharmonicity constants X/sub ij/. The resulting harmonic vibrational frequencies were used in a calculation of the general quadratic harmonic force field of SF/sub 6/. The most useful constraint in the F/sub 1//sub u/ symmetry block was the Coriolis constant zeta/sub 3/ = 0.693 +- 0.004 obtained from an analysis of ..nu../sub 3/ resolved with tunable diode lasers; but zeta/sub 4/ estimated from a similar analysis of ..nu../sub 4/, and the /sup 32/S--/sup 34/S isotope frequency shifts reported by Brunet and Perez for these fundamentals, are all in general agreement. The more important valence force constants are f/sub r/ = 5.45 +- 0.03, f/sub rr/(cis interaction) = 0.36 +- 0.01, f/sub rr'/ (trans interaction) = -0.02 +- 0.03, and f/sub ..cap alpha../ approximately 0.83, all in mdyn/A. From the absolute band intensities of Schatz and Hornig and the present force field, the S--F bond moment and its derivative are estimated tomore » be ..mu../sub 0/ = 2.44 +- 0.12 D and delta ..mu../delta r = 4.2 +- 0.3 D/A.« less

Vibrational constants and force field of sulfur hexafluoride

Optical emissions from long‐lived, excited states of CO+ and N2+ in ion beams have been studied CO or N2 is ionized by electron impact, and the ions are removed from the ion source to the observation region Spectra of emission from CO+ and N2+ are obtained for wavelengths in the ranges 3200–5200 A and 6000–8700 A, respectively All of the spectral features can be ascribed to the CO+ comet‐tail and N2+ Meinel emission systems The electronic transition moments, Re, for these two emission systems are determined and are found to vary rather slowly with wavelength over the wavelength ranges for which we have data Lifetimes of the v′=1, 2, 4, and 6 vibrational levels of CO+ (A 2π) are found to be 349, 278, 263, and 241 μsec, respectively Lifetimes of the v′=2, 3, 4, and 5 vibrational levels of N2+(A2πu) are found to be 1228, 1070, 1008, and 914 μsec, respectively The dependences of the measured lifetimes on v′ are consistent with the experimental values of Re and with available Franck‐Condon fact

Study of the A → X Transitions in N2+ and CO+

Octahedral fine-structure splittings in ν3 of SF6☆

High‐resolution spectra of the infrared‐active stretching fundamental ν3 of 238UF6 have been obtained between 620.6 and 633.5 cm−1 using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF6 in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground‐ and excited‐state values of the rotational constant B could be individually determined, and the U–F bond length in the ground vibrational state is r0=1.9962±0.0007 A. The Q branch of 235UF6 has also been analyzed and the 235UF6–238UF6 ν3 isotope shift measured to be 0.603 79±0.000 17 cm−1. The isotope shift and the Coriolis constant...

Measurement and analysis of the infrared‐active stretching fundamental (ν3) of UF6

A method for investigating radiation from long‐lived, excited states of ions is described. Emission spectra, apparent lifetimes of the excited states, and electron impact cross sections for producing the radiation can be studied. The spectrum of emission from long‐lived states of NO+ in an ion beam has been obtained for wavelengths between 1250 and 2200 A, but no NO+ emissions have been observed in spectral scans of the 2200–3400‐A region. Some of the spectral features are produced by transitions to the X 1Σ+ state of NO+ from the υ′ = 0 and 1 levels of a single electronic state having an internuclear distance 1.157−0.01+0.02 A and an apparent lifetime 135 ± 35 μsec. These two vibrational levels are 7.301 ± 0.006 and 7.511 ± 0.01 eV above the ground state of NO+. The other emissions observed can be characterized by two apparent lifetimes, 10.8 ± 2 and 160 ± 50 μsec, and appear to be produced by transitions to the X 1Σ+ state from several upper states. Tentatively, the energies of the upper states are set ...

Redus F. Holland

Papers

Vibrational constants and force field of sulfur hexafluoride

Study of the A → X Transitions in N2+ and CO+

Octahedral fine-structure splittings in ν3 of SF6☆

Measurement and analysis of the infrared‐active stretching fundamental (ν3) of UF6

Emission from Long-Lived States in Ion Beams. New Band Systems of NO +