Publications (129) View all
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Article: The route to high accuracy in ab initio calculations of Cu quadrupole-coupling constants.
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ABSTRACT: We report nonrelativistic and scalar-relativistic coupled-cluster calculations of the copper quadrupole-coupling constants for eleven small copper-containing compounds. It is shown to be necessary to treat both electron-correlation and scalar-relativistic effects on the same footing even for a qualitatively correct description, because both effects are significant and are strongly coupled in the case of Cu electric-field gradients. We show that the three scalar-relativistic schemes employed in the present study-the leading order of direct perturbation theory, the spin-free exact two-component theory in its one-electron variant, and the spin-free Dirac-Coulomb approach-provide accurate treatments of scalar-relativistic effects for the copper compounds under study. Furthermore, we demonstrate that results close to the basis-set limit can be obtained by augmenting large uncontracted standard basis sets for copper with additional steep functions. It is also shown that high-level correlation effects (those beyond the perturbative treatment of triple excitations) make important contributions in the present case.The Journal of chemical physics 12/2012; 137(22):224302. · 3.09 Impact Factor -
Article: Thermal decomposition of CH(3)CHO studied by matrix infrared spectroscopy and photoionization mass spectroscopy.
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ABSTRACT: A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.The Journal of chemical physics 10/2012; 137(16):164308. · 3.09 Impact Factor -
Article: Detection of the Far-IR ν(12) Bending Level in Propargyl: A Complete Set of Fundamentals for an Important Radical.
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ABSTRACT: Propargyl (HCCCH(2)) radicals have been generated by expanding precursors through a supersonic pyrolysis nozzle, and their infrared (IR) absorption spectra have been recorded in a rare gas matrix. Besides the 10 vibrational modes observed in our previous studies, this investigation detected the HCCCH(2) X̃ (2)B(1) in-plane bending mode (ν(12)) at 344.2 (±0.8) cm(-1) in a cryogenic argon matrix. This is the first experimental observation of ν(12) for the propargyl radical. In the previous study, ν(12) had been fixed at 333 (±10) cm(-1) based on detection of its overtone (2ν(12), 667.7 ± 1.0 cm(-1)) and a possible combination band (ν(10) + ν(12), 1339.0 ± 0.8 cm(-1)). The new observation is in reasonable agreement with this previous experimental estimation of ν(12). Ab initio coupled cluster anharmonic force field calculations were used to guide the analysis. Our finding completes the fundamental frequency database for propargyl.The Journal of Physical Chemistry A 09/2012; 116(42):10338-43. · 2.95 Impact Factor -
Article: Carbonyl Diazide, OC(N(3))(2): Synthesis, Purification, and IR Spectrum.
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ABSTRACT: Carbonyl diazide (1), OC(N(3))(2), is prepared by reaction of triphosgene and tetra-n-butylammonium azide in a solution of diethyl ether or dimethyl ether. The advantage of this synthetic method, relative to other procedures, is that the use of triphosgene, OC(OCCl(3))(2), mitigates the need to use highly poisonous reagents such as phosgene, OCCl(2), or chlorofluorocarbonyl, OC(Cl)F. The identity and purity of OC(N(3))(2) are established by gas-phase IR spectroscopy, which reveals the presence of both syn-syn and anti-syn conformers. Computed anharmonic vibrational frequencies and infrared intensities of carbonyl diazide (1) display excellent agreement with experiment, and reveal the presence of strong Fermi resonances.Inorganic Chemistry 08/2012; 51(18):9846-51. · 4.60 Impact Factor -
Article: High-accuracy extrapolated ab initio thermochemistry of the vinyl, allyl, and vinoxy radicals.
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ABSTRACT: Enthalpies of formation at both 0 and 298 K were calculated according to the HEAT (High-accuracy Extrapolated Ab initio Thermochemistry) protocol for the title molecules, all of which play important roles in combustion chemistry. At the HEAT345-(Q) level of theory, recommended enthalpies of formation at 0 K are 301.5 ± 1.3, 180.3 ± 1.8, and 23.4 ± 1.5 kJ mol(-1) for vinyl, allyl, and vinoxy, respectively. At 298 K, the corresponding values are 297.3, 168.6, and 16.1 kJ mol(-1), with the same uncertainties. The calculated values for the three radicals are in excellent agreement with the corresponding experimental values, but the uncertainties associated with the HEAT values for vinoxy are considerably smaller than those based on experimental studies.The Journal of Physical Chemistry A 07/2012; 116(29):7668-76. · 2.95 Impact Factor
