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  • 1
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    W4 theory for computational thermochemistry: In pursuit of confident sub-kJ/mol predictions
    AIP Publishing ; 2006
    In:  The Journal of Chemical Physics Vol. 125, No. 14 ( 2006-10-14)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 125, No. 14 ( 2006-10-14)
    Abstract: In an attempt to improve on our earlier W3 theory [A. D. Boese et al., J. Chem. Phys. 120, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations (T̂4), inclusion of connected quintuple excitations (T̂5), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the Active Thermochemical Tables thermochemical network. The recent CCSDT(Q) method offers a cost-effective way of estimating T̂4, but is insufficient by itself if the molecule exhibits some nondynamical correlation. The latter considerably slows down basis set convergence for T̂4, and anomalous basis set convergence in highly polar systems makes two-point extrapolation procedures unusable. However, we found that the CCSDTQ−CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)∕cc-pVTZ+CCSDTQ∕cc-pVDZ-CCSDT(Q)∕cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit T̂4 contribution. The T̂5 contribution converges very rapidly with the basis set, and even a simple double-zeta basis set appears to be adequate. The largest T̂5 contribution found in the present work is on the order of 0.5kcal∕mol (for ozone). DBOCs are significant at the 0.1kcal∕mol level in hydride systems. Post-CCSD(T) contributions to the core-valence correlation energy are only significant at that level in systems with severe nondynamical correlation effects. Based on the accumulated experience, a new computational thermochemistry protocol for first- and second-row main-group systems, to be known as W4 theory, is proposed. Its computational cost is not insurmountably higher than that of the earlier W3 theory, while performance is markedly superior. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1kcal∕mol on average, 95% confidence intervals narrower than 1kJ∕mol) with the latest experimental data obtained from Active Thermochemical Tables. Lower-cost variants are proposed: the sequence W1→W2.2→W3.2→W4lite→W4 is proposed as a converging hierarchy of computational thermochemistry methods. A simple a priori estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.2348881
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2006
    detail.hit.zdb_id: 3113-6
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  • 2
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    IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals. Part I
    AIP Publishing ; 2005
    In:  Journal of Physical and Chemical Reference Data Vol. 34, No. 2 ( 2005-06-01), p. 573-656
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    In: Journal of Physical and Chemical Reference Data, AIP Publishing, Vol. 34, No. 2 ( 2005-06-01), p. 573-656
    Abstract: This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.
    Type of Medium: Online Resource
    ISSN: 0047-2689 , 1529-7845
    URL: Article
    DOI: 10.1063/1.1724828
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2005
    detail.hit.zdb_id: 2006308-8
    SSG: 11
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  • 3
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    High-accuracy extrapolated ab initio thermochemistry. III. Additional improvements and overview
    AIP Publishing ; 2008
    In:  The Journal of Chemical Physics Vol. 128, No. 11 ( 2008-03-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 128, No. 11 ( 2008-03-21)
    Abstract: Effects of increased basis-set size as well as a correlated treatment of the diagonal Born-Oppenheimer approximation are studied within the context of the high-accuracy extrapolated ab initio thermochemistry (HEAT) theoretical model chemistry. It is found that the addition of these ostensible improvements does little to increase the overall accuracy of HEAT for the determination of molecular atomization energies. Fortuitous cancellation of high-level effects is shown to give the overall HEAT strategy an accuracy that is, in fact, higher than most of its individual components. In addition, the issue of core-valence electron correlation separation is explored; it is found that approximate additive treatments of the two effects have limitations that are significant in the realm of & lt;1kJmol−1 theoretical thermochemistry.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.2835612
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2008
    detail.hit.zdb_id: 3113-6
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  • 4
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    Characterization of nitrogen-containing radical products from the photodissociation of trimethylamine using photoionization detection
    AIP Publishing ; 2000
    In:  The Journal of Chemical Physics Vol. 113, No. 8 ( 2000-08-22), p. 3088-3097
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 113, No. 8 ( 2000-08-22), p. 3088-3097
    Abstract: The outcome of neutral and photoionized N(CH3)2 primary products of trimethylamine photodissociation at 193 nm is determined by combining photoionization detection with supporting G3 theoretical calculations. N(CH3)2 primary products with very little internal energy show an experimentally observed ionization onset of 9.1±0.2 eV, but do not appear at the parent ion (m/e=44). Instead, the parent ion is unstable and easily fragments to m/e=42, where the signal is observed. N(CH3)2 radicals with higher internal energies undergo H-atom loss from the neutral to give CH2NCH3, which has an observed ionization onset at parent (m/e=43) of & lt;9.3 eV. At slightly higher ionization energies, these secondary products also appear at m/e=42 (where their appearance energy is roughly 9.8–9.9 eV, uncorrected for internal energy). Finally, N(CH3)2 radicals with the highest internal energy in this study appear to undergo H2 loss as neutrals, giving rise to a species whose parent ion has m/e=42. The ionization onset of this species at m/e=42 is found to be in the range of 9.5–9.6 eV.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.1286973
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2000
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  • 5
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    Pulsed field-ionization photoelectron-photoion coincidence study of the process N2+hν→N++N+e−: Bond dissociation energies of N2 and N2+
    AIP Publishing ; 2005
    In:  The Journal of Chemical Physics Vol. 123, No. 7 ( 2005-08-15)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 123, No. 7 ( 2005-08-15)
    Abstract: We have examined the dissociative photoionization reaction N2+hν→N++N+e− near its threshold using the pulsed field-ionization photoelectron-photoion coincidence (PFI-PEPICO) time-of-flight (TOF) method. By examining the kinetic-energy release based on the simulation of the N+ PFI-PEPICO TOF peak profile as a function of vacuum ultraviolet photon energy and by analyzing the breakdown curves of N+ and N2+, we have determined the 0-K threshold or appearance energy (AE) of this reaction to be 24.2884±0.0010eV. Using this 0-K AE, together with known ionization energies of N and N2, results in more precise values for the 0-K bond dissociation energies of N–N (9.7543±0.0010eV) and N–N+ (8.7076±0.0010eV) and the 0-K heats of formation for N (112.469±0.012kcal∕mol) and N+ (447.634±0.012kcal∕mol).
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.1995699
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2005
    detail.hit.zdb_id: 3113-6
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  • 6
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    On the heats of formation of trifluoromethyl radical CF3 and its cation CF3+
    AIP Publishing ; 1997
    In:  The Journal of Chemical Physics Vol. 106, No. 1 ( 1997-01-01), p. 210-221
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 106, No. 1 ( 1997-01-01), p. 210-221
    Abstract: The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP0(CF3+C2F4)=13.721±0.005 eV and AP0(CF+/C2F4)=13.777±0.005 eV and an accurate difference in ionization potentials, IP(CF)−IP(CF3)=0.055±0.003 eV. With the existing photoelectron value IP(CF)=9.11±0.01 eV, this produces IP(CF3)=9.055±0.011 eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP0(CF3+/CF3Cl) & lt;12.867±0.008 eV has been found to be only an upper limit. The Analogous CF3+CF3+ fragment yield curves from CF3Br and CF3I produce AP0(CF3+/CF3Br)=12.095±0.005 eV and AP0(CF3+/CF3I)=11.384±0.005 eV, leading to D0(CF3−Br)=70.1±0.3 kcal/mol (70.8±0.3 kcal/mol at 298 K) and D0(CF3−I)=53.7±0.3 kcal/mol (54.3±0.3 kcal/mol at 298 K). Based on tabulated values for ΔHf∘(CF3Br) and ΔHf∘(CF3I), which appear to be inconsistent by ∼1 kcal/mol, a compromise value of ΔH298 f∘(CF3)=−111.4±0.9 kcal/mol (−110.7±0.9 kcal/mol at 0 K) is selected, resulting in ΔH298f∘(CF3+)=97.4±0.9 kcal/mol (98.1±0.9 kcal/mol at 0 K). Additionally, IP(CF4)≡AP0(CF3+/CF4)=14.67±0.04 eV can be inferred. From data on C2F4, ΔH298f∘(CF)=62.5±1.1 kcal/mol (61.7±1.1 kcal/mol at 0 K) can be deduced. Many earlier literature values for appearance potentials of CF3+ from CF3X, leading to very low ΔHf∘(CF3+) and/or IP(CF3) values, are demonstrated to be in error.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.473982
    Language: English
    Publisher: AIP Publishing
    Publication Date: 1997
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  • 7
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    Fourier transform photoelectron spectroscopy: The correlation function and the harmonic oscillator approximation
    AIP Publishing ; 1986
    In:  The Journal of Chemical Physics Vol. 85, No. 7 ( 1986-10-01), p. 3776-3784
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 85, No. 7 ( 1986-10-01), p. 3776-3784
    Abstract: The correlation function describes the time development of the wave packet placed by photoabsorption or photoionization onto the potential surface of an upper electronic state. The function can be obtained as a Fourier transform of the electronic band, and gives information about the features of the final state. The analytical expressions for the correlation function within the harmonic oscillator approximation are presented. Because of some unique properties of the correlation function, the expressions can be used to obtain accurate geometric details of the final state from experimental data. The approach is tested on some photoelectron spectra of diatomics and compared to known data. The method yields the equilibrium internuclear distance with an accuracy of ±0.0025 Å, and resolves the sign uncertainty present in the conventional harmonic Franck–Condon analysis. The comparison of the experimental data with the predictions of the harmonic model gives a deeper insight into the behavior of a wave packet in an anharmonic potential.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.450898
    Language: English
    Publisher: AIP Publishing
    Publication Date: 1986
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  • 8
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    Elaborated thermochemical treatment of HF, CO, N2, and H2O: Insight into HEAT and its extensions
    AIP Publishing ; 2021
    In:  The Journal of Chemical Physics Vol. 155, No. 18 ( 2021-11-14)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 155, No. 18 ( 2021-11-14)
    Abstract: Empirical, highly accurate non-relativistic electronic total atomization energies (eTAEs) are established by combining experimental or computationally converged treatments of the nuclear motion and relativistic contributions with the total atomization energies of HF, CO, N2, and H2O obtained from the Active Thermochemical Tables. These eTAEs, which have estimated (2σ) uncertainties of less than 10 cm−1 (0.12 kJ mol−1), form the basis for an analysis of high-level ab initio quantum chemical calculations that aim at reproducing these eTAEs for the title molecules. The results are then employed to analyze the performance of the high-accuracy extrapolated ab initio thermochemistry, or High-Accuracy Extrapolated Ab Initio Thermochemistry (HEAT), family of theoretical methods. The method known as HEAT-345(Q), in particular, is found to benefit from fortuitous error cancellation between its treatment of the zero-point energy, extrapolation errors in the Hartree-Fock and coupled cluster contributions, neglect of post-(T) core-correlation, and the basis-set error involved in higher-level correlation corrections. In addition to shedding light on a longstanding curiosity of the HEAT protocol—where the cheapest HEAT-345(Q) performs comparably to the theoretically more complete HEAT-456QP procedure—this study lays the foundation for extended HEAT variants that offer substantial improvements in accuracy relative to the established approaches.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/5.0069322
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2021
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  • 9
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    Unimolecular thermal fragmentation of ortho -benzyne
    AIP Publishing ; 2007
    In:  The Journal of Chemical Physics Vol. 126, No. 4 ( 2007-01-28)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 126, No. 4 ( 2007-01-28)
    Abstract: The ortho-benzyne diradical, o-C6H4 has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C6H4+Δ→ products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C6H4+Δ→HCCH+HCC–CCH. The experimental ΔrxnH298(o-C6H4→HCCH+HCC–CCH) is found to be 57±3kcalmol−1. Further experiments with the substituted benzyne, 3,6-(CH3)2-o-C6H2, are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)∕H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)] . Focal point extrapolations of the computational data yield a 0K barrier for the concerted, C2v-symmetric decomposition of o-benzyne, Eb(o-C6H4→HCCH+HCC–CCH)=88.0±0.5kcalmol−1. A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C6H6→H+[C6H5]→H+[o-C6H4] →HCCH+HCC–CCH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.2409927
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2007
    detail.hit.zdb_id: 3113-6
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  • 10
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    Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry
    AIP Publishing ; 2006
    In:  The Journal of Chemical Physics Vol. 124, No. 7 ( 2006-02-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 124, No. 7 ( 2006-02-21)
    Abstract: We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C3H3) formed by the 193nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5–9.9eV using high-resolution (energy bandwidth=1meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C3H3 thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C3H3 in excited autoionizing Rydberg states. The ionization energy (IE=8.674±0.001eV) of C3H3 determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C3H3+(X̃;νi,i=1–12)←C3H3(X̃). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C3H3 points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C3H3) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C3H3 and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.2168448
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2006
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