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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 114 (1992), S. 9136-9153 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 114 (1992), S. 6834-6849 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 109 (1987), S. 2902-2909 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 109 (1987), S. 1615-1621 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 329-344 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An ab initio investigation of the (CIIs) in-plane bent 3A‘ CH2CO→X˜ 3B1 CH2+X˜ 1∑+CO and the (CIs) out-of-plane bent 3A' CH2CO→X˜ 3B1 CH2+X˜ 1∑+CO dissociation paths has been performed. Geometrical structures, vibrational frequencies, and quadratic force constants have been determined at the DZP SCF and DZP CISD levels of theory for the X˜ 1A1, 3A‘, and 3A' states of ketene and for the 3A‘ and 3A' transition states for dissociation. The DZP CISD structure for A˜ 1A‘ ketene is also reported. Final energetic predictions for triplet ketene dissociation have been obtained from large-basis (QZ2P and QZ2P+f) UMP4(SDTQ) calculations at the DZP CISD geometries. The CIIs stationary point for 3A‘ ketene dissociation is a true transition state with r(C–C)=2.071 A(ring) at the DZP CISD level of theory. The corresponding CIs stationary point for 3A' ketene is actually a super transition state for the interconversion of two equivalent 3A‘CIIs transition states for dissociation. Final theoretical predictions of Te=19 400 cm−1 and T0=19 150 cm−1 are made for the adiabatic excitation energy of the a˜ 3A‘ state of ketene, and a value of 22.3 kcal/mol is proposed for the 3A‘ dissociation energy.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In an accompanying paper by Lee, Allen, and Schaefer [J. Chem. Phys. 87, 7062 (1987)], an efficient formulation for the analytic evaluation of two-configuration self-consistent-field configuration interaction (TCSCF-CI) energy first derivatives is presented. In this paper the TCSCF-CI gradient method is adapted and applied to singlet excited electronic states of the same symmetry as the ground state. Since single-configuration self-consistent-field configuration interaction (SCF-CI) wave functions are unreliable for such electronic states due to the possibility of variational collapse, the TCSCF-CI method based on excited-state orbitals is the simplest means by which dynamical electron correlation can be incorporated effectively. Geometrical structures, excitation energies, and harmonic vibrational frequencies obtained with double-zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets are reported for the 2 1A1 states of formaldehyde and ketene. The 2 1A1 state of H2CO is found to have two distinct minima with C2v symmetry: a (π→π*)1 valence minimum with re(C–O)=1.564 A(ring) and T0=8.50 eV, and an (n→3py)1 Rydberg minimum with re(C–O)=1.216 A(ring) and T0=7.93 eV. Some question exists on whether the (n→3py)1 state occurs experimentally at 8.11 or 7.96 eV, but in either case the agreement between theory and experiment is good. The 2 1A1 state of CH2CO is found to have only one C2v minimum with re(C–O) =1.129 A(ring) and re(C–C)=1.403 A(ring). This state is predominantly Rydberg in character and can be designated (π→3px)1, but evidence is presented which suggests a moderate amount of Rydberg–valence mixing involving the (π→π*)1 "state.'' The DZP+R CISD T0=6.94 eV prediction confirms previous assignments of the experimental band at 6.78 eV to the 2 1A1 state.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 7062-7075 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient formulation for the analytic evaluation of two-configuration self-consistent-field configuration interaction (TCSCF-CI) energy first derivatives is presented. Use is made of the Z-vector approach of Handy and Schaefer. The procedure outlined does not require the transformation of derivative integrals. We have applied the new procedure to the ground states of ozone and ethylene. For ozone, the (DZP) TCSCF-CI procedure yields a structure which is much improved relative to that obtained using a single-reference description. However, the incorrect ordering of the harmonic vibrational frequencies ω1(a1) and ω3(b2), which was first found using a TCSCF description of O3, remains at the TCSCF-CI level of theory. Anharmonic vibrational frequencies (utilizing TCSCF-CI harmonic frequencies and SCF anharmonic corrections) are reported for ethylene. With a DZP basis set, 11 of the 12 TCSCF-CI fundamental frequencies of C2H4 agree very well with experiment, the mean absolute error being 2.4%. However, the infrared inactive ν8=799 cm−1 is 15.0% below the accepted experimental value. The utilization of a TZ2P basis set resolves this discrepancy. The TZ2P TCSCF-CI ν8=922 cm−1 is 1.9% below experiment, and the average absolute error of all of the TZ2P fundamentals is only 2.6%.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4965-4975 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio predictions at the SCF, CISD, and CCSD levels are reported for the title compounds using DZP and TZP basis sets. The calculated geometries, rotational constants, dipole moments, fundamental frequencies, isotopic frequency shifts, vibration–rotation interaction constants, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with experimental data (if available). The best agreement is usually found for the CCSD results. The experimentally derived cubic force field of F2O is reproduced well by our results so that the predicted cubic and quartic force fields of HOF and the predicted quartic force field of F2O are also expected to be realistic. On the basis of our theoretical anharmonic constants, a new interpretation is suggested for the anomalous isotopic frequency shift of ν3 in HOF and DOF. Finally, an experimentally derived re structure with R(O–F)=1.4356 A(ring), r(O–H)=0.9664 A(ring), and α(H–O–F)=97.72° is proposed for HOF on the basis of the TZP CCSD vibration–rotation interaction constants.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5585-5592 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent photofragmentation experiments have observed stepwise increases in the dissociation rate for CH3CHO (T1)→CH3 (X˜ 2A2″)+HCO (X˜ 2A′) as a function of excitation energy. In accord with the Rice–Ramsperger–Kassel–Marcus (RRKM) form of transition-state theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional (DFT) methods with [C,O/H] atomic-orbital basis sets ranging in quality from [4s2p1d/2s1p] to [6s5p4d3f2g1h/5s4p3d2 f1g]. A high-level focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, D0=1583 cm−1, and the association barrier height, V0*=3149 cm−1. With a basis set of triple-ζ plus double-polarization plus f(TZ2Pf ) quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of 355i cm−1 and 516i cm−1, respectively, while the empirical value inferred from RRKM models is only 60i cm−1. The RRKM-derived frequencies for the degrees of freedom orthogonal to the reaction path are more reasonable but still not in convincing agreement with electronic structure theory. Thus, while the experimental steps in the dissociation rate of acetaldehyde (as well as ketene) have yet to be satisfactorily explained, proven ab initio methods provide strong evidence that simple RRKM fits to the k(E) profile provide misleading vibrational frequencies of the transition state on the corresponding triplet potential energy surface. © 2000 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2875-2878 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A series of high-quality conventional and explicitly correlated second-order Møller–Plesset energies, denoted as MP2 and MP2-R12/A, respectively, was computed in a state-of-the-art attempt to arrive at the ab initio limit for the barrier to linearity of water. [O/H]-centered Gaussian basis sets as large as [19s13p11d9f7g5h3i/13s11p9d7f5g3h] were employed in the study. We find that basis functions with high orbital angular momentum quantum number (h and i) are important for highly accurate predictions of the electronic energy, even when explicitly correlated approaches are utilized. This observation is at variance with previous claims that explicitly correlated methods require basis sets saturated only to the level of f functions for spectroscopic accuracy. Our best explicitly computed value for the MP2 contribution to the barrier is −357 cm−1. The resulting final ab initio estimate for the classical barrier to linearity of water is 11 119±15 cm−1, in good agreement with the most recently obtained empirical value of 11 105±5 cm−1. © 2001 American Institute of Physics.
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