ALBERT

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.

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.

Notes: Several complementary analyses have been performed in an investigation of the use of reference geometric structures which are not stationary at a given level of theory in the prediction of improved equilibrium anharmonic molecular force fields. Diatomic paradigms for the procedure were established by constructing empirical potential energy functions for the nitrogen and fluorine molecules which not only reproduce the available Rydberg–Klein–Rees data but also provide reliable derivatives through fourth order for ranges of 0.4 A(ring) or greater around the equilibrium bond distance. For comparison, analogous curves were determined at the double-ζ plus polarization (DZP) restricted Hartree–Fock (RHF) level of theory, and the quartic force fields for N2 and F2 were also obtained at the experimental re structures using a (8s5p3d2f1g) basis set and the coupled-cluster singles and doubles method augmented by a perturbative contribution from connected triple excitations [CCSD(T)].The results substantiate the ability of RHF theory to predict correlation-quality, higher-order force constants if an accurate reference geometry from experiment or a higher level of theory is employed. The theoretical foundations of this technique as applied to polyatomic molecular systems have been systematically explored. Mechanisms were analyzed which address the nonzero force dilemma by using various choices of internal coordinates to shift the equilibrium point of theoretical potential energy surfaces. Examples are presented in which the variations in predicted spectroscopic constants arising from different shift coordinate sets are non-negligible. A Cartesian projection scheme for higher-order force fields was developed and implemented to avert internal-coordinate dependences; formulas for higher-order projection matrices and higher-order derivatives of the external variables of a molecular system were concurrently derived. A formalism for the transformation of force fields between internal and Cartesian representations was also constructed which is applicable to arbitrary order. In addition to N2 and F2, case studies were performed on the F2O and N2O molecules, for which electron correlation effects are of unusual importance. Quartic force fields are reported for F2O and N2O at the DZP and TZ(2d1f) CCSD(T) levels of theory, respectively, which provide the best data sets currently available and facilitate the assessment of experimental force constants. The CCSD(T) results are reproduced remarkably well by RHF predictions at the experimental equilibrium structures of these molecules but not at the corresponding RHF optimum geometries. Finally, practical recommendations are made for predictions of higher-order force constants at nonstationary points.

Notes: The heat of formation of NCO has been determined rigorously by state-of-the-art ab initio electronic structure methods, including Møller–Plesset perturbation theory from second through fifth order (MP2–MP5) and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions were investigated to establish a consistent value for ΔHf,0(open circle)(NCO): (a) HNCO(X˜ 1A')→H(2S)+NCO(2Π), (b) HNCO(X˜ 1A')→H++NCO−, (c) N(4S)+CO→NCO(2Π), (d) HCN+O(3P)→H(2S)+NCO(2Π), and (e) NH(3Σ−)+CO→H(2S)+NCO(2Π). The one-particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2p1d] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p] to [8s6p4d] on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value ΔHf,0(open circle)(NCO)=31.4(5) kcal mol−1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, ΔHf,0(open circle)(HNCO)=−27.5(5) kcal mol−1, and hydrogen cyanide, ΔHf,0(open circle)(HCN)=31.9(5) kcal mol−1. The final proposals for ΔHf,0(open circle)(NCO) and ΔHf,0(open circle)(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.

Notes: The [FHCl]− molecular anion has been investigated in detail by means of state-of-the-art ab initio electronic structure methods, including restricted Hartree–Fock (RHF), Møller–Plesset perturbation theory (MP2–MP4), and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. The one-particle Gaussian basis sets ranged in quality from F[6s4p2d], Cl[10s7p2d], and H[4s2p] to F[18s13p6d4f], Cl[20s14p7d5f], and H[8s3p2d1f]. The first phase of the investigation focused on the prediction of thermochemical, spectroscopic, and bonding properties of [FHCl]− and the chemical interpretation thereof.The final proposals for the geometric structure and binding energy of the complex are re(H–F)=0.963±0.003 A(ring), Re(H–Cl)=1.925±0.015 A(ring), and D0(HF+Cl−)=21.8±0.4 kcal mol−1. A Morokuma decomposition of the ion-molecule bonding gave the following electrostatic (ES), polarization (PL), exchange repulsion (EX), dispersion (DISP), and charge-transfer plus higher-order mixing (CT+MIX) components of the vibrationless complexation energy: −27.3 (ES), −5.2 (PL), +18.3 (EX), −4.5 (DISP), and −5.0 (CT+MIX) kcal mol−1. The second phase of the work involved the construction of a CCSD global surface from 208 and 228 energy points for linear and bent conformations, respectively, these being fit to rms errors of only 3.9 and 9.3 cm−1, respectively, below 8000 cm−1. The surface was represented by a flexible analytic form which reproduces the quartic force field at equilibrium, exhibits the proper asymptotic properties, and is generally applicable to ion-molecule systems. The final phase of the study entailed the determination of converged J=0 and J=1 variational eigenstates of the [FHCl]− surface to near the HF+Cl− dissociation threshold by employing Jacobi coordinates and vibrational configuration interaction expansions in terms of natural modals.The fundamental vibrational frequencies given by the analysis were ν1=247, ν2=876, and ν3=2884 cm−1. The complete vibrational eigenspectrum was then analyzed in terms of several contemporary dynamical issues, including vibrational adiabaticity, anharmonic resonances, densities of high-lying states, and signatures of quantum ergodicity.

Notes: Characteristics of the ground electronic state of HNCO have been investigated theoretically in a series of eight ab initio analyses involving qualitative features of the electronic structure, the barrier to linearity, the NH(3Σ−)+CO fragmentation energy, the H–NCO bond dissociation energy, heats of formation of isomers of HNCO, fundamental vibrational frequencies and anharmonic force fields, the rovibrational spectrum of DNCO, and the precise Re structure of isocyanic acid. Sundry state-of-the-art electronic structure methods were employed in the study, including restricted and unrestricted Hartree–Fock (RHF and UHF), complete-active-space self-consistent-field (CASSCF), configuration interaction singles and doubles (CISD), Møller–Plesset perturbation theory through fourth and occasionally fifth order (MP2–MP5), coupled-cluster singles and doubles (CCSD), and CCSD augmented by a perturbative contribution from connected triple excitations [CCSD(T)]. The one-particle basis sets ranged in quality from (9s5p1d/4s2p1d) to (13s8p3d2f/6s5p3d2f ) on the heavy atoms and from (4s1p/2s1p) to (6s2p1d/4s2p1d) on hydrogen. Several revisions of thermochemical data are proposed, in particular, a larger barrier to linearity of 5.7(3) kcal mol−1, an enhanced bond energy of 85.4(10) kcal mol−1 for D0(NH–CO), and more reliable relative energies for the isomers of HNCO, viz., γe(HOCN)=25.5(10), γe(HCNO)=70(2), and γe(HONC)=84.5(15) kcal mol−1. In addition, the experimental value D0(H–NCO)=113.0(2) kcal mol−1 is confirmed. These results lead to several new proposals for heats of formation (ΔH°f,0, kcal mol−1): HNCO(−26.1), HOCN(−0.7), HCNO(+43.0), HONC (+57.6), and NCO(+35.3). A complete quartic force field has been constructed for HNCO by combining RHF third- and fourth-derivative predictions with CCSD quadratic force constants subjected to the scaled quantum mechanical (SQM) optimization scheme.This force field yields a set of ωi and χij vibrational constants which gives the following fundamental frequencies (with total anharmonicities in parentheses): ν1=3534(−186), ν2=2268(−45), ν3=1330(−9), ν4=778(−50), ν5=576(+9), and ν6=657(+21) cm−1, thus reproducing the observed band origins to 4 cm−1 or less. For DNCO the theoretical force field reveals misassignments of the low-frequency bending vibrations and predicts ν4(a')=727, ν5(a')=458, and ν6(a‘)=633 cm−1. Finally, the theoretical vibration–rotation interaction constants (αi) for five isotopic species of HNCO have been used in conjunction with empirical rotational constants and the Kraitchman equations to determine re(N–H)=1.0030(20) A(ring), re(N–C)=1.2145(6) A(ring), re(C–O)=1.1634(4) A(ring), θe(H–N–C)=123.34(20)°, and θe(N–C–O)=172.22(20)°.

Notes: A comprehensive anharmonic vibrational analysis of isotopic ketenes has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Møller–Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles (CCSD) approach, augmented for structural optimizations by a contribution for connected triple excitations [CCSD(T)]. The atomic-orbital basis sets of the study entailed C,O(10s6p/5s4p) and H(6s/4s) spaces multiply polarized in the valence region to give QZ(2d,2p) and QZ(2d1f,2p1d) sets. An iterative anharmonic vibrational refinement of a limited set of quadratic scaling parameters on 27 fundamentals of H2CCO, HDCCO, D2CCO, and H2C13CO generates a final quartic force field which reproduces the empirical νi data with an average absolute error of only 1.1 cm−1. This force field yields a complete and self-consistent set of Coriolis (ζij), vibrational anharmonic (χij), vibration–rotation interaction (αi), and quartic and sextic centrifugal distortion constants, providing a critical assessment of the assorted spectroscopic constants determined over many years and also facilitating future computations of vibrational state densities for detailed tests of unimolecular dissociation theories.The harmonic frequencies ascertained for H2CCO (in cm−1), with associated anharmonicities in parentheses, are ω1(a1)=3202.2(−129.2), ω2(a1)=2197.2(−44.4), ω3(a1)=1415.2(−25.9), ω4(a1)=1146.0(−29.7), ω5(b1)=581.9(+7.1), ω6(b1)=502.6(+26.3), ω7(b2)=3308.2(−141.3), ω8(b2)=996.0(−17.9), and ω9(b2)=433.6(+5.0). The large positive anharmonicity for the ν6(b1) C=C=O bending mode, which is principally a Coriolis effect, warrants continued investigation. Explicit first-order treatments of the strong Fermi interactions within the (ν4,2ν5,ν5+ν6,2ν6) manifold reveal resonance shifts for ν4(H2CCO, HDCCO, D2CCO) of (−12.1, −10.0, +12.2) cm−1, in order. The experimental assignments for this Fermi tetrad are confirmed to be problematic. From high-precision empirical rotational constants of six isotopomers and the theoretical anharmonic force field, the equilibrium structure of ketene is derived: re(C=O)=1.160 30(29) A(ring), re(C=C)=1.312 12(30) A(ring), re(C–H)=1.075 76(7) A(ring), and θe(H–C–H)=121.781(12)°. A natural bond orbital (NBO) analysis shows that the unusually large methylene angle is attributable to extensive in-plane π delocalization. © 1995 American Institute of Physics.

Notes: Diverse aspects of the potential surface for the proton-transfer reaction CH3OH+F−→CH3O−+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Møller–Plesset perturbation theory from second through fourth order (MP2–MP4), the configuration interaction and coupled-cluster singles and doubles methods (CISD and CCSD), and CCSD theory augmented by a perturbative correction for connected triple excitations [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2p1d;2s1p] to [14s9p6d4f;9s6p4d], including as many as 482 atomic orbitals for the CH3OHF− system. The ion–molecule complex on the proton-transfer surface is a tight, hydrogen-bonded structure of CH3OH⋅F− character, exhibiting a nearly linear -OHF− framework, an elongated O–H distance of 1.07(1) A(ring), and a small interfragment separation, r(H–F)=1.32(1) A(ring). Improved structural data for F−⋅H2O are obtained for calibration purposes. A large fluoride affinity is found for the CH3OHF− adduct, D0=30.4±1 kcal mol−1, and a bonding analysis via the Morokuma decomposition scheme reveals considerable covalent character. The harmonic stretching frequencies within the -OHF− moiety are predicted to be 421 and 2006 cm−1, the latter protonic vibration being downshifted 1857 cm−1 relative to ω1(O–H) of free methanol.A systematic thermochemical analysis of the reactants and products on the CH3OHF− surface yields a proton-transfer energy of 10.6 kcal mol−1, a gas-phase acidity for methanol of 381.7±1 kcal mol−1, and D0(CH3O–H)=104.1±1 kcal mol−1, facilitating the resolution of previous inconsistencies in associated thermochemical cycles. A minimum-energy path in geometric configuration space is mapped out and parametrized on the basis of constrained structural optimizations for fixed values of an aptly chosen reaction variable. The evaluation of numerous energy points along this path establishes the nonexistence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O−⋅HF type. The mathematical considerations for a classical multipole analysis of reaction path asymptotes are outlined for ion–dipole systems and applied to the CH3OHF− surface with due concern for bifurcations in the exit channel for the proton-transfer process. A global analytic surface for vibrational stretching motion in the -OHF− moiety of the CH3OHF− system is constructed, and a suitable dynamical model is tested which involves an effective, triatomic hydrogen pseudobihalide anion, [-OHF]−. Converged variational eigenstates of [-OHF]− to one-half its dissociation limit are determined using vibrational configuration interaction expansions in terms of self-consistent-field modals. The fundamental stretching frequencies of the CH3OHF− complex predicted by the [-OHF]− model are 504 (+84) and 1456 (−549) cm−1, the corresponding anharmonicities appearing in parentheses.

Notes: State-of-the-art ab initio quantum chemical techniques have been employed in the determination of the reaction path and attendant energetics for the singlet dissociation of CH2CO. Variational RRKM calculations implementing these results provide first principles predictions for the dissociation kinetics which are in quantitative agreement with the corresponding experimental data.