Ab initio vs molecular mechanics thermochemistry: homocubanes

The standard enthalpies of formation and strain energies for a series of homocubanes have been investigated by high-level ab initio G3(MP2)/B3LYP method. The relative stabilities of isomers are discussed. The comparison is made between the results of ab initio and molecular mechanics methods with the aim of assessing their performances. The usefulness of high-level calculations for generating thermochemical databases of relatively large molecules (e.g. C(11)H(14)) was also demonstrated.     

Ab initio spectroscopy and thermochemistry of the BN molecule

The lowest¹Σ⁺ and³Π states of the BN molecule have been studied using the quadratic configuration interaction method and (spdf) basis sets. The lowest¹Σ⁺ and³Π states lie extremely closely (T _e≈100 cm^?1) together; it is not clear which is the ground state. The very small separation should form a useful benchmark for basis sets and electron correlation methods. The dissociation energyD ₀ is computed to be 103.9±2 kcal/mol. A self-consistent set of spectroscopic constants is derived from a combination of ab initio and experimental data. JANAF-style thermodynamic functions in the range 100–6000 K, including anharmonic, rovibrational coupling, centrifugal stretching, and spin-orbit coupling effects are computed using direct numerical summation over the 25 lowest electronic states. A modified procedure for the latter is outlined that reduces computer time by one or two orders of magnitude without compromise in accuracy.     

Ab initio thermochemistry and kinetics for carbon-centered radical addition and beta-scission reactions

A quantitative comparison of ab initio calculated rate coefficients using five computational methods and five different approaches of treating hindered internal rotation and tunneling with experimental values of rate coefficients for nine carbon-centered radical additions/beta scissions at 300, 600, and 1000 K is performed. The high-accuracy compound methods, CBS-QB3 and G3B3, and the density functionals, MPW1PW91, BB1K, and BMK, have been evaluated using the following approaches: (i) the harmonic oscillator approximation; (ii) the hindered internal rotor approximation for the internal rotation about the forming/breaking bond in the transition state and product; and the hindered internal rotation approximation combined with (iii) Wigner, (iv) Skodje and Truhlar, and (v) Eckart zero-curvature tunneling corrections. The density functional theory (DFT) based values for beta-scission rate coefficients deviate significantly from the experimental ones at 300 K, and the DFT methods do not accurately predict the equilibrium coefficient. The hindered rotor approximation offers a significant improvement in the agreement with experimental rate coefficients as compared to the harmonic oscillator treatment, especially at higher temperatures. Tunneling correction factors are smaller than 1.40 at 300 K and 1.03 at 1000 K. For both the CBS-QB3 method, including the hindered rotor treatment but excluding tunneling corrections, and the G3B3 method, including hindered rotor and Eckart tunneling corrections, a mean factor of deviation with experimentally observed values of 3 is found.     

Halon thermochemistry: Ab initio calculations of the enthalpies of formation of fluoroethanes

The ab initio G2, G2(MP2), CBS-4 and CBS-Q quantum mechanical protocols and the parameterized BAC-MP4 procedure were used to calculate the enthalpies of formation (Δ_fH⁰) of ethane and the complete series of fluoroethanes, C₂H_xF_6−x, x = 0−5. Results from all methods exhibited significant negative deviations from experiment. With the exception of the CBS-4 and BAC-MP4 procedures, the negative errors in the calculated enthalpies were observed to be linearly dependent upon the number of CF bonds in the molecule. Application of a bond additivity correction (BAC) parameter, Δ_CF, derived in an earlier investigation of fluoro- and chlorofluoromethanes, removed some although not all of the systematic deviations. Introduction of a heavy atom interaction parameter, representing the effect of an attached carbon on the CF bond error, yielded corrected enthalpies which agree with experiment to within the reported uncertainties. The BAC-MP4 method, which has already been parameterized with generalized BACs, yields calculated enthalpies which average approximately 10 kJ mol⁻¹ below the experimental values of Δ_fH⁰ in the fluoroethanes.     

Ab initio aqueous thermochemistry: application to the oxidation of hydroxylamine in nitric acid solution

Ab initio molecular orbital calculations were performed and thermochemical parameters estimated for 46 species involved in the oxidation of hydroxylamine in aqueous nitric acid solution. Solution-phase properties were estimated using the several levels of theory in Gaussian03 and using COSMOtherm. The use of computational chemistry calculations for the estimation of physical properties and constants in solution is addressed. The connection between the pseudochemical potential of Ben-Naim and the traditional standard state-based thermochemistry is shown, and the connection of these ideas to computational chemistry results is established. This theoretical framework provides a basis for the practical use of the solution-phase computational chemistry estimates for real systems, without the implicit assumptions that often hide the nuances of solution-phase thermochemistry. The effect of nonidealities and a method to account for them is also discussed. A method is presented for estimating the solvation enthalpy and entropy for dilute aqueous solutions based on the solvation free energy from the ab initio calculations. The accuracy of the estimated thermochemical parameters was determined through comparison with (i) enthalpies of formation in the gas phase and in solution, (ii) Henry's law data for aqueous solutions, and (iii) various reaction equilibria in aqueous solution. Typical mean absolute deviations (MAD) for the solvation free energy in room-temperature water appear to be ~1.5 kcal/mol for most methods investigated. The MAD for computed enthalpies of formation in solution was 1.5-3 kcal/mol, depending on the methodology employed and the type of species (ion, radical, closed-shell) being computed. This work provides a relatively simple and unambiguous approach that can be used to estimate the thermochemical parameters needed to build detailed ab initio kinetic models of systems in aqueous solution. Technical challenges that limit the accuracy of the estimates are highlighted.     

Ab initio and density functional study of substituent effects in halogenated cations of alkenes

A theoretical study of the halogenated cations of mono-, di-, tri- and tetramethyl-substituted ethylenes, C₃H₆X⁺, C₄H₈X⁺, C₅H₁₀X⁺ and C₆H₁₂X⁺, X=F, Cl, Br, have been studied at the ab initio MP2 and density functional B3LYP levels of theory implementing 6-311++G(d,p) basis set. The potential energy surfaces of all molecules under investigation have been scanned and the ¹³C and ¹H NMR chemical shifts for all the bridged halonium ions studied have been calculated using the GIAO method at the B3LYP level. The calculated halogen binding energies in the halonium ions have been correlated with the experimental rates of chlorination and bromination of the corresponding alkenes. The computed hydride affinities and the NICS values for the bridged cations show that the bromo cations are more stable than the analogous chloro and fluoro cations.     

Ab initio study of the tautomeric forms of some quinolinediones

7,8-dihydroquinoline-4,5 (1H,6H)-dione (1) and 7,8-dihydroquinoline-2,5-(1H,6H)-dione (2) in their tautomeric oxo and hydroxy forms have been studied by ab initio Hartree-Fock calculations; tautomerization energies predict a more stable hydroxy structure having an intramolecular hydrogen bond for compound 1, whereas the oxo form is slightly-preferred for compound 2. Fourier Transform-Infra Red (FT-IR) spectra in CHCl₃ solution indicate that the predicted most stable tautomers in the vapour phase remain as such.     

Ab initio studies of two-photon absorption of some stilbenoid chromophores

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations.     

Ab initio calculation of some low-lying electronic excited states of methane

The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions.     

Ab initio study of some excited states of A1H

The propagator approach yields excitation energies (polarization propagator) and ionization potentials (electron propagator) without the computation of separate ground state and final state wavefunctions and is well suited for studying dynamical properties. These methods are applied to AlH molecule: excitation energies, ionization potential, optical and generalized oscillator strengths.Present adress: Laboratoire de Methodes Spectroscopiques, Université de Provence, Centre de Saint Jerôme, 13397 Marseille Cédex 4, France     

Ab initio calculation of the band structure of some boron-nitrogen polymers

Ab initio calculations using a Gaussian orbital basis set were performed on the two boron-nitrogen polymer systems polyaminoborane and polyboronimide. For the polyaminoborane system an alternating B-N bond model appears to be more stable than a symmetric B-N bond model. An electron drift from the NH₂ group to the BH₂ moiety was calculated for both models although the nitrogen atom was found to possess a negative charge stemming from polarization of the N-H bonds. The energy band diagrams derived from both models show rather featureless bands indicative of weakly interactive systems although that of polyboronimide indicates that it is a more delocalized system than its saturated counterpart. The conduction and valence bands at the X-point are composed of orbitals and the lowest electronic transition is predicted to be —* in nature. The electron distribution of polyboronimide indicates a movement of -electrons from the boron to the nitrogen coupled with a smaller -electron drift from the nitrogen to the boron.     

Ab initio calculation of the band structure of some boron polymers

Ab initio calculations using a STO-3G basis set have been performed on the polymer systems (HBX) _n where X = Be, BH, CH₂, NH, and O. Energy band diagrams and accompanying density-of-states plots have been obtained. The highest filled orbital of (HBNH) _n and (HBO) _n occurs at the X-point and possesses character while the - framework orbital at the X- point is the highest filled level for (HBBe) _n , (HBBH) _n , and (HBCH₂) _n . The conduction band for all five species has symmetry and the band gap of the (HBX) _n species increases in the order X = Be < BH < NH < O < CH₂. An estimate of the energy of polymerisation of the (HBX) _n systems suggests that HBNH is particularly stabilised by polymerisation. The electron distribution in (HBBe) _n shows a - electron drift towards boron, while in the other four systems the net electron transfer is directed away from boron. There is significant electron back-donation to boron in (HBO) _n and (HBNH) _n .     

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.     

High-accuracy extrapolated ab initio thermochemistry of vinyl chloride

Applying a modified "high accuracy extrapolated ab initio thermochemistry" (HEAT) scheme, the standard heat of formation of vinyl chloride at 0 K is computed to be 29.79 +/- 1 kJ/mol and at 298.15 K to be 20.9 +/- 2 kJ/mol, thus resolving earlier discrepancies among the available experimental values, which span a range from 21 up to 38 kJ/mol. The enthalpies of the reactions C2H4 + Cl2 --> CH2CHCl + HCl and C2H2 + HCl --> CH2CHCl at 298.15 K are determined to be -123.0 and -113.9 +/- 2 kJ/mol, respectively.     

Ab initio Ehrenfest dynamics

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.     

Ab initio evaluation of bond orders and valence numbers in some substituted benzenes

Mayer's definition of bond order and valence number of an atom in a molecule in the ab initio SCF theory have been applied to calculate bond orders and valence numbers using different ab initio methods for some fluorobenzenes. The results have been discussed in the light of Mulliken population analysis. The traditional view that the valency of an atom is directly related to the atomic charge is found to be invalid in the present ab initio calculations.     

Ab initio investigation of phloroglucinol

All-electron ab initio Hartree–Fock (RHF ) calculations have been carried out to investigate the keto/enol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5-benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-1-on, in the phenol system. © 1993 John Wiley & Sons, Inc.     

HEAT: High accuracy extrapolated ab initio thermochemistry

A theoretical model chemistry designed to achieve high accuracy for enthalpies of formation of atoms and small molecules is described. This approach is entirely independent of experimental data and contains no empirical scaling factors, and includes a treatment of electron correlation up to the full coupled-cluster singles, doubles, triples and quadruples approach. Energies are further augmented by anharmonic zero-point vibrational energies, a scalar relativistic correction, first-order spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The accuracy of the approach is assessed by several means. Enthalpies of formation (at 0 K) calculated for a test suite of 31 atoms and molecules via direct calculation of the corresponding elemental formation reactions are within 1 kJ mol(-1) to experiment in all cases. Given the quite different bonding environments in the product and reactant sides of these reactions, the results strongly indicate that even greater accuracy may be expected in reactions that preserve (either exactly or approximately) the number and types of chemical bonds.