The effect of stoichiometry on ab initio-based thermochemistry predictions

The effect of various reaction schemes used to convert total reaction enthalpies obtained via ab initio methods into enthalpies of formation has been studied employing a new optimization technique. The reaction schemes, conventional, isostoichiometric, and isodesmic have been defined and generated for 47 species for which highly accurate experimental enthalpies of formation in addition to total reaction enthalpies at 298 K at G2 and G3 levels of theory are known. The main finding is that the effect of reaction schemes in ab initio-based thermochemistry predictions is small, especially, for enthalpy predictions involving high-level ab initio methods.     

Dipole moment and rovibrational intensities in the electronic ground state of NH3: bridging the gap between ab initio theory and spectroscopic experiment

We report theoretical values for the transition moments of an extensive set of vibrational bands in the electronic ground state of (14)NH(3). For selected bands, we have further made detailed simulations of the rotational structure. The calculations are carried out by means of recently developed computational procedures for describing the nuclear motion and are based on a high-level ab initio potential energy surface, and high-level dipole moment surfaces, for the electronic ground state of NH(3). The reported theoretical intensity values are compared to, and found to agree very well with, corresponding experimental results. It is believed that the computational method, in conjunction with high-quality ab initio potential energy and dipole moment surfaces, can simulate rotation-vibration spectra of XY(3) pyramidal molecules prior to observation with sufficient accuracy to facilitate the observation of these spectra. By degrading the accuracy of selected elements of the calculations, we have also investigated the influence of customary approximations on the computed intensity values.     

CHF3…H2O complex revisited: a matrix isolation and ab initio study

Structural Chemistry - The structural and spectroscopic features of the CHF3…H2O complex have been investigated using high-level ab initio calculations and IR matrix isolation spectroscopy....     

Accuracy of recent potential energy surfaces for the He-N2 interaction. I. Virial and bulk transport coefficients

A new exchange-Coulomb semiempirical model potential energy surface for the He-N2 interaction has been developed. Together with two recent high-level ab initio potential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and N2. The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initio surface of Patel et al. [J. Chem. Phys. 119, 909 (2003)], but with slightly poorer agreement for the earlier ab initio surface of Hu and Thakkar [J. Chem. Phys. 104, 2541 (1996)].     

Ab initio thermochemistry of some halogenated cyclopropanes

The standard enthalpies of formation for a series of chloro- and fluoro-substituted cyclopropanes have been calculated by using high-level ab initio G3/B3LYP methods. The relative stabilities of isomers and the influence of substituents on thermochemistry in several classes of substituted derivatives are discussed.     

A systematic ab initio study of the equilibrium geometry and vibrational wave numbers of bismuthine

The equilibrium structure and the harmonic and anharmonic force fields of BiH(3) are determined by high-level ab initio calculations using a variety of correlation treatments, basis sets, and pseudopotentials, partly in combination with core polarization potentials. Spin-orbit effects are included by a configuration interaction treatment. This systematic study serves to establish a reliable computational protocol for such calculations and, in particular, to minimize basis set superposition errors through an improved new basis set and/or counterpoise corrections. Using the recommended procedures, the best ab initio results for the equilibrium geometry and the fundamental vibrational wave numbers are in good agreement with the available experimental data, which further supports the recent spectroscopic identification of BiH(3). The ground-state total atomization energy of BiH(3) is predicted to be 153.1 kcal/mol.     

Microwave spectra and molecular structures of (Z)-pent-2-en-4-ynenitrile and maleonitrile.

Accurate equilibrium structures have been determined for (Z)-pent-2-en-4-ynenitrile (8) and maleonitrile (9) by combining microwave spectroscopy data and ab initio quantum chemistry calculations. The microwave spectra of 10 isotopomers of 8 and 5 isotopomers of 9 were obtained using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants were adjusted for vibration-rotation interaction effects calculated from force fields obtained from ab initio calculations. The resultant equilibrium rotational constants were used to determine structures that are in very good agreement with those obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). The geometric parameters in 8 and 9 are very similar; they also do not differ significantly from the all-carbon analogue, (Z)-hex-3-ene-1,5-diyne (7), the parent molecule for the Bergman cyclization. A small deviation from linearity about the alkyne and cyano linkages is observed for 7-9 and several related species where accurate equilibrium parameters are available. The data on 7-9 should be of interest to radioastronomy and may provide insights on the formation and interstellar chemistry of unsaturated species such as the cyanopolyynes.     

Modeling mechanisms of unusual benzene imine n6 adduct formation in carcinogenic reactions of arylnitrenium ions with adenosine

Reaction mechanisms of the unusual benzene imine N6 adduct formation in carcinogenic reactions of arylnitrenium ions with adenosine have been investigated with density functional theory (DFT) and high-level ab initio methods. The DFT calculations indicate that the reaction pathways initiated by attack of adenine at the ortho C site of 4-biphenylylnitrenium ion are favored. However, high-level MP2 and QCISD calculations provide a contrary conclusion, that is the reaction pathways initiated by attack of adenine at the para C site of 4-biphenylylnitrenium ion are more feasible. Comparing with experimental results, the conclusion from high-level ab initio calculations is ultimately supported. The present study makes a theoretical prediction on the final products in the studied reaction, which is in agreement with experimental observations. In addition, this study provides some inspirations to the attacks of arylnitrenium ions at amino group of purines and pyrimidines in similar carcinogenic reactions.     

Conformational Analysis of 2,2′-bifuran: Correlated High-level Ab initio and DFT Results

The torsional potential for inter-ring rotation in 2,2′-bifuran has been systematically tackled using highly accurate ab initio calculations as well as cost-effective DFT methods. The successful convergence of the ab initio results allowed to confirm the presence of a shallow gauche minimum in the torsional potential curve. The standard DFT methods failed to capture such a tiny energy barrier but, interestingly, the results could be remarkably improved by a mixture of wavefunction and DFT energies in a multi-coefficient fashion; thus, accurate DFT-based and ab initio reference data also become available. Since the experimental evaluation of torsional potentials faces quantitative problems, the outcome of high-level theoretical calculations is expected to be reliably used in further investigation on structure and conformational distribution of this system.     

Computational study of pharmacophores: beta-sultams

The strain and resonance energies in beta-sultam derivatives have been calculated by using a high-level ab initio method (G3/B3LYP) in order to resolve the question of the principal driving force affecting solvolysis of these new antibiotics. We found that only the combined effect of stabilizing (via amide or sulfonamide resonance interactions) and destabilizing (ring strain) influences can account for the observed rates of solvolysis in beta-lactams and beta-sultams.     

Dynamics of the concerted triple proton transfer in cyclic water trimer using the multiconfiguration molecular mechanics algorithm

Cyclic water clusters are important molecular species to understand the nature of hydrogen bonded networks. Theoretical studies for the dynamics of triple proton transfer in the cyclic water trimer were performed. The potential energy surface (PES) of triple proton transfer is generated by the multiconfiguration molecular mechanics (MCMM) algorithm. We have used the MP2/6-31G(d,p) level for high-level ab initio data (energies, gradients, and Hessians), which are used in the Shepard interpolation. Eight high-level reference points were added step by step, including two points for the critical configurations of the large curvature tunneling paths. The more high-level points are used, the better the potential energy surfaces become. The rate constant and kinetic isotope effect (KIE) for the triple proton transfer at 300 K, which have been calculated by the canonical variational transition-state theory with microcanonical optimized multidimensional semiclassical tunneling approximation, are 1.6 x 10(-3) s(-1) and 230, respectively. Tunneling is very important not only for the triple proton transfer but also for the triple deuterium transfer. The MCMM results show good agreement with those from the direct ab initio dynamics calculations.     

Theory and range of modern semiempirical molecular orbital methods

Semiempirical molecular orbital methods have a long history. They serve to tackle large systems and complicated processes beyond the reach of ab initio or density functional methods. Although their setup is derived from Hartree–Fock theory, the design of approximate energy expressions and the empirical parameters are used to achieve higher accuracy than the underlying ab initio theory. In this way the effect of larger basis sets or correlation can be partially simulated. All widely used semiempirical methods establish their accuracy by error statistics for molecular properties with experimental and high-level ab initio or density functional theory calculations as a reference. Their computational efficiency makes them suitable for the study of biochemical systems and solid materials. The present review presents a variety of applications which demonstrate the need for and success of semiempirical methods.     

Hyperpolarizability of GaAs dimer is not negative

We present a systematic study of the static electric hyperpolarizability of Ga(2)As(2). The authors rely on finite-field high-level ab initio calculations with carefully optimized basis sets. Their best values for the mean and the anisotropy of the dipole polarizability are alpha=158.57 and Deltaalpha=130.33e(2)a(0) (2)E(h) (-1). For the hyperpolarizability we propose an estimate gamma=(155+/-15)x10(3)e(4)a(0) (4)E(h) (-3), which does not agree with the negative value predicted by Lan et al. [J. Chem. Phys. 124, 094302 (2006)]. Density functional theory based methods yield values close to those predicted by conventional ab initio methods. The (hyper)polarizability components are particularly enhanced along the direction defined by the Ga-Ga axis.     

Full-dimensional Time-dependent Quantum Dynamics in Unimolecular Dissociation of HCO

As an important species in atmospheric and combustion chemistry, unimolecular dissociation of HCO has attracted extensive experimental and theoretical interests ever since 1960's^[1-3], which include calculations of potential energy surfaces based on high-level ab initio results^[4].     

Calorimetric and computational study of the thermochemistry of phenoxyphenols

Thermodynamic properties of 3- and 4-phenoxyphenol have been determined by using a combination of calorimetric and effusion techniques as well as by high-level ab initio molecular orbital calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation in the condensed and gas states, Δ(f)H(m)°(cr or l) and Δ(f)H(m)°(g), at T = 298.15 K, of 3- and 4-phenoxyphenol were derived from their energies of combustion in oxygen, measured by a static bomb calorimeter, and from the enthalpies of vaporization or sublimation derived respectively by Calvet microcalorimetry for the 3-phenoxyphenol and by Knudsen effusion technique for the 4-phenoxyphenol. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies, and, finally, O?H bond dissociation enthalpies. The good agreement between the G3MP2B3-derived values and the experimental gas-phase enthalpies of formation for the 3- and 4-phenoxyphenol gives confidence to the estimate concerning the 2-phenoxyphenol isomer, which was not experimentally studied, and to the estimates concerning the radical and the anion. Additionally, the experimental values of gas-phase enthalpies of formation were also compared with estimates based on the empirical scheme developed by Cox.     

Cyclic-N3. I. An accurate potential energy surface for the ground doublet electronic state up to the energy of the 2A2/2B1 conical intersection

A sophisticated adiabatic ground electronic state potential energy surface for a pure nitrogen ring (cyclic-N3) molecule is constructed based on extensive high-level ab initio calculations and accurate three-dimensional spline representation. Most of the important features of the potential energy surface are presented using various reduced dimensionality slices in internal hyperspherical coordinates as well as full dimensional isoenergy surfaces. Very significant geometric phase effects are predicted in the spectra of rotational-vibrational states of cyclic-N3.     

Ring strain in [n]ladderanes

The ring strain energies in a series of [ n]ladderanes (n = 3-8) have been calculated by using high-level ab initio method (G3MP2//B3LYP-6-31G*) and the series of isodesmic, homodesmotic, and protobranching compensated reactions. The results show that various four-member rings incorporated into the ladderane chain have different strain energies and that the total strain in a ladderane molecule is smaller than the corresponding sum of strain energies of the cyclobutane rings.     

Ionic liquids. Combination of combustion calorimetry with high-level quantum chemical calculations for deriving vaporization enthalpies

In this work, the molar enthalpies of formation of the ionic liquids [C2MIM][NO3] and [C4MIM][NO3] were measured by means of combustion calorimetry. The molar enthalpy of fusion of [C2MIM][NO3] was measured using differential scanning calorimetry. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3MP2 theory. We have used a combination of traditional combustion calorimetry with modern high-level ab initio calculations in order to obtain the molar enthalpies of vaporization of a series of the ionic liquids under study.     

Thermochemistry of 2- and 3-acetylthiophenes: calorimetric and computational study

The relative stabilities of 2- and 3-acetylthiophenes have been evaluated by experimental thermochemistry and the results compared to high-level ab initio calculations. The enthalpies of combustion, vaporization, and sublimation were measured by rotating-bomb combustion calorimetry, Calvet microcalorimetry, correlation gas chromatography, and Knudsen effusion techniques and the gas-phase enthalpies of formation, at T = 298.15 K, were determined. Standard ab initio molecular orbital calculations at the G2 and G3 levels were performed, and a theoretical study on the molecular and electronic structures of the compounds studied has been conducted. Calculated enthalpies of formation using atomization and isodesmic reactions are compared with the experimental data. Experimental and theoretical results show that 2-acetylthiophene is thermodynamically more stable than the 3-isomer. A comparison of the substituent effect of the acetyl group in benzene and thiophene rings has been carried out.     

Molecular structures of the two most stable conformers of free glycine

The equilibrium molecular structures of the two lowest-energy conformers of glycine, Gly-Ip and Gly-IIn, have been characterized by high-level ab initio electronic structure computations, including all-electron cc-pVTZ CCSD(T) geometry optimizations and 6-31G* MP2 quartic force fields, the latter to account for anharmonic zero-point vibrational effects to isotopologic rotational constants. Based on experimentally measured vibrationally averaged effective rotational constant sets of several isotopologues and our ab initio data for structural constraints and zero-point vibrational shifts, least-squares structural refinements were performed to determine improved Born-Oppenheimer equilibrium (r(e)) structures of Gly-Ip and Gly-IIn. Without the ab initio constraints even the extensive set of empirical rotational constants available for 5 and 10 isotopologues of Gly-Ip and Gly-IIn, respectively, cannot satisfactorily fix their molecular structure. Excellent agreement between theory and experiment is found for the rotational constants of both conformers, the rms residual of the final fits being 7.8 and 51.6 kHz for Gly-Ip and Gly-IIn, respectively. High-level ab initio computations with focal point extrapolations determine the barrier to planarity separating Gly-IIp and Gly-IIn to be 20.5 +/- 5.0 cm(-1). The equilibrium torsion angle tau(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 +/- 2) degrees. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.