Thermochemical properties of polycyclic aromatic hydrocarbons (PAH) from G3MP2B3 calculations

In this article, we present a new database of thermodynamic properties for polycyclic aromatic hydrocarbons (PAH). These large aromatic species are formed in very rich premixed flames and in diffusion flames as part of the gas-phase chemistry. PAH are commonly assumed to be the intermediates leading to soot formation. Therefore, accurate prediction of their thermodynamic properties is required for modeling soot formation. The present database consists of 46 species ranging from benzene (C6H6) to coronene (C24H12) and includes all the species usually present in chemical mechanisms for soot formation. Geometric molecular structures are optimized at the B3LYP/6-31++G(d,p) level of theory. Heat capacity, entropy, and energy content are calculated from these optimized structures. Corrections for hindered rotor are applied on the basis of torsional potentials obtained from second-order M?ller-Plesset perturbation (MP2) and Dunning's consistent basis sets (cc-pVDZ). Enthalpies of formation are calculated using the mixed G3MP2//B3 method. Finally, a group correction is applied to account for systematic errors in the G3MP2//B3 computations. The thermodynamic properties for all species are available in NASA polynomial form at the following address: http://www.stanford.edu/group/pitsch/.     

Substituent effects on the thermochemistry of thiophenes. a theoretical (G3(MP2)//B3LYP and G3) study

Very good linear correlations between experimental and calculated enthalpies of formation in the gas phase (G3(MP2)//B3LYP and G3) for 48 thiophene derivatives have been obtained. These correlations permit a correction of the calculated enthalpies of formation in order to estimate more reliable "experimental" values for the enthalpies of formation of substituted thiophenes, check the reliability of experimental measurements, and also predict the enthalpies of formation of new thiophenes that are not available in the literature. Moreover, the difference between the enthalpies of formation of isomeric thiophenes with the same substituent in positions 2 and 3 of the ring has been analyzed. Likewise, a comparison of the substituent effect in the thiophene and benzene rings has been established.     

The performance of MP2.5 and MP2.X methods for nonequilibrium geometries of molecular complexes

Here we test the performance of the newly developed MP2.5 and MP2.X methods in terms of their abilities to generate accurate binding energies for noncovalently bound complexes at points away from their minimum energy structures and in terms of the accuracy of their potential energy minima. The MP2.X method is a scaled version of MP2.5 that allows for the use of smaller basis sets for the most computationally demanding (MP3) term, significantly reducing its computational cost. MP2.5 and MP2.X binding energy errors are compared to those of the reference CCSD(T)/CBS method on the dissociation curves associated with the S66 dataset of noncovalent complexes (S66x8). It is found that both the MP2.5 and MP2.X methods produce binding energy errors, as well as potential energy minima, that are significantly more accurate than those of MP2 methods. Thus, these methods are appropriate choices when very high quality geometries of noncovalent complexes are required.     

Symmetric‐group‐based methods in quantum chemistry

The eigenvalue problem of a Hamiltonian represented in a finite-dimensional model space being the N-electron subspace of the 2K-spinorbital Fock space is analyzed. It is pointed out that the permutation group S _N is a very convenient framework for this analysis. The resulting approach is known as the symmetric group approach to the N-electron problem. Its applications to construction of a basis in the model space, to the evaluation of matrix elements of spin‐independent and of spin‐dependent operators and, finally, to solution of the eigenvalue problem of the Hamiltonian are briefly reviewed. Recently developed applications of the symmetric group to studies of the Heisenberg Hamiltonian spectra and to evaluation of spectral density distribution moments are also dicussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared and Raman spectra and quantum chemistry calculations for 2,2,2-trifluoroethyl trichloromethanesulfonate, CCl3SO2OCH2CF3

The infrared spectra of CCl3SO2OCH2CF3 were obtained in the gaseous, liquid and solid states and complemented with the Raman spectrum of the liquid. Quantum chemistry calculations using the density functional theory (DFT) were used to predict the most stable geometry and conformation of the studied molecule. The harmonic vibrational frequencies and force field were also calculated. Comparison with related molecules and with the predicted frequencies was used as the basis for the assignment of the observed spectral features. Subsequently, a scaling of the original force field by means of a least square procedure was made in order to reproduce as well as possible the experimental frequencies, leading to a final root mean square deviation of 10.6 cm(-1).     

G3(MP2) ring strain in bicyclic phosphorus heterocycles and their hydrocarbon analogues

[Figure: see text] The ring strain present in 2-aza-1-phosphabicyclo[n.1.0]alka(e)nes (n = 1-5) is calculated at the G3(MP2) level using homodesmotic reactions. The influence of cyclopropa(e)nation and heteroatom substitution is analyzed by a comparison with the corresponding bicyclic hydrocarbons and separate ring systems. It is shown that the strain caused by fusion with a cyclopropane is the sum of the separate rings, whereas the strain resulting from fusion with cyclopropene leads to strain energies much larger than the sum of rings, as a result of the inverted nature of the bridgehead carbon. In all ring structures but one, cyclohexane, substitution by nitrogen and phosphorus is favorable and the effect is most pronounced for the most condensed structures. The calculated strain energies correlate very well with the experimental stability and reactivity of the bicyclic iron-amino phosphirane and phosphirene complexes.     

Reassessment of methyl rotation barriers and conformations by correlated quantum chemistry methods

Internal rotations of the methyl group in ortho‐substituted and 2,6‐disubstituted toluenes in their ground state have been investigated by means of various ab initio quantum chemistry methods. Computed barriers at the Hartree‐Fock (HF) level using medium sized basis sets agreed reasonably with experimental results in the case of the studied ortho‐substituted toluenes. However, this agreement worsens when using very large basis sets. Furthermore, the determination of the conformation and barriers of more weakly hindered methyl groups, that is, for 2,6‐dihalogenotoluenes or toluene itself, necessitates high level correlated computations, because of a possible failure of HF calculations in this case. Density functional theory (DFT) techniques required, in several cases, much more extended basis sets than the post‐HF Møller‐Plesset perturbation (MP2, MP4) ones, to insure the convergence of the computed barriers. Non‐negligible variations of the computed barriers when using different DFT functionals are observed for some systems. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 2093–2100, 2003     

Emergence and quantum chemistry

This paper first queries what type of concept of emergence, if any, could be connected with the different chemical activities subsumed under the label ‘quantum chemistry’. In line with Roald Hoffmann, we propose a ‘rotation to research laboratory’ in order to point out how practitioners hold a molecular whole, its parts, and the surroundings together within their various methods when exploring chemical transformation. We then identify some requisite contents that a concept of emergence must incorporate in order to be coherent from the standpoint of the scientific practices involved. In this respect, we finally propose a relational form of emergence which pays attention to the constitutive role of the modes of intervention and to the co-definition of the levels of organization. No metaphysical distinction between the higher and basic levels of organization is supposed, but only a plurality of modes of access. Moreover, these modes of access are not construed as mere ways of revealing intrinsic patterns of organization but, on the contrary, are considered to be active elements on which the constitution of those patterns depends. What is at stake in this paper is therefore not an ontological form of emergence but an agnostic one which fits what chemists do in their daily work.     

Advances in methods and algorithms in a modern quantum chemistry program package

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.     

Conformational analysis of 2-acetylcyclopentanone by the density functional and MP2 methods

Two-dimensional potential energy surfaces for internal rotation of the acetyl substituent and inversion of the five-membered ring in the gas phase and in acetonitrile were constructed by the B3LYP/6-31G(d) method for the diketo form of the 2-acetylcyclopentanone (2-ACPN) molecule. The diketo form of 2-ACPN exists in the gas phase and in low-polarity cyclohexane as a mixture of four rotamers K1, K2, K3, and K4. The fifth isomeric form K3a exists in polar acetonitrile. The estimation of the tautomeric composition of 2-ACPN taking into account the influence of the cyclohexane medium in the framework of the polarizable continuum model (PCM) using the MP 2/cc-pVTZ method agrees better with experimental data than the estimate obtained by the B3LYP/cc-pVTZ method. The constants of the enol-enol and keto-enol tautomeric equilibria of the 2-ACPN tautomers in solutions of aprotic solvents were estimated.     

Numerical integration techniques for quantum chemistry

Korobov theory for multidimensional numerical integration is used to evaluate electronic integrals. This paper shows the important role played by periodization techniques. Singularity (r ₁₂ ^?1 ) in the bielectronic six-dimensional integrals is removed through a twofold three-dimensional integration. Results are presented for atomic integrals involving Slater type atomic orbitals.     

G3(MP2) enthalpies of hydrogenation, isomerization, and formation of extended linear polyacetylenes

Motivated by our recent finding that, in contrast to their olefinic counterparts, linear alternant polyacetylenes (polyynes) show no appreciable thermodynamic evidence of conjugation stabilization, we have extended our G3(MP2) calculations of standard enthalpies of hydrogenation, delta(hyd), formation, delta(f), and isomerization, delta(isom), as far as isomeric dodecadiynes. We show that thermochemical stabilization of conjugated polyalkynes is about 1 kcal mol(-1) over most of this range, and that the progression from one polyalkyne to the next is regular and additive. The longest chain polyalkynes, however, begin to revert to classical conjugation stabilization energies. For example, 5,7-dodecadiyne has a thermochemical stabilization enthalpy of 3.1 kcal mol(-1), approaching that of 1,3-butadiene. We also point out some of the difficulties encountered when one departs from Kistiakowsky's operational definition of conjugation stabilization. A cautionary example is drawn from the recent literature in which arguments of hyperconjugation and "virtual states" are used to arrive at, among other things, a value of 8.5 kcal mol(-1) of conjugative stabilization in 1,3-butadiene.