Gaussian‐2 theory for molecular energies of first‐ and second‐row compounds

A systematic way for the cost reduction of density fitting methods

We present a simple approach for the reduction of the size of auxiliary basis sets used in methods exploiting the density fitting (resolution of identity) approximation for electron repulsion integrals. Starting out of the singular value decomposition of three-center two-electron integrals, new auxiliary functions are constructed as linear combinations of the original fitting functions. The new functions, which we term natural auxiliary functions (NAFs), are analogous to the natural orbitals widely used for the cost reduction of correlation methods. The use of the NAF basis enables the systematic truncation of the fitting basis, and thereby potentially the reduction of the computational expenses of the methods, though the scaling with the system size is not altered. The performance of the new approach has been tested for several quantum chemical methods. It is demonstrated that the most pronounced gain in computational efficiency can be expected for iterative models which scale quadratically with the size of the fitting basis set, such as the direct random phase approximation. The approach also has the promise of accelerating local correlation methods, for which the processing of three-center Coulomb integrals is a bottleneck.

Fast, accurate evaluation of exact exchange: The occ-RI-K algorithm

Construction of the exact exchange matrix, K, is typically the rate-determining step in hybrid density functional theory, and therefore, new approaches with increased efficiency are highly desirable. We present a framework with potential for greatly improved efficiency by computing a compressed exchange matrix that yields the exact exchange energy, gradient, and direct inversion of the iterative subspace (DIIS) error vector. The compressed exchange matrix is constructed with one index in the compact molecular orbital basis and the other index in the full atomic orbital basis. To illustrate the advantages, we present a practical algorithm that uses this framework in conjunction with the resolution of the identity (RI) approximation. We demonstrate that convergence using this method, referred to hereafter as occupied orbital RI-K (occ-RI-K), in combination with the DIIS algorithm is well-behaved, that the accuracy of computed energetics is excellent (identical to conventional RI-K), and that significant speedups can be obtained over existing integral-direct and RI-K methods. For a 4400 basis function C68H22 hydrogen-terminated graphene fragment, our algorithm yields a 14× speedup over the conventional algorithm and a speedup of 3.3× over RI-K.

Curious cases of 3,6-dinitropyrazolo[4,3-c]pyrazole-based energetic cocrystals with high nitrogen content: an alternative to salt formation

Two structurally interesting 3,6-dinitropyrazolo[4,3-c]pyrazole-based energetic cocrystals were prepared and confirmed by single crystal X-ray diffraction. As novel energetic materials, these two unusual neutral acid-base complexes possess high nitrogen content, good detonation properties, and improved impact and friction sensitivities which highlight their potential energetic applications.

Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: A Litmus Test for Cost-Effective Molecular Formation Enthalpies

The theoretical atomization energies of some 45 CxHyOz molecules present in the Active Thermochemical Tables compilation and of particular interest to the combustion chemistry community have been computed using five composite model chemistries as titled. The species contain between 1-8 "heavy" atoms, and a few are conformationally diverse with up to nine conformers. The enthalpies of formation at 0 and 298.15 K are then derived via the atomization method and compared against the recommended values. In general, there is very good agreement between our averaged computed values and those in the ATcT; those for 1,3-cyclopentadiene exceptionally differ considerably, and we show from isodesmic reactions that the true value for 1,3-cyclopentadiene is closer to 134 kJ mol(-1) than the reported 101 kJ mol(-1). If one is restricted to using a single method, statistical measures indicate that the best methods are in the rank order G3 ≈ G4 > W1BD > CBS-APNO > CBS-QB3. The CBS-x methods do on average predict ΔfH(⊖)(298.15 K) within ≈5 kJ mol(-1) but are prone to occasional lapses. There are statistical advantages to be gained from using a number of methods in tandem, and all possible combinations have been tested. We find that the average formation enthalpy coming from using CBS-APNO/G4, CBS-APNO/G3, and G3/G4 show lower mean signed and mean unsigned errors, and lower standard and root-mean-squared deviations, than any of these methods in isolation. Combining these methods also leads to the added benefit of providing an uncertainty rooted in the chemical species under investigation. In general, CBS-APNO and W1BD tend to underestimate the formation enthalpies of target species, whereas CBS-QB3, G3, and G4 have a tendency to overestimate the same. Thus, combining CBS-APNO with a G3/G4 combination leads to an improvement in all statistical measures of accuracy and precision, predicting the ATcT values to within 0.14 ± 4.21 kJ mol(-1), thus rivalling "chemical accuracy" (±4.184 kJ mol(-1)) without the excessive cost associated with higher-level methods such as W1BD.

Gas-phase lithium cation affinity of glycine

The gas-phase lithium cation binding thermochemistry of glycine has been determined theoretically by quantum chemical calculations at the G4 level and experimentally by the extended kinetic method using electrospray ionization quadrupole time-of-flight tandem mass spectrometry. The lithium cation affinity of glycine, ∆(Li)H°(298)(GLY), i.e. the∆(Li)H°(298) of the reaction GlyLi(+)→ Gly + Li(+)) given by the G4 method is equal to 241.4 kJ.mol(-1) if only the most stable conformer of glycine is considered or to 242.3 kJ.mol(-1) if the 298K equilibrium mixture of neutral conformers is included in the calculation. The ∆(Li)H°(298)(GLY) deduced from the extended kinetic method is obviously dependent on the choice of the Li(+) affinity scale, thus∆(Li)H°(298)(GLY) is equal to 228.7±0.9(2.0) kJ.mol(- 1) if anchored to the recently re-evaluated lithium cation affinity scale but shifted to 235.4±1.0 kJ.mol(-1) if G4 computed lithium cation affinities of the reference molecules is used. This difference of 6.3 kJ.mol(-1) may originate from a compression of the experimental lithium affinity scale in the high ∆(Li)H°(298) region. The entropy change associated with the reaction GlyLi(+)→Gly + Li(+) reveals a gain of approximately 15 J.mol(-) 1.K(-1) with respect to monodentate Li(+) acceptors. The origin of this excess entropy is attributed to the bidentate interaction between the Li(+) cation and both the carbonyl oxygen and the nitrogen atoms of glycine. The computed G4 Gibbs free energy,∆(Li)G°(298)(GLY) is equal to 205.3 kJ.mol(-1), a similar result, 201.0±3.4 kJ.mol(-1), is obtained from the experiment if the∆(Li)G°(298) of the reference molecules is anchored on the G4 results.

Aluminum monocation basicity and affinity scales

The experimental aspects of the determination of thermochemical data for the attachment of the aluminum monocation Al(+) to neutral atoms and molecules are reviewed. Literature aluminum cation affinities (enthalpy scale) and basicities (Gibbs energy scale) are tabulated and discussed. Ab initio quantum chemical calculations at the G4 level on 43 adducts provide a consistent picture of the energetics of the adducts and their structures. The Al(+)-ligand bonding is analyzed in terms of natural bond orbital and atom-in molecule analyses. A brief comparison of the Al(+) basicity scales and other gas- phase cation basicities is presented.

Theoretical study on homolytic C(sp2)–O cleavage in ethers and phenols

The C(sp²)–O BDEs and the substituent effect of ethers/phenols were investigated in detail by the wB97 method.

Distortion–interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes

The reactivity of uncatalyzed Alder-ene type reactions of hetero-substituted propylene is interpreted by distortion–interaction analysis of both the transition states and the complete reaction pathways.

An experimental and theoretical study of the gas phase kinetics of atomic chlorine reactions with CH3NH2, (CH3)2NH, and (CH3)3N

The first kinetic data for the gas phase reactions of amines with chlorine atoms.

Atmospheric reactivity of HCCCH2OH (2-propyn-1-ol) toward OH radicals: experimental determination and theoretical comparison with its alkyne analogue

The rate coefficient for the reaction of propargyl alcohol (2-propyn-1-ol,2P1OL) with OH radicals has been determined using gas chromatography with a flame ionization detector (GC/FID) at 298 K and atmospheric pressure.

Exceptional proton affinities of push–pull nitriles substituted by the guanidino and phosphazeno groups

The G2(MP2)-calculated PA(N-cyano) for (H₂N)₂CN–CN and (H₂N)₃PN–CN are larger than that of HCN by 186 and 250 kJ mol⁻¹, respectively.

Hydroxyalkoxy radicals: importance of intramolecular hydrogen bonding on chain branching reactions in the combustion and atmospheric decomposition of hydrocarbons

During both the atmospheric oxidation and combustion of volatile organic compounds, sequential addition of oxygen can lead to compounds that contain multiple hydrogen-bonding sites. The presence of two or more of these sites on a hydrocarbon introduces the possibility of intramolecular H-bonding, which can have a stabilizing effect on the reactants, products, and transition states of subsequent reactions. The present work compares the absolute energies of two sets of conformations, those that contain intramolecular H-bonds and those that lack intramolecular H-bonds, for each reactant, product, and transition state species in the 1,2 through 1,7 H-migrations and Cα-Cβ, Cα-H, and Cα-OH-bond scission reactions in the n-hydroxyeth-1-oxy through n-hydroxyhex-1-oxy radicals, for n ranging from 1 to 6. The difference in energy between the two conformations represents the balance between the stabilizing effects of H-bonds and the steric cost of bringing the two H-bonding sites together. The effect of intramolecular H-bonding and the OH group is assessed by comparing the net intramolecular H-bond stabilization energies, the reaction enthalpies, and barrier heights of the n-hydroxyalkoxy radical reactions with the corresponding alkoxy radicals values. The results suggest that there is a complex dependence on the location of the two H-bonding groups, the location of the abstraction or bond scission, and the shape of the transition state that dictates the extent to which intramolecular H-bonding effects the relative importance of H-migration and bond scission reactions for each n-hydroxyalkoxy radical. These findings have important implications for future studies on hydrocarbons with multiple H-bonding sites.

A theoretical investigation of the S2+ cation in the gas phase

The potential energy curves (PECs) of 18 lowest energy Λ-S states of the S2+ cation have been calculated using the complete active space self-consistent field method followed by the internally contracted multireference configuration interaction approach with the Davidson modification. When spin−orbit coupling is computed using the Breit−Pauli Hamiltonian with the cc-pCVTZ basis set, the 18 Λ-S states split into 56 Ω components. To improve the quality of the PECs, core−valence correlation and scalar relativistic corrections are included. All PECs are extrapolated to the complete basis set limit. Based on the PECs obtained by the icMRCI+CV+DK+Q/(CBS-56) calculations, the spectroscopic parameters of 17 Λ-S bound states and 50 Ω bound states are obtained. The transitions (including the Franck−Condon factors and radiative lifetimes) from two low-lying states to the ground state are calculated for several low vibrational levels. Analyses show that the spectroscopic parameters reported here can be expected to be reliably predicted.