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A thorough DFT investigation of over 800 small fullerenes reveals the ones with promising thermochemical properties for electronics applications.

Metal–ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]−1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report., In this study, the multifaceted electrochemistry of a bis(imino)bipyridine iron(II) complex is described. Spectroscopic studies demonstrate that throughout the various reduction processes, the oxidation state on the metal center remains constant (Fe(II); S = 1), whereas the ligand accesses four distinct oxidation states: [BDI]0 ⇄ [BDI]3−. The observed redox noninnocence of the ligand was investigated by computational methods, which is further discussed in this report.

The S66x8 noncovalent interactions benchmark has been re-evaluated at the "sterling silver" level, using explicitly correlated MP2-F12 near the complete basis set limit, CCSD(F12*)/aug-cc-pVTZ-F12, and a (T) correction from conventional CCSD(T)/sano-V{D,T}Z+ calculations. The revised reference value disagrees by 0.1 kcal/mol RMS with the original Hobza benchmark and its revision by Brauer et al, but by only 0.04 kcal/mol variety from the "bronze" level data in Kesharwani et al., Aust. J. Chem. 71, 238-248 (2018). We then used these to assess the performance of localized-orbital coupled cluster approaches with and without counterpoise corrections, such as PNO-LCCSD(T) as implemented in MOLPRO, DLPNO-CCSD (T1) as implemented in ORCA, and LNO-CCSD(T) as implemented in MRCC, for their respective "Normal", "Tight", and "very Tight" settings. We also considered composite approaches combining different basis sets and cutoffs. Furthermore, in order to isolate basis set convergence from domain truncation error, for the aug-cc-pVTZ basis set we compared PNO, DLPNO, and LNO approaches with canonical CCSD(T). We conclude that LNO-CCSD(T) with veryTight criteria performs very well for "raw" (CP-uncorrected), but struggles to reproduce counterpoise-corrected numbers even for veryVeryTight criteria: this means that accurate results can be obtained using either extrapolation from basis sets large enough to quench basis set superposition error (BSSE) such as aug-cc-pV{Q,5}Z, or using a composite scheme such as Tight{T,Q}+1.11[vvTight(T) - Tight(T)]. In contrast, PNO-LCCSD(T) works best with counterpoise, while performance with and without counterpoise is comparable for DLPNO-CCSD(T1). Among more economical methods, the highest accuracies are seen for dRPA75-D3BJ, {\omega}B97M-V, {\omega}B97M(2), revDSD-PBEP86-D4, and DFT(SAPT) with a TDEXX or ATDEXX kernel., Comment: Final published version with CC license

The computational modeling of fullerenes plays a fundamental role in designing low-dimension carbon nanostructures. Nevertheless, the relative energies of fullerenes larger than C

We have examined the use of systematic bond-separation reactions and purposely constructed chemistry-preserving isodesmic reactions for the thermochemical calculation of aromatic hydrocarbon species. The bond-separation approach yields somewhat disappointing accuracy even when the reaction energies are obtained with generally robust composite and double-hybrid (DH) density functional theory (DFT) methods. In contrast, for the purposely constructed reactions, we find a dramatic improvement in the accuracy for energies calculated with all methods examined. Notably, for medium-sized aromatic hydrocarbons, we find that an effective approach for formulating a well-balanced reaction is to split the target species into two halves with an aromatic overlapping region. Overall, the G4(MP2)-XK, MPW2PLYP, MN15, PBE, and DC-DFTB3 methods are reasonable within their respective classes of methods for the calculation of bond-separation as well as chemistry-preserving isodesmic reactions. We have further computed per-carbon atomization energy (AE) for a series of D6h benzene-type molecules, and thus obtained a formula for extrapolation to the graphene limit [AEn = 711.5 × (1 − 1/n0.640) kJ mol−1, where n = number of carbons]. It suggests that nano-graphene with a length larger than 10 nm would resemble properties of bulk graphene, and conversely, downsizing a nano-graphene beyond this point may lead to considerably altered properties from the bulk.

Two-dimensional nanoporous graphene (NPG) with uniformly distributed nanopores has been synthesized recently and shown remarkable electronic, mechanical, thermal, and optical properties with potent...

Eleven isomers of SiC4H2 lying within 50 kcal mol-1 have been theoretically investigated using density functional theory and high-level coupled-cluster methods. Among them, four isomers, 1-ethynyl-3-silacycloprop-1(2)-en-3-ylidene (1), diethynylsilylidene (2), 1-sila-1,2,3,4-pentatetraenylidene (4), and 1,3-butadiynylsilylidene (5), have already been identified in the laboratory. The current investigation reports three low-lying ( 1 eV) silylidenes [2-sila-(didehydrovinylidene)cyclopropene (8), an isomer with a planar tetracoordinate carbon (ptC) atom (10), and 1-ethynyl-1-silapropadienylidene (11)], which remain elusive in the laboratory to date. Isomer 9 also contains a ptC atom, which turned out to be a transition state at all levels. Though all isomers are polar (μ ≠ 0), rotational spectrum is available only for 4. Using matrix isolation, three isomers (1, 2, and 5) have been trapped in the laboratory at 10 K. Considering the astrochemical relevance of silicon-carbide clusters in the interstellar medium, the current theoretical data demand new molecular spectroscopic studies on SiC4H2. Surprisingly, unlike the isovalent C5H2 isomers, where the bent carbenes are yet to be identified in the laboratory, the bent silylidenes (2 and 5) have been trapped in the case of SiC4H2. In both the cases, molecules with transannular C-C and/or Si-C bonds remain elusive, though they lie in the low-lying region. Using suitable precursors, whether these peculiar geometries (especially 3 and 6) would be identified or not in the laboratory needs to be addressed by molecular spectroscopists. The present investigation documents structural and spectroscopic information of SiC4H2 isomers, which may compliment future molecular spectroscopic observations including radioastronomical searches.

Composite ab initio methods based on local coupled-cluster approaches calculate the CCSD(T)/CBS energy at a significantly reduced computational cost than the corresponding canonical methods. While showing promising performance for general thermochemistry, local composite ab initio methods have not been tested for fullerenes. Here we examine the performance of several local G4(MP2)-based methods for calculating the relative stability of a diverse set of C40 fullerenes. We use canonical G4(MP2) isomerization energies as reference data. Fullerenes provide a challenging problem for DLPNO-G4(MP2)-based methods. The DLPNObased methods result in overall root-mean-square deviations (RMSDs) of 28.6 (NormalPNO with CCSD(T0)), 23.0 (NormalPNO with CCSD(T1)), and 16.1 (TightPNO with CCSD(T0)) kJ mol–1. The local natural orbital LNO-G4(MP2) method provides the best overall performance with an RMSD of 4.9 kJ mol–1. The DLPNO-G4(MP2) and LNO-G4(MP2) methods systematically overestimate the canonical G4(MP2) isomerization energies. Therefore, they provide valuable upper limits of the fullerene isomerization energies., Computational and Theoretical Chemistry, 1217, art. no. 113874; 2022

The ability to accurately calculate reaction barrier heights is of central importance to many areas of chemistry. We report an extensive study examining the basis set convergence of post-CCSD(T) contributions (up to CCSDT(Q)) for a diverse set of 28 reaction barrier heights. In contrast to previous studies, we focus here on larger transition structures (TSs) involving 4-7 non-hydrogen atoms. The set of reaction barrier heights includes pericyclic, bipolar cycloaddition, cycloreversion, and multiple-proton transfer reactions. We find that in most cases post-CCSD(T) contributions converge rapidly toward the basis set limit, such that even double-ζ and truncated double-ζ basis sets provide useful estimates of the T-(T) and (Q) contributions, respectively. In addition, we find that due to the tendency of these small basis sets to systematically underestimate the T-(T) and (Q) components, scaling is an effective approach for improving performance. For example, scaling the T-(T)/cc-pVDZ contribution by 1.25 results in an RMSD of merely 0.4 kJ mol

The equilibrium geometries and spectroscopic properties of two key singlet carbenes, buta-1,3-diynylcarbene (6) and 2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (9), which have not been experimentally observed to date, are investigated using high-level coupled-cluster (CC) methods. The current theoretical study necessitates new experimental data on C5H2 isomers considering the relevance of these molecules to interstellar chemistry. Bent-pentadiynylidene (4) has been missing in the laboratory and the prime focus of our earlier theoretical work. The present theoretical study indicates that isomers 6 and 9 are also viable experimental targets. Apart from ethynylcyclopropenylidene (2), pentatetraenylidene (3), ethynylpropadienylidene (5), and 3-(didehydrovinylidene)cyclopropene (8), which are identified by Fourier transform microwave spectroscopy, the dipole moments of elusive 4, 6, and 9 are also nonzero (μ ≠ 0). The relative energies of these isomers, calculated at the CCSDT(Q)/CBS level of theory, with respect to linear triplet pentadiynylidene (1) reveal that they all lie within 25.1 kcal mol-1. Therefore, geometric, energetic, aromatic, and spectroscopic parameters are reported here, which may assist the efforts of molecular spectroscopists in the future. Anharmonic vibrational calculations on isomers 6 and 9 indicate that the former is loosely bound and would be challenging to be detected experimentally. Among the undetected carbenes, 9 may be considered as a potential target molecule considering its higher polarity and aromatic nature.

The enthalpies of formation and isomerization energies of P4Sn molecular cages are not experimentally (or theoretically) well known. We obtain accurate enthalpies of formation and isomerization energies for P4Sn cages (n = 3, 4, 5, 6, and 10) by means of explicitly correlated high-level thermochemical procedures approximating the CCSD(T) and CCSDT(Q) energies at the complete basis set (CBS) limit. The atomization reactions have very significant contribution from post-CCSD(T) correlation effects and, due to the presence of many second-row atoms, the CCSD and (T) correlation energies converge exceedingly slowly with the size of the one-particle basis set. As a result, these cage structures are challenging targets for thermochemical procedures approximating the CCSD(T) energy (e.g., W1-F12 and G4). Our best enthalpies of formation at 298 K (∆fH°298) are obtained from thermochemical cycles in which the P4Sn cages are broken down into P2S2 and S2 fragments for which highly accurate ∆fH°298 values are available from W4 theory. For the smaller P4S3 and P4S4 cages, the reaction energies are calculated at the CCSDT(Q)/CBS level and for the larger P4S5, P4S6, and P4S10 cages, they are obtained at the CCSD(T)/CBS level. Our best ∆fH°298 values are − 94.5 (P4S3), − 108.4 (α-P4S4), − 98.7 (β-P4S4), − 126.2 (α-P4S5), − 126.1 (β-P4S5), − 112.7 (γ-P4S5), − 144.7 (α-P4S6), − 153.9 (β-P4S6), − 134.4 (γ-P4S6), − 136.3 (δ-P4S6), − 118.7 (e-P4S6), and − 215.4 (P4S10) kJ mol−1. Interestingly, we find a linear correlation (R2 = 0.992) between the enthalpies of formation of the most stable isomers of each molecular formula and the number of atoms in the P4Sn cages. We use our best ∆fH°298 values to assess the performance of a number of lower-cost composite ab initio methods. For absolute enthalpies of formation, G4(MP2) and G3(MP2)B3 result in the best overall performance with root-mean-square deviations (RMSDs) of 10.6 and 12.9 kJ mol−1, respectively, whereas G3, G3B3, and CBS-QB3 result in the worst performance with RMSDs of 27.0–38.8 kJ mol−1. In contrast to absolute enthalpies of formation, all of the considered composite procedures give a good-to-excellent performance for the isomerization energies with RMSDs below the 5 kJ mol−1 mark.

The ability to accurately calculate relative energies of fullerenes is important in many areas of computational nanotechnology. Because of the large size of fullerenes, their relative energies cann...

The benzene...ethene and parallel-displaced (PD) benzene...benzene dimers are the most fundamental systems involving p-p stacking interactions. Several high-level ab initio investigations calculated the binding energies of these dimers at the CCSD(T)/CBS level of theory using various approaches such as reduced virtual orbital spaces and/or MP2-based basis set corrections. Here we obtain CCSDT(Q) binding energies using a Weizmann-3-type approach. In particular, we extrapolate the SCF, CCSD, and (T) components using large heavy-atom augmented Gaussian basis sets (namely, SCF/jul-cc-pV{5,6}Z, CCSD/jul-cc-pV{Q,5}Z, and (T)/jul-cc-pV{T,Q}Z). We consider post-CCSD(T) contributions up to CCSDT(Q), inner-shell, scalar-relativistic, and Born-Oppenheimer corrections. Overall, our best relativistic, all-electron CCSDT(Q) binding energies are Delta Ee,all,rel = 1.234 (benzene...ethene) and 2.550 (benzene...benzene PD), Delta H0 = 0.949 (benzene...ethene) and 2.310 (benzene...benzene PD), and Delta H298 = 0.130 (benzene...ethene) and 1.461 (benzene...benzene PD) kcal/mol. Important conclusions are reached regarding the basis set convergence of the SCF, CCSD, (T), and post-CCSD(T) components. Explicitly correlated calculations are used as a sanity check on the conventional binding energies. Overall, post-CCSD(T) contributions are destabilizing by 0.028 (benzene...ethene) and 0.058(benzene...benzene) kcal/mol, thus they cannot be neglected if 0.1 kcal/mol accuracy is sought., J. Chem. Phys., accepted with minor revision

Shape complementarity between catalyst and transition state structure is one of the cornerstones of chemical catalysis. Likewise, noncovalent interactions play a major role in catalysis. It has been predicted computationally and recently confirmed experimentally [Kroeger, A. A.; Hooper, J. F.; Karton, A. ChemPhysChem, 2020, 21, 1675-1681] that pristine graphene can efficiently catalyze chemical processes via π-interactions and shape complementarity. Here we show that other two-dimensional materials with different electronic structures and chemical compositions (h-BN and graphane) can also catalyze chemical processes that proceed via planar transition state structures. These include the bowl-to-bowl inversions in corannulene and sumanene and the rotation about the C-C bond in substituted biphenyls. This catalytic activity is achieved through shape complementarity between planar nanomaterial and planar transition state structure, enabling disproportionate stabilization of the transition state structures over the nonplanar reactants and products. A DFT-based energy decomposition analysis shows that this catalytic activity is mainly driven by dispersion and electrostatic forces, which together outweigh the Pauli repulsion term. These findings enrich and expand the concept of catalysis by pristine 2D materials.

11 pags., 8 figs., 1 tab., The relatively weak London dispersion forces are the only interactions that could cause aggregation between simple aromatic molecules. The use of molecular dynamics and high-levelab initiocomputer simulations has been used to describe the aggregation and interactions between molecular systems containing benzene, naphthalene and anthracene. Mixtures containing one type of molecule (homogenous) and more than one type of molecule (heterogenous) were considered. Our results indicate that as molecular weight increases so does the temperature at which aggregation will occur. In all simulations, the mechanism of aggregation is through small clusters coalescing into larger clusters. The structural analysis of the molecules within the clusters reveals that benzene will orient itself in T-shaped and parallel displaced configurations. Molecules of anthracene prefer to orient themselves in a similar manner to a bulk crystal with no T-shaped configuration observed. The aggregation of these aromatic molecules is discussed in the context of astrochemistry with particular reference to the dust formation region around stars., NAMD was developed by the Theoretical and Computational Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at UrbanaChampaign. This work was in part supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia, with the assistance of computational resources from the Pople high-performance computing cluster of the Faculty of Science at the University of Western Australia and with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. AK gratefully acknowledges an Australian Research Council (ARC) Future Fellowship (Project No. FT170100373). I. S. M. fellowship is funded by Australian Research Council (No. FT140100191). I. C. acknowledges funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007– 2013)/ERC2013-SyG, Grant Agreement No. 610256 NANOCOSMOS. DT acknowledges MEAE and MESRI for the financial support.

Despite versatile applications of functionalized graphene in catalysis, applications of pure, unfunctionalized graphene in catalysis are in their infancy. This work uses both computational and experimental approaches to show that single-layer graphene can efficiently catalyze the racemization of axially chiral BINOL in solution. Using double-hybrid density functional theory (DHDFT) we calculate the uncatalyzed and catalyzed Gibbs free reaction barrier heights in a number of representative solvents of varying polarity: benzene, diphenyl ether, dimethylformamide (DMF), and water. These calculations show that (i) graphene can achieve significant catalytic efficiencies (▵▵G≠cat ) varying between 47.2 (in diphenyl ether) and 60.7 (in DMF) kJ mol-1 . An energy decomposition analysis reveals that this catalytic activity is driven by electrostatic and dispersion interactions. Based on these computational results, we explore the graphene-catalyzed racemization of axially chiral BINOL experimentally and show that single-layer graphene can efficiently catalyze this process. Whilst the uncatalyzed racemization requires high temperatures of over 200 °C, a pristine single-layer graphene catalyst makes it accessible at 60 °C.

The equilibrium geometry of the singlet ground electronic state of the bent isomer of C5H2, bent-pentadiynylidene (4; X ∼ 1 A 1 ; C 2 v ), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C5H2, namely linear-pentadiynylidene (1; X ∼ 3 Σ g - ; D ∞ h ), ethynylcyclopropenylidene (2; X ∼ 1 A ′ ; C s ), pentatetraenylidene (3; X ∼ 1 A 1 ; C 2 v ), ethynylpropadienylidene (5; X ∼ 1 A ′ ; C s ), and 3-(didehydrovinylidene)cyclopropene (6; X ∼ 1 A 1 ; C 2 v ) had already been identified in the laboratory. With respect to 1, the relative energy difference calculated at the CCSDT(Q)/CBS level of theory including zero-point vibrational energy corrections are: 0.66 (2), 13.53 (3), 14.12 (4), 15.40 (5), and 20.01 (6) kcal mol−1, respectively. Isomers 2–6 are associated with a non-zero dipole moment ( μ ≠ 0 ), however, except 4, all the other four isomers were identified by Fourier Transform Microwave spectroscopy, including 5 and 6 which lie higher in energy. Isomer 4 remains elusive to date. We believe that the theoretical data such as, optimal geometry, dipole moment, rotational and centrifugal distortion constants, harmonic vibrational frequencies, infra-red intensities, and isotopic shifts (12C–13C) in harmonic vibrational frequencies presented here would assist experimentalists in the identification of this elusive molecule (4).

In this work, we use high-level ab initio procedures to show that the high-energy isomers of C7H2 with a planar tetracoordinate carbon (ptC) atom serve as reactive intermediate leading to the forma...

The potent antioxidant α-tocopherol is known to trap two hydroxyl radicals leading to the formation of the experimentally observed α-tocopherylquinone product. Based on double-hybrid density functional theory calculations, we propose for the first time, a reaction mechanism for the conversion of α-tocopherol to α-tocopherylquinone. We find that a water-catalysed ring-opening reaction plays a key role in this conversion. The water catalysts act as proton shuttles facilitating the proton transfers and reducing the ring strain in the cyclic transition structures. On the basis of the proposed reaction mechanism, we proceed to design an antioxidant with potentially enhanced antioxidant properties.

Efficient chemical gas detection is of great importance for various functionalities (such as leakage detection of hazardous and explosive gases in industrial safety systems). The recent discovery of 2D black phosphorene (BlackP) has created intensive interests toward nanosensors because of its maximized surface-to-volume ratio and exceptional carrier mobility that potentially deliver the superior performance than the conventional transition-metal oxides sensors. In this work, we have performed first-principles DFT calculations coupled with the statistical analysis to unravel the structural, electronic, and gas-sensing characteristics of pristine, defected, and metal-substituted BlackP toward toxic H2S and SO2 gas molecules. Our findings have revealed that pristine BlackP weakly interacts with both H2S and SO2 by van der Waals (vdW) forces characterized by the small binding energies. The analysis of electronic properties via the density of states (DOS) indicates that there is a negligible change in DOS aft...

The D5h → D10h isomerization in the C10 carbon cluster is investigated at the relativistic, all-electron CCSDT(Q)/CBS level. Previous high-level studies examined this isomerization at the valence CCSD(T)/CBS level. We show that capturing this isomerization energy requires accurate treatment of the CCSD(T)/CBS, post-CCSD(T), core-valence, scalar relativistic, diagonal Born–Oppenheimer, and zero-point vibrational energy components. Combining these components shows that the two structures are practically isoenergetic at 0 K (i.e., the D5h structure is more stable by merely +0.100 kcal mol−1). We also show that computationally economical composite protocols erroneously predict that the D10h structure is energetically more stable at 0 K.

Simultaneous loading of hydrophobic and hydrophilic drugs into a drug delivery system is a difficult task and challenges still remain due to opposite nature of drugs. So far, most of the drug delivery systems have been designed based on liposomes, dual emulsions or porous nano-silica cavities. The application of Graphene and its oxidized derivatives as nanocarriers (NCs) have grown rapidly in the past few years. The first part of this study, using molecular modeling methods, provides details on the possibility and the mechanism of simultaneous loading of two hydrophobic and hydrophilic drugs, Rifampicin and Isoniazid, into graphene oxide (GO). The results confirmed the possibility of simultaneous loading of drugs in GO. The binding energies, calculated at the B3LYP-D3/6-31G(d) level of theory, are: −46.5 and −14.0 kJ mol−1 for Isoniazid and Rifampicin, respectively. Drugs loading, as also evidenced in the second part of study experimentally. The drug-loaded NCs were coated with biopolymers of Chitosan and Gum Tragacanth. SEM results confirmed that GO-NCs have produced with a diameter

Accurate double-hybrid density functional theory and isodesmic-type reaction schemes are utilized to report accurate estimates of the heats of formation (ΔfH) for all 24 isolated-pentagon-rule isomers of the third most abundant fullerene, C84. Kinetic stabilities of these C84 isomers are also considered via C–C bond cleavage rates (Pcleav) calculated using density functional theory. Our results show that the relative abundance of C84 fullerene isomers observed in arc discharge synthesis is the result of both thermochemical and kinetic factors. This provides timely insight regarding the characterization of several C84 isomers that have been obtained experimentally to date. For instance, the established assignments of C84 isomers of (using the Fowler–Manolopoulos numbering scheme) 22 [D2(IV)], 23 [D2d(II)], 19 [D3d], 24 [D6h], 11 [C2(IV)], and 4 [D2d(I)] are consistent with the relative ΔfH and Pcleav values for these structures. However, our thermochemical and kinetic stabilities of Cs isomers 14, 15, and ...

We evaluate the performance of carbon force fields for 1811 C60 PW6B95-D3/Def2-QZVP isomerization energies. Several force fields (most notably the machine-learning GAP-20 potential) exhibit a high statistical correlation with the DFT isomerization energies. Therefore, linear scaling of the isomerization energies can significantly improve the accuracy. The best scaled force fields attain mean-absolute deviations of 8.5 (GAP-20), 12.3 (LCBOP-I and REBO-II), and 13.3 (ABOP) kcal mol−1, which translate to mean-absolute relative deviations of 4.7% (GAP-20), 6.5% (LCBOP-I), 6.6% (REBO-II) and 7.1% (ABOP). Therefore, these force fields offer a computationally economical way for exploring the relative energies of fullerenes.

We calculate the CCSD(T)/CBS complexation energies of C6X6⋯C2Xn complexes (X = F, Cl; n = 2, 4) by means of the W1-F12 and CCSD(T)/MP2(CBS) procedures. These complexes involve π–π stacking, charge-transfer, and van der Waals interactions and their complexation energies range between 11.1 (C6F6⋯C2F2) and 34.0 (C6Cl6⋯C2Cl4) kJ mol−1. We use our best CCSD(T)/CBS data to assess the performance of DFT, double-hybrid DFT (DHDFT), and standard/composite ab initio methods. The G4 and G4(MP2) composite methods show relatively poor performance with root-mean-square deviations (RMSDs) of 8.7 and 6.3 kJ mol−1, respectively. With the main exception of the Minnesota functionals, DFT functionals without a dispersion correction do not predict binding in these complexes. Most of the conventional DFT procedures attain RMSDs above the ‘chemical accuracy’ threshold. The best performing functionals with RMSDs ≤2.0 kJ mol−1 are: B3LYP-D3, PW6B95-D3, LC-ωPBE-D3, PWPB95-D3, B2GP-PLYP-D3, and B2-PLYP-D3.

We examine the reaction of ethanol with glyoxal to form hemiacetal by means of the high-level G4(MP2) procedure. In this reaction, an intermolecular proton transfer is coupled with the formation of a covalent C O bond between the two molecules. We find a novel catalytic reaction mechanism in which an H 2 SO 4 catalyst reduces the barrier height from ∆ H ‡ 298 = 140.2 to 16.3 kJ mol −1 . It is well established that H 2 SO 4 can effectively catalyse intramolecular proton transfers. This letter shows that H 2 SO 4 can catalyse an intermolecular proton transfer that is coupled with a covalent bond formation.

We use high-level ab initio CCSD(T) and CCSDT(Q) methods to investigate the energetic and spectroscopic properties of nine low-lying isomers of C7H2, which lie within 1 eV. Among these, heptatriynylidene (1), 1-(buta-1,3-diynyl)cyclopropenylidene (2) and heptahexaenylidene (9) have been detected experimentally. The other six isomers, 1,2-(diethynyl)cyclopropenylidene (3), bicyclo[4.1.0]hepta-1,2,4,5-tetraene-7-ylidene (4), cyclohepta-1,2,3,4-tetraen-6-yne (5), bicyclo[4.1.0]hepta-4,6-diene-2-yne-7-ylidene (6), bicyclo[4.1.0]hepta-1,5-diene-3-yne-7-ylidene (7) and 1-(buta-1,3-diynyl)propadienylidene (8), remain hypothetical to date. Except for 1, all of the isomers are associated with a non-zero dipole moment (μ ≠ 0). Although Fourier-transform microwave spectroscopy had detected 2 and 9, our study reveals that six hypothetical isomers (3–8) are thermodynamically sandwiched between the experimentally known and astronomically relevant isomers 2 and 9. The structural parameters, dipole moments, rotational and centrifugal distortion constants, harmonic vibrational frequencies, and infra-red intensities presented here may be useful for the laboratory detection of these previously unidentified isomers (3–8) and also all others (2–9) in astronomical sources.

In the present study, we have devised the G4(MP2)-XK composite method that covers species with up to fifth-row main-group elements (i.e., up to Rn). This new protocol is based on the previously published G4(MP2)-6X method, which has a general accuracy of ∼5 kJ mol–1 for a diverse range of first- and second-row systems. The main difference between G4(MP2)-6X and G4(MP2)-XK is that the Pople-type basis sets in the former are replaced by Karlsruhe-type basis sets, with adjustments to the standard Karlsruhe basis sets to mimic the ones that they replace. Generally, G4(MP2)-XK is comparable in accuracy to G4(MP2)-6X. It is somewhat computationally more efficient than G4(MP2)-6X for the larger species that we have examined (e.g., a pentaglycine peptide). Importantly, the accuracy of G4(MP2)-XK for heavier elements is similar to that for first- and second-row species, even though it contains parameters that are fitted only to systems of the first two rows. This is indicative of the transferability of G4(MP2)-XK,...

We evaluate the performance of standard and modified MP n procedures for a wide set of thermochemical and kinetic properties, including atomization energies, structural isomerization energies, conformational energies, and reaction barrier heights. The reference data are obtained at the CCSD(T)/CBS level by means of the W n thermochemical protocols. We find that none of the MP n -based procedures show acceptable performance for the challenging W4-11 and BH76 databases. For the other thermochemical/kinetic databases, the MP2.5 and MP3.5 procedures provide the most attractive accuracy-to-computational cost ratios. The MP2.5 procedure results in a weighted-total-root-mean-square deviation (WTRMSD) of 3.4 kJ/mol, whilst the computationally more expensive MP3.5 procedure results in a WTRMSD of 1.9 kJ/mol (the same WTRMSD obtained for the CCSD(T) method in conjunction with a triple-zeta basis set). We also assess the performance of the computationally economical CCSD(T)/CBS(MP2) method, which provides the best overall performance for all the considered databases, including W4-11 and BH76.

We have used computational quantum chemistry to investigate the thermochemistry of α-hydrogen abstraction from the full set of amino acids normally found in proteins, as well as their peptide forms, by •OH and •SH radicals. These reactions, with their reasonable complexity in the electronic structure (at the α-carbon), are chosen as a consistent set of models for conducting a fairly robust assessment of theoretical procedures. Our benchmarking investigation shows that, in general, the performance for the various classes of theoretical methods improves in the order nonhybrid DFT → hybrid DFT → double-hybrid DFT → composite procedures. More specifically, we find that the DSD-PBE-P86 double-hybrid DFT procedure yields the best agreement with our high-level W1X-2 vibrationless barriers and reaction energies for this particular set of systems. A significant observation is that, when one considers relative instead of absolute values for the vibrationless barriers and reaction energies, even nonhybrid DFT procedures perform fairly well. To exploit this feature in a cost-effective manner, we have examined a number of multilayer schemes for the calculation of reaction energies and barriers for the abstraction reactions. We find that accurate values can be obtained when a "core" of glycine plus the abstracting radical is treated by DSD-PBE-P86, and the substituent effects are evaluated with M06-2X. Inspection of the set of calculated thermochemical data shows that the correlation between the free energy barriers and reaction free energies is strongest when the reactions are either endergonic or nearly thermoneutral.

Composite ab initio methods are multistep theoretical procedures specifically designed to obtain highly accurate thermochemical and kinetic data with confident sub-kcal mol−1 or sub-kJ mol−1 accuracy. These procedures include all energetic terms that contribute to the molecular binding energies at these levels of accuracy (e.g., CCSD(T), post-CCSD(T), core–valence, relativistic, spin-orbit, Born–Oppenheimer, and zero-point vibrational energy corrections). Basis-set extrapolations (and other basis-set acceleration techniques) are used for obtaining these terms at sufficiently high levels of accuracy. Major advances in computer hardware and theoretical methodologies over the past two decades have enabled the application of these procedures to medium-sized organic systems (e.g., ranging from benzene and hexane to amino acids and DNA bases). With these advances, there has been a proliferation in the number of developed composite ab initio methods. We give an overview of the accuracy and applicability of the various types of composite ab initio methods that were developed in recent years. General recommendations to guide selection of the most suitable method for a given problem are presented, with a special emphasis on organic molecules. WIREs Comput Mol Sci 2016, 6:292–310. doi: 10.1002/wcms.1249 For further resources related to this article, please visit the WIREs website.

The CCSD(T) method is often considered as the gold standard in quantum chemistry for single-reference systems. Using W4 and W4lite theories, we calculate post-CCSD(T) contributions to the total atomization energies (TAEs) of n -alkanes and show that they reach up to 0.65 kcal/mol for n -hexane. Furthermore, we find that post-CCSD(T) contributions increase linearly with the size of the n -alkane, indicating that they will reach ∼1 kcal/mol for n -decane (C 10 H 22 ) and ∼2 kcal/mol for n -icosane (C 20 H 42 ). These results are significant since today CCSD(T)/CBS-type methods are being applied to hydrocarbons of increasing size and are assumed to give TAEs with chemical accuracy for these systems.

High-level heats of formation for C 60 are obtained through the use of reactions in which C 60 is broken down into its fundamental bowl-shaped aromatic fragments (corannulene and sumanene). The reaction enthalpies are obtained by means of the high-level G4(MP2) thermochemical protocol and reliable experimental (from the Active Thermochemical Tables, ATcT) or theoretical (from W1h theory) heats of formation are used for the molecules involved in these reactions. Our best theoretical estimate, Δ f H ° 298 [C 60 ( g )] = 2511.7 kJ mol −1 , suggests that the experimental value adopted by the NIST thermochemical database (Δ f H ° 298 [C 60 ( g )] = 2560 ± 100 kJ mol −1 ) should be revised downwards.

Reaction with ozone is a major atmospheric sink for α-phellandrene, a monoterpene found in both indoor and outdoor environments, however experimental literature concerning the reaction is scarce. In this study, high-level G4(MP2) quantum chemical calculations are used to theoretically characterise the reaction of ozone with both double bonds in α-phellandrene for the first time. Results show that addition of ozone to the least substituted double bond in the conjugated system is preferred. Following addition, thermal and chemically activated unimolecular reactions, including the so-called hydroperoxide and ester or 'hot' acid channels, and internal cyclisation reactions, are characterised to major first generation products. Conjugation present in α-phellandrene allows two favourable Criegee intermediate reaction pathways to proceed that have not previously been considered in the literature; namely a 1,6-allyl resonance stabilised hydrogen shift and intramolecular dioxirane isomerisation to an epoxide. These channels are expected to play an important role alongside conventional routes in the ozonolysis of a-phellandrene. Computational characterisation of the potential energy surface thus provides insight into this previously unstudied system, and will aid future mechanism development and experimental interpretation involving α-phellandrene and structurally similar species, to which the results are expected to extend.

The relative energies of the YMPJ conformer database of the 20 proteinogenic amino acids, with N- and C-termination, have been re-evaluated using explicitly correlated coupled cluster methods. Lower-cost ab initio methods such as MP2-F12 and CCSD-F12b actually are outperformed by double-hybrid DFT functionals; in particular, the DSD-PBEP86-NL double hybrid performs well enough to serve as a secondary standard. Among range-separated hybrids, ωB97X-V performs well, while B3LYP-D3BJ does surprisingly well among traditional DFT functionals. Treatment of dispersion is important for the DFT functionals; for the YMPJ set, D3BJ generally works as well as the NL nonlocal dispersion functional. Basis set sensitivity for DFT calculations on these conformers is weak enough that def2-TZVP is generally adequate. For conformer corrections to heats of formation, B3LYP-D3BJ and especially DSD-PBEP86-D3BJ or DSD-PBEP86-NL are adequate for all but the most exacting applications. The revised geometries and energetics for the YMPJ database have been made available as Supporting Information and should be useful in the parametrization and validation of molecular mechanics force fields and other low-cost methods. The very recent dRPA75 method yields good performance, without resorting to an empirical dispersion correction, but is still outperformed by DSD-PBEP86-D3BJ and particularly DSD-PBEP86-NL. Core-valence corrections are comparable in importance to improvements beyond CCSD(T*)/cc-pVDZ-F12 in the valence treatment.

Benchmark reaction barrier heights for the degenerate Cope rearrangements in the highly fluxional bullvalene and semibullvalene hydrocarbon cages are obtained at the CCSDT(Q) level close to the one-particle basis set limit and include inner-shell, scalar-relativistic, and Born–Oppenheimer corrections. Our best theoretical CCSDT(Q) Gibbs free reaction barrier height for semibullvalene (ΔG298‡ = 27.9 kJ mol−1) is in good agreement with the most recent experimental value of 25.9 kJ mol−1. However, our CCSDT(Q) reaction barrier height for bullvalene (ΔG298‡ = 62.2 kJ mol−1) indicates that the most recent gas-phase experimental value of 54.8 ± 0.8 should be revised upward.

It is well established that extrapolating the coupled-cluster single double triple [CCSD and (T)] correlation energies using empirically motivated extrapolation exponents can accelerate the basis set convergence. Here, we consider the extrapolation of coupled-cluster expansion terms beyond the CCSD(T) level to the complete basis set (CBS) limit. We obtain reference CCSDT-CCSD(T) [T3-(T)], CCSDT(Q)-CCSDT [(Q)], and CCSDTQ-CCSDT(Q) [T4-(Q)] contributions from cc-pV{5,6}Z extrapolations for a diverse set of 16 first- and second-row systems. We use these basis-set limit results to fit extrapolation exponents in conjunction with the cc-pV{D,T}Z, cc-pV{T,Q}Z, and cc-pV{Q,5}Z basis set pairs. The optimal extrapolation exponents result in noticeable improvements in performance (relative to α = 3.0) in conjunction with the cc-pV{T,Q}Z basis set pair; however, smaller improvements are obtained for the other basis sets. These results confirm that the basis sets and basis set extrapolations used for obtaining post-CCSD(T) components in composite thermochemical theories such as Weizmann-4 and HEAT are sufficiently close to the CBS limit for attaining sub-kJ/mole accuracy. The fitted extrapolation exponents demonstrate that the T3-(T) correlation component converges more slowly to the CBS limit than the (Q) and T4 terms. A systematic investigation of the effect of diffuse functions shows that it diminishes (i) in the order T3-(T) > (Q) > T4-(Q) and (ii) with the size of the basis set. Importantly, we find that diffuse functions tend to systematically reduce the T3-(T) contribution but systematically increases the (Q) contribution. Thus, the use of the cc-pVnZ basis sets benefits from a certain degree of error cancellation between these two components.

We use Gaussian-4 theory to investigate the reaction mechanism for the conversion of a 2-methylstyrene-based Criegee intermediate into o-toluic acid and 2-methylphenylformate. o-Toluic acid can be formed via an α-hydroxyalkyl-hydroperoxide intermediate with an activation energy of ΔG‡298 = 82.9 kJ mol−1 for the rate-determining-step (RDS). The RDS for the formation of 2-methylphenylformate has an activation energy of ΔG‡298 = 61.9 kJ mol−1. Formation of the o-toluic acid product is more exergonic by 67.4 kJ mol−1. Consistent with recent experimental results, our high-level calculations show that o-toluic acid is the thermodynamic product and 2-methylphenylformate is the kinetic product.

We introduce a database of 14 accurate bond dissociation energies (BDEs) of noble gas compounds. Reference CCSD(T)/CBS BDEs are obtained by means of W1 theory. We evaluate the performance of contemporary density functional theory (DFT), double-hybrid DFT (DHDFT), and composite ab initio procedures. A general improvement in performance is observed along the rungs of Jacob’s Ladder; however, only a handful of functionals give good performance for predicting the bond dissociation energies in the NGC14 database. Thus, this database represents a challenging test for DFT methods. Most of the conventional DFT functionals (71%) result in root-mean-square deviations (RMSDs) between 10.0 and 82.1 kJ mol−1. The rest of the DFT functionals attain RMSDs between 2.5 and 8.9 kJ mol−1. The best performing functionals from each rung of Jacob’s Ladder are (RMSD given in parenthesis): HCTH407 (30.9); M06-L (5.4); PBE0 (2.8); B1B95, M06, and PW6B95 (2.7–2.9); CAM-B3LYP-D3 (5.4); and B2T-PLYP (2.5 kJ mol−1).

We examine the basis-set convergence of post-CCSD(T) contributions to the dissociation energy of the highly multireference C2(1 Σ g + ) molecule. At the infinite basis set limit we obtain: CCSDT–CCSD(T)/cc-pV{6,7}Z = −2.268, CCSDT(Q)–CCSDT/cc-pV{6,7}Z = 3.420, CCSDTQ–CCSDT(Q)/cc-pV{5,6}Z = −1.151, CCSDTQ(5)–CCSDTQ/cc-pV{Q,5}Z = 0.412, CCSDTQ5–CCSDTQ(5)/cc-pV{T,Q}Z = −0.053, CCSDTQ5(6)–CCSDTQ5/cc-pV{D,T}Z = 0.060, CCSDTQ56–CCSDTQ5(6)/cc-pVTZ(1d) = 0.003, CCSDTQ56(7)–CCSDTQ56/cc-pVTZ(1d) = 0.002, and CCSDTQ567–CCSDTQ56(7)/cc-pVTZ(1d) = 0.001 kcal/mol. These post-CCSD(T) contributions add to 0.427 kcal/mol. Including the CCSD(T)/CBS energy, inner-shell, scalar relativistic, spin-orbit, DBOC, and ZPVE corrections from W4.3 theory results in a CCSDTQ567/CBS D0 value of 144.08 kcal/mol, which agrees to within overlapping uncertainties with the experimental ATcT value of 144.006 ± 0.06 kcal/mol.