M06-2X Levels Research Articles

Why Is the L-Shaped Structure of X2···X2 (X = F, Cl, Br, I) Complexes More Stable Than Other Structures?

Five different structures (L- and T-shaped (LS, TS), parallel (P), parallel-displaced (PD), and linear (L)) of (X2)2 dimers (X = F, Cl, Br, I, N) have been investigated at B97-D3, M06-2X, DFT-SAPT, and CCSD(T) levels. The Qzz component of the quadrupole moment of all dihalogens, which coincides with the main rotational axis of the symmetry of the molecule, has been shown to be positive, whereas that of dinitrogen is negative. All of these values correlate well with the most positive value of the electrostatic potential, which, for dihalogens, reflects the magnitude of the σ-hole. The LS structure is the most stable structure for all dihalogen dimers. This trend is the most pronounced in the case of iodine and bromine; for dinitrogen dimer, the LS, TS, and PD structures are comparably stable. The dominant stabilization energy for dihalogen dimers is dispersion energy, followed by Coulomb energy. In the case of dinitrogen dimer, it is only the dispersion energy. At short distances, the Coulomb (polarization) energy for dihalogen dimers is more attractive for the LS structure; at larger distances, the TS structure is more favorable, as dispersion and induction energies are systematically more stable for the TS structure. For all dimers and all distances, the long-range electrostatic energy covering the interactions of multipole moments is the most attractive for the TS structure. In the case of dihalogen dimers, the preference of the LS structure over the others, resulting from the concert action of Coulomb, dispersion, and induction energies, is explained by the presence of a σ-hole. In the case of dinitrogen, comparable stability of LS, TS, and PD structures is obtained, as all are dominantly stabilized by dispersion energy.

Influence of the physicochemical and aromatic properties on the chemical reactivity and its relation with carcinogenic and anticoagulant effect of 17β-aminoestrogens

Activity of steroid hormones is dependent upon a number of factors, as solubility, transport and metabolism. The functional differences caused by structural modifications could exert an influence on the chemical reactivity and biological effect. The goal of this work is to study the influence of the physicochemical and aromatic properties on the chemical reactivity and its relation with the carcinogenic risk that can associate with the anticoagulant effect of 17β-aminoestrogens using quantum-chemical descriptors at the DFT-B3LYP, BH&HLYP and M06-2X levels. The relative acidity of (H1) of the hydroxyl group increases with electron-withdrawing groups. Electron-donor groups favor the basicity. The steric hindrance of the substituents decreases the aromatic character and consequently diminution the carcinogenic effect. Density descriptors: hardness, electrophilic index, atomic charges, molecular orbitals, electrostatic potential and their geometric parameters permit analyses of the chemical reactivity and physicochemical features and to identify some reactive sites of 17β-aminoestrogens.

Theoretical study on the electronic structures and phosphorescent properties of five bis-cyclometalated iridium(III) complexes with 2-phenylpyridinato ancillary ligand

A DFT/TDDFT study was performed on five bis-cyclometalated iridium(III) complexes with the same ancillary ligand 2-phenylpyridinato (ppy) and main ligands with different substituted groups (CH(CH3)2, CH3, H, F, and NO2). The geometry structures, electronic structures, absorption, and phosphorescent properties of the five iridium(III) complexes have been investigated. Calculations of ionization potential and electron affinity were used to evaluate the injection abilities of holes and electrons into these complexes. The lowest energy absorption wavelengths are located at 408nm for 1, 407nm for 2, 401nm for 3, 381nm for 4, and 556nm for 5, respectively, in good agreement with the HOMO–LUMO energy gaps. The lowest energy emissions of these complexes are localized at 515, 514, 516, 518, and 567nm at M052X level, respectively. The calculated results indicate that the complex 4 possibly possesses the largest kr value among the five complexes. We hope that this theoretical work can provide constructive information for designing and synthesizing novel phosphorescent materials for use in the organic light-emitting diodes.

Theoretical investigation on the electronic structures and phosphorescent properties of seven iridium(III) complexes with the different substituted 2-phenylpyridinato ancillary ligand

The geometry structures, electronic structures, absorption, and phosphorescent properties of seven iridium(III) complexes have been investigated by using the DFT/TDDF methods. Calculations of ionization potential and electron affinity were used to evaluate the injection abilities of holes and electrons into these complexes. The lowest energy emissions of these complexes are localized at 662, 583, 502, 518, 527, 536, and 538nm, respectively, for complexes 1–7, simulated in CH2Cl2 medium at M052X level. The calculated results indicate that the complex 5 possibly possesses the largest kr value among the seven complexes.

A computational perspective on mechanism and kinetics of the reactions of CF3C(O)OCH2CF3 with OH radicals and Cl atoms at 298 K

Theoretical investigations are carried out on the gas-phase reactions of CF3C(O)OCH2CF3, 2,2,2-trifluoroethyl trifluoroacetate (TFETFA) with OH radicals and Cl atoms by means of modern DFT methods. Two conformers relatively close in energy have been identified for TFETFA, both of them are likely to be important in the temperature range of our study. Reaction profiles are modeled including the formation of two pre-reactive and post-reactive complexes. The optimized geometries, frequencies and minimum energy path are obtained at hybrid density functional model, M06-2X level using 6-31+G(d,p) basis set. The single point energy calculation was further refined by using 6-311++G(d,p) and Aug-cc-pVTZ basis sets. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation. Theoretically calculated rate constants at 298K and using the canonical transition state theory (CTST) are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CF3C(O)OCH2CF3 and radical generated by hydrogen abstraction, CF3C(O)OCHCF3, are also reported for the first time. The estimated atmospheric lifetime of TFETFA is expected to be 0.15 years.

Theoretical study on the mechanism and kinetics of atmospheric reactions NH2OH + OOH and NH2CH3 + OOH

Mechanism and kinetics of NH2OH + OOH and NH2CH3 + OOH reactions were studied at the B3LYP and M062X levels of theory using the 6-311++G(3df, 3pd) basis set. The NH2OH + OOH and NH2CH3 + OOH reactions proceed through different paths which lead to different products. Transition state structure and activation energy of each path were calculated. The calculated activation energies of hydrogen abstraction reactions were smaller than 25 kcal/mol and of substitution reactions are in the range of 50–70 kcal/mol. The rate constants were calculated using transition state theory (TST) modified for tunneling effect at 273–2000 K.

The benzoic acid–water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

Theoretical investigation on the atmospheric fate of the CF3C(O)OCH(O)CF3 radical: alpha-ester rearrangement vs. oxidation

A detailed quantum chemical study is performed on the unimolecular decomposition reaction of the alkoxy radical, CF3C(O)OCH(O)CF3 produced from CF3C(O)OCH2CF3, trifluoroethyl trofluoroacetate (TFETFA) at the MPWB1K and M06-2X level of theories using the 6-31+G(d,p) basis set. Five plausible decomposition pathways including reaction with O2, α-ester rearrangement and thermal decomposition (C–C, C–H and C–O bonds scission) have been considered in detail. Out of the five prominent decomposition channels, our results reveal that reaction with O2 is the dominant path for the decomposition of the CF3C(O)OCH(O)CF3 radical in the atmosphere involving the lowest energy barrier which is in accordance with recent experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid (TFA) has a negligible contribution for decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways are evaluated using Canonical Transition State Theory (CTST) at 298 K.

Degradation of methyl salicylate through Cl initiated atmospheric oxidation – a theoretical study

In the present study, a possible reaction mechanism for the degradation through Cl initiated atmospheric oxidation of methyl salicylate (MeSA) was studied using density functional theory at B3LYP and M06-2X levels of theory with 6-311++G(d,p) basis set. The degradation occurred through an abstraction process at the meta position of MeSA along the entire reaction path. The barrier height values indicated that the reaction of the benzoate radical of MeSA and NO was a favourable route for degradation, which was thermodynamically exothermic and exergonic in nature. The lifetime calculations indicated a short life span for MeSA at 1.14 days. The rate constant values displayed good Arrhenius character with respect to temperature. The activation hardness predicted a rapid reaction between MeSA and NO. The HOMO and LUMO values along with their plot indicated that the NO radical and benzene ring in the MeSA were highly reactive. Topological analysis revealed that during the reaction the interaction between reactants was covalent in nature.

Structures and stability of SCBO+/− and SBCO+/−: prediction of very short yet classical triple bonding of sulfur

Compounds containing the sulfur triple bonding have continued to attract chemists’ attention. In this article, with an attempt to predict intrinsically stable species with sulfur-related triple bonding, we report a thorough computational study of two charged systems [B,C,O,S]+ and [B,C,O,S]− at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPVE level for singlet and triplet potential energy surfaces, aug-cc-pVTZ-B3LYP, M06-2X, and CCSD(T) levels for critical structures, as well as the CCSD(T)/aug-cc-pVQZ and G4 levels for adiabatic bond dissociation energy (ABDE). A total of 26 isomers and 25 transition states were located. The cationic and anionic [B,C,O,S] have the singlet and triplet ground states, respectively. For both systems, the former low-lying isomers are the linear SCBO+/− 01 and SBCO+/− 02, both of which contain the S≡X (X = C, B) bonding and are kinetically very stable against interconversion and fragmentation. With the increased valence electron number in the order of SCBO/SBCO+ < SCBO/SBCO < SCBO/SBCO−, the SX bond distance elongates as 1.5653 < 1.6126 < 1.6924 A for X = B and 1.4715 < 1.5319 < 1.6100 A for X = C at the M06-2X/aug-cc-pVTZ level. Notably, SBCO+ bears the shortest S≡B bond known to date, while SCBO+ bears the shortest S≡C bond among the known classical and non-protonated compounds (the well-known F3SCCF3 and F3SCSF5 have been termed as “nonclassical” because of their unusually low ABDE of SC bond). Future mass spectroscopic studies are greatly appealed for the characterization of the cationic and anionic SCBO+/− 01 as well as SBCO+/− 02.

Strong halogen bonds between halo-perfluorobenzenes (C6F5X) and pyridine molecules: A combined theoretical and crystallographic data study

X⋯N interactions between halo-perfluorobenzenes and pyridine derivatives play a crucial role in halogen-bond-based crystal engineering and supramolecular architecture. In this work, a series of complexes of C6F5X (X=Br and I) with several pyridine molecules, which were extracted from crystal structures, have been calculated at the M05-2x level of theory. It was found that X⋯N interactions in the studied complexes are comparable in strength to medium strong hydrogen bonds. The introduction of electron-withdrawing substituents in pyridine attenuates X⋯N interactions, while the presence of electron-donating substituents enhances these halogen bonds. The quantum theory of atoms in molecules (QTAIM) and electrostatic potential (ESP) analyses were also employed to characterize these interactions. The results showed that electrostatic terms contribute significantly to the formation of X⋯N halogen bonds. Finally, some X-ray crystal structures retrieved from the Cambridge Structural Database (CSD) were selected to provide experimental evidence of present halogen bonds.

Quantum Chemical Study of B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Group

The hydrosilylation reaction of a carbonyl group catalyzed by tris(pentafluorophenyl)borane, B(C6F5)3, is investigated by using the DFT method. M06-2X level calculations suggest the presence of a stable complex between trimethylsilane and B(C6F5)3. The attack of the carbonyl group in acetone from the back side of the Si-H bond prompts the abstraction of the hydride ion by B(C6F5)3. This reaction path is lower in free energy than the conventional carbonyl-activation path via a four-membered cyclic transition state. The silane-activation mechanism is supported in this case, in agreement with experimental results reported by Piers and by Oestreich. The calculations show, on the other hand, that the silane-activation mechanism does not apply to the reaction catalyzed by BF3. This difference in mechanisms arises from a stronger electrophilicity of the boron center in B(C6F5)3 than in BF3 toward a hydride ion, as demonstrated by an analysis of reactive orbitals. Attractions between the silane part and the fluorine atoms at the ortho positions of C6F5 groups in the Lewis acid assist the path by making up for the destabilization of the reacting system that is caused by the distortion of the B(C6F5)3 framework in forming a bond with the hydrogen of silane.

Accurate prediction of polarised high order electrostatic interactions for hydrogen bonded complexes using the machine learning method kriging

As intermolecular interactions such as the hydrogen bond are electrostatic in origin, rigorous treatment of this term within force field methodologies should be mandatory. We present a method able of accurately reproducing such interactions for seven van der Waals complexes. It uses atomic multipole moments up to hexadecupole moment mapped to the positions of the nuclear coordinates by the machine learning method kriging. Models were built at three levels of theory: HF/6-31G**, B3LYP/aug-cc-pVDZ and M06-2X/aug-cc-pVDZ. The quality of the kriging models was measured by their ability to predict the electrostatic interaction energy between atoms in external test examples for which the true energies are known. At all levels of theory, >90% of test cases for small van der Waals complexes were predicted within 1kJmol−1, decreasing to 60–70% of test cases for larger base pair complexes. Models built on moments obtained at B3LYP and M06-2X level generally outperformed those at HF level. For all systems the individual interactions were predicted with a mean unsigned error of less than 1kJmol−1.

Zwitterionic 1,3-Shift Intermediates in the Interconversions of (Thio)carbamoyl Isocyanates and Carbamoyl Iso(thio)cyanates, R2N-CX-NCO ⇌ R2N-CO-NCX, and in Carbamoylketenes R2N-CO-CH=C=O

Cyclic zwitterionic intermediates 3 have been located as reactive intermediates in the thermal 1,3-shifts of dimethylamino and N- piperidinyl groups in carbamoyl isocyanates, carbamoyl isothiocyanates and carbamoylketenes at the B3LYP, M06-2X and MP2 computational levels of theory. The zwitterion formed in the course of the ketene ⇌ ketene rearrangement shows the highest stabilization (by about 3–5 kcal/mol). The interconversions of carbamoyl isocyanates and isothiocyanates should occur readily at or near room temperature with activation barriers in the range 23–30 kcal/mol, and the ketene ⇌ ketene interconversion is expected well below room temperature with an activation barrier of ca. 10 kcal/mol.

Conformational study of the structure of 18-thiacrown-6

Conformational analysis was performed for 18-thiacrown-6 (18t6) using the CONFLEX method and the MMFF94s force field. Computations were performed for some of the low energy conformations at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x, M06HF and MP2 levels. The computations were also performed using the DFT-D3 method along with the TPSS and PBE functionals. The study predicted a new C2 conformation as the ground state conformation of 18t6. This new C2 conformation is more stable than the experimentally known solid state conformation by 4.7kcal/mol at the MP2/6-311G** level. This conformation has all of the SCCS dihedral angles adopt exodentate structure. However, the experimentally known conformation of the solid phase has two of the SCCS dihedral angles violating this exodentate rule. It was concluded that for 18t6 stability a linear dihedral SCCS angle requirement is more important than a gauche CSCC dihedral angle requirement.

Density functional theory and time‐dependent density functional theory study on a series of iridium complexes with tetraphenylimidodiphosphinate ligand

A series of heteroleptic cyclometalated Ir(III) complexes for organic light‐emitting diodes (OLEDs) application have been investigated theoretically to explore their electronic structures and spectroscopic properties. The geometries, the electronic structures, the lowest‐lying singlet absorptions and triplet emissions of Ir(dfppy)2(tpip), Ir(tfmppy)2(tpip), and theoretically designed models of Ir(ppy)2(tpip) were investigated with the density functional theory (DFT)‐based approaches, where ppy = 2‐phenylpyridine, dfppy = 4,6‐difluorophenylpyridine, tfmppy = 4‐trifluoromethylphenylpyridine, and tpip = tetraphenylimidodiphosphinate. Their structures in the ground and their excited states have been optimized at the DFT/Becke 3‐parameter Lee Yang Parr (B3LYP)/Los Alamos National Laboratory 2‐double‐z (LANL2DZ) and time‐dependent DFT/B3LYP/LANL2DZ levels, and the lowest absorptions and emissions were evaluated at B3LYP and M062X level of theory, respectively. Furthermore, the energy transfer mechanism together with the advantage of low efficiency roll‐off for these complexes also can be analyzed here. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Tuning the electronic properties of C30B15N15 fullerene via encapsulation of alkali and alkali earth metals

Encapsulation of several alkali and alkali earth metals inside a C30B15N15 nanocage was investigated by using density functional calculations at B3LYP and M06-2X levels of theory. It was found that the encapsulation processes are spontaneous at room temperature and 1atm pressure. The encapsulation favorability increases in the series: K>Na>Ca>Li>Mg>Be. Alkali earth metals influence the electronic properties of cluster relatively more compared to alkali metals. A tunable HOMO–LUMO energy gap of up to ∼1.05eV was predicted for C30B15N15 by the controlled encapsulation of different alkali and alkali earth metals at B3LYP/6-31G (d). Electron emission density from C30B15N15 surface will be significantly increased upon the Ca encapsulation, while in the case of Li capsulation it will be decreased based on work function calculations in our work. The thermodynamic results of B3LYP and M06-2X functionals are somewhat similar, while those of electronic properties are significantly different.

Self-Catalyzed Mannich-Type Reaction of Enolizable Cyclic 1,3-Dicarbonyls to Acyclic Nitrones: An Entry to Functionalized β-Enamino Diones

A new method for the preparation of highly functionalized β-enamino diones has been developed. The protocol involves an initial self-catalyzed Mannich-type reaction of enolizable cyclic 1,3-dicarbonyls to nitrones, followed by a spontaneous intramolecular reorganization of the resulting nonisolated hydroxylamine to enamino derivatives. These compounds retain the features of unnatural α-amino acids. The ease of preparation makes them attractive intermediates for the synthesis of peptidomimetics, polyheterocycles, and other multifunctional compounds. All experimental results have been efficiently rationalized by in silico studies at the M06-2X level of theory, and a valid mechanistic pathway has been proposed.

Phenylacetylene dimer: Ab initio and DFT study

Phenylacetylene (PHA) consists of two substantially different subunits: the phenyl ring (aromatic) and the acetylenic (aliphatic) moiety. Both groups are represented by relatively high electron density caused by highly delocalized π-electron clouds. Here, we performed a detailed analysis of the phenylacetylene dimer which is characterized by a variety of non-covalent interactions like π–π stacking and H–π hydrogen bonds. We carried out geometry optimization of the phenylacetylene dimer at the counterpoise corrected MP2, MP2, B97D, ωB97X-D, M06-2X, and B3LYP levels of theory. For all the optimized structures, we calculated harmonic vibrational frequencies to assure that all the conformers are local/global minimum energy structures. Different levels of calculation predict different structures. Thus, for important structures, we computed binding energies at the complete basis set limit (CBS) of CCSD(T) theory. We conclude the antiparallel structure is the most stable. The formation of the phenylacetylene dimer is mainly driven by dispersion forces, however, the electrostatic interactions are also significant, which is confirmed by SAPT-DFT calculations. Hobza et al. reported counterpoise corrected MP2 results; however, we focus on DFT results.

Efficient blue‐emitting Ir(III) complexes with phenyl‐methyl‐benzimidazolyl and picolinate ligands: A DFT and time‐dependent DFT study

AbstractThe series of heteroleptic cyclometalated Ir(III) complexes for organic light‐emitting display application have been investigated theoretically to explore their electronic structures and spectroscopic properties. The geometries, electronic structures, and the lowest‐lying singlet absorptions and triplet emissions of Ir‐(pmb)3 and theoretically designed models Ir‐(Rpmb)2pic were investigated with density functional theory (DFT)‐based approaches, where pmb = phenyl‐methyl‐benzimidazolyl, pic = picolinate, and R = H/F. Their structures in the ground and excited states have been optimized at the DFT/B3LYP/LANL2DZ and TDDFT/B3LYP/LANL2DZ levels, and the lowest absorptions and emissions were evaluated at B3LYP and M062X level of theory, respectively. The mobility of holes and electrons were studied computationally based on the Marcus theory. Calculations of ionization potentials were used to evaluate the injection abilities of holes into these complexes. The reasons for the higher electroluminescence efficiency and phosphorescence quantum yields in Ir‐(Rpmb)2pic than in Ir‐(pmb)3 have been investigated. The designed moleculars are expected to be highly emissive in pure‐blue region. © 2013 Wiley Periodicals, Inc.