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Aryl acetylenes bearing electron-withdrawing groups have been found that can add to azulenes and yield 1,2-substituted heptalenes. Theoretical studies of the reaction mechanism helped to design the candidate molecules that have then been tested in the synthetic work. New heptalenes are obtained by reacting methyl (4-nitrophenyl)propiolate with either 4,6,8-trimethylazulene or guaiazulene, as well as 3-(4-nitrophenyl)-2-propyn-1-ol with guaiazulene. This opens a new synthetic route to 1-aryl-substituted heptalenes.

Control of isomerization of a receptor bearing multiple light-switchable subunits in a confined space is critical for the design of synthetic molecular machines. Toward this goal, a new azacryptand containing three azobenzene subunits has been developed, and its photoisomerization in an aqueous solution has been studied depending on anion coordination and recognition by a larger hostcucurbit[8]­uril (CB[8]). The cryptand in its hexaprotonated form shows considerable affinity for fluoride and perchlorate, which in turn affects the isomer distribution of the receptor under UV-light irradiation, stabilizing the isomers of the cryptand with Z-configurations. CB[8] was found to be able to encapsulate the isomers of the cryptand by forming a Matryoshka-type complex. The irradiation of a 10:1 CB[8]–cryptand mixture has led to a selective conversion of the cryptand to the E,E,Z isomer inside CB[8]. It has been demonstrated that the addition of fluoride to the resulted complex induces the release of the cryptand as a major E,E,E isomer, while other studied anions were ineffective in this reaction. To our knowledge, this work presents a first example of a host-controlled photoisomerization of an anion receptor bearing multiple switching azobenzenes that model the function of naturally occurring chaperones.

An attractive strategy for C-Se bond formation by Ullmann-type copper(I)-promoted cross-coupling is developed. A wide range of aryliodides reacts with various disubstituted 2-selenohydantoins under mild conditions and provides Se-arylated imidazolines in moderate to high yields. Computational mechanistic studies show the oxidative addition/intramolecular reductive elimination likely to be the lowest-energy pathway. Cytotoxic activity of all 43 reaction products has been tested in vitro against MCF7 and A549 cancer cell lines with VA13 and MCF10a control cells.

A general procedure for the optimization of atomic density-fitting basis functions is designed with the balance between accuracy and numerical stability in mind. Given one-electron wavefunctions and energies, weights are assigned to the product densities, modeling their contribution to the exchange and second-order correlation energy, and a simple weighted error measure is minimized. Generally-contracted Gaussian auxiliary basis sets are optimized to match the wavefunction basis sets [D. N. Laikov, Theor. Chem. Acc. 138, 40 (2019)] for all 102 elements in a scalar-relativistic approximation [D. N. Laikov, J. Chem. Phys. 150, 061102 (2019)].

Atomic effective one-electron potentials in a compact analytic form in terms of a few Gaussian charge distributions are developed, for Hydrogen through Nobelium, for starting molecular electronic structure calculations by a simple diagonalization. For each element, all terms but one are optimized in an isolated-atom Hartree--Fock calculation, and the last one is parametrized on a set of molecules. This one-parameter-per-atom model gives a good starting guess for typical molecules and may be of interest even on its own., Article and Supplementary Material

An approximate relativistic two-component Hamiltonian for use in molecular electronic structure calculations is derived in the form of a sum of fixed atom-centered kinetic and spin-orbit operators added to the non-relativistic Hamiltonian. Starting from the well-known zeroth-order regular approximation, further steps are taken to get rid of its nonlinearity in the potential, ending up with a simple formulation with easily computable integrals that can seamlessly work with any traditional electronic structure method. Molecular tests show a good accuracy of this approximation., To be published in a Journal

An explicit second-order numerical method to integrate the isokinetic equations of motion is derived by fitting circular arcs through every three consecutive points of the discretized trajectory, so that the tangent and the curvature satisfy the equations exactly at every central point. This scheme is reversible and robust, and allows an adaptive step size control. Its performance is tested by computing the thermodynamic properties of simple pair-potential models, and its chemical application is shown for the global search for stable structures, using canonical sampling and energy minimization, of hydrogen-bonded molecular clusters., The version accepted by Theoretical Chemistry Accounts, with comments

Electronic structure methods for accurate calculation of molecular properties have a high cost that grows steeply with the problem size, therefore, it is helpful to have the underlying atomic basis functions that are less in number but of higher quality. Following our earlier work [Chem. Phys. Lett. 416, 116 (2005)] where general correlation-consistent basis sets are defined, for any atom, as solutions of purely atomic functional minimization problems, and which are shown to work well for chemical bonding in molecules, we take a further step here and define a new kind of atomic polarization functionals, the minimization of which yields additional sets of diffuse functions that help to calculate better molecular electron affinities, polarizabilities, and intermolecular dispersion interactions. Analytical representations by generally-contracted Gaussian functions of up to microhartree numerical accuracy grades are developed for atoms Hydrogen through Nobelium within the four-component Dirac-Coulomb theory and its scalar-relativistic approximation, and also for Hydrogen through Krypton in the two-component nonrelativistic case. The convergence of correlation energy with the basis set size is studied, and complete-basis-set extrapolation formulas are developed., Comment: Sent to a journal

New organic molecules are designed, with synthesis in mind, to fit the shape of the thin and thick rhombs of the Penrose tiling and self-assemble by hydrogen bonding into nearly flat sheets (that may stack face-to-face) following the matching rules.

Density functionals with a range-separated treatment of the exchange energy are known to improve upon their semilocal forerunners and fixed-fraction hybrids. The conversion of a given semilocal functional into its short-range analog is not straightforward, however, and not even unique, because the latter has a higher information content that has to be recovered in some way. Simple models of the spherically averaged exchange hole as an interpolation between the uniform electron gas limit and a few-term Hermite function are developed here for use with generalized-gradient approximations, so that the energy density of the error-function-weighted Coulomb interaction is given by explicit closed-form expressions in terms of elementary and error functions. For comparison, some new nonoscillatory models in the spirit of earlier works are also built and studied; the energy densities from both kinds of models match rather closely (within less than 5%), but the latter slightly mismatches (by about 1%) the exact uniform electron gas limit.

Structure of radical anions of “bridged” amidoesters MeOOC(CH2)nCONMe2 (n=0–3) generated by X-ray irradiation was investigated by EPR, optical absorption spectroscopy and quantum-chemical calculations. Amidoester-0 (n=0) has positive calculated electron affinity and the corresponding delocalized radical anion is stabilized independent of matrix polarity. Meanwhile, for amidoesters with n=1–3 groups possessing apparently negative intrinsic electron affinity the role of matrix in stabilization of the corresponding radical anions becomes crucial. Two types of the localized radical anions were observed in the latter case.

The problem of finding an effective minimal atomic basis that spans the exact occupied wavefunctions of a mean-field theory at a given molecular geometry, which has a number of special properties, is studied and a new general procedure is developed that (1) solves for a raw minimal set of strongly atom-centered functions—products of spherical harmonics and molecule-optimized radial parts—that approximately span the occupied molecular wavefunctions and minimize the sum of their energies, (2) uses projection operators to get a new set of deformed atom-centered functions that exactly span the occupied space and fall into core and valence subsets, (3) applies a new zero-bond-dipole orthogonalization scheme to the core-orthogonalized valence subset so that for each two-center product of these functions the projection of its dipole moment along the line going through the two centers is zero. The resulting effective minimal atomic basis is intrinsic to the molecular problem and does not need a free-atoms input. Some interesting features of the zero-bond-dipole orthogonalization are showing up in the atomic population analysis of a diverse set of molecules. The new procedure may be useful for the interpretation of electronic structure, for the construction of model Hamiltonians in terms of transferable molecular integrals, and for the definition of active valence space in the treatment of electron correlation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

A new electronic structure model is developed in which the energies of both the ground and singly-excited states are eigenvalues of a dressed symmetric configuration interaction (CI) matrix in the space of the reference and singly-excited determinants. The effects of double and triple substitutions are approximately included into the CI matrix elements in the spirit of quasidegenerate second-order perturbation theory. The model correctly describes conical intersections between the ground and singly-excited states and appears to be the simplest single-reference correlated treatment for this class of problems. Test calculations on organic molecules are presented.

A new general approach is developed for obtaining systematic sequences of atomic single-particle basis sets for use in correlated electronic structure calculations of molecules. All the constituent functions are defined as the solutions of variational problems and are of three types: a minimal Hartree–Fock set, additional functions to represent low-lying excited configurations, and general functions for describing electron correlation. The latter are determined to minimize a functional derived from the closed-shell second-order correlation energy expression. Generally-contracted Gaussian expansions are developed to approximate these general functions in the non-relativistic case and within a scalar-relativistic approximation.

The 2-exo-substituted fenchol derivatives 1–7, easily prepared from (−)-fenchone in good-to-excellent yields, were pyrolyzed by dynamic gas-phase thermo-isomerization (DGPTI). At temperatures of ca. 620°, the substrates with a hydroxyallyl (1–4) or a hydroxypropargyl moiety (6) underwent an initial retro-ene reaction under cleavage of the C(2)C(3) bond to form enol-ene intermediates with no loss of optical activity. These intermediates then experience either tautomerization to the corresponding α,β-unsaturated ketones or subsequent Conia rearrangement under one-carbon ring expansion of the fenchone system to a bicyclo[3.2.1]octane framework. In the case of the isopropenyl substrate 3, the sterically crowded Conia product underwent a new type of ‘deethanation’ reaction by stepwise loss of two Me radicals, giving rise to the thermodynamically favored enone 21. A similar relaxation behavior was observed in the case of the ethynyl substrate 6, which showed a remarkable 1,3-Me shift after the Conia reaction, leading to the α,β-unsaturated cyclic ketone 25. The homolytic cleavage of the weakest single bond in 1–3 turned out to be a competing reaction pathway. Intramolecular H-abstraction within the generated diradical intermediates produced the monocyclic ketones 8, 16, and 19, besides the products obtained by tautomerization and Conia reaction. In contrast, a Ph substituent at C(2) in 7 allowed only the passage through a diradical species to provide phenone 26, which was converted by regioselective Baeyer–Villiger oxidation to the optically active cyclopentanol 29. Both reaction channels, the domino retro-ene–Conia rearrangement and the diradical-promoted H-transfer, have been shown to proceed highly stereoselectively. The absolute configuration of the newly formed stereogenic centers in all compounds was assigned by 1H-NOE experiments. The reaction mechanism of the novel domino retro-ene–Conia reaction was established by both a series of 2H- and 13C-labeling experiments, as well as by a detailed computational analysis.

A new method for quantification of the relative distribution of deuterium in molecules is proposed. The technique is based on the lineshape analysis in the 2H NMR spectra obtained at the natural abundance level of deuterium with allowance for inhomogeneity of the magnetic field. The equilibrium thermodynamic H/D isotope effects for hindered rotation about the C—N bond in the N,N-dimethylformamide molecule and for prototropic exchange in the cyclopentadiene molecule were determined. The results obtained agree with those of DFT calculations of the vibrational energies.

New [RhI(η5-azulene)(η4-diene)][BF4] complex salts 3–5 (diene=8,9,10-trinorborna-2,5-diene (nbd) and (1Z,5Z)-cycloocta-1,5-diene (cod)) were synthesized according to a known procedure (Scheme 1). All of these complexes show dynamic behavior of the diene ligand at room temperature. In the case of the [RhI(η5-azulene)(cod)]+ complex salts 3 and [RhI(η5-guaiazulene)(nbd)]+ complex salt 4a (guaiazulene=7-isopropyl-1,4-dimethylazulene), the coalescence temperature of the 1H-NMR signals of the olefinic H-atoms was determined. The free energy of activation (ΔG; Table 1) for the intramolecular movement of the diene ligands exhibits a distinct dependency on the HOMO/LUMO properties of the coordinated azulene ligand. The DFT (density-functional theory) calculated ΔG values for the internal diene rotation are in good to excellent agreement with the observed ones in CD2Cl2 as solvent (Table 2). Moreover, the ΔG values can also be estimated in good approximation from the position of the longest-wavelength, azulene-centered UV/VIS absorption band of the complex salts (Table 2). These cationic RhI complexes are stable and air-resistant and can be used, e.g., as precursor complexes in situ in the presence of (M)-6,7-bis[(diphenylphosphino)methyl]-8,12-diphenylbenzo[a]heptalene for asymmetric hydrogenation of (Z)-α-(acetamido)cinnamic acid with ee values of up to 68% (Table 4).

Model reactions of silica-supported zirconium hydrides (≡Si—O—)3ZrH and (≡Si—O—)2ZrH2 with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (≡Si—O—)3ZrCH3 and (≡Si—O—)2Zr(H)CH3 as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (≡Si—O—)2ZrH2 are more reactive toward the methane C—H bonds than zirconium monohydrides (≡Si—O—)3ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (≡Si—O—)2ZrH2 are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.

A DFT study of a model reaction [(η5-C5H5)2MCH3]+ + RH ⇌ [(η5-C5H5)2MR]+ + CH4 (M = TiIV, ZrIV; R = Me, Et, Pr, Pri) was carried out with the PBE density functional. Exchange of σ-bonded ligand proceeds through the formation of agostic complexes [Cp2M(RH)CH3]+ followed by their isomerization into complexes [Cp2M(CH4)R]+via an inner-sphere migration of a hydrogen atom. The calculated rate constants for such migrations involving the primary and secondary C--H bonds of propane molecule differ by 930 times for TiIV complexes and by 47 times for ZrIV complexes, which is due to the effect of steric factors.

The structures of silicon-containing organophosphorus betaines –S—SiR1 2—CR2 2—P+R3 3 and their ylide isomers were calculated using the density functional approach with the gradient-corrected PBE functional and extended TZ2P basis set. Three possible pathways of thermal decomposition of these betaines were analyzed. These are (i) cleavage of the central C—Si bond with the formation of a Wittig ylide and silanethione, (ii) intramolecular nucleophilic S N-substitution with elimination of phosphine PR3 3 and the formation of silathiirane (the Corey—Chaikovscky transformation), and (iii) a Wittig-type decomposition followed by the formation of substituted silaethylene. The structures of products and transition states of these reactions were calculated. The cis-gauche conformation of the –S—Si—C—P+ fragment of betaines was found to be the most stable. This is in agreement with the results of X-ray diffraction study and can be rationalized by strong Coulomb attraction between the cationic and anionic centers. The betaines are stable toward retro-Wittig thermal decomposition. The Corey—Chaikovscky formation of thiirane is preferable under conditions of thermal decomposition. Retro-Wittig-type decomposition of betaines followed by the formation of silanethione is favored by intra- and intermolecular coordination of donor ligands.

Polymerization of ethylene on zirconocene catalysts was studied with the density functional theory. The approach proposed previously for the model system Cp2ZrEt++C2H4 was extended to substituted zirconocene catalysts (RCp)2ZrEt+, which allowed us to compare the results of calculations with the experimentally found rate constants of chain propagation. The ethyl fragment was demonstrated to be an adequate model for a growing polymer chain. The order of decrease in the activation energy of chain propagation calculated for compounds with R=H, Me, Prn, and Bun is in qualitative agreement with the experimental data. However, the quantitative description gave underestimated theoretical rate constants of chain propagation compared to the experimental values. In the case of polyalkyl-substituted zirconocenes (R=1.2-Me2 or Me4), no correlation between the calculated and experimental characteristics was observed. The results were explained using the simplest model reaction of the replacement of the MeMAO− counterion by an ethylene molecule accompanied by displacement of the former to the outer coordination sphere of the Zr atom. This step was demonstrated to control the kinetics of the process. It was concluded that the isolated-cation model used in early investigations is not adequate and calls for modification with regard to the effect of the counterion.

A DFT study of ethylene polymerization by zirconocene catalysts was carried out. Stationary points corresponding to intermediates and transition states were located on the potential energy surface of the [Cp2ZrC2H5]++C2H4 model system. Three possible reaction mechanisms involving the formation of β-agostic complexes were considered. The energy and thermodynamic characteristics for different reaction pathways were calculated. Corresponding activation energies lie in the range 3.9–6.8 kcal mol−1.

The strongest vibrational feature of a radical anion produced from the diacetyl molecule under radiolysis in a low-temperature dimethyl ether matrix has been characterized for the first time by the analysis of IR- and EPR-spectroscopic data supported by quantum- chemical calculations.

A new electronic structure model is developed in which the ground state energy of a molecular system is given by a Hartree-Fock-like expression with parametrized one- and two-electron integrals over an extended (minimal + polarization) set of orthogonalized atom-centered basis functions, the variational equations being solved formally within the minimal basis but the effect of polarization functions being included in the spirit of second-order perturbation theory. It is designed to yield good dipole polarizabilities and improved intermolecular potentials with dispersion terms. The molecular integrals include up to three-center one-electron and two-center two-electron terms, all in simple analytical forms. A method to extract the effective one-electron Hamiltonian of nonlocal-exchange Kohn-Sham theory from the coupled-cluster one-electron density matrix is designed and used to get its matrix representation in a molecule-intrinsic minimal basis as an input to the paramtrization procedure -- making a direct link to the correlated wavefunction theory. The model has been trained for 15 elements (H, Li--F, Na--Cl, 720 parameters) on a set of 5581 molecules (including ions, transition states, and weakly-bound complexes) whose first- and second-order properties were computed by the coupled-cluster theory as a reference, and a good agreement is seen. The model looks promising for the study of large molecular systems, it is believed to be an important step forward from the traditional semiempirical models towards higher accuracy at nearly as low a computational cost., Revision 1 (with Comments) resubmitted 2011.08.28 to J Chem Phys

The radical anion resulting from electron capture by diacetonyl molecule has been characterized by EPR and optical absorption spectroscopy in glassy ether matrices at 77 K. In non-polar alkane glasses this species was not observed under the same conditions, which confirms the crucial role of matrix interactions in stabilizing this species. Calculations at the MP2 level show the vertical detachment energy to increase gradually from roughly zero for a bare anion to ∼1 eV for the complex involving six ether molecules.

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric C=C bonds of C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.

A new integral approximation for use in molecular electronic structure calculations is proposed as an alternative to the traditional neglect of diatomic differential overlap models. The similarity between the symmetrically orthogonalized and the original basis functions (assumed orthonormal within each atomic set but nonorthogonal between different centers) is used to construct a robust approximation for the two-electron integrals, with the error being quadratic in the deviation between the products of the functions. Invariance proper-ties of this procedure are rigorously proved. Numerical studies on a representative set of molecules at valence-only minimal basis Hartree-Fock level show that the approximation introduces relatively small errors, encouraging its future application in the semiempirical field.

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

Singlet excited state geometries of a set of medium sized molecules with different characteristic lowest excitations are studied. Geometry optimizations of excited states are performed with two closely related restricted open-shell Kohn–Sham methods and within linear response to time-dependent density functional theory. The results are compared to wave-function based methods. Excitation energies (vertical and adiabatic) calculated from the open-shell methods show systematic errors depending on the type of excitation. However, for all states accessible by the restricted methods a good agreement for the geometries with time-dependent density functional theory and wave-function based methods is found. An analysis of the energy with respect to the mixing angle for the singly occupied orbitals reveals that some states (mostly [ n → π ∗ ] ) are stable when symmetry constraints are relaxed and others (mostly [ π → π ∗ ] ) are instable. This has major implications on the applicability of the restricted open-shell methods in molecular dynamics simulations.