A new generation of B(n)N(n) rings as a supplement to boron nitride tubes and cages.

In B(n)N(n) cages or tubes, when the quasi-borazine rings are attached to each other through a pair of common atoms of B and N, the bonding structure is named class A. On the other hand, there are some B(n)N(n) rings including a covalent bond between two atoms of B and N, which are named class B. In all previous studies, both reports of synthesis and theoretical calculation of boron nitride tubes and cages, the quasi-borazine units are attached together like class A.

Pseudo Jahn-Teller effect and natural bond orbital analysis of structural properties of tetrahydridodimetallenes M2H4, (M = Si, Ge, and Sn).

The structural properties of ethene (1) and tetrahydridodimetallenes M(2)H(4) [M = Si (2), Ge (3), and Sn (4)] have been examined by means of CCSD(T)/Def2-TZVPP, MP4(SDTQ)/Def2-TZVPP, and B3LYP/Def2-TZVPP levels of theory and natural bond orbital analysis (NBO) interpretations. The results obtained showed the expected planar ground state structure for compound 1 (D(2h) symmetry) but trans-bent ground state structures for compounds 2-4 (C(2h) symmetry). The distortions of the high-symmetry configurations of compounds 2-4 are due to the pseudo Jahn-Teller effect (PJTE), which is the only source of instability of high-symmetry configurations in nondegenerate states.

On the covalent character of rare gas bonding interactions: a new kind of weak interaction.

At the averaged quadratic coupled-cluster (AQCC) level, a number of selected rare gas (Rg) containing systems have been studied using the quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and several other analysis methods. According to the criteria for a covalent bond, most of the Rg-M (Rg = He, Ne, Ar, Kr, Xe; M = Be, Cu, Ag, Au, Pt) bonds in this study are assigned to weak interactions instead of van der Walls or covalent ones. Our results indicate that the rare gas bond is a new kind of weak interaction, like the hydrogen bond for example.

Pseudo Jahn-Teller origin of nonplanarity and rectangular-ring structure of tetrafluorocyclobutadiene.

It is shown that the pseudo Jahn-Teller effect (PJTE) in combination with ab initio calculations explains the origin of instability of the planar configuration of tetrafluorocyclobutadiene, C(4)F(4), with respect to a puckered structure and square-to-rectangle distortion of the carbon ring, and rationalizes its difference from the planar-rectangular geometry of C(4)H(4) and nonplanar (puckered) structure of Si(4)H(4). The two types of instability and distortion of the high-symmetry D(4h) configuration in these systems emerge from the PJT coupling of the ground B(2g) state with the excited A(1g) term producing instability along the b(2g) coordinate (elongation of the carbon or silicon square ring), and with the excited E(g) term resulting in e(g) (puckering) distortion. A rhombic distortion b(1g) of the ring is also possible due to the coupling between excited A(1g) and B(1g) terms.

Structural evidence of anomeric effects in the anesthetic isoflurane.

The conformational and structural properties of the inhalational anesthetic isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. Two conformers of the isolated molecule were identified from the rotational spectrum of the parent and several (37)Cl and (13)C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anti carbon skeleton (τ(C(1)-C(2)-O-C(3)) = 137.

Pseudo Jahn-Teller origin of cis-trans and other conformational changes. The role of double bonds.

Based on the pseudo Jahn-Teller effect (PJTE) theory, an approach is developed to rationalize and predict the conformations and conformational changes in molecular systems with a common pattern, a double bond. It is shown that starting with the high-symmetry geometry of the environment (in many cases D(2d)), the double bond descends from an e(2) electronic configuration (e is a twofold degenerate MO) which produces a variety of PJT distortions, the main of which is the rotational (b(1)) transformation D(2d) → D(2h) accompanied by the formation of the double bond. Further PJT interactions with higher energy E-states may trigger additional distortions which in D(2h) symmetry are classified as in-plane (e(i)) cis and trans, and out-of-plane (e(o)) chair and boat.

Stereoelectronic interaction effects on the conformational properties of hydrogen peroxide and its analogues containing S and Se atoms: an ab initio, hybrid-DFT study and NBO analysis.

Ab initio molecular orbital (MP2/6-311+G**//MP2/6-31G+G**) and hybrid-density functional theory (B3LYP/6-311+G**//MP2/6-311+G**) methods and NBO analysis were used to study the stereoelectronic interaction effects on the conformational properties of hydrogen peroxide (1), hydrogen disulfide (2) and hydrogen diselenide (3). The results showed that the Gibbs free energy difference (G(T)-G(S)) values at 298.15K and 1atm between the skew (S) and trans (T) conformations (DeltaG(T-S)) increase from compound 1 to compound 2 but decrease from compound 2 to compound 3.

Relativistic ab initio study on PtF and HePtF.

The electronic structures and spectroscopic constants of the first three low-lying electronic states (Omega = 1/2, 3/2, and 5/2) of the linear HePtF complex were investigated by highly accurate relativistic ab initio methods, in which the spin-orbit coupling was taken into account, and compared with the results of PtF. It shows that the complex is significantly different from the typical van der Waals systems because of short He-Pt bond distances (1.80 approximately 1.

He@Mo(6)Cl(8)F(6): a stable complex of helium.

The electronic structure and chemical stability of the endo helium cluster, He@Mo(6)Cl(8)F(6), were investigated carefully by using density function theory. The results show that the cluster is significantly different from typical van der Waals systems: the bond distance between helium and molybdenum is only about 1.89 A.

Combined Jahn-Teller and Pseudo-Jahn-Teller Effect in the CO3 Molecule: A Seven-State Six-Mode Problem.

A complicated problem of seven electronic states in four terms, (1)A1', (1)E'', 1(1)E', and 2(1)E', interacting with six vibrational modes, a1', a2'', e', and e', was solved to take into account the combined two-mode Jahn-Teller (JT) plus two-mode pseudo JT effects and rationalize the electronic structure of the CO3 molecule. The JT and first-order pseudo JT effects in the E'' state are separated from the rest of the problem by symmetry; they do not influence the ground state properties. In the remaining five-state five-mode problem including the ground state, (A1' + 1E' + 2E') ⊗ (a1' + e' + e'), the JT two-mode problem is reduced to the one-mode one by means of coordinate transformations.

Symmetry breaking in the ground state of BNB: a high level multireference study.

A series of multireference approaches based on the SA-CASSCF wave function, i.e., CASPT2, MRCI, MRCI + Q, and MRAQCC with single- or multireference states, have been employed to investigate the symmetry breaking effect in the ground state X (2)Sigma(u)(+) of the triatomic BNB radical.

Theoretical study of the electronic states of CuCl2.

The electronic states of the CuCl(2) molecule are studied by several theoretical methods. We report geometries, excitation energies, vibrational frequencies, rotational constants, and transition dipole moments. With the purpose to describe the correlation energy accurately enough, a set of diffuse secondary 3d(') orbitals is introduced, thus resulting in a large active space of 21 electrons in 17 orbitals.

Do non-centro-symmetric linear X-Y-X molecules exist? The case for the (I)2Pi(u) state of CuCl2.

The potential energy surface of the low-lying excited state (I)(2)Pi(u) of CuCl(2) is constructed by using the ionization potential equation-of-motion coupled-cluster method and also the RASPT2 method with a large active space of 21 electrons in 17 orbitals to improve the results. It is found by the multiconfiguration calculation that this state has a barrier of 53 cm(-1) between two equivalent minima in which the linear molecule has a dipole moment. In our computations artifactual symmetry breaking is carefully avoided.

Comparative ab initio studies on the molecular structure and spectroscopic properties of FeF2: Single reference versus multireference methods.

The electronic excitation energies, molecular geometry, quadratic force fields, and vibrational frequencies in the ground (5)Delta(g) and low-lying excited (5)Sigma(g) (+) and (5)Pi(g) electronic states of iron difluoride are studied at sophisticated levels of theory. Two families of basis sets, nonrelativistic and Douglas-Kroll-Hess relativistic, are used that range in quality from triple-zeta to quintuple-zeta. These are augmented by additional diffuse functions (on fluorine atoms) and tight functions (on all atoms) for the description of core-valence correlation and utilized to determine complete basis set molecular properties.

Theoretical study on low-lying electronic states of NiH2.

The low-lying electronic states of the NiH2 molecule were investigated by using the MCQDPT2 method. In order to accurately describe the strong correlation derived from the nickel 3d9 super-configuration, a set of diffuse secondary 3d' orbitals were included in the active space, yielding a large active space of 12 electrons in 13 orbitals. It is shown that the absolute minimum energy configuration of NiH2 is bent, in agreement with the experimental observation.

Quantitative antidiabetic activity prediction for the class of guanidino- and aminoguanidinopropionic acid analogs based on electron-conformational studies.

The electron-conformational method has been employed to reveal the pharmacophore (Pha) and to predict antidiabetic activity, studying 154 compounds in the class of guanidino- and aminoguanidinopropionic acid analogs. The derived Pha consists of four sites with certain electronic and topological characteristics which are represented by two oxygen atoms of the carboxyl group and two nitrogens of the guanidine group but may be substituted with any other atoms that have the same electronic and geometric features. The Pha flexibility and the influence of out-of-Pha features are described by only three model descriptors that predict the experimental activities quantitatively within experimental uncertainty for a training set of 120 compounds.

Ab initio and experimental studies of the large-amplitude motion in BF2OH.

The ground state rotational spectrum of BF2OH was measured under high resolution by microwave Fourier transform spectroscopy (FTMW), and the small torsional splitting could be resolved for several lines. This splitting was analyzed using a phenomenological model previously developed for HNO3 [Coudert and Perrin, J. Mol.

Why are some ML2 molecules (M = Ca, Sr, Ba; L = H, F, Cl, Br) bent while others are linear? Implications of the pseudo Jahn-Teller effect.

The unexpected bent geometries of some alkaline earth dihalides and dihydrides, ML(2) (M = Ca, Sr, Ba; L = H, F, Cl, Br) have been explained in the literature using various models that attribute the effect to different phenomena like covalency, metal core polarization, sd-hybridization, and electron pair repulsion. We employ (based on first principles) the pseudo Jahn-Teller effect, as the only source of instability of high-symmetry configurations in nondegenerate states, to analyze the origin of the geometry of these systems and show that this approach explains all of their main structural features, including the topology of the Laplacian of the electron density and the vibrational frequencies. The main contribution to the distortion of the linear configuration is due to the pseudo Jahn-Teller mixing by bending of the sigma(u) HOMO formed by the ligand orbitals with the unoccupied pig orbitals of the metal (with main d(xz) and d(yz) character), resulting in new covalency which stabilizes the bent configuration.

Molecular spectroscopy beyond the born-oppenheimer approximation: a computational study of the CF(3)O and CF(3)S radicals.

This paper addresses some advances in the theoretical description of molecular spectroscopy beyond the Born-Oppenheimer adiabatic approximation. A solution of the nuclear dynamics problem complicated by the EE Jahn-Teller effect and spin-orbit coupling is considered for the case of the CF3O and CF3S radicals, all the model parameters being obtained solely from ab initio calculations without any adjustment to experimental numbers. Vibrational and vibronic model parameters were calculated at the equation-of-motion coupled cluster level of theory with basis sets of triple-zeta quality.

Ab initio simulation of the vibrationally resolved photoelectron spectrum of Si3-.

Electron photodetachment spectra provide a wealth of information about the electronic and vibrational level structures of neutral molecules that form stable anions. Experiments carried out for the smallest polyatomic silicon cluster anion (Si3-+hupsilon-->Si3*+e-) show vibrational progressions in six observed electronic bands (X-E) of the neutral species. The authors have performed ab initio calculations using the MRCI+D/aug-cc-pVQZ level for the corresponding electronic states followed by variational calculations of the vibronic levels associated with these adiabatic potential energy surfaces.

Orbital disproportionation and spin crossover as a pseudo Jahn-Teller effect.

It is shown that in systems with electronic half-closed-shell configurations of degenerate orbitals, e(2) and t(3) (which have totally symmetric charge distribution), ground state distortions from high-symmetry geometries may occur due to a strong pseudo Jahn-Teller effect (PJTE) in the excited states, resulting also in a novel phenomenon of PJT-induced spin crossover. There is no JTE neither in the ground state term nor in the excited terms (including degenerate terms) of these configurations but a strong PJT mixing between two excited states [((1)E+(1)A) [cross-filled circle] e and ((2)T(1)+(2)T(2)) [cross-filled circle] e in the e(2) and t(3) cases, respectively] pushes down the lower term to cross the ground state of the undistorted system and to form the global minimum with a distorted geometry. The analysis of the electronic structure of this distorted configuration shows that it is accompanied by orbital disproportionation: instead of proportional population of all degenerate orbitals by one electron each (as in the ground state of the undistorted system that follows Hund's rule), two electrons with opposite spins occupy one orbital, resulting in transformations of the type (e(theta);e(epsilon))-->(e(theta)e(theta)) for e(2) and (t(x);t(y);t(z))-->(t(x);t(x);t(z)) for t(3) systems.

Extensive ab initio study of the valence and low-lying Rydberg states of BBr including spin-orbit coupling.

The electronic states of the BBr molecule, including 12 valence states and 12 low-lying Rydberg states, have been studied at the theoretical level of MR-CISD+Q with all-electron aug-cc-pVQZ basis sets and Douglas-Kroll [Ann. Phys. (N.

Lost topological (Berry) phase factor in electronic structure calculations. Example: the ozone molecule.

It is shown that standard computations of electronic structures of polyatomic systems that yield the global minimum configuration and vibrational frequencies may be faulty if the symmetry of this configuration is lower than the highest possible one and the origin of this distortion, which is always due to the Jahn-Teller effect, is neglected; this may lead, in particular, to the loss of the Berry phase factor that changes the vibronic energy level spectrum and which we show to be present even when there are no apparent conical intersections. The general case and the ozone molecule are analyzed.

Pseudo-Jahn-Teller origin of geometry and pseudorotations in second row tetra-atomic clusters X4 (X=Na, Mg, Al, Si, P, S).

Experimentally determined or ab initio calculated molecular geometries carry no information about their origin. Employing the Jahn-Teller (JT) vibronic coupling effects as the only source of instability and consequent distortions of high-symmetry molecular configurations, we have worked out a procedure that allows us to trace the origin of particular geometries and determine the detailed electronic mechanism of their formation. This procedure is illustrated by considering a series of X(4) clusters with X=Na, Mg, Al, Si, P, and S.

Extensive ab initio study of the electronic states of SCl including spin-orbit coupling.

The effect of different basis sets for calculation of the spectroscopic constants of the ground state of sulfur monochloride (SCl) was analyzed using scalar relativistic multireference configuration interaction with single and double excitations plus Davidson correction. Then the generally contracted all-electronic correlation-consistent polarized valence quintuple zeta basis sets were selected to compute the electronic states of SCl including 12 valence and 9 Rydberg lambda-S states. The spin-orbit coupling effect was calculated via the state interaction approach with the full Breit-Pauli Hamiltonian.