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. There are no theoretical or experimental reports from class B compounds except for a brief study in our previous works (Struct. Chem. 2012, 23, 551-580; J. Phys. Chem. C 2010, 114, 15315.). In this study, we have used two kinds of boron nitride rings from a twisted BN sheet in the same chirality created by different mechanisms. For (4, 4) chirality, the molecules B(16)N(16) and B(15)N(15) are found to respectively represent class A and B, and for (5, 5) chirality the molecules B(20)N(20) and B(18)N(18) are respectively again of class A and B. The structure of class A rings is similar to boron nitride tubes, but we have shown that it is impossible to produce a macromolecule of class B form as tubes or cages, because there is much more instability and intermolecular tension in macro forms of class B. This is the main reason that the class B molecules are rare and, because of their small size, have not yet been synthesized, although we have some suggestions for the synthesis of these kinds of molecules. The stability and electromagnetic properties with hybrid density functional theory using the EPR-III and EPR-II basis sets for explanation of hyperfine parameters and spin densities, electrical potential, and isotropic Fermi coupling constant of these rings have been studied by the nonbonded interaction models. Normal mode analyses including aromaticity have been investigated by using the nucleus independent chemical shift values at the ring center. Interaction energy and gain in energy aid in describing the stability that is promoted upon gradual binding with molecular hydrogen, and a linear relationship occurred between them.

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. The distortions are due to the mixing of the ground A(g) and excited B(2g) states [i.e., HOMO(B(3u)) → LUMO + 3(B(1u)) for compound 1, HOMO(B(3u)) → LUMO + 2(B(1u)) for compound 2, and HOMO(B(3u)) → LUMO + 1(B(1u)) for compounds 3 and 4]. Importantly, the higher-lying B(1g), B(2u), and B(2g) states are not involved in the PJT interactions. The energy gaps between reference states (Δ) in the undistorted configurations decrease from compound 1 to compound 4, and the PJT stabilization energies increase. Therefore, the primary force constant of the ground state in the Q((b2g)) direction (K(0)) decreases from compound 1 to compound 4. This fact can be justified by the valence isoelectronic systems of these compounds (having similar vibronic coupling constants, F). For the purpose of more chemical transparency, the NBO results were analyzed, and their relation to the PJT interactions has been revealed. The NBO analysis showed that stabilization energy associated with π(M-H) (b(u)) → σ*(M═M) (b(u)) electron delocalization (i.e., the mixing of the distorted b(u) molecular orbitals along the b(2g) bending distortions) increases from compound 1 to compound 4. Also, by using the hybridized orbitals obtained, an n parameter is defined. The NBO results revealed that the n values in the mean hybrid orbitals (sp(n)) increase from compound 1 to 4. The correlations between the PJT stabilization energies, bond orders, n values, π(M-M) → σ*(M═M) electron delocalizations, and structural parameters of compounds 1-4 have been investigated.

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. For C(4)F(4), ab initio calculations of the energy profiles allowed us to evaluate the PJTE constants and to show that the two instabilities, square-to-tetragonal b(2g) and puckering e(g) coexist, thus explaining the origin of the observed geometry of this system in the ground state. The preferred cis-trans (e(g) type) puckering in C(4)F(4) versus trans-trans puckering (b(2u) distortion) in Si(4)H(4) follows from the differences in the energy gaps to their excited electronic E(g) and A(1u) terms causing different PJTE in these two cases.

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)) = 137.8(11)° or 167.4(19)°), differing in the trans (AT) or gauche (AG) orientation of the difluoromethyl group. The conformational abundances in the jet were estimated from relative intensity measurements as (AT)/(AG) ≈ 3:1. The structural preferences of the molecule have been rationalized with supporting ab initio calculations and natural-bond-orbital (NBO) analysis, which suggest that the molecule is stabilized by hyperconjugative effects. The NBO analysis of donor-acceptor (LP → σ*) interactions showed that these stereoelectronic effects decrease from the AT to AG conformations, so the conformational preferences can be accounted for in terms of the generalized anomeric effect.

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. The realization of these conformations depends on the positions of the excited E-states and the PJTE parameter values. The two emerging PJTE problems, ((3)A(2) + (3)E(1) + (3)E(2)) ⊗ (e(i) + e(o)) and ((1)A(1) + (1)B(1) + (1)B(2) + (1)E(1) + (1)E(2)) ⊗ (b(1) + e(i) + e(o)), are formulated in the matrix form and provide a general picture of the ground and excited adiabatic potential energy surfaces. Following this scheme in combination with ab initio calculations, the possible conformations and conformational transitions are analyzed for several specific systems including (in increasing complexity) N(2)H(2), C(2)H(4), N(2)(NH(2))(2) and N(2)(C(6)H(5))(2) (azobenzene). The family of molecular systems with a double bond is vast, but the importance of the PJT approach developed here is also in its general validity as it can be applied to any other systems.

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. The C conformations of compounds 1-3 are less stable than their S and T conformations. Based on these results, the racemization processes of the axial symmetrical (C(2) symmetry) conformations of compounds 1-3 take place via their T conformations. Based on the optimized ground state geometries using the MP2/6-311+G** level of theory, the NBO analysis of donor-acceptor (bond-antibond) interactions revealed that the stabilization (resonance) energy associated with LP(2)M2-->sigma*(M3-H4) electronic delocalization for the S conformations of compounds 1-3 are 1.35, 5.94 and 4.68 kcal mol(-1), respectively. There is excellent agreement between the variations of the calculated DeltaG(T-S) and stabilization (resonance) energies associated with LP(2)M2-->sigma*(M3-H4) electronic delocalization for the S conformations of compounds 1-3. The correlations between resonance energies, orbital integrals, dipole moments, bond orders, structural parameters and conformational behaviors of compounds 1-3 have been investigated. Test were made of complete basis set methods (CBS-QB3, CBS-4 and CBS-Q), the first two gave results essentially indistinguishable from those we used, but the CBS-Q results were in disagreement with experimental and other theoretical results.

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 1.87 A), large He-Pt stretching frequencies (500 approximately 600 cm(-1)), considerable binding energies (1400 approximately 2500 cm(-1) with corrections), and a small electron transfer from helium (about 0.06). However, the topological analysis of the electron density distribution indicates that there is strong van der Waals interaction in the He-Pt bond instead of weak covalent one.

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. Moreover, the bonding analysis clearly reveals considerable charge and bond order on the helium atom and bond order for He-Mo. The dissociation of He@Mo(6)Cl(8)F(6) to He + Mo(6)Cl(8)F(6) is prohibited by a barrier of 0.86 eV (19.8 kcal/mol), indicating that the cluster is chemically stable. However, no covalent He-Mo bonding was found so it is an analogue of He@adam. Comparison was also made with the isoelectronic system of [Mo(6)Cl(8)F(6)](2-).

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. We found that the mixing of the reference states contributes significantly to the dynamical correlation energy, which strongly affects the geometry of the ground state. Our results show that BNB in its ground state has a linear noncentrosymmetric structure with two equivalent global minima of the adiabatic potential energy surface and, respectively, two oppositely directed dipole moments of about 2 D. The barrier between the minima is about 20 cm(-1). The origin of the double-minimum potential in the ground state of BNB is explained as due to the pseudo-Jahn-Teller effect involving vibronic interaction with the first excited state A (2)Sigma(g)(+) via the asymmetric stretching vibrations.

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. By restricting the active space and selecting dominant configurations, the results of the general multireference second-order perturbation theory with this large active space agree very well with the experimental ones. It is found that the so-called (2)Pi(u) state is asymmetric linear and the (2)Sigma(u)(+) state is bent at the minima on their adiabatic potential energy surfaces, whereas the other five gerade states are centrosymmetric linear. After including the spin-orbit coupling, the (I)(2)Pi(g3/2)-(I)(2)Pi(g1/2) splitting is computed to be 415 cm(-1), in excellent agreement with the experimental value of about 480 cm(-1).

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. Several high-level ab initio calculations including all of the five states confirm the previously found coexistence of a central minimum of D3h symmetry and three equivalent minima with a distorted geometry of C2v symmetry in the ground state; the barrier between them is rather small, 0.2-0.3 eV, but large enough to distinguish them spectroscopically. Harmonic vibrational frequencies of the two configurations near the minima of the adiabatic potential energy surface are also evaluated. The calculations show how the distorted configurations are produced by the JT effect in one of the excited E states, similar to a previous finding in O3. Numerical data of ab initio calculations yield also the effective vibronic and primary force constants for all of the terms. An electronic structure problem of this complexity including a reduction of the two-mode problem to one mode with full interpretation of the origin of coexisting different geometries as due to the JTE in the excited state is presented here for the first time.

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. Further refinement, including consideration of interaction between the two excited (2)Pi states, yields a somewhat higher barrier between 100 and 500 cm(-1). The mechanism of formation of the double-minimum potential is explained by the pseudo-Jahn-Teller effect theory. Computed spectroscopic constants are in good agreement with experimental ones.

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. The quality of electron correlation treatment using conventional single reference coupled cluster methods CCSD and CCSD(T) is compared to that attained at the multiconfigurational quasidegenerate second-order perturbation theory (CASSCF+MCQDPT2) and the electron attachment equation-of-motion coupled cluster (EOMEA-CCSD) levels. Spin-orbit coupling effects are studied by the SO-MCQDPT2 method using the full Breit-Pauli spin-orbit operator. Effects of spin contamination in the coupled cluster molecular calculations are carefully analyzed. Results of the single reference CCSD(T) and multireference calculations are found to be in a remarkable agreement. The calculations indicate that the EOMEA-CC approach provides a suitable tool for an accurate treatment of FeF(2) and other systems where delicate electron correlation effects have to be carefully dealt with. The inclusion of relativistic effects is shown to be necessary for an accurate description of the molecular geometry and excitation energies of FeF(2). The results of calculations are in good agreement with the experimental data available. The predicted FeF(2) molecular properties are compared to those of the related FeF(3).

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. The global ground state is 1A1 (or A1 in the spin-orbit coupling case), whereas the lowest linear state is 3Deltag (or 3g). Some other cheaper single-configurational and multi-configurational methods were also used to study both states, and their shortcomings are discussed. Our theoretical results suggest that the arrangement of the experimental frequencies of NiH2 and NiD2 may be incorrect.

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. The quality of the derived Pha and the corresponding quantitative model of activity has been validated (and deemed acceptable) by cross-validation including many-fold cross-validations within the training set and against an independent test set of 34 additional analogs with known experimental activities out of the training set. At last, several dozen compounds never tested experimentally have been screened theoretically using this model, and statistically significant hypoglycemic activities for a few of them are predicted.

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. Spectrosc. 1995, 172, 352] and with the help of the geometries of the stationary points calculated ab initio. The torsional splitting was also calculated using the results of the calculations for the ground vibrational state, for the excited OH torsional states 91 and 92, and for the excited BOH bending state 41, and a satisfactory agreement with available experimental data was found.

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. We show that the model approaches to the problem, mentioned above, are either restricted particular cases of the pseudo Jahn-Teller interaction, or they yield very small contributions to the instability that do not explain the origin of the bending. All of our conclusions are supported by high-quality ab initio calculations.

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. The spin-orbit coupling in X 2E CF3O and CF3S was parametrized by means of a perturbative solution of the full Breit-Pauli spin-orbit operator. Spin-vibronic eigenvalues and eigenfunctions were computed in a basis set of products of electronic, electron spin, and vibrational functions. Results demonstrate the importance of explicit inclusion of the spin-orbit coupling and at least cubic Jahn-Teller terms in the model Hamiltonian for the high precision evaluation of spin-vibronic energy levels of CF3O and CF3S. The theoretical results support and complement the spectroscopic data observed for these species.

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. In contrast to previous approaches, the authors treat the nonadiabatic dynamics on the potential energy surfaces, which allows for a vastly improved reproduction of the experimental level structure and a corrected assignment for band A.