C2v Structure Research Articles

Structures and Rotational Barriers of a Diiodobinorsnoutane: Energetic Preference for Gauche Conformation.

The diiodobinorsnoutane, bi(5-iodopentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]non-4-yl) (5), exists in a sterically hindered gauche conformation rather than an anti or an averaged (freely rotating) C2v structure. Density functional theory (DFT) predictions place the gauche conformation 11 kcal/mol more stable than the anti conformation with a barrier of 17 kcal/mol connecting the minima. These are consistent with variable-temperature NMR (17.1 ± 0.8 kcal/mol) estimates and X-ray analysis. Predictions of the torsional profiles of the yet-unsynthesized bromo-, chloro-, and fluoro- analogues show a progressive lowering of the barriers.

The low-lying singlet electronic excited states of HfO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The vertical excitation energies for the low-lying singlet excited states of the HfO2 molecule were calculated at the SAC-CI/aug-cc-pVTZ-PP level, and were compared with those of ZrO2 and TiO2. The ground electronic state (X1A1) and ten selected singlet excited states (21A1, 31A1, 11A2, 21A2, 11B1, 21B1, 31B1, 11B2, 21B2, and 31B2 states) of HfO2 were optimized at the SAC-CI/def2-SVP and SAC-CI/def2-SVPD levels. Optimized C2v structures for the ten selected states and optimized Cs structures for the 11B1 (A″), and 21A1 (A′) states were obtained. The adiabatic excitation energies for the selected excited states were calculated at the SAC-CI/def2-TZVPPD//SAC-CI/def2-SVPD level. The unrestricted density functional theory (UBPW91) was also employed to study the X1A1, 11A2, 21A2, 11B1, 21B1, and 11B2 states for comparison.

Oxalate-templated synthesis of di-zinc macrocycles

Herein we report that oxalate serves as a template in the formation of di-zinc macrocycles featuring bis(imino)pyridine chelating units linked by p-xylylene or m-xylylenes linkers. The resulting complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography. NMR spectroscopy indicates highly symmetrical (C2v or D2h) structures of the macrocyclic complexes in solution. Solid-state structures, determined by X-ray crystallography (obtained only for the m-xylylene bridged macrocycles), revealed hexa-coordinate zinc centers with μ2-κ2,κ2-bound oxalate, meridional NNN chelates, and an additional ligand (DMF or H2O) bound to zinc centers. Cyclic voltammetry demonstrates series of reductions associated with the redox-active bis(imino)pyridine chelates; no oxidation was observed up to 1V. We have attempted to extract oxalate from the macrocycle using calcium bromide, ethylenediaminetetraacetic acid (EDTA), trimethyl silyl chloride (TMSCl), acetic acid, or hydrochloric acid (HCl). Of the above, EDTA, TMSCl, acetic acid, and HCl in ether failed to remove oxalate or to form a new isolable product, while calcium bromide transformed μ2-κ2,κ2-bound oxalate into the μ2-κ1,κ2-bound form. The addition of aqueous HCl enables observation of the oxalate-free complex by mass spectrometry as one of the major products.

Stress-induced phase and structural changes in KDP crystals

Potassium dihydrogen phosphate (KDP) is a nonlinear optical material which has been widely used in optical modulators and frequency converters. In making optical KDP components, however, the mechanical stresses in a manufacturing process often bring about unexpected surface integrity problems such as phase changes and cracks. This paper explores some possible phase transformations in monocrystalline tetragonal KDP under uniaxial, biaxial and triaxial compressive/tensile stresses externally applied through the use of ab initio Density Functional Theory (DFT) calculations. It shows that the change in crystal symmetry/phase transitions is due to hydrogen dynamics. Under a uniaxial tensile stress of ∼4GPa along the 〈110〉 direction, the paraelectric phase II, which has a tetragonal structure belonging to the I42d (D2d12) space group, will undergo an irreversible transformation to an orthorhombic (C2v) structure.

The low-lying singlet electronic excited states of ZrO2: A symmetry adapted cluster–configuration interaction (SAC–CI) study

The ground electronic state [X1A1(S0)] and nine low-lying singlet excited states [21A1(S2), 11A2(S3), 21A2(S6), 11B1(S4), 21B1(S7), 31B1(S11), 11B2(S1), 21B2(S5), and 31B2(S8) states] of ZrO2 were studied using SAC–CI theory. The geometries of the nine electronic excited states were optimized at the SAC–CI/def2-SVP, SAC–CI/def2-SVPD, and SAC–CI/LANL2TZ(f) levels. The vertical excitation energies and adiabatic excitation energies of the selected electronic excited states were calculated by single-point calculations at the SAC–CI/def2-TZVPPD and SAC–CI/LANL2TZ(f) levels. Stable C2v structures for the 11B2, 21B2, 31B2, and 11A2 states and stable Cs structures for the 11B1(A″), 31B1(A″), and 21A1(A′) states were determined. For comparison, the unrestricted density functional theory (UBPW91) was also applied to study the X1A1, 11A2, 11B1, and 11B2 states.

Potential energy surface of the CO2-N2 van der Waals complex.

Four-dimensional potential energy surface (4D-PES) of the atmospherically relevant CO2-N2 van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is mapped along the intermonomer coordinates. An analytic fit of this 4D-PES is performed. Our extensive computations confirm that the most stable form corresponds to a T-shape structure where the nitrogen molecule points towards the carbon atom of CO2. In addition, we located a second isomer and two transition states in the ground state PES of CO2-N2. All of them lay below the CO2 + N2 dissociation limit. This 4D-PES is flat and strongly anisotropic along the intermonomer coordinates. This results in the possibility of the occurrence of large amplitude motions within the complex, such as the inversion of N2, as suggested in the recent spectroscopic experiments. Finally, we show that the experimentally established deviations from the C2v structure at equilibrium for the most stable isomer are due to the zero-point out-of-plane vibration correction.

The Energy Difference between the Triply-Bridged and All-Terminal Structures of Co₄(CO)₁₂, Rh₄(CO)₁₂, and Ir₄(CO)₁₂: A Difficult Test for Conventional Density Functional Methods.

The M4(CO)12 molecules Co4(CO)12, Rh4(CO)12, and Ir4(CO)12 have two low-lying structures, the all-terminal structure with Td symmetry and the triply bridged structure with C3v symmetry. A total of 45 density functional theory (DFT) methods have been used to predict structures and vibrational frequencies for Co4(CO)12, Rh4(CO)12, and Ir4(CO)12. The different DFT methods show a broad range of energy differences ΔE = E(Td) - E(C3v). For Rh4(CO)12, none of the 45 DFT predictions is within 11 kcal/mol of the 2005 experimental value of 5.1 ± 0.6 kcal/mol reported by Allian and Garland (Dalton Trans. 2005, 1957 - 1965). For the challenging Ir4(CO)12 molecule, 21 DFT methods predict the correct Td structure, while 24 DFT methods predict the C3v structure to lie lower in energy. This research reveals many peculiar problems in the computation of the vibrational frequencies for the all-terminal structure.

Exploring non-Condon effects in a covalent tetracene dimer: how important are vibrations in determining the electronic coupling for singlet fission?

Singlet fission (SF) offers opportunities for wavelength-selective processing of solar photons with an end goal of achieving higher efficiency inexpensive photovoltaic or solar-fuels-producing devices. In order to evaluate new molecular design strategies and for theoretical exploration of dynamics, it is important to put in place tools for efficient calculation of the electronic coupling between single-exciton reactant and multiexciton product states. For maximum utility, the couplings should be calculated at multiple nuclear geometries (rather than assumed constant everywhere, i.e., the Condon approximation) and we must be able to evaluate couplings for covalently linked multichromophore systems. With these requirements in mind, here we discuss the simplest methodology possible for rapid calculation of diabatic one-electron coupling matrix elements-based on Boys localization and rediagonalization of molecular orbitals. We focus on a covalent species called BT1 that juxtaposes two tetracene units in a partially cofacial geometry via a norbornyl bridge. In BT1, at the equilibrium C2v structure, the "nonhorizontal" couplings between HOMOs and LUMOs (t(HL) and t(LH)) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of t(AB) coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the C2v point group) that turn on tHL and tLH values as well as for the patterns that emerge in constructive versus destructive interference of pathways to the SF product. For the best modes, calculated electronic coupling magnitudes for SF (at root-mean-squared deviation in position at 298 K), are within a factor of 2 of that seen for noncovalent tetracene dimers relevant to the molecular crystal. An overall "effective" electronic coupling is also given, based on the Stuchebrukhov formalism for non-Condon electron transfer rates.

Infrared spectra of M-η2-C2H2 and HM[sbnd]CCH produced in reactions of laser-ablated Fe and Os atoms with acetylene

The π- and CH insertion products (M-η2-C2H2 and HMCCH) are identified in the matrix infrared spectra from reactions of laser-ablated Fe and Os atoms with acetylene isotopomers, but the vinylidene product (H2CCM) is not, in contrast to the recently studied Ru case. The π-complex is produced in deposition and annealing, and it converts to the insertion complex during photolysis. While the vinylidene product is energetically comparable with the two primary products, the energy barrier is considerably higher in contrast to the Ru case. The relatively short CC bonds of the π-complexes indicate weak back-donations from the group 8 metals to the acetylene π∗ orbitals. The highly bent structure of HOsCCH evidently originates from the high d contributions to the COs and OsH bonds. The C2v structures of the vinylidene products arise from the p–d π bonding between C and M.

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C, Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX4−, with M = C, Si, Ge and X = F, Cl, Br, show that GeF4−, SiCl4−, GeCl4− and SiBr4− prefer a C2v conformation, but CCl4− is an elongated C3v structure. CBr4− has Td symmetry in MP2, but is slightly more stable in elongated C3v form with DFT and CCSD(T). GeBr4− has Td symmetry. CF4− and SiF4− are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl4 and SiCl4, and close to zero for GeF4 and SiBr4. Adiabatic EAs range from 0.47 eV for SiCl4 to 1.78 eV for GeBr4. The lowest excited states at Td symmetry are 2T2 resonances with energies of 2.1–3.5 eV, resulting from excitation of the a1 singly occupied molecular orbital to vacant t2 orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl4− is made. From dissociation energies of MX4, MX4−, MX3 and MX3−, appearance energies of X−, MX3−, X2− and MX2− were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl4−, SiCl4− and GeCl4−.

Partial Ionic Character beyond the Pauling Paradigm: Metal Nanoparticles

A canonical perspective on the chemical bond is the Pauling paradigm: a bond in a molecule containing only identical atoms has no ionic character. However, we show that homonuclear silver clusters have very uneven charge distributions (for example, the C2v structure of Ag4 has a larger dipole moment than formaldehyde or acetone), and we show how to predict the charge distribution from coordination numbers and Hirshfeld charges. The new charge model is validated against Kohn–Sham calculations of dipole moments with four approximations for the exchange–correlation functional. We report Kohn–Sham studies of the binding energies of CO on silver monomer and silver clusters containing 2–18 atoms. We also find that an accurate charge model is essential for understanding the site dependence of binding. In particular we find that atoms with more positive charges tend to have higher binding energies, which can be used for guidance in catalyst modeling and design. Thus, the nonuniform charge distribution of silver clusters predisposes the site preference of binding of carbon monoxide, and we conclude that nonuniform charge distributions are an important property for understanding binding of metal nanoparticles in general.

Hybrid density functional studies of cadmium vacancy in CdTe

The intrinsic defect of cadmium vacancy (VCd) in cadmium telluride (CdTe) has been studied by first-principles calculations using potentials with both the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) approximation and the generalized gradient approximation of the Perdew—Burke—Ernzerhof form (PBE-GGA). Both results show that the Td structure of the VCd defect for different charges is the most stable structure as compared with the distorted C3v structure with one hole localized at one of the four nearest Te atoms. This indicates that the John—Teller distortion (C3v) structure may be unstable in bulk CdTe crystal. The reason likely lies in the delocalized resonance nature of the t2 state of the VCd defect. Moreover, the formation energy obtained by the HSE method is about 0.6–0.8 eV larger than that obtained by the PBE method. The transition levels calculated by the PBE method and the HSE method are similar and well consistent with the experimental results.

Vibrational spectroscopy of Co⁺(CH₄)n and Ni⁺(CH₄)n (n = 1-4).

Vibrational spectra of M(+)(CH4)m(Ar)(3-m) and M(+)(CH4)n (M = Co, Ni; m = 1, 2; n = 3, 4) in the C-H stretching region (2500-3100 cm(-1)) are measured using photofragment spectroscopy, monitoring the loss of argon or methane. Interaction with the metal leads to large red shifts in the C-H stretches for proximate hydrogens. The extent of this shift is sensitive to the coordination (η(2) vs η(3)) and to the metal-methane distance. The structures of the complexes are determined by comparing measured spectra with those calculated for candidate structures at the B3LYP/6-311++G(3df,3pd) level. Binding energies are also computed using the CAM-B3LYP functional. In all cases, CH4 shows η(2) coordination to the metal. The m = 1 complexes show very large red shifts of 370 cm(-1) (for M = Co) and 320 cm(-1) (for M = Ni) in the lowest C-H stretch, relative to the symmetric stretch of free CH4. They adopt a C2v structure with the heavy atoms and proximate hydrogen atoms coplanar. The m = 2 complexes have slightly reduced red shifts, and Tee-shaped structures. Both Tee-shaped and equilateral (or quasi-equilateral) structures are observed for the n = 3 complexes. The measured photodissociation onset and significantly reduced intensity for low-frequency C-H stretches imply a value of 2650 ± 50 cm(-1) for the binding energy of Ni(+)(CH4)2-CH4. The Co(+)(CH4)4 complexes have two low-lying structures, quasi-tetrahedral and distorted square-planar, which contribute to the rich spectrum. In contrast, the symmetrical, square-planar Ni(+)(CH4)4 complex is characterized by a very simple vibrational spectrum.

Resorcin[4]arene‐Based Molecular Baskets and Water‐Soluble Container Molecules: Synthesis and 1H NMR Host–Guest Complexation Studies

AbstractResorcin[4]arene‐based molecular baskets, with four free or methylene‐bridged HO groups, and water‐soluble container molecules bearing poly(ethylene glycol) chains of different lengths on the lower rim and cap have been synthesized. These cavitands, topped with p‐xylylene bridges, feature well‐defined cavities capable of encapsulating heteroalicyclic guests. Association constants (Ka) were determined by 1H NMR spectroscopy for the organic‐soluble molecular baskets in CDCl3 and for the water‐soluble container molecules in D2O/CD3CN (2:1). Opposite guest selectivities were observed in the two environments. Upon complexation, the water‐soluble hosts show changes in their 1H NMR spectra. In the absence of guests, the p‐xylylene bridge rotates rapidly on the 1H NMR timescale, revealing a time‐averaged achiral C2v structure, whereas this rotation is hindered by guest inclusion, resulting in spectra showing a racemic C2‐symmetric host, indicative of planar chirality.

Concerted halogen bonding and orthogonal metal-halogen interactions in dimers of lithium formamidinate and halogenated formamidines: an ab initio study.

Dimers of lithium formamidinate, CH(NH)2Li, and halogenated formamidines, HN=CHNHX, (X=Cl, Br, or I) are used as model systems to investigate simultaneous N-X···N and N-Li···N interactions, in tandem with orthogonal Li···X interactions. Geometry optimizations and energy calculations for the dimers are examined with the MP2 method and the M06-2X hybrid functional and the aug-cc-pVTZ basis set (the aug-cc-pVTZ-PP basis set is used for the iodine atom). Both methods predict the formation of a planar structure of C2v symmetry, regardless of the identity of the halogen atom. In this structure, the identities of the constituent monomers are essentially lost. Accordingly, the N-X···N interactions emerge as a rather symmetric quasi-linear N···X···N, where the covalent N-X bond in the halogenated formamidine is replaced by a partly covalent N···X interaction. Formation of the C2v structure is also driven by a fairly linear N···Li···N interaction parallel to the N···X···N interaction, and a Li···X interaction orthogonal to both the N···X···N and N···Li···N interactions. The strength of the interactions increases with the size of the halogen. The robustness of the interactions suggests that the dimers studied here or suitable analogues may find diverse applications including their use as novel polymeric synthons.

Accurate equilibrium inversion barrier of ammonia by extrapolation to the one-electron basis set limit

A scheme based on treating uniform singlet-pair and triplet-pair interactions is suggested to extrapolate electron correlation energy of ammonia, calculated at two basis-set levels of ab initio theory in the infinite one-electron basis-set limit. The dual-level method is tested on the extrapolation of the full correlation in coupled-cluster singles and doubles and in the case also a noniterative perturbative correction for connected triple energies for the C3v and D3h structures of ammonia, with correlation-consistent basis sets of the type cc-pVXZ (X = D, T, Q, 5,6) and aug-cc-pVXZ (X = D, T, Q, 5). For testing and comparison purposes, the energies reported by Klopper [J. Comput. Chem. 22 1306 (2001)] have been taken. From a corresponding extrapolation of CCSD(T)/AVXZ energies for X = 4, 5, we obtain total inversion barriers of 1833.87 cm−1/1832.33 cm−1 for the two/three-parameter extrapolation rules, which are in good agreement with other theoretical extrapolation and empirical values in the literature.

Theoretical Study of Mononuclear Nickel(I), Nickel(0), Copper(I), and Cobalt(I) Dioxygen Complexes: New Insight into Differences and Similarities in Geometry and Bonding Nature

Geometries, bonding nature, and electronic structures of (N^N)Ni(O2) (N^N = β-diketiminate), its cobalt(I) and copper(I) analogues, and (Ph3P)2Ni(O2) were investigated by density functional theory (DFT) and multistate restricted active space multiconfigurational second-order perturbation (MS-RASPT2) methods. Only (N^N)Ni(O2) takes a C(S) symmetry structure, because of the pseudo-Jahn-Teller effect, while all other complexes take a C(2V) structure. The symmetry lowering in (N^N)Ni(O2) is induced by the presence of the singly occupied δ(d(xy)-π(x)*) orbital. In all of these complexes, significant superoxo (O2-) character is found from the occupation numbers of natural orbitals and the O-O π* bond order, which is independent of the number of d electrons and the oxidation state of metal center. However, this is not a typical superoxo species, because the spin density is not found on the O2 moiety, even in open-shell complexes, (N^N)Ni(O2) and (N^N)Co(O2). The M-O and O-O distances are considerably different from each other, despite the similar superoxo character. The M-O distance and the interaction energy between the metal and O2 moieties are determined by the d(yz) orbital energy of the metal moiety taking the valence state. The binding energy of the O2 moiety is understood in terms of the d(yz) orbital energy in the valence state and the promotion energy of the metal moiety from the ground state to the valence state. Because of the participations of various charge transfer (CT) interactions between the metal and O2 moieties, neither the d(yz) orbital energy nor the electron population of the O2 moiety are clearly related to the O-O bond length. Here, the π bond order of the O2 moiety is proposed as a good measure for discussing the O-O bond length. Because the d electron configuration is different among these complexes, the CT interactions are different, leading to the differences in the π bond order and, hence, the O-O distance among these complexes. The reactivity of dioxygen complex is discussed with the d(yz) orbital energy.

DFT Study on Structural Distortion and Vibronic Coupling of Vanadyl Porphyrin Anion and Cation

The geometries of one‐electron reduced/oxidized species ([TOP]−/[VOP]+) of vanadyl porphyrin (VOP) have been calculated with PBE1PBE method. The results show that for both [VOP]− and [VOP]+ the ground states are triplet, in which one of the two unpaired electron occupies the dxy orbital of the V atom while the other occupies the π‐orbital of porphyrin ring. Thus both [VOP]− and [VOP]+ can be considered as π‐radicals. The ground state of neutral VOP molecule is doublet with the unpaired electron occupying dxy orbital of V atom. In contract to the C4v symmetry of neutral VOP molecule, [VOP]− anion has a “rectangular” distorted C2v structure due to Jahn‐Teller effect. The linear vibronic coupling constants for the Jahn‐Teller active modes of [TOP]− were evaluated and the node patterns of frontier KS orbitals are used to explain the reason why the distortion occurs along specific modes. The ground state [VOP]+ has a porphyrin ring with pronounced bond length alternation due to pseudo‐Jahn‐Teller effect, causi...

C2V or C6V: Which is the most stable structure of the benzene–lithium complex?

The benzene–lithium complex is a prototype system to understand the interaction in lithium-doped nanocarbons. Herein, we have employed coupled-cluster theory and correlation consistent basis sets to study the C6v and C2v structures, of the C6H6·Li complex. We found that when the CCSD(T) method and large basis sets are employed, the C2V structure becomes lower in energy by 1.8kcal/mol. At the CCSD(T)/CBS level, and including corrections for core–valence and relativistic effects, the dissociation energy of the C2v (ionic) structure is 6.7kcal/mol whereas that corresponding to the C6v (nonionic) one is 4.9kcal/mol. Our results suggest that charge transfer in C6H6·Li does exist, in contrast with previous results.

Iron-Centered Ten-Vertex Germanium Clusters: The Ubiquity of Low Energy Pentagonal Prismatic Structures with Various Skeletal Electron Counts

One of the most significant recent developments (in 2009) is the discovery of the clusters M@Ge10(3-) (M = Fe, Co) in which the outer Ge10 polyhedron is a pentagonal prism rather than a deltahedral structure of the type predicted by the Wade-Mingos rules. Consistent with this experimental observation, density functional theory shows the lowest energy structures to be pentagonal prisms for the iron-centered clusters Fe@Ge10(z) in all nine charge states ranging from -5 to +3. This contrasts with the previously studied cobalt-centered germanium clusters Co@Ge10(z) for which the lowest energy structures are pentagonal prisms only for the electron richest systems where z ranges from -3 to -5. The C3v structures derived from the tetracapped trigonal prism found as lowest energy structures of the electron poorer Co@Ge10(z) (z = 0, -1, -2) systems are higher energy structures for the iron-centered germanium clusters Fe@Ge10(z) (z = 0, -1, -2). The strong energetic preference for pentagonal prismatic structures in the Fe@Ge10(z) clusters can be attributed to the need for the larger volume of the pentagonal prism relative to other 10-vertex closed polyhedra to accommodate the interstitial iron atom.