Qian-shu Li

Planar Hepta-, Octa-, Nona-, and Decacoordinate First Row d-Block Metals Enclosed by Boron Rings

Possible planar hypercoordinate molecules with first row d-block metal atoms in the centers of boron rings are explored comprehensively by density-functional theory (DFT) computations. Many optimized MB(n) (n = 7, 8, 9, and 10) neutral and charged clusters have local D(nh) minima, although these may not be the most stable isomers. The larger B(9) and B(10) rings are versatile in accommodating first row d-block metals, whereas the more compact B(8) ring only can enclose smaller transition metals (such as Mn, Fe, and Co) effectively. Delocalized pi and radial molecular orbitals involving boron are crucial in stabilizing these highly symmetrical planar hypercoordinate molecules. Early and middle transition metal d-orbitals participate in metal-boron covalent bonding, whereas partial ionic bonding is more important for the late d-block elements. Potential energy surface scans established several of these species to have planar hypercoordinate global minima: D(8h) FeB(8)(2-) was identified here, and D(8h) CoB(8)(-) and D(9h) FeB(9)(-) were identified in an earlier complementary study.

Aromaticity and relative stabilities of azines.

The most refined nucleus-independent chemical shift index (NICS(0)(πzz)) and the extra cyclic resonance energies (ECREs), based on the block localized wave function (BLW) method, show that the aromaticity of all azines is like that of benzene. The same is true for aza-naphthalenes relative to naphthalene. The lower relative energies of isomers with vicinal Ns are due to the weakness of NN bonds rather than to reduced aromaticity.

Structures, thermochemistry, and electron affinities of the germanium fluorides, GeFn/GeFn−(n=1–5)

Four different density functional methods have been employed to study the molecular structures, electron affinities, and first dissociation energies of the GeFn/GeFn−(n=1–5) molecules. The three types of electron affinities reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Ge–F dissociation energies De(Fn−1Ge–F), De(Fn−1Ge−–F), and De(Fn−1Ge–F−) of the GeFn/GeFn− species are also reported. The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, labeled as DZP++. Among the four density functionals used in this work, the BHLYP (which includes 50% exact exchange) method determines the molecular structures in best agreement with experiment, while other methods generally overestimated bond lengths. The theoretical Ge–F bond distances for the GeFn−(n=1–4) anions are predicted about 0.1 A longer than their corresponding neutral counterparts...

Molecules for materials: Germanium hydride neutrals and anions. Molecular structures, electron affinities, and thermochemistry of GeHn/GeH n− (n = 0–4) and Ge2Hn/Ge2H n− (n = 0–6)

The GeHn (n = 0–4) and Ge2Hn (n = 0–6) systems have been studied systematically by five different density functional methods. The basis sets employed are of double‐ζ plus polarization quality with additional s‐ and p‐type diffuse functions, labeled DZP++. For each compound plausible energetically low‐lying structures were optimized. The methods used have been calibrated against a comprehensive tabulation of experimental electron affinities (Chemical Reviews 102, 231, 2002). The geometries predicted in this work include yet unknown anionic species, such as Ge2H−, Ge2H  2− , Ge2H  3− , Ge2H  4− , and Ge2H  5− . In general, the BHLYP method predicts the geometries closest to the few available experimental structures. A number of structures rather different from the analogous well‐characterized hydrocarbon radicals and anions are predicted. For example, a vinylidene‐like GeGeH  2− structure is the global minimum of Ge2H  2− . For neutral Ge2H4, a methylcarbene‐like HGë‐GeH3 is neally degenerate with the trans‐bent H2GeGeH2 structure. For the Ge2H  4− anion, the methylcarbene‐like system is the global minimum. The three different neutral‐anion energy differences reported in this research are: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). For this family of molecules the B3LYP method appears to predict the most reliable electron affinities. The adiabatic electron affinities after the ZPVE correction are predicted to be 2.02 (Ge2), 2.05 (Ge2H), 1.25 (Ge2H2), 2.09 (Ge2H3), 1.71 (Ge2H4), 2.17 (Ge2H5), and −0.02 (Ge2H6) eV. We also reported the dissociation energies for the GeHn (n = 1–4) and Ge2Hn (n = 1–6) systems, as well as those for their anionic counterparts. Our theoretical predictions provide strong motivation for the further experimental study of these important germanium hydrides.

The germanium clusters Gen (n = 1-6) and their anions: structures, thermochemistry and electron affinities

Six different hybrid and pure density functional theory (DFT) methods have been employed to study the structures, electron affinities, and dissociation energies of the (n = 1–6) species. The basis set used is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Located were 20 structures of the neutral Ge n clusters and 12 structures of the anionic clusters. The ground states of Ge n and clusters in this work are in good agreement with available experiments. Three types of electron affinities reported are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Ge–Ge dissociation energies D e(Ge n −1–Ge) for Ge n and both D e( –Ge) and D e(Ge n −1–Ge−) for the species have also been reported. Previously observed trends in the prediction of bond lengths by DFT methods are also demonstrated for the germanium clusters and their anions. Generally, the Hartree–Fock/DFT hybrid methods predict shorter and more reliable bond lengths than the pure DFT methods. The most reliable adiabatic electron affinities, obtained at the DZP++ BP86 level of theory, are 1.55 (Ge), 1.97 (Ge2), 2.24 (Ge3), 2.08 (Ge4), 2.29 (Ge5) and 2.07 eV (Ge6), among which those for Ge2, Ge3 and Ge6 are in good agreement with experiment. However, for Ge5 the predicted electron affinity is somewhat smaller than the available experimental values. Average electron affinity differences with the best experiments are 0.10 eV for the B3LYP method and 0.12 eV for BP86. The first dissociation energies for the neutral germanium clusters predicted by the B3LYP method are 2.87 (Ge2), 3.22 (Ge3), 3.80 (Ge4), 3.12 (Ge5) and 3.37 eV (Ge6). Compared to the available experimental dissociation energies for Ge2, the theoretical predictions are very reasonable. For the vibrational frequencies of the Ge n series, the DZP++ B3LYP method produces reliable predictions with a relative error of ∼2% with respect to available experimental values. In addition to the earlier finding that the BHLYP method is the most reliable for geometries, we conclude that the BP86 or B3LYP methods do the best for electron affinities, with B3LYP preferable for dissociation energies and vibrational frequencies among the six DFT methods.

The arsenic clusters Asn (n = 1–5) and their anions: Structures, thermochemistry, and electron affinities

The molecular structures, electron affinities, and dissociation energies of the Asn/As  −n (n = 1–5) species have been examined using six density functional theory (DFT) methods. The basis set used in this work is of double‐ζ plus polarization quality with additional diffuse s‐ and p‐type functions, denoted DZP++. These methods have been carefully calibrated (Chem Rev 2002, 102, 231) for the prediction of electron affinities. The geometries are fully optimized with each DFT method independently. Three different types of the neutral‐anion energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first dissociation energies De(Asn−1‐As) for the neutral Asn species, as well as those De(As  −n−1 ‐As) and De (Asn−1‐As−) for the anionic As  −n species, have also been reported. The most reliable adiabatic electron affinities, obtained at the DZP++ BLYP level of theory, are 0.90 (As), 0.74 (As2), 1.30 (As3), 0.49 (As4), and 3.03 eV (As5), respectively. These EAad values for As, As2, and As4 are in good agreement with experiment (average absolute error 0.09 eV), but that for As3 is a bit smaller than the experimental value (1.45 ± 0.03 eV). The first dissociation energies for the neutral arsenic clusters predicted by the B3LYP method are 3.93 eV (As2), 2.04 eV (As3), 3.88 eV (As4), and 1.49 eV (As5). Compared with the available experimental dissociation energies for the neutral clusters, the theoretical predictions are excellent. Two dissociation limits are possible for the arsenic cluster anions. The atomic arsenic results are 3.91 eV (As  −2 → As− + As), 2.46 eV (As  −3 → As  −2 + As), 3.14 eV (As  −4 → As  −3 + As), and 4.01 eV (As  −5 → As  −4 + As). For dissociation to neutral arsenic clusters, the predicted dissociation energies are 2.43 eV (As  −3 → As2 + As−), 3.53 eV (As  −4 → As3 + As−), and 3.67 eV (As  −5 → As4 + As−). For the vibrational frequencies of the Asn series, the BP86 and B3LYP methods produce good results compared with the limited experiments, so the other predictions with these methods should be reliable.

Ab initio and density functional theory study of the mechanism of synthesis of the N5+ cation

Abstract The synthesis pathway of the N 5 + cation from N 2 F + and HN 3 has been investigated by ab initio and density functional theory (DFT) methods. Reactants, products, transition state (TS), and related ion–molecule complexes have been fully optimized up to the B3LYP/6-311++G ∗∗ level of theory. Relative energies are further calculated using MP4 and CCSD(T) methods. The optimized TS has a four-membered ring structure with C 1 symmetry. At the CCSD(T)/6-311++G ∗∗ //B3LYP/6-311++G ∗∗ level of theory, the energetic barrier height is predicted to be 12.5 kcal/mol, and the whole synthesis reaction is exothermic by 79.4 kcal/mol.

Bonding of Seven Carbonyl Groups to a Single Metal Atom: Theoretical Study of M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4)

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic metal-carbonyls of the group 4 elements, M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4) were predicted using density functional theory. Analogous M(CO)n structures were found for all three metals. The global minima for the 18-electron M(CO)7 molecules are all singlet C(3v) capped octahedra. The global minima for the 16-electron M(CO)6 species are triplet M(CO)6 structures distorted from O(h) symmetry to D(3d) symmetry. However, the corresponding singlet M(CO)6 structures lie within 5 kcal/mol of the triplet global minima. The global minima for M(CO)n (n = 5, 4) are triplet structures derived from the D(3d) distorted octahedral structures of M(CO)6 by removal of one or two CO groups, respectively. Quintet D(3h) trigonal bipyramidal structures for M(CO)5 and singlet T(d) tetrahedral structures for M(CO)4 are also found, as well as higher energy structures for M(CO)6 and M(CO)7 containing a unique CO group bonded to the metal atom through both M-C and M-O bonds. The dissociation energies M(CO)7 --> M(CO)6 + CO are substantial, indicating no fundamental problem in bonding seven CO groups to a single metal atom.

F + (H2O)2 reaction: the second water removes the barrier.

In light of controversy concerning the classical barrier for the F + H2O → HF + OH reaction, higher level theoretical methods are applied. Both aug-cc-pV5Z CCSD(T) and cc-pVQZ full CCSDT methods predict a low classical barrier of about 2 kcal/mol. For the analogous water dimer reaction, the presence of the second water molecule erases this barrier entirely. An entrance complex F···(H2O)2 is found lying 7.3 kcal/mol below separated F + (H2O)2. A barrier follows this complex on the reaction coordinate, but lying 3.1 kcal/mol below the reactants. There is also an exit complex HF···H2O3 lying 11.0 kcal/mol below the separated products HF + H2O3. Thus the reactions of atomic fluorine with the water monomer and dimer are qualitatively different.

Prospects for Making Organometallic Compounds with BF Ligands: Fluoroborylene Iron Carbonyls

The fluoroborylene ligand (BF), isoelectronic with CO, was recently (2009) realized experimentally by Vidović and Aldridge in Cp(2)Ru(2)(CO)(4)(mu-BF). In this research the related iron carbonyl fluoroborylene complexes Fe(BF)(CO)(n) (n = 4, 3), Fe(2)(BF)(CO)(8), and Fe(2)(BF)(2)(CO)(n) (n = 7, 6) are compared with the isoelectronic Fe(CO)(n+1) and Fe(2)(CO)(n+2) as well as the thiocarbonyls Fe(CS)(CO)(n) and Fe(2)(CS)(2)(CO)(n) using density functional theory. For Fe(BF)(CO)(4) the axially and equatorially substituted trigonal bipyramidal structures are predicted to be nearly degenerate as is the case for Fe(CS)(CO)(4). The lowest energy structures for Fe(BF)(CO)(3) are derived from the trigonal bipyramidal Fe(BF)(CO)(4) structures by removal of CO groups. For the binuclear derivatives Fe(2)(BF)(CO)(8) and Fe(2)(BF)(2)(CO)(n) (n = 7, 6) structures with BF bridges are preferred energetically over structures with CO bridges. However, no structures for the unsaturated Fe(2)(BF)(2)(CO)(6) are found with four-electron donor eta(2)-mu-BF groups. This differs from the corresponding Fe(2)(CS)(2)(CO)(6) where structures with eta(2)-mu-CS groups and formal Fe-Fe single bonds are preferred over structures with only two electron donor CO and CS groups and formal Fe=Fe double bonds. The lowest energy structure for Fe(2)(BF)(2)(CO)(7) is thus predicted to be similar to the well-known triply bridged Fe(2)(CO)(9) structure but with two bridging BF groups and one bridging CO group. However, the dissociation energy of Fe(2)(BF)(2)(CO)(7) into mononuclear fragments is much higher than that of Fe(2)(CO)(9). Removal of the bridging CO group from this lowest energy Fe(2)(BF)(2)(CO)(7) structure leads to the doubly BF-bridged global minimum structure for Fe(2)(BF)(2)(CO)(6).