Terminal Hydrogen Atoms Research Articles

Conformational stability of allylbenzene: A combined study by dispersed fluorescence spectroscopy and quantum chemistry calculation

Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic π electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the Cα–Cβ bond by 120° from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers.

Dichlorogallane (HGaCl(2))(2): its molecular structure and synthetic potential.

Dichlorogallane (HGaCl(2))(2) is readily prepared from gallium trichloride and triethylsilane in quantitative yield. Its crystal structure has been determined by single crystal X-ray diffraction. In the chlorine-bridged dimers of crystallographically imposed C(2h) symmetry, the terminal hydrogen atoms are in trans positions. In the reaction with 2 equiv of triethylphosphine, the mononuclear complex (Et(3)P)GaHCl(2) is formed. Thermal decomposition of (HGaCl(2))(2) affords hydrogen gas and quantitative yields of "GaCl(2)" as mixed-valent Ga[GaCl(4)]. Treatment of this product with triethylphosphine gives the symmetrical, Ga-Ga-bonded gallium(II) complex [GaCl(2)(PEt(3))](2) with an ethane-type structure and with the phosphine ligands in a single-trans conformation. The corresponding [GaBr(2)(PEt(3))](2) complex is prepared from Ga[GaBr(4)] and has an analogous structure. (Et(3)P)GaCl(3) has been synthesized and structurally characterized as a reference compound.

Synthesis and properties of some semi-fluoroalkoxy chain liquid crystals

Six compounds with semi-fluorinated chains have been synthesized. When the terminal hydrogen atom in the semi-fluoroalkoxy chain was changed to chlorine, both the clearing point and the melting point were enhanced. It was also found that the clearing points were decreased and melting points increased with the introduction of a triple bond into the cores.

Synthesis and liquid crystalline properties of several compounds with semi-fluorocarbon chains

Several molecules having long semi-fluoroalkyl chains have been synthesized, and their phase transition temperatures measured by texture observation in a polarizing microscope and DSC. The SmC phase is particularly favoured in this series, especially for compounds with a long semi-fluoroalkyl chain. It was found that when the terminal hydrogen atom of the semifluoroalkyl chain of one of the compounds (D) was substituted by a chlorine atom, the liquid crystalline sequence and ranges did not change; however, the clearing and melting points and TI-SmA and TSmA-SmC increased by nearly 10°C.

Modeling of the Tridentate Amorphous Silica Ligand

The 3-fold coordination of NiII cations to amorphous silica is modeled performing density functional theory (DFT) calculations on framework model clusters of increasing size. Using the nT notation of zeolite structures where n is the number of T atoms (T = Si or Al), four model clusters with the following structures are investigated: (i) (Si2O3H4)2- (2T) is made of two vicinal silanolate groups (SiO-) bonded through an oxygen bridge. The valence requirements of silicium are satisfied by addition of terminal hydrogen atoms. (ii) (Si3O6H4)2- (3T) is a six-membered ring made of two silanolate groups and one silanol group (SiOH) bonded through oxygen bridges. (iii) (Si4O7H6)2- (4T) is an eight-membered ring with two vicinal silanolate groups in positions 1 and 3 and a silanol group in position 5, and (iv) (Si5O8H8)2- (5T) is a flexible ten-membered ring with two vicinal silanolate groups in positions 1 and 3 and an isolated silanol group in position 7. DFT calculations are performed in order to estimate the ability of each model cluster to reproduce the experimental characteristics of previously described silica-supported NiII(O)3 species: (i) three-coordinated NiII of distorted C3v close to D3h symmetry and (ii) Ni−O distances in agreement with EXAFS measurements. (Si5O8H8)2- is preferred to the (Si2O3H4)2- model proposed earlier. Because of its flexibility, the larger framework model is able to best reproduce the experimental geometry of the NiII site. This model cluster may be assimilated to two vicinal silanolates and one neighboring isolated silanol or siloxane bridge on the real silica surface.

Thermal vibrations of Li + ion and Li atom at the circumference of graphite cluster model: a direct molecular dynamics calculation

For simulation on thermal behavior of the migrating species, Li + ion and Li atom, at the circumference of graphite in lithium battery, direct molecular orbital (MO) dynamics calculation was carried out at AM1 level, using the hydrogen terminated planar cluster model C 54H 18. According to the static MO calculation, both species stabilize above the segment joining two terminating hydrogen atoms, H1 and H2, which are bonded by the corner edge and the side edge carbon atoms, respectively. Although Li + ion is located 1.8425 Å above the middle point of the segment, Li atom 1.620 Å above the point shifted to H2 on the same segment. As results of the dynamics calculation at 300 K for these stabilized species, Li + ion shows the elliptic movement under the equilibrium coulombic potential energy ascribed to both the side edge and corner edge carbon atoms, however, Li atom circulates around the side carbon atom keeping the covalent bond composed of sp 3 hybrid orbitals. Migration trajectories of the species are reflected by the electronic density maps.

Iridium Cluster Chemistry: The Synthesis and the Solid State Structures of [HIr5(CO)12]2− and [HIr4(CO)10PPh3

The cluster [HIr5(CO)12]2- (1) was prepared by condensation of [HIr4(CO)11]- and [Ir(CO)4]- (molar ratio 1:1) in refluxing THF, with almost quantitative yields. Its solid state structure was determined by X-ray diffraction at low temperature on the salt [PPh3CH2Ph]2[HIr5(CO)12]. The metal atoms define a trigonal bipyramidal arrangement. The hydride ligand was located indirectly as a bridge between apical and equatorial metal atoms. The phosphine-substituted cluster [HIr4(CO)10PPh3]- (2) was synthetized by CO displacement on [HIr4(CO)11]-, in THF at room temperature. This reaction is selective, with no traces of polysubstitution products. In the solid state, the hydride and the triphenylphosphine are axially bound on basal iridium atoms; the terminal hydrogen atom was directly located by X-ray analysis at a Ir–H distance of 1.57(9) A. On the contrary, two isomers are present in THF solution, and they interconvert rapidly at room temperature, as shown by1H and 31P NMR spectra.

Structural characterisation of dimethylgallium tetrahydroborate and its adducts with diethyl ether and tetrahydrofuran

The structures of dimethylgallium tetrahydroborate and its 1∶1 adducts with diethyl ether and tetrahydrofuran have been investigated by single crystal X-ray diffraction at low temperatures. The structure of dimethylgallium tetrahydroborate has been shown to consist of discrete GaMe2(BH4) units in which each BH4 group exhibits bidentate ligation to a single Me2Ga unit, but in which there are long-range interactions between the Ga and the terminal hydrogen atom of a BH4 group in each of two neighbouring molecules. The structures of the adducts show that they are composed of molecules in which the Ga is 5-coordinate and asymmetrically bound to a bidentate BH4 unit. The adducts display enhanced thermal stability compared to that of uncoordinated GaMe2(BH4), but reversibly dissociate in the gas phase at ambient temperature. The infrared spectra of the compounds are discussed on the basis of their structures.

Hydrofluoropolyethers: a new “friendly” generation of fluorinated fluids

In this paper, some physico-chemical properties of a novel class of α–ω hydrofluoropolyethers (HFPEs) having terminal hydrogen atoms are presented and compared with those of similar perfluorinated structures. The effect of –CF2H end groups has been studied for the lower members of this family and its group contributions, determined for some physico-chemical properties, have been calculated and can be used to predict properties of new structures.

H-densities: a new concept for hydrated molecules.

It is common to represent molecules by "ball-and-stick" models that represent static positions of atoms. However, the vibrational states of water molecules involved in hydrogen bonding have wide amplitudes, even in their ground states. Here we introduce a new representation of this wide-amplitude vibrational motion: H-density plots. These plots represent the delocalized zero-point vibrational motion of terminal hydrogen atoms of water molecules weakly bound to other molecules. They are a vibrational analogy to electron densities. Calculations of the H-densities for complexes of water with water, benzene, phenol, and DNA bases are presented. These are obtained using the quantum diffusion Monte Carlo method. Comparisons of measured and calculated rotational constants provide experimental evidence of the new concept.

Structure and vibrational spectra of cis-diaquatetra-chloroplatinum(IV)-(18-crown-6)-water (1/1/2), cis-[PtCl4(H2O)2]·(18-cr-6)·2H2O

The title compound, cis-[PtCl4(H2O)2]·(18-cr-6)·2H2O(3), crystallizes from water in the orthorhombic space group Pbcn with lattice constants a= 7.9842(12) Å, b=16.550(3) Å, c= 18.027(4) Å, Z= 4. The crystal structure was solved by direct methods. The refinement converged to final wR2= 0.0485 for 2307 data (R1= 0.0187 for 1672 observed reflections). The platinum complex cis-[PtCl4(H2O)2] exhibits crystallographically imposed C2 symmetry with small deviations from C2v symmetry, only. Each of the two aqua ligands forms an O-H...O hydrogen bond to a water molecule in such a way that H4O2 units are yielded. Their three terminal hydrogen atoms form O-H...O hydrogen bonds to three oxygen atoms of the crown ether molecule. Thus, the crystal is threaded by chains built up of 18-crown-6/cis-[PtCl4(H2O)2]·2H2O units. The crown ether exhibits crystallographically imposed Ci symmetry and D3d symmetry in rough approximation. The Raman and IR spectra of 3 are as expected for a complex [PtCl4(H2O)2] with aqua ligands in mutual cis position.

Hypothesis: volatile anesthetics produce immobility by acting on two sites approximately five carbon atoms apart.

All series of volatile and gaseous compounds contain members that can produce anesthesia, as defined by the minimum alveolar anesthetic concentration (MAC) required to produce immobility in response to a noxious stimulus. For unhalogenated n-alkanes, cycloalkanes, aromatic compounds, and n-alkanols, potency (1 MAC) increases by two-to threefold with each carbon addition in the series (e.g., ethanol is twice as potent as methanol). Total fluorination (perfluorination) of n-alkanes essentially eliminates anesthetic potency: only CF4 is anesthetic (MAC = 66.5 atm), which indicates that fluorine atoms do not directly influence sites of anesthetic action. Fluorine may enhance the anesthetic action of other moieties, such as the hydrogen atom in CHF3 (MAC = 1.60 atm), but, consistent with the notion that the fluorine atoms do not directly influence sites of anesthetic action, adding -(CF2)n moieties does not further increase potency (e.g., CHF2-CF3 MAC = 1.51 atm). Similarly, adding -(CF2)n moieties to perfluorinated alkanols (CH2OH-[CF2]nF) does not increase potency. However, adding a second terminal hydrogen atom (e.g., CHF2-CHF2 or CH2OH-CHF2) produces series in which the addition of each -CF2- "spacer" in the middle of the molecule increases potency two- to threefold, as in each unhalogenated series. This parallel stops at four or five carbon atom chain lengths. Further increases in chain length (i.e., to CHF2[CF2]4CHF2 or CHF2[CF2]5CH2OH) decrease or abolish potency (i.e., a discontinuity arises). This leads to our hypothesis that the anesthetic moieties (-CHF2 and -CH2OH) interact with two distinct, spatially separate, sites. Both sites must be influenced concurrently to produce a maximal anesthetic (immobility) effect. We propose that the maximal potency (i.e., for CHF2[CF2]2CHF2 and CHF2[CF2]3CH2OH) results when the spacing between the anesthetic moieties most closely matches the distance between the two sites of action. This reasoning suggests that a distance equivalent to a four or five carbon atom chain, approximately 5 A, separates the two sites. Volatile anesthetics may produce immobility by a concurrent action on two sites five carbon atom lengths apart.

Rodalquilarite Revisited: The Hydrothermal Synthesis and Structural Reinvestigation of H3Fe2(TeO3)4Cl

The known mineral rodalquilarite, H3Fe2(TeO3)4Cl, was obtained as high quality single crystals via hydrothermal reactions. This material crystallizes in the triclinic space group,P1, with cell constants ofa=5.103(2) Å,b=6.653(2) Å,c=9.012(3) Å,α=73.40(2)°,β=78.03(2),γ=76.76(2),V=282.1(2) Å3and was obtained from an NH4Cl solution that was heated at 375°C for 4 days. A detailed structural characterization was performed (R=0.039,Rw=0.050) and showed that this material is based on layers consisting of edge-sharing FeO6octahedra which are interconnected by TeO3pyramids which are completed by the presence of a terminal hydrogen atom. Additionally, a second set of TeO3pyramids attached to the FeO6octahedra are linked across the layers by a shared hydrogen atom. The layers are held together only through these O–H–O interactions and additional weak Te–Cl interactions.In this study, we compare this structure to a previous report of rodalquilarite and a reported triclinic form of Fe2Te4O11, which may have been misidentified and is also the title compound. In addition, we have obtained the band gap of this material by diffuse reflectance spectroscopy and find it to be a wide band gap (Eg=2.51eV) material. The DC magnetic susceptibility was also obtained and showed that the title compound is antiferromagnetic with aTNof 29 K.

PCl 3 mediated cyclization: Synthesis, at room temperature, of N‐alkenyl derivatives of 1,4‐phthalazinedione

The use of a PCl3/hydrazone/dicarboxylic acid combination can be applied in an efficient one-pot procedure for the synthesis at room temperature of the title ring compounds that are also a new family of stable enamines. The phthalazinediones 7 were obtained in good yields by reaction of PCl3 with phthalic acid and hydrazones 1a,b,c containing hydrogen atoms only in one α position. Two structurally isomeric phthalazinediones (7 and 8) were obtained with hydrazones 1d,e,f,g containing hydrogen atoms in the two different α positions. When we used a methyl ketone hydrazone 1c,d,e,f,i, it was possible also to isolate the isomer containing an N-alkenyl group with two terminal hydrogen atoms. Where E,Z isomers are possible, the exclusive formation of the E isomer was always observed. As a rule, changing the order of addition of reagents gave almost identical results; however, in the case of phthalic acid, it is better to use the procedure in which PCl3 is added last to prevent the formation of the corresponding anhydride. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 291–296, 1999

PCl3 mediated cyclization: Synthesis, at room temperature, of N‐alkenyl derivatives of 1,4‐phthalazinedione

The use of a PCl3/hydrazone/dicarboxylic acid combination can be applied in an efficient one-pot procedure for the synthesis at room temperature of the title ring compounds that are also a new family of stable enamines. The phthalazinediones 7 were obtained in good yields by reaction of PCl3 with phthalic acid and hydrazones 1a,b,c containing hydrogen atoms only in one α position. Two structurally isomeric phthalazinediones (7 and 8) were obtained with hydrazones 1d,e,f,g containing hydrogen atoms in the two different α positions. When we used a methyl ketone hydrazone 1c,d,e,f,i, it was possible also to isolate the isomer containing an N-alkenyl group with two terminal hydrogen atoms. Where E,Z isomers are possible, the exclusive formation of the E isomer was always observed. As a rule, changing the order of addition of reagents gave almost identical results; however, in the case of phthalic acid, it is better to use the procedure in which PCl3 is added last to prevent the formation of the corresponding anhydride. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 291–296, 1999

2,4-Ethanotetraborane derivatives. 3.[1] determination of the molecular structure of 2,4-( t-butylethano)tetraborane(10), 2,4-(Bu tCHCH 2)B 4H 8, in the gas phase by electron diffraction

The molecular structure of 2,4-( t-butylethano)tetraborane(10), 2,4-(Bu tCHCH 2)B 4H 8, in the gas phase has been determined by analysis of electron diffraction data, restrained by MP2 6–31 G∗ ab initio calculations. The effect of the t-butyl group is mainly to twist the C-C bond of the C 2B 4 ‘basket’ by 6.6(14)°, so that the local symmetry of the C 2B 4 fragment is reduced from C 2v to C 2. The concomitant distortion of the B 4H 8 group from C 2v local symmetry is insignificant. Ab initio calculations at the same level are also reported for related compounds in which the basket ‘handle’ is trans-Bu tCHCHBu t, Me 3SiCHCH 2, trans-Me 3SiCHCHSiMe 3, cyclo-C 5H 8, cis-MeCHCHMe and Me 2CCMe 2, and for all possible compounds with one or two terminal hydrogen atoms of the C 2H 4B 4H 8 basket replaced by ethyl groups.

Determination of reaction geometries

Using polarized light the reaction geometry of selected species can be controlled even in bulk experiments. One reactant A is generated in a photodissociation process and its spatial distribution is completely described by the anisotropy parameter β. The other molecular reactant B is excited in a specific rovibrational state. Its spatial distribution is given by the J- and branch-dependent alignment parameter A0(2). Equations have been developed that allow a relatively easy conversion of experimental results to the angle of attack, γ. The unnormalized probability of an attack of A on B under an angle γ is given by the simple expression P(γ)∝[1+15βA0(2)P2(cos γ)P2(cos δ)] where δ is the angle between the E⃗ vectors of the dissociating and the exciting laser beam. As an example, we have studied the reaction of A+HCN→HA+CN with A=H,Cl. The experimental results prove a preferred linear reaction geometry, i.e, an end-on attack of atom A on the terminating hydrogen atom of the HCN reactant. However, the cone of acceptance is higher for the Cl+HCN reaction than for the H+HCN one.

Model atom approximation for estimating the localized bond energy based on the Morse potential

A method for solving the vibrational problem by using an effective energy operator containing Morse potentials and new model atom approximations is developed. The approach is explained by reference to the trifluoromethylacetylene molecule where the model atom is a CCC or CC atomic group. Using certain assumptions for parameters in the spectrum, we managed to confirm the known experimental result of bond strengthening of the terminal hydrogen atom when the CC bond order increases and to predict the CH bond energy (462.5±4.6 kJ/mole). This procedure seems to be also effective for more complex adsorption problems where the model atom is a crystal. Given that the surface effect is predicted by the parameters of the method, the corresponding model energy operator is constructed. With appropriate spectral data, the approach guarantees very accurate estimations of adsorption bond energies.

Dimethylindium octahydrotriborate, Me2InB3H3: synthesis, crystal structure and spectroscopic properties of a volatile indium hydride

Reaction between trimethylindium and tetraborane(10) at room temperature yields the volatile, viscous liquid dimethylindium octahydrotriborate, the first reported example of a volatile indium hydride; spectroscopic properties of the vapour indicate a molecular structure akin to that of Me2A1B3H8 although the crystal structure implies a more ionic formulation, [Me2In]+[B3H8]–, with the coordination at each indium centre being augmented via secondary intermolecular interactions with terminal hydrogen atoms.

Ab initio molecular orbital calculations on 1,2-hydrogen shifts in the ethene, allene and propyne radical cations

MRCI//ROHF/6-31G ∗∗ ab initio calculations on the 1,2-hydrogen shift in the ethylene radical cation show that the CH 3CH + product ion is not stable. CH 2CHCH + obtained by a 1,2-hydrogen shift from the propyne or the allene radical cation is only stable because the ion can relax to a more stable structure, which can be described as an allyl cation with one of the terminal hydrogen atoms removed.