pH2 Molecules Research Articles

Catalytic Hydrogenation of Trivinyl Orthoacetate: Mechanisms Elucidated by Parahydrogen Induced Polarization.

Parahydrogen (pH2) induced polarization (PHIP) is a unique method that is used in analytical chemistry to elucidate catalytic hydrogenation pathways and to increase the signal of small metabolites in MRI and NMR. PHIP is based on adding or exchanging at least one pH2 molecule with a target molecule. Thus, the spin order available for hyperpolarization is often limited to that of one pH2 molecule. To break this limit, we investigated the addition of multiple pH2 molecules to one precursor. We studied the feasibility of the simultaneous hydrogenation of three arms of trivinyl orthoacetate (TVOA) intending to obtain hyperpolarized acetate. It was found that semihydrogenated TVOA underwent a fast decomposition accompanied by several minor reactions including an exchange of geminal methylene protons of a vinyl ester with pH2. The study shows that multiple vinyl ester groups are not suitable for a fast and clean (without any side products) hydrogenation and hyperpolarization that is desired in biochemical applications.

Theoretical Study of Infrared Spectra of OCS-( pH2)2, OCS-( oD2)2, OCS-(HD)2, and Mixed OCS- pH2-He Trimers.

The calculated rovibrational energy levels and infrared spectra for OCS-( pH2)2, OCS-( oD2)2, OCS-(HD)2 and mixed OCS- pH2-He trimers are obtained by performing the exact basis-set calculations for the first time based on the newly developed potential energy surfaces ( J. Chem. Phys. 2017, 147, 044313). The "adiabatic-hindered-rotor" (AHR) method is used for reduced-dimension treatment of the hydrogen rotation. The predicted band origin shifts and the infrared spectra are in good agreement with the available experimental values: for the band origin shifts and infrared transitions, the root-mean-square(rms) deviations are smaller than 0.044 and 0.002 cm-1, respectively. We extend the assignments to the unrecorded infrared transitions for OCS-( pH2)2 and OCS-(HD)2 complexes and identify the infrared spectra for OCS-( oD2)2 and OCS- pH2-He for the first time. Three-dimensional density distributions for the ground states of OCS-( pH2)2, OCS- pH2-He, and OCS-(He)2 show that the pH2 molecules are localized in their corresponding global minimum regions, while the pronounced locations of the He atoms are missing in OCS- pH2-He and OCS-(He)2 with forming incomplete circles around the OCS axis. A clear tunneling splitting is observed for the torsional motion of the two hydrogen molecules ( pH2, HD, or oD2) on a ring about the OCS molecular axis, whereas no tunneling splitting is found in OCS- pH2-He or OCS-(He)2 due to a much lower torsional barrier.

Microwave spectroscopy of the seeded binary and ternary clusters CO-(pH2)2, CO-pH2-He, CO-HD, and CO-(oD2)(N=1,2).

We report the Fourier transform microwave spectra of the a-type J = 1-0 transitions of the binary and ternary CO-(pH2)2, CO-pH2-He, CO-HD, and CO-(oD2)N=1,2 clusters. In addition to the normal isotopologue of CO for all clusters, we observed the transitions of the minor isotopologues, (13)C(16)O, (12)C(18)O, and (13)C(18)O, for CO-(pH2)2 and CO-pH2-He. All transitions lie within 335 MHz of the experimentally or theoretically predicted values. In comparison to previously reported infrared spectra [Moroni et al., J. Chem. Phys. 122, 094314 (2005)], we are able to tentatively determine the vibrational shift for CO-pH2-He, in addition to its b-type J = 1-0 transition frequency. The a-type frequency of CO-pH2-He is similar to that of CO-He2 [Surin et al., Phys. Rev. Lett. 101, 233401 (2008)], suggesting that the pH2 molecule has a strong localizing effect on the He density. Perturbation theory analysis of CO-oD2 reveals that it is approximately T-shaped, with an anisotropy of the intermolecular potential amounting to ∼9 cm(-1).

RETRACTED ARTICLE: Quantum effects on translation and rotation of molecular chlorine in solid para-hydrogen

Structure and quantum effects of a Cl2 molecule embedded in fcc and hcp para-hydrogen (pH2) crystals are investigated in the zero-temperature limit. The interaction is modelled in terms of Cl2–pH2 and pH2–pH2 pair potentials from ab initio CCSD(T) and MP2 calculations. Translational and rotational motions of the molecules are described within three-dimensional anharmonic Einstein and Devonshire models, respectively, where the crystals are either treated as rigid or allowed to relax. The pH2 molecules, as well as the heavier Cl2 molecule, show large translational zero-point energies (ZPEs) and undergo large-amplitude translational motions. This gives rise to substantial reductions in the cohesive energies and expansions of the lattices, in agreement with experimental results for pure hydrogen crystals. The rotational dynamics of the Cl2 impurity is restricted to small-amplitude librations, again with high librational ZPEs, which are described in terms of two-dimensional non-degenerate anharmonic oscillators. The lattice relaxation causes qualitative changes of the rotational energy surfaces, which finally favour librations around the crystallographic directions pointing towards the nearest neighbours, both for fcc and hcp lattices. Implications on the reactant orientation in the experimentally observed laser-induced chemical reaction, Cl + H2 → HCl + H, are discussed.

Infrared-active spin-orbit transitions of halogen atom dopants in solid parahydrogen: The role of trapping site geometry

We present theoretical calculations of the (2)P(1/2) ← (2)P(3/2) spin-orbit transition of Cl dopants embedded as substitutional impurities in solid parahydrogen (pH2) matrices. In the lower-energy (2)P(3/2) spin-orbit level, the Cl atom's electron density distribution is anisotropic, and slightly distorts the geometry of the atom's trapping site. This distortion leads to a blue shift in the spin-orbit transition energy; the blue shift is enhanced when we account for the large-amplitude zero point motions of the pH2 molecules surrounding the Cl dopant. We also show that the intensity of the transition depends on the geometry of the trapping site. In the gas phase, the (2)P(1/2) ← (2)P(3/2) atomic transition is electric dipole forbidden. However, when the Cl atom resides in trapping sites that mimic the hexagonal close packed morphology of pure solid pH2, the transition becomes electric dipole allowed through interaction-induced transition dipole moments. These transition dipole moments originate in the anisotropic electron density distribution of the lower-energy (2)P(3/2) spin-orbit level.

Matrix Isolation Spectroscopy and Nuclear Spin Conversion of NH3 and ND3 in Solid Parahydrogen

We present matrix isolation infrared absorption spectra of NH3 and ND3 trapped in solid parahydrogen (pH2) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC) of NH3 and ND3 in freshly deposited pH2 samples as a tool to assign the sparse vibration-inversion-rotation (VIR) spectra of NH3 in the regions of the ν2, ν4, 2ν4, ν1, and ν3 bands and ND3 in the regions of the ν2, ν4, ν1, and ν3 fundamentals. Partial assignments are also presented for various combination bands of NH3. Detailed analysis of the ν2 bands of NH3 and ND3 indicates that both isotopomers are nearly free rotors; that the vibrational energy is blue-shifted by 1-2%; and that the rotational constants and inversion tunneling splitting are 91-94% and 67-75%, respectively, of the gas-phase values. The line shapes of the VIR absorptions are narrow (0.2-0.4 cm(-1)) for upper states that cannot rotationally relax and broad (>1 cm(-1)) for upper states that can rotationally relax. We report and assign a number of NH3-induced infrared absorption features of the pH2 host near 4150 cm(-1), along with a cooperative transition that involves simultaneous vibrational excitation of a pH2 molecule and rotation-inversion excitation of NH3. The NSCs of NH3 and ND3 were found to follow first-order kinetics with rate constants at 1.8 K of k = 1.88(16) × 10(-3) s(-1) and k = 1.08(8) × 10(-3) s(-1), respectively. These measured rate constants are compared to previous measurements for NH3 in an Ar matrix and with the rate constants measured for other dopant molecules isolated in solid pH2.

Time resolved dynamics of phonons and rotons in solid parahydrogen

We use femtosecond optical Kerr effect (OKE) spectroscopy to perform time- and wavelength-resolved pump-probe measurements on the energetics and lifetimes of transverse optical phonons and J = 2 rotons in solid para-hydrogen (pH2). By systematically studying the OKE spectroscopy of pH2 in the gas, liquid, and solid phases for delay times up to 300 ps, we can disentangle homodyne and heterodyne contributions in the solid to the signal that results from the slow phonon (900 fs) and fast roton (94 fs) dynamics. In solid pH2 at 8.5 K, the energies of the J = 2 Raman-active rotons are measured to be 351.98(8) cm(-1), 353.99(8) cm(-1), and 356.00(8) cm(-1) corresponding to the crystal field split MJ = ±1, ±2, and 0 substates. Consistent with the picture of quasi-free molecular rotation within the solid, we observe long-lived roton coherences with T2 lifetimes of 132(5), 114(5), and 82(5) ps for the MJ = ±1, ±2, and 0 substates. In contrast, similar measurements on normal-hydrogen (nH2) solids which nominally contain 75% ortho-hydrogen (oH2) and 25% pH2 molecules display qualitatively different roton dynamics; no persistent roton excitations are observed but rather overdamped librational excitations that decay within 3 ps. The measured low temperature T2 dephasing time of gaseous pH2 implies a collision cross section of 2.08(10) Å(2) which is close to the theoretical value for so-called elastic resonant collisions whereby rotational energy is exchanged between the two colliding partners, but the sum of the rotational energies is preserved. We argue that this same collisional process also determines the T2 dephasing lifetimes in the liquid and solid phases. Finally, OKE spectroscopy on pH2 solids with oH2 concentrations of 1-3% shows evidence for extremely long-lived rotational coherences which likely correspond to J = 2 rotons that are pinned next to single oH2 impurities within the pH2 solid.

Collective Bosonic Excitations in Doped para-H2 Clusters through the Full-Configuration-Interaction Nuclear Orbital Approach

The onset of collective rotational states as minima in the energy spectra of bosonic spin-less para-H2 (pH2) molecules confined in a belt around a molecular dopant is studied by analyzing excites states in (pH2)N-CO2 clusters (N ≤ 5). These minima result from a combined effect of a bosonic-symmetry-induced boundary periodic condition in cyclic arrangements of pH2 and the increasingly intensified hard-core of the effective pH2–pH2 interaction as N increases. The same also applies to doped 4He clusters in a contrast with the fermionic 3He case (N ≤ 4). The onset of the minima for pH2 and 4He marks a reversal in the apparent scaling of the rotational constant with N for the axial rotation around the dopant (from inversely proportional to proportional), whereas the 3He counterpart retains the regular dependence. The newly developed full-configuration-interaction nuclear orbital approach for bosons is presented here for the first time.

Simulation of the time dependent infrared ν2 mode absorptions of (oH2)n:H2O clusters in O2 doped solid hydrogen at 4.2K

Using Fourier transform infrared spectroscopy, we have analyzed the time evolution of the nu(2) mode of (oH(2))(n):H(2)O clusters (n = 11 to 1) embedded in solid normal hydrogen at 4.2 K over a period of 150 h using paramagnetic O(2) to speed up the ortho to para nuclear spin conversion process. For concentrations H(2)O/O(2)/H(2) = 1/20/4000, at time t = 0 right after the solid is prepared, all the H(2)O molecules are preferentially clustered by large numbers of oH(2). With time the cluster distribution irreversibly shifts toward smaller cluster sizes and also generates freely rotating H(2)O (n = 0) which is solvated completely by pH(2) molecules. From a spectral decomposition of the nu(2) (oH(2))(n):H(2)O cluster spectra, a phenomenological simulation of the time behavior of the clusters has been developed. The time evolution is modeled using coupled rate equations in a step by step n to n-1 cluster cascade fashion and analyzed over nine successive time periods. It shows that rotating H(2)O grows only at the expense of cluster n = 1 and that the process dramatically slows down as the conversion of orthohydrogen proceeds. At the end of the conversion process, it was found that cluster n = 1 remained with a very slow decrease.

Structuring molecular hydrogen around ionic dopants: Li+ cations in small pH2 clusters

The formation of clusters of molecular hydrogen around a cationic charge, the Li(+) ion, is modelled by treating the global interaction as a sum of potentials where the Li(+)-H(2) forces come from a full anisotropic potential energy surface produced earlier in our group. The H(2)-H(2) interaction is taken from the literature and treated as a spherical potential between structureless bosonic solvent molecules of para-H(2) (pH(2)). The optimization of geometries and the minimum energy values are obtained via a genetic algorithm treatment whose structures are modified at the end to include a modelling of quantum effects. The results of hydrogen clustering around the cationic dopant indicate the presence of marked shell structures which are initially completed by the octahedral arrangement of the first six solvent partners, while the next shells are dominated by the mainly dispersive interaction among pH(2) molecules and show, in larger clusters, less structured solvent collocations around the ionic impurity.

A dispersed fluorescence and ab initio investigation of the X̃B12 and ÃA12 electronic states of the PH2 molecule

In this work, the X̃B12 and ÃA12 electronic states of the phosphino (PH2) free radical have been studied by dispersed fluorescence and ab initio methods. PH2 molecules were produced in a molecular free-jet apparatus by laser vaporizing a silicon rod in the presence of phosphine (PH3) gas diluted in helium. The laser-induced fluorescence, from the excited ÃA12 electronic state down to the ground electronic state, was dispersed and analyzed. Ten (υ1υ2υ3) vibrationally excited levels of the ground electronic state, with υ1⩽2, υ2⩽6, and υ3=0, have been observed. Ab initio potential-energy surfaces for the X̃B12 and ÃA12 electronic states have been calculated at 210 points. These two states correlate with a Πu2 state at linearity and they interact by the Renner-Teller coupling and spin-orbit coupling. Using the ab initio potential-energy surfaces with our RENNER computer program system, the vibronic structure and relative intensities of the ÃA12→X̃B12 emission band system have been calculated in order to corroborate the experimental assignments.

Path-integral Monte Carlo simulation of the recombination of two Al atoms embedded in parahydrogen

We report the use of path-integral Monte Carlo (PIMC) simulations in the study of the stability against recombination of two Al atoms trapped in solid parahydrogen (pH2) at 4 K. The many-body interactions involving open-shell Al atoms are described with a pairwise additive Hamiltonian model. To estimate the lifetime against recombination, we use PIMC simulations to define an effective potential averaged over the position of the pH2 molecules, followed by a transition-state treatment. Different initial embedding sites are explored. If the initial substitution sites are within a distance of approximately 13 bohrs, the Al atoms will significantly distort the lattice structure to allow recombination, with an accompanying release of energy during the process. For substitution distances longer than approximately 14 bohrs, the dispersion of Al atoms is shown to be metastable, with lifetimes varying from approximately 30 min to several days. The electronic anisotropy is a factor that helps to stabilize the dispersion.

Atomic-scale observation and control of the reaction of phosphine with silicon

The ability to identify and control the pathway by which chemical reactions occur at the level of individual atoms will be of enormous benefit in the coming age of nanotechnology. Here, we present a detailed atomic-resolution scanning tunneling microscopy (STM) study of one such pathway: the adsorption, surface diffusion and dissociation of phosphine (PH3) on silicon (001). We show that PH3 dissociates on Si(001) to produce PH2 + H, PH + 2H and P + 3H moieties. In addition, we show that PH2 molecules are readily able to diffuse on Si(001) at room temperature, resulting in the formation of hemihydride dimers on low dosed Si(001) surfaces. In addition we show that control over the processes of diffusion and surface incorporation can be achieved using STM-based hydrogen lithography; a technique that is now used in many STM laboratories. [DOI: 10.1380/ejssnt.2006.609]

The structuring of a molecular dopant in a quantum solvent

The structural features of a varying number of pH2 molecules bound to one dopant molecule of OCS are examined using quantum calculations with the Diffusion Monte Carlo (DMC) approach. The interaction forces were obtained from ab initio calculations for the OCS–pH2 system and for the pH2 dimer. The overall potential energy surface is then constructed as a sum of interpartner potentials where the pH2 molecules are treated as spherical species. The calculations suggest that pH2 molecules indeed behave as a quantum solvent and that an initial “doughnut” of five to six equivalent pH2 partners is formed on the bisecting middle plane perpendicular to the OCS main axis, showing there strong spatial delocalization. Further added molecules move sequentially to either end of the dopant species, a process seen here to be increasingly more affected by the H2–H2 interactions contributing to clusters stability.

An ab initio based model for the simulation of multiple P2 atoms embedded in a cluster of spherical ligands, with application to Al in solid para-hydrogen

We present here a novel approach to the determination of the interaction between two atoms, each in a P2 electronic state, embedded in a cluster of spherical atoms. The model requires accurate ab initio potential energy curves for the M2 system, for all the 36 electronic states which correlate with dissociation into ground-state M(2P) atoms. Consequently, making use of a valence-bond-like model, we transform these 36 molecular orbital states into a set of 36 Cartesian (qaqb) states which correspond to assigning the two p electrons to Cartesian orbitals centered on either atom. It is then easy to use the earlier Balling and Wright model [L. C. Balling and J. J. Wright, J. Chem. Phys. 79, 2941 (1983)] to determine, in this 36 state basis, the matrix elements corresponding to the interaction of each P2 atom with any number of surrounding spherical ligands. The lowest eigenvalue of the resulting 36×36 matrix defines, in an adiabatic approximation, the potential governing the motion of the atoms. We apply this approach to the determination of the interaction of two Al atoms embedded in solid pH2, site-substituted in the center of two adjacent hexagons. We find the interaction between the two Al atoms to be significantly modified by the presence of the intervening pH2 molecules.

High-resolution spectroscopy of HCl and DCl isolated in solid parahydrogen: Direct, induced, and cooperative infrared transitions in a molecular quantum solid

The infrared spectroscopy and rovibrational dynamics of HCl and DCl dopants in solid parahydrogen (pH2) were investigated using high-resolution spectroscopic methods. The absorption spectra of HCl and DCl monomers in solid pH2 closely resemble the corresponding low temperature gas phase spectra, indicating that the gas phase vibrational and rotational quantum numbers of the dopant are conserved within the pH2 solid. Small deviations from gas phase behavior are observed, however, such as a reduced HCl rotational energy level spacing and splitting of the fivefold orientational degeneracy of HCl rotational states with J=2. In addition, the pure vibrational Q1(0) (v=1←0, J=0←0) H2 transition, which is infrared inactive in pure solid pH2, is detected in the HCl doped sample. We propose that this transition is induced in pH2 molecules by neighboring HCl molecules through a weak “overlap induction” mechanism that is the only induction mechanism operative for J=0 impurities in pH2. Rovibrational transitions are also detected near the induced Q1(0) H2 absorption; these are attributed to cooperative transitions involving single photon excitation of pH2–HCl pairs. Detailed isotopic analysis reveals that these cooperative transitions involve pure vibrational excitation of the pH2 and pure rotational excitation of the HCl. Two-molecule transitions have long been studied for isotopic and rotational dopants (e.g., D2, HD, orthohydrogen) in solid pH2, but this is the first time such cooperative transitions have been attributed to a chemical impurity in pH2 matrices.

Quantum molecular dynamics and spectral simulation of a boron impurity in solid para-hydrogen

Using path-integral molecular dynamics, we investigate the equilibrium properties of a boron impurity trapped in solid para-hydrogen. Because of its singly filled 2p orbital, the B atom interacts anisotropically with the pH2 molecules in the matrix. To assess the effect of this electronic anisotropy, we compare with similar simulations in which an orientation-averaged B–H2 potential is used. We investigate three matrices: (a) a single B atom site substituted for a pH2 molecule, (b) a similar site-substituted matrix with a nearest-neighbor vacancy, and (c) a B atom site substituted not in the bulk but near the pH2 surface. It is found that small distortions of the lattice occur to permit an energetically favorable orientation of the 2p orbital, even in the absence of a vacancy. When the B impurity is located near the surface, the spherically-averaged potential provides a noticeably different description from the case of the anisotropic potential. The 3s←2p absorption spectra of the B chromophore is also predicted by means of a semiclassical Franck–Condon technique using path integrals to sample the quantum lattice configurations. These spectral simulations provide additional insight into the interpretation of experimental observations of trapped B in a solid pH2 matrix.

Evidence for Superfluidity in Para-Hydrogen Clusters Inside Helium-4 Droplets at 0.15 Kelvin.

A linear carbonyl sulfide (OCS) molecule surrounded by 14 to 16 para-hydrogen (pH(2)) molecules, or similar numbers of ortho-deuterium (oD(2)) molecules, within large helium-4 ((4)He) droplets and inside mixed (4)He/(3)He droplets was investigated by infrared spectroscopy. In the pure (4)He droplets (0.38 kelvin), both systems exhibited spectral features that indicate the excitation of angular momentum around the OCS axis. In the colder (4)He/(3)He droplets (0.15 kelvin), these features remained in the oD(2) cluster spectra but disappeared in the pH(2) spectra, indicating that the angular momentum is no longer excited. These results are consistent with the onset of superfluidity, thereby providing the first evidence for superfluidity in a liquid other than helium.

Aluminum−Phosphorus Compounds with Low Coordination Numbers: Structures, Energies, and Vibrational Frequencies of the AlPH2, AlPH3, and AlPH4 Isomers and the H3Al−PH3 Adduct

Ab initio theoretical methods have been used to study small aluminum−phosphorus compounds. Geometries, relative energies, and harmonic vibrational frequencies of isomers of the formulas AlPH2, AlPH3, and AlPH4 and the H3Al−PH3 adduct have been determined at the self-consistent-field level (SCF), by the single- and double-excitation configuration interaction (CISD), and by the single- and double-excitation coupled cluster (CCSD) method. The lowest energy isomers for AlPH2, AlPH3, and AlPH4 are Al−PH2, H2Al−PH, and H2Al−PH2, respectively. The HAl−PH isomer is planar and has a short Al−P bond. An aluminum atom forms a weak complex with phosphine, and the barrier for insertion of Al into PH3 is estimated to be 12.0 kcal mol-1. Various conformations of the H2Al−PH2 molecule are investigated to give estimates of the energy of the π donor−acceptor bond. The energy of complexation for AlH3 and PH3 is obtained using triple-ζ-plus double-polarization function basis sets and the CCSD level. The theoretical energy of...

Bismuth(III) chloride complexation with diphosphine and diarsine oxide ligands: formation and crystal structures of [BiCl3{Ph2P(O)CH2P(O)Ph2}]2 and [BiCl3{As(O)MePh2}{Ph2As(O)Ch2CH2As(O)Ph2}] n

The reactions of bismuth(III) chloride and Ph2PCH2PPh2(dppm) and Ph2AsCH2CH2AsPh2-(dpae) have been studied. The yellow products isolated initially were characterised by spectroscopic (1H and 31P NMR, IR) and microanalytical data as the 1:1 addition compounds BiCl3·dppm and BiCl3·dpae respectively. Recrystallisation of these products from boiling acetonitrile/charcoal effected oxidation of the ligands and resulted in the formation of two new molecular adducts whose structures have been determined by X-ray crystallography. Crystals of [BiCl3{Ph2P(O)CH2P(O)Ph2}]21 are triclinic, space group P with Z= 2. There are two independent centrosymmetric molecules in the unit cell with similar edge–edge shared bioctahedral structures. Each bismuth centre is six-co-ordinate with approximal octahedral geometry being bonded to two terminal chlorine atoms, two bridging chlorine atoms and two oxygen (ligand) atoms. In situ ligand oxidation dppm → Ph2P(O)CH2P(O)Ph2 occurs and the resulting bidentate (OO′) chelation gives rise to puckered six-membered [graphic omitted] ring formation. Crystals of [BiCl3{As(O)MePh2}{Ph2As(O)CH2CH2As(O)Ph2}]n2 are monoclinic, space group P21/a with Z= 4. The structure consists of one-dimensional polymeric chains. Here the in situ ligand oxidation is more complex and results in the formation of two arsine oxide ligands, viz. dpae → Ph2As(O)CH2CH2As(O)Ph2+ As(O)MePh2 both of which are involved in co-ordination to BiIII. Each bismuth atom is bonded in an approximately mer-octahedral arrangement to three terminal chlorine atoms and three oxygen (ligand) atoms. Of the latter, one is provided by a monodentate As(O)MePh2 molecule and the other two by separate Ph2As(O)CH2CH2As(O)Ph2 molecules which, by virtue of their bidentate bridging mode, are linked to adjacent metal centres.