H2 Rotation Research Articles

Bending Relaxation of H2 O by Collision with Para- and Ortho-H2.

We extend our recent theoretical work on the bending relaxation of H2 O in collisions with H2 by including the three water modes of vibration coupled with rotation, as well as the rotation of H2 . Our full quantum close-coupling method (excluding the H2 vibration) is combined with a high-accuracy nine-dimensional potential energy surface. The collisions of para-H2 O and ortho-H2 O with the two spin modifications of H2 are considered and compared for several initial states of H2 O. The convergence of the results as a function of the size of the rotational basis set of the two colliders is discussed. In particular, near-resonant energy transfer between H2 O and H2 is found to control the vibrational relaxation process, with a dominant contribution of transitions with = , and being respectively the H2 initial and final rotational quantum numbers. Finally, the calculated value of the H2 O bending relaxation rate coefficient at 295 K is found to be in excellent agreement with its experimental estimate.

Quenching transitions for the rovibrational transitions of water: Ortho-H2O in collision with ortho- and para-H2.

We present here the first full computation of the rovibrational quenching of a polyatomic molecule (water) by a rotating molecular projectile (H2). The computation is performed for quenching from the first bending mode of water at ν ≃ 1595cm-1 with a rotation energy of up to ∼400cm-1 in the bending mode. Molecular hydrogen is in its para and ortho modifications; it is rotating with a rotational quantum number of up to 4 and 3, respectively. All computations are performed on a very reliable and fully tested potential water-hydrogen energy surface of full dimensionality. Dynamics is performed in the full coupled channel formalism in the rigid bender approximation with a decoupling of the water rotation and vibration bases. Rate coefficients are converged for a kinetic temperature range 50-500K. The crucial importance of the proper treatment of the projectile rotation is emphasized with orders of magnitude differences between the different channels for the H2 rotation. Sensitivity to the actual rovibrational initial state of water exists but in a weaker manner. Overall quenching rate coefficients are about 10-12cm3s-1, remaining one to three orders of magnitude lower than pure rotational quenching. They should be employed to model denser and warmer astrophysical media, such as high atmospheres or star and planet forming regions, which are to be explored by infrared space telescopes, such as JWST.

Predissociation spectroscopy of cold CN−H2 and CN−D2

The vibrational predissociation spectra of the CN–H2 and CN–D2 complexes are measured in regions between 450 and 3100 cm in an ion trap at different temperatures at the free electron lasers for infrared experiments (FELIX) Laboratory. Strong differences between the vibrational spectra of the two para and ortho nuclear spin isomers (CN–(p-H2) or CN–(o-H2) and CN–(p-D2) or CN–(o-D2)) are detected. In the case of CN–H2 we could assign the observed bands with the help of an accurate quantum calculation. The spectrum is dominated by the first overtone of the hindered H2 rotation in combination with the hindered CN rotation of the CN–(o-H2) nuclear spin isomer. The energetically higher-lying CN–(p-H2) isomer was not observed due to efficient ortho-para ligand exchange. The C–N stretching mode was recorded at 2040(1) cm. For the CN–D2 system several vibrational modes of both spin isomers overlap in their low-lying intermonomer streching and bending region, but the main observed bands are attributed to the more stable CN–(p-D2) spin isomer. The D–D stretching vibration was found to be at 2898(1) cm.

Collisional Excitation of CF+ by H2: Potential Energy Surface and Rotational Cross Sections

The CF+ molecule is considered one of the key species for the study of fluorine chemistry in the interstellar medium (ISM). Its recent detection, as well as its potential use as a tracer for atomic fluorine in the ISM, has increased the interest in the study of the physical and chemical properties of this cation. Accurate determination of the CF+ abundance in the ISM requires detailed modeling of its excitation from both radiation and collisions with the most dominant species, which are usually atomic and molecular hydrogen. Here, we report a new highly accurate potential energy surface (PES) describing the interaction between CF+ and the H2 molecule. Exact quantum time-independent calculations of the rotational excitation cross sections for collisions of CF+ with both para- and ortho-H2 are reported. Results obtained for collisions with para-H2 are compared to previous calculations performed using an approximate PES averaged over H2 rotation. Excitation data for collisions with ortho-H2 are provided for the first time.

Potential energy surface and rovibrational bound states of the H2-C3N- van der Waals complex.

Since their recent detection in the interstellar medium, anions have raised the question of their possible mechanisms of formation, destruction and excitation. This requires knowledge of their interaction with the most abundant interstellar species. In the present work, a four dimensional rigid rotor model of the potential energy surface is developed for the collision of C3N- with H2. Ab initio calculations are performed with explicitly-correlated coupled-cluster theory via CCSD(T)-F12/aug-cc-pVTZ. Two linear equilibrium structures are found, different in the orientation of C3N-. Two more equilibrium structures, symmetrically equivalent, are obtained by the permutation of H atoms. The vibrational dynamics is mainly controlled by the considerable difference between the two bending frequencies that correspond to the hindered rotations of C3N- and H2. This arises from the potential energy surface which is soft for rotation of C3N- and stiff for rotation of H2, and also from the large difference in mass between both monomers. Although a high potential barrier prevents the rotation of H2, a significant tunneling effect is observed which causes a splitting in the degenerate energy levels. On the contrary, the rotation of C3N- is allowed since the energy of the saddle points is lower than the energy of the bound states, but the wavefunctions remain localized around each linear structure unless a large excitation energy is available.

Electromagnetic nuclear spin conversion: Hydrogen on amorphous solid water

The interpretation of the conversion patterns of molecular hydrogen adsorbed on amorphous solid water confirms the electromagnetic nature of the process. Magnetic spin-orbit coupling and hyperfine contact introduce some electron spin-orbital excitation in the H2-ASW ground state. The molecule-solid electron repulsion evacuates the H2 rotation momenta. The theoretical rates fit the experimental ones in magnitude and steep temperature dependence. Fundamental discussion focus on the role of the surface electric field and on the dissipation of the rotational energy by the solid phonons.

UNCOVERING THE NUCLEUS CANDIDATE FOR NGC 253

NGC253 is the nearest spiral galaxy with a nuclear starburst which becomes the best candidate to study the relationship between starburst and AGN activity. However, this central region is veiled by large amounts of dust, and it has been so far unclear which is the true dynamical nucleus. The near infrared spectroscopy could be advantageous in order to shed light on the true nucleus identity. Using Flamingos-2 at Gemini South we have taken deep K-band spectra along the major axis and through the brightest infrared source. We present evidence showing that the brightest near infrared and mid infrared source in the central region, already known as radio source TH7 and so far considered just a stellar supercluster, in fact, presents various symptoms of a genuine galactic nucleus. Therefore, it should be considered a valid nucleus candidate. It is the most massive compact infrared object in the central region, located at 2.0" of the symmetry center of the galactic bar. Moreover, our data indicate that this object is surrounded by a large circumnuclear stellar disk and it is also located at the rotation center of the large molecular gas disk of NGC 253. Furthermore, a kinematic residual appears in the H2 rotation curve with a sinusoidal shape consistent with an outflow centered in the candidate nucleus position. The maximum outflow velocity is located about 14 pc from TH7, which is consistent with the radius of a shell detected around the nucleus candidate observed at 18.3 {\mu}m (Qa) and 12.8 {\mu}m ([NeII]) with T-ReCS. Also, the Br_gamma emission line profile is blue-shifted and this emission line has also the highest equivalent width at this position. All these evidences point out TH7 as the best candidate to be the galactic nucleus of NGC 253.

Free H₂ rotation vs Jahn-Teller constraints in the nonclassical trigonal (TPB)Co-H₂ complex.

Proton exchange within the M-H2 moiety of (TPB)Co(H2) (Co-H2; TPB = B(o-C6H4P(i)Pr2)3) by 2-fold rotation about the M-H2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP(iPr)3)Fe(H2) (Fe-H2) (SiP(iPr)3 = [Si(o-C6H4P(i)Pr2)3]). The g-values for Co-H2 and Fe-H2 show that both have the Jahn-Teller (JT)-active (2)E ground state (idealized C3 symmetry) with doubly degenerate frontier orbitals, (e)(3) = [|mL ± 2>](3) = [x(2) - y(2), xy](3), but with stronger linear vibronic coupling for Co-H2. The observation of (1)H ENDOR signals from the Co-HD complex, (2)H signals from the Co-D2/HD complexes, but no (1)H signals from the Co-H2 complex establishes that H2 undergoes proton exchange at 2 K through rotation around the Co-H2 axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co-H2 exhibits rotor-like behavior in solution because the underlying C3 molecular symmetry combined with H2 exchange creates a dominant 6-fold barrier to H2 rotation. Fe-H2 instead shows H2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H2(σ*) π-backbonding that becomes dependent on the H2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L2-M-E axis (E = Si for Fe-H2; E = B for Co-H2). Notably, the isotropic (1)H/(2)H hyperfine coupling to the diatomic of Co-H2 is nearly 4-fold smaller than for Fe-H2.

State-to-state, multi-collision, energy transfer in H–H2 gas ensembles

We use our recently developed computational model of energy flow in gas ensembles to study translation-to-internal energy conversion in an ensemble consisting of H2(0; 0) in a bath of H atoms. This mixture is found in plasmas of industrial importance and also in interstellar clouds. The storage of energy of relative motion as rovibrational energy of H2 represents a potential mechanism for cooling translation. This may have relevance in astrophysical contexts such as the post-recombination epoch of the early universe when hydrogenic species dominated and cooling was a precondition for the formation of structured objects. We find that conversion of translational motion to H2 vibration and rotation is fast and, in our closed system, is complete within around 100 cycles of ensemble collisions. Large amounts of energy become stored as H2 vibration and a tentative mechanism for this unequal energy distribution is suggested. The "structured dis-equilibrium" we observe is found to persist through many collision cycles. In contrast to the rapidity of excitation, the relaxation of H2(6; 10) in H is very slow and not complete after 10(5) collision cycles. The quasi-equilibrium modal temperatures of translation, rotation, and vibration are found to scale linearly with collision energy but at different rates. This may be useful in estimating the partitioning of energy within a given H + H2 ensemble.

Origin of the Enhanced Interaction of Molecular Hydrogen with Extraframework Cu+ and FeO+ Cations in Zeolite Hosts. A Periodic DFT Study

The interactions of hydrogen molecules with Cu+ and FeO+ extraframework cations in chabazite were studied using periodic DFT (B3LYP) calculations. Dispersion forces were accounted for by adding Grimme’s correction to the DFT energy at the B3LYP-optimized geometries. Two potential binding sites are found for FeO+ cations inside chabazite as for Cu+. The preferred site I is located in the middle of the six-membered ring, where FeO+ coordinates to three oxygen atoms of the zeolite framework, while in the less stable site IV FeO+ coordinates to two oxygen atoms of the eight-membered ring (ΔE = 17.3 kJ mol−1). The interaction of H2 with FeO+ (Eads = −17.5 kJ mol−1) is weaker than that with Cu+ (Eads = −85 kJ mol−1), so that binding is only favorable at the more open site IV, where no framework reorganization is needed. For both CuCHA and FeOCHA, the geometry of the adsorbed H2−M+ complex is governed by M d → H2 σ* back-donation, which also contributes to a large extent to the final adsorption energy and controls the barrier to H2 rotation. The M−H distance, the polarization of the metal d orbital involved in the back-donation toward the H2 molecule, as well as the H2 charge density show that the M d → H2 σ* back-donation is larger in H2−Cu+ than in H2−FeO+. Both the binding energy and the barrier to rotation are therefore much greater for H2−CuCHA than for H2−FeOCHA (15.1 vs 7.4 kJ mol−1), in agreement with the available experimental data. A comparison between site I and site IV along with results on the possible adsorption of a second H2 molecule per cation site suggests that the different adsorption sites observed by experiment are more likely due to the presence of different metal cation environments rather than binding of more than one H2 per cation site.

Accommodation of tmRNA–SmpB into stalled ribosomes: A cryo-EM study

In eubacteria, translation of defective messenger RNAs (mRNAs) produces truncated polypeptides that stall on the ribosome. A quality control mechanism referred to as trans-translation is performed by transfer-messenger RNA (tmRNA), a specialized RNA acting as both a tRNA and an mRNA, associated with small protein B (SmpB). So far, a clear view of the structural movements of both the protein and RNA necessary to perform accommodation is still lacking. By using a construct containing the tRNA-like domain as well as the extended helix H2 of tmRNA, we present a cryo-electron microscopy study of the process of accommodation. The structure suggests how tmRNA and SmpB move into the ribosome decoding site after the release of EF-Tu.GDP. While two SmpB molecules are bound per ribosome in a preaccommodated state, our results show that during accommodation the SmpB protein interacting with the small subunit decoding site stays in place while the one interacting with the large subunit moves away. Relative to canonical translation, an additional movement is observed due to the rotation of H2. This suggests that the larger movement required to resume translation on a tmRNA internal open reading frame starts during accommodation.

Quantum-mechanical calculations of vibrationally resolved cross sections for non-dissociative charge transfer of O3+ with H2

Charge transfer due to collisions of ground state O3+(2s22p 2P) ions with molecular hydrogen is investigated using the quantum-mechanical molecular-orbital close-coupling (MOCC) method，and electronic and vibrational state-selective cross sections along with the corresponding differential cross sections are calculated for projectile energies of 100，500，1000 and 5000eV/u at the orientation angles of 25°，45° and 89°. The adiabatic potentials and radial coupling matrix elements utilized in the QMOCC calculations were obtained with the spin-coupled valence-bond approach. The infinite order sudden approximation (IOSA) and the vibrational sudden approximation (VSA) are utilized to deal with the rotation of H2 and the coupling between the electron and the vibration of H2. It is found that the distribution of vibrationally resolved cross sections with the vibrational quantum number v′ of H+2(v′) varies with the increment of the projectile energy; and the electronic and vibrational state-selective differential cross sections show similar behaviors: there is a highest platform within a very small scattering angle，beyond which the differential cross sections decrease as the scattering angle increases and lots of oscillating structures appear，where the scattering angle of the first structure decreases as E-1/2p with the increment of the projectile energy Ep; and the structure and amplitude of the differential cross sections are sensitive to the orientation of molecule H2，which provides a possibility to identify the orientations of molecule H2 by the vibrational state-selective differential scattering processes.

Circumnuclear Gas in Seyfert 1 Galaxies: Morphology, Kinematics, and Direct Measurement of Black Hole Masses

(Abridged) The two-dimensional distribution and kinematics of the molecular, ionized, and highly ionized gas in the nuclear regions of Seyfert 1 galaxies have been measured using high spatial resolution (~0''.09) near-infrared spectroscopy from NIRSPEC with adaptive optics on the Keck telescope. Molecular hydrogen, H2, is detected in all nine Seyfert 1 galaxies and, in the majority of galaxies, has a spatially resolved flux distribution. In contrast, the narrow component of the BrG emission has a distribution consistent with that of the K-band continuum. In general, the kinematics of H2 are consistent with thin disk rotation, with a velocity gradient of over 100 km/s measured across the central 0''.5 in three galaxies, and across the central 1''.5 in two galaxies. The kinematics of BrG are in agreement with the H2 rotation, except in all four cases the central 0''.5 is either blue- or redshifted by more than 75 km/s. The highly ionized gas, measured with the [Ca VIII] and [Si VII] coronal lines, is spatially and kinematically consistent with BrG in the central 0''.5. Dynamical models have been fitted to the two-dimensional H2 kinematics, taking into account the stellar mass distribution, the emission line flux distribution, and the point spread function. For NGC 3227 the modeling indicates a black hole mass of Mbh = 2.0{+1.0/-0.4} x 10^7 Msun, and for NGC 4151 Mbh = 3.0{+0.75/-2.2} x 10^7 Msun. In NGC 7469 the best fit model gives Mbh < 5.0 x 10^7 Msun. In all three galaxies, modeling suggests a near face-on disk inclination angle, which is consistent with the unification theory of active galaxies. The direct black hole mass estimates verify that masses determined from the technique of reverberation mapping are accurate to within a factor of three with no additional systematic errors.

Driving Energies of Hydrogen Scrambling Motions in CH5+

A comprehensive quantum chemistry study with natural bond orbital analysis is performed to reveal the intrinsic properties of the floppy potential energy surface of the smallest methonium ion CH5+. In contrast to the low-energy barriers of Cs(I)-->Cs(II)-->Cs(I) and Cs(I)-->C2v-->Cs(I) that correspond to the H2 rotation and H-flip motions, the remarkably larger intramolecular interaction energies are the driving power to mimic the hydrogen scrambling in CH5+. As for the H2 rotation and H-flip motions, the hyperconjugative and electrostatic interactions compete strongly. They together with other interactions compensate for each other and lead to the floppy potential energy surface.

State to State to State Dynamics of theD+H2→HD+HReaction: Control of Transition-State Pathways via Reagent Orientation

The influence of reagent rotation on the dynamics of the D+H2 -->HD+H reaction is studied. The state-resolved differential cross section is measured using the Rydberg-atom scheme in a crossed beam experiment. It is found that the H2 rotation has a strong influence on the results. This effect was traced to the selection of the quantum bottleneck states through reagent orientation, thus suggesting a novel strategy to control the transition-state pathways in direct chemical reactions.

Ab initio configuration-interaction investigation of optical transitions in K+He and K+H2

The potassium resonance line (4s-->4p) centered around 770 nm is a major contributor to the optical extinction in the atmospheres of certain classes of brown dwarfs and extrasolar giant planets. The resonance line is significantly broadened by collisions with He and H2, and an accurate calculation of the line profile is needed for astrophysical models of these objects. As a first step, we report an accurate ab initio study of the K+He and K+H2 potential-energy curves correlating to the K 4s and 4p atomic energy levels, together with the dipole moments governing the transitions between these potential-energy curves. The molecular calculations have been carried out using a multireference configuration-interaction method, with the molecular orbitals expanded in a large Gaussian basis set. The transition dipole moments show significant variation with the molecular geometry. Calculations for the K+H2 system have been carried out for a range of H2 orientations and internuclear separations, so that the effect of H2 rotation and vibration may be explicitly included in future calculations of the pressure-broadened line profiles.

Ortho-Metalated Ruthenium Hydrido Dihydrogen Complexes: Dynamics, Exchange Couplings, and Reactivity

A series of ortho-metalated ruthenium hydrido dihydrogen complexes of the form RuH(H2)(X)(PiPr3)2 (X = 2-phenylpyridine (ph-py) (2), benzoquinoline (bq) (3), phenylpyrazole (ph-pz) (4)) were prepared by adding 1 equiv of the corresponding substrate X−H and 2 equiv of PiPr3 to Ru(COD)(COT) under 3 bar of H2. The analogous complex RuH(H2)(ph-py)(PCy3)2 (1) was prepared by adding 1 equiv of 2-phenylpyridine to RuH2(H2)2(PCy3)2. The ortho-metalated X ligand is coordinated to ruthenium via the nitrogen of one ring and the ortho carbon of a second ring as a result of C−H activation. 2 and 3 were characterized by X-ray diffraction. They are the first examples of complexes displaying exchange couplings between the hydride and the dihydrogen ligands. NMR studies and DFT calculations are used to understand this phenomenon. The series of ortho-metalated model complexes RuH(H2)(X)(PH3)2 was investigated by DFT at the B3PW91 level. The coherent (quantum-mechanical) as well as the incoherent (classical) exchange rates have been determined by line shape analysis, and the activation energies hardly depend on the nature of the X ligand: Ea(coherent) = ca. 10 kJ mol-1, Ea(incoherent) = ca. 40 kJ mol-1 (ca. 50 kJ mol-1 by DFT). The corresponding transition states (2qTSQEC−4qTSQEC) for the classical exchange process have been located by DFT at the B3PW91 level. The dihydrogen ligand is now trans to N and perpendicular to the plane of the chelating ligand. These states connect to the isomer with the dihydrogen trans to C through coupled rotation of H2 and proton transfer from H2 to H. The dihydrogen ligand can be substituted easily, and the corresponding complexes RuH(L)(ph-py)(PiPr3)2 with L = N2 (5), O2 (6), CO (7), C2H4 (8) have been isolated and fully characterized by NMR and by crystal structure analyses in the case of 6 and 8. The model systems RuH(L)(ph-py)(PH3)2 (5q−8q) have been optimized at the DFT level (B3PW91). In the case of 8, we could not detect any ethylene insertion in either the Ru−H or the Ru−C bond. Theoretical calculations explain the differences we observed with the Murai type catalysts, which are highly reactive to ethylene insertion.

State-to-State integral cross section for the H+H2O-->H2+OH abstraction reaction.

The initial state selected time-dependent wave-packet method was extended to calculate the state-to-state integral cross section for the title reaction with H2O in the ground rovibrational state on the potential energy surface of Yang, Zhang, Collins, and Lee. One OH bond length was fixed in the study, which is justifiable for the abstraction reaction, but the remaining 5 degrees of freedom were treated exactly. It was found that the H2 molecule is produced vibrationally cold for collision energy up to 1.6 eV. The OH rotation takes away about 4% of total available energy in the products, while the fraction of energy going to H2 rotation increases with collision energy to about 20% at 1.6 eV.

Hindered rotation of H2 adsorbed interstitially in nanotube bundles

A theoretical study of the rotational dynamics of H2 molecules trapped in the interstitial channels of a carbon nanotube bundle is presented. The potential used in this study is modeled as a sum of atom–atom (C–H) van der Waals interactions and electrostatic interactions of the molecule with the surrounding nanotubes. The rotational energy spectra is calculated using a product wave function, where the coupling between translational and rotational modes is treated in a mean-field manner. A molecular dynamics simulation study was performed for estimating the hydrogen rotational barrier. Both theoretical calculations and simulation results reveal the existence of a large rotational barrier (∼40 meV). The consequences of this rotational barrier for the rotational energy levels are worked out in detail.

Synthesis, X-ray crystal structure, NMR characterization and theoretical calculations on [Cp2Ta(η2-H2)(CO)]+, the first thermally stable group 5 dihydrogen complex

Protonation of Cp2TaH(CO) (Cp = C5H5, 1a; C5H4But, 1b) by HBF4·Et2O at −78 °C in CH2Cl2 affords [Cp2TaH2(CO)]BF4 (2, 3) as mixtures of 2 isomers. The minor ones (2a, 2b) contain the known trans-dihydride [Cp2TaH2(CO)]+ cations whereas the major ones (3a, 3b) are [Cp2Ta(η2-H2)(CO)]BF4, the first group 5 dihydrogen complexes. The crystal structure of the analogous complex 3a·BArf4 recorded at 120 K confirms the presence of the coordinated dihydrogen ligand, which displays an H–H separation of 1.09(2) A in agreement with distances calculated from NMR data. Protonation of Cp2TaH2(SiMe2Ph) by (Et2O)2 ·HBArf4 does not lead to an analogous silane derivative but to the new dinuclear complex [(Cp2TaH2)2(μ-H)](BArf4). Variable temperature NMR studies were carried out on the dihydrogen complex [Cp2Ta(H2)(CO)]+ (3) and its isotopomers. The high field signal of [Cp2Ta(HD)(CO)]+ (3-d) shows a decoalescence at 208 K in both 1H and 2D NMR, which allows us to calculate the barrier to rotation of HD (9.6 kcal mol−1). The absence of decoalescence in the signal of 3 down to 173 K and the absence of a large kinetic isotope effect for the classical rotation of H2 were demonstrated. These results are understood in terms of the presence of very large exchange couplings in a non-rotating dihydrogen ligand. The large barrier of rotation for the dihydrogen ligand in 3 was shown by DFT calculations to arise from a transition state in which the dihydrogen ligand is only coordinated through σ-donation from the H–H bond. The analogous phosphite and phosphine complexes {Cp2TaH2[P(OMe)3]}+ (4) and [Cp2TaH2(PMe2Ph)]+ (5) were shown to be cis dihydrides, in agreement with DFT calculations on a model compound, to display exchange couplings in NMR and no isotope effect for the classical exchange of the hydride ligands.