Spin Isomers Research Articles

Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II.

We measured infrared absorption spectra of crystalline II of CH4 and succeeded in detecting a prominent Q(2) peak in the ν3 vibrational region by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels. The two relaxation rates obtained by biexponential fitting were (0.48, 2.3), (1.1, 4.1), (2.3, 5.1), and (3.4, 15.3) in units of inverse hour (h-1) at 5.2, 6.0, 6.5, and 7.0 K, respectively.

Study of Hydration of Biomolecules and Nanoparticles in Aqueous Solutions and Suspensions Using Coherent Laser Spectroscopy

A considerable increase in the concentration of H2O ortho spin isomers and H2O2 molecules in the hydration layer at the interface of water and molecules of DNA, denatured DNA, and protein α- chymotrypsin is found using low-frequency laser spectroscopy of four-photon scattering (FPS). Spectra of rotational resonances of H2O ortho and para spin isomers in aqueous solutions of various biopolymers and in distilled water are obtained in the range of 0 to 100 cm−1 with a spectral resolution of ≈0.12 cm−1. Numerical analysis of the FPS spectra shows that concentration of H2O2 molecules in the DNA hydration layer increases by a factor of three after denaturation. In addition, FPS spectra of aqueous α-chymotrypsin solutions are obtained in the spectral range of ±7 cm−1 at concentrations of 0 to 20mgcm−3. It is found out that the speed of hypersound in the aqueous protein solution measured by the shift of the Brillouin components in the scattering spectrum increases and cubically depends on the concentration.

New assay method based on Raman spectroscopy for enzymes reacting with gaseous substrates.

Enzyme activity is typically assayed by quantitatively measuring the initial and final concentrations of the substrates and/or products over a defined time period. For enzymatic reactions involving gaseous substrates, the substrate concentrations can be estimated either directly by gas chromatography or mass spectrometry, or indirectly by absorption spectroscopy, if the catalytic reactions involve electron transfer with electron mediators that exhibit redox‐dependent spectral changes. We have developed a new assay system for measuring the time course of enzymatic reactions involving gaseous substrates based on Raman spectroscopy. This system permits continuous monitoring of the gas composition in the reaction cuvette in a non‐invasive manner over a prolonged time period. We have applied this system to the kinetic study of the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F. This enzyme physiologically catalyzes the reversible oxidation of H2 and also possesses the nonphysiological functions of H/D exchange and nuclear spin isomer conversion reactions. The proposed system has the additional advantage of enabling us to measure all of the hydrogenase‐mediated reactions simultaneously. Using the proposed system, we confirmed that H2 (the fully exchanged product) is concomitantly produced alongside HD by the H/D exchange reaction in the D2/H2O system. Based on a kinetic model, the ratio of the rate constants of the H/D exchange reaction (k) at the active site and product release rate (k out) was estimated to be 1.9 ± 0.2. The proposed assay method based on Raman spectroscopy can be applied to the investigation of other enzymes involving gaseous substrates.

Sub-Doppler electronic spectrum of the benzene-D2 complex.

Excitation spectrum of the benzene-D2 van der Waals complex in the vicinity of the S 1 ← S 0 60 1 vibronic transition of the monomer was recorded with sub-Doppler resolution by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Contrary to the previous report on the benzene-H2 complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117, 9819 (2013)], both spin isomers correlating to para and ortho D2 (with rotational angular momentum j = 1 and 0, respectively) are identified by using a gas sample of normal D2. Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho species, respectively, in addition to their origin bands. Comparison of the results for the two spin isomers has allowed us to make unambiguous band assignments, and vibrational frequencies of all the three vdW modes have been determined for benzene-H2 and -D2. Among the three modes, the two-dimensional vdW twist is correlated to the hindered internal rotation of H2/D2 and the barrier for the internal rotation has been evaluated: 72 and 66 cm-1 for benzene-H2 and -D2, respectively. Vibronic-state dependence of the intermolecular distance between benzene and H2/D2 is discussed on the basis of precisely determined rotational constants. Homogenous line broadening has been identified for all the observed vibronic bands, and the corresponding upper-state lifetimes are determined to be in the range of 0.3-0.7 ns.

Tipping the polaron–bipolaron balance: concentration and spin effects in doped oligo(aniline)s observed by UV-vis-NIR and TD-DFT

Computational and experimental study of UV-vis-NIR spectra, highlighting that doped oligo(aniline)s form mixtures of polaronic and bipolaronic spin isomers in solution.

Microwave spectra and nuclear quadrupole structure of the NH3-N2 van der Waals complex and its deuterated isotopologues.

The microwave spectrum of the NH3-N2 van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 - 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH3-(ortho)-N2 nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH3-(para)-N2 spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole 14N nuclei of NH3-N2 was resolved for both transitions, and the quadrupole coupling constants associated with the NH3 and N2 subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C 3 axis of NH3 and the N2 axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH3-(ortho)-N2 and (para)-NH3-(para)-N2 and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND3-N2, NHD2-N2, and NH2D-N2, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH3-N2 complex.

Photoexcitation of spin isomers of In and Cd nuclei in the pigmy resonance region

New experimental results on photo-excitation of the Cd and In spin isomers by real and virtual photons (electrons) in the threshold energy region are presented. Measurements have been performed using the INR electron linac LUE—8 MeV and, in parallel, the femtosecond laser facility of the Moscow State University. A detailed description of experimental methods based on the GEANT-4 simulation is done. The obtained data are discussed in frame of the phenomenological model which allows the theoretical evaluation of the isomeric ratio in this energy region. Contribution of the dipole and quadrupole mode excitations in Cd and In nuclei from measurements with real and virtual photons is discussed.

Fully Coupled Quantum Treatment of Nanoconfined Systems: A Water Molecule inside a Fullerene C60.

We implemented a systematic procedure for treating the quantal rotations by including all translational and vibrational degrees of freedom for any triatomic bent molecule in any embedded or confined environment, within the MCTDH framework. Fully coupled quantum treatments were employed to investigate unconventional properties in nanoconfined molecular systems. In this way, we facilitate a complete theoretical analysis of the underlying dynamics that enables us to compute the energy levels and the nuclear spin isomers of a single water molecule trapped in a C60 fullerene cage. The key point lies in the full 9D description of both nuclear and electronic degrees of freedom, as well as a reliable representation of the guest-host interaction. The presence of occluded impurities or inhomogeneities due to noncovalent interactions in the interfullerene environment could modify aspects of the potential, causing significant coupling between otherwise uncoupled modes. Using specific n-mode model potentials, we obtained splitting patterns that confirm the effects of symmetry breaking observed by experiments in the ground ortho-H2O state. Further, our investigation reveals that the first rotationally excited states of the encapsulated ortho- and para-H2O have also raised their 3-fold degeneracy. In view of the complexity of the problem, our results highlight the importance of accurate and computational demanding approaches for building up predictive models for such nanoconfined molecules.

Theoretical model for forbidden ortho-para absorption in nuclear spin isomers of molecules

A theoretical model for forbidden ortho-para absorption in molecules having weak intramolecular mixing of spin isomers is developed. The model is based on the quantum kinetic equation for the density matrix and uses the particular case of $^{13}\mathrm{CH}_{3}\mathrm{F}$ nuclear-spin isomers for numerical calculations. Forbidden ortho-para absorption appears to be very small, making its detection difficult in a standard approach. Two alternative methods for detection of forbidden ortho-para absorption are proposed. The methods use microwave enrichment of spin isomers and microwave speed up of the isomer conversion.

A magnetically focused molecular beam source for deposition of spin-polarised molecular surface layers.

Separating molecular spin isomers is a challenging task, with potential applications in various fields ranging from astrochemistry to magnetic resonance imaging. A new promising method for spin-isomer separation is magnetic focusing, a method which was shown to be capable of producing a molecular beam of ortho-water. Here, we present results from a modified magnetic focusing apparatus and show that it can be used to separate the spin isomers of acetylene and methane. From the measured focused profiles of the molecular beams and a numerical simulation analysis, we provide estimations for the spin purity and the significantly improved molecular flux obtained with the new setup. Finally, we discuss the spin-relaxation conditions which will be needed to apply this new source for measuring nuclear magnetic resonance signals of a single surface layer.

Determining the Humidity-Dependent Ortho-to- Para Ratio of Water Vapor at Room Temperature Using Terahertz Spectroscopy.

The origin of the water spin isomers observed under various physico-chemical conditions is of great interest, including that of H2O molecules in the gas phase. Here, terahertz time-domain spectroscopy (THz-TDS) was used to study the humidity-dependent ortho-to- para (O/P) ratio of water vapor at room temperature. The relative contents of para and ortho molecules were obtained by fitting the absorption lines of water vapor showing the relationship between the spin isomer contents and humidity. Larger O/P ratios with values of ∼3.2 were observed at lower humidity (<20%) due to the stronger attractive forces of para molecules. The concentration of the ortho isomers then began to decrease at higher humidity (>20%) due to the preferential formation of dimers and clusters at increasing concentrations. Thus, the ratio gradually decreased with increasing humidity.

On the impossibility of localised states for molecular rotors with cyclic potentials

We discover a surprising property of an important class of molecular rotors. These rotors have one (e.g. a methyl group) or two (e.g. the planar boron rotor ) moieties that consist of identical nuclei rotating in cyclic model potential energy surfaces with equivalent potential wells (e.g. for CH, for ). The familiar semiclassical picture of this contorsion assumes that the potential wells support equivalent global minimum structures with corresponding localised wave functions being embedded in the individual potential wells. In contrast, we show that the wave functions of these rotors can never be squeezed into a single potential well, and hence, global minimum structures do not exist. Our quantum mechanical derivation describes the rotors in the frame of the proper cyclic molecular symmetry group and makes use of the spin-statistics theorem and the hypothesis of nuclear spin isomers. We show that if the identical nuclei have zero spins, then a hypothetical localised state would violate the spin-statistic theorem. Otherwise, the hypothetical localised state is ruled out as unphysical superposition of different nuclear spin isomers of the molecular rotors.

Generating para-water from para-hydrogen: A Gedankenexperiment

A novel conceptual approach is described that is based on the transfer of hyperpolarization from para-hydrogen in view of generating a population imbalance between the two spin isomers of H2O. The approach is analogous to SABRE (Signal Amplification By Reversible Exchange) and makes use of the transfer of spin order from para-hydrogen to H2O in a hypothetical organometallic complex. The spin order transfer is expected to be most efficient at avoided level crossings. The highest achievable enrichment levels of para- and ortho-water are discussed.

Spin isomers in the ISIS TS1 cryogenic hydrogen moderator

As part of the ISIS TS1 project, work has been carried out on the cryogenic hydrogen moderator system, the main focus being understanding and improving the ratio of spin isomers in the liquid hydrogen moderator.

Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

We study the gravitational collapse of prestellar sources and the associated evolution of their chemical composition. We use the University of Grenoble Alpes Astrochemical Network (UGAN), which includes reactions involving the different nuclear--spin states of H2, H3+, and of the hydrides of carbon, nitrogen, oxygen, and sulfur, for reactions involving up to seven protons. In addition, species-to-species rate coefficients are provided for the ortho/para interconversion of the H3+ + H2 system and isotopic variants. The composition of the medium is followed from an initial steady state through the early phase of isothermal gravitational collapse. Both the freeze--out of the molecules on to grains and the coagulation of the grains were incorporated in the model. The predicted abundances and column densities of the spin isomers of ammonia and its deuterated forms are compared with those measured recently towards the prestellar cores H-MM1, L16293E, and Barnard B1. We find that gas--phase processes alone account satisfactorily for the observations, without recourse to grain-surface reactions. In particular, our model reproduces both the isotopologue abundance ratios and the ortho:para ratios of NH2D and NHD2 within observational uncertainties. More accurate observations are necessary to distinguish between full scrambling processes---as assumed in our gas-phase network---and direct nucleus- or atom-exchange reactions.

An Argon–Oxygen Covalent Bond in the ArOH+ Molecular Ion

AbstractThe OH+ cation is a well‐known diatomic for which the triplet (3Σ−) ground state is 50.5 kcal mol−1 more stable than its corresponding singlet (1Δ) excited state. However, the singlet forms a strong donor–acceptor bond to argon with a bond energy of 66.4 kcal mol−1 at the CCSDT(Q)/CBS level, making the singlet ArOH+ cation 3.9 kcal mol−1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH+ is the predicted singlet with a covalent Ar−O bond.

An Argon-Oxygen Covalent Bond in the ArOH+ Molecular Ion.

The OH+ cation is a well-known diatomic for which the triplet (3 Σ- ) ground state is 50.5 kcal mol-1 more stable than its corresponding singlet (1 Δ) excited state. However, the singlet forms a strong donor-acceptor bond to argon with a bond energy of 66.4 kcal mol-1 at the CCSDT(Q)/CBS level, making the singlet ArOH+ cation 3.9 kcal mol-1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH+ is the predicted singlet with a covalent Ar-O bond.

Hindered rotation and nuclear spin isomers separation of molecularly chemisorbed H2 on Pd(210)

We investigated the hindered rotation and nuclear spin isomer separation of H2 on Pd(210) for various pre-adsorbed atomic hydrogen coverages (Θ), by total energy calculations based on density functional theory. Our results revealed that H2 is in the molecularly chemisorbed state and the adsorption is characterized by a highly anisotropic potential energy surface. Further, we found that J = 1 degenerate level splitting is insensitive to the increase in Θ from 1 to 2 ML. This is due to the comparable potential strengths hindering/restricting the polar rotations in both coverages. On a fully H passivated (3 ML) Pd(210), H2 is in a weakly physisorbed state with a negligible potential anisotropy. Our findings suggest that the activation barrier for polar rotational motion does not strongly depend on the adsorption energy but rather on the surface-molecule bond. The estimated rotational state desorption energies show a separation of ortho and para isomers by around 7.0 meV.

Detection of Nuclear Spin Isomers of Water Molecules by Terahertz Time-Domain Spectroscopy

Nuclear spin isomers of water molecules, i.e., ortho- and para-H2O, are quantum states of one of the most important molecules. Presently, our understanding of the water isomer properties, especially in the gas phase, remains insufficient. Scientific and practical applications of water spin isomers require the development of efficient methods of their detection in gaseous samples. In this paper, we apply the method of wideband terahertz time-domain spectroscopy to study rotational absorption spectra of ortho and para water isomers and measure their relative content in the ambient atmosphere. We find the ortho/para ratio to be 3.03 ± 0.03 close to the expected value of 3 when using the measurement spectral range 0.15–1.05 THz.

Properties of Superheavy Isotopes $Z=120$ and Isotones $N=184$ Within the Skyrme--HFB Model

We study the nuclear properties of even-even superheavy $Z$=120 isotopes and $N$=184 isotones with the Skyrme Hartree-Fock-Bogoliubov (HFB)approach. Within this model we examine the deformation energy surfaces and two paths to fission: a reflection-symmetric path with elongated fission fragments (sEF) and a reflection-asymmetric path corresponding to elongated fission fragments (aEF). Furthermore, we explore the energy surfaces in the region of very large oblate deformations with toroidal nuclear density distributions. While the energy surfaces of toroidal $Z$=120 isotopes and $N$=184 isotones do not possess energy minima without angular momenta,local energy minima (toroidal high spin isomeric states) appear for many of these superheavy nuclei with specific angular momenta about the symmetry axis. We have theoretically located the toroidal high spin isomers (THSIs) of $^{302}$Og$_{184}$,$^{302}120_{182}$, $^{306}120_{186}$, and $^{306}122_{184}$.