The spin temperature of NH3 in Comet C/1999S4 (LINEAR)

Manipulating stereoselectivity of parahydrogen addition to acetylene to unravel interconversion of ethylene nuclear spin isomers

Symmetric molecules exist as distinct nuclear spin isomers (NSIMs). A deeper understanding of their properties, including interconversion of different NSIMs, requires efficient techniques for NSIM enrichment. In this work, selective hydrogenation of acetylene with parahydrogen (p-H2) was used to achieve the enrichment of ethylene NSIMs and to study their equilibration processes. The effect of the stereoselectivity of H2 addition to acetylene on the imbalance of ethylene NSIMs was experimentally demonstrated by using three different heterogeneous catalysts (an immobilized Ir complex and two supported Pd catalysts). The interconversion of NSIMs with time during ethylene storage was studied using NMR spectroscopy by reacting ethylene with bromine water, which rendered the p-H2-derived protons in the produced 2-bromoethan(2H)ol (BrEtOD) magnetically inequivalent, thereby revealing the non-equilibrium nuclear spin order of ethylene. A thorough analysis of the shape and transformation of the 1H NMR spectra of hyperpolarized BrEtOD allowed us to reveal the initial distribution of produced ethylene NSIMs and their equilibration processes. Comparison of the results obtained with three different catalysts was key to properly attributing the derived characteristic time constants to different ethylene NSIM interconversion processes: ∼3-6 s for interconversion between NSIMs with the same inversion symmetry (i.e., within g or u manifolds) and ∼1700-2200 s between NSIMs with different inversion symmetries (i.e., between g and u manifolds).

Separation and conversion dynamics of nuclear-spin isomers of gaseous methanol

All symmetrical molecules with non-zero nuclear spin exist in nature as nuclear-spin isomers (NSIs). However, owing to the lack of experimental information, knowledge is rare about interconversions of NSIs of gaseous molecules with torsional symmetry. Here we report our separation and conversion observations on NSI-torsion-specific transition systems of gaseous methanol from a light-induced drift experiment involving partially spatial separation of the ortho and para isomers. We find that vibrationally excited molecules of the methanol spin isomer have a smaller collision cross-section than their ground-state counterparts. Interconversion of the enriched ortho isomer with the para isomer, which is generally considered improbable, has been quantitatively studied by sensitive detections of the spectral intensities. Rather counterintuitively, this reveals that the interconversion is inhibited with increasing pressure. Our results suggest that the spin conversion mechanism in methanol is via a quantum relaxation process with the quantum Zeno effect induced by molecular collisions.

The conversion dynamics of nuclear-spin isomers has only been observed for a small number of molecules, generally with rotational symmetry. Here, the authors observe the separation of nuclear-spin isomers of gaseous methanol and show a decreased interconversion at higher pressures.

New trends in cometary chemistry

This paper presents some of the implications of new comet observations for cometary chemistry: recent observations of bright comets, space missions, and especially the first results of the Deep Impact experiment. Topics which are discussed are the molecular complexity of cometary material, the evidence for molecular diversity from the infrared observations by Deep Impact, possible relations between cometary nuclei and carbonaceous chondrites, the sites of ices in cometary nuclei, the problem of interpretation of the spin temperatures observed in cometary molecules.

Separation and Conversion Dynamics of Four Nuclear Spin Isomers of Ethylene

Molecules with three or more nuclei of nonzero spin exist as discrete spin isomers whose interconversion in the gas phase is generally considered improbable. We have studied the interconversion process in ethylene by creating a sample depleted in the B2u nuclear spin isomer. The separation was achieved through spatial drift of this isomer induced by resonant absorption of narrow-band infrared light. Evolution of the depleted sample revealed conversion between B2u and B3u isomers at a rate linearly proportional to pressure, with a rate constant of 5.5 (+/-0.8) x 10(-4) s(-1) torr(-1). However, almost no change was observed in the Ag isomer populations. The results suggest a spin conversion mechanism in C2H4 via quantum relaxation within the same inversion symmetry.