Heavier Isotopomers Research Articles

Evolution of CO on Titan

The early evolution of Titan's atmosphere is expected to produce enrichment in the heavy isotopomers of CO, 13CO and C 18O, relative to 12C 16O. However, the original isotopic signatures may be altered by photochemical reactions. This paper explains why there is no isotopic enrichment in C in Titan's atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange of C atoms between the CH 4 and CO reservoirs, mediated by the reaction 1CH 2+*CO→ 1*CH 2+CO, where *C is 13C. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 3.2×10 −12 cm 3 s −1 at the temperature appropriate for the upper atmosphere of Titan. We investigate the isotopic dilution of CO using the Caltech/JPL one-dimensional photochemical model of Titan. Our model suggests that the time constant for isotopic exchange through the above reaction is about 800 Myr, which is significantly shorter than the age of Titan, and therefore any original isotopic enhancement of 13C in CO may have been diluted by the exchange process. In addition, a plausible model for the evolution history of CO on Titan after the initial escape is proposed.

Precise measurements of 115/117/119Sn NMR frequencies and first observation of isotope-induced chemical shifts 1Δ117/119Sn( 15N), 1Δ115/119Sn( 15N), and 2Δ117/119Sn( 29Si) in N-trimethylstannyl amines

Tris(trimethylstannyl)amine 1 and trimethylsilyl-bis(trimethylstannyl)amine 2 (both ca. 13% 15N labelled) were studied by 115/117/119Sn-, 15N-, and 29Si NMR spectroscopy. Precise measurements of the Sn NMR frequencies of the respective Sn/ 15N isotopomers did not reveal any appreciable primary isotope effect, in agreement with the previous results. The standard frequencies Ξ( 119Sn) and Ξ( 117Sn), determined for tetramethyltin more than 30 years ago, were found to be very accurate, and Ξ( 115 Sn)=32.718781 MHz was measured in this work. In the 15N NMR spectra, small secondary isotope effects 1 Δ 117/119Sn( 15N) and 1 Δ 115/119Sn( 15N) could be detected for the first time, the 15N nucleus in the heavier isotopomer being more shielded. In the 29Si NMR spectrum of 2, the expected isotope-induced chemical shift 1 Δ 14/15N( 29Si) is shown, and in addition the so far unknown effect over two bonds 2 Δ 117/119Sn( 29Si) is visible.

Photolysis of Nitrous Oxide Isotopomers Studied by Time-Dependent Hermite Propagation

Nitrous oxide (N2O) plays an important role in greenhouse warming and ozone depletion. Yung and Miller's zero point energy (ZPE) model for the photolysis of N2O isotopomers was able to explain atmospheric isotopomer distributions without invoking in situ chemical sources. Subsequent experiments showed enrichment factors twice those predicted by the ZPE model. In this article we calculate the UV spectrum of the key N2O isotopomers to quantify the influence of factors not included in the ZPE model, namely, the transition dipole surface, bending vibrational excitation, dynamics on the excited state potential surface, and factors related to isotopic substitution itself. The relative cross-sections are calculated as the Fourier transform of the correlation function of the initial vibrational wave function and the time-propagated wave function, using a Hermite expansion of the time evolution operator. The model uses the electronic structure data recently published by Balint-Kurti and co-workers and makes several predictions. (a) The absolute values of the enrichment factors decrease with increasing temperature. (b) Photolysis of N2O will produce “mass-independent” enrichment in the remaining sample. (c) Much of the enrichment is due to decreased heavy isotopomer cross-section over the entire absorption band, in contrast to the wavelength shift predicted by the ZPE model. Consequently, to within the error of the calculation, we predict only minor enrichments at λ < 182 nm. The smaller bending excursion of heavy isotopomers combines with the transition dipole surface to produce a smaller integrated cross-section. This effect is partially countered by the larger fraction of heavy isotopomers in excited bending states; the first three bending states have an integrated intensity ratio of ca. 1:3:6. The model agrees with available experimental enrichment factors and stratospheric balloon infrared remote sensing data to within the estimated error.

Experimental constraints on the fractionation of 13C/ 12C and 18O/ 16O ratios due to adsorption of CO 2 on mineral substrates at conditions relevant to the surface of Mars

We report here measurements of fractionations of stable carbon and oxygen isotopes between CO 2 vapor and CO 2 adsorbed on kaolinite, basalt, or fluorite. Carbon-isotope fractionations between adsorbate and vapor (Δ 13C a-v = 1000 · ln (α a-v), where α = 13C/ 12C adsorbate)/( 13C/ 12C vapor); ∼δ 13C adsorbate-δ 13C vapor) exhibit a ∼1‰ enrichment of 13C in the vapor phase for all substrates and temperatures over the range 190 to 230 K. These fractionations are ‘reversed’ relative to the common expectation that heavy isotopomers should be concentrated into condensed phases relative to coexisting vapor (Lindemann, 1919). Oxygen-isotope fractionations between adsorbate and vapor are complex: Comparison of experiments using vacuum-baked substrates to those in which substrates were not vacuum-baked suggests that exchange between one or both phases of CO 2 and water adsorbed on the sample surfaces influences the 18O/ 16O fractionation between vapor and adsorbate and changes the 18O/ 16O composition of the total CO 2 in the experiment. In experiments on vacuum-baked substrates, oxygen results exhibited ‘normal’ fractionation of ∼1.8‰ (that is, 18O preferentially partitioned into the adsorbed CO 2). No evidence was found for measurable exchange of oxygen isotopes between structural oxygen in mineral substrates and CO 2 adsorbed on those substrates. The observation that 13C is enriched in the vapor phase may help account for subtle (per mil to tens of per mil) enrichments of 13C estimated for the Martian atmosphere as compared to bulk terrestrial carbon.

Formula omitted] isotope effect in the UV photodissociation of N 2O

14 N 2 16 O , 14 N 15 N 16 O and 15 N 14 N 16 O were photolyzed between 202 and 206 nm. The photodissociation products, O( 1 D 2) and N 2( X 1 ∑ g +) , were monitored by a resonance-enhanced multiphoton ionization technique. The O( 1 D 2) signal was weaker when the heavy isotopomers were photolyzed at 205.47 nm. The isotope effect was as large as 10% for 14 N 15 N 16 O , while it was 3% for 15 N 14 N 16 O . The absorption coefficients must be smaller for the heavy isotopomers. An inverse isotope effect was observed when N 2( X 1 ∑ g +) was monitored via the a″ 1 ∑ g + state. 14 N 2 signal was less than that for 14 N 15 N . There must be an isotope effect in the detection efficiency of highly rotationally excited N 2( X 1 ∑ g +) .

Ozone isotope enrichment: isotopomer-specific rate coefficients

Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection.

Stable isotope fractionation during ultraviolet photolysis of N2O

The biogeochemical cycling of nitrous oxide plays an important role in greenhouse forcing and ozone regulation. Laboratory studies of N2O:N2 mixtures irradiated between 193–207 nm reveal a significant enrichment of the residual heavy nitrous oxide isotopomers. The isotopic signatures resulting from photolysis are well modeled by an irreversible Rayleigh distillation process, with large enrichment factors of ε15,18(193 nm) = −18.4,‐14.5 per mil and ε15,18(207 nm) = −48.7,‐46.0 per mil. These results, when combined with diffusive mixing processes, have the potential to explain the stratospheric enrichments previously observed.

Faint OH (ν′ = 10), 17OH, and 18OH Emission Lines in the Spectrum of the Night Airglow1,1Lick Observatory Bulletin 1380. 22Based on observations obtained at the W. M. Keck Observatory, which is operated by the California Institute of Technology and the University of California.

ABSTRACTWe co‐added night‐sky spectra, recorded as by‐products of exposures with the high‐resolution echelle spectrograph (HIRES) of the Keck I 10 m telescope on stars and quasars over a period of approximately 21 / 2 years, to obtain a resultant spectrum with total exposure time ranging from 120 to 24 hr over the wavelength range 5720–8810 Å. On this very high signal‐to‐noise ratio spectrum, OH emission lines from the upper vibrational level ν′ = 10, previously undetected in the night airglow, were measured and identified. Estimates are made for the relative population of OH (ν′ = 10), and possible secondary mechanisms exciting this level are discussed. Also, lines of the isotopic molecules 18OH and 17OH were detected and measured, and the relative populations of these heavy isotopomers are discussed. Lines of the isotopic molecule OD were not detected, for reasons that are discussed, but predicted wavelengths of such lines were calculated and are listed for possible future use with even better signal‐to‐noise ratio spectra, preferably at longer wavelengths.

High resolution electronic spectroscopy of the R⋅SH complexes (R= Ne, Ar, Kr)

The high resolution laser induced fluorescence spectra of the à 2Π3/2 electronic transition of the R⋅SH/D (R= Kr, Ar, Ne) van der Waals (vdW) complexes are reported. Analysis of these bands requires the inclusion of rotation, fine and hyperfine structure, spin-rotation interactions, and parity splittings. A number of molecular parameters are determined, along with internuclear bond distances between the R and the SH moiety. Comparison of the present results for R⋅SH/D is made with the analogous R⋅OH/D species where applicable. In addition, the detailed “rotational” structure and the highly precise determinations of the band origins of the different heavy atom isotopomers are critical for absolute vibrational quantum number assignment in the à state.

The structure of 2,2-dimethyloxirane obtained by microwave spectroscopy: A comparison of oxiranes and thiiranes

The microwave spectra of the heavy atom isotopomers of 2,2-dimethyloxirane, (CH 3) 2COCH 2, were observed in natural abundance using a molecular beam Fourier transform-microwave spectrometer (MB-FTMW). The rotational constants were obtained and structures were derived using the r s, r 0, and r Δ0 methods. The r s (C–C) bond length in the ring is 1.444(6) Å, the r s (C–O) bond lengths are 1.437(2) and 1.419(7) Å, and the C(ring)–C(methyl) distance is 1.522(3) Å.

Isotope effect in gas—liquid chromatography of labelled compounds

The isotope effect in the gas—liquid chromatography (GLC) of 14C- and 2H-labelled compounds was examined experimentally and theoretically. Capillary column gas chromatographic—mass spectrometric (GC—MS) data indicated that the isotope effect was very small for 14C-labelled higher fatty acids and large for perdeuterated n-alkanes. Measurement of the temperature dependence of the corrected retention volumes and separation factors of C 15C 17 n-alkanes and their perdeuterated analogues yielded a relationship between the molar volume, enthalpy change and chromatographic retention: heavier isotopomers having lower molar volumes are eluted earlier when Van der Waals dispersion forces plays a dominant role in the solute—stationary phase interaction. The effect is proportional to the number of heavier atoms in the molecule and should be taken into account in the GLC and GC—MS of isotopically substituted compounds on efficient capillary columns.

Carbon‐13/carbon‐12 isotope effects on 119Sn nuclear shielding

AbstractSecondary 13C/12C isotope effects (Δδi) over one bond on the 119Sn nuclear shielding have been determined for 42 organotin compounds. Changes in the magnitude and sign of Δδi are related to the nature of the carbon atom and the nature of the other substituents at the tin atom. In addition to the rotational and vibrational effects connected with the Sn13C bond itself, Δδi is affected by the bonding of tin to the other substituents. In the case of polar bonds this leads to deshielding of the tin atom in the heavy isotopomer, which compensates the shielding effect normally associated with heavy isotopic substitution. Therefore, the magnitude of Δδi values and of the SnC bond distances are not related in a simple manner. A relationship exists between J(119Sn13C) and Δδi in alkynyltin compounds. The Δδi (13C/12C) values for 29Si and 207Pb shielding in some representative organo‐silanes and ‐plumbanes are reported for comparison.