195Pt NMR isotopologue and isotopomer distributions of [PtCln(H2O)6 − n]4 − n (n = 6,5,4) species as a fingerprint for unambiguous assignment of isotopic stereoisomers
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A detailed analysis of the (35)Cl/(37)Cl isotope shifts induced in the 128.8 MHz (195)Pt NMR resonances of [PtCl(n)(H(2)O)(6 - n)](4 - n) complexes (n = 6,5,4) in acidic solution at 293 K, shows that the unique isotopologue and isotopomer distribution displayed by the resolved (195)Pt resonances, serve as a fingerprint for the unambiguous identification and assignment of the isotopic stereoisomers of [PtCl(5)(H(2)O)](-) and cis/trans-[PtCl(4)(H(2)O)(2)].Keywords:
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The METLIN metabolite database has become one of the most widely used resources in metabolomics for making metabolite identifications. However, METLIN is not designed to identify metabolites that have been isotopically labeled. As a result, unbiasedly tracking the transformation of labeled metabolites with isotope-based metabolomics is a challenge. Here, we introduce a new database, called isoMETLIN (http://isometlin.scripps.edu/), that has been developed specifically to identify metabolites incorporating isotopic labels. isoMETLIN enables users to search all computed isotopologues derived from METLIN on the basis of mass-to-charge values and specified isotopes of interest, such as (13)C or (15)N. Additionally, isoMETLIN contains experimental MS/MS data on hundreds of isotopomers. These data assist in localizing the position of isotopic labels within a metabolite. From these experimental MS/MS isotopomer spectra, precursor atoms can be mapped to fragments. The MS/MS spectra of additional isotopomers can then be computationally generated and included within isoMETLIN. Given that isobaric isotopomers cannot be separated chromatographically or by mass but are likely to occur simultaneously in a biological system, we have also implemented a spectral-mixing function in isoMETLIN. This functionality allows users to combine MS/MS spectra from various isotopomers in different ratios to obtain a theoretical MS/MS spectrum that matches the MS/MS spectrum from a biological sample. Thus, by searching MS and MS/MS experimental data, isoMETLIN facilitates the identification of isotopologues as well as isotopomers from biological samples and provides a platform to drive the next generation of isotope-based metabolomic studies.
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The measurement of singly substituted, stable isotopologues, such as 13CO2, by mid-IR spectroscopy is well established. In addition, there is a great interest to exploit the information carried by more exotic isotopologues, i.e. of low abundance, multiply substituted (clumped) isotopic species or site-specific isotopomers. This information on isotopic composition can be used as proxy to constrain formation pathways, source attribution, temperature histories or dating (radioactive isotopes) of the respective molecules. The established method to perform such isotopic analysis is isotope ratio mass spectrometry (IRMS). This approach, however, in particular for rare isotopologues, typically requires very demanding instruments, several hours of analysis time and extensive sample preparation to separate isobaric interferences.Here, we demonstrate an alternative analytical method based on optical interrogation of the molecules by directly probing their ro-vibrational frequencies. This makes the method inherently suitable to distinguish between isotopomers (structural isomers). Furthermore, we propose a low temperature approach that substantially reduces the spectral interferences due to hot-band transitions of more abundant isotopologues. The effectiveness and versatility of this strategy are highlighted by three different applications: i) high-precision mid-IR measurements of clumped 12C18O2, ii) the detection of 14CO2 in enriched CO2 samples, and iii) a new scheme for determination of the intramolecular distribution (terminal and central positions) of 13C in propane.We developed a quantum cascade laser (QCL) spectrometer using a Stirling-cooled circular multipass absorption cell. The distributed feedback (DFB) QCL is driven in intermittent continuous wave (iCW) mode [1] with a repetition rate of 6.5 kHz. Its beam passes through a compact segmented circular multipass cell (SC-MPC) [2] with an optical path length of 6 m. The SC-MPC is placed in a vacuum chamber that is maintained at 5ּ 10-5 mbar and cooled down to 150 K.The precision in the ratios [12C18O2]/[12C16O2] and [12C16O18O]/[12C16O2] is 0.05 %₀ with 25 s integration time. Its accuracy is confirmed by agreement with literature values of the equilibrium constant, K, of the exchange reaction for CO2 samples equilibrated at 300 K and 1273 K [3].As proof of concept, we adapted the system to allow the detection of the radiocarbon 14C in enriched CO2 samples. Due to its ultra-low abundance (10-12), the absorption signatures of this isotopic species is completely hidden by the spectral contributions of the other, more abundant, CO2 isotopologues. Therefore, it is the perfect candidate for low-temperature spectroscopy. We present first results on 14CO2 with a precision of 50 ppt.And finally, we demonstrate the first high-resolution spectra of propane and its site-specific isotopomers (1-13C and 2-13C). We distinguish their individual contributions to the overall absorption spectrum and show a precision better than 0.1 ‰ for both isotopomer ratios (2-13C)/12C and (1-13C)/12C.  [1] M. Fischer et al., Opt. Express, 22(6), 7014–7027 (2014), doi: 10.1364/OE.22.007014.[2] M. Graf, L. Emmenegger, and B. Tuzson, Opt. Lett., 43(11), 2434-2437, (2018), doi: 10.1364/OL.43.002434.[3] A. Nataraj et al. Opt. Express 30, 4631-4641 (2022): doi.org/10.1364/OE.447172. 
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In order to investigate the HD isotope effect on a dihydrogen bonded cation system, we have studied NH+4...BeH2 and its isotopomers by ab initio path integral molecular dynamics. It is found that the dihydrogen bond can be exchanged by NH+(4) rotation. The deuterated isotopomer (ND+(4)...BeD(2); DD) can exchange the dihydrogen bond more easily than other isotopomers such as (NH+4...BeH2; HH). This unusual isotope effect is ascribed to the "quantum localization" which occurs when the effective energy barrier for the rotational mode becomes higher by the zero point energy of other modes. We also found that the binding energy of dihydrogen bonds for DD species is the smallest among the isotopomers.
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We report the first interstellar detection of DC$_7$N and six $^{13}$C-bearing isotopologues of HC$_7$N toward the dark cloud TMC-1 through observations with the Green Bank Telescope, and confirm the recent detection of HC$_5$$^{15}$N. For the average of the $^{13}$C isotopomers, DC$_7$N, and HC$_5$$^{15}$N, we derive column densities of 1.9(2)$\times$10$^{11}$, 2.5(9)$\times$10$^{11}$, and 1.5(4)$\times$10$^{11}$ cm$^{-2}$, respectively. The resulting isotopic ratios are consistent with previous values derived from similar species in the source, and we discuss the implications for the formation chemistry of the observed cyanopolyynes. Within our uncertainties, no significant $^{13}$C isotopomer variation is found for HC$_7$N, limiting the significance CN could have in its production. The results further show that, for all observed isotopes, HC$_5$N may be isotopically depleted relative to HC$_3$N and HC$_7$N, suggesting that reactions starting from smaller cyanopolyynes may not be efficient to form HC$_{n}$N. This leads to the conclusion that the dominant production route may be the reaction between hydrocarbon ions and nitrogen atoms.
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We investigate the mass dependent isotopic fractionation mechanisms, based on photolytic destruction and reaction with O( 1 D), to explain the 15 N/ 14 N and 18 O/ 16 O fractionation of stratospheric N 2 O and reconcile laboratory experiments with atmospheric observations. The Caltech/JPL two‐dimensional (2‐D) model is utilized for detailed studies of N 2 O and its isotopologues and isotopomers in the stratosphere. We compare model results with observations of isotopic enrichment using three different methods of calculating photolytic cross‐sections for each of the major isotopologues and isotopomers of N 2 O. Although the Yung and Miller [1997] successfully modeled the pattern of enrichments for each isotopologue or isotopomer relative to each other, their approach underestimated the magnitude of the enrichments. The ab initio approach by Johnson et al. [2001] provides a better fit to the magnitudes of the enrichments, with the notable exception of the enrichment for the 15 N 14 N 16 O. A simpler, semi‐empirical approach by Blake et al. [2003] is able to model the magnitude of all the enrichments, including the one for 15 N 14 N 16 O. The Blake et al. [2003] cross‐sections are temperature‐dependent, but adjustments are needed to match the measurements of Kaiser et al. [2002a] . Using these modified cross‐sections generally improves the agreement between model and mass spectrometric measurements. Destruction of N 2 O by reaction with O( 1 D) results in a small but nonnegligible isotopic fractionation in the lower stratosphere. On a per molecule basis, the rates of destruction of the minor isotopologues or isotopomers are somewhat less than that for 14 N 14 N 16 O. From our 2‐D model we infer the relative rates for isotopologues and isotopomers 14 N 14 N 16 O (446), 14 N 15 N 16 O (456), 15 N 14 N 16 O (546), 14 N 14 N 17 O (447) and 14 N 14 N 18 O (448), to be 1, 0.9843, 0.9942, 0.9949, and 0.9900, respectively. Thus the destruction of N 2 O in the atmosphere results in isotopic fractionations of (456), (546), (447) and (448) by 19.4, 9.5, 5.5 and 12.0‰. If we do not distinguish between the (456) and (546) isotopomers, the mean isotopic fractionation for 15 N is 14.5‰. If we assume that the mean tropospheric values for δ 456 , δ 546 , δ 15 N and δ 18 O are 16.35, −2.35, 7.0 and 20.7‰, respectively, we infer the following isotopic signature for the integrated sources of N 2 O: δ 456 = − 2.9‰, δ 546 = −11.7‰, δ 15 N = −7.3‰ and δ 18 O = 8.7‰.
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We have measured new submillimeter-wave data around 600 GHz and around 1.1 THz for the 13C isotopologue of formic acid and for the two deuterium isotopomers; in each case for both the trans and cis rotamer. For cis-DCOOH and cis-HCOOD in particular only data up to 50 GHz was previously available. For all species the quality and quantity of molecular parameters has been increased, providing new measured frequencies and more precise and reliable frequencies in the range of existing and near-future submillimeter and far-infrared astronomical spectroscopy instruments such as Herschel, SOFIA, and ALMA.
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We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling were applied using a fitted potential energy surface [J. Chen et al., J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval 4 ⋅ 10−20 to 4 ⋅ 10−17 cm3 s−1, demonstrating that even deuterated versions of the title reaction are possibly relevant to astrochemical processes in molecular clouds. The transferred hydrogen atom dominates the kinetic isotope effect at all temperatures.
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A detailed analysis of the (35)Cl/(37)Cl isotope shifts induced in the 128.8 MHz (195)Pt NMR resonances of [PtCl(n)(H(2)O)(6 - n)](4 - n) complexes (n = 6,5,4) in acidic solution at 293 K, shows that the unique isotopologue and isotopomer distribution displayed by the resolved (195)Pt resonances, serve as a fingerprint for the unambiguous identification and assignment of the isotopic stereoisomers of [PtCl(5)(H(2)O)](-) and cis/trans-[PtCl(4)(H(2)O)(2)].
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Nitrous oxide
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