Clumped-isotope signatures at equilibrium of CH₄, NH₃, H₂O, H₂S and SO₂

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• 作者：Qi Liu ; Yun Liu ; ^{liuyun@vip.gyig.ac.cn" class="auth_mail" title="E-mail the corresponding author}
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 February 2016
• 年：2016
• 卷：175
• 期：Complete
• 页码：252-270
• 全文大小：901 K

文摘

High precision Δ_i values at equilibrium determined by theoretical methods are imperatively needed as references for the development of new clumped-isotope thermometers (or tracers). In this study, quantum chemistry methods with corrections beyond the harmonic approximation are used to obtain the clumped-isotope signatures at equilibrium of several gas-phase molecules (i.e., CH₄, NH₃, H₂O, H₂S, and SO₂). Here, we consider as many corrections to the traditional Bigeleisen–Mayer equation as possible to obtain accurate Δ_i   values at equilibrium and their temperature dependences. The corrections include anharmonic correction for zero-point energy, anharmonic correction for vibrational excited states, vibration–rotation coupling correction for zero-point energy, vibration–rotation coupling correction for vibrational excited states, quantum mechanical correction to rotation, and centrifugal distortion correction, which are important for theoretical understanding of clumped-isotope signals. Specifically, molecular constants are calculated via second-order perturbative analysis at the MP2/aug-cc-pVTZ level. The CCSD/6-311+G(3df,3pd) and CCSD/aug-cc-pVTZ levels are further employed to ensure the precision of harmonic frequencies of methane. For methane, a polynomial fit of ${\mathrm{\Delta }}_{{}^{13}\text{CH}_3\text{D}}$ values over the temperature range of from 273.15 to 1000 K is obtained: Our results are slightly different from previous theoretical calculations, and may serve as new anchors for calibrating experimental observations.