Clumped-isotope signatures at equilibrium of CH4, NH3, H2O, H2S and SO2
Clumped-isotope signatures at equilibrium of CH4, NH3, H2O, H2S and SO2
复制标题
CH4、NH3、H2O、H2S 和 SO2 平衡时的聚集同位素特征
DOI：
10.1016/j.gca.2015.11.040
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发表时间：
2016-02
影响因子：
5
通讯作者：
Liu Yun
中科院分区：
文献类型：
--
作者：
Liu Qi;Liu Yun
用理论方法测定平衡态的高精度Δ i值是开发新的团块同位素温度计（或示踪剂）的必要参考。在这项研究中，量子化学方法的修正超出了谐波近似，以获得在平衡的几个气相分子（即ch4， nh3, h2o， h2s和so2）的团块同位素特征。在这里，我们考虑尽可能多地修正传统的Bigeleisen-Mayer方程，以获得准确的Δ平衡i值及其温度依赖性。这些校正包括零点能量的非调和校正、振动激发态的非调和校正、零点能量的振动-旋转耦合校正、振动激发态的振动-旋转耦合校正、旋转的量子力学校正和离心畸变校正，这些校正对团块同位素信号的理论理解具有重要意义。具体来说，通过MP2/aug-cc-pVTZ水平的二阶微扰分析计算分子常数。进一步采用CCSD/6-311+ G （3df, 3pd）和CCSD/ augg -cc- pvtz水平确保甲烷谐波频率的精度。对于甲烷，在273.15 ~ 1000 K的温度范围内，得到Δ CH 3 13 D值的多项式拟合：Δ CH 3 13 D= 0.00255 1000 T 4-0.11639 1000 T 3+ 1.01364 1000 T 2-0.43627 1000 T。我们的结果与以往的理论计算略有不同，可以作为校准实验观测值的新锚点。
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 (ie, CH 4, NH 3, H 2 O, H 2 S, and SO 2). 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 Δ CH 3 13 D values over the temperature range of from 273.15 to 1000 K is obtained: Δ CH 3 13 D= 0.00255 1000 T 4-0.11639 1000 T 3+ 1.01364 1000 T 2-0.43627 1000 T Our results are slightly different from previous theoretical calculations, and may serve as new anchors for calibrating experimental observations.
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