Theoretical estimation of 13C-D clumped isotope effects in methyl of several organic compound

Abstract:

Recent developments in mass spectrometry and tunable infrared laser direct absorption spectroscopy make it possible to measure ¹³C-D clumped isotope effects of methane. These techniques can be further applied to determine ¹³C-D clumped isotope effects of methyl fragments, therefore need accurate equilirbium Δ_i values to calibrate experimental measurements. In this study, we calculate temperature depandences of ¹³C-D clumped isotope signatures in methyl of several organic compounds including ethane, propane, acetic acid, etc. Our calculation are performed at CCSD/6-311+G(3df,3pd) by using Gaussian 03 program with no scale treament.

Our results show that the Δ_i values of ¹³C-D clumping in methyl fragments of different organic compounds yield similar signals (~5.5‰ at 25˚C, slightly lower than Δ_i value of ¹³C-D clumping in methane). For testing the calculated accuracy, theoretical treaments beyond the harmonic level by including several higher-order corrections to the Bigeleisen-Mayer equation are used. Contributions from higher-order corrections (e.g., AnZPE, AnEXC, VrZPE, VrEXC, QmCorr and CenDist) are estimated to repire the ignorings of the Bigeleisen-Mayer equation (the anharmonic effects of vibration, vibration-rotation coupling, quantum mechanics and centrifugal distortion for rotation, etc.) for the calculation of partition function ratios. The results show that the higher-order corrections contribute ~0.05‰ at 25˚C, which is similar to the contribution for calculating ¹³C-D clumped isotope signature of methane. By comparing our calculated frequencies to the measured ones, the uncertainty of our calculation of Δ_i values ¹³C-D clumping in methyl fragments is considered to be within ~0.05‰ at room temperature.