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Spin-spin coupling tensors in fluoromethanes
Chemistry (Weinheim an Der Bergstrasse, Germany)
|June 7, 2000
Summary
Theoretical calculations of spin-spin coupling tensors in fluoromethanes show good agreement with experimental data. The indirect contribution to couplings is generally small, allowing for structural determination without significant error.
Area of Science:
- Quantum chemistry
- Computational physics
- Nuclear Magnetic Resonance (NMR) spectroscopy
Background:
- Spin-spin coupling tensors (J) are crucial for understanding molecular structure and dynamics.
- Fluoromethanes (CH3F, CH2F2, CHF3) are important model systems for studying chemical bonding and intermolecular interactions.
- Experimental determination of J tensors, especially anisotropic components, can be challenging.
Purpose of the Study:
- To theoretically determine all spin-spin coupling tensors (J) for fluoromethanes using ab initio calculations.
- To compare theoretical results with experimental liquid crystal NMR (LC NMR) data.
- To assess the reliability of the multiconfiguration self-consistent field linear response (MCSCF LR) method for calculating J tensors.
Main Methods:
- Multiconfiguration self-consistent field linear response (MCSCF LR) ab initio calculations were performed.
- Experimental LC NMR data for CF and FF spin-spin couplings were obtained and analyzed.
- Contributions from molecular vibrations and their correlations were considered in the analysis of experimental data.
- Anisotropic indirect coupling (1/2J(aniso)) was determined by comparing experimental and calculated dipolar couplings.
Main Results:
- Theoretical J tensors for fluoromethanes were obtained, including principal values and orientations.
- Experimental LC NMR data for CF and FF couplings in CH3F, CH2F2, and CHF3 were reported.
- A fair agreement was found between theoretical ab initio results and experimental indirect coupling contributions.
- Relative indirect contributions (1/2J(aniso)/D(exp)) were found to be small (under 1.7%) and negative.
Conclusions:
- The MCSCF LR method provides reliable J tensors for fluoromethanes.
- The indirect contribution to CF and FF couplings is generally negligible for determining molecular orientation and structure.
- For high accuracy, partial correction for indirect contributions using transferable tensors is recommended.
- MCSCF LR is superior to density-functional theory for calculating J tensors in these systems.
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