Absolute ¹³C nuclear magnetic shielding of simple isolated molecules from gas phase measurements

The new experimental value of ¹³C absolute shielding constant in an isolated ¹³CO molecule was evaluated from the ¹³C and ³He gas phase NMR measurements performed for ³He/¹³CO mixtures. An absolute shielding constant σ₀(¹³CO)^300 K= 0.80(90) ppm was previously formally established for this reference molecule with uncertainty but much higher precision than ever before. This result serves as a reference value for the ¹³C NMR absolute shielding scale. Several earlier used shielding values were noted and then discussed. Taking this estimate into account, we also report the gas-phase ¹³C shielding constants obtained by extrapolation to the zero density limit for 57 carbon containing molecules and 86 different atom positions in the molecules. Our results are very precise experimental absolute shielding values which are a benchmark set of data suitable for comparison with the sophisticated theoretical calculations. They fully correlate with the experimental and advanced theoretical values reported earlier in the literature. The shielding constant of tetramethylsilane (TMS) of neat liquid used as an external standard is established as σ(¹³C)^300 K(TMS) = 186.43(90) ppm. For TMS used as internal standard, σ(¹³C)^300 K(0.05% TMS in CDCl₃) = 186.43 + 0.048(%) ppm and this value can be recommended as a secondary reference. With more than 20 years of achievements in this field, we report a large collection of carbon shielding constants and second virial coefficients from the gas phase experiments.