Effect of centrifugal distortion and anharmonic vibration on the 19F chemical shielding in CH3F

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Abstract
The temperature and density dependence of the 19F NMR frequency has been measured for CH3F gas. The NMR chemical shielding for 19F nucleus in the isolated CH3F molecule was obtained from 280 to 380 K by extrapolating the frequencies to zero density. This temperature dependence is interpreted in terms of the thermal average of powers of the normal coordinates. The linear term in this expansion is due to centrifugal distortion and anharmonic vibration. Using the cubic force constants and other molecular constants which have been reported for CH3F, an estimate of the dependence of the 19F shielding on the CF bond extension has been obtained.