The effect of anharmonic vibration and centrifugal distortion on nuclear shielding in linear triatomic molecules: NNO and CO2

Abstract

The temperature dependence of the mean bond displacement in linear triatomic molecules NNO and CO2 have been determined using the well-established anharmonic force fields for these molecules. The results are applied to explain the temperature dependence of the nuclear shielding observed in the zero-pressure limit for the 15N and 13C nuclei in 15N15NO and 13CO2. By fitting the observed σ0(T)−σ0(300 K) for T=250–350 K, empirical values of (∂σ/∂Δr)e=−220 ppm/Å (CO2), −1030 ppm/Å (NN*O), and −5190 ppm/Å (N*NO) are obtained. These derivatives are discussed in comparison to similar molecular types. The average shielding for the vibrational states (000) and (0110) are calculated for both 15N nuclei in NNO and compared with values obtained from experimental spin-rotational constants. The empirical derivatives are also used to calculate NMR isotope shifts from the mass-dependent 〈Δr〉.