Local‐mode effects on the rotational structure of the first stretching overtone band system of stibine, SbH3

A high‐resolution Fourier‐transform infrared spectrum, resolution about 0.006 cm−1, of the first stretching overtone band system of stibine has been measured and rotationally analyzed in detail up to J’=22 for both 121SbH3 and 123SbH3 isotopic species. The rotational levels of the A1 and E vibrational states have been analyzed simultaneously by including vibration–rotation coupling terms in the Hamiltonian for the two isotopic species. The A1/E vibrational energy‐level separation has been found to be 0.0727(8) cm−1 for 121SbH3 and 0.0763(8) cm−1 for 123SbH3 with the A1 state being higher in energy in both cases. Fits including 1445 transitions with the standard deviation of 0.0024 cm−1 for 121SbH3 and 1322 transitions with the standard deviation of 0.0022 cm−1 for 123SbH3 have been obtained by optimizing altogether 33 upper‐state parameters in both cases. A simple local‐mode model is shown to account very well for the vibrational dependence of the rotational constants and the coefficients of vibrationally...