Low resolution microwave spectroscopy. I. The structural significance of the band spacing

Abstract

Compounds which are nearly symmetric tops, have ∼20 mtorr vapor pressure and have a sizeable dipole moment along the a axis display band microwave rotational spectra under low resolution conditions. The frequencies of the band maxima are adequately represented by a symmetric rotor type equation, ν = (B + C)(J + 1). From calculated spectra it is shown that the B + C value defined above exceeds the ground state effective rotational constant B0 + C0. Low resolution B + C values and high resolution B0 + C0 values are compared for nineteen species. B + C differs from B0 + C0 by less than 2% in all cases and less than 0.8% for molecules with κ < −0.8. Empirically it is found that (B + C)/(B0 + C0) − 1 = ν/ν0 − 1 = 0.025(±0.005)(1 + κ), where k is the asymmetry parameter and ν0 is the hypothetical symmetric rotor frequency, (B0 + C0)(J + 1). This empirical relation allows B0 + C0 to be estimated from B + C with an uncertainty of only a few tenths of a percent.