Knowledge Bank

The barrier to internal rotation of p-fluorotoluene (pFT) and nitromethane (NM) are $4.77cm-1$ and $2cm-1$ respectively. Because of the nearly free internal rotation these molecules can be described by the Hamiltonian. $H=APa2+BPb2+CPC2+Fp2-2A\prime PaP$ together with centrifugal distortion constants for overall and internal rotation. FTIR spectra of a C-type band of pFT at $819cm-1$ and an A-type band of NM at $917cm-1$ are analysed for the $m=o$ state transitions to obtain the upper state rotational and centrifugal distortion constants. We have also assigned the vibration-rotation transitions belonging to the torsional states m-1 and 2. A method based on the splitting of R-branch transitions ( for C-type and for A-type) is proposed for determination of the constant A' and is used for both the molecules.