A study of quantum phase transitions and quantum monodromy in the bending motion of non-rigid molecules

The concepts of quantum phase transition (QPT) and excited state quantum phase transition (ESQPT) are applied to the study of bending motion in non-rigid molecules. Two key signatures of QPT and ESQPT, the Birge-Sponer (or anharmonicity) plot and the monodromy plot are discussed. A detailed analysis of bending motion in a series of molecules (HCNO, DCNO, BrCNO, ClCNO, CH3NCS, GeH3NCO, CH3NCO, OCCCS, NCCNO, NCNCS) spanning the rigidly-linear to rigidly-bent transition is presented, and potential functions in terms of angle variables are constructed. The effect of QPT and ESQPT on the rotational constant B and on rotation–vibration interactions through the splitting of the J = 1, Ka = ±1 components is evaluated and, finally, some anomalous features of mass scaling in non-rigid molecules are identified.