The rotational levels of the ground vibrational state of formaldehyde

A variational procedure for rovibrational energy levels and wavefunctions of centrally connected tetra-atomic molecules is extended to include high rotational states, and in particular, J less than or equal to 10 levels for the vibrational ground state of formaldehyde. It is very important to do this because it has made possible the calculation of the usual rotational spectroscopic constants which correspond to the forcefield and geometry. A direct comparison with the 'observed' spectroscopic constants is therefore possible. The geometry and forcefield are refined against 65 J = 0 levels of H2CO, 6 J = 0 levels of D2CO, 42 J = 1, 70 J = 2 and 98 J = 3 levels of the ground and fundamentals of H2CO and D2CO, using an iterative scheme. The mean absolute error of the J = 0 levels is 1.10 cm(-1) and that for J not equal 0 is 0.005 cm(-1), and the predicted geometry is CH = 1.10064 Angstrom, CO = 1.20296 Angstrom and HCO = 121.648 degrees. Finally, the rotational constants A, B, and C for the ground state are 281956, 38846 and 34003 MHz, compared with the observed values 281971, 38836, and 34002 MHz. The centrifugal distortion constants Delta(J), Delta(JK), Delta(K) and delta(J), are 77, 1275, 18 113 and 11 kHz compared with 75, 1291, 19 422 and 10 kHz. These results underline the accuracy of the new quartic forcefield.