AN ABINITIO PREDICTION OF THE SPECTROSCOPIC CONSTANTS OF MGNC - THE 1ST MG-BEARING MOLECULE IN SPACE

In collaboration with Kawaguchi et al.'s microwave identification of MgNC as the first Mg-bearing molecule in space (1993), we carried out ab initio molecular orbital calculations of spectroscopic constants at the correlated level for the tri-atomic molecules formed from Mg, C, and N. Three stationary points have been located by ROHF/TZ2P calculations: a linear MgNC (2SIGMA+), a linear MgCN (2SIGMA+), and the transition state between them. Relative energies calculated at the SDCI + Q/TZ2P level are 0 (MgNC), 510 (MgCN), and 2171 cm-1 (the transition state), respectively. The predicted SDCI/TZ2P values for the X 2SIGMA+ MgNC are as follows: rotational constant B0 = 5939.7 MHz, centrifugal distortion constant D0 = 0.0029 MHz, Coriolis coupling constants zeta21 = -0.9588 and zeta23 = 0.2842 MHz, dipole moment mu(e) = 5.22 D, and harmonic frequencies 2189 (omega1), 83.3 (omega2), and 539 cm-1 (omega3). The predicted B0 and D0 values agree well with the experimental measurements. The very low omega2 value in the bending motion indicates that X 2SIGMA+ MgNC is a floppy molecule.