The intermolecular potential between an inert gas and a halogen: Prediction and observation of transitions between the linear and T-shaped isomers of HeClF

The intermolecular potential surface of He and CIF is calculated with a large basis at the fourth-order Moller-Plesset level. The rotation-vibration levels calculated from the intermolecular potential surface serve as an excellent guide for finding the experimental spectra. Pure rotational transitions are observed for the lowest linear Sigma(0) state and for an excited T-shaped K = 0 Sigma(1) state of (HeClF)-Cl-35 and (HeClF)-Cl-37. Direct transitions between;the linear ground state and the T-shaped state are observed for: (HeClF)-Cl-35. The observed-energy difference between the J = 0 level of the linear state and the J = 0 level of the T-shaped state is 2.320 cm(-1). In addition, transitions into the two J = 1 levels and one J = 2 level of the K = 1 T-shaped state, Pi(1), are observed for (HeClF)-Cl-35. The He-ClF complex is highly nonrigid, undergoing large amplitude oscillation in both angular and radial coordinates. The effect of zero-point oscillation is seen in the large difference, 22.9 cm(-1), between the calculated potential energy minima of -58.1 (linear) and -35.2 cm(-1) (T-shaped) and the measured value (including zero-point energy) of 2.320 cm(-1). The potential surface is poorly represented as a sum of spherical atom-atom interactions. At both minima the He-Cl distance is shorter than the sum of van der Waals radii. The ab initio potential is too shallow since an appreciably better fit of the spectral transitions is obtained by uniformly increasing the magnitude of the interaction potential by 10%. Bound states calculated for a potential with the T-shaped minimum removed show significant differences from experiment, indicating that the T-shaped minimum does indeed exist. Spectroscopic constants for (HeClF)-Cl-35 are obtained in a fit to experimental data. For the ground state, Sigma(0), B = 5586.8312(34), D = 1.6595(10) MHz, H = 36.472(93) kHz, mu(a) =0.8780(14)D, and eq(eff) Q(J = 1) -133.659(18) MHz. For the T-shaped state, Sigma(1), nu = 69 565.023(35), B = 7056.161(17), D = 6.9523(24) MHz, mu(a) = 0.620(12) D, and eq(eff) Q(J=1) = -39.936(92) MHz. For the T-shaped Pi state, Pi(1), nu = 100 302.239(46), B = 7430.338(32), q(l) = 1380.622(46) MHz, mu(a) = 0.5621(99) D, and eq(eff) Q(Pi(1)(-) J=1)= -45.15(87) MHz. The large change in geometry between the Sigma(0) and Sigma(1) states is evidenced by the difference in rotational constants, dipole moments, and quadrupole coupling constants for each state. In addition, these values are consistent with a T-shaped Sigma(1) state rather than an antilinear Sigma(1) state. (C) 1998 American Institute of Physics. [S0021-9606(98)00632-1].