THE INTERMOLECULAR POTENTIAL BETWEEN AN INERT-GAS AND A HALOGEN - PREDICTION AND OBSERVATION OF TRANSITIONS BETWEEN THE LINEAR AND T-SHAPEDISOMERS 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-v ibration levels calculated from the intermolecular potential surface s erve as an excellent guide for finding the experimental spectra. Pure rotational transitions are observed for the lowest linear Sigma(0) sta te and for an excited T-shaped K = 0 Sigma(1) state of (HeClF)-Cl-35 a nd (HeClF)-Cl-37. Direct transitions between;the linear ground state a nd the T-shaped state are observed for: (HeClF)-Cl-35. The observed-en ergy 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, transiti ons into the two J = 1 levels and one J = 2 level of the K = 1 T-shape d state, Pi(1), are observed for (HeClF)-Cl-35. The He-ClF complex is highly nonrigid, undergoing large amplitude oscillation in both angula r and radial coordinates. The effect of zero-point oscillation is seen in the large difference, 22.9 cm(-1), between the calculated potentia l 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 pote ntial surface is poorly represented as a sum of spherical atom-atom in teractions. 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 a n appreciably better fit of the spectral transitions is obtained by un iformly increasing the magnitude of the interaction potential by 10%. Bound states calculated for a potential with the T-shaped minimum remo ved 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 sta te, Sigma(0), B = 5586.8312(34), D = 1.6595(10) MHz, H = 36.472(93) kH z, 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) M Hz. For the T-shaped Pi state, Pi(1), nu = 100 302.239(46), B = 7430.3 38(32), q(l) = 1380.622(46) MHz, mu(a) = 0.5621(99) D, and eq(eff) Q(P i(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 rotati onal constants, dipole moments, and quadrupole coupling constants for each state. In addition, these values are consistent with a T-shaped S igma(1) state rather than an antilinear Sigma(1) state. (C) 1998 Ameri can Institute of Physics. [S0021-9606(98)00632-1].