5314027783

POSTER SESSION, H42

fully ab initio force fields of near-spectroscopic

QUALITY FOR FIRST-ROW POLYATOMICS: c₂h₂ and fcch

JAN M. L. MARTIN. Department of Organie Chemistry, Weizmann Institute oj Science, IL-76100 Rehovot, Israel; TIMOTHY J. LEE, Thermo Sciences Dwision STC 230-3, NASA Ames Research Center, Moffett Field. CA 94035-1000. USA. B. JOAKIM PERSSON, National Supercomputer Centre, Linkóping University, 581 83 Linkóping, Sweden; PETER R. TAYLOR. Department of Chemistry and Bio-chemistry and San Diego Supercomputer Center,

Uniuersity of Califomia, San Diego. 9500 Gilman Drwe, MC 0505, San Diego. CA 92093-0505, USA.

The quartic force field of acetylene was determined¹ using the CCSD(T) (coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected triple excitations) met bod and a varicty of large one-particle basis sets of the the atomie natural orbital (ANO), correlation consistent (cc-pVnZ), and augmented corrclation consistent (aug-cc-pVnZ) types. The harmonie n_g bending frequency u>4 and the corresponding an-harmonicity uą - i/.\ are both found to be extremely sensitive to the basis set used, in particular to the presence of a sufficient complement of diffuse functions. (Due to sym-metry cancellation, the eorresponding effcct on tlie tt_u modę, i.e. u>s and - ^5, is much weaker.) Similar phenomena are obscrved morę generally in bending modes for inolecules that possess carbon-carbon multiple bonds. Tentative exp!anations are advaneed. Our l>est computed cjuartir force field, which combines CCSD(T)/[6.s5p4d3/2<//4s3p2dl/] an-harmonicitics with a geometry and harmonie frequencies that additionally include inner-shell correlation effeets. reproduces the observed fundamentals for HCCH. HCCD, DCCD, lł^l3CCH, and H^,3C^I3CH with a mean absolute error of 1.3 cm ¹, and the equilibrium rota-tional constant to four decimal places, without any empirical adjustment. Anharmonicity and quartic resonance constants are in excellent agreement with the recent deterinination of Temsamani and Herman [.1. Chem. Phys. 103. 6371 (1995)], except for the vibrational /-doubling constant /?45, for which an adjustment to the computed force field is proposed. The geometry, harmonie frequencies, and quartic force field of the FCCH molecule were computed² using similar methods. Both cubic and quartic resonances were included in the vibrational model Our finał force field, which does not involve any empirical adjustment, reproduces 36 low-lying vibrational band origins with a mean absolute error of 1.6 cm'¹. The bending anharmonieities exhibit a marked basis set sensitivity (albeit less pronounced than in C₂H₂), due almost entirely to the CCF bend. Evidence was found for the existence of an additional ąuartic resonance, u₂ % ^3 + 2Our best estimate, with conservative uncertainties, for the geometry is r_r(CF)=1.2768±0.0005, r*(CC) = 1.1964±0.0005, and r_f(CH)=1.0601±0.0005 A.

1

   .1. M. L. Martin. T. .1 Lee. and P. R. Taylor. J. Chem. Phys. 108, 676 (1998).

2

   B. .1. Persson. P. R. Taylor, and .1. M. L. Martin. J. Phys. Chem. A 102, 2483 (1998).