Rotationally resolved spectroscopy of a librational fundamental band of hydrogen fluoride tetramer

The rotationally resolved spectrum of a fundamental band of hydrogen fluori de tetramer has been recorded using a pulsed slit-jet, diode laser spectrom eter. The band has a parallel rotational structure and is assigned as the H -F out-of-plane libration fundamental with A(u) symmetry. Ninety-five groun d state combination differences were fit to a symmetric top Hamiltonian to give the following ground state rotational constants: B " = 0.132 081(7) cm (-1), D-J' = 7.1(7) x 10(-7) cm(-1), D-JK' = -9(2) x 10(-7) cm(-1), H-JJJ' = 6(2) x 10(-10) cm(-1), H-JJK' = 9(7) x 10(-10) cm(-1), H-JKK' = -1.3(8) x 10(-10) cm(-1). A total of 190 transitions were fit to determine the upper state spectroscopic constants: v(4) = 714.7849(1) cm(-1), B " = 0.129 634( 5) cm(-1), Delta(C-B) = 0.001 344 cm(-1), D-J' = 6.4(5) x 10(-7) cm(-1), D- JK' = -4.5(6) x 10(-7) cm(-1), Delta D-K = 2.92(8) x 10(-6) cm(-1), H-JJJ' = 3(1) x 10(-10) cm(-1), H-JKK' = -1.55(6) x 10(-8) cm(-1); Delta H-KKK = - 4.65(6)x10(-8) cm(-1). Furthermore, a perpendicular band centered at 752.7 cm(-1) was observed. The band has a rotational line spacing that gives an a pproximate B " value of 0.132 cm(-1); it has been assigned as the E-u symme try, H-F in-plane libration fundamental of the HF tetramer. Finally, a para llel band was observed at 741.0 cm(-1) with B " = 0.076 cm(-1) and has been assigned as the A " symmetry, H-F out-of-plane libration fundamental of th e HF pentamer. Structural parameters and harmonic vibrational frequencies a re estimated from first-principles, correlated MP2 and CCSD(T) calculations . These are the largest calculations performed to date for this system with respect to both orbital basis set and level of electron correlation. The C CSD(T) harmonic frequencies are, in particular, the first reported for the tetramer at this level of theory. Based on our results, we suggest "best es timates" of R-FF = 2.51 Angstrom, r(HF) = 0.947 Angstrom, and theta(HFF) = 9.7 degrees for the structural parameters and a range for D-e of 27.4 to 28 .1 kcal/mol (D-0 = 19.7 to 20.4 kcal/mol) for the parameters and for the en ergy of the tetramer dissociating into four monomers. (C) 2000 American Ins titute of Physics. [S0021-9606(00)01326-X].