Reassignment of the 11 537 cm-1 Band of Hydrogen Fluoride Dimer and Observation of the Intermolecular Combination Mode 3ν1 + ν4

High Resolution FTIR and Diode Laser Supersonic Jet Spectroscopy of the N = 2 HF Stretching Polyad in (HF)2 and (HFDF): Hydrogen Bond Switching and Predissociation Dynamics

We report Fourier transform infrared (FTIR) and high resolution diode laser spectra (∼ 1MHz instrumental bandwidth) obtained in cooled absorption cells as well as in a supersonic jet expansion for the N = 2 polyad region of the HF-stretching vibrations of (HF)2, HFDF and DFHF. Three vibrational transitions have been observed for (HF)2 and two for both monodeuterated isotopomers. For (HF)2 we have identified and analysed the observed transitions of the polyad member 22 of the type Δ K a = 0 and Δ K a = ± 1 up to rotational sublevel Δ K a = 3. Band centers as well as rotational constants of all four K a states have been determined. The tunneling splittings due to hydrogen bond switching for these four K a states have been investigated, with the Δ K a = 0 up to Δ K a = 2 sublevels having tunneling symmetry Γ vt = A + for the lower tunneling states, and switching periods ranging from 158ps for K a = 0 to 1.35ns for K a = 2. A tunneling level inversion is found at Δ K a = 3, leading to a symmetry Γ vt = B + for the lower tunneling state of this K a-sublevel. The vibrational assignment of the measured spectra of (HF)2 was established by comparison with the monodeuterated isotopomers HFDF and DFHF. For HFDF we have identified and analysed five subbands between 7600cm-1 and 7730cm-1. We have determined the spectroscopic constants of the rotational levels Δ K a = 0 and Δ K a = 1 for the vibrationally excited state and of the levels of Δ K a = 1 and Δ K a = 2 of the ground state, the latter from combination differences. From the measurements in a supersonic jet expansion we determined the predissociation line width of the N = 22, K a = 1 to be about 120MHz for the Γ vt = A + tunneling state of (HF)2 and about 90MHz for Γ vt = B +. For the Δ K a = 0 level of N = 22 we obtained predissociation line widths ranging around 100MHz, similar to those of the Δ K a = 1 level. In the case of HFDF, the predissociation line width of Δ K a = 1 is about 80MHz. Predissociation lifetimes for these levels with the unbonded HF stretching excited thus are in the range of about 1 to 2ns. The predissociation width in the N = 21 level is uncertain by about a factor three with lg(Δν/MHz) = (3 ± 0.5) and in N = 23 it is about 600MHz corresponding to rounded lifetimes of 0.1ns and 0.3ns when the bonded HF stretching is excited thereby demonstrating strongly mode selective predissociation rates in the N = 2 polyad. Under thermal equilibrium conditions we derived the pressure broadening coefficient for (HF)2 (γ = (6 ± 1) × 10-4cm-1/mbar in the wavenumber range between 7713cm-1 and 7721cm-1 for total gas pressures between 10 and 60mbar, all values as full widths half maximum). For absolute frequency calibrations we have remeasured the first overtone transitions of the monomer HF with much improved precision between P(5) (7515.80151cm-1) and R(7) (7966.22188cm-1).

The (4,0) mode of HF dimer at 14700cm−1

The deltaK = 0 and 1 subbands of the (4,0) <-- (0,0) transition of (HF)2, near 14,700 cm(-1), have been measured by molecular-beam intracavity laser-induced fluorescence. The hydrogen interchange tunneling is basically quenched in (4, 0) for both K = 0 and 1 levels, consistent with the early suggestion from a phenomenological model [H.-C. Chang and W. Klemperer, J. Chem. Phys. 104, 7830 (1996)]. The band origin upsilon0 = 14,700.458(7) cm(-1) and rotational constant (B + C)/2 = 0.22278(31) cm(-1) are determined for K = 0 of the (4, 0) mode. From the observed deltaK = 1 <-- 0 spectrum, we determined that A = 24.3 cm(-1), (B + C)/2 = 0.22296(20) cm(-1), and (B-C) = 4.5(2) x 10(-3) cm(-1). The predissociation linewidths of both K = 0 and 1 levels are 470(30) MHz with no apparent rotational dependence.

Thermodynamic calculations for reactions involving hydrogen halide polymers, ions, and Lewis acid adducts. 4. The temperature dependence of some reactions of HF and HCl

Ab initio MP2/6-311++G(d,p) structure optimizations and frequency calculations have been carried out on 10 polymers, (HX)n; on eight polyhalohydrogenate(1-) anions, HnXn+1-, and on four polyhalohydrogen(I) cations, Hn+1Xn+, (X = F and Cl) at a number of temperatures from 188 to 673 K, and at various total pressures. From the results, ΔG° and K values have been calculated for some reactions involving these species, and the relative amounts determined under a variety of conditions:

nHX –> (HX)n (n = 2-6) HX + HnXn+1- àHn+1Xn+2- (n = 0-3) HX + Hn+1Xn+ àHn+2Xn+1+ (n = 0, 1) There are some marked differences between the fluorine and chlorine series, but generally, the relative amounts of the large molecules and ions decrease as the temperature increases. For all reactions, ln K can be expressed as function to the fourth power in 1/T indicating that ΔH° is more or less temperature dependent over a range of 400-500°.Key words: ab initio, thermodynamics, HCl, HF.