The infrared spectrum of the F–H2 anion complex

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Abstract

The infrared spectrum of F–H2 is measured over the H–H stretch region. The νHH transition occurs at 3220 cm−1, displaced 940 cm−1 to lower energy from the H2 fundamental vibrational transition. A weaker band at 3640 cm−1 is assigned to νHH in combination with the intermolecular stretch vibration (νHH + νs). The experimental νHH frequency agrees with a prediction based on an ab initio H–H potential energy curve calculated at the CCSD(T)/aug-cc-pVQZ level. Absence of resolved rotational structure in the νHH band suggests that rovibrational lines are lifetime broadened and that F–H2 in the νHH = 1 level predissociates on a < 3 ps timescale.

Introduction

A novel application of anion-molecule complexes is as precursors for launching and characterizing neutral–neutral reactions in photoelectron studies. It is in this role that F–H2 has been used in a series of studies by Neumark and coworkers [1], [2], [3], [4] to probe the prototypical F + H2  HF + H triatomic rearrangement reaction. The F–H2 complexes are exposed to light that serves to detach the excess electron, with information on both the anion and neutral systems being encoded in the energy distribution of the photoejected electrons. The photoelectron spectra have been the subject of several quantum simulations using increasingly sophisticated potential energy surfaces (PESs) for describing the anion and neutral systems [3], [4], [5], [6], [7], [8]. The simulations require as their foundation, accurate potential energy surfaces for both the anion and neutral systems. We have undertaken the current infrared study of F–H2 with a view to obtaining empirical data, that can be used to test and refine the F + H2 PES.

The F–H2 complex is expected to share many of the general properties of Cl–H2, Br–H2 and I–H2 whose infrared spectra in the H–H stretch region have already been obtained [9], [10], [11], [12]. As shown by theoretical and spectroscopic studies, these species possess linear H-bonded equilibrium geometries with an intermolecular bond that contracts when the H–H stretch mode is excited. These characteristics are manifested in the appearance of νHH bands with Σ–Σ structures and prominent P branch heads. Hindered internal rotation of the H2 sub-unit between equivalent linear minima also plays an important role, and leads to tunnelling splitting of the energy levels. Lower and upper tunnelling levels are associated, respectively with F interacting with para and ortho modifications of H2. The consequences of tunnelling have been discerned in spectra of Cl–D2 and Br–D2 where, in each case, the νDD band consists of two overlapping Σ–Σ sub-bands.

The F–H2 complex has already received considerable theoretical and experimental attention. Nichols et al. [13] performed the first reported ab initio calculations, predicting a linear H-bonded structure. Subsequently, Boldyrev et al. [14] calculated bound rovibrational energy levels for F–H2 and its isotopomers using a PES computed at the MP2(full)/6-311++G(2df 2pd) level. This study highlighted the importance of hindered internal rotation of the H2 sub-unit, which leads to ground state tunnelling doublet levels separated by 0.029 cm−1. Harmonic vibrational frequencies were also reported (ωHH = 3607, ωs = 383 and ωb = 1004 cm−1). Perhaps the most reliable guide to the properties of F–H2 comes from recent ab initio and rovibrational calculations by Hartke and Werner [15], in which a 3-D PES, developed at the CCSD(T) level with large basis set, was used to calculate the anharmonic vibrational energies of the complex. The ground state vibrational level was predicted to be split by 0.05 cm−1 and to lie 1573 cm−1 below the F + H2 (vHH = 0) asymptote. The intermolecular stretch vibration (νs) was determined as 333 cm−1 (0.26 cm−1 tunnelling splitting), while the intermolecular bend νb was calculated as 1332 cm−1 (64 cm−1 tunnelling splitting). The energy of the quasi-bound νHH mode was not reported.

Previously our group has obtained spectra of the F–(D2)n 1  n  6 clusters in the 2500–2900 cm−1 range [16]. The spectrum for the F–D2 dimer exhibits a single broad feature centred at 2700 cm−1, which was assigned to the νDD + νS combination band. The fundamental νDD stretch band, which was not observed, presumably lies below 2500 cm−1, outside the range of our infrared OPO. Due to the higher stretching frequency of H2, we are here able to observe both the νHH band and the νHH + νS combination band.

Section snippets

Experimental methods

The infrared spectrum was obtained by irradiating mass selected F–H2 complexes with tuneable IR light in the H–H stretch region (3000–4000 cm−1) while monitoring production of F photofragments. The complexes were synthesised using a 1:20 mixture of H2/Ar (8 bar) seeded with traces of NF3. The gas was expanded from a pulsed nozzle (0.8 mm orifice diameter, 40 Hz repetition rate) and crossed by 200 eV electrons emitted from twin rhenium filaments adjacent to the nozzle orifice. Presumably, the F

Results and discussion

The mid-infrared spectrum of F–H2 (Fig. 1) features an intense, broad band with a maximum at 3245 ± 10 cm−1, and a weaker band at 3640 ± 20 cm−1. These two peaks can most convincingly be assigned as the νHH (H–H stretch) and νHH + νS bands (H–H stretch in combination with the intermolecular stretch). Both bands are anticipated to possess Σ–Σ structures since the F–H2 complex is predicted to be linear (due to interaction between the H2 quadrupole moment and the F negative charge). The steep lower

Conclusions

The F–H2 complex has been characterized via infrared vibrational predissociation spectroscopy. The dominant band at 3220 cm−1, assigned to the H–H stretch mode, is substantially red-shifted from the free H2 band (ΔνHH = −940 cm−1), consistent with the complex adopting a linear H-bonded structure. A second weaker band located 420 cm−1 to higher energy (at 3640 cm−1) has been assigned to the νHH + νS combination band. Rapid vibrational predissociation ensuing on a < 3 ps timescale, and consequent

Acknowledgements

We are grateful for financial support from the Australian Research Council and the University of Melbourne.

References (21)

  • C.L. Russell et al.

    Chem. Phys. Lett.

    (1996)
  • A.J.R. da Silva et al.

    Spectrochim. Acta A

    (1997)
  • B. Hartke et al.

    Chem. Phys. Lett.

    (1997)
  • A. Weaver et al.

    Faraday Discuss. Chem. Soc.

    (1991)
  • A. Weaver et al.

    J. Chem. Phys.

    (1990)
  • S.E. Bradforth et al.

    J. Chem. Phys.

    (1993)
  • D.E. Manalopoulos et al.

    Science

    (1993)
  • R.T. Skodje et al.

    J. Chem. Soc., Faraday Trans.

    (1997)
  • T. Takazanagi et al.

    Chem. Phys. Lett.

    (2001)
  • D.A. Wild et al.

    J. Chem. Phys.

    (2000)
There are more references available in the full text version of this article.

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Current address: M.P.I. für Biophysikalische Chemie, 37077, Göttingen, Germany.

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