IR Spectrum of FHF − and FDF − Revisited Using a Spectral Method in Four Dimensions Guillermo Pe ́ rez-Herna ́ ndez, † Jesú s Gonza ́ lez-Va ́ zquez, ‡ and Leticia Gonza ́ lez* ,§ † Institut fü r Mathematik, Freie Universitä t Berlin, Arnimallee 6, 14195 Berlin, Germany ‡ Instituto de Química Física Rocasolano, CSIC, C/Serrano 119, 28006 Madrid, Spain § Institute of Theoretical Chemistry, University of Vienna, Wä hringer Strasse 17, 1090 Vienna, Austria ABSTRACT: A four dimensional (4D) time-dependent calculation to obtain the first vibrational states of the hydrogen bifluoride ion, FHF − , and its deuterated counterpart, FDF − , has been performed using a spectral method in Cartesian coordinates. The corresponding potential energy surfaces have been computed at the CCSD(T)/aug- cc-pVTZ level of theory. The obtained values for the fundamental vibrational bands ν̃ 1 = 589 cm −1 , ν̃ 2 = 1305 cm −1 , and ν̃ 3 = 1372 cm −1 assigned to the symmetric stretch, bend, and asymmetric stretch modes, respectively (598, 943, and 972 cm −1 for FDF − , respectively) are in good agreement with available experimental and theoretical values. Selected overtones and mixed modes are also calculated. Infrared spectra have been simulated using the dipole approximation for two different polarization directions of the incident light. ■ INTRODUCTION The bifluoride ion (FHF − ) is the simplest representative of the hydrogen bihalides XHY − (X, Y = F, Cl, Br, or I), a family of compounds well characterized experimentally 1−32 as well as theoretically. 34−54 The ongoing interest in FHF − arises from the fact that it presents one of the strongest hydrogen bonds known. For the dissociation of the hydrogen bond toward F − − HF, energies (D 0 ) of up to ca. 45 kcal/mol have been measured 15−17,29 an exceptional value that places the bond (in energy terms) far from the typical hydrogen bond (D 0 of ca. 3−10 kcal/mol) and closer to a covalent bond. Additionally, from the molecular orbital theory viewpoint it has been regarded as a four-electron, three-center bonding situation 55,56 rather than a usual hydrogen bond of the form O−H···O. Moreover, hydrogen bihalides have been recently suggested as electron acceptors in the production of organic super- conductors, 53 given their large electron affinities (6.453 eV for FHF − ). Also interestingly, FHF − has been put forward as an apt candidate for laser induced asymmetrical dissociation 57 or the induction of a toroidal hydrogen bond 58 with circularly polarized light. Due to its interesting bonding nature and potential applications, the spectroscopical characterization of the FHF − anion dates back to 1941 when the crystal IR spectrum of potassium bifluoride was measured 1 and continues until today. Further solid state results include, e.g., those of Dawson, who examined both FHF − and FDF − in single KHF 2 and KDF 2 crystals using Raman spectroscopy and measured the symmetric stretch band ν̃ 1 of both isotopomers. 9 The ν̃ 2 (bending mode) and ν̃ 3 (asymmetric stretching mode) have been measured independently by Ault 14 and Hunt 19 for the anion FHF − in an argon matrix. In the gas phase, the most cited experiments on FHF − and its deuterated counterpart, FDF − , are those of Kawaguchi and Hirota, 20,21,59 who reported ν̃ 1 = 583 cm −1 , ν̃ 2 = 1286 cm −1 , and ν̃ 3 = 1331 cm −1 for FHF − . The fact that FHF − displays a large anharmonicity in its vibrational structure, especially in the asymmetric stretching mode (ν̃ 3 ) has also prompted a number of theoretical spectroscopic studies. Moreover, being a closed-shell singlet and having only 20 electrons, FHF − is the perfect candidate for high-level ab initio studies and quantum dynamical calculations. Interestingly, the vast majority of the theoretical studies aiming at obtaining vibrational frequencies use time-independent methods and none the time-dependent method we present here. One of the first theoretical studies on FHF − is from Almö f, 33 who provided 2D data for the symmetric and asymmetric stretching vibrations. The bending was first described by Thorson and co-workers in a series of papers, 37−39 where the fundamental symmetric, bending, and asymmetric stretching frequencies of FHF − were found at ν̃ 1 = 608 cm −1 , ν̃ 2 = 1363 cm −1 , and ν̃ 3 = 1565 cm −1 , respectively. S ̌ pirko and co-workers 51 attempted to provide transition frequencies in FHF − using a 3D model, which delivers ν̃ 1 = 603 cm −1 , ν̃ 2 = 1353 cm −1 (calculated as (1/2)2ν̃ 2 ), and ν̃ 3 = 1498 cm −1 . The latest study in FHF − was performed by Hirata et al. 54 who calculated the anharmonic vibrational frequencies by the vibrational self-consistent-field, configuration-interaction, and second-order perturbation methods with a multiresolution composite potential energy surface (PES) generated with the accurate coupled-cluster method. The obtained results, ν̃ 1 = Special Issue: Jö rn Manz Festschrift Received: June 14, 2012 Revised: August 14, 2012 Published: August 14, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 11361 dx.doi.org/10.1021/jp3058383 | J. Phys. Chem. A 2012, 116, 11361−11369