Article pubs.acs.org/JPCA
Experimental Evidence for Blue-Shifted Hydrogen Bonding in the Fluoroform−Hydrogen Chloride Complex: A Matrix-Isolation Infrared and ab Initio Study R. Gopi, N. Ramanathan, and K. Sundararajan* Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India S Supporting Information *
ABSTRACT: The 1:1 hydrogen-bonded complex of fluoroform and hydrogen chloride was studied using matrix-isolation infrared spectroscopy and ab initio computations. Using B3LYP and MP2 levels of theory with 6‑311++G(d,p) and aug‑cc‑pVDZ basis sets, the structures of the complexes and their energies were computed. For the 1:1 CHF3−HCl complexes, ab initio computations showed two minima, one cyclic and the other acyclic. The cyclic complex was found to have C−H···Cl and C−F···H interactions, where CHF3 and HCl sub-molecules act as proton donor and proton acceptor, respectively. The second minimum corresponded to an acyclic complex stabilized only by the C−F···H interaction, in which CHF3 is the proton acceptor. Experimentally, we could trap the 1:1 CHF3−HCl cyclic complex in an argon matrix, where a blue-shif t in the C−H stretching mode of the CHF3 sub-molecule was observed. To understand the nature of the interactions, Atoms in Molecules and Natural Bond Orbital analyses were carried out to unravel the reasons for blue-shifting of the C−H stretching frequency in these complexes.
1. INTRODUCTION The phenomenon of intermolecular interaction plays a significant role in the fields of biology, chemistry, and material science and has been a subject of interest for many years to researchers. Extensive studies were carried out using experimental techniques and theoretical methods on strong and weak hydrogen-bonding and van der Waals interactions. These interactions are believed to have a significant effect on a variety of chemical and biochemical processes and in crystal structures. Model systems help in understanding many biological reactions.1−13 Among several spectroscopic techniques that may be used to characterize the H-bonding interactions, infrared (IR) spectroscopy is one of the best. The IR technique probes the shift in the vibrational frequencies of the H-bonded complex with respect to the frequencies of the monomer. In the H-bonded complexes, bond lengthening is reflected by a red-shift in the vibrational frequencies. There have been a large number of studies reported on red-shifted H-bonding involving C−H···O, O−H···O, N−H···O, O−H···N, and N−H···N interactions, where a proton from the donor group is placed between two electronegative atoms. Apart from the studies of the usual redshifted H-bonds, many groups have studied the weak, unconventional red-shifted H-bonds from O−H···π and N− H···π interactions, where the π electrons act as donors. Studies of this unconventional H-bonding are important to explain many molecular reactions in chemistry, biology, and material science.14 Besides the above interactions, proton-shared/ionpair H-bonds have also been reported in the literature.15−24 In some systems, H-bonding results in unusual blue-shifting, especially when hydrogen is connected to carbon, as in CHCl3/ © 2014 American Chemical Society
CHF3/benzene. In contrast to bond lengthening and a red-shift under normal circumstances, the C−H bond exhibits bond shortening and a blue-shift. Hobza et al. first coined the term “anti-H-bond” for the blue-shifted H-bond and confirmed the existence of this unusual H-bonding in the benzene dimer, (C6H6)2, and in C6H6−CH4 and C6H6−CHCl3 complexes using correlated ab initio computations.25 Later, the term “antiH-bond” was renamed by the same group as “improper Hbond” or “blue-shifting H-bond”,26 and the latter term is now commonly used. Interest has grown rapidly since then in probing this blue-shifted H-bonding both theoretically and experimentally. Unlike for red-shifting, experimental reports on blue-shifted H-bonding are sparse. The reason could be the marginal intensity change of the C−H stretching mode on complexation, the detection of which demands sensitive techniques, which makes it difficult to probe this class of H-bonding. We have studied weak H-bonding in C2H2−CHCl3 and C2H2−CHF3 complexes, which are stabilized by C−H···π interaction, using matrix-isolation IR spectroscopy and ab initio computations.27−30 Even though computations indicated a blue-shift in the C−H stretching mode of CHCl3 and CHF3 sub-molecules in the C2H2−CHCl3 and C2H2−CHF3 complexes, in experiments we could not unambiguously discern the blue-shifted vibrational features. Ab initio computations on the 1:1 and the higher 1:2 complexes of C2H2 with CHCl3, CH2Cl2, Received: April 16, 2014 Revised: June 23, 2014 Published: June 30, 2014 5529
dx.doi.org/10.1021/jp503718v | J. Phys. Chem. A 2014, 118, 5529−5539
The Journal of Physical Chemistry A
Article
starting from the optimized monomer geometries without imposing any constraints in the structural parameters. Vibrational frequency calculations were performed for the optimized geometries to find out if they are minima on the potential energy surface and also to aid in assigning the experimental frequencies. A mode-by-mode scaling method, based on observed frequencies for the monomers, was used to predict the frequencies for the complex. Calculated stabilization energies were corrected for zero point energy (ZPE) and basis set superposition error (BSSE) separately using the method outlined by Boys and Bernardi.52 Bader’s atoms-in-molecules (AIM) theory was applied to study the nature of the interaction in the CHF3−HCl complexes.53 We searched for a (3,−1) bond critical point (BCP) between the CHF3 and HCl sub-molecules, which could be associated with the intermolecular interaction using the AIM package.54 The properties at this BCP, such as the electron density ρ(rc), the Laplacian of electron density ∇2ρ(rc), and the ratio of the eigenvalues |λ1|/λ3, were also examined using the AIM package. It is well known that weak interactions are characterized by small values of ρ(rc) and ∇2ρ(rc) > 0.55 Natural bond order (NBO version 3.1) analysis was performed to understand the nature of hyperconjugative interactions in the CHF3−HCl complexes.56
and CH3Cl also predicted a blue-shift of the C−H stretching frequency.31 The first experimental proof for blue-shifted H-bonding was reported by Trudeau et al.32 Later, Hobza et al., using doubleresonance IR ion-depletion spectroscopy, reported blue-shifted H-bonding in CHCl3−C6H5F complexes.33 Reimann et al. found experimental evidence for the blue-shift in C6H5F−CHF3 and other related complexes using IR ion-depletion spectroscopy.34,35 Later, the same group used a supersonic beam coupled with IR vibrational pre-dissociation spectroscopy to confirm the existence of the blue-shifted C−H stretching frequency of CHCl3/CHF3 sub-molecules in mixed clusters of derivates of benzene−CHCl3/CHF3 complexes. Van der Veken and co-workers, using IR spectroscopy, studied the blue-shifted H-bonding in HCCl3−xFx complexes with different oxygencontaining bases in cryosolutions.36−44 Using matrix-isolation IR spectroscopy, Ahokas et al. studied the blue-shifted Hbonding in formyl fluoride dimers.45 The first experimental proof for intramolecular blue-shifted H-bonding was reported by Matsuura et al.46 Using IR and Raman spectroscopy, Craig et al. observed a blue-shift in the C−H stretching region of cis-3,4-difluorocyclobutene in the liquid phase.47 Recently, Shirhatti and Wategaonkar, using IR−UV double-resonance spectroscopy, identified blue-shifted H-bonding in a variety of complexes, including 3-methylindole−CHX3, p-cresol−CHX3, and pcyanophenol−CHX3.48,49 With the help of theoretical calculations, a vast scope of blueshifted H-bonding remains to be explored experimentally, since not many systems have been reported in the literature. The aim of the present work is to identify the 1:1 H-bonded complexes between CHF3 and HCl in an argon matrix and to compare the experimental results with computations.
3. RESULTS AND DISCUSSION 3.1. Experimental Details. Figure 1 shows the IR spectra of CHF3 with and without HCl in an Ar matrix, spanning the region 3075−3025 cm−1. Block A and block B correspond to the spectra recorded at 12 K and after annealing at 35 K, respectively. In Ar matrix, the ν1 C−H stretching mode of CHF3 occurs at 3044.4 cm−1. The feature observed at 3058.9 cm−1 is assigned to the aggregates of CHF3, which agrees well with the reported literature value.57 When CHF3 and HCl were co-deposited in an Ar matrix, a new feature was observed in the C−H stretching region at 3051.1 cm−1, and the intensity of the feature increased on annealing the matrix. Figure 2 shows the IR spectra over the region 1390−1360 cm−1 (block A and block B). The feature observed at 1376.1 cm−1 and a site split feature at 1374.2 cm−1 in the Ar matrix are assigned to the doubly degenerate ν4 C−H bending mode of CHF3. Co-deposition of CHF3 and HCl in an Ar matrix and subsequent annealing produced new features at 1379.4 and 1371.8 cm−1. Figure 3 shows the IR spectra spanning the region 1160− 1100 cm−1 (block A and block B). The doubly degenerate ν5 C−F stretching mode in CHF3 appears as a doublet at 1148.0 and 1145.2 cm−1, while the ν2 C−F symmetric stretching mode is observed at 1136.0 cm−1 (Figure 3a). When CHF3 and HCl were co-deposited, new features were observed at 1141.7, 1149.9, and 1110.0 cm−1. The features observed at 1129.7 and 1122.4 cm−1 are due to aggregates of CHF3.57 Figure 4 (block A and block B) shows the IR spectral region 2960−2800 cm−1, corresponding to the H−Cl vibrational stretching region. The HCl molecule shows rotational fine structure in an Ar matrix.58 The feature observed at 2887.7 cm−1 (R branch) is assigned to the HCl stretch in the monomer in Ar matrix. The features at 2869.4 and 2854.4 cm−1 (Figure 4a) correspond to the Q and P branches of HCl, respectively. When CHF3 and HCl were co-deposited and annealed, a new feature appeared at 2863.6 cm−1 (Figure 4b,c). The feature observed at 2816.8 cm−1 is due to HCl dimer in an Ar matrix.
2. EXPERIMENTAL AND COMPUTATIONAL METHODS A closed-cycle helium-cooled cryostat RDK-408D2 (Sumitomo Heavy Industries Ltd.) was used to carry out matrix-isolation experiments. The whole cryostat unit along with the cold tip was housed in a vacuum chamber. The chamber was kept at a pressure of
