C–H⋯O interaction and water tunneling in the CHClF₂–H₂O dimer

Abstract

The rotational spectrum of the CHClF₂–H₂O weakly bound dimer has been measured using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5–18 GHz range. The structure of the complex has been determined by analysis of the moments of inertia of five isotopologues of the dimer. The primary interaction between the two monomers is a weak C–H⋯O contact (R_H⋯O = 2.332(3) Å) with a C–Cl⋯H–O contact also present (R_Cl⋯H = 2.749(13) Å). The observed structure is in reasonable agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level, although these predict a Cl⋯H distance that is significantly longer than the experimental results indicate. The rotational transitions of all isotopologues containing H₂O or D₂O were doubled, with relative intensities of the observed transitions consistent with an internal rotation of the water molecule leading to exchange of equivalent hydrogen atoms. Fitting the upper and lower components of the transitions using an effective Hamiltonian with the ERHAM program has yielded an energy difference between the tunneling states of 16.0(4) GHz, resulting in an estimate of the barrier to internal rotation of 195(5) cm⁻¹ (to be compared with an ab initio estimate of ∼117 cm⁻¹). The binding energy of the complex is estimated to be ∼5.5(2) kJ/mol (∼460 cm⁻¹) from a pseudo-diatomic approximation and assumption of a Lennard–Jones intermolecular potential.

Introduction

Weak C–H⋯X hydrogen bonds, where X is an electronegative element or a π-bond, have been increasingly studied in recent years, and it is thus desirable to learn as much as possible about the nature of these interactions [1], [2], [3], [4]. Alkyl halides are excellent candidates for these studies, since they may act as both weak C–H hydrogen bond donors and hydrogen bond acceptors (due to the presence of a relatively polar C–H bond and electronegative halogen atoms within the same molecule); therefore, weakly bound complexes of these molecules have recently been of spectroscopic interest. Dimers of alkyl halides with water are of particular interest, since water is capable of acting as a much stronger hydrogen bond donor and, again, as a hydrogen bond acceptor. Spectroscopic and/or ab initio investigations of several complexes of fluoromethanes, chloromethanes, and chlorofluoromethanes with water have recently been published, and most of these dimers have cyclic structures containing both C–X⋯H–O (X = Cl or F) and C–H⋯O contacts [5], [6], [7], [8], [9], [10], [11].

Microwave spectroscopic investigations of CH₂F₂–H₂O indicate a C–F⋯H–O hydrogen bond and a bifurcated C–H⋯O interaction [5], [6], and ab initio calculations on CH₂Cl₂–H₂O predict a similar structure [9]. Experimental gas-phase structural data are not available for CHX₃–H₂O and CH₃X–H₂O (X = Cl, F); however, matrix isolation infrared spectra and computational investigations (MP2 and B3LYP) of CH₃X–H₂O also give structures with both C–X⋯H–O and C–H⋯O interactions [7], [8], [9], [12]. There is more uncertainty about the structure of CHF₃–H₂O, with most studies showing a linear C–H⋯O angle [9], [10], [11], although at least one investigation indicates that the hydrogen atoms from the water molecule participate in two additional weak C–F⋯H–O interactions [8]. The structure of CHCl₃–H₂O is also likely to contain a near-linear C–H⋯O contact [9]. Of the CH_nX_4−n–H₂O complexes, CHX₃–H₂O is the only one that seems to lack a C–X⋯H–O interaction. Further high resolution spectroscopic studies of chloromethanes or fluoromethanes complexed with water are likely to be complicated by large amplitude motions of both monomer subunits within the complex; however, chlorofluoromethanes do not have sufficient symmetry to undergo internal rotation by tunneling, thus leading to potentially simpler spectroscopic analysis.

An extension of the previous studies of CH_nX_4−n–H₂O complexes to dimers containing both chlorine and fluorine will not only provide additional information on the balance between C–H⋯O and C–X⋯H–O interactions in weakly bound complexes, but will also provide information on whether chlorine or fluorine makes a better hydrogen bond acceptor. Caminati et al., recently investigated CH₂ClF–H₂O using microwave spectroscopy, observing a structure with both a very short C–Cl⋯H–O interaction (Cl⋯H = 2.655 Å) and a C–H⋯O interaction (H⋯O = 2.777 Å) (Fig. 1) [13]; thus, in this complex chlorine is clearly a preferable hydrogen bond acceptor compared to fluorine. A recent infrared matrix isolation and density functional theory study by Ito [14] presents computational evidence that in CHClF₂–H₂O there is again a preference for hydrogen bonding to chlorine over fluorine. This dimer also contains both C–Cl⋯H–O and C–H⋯O interactions; however, in contrast to CH₂ClF–H₂O, the H⋯O distance is predicted to be significantly shorter than the Cl⋯H distance [14].

The microwave spectroscopic studies of CH₂ClF–H₂O [13] and CH₂F₂–H₂O [6] both show doubling of the rotational transitions due to internal rotation of the water molecule. This type of large amplitude motion of water within weakly bound complexes is common [6], [13], [15], [16], [17], [18], and for both of these spectra, Caminati has analyzed the tunneling splittings to determine barriers to internal rotation of water of ∼330–340 cm⁻¹ [5], [13]. The observation of water internal rotation within these complexes could indicate that the C–X⋯H–O interactions are quite weak, since the X⋯H–O contact must be broken and reformed as the water rotates. It seems likely that the water molecule in CHClF₂–H₂O could undergo a low barrier internal rotation, similar to CH₂ClF–H₂O [13] and CH₂F₂–H₂O [5]. In addition to the possibility of internal rotation, Ito predicts that the “free” hydrogen atom from water lies slightly out of the symmetry plane of the rest of the complex [14], hinting at the additional possibility of a low barrier tunneling of the hydrogen atom between equivalent positions above and below the CHClF₂ symmetry plane.

In the present study, we have used Fourier-transform microwave (FTMW) spectroscopy and ab initio calculations to obtain more detailed information on the structure and dynamics of CHClF₂–H₂O. This will allow us to further probe the relative importance of C–H⋯O and C–X⋯H–O interactions and the preference of chlorine versus fluorine as a hydrogen bond acceptor, as well as comparing the structure with CH₂ClF–H₂O [13] and CH₂F₂–H₂O [5]. The high resolution afforded by FTMW spectroscopy should allow observation of splittings of the rotational transitions due to internal rotation of the water molecule and/or tunneling of the “free” hydrogen atom between the two sides of the symmetry plane of the complex. Finally, the study of CHClF₂–H₂O has potential atmospheric significance since CHClF₂ has been used as a coolant in refrigeration and air conditioning systems, and is thus a common pollutant. CHClF₂ is both an ozone depleter and a greenhouse gas; thus, it is important to have a knowledge of its interactions with ubiquitous atmospheric compounds such as water [19].