Halogen bond and internal dynamics in the σ–complex of pyridine-chlorotrifluoromethane: A rotational study

Highlights

• CCl···N halogen bond.

• Multiquadrupole problems.

• Internal dynamics.

Abstract

The rotational spectrum of the 1:1 complex pyridine–chlorotrifluoromethane shows that the two moieties are held together through a CCl···N halogen bond (R_Cl···N = 2.909(5) Å). The quadrupolar effects of the ³⁵Cl (or ³⁷Cl) and ¹⁴N nuclei combined with the high density of the rotational transitions give rise to a very congested spectrum. The multidimensional large amplitude internal dynamics is reflected in a two times larger rotational constant A, on an unexpected positive values of the inertial defect of the ground state, and on a negative value of the D_J centrifugal distortion constant. This comes from a combination of a free internal rotation and of a very low energy in plane oscillation of CF₃Cl with respect to pyridine. However, a coupled Hamiltonian including Coriolis interactions with an hypothetical (unobserved) v = 1 state solves the initial result of an unphysical negative value for the centrifugal distortion constant.

Introduction

The importance of halogen bond (HaB) has been increasingly noticed in the last years and found applications in the field of crystal engineering [1], nanomaterials [2], supramolecular chemistry [3], liquid crystal [4] and many others. For this reason, IUPAC formalized a definition of this non-bonding interaction: “a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity” [5].

Most of the investigations dedicated to the HaBs are based on solid-state X-ray diffraction [6]. However, more precise information on this kind of interaction, free from solvent effects or solid state linkages, comes from studies of isolated complexes with the subunits held together by such interaction in the gas phase. Accurate details of the HaB nature are provided by rotational spectroscopy of molecular complexes. An overview of properties of the HaB interaction has been given by Legon through Fourier transform microwave (FTMW) spectroscopy studies of a series of B···XY complexes, where B is the electron donor and XY is a dihalogen molecule [7]. It has been proved that in these cases the HaB is almost linear, with B···X-Y angles close to 180°.

Chlorotrifluoromethane (CF₃Cl) has been found to be a good prototype molecule to investigate HaBs. Because the fluorine atom is more electron-withdrawing than the chlorine atom, a positive electrostatic region (“σ-hole”) [8], [9] is formed on the outer surface of the chlorine atom in CF₃Cl (opposite to the CCl bond) and ready to interact with negative sites. CF₃Cl forms indeed, as “halogen donor”, a CCl···O HaB with H₂O [10], formaldehyde [11] and dimethyl ether [12], a CCl···N HaB with NH₃ [13], and a CCl···F HaB with CH₃F [14]. Strong dynamic effects considerably alter the rotational constants, with respect to the values of expected for a rigid rotor. The spectrum of CF₃Cl-H₂O, which is classically an asymmetric top, has been interpreted as that of a symmetric top [10]. The free internal rotation of the CF₃ group causes unexpectedly large values of the rotational constants A in the complexes of CF₃Cl with formaldehyde [11], dimethyl ether [12] and CH₃F [14].

Pyridine (PYR) is the best-known heterocyclic aromatic molecule, used extensively in coordination [15], [16] and surface chemistry [17]. For this reason, the non-covalent interactions between PYR with other molecules have been extensively investigated using high resolution spectroscopy. Depending on the chemical nature of the partner species, both π or σ type complexes could be formed in relation to its negative sites: either the π system of the aromatic ring, or the non-bonding orbital (n) at the nitrogen atom. Combined with laser–vaporization, ZEKE spectroscopy pointed out that Li and Ca complexes prefer σ bonding, whereas the complex with Sc has a π-type linkage [18].

PYR is the only aromatic molecule for which, thanks to its permanent dipole moment, the rotational spectra of all van der Waals complexes with rare gases (PYR-RG, except radon) have been reported, providing experimental evidences of the magnitude and directionality of the dispersive interactions [19], [20], [21], [22], [23], [24], [25]. In all the complexes, a π-type structure is observed, even for complexes with two RG atoms, which exhibit a “double π” arrangement with one atom above and one atom below the ring [24], [26], [27]. Conversely, most studies of molecular adducts of PYR with any other partners revealed σ-type arrangements [28], [29], [30], [31], [32], [33]. When reviewing the complexes of PYR with the family of CH_nF_4-n, one can note an interesting behaviour. In the case of PYR-CF₄ (n = 0), the two subunits are held together by a CF₃···N HaB, and CF₄ undergoes a free rotation with respect to PYR [34]. In PYR-CHF₃ [35], PYR-CH₃F [36] and CH₂F₂ [37], two kinds of weak hydrogen bonds (WHB), CH···N and CH···F, have been found to connect the two constituent moieties. The barriers to internal rotation of the CHF₃ and of the CH₃F groups have been determined from the A-E tunnelling splittings of the rotational transitions. PYR-CH₄ constitutes an exception: CH₄ prefers to link with PYR through the π electronic system rather than the n-orbital, with CH₄ acting as a pseudo rare gas [38].

Herein, we decided to investigate the rotational spectrum of PYR-CF₃Cl, which represents the largest molecular complex held by a HaB, to date, by using pulsed jet Fourier transform microwave (PJ-FTMW) spectroscopy. There are two interesting aspects: (1) which negative site of PYR, n-orbital or π-system, is favoured for the positive region of CF₃Cl? (2) How the internal dynamics will affect the value of the rotational constant of the complex with a much heavier molecule PYR than formaldehyde, CH₃F and dimethyl ether? The results are reported below.