Weak CH⋅⋅⋅F Bridges and Internal Dynamics in the CH₃F⋅CHF₃ Molecular Complex

Abstract

Eine Vorzugskonformation des intern hoch dynamischen Fluormethan‐Trifluormethan‐Molekülkomplexes folgt aus der Stabilisierung der beiden Untereinheiten durch drei schwache CH⋅⋅⋅F‐Wasserstoffbrücken und elektrostatische Dipol‐Dipol‐Wechselwirkungen. Die beiden Einheiten sind nicht starr fixiert, sondern weisen große Amplituden der Bewegung durch fast vollständig freies Drehen um ihre Symmetrieachsen auf (siehe Bild).

Understanding High‐Resolution Spectra of Nonrigid Molecules Using Group Theory

“…From this, the complete nuclear permutation-inversion (CNPI) group has been developed, which has its origins in the work of LonguetHiggins [10] and Hougen. [11] In the recent past, PI group theory has been successfully applied to a broad variety of complex nonrigid molecules, such as the water dimer (H 2 O) 2 (see also Section 3), [12,13] the methanol dimer (CH 3 OH) 2 , [14] 2-methylmalonaldehyde, [15,16] the dimer CH 3 FÀCHF 3 , [4] acetaldehyde CH 3 COH, [17,18] and the benzene dimer (C 6 H 6 ) 2 , [19] to mention just some to give an impression of its broad applicability. In a recent article, Hougen summarized strategies for advanced applications of PI groups to the microwave spectra of molecules with large-amplitude motions.…”

“…Depending on the barrier height for the large-amplitude motion and the corresponding moment of inertia of the moving moiety, the resulting spectral splittings can amount to several gigahertz, as for the case of CH 3 FÀCHF 3 . [4] From the spectra, not only information about the molecule's structure but also about its internal dynamics, such as the barrier heights, their dependence on the chemical bonds, or the orientation of several groups with respect to each other, can be obtained. Due to the nonrigidity of the molecule and the complexity of such spectra, their analysis is usually quite challenging.…”

“…[32] To group-theoretically describe the three tunneling pathways, we label the protons 1 and 2 and the oxygen nucleus A on one water moiety, and the protons 3 and 4 and the oxygen nucleus B on the other one. The equilibrium structure has reflection symmetry, and its point group is isomorphic to the PI group G 2 = {E,(34)*}, that is, this would be the appropriate group to describe an assumptive rigid water dimer structure [frameworks (1) to (4), and {E,(12)*} for frameworks (5) to (8)]. Each of the eight versions would have the same rotation-vibration energy levels, which would have an eightfold structural degeneracy (see Figure 5, left part).…”

Group Theory for High‐resolution Spectroscopy of Nonrigid Molecules

“…The high-resolution spectra of nonrigid molecules are usually highly complicated because of additional tunneling splittings of rovibronic transitions. Depending on the barrier height for the internal rotation and the corresponding moment of inertia of the internally rotating top, these splittings can amount to several gigahertz, as in the case of CH 3 F-CHF 3 (Caminati et al 2005). The corresponding spectra are usually very congested and rich in information about the molecule and its internal dynamics, such as the barrier heights, their dependence on the chemical bonds, or the orientation of several groups with respect to each other.…”

“…From this, the complete nuclear permutation inversion (CNPI) group has been developed, which has its origins in the work of Christopher (LonguetHiggins 1963) and Jon T. Hougen (1962). In the recent past, PI group theory has been successfully applied to a broad variety of complex nonrigid molecules, such as the water dimer (H 2 O) 2 (Dyke et al 1977, Coudert et al 2007, the methanol dimer (CH 3 OH) 2 (Ohashi and Hougen 1994), 2-methylmalonaldehyde Hougen 2006, Ilyushin et al 2008), the dimer CH 3 F-CHF 3 (Caminati et al 2005), acetaldehyde CH 3 COH (Chou et al 2004, Kleiner et al 1996, and the benzene dimer (C 6 H 6 ) 2 (Bunker, P., Schnell, M., Erlekam, U., Grabow, J.-U., von Helden, G., and Meijer, G. (2009) private communication. ), to mention just a few to give an idea of its broad applicability.…”