Conformational analysis of tetrahydrofuran and tetrahydrothiopene using 1H NMR spectroscopy and ab initio calculations

Abstract

Coupling constants provide important information on the conformation of molecules in solution. At present, four of the ten possible coupling constants n J HH have been experimentally determined for tetrahydrofuran (THF) and tetrahydrothiophene (THT) [1]. We have used these data to evaluate the parameters of the dynamic structure of the THF and TTF molecules in a new scheme for conformational analysis involving the quantum-chemical description of the dynamic systems in terms of large-amplitude oscillations. X = O, S Fig. 1. Conformational space of THF and THT. __________________________________________________________________________________________ 1

Synthesis and NMR spectra of tetrahydrofuran-2-13C

“…5. In this spectrum there are also relatively large isotopic chemical shifts of C-3 of form 1d, due to replacement of 12 C by 13 C at the C-2 carbon atom ( 1 ∆δ C-3 ), and of C-4 of form 1e due to replacement of 12 C by 13 C at the C-2 carbon atom ( 2 ∆δ C-4 ). Analogous signals are observed in the downfield part of the 13 C NMR spectrum in the region of the α-carbon atoms.…”

“…The results depend to a considerable degree on what model was used to describe the process and even on what methods were used to process the data. In our opinion [13], the conformational process in THF must be regarded as an exceptionally fast dynamic process taking place mainly in a scheme of pseudorotation as a vibration with large amplitude. In the early papers [4,5], the structure determination was conducted within the model with one fixed form, which was assigned the "twist" conformation 3 T 4 with C 2 symmetry (φ = 90° with the oxygen atom on the basal plane, see the designation in [13]).…”

“…In our opinion [13], the conformational process in THF must be regarded as an exceptionally fast dynamic process taking place mainly in a scheme of pseudorotation as a vibration with large amplitude. In the early papers [4,5], the structure determination was conducted within the model with one fixed form, which was assigned the "twist" conformation 3 T 4 with C 2 symmetry (φ = 90° with the oxygen atom on the basal plane, see the designation in [13]). Meyer et al [8] came to the conclusion that the global minimum of the pseudorotation potential is at the non-canonical conformation (C 1 symmetry, φ ≈ 50°), whereas Mamleev [7] and Mel'nik [9] concluded on the basis of data from the vibrational-rotational spectra that the most stable conformation is the "twist" conformation 3 T 4 .…”

“…The development of methods for nonempirical calculation of the spin-spin coupling constants [10,12] made it possible to raise the problem of characterizing the THF conformational state according to the NMR spectroscopy data. In a recent paper, we proposed a more complete scheme for the assessment of the parameters of the potential energy surfaces based on experimental data for the spin-spin coupling constants, on calculated conformational dependencies of the spin-spin coupling constants, and on solution of the reverse spectroscopic structural problem [13]. Having at our disposal a limited set of experimental spin-spin coupling constants (four vicinal constants from [20]) we came to the conclusion that the main conformation for THF is the 3 T 4 conformation.…”

“…The vicinal 3 J H,H constants from [20] and the long-range 4 J H,H constants for related derivatives of cyclopentane (see [33][34][35]) were used as the initial approximation. The final values of the spin-spin coupling constants n J H,H and n J C,H and the isotopic chemical shifts of the protons due to replacement of the 12 C nuclei by 13 C are presented in the Table 1.…”