Energy of Formation of an Acyclic N-Methyltrifluoromethanesulfonamide Dimer

Abstract

The energy of formation of an open-chain N-methyltrifluoromethanesulfonamide dimer stabilized by the N3H...O=S hydrogen bond is 20.1 kJ mol !1 (CH 2 Cl 2 ). This value exceeds by~12 kJ mol !1 the energy of formation of cyclic secondary methanesulfonamide self-associates per hydrogen bond.A specific property of trifluoromethanesulfonamide and its N-methyl derivative is their ability to form cyclic dimers in nonpolar inert solvents and in the gaseous phase [1,2]. According to the data of IR and Raman spectroscopy and electron diffraction, fluorosulfonic acid molecules in the gaseous phase and in solution also give rise to cyclic dimers [3,4]. The IR spectra of centrosymmetric dimers derived from secondary methanesulfonamides were discussed in [537]. Analogous structures formed by carboxylic acids were studied in detail in [8,9]. The heat of formation of carboxylic acid cyclic dimers reaches 40360 kJ mol !1 .N-Substituted carboxamides are involved in hydrogen bonding as s-cis or s-trans conformers. s-cis Conformers possess a stronger proton-donor ability, but s-trans conformers are more stable [10,11]. A combination of these factors determines the structure of self-associates, whether they are cyclic or open-chain dimers. The energy of self-association of carboxylic acid amides is also fairly high and is 15317 kJ mol !1 per hydrogen bond [8]. The corresponding energy found for secondary methanesulfonamides is 43 8 kJ mol !1 [5]. We anticipated that the energy of selfassociation of trifluoromethanesulfonamides per hydrogen bond should be considerably greater. Sulfonamides containing perfluoroalkyl groups exhibit a high acidity (pK a 6.39 [12] and 7.56 [13] for trifluoromethanesulfonamide and N-methyltrifluoromethanesulfonamide, respectively), and their cyclic dimers in the gas phase are stable up to 400 K. These data suggest formation of a fairly strong hydrogen bonds despite relatively weak electron-donor power of the S=O group in substituted methanesulfonamides [6,14]. In order to elucidate this problem, it is necessary to estimate the energy of hydrogen bond formation in different self-associates of trifluoromethanesulfonamides. The goal of the present work was to experimentally determine the energy of dimerization of N-methyltrifluoromethanesulfonamide (I) in an inert solvent by IR spectroscopy and to elucidate the dimer structure by quantum-chemical calculations in terms of the density functional theory (B3LYP) using 6-31G* and 6-311++G** basis sets.The energies of rotational isomers of compound I are almost similar (DE = 0.08 kJ mol !1 ), while its invertomers are degenerate [2]. Mutual orientation of the oxygen atoms in the SO 2 group and hydrogen atom on the nitrogen almost does not depend on the conformation. Therefore, the formation of either cyclic or open-chain dimers should be determined solely by the energy factors, i.e., by the strength of the N3H...O=S hydrogen bond and by the energy of nonspecific solvation. In fact, according to the IR data, compound I in carbon tetrachloride exists as an equilibriu...

Molecular Structure of Complexes with Bifurcated Hydrogen Bond: III. Solvate H-Complexes Formed by Trifluoromethanesulfonamide and Its Cyclic Dimer

“…As shown previously, this calculation procedure quite satisfactorily reproduces the energy of formation of cyclic homoassociates of trifluoroacetic acid and N-methylacetamide [7] and geometric parameters of the trifluoromethanesulfonic acid molecule, as well as of some other compounds with intra-and intermolecular hydrogen bonds [8,9].…”

Stereoelectronic structure and self-association of N-trimethylsilylsulfonamides RSO2NHSiMe3 (R = Me, CF3, Ph)

“…This is indicative of the formation of stronger hydrogen bonds typical of cyclic associates, just those which sulfonamides I and III form in CCl 4 solution, although in the crystal compound I, according to the XRD data, it forms chains by means of Н-bonds of 2.117 Å length [35], apparently, due to the packing effects. The comparison of the experimental and theoretical spectral shifts of N-trimethylsilyltriflamide II suggests that in CCl 4 solution it forms linear associates, unlike the earlier studied Nmethyltriflamide, which in CCl 4 solution forms two types of self-associates, both linear and cyclic [41]. EXPERIMENTAL IR spectra of solutions were recorded on a FTIR Varian 3100 spectrophotometer.…”

Trifluoromethanesulfonamide: X-ray single-crystal determination and quantum chemical calculations

“…Self-association of triflamide was studied theoretically and experimentally in the gas phase and in inert media. These studies have shown that because of a high NH-acidity of triflamide, the equilibrium between the monomeric molecules and various H-bonded self-associates (linear and cyclic dimers, trimers and tetramers) exists even in vapors up to 435 K. [3] Supramolecular structure of triflamide as a function of basicity and polarity/polarizability of the solvent was studied, [4] as well as transformation of its self-associates into the solvate H-complexes in protophilic media. [5] However, strange as it might seem, no direct determination of the structure of triflamide has been reported.…”