IR spectroscopy and quantum-chemical calculations (B3LYP/6-31G*) were used to show that N-methyltrifluoromethanesulfonamide in the gas phase and in a low-polarity inert solvent (CCl 4 ) exists as an equilibrium mixture of monomers and cyclic dimers. In a more polar solvent (CH 2 Cl 2 ), highly polar openchain dimers are stabilized.According to X-ray diffraction data, methanesulfonamide molecules in crystal are hydrogen-bonded to form infinite tapes [1] and are not associated in highly dilute CCl 4 solutions (IR spectral data [2]). At the same time, N-substituted sulfonamides in an inert medium exist as equilibrium mixtures of monomers and cyclic dimers [235]. Perfluoroalkyl substitution in sulfonamides much enhances their acidic properties. For example, trifluoromethanesulfonamide is much more acidic (pK a 6.39 [6]) than phthalimide (pK a 9.9) and even alloxan [2,4,5,6(1H,3H)-pyrimidinetetrone, pK a 6.64] or thiophenol (pK a 6.5). Such a high NH acidity results in that trifluoromethanesulfonamide self-associates exist in the gas phase up to 440 K. According to IR spectral data and ab initio quantumchemical calculations, these are trimers and cyclic dimers [7]. N-Methyl substitution slightly reduces the acidity, but N-methyltrifluoromethanesulfonamide (I) is still fairly acidic (pK a 7.56 [8]).Carboxamides in the solid state, too, are polymeric associates linked by hydrogen bonds of different type, but, unlike sulfonamides, they do not form associates in highly dilute solutions and in the gas phase. Thus, in terms of the structure of self-associates, sulfonamides are closer to carboxylic acids which form cyclic dimers in inert media even at high temperatures. However, in terms of the stability of cyclic dimers, these two classes of compounds differ substantially. The formation heats of carboxylic acid associates are 10315 kcal mol !1 [9], whereas those of N-substituted alkane-and benzenesulfonamides are no higher than 234 kcal mol !1 [4]. Probably, the reason for this phenomenon consists in that the equilibrium mixture in more complex in composition than proposed in [4] and comprises not only cyclic but also open-chain associates. The question is only what is the position of the open-chain dimer 76 47 cyclic dimer equilibrium. Sulfonamide self-association is best studied by IR spectroscopy, and we made use of this method to study trifluromethanesulfonamide in the gas phase. Structural assessment of trifluromethanesulfonamide self-associates in solution is complicated by the intricate spectral pattern in the N3H stretching vibration region and the strong dependence of band shapes on the polarity of the medium. 1 Therefore, as the object for study we chose N-methyltrifluoromethanesulfonamide (I) whose IR spectrum in the 360033100 cm !1 range is simplified due to the presence of only one N3H bond.A density functional study (B3LYP/6-31G*) of Nmethyltrifluoromethanesulfonamide and its possible associates showed that the potential energy minima correspond to monomer I, as well as cyclic (IIa) and and open-chain (IIb) di...
