Pyramidal nitrogen inversion hindered by a strong intramolecular hydrogén bond in 2‐diethylaminomethylphenols

Abstract

ln the 'H NMR spectra of 2-diethylaminomethyl-3,4,6-trichlorophenoi (1) below 260 K, an additional splitting of the CH, signal was found, which can be ascribed to the hindered nitrogen inversion. In the molecule of the 0-methylated derivative (2) this process is fast (on the NMR time-scale) down to 150 K. The frequencies and the activation parameters of the nitrogen inversion in (1) were measured by DNMR, which indicated that the inversion requires a preliminary stage of breaking an intramolecular hydrogen bond. The activation enthalpy, AH:",, of the inversion stage was evaluated as 28.5 f 6.7 kJ mol-'.

A nitrogen‐15 NMR study of hydrogen bonding in 1‐alkyl‐4‐imino‐1,4‐dihydro‐3‐quinolinecarboxylic acids and related compounds

“…Intrinsic effects have been examined by Limbach and co-workers, 7 who, from experiments on NÐ Ð ÐH hydrogen bonding (in HF-collidine), have shown that changes in solvent polarity (mixtures of CDF 3 /CDClF 2 were used as solvent) arising from temperature changes in the range 90-160 K induce a dipole moment in the solute (the F-H-N hydrogen-bonded system) that increases as the temperature is lowered. The induced dipole moment takes the form of charge transfer from N to HF accompanied by a displacement of the proton toward the nitrogen, shown by a large decrease in J( 19 1 H g becomes decreasingly negative and eventually positive as the temperature is raised (as noted above, Refs. 8, 10, 11), then it would seem most probable that the sign of the observed coupling fJ( 15 N, 1 H g for dimethylanthranilic acid is negative.…”

Dimerization and solvent-assisted proton dislocation in the low-barrier hydrogen bond of a Mannich base: a low-temperature NMR study

“…On lowering the temperature or increasing the concentration (unfortunately quantitative studies of concentration effects on NMR spectra were not possible owing to limited solubility at low temperatures), the intensity of the high-field signal rapidly grows which can, therefore, be assigned to the dimeric Mannich base. The signal consists, however, of two doublets of different intensities characterized by coupling constants 1 J 1 H, 15 N of 67 and 70 Hz, which may therefore be assigned to two dimers with different conformations, e.g. DA and DB.…”

“…Moreover, the dielectric constant of this solvent is strongly increased when the temperature is lowered. The dielectric constant of CHF 3 -CHF 2 Cl (1 : 1) increases from about 20 at 170 K to 45 at 95 K. 23 In addition, we chose to synthesize and study a 15 15 N) between the hydrogen bond proton and nitrogen for obtaining structural information about the hydrogen bond geometry. Second, we performed UV experiments under similar experimental conditions as chosen for the NMR experiments.…”

A third type of alkylamines possessing high nitrogen inversion-rotation barriers

“…Two types of compounds with high NIR barriers are known among a variety of alkylamines: (1) aziridines (with barriers of b16 kcal mol À1 ) 6 and (2) azanorbornanes (with barriers of b13 kcal mol À1 ). 9,10 In both cases specific effects cause this increase: abnormal angle strain in the aziridine case 8 and strain together with the 'bicyclic effect' in the azanorbornane case. 9,10 In general, azanorbornanes belong to the class of polycyclic amines whose backbone disallows a concerted ring inversion-nitrogen inversion-process.…”

“…7 Thus, when NIR barriers are b11 kcal mol À1 , in the absence of stabilization of a nitrogen pyramid via a strong N … H bond (e.g. an intramolecular H-bond 8 ), they may be defined as high NIR barriers. Two types of compounds with high NIR barriers are known among a variety of alkylamines: (1) aziridines (with barriers of b16 kcal mol À1 ) 6 and (2) azanorbornanes (with barriers of b13 kcal mol À1 ).…”