Theoretical energetic and vibrational analysis of amide-templated pseudorotaxanes

Four different minima on the potential energy surface of a pseudorotaxane were subject of this computational study. The most stable structure is one with three hydrogen bonds (60-80 kJ/mol) between axle and wheel. All functionals employed describe the next stable structure with a twofold hydrogen bond (30-50 kJ/mol) where the axle is still located inside the wheel. For almost all functionals the energy of a structure with the axle inside the wheel and with one hydrogen bond (20-40 kJ/mol) is only approximately another 10 kJ/mol above the second most stable structure. The calculated stability of the complex where the axle is located outside the wheel is functional dependent, but it lies for all calculations between the second most and the least stable pseudorotaxane structure. MP2 calculations performed additionally confirm this stability trend, and the estimation of the dispersion interaction points to contributions between 15 and 55 kJ/mol, corresponding to fractions of 20-50%. Thermochemical calculations show that a template as realized in this pseudorotaxane works as an entropic sink. From the analysis of the dipole moments we find that solvents could alter the situation tremendously. The frequency calculation reveals characteristic bands of which we analyze the N-H stretching modes found around 3500 cm−1, the CO stretching modes found around 1670 cm−1 and N-H bending modes found around 1480 cm−1. While the N-H stretching bands are shifted by between 100 and 180 cm−1, the CO modes, being more intense, are only shifted by up to 60 cm−1.