Geometric H/D Isotope Effects and Cooperativity of the Hydrogen Bonds in Porphycene

Abstract

We investigate the primary, secondary, and vicinal hydrogen/deuterium (H/D) isotope effects on the geometry of the two intramolecular hydrogen bonds in porphycene. Multidimensional potential energy surfaces describing the anharmonic motion in the vicinity of the trans isomer are calculated for the different symmetric (HH/DD) and asymmetric (HD) isotopomers. From the solution of the nuclear Schrödinger equation the ground-state wavefunction is obtained, which is further used to determine the quantum corrections to the classical equilibrium geometries of the hydrogen bonds and thus the geometric isotope effects. In particular, it is found that the hydrogen bonds are cooperative, that is, both expand simultaneously even in the case of an asymmetric isotopic substitution. The theoretical predictions compare favorably with NMR chemical-shift data.

Vibrations and hydrogen bonding in porphycene

“…In conclusion, the low, if not undetectable, intensity of the N-H stretching vibration in the experimental IR spectra results from the broadening caused by other modes. Coupling of the asymmetric 30 BU mode to the low frequency 7 AG mode was discussed previously by many authors 18,27,35,36 and our results confirm nicely their findings. The extended analysis presented here reveals other additional modes that contribute significantly to the broadening of the IR N-H stretching vibration (Fig.…”

“…Analysis based on this observation has been performed for porphycene using a limited number of modes. 35,36 Consequently the perturbation of the PES along one normal mode vector would affect the force constants, and thus vibrational frequencies associated with the remaining normal modes. In the following we apply the quantitative approach that takes into account couplings of all normal modes to the asymmetric N-H stretching mode 30 BU in porphycene (see inset in Fig.…”

Direct observation and control of hydrogen-bond dynamics using low-temperature scanning tunneling microscopy

“…6.5(b)), suggesting that the involved vibration mode may be directly associated with the reaction coordinate. Although it is hard to assign the mode unambiguously, it appears reasonable that the in-plane mode -including the displacement that reduces the distance between the N atoms -would be involved because such modes can lower the tautomerization barrier significantly through the reduction of the barrier for the H-atom transfer [266][267][268] and the process potentially takes place adiabatically (as described in Fig. 1.3(d)).…”

Hydrogen bond coupling in sodium dihydrogen triacetate

“…Therefore, an alternative theoretical method, based on quantum mechanical calculations including the anharmonicity in the potential energy surface, can be employed to highlight the coupling in double hydrogen bonded systems. HBC has been previously reported for simpler system like porphycene (more symmetric system with N-H…N hydrogen bond type) where coupled hydrogen bonds have been found [9,10]. In the current contribution, the previous work is extended to cover asymmetric, more complicated, systems such as sodium dihydrogen triacetate (SDHTA) as a model mimicking the O-H-O carbohydrate intermolecular hydrogen bonds.…”