Vibrations and hydrogen bonding in porphycene

Abstract

Combined use of IR, Raman, neutron scattering and fluorescence measurements for porphycene isolated in helium nanodroplets, supersonic jet and cryogenic matrices, as well as for solid and liquid solutions, resulted in the assignments of almost all of 108 fundamental vibrations. The puzzling feature of porphycene is the apparent lack of the N-H stretching band in the IR spectrum, predicted to be the strongest of all bands by standard harmonic calculations. Theoretical modeling of the IR spectra, based on ab initio molecular dynamics simulations, reveals that the N-H stretching mode should appear as an extremely broad band in the 2250-3000 cm À1 region. Coupling of the N-H stretching vibration to other modes is discussed in the context of multidimensional character of intramolecular double hydrogen transfer in porphycene. The analysis can be generalized to other strongly hydrogen-bonded systems.

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

“…6.9(b)). The distinct behavior from the N-D stretching may be attributed to a pronounced anharmonicity of the mode potential due to the H-bonds [253].…”

“…This effect may be attributed to vibrational broadening arising from the strong anharmonicity of the H-bonded N-H stretching mode. A recent theoretical study has predicted that the N-H stretching mode of a porphycene molecule in the gas phase can strongly couple with other low frequency modes through the anharmonicity of the potential, leading to a significant broadening of the vibrational density of states [253]. On the other hand, the broadening effect is less pronounced for the N-D stretching as the anharmonicity is less effective.…”

“…This rectangular geometry in porphycene results in a more linear arrangement of N-HÁ Á ÁN with an angle of 152°and a shorter distance of 2.63 Å between N atoms [250,251], which is 2.89 Å in porphine [252], along one direction. As a consequence, intramolecular H bonds (N-HÁ Á ÁN) are formed in the cavity [253], which stabilize the structure. Previous DFT calculations predicted that a porphycene molecule is the most stable among all other isomers including porphine in the gas phase [253].…”

“…As a consequence, intramolecular H bonds (N-HÁ Á ÁN) are formed in the cavity [253], which stabilize the structure. Previous DFT calculations predicted that a porphycene molecule is the most stable among all other isomers including porphine in the gas phase [253]. Formation of the H-bonds also results in a significant difference in thermodynamics and kinetics of tautomerization in porphycene.…”

Force-induced tautomerization in a single molecule

“…This molecule is a structural isomer of free-base porphyrin 27 and exhibits similar chemical properties. However, unlike porphyrin, porphycene has strong hydrogen bonds in the molecular cavity (the N-H···N moiety) 28 , resulting in a much faster tautomerization rate 29 . The porphycene molecule has three different twofold degenerate tautomers defined by the position of the inner hydrogen atoms ( Supplementary Fig.…”