IR-spectroscopic study of isotope effects on the NH/ND-stretching bands of meso-tetraphenylporphine and vibrational hydrogen tunneling

The IR spectra of meso-tetraphenylporphine (TPP) dissolved in CCl4 have been measured in the NH and ND stretching band region as a function of isotopic substitution using a newly constructed vacuum IR cell. Thus, it was possible for the first time to localize the NH and ND stretching bands of TPP–HD, which are characterized by the wave numbers 3335 and 2493 cm−1. TPP–H2 and TPP–D2 were found to absorb at lower frequencies, namely 3318 and 2482 cm−1. The observation of this band shift allows one to identify the observed NH and ND stretching bands in TPP–H2 and TPP–D2. They arise from antisymmetric stretching vibrations, the symmetric bands being forbidden in the IR spectra. NH/ND stretching bandwidths of the order of 20 cm−1 were observed, which is unusually small for proton transfer systems. This observation further confirms our previous conclusions from the primary HH/HD/DD kinetic isotope effects on the tautomerism in this molecule, namely that the motion of the hydrogen atoms takes place in a coupled manner from discrete vibrational NH stretching levels without substantial coupling to the motion of heavy atoms. Since the symmetric NH stretching vibration has a higher frequency than the antisymmetric stretching vibration, the interaction between the two inner hydrogen atoms in TPP is repulsive.