A DFT study of reversed isotope shifts in H/D substitution of free-base porphyrin and related free-base tetrapyrroles

Abstract

DFT/B3LYP calculations are used to analyse the occurrence of reverse isotope shift ratios (ISR) in H/D substitution of the free-base tetrapyrroles, in situations where the frequency ratio H / D is less than 1. The reverse ISR effect is found to be most evident in the out-of-plane bending modes (b 2g and b 3u symmetry) involving some N-H motion for the four molecules studied, viz., porphine (H 2 P), tetraaza-porphine (H 2 TAP), tetrabenzo-porphine (H 2 TBP), and phthalocyanine (H 2 Pc). It was analysed by following the evolution of the normal mode frequencies with incremental variation of the H atom masses from 1 to 2 amu. This method allows direct, unambiguous mode correlations to be established between the light and the heavy isotopologues. When the NH(D) motion is predominant, the H to D frequency evolution decreases in a continuous manner for a particular normal mode. In the case of two modes of the same symmetry and whose frequencies are similar, their frequency evolutions could cross, depending on the extent of NH(D) motion involved in them. The evolution diagrams may show avoided crossings of various extents, which thereby reflects the degree of the NH(D) motion in the modes. The reverse ISR effect is directly correlated to these avoided crossings. Because the isotope shifts are quite small (Ͻ10 cm -1 ) and occur in the congested 1500 -500 cm -1 spectral region, high-resolution methods yielding narrow line transitions are required for experimental analysis. The matrix isolation technique is particularly well suited for this work and is proposed for use in a search for this effect.Key words: vibrational spectroscopy, porphyrins, DFT calculations, matrix-isolation.Résumé : Des calculs DFT basés sur la fonctionnelle B3LYP ont été menés sur des molécules tétrapyrroliques afin d'analyser le cas de rapports de déplacements isotopiques (RDI) inverses reliés à la substitution H/D dans les bases libres, c'est-à-dire le cas de rapports de fréquences H / D inférieurs à 1. Ces rapports RDI inverses apparaissent clairement dans les modes de pliage hors plan (symétries b 2g et b 3u ) impliquant un mouvement N-H pour les quatre molécules étudiées : porphine (H 2 P), tétraazaporphine (H 2 TAP), tétrabenzoporphine (H 2 TBP) et phthalocyanine (H 2 Pc). Les RDI inverses ont été analysés en suivant l'évolution des fréquences des modes normaux lors d'une augmentation continue de 1 à 2 amu de la masse de l'atome d'hydrogène. Cette méthode permet d'établir de façon directe et non ambiguë les corrélations entre les fréquences des deux isotopologues. Lorsque le mouvement N-H(D) est prédominant dans un mode donné, l'évolution de la fréquence de H à D diminue continûment. Lorsque deux modes de même symétrie ont des fréquences voisines, leurs évolutions en fréquence de H à D peuvent se croiser, selon le degré d'implication du mouvement N-H(D) dans les deux modes considérés. Les diagrammes d'évolutions correspondants peuvent alors montrer des croisements évités plus ou moins prononcés, reflétant ainsi le poids du mouvement N...

Spectroscopic Study of Jet-Cooled Deuterated Porphycenes: Unusual Isotopic Effects on Proton Tunneling

“…This finding shows that deuteration affects not only the frequency of vibrations, but also the nature of displacement vectors of some modes, as already calculated for free base porphyrin. 62 Such an effect was also observed in deuterated tropolones (TRN), where the tunnelling splitting in mode ν 13 of 5D-TRN is larger than the corresponding mode in the parent TRN. 47 4.…”

Free base tetraazaporphine isolated in inert gas hosts: Matrix influence on its spectroscopic and photochemical properties

“…All of the present calculations were conducted with the Gaussian 03 suite of programmes 29 running, as described in Ref. 30, on a Linux workstation with two quad-core processors. When comparing with experimental results, the calculated vibrational frequencies were scaled by a uniform factor of 0.98, unless stated otherwise.…”