<sup>15</sup>N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene

Wehrle, Bernd; Limbach, Hans−Heinrich; Köcher, Matthias; Ermer, Otto; Vogel, Emanuel

doi:10.1002/anie.198709341

Cited by 140 publications

(102 citation statements)

References 38 publications

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“…The same is true for the ground state, as revealed by simulations of expected anisotropy values assuming various fractions of cis tautomers in the ground and lowest excited singlet states [30,82]. This is a very important result, given that previous structural assignments were mostly based on calculations, since they were impossible to obtain from X-ray data, or, in the case of NMR [40,68], IR and Raman measurements [42], not unequivocal.…”

Section: Tautomerization In the Lowest Excited Singlet Statementioning

confidence: 63%

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Tautomerization in Porphycenes

Waluk

2006

Hydrogen‐Transfer Reactions

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Porphyrins and metalloporphyrins are objects of intense studies in both basic and applied sciences. Owing to their crucial role in biological processes, such as photosynthesis, oxygen transport and activation, porphyrins have been labeled "Pigments of Life" [1]. The accuracy of this characterization is additionally confirmed by the use of porphyrins in medicine as phototherapeutic agents [2][3][4][5][6]. The potential of porphyrins as building blocks is enormous and includes, for example, artificial light-harvesting and photosynthetic systems [7-9], molecular memories [10,11], photovoltaic devices [12], and many other advanced materials [13][14][15][16][17][18]. The prospect of applications stimulated a rapid development of synthetic procedures that resulted in obtaining new classes of compounds, such as expanded, contracted, or inverted porphyrins [19,20]. Of particular interest in this area are constitutional isomers of porphyrin (also dubbed "reshuffled" porphyrins) [20], tetrapyrrole macrocycles which differ from the parent molecule in the way of linking the pyrrole rings by methine groups (Fig. 8.1). The research in this field started with the synthesis of porphycene in 1986 [21]. Since then, three more "nitrogen-in" isomers have been obtained: corrphycene [22], hemiporphycene [23,24], and isoporphycene [25,26]. Closely related to this class is an "inverted" ("N-confused") porphyrin [27,28].All these molecules have been shown to be planar and aromatic. They also exhibit electronic spectra characteristic for porphyrins, with the lower intensity Q transitions in the red part of the visible region followed by stronger Soret bands in the near-UV (Fig. 8.2). However, the relative intensities of the Q and Soret transitions differ significantly among the series. The lowest intensity of Q bands is observed in porphyrin and corrphycene. Hemiporphycene reveals stronger Q bands, and porphycene even stronger ones. This behavior can be rationalized upon inspecting the energies of the frontier p orbitals. The analysis of HOMO and LUMO splitting based on the perimeter model [29] has led to correct predictions not only for relative absorption intensities, but also for signs and patterns in magnetic circular dichroism. These predictions were experimentally confirmed for porphycene [30], corrphycene [31], and hemiporphycene [32].Hydrogen-Transfer Reactions. Edited by

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Section: Tautomerization In the Lowest Excited Singlet Statementioning

confidence: 63%

“…Comparison of the reaction for 1 and 2 in the crystalline state was performed using 15 N CPMAS NMR (Fig. 8.5) [40,68]. At elevated temperatures both molecules reveal only one peak, characteristic of a rapid inner hydrogen exchange.…”

Section: Structural Datamentioning

confidence: 99%

Tautomerization in Porphycenes

Waluk

2006

Hydrogen‐Transfer Reactions

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“…Unlike the porphyrin case, by perturbing the [20] The inner hydrogen migration between trans structures (see Figure 54, bottom) is common to porphyrin and porphycene [288]. Ground and excited state tautomerism in porphycene has been experimentally investigated by several techniques [323][324][325][326]. While this process has a stepwise mechanism in porphyrin [327], in porphycene the tautomerism has been described as a synchronous double hydrogen tunneling activated by a low frequency mode whose displacement affects the NH· · ·N distances [320].…”

Section: Porphycenesmentioning

confidence: 99%

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gellini

Salvi

2010

Symmetry

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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]-and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

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“…As compared to the latter, it provides a tighter cavity for the two H atoms which enhances the strength of the HBs. This statement is supported by X-ray diffraction, [18] infrared, [19] and NMR spectroscopy [20] (for a review, see also ref. [21]).…”

Section: Introductionmentioning

confidence: 96%

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

et al. 2007

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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.

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¹⁵N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene

Cited by 140 publications

References 38 publications

Tautomerization in Porphycenes

Tautomerization in Porphycenes

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

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

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15N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene

Cited by 140 publications

References 38 publications

Tautomerization in Porphycenes

Tautomerization in Porphycenes

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

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

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¹⁵N‐CPMAS‐NMR Study of the Problem of NH Tautomerism in Crystalline Porphine and Porphycene