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Porphyrins <jats:bold>S0<\/jats:bold>\u2013<jats:bold>S4<\/jats:bold> have been synthesised, characterised and studied in detail by spectroscopic and spectrometric techniques, and their isomeric purity verified by HPLC analysis. In particular, they exhibit perfectly well\u2010defined NMR spectra that display distinct patterns depending on their average symmetry at room temperature: <jats:italic>C<\/jats:italic><jats:sub>2<jats:italic>v<\/jats:italic><\/jats:sub>, <jats:italic>D<\/jats:italic><jats:sub>2<jats:italic>d<\/jats:italic><\/jats:sub>, <jats:italic>C<\/jats:italic><jats:sub>2<jats:italic>h<\/jats:italic><\/jats:sub>, <jats:italic>C<\/jats:italic><jats:sub>2<jats:italic>v<\/jats:italic><\/jats:sub>, and <jats:italic>D<\/jats:italic><jats:sub>2<jats:italic>h<\/jats:italic><\/jats:sub> for <jats:bold>S0<\/jats:bold>\u2013<jats:bold>S4<\/jats:bold>, respectively. NH tautomerism was evidenced by variable\u2010low\u2010temperature <jats:sup>1<\/jats:sup>H\u2005NMR experiments in [D<jats:sub>2<\/jats:sub>]dichloromethane performed on <jats:bold>S0<\/jats:bold> (\u0394<jats:styled-content>${G{{{\\ne}\\hfill \\atop {\\rm 298K}\\hfill}}}$<jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/tex2gif-ueqn-1.gif\" xlink:title=\"equation image\"\/><\/jats:styled-content>=48\u00b11\u2005kJ\u2009mol<jats:sup>\u22121<\/jats:sup>) and <jats:bold>S1<\/jats:bold> (\u0394<jats:styled-content>${G{{{\\ne}\\hfill \\atop {\\rm 298K}\\hfill}}}$<jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/tex2gif-ueqn-2.gif\" xlink:title=\"equation image\"\/><\/jats:styled-content>=55\u00b13\u2005kJ\u2009mol<jats:sup>\u22121<\/jats:sup>), which has led to an understanding of the average spectra observed for the five porphyrins at room temperature. On the other hand, <jats:bold>S2<\/jats:bold> and <jats:bold>S3<\/jats:bold> are stable atropisomers at room temperature, easily separated and characterised, as a result of restricted rotation of their strapped bridges due to their high rotational barrier energies. Upon heating to 82\u2009\u00b0C, they slowly equilibrate to a thermodynamic ratio of 64:36 in favour of the more stable <jats:bold>S2<\/jats:bold> isomer. This atropisomerisation process was evidenced by <jats:sup>1<\/jats:sup>H\u2005NMR spectroscopy and monitored by HPLC, from which high rotational energy barriers of 115.2 (\u0394<jats:styled-content>${G{{{\\ne}\\hfill \\atop {\\rm S2}\\rightarrow {\\rm S3}\\hfill}}}$<jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/tex2gif-ueqn-3.gif\" xlink:title=\"equation image\"\/><\/jats:styled-content>) and 116.9\u2005kJ\u2009mol<jats:sup>\u22121<\/jats:sup> (\u0394<jats:styled-content>${G{{{\\ne}\\hfill \\atop {\\rm S2}\\rightarrow {\\rm S3}\\hfill}}}$<jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/tex2gif-ueqn-4.gif\" xlink:title=\"equation image\"\/><\/jats:styled-content>) were deduced.<\/jats:p>","DOI":"10.1002\/chem.201403881","type":"journal-article","created":{"date-parts":[[2014,10,15]],"date-time":"2014-10-15T11:00:42Z","timestamp":1413370842000},"page":"16337-16349","source":"Crossref","is-referenced-by-count":22,"title":["Tautomerism and Atropisomerism in Free\u2010Base (<i>meso<\/i>)\u2010Strapped Porphyrins: Static and Dynamic 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