In Situ NMR Methods in Catalysis
Achemist,facedwiththeproblemofdeterminingthemechanismofachemical reaction, tries to identify a set of reactions that will account for the observed behavior:Ideally,asmallsetofknownreactionsshoulddescribeingreatdetail exactly what takes place at each stage of a chemical transformation. The fact that many reactions proceed in a stepwise fashion can most convincingly be demonstrated if intermediate species can be isolated and shown to proceed to the same products under otherwise identical reaction conditions. An - termediate is the reaction product of each of these steps, except for the last onethatformsthe?nalproduct. Someintermediatesarestablecompoundsin theirownright;someothers,however,aresoreactivethattheirisolationisnot possible. Occasionally, evidence for the existence of short-lived intermediates may be obtained, in particular by spectroscopic observation. The latter may - low a direct observation or an indirect inference from unusual phenomena occurring in the reaction products during in situ investigations of their c- responding chemical reactions. In NMR spectroscopy, for example, transient emissionandenhanced absorptionlinesmaybeobserved, andoneisinclined to believe that there is a universal and unambiguous reason for their appe- ance. Thisisnotnecessarilythecase,however,sincethisseeminglyidentical phenomenon may have a strikingly different origin: During free radical re- tions,aphenomenoncalledchemicallyinduced dynamicnuclear polarization (CIDNP) may give rise to virtually the same effect as occasionally observed duringhomogeneous(andpossiblyevenheterogeneous)hydrogenations:The latter phenomenon, called parahydrogen-induced polarization (PHIP), has a completely different physical basis. It was ?rst noticed twenty years later than CIDNP and occurs if there is an imbalance of the two spin isomers of symmetric molecules such as dihydrogen when hydrogenating unsaturated compoundsusingappropriatecatalysts. Thesetwoeffects,ifnotdifferentiated properly, can cause misinterpretations of reaction mechanisms, as occurred initially when their different origins had not yet been understood approp- ately.
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acid addition alkyl alkynes ALTADENA anion applications aromatic atoms Bargon benzene bond carbon Catal catalyst catalytically active cation Chem Commun Chem Rev Chem Soc chemical shift chemistry CIDNP coefﬁcients colloid complex compounds coupling density diffusion effect electron spin emission ester experiments ﬁeld cycling ﬁrst ﬂuorinated free radicals heteronuclei Homogeneous Catalysis hydroformylations hydrogenation product hyperﬁne hyperpolarized inﬂuence interactions intermediate ionic liquids kinetics Leitner Lett ligand Magn Reson magnetic ﬁeld membrane NMR spectra NMR spectrometer NMR spectroscopy nuclear Overhauser effect nuclear spin nuclei observed oleﬁn organic solvents Organometallics parahydrogen parahydrogen molecule parahydrogenation product parahydrogenation reaction PASADENA phase PHIP transfer PHIP-derived polarization transfer pressure probe protons pulse sequences radical pair reaction products reactive reactor reagent recycling scCO2 SCFs signal enhancement signiﬁcant singlet solubility speciﬁc spectrum styrene substrate supercritical supercritical carbon dioxide synthesis temperature tion transfer of PHIP transition metal triplet Volume Editor Wasserscheid Woelk yielding