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Conformational analysis of N , N ‐diisopropylamides by combined use of n.m.r. lanthanide‐induced shifts and conformational energy calculations
Author(s) -
Graham Laurine L.,
Vanderkooi Garret,
Getz Joseph A.
Publication year - 1977
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270090207
Subject(s) - lanthanide , chemistry , reagent , molecule , amide , crystallography , computational chemistry , stereochemistry , ion , organic chemistry
Abstract A new method of conformational analysis has been developed, in which energy minimization calculations are combined with lanthanide‐induced shift data. First, exhaustive energy calculations are carried out on the free molecules in order to determine the conformations of lowest energy. Then, the coordinates of all low energy conformations or pairs of conformations are used in the pseudocontact shift equation for lanthanide‐induced shifts in order to find which of the theoretically obtained conformation(s) gives the best agreement with experiment. The molecules complexed to the lanthanide shift reagent were N , N ‐diisopropylformamide (DIPF) and N , N ‐diisopropylacetamide (DIPA). Two different lanthanide shift reagents were used, Eu(fod) 3 Fod is the anion of 1,1,1,2,2,3,3‐heptafluoro‐7,7‐dimethyl‐4,6‐octanedione‐ d 27 . and Pr(fod) 3 , in order to check the validity of the method. Proton magnetic resonance spectra were taken at 6 °C in carbon tetrachloride solution. The principal conformation found was different for each amide. DIPF was found to exist as a mixture of I (39 mol%) and II (61 mol%) with Eu(fod) 3 , and a mixture of I (37%) and II(63%) with Pr(fod) 3 . DIPA was found to exist as a mixture of I (79%) and IV (21%) with Eu(fod) 3 and a mixture of I (87%) and IV (13%) with Pr(fod) 3 . For both molecules, the two conformations of lowest computed energy were also the pair which gave the best fit to the lanthanide shift reagent data. The location of the principal magnetic axis of the complex was found to lie between 0° and 14° from the lanthanide atom–oxygen atom bond axis. The technique of combining lanthanide shift reagent data with energy calculations shows great promise in conformational analysis.