Formamide rotational analysis

A similar rotational analysis has been done with formamide derivatives. It is known that thioformamide has a larger rotational barrier than formamide, which can... 

Other modifications include a formamide residue (191), a ring fragmentation product of thymine, which exists as either a cis or a trans conformer. Both isomers are rotated out of the helix, and the bases on either side of (191) occupy the space vacated by it. A pyrene abasic site base pair in DNA duplexes adopts the usual B-form duplex, with the pyrene residue within the duplex stacking on adjacent nucleobases. The abasic site folds back over the opposite strand to shelter the hydrophobic base from exposure to water.The photoresponsive azobenzene analogues (133, R- and 5-forms) have been incorporated into DNA for NMR analysis. Both isomers intercalate between neighbouring base pairs, and the 5-isomer exhibits more disturbance in its duplex structure which is refiected in lower Tms compared to the R-isomer. ... 

A similar conformational analysis has been done with formamide derivatives, with secondary amides, and for hydroxamide acids. It is known that thioformamide has a larger rotational barrier than formamide, which can be explained by a traditional picture of amide resonance that is more appropriate for the thioformamide than formamide itself. Torsional barriers in a-keto amides have been reported, and the C—N bond of acetamides, thioa-mides, enamides carbamates (R2N—C02R), and enolate anions derived... 

There has been a NPA analysis of the delocalization in amides. Both the planar and rotated forms of formamide and its =S, =Se, and =Te analogs were smdied by NPA and natural resonance theory. HF/6-31+G, MP2/6-31+G, and B3LYP/6-31+G calculations were employed. At the MP2/6-31+G level, the transfer of charge noted on going from the planar to rotated form of formamide was +0.105 at N, —0.088 at O, and -0.033 at C. This charge transfer is consistent with the resonance formulation. The shifts were in the same direction but somewhat larger for the heavier elements. 

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