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The vibrational spectra of amides—II. The force field and isotopic shifts of N,N-dimethyl formamide

✍ Scribed by D. Steele; A. Quatermain

Publisher
Elsevier Science
Year
1987
Tongue
English
Weight
612 KB
Volume
43
Category
Article
ISSN
1386-1425

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✦ Synopsis

The effects of' ho + 180 substitution on the vibrational frequencies of N,N-dimethylformamide have been studied. To understand these and the effects of previously measured shift data due to '%, *Hand 15N ab initio calculations of frequencies and intensities have been carried out at the 3-21G level. Accord between theory and experiment is generally good. A surprising result is the prediction of a weak band near 2000 cm-' in DMF due to in-plane interaction between the methyl umbrella modes and the anti-symmetric CN stretch. This abnormally high frequency is explained as arising due to the planar trigonal C,N enttty. Previous problems in reproducing isotope shifts are shown to be due to this mode being previously assigned near lSOOcm_ I. The effects of suppressing reference to one of a set of internal valence angles involved in a redundancy are explored. It is shown that the principal effect is to add the diagonal quadratic constant for that coordinate to all other quadratic terms involving pairs of the angles involved in the redundancy. This

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