ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Quadrupole coupling parameters (QCPs), χQ, for nitrogen, oxygen, the carbonyl deuteron, and both amide deuterons in neat, liquid formamide were measured via NMR relaxation time experiments as a function of temperature. At room temperature the experimental values obtained for nitrogen, oxygen, carbonyl deuteron, and cis and trans amide deuteron were 2.84, 9.18, 0.170, 0.280, and 0.233 MHz, respectively. These parameters and also the quadrupole coupling asymmetry parameters, ηQ, were calculated using standard ab initio self-consistent field methods at the 6-31G* level for eight different clusters of formamide molecules. The cluster sizes varied from one to six molecules and include linear and cyclic structures. At room temperature the theoretical calculations indicate that the dominant (95%) species is a six-membered ring of formamide molecules involving hydrogen bonding between the trans amide deuteron and the oxygen. This ring shows strong cooperative effects. The ab initio values obtained for the QCPs and the asymmetry parameters of this ring are 2.94 MHz and 0.394 for the nitrogen, 9.28 MHz and 0.402 for the oxygen, 0.182 MHz and 0.038 for the carbonyl deuteron, 0.285 MHz and 0.147 for the cis, and 0.221 MHz and 0.223 for the trans deuteron, respectively. These results are in excellent agreement with the experimental NMR results; both show clearly that primarily the trans amide proton is involved in hydrogen bonding. Other structural data from x-ray, electron, and neutron diffraction, low-frequency Raman and far-IR spectra are consistent with the conclusion that cyclic hexamers are the dominant species of liquid formamide under standard state conditions. © 1995 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.469237
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