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Cyclopropenone: Normal coordinate calculation and the form of the normal modes

Spectrochimica Acta Part A Molecular Spectroscopy
Publication Date
DOI: 10.1016/0584-8539(75)80167-x


Abstract A normal coordinate calculation has been carried out for cyclopropenone- d 0 and - d 2 to try to understand the origin of their strong bands at 1483 and 1409 cm −1 respectively. The conclusions are as follows. (1) The five a 1 frequencies for each molecule (i.e. 10 in all) can be reproduced well with only five diagonal and one off-diagonal force constants. (2) The 1483 and 1409 cm −1 bands are the CC stretches. About 80% of their potential energy is in this coordinate. They are the lowest localized CC stretches known to the authors. (3) The force constant for the CC stretch is lower than usual, whereas that for the CC stretch is higher. (4) In disubstituted cyclopropenones, the intense bands near 1850 and 1650 cm −1 are strongly mixed CO and CC stretches. The 1650 cm −1 one is not a pure CC stretch at all; it is an in-phase mixture of approximately equal amounts of the above two coordinates. The 1850 cm −1 band is the out-of-phase mixture, with some added CC symmetric stretch. (5) The striking difference between the apparent “CC” frequencies in cyclopropenone- d 0 and - d 2 (<1500 cm −1) and those in disubstituted derivatives (∼1650 cm −1) is due almost entirely to a mass effect.

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