In this contribution we submit a theory of vibrations of a tetra-atomic symmetric
bent molecule ABBA in cis-conformation. Compared to the previous calculations, where some
approximations have been made, now we gave a more rigorous version. For the molecule
studied we have calculated frequencies of all five vibrations. Among them three vibrations,
ax1, ay1, and sy1, are normal or nearly normal. The longitudinal vibrations are of two types:
(→← →←) and (→→ ←←).
Keywords: vibrations; tetra-atomic symmetric bent molecule; calculated frequencies; vibration classification; normal (or nearly normal) vibrations: ax1, ay1, sy1; longitudinal vibrations of two types
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