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timized with 6-31 g* basis set. The optimized geometries
were verified for the minima by careful analysis of the cal-
culated vibrational frequencies (second derivative of the
energy). The use of such inexpensive basis set as 3-21 g*
was dictated mainly by two reasons: The computational
limitations (e.g., the real computational time for the full
optimization of the geometries at each conformational step
for the calculated rotational barrier in case of M4 on our
fastest processor (3.2 GHz) approaches 5 weeks) and the
fact that we are estimating the height of the barriers which
involves the calculations of the differences in energies only
between different optimized conformations. Our primary
objective was to obtain information on the relative values
of the rotational barriers rather than highly accurate values
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8. Supplementary information
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publications CCDC 292518 and 252519 upon application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
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Acknowledgement
We are grateful to the National Science Foundation and
UC MEXUS CONACYT for support.
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