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Title: "Isolation and Structure of the OCNCO+ Ion"

Authors: Bernhardi, I., Drews, T., Seppelt, K.

Reference: Angew. Chem. Int. Ed. 1999, 38(15), 2232-2233.

Goals and objectives: The goal was to isolate in substance the ion
O=C=N=C=O+ and to establish its structure by experimental and
computational methods.
Objectives were to generate the ion in the form of a salt by Cl- or F-
abstraction with strong Lewis acids AsF5 and SbF5 and to determine its
structure and vibrational data experimentally and thereafter also
theoretically, the main emphasis being on the central nitrogen atom being
the center of the bending of the ion.

Methods: Experimental values were obtained by Raman spectroscopy and in
part by cristallography.  Solution and refinement of the structure was
done with the shelx programs.
Theoretical values were determined by basis set 6-31G(d,p), in
Hartree-Fock (HF) and Moller-Plesset (MP2)
approximations and in coupled cluster calculation (CCD) with double
substitution of the Hartree-Fock determinant.

Discussion: Both, data from experiments and calculations showed bent
structure for OCNCO+ with the angle of 130.7 degrees and
in the range of 133-138 degrees correspondingly. The calculation of the
partial charges according to Mullikan at the MP2 level for the central
nitrogen atom gave the value of -0.36. According to the analysis of the
natural bond orbitals this was a result of the two nonbonding electron
pairs at nitrogen with an occupation of 1.6 and 1.5e-, the former had 23%
s character and was responsible for the bending. Regardless
of the basis set and method of approximation of the electron correlation,
the change in energy between linear and bent arrangements was only a few
kJ/mol.
Finally similar studies have been made on C3O2 and N5+ ion, which also
have been found to be bent at the central atoms according to minimum
energy.

I liked the paper because it was short and it had a goal quite easy to
stay focused at and it enabled to compare three approximations plus the
experimental values for the ion. (Although none of them seems to have much
regularity in the size or direction of the error compared to the
experiment. HF seems to be good for calculating bond lengths and CCD for
angles.)
It was also quite astonishing to read about the details of synthesis and
stabilizing the product.

I did not understand what was meant by the term 'potential of the
deformation mode' and also the meaning of the vibrational data in Table 1
in terms of the ion. (This is the topic in our current lectures, I 
believe.)
Also the nature of the CCD approximation is not clear to me.