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Basic Salts with »Very Short« Hydrogen Bonds; the Crystal Structure of a-Picoline-N-oxide Hemihydrochloride Sesquihydrate (Dunlop's Salt)

J. Clare Speakman ; Chemistry Department, The University, Glasgow G12 SQQ, Scotland, U.K.
Kenneth W. Muir ; Chemistry Department, The University, Glasgow G12 SQQ, Scotland, U.K.

Puni tekst: engleski pdf 16.779 Kb

str. 233-248

preuzimanja: 84



Type A acid salts of many simple carboxylic acids (HX) crystallise
with anions, XHx-, which are symmetrical and which
contain »very short« OHO-bonds with O . .. 0 less than 250 pm.
Such compounds also reveal anomalous IR spectra. The Glasgow
work on such crystal structures has benefited from a long collaboration
with Hadzi's spectroscopic group at Ljubljana. The importance
of such joint studies in helping the understanding of
strong hydrogen bonding is discussed.
Some organic bases form analogous basic salts, which have
similarly anomalous spectra. An example, discovered by Hadzi in
1962, is the hemihydrobromide of a.-picoline-N-oxide, B · 1/2HBr,
where B = CoH7NO, for which he predicted the formula BHB+ Br-.
Preliminary X-ray work confirmed this and the presence of a
»very short« OHO-bond. Hitherto unpublished crystallographic
work is summarised.
Dunlop's salt is the sesquihydrated hemihydrochloride of the
same base. A careful X-ray study has been made (1040 reflexions;
R = 3.70/o): B · 1/2HCl · 3/2H20 has a structure corresponding to
BHB+ c1- · 3H20; the cation lies across a centre of symmetry, with
O ... O = 241.4(3) pm. At one stage the salt was supposed to
include a symmetrical ClHc1- anion; but our more precise work
shows the anion to be c1- ... H-OH (or HO-H ... en symmetrised
by disorder across a twofold axis of the crystal, with c1- ... O =
= 297(1) pm. The spectrum of DSALT is almost identical with that
of the hemihydrobromide.

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