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was close to that of sulfur (mp. 115 °C). The reaction was continued at
room temperature until the evolution of gas and further precipitation
ceased (2–3h). The precipitate obtained was confirmed as sulfur from
the unit cells obtained from single crystal X-ray diffractometer [14].
This observation is in consistent with the earlier report [10].
The precipitate was filtered off and the clear solution was kept in
refrigerator maintained at 7 °C. After two days, some more sulfur
precipitated which was removed by filtration. After four days a white
crystalline solid also precipitated from the reaction mixture. The solid
was washed with ethylacetate, dried under vacuum. NMR (in DMSO-
D6) and IR spectra of this solid sample was obtained which indicated
the formation of ammonium thiocyanate [15]. Later, the formation of
ammonium thiocyanate was also confirmed by single crystal X-ray
diffraction technique.
To find any of the species involved in the plausible mechanism, the
progress of the reaction was monitored by liquid chromatography
coupled with electron spray mass spectroscopy (LCMS) at different
time intervals. The recorded LCMS at first and second hours showed
strong signals corresponding to m/z values of 75 and 59. This may be
due to SCNOH and protonated SCN respectively. The LCMS recorded
after 5h showed few more less intense signal with higher m/z value
(see Supplementary information). Based on our observations as well
as reports on similar reactions [7,16], we propose to explain the
formation of ammonium thiocyanate and sulfur as shown in Scheme 1.
The formation of hydroxylthiourea shown in the step 1 of the
mechanism is well established in similar reactions [7,11,16]. Ammo-
nia being a very good leaving group, could facilitate the formation of
hydroxy isothiocyanate, S C N–OH (1). The corresponding mass
observed in the LCMS of the reaction mixture also supports the
formation of compound 1.
Alkoxy and aryloxy isothiocyanates (RO–N C S) were unknown
until a recent report on preparation of methoxy isothiocyanate (CH3–
O–N C S) by flash vacuum thermolysis at 400 °C and isolation of it in
argon matrix. The methoxy isothiocyanate was reported to be stable
only up to −153 °C (120 K) [17]. The fragmentation pattern of this
compound in collision induced mass spectrum and charge reversal
mass spectrum have also been reported recently in which fragment
peak corresponding to (SCNO)− ion is observed predominantly [18].
Hence it can be presumed that compound 1 is the one of the species
involved in the mechanism.
The formation of oxathiranethione (COS2) proposed in this
mechanism is already been described in literature [8,9]. The IR
spectrum of collected gas from our reaction also suggested the same.
Albeit the structure of COS2 is unknown, it is known to decompose to
yield sulfur and carbonylsulfide (COS). Thus the proposed mechanism
shown in Scheme 1 could explain the formation of ammonium
thiocyanate as well as sulfur.
The ammonium thiocyanate obtained in this reaction afforded an
opportunity to reinvestigate the structure of it [19]. The ammonium
thiocyanate crystallizes in a cetrosymmetric monoclinic P 21/c space
group with all atoms located in general positions. The X-ray crystal
structure (Fig. 1) of ammonium thiocyanate shows a ammonium
cation and a thiocyanate anion in the asymmetric unit and there are
four such units in the unit cell. The crystal structure analysis of
ammonium thiocyanate reveals that, the two components are
interlinked by strong N–H∙∙∙N and weak N–H∙∙∙S types of hydrogen
bonding interactions [20]. The relevant hydrogen bonding parameters
of ammonium thiacyanate with symmetry codes are summarized in
the supplementary information.
In conclusion we have studied the reaction of aqueous hydroxyl-
amine with carbon disulfide in three different mole ratios and
proposed a mechanism for the formation of ammonium thiocyanate
and sulfur from these reactions. The structure of ammonium
thiocyanate has also been reinvestigated.
Acknowledgement
The authors gratefully acknowledge School of Chemistry, Univer-
sity of Hyderabad for infrastructure and support of the DRDO, India in
the form of grant to ACRHEM. The authors also acknowledge Dr. R.
Balamurugan and Dr. V. Baskar, School of Chemistry, University of
Hyderabad for useful discussions.
Appendix A. Supplementary material
Crystallographic data reported in this manuscript have been
deposited with Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC-770160. Copies of the data can be
(or from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge, CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc.cam.
ac.uk). Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.03.003.
Fig. 1. Thermal ellipsoidal plot of compound [SCN][NH4] with atom labeling scheme.
Displacement ellipsoids are drawn at 30% probability level except for the H atoms,
which are shown as circles of arbitrary radius. [Bond Lengths: C–S; 1.641 Å and C–N;
1.152 Å].
Scheme 1. Plausible mechanism for the formation of ammonium thiocyanate.