13C NMR and FT-IR spectra of thiocyanogen, S2(CN)2, selenocyanogen, Se2(CN)2, and related compounds

Article abstract of DOI:10.1016/S0277-5387(00)00304-1

The pseudohalogens thiocyanogen [(SCN)₂], sulfur dicyanide [S(CN)₂], selenocyanogen [(SeCN)₂], selenium dicyanide [Se(CN)₂] and selenium diselenocyanate [Se(SeCN)₂] have been prepared and studied by ¹³C NMR spectroscopy for the first time. The ¹³C NMR results confirm the structure previously assigned to these compounds on the basis of vibrational spectroscopy. Thiocyanogen, sulfur dicyanide, selenocyanogen and selenium dicyanide have been also studied as liquid film on KBr plate in the solid state (KBr pellet) by FT-IR spectroscopy and the data collected have been compared and discussed with previous results recorded on solutions in organic solvents. Good agreement with early results has been obtained. It has been confirmed by solid state ¹³C NMR-MAS that under mild conditions selenocyanogen disproportionates into selenium dicyanide and selenium diselenocyanate. By heating selenocyanogen above 180°C it polymerizes irreversibly into a brown polymer known as paraselenocyanogen whose FT-IR spectrum is surprisingly very similar to that of parathiocyanogen or polythiocyanogen, the product of self-polymerization of thiocyanogen. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00304-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 681–688
13C NMR and FT-IR spectra of thiocyanogen, S2(CN)2, selenocyanogen,
Se2(CN)2, and related compounds
*
F. Cataldo
Progega snc, Via Casilina 1626/A, 00133 Rome, Italy
Received 9 November 1999; accepted 13 January 2000
Abstract
The pseudohalogens thiocyanogen [(SCN)2], sulfur dicyanide [S(CN)2], selenocyanogen [(SeCN)2], selenium dicyanide [Se(CN)2]
and selenium diselenocyanate [Se(SeCN)2] have been prepared and studied by 13C NMR spectroscopy for the first time. The 13C NMR
results confirm the structure previously assigned to these compounds on the basis of vibrational spectroscopy. Thiocyanogen, sulfur dicyanide,
selenocyanogen and selenium dicyanide have been also studied as liquid film on KBr plate in the solid state (KBr pellet) by FT-IRspectroscopy
and the data collected have been compared and discussed with previous results recorded on solutions in organic solvents. Good agreement
with early results has been obtained. It has been confirmed by solid state 13C NMR–MAS that under mild conditions selenocyanogen
disproportionates into selenium dicyanide and selenium diselenocyanate. By heating selenocyanogen above 1808C it polymerizes irreversibly
into a brown polymer known as paraselenocyanogen whose FT-IR spectrum is surprisingly very similar to that of parathiocyanogen or
polythiocyanogen, the product of self-polymerization of thiocyanogen.
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Pseudohalogens; Selenocyanogen; Thiocyanogen
1. Introduction
R–CH_CH–RqS2(CN)2™
NC–S–C(R)–C(R)–S–CN (3)
qNC–S–C(R)–C(R)–N_C_S (4)
The chemistry of pseudohalogens has always been a fas-
cinating and rather briefly treated subject in textbooks of
inorganic chemistry [1–4]. Thiocyanogen [S2(CN)2] was
discovered by Soderback 81 years ago [1,5], whereas
selenocyanogen [Se2(CN)2] was prepared in pure form only
in 1925 [1,6,7].
Infrared (IR) and Raman spectra of thiocyanogen [8–10]
strongly suggest structure 1, which has a disulfide bridge,
NC–S–S–CN, rather than structure 2, which has the isothio-
cyanate form S_C_N–N_C_S, even though, according to
certain authors [9], on the basis of the position of the IR
band due to a –SCN or –NCS group, structure 2 is favoured
in polar solvents such as ether while structure 1 is largely
predominantin non-polaraproticsolvents.Thisconsideration
is not negligible because the addition of thiocyanogen to
ethylenic double bonds of organic substrates involves the
addition of both –SCN and –NCS groups [11]:
However, in contrast to the above suggestion, the product
with structure 4 can be formed in both polar and non-polar
solvents [11,12]. Recently, the matter has been rationalized
[12], and it has been shown that the IR band shift of thio-
cyanogen observed previously in ether could reasonably be
due to a charge-transfer interaction of thiocyanogen with
solvent rather than to the presence of isomer 2.
Moreover, it has been pointed out [12] that the heat of
formation of thiocyanogen found experimentally coincides
with the value theoretically calculated for isomer 1 rather
than compound 2 [13], and similar conclusions were
obtained from the gas-phase UV spectrum of thiocyanogen,
in agreement with theoretical calculations [14].
Thus, to explain the different addition behaviour which
leads simultaneously to structures 3 and 4, it is possible that
the thiocyanogen radical derived from S–S homolysis has a
bifunctional behaviour, as has been previously postulated
[11,15–17]:
* Tel.: q39-06-205 5084; fax: q39-06-205 0800; e-mail: cdcata@
flashnet.it
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(00)00304-1
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xS–CN (5)lxN_C_S (6)
73 kcal moly1 [24], which is larger than that for a S–S bond,
which requires 63 kcal moly1 [26]; in any case, these data
should be considered only as approximations. Taking into
account the rich organic chemistry of selenium [23,24], the
C–Se bond should be as stable as the C–S bond; moreover,
the Pauling electronegativity of selenium is slightly lower
than that of sulfur [24], thus the C–Se bond should have at
least the same degree of covalency as the C–S bond.
Bhatnagar et al. [18] have shown by magnetic suscepti-
bility measurements that freshly prepared solutions of
thiocyanogen in carbon disulfide and other solvents are
diamagnetic but that on standing they become paramagnetic.
In certain solvents such as cyclohexane, thiocyanogen is less
stable and paramagnetism is observedalsoinfreshlyprepared
solutions. The paramagnetism can be attributed to the for-
mation of thiocyanogen radicals due to homolysis of S–S
bonds during solution ageing. Probably these radicals induce
the irreversible self-polymerization of thiocyanogen to a
brick-red solid known as parathiocyanogen or polythiocyan-
ogen whose structure seems to be very peculiar [15–17,19].
Selenocyanogen [Se2(CN)2]wasisolatedforthefirsttime
in 1925 thanks to the work of Birkenback and Kellermann
([6,7]). Fundamental studies of the structure of seleno-
cyanogen and related compounds have already been pub-
lished [20,21] and are based on IR spectroscopy; some short
reviews on selenocyanogen and related compounds are also
available [22–24]. Selenocyanogen and related compounds
belong to the class of pseudohalides and are the selenium
analogues of the thiocyanogen family of compounds. In com-
parison to thiocyanogen, selenocyanogen and itshomologues
have received much less attention by investigators, but the
available data show that the selenocyanogen family has the
same chemical structure as the thiocyanogen family of
compounds.
Some unexpected differences between thiocyanogen and
selenocyanogen can be immediately outlined here. While
thiocyanogen is unstable and cannot be isolated in the pure
state unless special conditions at low temperatures are
adopted, selenocyanogen is rather more stable and can be
isolated at room temperature as a yellow powder; it can also
be re-crystallized from CH2Cl2 solutions and remains
unchanged for months if stored in a closed flask at q58C in
the absence of humidity or solvent impurities. Thiocyanogen
heated above 08C or isolated by solvent evaporation at room
temperature quickly turns into a brick-red solid [17]; how-
ever, selenocyanogen when heated in solution with carbon
disulfide undergoes a unique disproportionation reaction as
follows [21–24]:
2. Experimental
13C NMR spectra in CDCl3 solution were performed with
a Bruker AMX-600 spectrometer operating at 150.9 MHz;
7 s relaxation delay with 0.7 s of acquisition time and 908
pulse angle were applied. Spectra were obtained with 64 000
words. All reagents and solvents used were from Aldrich or
Fluka.
Solid state 13C MAS–NMR spectra were recorded at room
temperature on a Bruker AC-200 spectrometer, equipped
with an HP amplifier, 1H 200 MHz, 120 W continuous wave
and a pulse amplifier M3205. Samples were packed into
4-mm zirconia rotors and sealed with Kel-F caps. The spin
rate was 8 kHz. Spectra were obtained with 512 words in the
time domain, zero-filled and Fourier-transformed with a size
of 1024 words; 20 000 scans were performed.
FT-IR spectra were recorded on a Perkin-Elmer1710spec-
trometer. Samples were studied embedded in KBr pellets. All
reagents and solvents were from Aldrich or Fluka and were
used as received.
2.1. Lead thiocyanate preparation
Lead thiocyanate was prepared by adding ammonium
thiocyanate solution to an aqueous solution of lead(II)nitrate
in stoichiometric amounts. The white precipitate was col-
lected by filtration, washed with water and dried in an oven
at 1058C.
2.2. Silver cyanide preparation
Similarly, silver cyanide was prepared by precipitation
from silver nitrate and sodium cyanide in stoichiometric
amounts. The white precipitate was collected by filtration,
protected totally from light and dried in a desiccator under
dynamic vacuum.
2Se2(CN)2™Se(CN)2qSe3(CN)2
(1)
When heated at higher temperatures, selenocyanogen lib-
erates a sublimate (see below) and is polymerized into a
brownish material. Practically nothing is known about the
selenocyanogen polymerization products or about the strong
sensitivity of selenocyanogen to certain molecules and sol-
vents which cause itspolymerizationtoadeep-redsolid[25].
The difference in stability between thiocyanogen and
selenocyanogen is quite striking notwithstanding that the two
molecules should have the same geometrical structure, and
in general it is expected that the S–S bond strength is com-
parable to that of the Se–Se bond strength. However, the
dissociation energy of a Se–Se bond in diselenides is about
2.3. Thiocyanogen preparation
Thiocyanogen [S2(CN)2] was prepared as previously
reported [17], by suspending 6.8 g of dry lead(II) thiocya-
nate in 50 ml of CDCl3 (deuterated chloroform) by magnetic
stirring and adding dropwise 3.36 g of bromine. The freshly
prepared thiocyanogen solution was decanted and immedi-
ately used to record the 13C NMR spectrum. On standing the
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683
solution polymerized to the insoluble and brick-red solid
polythiocyanogen [15–17].
refrigerator, a discrete crystalline sublimate was deposited;
this was collected and identified as selenium dicyanide by its
IR spectrum. The dark-brown precipitate formed in the flask
was easily separated from decalin by settlinganddecantation.
The collected precipitate was washed several times with
dichloromethane and then dried in air. The precipitate was
paraselenocyanogen or polyselenocyanogen.
2.4. Sulfur dicyanide preparation
Sulfur dicyanide [S(CN)2] was prepared by suspending
4.5 g of silver cyanide in 50 ml of CDCl3 by vigorous stirring
and by adding dropwise 1.0 ml of sulfur dichloride (SCl2).
The mixture was stirred for 30 min at room temperature and
then the silver chloride formed was separated by decantation.
The CDCl3 solution of S(CN)2 was used to record the 13C
NMR spectrum.
2.9. Stability of selenium dicyanide
The white and tabular crystals of selenium dicyanide are
stable for months when stored in a tightly closed flask at room
temperature. Exposure to light causes the formation of an
orange colour. Also moisture and certain solvents cause a
change in colour.
2.5. Preparation of silver selenocyanate
Silver selenocyanate was prepared by adding a solution of
potassium selenocyanate to a solution of silver nitrate in stoi-
chiometric amounts. The precipitate formed was collectedby
filtration, washed with water and dried into a desiccatorunder
dynamic vacuum. Care should be taken to avoid exposure to
light.
3. Results and discussion
In previous work we have concentrated our efforts on the
study of the structure of parathiocyanogen or polythio-
cyanogen [(SCN)x], the inorganic polymer resulting from
spontaneous polymerization of thiocyanogen [15–17]. In
these studies we reported evidence about the peculiar and
probably linear structure of parathiocyanogen. With FT-IR
and electronic spectroscopy we have also studied thio-
cyanogen [(SCN)2] with the aim of understanding some
aspects of its spontaneous polymerization [17].
2.6. Preparation of selenocyanogen
Selenocyanogen was prepared by suspending 13.5 g of
silver selenocyanate [Ag(SeCN)] in 100 ml of dichloro-
methane with vigorous stirring. Then an iodine solution in
dichloromethane was added (6.0 g of iodine per 160 ml of
dichloromethane). Very rapid discolouration of the iodine
was observed. The mixture was stirred for 2 h at q138C and
then it was decanted. The silver iodide collected was yellow
while the Se2(CN)2 solution was orange. Dichloromethane
was then distilled off under reduced pressure in a water-bath
to leave a canary-yellow powder. Yield: 2.77 g (55.9% over
iodine used).
3.1. 13C NMR spectrum of thiocyanogen
The 13C NMR spectrum of freshly prepared thiocyanogen
(see Table 1) is characterized by a sharp singlet at 107.36
ppm. The thiocyanate ion of potassium thiocyanate is
reported to show a 13C NMR signal at about 133.3 ppm
[27,28], but alkyl thiocyanates such as methyl-, ethyl- and
butylthiocyanate give an NMR signal at higher fields, namely
at 113.5, 112.1 and 112.1 ppm, respectively [27]. On the
other hand, organic isothiocyanates are detected at 128–136
ppm [27,28], in the same range where the inorganic thiocy-
anate ion of KSCN gives an NMR signal, hence at consid-
erably lower fields than organic thiocyanates (see Table 1).
Based on these data, it can be concluded that thiocyanogen
has structure 1, with a disulfide bridge connecting the two
cyanide groups because its NMR signal is comparable to that
of alkylthiocyanates. In fact, for isomer 2 of ‘isothiocyano-
gen’ we should expect a singlet NMR signal at 128–132 ppm,
while for isomer 7 of ‘thiocyanogen–isothiocyanogen’(NC–
S–N_C_S) we would expect two different signals, one due
to the thiocyanate group at about 107 ppm and the other due
to the isothiocyanate group at 128–132 ppm. Since we
observe a unique signal at 107.36 ppm we can conclude that
structure 1 is the correct structure of thiocyanogen.
Selenocyanogen was dissolved in CDCl3 to record the 13
C
NMR spectrum in solution immediately after preparation.
The solid-state 13C NMR spectrum was recorded 3 weeks
after preparation.
2.7. Stability of selenocyanogen
Selenocyanogen is rather stable when stored in closed
flasks in the dark at q58C. When stored in this way no
decomposition can be observed (visually and by IR spec-
troscopy) even after months. However, humidity, heat and
certain solvents greatly accelerate the decomposition and
polymerization of selenocyanogen [10].
2.8. Preparation of selenium dicyanide and
paraselenocyanogen
Selenocyanogen (0.217 g) was mixed with 15 ml of
decalin and heated in a oil-bath for 2 h at 2108C and 1 h at
1808C. The initially yellow decalin solution quickly became
orange and then during heating turbid and brown. In the
Additionally, our NMR spectrum does not show any evi-
dence for the presence of thiocyanate (5) and isothiocyanate
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Table 1
13C NMR synopsis
Compound name
Structure (see text)
NMR signal (ppm)
Reference
Thiocyanogen
Sulfur dicyanide
Selenocyanogen
Selenium dicyanide
Selenium diselenocyanate
Potassium thiocyanate
Methyl thiocyanate
Ethyl thiocyanate
Potassium cyanide
Alkyl nitrile
1
8
10
107, 36 (singlet, in CDCl3)
126 (singlet, in CDCl3)
89, 92 (singlet, in CDCl3)
130 (broad, solid-state MAS–NMR)
98,3 (broad, solid-state MAS–NMR)
133.3
113.5
112.1
this work
this work
this work
this work
this work
[27,28]
[27]
[27]
[28]
[28]
[27,28]
[27,28]
see Eq. (2)
see Eq. (2)
KSCN
Me–SCN
Et–SCN
KCN
R–CN
R–N_C
R–N_C_S
168.5
112–126
157–167
128–136
Alkyl isonitrile
Alkyl isothiocyanate
(6) radicals, which means that these radicals are only present
at very low concentrations in our system.
3.2. 13C NMR spectrum of sulfur dicyanide
We have also studied by 13C NMR spectroscopy the lower
thiocyanogen homologue sulfur dicyanide, which has the
structure NC–S–CN (8) (V-shaped, not linear), which was
assigned on the basis of vibrational spectroscopy and X-ray
studies [22,23], rather than NC–N_C_S (9) or other pos-
sible structures.
Ageing of thiocyanogen solution in CDCl3 causes the for-
mation of an orange-coloured solution, and also a brick-red
precipitate starts to appear in suspension and at the bottom of
the solution. As mentioned in Section 1, Bhatnagar et al. [18]
have shown that freshly prepared thiocyanogen solutions are
diamagnetic but become paramagnetic upon ageing due to
free radical formation. The 13C NMR spectrum of aged thio-
cyanogen solution still shows the signal at 107 ppm due to
thiocyanogen but a new feature at 169.6 ppm starts to appear.
We have already shown [17] that thiocyanogen polymer
(polythiocyanogen) has a unique feature at 186 ppm in di-
methylformamide (DMF) solution, where it hassomedegree
of solubility; however, polythiocyanogen is not soluble in
chloroform. Thus the signal at 169.6 ppm (in CDCl3) could
be due to a low molecular weight and soluble oligomer of
thiocyanogen.
Other possible interpretations involve the presence of iso-
thiocyanate ion, which we suppose should absorb in this field
from the observation that isocyanides give NMR signals at
157–165 ppm (see Table 1). If the assignment of the feature
at 169.6 ppm to the isothiocyanate group is correct, we have
the first direct indication that thiocyanogen undergoes the
scission of its S–S bond followed by a rearrangement to an
isothiocyanate group. This could explain the simultaneous N
and S addition to the double bonds of organic substrates (see
structures 3 and 4). However, a simpler and more convincing
explanation exists for the feature at 169.6 ppm. In fact, cya-
nide ion absorbs at 168.5 ppm (see Table 1); hence, instead
of scission of the S–S bond, we have evidence of scission of
the C–S bond which leads to the cyanide group. Scission of
the C–S bond of thiocyanogen would imply the formation of
thiocyanate ion and free sulfur. Unfortunately, there is no
evidence of the presence of free thiocyanate in our NMR
spectrum because no signal at 113.5 ppm wasobserved.Thus,
the NMR spectroscopy data suggest that the degradation of
thiocyanogen in chloroform involves the formation of cya-
nide ions and free sulfur.
Freshly prepared sulfur dicyanide shows a unique and
sharp singlet at about 126 ppm (see Table 1) in CDCl3 solu-
tion. Even if the 13C NMR signal of this molecule appears at
lower fields than that observed for thiocyanogen, we can
immediately exclude structure 9, which requirestwodifferent
signalsat lowerfields, namelyoneat leastat157ppm[27,28]
for the isocyanide group, which was not observed, and the
other above 130 ppm for the isothiocyanide group [27,28].
Since only one signal was detected we can conclude that
structure 8 is the correct structure of sulfur dicyanide. To
explain why the signal of sulfur dicyanide appearsat 126 ppm
and that of thiocyanogen at 107 ppm we must assume that
some conjugation between the triple bonds of the two cyano
groups occurs through the sulfur bridge in the case of sulfur
cyanide, while this is absent in the case of thiocyanogen,
which has two sulfur atoms per bridge. Therefore, conjuga-
tion causes an NMR signal closer to the thiocyanate ion in
the case of sulfur dicyanide (133.3 ppm for thiocyanate ver-
sus 126 ppm for sulfur dicyanide), while for thiocyanogen
the NMR signal at 107 ppm is comparable to that of organic
alkyl thiocyanateswhichinfactabsorbinthatregion[20,21].
The general observation that sulfur dicyanide is more stable
than thiocyanogen could be ascribedtothefactthattheformer
is stabilized also by conjugation while the latter is not. In fact,
sulfur dicyanide can be isolated in the form of crystals by
sublimation [22], whereas thiocyanogen can be isolated in
crystal form only under more complex conditions at y508C
[10] and at temperatures above 08C shows a certain stability
only in solution. When thiocyanogen in solution is concen-
trated by evaporating the solvent at room temperature under
reduced pressure, it polymerizes very quickly [17], some-
times with detonation [17]. Upon prolonged ageing in solu-
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685
tion at room temperature, sulfur dicyanide begins self-
polymerization. The 13C NMR spectrum shows a resonance
at 129 ppm, but also a new line appears at 124 ppm and
complicated resonances are observed in the 147–165 ppm
region with the main peak at 155.5 ppm. Additional features
are observed in the 171–182 ppm region. These aged spectra
will be interpreted and discussed elsewhere [25].
the Se–Se–C angle is different from the S–S–C angle this
difference could affect the electronic shielding of the carbon
nucleus leading to a difference in chemical shift.
As mentioned in Section 1, selenocyanogen is relatively
stable at room temperature and we have tried to record also
its solid-state 13C NMR spectrum. This spectrum (see Table
1), was recorded only 3 weeks after selenocyanogen prepa-
ration. It is characterized by two main resonance peaks, one
lying at 98.3 ppm and a doublet at about 130 ppm. Since only
one resonance was detected in freshly prepared selenocyan-
ogen solution, it is likely that some disproportionation or
decomposition occurred in the solid during the 3 weeks after
the preparation so that the solid-state spectrum now shows at
least two resonance peaks. Since the well-known dispropor-
tionation reaction of selenocyanogen described in the litera-
ture [22–24] involves the reaction (Eq. (1)) showing that
selenocyanogen is transformed into selenium dicyanide and
selenium diselenocyanate in mild conditions, we tentatively
assign the resonance due to selenium diselenocyanate the
peak at 98.3 ppm, while the peak at about 130 ppm is assigned
to selenium dicyanide by observing that sulfur dicyanide has
a 13C NMR signal (in solution) at 126 ppm.
3.3. 13C NMR spectra of selenocyanogen and selenium
dicyanide
As reported in Table 1, the 13C NMR spectrum of freshly
prepared selenocyanogen [Se2(CN)2] in CDCl3 solution
shows a sharp singlet at 89.92 ppm. The presence of a unique
singlet is in complete agreement with the fact that the correct
structure of selenocyanogen is analogous to that assigned to
thiocyanogen, NC–Se–Se–CN (10).
All other possible structures involving isoselenocyanide,
Se_C_N–N_C_Se (11), and selenocyanide isoseleno-
cyanide, Se_C_N–Se–CN (12), can be excluded com-
pletely. What is somewhat surprising is that the resonance of
selenocyanogen appears at higher fields than that of thio-
cyanogen (89.92 ppm for the former versus 107.36 ppm for
the latter), notwithstanding that these two compounds have
the same general structure and that selenium and sulfur are
very close in their chemical properties. At present we have
no clear explanation for this difference, but it may be that if
3.4. FT-IR spectra of thiocyanogen and sulfur dicyanide
As shown in Table 2, the strong thiocyanogen band due to
antisymmetric stretching of the –CN group has beenobserved
Table 2
Infrared band summary; values in cmy1
Thiocyanogen film
on KBr plate a
Thiocyanogen solution
in carbon tetrachloride b
Thiocyanogen solution
in carbon disulfide b
Thiocyanogen solution
in ether b
Sulfur dicyanide c
Sulfur dicyanide
(KBr pellet) d
2162 vs
2073 m
2175 s
1990 m
2171 s
2177 ms
2030 m
2179 s (with 2188 sh)
2180 s
1735 vw, br
1660 s
1415 mw
1350 vw, br
1300 sh
1220 m
1140 br
1060 mw
780 sh
790 mw
740 mw
685 m
670 m
600 w
668 s
673 s
667 s
684 s
665 vs
635 m
620 m
600 w
550 w
525 w
465 w
420 vw
440 vw
490 mw
359 vw
499 vvw
368 w
374 w
329 w
a From [17].
b From [9,10].
c From [8].
d This work.
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in the range 2171–2177 cmy1 by various authors when the
pseudohalogen was dissolved in organic solvents. By leaving
some drops of thiocyanogen solution in CH2Cl2 to evaporate
onto a KBr disk, it is possible to record the spectrumofalmost
‘dry’ and still not polymerized thiocyanogen [17]. Under
these special conditions, thestrongbandduetoantisymmetric
stretching appears at 2162 cmy1 while the band presumably
due to symmetric stretching is detected at 2073 cmy1. This
is at a significantly higher frequency than the value detected
in CCl4 solution (1990 cmy1) and in ether solution (2030
cmy1) reported by other authors [9,10]. This suggests that
this band is very sensitive to the nature of the solvent in which
the molecule is dissolved [12]. The intense band at about
670 cmy1 is due to a C–S stretching band, confirming struc-
ture 1.
bending at 436 cmy1. Additionally, we have recorded other
intense bands in the FT-IR spectrum of selenocyanogen in
KBr. These bands were never reported previously because
the spectra were recorded in organic solvents so that some
spectral regions were buried by the intense absorption of the
solvent, or at the time of the early measurements certain
spectral regions were not accessible to the spectrophotome-
ters or were not considered by the researchers. In fact, we
observe additional bands at 3650, 3540 and 3434 cmy1; these
bands are strong and may be are only overtones or combi-
nation bands, but additionally we observe other two bands at
1640 and 1610 cmy1 which, however, remain unassigned.
There are several other weak and very weak infrared bands
(see Table 1) which were detected for the first time but which
could also be due to impurities.
An alternative and reasonable interpretation suggests that
the band at 2073 cmy1 could be assigned to cyanide ion. In
fact, sodium cyanide shows an IR band at 2060 cmy1 [15].
This assignment seems to confirm the NMR interpretation
we have given in the preceding section about the decompo-
sition of thiocyanogen in chloroform.
Also for selenium dicyanide (see Table 1), for the IR
spectrum (in KBr) recorded on the sublimate formed by
thermal decomposition of selenocyanogen there is a good
general agreement between early reported spectraandtheFT-
IR reported here. The nitrile stretching appears in KBr pellet
as a sharp peak at 2181 cmy1, hence at higher frequency than
selenocyanogen, in agreement with previous values reported
at 2171 and 2183 cmy1 [20,21]. In agreement with previous
measurements [20,21] we observe the antisymmetric C–Se
stretching at 605 cmy1 and the symmetric C–Se stretching at
507 cmy1. At 462 and 436 cmy1 we observe the C–Se bend-
ing mode. Again also in the case of selenium dicyanide we
observe new IR bands never reported before, namely at 3649,
Table 2 shows also that the FT-IR spectrum of sulfur
dicyanide is in reasonable agreement with previously pub-
lished spectra. In particular, only one nitrile stretching band
is observed at 2180 cmy1 in the solid state (KBr pellet).
Thermodynamic calculations using the group increment
method [29] give for thiocyanogen DGofs63.7 kcal moly1
and DHofs69.8 kcal moly1 (literature experimental data:
DHofs74.3 kcal moly1). Sulfur dicyanide is a slightly less
endothermic compound, with DGofs61.4 kcal moly1 and
3589, 3581, 3453, 2922, 2851 cmy1 and at 1638, 1612 cmy1
.
These bands remain unassigned; some of them could be due
to impurities.
DHofs64.0 kcal moly1
.
3.5. FT-IR spectra of selenocyanogen and selenium
dicyanide
3.6. FT-IR spectrum of paraselenocyanogen
Concerning paraselenocyanogen or polyselenocyanogen
there are no precise data available in the literature. In Section
2 we have shown that selenocyanogen undergoes a thermal
decomposition reaction when heated at hightemperaturewith
liberation of a sublimate which was identified as selenium
dicyanide by IR spectroscopy as just discussed, and the for-
mation of a dark-brown insoluble product which is parase-
lenocyanogen and can be described by the general formula
of [Sey(CN)2]x with 0-y-3 and x very large. We will
examine in more detail the paraselenocyanogen formation
mechanism and its structural aspects elsewhere [25]. How-
ever, here we would like to report that the IR spectrum of
paraselenocyanogen is very peculiar and is very similar to
the spectrum of parathiocyanogen [17].
In fact, the spectrum of paraselenocyanogen is character-
ized by a very intense and broad band at 1200 cmy1 with a
shoulder at 1263 cmy1, two medium bands at 1444 and 1412
cmy1 and medium-weak bands at 1619 and 881 cmy1. Weak
bands are detectable at 801, 638, 609, 525 cmy1 as well as
at 2262, 2184 and 2148 cmy1. Since selenium diselenocyan-
ate [Se3(CN)2] has only three main IR lines (see Table 3)
which do not coincide at all with the spectrum of para-
The IR spectrum of selenocyanogen in solution has been
studied by several authors [20,21,23]. In this work we have
recorded for the first time the FT-IR spectrum of seleno-
cyanogen in KBr pellet. As shown in Table 3, there is gen-
erally good agreement between the main bands of Se2(CN)2
measured in solution and in the solid state. The nitrile stretch-
ing band of selenocyanogen was reported at 2152 cmy1 when
measured in organic solvents [20], while it is shifted to 2143
cmy1 in KBr pellet followed by a weaker band at 1992 cmy1
.
If the former stronger band is assigned to nitrile antisymme-
tric stretching then the weaker band at lower frequency is
assignable to nitrile symmetric stretching. There are no indi-
cations about the presence of an isoselenocyanate group,
since it is characterized by out-of-phase stretching at 2182
cmy1 and in-phase stretching mode at 983 cmy1 [30], which
are completely absent from our spectra. The symmetric Se–
C(N) stretching is reported as a strong band at 521 cmy1 in
organic solvents whereas in KBr pellet it appears at 505
cmy1; in the solid state we observe also another band at 580
cmy1 which could be due to antisymmetric Se–C(N)stretch-
ing. In the solid state we were able to observe also Se–CN
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687
Table 3
Infrared band summary; values in cmy1
Selenium diselenocyanate
Selenocyanogen
Selenocyanogen
Selenium dicyanide
Selenium
Selenium
(KBr) a
(KBr pellet) b
(chloroform) a
(KBr pellet) b
dicyanide a
dicyanide c
3650 ms
3625 sh
3583 sh
3540 vs
3649 vs
3627 sh
3589 vs
3544 sh
3518 vs
3435 m
3434 vs
2935 mw
2867 w
2922 vs
2851 ms
2656 w
2523 vw
2500 vw
2181 s
2171 s (KBr) d
2183
2131 ms
2143 vs
1992 br
2152 vs
1721 sh
1640 m
1610 s
1638 m
1612 vs
1503 vw
1460 vw
1449 ms
1345 w
1308 w
1280 m, br
1260 w
1228 w
1140 w
1055 w
1020 w
973 mw
925 mw
1120 vw
1080 vw
1020 w
990 sh
870 sh
839 mw
745 vw
580 br
505 vvs
605 br
507 vvs
462 w
436 w
608 m
516 vs
608
516
513 s
521 vs
436 mw
412 w
436 w
345 w
336 w
312 w
302 w
364 and 360 w
357 w
333 w
345
310
a From [20].
b This work.
c From [21].
d In acetonitrile the bands are resolved at 2183 (sh) and 2175 (m).
selenocyanogen, we can conclude that the disproportionation
reaction (Eq. (1)) happensunderverymildconditions,while
under strong heating the decomposition reaction of seleno-
cyanogen is as follows:
cyanogen. Since we have proposed [15–17] that parathio-
cyanogen could have a linear structure rather than a
triazine-based structure, we can conclude that paraseleno-
cyanogen also has a linear structure because its spectrum is
impressively similar to that of parathiocyanogen.
2xSe2(CN)2™xSe(CN)2q[Sey(CN)2]xqxSe
(2)
(with 0-y-3)
Acknowledgements
The coincidence between the FT-IR spectra of para-
selenocyanogen and parathiocyanogen shows only that
selenocyanogen polymerizes like thiocyanogen by forming a
polymer which should have the same structure of parathio-
We wish to thank Dr Donatella Capitani from Istituto di
Chimica Nucleare, CNR, Monterotondo Stazione, Rome, for
assistance in recording the 13C NMR spectra.
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688
F. Cataldo / Polyhedron 19 (2000) 681–688
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