Gas phase infrared spectrum and ab initio calculations of phosphorus(III) thiocyanide, SPCN

Article abstract of DOI:10.1016/j.saa.2004.12.041

An attempt has been made to record the gas phase infrared spectrum of phosphorus(III) thiocyanide, SPCN, for the first time. The molecule was generated by an on-line process using phosphorus(III) thiochloride, SPCl, as a precursor passed over heated silver cyanide at about 350 °C. The products were characterized by the infrared spectra of their vapors. The low resolution gas phase Fourier transform infrared spectrum shows three of six characterized fundamental modes of SPCN within the range of the spectrometer used at 2151, 743 and 622 cm^-1 These three bands were assigned to ν₁(CN stretch), ν₂(SP stretch), and ν₃(CP stretch), respectively. Ab initio self-consistent-field (SCF) molecular orbital (MO) and Moller-Plesset second-order perturbation theory (MP2) calculations were performed to determine the geometry, total energy and the vibrational frequencies of SPCN.

Full text of DOI:10.1016/j.saa.2004.12.041

Spectrochimica Acta Part A 62 (2005) 282–286
Gas phase infrared spectrum and ab initio calculations
of phosphorus(III) thiocyanide, SPCN
∗
Abdul W. Allaf , Mohammed N. Odeh
Atomic Energy Commission of Syria, Department of Chemistry, P.O. Box 6091, Damascus-Syria, Syrian Arab Republic
Received 24 June 2004; accepted 6 December 2004
Abstract
An attempt has been made to record the gas phase infrared spectrum of phosphorus(III) thiocyanide, SPCN, for the first time. The molecule
was generated by an on-line process using phosphorus(III) thiochloride, SPCl, as a precursor passed over heated silver cyanide at about
◦
3
50 C. The products were characterized by the infrared spectra of their vapors. The low resolution gas phase Fourier transform infrared
−
1
spectrum shows three of six characterized fundamental modes of SPCN within the range of the spectrometer used at 2151, 743 and 622 cm
These three bands were assigned to ν
molecular orbital (MO) and Møller–Plesset second-order perturbation theory (MP2) calculations were performed to determine the geometry,
total energy and the vibrational frequencies of SPCN.
1
(C≡N stretch), ν
2 3
(S P stretch), and ν (C P stretch), respectively. Ab initio self-consistent-field (SCF)
©
2005 Elsevier B.V. All rights reserved.
Keywords: Gas phase infrared spectrum; Ab initio; SPCN
1
. Introduction
[4–7]. Binnewies and Schn o¨ ckel reviewed molecules of the
nitrosyl halides type containing elements P, As and Sb in
the central position [8]. These molecules were compared to
NOX as well as to other isoelectronic molecules. The review
also reported the formation of molecules and their vibrational
spectraincludingquantumchemicalcalculationsandthermo-
dynamic stability. Leszczy n´ ski and co-workers reported very
advanced ab initio calculations, electronic structure and rela-
tive stability of the halogenated thiophosphorus compounds
SPX where X is an F, Cl and Br and their XSP isomers using
ab initio post-Hartree–Fock (HF) methods [9]. Zidan and Al-
laf have reported the gas phase on-line production of phos-
phorus thiotrihalides, SPX3 (X = F, Cl, Br and I) and their
identifications by infrared spectroscopy [10]. Doroko and
Buelow reported the vapor-phase infrared spectrum of ni-
trosyl cyanide, NOCN, using nitrosyl chloride, NOCl, as a
starting compound over heated silver cyanide [11]. Similar
route was followed to record the gas phase infrared spectrum
[12] and HeI photoelectron spectrum [13] of thiazyl cyanide,
NSCN, using thiazyl chloride, NSCl, as a starting material
passed over heated AgCN. Thionitrosyl cyanide, NCNS, and
its radical cation were generated by dissociative ionization
Gas phase vibrational spectra of small polyatomic
molecules are of large interest. They are free from inter-
molecular interaction and exhibit rotational fine structure
[1,2]. Fourier transform infrared spectroscopy (FTIR) is one
ofthemostimportanttoolsinmeasuringgasphasevibrational
spectra. Thiophosphorus compounds have very important ap-
plications due to the presence of P S bond in their struc-
tures. The vast applications of P S containing compounds
are as a component of matches and as lubricating agents,
oil additives, floating agents, insecticides and vulcanization
accelerators [3]. The simplest triatomic of halogenated thio-
phosphorus species have the S P X moiety bent structure,
where X is a halogen. SPF, SPCl and SPBr, were synthe-
sized via the reaction of phosphoryl halides, PX3 with silver
at high temperatures at above 1000 K and recorded the ma-
trix isolated infrared spectra of these species, all the three
previous reactions were found to be strongly endothermic
∗
Corresponding author. Fax: +963 11 6112289.
E-mail address: scientific@aec.org.sy (A.W. Allaf).
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.12.041

A.W. Allaf, M.N. Odeh / Spectrochimica Acta Part A 62 (2005) 282–286
283
of (1,2,5)-thiadiazolo 3,4-C (1,2,5)-thiadiazole and charac-
terized by a collisional activation (CA) and neutralization
reionization (NR) mass spectrometry [14]. These mass spec-
trometric experiments also confirmed the reaction product
NSCl + AgCN→AgCl + [C, N2, S] and the last term is a sta-
ble species in the gas phase and corresponds to the isomeric
thiazyl cyanide different to thionitrosyl cyanide [14].
On the other hand, the oxygen analogue species, OPCN,
was generated by similar route using OPCl as precursor
passed over heated AgCN. Both gas phase infrared spectrum
and ab initio calculations were carried out [15]. However,
the asymmetric top SPCN molecule is a bent tetra atomic
species of Cs symmetry and posses six normal infrared active
modes of vibrations. This paper describes the assignment of
three fundamental vibration modes using the gas/solid reac-
tion of phosphorus(III) thiocyanide, SPCl, passed over heated
AgCN. The remaining bands and some other properties were
determined using ab initio quantum chemical calculations
method.
(99.9%), respectively. The synthesis was carried out as fol-
lows:
SPCl3(g) + 2Ag → S P Cl + 2AgCl(g)
(s)
(g)
The other experimental conditions for this step were slow
−
1
flow reaction, resolution of 2 cm and under about 0.2 Torr
pressure. The distance between the furnace containing silver
wool and the IR cell was about 15 cm. In order to avoid any
disproportionation or polymerization, the IR cell was heated
◦
to about 200 C with heating tape.
The last experiment was the combinations of the two pre-
vious experiments by on-line procedures. OPCN was gener-
ated using an on-line process passing SPCl gas, which was
obtained as mentioned in the second experiment through an-
other furnace containing dried silver cyanide (99.9%, Merck)
◦
heated at 350 C. The distance between the second furnace
and the IR cell was about 3 cm. The synthesis reaction is as
follows:
S P Cl(g) + AgCN(s)→ SPCN(g) + AgCl(g)
2
. Experimental
The remaining conditions for this step were also slow flow re-
action, resolution of 2 cm and under about 0.20–0.30 Torr
pressure.
The first procedure was to optimize the production
−
1
conditions of SPCl3, and recording its gas phase infrared
spectrum before attempting to observe the SPCl and SPCN
infrared spectra. SPCl3 has enough vapor pressure at room
temperature. Therefore, the ampoule containing SPCl3 was
first degassed in the vacuum line using liquid nitrogen and
then the valve of the ampoule was loosed very slightly in
order to obtain the vapor pressure of SPCl3 and then the
vapor was passed directly via the vacuum line into the in-
frared cell which is fitted with two KBr windows. The vac-
uum system was pumped out via a liquid nitrogen trap with
3. Computational details
In order to facilitate the interpretation of the infrared spec-
tra, ab initio molecular orbital calculations were performed
to determine the optimization structure and geometry, total
energy and the vibrational frequencies of SPCN. The opti-
mized structure was determined at the Hartree–Fock level
using 6-31G(d) basis sets as well as at the second-order
Møller–Plesset perturbation theory (MP2) using 6-311+G(d)
and cc-pvDZ [16–18] (coupled-cluster-polarization valence
double zeta) basis sets respectively. All computations were
carried out using Gaussian 98 package on INDI silicon graph-
ics computers at the University of Birmingham, England. The
3
−1
a rotary pump (RV8, Edwards) displacing 8.5 m h . The
infrared spectra for this procedure and the following experi-
ments were recorded on a JASCO 300E FTIR spectrometer.
The remaining conditions for this step were slow flow reac-
−
1
tion, resolution of 2 cm and under about 0.25 Torr pres-
sure.
The second experiment was focused on the optimization
and production conditions of phosphorus(III) thiochloride,
SPCl, in an on-line process. We started from SPCl3 as a pre-
cursor with the conditions mentioned above and then passed
◦
˚
optimized bond length (A), angles ( ), total energy and dipole
moment of SPCN as calculated at the HF-self-consistent field
(SCF) level using 6-31G(d) basis set and at MP2 level using
◦
6-311+G(d) and cc-pvDZ basis sets, respectively, have been
summarized in Table 1.
directly over heated silver wool used as a reductant at 900 C.
SPCl3 and silver wool were obtained from Fluka and Merck
Table 1
Optimized bond length ( A˚ ) and angles ( ) of SPCN as estimated theoretically
◦
r(S P) ( A˚ )
r(P C) ( A˚ )
r(C≡N) ( A˚ )
1.138
1.183
1.193
∠SPC ( )
◦
∠PCN ( )
◦
Dipole moment (Debye)
Molecule
SPCN
Methods
HF-SCF
Total energy (a.u.)
−830.52071
−831.12013
−831.08672
1.902
1.939
1.960
1.815
1.806
1.827
105.30
104.04
104.02
185.37
188.13
184.74
2.399
2.535
2.578
a
MP2
b
MP2
a
6
-311+G(d).
b
cc-pvDZ basis sets.

2
84
A.W. Allaf, M.N. Odeh / Spectrochimica Acta Part A 62 (2005) 282–286
Fig. 3. Gas phase infrared spectrum of SPCN in the range 400–2300 cm 1.
−
Fig. 1. Gas phase infrared spectrum of SPCl3 in the range 400–1000 cm 1.
−
at 716 cm 1, assigned to ν3, is the S P stretching mode of
−
−
1
SPCl. The second band at 462 cm is assigned to ν2 and
corresponds to P Cl stretching mode. It can be seen from
Fig. 2 that the P 35Cl/P Cl vibrational stretches ratio is
about 3:1 and that lies very nicely with the expectation. The
37
−1
35
observed frequencies were at 462 and 445 cm for P Cl
stretch/P 37Cl stretch, respectively, with a shift of 17 cm
−1
.
The third expected band of SPCl corresponds to a bend mode,
−1
lies out the range spectrometer used (400–4000 cm ).
These recorded results can be compared with those
observed by the matrix isolation technique at 716.1 and
−
1
4
62.4 cm for S P and P Cl stretching modes, respectively,
while the bend mode of SPCl was observed by Raman tech-
nique at 229 cm [6,19]. Note that identical vibrations were
Fig. 2. Gas phase infrared spectrum of SPCl in the range 400–1000 cm 1.
−
−1
observed by our gas phase route with those observed by the
matrix isolation technique.
4
. Results and discussion
−
1
The band observed at 692 cm has been assigned to a
combination band (ν1 + ν2). The remaining bands in Fig. 2
could be related to the impurities or to the starting material
(SPCl3). Full assignments of the bands in comparison with
those related results can be seen in Table 2.
Fig. 3 shows a mixture of three IR spectra for SPCN, SPCl
andSPCl3. Itshouldbementionedatthispointthattheprepar-
ing of the second reaction by an on-line process is in general
far from being trivial because of the difficulties of obtaining
pure SPCN. Nevertheless, we could observe three of six IR-
modes of SPCN. It should be pointed out here that there are
six fundamental IR active modes of vibrations correspond-
ing to SPCN. Five of them are in plane and divided into three
Fig. 1 shows the infrared spectrum of SPCl3 which is the
first precursor used to generate SPCl and SPCN by an on-
line process. This spectrum was the result of passing the con-
trolled vapor pressure of SPCl3 only in the vacuum line into
theIRcell. Threestrongbandswereobservedwithinthespec-
trometer range at 762, 542 and 435 cm . The assignments
of the bands can be seen in Table 2 in comparison with the
related results. For more details, the reader can be referred to
reference [10].
−
1
Fig. 2 shows the result of passing SPCl3 gas over heated
◦
silver wool used as a reductant at 900 C. Two new-centered
bands at 716 and 462 cm were observed. The first band
−
1
Table 2
Observed gas phase IR frequencies of SPCl3 and SPCl in comparison with related results
Observed gas phase (cm 1)
−
Related work (matrix isolated)
Molecule
Mode
Assignments
SPCl3 [10]
ν1
ν4
ν2
ν3
ν5
ν6
A1 (S P) stretch
E (P Cl) stretch
A1 (PCl3) umbrella
A1 (ClPS) deformation
E (S PCl3) rock
762 [10]
753 [27]
542
435
350
250
542
435
–
–
–
E (PCl3) deformation
167
´
´
´
SPCl
ν1
ν2
ν3
A (SPCl) bend
–
462
716
229 [6]
462.4
716.1
A (P Cl) stretch
A (S P) stretch
[27] and [6] are the liquid phase and the matrix isolated infrared results of SPCl3 and SPCl, respectively.

A.W. Allaf, M.N. Odeh / Spectrochimica Acta Part A 62 (2005) 282–286
285
Table 3
Observed and calculated frequencies of SPCN (cm 1)
−
Molecule Mode Assignment
IR gas phase HF-SCF
observed)
Force constant
(HF-SCF)
MP2
Force constant
MP2
cc-pvDZ
Force constant
(MP2)/cc-pvDZ
(mDyn/A)
(
6-311+G(d) (MP2)/6-311+G(d)
(mDyn/A)
(mDyn/A)
ꢀ
SPCN
ν1(A ) C≡N stretch
2151
743
622
–
2545.0
782.65
638.68
454.20
308.10
171.10
48.7911
11.3126
4.2561
1.6409
0.7098
0.3320
2071.20
666.50
601.50
371.60
238.10
131.65
32.2696
8.1631
3.8169
1.1255
0.4231
0.1881
2068.80
670.90
587.95
389.10
251.60
136.20
32.1979
8.2835
3.6662
1.2117
0.4740
0.2052
ꢀ
ν2(A )
S
P
P stretch
C stretch
ꢀ
ν3(A )
ꢀ
ν4(A ) N≡C P bend
ꢀ
ꢀ
ν5(A )
C
P
S bend
–
–
ꢀ
ν6(A ) Out of plane wag
range 700–800 cm 1. Therefore, the reported S P frequency
−
stretching bonds and two angle deformation modes. Since
−
1
the ∠PCN mode is nearly linear as estimated theoretically
at 743 cm lies within an acceptable range [2,19]. The C P
stretching band at 622 cm can be compared with the same
mode observed in OPCN which has a value of 620 cm
15]. It should be mentioned here that the C P stretching
frequency was observed at 1240 cm in the chlorophos-
phaethyne, ClCP [23]. Therefore, the assignment of C P
stretching band at 622 cm is justified.
The remaining bands shown in Fig. 3 at 2200, 1980, 1273,
200, 1056, 1023, 927, 837 and 716 cm have been assigned
−
1
(see Table 1), the sixth mode of vibration is assigned to an
−1
out-of-plane wag rather than a molecular torsion. The three
fundamental modes of SPCN within the range of the spec-
[
−
1
−
1
trometer used were observed at 2151, 743 and 622 cm
.
These three bands were assigned to ␯1(C N stretch),
ν2(S P stretch) and ν3(C P stretch), respectively.
−
1
−
1
The C N stretching band at 2151 cm can be compared
with the same mode in NOCN [11] and OPCN [15] which
−
1
1
−
1
−1
haveavalueof2177.5and2165 cm , respectively, withsim-
as follows: the first two bands at 2200 and 1980 cm were
assigned to the C N stretching modes of C2N2 [24,25] and
−
1
ilar PR type structure. The S P stretching band at 743 cm
has also typical PR type structure and lies to the left of the
S P band observed in SPCl. This band can be compared
with the same gas phase infrared mode observed in SPCl3
and SPCl which have values of 762 [10] and 716 (this work)
cm , respectively. Similar trend was observed between the
gas phase infrared spectra of OVF3 and OVF for the O V
(
1
SCN)2 [26], respectively. The bands at 1273, 1200, 1056,
−
1
023, 927 and 837 cm have been not assigned and could
−1
be due to the impurities. The last band at 716 cm has been
explained in Fig. 2.
The remaining three fundamental modes of SPCN are as-
signed and estimated theoretically and listed in Table 3 as
calculated at the HF-SCF and MP2 level using 6-31G(d) and
−1
−
1
stretching frequency which have values of 1058 cm [20]
−
1
and 1028 cm [21], respectively. By the way, the O V
stretchingfrequencyintheanaloguesspeciesOVCNhasbeen
observed at around 1040 cm [22]. Finally, it is known from
the literature that the S P stretching frequency lies in the
(6-311+G(d) and cc-pvDZ) basis sets, respectively. Table 3
is also included the force constants of SPCN as calculated
at the three previous basis sets. Table 4 shows also the full-
calculated quantities of SPCN.
−
1
Table 4
Summary of the calculated quantities of SPCN
SPCN
HF-SCF/6-31G(d)
MP2/(6-311+G(d)
MP2/cc-pvDZ
Zero-point energy (a.u)
0.01116
0.00929
0.00935
Conditions
Temperature (K)
Pressure (atm)
298
1
298
1
298
1
Internal energy (a.u)
Enthalpy
Free energy (a.u)
Heat capacity, Cv (cal/(mol K))
Entropy cal/(mol K)
−830.50521
−830.50427
−830.53760
12.482
−831.10619
−831.10525
−831.13930
13.129
−831.07277
−831.07182
−831.10580
13.085
70.164
71.659
71.521
Rotational constants (GHz)
A
B
C
14.90632
2.78318
2.34529
14.37030
2.73304
2.29631
13.65573
2.70540
2.25805
Partial charges
P
S
C
N
0.52
−0.19
−0.02
−0.30
0.20
−0.14
0.13
−0.19
0.43
−0.16
−0.15
−0.13

2
86
A.W. Allaf, M.N. Odeh / Spectrochimica Acta Part A 62 (2005) 282–286
From the theoretical point of view, the calculated vibra-
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−
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−
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