4313
J. Phys. Chem. 1993, 97, 4313-4319
Infrared Spectra of HSPH2, HPS2, and HSW2 in Solid Argon Zofia Mielke and Lester Andrews’ Chemistry Department, University of Virginia, Charlottesville, Virginia 22901, and Institute of Chemistry, Wroclaw University, 50-383 Wroclaw, Poland Received: October 2, 1992
Argon/phosphine samples were reacted with discharged Ss/Ar mixtures and the products were trapped in solid argon for infrared spectroscopic analysis. Isotopic substitution, photolysis, and annealing behavior have characterized three novel phosphorus thiohydrides: HSPH2, HPSS, and HSPS2. The chain HPSS isomer was the product of reaction between S2 and PH radical whereas HSPS2 was the product of reaction between S3 and PH. There is spectroscopic evidence that HSPH radical was also trapped in the matrix.
Introduction There have been few experimental studies of simple thiohydrides of phosphorus. The evidence is given for H2PSH formed in electric discharge of PH3 and H2S mixtures.l The reaction of HCl with Na3PS4leads to the formation of tetrathiophosphoric acid, H3PS4.2 Both H2PSH and H3PS4 species are unstable at room temperature and were stabilized by condensation at low temperatures. No other simplethiohydridesof phosphorus have been observed although there exists a number of organic derivatives of simple phosphorus thiohydrides and a number of metal salts of simple phosphorus thioacids. Theoretical attempts have been undertaken to determine the structure of some of the simplest phosphorus thiohydrides and to understand the nature of the PS In recent ab initio SCF studiesthe geometriesand vibrational IR spectra of the phosphine sulfide, H3PS, and its isomer, phosphinothious acid, H2PSH, have been calculated.6 Recent matrix infrared work on phosphorus oxides and hydroxides in this laboratory7,8has characterized a number of small phosphorus hydroxides using matrix reactions of 0 atoms or O3 molecules with PH3. It was desired to prepare and characterize the analogous small phosphorus thiohydridesspecies. However the sulfur atoms in the ground state are less reactive than oxygen atoms; in contrast to many oxygen atom reactions known to take place in low temperature matrices, there are only few reports of analogous reactions involving sulfur atoms. Photolysisof an argon matrix containing PH3 and COS gave no evidence for any product reaction between PH3 and sulfur atoms generated from COS photoly~is.~The small phosphorus thiohydrides species were synthesized by reacting discharged Ar/Ss with PH3 followed by trapping the reaction products in low temperature matrices. This article describes three novel phosphorus thiohydrides synthesized in this way. Experimental Section
The cryogenicrefrigeration system,vacuum vessel, and coaxial discharge tube used in this study have been previously described.I0J1 The elemental sulfur was loaded in the side arm of the coaxial tube and heated with resistance wire to entrain sufficient s8 vapor in the center of the discharge tube. The Ar/ Sa mixture flowing through the center tube was subjected to microwave discharge sustained by a Burdick M W 2 0 0 diathermy with an Evenson-Broida cavity. Pure phosphine or Ar/PH3 = 2/1 mixture was introduced into the surrounding tube, mixed with discharged Ar/Ss stream, and collected on a CsI target at 12 f 1 K. The Argon flow rate in the center tube was between 3 and 5 mmol/h, and the estimated molar Ar/Ss ratio was from 200 to 2000 depending on the experiment. The phosphine or phosphine/argon flow rate in the surrounding tube was between
5 X and 5 X 1W mmol/h. The estimated overall concentration of the Ar/PH3/Ss mixture deposited on the CsI target varied between 2000/20/1 and 2000/1/10;within this concentration range matrices of different composition and different excess of Sa or PH3 were studied. Two different microwave discharge conditionswere employed. The first set of experiments employed lower microwave power (50%)and maintained the discharge to the end of the inner tube. The second studies used high microwave power (7&80%) and the discharge glow was extended from the inner tube into the last 1 cm of the outer tube. In the second case the Ar/PH3 stream was mixed with Ar/& stream in the discharge plume. Natural isotopic sulfur (Electronic Space Products, Inc., recrystallized) and enriched sulfur (85% 34S,Oak Ridge National Laboratory) were used as received, a 32S/34S= 0.7/1 mixture of the two samples was also studied. PH3 (Matheson) was used directly and PD3was prepared by the reactionof D20with calcium phosphide. FTIR spectra were recorded on a Nicolet 7199 at 1 cm-I resolution or on a Nicolet 60 SXB at 0.5 cm-’ resolution (KBr beamsplitter) from 4000 to 450 cm-I. Spectra were recorded after sample deposition, after photolysis with a high-pressure mercury arc (filtered by water and glass filters), and after annealing sequences.
Results The first series of experiments passed Ar/Ss mixtures through a mild microwave discharge maintained in the center tube and codeposited this stream with PH3 orAr/PH3 from the outer tube. Infrared spectra of obtained matrices exhibited very strong PH3 absorptions at 994,1 1 14,2340,and 2345 cm-I. This indicates that bulk of PH3 was not decomposed in these experiments and that S, reactions ( n = 1-8) with PH3 itself were dominant here. In the second series of experiments the discharge glow was extended into the last 1 cm of the outer tube and the Ar/PH3 stream was mixed with Ar/Ss in the discharge plume. The infrared spectra exhibited weak PH3 absorptions at 994and 2340 cm-l which showed that PH3 was mostly destroyed in this experiment. In both series of experiments the same product absorptions were observed but with different relative intensities. The products formed in the performed experiments belong to one of the three types of species: S,, Pay, or H,PSy. The S, species are the products of the Ar/Sa discharge reaction which is described elsewhere.” The P S y species produced in these experiments are also formed in analogous experiments with P4 and sulfur as was reported earlier.I0 The H,P& species are characteristic of a PH3 s8 discharge reaction and will be described in the present paper.
+
0022-3654/93/2097-43 13$04.00/0 @ 1993 American Chemical Society
Mielke and Andrews
4314 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 ss s
I
875 850
I
I
I
I
7L5 720 695 670 6A5
-
0
L75 450
WAVENUMBER
a
WAVENUMBE R
'1
Figure 1. Infrared spectra in the P=S, P-S stretching regions of discharged Ar/Ss sample codeposited at 12 K with PH3/Ar = 1/2 sample, sulfur evaporated at t = 95 "C: (A) mild discharge experiment; (B) extended discharge experiment; (C) the same matrix as (B) after 30 min full arc photolysis. Absorptions marked by P and S are due to P,S, and S , species, respectively, bands marked by arrows aredue to H,P& species (solid arrow, HSPS,; dashed arrow, HPSS, dashed dotted arrow, HSPH2, dotted arrow, unidentified species).
Phosphine and Sulfur. Figure 1 presents the infrared spectra of phosphine-sulfur discharge products in the region of P.=S and P-S(H) stretchingvibrations. In addition to bands Characteristic of S, and P,S, species, new absorptions are observed for H,P,S, species. The absorptionsinclude a characteristic doublet at 690.6 and 682.4 cm-I, three bands at 861.2,765.7, and 647.2 cm-I, and a number of absorptions in the P-S(H) stretching region at 484.1, 480.2, 474.1 (sh), 463.7, and 461.2 cm-I. The H,P,S, absorptions in the P=S and P-S(H) stretching regions are accompanied by five absorptions in the high-frequency region at 2571.3, 2563.9, 2187.0, 2172.6, and 967.5 cm-I. The relative band intensities depend critically on experimental conditions. The484.1 and461.2cm-I bandsinPS(H) stretching region and the 2563.9 and 21 87.0 cm-I bands in the high-frequency region diminish stronglywith respect to other H,P,S, absorptions in extended discharge experiments. The extended discharge experiments favor the 480.2-cm-1 band which becomes the strongest one of all H,P,S, absorptions in PS stretching region. The relative intensities of the 2571.3, 861.2, 765.7, and 647.2 cm-I bands increase clearly and the relative intensities of the 2172.6,690.6, and 682.4cm-I bands increase slightly with respect to 484.1 and 461.2 cm-1 bands with an increase of sulfur concentration in both mild and extended discharge experiments. Several matrices were subjected to high-pressure mercury arc photolysis. Figure IC shows the effect of a 30-min photolysis with visible radiation (A > 420 nm) on a matrix obtained in an extended discharge experiment. The radiation increased the 861.2,765.7, and 647.2 cm-I bands by ca. 180%,decreased the 690.6 and 682.4 cm-I bands by ca. 40% and the 463.7 cm-I band by ca. 15%, and left the other bands unchanged within experimental error. In the high-frequency region photolysis increased the2571.3 cm-l bandanddecreased the2172.6cm-1 bandleaving the other bands unchanged (within experimental error). Photolysis at X > 290 nm and X > 220 nm continued increasing the 2571.3, 861.2, 765.7, and 647.2 cm-I bands and decreasing the 2172.6, 690.6, 682.4, and 463.7 cm-1 bands. Sample annealing to 32 & 2 K markedly increased the 2571.3, 861.2,765.7,and 647.2cm-I bands, virtually destroyed the 21 87.0 and 461.2 cm-l bands and slightly decreased the 2172.6, 690.6, and 682.4 cm-' bands. The performed experiments allow characterization of four different band sets which respond in different ways to photolysis and annealing processes or are formed with different yield in mild and extended discharge experiments. These are the bands at 2563.9, 967.5, and 484.1 cm-I, at 3172.6, 690.6, and 682.4
E5
*z 90
WAVENUMBER
Figure 2. Infrared spectra in the S-H and P S H stretching regions of discharged Ar/& codeposited with PHJAr: (A) (B) 34S~; (C) 32.34S8,
cm-I, at 2571.3,861.2,765.7 and 647.2 cm-I, and at 2187.0 and 46 1.2 cm-I. The relative intensitiesof all bands within each band set are approximatelyconstant in all performed experimentswithin experimental error. Phosphine and Isotopic Sulfur. In order to establish the character of the mode corresponding to particular absorption and to determine the number of sulfur atoms present in each species characterizedby the four absorptionsets, some experiments were done with 50%and 85% 34Senriched samples. The infrared spectra are presented in Figures 2-4 and the frequencies are collected in Table I. The two bands observed in the SH stretching region at 2571.3 and 2563.9 cm-' shifted to 2569.3 and 2561.8 cm-I, respectively, with j4S. The absorptions at 484.1, 480.2, and 461.2 cm-I were shifted to 477.3, 473.9, and 454.8 cm-I, respectively, upon 34Ssubstitution. The characteristic doublet at 690.6 and 682.4 cm-l shifted to 680.4 and 666.1 cm-I and the threeabsorptions at 861.2,765.7, and 647.2cm-I shifted to 856.7, 760.1, and 639.3 cm-I, respectively. The two bands at 2187.0 and 2172.6 cm-l in the PH stretching region showed a small 34S shift to 2186.7 and 2172.4 cm-I. In the spectra of 50%34Senriched experimentsthe 2563.9 and 484.1 cm-I bands gave isotopic doublets with equal intensity components centered on the pure 32Sand pure 3% peak locations. The 690.6 cm-I component of the characteristic doublet exhibited an isotopic 1/ 1/ 1/ 1 quartet as presented in Figure 3 and Table I; the 861.2 and 765.7 cm-I bands gave characteristic multiplets shown in Figure 4 and Table I. The mixed isotopic components of the 682.4 and 647.2 cm-l bands in the spectra of 50%enriched experiments are masked by isotopic S, absorptions, but four components corresponding to 647.2 cm-1 band can be clearly seen at 647.2, 646.9, 643.6, and 639.8 cm-1. Deuterated Phosphine (PH,Ds,) and Sulfur. Deuterium substitution shifted many of the bands which are listed in Table I. In the SD stretching region the two absorptions observed at 1877.1 arid 1860.3 cm-l exhibited an H/D ratio of 1.36 and 1.38 with their protonated counterparts at 2563.9 and 2571.3 cm-I, respectively. The two bands observed at 8 13.6 and 687.4 cm-I in deuterated phosphine experiments are tentatively identified as
IR Spectra of HSPH2, HPS2, and HSPS2 in Argon
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4315
TABLE I: Observed Vibrational Frequencies (cm-I) and Assignments for Pbosphorus Tbiobydrides
0
4
H2PSH
H2P34SH
H2P32J4SH
D2PSH
assignment
2563.9 967.5 484.1
2561.8
2563.9,2561.8
1877.1
477.3
484.1,477.3
48 1.2
u(S-HI a(PSH)? u(P-SH)
HPS2
HP34S2
2172.6 690.6
2172.4 680.4
682.4
666.1
HSPS2
HP34S2
2571.3 861.2
2569.3 856.7
765.7
760.1
641.2
639.3
n
W UN
z . Q
m
U 0
cn
m-
C '
0
-6 WAVENUMBER
Figure 3. Infrared spectra in the 695-655 cm-' region of discharged (C) 32,34S8.For Ar/S8 codeposited with PH,/Ar: (A) 32S8;(B) 34S~; comparison the spectra of discharged Ar/& samples deposited at 12 K are presented (A', 3 2 S ~B',; 3 4 S ~C',; 32,34Sg).
e 60
855
WAVENUMBER
Figure 4. Infrared spectra in the 865-855 and 770-760 cm-I regions of discharged Ar/32-34S8sample codeposited with PH3/Ar.
deuterium counterparts of the absorptions at 861.2 and 765.7 cm-1. The hydrogenatedcounterpart of the 588.1 cm-' absorption is not observed; the band shows the same behavior after photolysis and matrix annealing as the 2172.6,690.6, and 682.4 cm-l band set in the PH3 experiment and is assigned to the same species. In the P-S stretching region the band at 484.1 cm-I showed a 2.9 cm-1 red shift to 481.2 cm-i on deuteriation.
Discussion HzPSH. The three bands observed at 2563.9,967.5, and 484.1 cm-1 in PH3 SS discharge experiments are assigned to phosphinothious acid, H2PSH, as is now discussed. The 484.1
+
HP32i34S2
DPS2
2172.5 690.6,687.1 682.3, 680.4
690.3
u(P-H) u(P=S)
682.3 588.1
u(S=S) b(HPS)
HP32.34S3 2570.3 861.2,860.6, 859.2, 858.6, 857.4,856.7 765.7, 765.0, 763.9, 763.0, 762.2, 760.6, 760.1 647.2,646.9, 643.6, 639.8
assignment
DSPS2
+ u(S=S) + u(P-S)
assignment
1860.3 813.6
v(S-H) vas(PS2)+ 6(PSH)
687.4
vaS(PS2) 6(PSH) u(P-SH)
+
+
vs(PS2) + u(P-SH)
3 2 s
3 4 s
assignment
2187.0 480.2 474.1 sh 463.7 461.2
2186.7 473.9
u(PH) in HPSH? ? ?
454.8
u(SH) in HPSH?
cm-I band occurs in the region characteristic of the P-S stretching vibrations;12 the 32S/34Sisotopic ratio 484.11477.3 = 1.0142 is in very good agreement with the harmonic ratio for a P-S(H) stretching mode (1.014). A weak band at 2563.9 cm-' with 2.1 cm-1 34Sisotopicshift and large deuterium shift demonstrates the presence of the S H group. The sulfur isotopic doublets observed for P S and S-H stretching modes in 32,34S experiments (see Figure 2) indicate that only one sulfur atom is present in this species. The 967.5 cm-' band is in the region characteristic of the PH2 wagging mode; the corresponding mode is observed at 915 cm-I in the spectrum of phosphinous acid, HIPOH, and is the most intense one of the three PH2 deformation modes. The lack of the sensitivity of the 967.5 cm-I absorption to 34S substitution excludes its assignment to PSH bending vibration which may also occur in this region. Although the presence of the PH2 group is not spectroscopically well documented, the experimental evidence leaves no doubts as regards an identification of the species as phosphinothious acid. The yield of the species is higher in experimentswith coaxial tube and mild discharge than in experiments with extended discharge. In experiments with mild Ar/& discharge the bulk of PH3 does not decompose (as shown by the spectra) and the major products of PH3 decomposition are PH2 and PH radicals whereas in experiments with extended discharge the major products are PH radicals and P atoms. PH3 and PHI can react readily with S atoms or SH radicals to form H2PSH. The scrambled 32,34S experiments prove that only one S atom is present in the species, in the PSH group. The other possible species with one S atom and one P atom are HPSH and PSH radicals. The HPSH and PSH radicals are expected to be very sensitive to annealing whereas the three absorptions at 2563.9, 484.1, and 967.5 cm-1 show little sensitivity to the annealing process. The correspondingPOH radical has not been identified in the PH3 + 0 atom reaction, instead its isomer, HPO, was observed.7.8 So, the threeabsorptions at 2563.9,967.5, and 484.1 cm-1 are assigned to the H2PSHmolecule. Phosphinothiousacid, H2PSH,was reported to be formed in electric discharge of a PH3 H2S mixture and the P-S stretching vibration was identified
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4316 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
at 477 cm-1.1 This is in good agreement with our data; the 7.1 cm-I frequency difference between the reported value and our data is most probably due to a matrix effect. Blue frequency shifts of stretchingvibrations areoften observed for s p i e s isolated in an argon matrix as compared to the solid or liquid phase. Despiteconsiderableeffortto detect other H2PSH absorptions, only the three reported bands were identified. The yield of the species in discharge experiment is very low, below 1%. It is worth noting that the phosphinous acid, H2POH, an oxygen analogue of phosphinothious acid, was also formed in small yield in the gas phase reaction of PH3with 0 atoms from the discharge; the yield of this species was much higher in the photochemical matrix reaction of PH3 and 03.7 The PH2 stretching, deformation, and wagging modes in HzPSH are expected to have similarfrequencies as the correspondingmodes in PH3 which is the case for H2POH. The strong absorptionsof the parent PH3 molecule in the studied spectra probably mask the weak PHI absorptions of H2PSH molecule. The phosphinothious acid, H2PSH, can exist in two isomeric forms,cis and trans, and both formswere observed for phosphinous acid in similar studies. The present spectroscopic data do not provide a basis for identification of a specific HzPSH isomer. The ab initio SCF/G-3 lG* calculations were performed for both HzPSH cis and trans isomer and the calculated vibrational spectra were reported.6 The reported frequencies for the S-H and P-S stretching vibrations (2629 and 479 cm-I for cis and 2655 and 480 cm-i for trans isomer) agree fairly well with the experimental data. Very good agreement of the calculated 34Sand D shifts with observed isotopic shifts of the S-H and P-S stretching vibrations additionally confirmed identification of the H2PSH molecule. HPS2. The three bands observed at 2172.6,690.6, and 682.4 cm-I show approximately thesame relative intensities in allstudied spectra; particularly characteristic is the 690.6 and 682.4 cm-l doublet. These absorptionsdecrease on photolysis with a 420 nm filter cutoff (see Figure 1) and after matrix annealing. The 2172.6 cm-I absorption exhibits 0.2 cm-I 34Sisotopic shift and is assigned to PH stretching mode. The band appears to the red of the PH stretching absorption of free PH3. This suggests that it is characteristic of trivalent phosphorus compound as the PH stretching frequencies of trivalent phosphorus compounds are generally lower than those of pentavalent phosphorus compound^.^^^^ For example the PH stretches of HP(02)0, H(HO)zPO, and H 3 P 0 appear to the blue of those of free PH3 whereas those of HzPOH and HOP come to lower energy. The band at 690.6 cm-i shifting to 680.4 cm-1 with 34Sis assigned toP=S stretch since it comes in this characteristic region and has a 34Sshift of 10.2 cm-I. This is slightly more than the shift 10.1 cm-I predicted for a P=Soscillator in this region. In the 32,34S experiment the 690.6 cm-' band gave a quartet with components of approximately equal intensity at 690.6, 687.1, 682.3, and 680.4 cm-I. This multiplet points to the second S atom in the species which perturbs the P=S stretch on 34S substitution. The presence of a second S atom is also evidenced by the 682.4 cm-I absorption. The 682.4 cm-I band occurs in the region characteristic of S=S stretching vibrations and shows a 16.3cm-134Sshift, which confirms its assignment. Its components in the 32,34S experiment coincide with numerous S, absorptions occurring in this region. The low isotopic shift ratio of the S=S vibration 32S/34S = 1.0245 is accompanied by high isotopic shift for the P=S vibration and strong perturbation of the P=S stretch by another S atom present in the molecule. These facts indicate the presence of the PSS group in the species where the PS and SS stretching vibrations interact with each other. Identification of PH and PSS groups points to the HPSS molecule responsible for the three absorptions. The two deformation modes of the HPSS species have not been identified. The PSS bending mode is expected to occur
Mielke and Andrews
TABLE II:
Comparison between Observed and Calculated Frequencies (cm-1) of HPSS Isotopesa
4PH)
obsd calcd 6(HPS) obsd calcd u(PS) + v(SS) obsd calcd u(SS) + u(PS) obsd calcd b(PSS) calcd
2172.6 2172.5
2172.4 2172.5
2172.5 2172.5
2172.5 2172.5
809.2 690.6 690.1 682.2 682.2 201.3
808.4 680.4 679.3 666.1 664.5 197.8
809.1 687.1 687.6
808.4 682.3 682.3
675.0 199.1
671.2 199.9
1562.5 588.1 586.1 690.6 691.2 682.3 682.4 196.9
The structural parameters employed were r(PH) = 1.4 A, r(PS) = 1.97 A, r(SS) = 1.96 A, a(HPS) = loo', and a(PSH) = 112'. Best-fit force constants were F(P-H) = 2.71, F(P=S) = 4.6, F(S=S) = 4.55, F(HPS) = 0.72, F(PS,SS) = 0.78, F(PH,PS) = -0.13, F(PS,HPS) = 0.28, and F(SS,HPS) = 4 . 1 2 . In this and succeeding tables bond stretching and stretch-stretch interaction force constants are in units lo2 N m-I, stretch bend interaction force constants are in units N, and bending and bend-bend interaction force constants are in units 10-l8 N m.
below 400 cm-I which was the observation limit. The torsion vibration is also anticipated to appear at low frequencies, probably below 450 cm-1, and may be covered by numerous P-S stretching absorptions observed in this region. The HPS bending vibration was not identified in the hydrogen experiment but the absorption at 588.1 cm-' in deuterium experiment is tentatively assigned to its deuterium counterpart. In order to reproduce isotopic shifts and splittings, a normal coordinate analysis was performed using the Schachtschneider program. Displacements in the following five coordinates were taken as the internal coordinates
The torsion and PSS bending vibrations were not identified; the internal coordinate corresponding to torsion was excluded from vibrational analysis. The PSS bending vibration was assumed to lie in the range 180-250 cm-1, and the corresponding F(PSS) force constant was constrained at 0.85 X N m-I. The bond lengths of the optimized HPSS geometry obtained in preliminary ab initio calculation^'^ were used: rl = 1.4 A, rz = 1.97 A, r3 = 1.96A. Various values of a1and a2 were tried and the magnitudes of the calculated isotopic shifts were compared with the observed values for each geometry. Better agreement was obtained for cis than for trans geometry and the optimum values of aIand a2 were found to be looo and 112O. The observed and calculated fundamentals are compared to Table 11, and the best-fit force constants are also given in the table. There is reasonably good agreement between the calculated and observed frequencies.An interesting feature of the force field is relatively large value of F(P=S,S=S) force constant. It was necessary to introduce this in order to obtain the small frequency difference (8.3 cm-I) between the 690.6 and 682.4 cm-I modes. HPSS is most probably formed in the reaction of PH radicals with Sz.
PH + S,
-
HPSS The previous studies demonstrated the ease of formation of PH and Sz in low temperature matrices; the concentration of both reactants in performed experiments is high and consequently the HPSS molecule is also formed with reasonable yield. The PH radical is highly reactive, as was shown earlier that it reacts at similar conditions with 0 2 to give HOPO* and with CO to give HPC0.I4
IR Spectra of HSPH2, HPS2, and HSPS2 in Argon One can presume that the molecule of general formula HPS2
can exist in three chemically stable isomeric forms: HSPS, HP-
(S)S,and HPSS. The type of S=S bond exhibited in HPSS, viz., the bond in which sulfur forms only one bond to sulfur and none to another atom, is rare. The known examples of neutral molecules containing such a bond are S=S=O, S=SF2, and polysulfide chains ( S ) , . I 5 The S2F2 molecule exists in two isomeric forms, with the more symmetrical FSSF spontaneously rearranging at room temperature to SSF2.15 The possible conversion of HSSH to H2S-S has been also recently investigated by using SCF and correlated (MP3) theoretical methods. SSH2 was found to be less stable than HSSH by a large margin of energy, but a true local minimum correspondedto SSH2 with all real vibrational frequencies.I6 The results of recent ab initio calculations for HP(S)S and HPSS isomers also indicate that HP(S)S branched is more stable than HPSS chain by more than 150 kJ m ~ l - ~ The . ' ~ stabilization of HPSS isomer in the matrix may be due to a relatively high potential energy barrier for conversion of HPSS chain to HP(S)S branched as the process requiresthedissociationof theS=S bond. TheHP(S)S branched isomer was not identified in the performed experiments. It is worthy of notice that the oxygen analogue of the branched isomer, HP(O)O, was not identified in an analogous PH3 + 0 atom reaction. The stabilization of a particular isomer in the matrix is conditioned by many factors, the most important being the energy of the molecule at birth and barrier for conversion and relaxation processes. For example the cyclic P40 isomer was found to be more stable than the tetrahedral P 4 0 isomer with terminal PO bond by more than 130 kJ mol-' but the matrix addition reaction of oxygen atom to tetrahedral P4gave the P 4 0tetrahedral isomer as the major product.18 HSPS2, Thiometaphosphoric Acid. The species characterized by four absorptions at 2571.3, 861.2, 765.7, and 647.2 cm-I is identified as thiometaphosphoricacid, HSPS2, on the basis of the following data. The 2571.3 cm-l band with 2 cm-I 34Sshift is characteristic of the SH stretching mode; the correspondingband was not identified in experimentswith deuterated phosphine. The trio of bands at 861.2, 765.7, and 647.2 cm-I demonstrate the presence of the SPS2 group as follows. The three bands 861.2, 765.7, and 647.2 cm-1 occur in the P=S stretching region and show 32S/34S isotopic shift ratios 1.0053, 1.0074, and 1.0124, respectively. The 765.7 cm-I band is ca. 21 cm-' blue shifted from the antisymmetric stretching vibration of PSI radical. In 32734s experiments the 765.7 cm-I band is split into seven componentsof approximately equal intensity, except the central component is more intense than the others (see Figure 4 and Table I). This suggests that the middle component consists of two coinciding bands and, in fact, the 765.7 cm-I band is split into an octet. The octet consists of a quartet with the j2Sj4Sand 34S32S component having close peak locations, which is characteristic of two nearly equivalentS atoms. Each of the components of this quartet is split into a further doublet by the perturbing influence of the third, inequivalent sulfur atom which is joined to phosphorus by a single bond. Such an isotopicpattern resembles that observed for uaS(PO2)and ,,(PO,) vibrations of HOP02 in a 16,l8O3experiment and proves that the PS2 antisymmetric stretching vibration of the SPS2 group contributes to the 765.7 cm-1 mode. The 861.2 cm-I band gave a triplet in 32,34S experiment with relative intensities of 1:2:1. The side components of the triplet are further split into doublets. The central component of the triplet is broader than the two side componentswhich suggests that it consists of two or more coinciding bands. In fact, the 861.2 cm-I band, like the 765.7 cm-I band, is probably split into an octet but the four central components are not resolved due to the small difference in frequencies between them. So, one can
The Journal of Physical Chemistry, Vol. 97, NO. 17. 1993 4317 conclude that the 861.2 cm-l vibration, like to the 765.7 cm-I vibration, contributes to the PS2 antisymmetric stretching in the SPS2 group. The small isotopic shift ratios of 861.2 and 765.7 cm-1 bands (1.0053 and 1.0074, respectively) indicate that they do not correspond to the pure phosphorus-sulfur stretching vibrations. The spectra of experiments with deuterated phosphine suggest that their deuterated counterparts occur at 8 13.6 and 687.4 cm-1. The large shifts of the 861.2 and 765.7 cm-' absorptions after deuterium substitution indicate that PSH bending vibration contributes to these modes. So, the 861.2 and 765.7 cm-l absorptions correspond to mixed PS2 antisymmetric stretching and PSH bending modes. This is confirmedby normal coordinate analysis as discussed later. The 647.2 cm-I absorption is assigned to mixed PS2 symmetric stretching and P-S(H) stretching modes. The band occurs in the region characteristic for both mentioned above modes. In the 32,34S experiment the 650-600 cm-' region is obscured by S, absorption, but four componentscorrespondingto the 647.2 cm-' vibration in 32Sexperiments are evident: 647.2, 646.9, 643.6, and 639.8 cm-I. The appearance of the central component at 643.6 cm-I proves that the PS2 symmetric stretch contributes to the 647.2 cm-l vibration. The intensity of the 647.2 cm-1 absorption is comparable to the intensity of the 765.7 cm-l band to which major contribution gives uas(PS2)as discussed earlier. The band due to antisymmetric stretch in the PS2 triatomic is expected to be much more intense than the absorption due to pure symmetric stretch and the comparable intensity of the 765.7 and 647.2 cm-I bands is accounted for by mixing of the U,(PS)~ and u,(P-SH) modes. The 647.2/639.3 = 1.0123 isotopic shift ratio, which is less than the ratio expected for symmetric PS2 stretch and slightly less than that expected for P S H stretch, may indicate that the PSH bending mode also contributes to the 647.2 cm-I vibration. The second component corresponding to mixed v,(PS2) and u(PSH) vibration was not identified, this may be obscured by S, or PS stretch absorptions occurring in this region. The bending modes of the S P S 2 group are expected to lie below 400 cm-I, which was the low observation limit. The identification of vibrational bands characteristic of S H and S P S 2 groups argues strongly that the species described by the four absorptionsis thiometaphosphoricacid, HSPS2. Normal coordinate analysis and experimental facts, as discussed below, confirm this identification. A normal coordinate analysis was performed using the Schachtschneiderprogram. The four deformationmodes, ~ ( P S Z ) , w(SPS2),p(PS2),and r(SH), werenotobservedandwereexcluded from the vibrational analysis. The four modes are expected to be fairly well separated from the higher frequency modes. Displacements in the five parameters given below were taken as the internal coordinates.
HSPS2was assumed to be planar like the HOP02 and HONO2 molecule^.^ The structural parameters employed in the geometry of the molecule are listed in Table 111, they were transferred from PS2 species (SPS angle)1° and H3PS and HSPH2 molecules (HS, P-S, P - S bond lengths and PSH angle).6J3 Various values of rl, r3 = r4, and a1 parameters were also tried, but small changes of the structural parameters did not have significant influenceon the overall agreement between the calculated and observed frequencies. Calculated and observed frequencies of the five modes of the H32SP32S2, H34SP34S2, and D32SP32S2 isotopes are listed in Table 111; in addition the observed and calculated frequencies of the mixed PSIantisymmetricstretchingand PSH bending coordinates
Mielke and Andrews
4318 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
TABLE III: Observed and Calculated Frequencies (cm-I) of HJ2SPJ2S2,H34SPJ4Sz,and D3%PJ2SzIsotopess mode v(S-H) WSH)
obsd calcd obsd calcd obsd calcd obsd calcd calcd
+ vas(PS2)
va,(PS2)
+ b(PSH) + u(PS)
uS(PS2)+ 6(PSH)
+ v(PS)
v(PS) + US(PS2)
H32SPY32 2571.3 2571.5 861.2 860.9 765.7 766.4 647.2 647.3 470.4
H34SP'2S2 2569.1 2569.3 856.7 856.7 760.1 759.6 639.3 639.4 464.1
D32SP32S2 1860.3 1844.5 (813.6) 807.4 (687.4) 680.1 567.9 462.6
"The structural parameters employed were r(S-H) = 1.33 A, r(P-SH) = 2.13 A, r(P=S) = 1.97 A, a(SPS) = 127.6,and a(SPSH) = 116.5O. Best-fit force constants were F(S-H) = 3.807, F(P-SH) = 3.1, F ( P = S ) b = 5.19, F(+S)'= 4.54, F(PSH) = 0.66,F(P-S,P-S) = 0.32,F(P=S,P-S) = 0.46,andF(P=S,PSH) = 0.197. b.c In position cis and trans to SH bond, respectively.
TABLE I V Observed and Calculated Frequencies (cm-I) of Mixed v,(PS2) and G(PSH) Modes of Isotopes of HSPS2 in Solid Argon 329345
obsd
calcd
assignt
obsd
calcd
assignt
765.7 765.0 163.9 763.0 762.2
766.4 765.9 763.7 763.2 762.9 762.5 760.0 759.6
32-32-32" 34-32-32 32-32-34 34-32-34 32-34-32 34-34-32 32-34-34 34-34-34
861.2 860.6
860.9 860.8 859.6 859.5 858.1 858.1 856.8 856.7
32-32-32 32-32-34 34-32-32 34-32-34 32-34-32 32-34-34 34-34-32 34-34-34
760.6 760.1
859.2 858.6 857.4 856.7
The first sulfur atom belongs to the SH group, the second is the sulfur atom of PS2 group in position cis to the SH group, and the third one is the sulfur atom of the PS2 group in position trans to the SH group.
of the eight HSPS2 isotopes containing scrambled 32,34Sare compared in Table IV. The overall agreement between the calculated and observed frequencies is good. The normal coordinate analysis confirms strong coupling between PSH bending, PS2 antisymmetric and symmetric stretching, and P-S(H) stretching coordinates. The deuterium counterparts of the 861.2 and 765.7 cm-I modes were only tentatively identified on the basis of the experiment (due to the complexity of the spectra) and were not included in the iteration procedure of force constant adjustment. The best-fit forceconstantscalculated from frequencies of hydrogenated HSPS2 isotopes predict 807.4 and 680.1 cm-I frequencies for coupled PS2 asymmetric stretching and PSD bending modes in deuterated DSPS2 which agree fairly wellwith813.6and687.4cm-labsorptionsobservedin thespectra. The 813.6 and 687.4 cm-I bands increase on annealing which confirms their assignment to DSPS2. It is interesting to notice that the two sulfur atoms of the PS2 group are inequivalent similar to the two oxygen atoms of the phosphoryl group in HOP02. The best agreement between the observed and calculated frequencies was obtained when the force constants of both P=S oscillators were allowed to float in the iteration process of the F-G matrix calculations which gave a difference of 0.65 X lo2 N m-1 between the diagonal force constants. HSPS2 was formed in reasonable yield in PH3 + Ss discharge experiments in which the S, concentration was relatively high. The species is most probably produced in the reaction of PH radical with S3 and higher polysulfide S, chains (where n is an odd number) according to the scheme
HP
-
+ S,
HP + S,
HSPS,
HSPS,
+ S,-3
The yield of HSPS2 species increases after matrix annealing.
The absorptions characteristic of HSPS2 were also observed in the spectra of thermolysis products of P& and were identified to be the volatile impurities formed in the hydrolysis reaction of P 4 S l ~ . 1It9 is known that an oxygen analogue of thiometaphosphoric acid, HOP02, can be formed in a decomposition reaction of tetrametaphosphoric acid, H4P4012, which is the hydrolysis product of P~OIO.~O The thiometaphosphoric acid, HSPS2, can be formed in an analogous reaction of P ~ S Ihydrolysis O
+
P4Slo 2H,O
-
-
H4P4SI,O2 ZHSPS,
+ 2HOPS,
The formation of HSPS2 in the P&o hydrolysis reaction strongly supports the correct identification of this species. Other Species. A number of absorptions is observed in the 450-500 cm-I region which is characteristic of P S and bridged P-S-P stretching vibrations. Beside the 484.1 cm-I'absorption assigned to H2PSH species the bands are observed at 480.2,474.1, 463.7, and 461.2 cm-I (shoulder). The 463.7 and 461.2 cm-I bands are relatively intense in the spectra of mild discharge experiments and in experiments with an excess of PH3. The 461.2 cm-I shoulder is sensitive to both photolysis and annealing and seems to track together with the 2187 cm-I absorption in the PH stretching region which shows similar behavior. The 32S/34S isotopic shift ratio 461.2 f 454.8 = 1.014 is in good agreement with the harmonic ratio for P-S(H) diatomic. The 2187 cm-I band due to the PH stretching mode shows a 0.3 cm-I isotopic shift with 34Ssubstitution. Unfortunately, the corresponding bands were not identified in the 3%/ 34Sexperiment and in deuterated experiments as the yield of the species was too small and the region was relatively noisy. But the obtained data allow for tentative assignment of the 2187 and 461.2 cm-I absorptions to the HPSH radical. An analogous HPOH radical was formed in relatively high yield in the PH3-O atom reaction.* The463.7 cm-I bandis not sensitive to photolysis and annealing and must be due to more stable species. It shows 34Sisotopic shift characteristic of the P S ( H ) stretching mode but the present data do not allow identification of the corresponding species. The intensities of the 480.2 and 474.1 cm-I absorptions increase relative to 463.7 and 461.2 cm-I absorptions in an extended discharge experiment and in experiments with an excess of sulfur. The 480.2 cm-1 absorption shows little or no sensitivity to matrix photolysis and annealing. Mixed sulfur isotopes produced a doublet of triplets indicating a single S atom vibration perturbed by two (or more) equivalent sulfur atoms. Sulfur-34 shifted the 480.2 cm-I band to 473.9 cm-I giving an isotopic shift ratio equal to 1.0154. This value is very close to the isotopic shift ratio observed for a PSP antisymmetric stretching vibration in cyclic P4S (1.0155).1° The obtained data point to assignment of the 480.2 cm-I absorption to PSP antisymmetricstretching vibration of a fragment species which cannot be identified from the present data. The remaining absorptions in the 450-500 cm-I region are likely to be due predominantly to P-S(H) vibrations in other fragments which cannot be identified. Conclusions
The reaction of discharge-produced S atoms and S, radicals with PH3molecules and its photolysis products produced a number of phosphorus sulfides and thiohydrides that were isolated in solid argon for infrared spectroscopic studies. The use of enriched 3 4 s provided isotopic shifts and multiplets to define the sulfur stoichiometry in the product species. The phosphorus sulfides formed are in common with those from previous phosphorussulfur discharge reactions.lO Three novel phosphorus thiohydrides were identified in this way, HSPH2, HPS2 and HSPS2; there was some evidence that HPSH species was also formed in this reaction. The phosphinothious acid, HzPSH, is characterized by three absorptions at 2563.9, 967.5, and 484.1 cm-I assigned to SH
IR Spectra of HSPH2, HPS2, and HSPS2 in Argon
The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 4319
stretching, PHI wagging, and P-S(H) stretching modes, respectively. The chain-HPSS isomer was identified by the-PH stretching mode at 2172.6 cm-’ and two coupled PS SS stretching vibrations at 690.6 and 682.4 cm-I. The HPSS species is the major product of the reaction between S2 and PH radicals; it is interesting to notice that an analogous reaction between 0 2 and PH radicals formed the H O P 0 isomeric species. The thiometaphosphoricacid, HSPS2,is characterized by the 257 1.3 cm-I band which is due to the SH stretching mode, by two bands at 861.2 and 765.7 cm-I corresponding to mixed PS2 antisymmetric stretching and PSH bending coordinates, and a band at 647.2 cm-I due to mixed PS2 symmetric stretching and P-S(H) stretchingvibrations. The two absorptionsobserved in the studied spectra at 2187 and 461.2 cm-l and assigned to P-H and P-S(H) stretching vibrations, respectively, are very sensitive to both photolysis and annealing and suggest a formation of a HPSH radical in the matrix. Some other phosphorus thiohydrides formed in the performed experiments cannot be identified from the present data.
+
Acknowledgment. We are very grateful to Dr. J. S.Kwiatkowski and Dr. J. L. Leszczynski for sending us the results of ab initio calculations for H2PSH and HPSS molecules prior to publication and toDr. J. S.Kwiatkowski for his helpfulcomment. Support for this research by N.S.F.Grant CHE88-22556 is gratefully acknowledged.
References and Notes (1) Schenk, P. W.; Leutner, B. Angew. Chem. 1966, 78, 942. (2) Schmidt, M.; Wieber, M. Z . Anorg. Allg. Chem. 1963, 326, 182. (3) Schmidt, M. W.; Gordon, M. S.Can. J . Chem. 1985,63, 1609. (4) Ewig, C. S.;Van Wazer, J. R. J . Am. Chem. SOC.1985,107, 1922. (5) Boatz, J. A.; Gordon, M. S. J . Comput. Chem. 1986, 7, 306. (6) Kwiatkowski, J. S.;Leszczynski, J. J . Phys. Chem. 1992,96, 6636. (7) Withnall, R.; Andrews, L. J . Phys. Chem. 1989, 91, 784. (8) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92, 4610. (9) Hawkins, M.; Almond, M. J.; Downs, A. J. J . Phys. Chem. 1985,89, 3326. The analogous experiment was repeated also in our laboratory with the same results. (10) Mielke, Z.; Brabson, G. D.; Andrews, L. J . Phys. Chem. 1991, 95, 75. (11) Brabson, G. D.; Mielke, Z.; Andrews, L. J . Phys. Chem. 1991, 95, 79. (12) Corbridge, D. E. C. In Topics in Phosphorus Chemistry; Grayson, M., Griffith, E. J., Eds.; Wiley-Interscience: New York, 1969; Vol. 6, p 242. (13) Kwiatkowski, J. S.;Leszczynski, J. Private information. (14) Mielke, Z.; Andrews, L. Chem. Phys. Lett. 1991, 181, 355. (15) Kuczkowski, R. L. J . Am. Chem. SOC.1964, 86, 317. (16) Marsden, C. J.; Smith, B. J. J . Phys. Chem. 1988, 92, 347. (17) Lohr, L. L. J . Phys. Chem. 1990, 94, 4832. (18) Andrews, L.; Withnall, R. J . Am. Chem. SOC.1988, 110, 5611. (19) Andrews, L.; Reynolds, G.G.; Mielke, Z.; McCluskey, M. Inorg. Chem. 1990,29,5222. Andrews, L.; Thompson, C.; Dtmarq, M. C. Inorg. Chem. 1992, 31, 3173. (20) Toy, A. D. F. In Comprehensive Inorganic Chemistry; Pergamon Press: 1973, Vol. 2, pp 389-545.
