Spectra of Heptasulfur Imide
343
Vibrational Spectra and Force Constants of Heptasulfur Imide Ralf Steudel Institut iur Anorganische und Analytische Chemie, Technische Universiiat Berlin, Berlin-Charlottenburg, West Germany (Received August 20, 1976) Publication costs assisted by Verband der Chemischen Industrie
Raman, infrared, and far-infrared spectra of solid and dissolved S7NH,STND, S716NH,and S715NDhave been recorded. All fundamental frequencies of heptasulfur imide have been observed and assigned in accordance with the molecular symmetry C,. A normal-coordinatetreatment was carried out using a modified Urey-Bradley force field with 16 independent force constants. Good agreement between observed and calculated frequencies was obtained and both Urey-Bradley and valence force constants are reported.
Introduction In recent years the vibrational spectra of certain sulfur rings such as SS,' and S123have been definitely assigned. These rings belong to the degenerate point groups D3d and D4d, respectively, and since there are no substituents the spectra consist of relatively few absorptions and Raman lines, respectively. It has been shown that the spectra can be understood by means of very simple Urey-Bradley force fields with six or seven independent force constants only. For these reasons sulfur rings are ideal molecules to study the dependence of certain fundamental vibrations on ring size and molecular symmetry as well as possible relationships between force constants and structural parameter^.^^^ Oxidation of Sa with trifluoroperacetic acid yields S806 whose molecules still contain eight-membered puckered rings but with C, ~ymrnetry.~ The oxygen is linked to one of the sulfur atoms by a double bond in an axial position. The vibrational spectra of S80 have been investigatedsand force constants have been cal~ulated.~ The results show that the lowering of ring symmetry in S80 compared with S8 causes all bending and torsional modes degenerate in Sa to split into their components but without much change in average frequency and Raman i n t e n ~ i t y . ~ Another possibility to lower the symmetry of the Saring is substitution of one sulfur atom by a heteroatom. The simplest compound of this type whose structure is known and which can be prepared in high purity is heptasulfur imide, S7NH. S7NHforms almost colorless crystals which can be prepared, for example, from Sa by treatment with NaN3 in tris(dimethy1amino)phosphineoxide, subsequent hydrolysis in aqueous hydrochloric acid, and purification of the crude product by repeated recrystallization or column chromatography.1° The molecular structure of S7NH has been investigated several times11J2but only recently was it possible to determine the positions of all atoms including the hydrogen by x-ray diffraction on single crystals at -160 OC.13 The molecules consist of crownshaped STN rings containing almost planar groups S2NH. The molecular and site symmetry is C, and the centrosymmetric unit cell contains four molecules. The geometrical parameters are given in Table I; the numbering of atoms and bonds is shown in Figure 1. The vibrational spectra of S7NH were first investigated by Nelson14 who recorded the Raman spectrum in the region 40-3500 cm-l and the infrared spectrum in the region 200-4000 cm-l using solid S7NH and solutions in CS2. However, Nelson did not observe all fundamental frequencies and made only a few assignments. Furthermore, some of the wavenumbers given in Tables 2 and 3 of Nelson's paper do not agree with the values which can
TABLE I: Bond Distances ( r , A ) , Valence Angles ( a , p, deg), and Dihedral Angles ( 7 , deg) of Heptasulfur Imidea rl, r 2 2.048 r 3 , r4 2.062 r s , r6 2.049 r , , r8 1.676 R , 0.91
a, 107.2 a,, a 3 106.8 a 4 , u 5 108.3 U6,
a8
a , 110.1 123.8
93.5 99.4 T ~ T~, 94.8 T,, r 8 96.5 p,, p, 117.1 T,, T,
r 3 ,r 4
a The angle at atom i is termed a i , the two angles SNH are termed p , and p,, and the torsion angle at the bond
ri is called ri.
be obtained from her Figures 2a and 2b. To remove these discrepancies and to determine all fundamental frequencies of the heptasulfur imide molecule we recorded the Raman, infrared, and far-infrared spectra of S7NH,S7ND,S715NH,and S715NDin the solid state as well as in CS2 solutions and made a normal-coordinate analysis.
Experimental Section S7NH was prepared from Sa and NaN310 since this method yields only traces of Ss(NH)zand S5(NH), which can be separated from S7NHby chromatography only.15 The crude product was extracted with boiling methanol from which on cooling Sa crystallizes first followed by S7NHon evaporation. The samples used for spectroscopy were purified by repeated recrystallization from CH30H and CC14 and in some cases by chromatography and showed melting points between 109 and 113 "C. All samples were free of SG(NHI2as checked by thin-layer chromatography. Only traces of Sa were present in some cases but since all strong IR absorptions and Raman lines of S8 coincide with strong IR absorptions and Raman lines, respectively, of S7NH no differences in the spectra of samples from different preparations were observed. Since the hydrogen in S7NHis fairly acidic (pK, = 5 9 S7ND was easily obtained by recrystallization of S7NH from excess methanol-dl (199 atom % CH30D) or alternatively by very slow precipitation of 2 g of S7NH dissolved in 120 ml of dry acetone with 50 ml of DzO (199.7 atom % DzO) and subsequent drying under high vacuum for several hours. On standing in air S7NDchanges to S7NHwithin 24 h. Therefore, due to the conventional sample preparation the IR spectra of S7NDexhibited additional weak absorptions belonging to S7NH. S715NHwas prepared from Kl5NNz(95 atom 5% 15Nat one terminal atom of the azide anion) which first had to be converted into Na15NN2by ion exchange since KN3 does not react with S8 to give S7NH in good yields. 1.6 g of NaI5NN2was stirred with 1.7 g of Sa in 33 ml of The Journal of Physical Chemistty, Vol. 81, No. 4, 1977
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Ralf Steudel
Figure 1. Numbering of atoms and bonds in heptasulfur imide.
I
I
I
I
I
I
I
1400
1200
1000
800
600
400
200
FREQUENCY (CM- l ) Flgure 3. Infrared spectrum of solid S,ND at 25 " C (containing approximately 10% S,NH).
I
I
I
I
I
I
800
700
600
500
400 300 FREOUENCY ICM")
200
100
0
Figure 4. Raman spectrum of solid S,NH at 25 " C (spectral bandwidth 1.5 cm-').
fundamental vibrations are to be expected, 12 of which belong to the species a' and 9 to an and all of which are infrared and Raman active. There must be 9 stretching, 10 bending, and 2 torsion vibrations (cf. S809). The NH stretching wavenumber of S7NHat 3334 cm-l in CS2shifts to 3270 cm-l in the solid due to weak intermolecular H.43 hydrogen bonding.13 In S7ND it occurs at 2475 cm-' corresponding to a H/D ratio of 1.35. S7*NH shows"U at 3331 cm-' (in CS2)but no splitting was observed which, according to the mass difference between I4N and I5N, should amount to 7 cm-'. The SN stretching wavenumbers can be expected in the region 600-1000 cm-I since the planar HNS2group can be compared with the molecules OClz (673, 63418),H11BC12 (892, 74019),and HNC12 (687, 66620)whose XC1 stretching wavenumbers are given in parentheses. The antisymmetric SN vibration is of high IR intensity but does not show up in the Raman spectrum. It shifts on deuteration much more than on 15Nsubstitution because of some coupling with 6ND(aff)in S7ND. This mode occurs at 1288 cm-I in S7NHand at 970 cm-l in S7ND. The coupling also causes a substantial increase in IR intensity of 6m. In HNClzthe corresponding vibration occurs at 1295 cm-1.20 The symmetric SN vibration of S7NH does also not occur in the Raman spectrum and is of very low IR intensity at 25 "C but can be easily detected in IR spectra taken at -185 "C. The wavenumbers are listed in Table IV. The six SS stretching wavenumbers (3af, 3af9 can be expected in the region 400-500 cm-' since there is a linear relationship between SS bond distances and average SS frequency5 from which uss = 458 cm-' is obtained. Since The Journal of Physical Chernistv, Val. 81, No. 4, 1977
Spectra of Heptasulfur Imide
345
TABLE 11: Vibrational Wavenumbers of S,NH (cm-' )' Raman Solid In CS, Solid
Infrared In CS,
21 m 47 501 vs
Assignment
Lattice 52 vw
56 w,sh 71 s 91 vs 105 w s h 158 vs 162 vs 171 w
1
215 vvs 220 vw,sh 247 w 251 vw 261 w 282 s
obsc 89 s 160 vs,dp
63 w 75 w 96 vw 107 m 168 m
a' T a" r
91 t 21 or 63 t 52 a' t a" t i s s ~ 2.91
170 vw,sh 212 s
204 s 211 m
250 m
249 m
212 vvs,p 249 w 272 m,p
424 w,sh 433 m 456 m
440 m 460 w
473 s 496 m
478 m 487 m
260 w 276 s 295 w 356 m 427 s
272 m 358 w-m 424 s
456 s
446 s 467 w,sh
500 m 522 w,sh 552 vw 660 vw,sh 694 w 718 vw 740 vw 816 ws 1274 vw 3270 m
494 m 517 w,sh
a' 6sss 160 t 91 a:' S S S N a GSNS 212 t 9 1 a:,SS S N
a vss 272 + 249 500 t 52 or 500 t 63 500 t 160 or 456 + 212 680 w a' V S N 456 t 261 or 2.356 456 t 279 805.8 s a:: V S N 1288 m a, ~ S N H 3334 m 3258 m-s a VNH a v, very; s, strong; m, medium; w, weak; sh, shoulder; b, broad; p, polarized; dp, depolarized; obsc, obscured by CS,; u , stretching; t i , bending; 7,torsion vibration.
the bond distances vary by only 0.014 A, vibrational coupling only causes the vss to be spread over a certain region. The spectra of S7NHand S7NDshow considerable differences in the 400-500-cm-' region. In the case of S7ND five frequencies of low or medium intensity are found in the IR and Raman spectra and can be assigned to the six SS stretching fundamentals assuming one incidental degeneracy at 463 cm-'. Since all the calculations show the SS stretching modes distributed in alternating order to the two symmetry species (a' > a" = a' > a" > a' > a") we assign the wavenumbers in the 420-480-cm-' region accordingly. S7NH exhibits only three strong IR absorptions in the 400-500-cm-' region. These are absent not only in S7ND but also in the spectrum of (S7N)zS21in which two S7N rings are connected by a sulfur atom. The same holds for S7NCH3." Therefore, we assume that the wavenumbers at 427, 456, and 498 cm-l in the spectra of S7NH are connected with the symmetric NH bending vibration (NH wagging). In S7NDthis mode occurs as a broad structured absorption at 367 cm-'. In S7NH it must be coupled to some extent with the SS stretching modes and we assign the 500-cm-l frequency which occurs in the Raman spectrum at 496 cm-' and is polarized to the highest SS stretching mode which is intensified and shifted from 474 cm-l in S7ND by mixing with aNH(a'). This assumption is supported by the normal-coordinateanalysis (see below). The two remaining wavenumbers at 456 and 427 cm-I are assigned to 6"(a') assuming a splitting caused by a slightly asymmetric double minimum potential (DMP) as has been observed for NH stretching frequencies previou~ly.~~ The
Figure 5. Assumed double minimum potential for the NH wagging mode of heptasulfur imide.
x-ray structure analysis of S7NH showed the hydrogen in an axial position with respect to the S7Nring but the angle between the NH bond and the plane formed by the neighboring atoms 6, 7, and 8 (see Figure 1)amounts to only 14' (in NH3, 56"). Since there must be another position for the hydrogen on the other side of that plane (equatorial position) differing in energy only slightly (due to the different longe range SH interaction with atoms 4 and 5) the assumption of a DMP for the wagging vibration seems reasonable. The barrier between the axial and equatorial positions should be quite low causing the first vibrationally excited 8" level to be located near the top of the barrier which results in a splitting of this level (see Figure 5). In this case the relative IR intensities of the transitions 0-2 and 0-3 can be of comparable magnit ~ d e . On ~ ~deuteration , ~ ~ the levels 0 to 3 decrease in energy and the splitting of levels 2 and 3 becomes much smaller leading to one broad but structured absorption The Journal of PhysicaJ Chemistry, Vol. 81,
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Ralf Steudel
346
TABLE 111: Vibrational Wavenumbers of S,ND (cm-’p Infrared Raman solid Solid In CS, Assignment
I
22 m 46 m 49 m 56 w,sh 70 m 91 s 105 vw,sh
:::b 171 w 214 vs 221 w 247 w 252 w 262 w 281 m
460 w,b 472 s
709 vvw 715 vvw 721 vvw 729 vvw
a
203 vs 210 m
247 m
248 s
262 w 272 vs 327 m 367 vs,b 426 w 435 vw 455 s 464 m 475 m 501 w
439 w
2425 w
211 vs
270 s 312 vs 368 vs 427 m
Lattice a’ r a” r 91 + 22 a‘ + a“ F S S S 2.91 a:’ s s s s a ssss 160 + 70 a‘ s s s s 160 + 91 a;’SSSN 8,
&SNS
a SSSN ~:,YND
658 w 680 w-m
517 w 655 w 676 vw 697 w
a, v s s a,,vss a vss a’ + a“ v s s a’ v s s S,NH 270 + 248 461 + 204 a’ V S N 427 + 270
723 w
713 w
455
461 m 469 m 478 m
776 vvs 966 vs 1055 vw,b
774.5 ws 970 vs 1050 vw
2433 s
2475 s
+ 272
a;: ” S N a ~ S N D 970 + 91 or 776 + 272 a‘ VND
For abbreviations see Table 11.
TABLE IV: SN Stretching Wavenumbers of Heptasulfur Imide (cm-’) Species State a’ Solid a” In CS,
S,I4NH S,14ND S,”NH 694 680 805.8 774.5 787.0
S,”ND 762.0
band for 6ND(a’). This explanation for the strong IR absorptions of S7NH in the region 400-500 cm-l is supported by the observation that these bands are observed in CS2solution too but shifted to lower wavenumbers due to the cleavage of the weak hydrogen bonds in solid S7NH. Furthermore, S7NH reacts with tris(dimethy1amino)phosphorus oxide (TDPO) to form the solid adduct TDP0.2S7NH in which according to a x-ray structure analysis the two NH groups are hydrogen bonded to the oxygen atom.26 In this compound aNH(a’)occurs at 671 cm-l and 6ND(a’)at 545 cm-‘ which in both cases corresponds to a shift from S7NH(D)by a factor of 1.5 provided 6”(a’) is located near 450 cm-l in SYNH. The assignment of the 10 ring bending and torsional modes is more difficult. The highest frequency at 356 cm-l can be assigned to the symmetric SSN bending mode which according to the lower mass of nitrogen must be the highest ring bending mode.27 No l4N/I5N splitting has been observed for this mode which due to the calculations should amount to 6 cm-l. In S7NDGssN(a’)is repelled by 6ND(a’)and therefore shifted to 327 cm-l. The remaining nine modes can be expected between 300 and 60 cm-’ since all corresponding vibrations of S8and SsO are found in this region. The two torsional modes degenerate in S8 (85 cm-l) and split in S80 (84,67 cm-l) The Journal of Physical Chemistry, Vo/.81, No. 4, 1977
TABLE V: Comparison of Bending and Torsional Modes of Eight-Membered Rings (Symmetry Species, Wavenumbers, Raman Intensities) sRo
(‘8)
a’ a’
219vs 250vw
a’
‘’Ow 197 w
a”
a’ a”
67 84 s
a’ l 4 O s a” 157 vs a’ (219)
-
s~
b,
216s 243w
e,
184w
a,
> > > >
e, e,
235 vw
S,” (C,) a‘ 215vvs a’ 2 8 2 s a’ 3 5 6 -
<
