Vibrational spectra, conformations and intramolecular interactions of the Cl2P–O–(CH2)2SCN molecule

Journal of Molecular Structure 435 (1997) 281–288

Vibrational spectra, conformations and intramolecular interactions of the Cl 2P–O–(CH 2) 2SCN molecule S.A. Katsyuba*, R.M. Kamalov, O.N. Scherba, G.S. Stepanov, V.A. Alfonsov A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of Russian Academy of Sciences, Arbuzov str. 8, 420083 Kazan, Russia Received 28 October 1996; revised 1 April 1997; accepted 21 April 1997

Abstract IR and Raman spectroscopy, normal coordinate analysis, and molecular mechanics were used to study the conformations of the Cl 2P–O–(CH 2) 2SCN molecule. It is shown that in the low temperature crystals the molecule exists in two or more conformations, and in the liquid in at least three conformations. In both liquid and solid phases there exist the conformers with the O…S and/or P…S intramolecular distances comparable with the sums of the corresponding van der Waals radii. The substantial energy preference of the gauche conformation about the CC bond in the liquid state is assigned to certain attractive intramolecular interactions between the sulfur and phosphorus (or oxygen) atoms. The energy of the interaction may amount to about 2–4 kcal mol −1. q 1997 Elsevier Science B.V. Keywords: Vibrational spectroscopy; normal coordinate analysis; molecular mechanics; conformational isomerism; intramolecular interactions

1. Introduction The intramolecular interactions of nonbonded heteroatoms attract considerable attention in the context of present interest in hypervalency at phosphorus, silicon, etc. [1]. They could be regarded as the first step in chemical bond formation. The compounds exhibiting these interactions may be used as models of prereaction and transition states of nucleophilic displacement reactions. Several examples of such a kind of interactions between P(III) and O or N atoms of Cl 2P–X–CH 2CH 2Y molecules (X = NMe, O; Y = OMe, NMe 2) was reviewed recently [2]. The examination of a similar Cl 2P–O–CH 2CH 2SCN * Corresponding author.

molecule (1) may allow us to reveal possible P(III)…S(II) interactions and to compare them with previously reported ones.

2. Experimental The compound 1 was synthesized by the reaction of an excess of PCl 3 with 2-trimethylsiloxyethyl thiocyanate [3] and was investigated immediately after a few distillations in a dry argon atmosphere (b.p. 94– 958C/0.08 mmHg); the purity control was performed by NMR 1H and 3 1P spectroscopy. Raman spectra were recorded on a Coderg spectro˚ , power meter with a He–Ne LG-38 laser (l = 6328 A 50 mW). The liquids were sealed in a glass capillary.

0022-2860/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 7 )0 0 18 6 -5

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Crystal samples were prepared by cooling the capillary in a standard refrigerator unit with liquid nitrogen vapor. IR spectra of pure compound 1 were recorded on spectrometer UR20 (Karl Zeiss) in the 400–4000 cm − 1 range. Liquid samples were prepared as films between KBr plates. Crystal films were obtained by cooling the liquid ones with liquid nitrogen in a refrigerator unit. The crystal growth was visually controlled by watching samples through crossed polaroids. IR spectra of the solutions in CCl 4 (C < 0.1–0.01 M) were recorded on Fourier spectrometer IFS-113v (Bruker). The spectrum of CCl 4 was subtracted from the spectra of the solutions.

Fig. 1. GG9G9G conformation of the Cl 2P–O–(CH 2) 2SCN mole˚; cule (the result of molecular mechanics modeling): O…S = 3.4 A ˚. P…S = 4.3 A

2.1. Computations For detailed interpretation of the experiment the normal coordinate analysis was carried out, following the well-known GF-method [4]. The F-matrices of all possible conformers were additively composed of force constants transferred from CH 3CH 2OPCl 2 (2) [5] and CH 3CH 2SCN (3) [6] molecules. The program [7] was used for the least-squares refinement of initial force field. The corrections, Df, to the force constant matrix were calculated according to the equation: Df = (J t WJ + bE) − 1 J t W Dl where J is the Jacobian matrix, W is a weight matrix, and E represents the unit matrix. Weight factors, W i i, were assumed equal to 1/l i, where l i are the eigenvalues of the GF-matrix. The damping factor b was calculated by the equation b = n=[Sp(J t WJ + bE) − 1 J t W + Dft Df] where n is the number of elements of the correction vector Df. Structural parameters of all possible conformers of molecule 1 were evaluated in such a way. The values of bond lengths and bond angles for CCH 2SCN moiety were taken from those determined in the microwave study [8] of the molecule 3, and for Cl 2P–O–CH 2 moiety from those determined in the electron diffraction study [9] of the CH 3OPCl 2 molecule. OCC, OCH and H10CH11 angles were assumed to be equal to 109.58 (for numbering scheme see Fig. 1). All dihedral angles of the model conformers were estimated by molecular mechanics calculations. For that purpose the molecular mechanics force

field [10], successfully used for conformational analysis of the dichlorophosphite 2 [5] and many other organophosphorus molecules (e.g. [11–13]), was applied. The torsion term parameters necessary to deal with the CSCN fragment were optimized. The torsional contribution to the potential energy function for CC–SC(N) axis was calculated as Utors = 0:5V1 (1 + cos(3w)) + 0:5V2 (1 − cos(w)) V 1 = 1.0 and V 2 = 1.5 kcal mol −1 allowing to reproduce the barrier to internal rotation in CH 3SCN [14] and the enthalpy difference between anti and gauche rotational isomers of the molecule 3 [6].

3. Results and discussion For the purpose of the conformational analysis of 1 we have studied its vibrational spectra (Table 1). Our assignments in the frequency region 1200–3000 cm − 1 are based on the interpretation of the spectra of the molecules 2 [5], CH 3CH 2SCN 3 [6], and CH 3O(CH 2) 2SCH 3 5 [15]. The assignments of POC stretch and PCl 2 scissor modes are rather obvious because the corresponding bands have typical frequencies, depolarization ratios and high intensities [5,16]. The same is true for SC stretches (,690, 662, ,640 cm − 1) and SCN out-of-plane bend (,410 cm − 1). They appear to be at practically the same frequencies as in the case of the molecule 3, the very weak 662 cm − 1 line being assigned to minor form with anti conformation of CCSC(N) fragment [6]. All other bands are interpreted mainly on the basis of our

S.A. Katsyuba et al./Journal of Molecular Structure 435 (1997) 281–288

283

Fig. 2. Infrared spectra of liquid Cl 2P–O–(CH 2) 2SCN at: (a) 298 K; (b) 220 K.

normal coordinate calculations, which will be discussed below. The presence of conformers is apparent in the frequency region below 1450 cm − 1 where at least three Raman lines at 189, 360, 718 cm − 1, and three IR bands 715, 805, 1423 cm −1 are present in the spectra of the liquid but absent in the spectra of the solid. These bands exist in the spectra of dilute CCl 4 solutions of the compound 1 and, hence, cannot be assigned to intermolecular associates. According to molecular mechanics computations the molecule 1 is able to exist in 23 stable conformations (Table 2), which can be distinguished by vibrational spectroscopy. As stated previously, we calculated the normal modes of all possible conformers in order to make spectra interpretation more clear. According to well known rules [17], force constants of a certain atomic group depend only on atoms (or groups) immediately adjacent to this former group. Hence, the transferability of force constants of Cl 2P–O–CH 2 and CH 2 –S–CN moieties of molecule 1 is bound to be fairly good, though minor changes of OCH 2C and CCH 2S intragroup force constants in comparison with the corresponding parameters of parent molecules are possible. So, first of all the program [7] was used to obtain the best fit to the observed frequencies of CH 2 vibrations (,1230–1460 cm −1). The general improvement of the calculated spectra was achieved simultaneously for all the model

conformers by means of slight variation of force constants of CH 2 groups.1 The frequencies of the GG9G9A, GG9G9G and GG9G9G9 model conformers of the molecule 1 (for notation see Table 2), calculated on the basis of the discussed evaluations of the force constants, fit associated observed values quite satisfactorily (Table 1). To achieve the better fit for all other conformers, we had to adjust the force constants of the POCH 2 –CH 2S moiety more substantially. But none of the calculated frequencies of anti conformations about the CC bond could be fitted to the strong IR band 900 cm − 1 even with the initial force constants changed severely. So, this band may be assigned only to gauche conformations. It is present in the spectra of crystalline 1, so the molecule 1 in the low temperature crystals adopts at least one of the twelve conformations with mutual gauche orientation of OC and C 4 –S bonds (F 3 < 6708 in Table 2). It should be noted, that despite the smaller number of bands in the spectra of solid samples (Table 1), it was still larger than it should be for one conformation. This points to the existence of at least two conformers in the solid state and, hence, at least three forms in the liquid state. The presence of more than a single conformer in crystals of organophosphorus compounds 1 The force constants and calculated frequencies of all the model conformers are available from the authors upon request.

s

w, sh

m sh w vw s vvs m s

2158

813

730 718 687 662 640 512 480 455

w

823

735 690 638 518 482 440

p 0.7 p p p? 0.7

vw m s vs

w

m

w

w

1413

805

vs w

0.4 2155 1455

s

900

638 510 482 448

770 730 715 686 m s vvs s

vw s sh w

w

w s m sh sh m m m vw vw w, sh vs vs vs vs s

2890 2156 1455 1423 1415 1400 1373 1295 1270 1240 1090 1063 1030 1020 1010 963

820

w m

I

3000 2945

n/cm −1

n/cm −1

n/cm −1 I I

Liquid

Solid

Liquid r

IR a

Raman a

Table 1 Vibrational spectra of the Cl 2P–O–(CH 2) 2SCN molecule

637 520 482 449

686

777 735

1411 1405 1373 1292 1270 1236 1090 1075 1035 ,1020 ,1010 970 960 920 900 880 821

2889 2155 1452

3000 2948

n/cm −1

Solid

w s m

w m

m s vvs vs

m

vw s

m w, sh m s m vw sh s s br, sh br, sh m sh w, sh vs w, sh vw

I

1093

997

1092

997

497 484 459 449

678 651

727

819

626 518 488 455 449

677

727

829

918

1300 1256 1243

1300 1256 1243

921

1411 1405

3014 2933 2932 2860 2171 1452

n/cm −1

1411 1405

3014 2933 2932 2860 2171 1452

n/cm −1

623 511 488 460 449

683

726

824

909

997

1092

1300 1256 1243

1411 1405

3014 2933 2932 2860 2171 1452

n/cm −1

487 459 443

640

704

807

810

967

1079 1018

1403 1383 1305 1269 1235

3014 2933 2932 2860 2171 1452

n/cm −1

GG9G9A b GG9G9G b GG9G9G9 b GAAG9 b

Calculated for the conformers

487 447 443

655 611

721

895

957

1003

1299 1272 1218 1100

1399

3014 2933 2932 2860 2171 1456 1421

n/cm −1

GAGG b

n sPOC nC4S, nPO GAAG9 n a sCSC n sS–C6 n sCSC dSCN9 d n sPCl 2 dSCN9 n a sPCl 2

rCH 2(S) GAAG9

rCH 2(S)

n a sOCC

n a sCH 2(S) n a sCH 2(O) n sCH 2(S) n sCH 2(O) nCN dCH 2(O) dCH 2(S) GAGG dCH 2(S) qCH 2(O) qCH 2(O) GAAG9 tCH 2(O) qCH 2(S) tCH 2(S) n sOCC GAGG n sOCC n a sPOC GAAG9 rCH 2 GAGG n a sCCS, rCH 2(O) n a sPOC GAGG

Assignment for GG9G9G9 conformer c

284 S.A. Katsyuba et al./Journal of Molecular Structure 435 (1997) 281–288

m sh vw m vw

189 170 147

141 129

dp 125

vw

m vw

0.5

175 150

w vw vvw s

310 275 253 210

dp p?

w

382

0.5

s vw

432 419

p? 405

w

407

w

125 101 44 31 22

140

220 191

321

405 383

128 80 38 31 20

161

227 191

326

407 383

110 87 46 27 22

173

232 190

326

406 383

97 60 41 26

137

198 176 169

240

410 388 378

89 66 27 23

132

161

188

306 239

407

438

dPCl 2 d,t,qPCl 2 GAAG9 dCSC, dSCN9 tPCl 2, dCSC xCC GAAG9 dCSC GAGG dCSC, tPCl 2 dCSC xCS xOC xCC

t,q,dPCl 2

dSCN0 e dPOc dPOC GAAG9 qPCl 2

dOCC GAGG

b

w, weak; m, medium; s, strong; v, very; sh, shoulder; br, broad; p, polarized; dp, depolarized. For notation see Table 2. c n, stretch; d, bend; q, wagging; t, twisting; r, rocking; xAB, torsion about AB bond; s, symmetrical (i.e. in-phase); as, antisymmetrical (i.e. out-of-phase). d In CSC plane. e Out of CSC plane.

a

w vw, sh w sh w vw vw s

415 405 375 360 305 273 253 210 S.A. Katsyuba et al./Journal of Molecular Structure 435 (1997) 281–288

285

286

S.A. Katsyuba et al./Journal of Molecular Structure 435 (1997) 281–288

Table 2 Energy and geometry characteristics of stable conformations of the Cl 2P–O–(CH 2) 2SCN molecule, calculated by molecular mechanics method Name a

f b1deg

f b2/deg

f b3/deg

f b4/deg

AAAA AAAG AAGA AAGG AAGG9 GAAA GAAG GAAG9 GAGA GAGG GAGG9 GAG9A GAG9G GAG9G9 GGAA GGAG GGAG9 GG9AA GG9AG GG9AG9 GG9G9A GG9G9G GG9G9G9

180 181 173 174 174 61 61 61 63 63 64 68 68 67 35 36 34 59 50 50 61 61 60

180 183 167 167 169 184 186 183 180 180 183 196 194 195 76 75 75 − 74 − 75 − 75 − 77 − 76 − 78

180 179 68 65 69 179 177 181 70 67 71 − 69 − 69 − 67 174 172 176 184 180 180 − 71 − 73 − 69

180 62 182 61 − 69 180 62 − 62 183 61 − 67 177 69 − 62 181 59 − 62 179 60 − 60 178 64 − 61

U c/kcal mol −1 1.08 1.13 2.99 1.72 2.24 1.02 0.09 0.00 3.26 1.98 2.67 3.81 3.14 2.50 2.19 1.17 1.07 1.86 0.75 0.92 3.94 3.22 2.64

a

Here A stands for anti, and G stands for gauche. f 1, f 2, f 3, f 4 are dihedral angles of rotation around P–O, O–C, C–C and C–S bonds respectively. They are equal to zero in the eclipsed position of the phosphorus lone pair and OC bond (f 1), PO and CC bonds (f 2), OC and CS bonds (f 3), CC and CS bonds (f 4). c Relative conformational energy (potential energy of global minimum of internal rotation potential energy surface is equal to zero). b

was noted repeatedly (e.g. [13]). In our case it is especially apparent in the spectral region ,900–1100 cm − 1, where at least six strong bands (900, 970, ,1010, ,1020, 1035, and 1075 cm − 1) are observed, whereas any conformer should not exhibit more than three bands here (Table 1). When the crystals are melted and the liquid is further heated, the relative intensity of the band 900 cm − 1 decreases dramatically. And vice versa it grossly increases upon cooling of liquid 1 (Fig. 2). This is indicative of substantial energetical preferability of at least one of the above-mentioned gauche conformations about the CC bond in respect to all other conformers. The last result does not agree with molecular mechanics data (Table 2), according to which the energy of the gauche conformations about CC bond is much higher than that of the corresponding anti conformations. It is worth noting, that nothing of the kind was revealed in conformational behaviour of the

similar molecule CH 3O(CH 2) 2SCH 3 5 [15]. Anti conformations about CC bond was slightly energetically preferable in this case, anti,anti,gauche conformer being the most energetically stable. According to our molecular mechanics computations anti,anti,gauche conformation of the molecule 5 corresponds to global minimum of the conformational energy surface, and the energies of all the gauche conformations about CC bond are at least 0.4 kcal mol −1 higher. So, contrary to the case of the molecule 1, the predictions of the force field [10] for the molecule 5 are in agreement with the observations [15]. The disaccord in the case of the molecule 1 is most probably caused by certain specific intramolecular interactions which was apparently not taken into account in our molecular mechanics calculations. In particular, gauche orientation of OC and CS bonds leads to close intramolecular contact between O and S atoms of the molecule 1 (Fig. 1). If any kind of attractive O…S intramolecular interactions could

S.A. Katsyuba et al./Journal of Molecular Structure 435 (1997) 281–288

arise, it would cause the energetical preferability of the conformers, contrary to what might be expected from molecular mechanics consideration. A comparison of the molecular mechanics evaluations of the relative energy of the gauche and anti conformations about CC bond (Table 2) allows to estimate the lower limit of the energy of possible O…S interactions, neglected in our computations. It amounts to about 2 kcal mol −1 on the average. Close intramolecular contacts are also possible between P and S atoms of the molecule 1 in the GG9G9A and GG9G9G, and GG9G9G9 conformations (Fig. 1). According to the molecular mechanics estimates, the distance between the atoms is comparable with the sum of their van der Waals radii. So, it is probable that there arises some weak attractive P…S interaction, which leads to energetic stabilization of the GG9G9A, GG9G9G, and GG9G9G9 conformations. The possibility of such a kind of interactions was discussed in the papers [18], devoted to investigation of the compounds R nP[SE(X)R m9] 3−n where E = C,P; X = S,O; R = Ph,SE(X)R m9; R9 = Alk,Ph,AlkO,Alk 2N; n, m = 1,2. The molecules of the compounds exist, like the molecule 1, in such a conformation, that terminal X atoms are closed tightly to the central phosphorus atom. Some contribution into stabilization of the observed conformations is probably made by hyperconjugative interaction of the lone electron pair of atom X with the antibonding orbital of the opposite P–S or P–C bond. The energy of this interaction was estimated at about 3–5 kcal mol −1 The energy of the P…S attraction in the case of the molecule 1 may be evaluated in the same manner as for O…S interaction. If it is granted that energetical preferability of the gauche conformations about CC bond is accountable mainly to the P…S attraction, neglected in our molecular mechanics computations, then the energy of this interaction must exceed the conformational energy of the GG9G9A, GG9G9G, or GG9G9G9 conformations, given in Table 2. So, it averages between 2.5 and 4 kcal mol −1. The discussed attractive forces are probably of the similar nature as in the case of the Cl 2PN(Me) CH 2CH 2NMe 2 molecule [2], where the phosphorus atom appears as acceptor atom and the nitrogen atom is donor. More powerful P…N interactions cause the convoluted conformation of this molecule, akin GG9G9G discussed above, and even lead to the

287

formation of the intramolecular coordinative P ← N bond. But the sulfur atom of the thiocyanato group of the molecule 1 possess much less pronounced donor abilities, than the amino group. So, as would be expected under these conditions the effect of the corresponding interaction is more subtle. It influences perceptibly only the relative energy of the conformers with and without intramolecular contacts between heteroatoms. Thus, we are going to undertake the conformational analysis of the compounds Cl 2P–X– (CH 2) nY (X = O, NMe, S, etc.; Y = OMe, SMe, Et, NMe 2, etc) to evaluate and to compare the effects of the intramolecular interactions between different heteroatoms in the different chemical environment. We consider possible to change the distance between the heteroatoms up to formation of the chemical bond. Thus it may be possible to follow the elementary chemical act and estimate the dependence of energy of the interaction on the distance between the interacting centers. The studies are now in progress and will be the subject of our future publications. References [1] R.R. Holmes, Chem. Rev., 96 (1996) 927. [2] T. Kaukorat, I. Neda, R. Schmutzler, Coord. Chem. Rev., 137 (1994) 53. [3] R.M. Kamalov, G.S. Stepanov, D.V. Ryzhikov, M.A. Pudovik, A.N. Pudovik, Zh. Obshch. Khim., 61 (1991) 1754. [Russ. J. Gen. Chem. (Engl. Transl.)]. Chem. Abstr., 116 (1992) 194439g. [4] E.B. Wilson, J.C. Decius, P.C. Cross, Molecular Vibrations, McGraw-Hill, New York, NY, 1955. [5] S.A. Katsyuba, O.N. Nadiseva, V.N. Shegeda, G.S. Stepanov, Zh. Prikl. Spektrosk., 56 (1992) 725. [J. Appl. Spectroscopy (Engl. Transl.)]. [6] J.R. Durig, J.F. Sullivan, H.L. Heusel, J. Phys. Chem., 88 (1984) 374. [7] S.A. Katsyuba, M.K. Mikhailova, M.Kh. Salakhov, Zh. Prikl. Spektrosk., 47 (1987) 237. [J. Appl. Spectrosc. (Engl. Transl.)]. [8] A. Bjorseth, K.M. Marstokk, J. Mol. Struct., 11 (1972) 15. [9] N.M. Zaripov, Zh. Strukt. Khim., 23 (1982) 142. [10] A.Kh. Plyamovatyi, V.G. Dashevskyi, M.I. Kabachnik, Dokl. Akad. Nauk SSSR, 234 (1977) 1100; 235 (1977) 124. V.G. Dashevskyi, Konformatsii Organicheskikh Molekul (Russ.), Khimiya, Moscow, 1974. [11] N.I. Monakhova, S.A. Katsyuba, L.Kh. Ashrafullina, R.R. Shagidullin, Zh. Prikl. Spektrosk., 51 (1989) 944. [J. Appl. Spectrosc. (Engl. Trans.)]. [12] A.B. Remizov, S.A. Katsyuba, Zh. Strukt. Khim., 30 (1989) 38.

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[13] S.A. Katsyuba, N.I. Monakhova, L.Kh. Ashrafullina, R.R. Shagidullin, J. Mol. Struct., 269 (1992) 1. [14] R.G. Lett, W.H. Flygare, J. Chem. Phys., 47 (1967) 4730. [15] M. Ohta, Y. Ogawa, H. Matsuura, I. Harada, T. Shimanouchi, Bull. Chem. Soc. Jpn., 50 (1977) 380. [16] R.R. Shagidullin, A.V. Chernova, V.S. Vinogradova, F.S. Mukhametov, Atlas of IR Spectra of Organophosphorus Compounds (interpreted spectrograms), Nauka, Moscow, 1990. [17] H. Matsuura, M.Tasumi, in: J.R. Durig (Ed.), Vibrational

Spectra and Structure, vol. 12, Elsevier, Amsterdam, 1983, Chapter 2, pp. 69–143. [18] V.A. Alfonsov, I.A. Litvinov, O.N. Kataeva, S.A. Katsyuba, D.A. Pudovik, XIIIth International Conference on Phosphorus Chemistry, 1995, Israel, Jerusalem, Abstracts, p. 46. V.A. Alfonsov, D.A. Pudovik, I.A. Litvinov, S.A. Katsyuba, E.A. Phylippova, M.A. Pudovik, V.K. Khairullin, E.S. Batiyeva, A.N. Pudovik, Dokl. Akad. Nauk SSSR., 296 (1987) 103 [Dokl. Chem. (Engl. Transl.)].