Inorg. Chem. 1991, 30, 59-62
59
Contribution from the lnstitut fur Physikalische und Theoretische Chemie, Universitat Tubingen, 7400 Tubingen, West Germany, and lnstitut fur Anorganische und Analytische Chemie, Freie Universitat Berlin, 1000 Berlin, West Germany
Gas-Phase Structure of Sulfur Cyanate Pentafluoride, SFSOCN. Electron Diffraction and ab Initio Study Petra Zylka,la Hans-Georg Mack,la Arno Schmuck,Ib Konrad Seppelt,*Jb and Heinz Oberhammer*Vla Received March I S , 1989
The geometric structure of SF50CN was determined by gas-phase electron diffraction. The following parameters (ravalues with 30 uncertainties) were obtained: C=N = 1.162 (13) A, 0-C = 1.271 (13) A, (S-F)man = 1.554 (2) A, S-0 = 1.653 (6) A, LSOC = 120.4 ( I 3)O, LOCN = 175.3 (36)O, LF,SF, = 90.4 (6)', and tilt = 3.5 ( 1 1)' (tilt angle between the C4 axis of the SF5 group and the S - 0 bond, away from OCN group). These results are compared with the gas-phase structure of the analogous isocyanate SF5NC0. The cyanate and isocyanate group have different substituent effects on the SF5 moiety. Ab initio calculations (HF/4-31G*) reproduce the experimental geometries of both isomers satisfactorily and predict the energy of the cyanate to be 26 kcal/mol above that of the isocyanate.
Introduction Remarkably little is known about geometric structures of covalent cyanates and about their stability relative to the corresponding isocyanates. A comprehensive review of the chemical and physical properties of these isomers is given in ref 2. The parent isocyanate, HNCO, has been known for more than 150 years,3 and its geometry has been characterized extensively by infrared4 and microwaves spectroscopy. The existence of cyanic acid, HOCN, however, has been disputed for many years2 It could be proven spectroscopically only by matrix infrared technique, being produced by photolysis of isocyanic acid6 or fulminic acid, HCNO.' Alkyl- and aryl-substituted cyanates exhibit much higher stability and have been characterized by infrared, Raman, and NMR spectroscopy,8but no gas-phase structural studies have been reported. An X-ray investigation of (4-chloro-3,5-dimethylpheny1)cyanate resulted in mean bond lengths of C=N = 1.14 A and 0-C = 1.27 The only cyanate whose gas-phase structure has been determined is SeFsOCN, which has been studied by electron diffraction.I0 Since C, N, and 0 make very similar contributions to the scattering intensities, the cyanate and isocyanate structures fit the electron diffraction intensities equally well. The original model was selected on the basis of bond distances, which made the isocyanate structure more likely." Subsequent NMR data, however, proved unambiguously, that this substance was the cyanate.I0 In the present study we report an electron diffraction investigation of SF50CN,whose synthesis has been reported recently.I2 Spectroscopic datat2 unambiguously demonstrate its cyanate structure. Since the geometry of the analogous isocyanate, SF5NC0, has previously been studied by the same method,l' these two compounds present the first example where geometric structures of corresponding isomers can be compared. The experimental investigation is supplemented by ab initio calculations, which also give information about their relative stability. Ab Initio Calculations In a preceding theoretical study on geometric structures and ( I ) (a) Universitat Tubingen. (b) Freie Universitat Berlin. (2) Patai, S., Ed. The Chemistry of Cyanares and Their Thio Derivatives; John Wiley: New York, 1977; Parts I and 2. (3) Licbig, J.; Gay-Lursac, L. Ann. Chem. Phys. 1824, 25, 285. (4) Ashby. R. A.; Werner, R. L. J . Mol. Spectrosc. 1%5, 18, 184. Krakow, B.; Lord. R. C.;Neely. G. 0. J . Mol. Spectrosc. 1968, 27, 148. (5) Hocking, W. If.; Gerry, M. C. L.; Winnewisser, G. Can J . Phys. 1975, 53, 1869. Yamada, K.; Winnewisser, M. J . Mol. Specrrosc. 1977, 68, 307. Yamada, K. J. Mol. Specfrosc. 1980, 79, 323. ( 6 ) Jacox, M. E.: Milligan. D. E. J . Chem. Phys. 1964, 40, 2457. (7) Bondybey, V. E.: English, J. H.; Mathews, C . W.; Contolini, R. J. J . Mol. Spectrosc. 1982, 92, 43 I . ( 8 ) Ben-Efraim, D.A . In The Chemistry of Cyanafes and Their Thio Dcriuatiaes: John Wilcy: New York, 1977; Part I , Chapter 5 , p 191 and references therein. (9) Kutschabsky, I..; Schrauber, H . Krist. Tech. 1973, 8, 217. (IO) Seppelt, K.; Oberhammer, H. Inorg. Chem. 1985, 24, 1227. ( I I ) Oberhammer. f-I.:Seppelt, K.; Mews, R. J . Mol. Srrucf. 1983, 101, 325. ( I 2) Schmuck, A.; Seppelt, K. Angew. Chem. 1987.99, 138; Angew. Chem., I n ! . kd. Engl. 19117, 26. 134.
Table 1. Experimental and Calculated Geometric Parameters (in A and deg) for S F 5 0 C N and S F 5 N C 0 SF5OCN SF5NCO ed" aib edc aib C=N; N=C 1.162 (13) 1.131 1.234 (8) 1.203 0-c; c=o 1.271 (13) 1.289 1.179 (7) 1.136 1.546 1.567 (2) 1.557 (S-F) m a n 1.554 (2) A = r(SF,) - r(SF,) Od 0.005 od 0.002 S-0; S-N 1.653 (6) 1.648 1.668 (6) 1.661 LSOC; LSNC 120.4 (13) 122.2 124.9 (12) 130.4 LOCN; LNCO' 175.3 (36) 176.9 173.8 (37) 174.2 LF,SF, 90.4 (6) 90.3 90d 88.8 tilt/ 3.5 (11) 2.0 Od 1.3 " r , values from tlectron diffraction experiment. Estimated uncertainties are 3a values and include a possible scale error of 0.1%. b r , values from ab initio calculations (HF/4-31G*). cr, values from ref I 1 . dNot refined. Bending of OCN or NCO group in trans direction with respect to sulfur. /Tilt angle between C4 axis of the SF5 group and SO/SN bond direction. Tilt is away from the OCN or NCO
group. relative stabilities of cyanates and isocyanates, ab initio calculations with different basis sets and at various levels of theory ( H F or MP2) were performed.13 In the isocyanates polarization functions have a very strong effect on the nitrogen bond angles. Basis sets without polarization functions (3-21G) result in linear XNCO structures, whereas experimental nitrogen bond angles in HNCO and ClNCO are reproduced correctly if polarization functions are included. In the cyanates, XOCN, polarization functions show only minor effects on the oxygen bond angles (decrease of ca. 5 O ) . Bond distances in the NCO group are predicted very well with 3-21 G basis sets, whereas 6-3 1G* basis sets result in bond lengths that are too short. This excessive shortening is compensated if electron correlation effects are taken into account. Obviously, it is not feasible to apply the MP2/6-31G* procedure to the compounds of this study and HF/4-31G* calculations were considered to be an adequate compromise for a structure optimization. The theoretical results are prcsented in Table I together with thc experimental structures. Since the SF, groups were constrained to C,, symmetry in the experimental analyses for the cyanate and the isocyanate, the same constraint was applied to the ab initio optimizations. Test calc~lations'~ for XOCN and XNCO show that basis sets without polarization functions ( H F/3-21 G ) underestimate thc energy differences between cyanates and isocyanates compared to basis sets with polarization [unctions ( H F / 6 - 3 1 G * ) by about 3 kcal/mol for X = H and by about 19 kcal/mol for X = CI. Inclusion of electron correlation in the MP2 approximation changes the relative energies only by 1-2 kcal/mol for X = H, CH,, CI, or F. These calculations suggest that the H F / 4 - 3 1 G * method gives a realistic estimate of the energy difference between SFSOCN and SFSNCO. All calculations were performed with the GAUSSIAN 82 program ( 1 3 ) Mack. H. G.;Oberhammer, H. Chem. Phys. Lerf. 1989, 157, 436.
0020-1669/91/ 1330-0059$02.50/0 0 1991 American Chemical Society
60 Inorganic Chemistry, Vol. 30, No. 1 , 1991
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Zylka et al.
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R in Rngstrom Figure 1. Experimental radial distribution function and difference curve. Table 11. Vibrational Amplitudes (in A) for SF,OCNa C=N 0.046 (23) ( / I ) }0.059 (12) ( I s ) 0.043 (24) (12) S**C 0.070 (17) (16)
t:;F
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0.045 (3)
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Error limits are 3u values. For atom labeling, see Figure I . b N o t rcfincd. packagef4on a BASF 7/88 computer. Experimental Structure Analysis A preliminary structural model for SFSOCNwas derived from the radial distribution function (Figure 1) and was then refined by a least-squares analysis of the molecular intensities. The impurity that was observed in the liquid phase (
