Vibrational spectra and gas phase structure of N-trifluoroacetylimidosulfurous difluoride, CF3C(O)NSF2

Journal of Molecular Structure 607 (2002) 207±215

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Vibrational spectra and gas phase structure of N-tri¯uoroacetylimidosulfurous di¯uoride, CF3C(O)NySF2 Marta I. Mora Valdez a, Edgardo H. Cutin a, Carlos O. Della Vedova b, Ruediger Mews c, Heinz Oberhammer d,* a

Facultad de BioquõÂmica, QuõÂmica y Farmacia, Instituto de QuõÂmica FõÂsica, Universidad Nacional de TucumaÂn, San Lorenzo 456, (4000) TucumaÂn, Argentina b CEQUINOR (CONICET) and Laboratorio de Servicios a la Industria y al Sistema Cientõ®co (UNLP-CIC-CONICET), Departamento de QuõÂmica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Argentina c Institut fuÈr Anorganische und Physikalische Chemie, UniversitaÈt Bremen, 28334 Bremen, Germany d Institut fuÈr Physikalische und Theoretische Chemie, UniversitaÈt TuÈbingen, 72076 TuÈbingen, Germany Dedicated to Professor Hans BuÈrger on the occasion of his 65th birthday Received 17 September 2001; revised 23 October 2001; accepted 23 October 2001

Abstract The conformational and structural properties of N-tri¯uoroacetylimidosulfurous di¯uoride, CF3C(O)NySF2, have been studied by vibrational spectroscopy (IR(gas) and Raman(liquid)), by gas electron diffraction and by quantum chemical calculations (HF, MP2 and B3LYP with 6-31G p and 6-311G p basis sets). Only a single conformer with a syn±syn structure and CS symmetry was observed in the gaseous and liquid states. The CF3C(O) group is oriented syn relative to the SF2 group and the CyO and NyS double bonds are synperiplanar to each other. The preference of this conformation has been rationalized based on a natural bond orbital analysis. Conformations with anti orientations around the NyS and N±C bonds correspond to stable structures according to quantum chemical calculations, but are higher in energy by 2.4 kcal/mol or more. The skeletal para N±C ˆ 1:406…16† A;  S±F ˆ 1:585…2† A;  C±NyS ˆ 128:2…16†8; NyS±F ˆ 110:4…12†8 and meters, NyS ˆ 1:481…7† A; F±S±F ˆ 89:3…11†8 (ra values with 3s uncertainties), are very similar to those in other imidosulfurous di¯uorides. Whereas the HF approximation reproduces the geometric parameters satisfactorily, the MP2 and B3LYP methods predict some bonds Ê . q 2002 Elsevier Science B.V. All rights reserved. too long by up to 0.07 A Keywords: Vibrational spectroscopy; Imidosulfurous compounds; Gas electron diffraction; Quantum chemical calculations

1. Introduction All imidosulfurous compounds of the type RNySF2 with RyCl [1], CF3 [2,3], SF5 [4], CN [5] FC(O) [6] * Corresponding author. Tel.: 149-7071-29-76907; fax: 1497071-29-5490. E-mail address: [email protected] (H. Oberhammer).

and FSO2 [7] whose gas phase structures have been determined possess syn con®guration around the NyS bond (Scheme 1). In addition to being sterically unfavorable, the two electron lone pairs of nitrogen and sulfur eclipse each other in this con®guration. syn structures were also observed for CF3NySCl2 [8] and for ClNyS(CF3)2 [9]. The only exception found among imidosulfurous compounds so far is [(¯uoroformyl)imido](tri¯uoromethyl)sulfurous ¯uoride,

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022- 286 0( 01) 00926-7

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M.I. Mora Valdez et al. / Journal of Molecular Structure 607 (2002) 207±215 Table 1 Relative energies (kcal/mol) of the four possible conformers of CF3C(O)NySF2

Scheme 1.

Method

syn±syn a

syn±anti b

anti±syn b

anti±anti a

HF/6-31G p MP2/6-31G p B3LYP/6-31G p

0.00 0.00 0.00

3.46 3.14 2.43

5.55 4.68 4.06

10.30 8.33 6.91

a b

FC(O)NyS(F)CF3, where the predominant conformer (79(12) or 86(8)% according to gas electron diffraction or IR spectroscopy, respectively) possesses anti con®guration around the NyS bond [10]. This unexpected result suggested a study of its structural isomer N-tri¯uoroacetylimidosulfurous di¯uoride, CF3C(O)NySF2. This compound has been prepared for the ®rst time by Glemser et al. in 1969 and a short report of its IR spectrum was given [11]. As a part of our vibrational and structural studies of imidosulfurous compounds, we report now IR spectra of the vapor phase, Raman spectra of the liquid phase, and the gas phase structure as determined by gas electron diffraction (GED). These experimental studies have been supplemented by quantum chemical calculations. 2. Experimental Tri¯uoroacetylimidosulfurous di¯uoride, CF3C(O)NySF2, was obtained by reaction between CF3C(O)NH2 and SF4, in the presence of NaF [11]. The product was puri®ed by several trap-to-trap distillations at reduced pressure.

Scheme 2.

CS symmetry. C1 symmetry.

A Perkin±Elmer 1600 FTIR spectrometer was used to record the FTIR spectra of the vapor phase between 4000 and 400 cm 21 (resolution was 2 cm 21). Raman spectra between 2000 and 50 cm 21 were obtained for the liquid phase using a Spex Ramalog spectrometer equipped with an Ar (Spectra-Physics Model 165) laser. The 457.9 and 514.5 nm excitation lines were used. All spectra were recorded at room temperature. The resolution was 5 cm 21 and the power was 100 mW for both excitation lines. GED intensities were recorded with a Gasdiffraktograph KD-G2 [12] at 25 and 50 cm nozzle-to-plate distances and with an accelerating voltage of about 60 kV. The sample reservoir was kept at 215 8C and the inlet system and gas nozzle were at room temperature. The photographic plates (Kodak Electron Image plates 13 £ 18 cm 2 ) were analyzed with the usual methods [13]. 3. Quantum chemical calculations Due to internal rotations around the N±C single and the NyS double bond, four different conformations are feasible for CF3C(O)NySF2 with syn/anti orientation of the N±C bond relative to the bisector of the SF2 group and syn/anti orientation of the CyO bond relative to the NyS bond.(Scheme 2). Geometry optimizations of these conformations were performed with ab initio methods (HF and MP2) and with the hybrid method B3LYP using 6-311G p basis sets. Frequency calculations were performed for all four conformations with the HF and B3LYP method using 6-31G p basis sets. All three methods predict the syn±syn form in which the CF3CO group is synperiplanar to the FSF bisector and the CyO bond is synperiplanar with respect to the NyS bond to be the most stable

M.I. Mora Valdez et al. / Journal of Molecular Structure 607 (2002) 207±215

209

Fig. 1. IR(gas) spectrum for CF3C(O)NySF2.

structure. The calculated relative energies of the other three conformers are listed in Table 1. The syn±syn and anti±anti conformers possess CS symmetry, whereas the structures of the syn±anti and anti±syn forms are distorted to C1 symmetry due to torsions around the N±C and NyS bonds. Structures with exact CS symmetry possess one imaginary frequency and correspond to transition states. In the syn±anti form, the distortion from CS symmetry is due to F´ ´´F repulsion between the CF3 and SF2 groups and

occurs predominantly around the N±C bond (t…N±C† ˆ 41:18 and t…NyS† ˆ 3:28 according to MP2 method). In the anti±syn conformer, the distortion is due to repulsion between the sulfur lone pair and the CyO bond and occurs around both bonds (t…N±C† ˆ 23:68 and t…NyS† ˆ 18:38 according to MP2 method). The predicted energy differences between the four conformers depend on the computational method. The highest values are obtained with the HF approximation and the lowest values with the

Fig. 2. Raman(liquid) spectrum of CF3C(O)NySF2.

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Table 2 Assignment of fundamental modes, de®nition of symmetry coordinates, experimental and calculated wavenumbers for CF3C(O)NySF2 Mode A0 n( n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 A 00 n 16 n 17 n 18 n 19 n 20 n 21 n 22 n 23 n 24 a b

Approximate description a

Symmetry coordinate

IR wavenumbers (cm 21)

HF/6-311G p (cm 21) b

B3LYP/6-31G p (cm 21)

CyO stretch NyS stretch C±C stretch CF3 sym. stretch CF3 sym. stretch C±N stretch SF2 sym. stretch CF3 sym. def. CF3 sym. def. SF2 sym. def. CC(O) rocking SF2 wagging CF3 sym. def. CC(O) sym. def. C±NyS def.

r5 r7 r4 6 21/2(2r1 2 r2 2 r3) 3 21/2(r1 1 r2 1 r3) r6 2 21/2(rP8 1 r9)P 6 21/2( a 2 b ) 6 21/2(2a 1 2 a 2 2 a 3) 6 21/2(2g 3 2 g 1 2 g 2) 2 21/2(d 1 2 d 2) h 6 21/2(2b 1 2 b 2 2 b 3) 6 21/2(2d 1 2 d 2 2 d 3) u

1779 (IR) 1361 (IR) 1272 (IR) 1232 (IR) 1021 (IR) 855 (Ra) 749 (IR) ± 580 (IR) 498 (IR) 417 (Ra) 382 (Ra) 324 (Ra) 197 (Ra) 118 (Ra)

1830 1379 1272 1230 1018 821 780 731 565 488 410 391 325 184 113

1827 1361 1278 1237 1031 816 759 732 568 474 411 363 321 181 109

CF3 asym. stretch CC(O) oop. def. SF2 asym. stretch CF3 asym. def. SF2 twisting CF3 asym. def. Skeletal torsion N±C torsion CF3 torsion

2 21/2(r2 2 r3) p 2 21/2(r8 2 r9) 2 21/2(a 2 2 a 3) 2 21/2(g 1 2 g 2) 2 21/2(b 2 2 b 3) t ((yS) t (N±C) t (CF3)

1187 (IR) 803 (Ra) 728 (IR) 527 (Ra) 398 (Ra) 237 (Ra) ± ± ±

1217 790 769 515 400 234 110 41 34

1215 778 745 515 375 219 96 33 20

def.: deformation; sym.: symmetric; asym.:asymmetric; oop.:out-of-plane. scaled with 0.90.

Fig. 3. Molecular model of CF3C(O)NySF2 with internal coordinates.

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hybrid method. All methods, however, predict the same sequence in relative stability, syn±syn . syn± anti . anti±syn . anti±anti. All calculations indicate, that only the syn±syn conformer is observable in our experiments. The cartesian force constants obtained with the B3LYP method for the most stable syn±syn conformer were transformed to symmetry force constants and used for calculation of vibrational amplitudes with the program ASYM40 [14]. It was not attempted to derive vibrational amplitudes from a force ®eld based on experimental frequencies. It is well known that amplitudes for bonded and 1,3-nonbonded distances are reproduced very well with calculated force ®elds. Amplitudes for longer non-bonded distances, however, depend strongly on torsional vibrations which have not been observed in the experimental spectra. All quantum chemical calculations were performed with the gaussian 98 program suit [15]. 4. Vibrational analysis The IR(gas) and Raman(liquid) spectra are shown in Figs. 1 and 2, the observed and calculated wavenumbers, together with tentative assignments are collected in Table 2. Internal coordinates are de®ned in Fig. 3. In the case of CS symmetry, 24 normal modes of vibration can be classi®ed as 15 A 0 inplane modes and nine A 00 out-of-plane modes, all of which are active in IR and Raman. The fundamental modes were assigned by comparison with spectra of related molecules, such as FC(O)NySF2 [16], ClSO2NySF2 [17], CF3C(O)NCO [18], FC(O)NyS(F)CF3 [19], CF3SO2NCO [20], and CF3SO2N3 [21], and by comparison with calculated vibrational frequencies and intensities. The band with medium intensity centered at 1779 cm 21 in the IR spectrum of the vapor phase and at 1750 cm 21 in the Raman spectrum of the liquid can immediately be assigned to the CyO stretching fundamental mode. The reported values for this mode are 1841 cm 21 (IR) in FC(O)NSF2 [16], 1796 cm 21 (IR) in FC(O)NyS(F)CF3 [19] and 1781 cm 21 (IR) in CF3C(O)NCO [18]. The observation of a single band in the CyO stretching region in the IR and Raman spectra demonstrates the presence of a single conformer in the gas phase and in the liquid state. The calculations predict shifts of this vibration

211 21

in any of the other stable conformers by 20 cm or more and such shifts would be observed if other conformers were present. Three deformation and rocking (in and out-ofplane) vibrations are expected for the CC(O) group. The out-of-plane CC(O) fundamental deformation was assigned to the band at 803 cm 21 appearing in the Raman spectrum. This wavenumber is fairly well comparable with the same vibration found in CF3C(O)NCO [18]. Rocking and in-plane modes appear in the Raman spectrum at 417 and 197 cm 21. By comparison with the vibrational spectra of FC(O)NySF2 [16], ClSO2NySF2 [17] and FC(O)NyS(F)CF3 [19], the strong band appearing at 1361 cm 21 in the IR spectrum of the vapor phase and at 1379 cm 21 in the Raman spectra of the liquid can be assigned to the NyS stretching mode. The most intense IR band at 1272 cm 21 (1254 cm 21 in Raman) can be attributed to the C±C stretching fundamental mode. The high intensity of this band is in part due to a strong contribution of the symmetric CF3 stretching vibration. The C±N stretching fundamental mode can be assigned to the band at 855 cm 21 in Raman spectra, a position which compares well with the same mode in CF3C(O)NCO [18]. The three stretching modes for the CF3 group were assigned to the very intense bands appearing in 1232, 1187 and 1021 cm 21 in the IR spectrum and to the weak bands in the Raman spectrum at 1223, 1175 and 1024 cm 21 which correspond to the A 0 antisymmetric, A 00 antisymmetric, and A 0 symmetric stretchings, respectively. These values agree closely with such vibrations in similar molecules [19±21]. Four deformations (two symmetric and two antisymmetric) are expected for the CF3 group but only three can be observed in the IR and Raman spectra. The feature appearing at 580 cm 21 in both IR and Raman spectra can be assigned to the symmetric deformation, while those at 527 and 237 cm 21 of the Raman spectrum are related to the antisymmetric deformations. The A 0 antisymmetric deformation mode is expected to appear near 730 cm 21, as predicted by theoretical calculations. We assume this band to overlap with the bands assigned to the SF2 stretching modes. The SF2 symmetric and antisymmetric fundamental modes appear at 749 and 728 cm 21 in the IR of the vapor phase and at 752 and 721 cm 21 in the Raman spectrum of the liquid; a similar behavior was

212

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but none of them can be observed in Raman spectrum.

5. Structure analysis

Fig. 4. Averaged experimental (dots) and calculated (full line) molecular intensities for 50 cm (above) and 25 cm (below) nozzle-toplate distances and residuals.

observed in FC(O)NySF2 [16]. The symmetric deformation of this group was assigned to the IR band at 498 cm 21 and to the Raman band at 495 cm 21. The twisting and wagging fundamental modes appear at 398 and 382 cm 21 in the Raman spectrum. At 118 cm 21, the Raman spectrum exhibits an absorption signal, which is assigned to the CNS fundamental deformation mode. Three torsional vibrations are expected for this molecule,

Averaged molecular intensities in the s ranges of 2± Ê 21 in steps of Ds ˆ 0:2 A 21 …s ˆ 18 and 8±35 A …4p=l†sin u=2; where l is the electron wavelength and u the scattering angle) are shown in Fig. 4. The radial distribution function (RDF) was derived by Fourier transformation of these intensities, applying an arti®cial damping function exp…2gs2 † with g ˆ 0:0019 A 2 (Fig. 5). The experimental RDF can be ®tted reasonably well only with a syn±syn structure. The geometric parameters were re®ned by least squares ®tting of the molecular intensities. The overall symmetry was constrained to CS and local C3v symmetry was assumed for the CF3 group. Both assumptions are justi®ed by the quantum chemical calculations. The calculated values for the C±C±F angles and those for the F±C±F angles differ by less than 0.58. Vibrational amplitudes which cause large correlations between geometric parameters or which are poorly determined in the experiment were set to calculated values. With these

Fig. 5. Experimental RDF and difference curve. The positions of interatomic distances are indicated by vertical bars.

M.I. Mora Valdez et al. / Journal of Molecular Structure 607 (2002) 207±215

213

Table 3 Ê and 8. For atom numbering see Fig. 5) Experimental and calculated geometric parameters for CF3C(O)NSF2 (parameters in A GED a

HF/6-311G p

B3LYP/6-311G p

MP2/6-311G p

1.488 1.39 1.534 1.173 1.310 b 1.572 127 127.4 109 108.5 109 92.2

1.51 1.408 1.55 1.195 1.331 b 1.652 129.4 127.3 110.3 108.6 b 109.5 92.1

1.506 1.416 1.539 1.203 1.327 b 1.638 126.6 127.3 109.7 108.7 b 109.5 91.6

assumptions, 12 geometric parameters (p1±p12) and eight vibrational amplitudes (l1±l8) were re®ned simultaneously. The following correlation coef®cients had values larger than u0.6u: p1=p5 ˆ 0:89; p1=p2 ˆ 20:63; p1=p10 ˆ 0:78; p5=p10 ˆ 0:84; p7=p8 ˆ 20:67; p1=l1 ˆ 0:73; p2=l1 ˆ 20:89; p5=l1 ˆ 0:66 and p12=l2 ˆ 20:74: The results of the least squares analysis are given in Table 3 (geometric parameters) and Table 4 (vibrational amplitudes) together with the calculated values.

6. Discussion

NyS N±C C±C CyO C±F S±F SyN±C N±CyO N±C±C F±C±F NyS±F F±S±F a b

1.481(7) 1.406(16) 1.517(10) 1.201(6) 1.327(4) 1.585(2) 128.2(16) 128.4(17) 109.6(22) 108.7(4) 110.4(12) 89.3(11)

p1 p2 p3 p4 p5 p6 p7 p8 p9 p10 p11 p12

ra values with 3s uncertainties. Mean value.

According to vibrational spectra and GED, CF3C(O)NSF2 adopts a syn±syn conformation and contributions of any other conformer larger than about 5% can be excluded. This result is in agreement with quantum chemical calculations which predict the second-lowest conformer (syn±anti) to be higher in energy by 2.4 kcal/mol or more (see Table 1). A structure with anti orientation around the NyS bond (anti±

Table 4 Ê , for atom numbering see Fig. 5) Interatomic distances, experimental and calculated (B3LYP/6-31G p) vibrational amplitudes (values in A Distance CyO C±F N±C NyS C±C S±F F3´´´F4 F1´´´F2 C1´´ ´F3 N´´´O C2´´ ´O N´´´C2 N´´´F1 S´´ ´C1 a b

1.20 1.33 1.40 1.48 1.51 1.59 2.16 2.23 2.33 2.35 2.38 2.39 2.52 2.60

3s uncertainties. Not de®ned.

GED a

B3LYP

b

0.037 0.044(3) 0.048 b 0.039 b 0.051 b 0.044(3) 0.059(4) 0.076 b 0.065 b 0.059(4) 0.059(4) 0.066 b 0.077 b 0.099(19)

l1

l1 l2 l2 l2 l3

0.037 0.045 0.048 0.039 0.051 0.045 0.057 0.076 0.065 0.054 0.06 0.066 0.077 0.064

Distance O´´´F3 N´´´F4 S´´ ´O C1´´´F1 O´´´F1 O´´´F4 N´´´F3 S´´ ´C2 S´´ ´F4 C2´´´F1 F1´´´F4 S´´ ´F3 F1´´´F5 F1´´´F3

2.62 2.77 3.07 3.12 3.13 3.27 3.55 3.84 4.17 4.5 4.71 4.91 5.19 5.33

GED a

B3LYP

b

0.100 0.238(24) 0.148(28) 0.226(89) 0.238(24) 0.238(24) 0.063 b 0.099(19) 0.238(24) 0.248(80) 0.374 b 0.159(44) 0.248(80) 0.238(24)

l4 l5 l6 l4 l4 l3 l4 l7 l8 l7 l4

0.100 0.258 0.108 0.147 0.240 0.246 0.063 0.064 0.232 0.188 0.374 0.072 0.186 0.251

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Table 5 Skeletal geometric parameters of imidosulfurous di¯uorides RNySF2 and of FC(O)NyS(F)CF3 Compound

NyS

N±R

S±F

R±NyS

F±S±F

NyS±F

ClNySF2 a CF3NySF2 b NCNySF2 c FSO2NySF2 d FC(O)NySF2 e CF3C(O)NySF2 f FC(O)NyS(F)CF3 g

1.476(4) 1.477(6) 1.484(3) 1.487(5) 1.479(4) 1.481(7) 1.549(5)

1.703(4) 1.409(8) 1.368(4) 1.638(5) 1.395(6) 1.406(16) 1.391(8)

1.596(2) 1.594(2) 1.593(2) 1.575(3) 1.586(2) 1.585(2) 1.599(4)

120.0(2) 127.2(11) 126.2(15) 129.9(8) 126.1(11) 128.2(16) 112.4(11)

89.3(3) 92.8(4) 90.5(3) 92.2(9) 93.4(3) 89.3(11) 90.3(14) h

111.1(2) 112.7(10) 108.8(8) 109.7(8) 110.4(8) 110.4(12) 108.8(33)

a b c d e f g h

Ref. [1]. Ref. [3]. Ref. [5]. Ref. [7]. Ref. [6]. This work. Ref. [10]. Angle F±S±C.

syn conformer) is predicted to be higher in energy by 4.1 kcal/mol or more. The strong preference for syn orientation around the NyS bond is in accordance with the conformational properties of all other imidosulfurous compounds whose gas phase structures are known, but it is in contrast to those of the bond isomer FC(O)NS(F)CF3. In the latter compound, the anti±syn form is favored over the syn±syn conformer by 0.79(36) or 1.09(35) kcal/mol according to GED and IR spectroscopy, respectively. A natural bond orbital (NBO) analysis [22] based on the MP2 wave function reveals that the sterically unfavorable syn orientation around the NyS bond is stabilized by orbital interactions between the sulfur lone pair and the antiperiplanar oriented N±C bond and between the nitrogen lone pair and the anticlinal oriented S±F bonds. The total stabilization due to these anomeric effects amounts to 229.5 kcal/mol (29.7 kcal/mol for n…S† ! s p …N±C† and 29.9 kcal/mol for each n…N† ! s p …S±F† interaction). Likewise, the preference for syn orientation of the CyO bond with respect to the NyS bond can be rationalized with an anomeric interaction between the nitrogen lone pair and the s(CyO) orbital. This interaction energy for syn orientation of the CyO bond (210.9 kcal/mol) is appreciably larger than the n…N† ! s p …C±C† interaction energy (27.9 kcal/mol) for anti orientation of the two double bonds. Furthermore, strong steric repulsions between the ¯uorine

atoms of the CF3 and SF2 groups destabilize the syn±anti form. These repulsions lead to distortion of this conformer to C1 symmetry and rotation of the CF3C(O) group around the N±C bond by about 408 (MP2), away from the ideal CS symmetry. Table 5 compares skeletal geometric parameters of some imidosulfurous di¯uorides RNySF2 whose gas phase structures are known and of FC(O)NyS(F)CF3. The NyS bond lengths in all di¯uorides with syn con®guration around this bond possess very similar Ê , nearly independent of the values of 1.48 ^ 0.01 A substituent R. These short bonds can be rationalized by the anomeric effects between the lone pairs of sulfur and nitrogen and the opposite N±X (X ˆ C, Cl or S) and S±F bond orbitals. These orbital interactions lead to partial triple bond character of the NyS bond. FC(O)NyS(F)CF3 possesses anti con®guration around the NyS bond which does not allow anomeric interactions. As a consequence, the NyS bond is Ê ) in this compound. considerably longer (1.549(5) A The R±NyS angles in the RNySF2 compounds are large …128 ^ 28†; with the exception of ClNySF2 (120.0(2)8), and re¯ect strong steric repulsions which exist in the syn con®guration between the substituents R and the ¯uorine atoms of the SF2 group. This angle is much smaller (112.4(11)8) in the anti conformation of FC(O)NyS(F)CF3. Neglecting systematic differences between vibrationally averaged ra distances derived from the GED

M.I. Mora Valdez et al. / Journal of Molecular Structure 607 (2002) 207±215

experiment and equilibrium re values from calculaÊ ), tions (re is smaller than ra by about 0.005±0.010 A the HF approximation reproduces all bond lengths satisfactorily (see Table 3). However, NyS, C±C and S±F bond distances predicted by the MP2 and B3LYP methods are longer than the experimental Ê . The bond angles predicted values by up to 0.07 A by all three methods agree with the experimental values to within about ^28. The vibrational frequencies are reproduced satisfactorily with the B3LYP and HF methods if the values derived with the latter method are scaled with a factor of 0.9. Acknowledgements EHC and CODV thank the Universidad Nacional de Tucuman and CONICET, RepuÂblica Argentina, for ®nancial support. We thank the FundacioÂn Antorchas (RepuÂblica Argentina), Alexander von Humboldt Stiftung and DAAD (Deutsche Akademische Austauschdienst, Germany) for ®nancial support and for the DAAD-FundacioÂn Antorchas and Alexander von Humboldt Stiftung-FundacioÂn Antorchas Awards to the German±Argentine cooperation. CODV also thanks the Consejo Nacional de Investigaciones Cientõ®cas y TeÂcnicas (CONICET), ANPCYT (PICT 122) (Argentina), and the ComisioÂn de Investigaciones Cientõ®cas de la Provincia de Buenos Aires (CIC), RepuÂblica Argentina, for ®nancial support. He is indebted to the Facultad de Ciencias Exactas, Universidad Nacional de La Plata, RepuÂblica Argentina for ®nancial support and to the FundacioÂn Antorchas for the National Award to the Argentinean cooperation. References [1] J. Haase, H. Oberhammer, W. Zeil, O. Glemser, R. Mews, Z. Naturforsch. 23a (1970) 153. [2] R.R. Karl, S.H. Bauer, Inorg. Chem. 14 (1975) 1859. [3] F. Trautner, D. Christen, R. Mews, H. Oberhammer, J. Mol. Struct. 525 (2000) 135.

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