Gas phase structure of trifluoromethyliminosulfurdifluoride, CF3NSF2, revisited

Journal of Molecular Structure 525 (2000) 135–139 www.elsevier.nl/locate/molstruc

Gas phase structure of trifluoromethyliminosulfurdifluoride, CF3NySF2, revisited F. Trautner a, D. Christen a, R. Mews b, H. Oberhammer a,* a

Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Tu¨bingen, 72076 Tu¨bingen, Germany b Institut fu¨r Anorganische und Physikalische Chemie, Universita¨t Bremen, 28334 Bremen, Germany Received 15 November 1999; accepted 15 December 1999

Abstract The gas phase structure of trifluoromethyliminosulfurdifluoride, CF3NySF2, was reinvestigated by a joint analysis of gas electron diffraction (GED) and microwave spectroscopy. Quantum chemical calculations were performed with the HF, MP2 and B3LYP methods using 6-31G ⴱ basis sets. The GED intensities and MW rotational constants B and C were fitted best with a syn configuration (CF3 group syn with respect to SF2) and staggered orientation of the CF3 group with large amplitude torsional motion around the C–N bond. The following skeletal geometric parameters (rg distances and ra angles with 2s uncertainties) ˚ , S–F 1.594(2) A ˚ , N–C 1.409(8) A ˚ , C–NyS 127.2(11)⬚, NyS–F 112.7(10)⬚ and F–S–F 92.8(4)⬚. were derived: SyN 1.477(6) A The values for the SyN and N–C distances and for the F–S–F angle differ appreciably from those derived in an earlier GED ˚ and the MP2 and B3LYP calculations investigation. The HF approximation reproduces all bond lengths to within ^0.02 A predict the SyN and the S–F bonds to be longer. The bond angles are reproduced very well with all three methods. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Trifluoromethyliminosulfurdifluoride; Gas electron diffraction; Microwave spectroscopy; Quantum chemical calculation

1. Introduction Gas phase structures of several compounds of the type RNySF2 have been reported in recent years. All compounds of this type possess syn configuration with the substituent R syn with respect to the SF2 group and the nitrogen and sulfur electron lone pairs eclipsing each other. For substituents RyCl [1], FC(O) [2], SF5 [3] and NC [4] the SyN bond lengths are very similar ˚ in SF5NySF2 to 1.484(3) A ˚ in from 1.470(10) A NCNySF2. However, a much shorter SyN bond ˚ has been derived in a gas elecdistance of 1.447(6) A * Corresponding author. Tel.: ⫹ 49-7071-296907; fax: ⫹ 497071-296910. E-mail address: [email protected] (H. Oberhammer).

tron diffraction study (GED) for CF3NySF2 [5]. Furthermore, an unusually long N(sp 2)–C bond length ˚ and a very small F–S–F angle of of 1.469(10) A 81.1(16)⬚ were reported for this compound. These structural parameters are not compatible with the general bonding properties of iminosulfur compounds. Therefore, we decided to reinvestigate the structure of this compound by applying a joint analysis of GED intensities and microwave (MW) rotational constants. The experimental investigations are supplemented by quantum chemical calculations.

2. Experimental CF3NySF2 was synthesised by the reaction of

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Table 1 Measured rotational transitions …J 0 K 0a K 0c ← J 00 K 00a K 00b † in CF3NySF2 (MHz) Transition

n

Transition

n

Transition

n

643 –542 642 –541 761 –660 762 –661 753 –652 752 –651 744 –643 743 –642 818 –717 827 –726 863 –762 862 –761

14487.35 14487.35 16895.57 16895.57 16899.00 16899.00 16904.53 16904.53 18880.00 19261.60 19309.87 19309.87

854 –753 853 –752 845 –744 844 –743 836 –735 826 –725 928 –827 973 –872 972 –871 964 –863 963 –862 954 –853

19314.89 19314.89 19322.56 19323.04 19331.11 19506.35 21654.40 21720.29 21720.29 21725.43 21725.43 21732.09

955 –854 946 –845 937 –836 936 –835 1074 –973 1073 –972 1065 –964 1064 –963 1056 –955 1055 –954 1047 –946

21732.09 21742.53 21751.10 21783.73 24134.74 24134.74 24140.81 24140.81 24149.28 24149.28 24163.47

(FCN)3 and SF4 in the presence of CsF as catalyst [6]. The purity of the sample was checked by IR spectroscopy. The electron diffraction intensities were recorded with a Gasdiffraktograph KD-G2 [7] at 25 and 50 cm nozzle-to-plate distances and with an accelerating voltage of about 60 kV. The sample reservoir was cooled to ⫺60⬚C and the inlet system and nozzle were at room temperature. The photographic plates were analysed with the usual methods [8] and averaged molecular intensities in the s-ranges 2–18 and ˚ ⫺1, in intervals of Ds ˆ 0:2 A ⫺1 ; are shown in 8–35 A Fig. 1. The microwave spectrum was recorded with a conventional Stark spectrometer, modulated at 50 kHz, using phase-locked BWOs as sources. The

5 m cell was cooled to ⫺40⬚C and most transitions were measured at a pressure of 8 mTorr. Trifluoromethyliminosulfurdifluoride is a near prolate rotor …k ˆ ⫺0:96† and theoretical calculations (see below) predicted the ma-component of the dipole moment to be approximately 10 times as strong as the mc-component. (The mb-component is zero for symmetry reasons.) Only a ma-spectrum could be assigned. A later attempt to identify mc-transitions using a double resonance technique with the spectrometer described in Ref. [9] and an already identified ma-transition as pump frequency failed, probably because of the weakness of the mc-transitions. The key to the assignment of the ma-spectrum were the high J/high Ka transitions that, due to the near degeneracy, show a fast Stark effect and appear at Stark fields below 20 V cm ⫺1. Thus, the first lines to be assigned were the K a ˆ 7; 6, 5 and 4 of the 9 ← 8 R-branch transition in the Kband. Only R-branch transitions could be identified, with 5 ⱕ J ⱕ 10; 1 ⱕ Ka ⱕ 7 and 0 ⱕ Kc ⱕ 7:35 transitions were identified in the Ku- and K-bands (Table 1) and a quartic Watson Hamiltonian in the A-reduction and I R-representation was fitted to the lines. The rotational constant A was not fitted and Table 2 Experimental and fitted rotational constants in MHz

Fig. 1. Experimental (dots) and calculated (full line) molecular intensities for long (above) and short (below) nozzle-to-plate distances and residuals.

A B C

Bi0 (exp)

Biz (exp)

Biz (GED/MW)

– 1252.30 (14) 1160.85 (16)

– 1252.0 (2) 1161.4 (3)

3226.6 1252.0 1161.4

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state structure were obtained with the B3LYP method and transformed to symmetry coordinates. All force constants, except those for torsions around the N–C and SyN bonds, were scaled with a factor of 0.9. The vibrational amplitudes and vibrational corrections for interatomic distances, Dr ˆ ra ⫺ rz ; and for the rotational constants, DBi ˆ Bi0 ⫺ Biz (Bi ˆ A; B or C), were calculated with this force field using the program ASYM40 [11]. In the case of large amplitude vibrations the concept of perpendicular vibrations leads to unrealistically large corrections Dr for distances that do not depend on this vibration. Therefore, the contributions of the CF3 torsional vibration (nt ˆ 22 cm⫺1 according to B3LYP) to the corrections for the C–F, F·· ·F and N· ··F distances were neglected. Fig. 2. Experimental radial distribution function and difference curve for GED/MW structure. Interatomic distances are shown by vertical bars.

DK as well as d K was set to zero. The rotational constants of this fit are collected in Table 2 and the centrifugal distortion constants are DJ ˆ 1:701…72† kHz; DJK ˆ 6:970…129† kHz and dJ ˆ 0:366…656† kHz: The transitions showed no hyperfine structure and molecular quadrupole coupling was not considered in the analysis. 3. Theoretical calculations The geometry of CF3NySF2 was fully optimised with the HF, MP2 and B3LYP methods and the 631/G ⴱ basis set using the program system Gaussian 94 [10]. All three methods predict a ground state structure in which the CF3 group lies syn with respect to the SF2 groups and staggers the SyN double bond. Low barriers to internal rotation of the CF3 group, 0.30–0.55 kcal mol ⫺1, were derived. Considerably higher energies between 5.3 kcal mol ⫺1 (HF) and 6.2 kcal mol ⫺1 (MP2) were obtained for the anti configuration. According to the HF and B3LYP method the anti structure with eclipsed orientation of the CF3 group corresponds to a minimum on the energy hyperface (no imaginary frequency), whereas the MP2 calculations predict imaginary frequencies for staggered and eclipsed orientation of the CF3 group. The cartesian force constants for the ground

4. Structure analysis The experimental radial distribution function (RDF) was derived by Fourier transformation of the molecular intensities. The RDF (Fig. 2) can be reproduced reasonably well only with a syn conformation and staggered orientation of the CF3 group. In the least-squares fitting of the molecular intensities local C3v symmetry for the CF3 group with a tilt angle between the C3 axis and the N–C bond direction was assumed. The intensities could not be fitted satisfactorily using a rigid model with an exactly staggered CF3 group (Cs overall symmetry). Large differences between experimental and calculated RDFs occurred  The fit improved considerably, in the region r ⬎ 3 A: if an “effective” torsional angle for the CF3 group was introduced. A value of 14.9(15)⬚ away from the exact staggered orientation was obtained. This suggested the use of a dynamic model with a large amplitude torsional motion of the CF3 group around the N–C bond. With a three-fold cosine potential and a barrier to internal rotation of V ˆ 0:60…24† kcal mol⫺1 the best fit of the intensities and RDF was obtained (see residuals in Figs. 1 and 2). Vibrational amplitudes which caused high correlations or which were poorly determined in this analysis were constrained to the theoretical values. For the dynamic model these were calculated without contribution from the torsional vibration. The following correlation coefficients had values larger than 兩0.6兩: SN/ CN ˆ ⫺0.71, SN/CF ˆ ⫺0.69, CN/CF ˆ ⫺0.63,

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Table 3 Experimental and calculated geometric parameters of CF3NySF2

SyN S–F N–C C–F C–NyS NyS–F F–S–F F–C–F Tilt (CF3) d V3 (CF3) e a b c d e

GED/MW a

GED b

HF c

MP2 c

B3LYP c

1.477(6) 1.594 (2) 1.409 (8) 1.331 (3) 127.2 (11) 112.7 (10) 92.8 (4) 108.1 (4) 4.0 (8) 0.60 (24)

1.447 (6) 1.583 (4) 1.469 (10) 1.332 (5) 130.4 (7) 112.6 (5) 81.1 (16) 108.6 (4) – –

1.480 1.580 1.405 1.316 128.1 109.5 90.7 107.7 2.0 0.30

1.507 1.638 1.423 1.344 125.5 110.0 90.0 108.1 1.9 0.55

1.509 1.641 1.424 1.339 127.3 110.0 90.6 108.0 2.0 0.43

˚ ) and a z-angles (⬚) with 2s uncertainties. rg-distances (A Ref. [5]. 6-31 G ⴱ basis set. Mean values are given for parameters which are not unique. Tilt between C3 axis of CF3 group and N–C bond for exact staggered orientation. The tilt is toward the nitrogen lone pair. Barrier to internal rotation of CF3 group around the N–C bond in kcal mol ⫺1.

CN/FSF ˆ ⫺0.61, CN/FCF ˆ ⫺0.65 and CF/ FCF ˆ 0.76. In the second step the two rotational constants Bz and Cz were included in the fit. The relative weight for the rotational constants was increased until they were reproduced within their experimental uncertainties. These uncertainties for the Biz constants are estimated to be larger than those for the Bi0 constants due to uncertainties in the corrections DB i. The rz bond lengths obtained in this analysis were converted to rg parameters for a direct comparison with the values reported by Karl and Bauer [5]. The geometric parameters of the joint analysis of GED and MW data are listed in Table 3, together with the values of Ref. [5] and calculated values. Experimental

and calculated vibrational amplitudes for the exact staggered conformation and without contribution of the torsional vibration are given in Table 4 and the fitted rotational constants are included in Table 2.

5. Discussion Some geometric parameters of the joint GED/MW study differ appreciably from those reported earlier (see Table 3). The GED/MW value for the SyN ˚ longer and that for the N–C bond bond is 0.03 A ˚ shorter. Furthermore, the F–S–F angle derived 0.06 A in the joint analysis is nearly 12⬚ larger than that of

Table 4 ˚ with 3s Interatomic distances and experimental and calculated vibrational amplitudes for staggered orientation of CF3 group (values in A uncertainties. The amplitudes were derived with a dynamic model and do not contain contributions from the torsional vibration. For atom numbering see Fig. 2)

C–F N–C SyN S–F F···F N···F F4···F5 N···F4 a

Not refined.

Distance

GED

B3LYP

1.33 1.41 1.48 1.59 2.15 2.20–2.28 2.31 2.56

0.046 a 0.050 a 0.039 a 0.047 (2) 0.059 a 0.052 (6) 0.078 a 0.076 a

0.046 0.050 0.039 0.047 0.059 0.061 0.078 0.076

S···C F2···F4 S···2 C· ··F4 S···F1 F2···F5 F1···F4

Distance

GED

B3LYP

2.59 2.95 3.07 3.17 3.64 3.70 4.36

0.051 (14) 0.234 (45) 0.130 (8) 0.140 a 0.063 (9) 0.220 a 0.127 (15)

0.064 0.162 0.110 0.140 0.058 0.220 0.138

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Table 5 Geometric parameters of RNySF2 compounds

CF3NySF2 a ClNySF2 b SF5NySF2 c FC(O)NySF2 d NCNySF2 e a b c d e

NyS

N–R

S–F

R–NyS

F–S–F

NyS–F

1.477 (6) 1.476 (4) 1.470 (10) 1.479 (4) 1.484 (3)

1.409 (8) 1.703 (4) 1.679 (10) 1.395 (6) 1.358 (4)

1.594 (2) 1.596 (2) 1.603 (10) 1.586 (2) 1.593 (2)

127.2 (11) 120.0 (2) 141.9 (1) 126.7 (11) 126.2 (15)

92.8 (4) 89.3 (3) 87.6 (1) 93.4 (3) 90.5 (3)

112.7 (10) 111.2 (2) 103.4 (1) 110.4 (8) 108.8 (8)

This work. Ref. [1]. Ref. [3]. Ref. [2]. Ref. [4].

Karl and Bauer. Table 5 demonstrates that the SyN bond length and the F–S–F angle of the joint analysis agree very closely with those in analogous RNySF2 compounds. The SyN bond lengths vary from ˚ in NCNSF2, ˚ in SF5NSF2 to 1.484(3) A 1.470(10) A and the F–S–F angles from 89.3 (3)⬚ in ClNSF2 to 93.4 (3)⬚ in FC(O)NSF2. The N–C bond length of ˚ is very similar to such N(sp 2)–CF3 bond 1.409(8) A ˚ [12]), CF3N3 lengths in CF3NCO (1.394 A ˚ [13]) or in CF3NyCHF (1.414 (7) A ˚ (1.425(5) A [14]). The HF approximation reproduces all bond ˚ and bond angles to within lengths to within ^0.02 A ^3⬚. The MP2 and B3LYP calculations, however, ˚ and predict the SyN bond to be longer by 0.03 A ˚ . The bond the S–F bonds to be longer by 0.04 A angles are reproduced very well with these methods. Acknowledgements We thank the students M. Krucherz, U. Baisch, M. Bayer and J. Wissler who recorded the microwave spectra and helped analysing the electron diffraction intensities and microwave spectra. We are grateful for the financial support from the Fonds der Chemischen Industrie. References [1] J. Haase, H. Oberhammer, H.K. Zeil, O. Glemser, R. Mews, Z. Naturforsch. 25a (1970) 153.

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