Journal of Molecular Structure 484 (1999) 249–257
Gas phase structures of N-(fluorosulfonyl)imidosulfurous difluoride, FSO2NySF2, and N-(fluorosulfonyl)imidosulfuryl fluoride, FSO2NyS(O)F2 R. Haist a, R.S.M. Alvare´z b, E.H. Cutin b, C.O. Della Vedova c, H. Oberhammer a,* a Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Tu¨bingen, 72076 Tu¨bingen, Germany Instituto de Quı´mica Fı´sica, Facultad de Bioquı´mica, Quı´mica y Farmacia, Universidad Nacional de Tucuma´n, Ayacucho 491, (4000) Tucuma´n, Argentina c CEQUINOR (CONICET) and Laboratorio de Servicios a la Industria y al Systema Cientı´fico (UNLP-CIC-CONICET), Departamento de Quı´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Argentina b
Received 5 October 1998; received in revised form 4 December 1998; accepted 4 December 1998
Abstract The geometric structures of N-(fluorosulfonyl)imidosulfurous difluoride, FSO2NySF2 (1), and N-(fluorosulfonyl), FSO2NyS(O)F2 (2) were determined by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G*, B3LYP/6-31G* and MP2/6-31G*). In both compounds the SF2 groups are syn with respect to the N–S bond. The S–F bond of the FSO2 groups is oriented anticlinal relative to the NyS double bond with f (F–S–NyS) 113(7)8 in 1 and 107(4)8 in 2. ˚ , NyS 1.487(5) A ˚, The following skeletal parameters (ra values with 3s uncertainties) were obtained for 1: N–S 1.638(5) A ˚ ˚ S–NyS 129.9(8)8; for 2: N–S 1.631(5) A, NyS 1.475(5) A, S–NyS 125.9(8)8. The HF approximation predicts the orientation of the FSO2 group in 1 and 2 correctly (f 1148 and 1108) and reproduces bond lengths and angles satisfactorily. The B3LYP and MP2 methods predict the dihedral angles about 208–308 smaller than the experimental values and calculate all ˚ . q 1999 Elsevier Science B.V. All rights reserved. bonds to be too long by 0.03–0.06 A Keywords: N-(fluorosulfonyl)imidosulfurous difluoride; N-(fluorosulfonyl)imidosulfuryl fluoride; Gas phase structures; Quantumchemical calculations
1. Introduction All imidosulfurous difluorides of the type RNySF2 with R Cl [1], CF3 [2], FC(O) [3], and SF5 [4], whose structures were determined in the gas phase, possess a syn conformation of the molecular skeleton. Similarly, N-sulfinylimines, RNySyO, with R H * Corresponding author. Tel.: 149-70-71-296907; fax: 149-7071-296910. E-mail address:
[email protected] (H. Oberhammer)
[5], Cl [6], SiH3 [7], SiMe3 [8], CF3S [9], and FC(O)S [10] adopt syn conformations (Scheme 1). This raises the question about the conformational properties of imidosulfuryl fluorides RNyS(O)F2, where either the SF2 group or the SyO double bond can be syn with respect to the R–N bond. Only two gas phase structures of compounds of this type were reported in the literature. Both compounds, ClNyS(O)F2 [11] and NCNyS(O)F2 [12], adopt structure I with the SF2 group syn and the SyO double bond anti with respect to the R–N bond (Scheme 2). In continuation of vibrational and structural investigations of compounds
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00900-4
250
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
Averaged molecular intensities in the s-ranges 2–18 ˚ 21, in steps of Ds 0.2 A ˚ 21 are shown in and 8–35 A Figs. 1 and 2. Scheme 1.
which contain the FSO2 or CF3SO2 group bonded to sp 2-hybridized nitrogen, such as FSO2NCO [13], CF3SO2NCO [14] or CF3SO2NNN [14], we report in the present study gas phase structure determinations for FSO2NySF2 and FSO2NyS(O)F2 by gas electron diffraction (GED) and quantumchemical calculations. Preliminary analyses of the vibrational spectra of these two compounds do not allow any conclusions about their conformational properties [15]. In both cases these spectra indicate the presence of a single conformer only. In addition to the conformation of the RNyS(O)F2 skeleton, we are also interested in the the orientation of the FSO2 group around the S–N bond. Previous studies of the compounds FSO2NyY and CF3SO2NyY (Y CO or NN) resulted in conformations in which one SyO double bond nearly eclipses the NyY double bond.
2. Experimental FSO2NySF2 was synthesized by reacting FSO2NH2 with SF4 at room temperature and in the presence of NaF to remove HF [16]. FSO2NyS(O)F2 was prepared according to the literature method, using SOF4 instead of SF4 [17]. The compounds were purified by repeated distillation and the purity was checked by IR spectra. The electron diffraction intensities were recorded with a Gasdiffraktograph KD-G2 [18] at 25 and 50 cm nozzle-to-plate distances and with an accelerating voltage of ca. 60 kV. The sample reservoirs were kept at 158C (FSO2NySF2) and 2 108C (FSO2NyS(O)F2), respectively, and the inlet system and gas nozzle were at room temperature. The photographic plates (Kodak Electron Image Plates 13 × 18 cm) were analyzed with the usual methods [19].
Scheme 2.
3. Quantumchemical calculations 3.1. FSO2NySF2 The geometries were optimized for various torsional angles around the S–N bond, f (F–S– NyS), with the HF/3-21G*, B3LYP/6-31G* and MP2/6-31G* methods. For the 3-21G* basis an additional set of d-functions was added for nitrogen. The structures corresponding to minima of the potential functions were fully optimized. The HF method predicts two minima at f (F–S–NyS) 08 and 1148. The former minimum is higher in energy by 1.6 kcal mol 21 than the latter one. According to this method internal rotation around the S–N single bond is hindered with a barrier of 2.0 kcal mol 21 at f (F– S–NyS) 1808. The potential functions derived with the B3LYP and MP2 calculations are different from the HF result. They contain only one minimum in the range 08 to 1808 at f (F–S–NyS) 948 and 818, respectively, and the barriers to internal rotation are only about 0.6 kcal mol 21. The geometric parameters of the ground state structure which were obtained by the three methods are given in Table 1. 3.2. FSO2N S(O)F2 Quantumchemical calculations analogous to those described before were performed for structure I of this compound, with the SyO bond of the S(O)F2 group anti with respect to the S–N single bond. Again, the HF method predicts two minima at f (F–S–NyS) 08 and 1108. The former minimum is higher in energy by 2.3 kcal mol 21 and internal rotation is strongly hindered according to this method. The potential function obtained with the B3LYP and MP2 methods contains only one minimum at f 818 and 808, respectively, and barriers of 1.8 kcal mol 21. Additional calculations (HF and B3LYP) were performed for structure II in which the SyO bond of the S(O)F2 group is oriented syn to the S–N bond. Both methods predict two minima for this type of structure with f (F–S–NyS) 1808 (structure IIa) and 658 (HF) or 688 (B3LYP) (structure IIb). Structure IIa is
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
251
Fig. 1. FSO2NySF2: Experimental (dots) and calculated (full line) molecular intensities and differences for long (upper curves) and short nozzle-to-plare distances (lower curves).
calculated to be higher in energy than the global minumum (structure I) by DE 2.0 (HF) and 2.4 kcal mol 21 (B3LYP) and structure IIb by DE 1.7 kcal mol 21 (HF and B3LYP) (see Scheme 3).
Vibrational amplitudes for both compounds were calculated with the program ASYM40 [20]. Cartesian force constants (HF/3-21G* values) for the global minima at f (F–S–NyS) 1148 (FSO2NySF2) and
Fig. 2. FSO2NyS(O)F2: Experimental (dots) and calculated (full line) molecular intensities and differences for long (upper curves) and short nozzle-to-plare distances (lower curves).
252
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
Table 1 Experimental and calculated geometric parameters for FSO2NySF2 GED a SyO NyS2 (S–F)mean DSF (S2–F) 2 (S1–F) S1–F S2–F N–S1 S1–NyS2 N–S1–F1 (N–S1yO)mean DNSO (NSO1) 2 (NSO2) N–S1yO1 N–S1yO2 F1–S1yO NyS2–F F–S2–F f 0(F1–S1–NyS2)
1.407 (2) 1.487 (5) 1.568 (2) 0.022 [5] b 1.553 (4) 1.575 (3) 1.638 (5) 129.9 (8) 95.5 (24) 110.6 (13) 4.0 [20] b 112.6 (17) 108.6 (17) 107.4 (21) 109.7 (8) 92.2 (9) 113(7)
p1 p2 p3
p4 p5 p6 p7
p8 p9 p10 p11
HF/3-21G*
B3LYP/6-31G*
MP2/6-31G*
1.411 1.494 1.557 0.020 1.544 1.564 1.618 127.3 97.7 110.3 1.2 110.9 109.7 106.7 108.7 92.6 114.4
1.443 1.524 1.620 0.025 1.603 1.628 1.693 127.0 96.2 109.9 4.1 111.9 107.8 106.8 109.6 91.7 94.3
1.441 1.520 1.620 0.022 1.605 1.627 1.686 126.2 96.9 109.1 4.4 111.3 106.9 106.7 109.8 91.1 80.5
˚ and degrees, uncertainties are 3s values and include possible systematic errors owing to geometric assumptions. For The ra structure in A atom numbering see Fig. 3. b Value not refined but varied within the estimated uncertainty in brackets. a
f (F–S–NyS) 1108 (FSO2NyS(O)F2) were multiplied with a scaling factor of 0.85 and transformed into symmetry force constants which were then used to derive vibrational amplitudes for different dihedral angles f (F–S–NyS). Amplitudes for the dynamic analysis of the GED intensities of FSO2NySF2 (described later) were calculated without contributions from the torsional vibration around the S–N single bond. Vibrational amplitudes are given in Tables 2 and 4. All quantumchemical calculations were performed with the Gaussian 94 program [21]. 4. Gas phase Structures The radial distribution functions (RDF) were calculated by Fourier transformation of the molecular
intensities. An artificial damping function ˚ 2 was applied to the exp(2g s 2) with g 0.0019 A intensities. Preliminary molecular models which were derived from the RDFs (Figs. 3 and 4) were refined by least-squares fitting of the intensities. The intensities were multiplied with a diagonal weight matrix. Local CS symmetry was assumed for the SF2 and S(O)F2 groups, in accordance with the theoretical calculations which predict only small deviations from this symmetry. Differences between the closely spaced S–F distances (DSF (S1–F) 2 (S2–F)), between the SyO distances in FSO2NyS(O)F2 (DSO (S1yO) 2 (S2yO)) and between the N– SyO angles (DNSO (N–SyO1) 2 (N–SyO2)) were fixed to calculated values. Uncertainties of ˚ and ^28 were estimated for these differ^0.005 A ences and taken into account in the error limits for refined parameters. 4.1. FSO2NySF2
Scheme 3.
In the first step rigid molecular models with the SF2 group syn or anti to the S–N single bond were used in the least-squares analysis. Two structures with syn configuration of the SF2 group and dihedral angles
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
253
Fig. 3. FSO2NySF2: Experimental radial distribution function and difference curve. The positions of important interatomic distances are shown by vertical bars.
Fig. 4. FSO2NyS(O)F2: Experimental radial distribution function and difference curve. The positions of important interatomic distances are shown by vertical bars.
254
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
Table 2 Interatomic distances and vibrational amplitudes of FSO2NySF2 a
SyO NyS2 S–F N–S1 F2…F3 N…F1 F1…O O1…O2 N…F2 N…O S1…S2
Distance
Amplitude GED
Amplitude HF b
1.41 1.49 1.55–1.58 1.64 2.27 2.36 2.39 2.46 2.51 2.48–2.54 2.83
0.035 c 0.038 c 0.041 c 0.044 c 0.065 c 0.069 c 0.061 c 0.054 c 0.068 c 0.063 c 0.062 (4) l1
0.035 0.038 0.041 0.044 0.065 0.069 0.061 0.054 0.068 0.063 0.056
O1…F3 O1…F2 S2…O1 S1…F2 S2…F1 F1…F3 S2…O2 O2…F2 F1…F2 O2…F3
Distance
Amplitude GED
Amplitude HF b
3.08 3.10 3.14 3.29 3.55 3.63 3.82 4.11 4.44 4.59
0.206 c 0.218 c 0.099 (23) l2 0.111 (24) l3 0.165 (38) l4 0.425 c 0.099 (23) P2 0.343 c 0.170 c 0.114 c
0.206 0.218 0.111 0.134 0.178 0.425 0.123 0.343 0.170 0.114
˚ , uncertainties are 3s values. For atom numbering see Fig. 3. Values in A Calculated with HF/3-21G* method. c Value not refined. a
b
f (F–S–NyS) of 164(11)8 and 113(7)8 fitted the experimental intensities almost equally well. These two models are rather similar if the fluorine atom F1 in the FSO2 group is exchanged with one oxygen atom. As fluorine and oxygen possess similar atomic numbers, both models lead to almost identical molecular intensities and the GED experiment cannot distinguish between these two models. On the basis of the three different theoretical calculations which resulted in ground state structures with f (F–S– NyS) between 808 and 1148 we chose the model with f (F–S–NyS) 113(7)8 to be the correct one. As B3LYP and MP2 methods predict a barrier to internal rotation of the FSO2 group around the S–N single bond of only about 0.6 kcal mol 21, we applied in a second step a dynamic model. We used a simple cosine potential V V0[1 2 cos 2(u 0 2 u )], which reproduces the shape of the B3LYP and MP2 potential functions closely. This analysis resulted in f 0 108(8)8 and V0 0.8(6) kcal mol 21. The dihedral angle f 0 which corresponds to the minimum of the potential function agrees within the experimental uncertainty with the value derived in the rigid analysis. The agreement factor for the molecular intensities of the long nozzle-to-plate distance was slightly larger for the dynamic model (R50 3.45%) than for the rigid model (R50 3.38%). All bond lengths and bond angles from the rigid and dynamic analyses agree with each other within their standard deviations.
In the least-squares analysis for the rigid model 11 geometric parameters pi and four vibrational amplitudes lk were refined simultaneously. Amplitudes which caused high correlations or which were badly determined by the GED experiment were fixed to their calculated values. The following correlation coefficients had values larger than u0.6u: p6/p8 2 0.94, p7/p8 0.61, p7/p11 2 0.63, p8/p11 0.88, l2/l3 0.72, l2/l4 0.69 and l3/l4 0.85. The final results for the rigid model are given in Tables 1 and 2. 4.2. FSO2NyS(O)F2 As all three theoretical methods predict high barriers to internal rotation around the S–N single bond (1.8 kcal mol 21 or larger), only a rigid model was used for this compound. In addition to the constraints described before, the NyS1–F1 angle was set to the calculated value, as this parameter caused large correlations between the various bond angles. The refinement of a structure of type I (SyO anti to N–R) resulted again in two possible models with dihedral angles of f (F–S–NyS) 158(8)8 and 107(4)8. The agreement factor R50 for f 107(4)8 is slightly smaller than that for f 158(8)8 (3.07% vs. 3.18%) and the correct model was chosen again on the basis of the theoretical calculations which predict dihedral angles between 808 and 1108. Refinements of structures of type II (SyO syn to N–R) did not result in a satisfactory fit of the experimental
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
255
Table 3 Experimental and calculated geometric parameters for FSO2NyS(O)F2 GED a (SyO)mean DSO (S1yO) 2 (S2yO) S1yO S2yO NyS2 (S–F)mean DSF (S1–F) 2 (S2–F) S1–F S2–F N–S1 S1–NyS2 N–S1–F1 (N–S1yO)mean DNSO (NSO1) 2 (NSO2) N–S1yO1 N–S1yO2 F1–S1yO NyS2yO3 NyS2–F F–S2–F F–S2yO3 f (F1–S1–NyS2)
1.400 (3) 0.012 [5] b 1.404 (5) 1.392 (5) 1.475 (5) 1.535 (2) 0.019 [5] b 1.548 (4) 1.529 (3) 1.631 (6) 125.9 (8) 97.5 c 110.5 (12) 4.8 [20] b 112.9 (16) 108.1 (16) 105.6 (11) 116.8 (22) 111.1 (8) 95.3 (21) 110.2 (14) 107 (4)
p1
p2 p3
p4 p5 p6
p7 p8 p9 p10 p11
HF/3-21G*
B3LYP/6-31G*
MP2/6-31G*
1.406 0.013 1.410 1.397 1.474 1.533 0.012 1.541 1.529 1.613 127.2 97.9 109.8 0.2 109.9 109.7 106.9 119.5 110.4 95.1 109.2 110.1
1.438 0.011 1.442 1.431 1.521 1.594 0.018 1.606 1.588 1.684 124.4 98.1 109.0 5.4 111.7 106.3 106.8 118.1 111.7 94.2 109.4 81.2
1.437 0.011 1.441 1.430 1.514 1.593 0.019 1.606 1.587 1.675 123.6 97.5 108.9 4.8 111.3 106.5 106.8 118.8 111.4 93.9 109.6 79.5
a ˚ and degree, uncertainties are 3s values and include possible systematic errors owing to geometric constraints. For atom The ra structure in A numbering see Fig. 4. b Value not refined but varied within the estimated uncertainty in brackets. c Value not refined.
Table 4 Interatomic distances and vibrational amplitudes of FSO2NyS(O)F2 a
SyO NyS2 S–F N–S1 F2…F3 F1…O1 N…F1 F2…O3 N…O3 N…F2 N…O1,2 O1…O2 S1…S2 O1…F2
Distance
Amplitude GED
Amplitude HF b
1.39–1.40 1.48 1.53–1.55 1.64 2.26 2.35 2.39 2.40 2.44 2.48 2.45–2.53 2.48 2.77 2.99
0.035 c 0.038 c 0.040 c 0.044 c 0.065 c 0.062 c 0.070 c 0.061 c 0.056 c 0.065 c 0.064 (5) l1 0.055 c 0.064 (5) l1 0.225 c
0.035 0.038 0.040 0.044 0.065 0.062 0.070 0.061 0.056 0.065 0.064 0.055 0.061 0.225
S2…O1 O1…F3 S1…F2 S2…F1 F1…F3 S2…O2 S1…O3 O2…F2 F1…F2 O1…O3 O2…F3 F1…O3 O2…O3
˚ , uncertainties are 3s values. For atom numbering see Fig. 4. Values in A Calculated with HF/3-21G* method. c Value not refined. a
b
Distance
Amplitude GED
Amplitude HF b
3.04 3.05 3.23 3.49 3.51 3.79 3.99 4.13 4.35 4.43 4.54 4.56 4.83
0.086 (29) l2 0.223 c 0.115 (28) l3 0.143 (61) l4 0.368 c 0.079 (19) l5 0.064 (5) l1 0.285 c 0.164 c 0.116 c 0.119 c 0.173 c 0.115 c
0.116 0.223 0.143 0.164 0.368 0.103 0.061 0.285 0.164 0.116 0.119 0.173 0.115
256
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
Table 5 ˚ and degree) of RNySF2 and RNyS(O)F2 Skeletal parameters (A compounds
b
ClNySF2 FSO2NySF2 c ClNyS(O)F2 d FSO2NyS(O)F2 c
NyS
S–F
SyO a
R–NyS
1.476 (4) 1.487 (5) 1.484 (7) 1.475 (5)
1.596 (2) 1.575 (3) 1.548 (3) 1.529 (3)
— — 1.394 (3) 1.392 (5)
120.0 (6) 129.9 (8) 114.7 (8) 125.9 (8)
a
In S(O)F2 group. Ref. [1]. c This work. d Ref. [11]. b
intensities. Eleven geometric parameters and five vibrational amplitudes were refined simultaneously and the following correlation coefficients had values larger than u0.6u: p7/p8 0.63, p7/p9 0.61, p7/p10 2 0.83, and p7/p8 2 0.85. The final results are given in Tables 3 and 4.
5. Discussion FSO2NySF2 possesses a structure in which the SF2 group is oriented syn with respect to the N–S single bond. This result is in line with all other compounds of the type RNySF2 (see Section 1) and with ab initio calculations for such a compound, which predict the energy of the anti conformer to be higher by several kcal mol 21 [3]. The same orientation of the SF2 group was determined for FSO2NyS(O)F2. This implies antiperiplanar orientation of the SyO bond relative to the N–S bond (structure I, Scheme 2). The configuration of this compound is equal to those of ClNyS(O)F2 and NCNyS(O)F2. It is confirmed by our quantumchemical calculations which predict the energy of structure II to be higher by 1.7 kcal mol 21 than that of structure I. The torsional orientation of the FSO2 group could not be determined unambiguously by the GED experiment. For both compounds models with f (F–S– NyS) around 1108 and around 1608 fit the experimental intensities almost equally well. The choice of the model has to be based on quantumchemical calculations. All three methods (HF/3-21G*, B3LYP/631G* and MP2/6-31G*) which were applied for the two compounds result in dihedral angles between 808
and 1108. Thus, the GED results of f (F–S–NyS) 113(7)8 in 1 and 107(4)8 in 2 are chosen as “correct” dihedral angles. These parameters which were derived with a rigid model are “effective” dihedral angles owing to a large amplitude torsional vibration. In the case of 1, for which the calculations predict a considerably lower barrier to internal rotation than for 2, a dynamic model leads to a dihedral angle f 0(F–S–NyS) 108(8)8 which coincides with the result from the rigid analysis. Therefore, these “effective” dihedral angles do not deviate drastically from the respective equilibrium values. The torsional orientation of the FSO2 groups in 1 and 2 is such that one SyO double bond (S1yO1) staggers the SF2 group, i.e. eclipses the SyN double bond. The dihedral angles f (O1yS–NyS) are 1(7)8 and 3(4)8 in 1 and 2, respectively. Such eclipsed or nearly eclipsed configuration of SyO and NyY double bonds was observed also for FSO2NCO [13], CF3SO2NCO and CF3SO2N3 [14]. The low level HF/3-21G* approximation reproduces the experimental bond lenghts in both ˚ or better. The high compounds to within ^ 0.02 A level B3LYP and MP2 methods, however, calculate ˚ . The theoall bond lenghts too long by 0.03–0.06 A retical equilibrium distances re should actually be shorter than the experimental mean values ra by ˚ . Considering the experimental about 0.005–0.010 A uncertainties, all three theoretical methods reproduce the bond angles very well. The calculated dihedral angles f (F–S–NyS), however, differ appreciably by about 308. The HF values agree with the experimental results, whereas the B3LYP and MP2 values are too small be 208–308. The skeletal geometric parameters of two RNySF2 and RNyS(O)F2 compounds with R Cl and FSO2 are given in Table 5. Increase of the sulfur oxidation number from S(IV) to S(VI) does not affect the SyN bond lengths which are very similar in these ˚ ). The S–F bond lengths compounds (1.480 ^ 0.005 A ˚ and the R–NyS angles shorten by about 0.04–0.05 A decrease by 48–58 upon increase of the oxidation number.
Acknowledgements H.O. is grateful for financial support by the Fonds
R. Haist et al. / Journal of Molecular Structure 484 (1999) 249–257
der Chemischen Industrie. We thank the Fundacio´n Antorchas (Repu´blica Argentina), Alexander von Humboldt Stiftung and DAAD (Deutscher Akademischer Austauschdienst, Germany) for financial support and for the DAAD-Fundacio´n Antorchas and Alexander von Humboldt Stiftung-Fundacio´n Antorchas Awards to the German-Argentine cooperation. C.O.D.V. also thanks the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), and the Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires (CIC), Repu´blica Argentina, for financial support. He is indebted to the Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Repu´blica Argentina for financial support and to the Fundacio´n Antorchas for the National Award to the Argentinean cooperation.
References [1] J. Haase, H. Oberhammer, W. Zeil, O. Glemser, R. Mews, Z. Naturforsch. 25a (1970) 153. [2] R.R. Karl, S.H. Bauer, Inorg. Chem. 14 (1975) 1859. [3] C. Leibold, E.H. Cutin, C.O. Della Vedova, H.-G. Mack, R. Mews, H. Oberhammer, J. Mol. Struct. 375 (1996) 207. [4] R.M. White, S.R. Bailey, J.D. Graybeal, J.S. Thrasher, M.H. Palmer, J. Mol. Spectrosc. 129 (1988) 243. [5] W.H. Kirchhoff, J. Am. Chem. Soc. 91 (1969) 2437. [6] H. Oberhammer, Z. Naturforsch. 25a (1970) 1497. [7] S. Cradock, E.A.V. Ebsworth, G.D. Meikle, D.W.H. Rankin, J. Chem. Soc. Dalton Trans. (1975) 805.
257
[8] K.I. Gobbato, C.O. Della Vedova, H. Oberhammer, J. Mol. Struct. 350 (1995) 227. [9] R.M. Romano, C.O. Della Vedova, H.-G. Mack, H. Oberhammer, J. Mol. Struct. 440 (1998) 43. [10] H.-G. Mack, H. Oberhammer, C.O. Della Vedova, J. Mol. Struct. 265 (1992) 347. [11] H. Oberhammer, O. Glemser, H. Klu¨ver, Z. Naturforsch. 29a (1974) 901. [12] E.H. Cutin, C.O. Della Vedova, H.-G. Mack, H. Oberhammer, J. Mol. Struct. 354 (1995) 165. [13] C.O. Della Vedova, E.H. Cutin, H.-G. Mack, H. Oberhammer, J. Mol. Struct. 380 (1996) 167. [14] R. Haist, H.-G. Mack, C.O. Della Vedova, E.H. Cutin, H. Oberhammer, J. Mol. Struct. 446 (1998) 197. [15] R.M.S. Alvarez, E.H. Cutin, R. Romano, C.O. Della Vedova, manuscript in preparation. [16] O. Glemser, H.W. Roesky, R.R. Heinze, Angew. Chem. 79 (1967) 153; Angew. Chem. Int. Ed. Engl. 6 (1967) 179. [17] O. Glemser, H.W. Roesky, R.R. Heinze, Angew. Chem. 79 (1967) 723; Angew. Chem. Int. Ed. Engl. 6 (1967) 709. [18] H. Oberhammer, Molecular Structure by Diffraction Methods, Vol. 4, The Chemical Society, London, 1976, p. 24. [19] H. Oberhammer, W. Gombler, H. Willner, J. Mol. Struct. 70 (1981) 273. [20] L. Hedberg, I.M. Mills, J. Mol. Spectrosc. 160 (1993) 117. [21] GAUSSIAN 94 (Revision B.1), M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Peterson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian Inc., Pittsburgh, PA, 1955.
