Molecular structures of tetrabromothiophene and -selenophene as determined by gas-phase electron diffraction and high-level quantum chemical calculations

Journal of Molecular Structure 1030 (2012) 75–82

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Molecular structures of tetrabromothiophene and -selenophene as determined by gas-phase electron diffraction and high-level quantum chemical calculations Yuriy A. Zhabanov a, Christophe M.L. Vande Velde b,c, Frank Blockhuys b, Sergey A. Shlykov a,⇑ a

Department of Physics, Ivanovo State University of Chemistry and Technology, F. Engels Ave. 7, 153000 Ivanovo, Russian Federation Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium c Karel de Grote University College, Department of Applied Engineering, Salesianenlaan 30, B-2660 Antwerp, Belgium b

h i g h l i g h t s " A combined GED/MS experiment was applied for molecular structures study of, C4Br4S and C4Br4Se, Planar structures of C2v equilibrium symmetry were

found for these molecules. Calculations of DFT/B3LYP, MP2 and CCSD levels of theory and molecular dynamics simulations were performed.

a r t i c l e

i n f o

Article history: Received 3 May 2012 Received in revised form 1 July 2012 Accepted 3 July 2012 Available online 10 July 2012 Dedicated to the memory of Prof. Dr. Herman J. Geise and Prof. Dr. Lev V. Vilkov Keywords: Tetrabromothiophene Tetrabromoselenophene Molecular structure Gas-phase electron diffraction Quantum chemical calculation

a b s t r a c t The molecular structures of tetrabromothiophene and tetrabromoselenophene were studied by gasphase electron diffraction and quantum chemical calculations. Calculations at the DFT/B3LYP and MP2 levels of theory confirm that the molecules possess a planar structure in the gas phase. MD simulations were performed for both molecules. Definitive gas-phase molecular structures of tetrabromothiophene and tetrabromoselenophene are reported, based on refinements of electron diffraction data starting from different input geometries. The single-crystal structure of tetrabromoselenophene is also reported. Geometries of all tetrabromochalcogenophenes, C4Br4X (X = O, S, Se, Te), were calculated and trends in the geometrical parameters along the series are discussed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction For several decades, small aromatic rings containing a heteroatom of the chalcogen group have been used as the basic components of conjugated organic materials, and early examples include electrically conducting polymers based on furan [poly(2,5-furanylene vinylene), PFV], thiophene [poly(2,5-thienylene vinylene), PTV] and selenophene [poly(2,5-selenylene vinylene), PSV] [1–3]. PFV being chemically unstable, the introduction of functional groups on the rings or the vinyl spacers of the latter two later became the tools of choice to produce materials with a greater solubility, stability and processibility: in this respect methoxy groups proved particularly successful for PTV [4–6], while alkyl groups were used for PSV [7]. The precursors to ring-substituted ⇑ Corresponding author. Tel.: +7 4932 35 98 74. E-mail address: [email protected] (S.A. Shlykov). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.07.004

PTVs were 3-methoxythiophene and 3,4-dimethoxythiophene, which were prepared from 3-bromothiophene and 3,4-dibromothiophene, respectively, using a copper(I)-catalysed reaction [8]. The latter brominated thiophenes were obtained from 2,3, 5-tribromothiophene and tetrabromothiophene, respectively, via a,a0 -didebromination. Likewise, the procedures to obtain alkylsubstituted PSVs started from brominated selenophenes [7]. As such, these brominated chalcogenophenes constituted the fundamental building blocks of new organic materials with interesting electrical and/or optical properties. The corresponding polymer based on tellurophene is not known, but the ring has been used in tellurophene/benzene co-polymers [9], the preparation of which is, however, not based on brominated tellurophenes. Apart from their usefulness in materials chemistry, perbrominated aromatic rings such as tetrabromothiophene (C4Br4S) and tetrabromoselenophene (C4Br4Se) have also attracted attention from physical chemists due to their somewhat unusual molecular

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composition: indeed, these are two organic molecules of which the usual outside lining of small, hard hydrogen atoms has been replaced by one of large, soft, electron-rich bromine atoms. For C4Br4S in particular, the possible influence of the thus screened p-electron system on the properties has attracted considerable attention and the list of its material characteristics investigated to date is unexpectedly impressive [10–21]. The experimental molecular structure of C4Br4S, however, is conspicuously absent from this list. Its solid-state structure was reported recently [21] but since it was based on X-ray powder diffraction (XRPD) measurements, it proved difficult to obtain a detailed molecular structure. As was discussed in that paper, the particular positioning of the bromine atoms hampers the creation of the long-range order needed to form high-quality single crystals and only a powder could be obtained. Consequently, it seems unlikely that single crystals of C4Br4S and, therefore, an accurate solid-state molecular structure, will ever be obtained. The next best opportunity to obtain an experimental geometry for C4Br4S then involves the molecular structure of its isolated molecules and the latter is reported in this paper. Initially, similar problems were encountered with C4Br4Se, as high-quality single crystals could not be obtained and the compound was included in the same XRPD study [21]. Since then, however, further attempts did lead to crystals of sufficient quality and the solid-state single-crystal molecular structure of C4Br4Se is reported here, together with its gas-phase molecular structure. Gas-phase electron diffraction (GED) and rotational spectroscopy have already been applied to a number of chalcogenophenes, such as furan (C4H4O) [22], the parent thiophene (C4H4S) [23,24] and a number of the latter’s methyl derivatives [25–27], but the gas-phase structures of the four tetrabromochalcogenophenes, C4Br4X (X = O, S, Se, Te), remain unknown. The geometries of tetrabromofuran and C4Br4S have been studied using quantum chemical calculations [20], but their heavier counterparts have again remained uninvestigated. In this work we present the definitive molecular structures of C4Br4S and C4Br4Se in the gas phase, using a combined gas-phase electron diffraction and mass spectrometry approach, in conjunction with high-level quantum chemical calculations. Where possible, a comparison between the gas-phase and solid-state molecular structures will be made.

ment [DsM(s) and Df(r), respectively] are shown in Figs. 1 and 2, respectively. The least-squares analysis of the experimental sM(s) function was performed with a modified version of the KCED program [31]. For both compounds, the mass spectra recorded simultaneously with the diffraction patterns were very similar to those reported for C4Br4S [14,15]. In the electron impact (Uioniz. = 50 V) mass spectra, the most intense peaks were those of the molecular ions C4Br4X+ (X = S or Se). Dissociative ionisation under electron impact was mostly expressed by the removal of one to four bromine atoms from the molecule and ions of composition C4BrnX+ (with n = 0–3) were the most abundant, which is typical for halides. No peaks with masses exceeding that of monomeric C4Br4X species were found in the mass spectra of either compound. 2.2. X-ray crystallography The single-crystal structure of C4Br4Se was determined on a Bruker APEX II area detector diffractometer using Mo Ka radiation (k = 0.71073 Å) and x and u scans; the data reduction and multiscan absorption correction were performed with the SADABS software [32]. The structures were solved by direct methods using SHELXS-97 [33] and refined using SHELXL-97 [33]. CCDC-844933 contains the supplementary crystallographic data for this paper; these can be obtained free of charge from the Cambridge Crystallographic Data Centre. Crystal data for C4Br4Se. M = 446.64, monoclinic, a = 14.7882(9), b = 4.0367(2), c = 13.8203(9) Å, b = 96.330(2)°, V = 819.98(8) Å3, T = 100(2) K, space group P21/c, Z = 4, Z0 = 1, Dc = 3.618 Mg m3, l(Mo Ka) = 23.979 mm1, 11618

(a) C4Br4S Rf =3.52 %

sM(s)

2. Experimental and computational details

ΔsM(s)

Tetrabromothiophene [20] and tetrabromoselenophene [21] were prepared as previously reported. 2.1. Ged/MS The combined gas-phase electron diffraction and mass spectrometric (GED/MS) experiment was carried out using the technique described earlier [28,29]. The vapour effused from a graphite cell with an orifice with internal dimensions of 0.6 mm  1.2 mm (diameter  length). The ratio of the evaporation/effusion areas exceeded 500. The temperatures as measured by a W/Re (5/20) thermocouple were 347(3) and 345(3) K for C4Br4S and C4Br4Se, respectively. The scattered electrons were collected on Kodak Electron Image films of 9  12 cm. Up to six films for each of the two camera distances, L1 = 598 and L2 = 338 mm, were recorded for analysis. The optical densities were measured by a computer-controlled MD-100 (Carl Zeiss, Jena) microdensitometer [30]. The molecular scattering function was evaluated as sM(s) = [I(s)/ G(s)  1]s, where I(s) is the total electron scattering intensity and G(s) the experimental background. Experimental and theoretical molecular scattering intensities sM(s) and radial distribution curves f(r) along with the differences between theory and experi-

0

5

10

15

20

25

30

-1

s, Å

(b) C4 Br4 Se Rf =3.55 %

sM(s)

ΔsM(s)

0

5

10

15

20

25

-1

s, Å

Fig. 1. Experimental (dots) and theoretical (line) molecular scattering intensities sM(s) and the differences (experiment–theory) DsM(s) for (a) C4Br4S and (b) C4Br4Se for the best fits (see Tables 1 and 2).

Yu.A. Zhabanov et al. / Journal of Molecular Structure 1030 (2012) 75–82

(a)

[41], using the CP2K software [42] at the DFT level (PADE/DZVP for C4Br4S and BLYP/6-311G for C4Br4Se). Selected calculated geometrical parameters are listed in Tables 1 and 2 for C4Br4S and C4Br4Se, respectively, and in Table 6 for all tetrabromochalcogenophenes; the numbering of the compounds is shown in Fig. 3.

14

12

13

3

9,10,11 8 6 7

1 2

15,16

C4 Br4S

4,5

19

20

18 17

f(r)

1

2

3

4

5

6

7

r, Å

(b)

C4Br4Se

12

18

3,4,5 8 2

9,10,11

19

1

14 20 13 15,16 17

67

f(r) Δf(r)

0

1

2

3

4

3. Results and discussion 3.1. Quantum chemical calculations

Δ f(r)

0

77

5

6

7

r, Å Fig. 2. Experimental (dots) and theoretical (line) radial distribution curves f(r) and the differences (experiment–theory) Df(r) for (a) C4Br4S and (b) C4Br4Se. The contribution of the individual terms is shown by vertical bars with the numbering given in Tables 3 and 4.

unique reflections (Rint 0.0723) measured. F P 2r(I)] = 0.0623, wR(all F2) = 0.0924.

Final

R1[2346

2.3. Computational details Quantum chemical calculations on the molecules in C2v symmetry were carried out using the PC GAMESS 7.0 version [35] of the GAMESS program [34]. The all-electron aug-cc-pVTZ basis sets supplemented with diffuse s, p, d, and f functions [36,37] were used for the carbon, oxygen and sulphur atoms. For bromine, selenium and tellurium, the core shells [1s22s22p63s23p63d10 (for Br and Se), and 1s22s22p63s23p63d104s24p64d10 (for Te)] were described by relativistic effective core potentials [38], and the aug-cc-pVTZ [39] basis set was used for the description of the valence shells. Moreover, for selenium, apart from the 28-electron core mentioned above, a smaller, 10-electron core [40] was used. The calculations were performed using Density Functional Theory (DFT/B3LYP) and the MP2 method. For C4Br4S and C4Br4Se, the MP2 calculations were carried out in two ways: (i) using all electrons and all molecular orbitals in the calculations (Full) and (ii) using the frozen-core approximation (FC). For C4Br4O and C4Br4Te, only the Full option was used in the MP2 calculations. An additional calculation at the CCSD/cc-pVTZ level using the FC approximation was performed for C4Br4S. For all DFT and MP2 calculations, frequency calculations were performed to verify that the structures are minima on the potential energy surface. Finally, for both C4Br4S and C4Br4Se, geometrical and vibrational parameters were calculated using molecular dynamics (MDs) simulations, a novel method of calculating vibrational corrections which has been recently reported

The geometries of C4Br4S and C4Br4Se calculated using different quantum chemical approaches and shown in Tables 1 and 2 differ from each other depending on the method/basis set combination applied. Addition of diffuse functions, i.e., when aug-cc-pVTZ is used instead of cc-pVTZ, does not lead to significant changes in the geometries of neither C4Br4S nor C4Br4Se, and neither for the DFT nor the MP2 calculations, as the differences do not exceed 0.002 Å for the bond distances and 0.2° for the valence angles; therefore, the results of the DFT and MP2 calculations with the cc-pVTZ basis set are not displayed in Tables 1–4 and 6. In the case of the MP2/aug-cc-pVTZ calculations, the results of those using the FC approximation do display a number of noticeable differences from those obtained with all atomic and molecular orbitals taken into account (the calculations marked ‘Full’ in Tables 1 and 2). For C4Br4S, the FC approximation leads to a larger size of the molecule and the largest differences (0.009 Å) are observed for the C1– C2 and C2–C4 distances, which seems unexpected to some extent. In contrast, the valence angles are not influenced by the use of the FC approximation. For C4Br4Se, similar trends are observed: the C1–C2 and C2–C4 distances increase by 0.008 Å and the valence angle at the selenium atom increases by 0.3°. Recently published results on SeBr2 [43] indicate that, when for heavy atoms (such as, in this case, Se, Br and Te) effective core potentials are used, the size of the core shell may affect the values of the geometrical parameters. For selenium, a small (10 electrons) and a large (28 electrons) core shell have been used in this work in the MP2 calculations on C4Br4Se (Table 2). The data obtained shows that the geometry changes only for the Se–C bond distance (which is reduced by 0.014 Å) and the C–Se–C valence angle (which is increased by 0.5°) when the small core is substituted for the large one. The influence of the computational method on the geometry is considerably larger. Assuming that for C4Br4S the CCSD results can be used as a reference for the less sophisticated DFT and MP2 methods, it can be clearly seen from the three sets of results with the aug-cc-pVTZ basis that while B3LYP overestimates the carbon– bromine bond lengths, the carbon–carbon distances produced by MP2 are much more similar than for either CCSD or B3LYP; both B3LYP and MP2 yield carbon–sulphur distances which differ from the CCSD value by about 0.01 Å. Also, B3LYP produces valence angles which are closer to those from CCSD, even though the largest difference between the latter and MP2 is limited to 0.7°. Similar conclusions can be drawn from the comparison between B3LYP and MP2 for C4Br4Se: there is a larger difference between the carbon–carbon distances and the carbon–bromine and carbon–selenium bonds are considerably longer for B3LYP than for MP2. The values of the angles display the same trends as for C4Br4S. For C4Br4S, the calculated vibrational corrections as obtained from the B3LYP and MP2 force fields appear to be very close to each other for the bonded distances, but those associated with the non-bonded atom pairs differ significantly (Table 3). For C4Br4Se, the agreement between two quantum chemical approaches (DFT and MP2) is much better (Table 4). Note that the vibrational amplitudes and corrections calculated for C4Br4Se (Table 4) are not influenced by the size of the core shell.

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Table 1 Selected geometrical parameters (distances in Å and angles in degrees) of C4Br4S from the results of different computational methods and GED. Calculationsc DFT

MP2

r e , \e 1.734 1.436 1.362 1.872 1.880 91.2 119.7 112.5 123.9 0.074 0.008

S–C4 C1–C2 C2–C4 C4–Br7 C2–Br9 C–S–C S–C3–Br6 C1–C2–C4 C3–C1–Br8 D1a D2a Rf (%)

GEDe b

CCSD

MD

Full

FC

FC

1.714 1.412 1.372 1.852 1.856 91.5 120.5 112.3 123.6 0.040 0.004

1.720 1.421 1.381 1.854 1.858 91.5 120.5 112.3 123.6 0.040 0.004

1.724 1.431 1.353 1.854 1.858 90.9 119.7 112.3 124.0 0.078 0.004

DFT

MP2

MD

Full re 1.762 1.452 1.384 1.902 1.911 – – – – 0.068 0.009

ra 1.767 1.456 1.387 1.908 1.918 – – – – 0.069 0.010

rh1, \h1 1.721(4) 1.444(12) 1.358(6) 1.863(5) 1.872(5) 91.1d 119.9(3) 111.9(4) 124.7(6) 0.086(17) 0.009(7) 3.52

1.725(4) 1.455(14) 1.361(7) 1.871(6) 1.867(6) 90.9d 119.6(4) 111.7(5) 125.5(7) 0.095(20) 0.004(9) 4.07

1.718(5) 1.410(22) 1.367(12) 1.849(7) 1.874(7) 91.4d 120.7(5) 112.6(5) 123.0(9) 0.043(33) 0.025(12) 5.69

a

D1 = r(C2–C4)  r(C1–C2) and D2 = r(C2–Br9)  r(C4–Br7). Molecular dynamics calculations. The equilibrium parameter (re) was calculated using the CP2K software [42] at the DFT/PADE/DZVP level of theory. The effective parameter (ra) was calculated based on the MD results using the approach described in Ref. [41]. c These calculations were performed using the aug-cc-pVTZ (for DFT and MP2) and cc-pVTZ (for CCSD) basis sets. d Dependent parameter. e Least-squares results were obtained using the three different sets of calculated vibrational corrections, starting geometries and vibrational amplitudes. The latter were obtained from the SHRINK program [44] (second approximation). Uncertainties given in parentheses were taken as: [r2scale + (2.5rLS)2]½ for bond distances and 3rLS for bond angles, where rscale = 0.002r and rLS is a standard deviation in least-squares refinement. b

Table 2 Selected geometrical parameters (distances in Å and angles in degrees) of C4Br4Se from the results of different computational methods and GED. Calculationsc DFT

Se–C4 C1–C2 C2–C4 C4–Br7 C2–Br9 C–Se–C Se–C3–Br6 C1–C2–C4 C3–C1–Br8 D1a D2a D3a Rf (%)

r e , \e 1.880 1.444 1.357 1.873 1.887 86.6 120.0 114.7 122.9 0.087 0.007 0.007

GEDe b

MP2

MD

Fullf

Fullg

FCg

1.842 1.416 1.370 1.853 1.860 87.4 121.0 114.4 122.6 0.046 0.011 0.018

1.856 1.417 1.370 1.853 1.861 86.9 120.8 114.5 122.5 0.047 0.003 0.005

1.859 1.425 1.378 1.855 1.863 87.2 120.9 114.4 122.6 0.047 0.004 0.004

re 1.910 1.452 1.373 1.891 1.912 – – – – 0.079 0.019 0.002

DFT

ra 1.913 1.457 1.376 1.904 1.916 – – – – 0.081 0.009 0.003

rh1, \h1 1.867(4) 1.457(3) 1.377(3) 1.874(4) 1.870(4) 86.9d 120.8(3) 113.8(3) 123.8(3) 0.080(10) 0.006(5) 0.003(3) 3.59

XRDh MP2

f

g

MP2

Full

Full

1.868(4) 1.449(3) 1.381(3) 1.872(4) 1.871(4) 86.9d 120.8(3) 113.9(3) 123.4(3) 0.068(11) 0.004(5) 0.002(3) 3.55

1.863(4) 1.446(3) 1.382(3) 1.880(4) 1.869(4) 86.9d 120.6(3) 113.8(3) 124.0(3) 0.064(13) 0.016(6) 0.005(3) 4.00

MD

1.865(7) 1.451(22) 1.374(11) 1.861(10) 1.863(7) 86.8d 120.4(4) 113.9(7) 125.0(9) 0.077(32) 0.004(14) 0.003(8) 4.25

1.886(6)/1.876(5) 1.429(7) 1.342(8)/1.348(6) 1.861(6)/1.855(5) 1.878(5)/1.884(5) 86.4(2) 119.6(3)/120.5(3) 115.6(4)/114.8(5) 122.9(4)/122.1(4)

a

D1 = r(C2–C4)  r(C1–C2), D2 = r(C4–Br7)  r(Se–C4) and D3 = r(C2–Br9)  r(Se–C4). Molecular dynamics calculations. The equilibrium parameter (re) was calculated using the CP2K software [42] at the DFT/PADE/DZVP level of theory. The effective parameter (ra) was calculated based on the MD results using the approach described in Ref. [41]. c These calculations were performed using the aug-cc-pVTZ basis set. d Dependent parameter. e Least-squares results were obtained using the three different sets of calculated vibrational corrections, starting geometries and vibrational amplitudes. The latter were obtained from the SHRINK program [44] (second approximation). Uncertainties given in parentheses were taken as: [rscale2 + (2.5rLS)2]½ for bond distances and 3rLS for bond angles, where rscale = 0.002r and rLS is a standard deviation in least-squares refinement. f 28-Electron core. g 10-Electron core. h C4Br4Se has C1 site symmetry. b

The geometrical and vibrational parameters (amplitudes and vibrational corrections) obtained from the MD simulation appear to be considerably different from those obtained from DFT and MP2 (see Tables 1–4) for both molecules. A possible reason is the smaller basis sets applied as compared with those used for the two other computational methods. 3.2. Gas-electron diffraction All refinements were carried out assuming that C4Br4S and C4Br4Se are planar, as suggested by all quantum chemical calculations. The z-matrices of both molecules were built in such a way as

to obtain from the refinement not only the actual geometric parameters (internuclear distances and valence angles) and vibrational amplitudes, but also differences between similar bond distances. As such, eight independent parameters were refined for each molecule: three bond distances (C1–C2, S5–C4 and C4–Br7), three valence angles (C1–C2–C4, S5–C3–Br6 and C3–C1–Br8) and two differences [D1 = ra(C2–C4)  ra(C1–C2) and D2 = ra(C2– Br9)  ra(C4–Br7)] for C4Br4S, and two bond distances (C1–C2 and Se5–C4), three valence angles (C1–C2–C4, Se5–C3–Br6 and C3–C1–Br8) and three differences [D1 = ra(C2–C4)  ra(C1–C2), D2 = ra(C4–Br7)  ra(Se5–C4) and D3 = ra(C2–Br9)  ra(Se5–C4) for C4Br4Se.

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7

6 X 4

3 1 2 8

C4Br4O

C4Br4S

9

C4Br4Se

C4Br4Te

Fig. 3. Models of the four free tetrabromochalcogenophenes C4Br4X (X = O, S, Se, Te) with the numbering of the atoms; carbon atoms are represented in yellow, bromine atoms in green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 Vibrational amplitudes obtained from the different quantum chemical calculations and from the analysis of the GED data, as well as calculated vibrational corrections for C4Br4S (in Å). Parameter numbera

Terma

GEDd Group number

Calculationsc b

DFT

MP2

MD

l 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

C2–C4 C1–C2 S–C4 C4–Br7 C2–Br9 C1–C4 C3–C4 S–C2 C4–Br9 C2–Br7 C2–Br8 S–Br7 Br8–Br9 Br7–Br9 C4–Br8 C2–Br6 C4–Br6 S–Br9 Br7–Br8 Br6–Br7

1 1 2 2 2 2 3 4 4 4 4 4 5 5 5 6 6 6 6 7

0.044(11) 0.048(11) 0.043(3) 0.042(3) 0.043(3) 0.062(6) 0.067(6) 0.059(6) 0.063(2) 0.064(2) 0.066(2) 0.073(2) 0.118(2) 0.122(2) 0.068(3) 0.067(3) 0.070(3) 0.064(3) 0.086(2) 0.084(2)

0.044(14) 0.045(14) 0.041(3) 0.042(3) 0.042(3) 0.063(8) 0.071(8) 0.061(8) 0.065(3) 0.067(3) 0.067(3) 0.072(3) 0.108(3) 0.123(3) 0.071(3) 0.066(3) 0.068(3) 0.063(3) 0.086(2) 0.085(2)

0.057(11) 0.054(11) 0.038(6) 0.036(6) 0.044(6) 0.055(10) 0.058(10) 0.063(10) 0.070(3) 0.065(3) 0.064(3) 0.069(3) 0.094(2) 0.143(2) 0.057(4) 0.049(4) 0.048(4) 0.067(4) 0.086(3) 0.078(3)

DFT

MP2

MD

l

Corr.e

l

Corr.e

l

Corr.e

0.044 0.049 0.050 0.049 0.049 0.055 0.060 0.052 0.064 0.065 0.066 0.074 0.117 0.121 0.063 0.062 0.065 0.060 0.079 0.077

0. 0. 0.001 0. 0. 0.002 0.001 0.004 0.004 0.005 0.004 0.006 0.005 0.009 0.009 0.009 0.010 0.011 0.018 0.020

0.046 0.047 0.049 0.050 0.050 0.056 0.063 0.054 0.071 0.072 0.072 0.078 0.147 0.163 0.071 0.066 0.068 0.064 0.084 0.083

0. 0. 0.001 0.001 0. 0.002 0.003 0.009 0.007 0.005 0.011 0.012 0.005 0.025 0.013 0.018 0.019 0.020 0.026 0.039

0.055 0.074 0.044 0.052 0.063 0.060 0.049 0.049 0.065 0.062 0.117 0.050 0.077 0.060 0.066 0.121 0.064 0.049 0.079 0.065

0.002 0.006 0. 0.004 0.009 0.001 0. 0. 0.005 0.009 0.005 0.001 0.020 0.011 0.004 0.009 0.004 0. 0.018 0.010

a

See Fig. 3 for the numbering of the atoms and Fig. 2 for the positions of the individual terms in the radial distribution curves. The numbers of the groups containing the vibrational amplitudes of the closely related internuclear distances. The differences between the amplitudes within each group were constrained to the calculated values. c The calculated amplitudes and vibrational corrections, Dr = rh1  ra, were obtained from the force fields produced for the different quantum chemical calculations by the SHRINK program [44], second approximation, at the temperature of the GED experiment. d Vibrational corrections and starting values for the amplitudes were taken from the corresponding calculated values given in the ‘‘Calculations’’ section of this Table. Uncertainties given in parentheses were taken as 3rLS where rLS is a standard deviation in least-squares refinement. e Vibrational corrections which are smaller than 0.0005 are given as zeros. b

The vibrational corrections (rh1  ra) and starting values of vibrational amplitudes were calculated using the SHRINK program [44], using the so-called ‘second approximation’ in which the harmonic approximation is combined with curvilinear coordinates. The force fields obtained from the quantum chemical calculations were used as input for the SHRINK program. The vibrational parameters (amplitudes and corrections) were calculated at the temperatures of the GED experiments. The amplitudes were refined in groups, while the differences between the amplitudes were kept at the values estimated from the specific quantum chemical calculations. The theoretical and refined amplitudes, vibrational corrections and group numberings are listed in Tables 3 and 4 for C4Br4S and C4Br4Se, respectively. Since the different quantum chemical calculations performed in this work yielded somewhat different geometries as well as vibrational parameters (see Tables 1–4), a series of least-squares refinements of GED data were carried out for which the input information, i.e., the vibrational corrections, starting amplitudes and starting geometries (including differences between similar distances), was adopted from the corresponding quantum chemical calculation. The results of the refinements, listed in Tables 1 and 2 for the geometrical parameters and in Tables 3 and 4 for the

root-mean-square vibrational amplitudes, are marked in columns according to the specific method (DFT, MP2 or MD) from which the input information for the refinements was obtained. For C4Br4S, the best fit with the electron diffraction data was obtained with the DFT-input (Rf = 3.52%) and poorer agreements were found for the refinements with the MP2- and MD-inputs (4.07% and 5.69%, respectively) (Table 1). For C4Br4Se, both the DFT- and MP2-inputs (large core) yielded practically the same disagreement factors (3.59% and 3.55%, respectively) (Table 5). Refinement with MP2- (small core) and MD-inputs led to poorer fits, with Rf-values of 4.00% and 4.25%, respectively. All experimentally determined valence angles are in good agreement with the calculated values for both molecules, regardless of the approach applied. Note that a comparison between the results of the MP2 calculations and those of the GED refinements does not allow drawing any evident conclusions about whether calculations using the FC approximation lead to a worse agreement with the experiment than those without (Full). Application of the small 10-electron effective core potential on the selenium atom does not seem to produce more reliable results. Using the associated vibrational corrections and starting vibrational amplitudes in the constraints scheme did not improve the

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Table 4 Vibrational amplitudes obtained from the different quantum chemical calculations and from the analysis of the GED data, as well as calculated vibrational corrections for C4Br4Se (in Å). Par. Noa

Terma

GEDd

Calculationsc b

Gr. no.

f

DFT

MP2–28e

MP2–10e

f

MD

l 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

C2–C4 C1–C2 C4–Br7 Se–C4 C2–Br9 C1–C4 C3–C4 Se–C2 C4–Br9 C2–Br8 C2–Br7 Se–Br7 Br8–Br9 Br7–Br9 C4–Br8 C2–Br6 C4–Br6 Se–Br9 Br7–Br8 Br6–Br7

1 1 2 2 2 3 4 4 4 4 4 5 5 5 6 6 6 6 7 8

0.031(12) 0.036(12) 0.049(2) 0.050(2) 0.050(2) 0.065(20) 0.063(3) 0.054(3) 0.065(3) 0.065(3) 0.067(3) 0.079(2) 0.115(2) 0.120(2) 0.063(2) 0.062(2) 0.066(2) 0.058(2) 0.085(2) 0.079(5)

0.037(10) 0.040(10) 0.049(2) 0.050(2) 0.049(2) 0.064(19) 0.064(3) 0.054(3) 0.065(3) 0.066(3) 0.066(3) 0.078(2) 0.115(2) 0.122(2) 0.063(2) 0.062(2) 0.066(2) 0.058(2) 0.084(2) 0.078(5)

0.037(11) 0.039(11) 0.048(2) 0.051(2) 0.049(2) 0.069(22) 0.066(4) 0.059(4) 0.065(4) 0.066(4) 0.071(4) 0.074(2) 0.085(2) 0.113(2) 0.064(2) 0.064(2) 0.061(2) 0.058(2) 0.085(3) 0.078(5)

MP2–28ef

DFT

0.023(35) 0.028(35) 0.049(3) 0.050(3) 0.046(3) 0.060(4) 0.069(4) 0.063(4) 0.066(4) 0.062(4) 0.073(4) 0.068(2) 0.059(2) 0.096(2) 0.053(3) 0.063(3) 0.067(3) 0.059(3) 0.085(3) 0.077(6)

e

MP2–10ef e

MD

l

Corr.

l

Corr.

l

Corr.e

l

Corr.e

0.044 0.049 0.049 0.050 0.050 0.056 0.062 0.053 0.064 0.064 0.066 0.076 0.113 0.118 0.064 0.063 0.067 0.059 0.080 0.076

0. 0. 0. 0.001 0. 0.002 0.001 0.004 0.004 0.005 0.004 0.006 0.004 0.008 0.009 0.009 0.010 0.011 0.018 0.020

0.045 0.047 0.047 0.048 0.048 0.056 0.062 0.052 0.063 0.064 0.064 0.074 0.112 0.118 0.062 0.062 0.066 0.058 0.078 0.073

0. 0. 0. 0.001 0. 0.002 0.001 0.005 0.004 0.004 0.005 0.007 0.005 0.008 0.009 0.010 0.010 0.011 0.018 0.021

0.045 0.047 0.050 0.047 0.048 0.055 0.060 0.052 0.058 0.059 0.064 0.060 0.072 0.099 0.062 0.062 0.059 0.056 0.070 0.053

0. 0.001 0.002 0. 0. 0.001 0.001 0.004 0.004 0.004 0.006 0.007 0.006 0.007 0.006 0.008 0.008 0.009 0.015 0.019

0.028 0.032 0.039 0.038 0.036 0.039 0.048 0.042 0.045 0.041 0.052 0.058 0.049 0.086 0.041 0.051 0.055 0.047 0.050 0.064

0.004 0.005 0.003 0.013 0.004 0.008 0.013 0.002 0.004 0.012 0.014 0.008 0.033 0.005 0.011 0.015 0.017 0.005 0.023 0.023

a

See Fig. 3 for the numbering of the atoms and Fig. 2 for the positions of the individual terms in the radial distribution curves. The numbers of the groups containing the vibrational amplitudes of the closely related internuclear distances. The differences between the amplitudes within each group were constrained to the calculated values. c The calculated amplitudes and vibrational corrections, Dr = rh1  ra, were obtained from the force fields produced for the different quantum chemical calculations by the SHRINK program [44], second approximation, at the temperature of the GED experiment. d Vibrational corrections and starting values for the amplitudes were taken from the corresponding calculated values given in the ‘‘Calculations’’ section of this Table. Uncertainties given in parentheses were taken as 3rLS where rLS is a standard deviation in least-squares refinement. e Vibrational corrections which are smaller than 0.0005 are given as zeros. f Calculations with small (10-electron) and large (28-electron) effective core potentials for the selenium atom. b

Table 5 Details (distances d in Å, angles h in degrees and symmetry codes) of the short intermolecular C–Br  A contacts in the crystal structure of C4Br4Se. C C1 C1 C2 C2 C4

Br Br8 Br8 Br9 Br9 Br7

A Br8 Br8 Se Se Se

dBr  A 3.5614(8) 3.5614(8) 3.6361(8) 3.5373(8) 3.6868(8)

h 165.15(15) 125.39(15) 170.95(15) 120.20(15) 128.27(16)

Table 6 Selected calculated equilibrium geometrical parameters (distances in Å and angles in degrees) for the four tetrabromochalcogenophenes. C4Br4O

Symmetry code x, ½ + y, 3/2  z x, ½ + y, 3/2  z x, ½  y, ½ + z x, ½  y, ½ + z 1  x, y, 2  z

fit of the GED intensities with the theoretical model, but rather the opposite, as Rf increased from 3.55% to 4.00% (Table 2). This might indicate that relativistic effects for the heavy selenium atom in the case of small core potential should be taken into account. The S–C4 term in C4Br4S (No. 3 in Table 3) and the C2–C4 and C1–C2 terms in both molecules (Nos. 1 and 2 in Tables 3 and 4) contribute very little to the diffraction scattering, as can be seen from the radial distribution curves in Fig. 2. The C4–Br7, Se–C4 and C2–Br9 terms in C4Br4Se (Nos. 3, 4 and 5 in Table 4) are very close to each other, within a range of less than 0.01 Å. As expected, all this introduced additional difficulties into the data refinement, and an attempt was made to minimise the uncertainties by introducing a number of constraints, defined as the differences between closely resembling bond distances. These values, Dr, were added as independent parameters in the least-squares procedures (see above). For C4Br4S, two such differences were refined (Table 1). Despite the large uncertainties on the terms involved in D1 and considering their small contribution to the scattering, one may state that there is agreement between calculations and experiment. The calculated values for D2 are rather small (less than 0.01 Å) but this is again in compliance with the GED values. For C4Br4Se, three such differences were refined. The calculated values for D1

X–C4 C1–C2 C2–C4 C2–Br9 C4–Br7 C1–C2–C4 X–C3–Br6 C3–C1– Br8 C–X–C

C4Br4S a

C4Br4Se a

C4Br4Te a

DFT

MP2

DFT

MP2

DFT

MP2

DFT

MP2a

1.362 1.438 1.357 1.867 1.855 106.0 117.0 127.1

1.356 1.416 1.365 1.845 1.835 106.2 117.7 126.6

1.734 1.436 1.362 1.880 1.872 112.5 119.7 123.9

1.714 1.412 1.372 1.856 1.852 112.3 120.5 123.6

1.880 1.444 1.357 1.887 1.873 114.7 120.0 122.9

1.856 1.417 1.370 1.861 1.853 114.5 120.8 122.5

2.064 1.453 1.356 1.894 1.879 117.2 121.2 121.7

2.045 1.423 1.370 1.868 1.858 117.1 122.1 121.3

106.9

106.8

91.2

91.5

86.6

86.9

81.4

81.6

a

MP2 results were obtained with all electrons and all molecular orbitals used (Full), except for the heavy atoms for which 28-electron (for selenium and bromine) and 46-electron (for tellurium) core potentials were used.

vary from 0.087 to 0.046 Å while the refinements yielded values varying from 0.064(13) to 0.080(10) Å (Table 2). The calculated values for D2 vary from 0.009 Å in the case of MD to +0.011 Å for MP2(Full)/aug-cc-pVTZ (small core). The refinements reproduce this difference. Finally, the calculated values for D3 are small, except the 0.018 Å from MP2(Full)/aug-cc-pVTZ (small core). All the refined values are close to zero. The vibrational amplitudes estimated on the basis of the force fields from the different quantum chemical methods do not differ dramatically from each other in the case of the bonded pairs of atoms (Tables 3 and 4). The refined amplitudes are, in general, in agreement with the calculated values. The most noticeable discrepancies in the amplitudes (between the different calculations and between the calculated and the refined values) were observed

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81

for the non-bonded pairs of atoms, especially those involving the bromine atoms. These heavy atoms, due to their high scattering power, contribute a great deal to the diffraction patterns (see the radial distribution curves in Fig. 2) and have, therefore, a considerable influence on the refinement convergence. Exceptions to this are found for the MD results for both compounds: in general, both the amplitudes and the vibrational corrections estimated from the MD simulations differ considerably from the predictions of other theoretical methods applied (Tables 3 and 4). The largest differences are found for C4Br4Se: even the signs of the vibational corrections are different from those produced by DFT and MP2. Probably for this reason the MD constraints applied in the GED refinements yielded higher R factors and larger uncertainties on the parameters (Tables 1 and 2). 3.3. The molecular and crystal structure of C4Br4Se Previous X-ray diffraction studies of C4Br4S and C4Br4Se were carried out on the powders only [21]. In the present work single crystals have been successfully grown for C4Br4Se, but still no positive results could be obtained for C4Br4S. The solid-state structural data of C4Br4Se have been presented in Table 2. Since C4Br4Se has C1 site symmetry in the crystal, the molecules are somewhat distorted with respect to the C2v structure found in the gas phase, but the differences in bond length between the formally equivalent bonds amounts to no more than 0.010 Å for the Se–C4 bond; the differences for the formally equivalent angles amount to no more than 0.9°. The molecules are quasi-planar: the torsion angles within the ring are limited to about 1° and to about 4° for the bromine atoms. The bond angles determined experimentally by GED and XRD are quite close and quite similar to those obtained by the quantum chemical calculations. The solid-state bond distances, however, differ more substantially from the GED and calculated values: while the Se–C bonds are considerably longer in the crystal than in the gas phase, all other bonds are shorter. It is clear that the latter differences are due to the crystal environment and the specific intermolecular interactions between the molecules. platon [45] was used to calculate all intermolecular contacts and the results have been presented in Table 5; Fig. 4 was prepared using mercury [46]. The bromine atoms in the bpositions of the ring (Br8 and Br9) are each involved in two intermolecular contacts: Br8 contacts two other Br8 atoms [47] from two different neighbouring molecules in type II halogen  halogen interactions [48], while Br9 interacts with two selenium atoms from another two neighbouring molecules. Br7 is intermolecularly involved in a reciprocal contact with a selenium atom of a fifth neighbouring molecule. Interestingly, the other a-bromine atom, Br6, is not involved in intermolecular contacts, most likely due to steric factors. This and the differences between the supramolecular organisations of C4Br4S and C4Br4Se have been discussed previously [21]. 3.4. The influence of the chalcogen atom Finally, we present the results of the quantum chemical calculations on the molecular structures of the four tetrabromochalcogenophenes C4Br4X (X = O, S, Se, Te) at levels which are higher than previously published; the results are listed in Table 6. The four molecules are planar in their free state. From a comparison of the geometrical parameters of the four C4Br4X it may be noted that the ring proportions are changed in the series O ? Te: the 0 C–X bond length increases in the series O ? Te by about 0.7 Å A, while the other ring distances are almost unchanged. The valence angles change continuously in the chalcogen series: \(C–X–C) decreases by about 22°, \(C1–C2–C4) increases from 106° to 117°,

Fig. 4. Weak intermolecular Br  Br and Br  Se interactions in the solid-state structure of C4Br4Se; see Table 5 for details.

\(X–C3–Br6) and \(C3–C1–Br8) change by about 5°, increasing and decreasing, respectively. 4. Conclusions The gas-phase molecular structures of tetrabromothiophene (C4Br4S) and tetrabromoselenophene (C4Br4Se) have been determined experimentally using GED/MS. The molecules possess planar gas-phase structures of C2v equilibrium symmetry and are stable in the gas phase up to at least 347(3) and 345(3) K, respectively, i.e., the temperatures at which the measurements have been performed. Likewise, the solid-state molecular structure of C4Br4Se has been determined from single-crystal XRD: in the solid the molecules display small deviations from the planar structure due to a number of intermolecular Br  Br and Br  Se contacts. In addition, the structures of the isolated molecules, as well as those of tetrabromofurane (C4Br4O) and tetrabromotellurophene (C4Br4Te), were studied at the DFT/B3LYP, MP2 and CCSD levels of theory and using molecular dynamics simulations; all structures are planar and the effects of the chalcogen on the geometrical parameters have been discussed. The perbrominated chalcogenophenes considered in this work may be also compared with the parent compounds in terms of the distortion of the rings by the presence of the heavy substituents. The approach combining data from GED, MW and liquid crystal NMR applied for furane [22] and thiophene [24], and the MW data on selenophene [49] and tellurophene [50] all demonstrate the absence of a noticeable influence of the bromine atoms on the bond lengths and angles in the rings. This paper represents the final chapter in our research on the structure of tetrabromothiophene [20,21]. Acknowledgements Financial support by the University of Antwerp under Grant GOA-2404 and by the Ministry of Education of the Russian Federation, federal target program ‘‘Scientific and scientificpedagogical personnel of innovative Russia’’ for 2009 – 2013, grant no. 2012-1.3.2-12-000-1014-688 are gratefully acknowledged. The authors thank Dr. A.V. Zakharov and Dr. N.V. Belova (ISUCT) for the helpful discussions and Dr. L. Straver of Bruker AXS for collecting the single-crystal data set. References [1] T. Dierick, D. De Schrijver, W. Eevers, H.J. Geise, Bull. Soc. Chim. Belg. 100 (1991) 631.

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