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Molecular structure and conformational composition of thiophene-2-carbonyl chloride as determined by gas phase electron diffraction
Journal Elsevier
of Molecular Structure, 145 (1986) Science Publishers B.V., Amsterdam
359-366 - Printed
in The Netherlands
MOLECULAR STRUCTURE AND CONFORMATIONAL COMPOSITION OF THIOPHENE-Z-CARBONYL CHLORIDE DETERMINED BY GAS PHASE ELECTRON DIFFRACTION
KOLBJQRN Department (Received
AS
HAGEN of Chemistry, 3 February
AVH,
University
of Trondheim,
N-7000
Trondheim
(Norway)
1986)
ABSTRACT Thiophene-2carbonyI chloride has been investigated by electron diffraction from the gas at a nozzle temperature of 363 K. The more stable of the two conformers identified is the planar form with the carbonyl oxygen and the sulfur atom of the thiophene ring syn to each other; the less stable conformer is the planar (or near planar) anti form. Assuming that the only geometrical difference between the two forms is their S-C-C=0 torsion angle and assuming the thiophene ring to have C,, s y mmetry, the results for the most important distances (t-,-values) and angles (ia) are: r(C-H) = 1.070(38) A, r(C=O) = 1.194(7) A, r(C=C) = 1.380(11) A, r(C-C) (in the ring) = 1.410(23) a, r(C-COCI) = 1.475(20) A, r(C-S) = 1.713(6) a, r(C-Cl) = 1.796(11) a, IC=C-COCl = 127.0(1.6)“, LC=C-H = 131.0(8.2)“, IC-C=O = 126.5(1.3)“, LC+C--CI = LC=C-s = 112.2(8)“, 112.9(1.2)“, LC-S-C = 91.4(7)“; the quantities in parentheses are estimated 2 e values. At 363 K the observed proportion of the conformer with S and 0 syn is 59(11)%, and the syn conformer has a r.m.s. torsional amplitude of 7 = 20.6(6.7)“. The remaining vibrational amplitudes are calculated from an assumed force field. The results are compared with those for related thiophene and furan compounds. INTRODUCTION
Different conformational compositions have been observed in 2-substituted furan and thiophene carbonyl compounds. In furan-2aldehyde (furfural), the conformer with the carbonyl oxygen anti to the oxygen atom of the furan ring is reported to be the major form in the gas phase with 69 + 9% present at 50°C [l] . For the corresponding acid chloride, furan-2-carbonyl chloride (2-furoyl chloride), the relative stability of the anti and syn conformers is reversed with 70 k 14% of the molecules observed as having the two oxygen atoms syn at 85°C [2]. Although several spectroscopic investigations [ 3-81 on thiophene-2-aldehyde report only one conformer, both NMR [9], matrix isolation IR [lo] and gas phase electron diffraction (ED) [lo] have shown that this compound also is a conformational mixture. Here the major conformer was observed to have 0 and S syn with 81 + 8% of this form being present in gas phase at 94°C. Substituting the aldehyde proton with chlorine increased the stability of 0022-2860/86/$03.50
o 1986
Elsevier
Science
Publishers
B.V.
360
the syn conformer in the fur-an compounds. If this is true also for the thiophenes, hardly any of the anti form would be expected to be found in thiophene-2carbonyl chloride. However, there are indications that this compound also contains a mixture of conformers [ 111. (See Fig. 1 for description of the possible conformers). We have investigated thiophene-2-carbonyl chloride using gas phase electron diffraction and our results are reported here. EXPERIMENTAL
AND
DATA
REDUCTION
Thiophene-2-carbonyl chloride was kindly provided by D. J. Chadwick, University of Liverpool. The purity was shown by GC to be >99%. Electron diffraction photographs were recorded with the Oslo electron diffraction unit [12] on Kodak Electron Image plates with a nozzle temperature of about 90°C. The electron wavelength (0.06475 a) was calibrated against diffraction patterns of benzene [ 131. Optical densities were measured by a Joyce Loebl microdensitometer. For nozzle-to-plate distances of 48.498 and 20.498 cm, 5 and 6 plates, respectively, were selected for analysis. Data were reduced in the usual way [ 14-161, and a calculated background [ 171 was subtracted from the data for each plate to yield experimental molecular intensity curves in the form sl, (s). Data from the long and short camera distances were obtained over the ranges 2.00 < s < 19.00 and 8.00 < s < 38.00 P, respectively, at intervals As = 0.25 8-l. The average experimental intensity curves are shown in Fig. 2; the individual curves and backgrounds are available as supplementary material [ 181. The atomic scattering and phase factors used were obtained from the tables of Schafer et al. [19]. STRUCTURE
ANALYSIS
An experimental radial distribution (RD) the usual way by Fourier transformation of 2, l A_d l A2 lexp (-Bs’) with B = 0.0015 calculated for conformers with the oxygen
curve (Fig. 3) was calculated in the function 1; (s) = sl;, (s) -2, A*. Theoretical RD curves were and sulfur atoms syn (@ = 0”)
Hz
Hz
I H3\
Cl
C3/
/ H3\
“‘\\
c3/
“‘21
O
C! G-c
Cl
II
\
/
H/c4-s 4 Fig. 1. Diagrams of atomic numbering.
SYn
!I
5
/
;
0 H/+s 4
the syn and anti conformers
AntI
Cl
of thiophene-2-carbonyl
chloride
with
361
Fig. 2. Intensity camera distances.
curves, sl,(s). Experimental curves are averages of all plates for the two The theoretical curves were calculated from the parameters in Table 1.
or anti (@ = 180”) to each other, using reasonable values for bond distances and valence angles. The torsion sensitive distances are all larger than 2.9 A, and the outer part of the experimental RD curve is shown in Fig. 4 together with theoretical curves for the syn and anti conformers and for a mixture of the two conformers. From these curves it is obvious that thiophene-2carbonyl chloride at 363 K is a mixture of the syn and anti isomers in a ratio of approximately 60:40. Refinements of the structure were carried out by the least squares method [20] based on intensity curves by adjusting one theoretical curve to the two
b---,
0
,I,
1,
I
I,,
)“/‘/I
,“,
2
,I
_m_r_-r7
3
TT
4
,-I
5
r_--j r,A
6
Fig. 3. Radial distribution curves calculated from the intensity curves multiplication of sZm(s) by 2, -Z,-,/Ac-Acl lexp (-B.9) with B = 0.0015 intensity data were used in the experimental curve for s < 2.0 A-‘.
of Fig. 1 after A2. Theoretical
362
i 6 Fig. 4. Theoretical radial distribution curves for conformers with S and 0 either syn or anti to each other and for a mixture of the two conformers together with the experimental RD curve. Only the conformationally important part of the curves is shown.
average experimental curves using a unit weight matrix. Assuming the two conformers to be different in their geometry only in the S-C-C=0 torsion angle 4, and assuming the thiophene ring to have Czv symmetry, the structure of thiophene-2carbonyl chloride can be described by 13 geometrical parameters, in our refinements taken as: r(C-H), r(C=O), r(C-S), r(C,-CS), (r(C-C)) = 0.5(r(C1=C2) + r(C,-C,)), Ar(C-C) = r(C2-C,) - r(C,=&), r(C-Cl), LC2=C1-C5, LC=C-S, LC=C-H, LC-C=O, LC-C-Cl and 4, the S-C-C=0 torsion angle. Vibrational amplitudes (I), perpendicular amplitude corrections (K) and centrifugal distortion constants (6,) were calculated using a valence force field developed for thiophene-2-aldehyde [lo] with the necessary modification for the chloride substitution. The vibrational amplitudes were kept constant at the calculated values in the least squares refinements. For the syn conformer a dynamic model was used where this conformer was represented by five distinct forms with different torsion angles, each weighted according to a Gaussian potential function. The r.m.s. amplitude for the torsional vibration (7) was introduced as a parameter and it was determined in the least-squares refinements. The final results of the refinements are given in Table 1, and theoretical intensity and RD curves calculated from these results, together with experimental and difference curves, are shown in Figs. 2 and 3. Table 2 gives the correlation matrix for the parameters refined.
363 TABLE
1
Final parameter
values for the structure
Parameter
r, or i,
1CdC
Parameter
r( C-H) r(C=O) r( C-S)
1.070( 38) 1.194(7) 1.713(6) 1.475(20) 1.395(8) 0.030( 33) 1.796(11) 127.0( 1.6) 112.2(8) 131.0(8.2) 126.5( 1.3) 112.9(1.2) 20.6(6.7) 166.1( 15.9) 58.9(11.0)
0.077 0.038 0.050 0.049
Selected f.c,=c,-c, L C-S-C f.s--C,-c, r(C=C)
r(C,-C,) W(C-C))b A r( C-C)c r( C-Cl) LC,=C,-c, LC=C-s LC=C-H LC-c=o L c-c-c1 r
d
Lee % syn
0.052
of thiophene-2carbonyl
chloridea
ra or L, dependent
r(C,-C,) r(C, . ..C.) r(C,***C,) r(C, . ..S) r(C,..aS) r(C, ..*C,) r(C, . ..O) r(C,*.*Cl) r(C, ..*C,) r(C,. . *C,) r(O***Cl) r(C, ..*Cl) r(C, *..Cl) r(C,. . ‘Cl) r(S*..Cl) r(C, . ..O) r(C,* * *O) r(C,* * 0 0) r(S...O) r(C, ..*Cl) r(C, ..*Cl) r(C, ..*Cl) r(S.**Cl) r(C, . ..O) r(C,.*.O) r(C,. * * 0) r(S*. -0)
>
>
J
angles and distances 112.2(5) 91.4(7) 120.811.2) 1.380( 11) 1.410(23) 2.315(8) 2.553(33) 2.571( 10) 2.770(17) 2.451( 16) 2.386(18) 2.729( 19) 3.739( 16) 3.891( 17) 2.610( 19) 3.047( 35) 4.453(33) 5.076(25) 4.361(12) wn 3.651(19) 4.692(15) 4.559(19) 3.097( 11) 4.059( 30) 4.967(20) 4.657(22) 3.072(44) 3.026(27) anti 4.384(22) 4.815(22) 3.896(20)
1c ale
0.044 0.049 0.055 0.070 0.051 0.073 0.059 0.058 0.074 0.067 0.068 0.064 0.147 0.147 0.110 0.075 0.062 0.072 0.105 0.128 0.075 0.095 0.131 0.148 0.106 0.101 0.078 0.068
aDistances (ra) and vibrational amplitudes (I) in Angstroms, angles in degrees. Parenthesized uncertainties are 20 and include estimates of systematic errors and correlation in the experimental data. bW(C+)) = O.S(r(C,=C,) + r(C,-C,)). ‘Ar(C--C) = r(C,-C,) do is the r.m.s. torsional amplitude for the syn conformer. Ed@ is the S-C-C=0 r(C,=C,). torsion angle for the form with 0 and S anti to each other.
DISCUSSION
Our investigation shows that thiophene-2-carbonyl chloride in the gas phase is a mixture of substantial amounts of both syn and anti conformers. This concurs with the results observed in solutions by Chadwick et al. [9]. In Table 1 we have reported a torsion angle of 166 + 16” for the anti form. This was the result obtained with a torsionally stiff model. Our results are also consistent with a planar conformer undergoing large torsional motion (like the syn form) or with a molecule having a low torsional barrier at the planar
2
0.0011 0.0093 0.0047 0.0017 0.0026 0.53 0.25 2.61 0.42 0.37 2.16 6.33 3.56
r( C-S) r( C-H)
aStandard
deviations
LC,=C,-Cl LC=C-s L C=C-H LC-c=o L c-c-c1 7 Lo % syn
r( C=O) r( C-Cl)
tic,-C,)
0.0080
Ar(c--c)
2 3 4 5 6 7 8 9 10 11 12 13 14 15
100
r,
82 100
from least squares.
0.0018
W(C-C))
1
‘7L.$
x 100 for the parameters
Parameter
matrix
No.
Correlation
TABLE
2 100
-2
-10 -12 10 100
-26 -42 -22 -16 100
for thiophene-2-carbonyl
-1 -5 -4 30 2 100
12 22 -18 -21 14 -3 100
chloride
-28 -42 10 -6 58 11 6 100
47 72 -13 -6 -50 -5 11 -61 100 0 -21 23 -1 4 15 -12 100
-3 -5
10 20 -3 -11 -14 -4 35 -17 38 32 100
6 9 8 13 -31 21 -18 9 27 -29 -44 100
8 11 -2 -3 6 3 17 -11 7 3 -3 1 100
-3 -9 -12 -18 37 10 25 39 -12 34 14 2 9 100
-10 16 -2 I 15 -8 3 1 -6 27 16 100
8 3 5
;;: &
365
position. The ED experiment cannot easily distinguish between these possibilities. When a dynamic model, like the one described for the syn conformer, was used also for anti, a value of 7,,ti = 18 f 14” was obtained. The fit between experimental and theoretical curves was equally good for this as for the alternative model. If we assume the two conformers of thiophene-2-carbonyl chloride to have the same entropy, a not unreasonable assumption since 7anti = rSsyn,the obseved conformational composition corresponds to an energy difference between anti and syn of 1.1 + 1.4 kJ mol-‘. This value can be compared with 4.3 * 1.6 kJ mol-’ for thiophene-2aldehyde [lo], 2.1 A 1.6 kJ mall’ for furan-2-carbonyl chloride [2] and -4.2 f 0.8 kJ mol-’ for furan-2aldehyde [ 11. In the furan compounds increased stability was observed for the syn conformer when the aldehyde proton was substituted by a chlorine atom, while the opposite effect was found for the thiophenes. Part of the reason for this difference may be the existence of an attraction between the aldehyde proton and the furan oxygen. The corresponding interaction between the thiophene sulfur and the aldehyde proton would be much smaller, and the anti conformer would therefore be relatively more stable in thiophene-2aldehyde. In Table 3 some of the geometrical parameters obtained for thiophene-2carbonyl chloride are compared with those for the related thiophene and furan compounds. The thiophene rings have similar geometry in both the acid chloride and the aldehyde; bond distances and valence angles are all equal within error limits. The parameter values are also very similar to those in thiophene itself [ 21-231. TABLE Parameter
3 values for thiophene-2-carbonyl
Parameter
Thiophene-2carbonyl chloride
r(C,---Cd r( C=C) r(C,-C,)
r(C=O) r(C-X) r( C-Cl) LC,-c,=c, IC,=C,-x IC-c=o L c-c-c1 LC-x-c Mb % syn Ref.
(X = S, 0)
1.410(23) 1.380( 11) 1.475(20) 1.194(7) 1.713(6) 1.796( 11) 127.0(1.6) 112.2(8) 126.5(1.3) 112.9( 1.2) 91.4(7) 166.1(15.9) 59(11) This work
chloride Thiophene-2aldehyde 1.431(15) 1.375(7) 1.466(16) 1.224(7) 1.7 17(4) 126.4(1.3) 111.8(4) 123.7(9) 92.0(4) 158.5(23.1) 81(8) 10
and some related Furan-2carbonyl
moleculesa
chloride
1.426(8) 1.357(8) 1.465( 13) 1.207(6) 1.378(10) 1.787(6) 131.6(g) 110.9(4) 125.8(8) 111.8(6) 105.6(8) 153.2(11.6) 70(14) 2
aDistances (ra) in Angstroms, angles in degrees. Quantities in square brackets in the refinements. b~~$ is the X-C-C=0 torsion angle of the anti conformer
Furan-2aldehyde [ 1.4311 [1.361] 1.435(7) 1.212(4) [1.362] 131.7(4) [110.7] 122.7(8) [106.5] [lo81 31(9) 1 were assumed (see text).
366
The two acid chlorides have a larger LC-00 and a shorter r(C=O) than the two aldehydes, but this is a difference observed in aimost all molecules of this type. Because of the difference between r-(C-S) and r(C-0), the angles in the thiophene rings are of course quite different from the corresponding angles in the furans. A significant difference is also observed for LC2=C1-C5 where the substituted furans have a larger angle (131.6-131.7”) than the thiophene carbonyl compounds (126.4-127.0”). When comparing the geometrical parameters of these four molecules, it should, however, be kept in mind that our model assumes that the syn and anti conformers have the same geometry apart from the C-C torsion angle and this assumption may have some effect on the values of the parameters. ACKNOWLEDGEMENTS
I am grateful to Dr. D. J. Chadwick, University of Liverpool, for providing the sample of thiophene-2-carbonyl chloride and to Siv.ing R. Seip and Ms. S. Gundersen, University of Oslo, for help with the electron diffraction experiment. Financial support from the Norwegian Research Council for Science and the Humanities is acknowledged. REFERENCES 1 G. Schultz, I. Fellegvari, M. Kolonits, A. I. Kiss, B. Pete and J. Banki, J. Mol. Struct., 50 (1978) 325. 2 K. Hagen, J. Mol. Struct., 130 (1985) 255. 3 F. Monnig, H. Dreizler and H. D. Rudolph, Z. Naturforsch., Teil A, 20 (1965) 1323. 4 J. F. Bertran, E. Ortiz and L. Ballester, J. Mol. Struct., 17 (1973) 161. 5 M. L. Martin, C. Andrieu and G. J. Martin, Bull. Sot. Chim. Fr., (1968) 698. 6 B. Roques and M. C. Fournie-Zaluski, Org. Magn. Reson., 3 (1971) 305. 7 S. Combrisson, B. Roques, P. Rigny and J. J. Bassilier, Can. J. Chem., 49 (1971) 905. 8 S. Nagata, Y. Yamabe, K. Yoshikawa and H. Kato, Tetrahedron, 29 (1973) 2545. 9 D. J. Chadwick, G. D. Meakins and E. E. Richards, Tetrahedron Lett., 36 (1974) 3183. 10 G. 0. Braathen, K. Kveseth, C. J. Nielsen and K. Hagen, J. Mol. Struct., 145 (1986) 45. 11 D. J. Chadwick, J. Chambers, G. D. Meakins and R. L. Snowden, J. Chem. Sot., Perkin Trans. 2, (1976) 1. 12 0. Bastiansen, 0. Hassel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. 13 K. Tamagawa, T. Iijima and M. Kimura, J. Mol. Struct., 30 (1976) 243. 14 K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 1003. 15 G. Gundersen and K. Hedberg, J. Chem. Phys., 51 (1969) 2500. 16 B. Andersen, H. M. Seip, T. G. Strand and R. Stdlevik, Acta Chem. Stand., 23 (1969) 3224. 17 L. Hedberg, Abstr. Fifth Austin Symp. Gas Phase Mol. Struct., Austin, TX, March 1974, p. 37. 18 Available from BLLD as Supplementary Publication No. SUP 26310 (7 pages) (see J. Mol. Struct., 34 (1976) 322 for further details of B.L.L.D.) 19 L. Schafer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 55 (1971) 3055. 20 K. Hedberg and M. Iwasaki, Acta Crystallogr., 17 (1964) 529. 21 B. Bak, D. Christensen, L. Hansen-Nygaard and J. Rastrup-Andersen, J. Mol. Spectrosc., 7 (1961) 58. 22 R. A. Bonham and F. A. Momany, J. Phys. Chem., 67 (1963) 2474. 23 W. R. Harshbarger and S. H. Bauer, Acta Crystallogr., Sect. B, 26 (1970) 1010.
of Molecular Structure, 145 (1986) Science Publishers B.V., Amsterdam
359-366 - Printed
in The Netherlands
MOLECULAR STRUCTURE AND CONFORMATIONAL COMPOSITION OF THIOPHENE-Z-CARBONYL CHLORIDE DETERMINED BY GAS PHASE ELECTRON DIFFRACTION
KOLBJQRN Department (Received
AS
HAGEN of Chemistry, 3 February
AVH,
University
of Trondheim,
N-7000
Trondheim
(Norway)
1986)
ABSTRACT Thiophene-2carbonyI chloride has been investigated by electron diffraction from the gas at a nozzle temperature of 363 K. The more stable of the two conformers identified is the planar form with the carbonyl oxygen and the sulfur atom of the thiophene ring syn to each other; the less stable conformer is the planar (or near planar) anti form. Assuming that the only geometrical difference between the two forms is their S-C-C=0 torsion angle and assuming the thiophene ring to have C,, s y mmetry, the results for the most important distances (t-,-values) and angles (ia) are: r(C-H) = 1.070(38) A, r(C=O) = 1.194(7) A, r(C=C) = 1.380(11) A, r(C-C) (in the ring) = 1.410(23) a, r(C-COCI) = 1.475(20) A, r(C-S) = 1.713(6) a, r(C-Cl) = 1.796(11) a, IC=C-COCl = 127.0(1.6)“, LC=C-H = 131.0(8.2)“, IC-C=O = 126.5(1.3)“, LC+C--CI = LC=C-s = 112.2(8)“, 112.9(1.2)“, LC-S-C = 91.4(7)“; the quantities in parentheses are estimated 2 e values. At 363 K the observed proportion of the conformer with S and 0 syn is 59(11)%, and the syn conformer has a r.m.s. torsional amplitude of 7 = 20.6(6.7)“. The remaining vibrational amplitudes are calculated from an assumed force field. The results are compared with those for related thiophene and furan compounds. INTRODUCTION
Different conformational compositions have been observed in 2-substituted furan and thiophene carbonyl compounds. In furan-2aldehyde (furfural), the conformer with the carbonyl oxygen anti to the oxygen atom of the furan ring is reported to be the major form in the gas phase with 69 + 9% present at 50°C [l] . For the corresponding acid chloride, furan-2-carbonyl chloride (2-furoyl chloride), the relative stability of the anti and syn conformers is reversed with 70 k 14% of the molecules observed as having the two oxygen atoms syn at 85°C [2]. Although several spectroscopic investigations [ 3-81 on thiophene-2-aldehyde report only one conformer, both NMR [9], matrix isolation IR [lo] and gas phase electron diffraction (ED) [lo] have shown that this compound also is a conformational mixture. Here the major conformer was observed to have 0 and S syn with 81 + 8% of this form being present in gas phase at 94°C. Substituting the aldehyde proton with chlorine increased the stability of 0022-2860/86/$03.50
o 1986
Elsevier
Science
Publishers
B.V.
360
the syn conformer in the fur-an compounds. If this is true also for the thiophenes, hardly any of the anti form would be expected to be found in thiophene-2carbonyl chloride. However, there are indications that this compound also contains a mixture of conformers [ 111. (See Fig. 1 for description of the possible conformers). We have investigated thiophene-2-carbonyl chloride using gas phase electron diffraction and our results are reported here. EXPERIMENTAL
AND
DATA
REDUCTION
Thiophene-2-carbonyl chloride was kindly provided by D. J. Chadwick, University of Liverpool. The purity was shown by GC to be >99%. Electron diffraction photographs were recorded with the Oslo electron diffraction unit [12] on Kodak Electron Image plates with a nozzle temperature of about 90°C. The electron wavelength (0.06475 a) was calibrated against diffraction patterns of benzene [ 131. Optical densities were measured by a Joyce Loebl microdensitometer. For nozzle-to-plate distances of 48.498 and 20.498 cm, 5 and 6 plates, respectively, were selected for analysis. Data were reduced in the usual way [ 14-161, and a calculated background [ 171 was subtracted from the data for each plate to yield experimental molecular intensity curves in the form sl, (s). Data from the long and short camera distances were obtained over the ranges 2.00 < s < 19.00 and 8.00 < s < 38.00 P, respectively, at intervals As = 0.25 8-l. The average experimental intensity curves are shown in Fig. 2; the individual curves and backgrounds are available as supplementary material [ 181. The atomic scattering and phase factors used were obtained from the tables of Schafer et al. [19]. STRUCTURE
ANALYSIS
An experimental radial distribution (RD) the usual way by Fourier transformation of 2, l A_d l A2 lexp (-Bs’) with B = 0.0015 calculated for conformers with the oxygen
curve (Fig. 3) was calculated in the function 1; (s) = sl;, (s) -2, A*. Theoretical RD curves were and sulfur atoms syn (@ = 0”)
Hz
Hz
I H3\
Cl
C3/
/ H3\
“‘\\
c3/
“‘21
O
C! G-c
Cl
II
\
/
H/c4-s 4 Fig. 1. Diagrams of atomic numbering.
SYn
!I
5
/
;
0 H/+s 4
the syn and anti conformers
AntI
Cl
of thiophene-2-carbonyl
chloride
with
361
Fig. 2. Intensity camera distances.
curves, sl,(s). Experimental curves are averages of all plates for the two The theoretical curves were calculated from the parameters in Table 1.
or anti (@ = 180”) to each other, using reasonable values for bond distances and valence angles. The torsion sensitive distances are all larger than 2.9 A, and the outer part of the experimental RD curve is shown in Fig. 4 together with theoretical curves for the syn and anti conformers and for a mixture of the two conformers. From these curves it is obvious that thiophene-2carbonyl chloride at 363 K is a mixture of the syn and anti isomers in a ratio of approximately 60:40. Refinements of the structure were carried out by the least squares method [20] based on intensity curves by adjusting one theoretical curve to the two
b---,
0
,I,
1,
I
I,,
)“/‘/I
,“,
2
,I
_m_r_-r7
3
TT
4
,-I
5
r_--j r,A
6
Fig. 3. Radial distribution curves calculated from the intensity curves multiplication of sZm(s) by 2, -Z,-,/Ac-Acl lexp (-B.9) with B = 0.0015 intensity data were used in the experimental curve for s < 2.0 A-‘.
of Fig. 1 after A2. Theoretical
362
i 6 Fig. 4. Theoretical radial distribution curves for conformers with S and 0 either syn or anti to each other and for a mixture of the two conformers together with the experimental RD curve. Only the conformationally important part of the curves is shown.
average experimental curves using a unit weight matrix. Assuming the two conformers to be different in their geometry only in the S-C-C=0 torsion angle 4, and assuming the thiophene ring to have Czv symmetry, the structure of thiophene-2carbonyl chloride can be described by 13 geometrical parameters, in our refinements taken as: r(C-H), r(C=O), r(C-S), r(C,-CS), (r(C-C)) = 0.5(r(C1=C2) + r(C,-C,)), Ar(C-C) = r(C2-C,) - r(C,=&), r(C-Cl), LC2=C1-C5, LC=C-S, LC=C-H, LC-C=O, LC-C-Cl and 4, the S-C-C=0 torsion angle. Vibrational amplitudes (I), perpendicular amplitude corrections (K) and centrifugal distortion constants (6,) were calculated using a valence force field developed for thiophene-2-aldehyde [lo] with the necessary modification for the chloride substitution. The vibrational amplitudes were kept constant at the calculated values in the least squares refinements. For the syn conformer a dynamic model was used where this conformer was represented by five distinct forms with different torsion angles, each weighted according to a Gaussian potential function. The r.m.s. amplitude for the torsional vibration (7) was introduced as a parameter and it was determined in the least-squares refinements. The final results of the refinements are given in Table 1, and theoretical intensity and RD curves calculated from these results, together with experimental and difference curves, are shown in Figs. 2 and 3. Table 2 gives the correlation matrix for the parameters refined.
363 TABLE
1
Final parameter
values for the structure
Parameter
r, or i,
1CdC
Parameter
r( C-H) r(C=O) r( C-S)
1.070( 38) 1.194(7) 1.713(6) 1.475(20) 1.395(8) 0.030( 33) 1.796(11) 127.0( 1.6) 112.2(8) 131.0(8.2) 126.5( 1.3) 112.9(1.2) 20.6(6.7) 166.1( 15.9) 58.9(11.0)
0.077 0.038 0.050 0.049
Selected f.c,=c,-c, L C-S-C f.s--C,-c, r(C=C)
r(C,-C,) W(C-C))b A r( C-C)c r( C-Cl) LC,=C,-c, LC=C-s LC=C-H LC-c=o L c-c-c1 r
d
Lee % syn
0.052
of thiophene-2carbonyl
chloridea
ra or L, dependent
r(C,-C,) r(C, . ..C.) r(C,***C,) r(C, . ..S) r(C,..aS) r(C, ..*C,) r(C, . ..O) r(C,*.*Cl) r(C, ..*C,) r(C,. . *C,) r(O***Cl) r(C, ..*Cl) r(C, *..Cl) r(C,. . ‘Cl) r(S*..Cl) r(C, . ..O) r(C,* * *O) r(C,* * 0 0) r(S...O) r(C, ..*Cl) r(C, ..*Cl) r(C, ..*Cl) r(S.**Cl) r(C, . ..O) r(C,.*.O) r(C,. * * 0) r(S*. -0)
>
>
J
angles and distances 112.2(5) 91.4(7) 120.811.2) 1.380( 11) 1.410(23) 2.315(8) 2.553(33) 2.571( 10) 2.770(17) 2.451( 16) 2.386(18) 2.729( 19) 3.739( 16) 3.891( 17) 2.610( 19) 3.047( 35) 4.453(33) 5.076(25) 4.361(12) wn 3.651(19) 4.692(15) 4.559(19) 3.097( 11) 4.059( 30) 4.967(20) 4.657(22) 3.072(44) 3.026(27) anti 4.384(22) 4.815(22) 3.896(20)
1c ale
0.044 0.049 0.055 0.070 0.051 0.073 0.059 0.058 0.074 0.067 0.068 0.064 0.147 0.147 0.110 0.075 0.062 0.072 0.105 0.128 0.075 0.095 0.131 0.148 0.106 0.101 0.078 0.068
aDistances (ra) and vibrational amplitudes (I) in Angstroms, angles in degrees. Parenthesized uncertainties are 20 and include estimates of systematic errors and correlation in the experimental data. bW(C+)) = O.S(r(C,=C,) + r(C,-C,)). ‘Ar(C--C) = r(C,-C,) do is the r.m.s. torsional amplitude for the syn conformer. Ed@ is the S-C-C=0 r(C,=C,). torsion angle for the form with 0 and S anti to each other.
DISCUSSION
Our investigation shows that thiophene-2-carbonyl chloride in the gas phase is a mixture of substantial amounts of both syn and anti conformers. This concurs with the results observed in solutions by Chadwick et al. [9]. In Table 1 we have reported a torsion angle of 166 + 16” for the anti form. This was the result obtained with a torsionally stiff model. Our results are also consistent with a planar conformer undergoing large torsional motion (like the syn form) or with a molecule having a low torsional barrier at the planar
2
0.0011 0.0093 0.0047 0.0017 0.0026 0.53 0.25 2.61 0.42 0.37 2.16 6.33 3.56
r( C-S) r( C-H)
aStandard
deviations
LC,=C,-Cl LC=C-s L C=C-H LC-c=o L c-c-c1 7 Lo % syn
r( C=O) r( C-Cl)
tic,-C,)
0.0080
Ar(c--c)
2 3 4 5 6 7 8 9 10 11 12 13 14 15
100
r,
82 100
from least squares.
0.0018
W(C-C))
1
‘7L.$
x 100 for the parameters
Parameter
matrix
No.
Correlation
TABLE
2 100
-2
-10 -12 10 100
-26 -42 -22 -16 100
for thiophene-2-carbonyl
-1 -5 -4 30 2 100
12 22 -18 -21 14 -3 100
chloride
-28 -42 10 -6 58 11 6 100
47 72 -13 -6 -50 -5 11 -61 100 0 -21 23 -1 4 15 -12 100
-3 -5
10 20 -3 -11 -14 -4 35 -17 38 32 100
6 9 8 13 -31 21 -18 9 27 -29 -44 100
8 11 -2 -3 6 3 17 -11 7 3 -3 1 100
-3 -9 -12 -18 37 10 25 39 -12 34 14 2 9 100
-10 16 -2 I 15 -8 3 1 -6 27 16 100
8 3 5
;;: &
365
position. The ED experiment cannot easily distinguish between these possibilities. When a dynamic model, like the one described for the syn conformer, was used also for anti, a value of 7,,ti = 18 f 14” was obtained. The fit between experimental and theoretical curves was equally good for this as for the alternative model. If we assume the two conformers of thiophene-2-carbonyl chloride to have the same entropy, a not unreasonable assumption since 7anti = rSsyn,the obseved conformational composition corresponds to an energy difference between anti and syn of 1.1 + 1.4 kJ mol-‘. This value can be compared with 4.3 * 1.6 kJ mol-’ for thiophene-2aldehyde [lo], 2.1 A 1.6 kJ mall’ for furan-2-carbonyl chloride [2] and -4.2 f 0.8 kJ mol-’ for furan-2aldehyde [ 11. In the furan compounds increased stability was observed for the syn conformer when the aldehyde proton was substituted by a chlorine atom, while the opposite effect was found for the thiophenes. Part of the reason for this difference may be the existence of an attraction between the aldehyde proton and the furan oxygen. The corresponding interaction between the thiophene sulfur and the aldehyde proton would be much smaller, and the anti conformer would therefore be relatively more stable in thiophene-2aldehyde. In Table 3 some of the geometrical parameters obtained for thiophene-2carbonyl chloride are compared with those for the related thiophene and furan compounds. The thiophene rings have similar geometry in both the acid chloride and the aldehyde; bond distances and valence angles are all equal within error limits. The parameter values are also very similar to those in thiophene itself [ 21-231. TABLE Parameter
3 values for thiophene-2-carbonyl
Parameter
Thiophene-2carbonyl chloride
r(C,---Cd r( C=C) r(C,-C,)
r(C=O) r(C-X) r( C-Cl) LC,-c,=c, IC,=C,-x IC-c=o L c-c-c1 LC-x-c Mb % syn Ref.
(X = S, 0)
1.410(23) 1.380( 11) 1.475(20) 1.194(7) 1.713(6) 1.796( 11) 127.0(1.6) 112.2(8) 126.5(1.3) 112.9( 1.2) 91.4(7) 166.1(15.9) 59(11) This work
chloride Thiophene-2aldehyde 1.431(15) 1.375(7) 1.466(16) 1.224(7) 1.7 17(4) 126.4(1.3) 111.8(4) 123.7(9) 92.0(4) 158.5(23.1) 81(8) 10
and some related Furan-2carbonyl
moleculesa
chloride
1.426(8) 1.357(8) 1.465( 13) 1.207(6) 1.378(10) 1.787(6) 131.6(g) 110.9(4) 125.8(8) 111.8(6) 105.6(8) 153.2(11.6) 70(14) 2
aDistances (ra) in Angstroms, angles in degrees. Quantities in square brackets in the refinements. b~~$ is the X-C-C=0 torsion angle of the anti conformer
Furan-2aldehyde [ 1.4311 [1.361] 1.435(7) 1.212(4) [1.362] 131.7(4) [110.7] 122.7(8) [106.5] [lo81 31(9) 1 were assumed (see text).
366
The two acid chlorides have a larger LC-00 and a shorter r(C=O) than the two aldehydes, but this is a difference observed in aimost all molecules of this type. Because of the difference between r-(C-S) and r(C-0), the angles in the thiophene rings are of course quite different from the corresponding angles in the furans. A significant difference is also observed for LC2=C1-C5 where the substituted furans have a larger angle (131.6-131.7”) than the thiophene carbonyl compounds (126.4-127.0”). When comparing the geometrical parameters of these four molecules, it should, however, be kept in mind that our model assumes that the syn and anti conformers have the same geometry apart from the C-C torsion angle and this assumption may have some effect on the values of the parameters. ACKNOWLEDGEMENTS
I am grateful to Dr. D. J. Chadwick, University of Liverpool, for providing the sample of thiophene-2-carbonyl chloride and to Siv.ing R. Seip and Ms. S. Gundersen, University of Oslo, for help with the electron diffraction experiment. Financial support from the Norwegian Research Council for Science and the Humanities is acknowledged. REFERENCES 1 G. Schultz, I. Fellegvari, M. Kolonits, A. I. Kiss, B. Pete and J. Banki, J. Mol. Struct., 50 (1978) 325. 2 K. Hagen, J. Mol. Struct., 130 (1985) 255. 3 F. Monnig, H. Dreizler and H. D. Rudolph, Z. Naturforsch., Teil A, 20 (1965) 1323. 4 J. F. Bertran, E. Ortiz and L. Ballester, J. Mol. Struct., 17 (1973) 161. 5 M. L. Martin, C. Andrieu and G. J. Martin, Bull. Sot. Chim. Fr., (1968) 698. 6 B. Roques and M. C. Fournie-Zaluski, Org. Magn. Reson., 3 (1971) 305. 7 S. Combrisson, B. Roques, P. Rigny and J. J. Bassilier, Can. J. Chem., 49 (1971) 905. 8 S. Nagata, Y. Yamabe, K. Yoshikawa and H. Kato, Tetrahedron, 29 (1973) 2545. 9 D. J. Chadwick, G. D. Meakins and E. E. Richards, Tetrahedron Lett., 36 (1974) 3183. 10 G. 0. Braathen, K. Kveseth, C. J. Nielsen and K. Hagen, J. Mol. Struct., 145 (1986) 45. 11 D. J. Chadwick, J. Chambers, G. D. Meakins and R. L. Snowden, J. Chem. Sot., Perkin Trans. 2, (1976) 1. 12 0. Bastiansen, 0. Hassel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. 13 K. Tamagawa, T. Iijima and M. Kimura, J. Mol. Struct., 30 (1976) 243. 14 K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 1003. 15 G. Gundersen and K. Hedberg, J. Chem. Phys., 51 (1969) 2500. 16 B. Andersen, H. M. Seip, T. G. Strand and R. Stdlevik, Acta Chem. Stand., 23 (1969) 3224. 17 L. Hedberg, Abstr. Fifth Austin Symp. Gas Phase Mol. Struct., Austin, TX, March 1974, p. 37. 18 Available from BLLD as Supplementary Publication No. SUP 26310 (7 pages) (see J. Mol. Struct., 34 (1976) 322 for further details of B.L.L.D.) 19 L. Schafer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 55 (1971) 3055. 20 K. Hedberg and M. Iwasaki, Acta Crystallogr., 17 (1964) 529. 21 B. Bak, D. Christensen, L. Hansen-Nygaard and J. Rastrup-Andersen, J. Mol. Spectrosc., 7 (1961) 58. 22 R. A. Bonham and F. A. Momany, J. Phys. Chem., 67 (1963) 2474. 23 W. R. Harshbarger and S. H. Bauer, Acta Crystallogr., Sect. B, 26 (1970) 1010.