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The structure of thiophene
JOURNAL
OF MOLECULAR
SPECTROSCOPY
7, 58-63 (1961)
The Structure
of Thiophene
BJdRGE BAK, DANIEL CHRISTENSEN, LISE HANSEN-~YGAARD, AKD JOHN RASTRUP-ANDERSEN
Chemical Laboratory o-f the University of Copenhagen,
Copenhagen,
Denmark
The microwave spectra of 2-CWhiophene and of 3-C13-thiophene have been recorded and analyzed. In addition, t,hree lines from the S”-thiophene were found and identified. Taken together with earlier published data for thiophene, thiophene-2d, and thiophene-3d, an unambiguous calculation of t,he (‘T,‘-) structure of thiophene is now possible, the structural parameters being: (distances) C-s = 1.714 A; C=C = 1.370A; C-C = 1.423 A; C(2)-H(2) = 1.078 A; C(3)-H(3) = 1.081 A; (angles) C(5)--S-C(2) = 92”lO’; S-C(Z)--C (3) = lll”28’; C(2)--c(3)-C(-1) = 112”27’; S-C(2)-H(2) = 119”51’; C!1)--c: (3) - H(3) = 121”lti’.
In a previous publication (1) tbe literature on the struct,urr of thiophrnc was reviewed, and microwave da& for thiophenc, thiophene-2d, thiophcnc-:M, thiophene-3,3’& , and thiophene-dr were given. Lines from the 2-c”“-, t,he :3-C’“-, and the S34-isotopic species were also observed. l:ormally, a complete strurture of thiophene resulted, but’ the scar&y of lines from the Cl”- and the SZ4-species (st,udied in t,heir natural abundance) made further investigation highly desirable. The present paper brings t’he necessary and sufficient, mat#erial for a calculat,ion of the so-called r,-structure of thiophene (I?), complete dat,a for all necessary isot’opic species now being available. This structure suffers from t’he usual limit,ations of all r&ructures in, that presumably it is a structure somewhere in bet,ween t’he five ground-st’at,e struct’ures of the isot,opic species c*onsidercd on one side, and their common hypot,heGcal vibrationless state st,rurture on the ot,her side. The authors realize that by proper use of data from t,he vibrational spectra of isotopic thiophenes it will be possible to correct’ the r,-st#ructure into t’he direct,ion of the hypothekal vibrationless state. By inspec%ing the literature we have found, however, that the vibrational spectrum of t,hiophene has hren far less studied t,han one would assume, considering the fundament,al importance of the subst#ance. It is felt, that publication of our microwave results should not, await a clarification of t’he vibrational analysis, since t’he prrsent, r,-st,ruct,ure probably is not going t’o deviat’e so dist’inctly from the vibrationless st#ate st,ruct’ure as to alter our main conclusions. 58
THE
STRUCTURE
OF THIOPHESE;
TABLE OBSERVED
AND CALCULATED
2-C’3-THIOPHExE,
MICROW.~VE
3-C’3-THIOPHENE
xl
I ABSORPTION LINES
AND S”-THIOPHENE
(IN MHz)
FOR
IS THE 12ooo-.WJ
MHz REGION
Transition
Z-P3-thiophene Ohs..
3-P-thiophene
Calc.
Ohs.
~
Calc.
15032.8 16275.2
14?391.3* 16157.7*
1 111-t 211 %I? + 4,:j
19459.8” 22031.6”
19459.8 22031.4
212 4 22) -
30:, 3,,
2,, *
3,r
22802.8” 25869.4” 28373. P 28935 X* 20321 .3
22803.0 25869.5 28373 4 28935.9 20321.2
19118.3” 21873.0” 22730.9” 25529. -18 28018.5” 28328.0”
24803 (I 20047.7 28931.2 23093.6 25894.3 IiRSr(.S
24803.1 20047.6 28930.7 23094.0 25896.2 17860.0
2,> 2”Z
22” -
32,
-I,, -
-li<
5,: t&T,+ i:,: $6 4 !)fii +
51, 624 LJ S,, 9J.j 11,; ---*lli, 1”s: - 126, :* Idrntificvl
Ok
C:tlv.
-.
15032.8” 16275.2%
1,) 4 10, -
P-thiophene
27113.5 23724.5 18302.0 27132.7 20448.7
14891.1 16157.7 19148.8 21873.2 2273 1 1 25529.6 28018.4 28.128.1
19006.4
lrn6.4
25.3713S’
25373.
x
2X05-l
28054
2
?!a
2i113.7 23724.6
18301 .-I * IY 27182. 2044X. 6
13882.7
))y its Ht:trk-pattern.
‘I’hc preparation of the enriched samples of 2- and 3-C”“-thiophene (:!A P; ~ rwp . 22 5%enriched in (,‘“‘I will he described elsewhere (3). Both samples were of high purity. The microwave spectrograph employed was of the conventional Stnrk-modalatcd type with a pen-and-ink recorder and calibrated against spwtral lines of known frequcnc~y. ;\Iost of the measured frquenrirs are WIIsidrrcd rrlinblc to f0.05 MHz. _A suitable numhcr of lines were identified b> their rtwl\.cd Stark-pattern as indicated in Table I III our prwious publication ( 1 ) the Zrl -+ Srz transit,ion was given as 2.5,8ti!).O :md 25,52!).2 1\IHz for 2-~?‘~- and 3-C’“-thiophene, rcspcctivt4y. Due t.o thr larger isotopic :Il)undalwc now available more acwratc~ dct.erminutions were no\? possible. ‘I’hcl cwrrcactions for the S”4-species are dw to slightly increased scnsiI ivity of t hc spectrograph. WTCbelieve that the S”“-lines of Table I really belong to the molccwlar ground state and not to some vibrationally ewitcd Ieve ol the rcsultiiig rotational wnstnnt .-I (‘I’ablc~ I1 1 ordinary t hiophcne bwauw agrrra cswllcntly with the .4 constant of ordinary thiophcnc. (‘AT,CI!I,ATI0N
OF
ROTATIONAL
(WSSTrlSTS
The rotnt ional cwnst,ants of the three ‘new isot.opic species wcrc r*alrulat~c4 by the s:mw procrdtlrr as indiwtcd carlirr ( 1 ). I-silqq thr ‘vonvrrsion fwtor’
60
BAK
ET AL.
TABLE
II
ROTATIONAL CONSTANTS d, B, c IN MHz, ASYMMETRY PAR.WETER K, ANLI PRINCIPAL MOS~EXTS OF INERTIA I,, I*, I, IN amu.4* TOGETHER WITH THE INERTIAL DEFECT (I.D.) FOR THIOPHENE ANI) 7 ISOTOPIC SPEPIES 3_C’35‘3’. 2.((11 thiophene thiophene thiophene
ThiopheneThiophene Thiophene3d 2d d H (’
K 1, Zb Z, I.D.
7437.32 8041.77 5413.61 5418.12 3131.82 3235. ST to. 05994 -0.09182 62.8632 67.9722 93.3815 93.3038 161.4176 156.2321 0.0639 0.0651
7856.13 5138.14 3105.23 -0.14420 64.3486 98.3879 162.7999 0.0634
7616.99 4914.50 2985.99 -0.16713 66.3689 102.8652 169.3010 0.0669
7981.43 8042.29 7852.89 5319.23 5274.23 5418.34 3190.63 3183.70 3204.81 -0.13945 -0.04757 -0.11138 63.3384 62.8591 64.3752 95.0384 95.8493 93.3000 157.7413 158.4424 158.7873 0.0656 0.0789 0.0661
6587.67 4905.66 2810.88 +0.10929 76.7390 103.0506 179.8479 0.0583
TABLE
III
CARTESUN COORDINATES FOR THE ATOMS OF THIOPHENE (IN A UNITS) Atom
xs
s C(2) C(3) H(2) H(3)
-1.1426 ? 1.3121 -0.2550 2.2050
Ye 0” 1.2346 0.7116 2.2692 1.3201
* By symmetry.
505,531 for converting observed rotational constants (A, B, C in MHz) into moments of inertia (I, , Ib , I, in amuA*), we got the results of Table II. Frequencies, recalculated from the rotational constants of Table II, are given ;n Table I. CALCULATION
By using the ‘Kraitchman
OF THE
MOLECULAR
formulas
for a planar
STRUCTURE
molecule:
moments of inertia of the ‘parent’ where 1,’ = I, , I,’ = Ib (ground-state molecule), AI,’ and AI,’ are the changes caused by isotopic mono-substit)ution, and H is the corresponding reduced mass MAM/(M + AM), we have calculated the Cartesian coordinates r, , yS of the atoms of thiophene in a system with its
THE STRUCTURE
OF THIOPHENE
61
TABLE: IV C;EOMETRI(*AI,PARAMETERS OF THIOPHEXJZFROM THE PREVIOT;S _.INI)PRESENT MICROWAVE IYVESTI(GATION (INSTANCES IN A UXITS) Experimental uncertainty Distances l.Oi76
C(2)H(2) C(B,H(S) C!Q)S (‘(2,C(3) (‘(3iC(4) Angles C(5)SC(21 SC(2)C(3) (‘12)C(3)C(4) SC(2,H(2) (‘ c.4IC c.3) H (3)
1.0805 1.7110 1.3696 1 .-I232
rto.0015 *0.0011 sto.0014 *0.0017 *o ,002:~
92"lO' L11°28’ 112’“7 I ll!PX 124”16’
origin
in t,he center of mass of ordinary thiophene and with axes coinciding with t,hc inert.ial axes. The s-axis is identical with t.he a-axis of l’ig. 1 in our prcviolw puhlicat~ion ( I ) . The results are given in Table III. The interrogation mark in Table III refers to the fact that the .rs of C’(Z) calculat,ed by means of the Kraitchman formula is imaginary ( = O.OtiO:
0.67e
FIN. 1. Approximate
distribut,ion
of mobile
charge irl thiophene
62
BAH ET AL.
we have considered t#he uncertainty of the rotat,ional con&ants somewhat more carefully than in Ref. 1. In Ref. 1, B = 5418.12 f 0.3 MHz (thiophenr). A less pessimist#ic and quite realistic estimate gives B = 5418.12 f 0.1 MHz. Ll = 8041.77 f 0.2 MHz and C = 3235.77 f 0.2 MHz remain unchanged. Now, by varying the measured rotational c*onst,ant,s of all the isotopic species within t,he indicated limit’s (except for t,he SS-species where AA = f0.4 and AB = f0.2 MHz was supposed) we have calculated the experimcnt,al uncertainties given in Table IV. Because of its inherent difliwlty the procedure applied is probably not e&rely correct, but the results serve t)o illust)rate the order of magnit,ude of t,he experimental uncert,aint#y, not including t,he error, so far uncontrollable, that’ may have been committed by using the relat,ion C mixci = 0. DISCUSSI0N
In the improved t,hiophene model now given, the carbon-carbon dist,ances in thiophene are closer t,o the benzenoid carbon--carbon dist,ancc t’han in our earlier model, i.e., thiophene is more aromatic than might, be seen from the older structure. From what is krlowll of the struct’ure of another five-membered heterocyclic, f wu~~ (4 ) , t,he carbon--carbon double-bond dist,ance in t#his compomid is likely to be close to 1.340 A, i.e., the et,hylenic value. Obviously, the statement, that furan, pyrrole, and thiophene are similar quasi-aromatic compounds because in all three cases six mobile electrons are available (4 from the double bonds and 2 from t,he heteroatom lone-pair orbital), must now be refined, since the differences in electronegativity of 0, K, and S make t,hemselves distinctly felt. The electronegativity of sulfur is close t,o that’ of carbon so that t’he sulfur atom of t,hiophene is a far less efficient’ ?r-el&ron ‘trap’ t#han the oxygen at)om in furan. This latt.er molecule can probably be described correctly as an unsaturated, cyclic ether with very little aromatic character, judging from it,s geometry. The conclusion is in harmony with the chemical properties of furan. A rough est)imate of the x-elect,ron distribut’ion in thiophene may be oht,ained as follows: Wit,hin the limits of error the lengths of the C(Y)C(B) and the C(3)C(4) bonds are symmetrically displaced with respect to the carbon-carbon bond length in benzene (1.397 A). Furthermore, the C( S)C(4) bond length is so close to the graphite carbon--carbon bond length (1.420 A) that, it is reasonable to assume t’hat, t,he ‘mobile’ charge of the t’wo bonds is the same, viz., 0.67 P. If we assume that, a linear dependence holds between mobile charge and bond length, t’he C( 2)C(3) bond gets the mobile charge 1.33 e. The measured bond length of t’he C, S bond in thiophene is almost, midway bctJween a pure single bond (1.809 A (5)) and a pure double bond (l..i.%A). Its double-bond character is estimat,ed to 40% (mobile charge 0.8 e). Consequent,ly, in a first rough analysis, t’he dist’ribution of mobile charge in t,hiophene is as indicated in li’ig. 1. The total mobile charge is 0.67 + 2 X 1. 33 + 2 X 0.80 = 4 93 e in harmony
THE
STRUCTURE
OF THIOPHENE
fi:!J
with t,he fact, t’hat carbon and sulfur have the same electronegativity. the four carbons and the sulfur atom may each contribute one negative
Hence,
charge unit to the ‘cloud’ of mobile charges whereas the second sulfur ‘r’-electron charge unit’ has a much larger charge barrier t,o pass. The ‘x’moment calculated from the charge distribution of E’ig. 1 is 0.80 debye, pointming from a positive sulfur atom ‘upwards’ towards the C(3)(‘(4)-bond. The generally accepted ‘J-moment (mainly from the two C-S bonds, or from the sulfur in-plane lone-pair) is 0.9 fl = 1.2A debye, pointing in t(he opposite direction (6). The resulting calculated dipole moment, is 1.26 - 0.80 = 0.46 debye ( with it,s negative end at, sulfur i . The measured value is 0.6 debye. The higher double-bond charackr of the C, S bond as cwmpared to the charant’er of the C(S)C(J)-bond probably can be cwrrelat,ed with the greater c*hcmic*al ac%ivit’y of the t’hiophenic cY-carbons in electrophilic rcact,ions. .J somewhat, more elaborate but still very primit.ivr t.rratment, of tShiophcur may be given using our knowledge of the ~alcncc angles (,7i. ?;o cl\lalitatirr features arc hereby changed.
RFFFItFYCl+‘c; i i 1* 2, 1. IS. I~AK, I>. (:HRISTEXSEN, Php.
,'.('.<'. ('OSTAIN, J. Chem.
.3. 13. RAK.J. /t. 5. h’. 7.
6. RASTRI-P-AXIIERSEN,
ASD
J<. TANNENRAm,
.1. C'hrrrr.
26, 892 (19561. CHRISTIANSEN,
Phys.
23, 864 (1958).
AND J.T.
NIELSEN,.-l~tn
chenl. Stand.
14, 1865 (1960).
13. 13.4~~1,.HANSEN, ASD J. RASTRTP-kDERSEN, Disc. Faraday fk. 19, 30 (19551. H. D. RIWILPH, H. ~REIZLER, AND W. M~IER, Z. A~;trturforsch. iSa, 742 (1960). ,\Ic(+raw-Hill, Sew York. l!K;i. (:. 1’. SMYTH. “Dielectric Behaviour and Structure.” 1%. 1s.t~ .ANI) I,. FIBNSEN-NYGAARII, .I.Chet/l.Phi/s. 33, -118 (1960).
OF MOLECULAR
SPECTROSCOPY
7, 58-63 (1961)
The Structure
of Thiophene
BJdRGE BAK, DANIEL CHRISTENSEN, LISE HANSEN-~YGAARD, AKD JOHN RASTRUP-ANDERSEN
Chemical Laboratory o-f the University of Copenhagen,
Copenhagen,
Denmark
The microwave spectra of 2-CWhiophene and of 3-C13-thiophene have been recorded and analyzed. In addition, t,hree lines from the S”-thiophene were found and identified. Taken together with earlier published data for thiophene, thiophene-2d, and thiophene-3d, an unambiguous calculation of t,he (‘T,‘-) structure of thiophene is now possible, the structural parameters being: (distances) C-s = 1.714 A; C=C = 1.370A; C-C = 1.423 A; C(2)-H(2) = 1.078 A; C(3)-H(3) = 1.081 A; (angles) C(5)--S-C(2) = 92”lO’; S-C(Z)--C (3) = lll”28’; C(2)--c(3)-C(-1) = 112”27’; S-C(2)-H(2) = 119”51’; C!1)--c: (3) - H(3) = 121”lti’.
In a previous publication (1) tbe literature on the struct,urr of thiophrnc was reviewed, and microwave da& for thiophenc, thiophene-2d, thiophcnc-:M, thiophene-3,3’& , and thiophene-dr were given. Lines from the 2-c”“-, t,he :3-C’“-, and the S34-isotopic species were also observed. l:ormally, a complete strurture of thiophene resulted, but’ the scar&y of lines from the Cl”- and the SZ4-species (st,udied in t,heir natural abundance) made further investigation highly desirable. The present paper brings t’he necessary and sufficient, mat#erial for a calculat,ion of the so-called r,-structure of thiophene (I?), complete dat,a for all necessary isot’opic species now being available. This structure suffers from t’he usual limit,ations of all r&ructures in, that presumably it is a structure somewhere in bet,ween t’he five ground-st’at,e struct’ures of the isot,opic species c*onsidercd on one side, and their common hypot,heGcal vibrationless state st,rurture on the ot,her side. The authors realize that by proper use of data from t,he vibrational spectra of isotopic thiophenes it will be possible to correct’ the r,-st#ructure into t’he direct,ion of the hypothekal vibrationless state. By inspec%ing the literature we have found, however, that the vibrational spectrum of t,hiophene has hren far less studied t,han one would assume, considering the fundament,al importance of the subst#ance. It is felt, that publication of our microwave results should not, await a clarification of t’he vibrational analysis, since t’he prrsent, r,-st,ruct,ure probably is not going t’o deviat’e so dist’inctly from the vibrationless st#ate st,ruct’ure as to alter our main conclusions. 58
THE
STRUCTURE
OF THIOPHESE;
TABLE OBSERVED
AND CALCULATED
2-C’3-THIOPHExE,
MICROW.~VE
3-C’3-THIOPHENE
xl
I ABSORPTION LINES
AND S”-THIOPHENE
(IN MHz)
FOR
IS THE 12ooo-.WJ
MHz REGION
Transition
Z-P3-thiophene Ohs..
3-P-thiophene
Calc.
Ohs.
~
Calc.
15032.8 16275.2
14?391.3* 16157.7*
1 111-t 211 %I? + 4,:j
19459.8” 22031.6”
19459.8 22031.4
212 4 22) -
30:, 3,,
2,, *
3,r
22802.8” 25869.4” 28373. P 28935 X* 20321 .3
22803.0 25869.5 28373 4 28935.9 20321.2
19118.3” 21873.0” 22730.9” 25529. -18 28018.5” 28328.0”
24803 (I 20047.7 28931.2 23093.6 25894.3 IiRSr(.S
24803.1 20047.6 28930.7 23094.0 25896.2 17860.0
2,> 2”Z
22” -
32,
-I,, -
-li<
5,: t&T,+ i:,: $6 4 !)fii +
51, 624 LJ S,, 9J.j 11,; ---*lli, 1”s: - 126, :* Idrntificvl
Ok
C:tlv.
-.
15032.8” 16275.2%
1,) 4 10, -
P-thiophene
27113.5 23724.5 18302.0 27132.7 20448.7
14891.1 16157.7 19148.8 21873.2 2273 1 1 25529.6 28018.4 28.128.1
19006.4
lrn6.4
25.3713S’
25373.
x
2X05-l
28054
2
?!a
2i113.7 23724.6
18301 .-I * IY 27182. 2044X. 6
13882.7
))y its Ht:trk-pattern.
‘I’hc preparation of the enriched samples of 2- and 3-C”“-thiophene (:!A P; ~ rwp . 22 5%enriched in (,‘“‘I will he described elsewhere (3). Both samples were of high purity. The microwave spectrograph employed was of the conventional Stnrk-modalatcd type with a pen-and-ink recorder and calibrated against spwtral lines of known frequcnc~y. ;\Iost of the measured frquenrirs are WIIsidrrcd rrlinblc to f0.05 MHz. _A suitable numhcr of lines were identified b> their rtwl\.cd Stark-pattern as indicated in Table I III our prwious publication ( 1 ) the Zrl -+ Srz transit,ion was given as 2.5,8ti!).O :md 25,52!).2 1\IHz for 2-~?‘~- and 3-C’“-thiophene, rcspcctivt4y. Due t.o thr larger isotopic :Il)undalwc now available more acwratc~ dct.erminutions were no\? possible. ‘I’hcl cwrrcactions for the S”4-species are dw to slightly increased scnsiI ivity of t hc spectrograph. WTCbelieve that the S”“-lines of Table I really belong to the molccwlar ground state and not to some vibrationally ewitcd Ieve ol the rcsultiiig rotational wnstnnt .-I (‘I’ablc~ I1 1 ordinary t hiophcne bwauw agrrra cswllcntly with the .4 constant of ordinary thiophcnc. (‘AT,CI!I,ATI0N
OF
ROTATIONAL
(WSSTrlSTS
The rotnt ional cwnst,ants of the three ‘new isot.opic species wcrc r*alrulat~c4 by the s:mw procrdtlrr as indiwtcd carlirr ( 1 ). I-silqq thr ‘vonvrrsion fwtor’
60
BAK
ET AL.
TABLE
II
ROTATIONAL CONSTANTS d, B, c IN MHz, ASYMMETRY PAR.WETER K, ANLI PRINCIPAL MOS~EXTS OF INERTIA I,, I*, I, IN amu.4* TOGETHER WITH THE INERTIAL DEFECT (I.D.) FOR THIOPHENE ANI) 7 ISOTOPIC SPEPIES 3_C’35‘3’. 2.((11 thiophene thiophene thiophene
ThiopheneThiophene Thiophene3d 2d d H (’
K 1, Zb Z, I.D.
7437.32 8041.77 5413.61 5418.12 3131.82 3235. ST to. 05994 -0.09182 62.8632 67.9722 93.3815 93.3038 161.4176 156.2321 0.0639 0.0651
7856.13 5138.14 3105.23 -0.14420 64.3486 98.3879 162.7999 0.0634
7616.99 4914.50 2985.99 -0.16713 66.3689 102.8652 169.3010 0.0669
7981.43 8042.29 7852.89 5319.23 5274.23 5418.34 3190.63 3183.70 3204.81 -0.13945 -0.04757 -0.11138 63.3384 62.8591 64.3752 95.0384 95.8493 93.3000 157.7413 158.4424 158.7873 0.0656 0.0789 0.0661
6587.67 4905.66 2810.88 +0.10929 76.7390 103.0506 179.8479 0.0583
TABLE
III
CARTESUN COORDINATES FOR THE ATOMS OF THIOPHENE (IN A UNITS) Atom
xs
s C(2) C(3) H(2) H(3)
-1.1426 ? 1.3121 -0.2550 2.2050
Ye 0” 1.2346 0.7116 2.2692 1.3201
* By symmetry.
505,531 for converting observed rotational constants (A, B, C in MHz) into moments of inertia (I, , Ib , I, in amuA*), we got the results of Table II. Frequencies, recalculated from the rotational constants of Table II, are given ;n Table I. CALCULATION
By using the ‘Kraitchman
OF THE
MOLECULAR
formulas
for a planar
STRUCTURE
molecule:
moments of inertia of the ‘parent’ where 1,’ = I, , I,’ = Ib (ground-state molecule), AI,’ and AI,’ are the changes caused by isotopic mono-substit)ution, and H is the corresponding reduced mass MAM/(M + AM), we have calculated the Cartesian coordinates r, , yS of the atoms of thiophene in a system with its
THE STRUCTURE
OF THIOPHENE
61
TABLE: IV C;EOMETRI(*AI,PARAMETERS OF THIOPHEXJZFROM THE PREVIOT;S _.INI)PRESENT MICROWAVE IYVESTI(GATION (INSTANCES IN A UXITS) Experimental uncertainty Distances l.Oi76
C(2)H(2) C(B,H(S) C!Q)S (‘(2,C(3) (‘(3iC(4) Angles C(5)SC(21 SC(2)C(3) (‘12)C(3)C(4) SC(2,H(2) (‘ c.4IC c.3) H (3)
1.0805 1.7110 1.3696 1 .-I232
rto.0015 *0.0011 sto.0014 *0.0017 *o ,002:~
92"lO' L11°28’ 112’“7 I ll!PX 124”16’
origin
in t,he center of mass of ordinary thiophene and with axes coinciding with t,hc inert.ial axes. The s-axis is identical with t.he a-axis of l’ig. 1 in our prcviolw puhlicat~ion ( I ) . The results are given in Table III. The interrogation mark in Table III refers to the fact that the .rs of C’(Z) calculat,ed by means of the Kraitchman formula is imaginary ( = O.OtiO:
0.67e
FIN. 1. Approximate
distribut,ion
of mobile
charge irl thiophene
62
BAH ET AL.
we have considered t#he uncertainty of the rotat,ional con&ants somewhat more carefully than in Ref. 1. In Ref. 1, B = 5418.12 f 0.3 MHz (thiophenr). A less pessimist#ic and quite realistic estimate gives B = 5418.12 f 0.1 MHz. Ll = 8041.77 f 0.2 MHz and C = 3235.77 f 0.2 MHz remain unchanged. Now, by varying the measured rotational c*onst,ant,s of all the isotopic species within t,he indicated limit’s (except for t,he SS-species where AA = f0.4 and AB = f0.2 MHz was supposed) we have calculated the experimcnt,al uncertainties given in Table IV. Because of its inherent difliwlty the procedure applied is probably not e&rely correct, but the results serve t)o illust)rate the order of magnit,ude of t,he experimental uncert,aint#y, not including t,he error, so far uncontrollable, that’ may have been committed by using the relat,ion C mixci = 0. DISCUSSI0N
In the improved t,hiophene model now given, the carbon-carbon dist,ances in thiophene are closer t,o the benzenoid carbon--carbon dist,ancc t’han in our earlier model, i.e., thiophene is more aromatic than might, be seen from the older structure. From what is krlowll of the struct’ure of another five-membered heterocyclic, f wu~~ (4 ) , t,he carbon--carbon double-bond dist,ance in t#his compomid is likely to be close to 1.340 A, i.e., the et,hylenic value. Obviously, the statement, that furan, pyrrole, and thiophene are similar quasi-aromatic compounds because in all three cases six mobile electrons are available (4 from the double bonds and 2 from t,he heteroatom lone-pair orbital), must now be refined, since the differences in electronegativity of 0, K, and S make t,hemselves distinctly felt. The electronegativity of sulfur is close t,o that’ of carbon so that t’he sulfur atom of t,hiophene is a far less efficient’ ?r-el&ron ‘trap’ t#han the oxygen at)om in furan. This latt.er molecule can probably be described correctly as an unsaturated, cyclic ether with very little aromatic character, judging from it,s geometry. The conclusion is in harmony with the chemical properties of furan. A rough est)imate of the x-elect,ron distribut’ion in thiophene may be oht,ained as follows: Wit,hin the limits of error the lengths of the C(Y)C(B) and the C(3)C(4) bonds are symmetrically displaced with respect to the carbon-carbon bond length in benzene (1.397 A). Furthermore, the C( S)C(4) bond length is so close to the graphite carbon--carbon bond length (1.420 A) that, it is reasonable to assume t’hat, t,he ‘mobile’ charge of the t’wo bonds is the same, viz., 0.67 P. If we assume that, a linear dependence holds between mobile charge and bond length, t’he C( 2)C(3) bond gets the mobile charge 1.33 e. The measured bond length of t’he C, S bond in thiophene is almost, midway bctJween a pure single bond (1.809 A (5)) and a pure double bond (l..i.%A). Its double-bond character is estimat,ed to 40% (mobile charge 0.8 e). Consequent,ly, in a first rough analysis, t’he dist’ribution of mobile charge in t,hiophene is as indicated in li’ig. 1. The total mobile charge is 0.67 + 2 X 1. 33 + 2 X 0.80 = 4 93 e in harmony
THE
STRUCTURE
OF THIOPHENE
fi:!J
with t,he fact, t’hat carbon and sulfur have the same electronegativity. the four carbons and the sulfur atom may each contribute one negative
Hence,
charge unit to the ‘cloud’ of mobile charges whereas the second sulfur ‘r’-electron charge unit’ has a much larger charge barrier t,o pass. The ‘x’moment calculated from the charge distribution of E’ig. 1 is 0.80 debye, pointming from a positive sulfur atom ‘upwards’ towards the C(3)(‘(4)-bond. The generally accepted ‘J-moment (mainly from the two C-S bonds, or from the sulfur in-plane lone-pair) is 0.9 fl = 1.2A debye, pointing in t(he opposite direction (6). The resulting calculated dipole moment, is 1.26 - 0.80 = 0.46 debye ( with it,s negative end at, sulfur i . The measured value is 0.6 debye. The higher double-bond charackr of the C, S bond as cwmpared to the charant’er of the C(S)C(J)-bond probably can be cwrrelat,ed with the greater c*hcmic*al ac%ivit’y of the t’hiophenic cY-carbons in electrophilic rcact,ions. .J somewhat, more elaborate but still very primit.ivr t.rratment, of tShiophcur may be given using our knowledge of the ~alcncc angles (,7i. ?;o cl\lalitatirr features arc hereby changed.
RFFFItFYCl+‘c; i i 1* 2, 1. IS. I~AK, I>. (:HRISTEXSEN, Php.
,'.('.<'. ('OSTAIN, J. Chem.
.3. 13. RAK.J. /t. 5. h’. 7.
6. RASTRI-P-AXIIERSEN,
ASD
J<. TANNENRAm,
.1. C'hrrrr.
26, 892 (19561. CHRISTIANSEN,
Phys.
23, 864 (1958).
AND J.T.
NIELSEN,.-l~tn
chenl. Stand.
14, 1865 (1960).
13. 13.4~~1,.HANSEN, ASD J. RASTRTP-kDERSEN, Disc. Faraday fk. 19, 30 (19551. H. D. RIWILPH, H. ~REIZLER, AND W. M~IER, Z. A~;trturforsch. iSa, 742 (1960). ,\Ic(+raw-Hill, Sew York. l!K;i. (:. 1’. SMYTH. “Dielectric Behaviour and Structure.” 1%. 1s.t~ .ANI) I,. FIBNSEN-NYGAARII, .I.Chet/l.Phi/s. 33, -118 (1960).