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Microwave spectrum, dipole moment and structure of 1,1-difluorocyclohexane
Volume 24, number 3
CHEMICAL PHYSICS LETIERS
1 February 1974
MICR~WAVESPE~~RUM,DIPOLEMOMENT AND STRUCTURE OF l,l-DIFLUOROCYCLOHEXANE D. DAMJANI and L. FERRETTI Laboratorio di Spettronwpi~ Molecolare. C.N.R.. 40126 Bologna. Italy and Istituto ~~imico “G. Ciamickw“, University of Bolognn. 40126 Bolognn. Italy Received 29 October 1973
From the
analysis of the miaowave
rotationa spectrum of IJdifluorocyclohexane
in the frequency region
8-40 GHz the rotational constants (.4 = 3191.781, B = 1759.768, C= 1512.293 nlHzj, the centrifugal distortion coefficients @J= 124 X 104,DJ~- 2.92 X 104, 0~ = -8.20 X 10d3) and the dipole moment and its components (p = 2556, iia = 2.415, pb = 0, fit = 0.837 debye) have been derived. Assuming that the CC and CH distances are the same as those of propane and that the ring structue in the chair form is symmetrical, the following structural palmeters are obtained: LCCC = 111.95”, LFCF = 108.68”, C-F = 1.356 A.
1.Introduction The 1,ldifluorocyclohexane (C6HIOF2) molecule, in the energetically preferred chair form, can have a uuiqueconformational structure since the two substituents are identical and are located in the axial and equatorial positions df the same c&bon atom. This structure as&mes the existence of a plane of symmetry which contains the FCF p&r group. The.prelimi&ry rotational constants and the orientation.of the molecule referred to the principaiaxis system h&i been cal&lated usipg the C-F distance +ndFCF&@e&fl,ld$h~oroethand [l] and the other structural parameters of chlorocyciohexane [2]. These calculationsindicatethat_ the $oIecule is,a proIate asymm+ic top with’K-=‘ -0.7. Using the appro-ate structure obtained from these data and the tabulated tial~es of t$e bond dipble morr@nts .[?I ;the magnitude of the dipole motient comp$nents@s
weak ‘%” type transitions.
2. Experimental A commercial samtile of 1 ,ldifluorocyclohextii, 99.9% pure, has-been used without further purification. From-the spectral analysis no evidence has been found for decomposition of the sample in the gold plated absorption cell over periods of several hours. The spectra have been recorded with a HewlettPackard 8400 C microwave eectromet@ in__& frequency .range 8-40. GIjz. The measuiemegtshavebe+ carried out with t?u5 Star&‘cell cooled at dry ice tern-. perature, at Very I?W gas &nple pressure a+d .&ir@ Stark fields up to 3 kV/c.m.-The.tiansition frequency measurements are assumed to 6e ac&rate,$t$ .: ___.. :_ io.?2 MHz.. I .. ,. -1 ., ] : __ -..__.-..;.. _i_:~ ;
VoIume 24, number 3
i February 1974
CHEMICAL PHYSICS LE-l-l-ERS Table 1 Rotational transition frequencies(htHz) of 1 .l-dinuorocycIohexane
Transition
313 303 +
Frequency
212 202.
322 + 228 321 312 414 404 51s 50s 524 523
- 220 - 211 - 313 + 303 + 414 + 404 -423 - 422 514 +- 413 331 + 221 616
+
515
605 - 505 625 * 524 615-514 188*11c-IS,.,, 81~ * 717. 83s t 734
obs.
obs. - talc.
9427.18 9701.38 9816.31 9931.19 10167.89 12540.70 12816.52 15634.04 15867.03 16295.29 16787.67 16826.63 17611.88 18708.69 18878.67 19501.68 20078.38 22492.82 24813.77 26750.21
0.05 -0.04 0.01 -0.01 -0.02 -0.04 0.00 -0.07 0.10 0.04 -0.06 9.07 -0.02 -0.05 -0.01 0.08 0.07 0.00 -0.03 -0.05
;;I
in table 1, par&f the tikasured frequencies of bb- :
&&
By I: ;&.;
:
~s~&&:are-fis~~;
~Iekts&a&~
-.
ofau
_..
946
+
845
ohs.
27086.57 2785151 27889.99 2895630 29691.70 2978857 30262.33 30418.58 30883.61 30904.83 32055.07 3237168 33912.29 33923.63 38190.04 38326.96 39182.47 399119.81 39964.35 39967.35
0.02 -0.03 -0.02 -0.03 0.04 -0.01 -0.04 0.06 -0.0 1 -0.02 -0.02 0.06 0.04 0.01 0.01 0.02 -0.05 O.Ol 0.05 -0.04
- talc.
4. Dipole moment
: an “u” type spectrum. Owing to the fact that the rnofeculehas several different vibrational states, each t&&ion i$ a group of at least eight lines, t&e most intense of which has been supposed to correspond to the &und state. Thk ~ssiprnent h&s been made by least squares fitting the frequenci& of the low .T @&ions, which could be uriar&iguousiy identified from-thbir relative positionSin the spectrum. Measuremerkof 52’: type -and higher J %” ty$e’tran&ons have c&feed the f~~~~&j&n~~ ;, ‘. .‘. 1
* 725 t 81s + 80s + 827
ObS.
the values of the rotational constants and centrifugal distortion coefficients have been obtained. These are listed in table 2.
3191.781iO.013MHz 1759.768+0.005MHz 1512.293t0.003MHz -0.70534 1.24x1cl-4 t 1.0x10-5MHZ 2.92XIOc4t9.2XlW5 MHz -%~OXlO-~r3.4~10~ MHz
B c
CD5 919 909 928
945 + 844 936 - 835 927 * 826 101,10+919 100,10+909 1029 + 928 1519 - 91s ~~l,l!~fOl,lO ~1o,Il+l%,lo &,Ll+-112,10 121,11+111,10 &,JO+1139 f22,10C1129 f31,13+l2~,~2 I30,13~l20,12
Table2 Rotational constants of l.l-ditluorocyc!ohexane A
Frequency
Transition
f
73e Stark displacements of theM= 1,2,3 components of the 6,, + S,, transition and of the M = 0 component of the 616 + S1s transition have been measured in order to determine the “a” and “c” components, fin and IZ,, of the electric dipole moment. The other components of the 6,, + S,, transition do not show the normal second order Stark effect. .Using electric fieIds,up to 3 kVfcm, Stark @placetients greater than 10 Mti hatie been obtained. The absorption cell has;beeh caliirated using the J : = I.‘+ 0 transiti& vf OCS and assuming JEW- = 0.71521 [4]: : By least squares fitting these,,exp$i&erkaf data; the dipole moment tid their comp&ent$ ‘&ted ii~’ :: .’ ‘table 3 have been obtained. & is the a&e between p : .~‘1 and +e :u’: axis ditem+ied .from.& :+ndfi, valu& ’l. :. By t+ipg ihio,~kco~~~~$itifact @&&e GF bond ,: f
fit&g. tieti$ured fr~q~e~&s .. _. .‘,‘. “... ._ _;__, :. ._., _ ., : ; ‘. ‘. -: ;: .,’ ..J :
_-.: ‘- --.
::
..
Volume 24, number 3
Table 3 Dipole moment parameters of 1 Jdifluorocyclohexane (debye) Pa
pb PC
ir lx a)
2.415 0 0.837 2.556 19.12”
1 February 1974
CHEMICAL PHYSICS LETTERS
f 0.007 f 0.010 + 0.010 t 0.26”
a) Q is the angIe between p vector and the “n” axis. moments
are the largest contributors to the dipole moment, the direction of this vector is unequivocally determined.
Table 4 Structure parameters of 1 ,L-difluorocyclohexane spending quantities of analogue molecules Parameter
c-c C-H C-F LFCF LCCC LHCH Re)
and ccrre-
1 .l-difluoro-
other
cyclohesane
motecutes
1526 Aa) 1.096 Aa) 1.356 + 0.007 A
1.526 Ab) 1.096 Ab) 1.345 * 0.001 A c)
a) Assumed; b) propane [ 111; c) l,l-difluoroethane 111; 1 e) p is the angle between the bisecting d) cyclohexane [9]; axis of the FCF angle and the “‘a” axis.
5. Structure These three pieces of information, that is, the moments of inertia of the moIecule, which are the only ones obtainable from experimental results, are quite insufficient to make a complete structural analysis. Nevertheless, it should be noted that the molecular structures of some halo-cyclohexanes [2,5-81 are essentially the same as that reported for cyclohexane [9], especially if the fluorine atom is in the axial position [6] and consequentty subjected to strong interactions. However, the assumptions made in order to find the molecular structures of these halocylcohexanes [2,5,6] can be accepted also for the 1, I-difluorocyclohexane, reducing (at least) the number of structural variables. These assumptions are: (1) molecular symmetry is the same as that of cyclohexane; (2) C-H and C-C distances are the same as those of propane [lo]; (3) the sum of the HCH and CCC angles is twice the tetrahedral angle, as is required if the carbons’ bonding orbitals are sp3 hybrids. By these assumptions the structure of the whole molecule, omitting the FCF group, can be established by finding the CCC angle. This angle can be computed from the quantity
which is independent.&f the principal axis coordinates of the fluorine atoms, since these atoms are located In the plane-of symmetry a/i. Finally Paiand Pee-. may be used to determine the C-F bond distance and. .. FCFangle. : :-_, :-. . -.. -_ :
The structure of the whole molecule has been calculated following this procedure. In the first column of table 4 the calculated CCC, HCH, FCF angles and the C-F distance are listed; the second column shows for comparison the corresponding values observed for propane, cyclohexane and I ,I-difluoroethane. fl is the angle between the bisecting axis of the FCF angle and the “a” axis. The errors listed in the first column of table 4 have been computed assuming that the C-C and C-H distance values are precise to no more than ti.005 A and by taking into account the experimental errors of the rotational constants. It can be noted that the experimental values for the CCC, HCH and FCF angles and the C-F distance are essentially the same as the corresponding parameters of cyclohexane and of I,l-difluoroethane. Finally, one can observe from the OLand fi values that the direction of the dipole moment vector does not coincide with that of the bisecting axis of the FCF angle, as one might expect from the molecular syrnmetry and the composition of CH and CF bond moments. The difference between the two directions (about 10’) cannot be compentited for by changes in the obtained structure by a modification of the initial parameters within a’reasonable range. By caiculating the dipole moment followingthe... method suggested by Smith and Eyrmg [ 1 I], which takes into accountthe influence of the inductive ef‘feet on all the bond moments, one obtains rr, and cc, .. .valueSwbich~are’near enough to the experimentalones .,.
. .:
: -I ::_..
.'
,. .~~?59. :-- .:.
Volume
24, number 3
CHEMICAL PHYSICS LETTERS
and from which one deduces that the angle between the calculated dipole moment and the “a” axis is 2 l”20’ and practically coincides with the angle “a”_
References [ 1 j N. Solimene and B.P. Dailey, J. Chem. Phys. 22 (1954) 2042. [2] D. Damiani and L. Ferretti, Chem. Phys. Letters 21 (1973) 592. [3 ] V.I. IMinkin, O.A. Ossipov and Yu.A. Zhdanov. Dipole moments in organic chemistry (Plenum Press, New York, 1970).
1 February 1974
[4] J-S. hluenter, J. Chem. Phys. 48 (1968) 4544. [5] L. Pierce and R. Nelson, J. Am. Chem. Sot. 88 (1966) 216. [6] L. Pierce and R. Nelson, J. Am. Chem. Sot. 88 (1966) 5406. [71 P. Andersen. Acta Chem. Stand. 16 (1962) 2337.
[8] V.A. Atkinson, Acta Chem. Stand. 15 (1961) 599_ 191 M. Davis and 0. Hassel, Acta Chem. Stand. 17 (1963) 1181. [lOI D-R. Lide Jr., J. Chem. Phys. 33 (1960) 1514. [ll] R-P. Smith, T. Ree, J.L. Magetand H. Eyring, J. Am. Chem. Sot. 73 (1951) 2263; R.P. Smith and H. Eyring. J. Am. Chem. Sot. 74 (1952) 229; R-P. Smith and E-M. Mortensen, J. Am. C&em. Sot. 78 (1956) 3932.
CHEMICAL PHYSICS LETIERS
1 February 1974
MICR~WAVESPE~~RUM,DIPOLEMOMENT AND STRUCTURE OF l,l-DIFLUOROCYCLOHEXANE D. DAMJANI and L. FERRETTI Laboratorio di Spettronwpi~ Molecolare. C.N.R.. 40126 Bologna. Italy and Istituto ~~imico “G. Ciamickw“, University of Bolognn. 40126 Bolognn. Italy Received 29 October 1973
From the
analysis of the miaowave
rotationa spectrum of IJdifluorocyclohexane
in the frequency region
8-40 GHz the rotational constants (.4 = 3191.781, B = 1759.768, C= 1512.293 nlHzj, the centrifugal distortion coefficients @J= 124 X 104,DJ~- 2.92 X 104, 0~ = -8.20 X 10d3) and the dipole moment and its components (p = 2556, iia = 2.415, pb = 0, fit = 0.837 debye) have been derived. Assuming that the CC and CH distances are the same as those of propane and that the ring structue in the chair form is symmetrical, the following structural palmeters are obtained: LCCC = 111.95”, LFCF = 108.68”, C-F = 1.356 A.
1.Introduction The 1,ldifluorocyclohexane (C6HIOF2) molecule, in the energetically preferred chair form, can have a uuiqueconformational structure since the two substituents are identical and are located in the axial and equatorial positions df the same c&bon atom. This structure as&mes the existence of a plane of symmetry which contains the FCF p&r group. The.prelimi&ry rotational constants and the orientation.of the molecule referred to the principaiaxis system h&i been cal&lated usipg the C-F distance +ndFCF&@e&fl,ld$h~oroethand [l] and the other structural parameters of chlorocyciohexane [2]. These calculationsindicatethat_ the $oIecule is,a proIate asymm+ic top with’K-=‘ -0.7. Using the appro-ate structure obtained from these data and the tabulated tial~es of t$e bond dipble morr@nts .[?I ;the magnitude of the dipole motient comp$nents@s
weak ‘%” type transitions.
2. Experimental A commercial samtile of 1 ,ldifluorocyclohextii, 99.9% pure, has-been used without further purification. From-the spectral analysis no evidence has been found for decomposition of the sample in the gold plated absorption cell over periods of several hours. The spectra have been recorded with a HewlettPackard 8400 C microwave eectromet@ in__& frequency .range 8-40. GIjz. The measuiemegtshavebe+ carried out with t?u5 Star&‘cell cooled at dry ice tern-. perature, at Very I?W gas &nple pressure a+d .&ir@ Stark fields up to 3 kV/c.m.-The.tiansition frequency measurements are assumed to 6e ac&rate,$t$ .: ___.. :_ io.?2 MHz.. I .. ,. -1 ., ] : __ -..__.-..;.. _i_:~ ;
VoIume 24, number 3
i February 1974
CHEMICAL PHYSICS LE-l-l-ERS Table 1 Rotational transition frequencies(htHz) of 1 .l-dinuorocycIohexane
Transition
313 303 +
Frequency
212 202.
322 + 228 321 312 414 404 51s 50s 524 523
- 220 - 211 - 313 + 303 + 414 + 404 -423 - 422 514 +- 413 331 + 221 616
+
515
605 - 505 625 * 524 615-514 188*11c-IS,.,, 81~ * 717. 83s t 734
obs.
obs. - talc.
9427.18 9701.38 9816.31 9931.19 10167.89 12540.70 12816.52 15634.04 15867.03 16295.29 16787.67 16826.63 17611.88 18708.69 18878.67 19501.68 20078.38 22492.82 24813.77 26750.21
0.05 -0.04 0.01 -0.01 -0.02 -0.04 0.00 -0.07 0.10 0.04 -0.06 9.07 -0.02 -0.05 -0.01 0.08 0.07 0.00 -0.03 -0.05
;;I
in table 1, par&f the tikasured frequencies of bb- :
&&
By I: ;&.;
:
~s~&&:are-fis~~;
~Iekts&a&~
-.
ofau
_..
946
+
845
ohs.
27086.57 2785151 27889.99 2895630 29691.70 2978857 30262.33 30418.58 30883.61 30904.83 32055.07 3237168 33912.29 33923.63 38190.04 38326.96 39182.47 399119.81 39964.35 39967.35
0.02 -0.03 -0.02 -0.03 0.04 -0.01 -0.04 0.06 -0.0 1 -0.02 -0.02 0.06 0.04 0.01 0.01 0.02 -0.05 O.Ol 0.05 -0.04
- talc.
4. Dipole moment
: an “u” type spectrum. Owing to the fact that the rnofeculehas several different vibrational states, each t&&ion i$ a group of at least eight lines, t&e most intense of which has been supposed to correspond to the &und state. Thk ~ssiprnent h&s been made by least squares fitting the frequenci& of the low .T @&ions, which could be uriar&iguousiy identified from-thbir relative positionSin the spectrum. Measuremerkof 52’: type -and higher J %” ty$e’tran&ons have c&feed the f~~~~&j&n~~ ;, ‘. .‘. 1
* 725 t 81s + 80s + 827
ObS.
the values of the rotational constants and centrifugal distortion coefficients have been obtained. These are listed in table 2.
3191.781iO.013MHz 1759.768+0.005MHz 1512.293t0.003MHz -0.70534 1.24x1cl-4 t 1.0x10-5MHZ 2.92XIOc4t9.2XlW5 MHz -%~OXlO-~r3.4~10~ MHz
B c
CD5 919 909 928
945 + 844 936 - 835 927 * 826 101,10+919 100,10+909 1029 + 928 1519 - 91s ~~l,l!~fOl,lO ~1o,Il+l%,lo &,Ll+-112,10 121,11+111,10 &,JO+1139 f22,10C1129 f31,13+l2~,~2 I30,13~l20,12
Table2 Rotational constants of l.l-ditluorocyc!ohexane A
Frequency
Transition
f
73e Stark displacements of theM= 1,2,3 components of the 6,, + S,, transition and of the M = 0 component of the 616 + S1s transition have been measured in order to determine the “a” and “c” components, fin and IZ,, of the electric dipole moment. The other components of the 6,, + S,, transition do not show the normal second order Stark effect. .Using electric fieIds,up to 3 kVfcm, Stark @placetients greater than 10 Mti hatie been obtained. The absorption cell has;beeh caliirated using the J : = I.‘+ 0 transiti& vf OCS and assuming JEW- = 0.71521 [4]: : By least squares fitting these,,exp$i&erkaf data; the dipole moment tid their comp&ent$ ‘&ted ii~’ :: .’ ‘table 3 have been obtained. & is the a&e between p : .~‘1 and +e :u’: axis ditem+ied .from.& :+ndfi, valu& ’l. :. By t+ipg ihio,~kco~~~~$itifact @&&e GF bond ,: f
fit&g. tieti$ured fr~q~e~&s .. _. .‘,‘. “... ._ _;__, :. ._., _ ., : ; ‘. ‘. -: ;: .,’ ..J :
_-.: ‘- --.
::
..
Volume 24, number 3
Table 3 Dipole moment parameters of 1 Jdifluorocyclohexane (debye) Pa
pb PC
ir lx a)
2.415 0 0.837 2.556 19.12”
1 February 1974
CHEMICAL PHYSICS LETTERS
f 0.007 f 0.010 + 0.010 t 0.26”
a) Q is the angIe between p vector and the “n” axis. moments
are the largest contributors to the dipole moment, the direction of this vector is unequivocally determined.
Table 4 Structure parameters of 1 ,L-difluorocyclohexane spending quantities of analogue molecules Parameter
c-c C-H C-F LFCF LCCC LHCH Re)
and ccrre-
1 .l-difluoro-
other
cyclohesane
motecutes
1526 Aa) 1.096 Aa) 1.356 + 0.007 A
1.526 Ab) 1.096 Ab) 1.345 * 0.001 A c)
a) Assumed; b) propane [ 111; c) l,l-difluoroethane 111; 1 e) p is the angle between the bisecting d) cyclohexane [9]; axis of the FCF angle and the “‘a” axis.
5. Structure These three pieces of information, that is, the moments of inertia of the moIecule, which are the only ones obtainable from experimental results, are quite insufficient to make a complete structural analysis. Nevertheless, it should be noted that the molecular structures of some halo-cyclohexanes [2,5-81 are essentially the same as that reported for cyclohexane [9], especially if the fluorine atom is in the axial position [6] and consequentty subjected to strong interactions. However, the assumptions made in order to find the molecular structures of these halocylcohexanes [2,5,6] can be accepted also for the 1, I-difluorocyclohexane, reducing (at least) the number of structural variables. These assumptions are: (1) molecular symmetry is the same as that of cyclohexane; (2) C-H and C-C distances are the same as those of propane [lo]; (3) the sum of the HCH and CCC angles is twice the tetrahedral angle, as is required if the carbons’ bonding orbitals are sp3 hybrids. By these assumptions the structure of the whole molecule, omitting the FCF group, can be established by finding the CCC angle. This angle can be computed from the quantity
which is independent.&f the principal axis coordinates of the fluorine atoms, since these atoms are located In the plane-of symmetry a/i. Finally Paiand Pee-. may be used to determine the C-F bond distance and. .. FCFangle. : :-_, :-. . -.. -_ :
The structure of the whole molecule has been calculated following this procedure. In the first column of table 4 the calculated CCC, HCH, FCF angles and the C-F distance are listed; the second column shows for comparison the corresponding values observed for propane, cyclohexane and I ,I-difluoroethane. fl is the angle between the bisecting axis of the FCF angle and the “a” axis. The errors listed in the first column of table 4 have been computed assuming that the C-C and C-H distance values are precise to no more than ti.005 A and by taking into account the experimental errors of the rotational constants. It can be noted that the experimental values for the CCC, HCH and FCF angles and the C-F distance are essentially the same as the corresponding parameters of cyclohexane and of I,l-difluoroethane. Finally, one can observe from the OLand fi values that the direction of the dipole moment vector does not coincide with that of the bisecting axis of the FCF angle, as one might expect from the molecular syrnmetry and the composition of CH and CF bond moments. The difference between the two directions (about 10’) cannot be compentited for by changes in the obtained structure by a modification of the initial parameters within a’reasonable range. By caiculating the dipole moment followingthe... method suggested by Smith and Eyrmg [ 1 I], which takes into accountthe influence of the inductive ef‘feet on all the bond moments, one obtains rr, and cc, .. .valueSwbich~are’near enough to the experimentalones .,.
. .:
: -I ::_..
.'
,. .~~?59. :-- .:.
Volume
24, number 3
CHEMICAL PHYSICS LETTERS
and from which one deduces that the angle between the calculated dipole moment and the “a” axis is 2 l”20’ and practically coincides with the angle “a”_
References [ 1 j N. Solimene and B.P. Dailey, J. Chem. Phys. 22 (1954) 2042. [2] D. Damiani and L. Ferretti, Chem. Phys. Letters 21 (1973) 592. [3 ] V.I. IMinkin, O.A. Ossipov and Yu.A. Zhdanov. Dipole moments in organic chemistry (Plenum Press, New York, 1970).
1 February 1974
[4] J-S. hluenter, J. Chem. Phys. 48 (1968) 4544. [5] L. Pierce and R. Nelson, J. Am. Chem. Sot. 88 (1966) 216. [6] L. Pierce and R. Nelson, J. Am. Chem. Sot. 88 (1966) 5406. [71 P. Andersen. Acta Chem. Stand. 16 (1962) 2337.
[8] V.A. Atkinson, Acta Chem. Stand. 15 (1961) 599_ 191 M. Davis and 0. Hassel, Acta Chem. Stand. 17 (1963) 1181. [lOI D-R. Lide Jr., J. Chem. Phys. 33 (1960) 1514. [ll] R-P. Smith, T. Ree, J.L. Magetand H. Eyring, J. Am. Chem. Sot. 73 (1951) 2263; R.P. Smith and H. Eyring. J. Am. Chem. Sot. 74 (1952) 229; R-P. Smith and E-M. Mortensen, J. Am. C&em. Sot. 78 (1956) 3932.