- Email: [email protected]
Microwave spectrum of cyclohexanecarbonitrile
Volume 47, number 2
CHEMICALPHYSICSLE’ITERS
15 April 1977
MICROWAVE SPECTRUM OF ~YCLOHEX~EC~BO~I~I~E D. DAMIANI, L. FERRETTF and G. CORBELLI Lnborotorio di Spettroscopia Moiecofare det CXR.. 40126
Bologtro, ftafy Itafy and Istituto chimico “G. Ciamician“* finiversity of Bologna, 401.26 BoIogna,
Received 6 January 1977
The microwave rotational spectrum of cyclohexanecarbonitriile was investigated in the frequency region S-40 GHz. From the measured transition frequencies the rotational constants of the two molecular conformers were derived (equatorial fsomer: A = 4238.77,B = 1399.172, C = 1128.845 MHz; axial isomer: A = 3005.58, B= 1763A83,C= 1558.6iS MHz). Assuming the values of 1.53 1, 1.096 and 1.159 A, respectively. for the C-C, C-H and C-N distances, and supposing that the ring structure has the sarre symmetry as in cyclohexane, the following structural parameters were also obtained: equatorial isomer C-C-C (carbon ring) = 11 1.40a, r(C-CN) = 1.489 A, H-C-CN = 107.42°; axial isomer C-C-C (carbon ring) = 1 11.6.S”, r:C-CN) = 1.489 A, H-C-CN = 105.53°.
1. Introduction Conformational studies have shown that cyclohexanecarbonitrile, like other monosubstituted cyclohexanes, can exist in axial and equators conformations, and that the Gibbs free energy difference corr~spo~d~g to the ~~~ibriurn conformationat constant is AG& = -0.15 kcal/moIe [I ,Z] _This feature may be attributed to the fact that the geometry makes the van der Waals repulsions between the CN group and the nearest hydrogen atoms in the axial isomer more important than in the equatorial one. The low AG:N value suggests that these repulsions have Iittie importance and can be easily compensated for by moderate molecular deformations. The purpose of the present work is to determine some structural parameters of the cyclohexane carbonitrile conformers from the microwave spectral data relative to its most common isotopic species. To have an idea about the expected qualitative features of the microwave spectra, preliminary rotational c;onstants and the components of the dipole moment along the principal axis of inertia have been calculated using the tabulated values of the bond dipoIe moments [3], the stuctural parameters of the CN group in ~-bu~lc~bonit~e [4] and those of the ring in the ~~orocyclohex~e [5,6]. Such a calculation shows that the molecules of the
two isomers are prolate asymmetric tops (Rays’ coefficient X:= -0.7 + -0.8) and that the spectrum should consist mainly of a-type transitions of the equatoriaf conformer and of less intense Q- and c-type transitions of the axial conformer.
2. Experimental
A commercial sample of cyctohexanecarbonitr@, 99% pure, was used and the spectra were recorded by a Hewlett-Packard 8400 microwave Stark spectrometer in the frequency range 840 GHz. The cell was cooled at about -30°C and the gas sample pressure was kept at about 30-40 PHg. The values of the reported transition frequencies are assumed to be accurate within O_Q1MHz for the equatorial form and within 0.08 MHz for the axial one.
3. Microwave spectrum The observed spectrum appears very dense because of the presence of the two conformers and the occurrence of severaI transitions beionging to vibrationatly excited states, Despite of the high v&e of the dipole moment of the two isomers (: 3.8 D) [7], the intensity of the spectrum at Iower frequencies is too weak 335
Volume 47, number 2
CHEMICAL PHYSICS LETTERS
to measure the frequencies of the transitions with sufficient accuracy. The quadrupole hyperfine structure due to the nitrogen atom was not resolved. For these reasons any attempt to make an analysis of the Stark effect proved unsuccessfu1; thus no information on the dipole moments of the conformers was obtained. The a-type transitions of the equatorial isomer are prominent in the spectrum also because they pile up
in bands. The axial isomer bands are observable at the highest frequencies of the range even if they are weaker and more dispersed. The characteristic line pattern in the band was very helpful in assigning the equatorial conformer transitions, whereas the axial conformer spectrum was assigned only using the radio frequency-microwave double resonance technique. Table 1 lists the observed transitions and the frequencies which have been measured with greater accuracy for the two conformers. In view of the relative intensities in the spectrum, it can be safely assumed that the listed frequencies refer to the vibrational ground state. The values of the rotational constants for the two conformational isomers were obtained by a least squares fitting of the frequencies of each set, in the rigid rotor approximation (table 2).
I5 April 1977
Table 1 Rotationa! transition frequencies (MHz) of cyclohexanecarbonitrile Transition
rrequency obs.
obs. - talc. --
14601.09 15809.44 1565 1.68 16553.36 17554.27 17803.98 20840.41 21115.56 23268.35 22429.50 23827.48 29572.38 30289.75 30255.47 34229.25 36650.40 34779.39 36799.33 36532.94
0.11 0.06 -0.03 -0.10 0.10 0.06 0.05 0.04 0.04 0.15 -0.12 0.06 0.06 0.03 -0.12 -0.10 -0.08 0.05 0.06
6sHllCN 60,6
+
50,s
61.5
+
51.4
62,4
+
52,3
71.7
+
61,6
72,6
+
62,s
73.5 -
63.4
81,7
c
7l,6
82,6
t-
72,~
91,8
-
a1,7 82,7
92,s
-
92,7
+
82,6
=2,11
-
1 i2,lO
130.13 + 120.12 131,13 - q12 142,13 + t32,12 142,1~ + 132,11 151,15 + 141,14 151,14 e
141.13
152,14 c
142,13
a<&llCN
4. StnIctllre
61.5 62,s -
51,4
20313.89
52,4
19828.85 2010750 22420.26 23077.46 23359.74 27330.72 26728.25 26759.05 28585.39 29506.68 31709.81 32976.18
about the structure of the two conformers it was necessary to make some assumptions due to the paucity of the
63,3
-
53,2
70,7
-
60.6
72,6
+
62,s
available experimental data. Following the procedure already adopted for other monosubstituted cyclohexanes [6,8-101 we assumed
73,s
+
63~
82,6
-
72,~
a4,5
-
74p
a,,4
+
74,3
%,P
+
81.8 82,7
In order to obtain some information
that: (I) the CN group, which lies in the Q/Cmolecular symmetry plane does not change the cyciohexane ring symmetry; (2) the C-C distance is the same as that in cyclohexane [S] ; (3) the C-H distance is equal to that in the me’thylene group in propane [ 1 I] ; (4) the sum
92,s
-
101,lO
-
91,P
101,9
+
%,8
0.14 0.22 0.13 -0.00 0.15 -0.22 -0.06 -0.02
-0.10 -0.10 0.01 0.06 -0.09
of the H-C-H and C-C-C angles in the cyclohexane frame is twice the tetrahedral angle, as required by the
sp3 hybridization of the carbon atom; the EN distance is equal to that found for the r-butylcarbonitrile. Using these assumptions and a mean square fitting 336
of the rotational constants of the two conformers we obtained the structural parameters listed in table 3. The errors listed in this table were computed taking
CHEMICAL PHYSICS LETTERS
Volume 47, number 2 Tabfe 2 Rotational
constants
(MHz) of cyclobexanec~bo~~t~~e aC6H r rCN
e-%HrtCN
Table 3 Structural
A 4238.766 = 0.134 B 1399.172 5 0.003
3005.584 + 0.219 1763.483 f 0.008
C K
lS58.6L5 + 0.007 -0.71683
1128845
t 0.002
-0.82615
parameters
of cyclohexanecarbonitrile
e-Cefi~rCN
J.5 AprR L97T
C-CR distance is equal for the two isomers anutd in has a value ~u~e~edia~e between that of ethykyauide (1.474 A) [ 121 and t-butylcarbonitrile (I.495 A)_ The H-C-CN angle for the axial isomer is srnaher than that for the equatorial isomer, similar to the R-C-F an$e in tIuorocycIohexane [8,9J. On the contrary the C-CC angle in the carbon atom ring is sIightIy greater in the axial than in the equatorial isomer. As a result she carbon atom ring of the axial conformer, as a whole, is slightly flattened and the CN group is bent outward, exactly as is expected from conformational studies on monosubstituted cyclohexane.
aCoH, rCN
References
C-C C-H C=N C-CN L c-c-cd) f. H-C-H
1.531 Aa) 1.096 A b, 1.159 AC) I.489 + 0.009 A 111.40” +-0.37O ‘Io?.s3o * O_3S0
1.489+0.00911 f I l.6S0r0.380 107.28°%0.360
L H-C-CN
107.42* f O.SOc
105.s3°t0.890 I41
‘) Assumed, cyclohexane [ 5f . b)Assnmed, propane [lo]. c) Assumed, t-butylcarbonitrile [4] d, Carbon atom ring.
IS] I61 171
account both the errors reported for the assumed parameters and the exper~ent~ errors on the rotational constants. By comparison of the two sefs of parameters some comments about the structure of the conformers can be made. First, it appears that the values of the corresponding parameters of the two sets, except the N-C-CN angles, are quite sirniiar. In particular the into
I81
E-L- Eiiel, N-L. AIiinger, S.J. Au:aal and G-A. Morrison. Conformational analysis (htterscience. New York. 1965). B. Rickborn and F.R. Jensen, 3. Org Chem. 27 (E962) 4606. V.I. Minkii, 0-A. Ossipov and Yu.A_ Zhdsnov, Dipole moments in organic chemistry (Plenum Press, New York. 1970). L.J. Nugent, D.E. hfann and DR. Lide Jr., 2. Chem. P&s. 36 (1962) 965.. 0. Bastiansen, L. Fernholt, HM. Seip, W. Kambaza and K. Kuchitsu, J. Mol. Struct. 18 (1973) 163. D. Damiani and L. Ferret& Chem. Phys. Letters 2L (1973) 592. C.W.N. Cumper, SK. Dev and S.R. Landor, I. Chcm. Sot. Perkin Trans. If (1973) 537.. L. Pierce and R. Nelson, J. Am. f&m. SCIC.88 Q966j 216.
191 I_.. Pierce and J-F, Reecher, J. Am. Chem. Sot. 88 (I9661 5406. [IO] D. Damiani and L. Ferretti, Chem. Phys. Letters 24 (L9745 357. Ill] D-R. tide Jr., J. Chem. Whys. 33 (i960) 1514_ 1121 R-G. Lerner and B.P. Dailey, J. Chem. Phys. 26 Cf95-I) 678.
337
CHEMICALPHYSICSLE’ITERS
15 April 1977
MICROWAVE SPECTRUM OF ~YCLOHEX~EC~BO~I~I~E D. DAMIANI, L. FERRETTF and G. CORBELLI Lnborotorio di Spettroscopia Moiecofare det CXR.. 40126
Bologtro, ftafy Itafy and Istituto chimico “G. Ciamician“* finiversity of Bologna, 401.26 BoIogna,
Received 6 January 1977
The microwave rotational spectrum of cyclohexanecarbonitriile was investigated in the frequency region S-40 GHz. From the measured transition frequencies the rotational constants of the two molecular conformers were derived (equatorial fsomer: A = 4238.77,B = 1399.172, C = 1128.845 MHz; axial isomer: A = 3005.58, B= 1763A83,C= 1558.6iS MHz). Assuming the values of 1.53 1, 1.096 and 1.159 A, respectively. for the C-C, C-H and C-N distances, and supposing that the ring structure has the sarre symmetry as in cyclohexane, the following structural parameters were also obtained: equatorial isomer C-C-C (carbon ring) = 11 1.40a, r(C-CN) = 1.489 A, H-C-CN = 107.42°; axial isomer C-C-C (carbon ring) = 1 11.6.S”, r:C-CN) = 1.489 A, H-C-CN = 105.53°.
1. Introduction Conformational studies have shown that cyclohexanecarbonitrile, like other monosubstituted cyclohexanes, can exist in axial and equators conformations, and that the Gibbs free energy difference corr~spo~d~g to the ~~~ibriurn conformationat constant is AG& = -0.15 kcal/moIe [I ,Z] _This feature may be attributed to the fact that the geometry makes the van der Waals repulsions between the CN group and the nearest hydrogen atoms in the axial isomer more important than in the equatorial one. The low AG:N value suggests that these repulsions have Iittie importance and can be easily compensated for by moderate molecular deformations. The purpose of the present work is to determine some structural parameters of the cyclohexane carbonitrile conformers from the microwave spectral data relative to its most common isotopic species. To have an idea about the expected qualitative features of the microwave spectra, preliminary rotational c;onstants and the components of the dipole moment along the principal axis of inertia have been calculated using the tabulated values of the bond dipoIe moments [3], the stuctural parameters of the CN group in ~-bu~lc~bonit~e [4] and those of the ring in the ~~orocyclohex~e [5,6]. Such a calculation shows that the molecules of the
two isomers are prolate asymmetric tops (Rays’ coefficient X:= -0.7 + -0.8) and that the spectrum should consist mainly of a-type transitions of the equatoriaf conformer and of less intense Q- and c-type transitions of the axial conformer.
2. Experimental
A commercial sample of cyctohexanecarbonitr@, 99% pure, was used and the spectra were recorded by a Hewlett-Packard 8400 microwave Stark spectrometer in the frequency range 840 GHz. The cell was cooled at about -30°C and the gas sample pressure was kept at about 30-40 PHg. The values of the reported transition frequencies are assumed to be accurate within O_Q1MHz for the equatorial form and within 0.08 MHz for the axial one.
3. Microwave spectrum The observed spectrum appears very dense because of the presence of the two conformers and the occurrence of severaI transitions beionging to vibrationatly excited states, Despite of the high v&e of the dipole moment of the two isomers (: 3.8 D) [7], the intensity of the spectrum at Iower frequencies is too weak 335
Volume 47, number 2
CHEMICAL PHYSICS LETTERS
to measure the frequencies of the transitions with sufficient accuracy. The quadrupole hyperfine structure due to the nitrogen atom was not resolved. For these reasons any attempt to make an analysis of the Stark effect proved unsuccessfu1; thus no information on the dipole moments of the conformers was obtained. The a-type transitions of the equatorial isomer are prominent in the spectrum also because they pile up
in bands. The axial isomer bands are observable at the highest frequencies of the range even if they are weaker and more dispersed. The characteristic line pattern in the band was very helpful in assigning the equatorial conformer transitions, whereas the axial conformer spectrum was assigned only using the radio frequency-microwave double resonance technique. Table 1 lists the observed transitions and the frequencies which have been measured with greater accuracy for the two conformers. In view of the relative intensities in the spectrum, it can be safely assumed that the listed frequencies refer to the vibrational ground state. The values of the rotational constants for the two conformational isomers were obtained by a least squares fitting of the frequencies of each set, in the rigid rotor approximation (table 2).
I5 April 1977
Table 1 Rotationa! transition frequencies (MHz) of cyclohexanecarbonitrile Transition
rrequency obs.
obs. - talc. --
14601.09 15809.44 1565 1.68 16553.36 17554.27 17803.98 20840.41 21115.56 23268.35 22429.50 23827.48 29572.38 30289.75 30255.47 34229.25 36650.40 34779.39 36799.33 36532.94
0.11 0.06 -0.03 -0.10 0.10 0.06 0.05 0.04 0.04 0.15 -0.12 0.06 0.06 0.03 -0.12 -0.10 -0.08 0.05 0.06
6sHllCN 60,6
+
50,s
61.5
+
51.4
62,4
+
52,3
71.7
+
61,6
72,6
+
62,s
73.5 -
63.4
81,7
c
7l,6
82,6
t-
72,~
91,8
-
a1,7 82,7
92,s
-
92,7
+
82,6
=2,11
-
1 i2,lO
130.13 + 120.12 131,13 - q12 142,13 + t32,12 142,1~ + 132,11 151,15 + 141,14 151,14 e
141.13
152,14 c
142,13
a<&llCN
4. StnIctllre
61.5 62,s -
51,4
20313.89
52,4
19828.85 2010750 22420.26 23077.46 23359.74 27330.72 26728.25 26759.05 28585.39 29506.68 31709.81 32976.18
about the structure of the two conformers it was necessary to make some assumptions due to the paucity of the
63,3
-
53,2
70,7
-
60.6
72,6
+
62,s
available experimental data. Following the procedure already adopted for other monosubstituted cyclohexanes [6,8-101 we assumed
73,s
+
63~
82,6
-
72,~
a4,5
-
74p
a,,4
+
74,3
%,P
+
81.8 82,7
In order to obtain some information
that: (I) the CN group, which lies in the Q/Cmolecular symmetry plane does not change the cyciohexane ring symmetry; (2) the C-C distance is the same as that in cyclohexane [S] ; (3) the C-H distance is equal to that in the me’thylene group in propane [ 1 I] ; (4) the sum
92,s
-
101,lO
-
91,P
101,9
+
%,8
0.14 0.22 0.13 -0.00 0.15 -0.22 -0.06 -0.02
-0.10 -0.10 0.01 0.06 -0.09
of the H-C-H and C-C-C angles in the cyclohexane frame is twice the tetrahedral angle, as required by the
sp3 hybridization of the carbon atom; the EN distance is equal to that found for the r-butylcarbonitrile. Using these assumptions and a mean square fitting 336
of the rotational constants of the two conformers we obtained the structural parameters listed in table 3. The errors listed in this table were computed taking
CHEMICAL PHYSICS LETTERS
Volume 47, number 2 Tabfe 2 Rotational
constants
(MHz) of cyclobexanec~bo~~t~~e aC6H r rCN
e-%HrtCN
Table 3 Structural
A 4238.766 = 0.134 B 1399.172 5 0.003
3005.584 + 0.219 1763.483 f 0.008
C K
lS58.6L5 + 0.007 -0.71683
1128845
t 0.002
-0.82615
parameters
of cyclohexanecarbonitrile
e-Cefi~rCN
J.5 AprR L97T
C-CR distance is equal for the two isomers anutd in has a value ~u~e~edia~e between that of ethykyauide (1.474 A) [ 121 and t-butylcarbonitrile (I.495 A)_ The H-C-CN angle for the axial isomer is srnaher than that for the equatorial isomer, similar to the R-C-F an$e in tIuorocycIohexane [8,9J. On the contrary the C-CC angle in the carbon atom ring is sIightIy greater in the axial than in the equatorial isomer. As a result she carbon atom ring of the axial conformer, as a whole, is slightly flattened and the CN group is bent outward, exactly as is expected from conformational studies on monosubstituted cyclohexane.
aCoH, rCN
References
C-C C-H C=N C-CN L c-c-cd) f. H-C-H
1.531 Aa) 1.096 A b, 1.159 AC) I.489 + 0.009 A 111.40” +-0.37O ‘Io?.s3o * O_3S0
1.489+0.00911 f I l.6S0r0.380 107.28°%0.360
L H-C-CN
107.42* f O.SOc
105.s3°t0.890 I41
‘) Assumed, cyclohexane [ 5f . b)Assnmed, propane [lo]. c) Assumed, t-butylcarbonitrile [4] d, Carbon atom ring.
IS] I61 171
account both the errors reported for the assumed parameters and the exper~ent~ errors on the rotational constants. By comparison of the two sefs of parameters some comments about the structure of the conformers can be made. First, it appears that the values of the corresponding parameters of the two sets, except the N-C-CN angles, are quite sirniiar. In particular the into
I81
E-L- Eiiel, N-L. AIiinger, S.J. Au:aal and G-A. Morrison. Conformational analysis (htterscience. New York. 1965). B. Rickborn and F.R. Jensen, 3. Org Chem. 27 (E962) 4606. V.I. Minkii, 0-A. Ossipov and Yu.A_ Zhdsnov, Dipole moments in organic chemistry (Plenum Press, New York. 1970). L.J. Nugent, D.E. hfann and DR. Lide Jr., 2. Chem. P&s. 36 (1962) 965.. 0. Bastiansen, L. Fernholt, HM. Seip, W. Kambaza and K. Kuchitsu, J. Mol. Struct. 18 (1973) 163. D. Damiani and L. Ferret& Chem. Phys. Letters 2L (1973) 592. C.W.N. Cumper, SK. Dev and S.R. Landor, I. Chcm. Sot. Perkin Trans. If (1973) 537.. L. Pierce and R. Nelson, J. Am. f&m. SCIC.88 Q966j 216.
191 I_.. Pierce and J-F, Reecher, J. Am. Chem. Sot. 88 (I9661 5406. [IO] D. Damiani and L. Ferretti, Chem. Phys. Letters 24 (L9745 357. Ill] D-R. tide Jr., J. Chem. Whys. 33 (i960) 1514_ 1121 R-G. Lerner and B.P. Dailey, J. Chem. Phys. 26 Cf95-I) 678.
337