- Email: [email protected]
MO studies of the nature of the bifurcated hydrogen bond. Rotational barriers in cyclohexanol and 1,3-dioxan-5-ol
CilEhliCAL
MO STUDIES OF THE NATURE ROTATIONAL J. ANDRIk
BARRIERS
OF THE BIFURCATED
IN CYCLOHEXANOL
HYDROGEN
BOND.
AND 1,3-DIOXAN&OL
J-L. I’ASCUAL and E. SILLA
Dtrpor&wmw fo de Q~iinica-I-i’sicd. Facultad Um$asur (I’alem%7j. Spain IIwcivcd
31 z\ugu.st 1983
LETTXRS
PHYSICS
6 \larch
dc @thica.
Lkiwrsity
of Valemio.
1984: in final form 25 JUIIC 1984
I _ Introduction It is well ki10w11 ticit I,3diox:w5-ol and sowe of its derivatives subsrituted at the C, atom exist as a rnisturc of chair conformers with rile hydrosyl group itI the zhd or equatorial position [I >?I. A greater stability for the asial coaformcr has been established. this being iuterprcted as due to the fornmtion of m ~?~tr~I~~ol~~ul~~ bifurcated hydrogm bond [3-91. I-Iowever, the nature of this hydrogen bond is not well understood because its srructurt‘ is far from the classicaf description. As f3r 3s we know. only one theoretica study 0T this system has been carried out [IO]. Naimushin et al.
caiculatcd the rotationnl barrier of the axiai conl~ornicr by esrended f4iickel and CNDO 111eth0ds witl1out my p.meiry optimization, the results obtained by the two mctliods being contradictory. In order to clarify the nature and disposition of this ~Ii~~tllole~~lIar bifurcated hydrogen bond, in this paper a conipara tive quantum-cllexllical nnalysis of the rotational barriers in the asiaf arxi equatorial conformers ofcyclol~cxanol artd 1.3-dioxan-S-01 is undertaken.
2. Methods of calculation
INDO [ 1 i] computations 468
were carried out initially
in order to get a semi-empirical description of the system. This is needed prior to the ab initio study because of the large size of these systems. Computatians were carried out using Rinaidi’s method for geometry optimization [ill] (GEOMO program [ 131). Optimizations were terminated when the gradient of the unconstrained parameters was reduced to below 10e6 hartrcc/bollr or liartree~~dia~l_ The dihedral angle B (see @.I) was chosen as the independent variable, the energy being minimized with respect to the remaining geometric coordinates both in axial and equatorial conformers. The rotational harriers were then calculated using the STO-3G basis set [ 141 with tlw INDO optimized structures. 0 0092614/84/S ~or~l-Ho~and
03.00 0 Elsevier Science Publishers Physics Publishing Division)
B.V.
Volwne 109. number 5
CHEMICAL
PHYSICS
3. Results and discussion
31 _4ugusr 1981
LEl-l-ERS
E (kJ/mol) 2*
lND0 rotational barriers for axial and equatorial forms of 1,3-dioxan-5-01 arc shown in fig. 2. In both cases, there are three minima: two, at 0 = 120” and 240°, are symmetrical, and the third, which corresponds to the possible bifurcated hydrogen bond in the axial conformer, appears at B = O”. The axial conformation is the most stable for all values of 0, the presence of the hydrogen bond not being the decisive factor. However, the energy difference between the curves depends on the value of 8: the largest difference being in the 0 = O-40” zone. The energy difference between the rotamers 0 = 0 and 120” is 9.4 kJ/mol for the equatorial conformer, and only 6.2 kJ/mol for the axial conformer_ Rotational barriers for cyclohexanol show that the axial conformer is also the most stable, the energy difference between the rotamers at 8 = 0 and 120” being 9.4 and 9.3 kJ/mol for the equatorial and axial conformersl respectively. Rotational barriers in 1,3-dioxan-5-01 obtained with the STO-3G basis set are depicted in fig. 3. Both curves display the same minima as the INDO results, the relative stability of the minima, however, being different. In these cases, the absolute minimum occurs with the axial conformer at 0 = 0”. At this angle, an intramolecular bifurcated hydrogen bond may be formed_ which could explain the high stability of this
t=_;=~;-.---_--L_-i-; 0
LO
80
120 160 6 (DEGREES)
Rotational barriers in axial (---I and equatorial conformers of 1,3-diosan-5-01 by the INDO method. Energy relative to the wial rotamer 3t 0 = 120”. (Tot31
Fig. 1. (- --)
energy = -227374.9
kJ/mol.)
-__-
I
20
:‘
18 16I ‘L
I
I / -i
i2 ?O
a 6 L
I
I
I
2 3. i --I)
MY-.-
-
-
LO
a0
~-.-T
120 9
160
K?EGAEES!
I-is. 3. Rot;ltionaI barriers in xsial (--) and equatorial (---j conformers of 1,3-diosan-5-01 obtained with due STO-3G basis WI. Energy relative IO the sxiltl roramer ar 0 = 0”. (Total encr:y = -985996.7 kJ,‘mol.j conformation. For cyclohexanol, STO-3G results show that, at 0 = O”, the equatorial conformer is 12.3 kJ/mol more stable than the axial conformer, in qualitative agreement with experimental results in several solvents [ 15 1. However, there are no experimental values in the gas phase. The nature of the interaction between the hydrogen atom of the hydroxyl group and the osygen atoms of the ring is essentially electrostatic, as the following facts seem to show: (i) The overlap population between these atoms, obtained using a Mulliken population analysis with the ET03G basis set, is 0.0005. (ii) The H...O distance is 2.59 a, equal to the value calculated from microwave spectroscopy [S]. (iii) The O-H...0 a&e is 9% lo, the esperimental value being 100.4” [S] _ Both methods indicate that the axial form is the most stable conformation of 1,3-diosan-S-01 and that an electrostatic interaction exists between the hydro469
\rolttntc
109. ntttnbcr
5
CHEMICAL
1 atomic charges (in au) ior the axial conformation 1.3-diosan-S-o1 at 0 = 0” At0111
INDO
STO-3G
-0.29 0.45 -0.29 0.24 0.18 0.24 -0.29 0.16
-0.27 0.15 -0.27 0.01 0.06 0.01
LETTERS
31 August
1984
Acknowledgement
Table Net
PHYSICS
of
We would J. Figueruelo
like to express our appreciation for his helpful comments.
to Professor
-01 C?_ 03
C, CS C6 0 H
gen
atotn
of the
hydroxpl
group
References
-0.28
0.17
and the oxygen
atoms
of the ring for this conformation at 0 = 0”. However. differences beween the methods arise when attempting to quantitatively evaluate that interaction. These differences may be explained by taking into account that, at 0 = O”, the hydrogen atom of the hydroxyl group is located 2.59 ;i from the oxygen atoms of the ring and 2.57 ,% from the C4 and C(5 atOnlS. In table I, ths net effective charges on the ring centers and on the hydroxyl group, obtained by applying both mcthods, are indicated. As can be seen, the charges on the C, and C6 a1oms according to the INDO method are similar in magnitude but opposite in sign to those on rhe 0, and OS atoms, and, as a result, the net interaction between the H atom of the hydroxyl group and those four centers is negligible. However, according to the ab initio method, the charges on C4 and C6 are negligible, and therefore a net stabilizing interaction esisls.
470
S.A. Barker. J.S. Britnxombe, A.B. Foster, D.11. Whiffen and G. Zweifcl, Tetrahedron 7 (1959) 10. 121 EL. Eliei and h1.K. Kaloustian, Chem. Comtnun. (1970) 290. and 131 N. Baggctt, h1.A. Bukbari. A.B. Foster, J. Lehmann J.M. Webbcr. J. Chem. Sot. (1063) 4157. I41 S.A. Barker, A.B. Foster. A.H. Hnines. J. Lchmnnn. J.M. Wcbber and G. Zweifel, J. Chcm. Sot. (1963) 4161. ISI J. Gelas nnd K. Rtmlbaud. Uull. Sot. Chin>. France (1969) 1300. I61 I’. Calinaud and J. Celns. Bull. Sot. Chim. I’rttncc (1975) 1237. Tetrahedron Lcttcrs [71 J.C. Jochitns and Y. Kobayashi. (1976) 2065. 181 J.L. Alonso and E.B. Wilson. J. Am. Chern. Sot. 102 ( 1960) 1218. 191 J.L. Alonso. Analcs Quitn. 77 (1961) 76. V.L. Levcdev. R.T. Akltmadinov, EA. 1101 A.I. Naimushin, Kantor, D.L. Rachmankulov and Y.&l. Pnushkin, Dok:. Akad. Nauk. SSSR 25 I (I 980) 1420. 1111 J.A. Pople, D-L. Bcvcridgc and P.A. Dobosh, J. Chsm. I’hys. 47 (1967) 2026. Chem. 1 (1976) 109. 1121 D. Rinaldi. Computer 1131 D. Rinaldi, Thesis. Nancy (1975), QCPE no. 290. I141 W.J. Heltrc. R.F. Stewart and J.A. Poplc. J. Chem. Phys. 5 I (1969) 2657. J. Am. Chctn. Sot. 87 1151 EL. EIiel and S.H. Schroctcr. (1965) 5031.
Ill
MO STUDIES OF THE NATURE ROTATIONAL J. ANDRIk
BARRIERS
OF THE BIFURCATED
IN CYCLOHEXANOL
HYDROGEN
BOND.
AND 1,3-DIOXAN&OL
J-L. I’ASCUAL and E. SILLA
Dtrpor&wmw fo de Q~iinica-I-i’sicd. Facultad Um$asur (I’alem%7j. Spain IIwcivcd
31 z\ugu.st 1983
LETTXRS
PHYSICS
6 \larch
dc @thica.
Lkiwrsity
of Valemio.
1984: in final form 25 JUIIC 1984
I _ Introduction It is well ki10w11 ticit I,3diox:w5-ol and sowe of its derivatives subsrituted at the C, atom exist as a rnisturc of chair conformers with rile hydrosyl group itI the zhd or equatorial position [I >?I. A greater stability for the asial coaformcr has been established. this being iuterprcted as due to the fornmtion of m ~?~tr~I~~ol~~ul~~ bifurcated hydrogm bond [3-91. I-Iowever, the nature of this hydrogen bond is not well understood because its srructurt‘ is far from the classicaf description. As f3r 3s we know. only one theoretica study 0T this system has been carried out [IO]. Naimushin et al.
caiculatcd the rotationnl barrier of the axiai conl~ornicr by esrended f4iickel and CNDO 111eth0ds witl1out my p.meiry optimization, the results obtained by the two mctliods being contradictory. In order to clarify the nature and disposition of this ~Ii~~tllole~~lIar bifurcated hydrogen bond, in this paper a conipara tive quantum-cllexllical nnalysis of the rotational barriers in the asiaf arxi equatorial conformers ofcyclol~cxanol artd 1.3-dioxan-S-01 is undertaken.
2. Methods of calculation
INDO [ 1 i] computations 468
were carried out initially
in order to get a semi-empirical description of the system. This is needed prior to the ab initio study because of the large size of these systems. Computatians were carried out using Rinaidi’s method for geometry optimization [ill] (GEOMO program [ 131). Optimizations were terminated when the gradient of the unconstrained parameters was reduced to below 10e6 hartrcc/bollr or liartree~~dia~l_ The dihedral angle B (see @.I) was chosen as the independent variable, the energy being minimized with respect to the remaining geometric coordinates both in axial and equatorial conformers. The rotational harriers were then calculated using the STO-3G basis set [ 141 with tlw INDO optimized structures. 0 0092614/84/S ~or~l-Ho~and
03.00 0 Elsevier Science Publishers Physics Publishing Division)
B.V.
Volwne 109. number 5
CHEMICAL
PHYSICS
3. Results and discussion
31 _4ugusr 1981
LEl-l-ERS
E (kJ/mol) 2*
lND0 rotational barriers for axial and equatorial forms of 1,3-dioxan-5-01 arc shown in fig. 2. In both cases, there are three minima: two, at 0 = 120” and 240°, are symmetrical, and the third, which corresponds to the possible bifurcated hydrogen bond in the axial conformer, appears at B = O”. The axial conformation is the most stable for all values of 0, the presence of the hydrogen bond not being the decisive factor. However, the energy difference between the curves depends on the value of 8: the largest difference being in the 0 = O-40” zone. The energy difference between the rotamers 0 = 0 and 120” is 9.4 kJ/mol for the equatorial conformer, and only 6.2 kJ/mol for the axial conformer_ Rotational barriers for cyclohexanol show that the axial conformer is also the most stable, the energy difference between the rotamers at 8 = 0 and 120” being 9.4 and 9.3 kJ/mol for the equatorial and axial conformersl respectively. Rotational barriers in 1,3-dioxan-5-01 obtained with the STO-3G basis set are depicted in fig. 3. Both curves display the same minima as the INDO results, the relative stability of the minima, however, being different. In these cases, the absolute minimum occurs with the axial conformer at 0 = 0”. At this angle, an intramolecular bifurcated hydrogen bond may be formed_ which could explain the high stability of this
t=_;=~;-.---_--L_-i-; 0
LO
80
120 160 6 (DEGREES)
Rotational barriers in axial (---I and equatorial conformers of 1,3-diosan-5-01 by the INDO method. Energy relative to the wial rotamer 3t 0 = 120”. (Tot31
Fig. 1. (- --)
energy = -227374.9
kJ/mol.)
-__-
I
20
:‘
18 16I ‘L
I
I / -i
i2 ?O
a 6 L
I
I
I
2 3. i --I)
MY-.-
-
-
LO
a0
~-.-T
120 9
160
K?EGAEES!
I-is. 3. Rot;ltionaI barriers in xsial (--) and equatorial (---j conformers of 1,3-diosan-5-01 obtained with due STO-3G basis WI. Energy relative IO the sxiltl roramer ar 0 = 0”. (Total encr:y = -985996.7 kJ,‘mol.j conformation. For cyclohexanol, STO-3G results show that, at 0 = O”, the equatorial conformer is 12.3 kJ/mol more stable than the axial conformer, in qualitative agreement with experimental results in several solvents [ 15 1. However, there are no experimental values in the gas phase. The nature of the interaction between the hydrogen atom of the hydroxyl group and the osygen atoms of the ring is essentially electrostatic, as the following facts seem to show: (i) The overlap population between these atoms, obtained using a Mulliken population analysis with the ET03G basis set, is 0.0005. (ii) The H...O distance is 2.59 a, equal to the value calculated from microwave spectroscopy [S]. (iii) The O-H...0 a&e is 9% lo, the esperimental value being 100.4” [S] _ Both methods indicate that the axial form is the most stable conformation of 1,3-diosan-S-01 and that an electrostatic interaction exists between the hydro469
\rolttntc
109. ntttnbcr
5
CHEMICAL
1 atomic charges (in au) ior the axial conformation 1.3-diosan-S-o1 at 0 = 0” At0111
INDO
STO-3G
-0.29 0.45 -0.29 0.24 0.18 0.24 -0.29 0.16
-0.27 0.15 -0.27 0.01 0.06 0.01
LETTERS
31 August
1984
Acknowledgement
Table Net
PHYSICS
of
We would J. Figueruelo
like to express our appreciation for his helpful comments.
to Professor
-01 C?_ 03
C, CS C6 0 H
gen
atotn
of the
hydroxpl
group
References
-0.28
0.17
and the oxygen
atoms
of the ring for this conformation at 0 = 0”. However. differences beween the methods arise when attempting to quantitatively evaluate that interaction. These differences may be explained by taking into account that, at 0 = O”, the hydrogen atom of the hydroxyl group is located 2.59 ;i from the oxygen atoms of the ring and 2.57 ,% from the C4 and C(5 atOnlS. In table I, ths net effective charges on the ring centers and on the hydroxyl group, obtained by applying both mcthods, are indicated. As can be seen, the charges on the C, and C6 a1oms according to the INDO method are similar in magnitude but opposite in sign to those on rhe 0, and OS atoms, and, as a result, the net interaction between the H atom of the hydroxyl group and those four centers is negligible. However, according to the ab initio method, the charges on C4 and C6 are negligible, and therefore a net stabilizing interaction esisls.
470
S.A. Barker. J.S. Britnxombe, A.B. Foster, D.11. Whiffen and G. Zweifcl, Tetrahedron 7 (1959) 10. 121 EL. Eliei and h1.K. Kaloustian, Chem. Comtnun. (1970) 290. and 131 N. Baggctt, h1.A. Bukbari. A.B. Foster, J. Lehmann J.M. Webbcr. J. Chem. Sot. (1063) 4157. I41 S.A. Barker, A.B. Foster. A.H. Hnines. J. Lchmnnn. J.M. Wcbber and G. Zweifel, J. Chcm. Sot. (1963) 4161. ISI J. Gelas nnd K. Rtmlbaud. Uull. Sot. Chin>. France (1969) 1300. I61 I’. Calinaud and J. Celns. Bull. Sot. Chim. I’rttncc (1975) 1237. Tetrahedron Lcttcrs [71 J.C. Jochitns and Y. Kobayashi. (1976) 2065. 181 J.L. Alonso and E.B. Wilson. J. Am. Chern. Sot. 102 ( 1960) 1218. 191 J.L. Alonso. Analcs Quitn. 77 (1961) 76. V.L. Levcdev. R.T. Akltmadinov, EA. 1101 A.I. Naimushin, Kantor, D.L. Rachmankulov and Y.&l. Pnushkin, Dok:. Akad. Nauk. SSSR 25 I (I 980) 1420. 1111 J.A. Pople, D-L. Bcvcridgc and P.A. Dobosh, J. Chsm. I’hys. 47 (1967) 2026. Chem. 1 (1976) 109. 1121 D. Rinaldi. Computer 1131 D. Rinaldi, Thesis. Nancy (1975), QCPE no. 290. I141 W.J. Heltrc. R.F. Stewart and J.A. Poplc. J. Chem. Phys. 5 I (1969) 2657. J. Am. Chctn. Sot. 87 1151 EL. EIiel and S.H. Schroctcr. (1965) 5031.
Ill