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Rotational isomerism and internal rotation in some molecules containing carbon-oxygen or carbon-sulphur bonds
Journal of Molecular Structure, 32 (1976) 0 Elsevier Scientific Publishing Company,
35!5-363 Amsterdam’-
Printed in The Netherlands
ROTATIONAL ISOMERISM AND INTERNAL MOLEC~ES ALONG C~O~~XYGEN CARBON-SULPHUR BONDS
ROTATION OR
IN SOME
D. G. LISTER* School of Physical and Molecular
Sciences,
University
College
of North
Wales,
4, 40136
Bologna
Bangor
[Gt. Britain) P. PALMIERI Istituto di Chimica (First received
Fisiea e Spettroscopia,
ViaIe Risorgimento
30 May 1975; in final form 20 October
(Ituiy)
1975)
ABSTRACT SCF computations of the equilibrium conformations, electric dipole moments and barriers to internal rotation have been made for a number of molecules containing carbon-oxygen or carbon-sulphur bonds The computed and experimental results are compared and the significance of the differences discussed in detail. INTRODUCTION
The rotational isomerism and barriersto internal rotation in a number of analogous oxygen and sulphur compounds have now been investigated by microwave spectroscopy [l-16]. There does not seem to be any simple relationship between the barriers to internal rotation in the two series of molecules. Since knowledge of rotational isomerism and barriers to internal rotation is often useful in discussions about the differences in stereochemistry and reactivity between oxygen and sulphur compounds we have attempted to make a rational&&ion of the available experimental data in terms of quantum-mechanical SCF computations. AU of the molecules studied here may be considered to be of the general type R-X-R’ with R=H, Me, R’ an organic or inorganic group and X=0, S. Standard minimal basis sets of ST0/3G [17] and extended basis sets of STO/4.31G [lS] at omit orbitals have been used since these are now extensively used to interpret the stereochemistry of organic compounds [lS, 201. The SCF energies have been computed using the Gaussian 70 series of programs [Zl] _ The scheme of the computations has been to optimize in each molecule certain geometrical parameters which are likely to be sensitive *Present address: Department Dar Es Salaam, Tanzania.
of Chemistry,
University
of Dar Es Salaam,
P-0.
Box
35091,
to the rotational isomerism and also to compute electric dipole moments. As structural data and electric dipole moments are available from microwave investigations it is thus possible to make a check on the accuracy of the quota-mech~ic~ descriptions of these molecules tibtained with STO/3G and STOf4,31G atomic orbit&. RESULTS
Molecular geometries The R-X, X-R bond lengths and the R%R’ bond angles were optimized for various possible rotamers by a variational treatment of the molecular energies. The remaining geometrical parameters were either taken from the molecular structures given in the literature or estimated from the structures of related molecules. The equilibrium conformations and molecular geometries obtained in this way are collected in Table 1, All of the molecules containing methyl groups are seen to have equilibrium conformations in which the methyl group staggers the adjacent C-X bond. This is in agreement with the results of some of the microwave studies, where partially deuterated methyl groups have been used to determine its equilibrium orientation. In the CHs--X-CHCH2 and CH,-X-CHO molecules additional rotational isomerism associated with the X-R’ bond is possible, and in each case the correct equilibrium conformation is predicted by the computations. For the dimethyl ethers the equilibrium conformation is predicted to be that in which both methyl groups stagger the C-X bonds. In general, the computed bond lengths are slightly larger and the bond angles slightly smaller than the observed values. The bond angles appear most affected by the internal rotation, e.g. in CH,-O-CH=CH, and CH,--S-CH=CH, the bond angles are 5” greater in the eclipsed rotamers. Propargyl alcohol and propargyl mercaptan may be regarded as being derived from the parent CH,XH compounds by replacing one of the methyl hydrogens by an acetylene group. The potential hindering internal rotation no longer has threefold symmetry and the two different kinds of staggered rotamers shown in Fig. 1 may occur. In both cases the guuche rotamers (Fig. 1 (3)) are found to be more stable and the computed equilibrium dihedral angles of 121” for HOCH,CCH, and 126” for HSCH,CCH, as measured from the trans position are in excellent agreement with the experimental resuhs. Both phenol and t~ophenol are computed to be planar molecules.
Electric dipole moments The electric dipole moment and its components along the principal inertiaI axes for the most stable rotameric forms of the various molecules
Staggereda Staggered Staggered Staggered Staggered Staggered Cis t staggeredb Cis t staggered Cis t staggered Cis t staggered StaggerecY Staggered Gauched Gauche Planar Planar
1.44 1.81 1.45 1.82 1.46 1.81 1.44 1.81 1.44 1.80 1.44 1.81 0.99 1.34 0.99 1.33
1.43 1.82 1.43 1.82 1.39 1.82 1.45 1.81 1.44 1.80 1.41 1.80 0.95 1.34 0.96 1.30
0.99 1.34 1.36 1.72 1.75 2.09 1.39 1.78 1.33 1.77 1.44 1.81 1.45 1.83 1.40 1.78
(a)
(a)
(b)
R”X (A)
R-X (A)
1.34 1.31 1.68 1,67 2.01 1.33 1.77 1.33 1.77 1.41 1.80 1.43 1.82 1.36 1.77
0.95
(b) 103.8 94.9 112.2 98.6 109.4 98.0 116.1 103.1 113.4 99.1 110.5 98.6 104.0 94.7 106.2 96.7
(a)
R-X-R’ (A)
108.5 96.5 113.4 101.0 112.8 100.4 120.0 107.0 114.8 100.1 111.7 98.9 108.5 96.5 109.0 99.0
(b)
aConformation of the methyl group with respect to the X-R bond. bCis refers to the conformation of the methyl group with respect to the vinyl or carbonyl group and staggered gives its conformation with respect to the X-R’ bond, CBoth methyl groups are staggered with respect to the adjacent C-X bond. dGauche refers to the conformation shown in Fig, lB,
Methanol [l] CH,OH Methane thiol [ 21 CH,SH Methoxyethyne [ 31 CH,OCCH Methyl thioethyne [4] CH,SCCH Methyl hypochlorite [S] CH,OCI Methyl thiohypochlorite [6] CH,SCl Methyl vinyl ether [7] CH,OCHCH, Methyl vinyl sulphide [8] CH,SCHCH, Methyl formate [9] CH,OCHO Methyl thiolformate [lo] CH,SCHO Dimethyl ether [ll] (CH,),O Dimethyl sulphide [12] (CH,),S Ropargyl alcohol [ 131 HOCH,CCH Ropargyl mercaptan ] 141 HSCH,CCH Phenol [ 15 ] HOC,H, Thiophenol [ 161 HSC,H,
Equilibrium conformation
(a) Computed and (b) experimental geometries for some R-X-R’ molecules
TABLE 1
CA)
(E)
Fig. 1. (A) Truns and (B) gauche isomers of propargyl
alcohol
and mercaptan.
computed using the ST0/3G basis sets are compared with the values obtained from the Stark effect measurements in Table 2. On the whole, charge distributions in the oxygen compounds appear to be well represented, e.g. in CH30C1 the chlorine atom has a small positive charge in agreement with the high chlorine nuclear quadrupole coupling constant found in the direction of the O-Cl bond. An exception is the p, component of CH,OCCH where the polarisation effects induced in the triple bond by the adjacent oxygen atom are probably underestimated in the SCF charge distribution. In contrast ST0/3G orbitals appear to give rather a poor representation of the charge distributions in the sulphur compounds. All of the dipole moments are much lower than the observed values and an analysis reveals that the polarity of the C-S bonds is far too low. Probably only the S-H charge densities, pi., in CH$H and cc,in HSCH,CCH, are well represented. Electric dipole moments computed using the extended basis set are also given for some of the molecules in Table 2. For both oxygen and sulphur compounds the computed electric dipole moments are too high. Rotational
barriers
Computed and experimental barriers to internal rotation are compared in Table 3. In the ST0/3G computations, with the exception of CH3SCCH,CH3SCl and CHSSCHO, the computed barriers are all higher than the experimental values. Also, with the exception of HSCH,CCH, the barriers computed using these orbitals are lower for the sulphur compounds compared to those for the corresponding oxygen molecules. For some molecules barriers have also been derived using ST0/4_31G orbitals, and these are all lower than for the computations with the ST0/3G orbit&, with the exception of CHSOCHO. In several cases it may be noted that the experimental barriers fall between the computed values.
359
TABLE
2
(I) Experimental and computed, (II) ST0/3G, (III) ST0/4.31G for some R-X-R molecules
r,(D)
P,,(D)
r,(D)
P@)
0.89
1.44 1.27 1.93
0.0
0.82 1.56
0.0
0.0
1.69 1.52 2.48
CH,SH [23 ]
1.33 0.72 1.52
0.71 0.68 1.15
0.0 0.0 0.0
1.51 0.99 1.91
CH,OCCH
[ 31
1.41 0.75
1.32 1.39
0.0 0.0
1.93 1.58
CH,SCCH
[4]
1.00 0.68
1.36 0.90
0.0 0.0
1.69 1.13
CH,OCI
2.26 1.66
0.87 2.02
0.0 0.0
2.43 2.62
CH,SCl [S]
1.76 2.85
0.95 0.15
0.0 0.0
2.00 2.86
CH,OH 1221
I II
CH,OCHCH,
[7]
0.29 0.06
0.92 0.90
0.0 0.0
0.96 0.90
CH,SCHCH,
[ 81
0.07 0.01
1.13 0.64
0.0 0.0
1.14 0.64
CH,OCHO
[9]
1.42 1.09 1.76
0.68 0.30 0.65
0.0 0.0 0.0
1.77 1.12 1.88
CH,SCHO
[lo]
1.52 0.95 1.50
0.43 0.49 0.74
0.0 0.0 0.0
1.58 1.06 1.67
1.30 1.33 2.16
0.0 0.0
III
0.0 0.0 0.0
1.30 1.33 2.16
I II III
0.0 0.0 0.0
0.0 0.0
I II III
1.17 0.89 1.69
1.50 0.89 1.77 -
0.0 -
1.50 0.89 1.77 -
0.19 0.53
1.00 1.49
1.36 2.31
I II
0.72 0.27
0.50 0.25
0.81 0.62
1.19 0.72
HOC,H, [ 151
I II
0.13 0.36
1.26 1.19
0.0 0.0
1.27 1.24
HSC,H,
II
0.25
0.70
0.0
0.74
(CH,),S
1121
HOCH,CCH
HSCH,CCH
[13]
[24]
0.0
electric dipole momentsa
a~t is the modulus of the dipole moment and pa, cl,,, cc,the absolute values of the components along the inertial axes.
TABLE
3
(I) Experimental and computed, (II) ST0/3G, (III) ST0/4.31G
CH,OH [l] CH,SH [ 21 CH,OCCH [3] CH,SCCH [ 4 ] CH,OCI 153 CH,SCI [63 CH,OCHCH, [7] CH,SCHCH, [ 81 CH,OCHO [ 9 ] CH,SCHO [ 101 tCH,),O 1111 (CH,),S [121 HOCH,CCH [ 131” HSCH,CCH [14]= HOC,H, [ 151 HSC,H, [ 161
I (kJ mol-‘)
II (kJ mol-‘)
4.5 5.3 6.0 7.3 12.7 10.9 16.0 13.5 5.0 >10.5 11.4 8.9 1.1 6.4 14.0 -
7.8 5.7 7.7 5.7 13.7 7.5 23.5 21.5 3.6 2.4 26.1 18.5 3.3 5.6 17.8 13.9
ZJ mol-
barriers to internal rotation
’)
3.6 3.8 4.1 9.8 14.2 6.6 1.2 16.0 15.1 2.2
aThe barrier at the cis position.
DISCUSSION
The present computations are not close enough to the Hartree-Fock limit to allow a detailed quantitative analysis of the origins of the rotational barriers but they do permit a certain amount of rationalisation of the experimental results. It has been shown that the barrier to internal rotation in methanol 1‘25~-271 mainly arises from repulsive interactions between the bonding pairs of electrons in the methyl group and the hydroxyl bonding and lone pairs on the oxygen. Indirect evidence for this type of interaction comes from the differences in the charge distributions [28] between the eclipsed and staggered conformations. The computations indicate that the barriersin CH,SH, CH,OCCH and CH,SCCH are of a similar nature. In the sulphur compounds the increased X-R bond distance is balanced by an increase in the size of the sulphur lone pairs and therefore the observed and computed barriers for these molecules are similar to those for the oxygen compounds. In the split valence shell description (ST0/4_31G) of these molecules the interelectronic repulsions are reduced by suitable polarisation of the electronic charge densities, thus accounting for the low barriers obtained using these orbitals. SCF computations of near Hartree-Fock quality have been reported for methanol [ 291. Conformational energies have also been obtained for CH&H using double zeta basis set of contracted gaussianorbit& and with the inclusion
361
of d orbitals on the sulphur atom 130,311. A common feature to the present
and previous, more accurate, computations on methanol and methane thiol is that the trend of the computed rotational barriers (CH,OH > CH,SH) is contrary to the experimental cl,21 values. In CH,XCl molecules the sharp increase in the barriers is due, according to the computations, to repulsive interactions between the electrons of the methyl group and the charge density surrounding the chlorine atom. In the cis barrier of HOCH&CH these repulsive interactions are partially off set by a bonding interaction between the positive hydroxylic hydrogen atom and the electrons of the triple bond. In HSCNzCCW this interaction is absent because of the reduced polarity of the SH bond. In all of the remaining molecules these computations indicate that the main contributions to the barriers come from bonding interactions. In the CI13XCHCH2 and CH3XCH0 molecules the stability of the staggered with respect to the eclipsed rotamers is explained in terms of a conjugative interaction between the methyl hydrogen 1s orbit& and the 7rorbit& of the double bond as shown in E’ig. Z(A) and 2(B). The equilibrium conformations of these molecules, in which the methyl group is ck to the double bond, is also due to these interactions_ In this conformation the occupied 7~molecular orbitals bear some formal resemblance to those of a five-membered ring. In the dimethyl ethers the hydrogen Is orbitals of both methyl groups participate
(A)
Fig. 2. Conjugative interactions sf 1s methyl hydrogen orbitals in the staggered forms of: (A) methyl vinyl ether and tbioether; (B) methyl formate and thiofformate and (C) dimethyl ether and thioether.
in such an interaction (Fig. 2(C)). Similarly, the staggered forms of several double methyl rotors have been explained to be more stable than the corresponding staggered-eclipsed conformers by an %romatic” stabilisation involving bonding between the methyl groups [ 321. These conjugative effects appear to be exaggerated in the ST0/3G computations but more correctly described by the STO/4.31G computations. The lower barriersin the sulphur compounds, with the exception of CH,SCHO, are explained by the poorer conjugating properties of sulphur 3p, orbitals compared to oxygen 2p, orbitals. In the C6H5XH molecules conjugation of the X atom with the aromatic ring stabilizes the planar form, gives some double bond character to the C-X bond and accounts for the high barriers in these molecules [33]. Once again the difference in the barriers reflects the difference in the conjugative properties of sulphur and oxygen atoms. Similar computations have recently been made on CH,OC,H, and CH,SC,H, 1341 and indicate differences in the rotational isomerism of these molecules. Anisole is predicted to have a planar heavy atom skeleton while in thioanisole the computations indicate nearly equal energies for the planar and o&of-plane conformations. In the latter molecule steric interactions between the methyl and ortho hydrogen atoms could lead to the existence of non planar rotamers. ACKNOWLEDGEMENTS
D. G. L. thanks the University College of North Wales and the Royal Society, and P. P. the C.N.R. of Italy for financial support.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
R. M. Lees and J. G. Baker, J. Chem. Phys., 48 (1968) 5299. T. Kojima, J. Phys. Sot. Jap., 15 (1960) 1284. D. Den Engelesen, H. A Dykerman and J. Kerresen, Rec. Trav. Chim., 84 (1965) D. Den Engelesen, J. Mol. Spectrosc., 22 (1967) 426. J. R. Rigden and S. S. Butcher, J. Chem. Phys., 40 (1964) 2109. A. Guarneri, Z. Naturforsch., 18 (1970) 299. P. Cahill, P. Gold and N. L. Owen, J. Chem. Phys., 48 (1968) 1620. R. E Penn and F_ A Curl, J_ MoL Spectrosc., 24 (1967) 235. F. A. Curl, J. Chem. Phys, 30 (1959) 1529. M. C. L. Gerry, G. I. L. Jones, D. G. Lister, N. L. Owen and P. Palmieri, J. Mol. Spectrosc., in press. W. Benkis, P. H. Kasai and R. J. Meyers, J. Chem. Phys., 38 (1963) 2753. L. Pierce and H. Hayashi, J. Chem. Phys., 35 (1961) 479. E. Hirota, J. Mol. Spectrosc., 26 (196g 335. F. Scappini, H. Mader and J. Sheridan, Z. Naturforsch. A, 28 (1973) 77. T. Pederson, N. W. Larsen and L. Nygaard, J. Mol. Struct., 4 (1969) 59. K. I. Johannson, H. Oldenberg and I-L Selen, Ark. Phys., 33 (1966) 313. W_ J. Hehre, R. F. Stewart and J. A. Pople, J. Chem. Phys., 51 (1969) 2.657. R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 54 (1971) 7.
1357.
363 19 L. Radom and J. A. Popie, J. Amer. Chem. Sec., 82 (1970) 4786. 20 L. Radom, W. J. Hehre and J. A. Pople, J. Amer. Chem. Sot., 83 (1971) 289. 21 W. J. Hehre, W, A Lathan, R. Ditchfield, M, D. Newton and J. A. Popie, Gaussian 70: ab initio SCF-MO Calculations on Organic Molecules, Program no. 236, Quantum Chemistry Program Exchange, Indiana University, U.S.A. 22 D. G. Burzhard and D. M. Dennison, Phys. Rev., 84 f1951) 408, 23 T. Kojima and T. Nishikawa, J. Phys. Sot. Jap,, 12 (1957) 680. 24 H. Mader, A. M. Mirri and F, Scappini, J. Mol. Spectrosc., 57 (19’75) 264. 25 W. H. Fink and L. C. Allen, J. Chem. Phys., 46 (1967) 2261. 26 L. Pedersen and K. Morokuma, 3. Chem. Phys., 46 (1967) 3941. 27 L. C. Allen, Chem. Phys. Lett., 2 (1968) 587. 28 W. L. Jorgensen and L. C. Allen, J. Amer. Chem, Sot., 93 (1971) 567. 29 L. M, Tell, S. Wolfe and I. G. Csizmadia, J. Chem. Phys., 59 (1973) 4047. 30 S. Wolfe, L. M, Tell and I. G. Csizmadia, Theor. Chim. Acta, 31 (1973) 355. 31 F. Bernardi, I. G. Csizmadia, A, Mangini, H. B. Schiegel, M. H. Whangbo and S. Wolfe, J. Amer. Chem. Sot., 97 (1975) 2209. 32 D. Cremer, J. J. Binkley, J. A. Pople and W. J. Hehre, J. Amer.Chem. Sot., 96 (1974) 6900. 33 W. J. Hehre, L. Radom and J. A. Pople, J. Amer. Chem. Sot., 94 (1972) 1496. 34 P. Palmieri, F. Tuflini, B. Velino and C_ Zauh, Gazz. Chim. Ital., 105 (1975) 919.
35!5-363 Amsterdam’-
Printed in The Netherlands
ROTATIONAL ISOMERISM AND INTERNAL MOLEC~ES ALONG C~O~~XYGEN CARBON-SULPHUR BONDS
ROTATION OR
IN SOME
D. G. LISTER* School of Physical and Molecular
Sciences,
University
College
of North
Wales,
4, 40136
Bologna
Bangor
[Gt. Britain) P. PALMIERI Istituto di Chimica (First received
Fisiea e Spettroscopia,
ViaIe Risorgimento
30 May 1975; in final form 20 October
(Ituiy)
1975)
ABSTRACT SCF computations of the equilibrium conformations, electric dipole moments and barriers to internal rotation have been made for a number of molecules containing carbon-oxygen or carbon-sulphur bonds The computed and experimental results are compared and the significance of the differences discussed in detail. INTRODUCTION
The rotational isomerism and barriersto internal rotation in a number of analogous oxygen and sulphur compounds have now been investigated by microwave spectroscopy [l-16]. There does not seem to be any simple relationship between the barriers to internal rotation in the two series of molecules. Since knowledge of rotational isomerism and barriers to internal rotation is often useful in discussions about the differences in stereochemistry and reactivity between oxygen and sulphur compounds we have attempted to make a rational&&ion of the available experimental data in terms of quantum-mechanical SCF computations. AU of the molecules studied here may be considered to be of the general type R-X-R’ with R=H, Me, R’ an organic or inorganic group and X=0, S. Standard minimal basis sets of ST0/3G [17] and extended basis sets of STO/4.31G [lS] at omit orbitals have been used since these are now extensively used to interpret the stereochemistry of organic compounds [lS, 201. The SCF energies have been computed using the Gaussian 70 series of programs [Zl] _ The scheme of the computations has been to optimize in each molecule certain geometrical parameters which are likely to be sensitive *Present address: Department Dar Es Salaam, Tanzania.
of Chemistry,
University
of Dar Es Salaam,
P-0.
Box
35091,
to the rotational isomerism and also to compute electric dipole moments. As structural data and electric dipole moments are available from microwave investigations it is thus possible to make a check on the accuracy of the quota-mech~ic~ descriptions of these molecules tibtained with STO/3G and STOf4,31G atomic orbit&. RESULTS
Molecular geometries The R-X, X-R bond lengths and the R%R’ bond angles were optimized for various possible rotamers by a variational treatment of the molecular energies. The remaining geometrical parameters were either taken from the molecular structures given in the literature or estimated from the structures of related molecules. The equilibrium conformations and molecular geometries obtained in this way are collected in Table 1, All of the molecules containing methyl groups are seen to have equilibrium conformations in which the methyl group staggers the adjacent C-X bond. This is in agreement with the results of some of the microwave studies, where partially deuterated methyl groups have been used to determine its equilibrium orientation. In the CHs--X-CHCH2 and CH,-X-CHO molecules additional rotational isomerism associated with the X-R’ bond is possible, and in each case the correct equilibrium conformation is predicted by the computations. For the dimethyl ethers the equilibrium conformation is predicted to be that in which both methyl groups stagger the C-X bonds. In general, the computed bond lengths are slightly larger and the bond angles slightly smaller than the observed values. The bond angles appear most affected by the internal rotation, e.g. in CH,-O-CH=CH, and CH,--S-CH=CH, the bond angles are 5” greater in the eclipsed rotamers. Propargyl alcohol and propargyl mercaptan may be regarded as being derived from the parent CH,XH compounds by replacing one of the methyl hydrogens by an acetylene group. The potential hindering internal rotation no longer has threefold symmetry and the two different kinds of staggered rotamers shown in Fig. 1 may occur. In both cases the guuche rotamers (Fig. 1 (3)) are found to be more stable and the computed equilibrium dihedral angles of 121” for HOCH,CCH, and 126” for HSCH,CCH, as measured from the trans position are in excellent agreement with the experimental resuhs. Both phenol and t~ophenol are computed to be planar molecules.
Electric dipole moments The electric dipole moment and its components along the principal inertiaI axes for the most stable rotameric forms of the various molecules
Staggereda Staggered Staggered Staggered Staggered Staggered Cis t staggeredb Cis t staggered Cis t staggered Cis t staggered StaggerecY Staggered Gauched Gauche Planar Planar
1.44 1.81 1.45 1.82 1.46 1.81 1.44 1.81 1.44 1.80 1.44 1.81 0.99 1.34 0.99 1.33
1.43 1.82 1.43 1.82 1.39 1.82 1.45 1.81 1.44 1.80 1.41 1.80 0.95 1.34 0.96 1.30
0.99 1.34 1.36 1.72 1.75 2.09 1.39 1.78 1.33 1.77 1.44 1.81 1.45 1.83 1.40 1.78
(a)
(a)
(b)
R”X (A)
R-X (A)
1.34 1.31 1.68 1,67 2.01 1.33 1.77 1.33 1.77 1.41 1.80 1.43 1.82 1.36 1.77
0.95
(b) 103.8 94.9 112.2 98.6 109.4 98.0 116.1 103.1 113.4 99.1 110.5 98.6 104.0 94.7 106.2 96.7
(a)
R-X-R’ (A)
108.5 96.5 113.4 101.0 112.8 100.4 120.0 107.0 114.8 100.1 111.7 98.9 108.5 96.5 109.0 99.0
(b)
aConformation of the methyl group with respect to the X-R bond. bCis refers to the conformation of the methyl group with respect to the vinyl or carbonyl group and staggered gives its conformation with respect to the X-R’ bond, CBoth methyl groups are staggered with respect to the adjacent C-X bond. dGauche refers to the conformation shown in Fig, lB,
Methanol [l] CH,OH Methane thiol [ 21 CH,SH Methoxyethyne [ 31 CH,OCCH Methyl thioethyne [4] CH,SCCH Methyl hypochlorite [S] CH,OCI Methyl thiohypochlorite [6] CH,SCl Methyl vinyl ether [7] CH,OCHCH, Methyl vinyl sulphide [8] CH,SCHCH, Methyl formate [9] CH,OCHO Methyl thiolformate [lo] CH,SCHO Dimethyl ether [ll] (CH,),O Dimethyl sulphide [12] (CH,),S Ropargyl alcohol [ 131 HOCH,CCH Ropargyl mercaptan ] 141 HSCH,CCH Phenol [ 15 ] HOC,H, Thiophenol [ 161 HSC,H,
Equilibrium conformation
(a) Computed and (b) experimental geometries for some R-X-R’ molecules
TABLE 1
CA)
(E)
Fig. 1. (A) Truns and (B) gauche isomers of propargyl
alcohol
and mercaptan.
computed using the ST0/3G basis sets are compared with the values obtained from the Stark effect measurements in Table 2. On the whole, charge distributions in the oxygen compounds appear to be well represented, e.g. in CH30C1 the chlorine atom has a small positive charge in agreement with the high chlorine nuclear quadrupole coupling constant found in the direction of the O-Cl bond. An exception is the p, component of CH,OCCH where the polarisation effects induced in the triple bond by the adjacent oxygen atom are probably underestimated in the SCF charge distribution. In contrast ST0/3G orbitals appear to give rather a poor representation of the charge distributions in the sulphur compounds. All of the dipole moments are much lower than the observed values and an analysis reveals that the polarity of the C-S bonds is far too low. Probably only the S-H charge densities, pi., in CH$H and cc,in HSCH,CCH, are well represented. Electric dipole moments computed using the extended basis set are also given for some of the molecules in Table 2. For both oxygen and sulphur compounds the computed electric dipole moments are too high. Rotational
barriers
Computed and experimental barriers to internal rotation are compared in Table 3. In the ST0/3G computations, with the exception of CH3SCCH,CH3SCl and CHSSCHO, the computed barriers are all higher than the experimental values. Also, with the exception of HSCH,CCH, the barriers computed using these orbitals are lower for the sulphur compounds compared to those for the corresponding oxygen molecules. For some molecules barriers have also been derived using ST0/4_31G orbitals, and these are all lower than for the computations with the ST0/3G orbit&, with the exception of CHSOCHO. In several cases it may be noted that the experimental barriers fall between the computed values.
359
TABLE
2
(I) Experimental and computed, (II) ST0/3G, (III) ST0/4.31G for some R-X-R molecules
r,(D)
P,,(D)
r,(D)
P@)
0.89
1.44 1.27 1.93
0.0
0.82 1.56
0.0
0.0
1.69 1.52 2.48
CH,SH [23 ]
1.33 0.72 1.52
0.71 0.68 1.15
0.0 0.0 0.0
1.51 0.99 1.91
CH,OCCH
[ 31
1.41 0.75
1.32 1.39
0.0 0.0
1.93 1.58
CH,SCCH
[4]
1.00 0.68
1.36 0.90
0.0 0.0
1.69 1.13
CH,OCI
2.26 1.66
0.87 2.02
0.0 0.0
2.43 2.62
CH,SCl [S]
1.76 2.85
0.95 0.15
0.0 0.0
2.00 2.86
CH,OH 1221
I II
CH,OCHCH,
[7]
0.29 0.06
0.92 0.90
0.0 0.0
0.96 0.90
CH,SCHCH,
[ 81
0.07 0.01
1.13 0.64
0.0 0.0
1.14 0.64
CH,OCHO
[9]
1.42 1.09 1.76
0.68 0.30 0.65
0.0 0.0 0.0
1.77 1.12 1.88
CH,SCHO
[lo]
1.52 0.95 1.50
0.43 0.49 0.74
0.0 0.0 0.0
1.58 1.06 1.67
1.30 1.33 2.16
0.0 0.0
III
0.0 0.0 0.0
1.30 1.33 2.16
I II III
0.0 0.0 0.0
0.0 0.0
I II III
1.17 0.89 1.69
1.50 0.89 1.77 -
0.0 -
1.50 0.89 1.77 -
0.19 0.53
1.00 1.49
1.36 2.31
I II
0.72 0.27
0.50 0.25
0.81 0.62
1.19 0.72
HOC,H, [ 151
I II
0.13 0.36
1.26 1.19
0.0 0.0
1.27 1.24
HSC,H,
II
0.25
0.70
0.0
0.74
(CH,),S
1121
HOCH,CCH
HSCH,CCH
[13]
[24]
0.0
electric dipole momentsa
a~t is the modulus of the dipole moment and pa, cl,,, cc,the absolute values of the components along the inertial axes.
TABLE
3
(I) Experimental and computed, (II) ST0/3G, (III) ST0/4.31G
CH,OH [l] CH,SH [ 21 CH,OCCH [3] CH,SCCH [ 4 ] CH,OCI 153 CH,SCI [63 CH,OCHCH, [7] CH,SCHCH, [ 81 CH,OCHO [ 9 ] CH,SCHO [ 101 tCH,),O 1111 (CH,),S [121 HOCH,CCH [ 131” HSCH,CCH [14]= HOC,H, [ 151 HSC,H, [ 161
I (kJ mol-‘)
II (kJ mol-‘)
4.5 5.3 6.0 7.3 12.7 10.9 16.0 13.5 5.0 >10.5 11.4 8.9 1.1 6.4 14.0 -
7.8 5.7 7.7 5.7 13.7 7.5 23.5 21.5 3.6 2.4 26.1 18.5 3.3 5.6 17.8 13.9
ZJ mol-
barriers to internal rotation
’)
3.6 3.8 4.1 9.8 14.2 6.6 1.2 16.0 15.1 2.2
aThe barrier at the cis position.
DISCUSSION
The present computations are not close enough to the Hartree-Fock limit to allow a detailed quantitative analysis of the origins of the rotational barriers but they do permit a certain amount of rationalisation of the experimental results. It has been shown that the barrier to internal rotation in methanol 1‘25~-271 mainly arises from repulsive interactions between the bonding pairs of electrons in the methyl group and the hydroxyl bonding and lone pairs on the oxygen. Indirect evidence for this type of interaction comes from the differences in the charge distributions [28] between the eclipsed and staggered conformations. The computations indicate that the barriersin CH,SH, CH,OCCH and CH,SCCH are of a similar nature. In the sulphur compounds the increased X-R bond distance is balanced by an increase in the size of the sulphur lone pairs and therefore the observed and computed barriers for these molecules are similar to those for the oxygen compounds. In the split valence shell description (ST0/4_31G) of these molecules the interelectronic repulsions are reduced by suitable polarisation of the electronic charge densities, thus accounting for the low barriers obtained using these orbitals. SCF computations of near Hartree-Fock quality have been reported for methanol [ 291. Conformational energies have also been obtained for CH&H using double zeta basis set of contracted gaussianorbit& and with the inclusion
361
of d orbitals on the sulphur atom 130,311. A common feature to the present
and previous, more accurate, computations on methanol and methane thiol is that the trend of the computed rotational barriers (CH,OH > CH,SH) is contrary to the experimental cl,21 values. In CH,XCl molecules the sharp increase in the barriers is due, according to the computations, to repulsive interactions between the electrons of the methyl group and the charge density surrounding the chlorine atom. In the cis barrier of HOCH&CH these repulsive interactions are partially off set by a bonding interaction between the positive hydroxylic hydrogen atom and the electrons of the triple bond. In HSCNzCCW this interaction is absent because of the reduced polarity of the SH bond. In all of the remaining molecules these computations indicate that the main contributions to the barriers come from bonding interactions. In the CI13XCHCH2 and CH3XCH0 molecules the stability of the staggered with respect to the eclipsed rotamers is explained in terms of a conjugative interaction between the methyl hydrogen 1s orbit& and the 7rorbit& of the double bond as shown in E’ig. Z(A) and 2(B). The equilibrium conformations of these molecules, in which the methyl group is ck to the double bond, is also due to these interactions_ In this conformation the occupied 7~molecular orbitals bear some formal resemblance to those of a five-membered ring. In the dimethyl ethers the hydrogen Is orbitals of both methyl groups participate
(A)
Fig. 2. Conjugative interactions sf 1s methyl hydrogen orbitals in the staggered forms of: (A) methyl vinyl ether and tbioether; (B) methyl formate and thiofformate and (C) dimethyl ether and thioether.
in such an interaction (Fig. 2(C)). Similarly, the staggered forms of several double methyl rotors have been explained to be more stable than the corresponding staggered-eclipsed conformers by an %romatic” stabilisation involving bonding between the methyl groups [ 321. These conjugative effects appear to be exaggerated in the ST0/3G computations but more correctly described by the STO/4.31G computations. The lower barriersin the sulphur compounds, with the exception of CH,SCHO, are explained by the poorer conjugating properties of sulphur 3p, orbitals compared to oxygen 2p, orbitals. In the C6H5XH molecules conjugation of the X atom with the aromatic ring stabilizes the planar form, gives some double bond character to the C-X bond and accounts for the high barriers in these molecules [33]. Once again the difference in the barriers reflects the difference in the conjugative properties of sulphur and oxygen atoms. Similar computations have recently been made on CH,OC,H, and CH,SC,H, 1341 and indicate differences in the rotational isomerism of these molecules. Anisole is predicted to have a planar heavy atom skeleton while in thioanisole the computations indicate nearly equal energies for the planar and o&of-plane conformations. In the latter molecule steric interactions between the methyl and ortho hydrogen atoms could lead to the existence of non planar rotamers. ACKNOWLEDGEMENTS
D. G. L. thanks the University College of North Wales and the Royal Society, and P. P. the C.N.R. of Italy for financial support.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
R. M. Lees and J. G. Baker, J. Chem. Phys., 48 (1968) 5299. T. Kojima, J. Phys. Sot. Jap., 15 (1960) 1284. D. Den Engelesen, H. A Dykerman and J. Kerresen, Rec. Trav. Chim., 84 (1965) D. Den Engelesen, J. Mol. Spectrosc., 22 (1967) 426. J. R. Rigden and S. S. Butcher, J. Chem. Phys., 40 (1964) 2109. A. Guarneri, Z. Naturforsch., 18 (1970) 299. P. Cahill, P. Gold and N. L. Owen, J. Chem. Phys., 48 (1968) 1620. R. E Penn and F_ A Curl, J_ MoL Spectrosc., 24 (1967) 235. F. A. Curl, J. Chem. Phys, 30 (1959) 1529. M. C. L. Gerry, G. I. L. Jones, D. G. Lister, N. L. Owen and P. Palmieri, J. Mol. Spectrosc., in press. W. Benkis, P. H. Kasai and R. J. Meyers, J. Chem. Phys., 38 (1963) 2753. L. Pierce and H. Hayashi, J. Chem. Phys., 35 (1961) 479. E. Hirota, J. Mol. Spectrosc., 26 (196g 335. F. Scappini, H. Mader and J. Sheridan, Z. Naturforsch. A, 28 (1973) 77. T. Pederson, N. W. Larsen and L. Nygaard, J. Mol. Struct., 4 (1969) 59. K. I. Johannson, H. Oldenberg and I-L Selen, Ark. Phys., 33 (1966) 313. W_ J. Hehre, R. F. Stewart and J. A. Pople, J. Chem. Phys., 51 (1969) 2.657. R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 54 (1971) 7.
1357.
363 19 L. Radom and J. A. Popie, J. Amer. Chem. Sec., 82 (1970) 4786. 20 L. Radom, W. J. Hehre and J. A. Pople, J. Amer. Chem. Sot., 83 (1971) 289. 21 W. J. Hehre, W, A Lathan, R. Ditchfield, M, D. Newton and J. A. Popie, Gaussian 70: ab initio SCF-MO Calculations on Organic Molecules, Program no. 236, Quantum Chemistry Program Exchange, Indiana University, U.S.A. 22 D. G. Burzhard and D. M. Dennison, Phys. Rev., 84 f1951) 408, 23 T. Kojima and T. Nishikawa, J. Phys. Sot. Jap,, 12 (1957) 680. 24 H. Mader, A. M. Mirri and F, Scappini, J. Mol. Spectrosc., 57 (19’75) 264. 25 W. H. Fink and L. C. Allen, J. Chem. Phys., 46 (1967) 2261. 26 L. Pedersen and K. Morokuma, 3. Chem. Phys., 46 (1967) 3941. 27 L. C. Allen, Chem. Phys. Lett., 2 (1968) 587. 28 W. L. Jorgensen and L. C. Allen, J. Amer. Chem, Sot., 93 (1971) 567. 29 L. M, Tell, S. Wolfe and I. G. Csizmadia, J. Chem. Phys., 59 (1973) 4047. 30 S. Wolfe, L. M, Tell and I. G. Csizmadia, Theor. Chim. Acta, 31 (1973) 355. 31 F. Bernardi, I. G. Csizmadia, A, Mangini, H. B. Schiegel, M. H. Whangbo and S. Wolfe, J. Amer. Chem. Sot., 97 (1975) 2209. 32 D. Cremer, J. J. Binkley, J. A. Pople and W. J. Hehre, J. Amer.Chem. Sot., 96 (1974) 6900. 33 W. J. Hehre, L. Radom and J. A. Pople, J. Amer. Chem. Sot., 94 (1972) 1496. 34 P. Palmieri, F. Tuflini, B. Velino and C_ Zauh, Gazz. Chim. Ital., 105 (1975) 919.