J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1211-1215 121 1 Molecular Conformations and Rotation Barriers of 2-Halogenoethanethiols and 2-Halogenoethanols :An arb initio Study Giuseppe Buemi Dipartimento di Scienze Chimiche, Universita di Catania, Viale A. Doria nr. 6,95125,Catania, Italy The molecular geometries of all the possible conformations of 2-halogenoethanethiols and 2-halogenoethanols (F, CI, Br, I) have been fully optimized at the ab initio MP2/6-31G** level (for Br and I the LANLlDZ basis was adopted). Whilst for all 2-halogenoethanols and for 2-fluoroethanethiol the Gg' structure was found to be the most stable, the Tg rotamer is favoured for 2-chloro-, 2-bromo- and 2-iodo-ethanethiols. In these latter molecules the hydrogen-bond strength seems to be insufficient to overcome the non-bonded repulsive interactions and to stabilize the Gg' conformer.The rotation barriers around the C(l)-C(2) bond are rather high, whilst those concerning the SH and OH groups are generally small and of similar size. Experimental measurements (ref. 1-9 and references cited therein) and ab initio calculations performed by using various basis sets (ref. 10-12 and references cited therein) have shown that a gauche-gauche (Gg) conformation is the most stable rotamer of halogenoethanols, in both the gas and liquid phases. This stability is usually attributed to formation of a Y. .H-0 intramolecular hydrogen bridge (which cannot occur in the trans structures) although doubts about this interpretation arise from the prevalence of the Gg rotamer in 2-fluoroethylacetate and 2-fluoroethyltrichloroacetate,where no hydrogen bond is po~sible.'~*'~ The related hydrogen- bond energy, if any, is expected to be reasonably small.In 2-fluoroethanol it was evaluated to be 9.2 and 7.95 kJ mol- ' from electron diffraction2' and ab initio calculations,' respectively, with negligible contribution from the gauche effect.I2 A strength of ca. 10 kJ mol-' is suggested for 2- chloroethanol,2b in contrast with a previous study," where it was estimated to be ca. 2.7 kJ mol-' lower than in 2- fluoroethanol. Since the strength of a hydrogen bridge depends highly on the electronegativity of the atoms involved, its energy in 2-halogenoethanethiols must be lower than in 2-halogenoethanols, so that in the sulfurated mol- ecules the Tt and/or the Tg conformations would be pre- ferred.Experimental data on these latter compounds are scarce and not conclusive. The microwave spectrum of 2-chloroethanethiol, although largely intractable,' allowed identification of only one rota- tional isomer, which exists in a heavy atoms trans conforma-tion, but also showed numerous lines arising, probably, from gauche species. IR spectral6 suggest that the gauche con-former is ca. 0.5 kcal mol-' less stable than the trans con-former. Ab initio calculations (3-21G and 6-31G*)17*18 indicate that the most stable rotamer is the trans-gauche structure, which was found to be 0.79 and 1.32 kcal mol- more stable than the Gg' and Tt conformers,? respectively.More recent 3-21G cal~ulations'~ confirm the gauche accommodation of the S-H group in both 2-fluoro- and 2-chloro-ethanethiol. No theoretical study was found in the literature for 2-bromo- and 2-iodo-e t hanet hiols. The aim of the present paper is a systematic ab initio study of the title compounds in order to calculate geometries and energies of all the possible conformations of 2-fluoro-, 2-chloro-, 2-bromo- and 2-iodo-ethanethiol, and to estimate roughly the hydrogen-bridge energy if any (or at least, the t The Gg and Gg' rotamer differ from each other by having differ- ent torsion angles around the C-C bond (in the former ca. 60°, in the latter ca. -60"). existence of favourable accommodation for hydrogen-bridge formation). A further goal is to evaluate the rotation barriers in the various interconversion pathways of the most stable rotamers.For comparison purposes, calculations were carried out also on the corresponding oxygenated com-pounds, 2-halogenoethanols, for which more experimental and theoretical data are available in the literature. Calculations All calculations were carried out by means of the GAUSSIAN92 program,20 running on a DIGITAL ALPHA- 3400 workstation as a translated image of VAX executable sources. For building the energy curves for 2-fluoro- and 2-chloro-ethanethiols the 3-21G* basis set, with fully geometry optimization, was adopted. Then all the minima and maxima points were fully optimized at the MP2/6-31G** level.Since such bases are not available for bromine and iodine, calcu- lations on the 2-bromo- and 2-iodo-derivatives were per- formed using the LANLlDZ basis set," which uses the Dunning-Huzinaga valence double zeta (D95V) basis22 for the first-row elements and the Los Alamos ECP +DZ basis23 (ECP = effective core potentials) for the elements from Na to Bi. Here also, minima and maxima were then reoptimized taking into account the correlation energy (MP2/ LANLlDZ). Fig. 1 and 2 were produced by means of the Harvard Graphics program, version 2.0. Results and Discussion The energy curves of 2-halogenoethanethiols, calculated at the 3-21G* level for rotation from 0" to 360" around the C-C and C-S bonds are shown in Fig. 1. Those of the corresponding 2-halogenoethanols were assumed to have analogous trends.Such an assumption is supported by the energy curves for 2-fluoro- and 2-chloro-ethanols, shown in Fig. 2, which were calculated at both the MP2/3-21G* and the MP2/LANLlDZ level (with fully geometry optimization) in order to check the limit of comparability of the results from the two bases. The optimized geometrical parameters obtained by MP2/LANLlDZ calculations are not reported here, but are available upon request. The main differences from those of the MP2/3-21G* basis are: (i) The energy curves concerning the rotation around the C-C bond show analogous trends whilst differences are noted in those concerning the rotation of the OH group. In particular the Gt conformation is no longer found when the LANLlDZ basis is adopted (a flex point appears instead of a minimum-energy point); optimization of the geometry of 1212 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 such a rotamer evolves towards the Gg' conformer. This does not occur for 2-bromo- or 2-iodo-ethanol or any of the 2- --halogenoethanethiols. ;20 g 20 (ii) The bond lengths predicted by the LANLlDZ basis are 7 7 longer than those from the 3-21G* and 6-31G** (with and $1 0 $10 without inclusion of correlation energy); in some cases the d 0 d 0 difference can reach ca. 0.1 A. Bond angles are much less affected, differences being limited to one degree or less. 0 60 180 -60 0 0 60 180 -60 0 120 -120 120 -120 (iii) Inversion of the stability order can occur when AE values are very small.In the light of the above remarks we must remember that in the following discussion only qualitative comparison can be made between the results for bromo- and iodo-derivatives I-(MP2/LANLlDZ) and those for fluoro-and chloro-z 20 derivatives (MP2/6-3 lG**). 7 From Fig. 1 it can be seen that rotation around C-C $10 gives rise to three minima, whilst rotation around C-S determines two other minima, so that five rotamers are pos- n" 0 60 180 -60 0 0 60 180 -60 0 sible for each molecule. Following ref. 10, they are labelled as 120 -120 120 -120 Gg, Tg, Gg', Gt and Tt (Fig. 3), where the upper-case letter torsion ang le/deg rees torsion angle/degrees refers to the torsion angle around the C-C bond and the Fig.1 Potential-energy curves calculated at 3-21G* (F and C- lower-case letter to the torsion angle around the C-X bondderivatives) and LANLlDZ (Br and I derivatives) levels. (0)Series 1 : rotation around the C-C bond; (+) series 2: rotation around the (rotation of the SH or OH groups). All except the Tt form, C-X bond in the Gg structure; (*) series 3: rotations around the have statistical weight, g = 2. C-X bond in the Tg structure. (a) 2-Fluoroethanethiol, (b) 2-The main geometrical parameters of the possible rotamers chloroethanethiol, (c) 2-bromoethanethiol, (d) 2-iodoethanethiol. of 2-halogenoethanethiols are reported in Table 1; those of Rotation around the C-X bond in the Gg' isomer produces a curve 2-halogenoethanols are omitted to save space whilst the sta- symmetrical with that of the Gg rotamer.On going from left to right, bility order and dipole moments are summarized in Table 2. the three minima correspond to Gg, Tg, G'g (series l), Gg, Gt, Gg' (series 2), Tg, Tt and Tg' (series 3) structures. Note that G'g and Tg However, the complete optimized geome!ries of all the title are equivalent to the Gg' and Tg' conformers, respectively. compounds are available upon request. A first analysis of Table 1 Calculated and experimental geometries of 2-halogenoethanethiols (distances in A, angles in degrees, energies in kJ mol-l) 2-fluoroethanethiol" 2-chloroethanethiol" 1.513 1.510 1.513 1.514 1.510 1.518 1.516 1.515 1.515 1.515'C( 1) -C(2) 1.816 1.816 1.816 1.822 1.823 1.815 1.815 1.817 1.825 1.824'C(2)-S(3) 'C(l)-Y 1.389 1.398 1.396 1.395 1.395 1.780 1.786 1.787 1.783 1.783 'S(3)-H(5) 1.332 1.331 1.332 1.330 1.331 1.332 1.331 1.332 1.331 1.331 'C(2) -H 1.089 1.089 1.088 1.088 1.083 1.089 1.091 1.088 1.088 1.089 'C(1) -H 1.092 1.090 1.090 1.090 1.090 1.090 1.089 1.086 1.087 1.087 'S(3)-Y 3.548 3.109 4.003 3.967 2.993 3.478 3.490 4.386 4.347 3.352 'H(5) -Y 3.150 2.484 4.199 4.718 4.182 3.995 2.877 4.576 5.076 4.542 6Y-C(U-C(2) 110.2 109.1 108.7 108.3 109.3 112.7 113.1 110.6 110.4 112.3 6C(1)-C(2)-s 114.4 112.8 112.2 108.8 109.6 115.5 114.9 111.8 108.0 111.0 'C(2) -S-H(5) 95.8 94.7 96.1 95.8 95.8 95.7 95.9 96.0 95.7 95.3 my-c-c-s 61.8 -63.0 178.6 179.2 58.1 66.0 -68.7 178.2 179.9 63.8 wH(5) -S-C -C 64.0 54.1 67.8 175.9 152.8 68.7 63.8 67.9 179.0 156.1 AE 8.14 0.00 3.01 9.72 8.88 8.51 1.81 0.00 6.53 10.69 P 3.06 2.16 1.10 1.20 3.41 3.15 2.05 1.25 1.37 3.44 w(%) 3 72 22 1 2 2 31 64 2 1 2-br~moethanethiol~ 2-i~doethanethiol~ 'C( 1 ) -C(2) 1.546 1.545 1.542 1.542 1.543 1.550 1.549 1.544 1.545 1.547 'C(2) -S(3) 1.904 1.903 1.912 1.92 1 1.915 1.903 1.903 1.913 1.922 1.914 'C(1) -Y 2.032 2.040 2.046 2.040 2.037 2.200 2.206 2.210 2.205 2.204 'S(3)-H(5) 1.374 1.372 1.374 1.373 1.374 1.374 1.372 1.374 1.374 1.374 'C(2) -H 1.103 1.103 1.101 1.101 1.103 1.101 1.103 1.101 1.101 1.103 'C(1)-H 1.101 1.100 1.099 1.099 1.100 1.090 1.101 1.100 1.100 1.100 'S(3) -Y 3.734 3.761 4.724 4.687 3.620 3.865 3.924 4.893 4.857 3.779 'H(5)-Y 4.304 3.1 16 4.95 1 5.424 4.855 4.101 3.265 5.112 5.587 4.984 6, -C(l)-C,2) 112.6 112.0 110.0 109.8 112.4 113.6 113.1 110.8 110.5 113.6 dC(1)-C(2)-s 115.2 114.7 111.1 107.5 110.8 115.3 115.0 11 1.2 107.6 110.8 'C(2) -S-H(5) 96.2 96.7 96.6 96.3 95.9 96.2 96.9 96.5 96.4 96.0 "Y-c-c-s 68.3 -71.7 178.2 180.0 66.8 67.9 -72.9 177.8 180.0 68.5 OH(5)-S -C -C 68.5 66.0 70.7 180.0 155.8 66.1 67.8 69.4 180.0 164.2 AE 9.58 2.90 0.00 6.05 12.48 10.04 5.10 0.00 6.31 13.51 cc 3.77 2.35 1.48 1.63 4.11 3.27 2.02 1.17 1.38 3.55 w(%) 2 23 72 3 2 11 84 3 " MP2/6-31G** basis.MP2/LANLlDZ basis. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 c 1.20z 3 $10 0 I Ii II I I I I 0 60 120 180 -120 -60 0 torsion angle/degrees I JI I I I I 0 60 120 180 -120 -60 0 torsion angle/degrees Fig.2 Energy curves for (a)2-fluoroethanol and (b)2-chloroethanol calculated at MP2/3-21GS (series 1 and 2) and MPYLANLIDZ (series 3 and 4). Series 1 and 3: rotation around the C-C bond. Series 2 and 4: rotation around the C-X bond in the Gg structure. On going from left to right, the three minima correspond to Gg, Tg, Gg (series 1 and 3), Gg, Gt and Gg' (series 2 and 4). Gg and Gg' are equivalent conformers. such geometries evidences that the C-Y (Y = halogen) bond length is practically constant in the sulfurated as well as in the oxygenated compounds. The C-S-H angle ranges from 95" to 97", independent of the basis set used for the calcu- lations, whilst the C-0-H angle undergoes more impor- tant changes (at least for the most stable conformer) on passing from the less to the most bulky halogen.Calculated and experimental data agree rather well for 2-fluoro- and 2- chloro-ethanols; the agreement is poor for 2-bromoethanol, but we must bear in mind that several geometrical param- eters in the cited microwave studies are assumed values and that the LANLlDZ basis overestimates bond lengths. Gt Tg Tt Fig. 3 Possible conformers of 2-halogenoethanethiols (Y = halogen, X = S). The Gg.' rotamer is similar to the Gg rotamer, but MY-C-C-X) is near to -60". The stability order found for the various rotamers of 2- fluoro- and 2-chloroethanols is analogous to that reported by Murto et al.," but 6-31G** and MP2 energies reported there are different from ours because of the partial geometry opti- mization performed by those authors.Use of a triple-zeta basis set augmented by two sets of polarization functions on C, F and 0, and one set of polarization functions on the H atoms12 produces AE values lower than those obtained in the present paper for 2-fluoroethanol. Our results indicate that the Gg' is always the most stable and the Gg the less stable conformation of 2-halogenoethanols. According to the Boltz- mann equation, and in agreement with experimental find- ings, the Gg' is therefore the prevailing rotamer at room temperature; its percentage decreases on increasing the size of the halogen atom, so that in 2-iodoethanol an equilibrium between the Gg' and Tg conformers (56% and 31%, respectively) is predicted. It is noteworthy that AE between the Gg' and Tg forms of 2-fluoroethanol (10.83 kJ mol-') is in excellent agreement with the value of 11.3 kJ mol-' esti-mated from electron diffraction studies.2b Electron diffraction measurements suggest also that the energy difference between the gauche (our Gg') and trans (our Tg) forms of 2-chloroethanol is 10.04 kJ mol-'.Our calcu- lations predict a value of 6.31 kJ mol-', which is lower than that figure, but very close to the value of 5.02 kJ mol-', esti-Table 2 Calculated stability order, dipole moments and percentages of 2-halogenoethanol conformations (energies in kJ mol-', p in D") Gg Gg' Tg Tt Gt Gg Gg' Tg Tt Gt 2-flu~roethanol~ 2-chl~roethanol~ AE cc w (Yo) 13.63 3.33 - 0.00 1.89 97 10.83 1.79 1 10.80 2.03 1 12.46 3.15 1 10.90 3.44 1 0.00 1.73 88 6.3 1 1.94 7 7.16 2.29 3 10.37 3.35 1 2-bromoethanol' 2-iodoethanol' AE cc w (Yo) 11.58 4.11 1 0.00 2.13 76 4.24 2.20 14 4.09 2.63 7 10.02 3.89 1 9.01 3.89 2 0.00 1.87 56 1.51 1.95 31 2.83 2.37 2 8.1 1 3.65 9 1 D x 3.33564 x lo-'' C m.MP2/6-31G** basis. MP2/LANLlDZ basis. mated from the relative intensities of the C-Cl stretching bands of the gauche and trans isomers in the vapour phase, measured as a function of the temperature up to 165"C.7 Acceptable agreement is found also for the corresponding AE for 2-bromoethanol since the gauche-trans enthalpy differ- ence, evaluated by the same technique is 6.07 kJ mol-', whilst our results give 4.24 kJ mo1-'.The agreement improves if comparison is made with the AE of 5.44 kJ mol-' cited in ref. 5(b). The Tg and Tt rotamers are suggested by calculation to be nearly isoenergetic isomers. The situation with the 2-halogenoethanethiols is rather dif- ferent than with the 2-halogenoethanols. In fact, the Gg' is predicted to be the most stable rotamer only for the 2- fluoroderivative whilst the Tg form is the most stable confor- mation for 2-chloro-, 2-bromo- and 2-iodo-ethanethiols ;its percentage increases on passing from C1 (64%) to Br (72%) and I (84%), i.e. on increasing the size of halogen atom (and on decreasing its electronegativity). As far as geometrical parameters are concerned, bond lengths and bond angles of the left-side moiety undergo negli- gible variations on passing from halogenoethanols to halogeno- ethanethiols. The halogenoethanol rotamers are always more polar than the corresponding halogenoethanethiols; in any case the Gg and Gt forms show the largest dipole moment values in both series of compounds.Tg and Tt are generally the less polar halogenoethanethiol conformers. The theoretical results for 2-chloroethanethiol agree very well with the experimental findings from microwave spectros- copy.15 The energy difference between the Tg and Gg' con-formers is 1.81 kJ mol-', to be compared with the 2.09 kJ mol-suggested by IR studies.16 The stability order is analo- gous to that obtained at the 3-21GI7 and 6-31G* levels," but the energy differences among the various conformers decreases when correlation is considered.Hydrogen Bonding In the Gg' conformations of 2-halogenoethanols and 2-halogenoethanethiols, the distances between the halogen and oxygen (or sulfur) atoms and between the halogen and H(5) atoms are constantly lower than the sum of the related Van der Waals radii, so that the formation of a hydrogen bridge is conceivable. This fact has been invoked to justify the greater stability of the Gg' isomer with respect to the other rotamers of 2-halogenoethanols and it could also determine the stabil- ity order of 2-halogenoethanethiols. Calculations show that when oxygen is substituted with sulfur the hydrogen-bond strength decreases and, consequently, the Gg' structure is destabilized.Bearing in mind the electronegativity of halo- gens, we must expect that the strength of the hydrogen bridge weakens on going from F to I, and becomes no longer suffi- cient to overcome the repulsive interactions. Indeed, no evidence of intramolecular hydrogen bonding was found in b-chloro- and b-bromo-ethylmercaptans when IR measurements, with different solvents, were made for the S-H stretching modes.16 On the other hand, a more careful analysis of geometrical data evidences that the S.* .Yand *0..Y(Y = halogen) distances are lower than the sum of the van der Waals radii also in the Gg and Gt structures, i.e. also when the Y..aH(5) distance is not suitable for hydrogen-bond formation. This could mean that the total energies of such rotamers could be affected by interactions between the sulfur (or oxygen) and the halogen atoms similar to those between J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Hydrogen-bond strength (kJmol-') of the title compounds evaluated with respect to the various rotamers assumed to be hydrogen bond free 2-Y-ethanethiols 3.01F 8.88 8.14 C1 8.88 6.70 -1.81 Br 9.58 6.68 -2.90 I 8.41 4.94 -5.10 2-Y-ethanoIs F 12.46 13.63 10.83 C1 10.37 10.90 6.3 1 Br 10.02 11.58 4.24 I 8.11 9.01 1.51 9.72 4.72 4.24 1.21 10.80 7.16 4.09 2.83 between the energy of the examined isomer and the energy of another conformation where no hydrogen bridge is present. The obtained figures are not absolute values because they are more or less affected by the different geometries of the hydrogen-bonded structure and of the minimum-energy refer- ence structure assumed to be hydrogen bond free.In the present case we could select each of the reported conformers, except the Gg' one, as hydrogen bond free reference structure. Inspection of the results obtained (see Table 3) evidences that E,, for 2-fluoroethanol ranges from 10.80 to 13.6 kJ mol-'. The lower datum agrees well with the values of 9.20 and 7.95 kJ mol- ' suggested by the most recent electron diffractionZb and ab initid2 investigations, respectively. Moreover, EHB of 2-substituted ethanethiols is always lower than that of the corresponding 2-substituted ethanols, and decreases on going from F to I, except in the ethanethiol series when it is calcu- lated with respect to the Gt rotamer (in this case equal strengths are found for 2-fluoro- and 2-chloro-ethanethiols).This lowering of EHB is in line with the lower electronega- tivity of sulfur with respect to oxygen and could justify the inversion of the Gg' and Tg stability order found on passing from 2-fluoro- to 2-chloro-ethanethiol. Since EHB for F and C1 derivatives comes from a different basis set from that adopted for Br and I derivatives, compari- son between these results and the previous values may be meaningless. Rotation Barriers The high flexibility of the compounds studied allows several interconversions between two or more conformations by rotation around the C-C and/or C-X bonds.The rotation barriers to be overcome in such pathways are reported in Table 4. The Tg e Gg' interconversion pathway implies rota- tion around the C(l)-C(2) bond which shows its maximum energy value (see Fig. 1) is at ca. -120". Starting from the Gg' conformer it is possible to reach the Gt and Gg rotamers by rotation of the SH or OH groups. For the Gg'e Gt pathway the maximum energy point (3- 21G* calculations) lies near -135" for F and C1 derivatives and near -150" for Br and I derivatives (the energies at -135" and -150" are not appreciably different in each compound). For the GteGg pathway the maximum is centred at 120". Rotation of the SH or OH group allows interconversion between the Tg and Tt conformations, cross- sulfur and oxygen noted by Kucsman and co-~orkers.~~-~~ ing through an energy maximum centred at 135" for all com- To understand better the reason for the different order of pounds.As can be seen from data reported in Table 4, the SH stability of 2-halogenoethanols and 2-halogenoethanethiols and OH rotation barriers are of the same order in both series we need a quantitative evaluation of the hydrogen-bond of compounds. The highest values are those of the Gg' e Gt energy (EHB), which is usually assumed to be the difference interconversion pathway; most of the remaining ones are J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Barriers (kJ mol-') in some interconversion pathways 2-halogenoethanethiols 2-halogenoethanols F C1 Br I F CI Br I Gg'+ Tg 21.84 19.09 16.95 16.10 24.37 23.75 19.82 19.59 Tg + Gg' Gg + Gt Gt + Gg' Gg' + Gt Gt + Gg Tg + Tt Tt -+ Tg 18.83 2.32 4.2 1 13.10 1.58 6.72 0.01 20.90 3.15 3.19 12.07 0.97 6.86 0.33 19.85 3.21 1.77 11.34 0.32 6.12 0.07 21.20 4.03 1.37 9.78 0.55 6.48 0.17 13.54 4.91 1.13 13.58 4.9 1 2.87 2.91 17.43 5.75 0.44 10.80 6.28 3.48 2.63 15.58 5.26 0.07 10.10 6.82 2.20 2.35 18.08 5.48 0.30 8.42 6.38 3.37 2.05 extremely small so that the related interconversions may occur easily at room temperature, especially from the less to the most stable rotamers.On the whole, the 2-halogenoethanol barriers are in line with those reported in ref. 10. 5 6 7 (a) A. Almenningen, 0.Bastiansen, L. Fernholt and K.Edberg, Acta Chem. Scand., 1971,25, 1946; (b) A. Almenningen, L. Fern-holt and K. Kveseth, Acta Chem. Scand., 1977,31,297. J. Pourcin, G. Davidovics, H. Bodot, L. Abouaf-Marguin and B. Gauthier-Roy, Chem. Phys. Lett., 1980,74, 147. P. Buckley, P. A. Giguere and M. Schneider, Can. J. Chem., Conclusions 8 1969,47,901. P. Buckley, P. A. Giguere and D. Yamamoto, Can. J. Chem., The present study has shown that 2-halogenoethanethiols, analogously to 2-halogenoethanols, exist mainly in the Gg' and Tg conformations. At room temperature, the former rotamer prevails in the oxygenated molecules and in 2-fluoroethanethiol, whilst the latter is the most stable one in the remaining sulfurated compounds. In Br and I derivatives an equilibrium with remarkable percentages of the two struc- tures is predicted; however, their interconversion should be 9 10 11 12 13 1968,46,2917.L. Radom, W. A. Lathan, W. J. Here and J. A. Pople, J. Am. Chem. SOC.,1973,95,693. J. Murto, M. Rasanen, A. Aspiala and L. Homanen, J. Mol. Struct. (Theochem), 1983,92,45. J. Murto, M. Rasanen, A. Aspiala and T. Lotta, J. Mol. Struct. (Theochem),1984,108,99. D. A. Dixon and B. E. Smart, J. Phys. Chem., 1991,95,1609. R. C. Griflith and J. D. Roberts, Tetrahedron Lett., 1974, 39, 3499. difficult owing to the relatively high barriers to be overcome. Lower barriers are found for the SH and OH groups, which in some cases can undergo free rotation very easily. The different stabilities of the Gg' and Tg rotamers in the oxygenated and sulfurated molecules can be attributed to the different strength of the hydrogen bridge present in the 14 15 16 17 R.J. Abraham and J. R. Monasterios, Org. Magn. Reson., 1973, 5, 305. R. N. Nandi, M. F. Boland and M. D. Harmony, J. Mol. Spec- trosc., 1982,92,419. M. Hayashi, Y. Shiro, M. Murakomi and H. Murata, Bull. Chem. SOC.Jpn., 1965,38,1740. M. Osaku, J. Mol. Struct. (Theochem), 1986,138,283. former structure, owing to the lower electronegativity of sulfur with respect to oxygen. Although it is not possible to obtain absolute energy values for the bridge strength because it is highly dependent on the conformation assumed to be hydrogen bond free, the numerical values here obtained are in line with the previous justification. Moreover, since elec- tronegativity (as well as the hydrogen-bond energy) decreases on going from F to I, in both series of molecules the percent- age of the Tg rotamer increases on increasing the size of the halogen atom.Financial contribution from the Italian Minister0 dell'universita e della Ricerca Scientifica e Tecnologica (MURST),Roma, is gratefully acknowledged. 18 19 20 21 22 23 24 R. Benassi and F. Taddei, J. Mol. Struct., 1990,205, 177. S. L. Emery, G. R. Famini, J. 0. Jensen, J. M. Leonard and D. J. Reutter, Phosphorus, Sulfur Silicon Relat. Elem., 1990, 53, 373. Gaussian 92, Revision B, M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gom- perts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzales, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart and J. A. Pople, Gaussian Inc., Pittsburgh, PA, 1992. M. Frisch, J. Foresman and A. Frisch, Gaussian 92 User's Guide. T. H. Dunning and P. J. Hay, in Modern Theoretical Chemistry, Plenum, New York,1976, ch. 1, pp. 1-28. (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b) W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985,82,284; (c)P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985,82,299. J. G. Angyan, R. A. Poirier, A. Kucsman and I. G. Csizmadia, J. References 25 Am. Chem. SOC.,1987,109,2237. A. Kucsman, I. Kapovits, M. Czugler, L. Parkanyi and A. K. S. Buckton and R. G. Azrak, J. Chem. Phys., 1970,52,5652. (a)K. Hagen and K. Hedberg, J. Am. Chem. SOC.,1973,95,8263; (b) J. Huang and K. Hedberg, J. Am. Chem. SOC.,1989,111,6909. 26 Kalman, J. Mol. Struct., 1989, 198, 339. L. Parkanyi, A. Kalman, A. Kucsman and I. Kapovits, J. Mol. Struct., 1989,198, 339. R. G. Azrak and E. B. Wilson, J. Chem. Phys., 1970,52,5299. K. G. R. Pachler and P. L. Wessels, J. Mol. Struct., 1970, 6, 5299. Paper 3/06863C ; Received 16th November, 1993