Journal of Molecular Structure (Theochem), 283 (1993) 141-149 0166-1280/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

141

The molecular structure of 1,3-butadien- l-01 and 1,3-butadien-2-01: an ab initio SCF-MO study

Paulo J.A. Ribeiro-Claro

Departamento de Q&mica, Universidade de Coimbra, P-3049 Coimbra, Portugal

(Received 9 November 1992)

Abstract

Ab initio SCF-MO calculations at the 3-21G level with full geometry optimization were performed for the three isomers of 1,3-butadienol. Improved energy estimates were obtained with single-point calculations at the 6-31G* and MP2/6-31G* levels of theory. The critical points for rotation about the central C-C bond were located for both configurations of the OH group. trans-Butadien-l-01 has stable gauche forms (0 = 39” ), with gauche-trans energy differences of ca. 10 kJ mol-‘, very close to the reported values for butadiene itself. The energies and geometries of the less stable conformers of ckbutadien-l-01 and butadien-2-01 are strongly dependent on the OH group configuration, lying ca. 1.3-15.5 kJmol_’ above the s-trans minima, with 0 values ranging from 0” to 43.5”.

Introduction

Being the simplest conjugated system, 1,3-buta-

diene can be used as a model for conjugated poly-

enes, and the factors governing its structure are

important for understanding larger polyene sys-

tems. The structure of the less stable conformer

of 1,3-butadiene has been a matter of some contro-

versy. While ab initio studies at various levels of

calculation gave a non-planar gauche structure for

the second stable conformer of this molecule [l-4],

the most recent experimental results have been

interpreted as supporting either a planar

s-cis structure [5] or a non-planar gauche structure

[6,7]. In a previous study [8], ab initio MO calcula-

tions were performed on fluorine substituted

butadiene molecules, with the aim of obtaining

additional information on the structure of buta-

diene like molecules. It was found that the conju-

gative stabilization in the planar s-cis forms is

limited and is overwhelmed by steric interactions

between the terminal hydrogen atoms, between the

vicinal hydrogen atoms and also between the

double bond moieties. In the present work, the

study of substituted butadienes was extended to

the OH substitution. To this end, ab initio

SCF-MO calculations at the 3-21G level (with full geometry optimization) and at the 6-31G* and MP2/6-31G* levels (single-point calculations) were performed on the three isomers of buta- dienol: trans-1,3-butadien-l-01, cis-1,3-butadien-l- 01 and 1,3-butadien-2-01 (hereafter referred to as tlBOH, clBOH and 2BOH respectively). Loca- tion of the critical points for the rotation around the central C-C bond (minima and maxima) and energy differences between them are discussed in relation to butadiene and fluorobutadienes.

Computational methods

Calculations were performed using the GAUSSIAN 82 program package [9] adapted for use on a Digital/Vax VMS 8530 computer. Molecular geo- metries were fully optimized using the 3-21G split

142 P.J.A. Ribeiro-ClarojJ. Mol. Struct. (Theochem) 283 (1993) 141-149

valence basis set [lo]. Previous results [ 1 l- 131 have shown that the 3-21G basis set allows a reasonable compromise between the computer time available and the quality of geometry optimization. Geo- metry optimizations were performed by the force gradient method with an analytical gradient [14]. The final root mean square (r.m.s.) gradient length was always less than 1 x 10m5 hartree bohr-’ or hartree rad-‘, yielding geometries accurate to 0.01 pm or 0.1” and energies stable below the microhartree level (less than 3 J mol-I). Optimiza- tion to transition states (saddle points) generally required the initial evaluation of the curvature by means of force constant calculations. The order A of each critical point (X = 0 for a minimum, X = 1 for a first-order saddle point) was obtained by computing the harmonic vibrational frequencies [I 51, using analytic second derivatives.

Additional single-point calculations were per- formed using the 6-31G* basis set [16] at the SCF and second-order Moller-Plesset (MP2) perturba- tion [17] levels of theory.

Results and discussion

Figure 1 shows the nomenclature and the numbering of the atoms used throughout this paper. The two stable configurations of the OH

(a) 0-H 6 (b)

Fig. 1. Nomenclature and numbering of the atoms for the butadienol molecules (shown for (a) tlBOH s-cis OH-syn and for (b) 2BOH s-cis OH-anti).

group are referred to as OH-syn (4 ZY 0”) and OH-anti (4 M 180”), while s-cis and s-trans are used to designate the planar configurations of the double bonds relative to each other. Tables 1 and 2 present the energies and dihedral angles of critical points in the potential curve for the rotation around the C-C central bond in the butadienol molecules, for both configurations of the OH group. The order of each critical point is shown in Table 2 by the harmonic frequency of the CC-CC torsional mode (X = 1 for one imaginary frequency). The energies of four of the critical points in Table 1 had already been pre- sented in the literature [18], without geometry information.

As it has been pointed out elsewhere [8], the results obtained for 1,3-butadiene with the 3-21G basis set compare favourably with the results obtained at higher levels, either SCF or post- SCF. In addition, for the fluorine substituted buta- dienes [8], the single-point calculations performed at the 6-31G* and MP2/6-31G* levels generally confirm the 3-21G energy differences. This quality of the 3-21G results supports the comparison between 1,3-butadiene and its substituted deriva- tives at this level of theory. Nevertheless, the results given in Table 1 show that the relative energies of some configurations of the butadienol isomers are significantly affected by an increase in the basis-set dimension and/or the inclusion of an electron-correlation term. This is particularly evi- dent for 2BOH, but can also be observed for a few forms of the remaining molecules.

An interesting feature of Table 1, emphasized in Fig. 2, is that the OH-syn configuration (OH bond towards the C-C double bond) is clearly more stable than the OH-anti configuration, following the trend already observed for ethenol (vinyl alcohol) [19,20]. In fact, the only exception to this feature is the s-cis form of cl BOH at the 3-21G and MP2/6-31G* levels.

As in the case of the fluorinated butadiene molecules [8], the substitution in the trans-1 posi- tion leaves the conformational structure of the parent compound almost unaffected, while the

P.J.A. Ribeiro-Clara/J. Mol. Struct. (Theo&m) 283 (1993) 141-149 143

Table 1 Relative energies of the critical pointsa in the potential curve for the internal rotation about the C2-C3 bond in butadienol molecules

OH-syn

s-trans

E (hartree)

TS

OH-anti

gauche s-cis s-trans TS gauche s-cis

A&sb E (hart@ (kJ mol-‘)

ABcpb (kJ mol-‘)

tlBOH 3-21G 6-31G*C MP2/6-3 1 G*’

clBOH 3-21G 6-31G*’ MP2/6-3lG+’

2BOH 3-21G 6-31GSC MP2/6-3 lG*C

-228.499106 22.93 10.15 13.84 3.69 -228.494104 22.72 9.33 13.88 4.55 -229.713799 24.88 11.46 16.03 4.57 -229.170119 25.21 11.27 16.58 5.31 -230.454071 24.80 9.99 15.02 5.03 -230.450971 25.68 10.07 15.06 4.99

-228.498044 12.30 1.31 15.61 14.30 -228.497331 29.16 10.69 -229.712122 13.81 4.40 15.75 11.35 -229.770939 25.99 13.63 -230.452581 12.87 3.91 14.18 10.27 -230.451045 25.82 10.18

-228.504086 33.93 15.48 16.45 0.97 -228.499340 24.79 12.30 19.46 7.16 -229.775969 27.12 10.79 13.19 2.40 -229.771968 17.46 5.19 13.71 7.92 -230.455693 24.39 8.98 11.59 2.61 -230.451861 15.72 5.19 13.02 7.83

a Hartree = 2625.5 kJ mol-‘; s-trans, gauche and s-cis refer to different conformations of the two double bonds relative to the rotation about the C-C central bond, as shown in Fig. 1. TS stands for the transition state between two different minima in the potential function for this rotation. b AEcg = Eks - Egawhe. ’ Energies determined for the 3-21G optimized geometry.

Table 2 Geometries and frequencies of the CC-CC torsional mode of the critical points in the potential curve for the internal rotation about the C2-C3 bond in butadienol molecules

OH-syn

s-trans TS gauche s-cis

OH-anti

s-trans TS gauche s-cis

rIBOH HOCC (deg) CCCC (deg) Frequency (cm-‘)

clBOH HOCC (deg) CCCC (deg) Frequency (cn-‘)

2BOH HOCC (deg) CCCC (deg) Frequency (cm-‘)

0.0 0.0 2.6 0.0 180.0 160.9 -158.0 180.0 180.0 103.4 38.8 0.0 180.0 103.8 39.1 0.0 152 158i 165 147i 153 158i 168 154i

0.0 -7.9 8.9 0.0 163.5 -171.9 180.0 180.0 110.8 43.5 0.0 179.5 90.2 0.0 102 109i 136 262i 147 107i 81

0.0 0.1 1.7 0.0 130.0 -174.9 -147.3 180.0 180.0 110.5 28.6 0.0 167.0 100.1 40.8 0.0 157 152i 101 81i 150 143i 131 185i

144 P.J.A. Ribeiro-Clara/J. Mol. Struct. (Theochem) 283 (1993) X41-149

0 0 60 120 160

(a)

40 .

0

(b)

1 60 120 160

CCCCY

0 60 120 160

(c) cccw

Fig. 2.3-21G potential functions for the internal rotation about the central C-C bond in (a) tlBOH, (b) clBOH and (c) 2BOH; 0, OH-anti; 0, OH-syn.

substitution in the cis-1 and cis-2 positions causes a s-trans minimum. In the case of 2BOH, both larger perturbation. Thus, the potential energy OH-syn and OH-anti configurations have gauche functions for the rotation about the central C-C minima, but in the former it is very close to bond of tlBOH shown in Fig. 2 clearly resemble the s-cis form, both in energy (less than 1 kJ mol-’ that of 1,3-butadiene, in both the location and the at the 3-21 G level) and in geometry (0 = 29”). energetics of the critical points. In fact, for l,Zbuta- The OH-anti s-trans configuration is actually diene at the 3-21G level, es,,,,, = 38”, 0rs = 102”, AErs = 23.6 kJmol_‘,

an OH-skew form, with both OH and vinyl AEsauche = 10.2 kJ mol-’ groups moving out of the plane in opposite

and AE,_,, = 13.3 kJmol_’ [8], which are very directions (0 = 167”, q5 = 130”). These observa- close to the values obtained for t 1BOH with both tions can be taken as evidence of the minor configurations of the OH group (see Tables 1 and importance of substituent through-bond elec- 2). However, clBOH and 2BOH exhibit a mark- tronic effects in the conformational structure, rela- edly different behaviour. In particular, for clBOH, tive to the “steric” through-space interactions, the OH-anti configuration presents a minimum at which increase on moving the substituent from an the s-cis form, while the OH-syn configuration has outer position (in tlBOH) to an inner position (in a gauche minimum only 2-4 kJmol_’ above the 2BOH).

P.J.A. Ribeiro-Clara/J. Mol. Struct. (Theo&em) 283 (1993) 141-149 145

tram-Butadien-l-01 (tlBOH) [21]. In addition, all the geometrical parameters around the hydroxyl group (CO and OH bond

Table 3 presents the 3-21G optimized geometry lengths, and CC0 and COH bond angles) compare for the critical points of t 1 BOH. To the best of our well with those obtained for ethenol at the same knowledge, there are no previously reported level [19]. As in 1,3-butadiene, while the Cl-C2 experimental results for this molecule. Thus, the and C3-C4 bond lengths are indicative of double present results can only be compared with those bonds, the central C2-C3 bond length (146-149 pm) of similar systems. In a general evaluation, the is significantly shorter than a typical single bond calculated values agree with typical values on simi- (154pm) and considerably longer than a conju- lar molecules, in particular, ethenol [19,20], 1,3- gated C-C bond in the aromatic ring (138 pm), butadiene [3,4], and trans-1-fluorobutadiene [8]. thus indicating a limited extent of conjugation. The calculated C-O bond length (ca. 138 pm) is The geometries of the two minima (gauche and close to the microwave value for ethenol (137 pm) s-trans) differ much more than in trans-l-fluoro-

Table 3 3-21G optimized geometries for tlBOH at the critical points of rotation about the C2-C3 bond

OH-syn

s-trans TS gauche s-cis

OH-anti

s-trans TS gauche s-cis

Bond length (pm) Cl-C2 C2-C3 c3-c4 Cl-0 O-H Cl-HI C2-H2 C3-H3 C4-H4 C4-H5

Bond angle (deg) Cl-C2-C3 C2-C3-C4 O-Cl-C2 H-O-Cl HI-Cl-C2 H2-C2-Cl H3-C3-C4 H4-C4-C3 HS-C4-C3

Dihedral angle (deg) Cl-C2-C3-C4 0-Cl-C2-C3 H-O-Cl-C2 Hl-Cl-C2-C3 H2-C2-Cl-C3 H3-C3-C4-C2 H4-C4-C3-C2 H5-C4-C3-C2

132.0 131.5 131.9 132.0 131.7 131.2 131.6 131.7 146.2 148.9 147.2 147.2 146.2 148.8 147.2 147.2 132.1 131.7 132.0 132.0 132.1 131.7 132.1 132.2 137.4 137.8 137.5 137.4 138.2 138.7 138.4 138.2 96.6 96.6 96.6 96.6 96.2 96.3 96.4 96.2

107.0 107.0 106.9 106.9 107.5 107.4 107.4 107.3 107.6 107.7 107.6 107.6 107.3 107.4 107.3 107.3 107.6 107.7 107.6 107.5 107.7 107.7 107.6 107.5 107.4 107.4 107.4 107.3 107.4 107.4 107.4 107.4 107.2 107.4 107.3 107.2 107.2 107.4 107.3 107.3

122.7 122.7 123.4 125.6 122.8 122.8 123.4 125.9 124.6 123.7 125.3 127.8 124.5 123.6 125.4 128.0 126.9 126.8 126.8 126.4 121.5 121.5 121.4 121.0 113.0 112.8 113.0 113.0 112.7 112.3 112.4 112.7 122.7 122.7 122.6 123.4 121.8 121.9 121.8 122.6 120.2 120.2 119.2 118.8 118.7 118.7 118.4 117.2 119.3 119.5 119.2 118.2 119.2 119.5 119.2 118.1 121.9 121.7 122.0 123.1 121.8 121.7 121.9 123.1 121.6 121.7 121.5 121.0 121.7 121.7 121.5 121.1

180.0 103.4 38.8 0.0 180.0 103.8 39.1 0.0 180.0 179.7 181.1 180.0 180.0 177.4 -176.6 180.0

0.0 0.0 2.7 0.0 180.0 160.9 -157.9 180.0 0.0 -0.5 2.3 0.0 0.0 -1.1 2.5 0.0

180.0 -179.7 -178.5 180.0 180.0 -179.0 -179.0 180.0 180.0 -178.0 -179.8 180.0 180.0 -178.9 -179.6 180.0

0.0 -1.2 3.1 0.0 0.0 -1.1 3.3 0.0 180.0 178.8 -177.7 180.0 180.0 178.8 -177.9 180.0

146 P.J.A. Ribeiro-Clara/J. Mol. Struct. (Theochem) 283 (1993) 141-149

butadiene. Nevertheless, the largest differences relative to the most stable form are observed for the two maxima (TS and s-cis). In the TS form, while the Cl-C2 and C3-C4 bonds are shorter than in any other form, the central C2-C3 bond is longer than in the s-trans form by 2.7pm (see Table 3). However, in the s-cis form, the bond angles Cl-C2-C3 and C2-C3-C4 are larger than in the s-trans form by almost 3”. These geo- metrical trends emphasize the two effects dominat- ing the rotation around the central C-C bond: the electron delocalization over the atoms of the skeleton (reduced in the TS) and the repulsive steric interactions between the two vinyl groups (maximized in the s-cis form).

With regard to the rotation around the C-O bond, it should be noted that the calculated energy differences between the OH-anti and OH-syn configurations (3-21G 12 kJ mol-‘; MP2/6-31G* 7 kJ mol-‘) are very similar to those obtained for vinyl alcohol at the same levels of calculation [19]. In addition, Table 2 and Fig. 1 show that these energy differences are nearly constant during the rotation around the central C-C bond. Thus, one can conclude that in t 1BOH the rotations around the C-O and the C-C bonds are independent. This conclusion is supported by an analysis of the geo- metrical parameters in Table 3. The changes in geometry between the OH-syn and OH-anti configurations follow those observed for vinyl alcohol and are nearly independent of the rotation around the central C-C bond. In going from the OH-anti configuration to the more stable OH-syn configuration, there is a decrease in the CO bond length (1 pm) and a large increase in the C-C-O bond angle (SO), followed by smaller increases of ca. 1” in the C-O-H, C2-Cl-H and Cl-C2-H bond angles. The increase in the C-C-O, C-O-H and Cl-C2-H bond angles clearly indicate an increase in the steric repulsion in the OH-syn con- figuration. The greater stability of this form should then be ascribed to an electronic effect, such as lone-pair delocalization, as already suggested for ethenol [19]. In fact, the decrease in the C-O bond length supports this interpretation.

cis-Butadien-l-01 (clBOH)

The optimized geometries for the critical points of clBOH in both configurations of the OH group are presented in Table 4. The general evaluation of the geometrical parameters already done for t 1 BOH applies to this case also.

The main difference between t 1 BOH and cl BOH

is the totally different shapes of the potential curves (Fig. 2) for the two OH configurations. In the OH-anti configuration, with the hydroxyl hydro- gen atom pointing out of the molecule, the poten- tial function for clBOH is very similar to that of cis-1-fluorobutadiene [8]. In analogy with this molecule, the substitution of the Cl-H...H-C4 repulsive interaction (in 1,3-butadiene) by a less repulsive 0. . .H4 interaction (in cl BOH) accounts for the stability of the s-cis form. The Cl-C2-C3, C2-C3-C4 and O-Cl -C2 bond angles are similar to the corresponding ones in cis-1-fluorobutadiene. In addition, there is a decrease in the C2-C3-C4 and C3-C4-H bond angles relative to the corre- sponding form of tlBOH, supporting the above interpretation.

In the OH-syn configuration, the energy of the planar forms is increased dramatically; the gauche-gauche barrier becomes higher than the TS barrier (at all levels of calculation herein reported), while the energy of the s-trans minimum approaches that of the corresponding OH-anti form. In fact, for the s-trans form, AEanti_s,,n ranges from 1.9 kJmol_’ (3-21G) to 4.0kJmol-* (MP2/6-31G*), well below the 12-8 kJ mol-’ observed for ethenol and tlBOH. These features can be easily interpreted in terms of steric repul- sions between the OH group and the C4-H4 (in the s-cis form) and C3-H3 (in the s-trans form) moi- eties. The features are evident from the data given in Table 4, particularly from the bond angles of the skeleton. In the case of the s-cis form, there are three bond angles (Cl-C2-C3, C2-C3-C4 and 0-Cl-C2) larger than 130”. In the s-trans form, the same three angles are larger than the corre- sponding ones in the unsubstituted compounds by ca. 2.5”, on average. In both planar forms the

P.J.A. Ribeiro-Clara/J. Mol. Strut. (Theo&em) 283 (1993) 141-149 147

Table 4 3-21G optimized geometries for clBOH at the critical points of rotation about the C2-C3 bond

OH-syn

s-trans TS gauche s-cis

OH-anti

s-tram TS s-cis

Bond length (pm) Cl-C2 C2-C3 c3-c4 Cl-0 O-H Cl-H1 C2-H2 C3-H3 C4-H4 C4-H5

Bond angle (deg) Cl-C2-C3 C2-C3-C4 O-Cl-C2 H-O-Cl Hl-Cl-C2 H2-C2-Cl H3-C3-C4 H4-C4-C3 H5-C4-C3

Dihedral angle (deg) Cl-C2-C3-C4 0-Cl-C2-C3 H-O-Cl-C2 Hl-Cl-C2-C3 H2-C2-Cl-C3 H3-C3-C4-C2 H4-C4-C3-C2 HS-C4-C3-C2

132.2 131.6 132.1 132.4 131.7 131.2 131.8 146.2 148.8 147.4 146.8 146.2 148.5 146.8 132.1 131.8 132.3 132.4 132.1 131.6 132.3 137.4 137.6 137.3 136.8 138.5 138.3 138.3 96.4 96.7 96.7 96.0 96.3 96.3 96.3

106.9 106.9 106.9 107.0 107.3 107.4 107.3 107.2 107.2 107.2 107.2 107.3 107.7 107.3 107.7 107.8 107.6 107.6 107.3 107.7 107.6 107.4 107.4 107.4 107.1 107.5 107.3 106.7 107.2 107.4 107.3 107.2 107.3 107.4 107.4

125.8 122.9 124.9 130.3 123.1 123.6 127.7 124.3 124.1 125.4 130.6 124.1 123.9 127.2 128.4 126.5 127.4 130.1 121.4 122.4 123.2 114.2 112.4 112.4 114.6 112.9 112.5 112.8 121.6 122.8 121.9 120.3 121.9 121.1 120.7 117.3 118.9 117.8 115.0 118.6 118.4 116.3 118.1 119.3 118.9 116.5 120.4 119.8 118.6 121.8 121.7 121.9 124.9 122.1 121.7 121.7 121.7 121.7 121.5 120.5 121.6 121.6 120.6

180.0 110.8 43.5 0.0 179.5 90.2 0.0 0.0 -1.7 1.1 0.0 -1.6 1.1 0.0 0.0 -7.9 8.9 0.0 163.5 -171.9 180.0

180.0 178.5 179.8 180.0 179.9 179.7 180.0 180.0 -179.6 -177.8 180.0 179.7 -179.8 180.0 180.0 - 178.7 -179.4 180.0 180.0 -179.9 180.0

0.0 -1.0 3.5 0.0 -0.2 -1.3 0.0 180.0 178.6 -179.1 180.0 179.8 179.3 180.0

C-O-H bond angle increases from its typical value of ca. 112” to more than 114”.

Butadien-2-01 (2BOH)

Table 5 presents the optimized geometry for 2BOH at the critical points of rotation about the central C-C bond. For this molecule there are no experimental data available and the general dis- cussion of calculated values already given for the previous molecules also applies. The geometrical structure of 2BOH does not present deviations from 1,3-butadiene larger than those exhibited by

cl BOH and t 1 BOH, despite the “inner” position of the substituent. In fact, both bond lengths and bond angles follow the trends already observed for the other molecules studied. For instance, although in 2BOH the O-H bond eclipses a C-C bond in both the syn and the anti configurations, the main geometrical change in going from one form to another is still the decrease of the 0-C2-Cl bond angle by ca. 6”.

The OH-syn forms display some similarities with 2-fluorobutadiene, both energetically and geo- metrically. As in 2-fluorobutadiene, the substitu- tion of the hydrogen atom at C2 reduces the

148

Table 5

P.J.A. Ribeiro-ClarojJ. Mol. Struct. (Theo&em) 283 (1993) 141-149

3-21G optimized geometries for 2BOH at the critical points of rotation about the C2-C3 bond

OH-syn

s-trans TS gauche s-cis

OH-anti

s-tram TS gauche scis

Bond length (pm) Cl-C2

C2-C3

c3-C4

c2-0

O-H

Cl-H1

Cl-H2

C3-H3

C4-H4

C4-H5

Bond angle (deg) Cl-C2-C3

C2-C3-C4

o-c2-Cl

H-O-Cl

HI-Cl-C2

H2-Cl-C2

H3-C3-C4

H4-C4-C3

H5-C4-C3

Dihedral angle (akg) Cl-C2-C3-C4

O-C2-Cl-H2

H-0-CZ-Cl

Hl-Cl-C2-C3

H2-Cl-C2-C3

H3-C3-C4-C2

H4-C4-C3-C2

H5-C4-C3-C2

132.1 131.6 132.0 131.9 131.7 131.3 131.6 131.7

146.8 148.8 147.4 147.6 147.0 149.1 147.6 147.9 131.8 131.4 131.7 131.7 131.9 131.7 131.8 131.8

137.8 138.1 138.2 138.2 139.3 139.0 139.3 138.8

96.5 96.6 96.7 96.5 96.8 96.4 96.6 96.3

107.0 106.9 106.9 106.8 107.1 107.0 106.9 106.9

107.3 107.4 107.3 107.3 107.0 107.1 107.0 107.0 107.3 107.4 107.3 107.2 107.3 107.5 107.6 107.6 107.1 107.3 107.3 107.3 107.3 107.4 107.3 107.2 107.2 107.3 107.2 107.2 107.2 107.3 107.3 107.2

123.2 123.2 125.5 126.6 123.5 123.6 124.8 125.8

123.3 123.0 125.2 126.5 123.5 122.9 124.5 126.7

124.4 124.9 124.4 124.0 119.9 120.0 119.6 118.4

112.8 112.2 112.7 112.7 111.5 111.7 112.0 113.2

120.6 120.5 120.9 121.3 121.1 121.0 121.2 121.9

122.3 122.3 122.0 121.9 120.5 120.4 120.2 119.9

121.3 120.9 121.0 120.6 120.9 120.5 120.4 118.3

121.2 121.8 122.3 122.9 121.7 121.8 121.6 122.6

121.4 121.5 121.2 120.9 121.7 121.6 121.6 121.1

180.0

0.0

0.0

0.0

180.0

180.0

0.0

180.0

-

100.5 28.6 0.0 167.5 100.5 40.8 0.0

-0.2 2.5 0.0 -2.1 0.4 4.4 0.0

0.7 1.6 0.0 130.0 -174.7 -147.3 180.0

-0.9 1.7 0.0 -1.5 -0.4 2.9 0.0

179.2 -179.1 180.0 -179.9 179.6 -179.3 180.0

178.6 -179.6 180.0 179.1 -178.2 -179.9 180.0

-0.7 1.7 0.0 -1.6 -2.4 1.9 0.0

179.2 -179.2 180.0 -179.8 178.7 -178.4 180.0

C2H. . .H3 repulsion, leading to a small gauche- gauche barrier and a gauche minimum at 19 = 29”. The same effect allows for a slight decrease in the H4-C4-C3 and C2-C3-C4 bond angles in the s-trans form, relative to tlBOH and cl BOH.

In the less stable OH-anti forms, with the hydro- xylic hydrogen atom now pointing “inwards”, the interactions with the second vinyl group become more important. Due to the additional OH.. .HC3 repulsion, the gauche minimum shifts to a larger t9 value and the gauche-gauche barrier increases to 7-8 kJ mol-’ . In fact, in the s-cis form the H-O-C2 and H-C3-C2 bond angles are larger

than in all the other forms. However, the strong non-planarity of the s-trans form (0 = 167.5”, 4 = 130”) can be ascribed to an attractive hydro- gen-bond-type interaction between the OH proton and the r-system on C3C4. The average distance between the proton and the two carbon atoms is 268 pm, close to the value observed for the similar situation in 1,4-pentadien-3-01 [22].

References

1 J.E. Rice, B. Liu, T. Lee and C.M. Rohlfing, Chem.

Phys. Lett., 161 (1989) 277.

P.J.A. Ribeiro-ClarolJ. Mol. Struct. (Theochem) 283 (1993) 141-149 149

2 IL. Alberts and H.F. Schaefer III, Chem. Phys. Lett., 161 (1989) 375.

3 C.W. Bock, P. George and M. Trachtman, Theor. Chim. Acta, 64 (1984) 293.

4 G.R. De Mare and D. Neisius, .I. Mol. Struct. (Theo- them), 109 (1084) 103.

5 J.J. Fisher and J. Michl, J. Am. Chem. Sot., 109 (1987) 1056. B.R. Arnold, V. Balaji and J. Michl, J. Am. Chem. Sot., 112 (1990) 1808.

6 R. Engeln, D. Consalvo and J. Reuss, Chem. Phys., 160 (1992) 427.

7 M. Kofranek, A. Karpfen and H. Lischka, Chem. Phys. Lett., 189 (1992) 281.

8 P.J.A. Ribeiro-Claro, Chem. Phys. Lett., 188 (1992) 303.

9 J.S. Binkley, M.J. Frisch, D.J. DeFrees, K. Raghava- chari, R.A. Whiteside, H.B. Schlegel, E.M. Fleuder and J.A. Pople, GAUSSIAN 82, Carnegie-Mellon University, Pittsburgh, PA, 1983.

10 J.S. Binkley, J.A. Pople and W.J. Hehre, J. Am. Chem. Sot., 102 (1980) 939.

11 P.J.A. Ribeiro-Claro, Ph.D. Thesis, University of Coimbra, 1987.

12 R. Fausto, J.J.C. Teixeira-Dias and P.R. Carey, J. Mol. Struct. (Theochem), 152 (1987) 119; 168 (1988) 179.

13 L.A.E. Batista de Carvalho, A.M. Amorim da Costa and J.J.C. Teixeira-Dias, J. Mol. Struct. (Theochem), 205 (1990) 327.

14 H.B. Schegel, Ph.D. Thesis, Queen’s University, King- ston, Ont., 1975.

15 H.B. Schegel, Adv. Chem. Phys., 67 (1987) 249. 16 P.C. Hariharan and J.A. Pople, Theor. Chim. Acta, 28

(1973) 213. 17 C. Moller and M.S. Plesset, Phys. Rev., 46 (1934) 618. 18 J.F. Marco&a, I.G. Czismadia, P. Yates and J.J. Kre-

pinsky, J. Mol. Struct. (Theochem), 167 (1988) 359. 19 C. Plant, K. Spencer and J.N. Macdonald, J. Mol.

Struct., 140 (1986) 317. 20 R.H. Nobes, L. Radom and N.L. Allinger, J. Mol.

Struct. (Theochem), 85 (1981) 185. 21 S. Saito, Chem. Phys. Lett., 42 (1976) 399. 22 T. Freire, J.J.C. Teixeira-Dias and P.J.A. Ribeiro-

Claro, in preparation,