Reinvestigation of the molecular structures of some organo-sulfur compounds

Journal of Molecular Structure (Theochem), 138 (1986) 283-297 Klsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

REINVESTIGATION OF THE MOLECULAR ORGANO-SULFUR COMPOUNDS

STRUCTURES

OF SOME

MASARU OHSAKU Department of Chemistry, Faculty of Science, Hiroshima University, Higashisenda l-1-89, Hiroshima 730 (Japan) (Received 18 October 1985)

ABSTRACT The results of ab initio SCF MO calculations at the HF level on HSCH,SH, CH,SCH,SH, ClCH,CH,SH and HSCH,CH,SH using two kinds of basis sets, 3-21G and 3-21G + d(0.8C + 0.6s + 0.7Cl) are presented. Reasonable geometrical parameters for these molecules were reproduced by the latter set. Valuable information on rotational isomerism of these molecules is proposed. The barrier heights from the gauche to the tmns forms in relation to the rotation around the C-S and C-C bonds are analyzed. The role of hydrogen bonding in governing the stable molecular geometry is discussed for CH,SCH,SH and ClCH,CH,SH. INTRODUCTION

Molecules including sulfur atom(s) act uniquely in chemical reactions or in their molecular structures. Some of these compounds were investigated up to the present time by using the MO calculation procedures [l-5]. Among them the EtSMe molecule was treated extensively by using a variety of basis sets [5]. In that paper we have found: first, the 3-21G set is economical to use and very helpful for a rough estimation of the energy difference between the rotational isomers; second, the 3-21G + d(0.W + 0.6s) set is good for geometry reproducibility and, third, the 3-21G + d(0.8C + 0.6s) set plus M@ller-Plessett (MP2) perturbation calculations [36, 371 explain reasonably both molecular geometry and energy difference between the rotational isomers. Preliminary calculations by using the semi-empirical and ab initio MO procedures for thiols, HSCH,SH, CHBSCHzSH and HSCH&H#H, were performed [ lc, 21. All of these calculations, however, were carried out without any levels of geometry optimization. In the present article, we have reinvestigated/investigated the molecular species, HSCH$H, CH$CH$H, CICH&H1SH and HSCH&H$H to obtain theoretical information of the molecular structures. All of the calculations were performed using the GAUSSIAN 80 program package [6] ; geometry optimization was performed with the energy gradient method [ 71. 0166-1280/86/$03.50

o 1986 Klsevier Science Publishers B.V.

284 OPTIMIZED GEOMETRY

The optimized geometries are summarized in Tables l-4 for the molecules HSCH,SH, CH$CH$H, ClCH&H$H and HSCH$H$H, respectively. Schematic structure and notation of geometrical parameters are given in Fig. 1. As was already shown in the previous paper [5], the 3-21G set reproduced bond lengths somewhat too long in relation to the S atom while the 3-21G + d(0.8C + 0.6S/O.7Cl) set gave reasonable lengths. The observed r(C-S) and r(S-H) of EtSH [8, 91, for example, are (in A) 1.814 (1.829) and 1.336 (1.328). TABLE 1 Optimized structural parameters of methanedithiola b

3-216 TT

Rl R2 83 Al A2 A3 Bl B2

1.364 1.883 1.076 96.35 107.63 109.06 (180.0) (180.0)

3-21G + d(C,S) TG(120)c

TG(6O)c 1.353 1.879 1.076 9714 111.13 109.70 (180.0) (60.0)

1.363 1.883 1.076 96.99 109.38 109.46 (180.0) (120.0)

TC

GG 1.361 1.879 1.075 97.34 114.86 108.06 68.85 59.42

cc

1.352 1.885 1.076 98.12 111.40 109.16 (180.0) (0.0)

1.360 1.893 1.073 99.63 117.33 10718 (0.0) (O-0)

TT 1.329 1.819 1.083 96.03 107.70 109.61 (180.0) (180.0)

GG 1.328 1.812 1.083 96.91 115.69 108.27 58.57 60.51

aBond lengths are in A, angles in degrees, figures in parentheses are assumed. bSee Fig. 1. CTrans-gauche (60” ) and other tram-gauche (120” ) forms. TABLE 2 Optimized structural parameters of (methylthio)methanethiola b

3-21G TT

Rl R2 R3 R4 R5 R6 Al A2 A3 A4 A5 A6 Bl B2

1.878 1.881 1.354 1,076 1,888 1.079 108.24 96,66 108.99 97.54 106-84 109.22 (180.0) (180.0)

3-21G + d(C, S) TG 1.879 1.872 1.352 1.076 1.888 1.079 111.51 97.09 109.60 98.34 106.90 109.27 (180.0) 65.14

GG 1.870 1.887 1.351 1.076 1.885 1.078 114.75 97.00 108.60 99.08 106.68 108.94 65.87 59.79

TC 1.879 1.885 1.351 1.076 1.888 1.078 112.09 98.80 109.12 98.35 106.80 109.11 (180.0) (0.0)

cc 1.899 1.897 1.345 1.073 1.886 1.078 120.71 101.04 106.46 107.24 105.16 110.04 (0.0) (0.0)

TT 1.811 1.818 1.329 1.084 1.808 1.086 108.25 96.23 109.52 98.10 107.84 110.56 (180.0) (180.0)

GG 1.805 1.824 1.328 1.083 1.807 1.085 115.29 96.58 108.91 99.69 107.64 110.30 66.52 59.43

aBond lengths are in A, angles in degrees, figures in parentheses are assumed. bSee Fig. 1.

285 TABLE 3 Optimized structural parameters of chloro-ethanethioP b

0bS.C

TT Rl R2 R3 R4 R5 Al A2 A3 A4 A5 A6 A7 Bl B2

1.789 1.082 1.519 110.1 63.3 58.4

3-21G + d(C, S,Cl)

3-21G

1.894 1.076 1.517 1.894 1.352 107.62 105.34 105.34 110.48 110.48 107.76 97.38 (180.0) (180.0)

TG 1.901 1.076 1.515 1.889 1.352 107.76 104.91 105.37 111.63 110.84 110.51 97.17 (180.0) 75.80

GT 1.888 1.077 1.518 1.892 1.352 110.21 105.06 105.28 110.97 108.76 110.12 96.97 70.48 (180.0)

GG

GG

1.886 1.077 1.518 1.885 1.354 109.97 105.12 105.12 111.00 109.58 114.01 97.16 68.72 74.23

1.898 1.077 1.516 1.884 1.351 109.54 104.47 105.02 111.54 109.03 113.59 97.72 71.19 -67.14

TT

TG

1.791 1.082 1.522 1.825 1.328 109.16 107.22 107.22 109.69 109.69 108.10 97.02 (180.0) (180.0)

1.797 1.082 1.521 1.820 1.329 109.30 107.02 107.41 110.86 110.19 111.16 96.88 (180.0) 73.98

aBond lengths are in A, angles in degrees, figures in parentheses are assumed. bSee CFrom ref. 11.

Fig. 1.

TABLE 4 Optimized structural parameters of ethanedithiola b

Obs.C

TTT Rl R2 R3 R4 Al A2 A3 Bl B2 B3

1.821 1.538 1.340 1.094 112.5 96.5 110.7 136.6 64.8 162.9

3-21G-+ d(C, S)

3-21G

1.895 1.523 1.353 1.078 107.82 97.80 110.69 (180.0) (180.0) (180.0)

GTG 1.892 1.520 1.353 1.079 111.81 97.11 llL33 69.99 (180.0) 69.76

GTG’ 1.892 1.520 1.352 1.079 111.77 96.94 111.37 71.67 (180.0) -71.21

TG@ 1.898 1.523 1.353 1.079 109.95 96.96 110.33 169.12 67.02 186.77

TGF 1.897 1.524 1.353 1.079 110.10 96.97 110.33 (180.0) 68.57 (180.0)

GGG 1.889 1.524 1.354 1.080 113.44 96.88 111.06 75.33 68.77 75.33

TTT 1.822 1.527 1.329 1.085 108.40 97.32 109.76 (180.0) (180.0) (180.0)

GTG’ 1.823 1.524 1.329 1.085 112.28 96.71 110.68 70.04f (180.0) 70.04f

aBond lengths are in a, angles in degrees, figures in parentheses are assumed. bSee Fig. 1. CFrom ref. 12. dThe form corresponds to that reported in ref. 12. Wsual form. fAssumed as equal.

HSCH,SH

Let us now examine in some detail the geometrical obtained parameters. As for the bond lengths, the amounts of variation in relation to the rotational isomers are in the order, R2 > Rl > R3. The value of the R3 scarcely varied under the change of the internal rotation angles around the C-S bonds, Bl

b

a Ii

H

H

H

H

C

H

d

Fig. 1. Schematic structure and notations of geometrical parameters in: (a) methanedithiol; Bl, B2: r(HS-CS), T(SC-SH); (b) (methylthio)methanethiol; Bl, B2: T(CS-CS), T(SC-SH); (c) chloro-ethanethiol; Bl, B2: T(CIDCS), T(CC-SH) and (d) ethanedithiol; Bl, B2, B3: r(HS-CC), ~(SC-CS), T(CC-SH).

and B2. As for the bond angles, the variations are in the order, A2 > Al > A3. The angle A3 is closely related to the magnitude of A2, i.e., the A2 increased while the A3 decreased. Variation of A2. In order to explain the variation of the A2 in relation to the lone-pairs on the S atoms, we performed several test calculations attaching protons onto the sulfur lone-pairs. By addition of a bare proton, a lonepair orbital will be spoiled. The A2 optimization was performed fixing the other geometrical parameters constant as those obtained for the GG form. The set used was the 3-21G. There are two ways to attach one proton on an S atom and there are four ways for two protons, as shown in Fig. 2. In the case of one-proton addition, when the proton occupied the gauche position in relation to the S-C-S group (a) the A2 widened its value from the parent molecule, on the other hand when it occupied the truns position (b) the reverse was true. In the case of the two-proton addition, the following cases are considered: (i) both protons are on one sulfur atom (c), and (ii) one proton on each S atom, gg (d), gt (e) and tt (f). In case (i), the vlaue of the A2 obtained (c) was wider than the case of one-proton addition on the S atom (a). In case (ii) the A2 of the gg (d) position was the widest of all three and those of the gt (e) and tt (f) followed it. As a result, we can recognize that the trans position proton narrows the A2, on the other hand the gauche position proton widens it. The gg and tt cases widen or narrow the A2 more than single g or t attachment. We can summarize here the effect that: a lonepair orbital on the sulfur atom which is located at the gauche position in relation to the S-C-S group plays a role to narrow the A2, but one on the sulfur atom which is located at the trms position widens the A2 from its parent molecule in the GG skeletal conformation. The former prevails the latter and there is a rough sum rule among them.

287

a

b

113,4

C

117,8

d

121,2

e

116.6

f

112.4

Fig. 2. Variation of the A2 (degrees) calculated with a variety of proton addition on the sulfur atom(s) in the GG form of methandithiol(3-21G). The attached protonic orbital is shown with the oblique lines. The parent molecule, the optimized A2 is 114.86”.

CH$CH,SH

As for the valence angles, the values of the Al, A2 and A4 varied much under the influence of the rotation around the CS-CS and SC-SH bonds. Up to the present time, we are unaware of the reported geometry. Therefore we cannot make a further discussion at present. However the geometry obtained by the 3-21G + d(C, S) basis set will be helpful should an experiment be done in the future. Cl CH, CH,SH In this molecule, similar basis set dependency as discussed above is seen. The 3-21G set reproduced r(C-Cl) somewhat too long as well as r(C-S) and r(S-H). A preliminary microwave study was performed on this species [lo] , but we are not aware of the precise geometries reported. We can then compare some of the geometrical parameters for the reported oxygen analog, such as CICHzCHzOH [ 111. The 3-21G + d(C, S, Cl) basis set reproduced

288

reasonable bond lengths, Rl, R2 and R3. In this species the Al and A6 varied greatly under the influence of the skeletal rotation angles. In the present molecule, both of the #(ClCH), A2 and A3, and G(CCH), A4 and A5, were optimized independently as a test case. From these we can see the variation of these angles under the non-equivalent situation. It seems, however, that the differences between the two are not considerable. HSCH,CH,SH

In Table 4 a recently reported geometrical parameter set [12] is also listed for comparison. With the bond lengths there is excellent reproducibility as seen in those by the 3-21G + d basis set. As for the r(C-C) and r(C-H) values we can see little basis set dependency. The Al varies largely in relation to the rotational isomers. This gives us important information in relation to the rotational isomerism of the molecule. The TTT form produced the narrowest angle of the Al and the TGT form follows this. The form GTG or GTG’ explains reasonably the observed Al. The calculated value of the GGG form is slightly wider than the observed. ROTATIONAL

ISOMERISM

Rotational isomerism of the sulfur containing molecule was extensively discussed, for example by the present author and co-workers [13-211. The experimental determination of the stable form is, however, sometimes difficult because the energy difference between or among the rotational isomers around the C-S bond is, in general, not very much. In such a case an ab initio MO calculation will be helpful. HSCH,SH

This is the simplest dithiol except for hydrogen persulfide HSSH but we do not have the reported experimental geometrical and/or spectroscopic data at the present time. The present analysis, however, will be helpful to get information in relation to the other dithiols/thiols. As summarized in Table 5, the stability of the three forms, e.g., was calculated to be in the order: GG > TG > TT. The order is already proposed [2] and the agreement is obvious. The present stability agrees well with the oxygen analog [35]. We found an interesting fact in the present molecule. In the tmns form, the energy TABLE 5 Calculated

energies (kcal mol-‘) of methanedithiol TT

3-21G 381G + d(C.S)

-621612.64 -621670.69

TG (60’

)

-621514.49

TG (120") -621612.82

GG -621616.26 -521674.04

TC -521512.44

cc -621610.21

289

minimum does not appear around the dihedral angle of ISO”, but around 150” (see also Fig. 3). This may be due to the role of the lone-pair electrons on the S atoms. CH,SCH,sH The results obtained are summarized in Table 6. In this molecule the GG form is estimated to be the most stable form. This is the same as in HSCH2SH. The agreement with that observed [ 141 is excellent. ClCH,CH,SH The total energies calculated are summarized in Table 7. The rotational isomerism of this molecule was previously analyzed by Hayashi et al. [22] based on infrared spectroscopy and they proposed that the tmns and gauche forms about the C-C as well as the C-S bonds coexist in the gaseous and liquid states. They also concluded that the tram form is more stable than the gauche form by ca. 0.5 + 0.3 kcal mol-’ around the C-C bond. From the microwave spectroscopy however, this molecule would be considered to

1 -521510.

0

I

I

I

I

I

30

60

90

120

150

1 10

Dihedral on9le (de@ Fig. 3. Total energy as a function of the rotational angle around the C-S bond in methane. -): Rotation around the SC-SH bond; (-A -): simultaneous dithiol (3-21G). (rotation around the two SC-SH bonds.

- -545874

+ 0

I 30

I 60

I 90

I 120

I 150

-

877

-

878

180

Dihedral angle (de@ Fig. 4. Total energy as a function of the rotational angle around the C-S bond in (methylthio)methanethiol (3-21G). (e-): Rotation around the SC-SH bond; (-A -): rotation around the CS-CS bond; (-0 -): simultaneous rotation around the SC-SH and CS-CS bonds. Arrow sign shows the optimized point for the GG form: Bl = 65.87” and B2 = 59.79”. TABLE 6 Calculated energies (kcal mol-‘) of (methylthio)methanethiol TT 3-21G 3-21G + d(C, S)

-545875.14 -545941.64

TG

-545877.13

GG

-545879.18 -545944.98

TC -545875.24

cc -545870.12

exist in only one form [lo]. The microwave work no longer says whether or not the molecule exists in astructure with an SH* **Cl hydrogen bond, which . Examinawould be analogous to the OH*.. Cl bond in chloroethanol [lla] tion of the rotation about the C-S bond shows there are two minima (see also Fig. 5): these correspond to the gauche and trans forms about the C-S bond. There is little energy difference between the gauche and tmns forms around this bond. In this molecular species it can also be considered that there exist rotational isomers around the C-C bond. We have two minima

291 TABLE 7 Calculated energies (kcal mol-’ ) of chloro-ethanethiol

3-21G 3-21G + d(C, S, Cl)

GT

TG

TT

-584260.89 -584317.30

-584262.09 -584318.69

-584258.65

GG

-584259.16

GG

-584261.46

-584258

260 261 262 0

30

60

90

120

150

180

Dihedral angle (deg) Fig. 5. Total energy as a function of the rotational angle around the C-S and C-C bonds in chloro-ethanethiol (3-21G). (- l -): Rotation around the CC-SH bond; (--A -): rotation around the CIC-CS bond.

formations. With this molecule the TG form is calculated to be the most stable form and the GG’ and TT forms follow this. The present calculation offers a good explanation of the observed relative stability [22]. If the TG and GG’ forms coexist the energy difference calculated by the 3-21G set between them can be estimated to be 0.63 kcal mol-‘. This difference also agrees excellently with the observations of Hayashi et al. [22] ,0.5kcal mol-I.

292

HSCH,CH,SH The total energies calculated are summarized in Table 8. This molecule is very important as a suitable model molecule for the analysis of the cystinecysteine interconversion in vivo. From this point of view, a large number of reports in relation to the molecular structure appear [12,21,23, 241. However, in the molecular structure reported the skeletal torsional angles especially are still not completely clarified. For the rotation about the C-SH bond, there is no actual energy difference between the gauche and truns forms, as shown in Fig. 6. This is different from the former two molecules, CH3CH2SH and CH,SCH,SH; this may make it difficult to estimate the exact skeletal form from the experiment. For the internal rotation about the C-C bond, the truns conformation is more stable than the gauche form, and the energy difference between them is about 3 kcal mol”. With this molecule a new geometry appeared recently [12]. This geometry is nearly the TGT form. However, in relation to the two C-S bonds, the proposed conformation is not the exact tmns, 180”, but the Bl and B2 are rotated by 17.1” and -43.4”) respectively, from 180”. With the aid of the ab initio MO treatment, the total energy estimated is the smallest in the GTG’ form and the GTG form follows this. By using the 3-21G set, the newly reported geometry does not reproduce the smallest energy, but more like the energy of the exact TGT form. Hence these revised skeletal internal rotation angles are rather implausible. Further examination of the total energies obtained, shows the energies of the GTG and GTG’ forms have nearly the same values. Of note is the large stability of the GTG’ form. This corresponds well with the earlier experimental [ 231 and the theoretical work [2]. ROTATIONAL

BARRIER

HEIGHT

It is indeed of importance to analyze the barrier height for the internal rotation around the C-S bond.. Around the S-S bond the barrier height was previously estimated for the molecules HSSH and CH$SCHB [3], for example. We estimated the barrier height for the internal rotation by using the 3-21G basis set. The potential energy curve in relation to the skeletal dihedral angles of HSCH,SH, CH$CH$H, ClCH&H$H and HSCH&H$H are summarized in Figs. 3-6, respectively. TABLE 8 Calculated energies (kcal mol-‘) TTT

3QlG 3-21G + d(C,S)

-54587599 -545945.71

of ethanedithiol GTG

-546880.03

GTG’

-545880.10 -545947.30

TGTa

-645876.66

VI+he form corresponds to that reported in ref. 12. bUsual form.

TGTb

-546876.61

GGG

-545877.38

293

, -545870

-5458571

871 872 873 874 875 876 875-

877

8770

I

I

1

I

I

30

60

90

120

150

Dihedral anale

180

(de@

Fig. 6. Total energy as a function of the rotational angle around the C-S and C-C bon& in ethanedithiol (3-21G). (-•-): Rotation around the CC-SH bond; (--A---): rotation around the SC-CS bond; (-0 -): simultaneous rotation around the two CC-SH bonds.

With the molecule HSCH$H, the barrier for rotation about the C-S bond from the gauche to the trans forms is estimated as ca. 2 kcal mol-‘, while there is indeed a low barrier height from the tram to the gauche rotation. Here in this molecule, we carried out the full geometrical optimization for the four points: B2 = 0” (TC), B2 = 60” (TG), B2 = 120” (Z’G) and B2 = 180” (TT), where Bl = 180”. With the other points, the single-point calculations were performed with the use of the optimized TT geometry varing the B2. Therefore in this case the rotation about the C-S bond is not completely flexible. Hence too great an energy around B2 = 150” might be reproduced. So the question arises of what happens when the rotation of the SH groups on both sides of the molecule takes place simultaneously around the C-S bonds. In this case the barrier for the rotation from thegauche to the tmns forms becomes higher and reaches ca. 5 kcal mol”, also in this case lower barrier appeared from the tram to the gauche forms, corresponding to ca. 1.3 kcal mol-‘. With the molecule CH$CH$H the barrier for rotation around the CS-SH bond is about 2 kcal mol”, and this value is similar to that obtained for HSCH$H. With this molecular species the full geometrical optimization was

294

performed at the three points, B2 = 0” (TC), B2 = 60” (TG) and B2 = 180” (TZ’), where Bl = 180”. The GG form is also fully geometry optimized. With the other points we carried out the single-point calculations using the optimized TT geometry varying the B2. In this case the barrier might be overestimated. The cis form is slightly smaller in energy than the trans form. This is not the same case as in the molecule HSCH$H. The barrier for the rotation about the MeS-CSH bond from the gauche to the buns forms is estimated to be ca. 2 kcal mol-‘; the energy of the G form is smaller than that of the T form, and there is a low barrier from the bans to the gauche forms. Interesting phenomena can be seen when the rotations take place about the CS-CS and SC-SH bonds simultaneously. With these rotations the energy lowers ca. 2 kcal mol-l around the G form; then appeared the barrier of ca. 2 kcal mol” high from the truns to the gauche rotation. The barrier height from the gauche to the tmns forms around the C-S bond of ClCH&H$H is indeed very low (ca. 1.3 kcal mol-I). The barrier for rotation around the C-C bond from the gauche to the truns forms was estimated to be ca. 2.9 kcal mol-‘. With the molecule HSCH,CH,SH in relation to the rotation about the C-S bond, the energies of the tmns and gauche forms are nearly the same. In order to estimate the barrier heights, only the single-point calculations were performed by using the geometry of the TTT form optimized by the 3-21G + d(C, S) set except for the skeletal dihedral angles. The barrier between the truns and gauche forms is ca. 1 kcal mol”. The barrier height for the rotation about the C-C bond from the gauche to the trans forms is ca. 2.2 kcal mol” and the reverse direction is ca. 5 kcal mol-‘. There are two minima corresponding to the trans and gauche forms for this rotation. The T form is lower by ca. 2 kcal mol-’ than the G form. With the simultaneous rotation about two C-S bonds, the barrier from the gauche to the trans forms is ca. 1.7 kcal mol-‘.

Table 9 summarizes the rotational barrier heights around the C-S and C-C bonds for the molecules treated together with those of their related analogs. With the C-S rotational barrier heights, the estimated values are ca. l-2 kcal mol-‘. There is a very good agreement between the calculated and the observed values [Sb, 33, 341. These values decrease in the order around the bond, C-C, C-C and C-S. From this we can recognize that rotation around the C-S bond is much softer than that around the C-C or C-O bond. In the case of the thiol group around the C-S(H) bond, there is a very low barrier from the trans to the gauche rotation. The calculated barrier heights from the gauche to the trans rotation for the species HSCH#H and CHBSCHzSH are slightly higher than those of the other molecules, CICHzCHzSH and HSCH&H2SH. With the C-C torsional barrier heights, the calculated values are ca. 2-3 kcal mol-‘. These values correspond well with the observed ones for EtSMe [ 331 and EtSH [Sb] . In the case of the molecular species HSCH&H$H, the

291 TABLE 9 Rotational barrier of sulfur containing molecules together with some related molecule (kcal mol -’ ) Molecules

STO-3G CH,CH,CH,CH, (u(CC-CC)

Obs.

Calc. 3-21G

4-31G

3 64s 3.5ob

3.5W

3.21d, 3.4e, 3.10

CH,CH,OCH, or(CC-OC) or(H,C-CO)

3.15a

3.31a

2.61h 3 ,08h

CH,CH,SCH,’ (Y(CC-SC) a( H,C-CS)

1.77a

2.05:. 1.71i 3.331

HSCH,SH or(SC-SH)

2.1’

CH,SCH,SH a(SC-SH) Q.(CS-CS)

2.1’ 2.1m

CiCH,CH,SH or(CC-SH) cr(c1c-Cs)

1.3m 2.9m*n

HSCH,CH,SH a( CC-SH) (r(SC--cS)

1.0m 2.2m.n

CH,CH,SH (r(CC-SH) a( H,C-CS)

1 .24k*n 3.26r=

*From ref. 25. bFrom ref. 26. CFrom ref. 27. dFrom ref. 28. eFrom ref. 29, from th gauche to the trans forms. fFrom ref. 30. aFrom ref. 31. hFrom ref. 32. ‘From ref. 32 jFrom ref. 34. kFrom ref. Sb. ‘Flexible rotation, see also text. mRigid rotation. “Fror the gauche to the trans forms.

barrier around the C-C bond is fairly low. The lower barrier heights arounc the bond make it easier to flex the molecule. Flexing is one of the importan factors considered when the chemical reaction is carried out and plays a important role in the function of the molecules.

296 HYDROGEN

BONDING

In the case of ClCH&H$H and CH3SCH2SH, the hydrogen bonding may play a role in governing the molecular structures. In the case of ClCH&H20H, the hydrogen bonding between the chlorine atom and the alcoholic proton was proved [ 111. However no reports to prove the hydrogen bonding between the chlorine and the thiol proton in ClCH&H#H have yet appeared. In the case of ClCH,CH,SH, we obtained very interesting calculated results; that is, on comparison of the energy obtained for the GG and GG’ forms, the GG’ form was found to be lower by ca. 2.3 kcal mol” than the GG form. Here the distance between Cl* H(SH) is 2.96 8, in the GG’ case. With the molecule CHBSCH2SH, remarkably the energy of the TC form is lower than that of the TT form. In the case of the TC form, the distance between the sulfur and the thiol proton is 2.83 A. This is not close enough for strong hydrogen bonding to occur. However there must be a certain weak attractive interaction between S* H(SH) in the molecule. This attraction seems to be much stronger than that in HSCH$H. The methyl group in this molecular species may take part and stabilize the TC form more than that in HSCH$H. Anyway, in these two molecular species the distances described above are within the sum of the van der Waals radii of the related atoms. These may govern the molecular structures of these compounds. Of course in the case of HSCHzSH in the TC form the distance between S* l*H(SH), which is calculated to be 2.80 8, is within the sum. l

l

l

l

CONCLUSIONS

For the geometry, the 3-21G + d(0.8C + 0.6s + 0.7Cl) basis set at the HF level was enough to reproduce those observed. From the energies and the potential energy curves, the 3-21G set explained reasonably or might explain satisfactorily the observed stability of the molecular conformations. The weak hydrogen-bonding-like attraction is abstracted from these ab initio MO calculations for the molecules such as CHBSCHzSH and ClCHJJH#H. ACKNOWLEDGEMENTS

The author thanks Profs. Akira Imamura and Michiro Hayashi for valuable comments. Thanks are also due to the Computer Center of the IMS and the Information Processing Center of Hiroshima University for the use of the HITAC M-200H systems. REFERENCES 1 (a) M. Ohsaku, 2627.

N. Bingo,

K. Eida and H. Murata, Bull. Chem. Sot. Jpn., 49 (1976)

297 (b) M. Ohsaku, N. Bingo, H. Fujiwara and H. Murata, Bull. Chem. Sot. Jpn., 51(1978) 1539. (c) M. Ohsaku, N. Bingo, W. Sugikawa and H. Murata, Bull. Chem. Sot. Jpn., 52 (1979) 355. 2M. Ohsaku and H. Murata, J. Mol. Struct. (Theochem), 85 (1981) 125. 3(a) A. Bauk, J. Am. Chem. Sot., 106 (1984) 6517. (b) F. Grein, Chem. Phys. Lett., 116 (1985) 323. 4 M. Ohsaku, T. Ichiishi, A. Imamura and M. Hayashi, Bull. Chem. Sot. Jpn., 57 (1984) 2791. 5 M. Ohsaku and A. Imamura, Mol. Phys., 55 (1985) 331. 6 J. A. Pople, J. S. Binkley, R. A. Whiteside, R. Kriihnan, R. Seeger, D. J. Defrees, H. B. Schlegel, S. Topiol and L. R. Kahn, Q.C.P.E., 13 (1981) 406. 7 H. B. Schlegel, J. Comp. Chem., 3 (1982) 214. 8 (a) M. Hayashi, H. Imaishi and K. Kuwada, Bull. Chem. Sot. Jpn., 47 (1974) 2382. (b) J. Nakagawa, K. Kuwada and M. Hayashi, Bull. Chem. Sot. Jpn., 49 (1976) 3420. 9 R. E. Schmidt and C. R. Quade, J. Chem. Phys., 62 (1975) 3864. 10 R. N. Nandi, M. F. Boiand and M. D. Harmony, J. Mol. Spectrosc., 92 (1982) 419. 11 (a) R. G. Azrak and E. B. Wilson, J. Chem. Phys., 52 (1970) 5299. (b) A. Aimenningen, 0. Bastiansen, L. Fernholt and K. Hedberg, Acta Chem. Stand., 25 (1971) 1946. 12 R. N. Nandi, C.-F. Su and M. D. Harmony, J. Chem. Phys., 81 (1984) 1051. 13 M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Jpn., 45 (1972) 113. 14 M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Jpn., 45 (1972) 3035. 15 M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Jpn., 46 (1973) 1399. 16 M. Ohsaku, Bull. Chem. Sot. Jpn., 47 (1974) 965. 17 M. Ohsaku, Spectrochim. Acta, Part A: 31 (1975) 1271. 18 M. Ohsaku, H. Murata and Y. Shiro, Spectrochim. Acta, Part A: 33 (1977) 467. 19M. Ohsaku, H. Murata and Y. Shiro, J. Mol. Struct., 42 (1977) 31. 20 M. Ohsaku and H. Murata, Spectrochim. Acta, Part A: 34 (1978) 869. 21 M. Ohsaku and H. Murata, J. Mol. Struct., 52 (1979) 143. 22M. Hayashi, Y. Shiro, M. Murakami and H. Murata, Bull. Chem. Sot. Jpn., 38 (1965) 1740. 23 M. Hayashi, Y. Shiro, M. Murakami and H. Murata, Bull. Chem, Sot. Jpn., 38 (1965) 1734. 24 I. Hargittai and Gy. Schultz, J. Chem. Sot. Chem. Commun., (1972) 323; Acta Chim. Acad. Sci. Hung., 75 (1973) 381. 25 J. L. Bredas, M. Dufey, J. G. Fripiat and J. M. Andre, Mol. Phys., 49 (1983) 1451. 26 L. Radom and J. A. Pople, J. Am. Chem. Sot., 92 (1970) 4786. 27 L. Radom, W. A. Lathan, W. J. Hehre and J. A. Pople, J. Am. Chem. Sot., 95 (1973) 693. 28 J. R. Durig and D. A. C. Compton, J. Phys. Chem., 83 (1979) 265. 29 J. E. Piercy and M. G. S. Rao, J. Chem. Phys., 46 (1967) 3951. 30 K. Ito, J. Am. Chem. Sot., 75 (1953) 2430. 31 U. Burkert, J. Comp. Chem., l(l980) 285. 32 J. R. Durig and D. A. C. Compton, J. Chem. Phys., 69 (1978) 4713. 33 J. R. Durig, D. A. C. Compton and M.-R. JaIiIian, J. Phys. Chem., 83 (1979) 511. 34 M. Adachi, J. Nakagawa and M. Hayashi, J. Mol. Spectrosc., 91 (1982) 381. 35 H. Matsuura, M. Yamamoto and H. Murata, Spectrochim. Acta, Part A: 36 (1980) 321. 36 C. MdiIer and M. S. PIessett, Phys. Rev., 46 (1934) 618. 37 J. S. Binkley and J. A. Pople, Int. J. Quantum Chem., 9 (1976) 229.