Ab initio and indo molecular orbital calculations of the geometries and electronic structures of substituted sulfines

Tetrahedron.

Vol. 33, pp. 539 to 545. Pergamon Press 1977. Printed in Great Britain

AB INITIO AND INDO MOLECULAR ORBITAL CALCULATIONS OF THE GEOMETRIES AND ELECTRONIC STRUCTURES OF SUBSTITUTED SULFINES JAN VAN LIEROP* and AD VAN DER AVOIRD Institute of Theoretical Chemistry, University of Nijmegen, Toernooiveld, Nijmegen, The Netherlands and BINNE ZWANENBURG Department of Organic Chemistry, University of Nijmegen, Toernooiveld, Nijmegen, The Netherlands (Received in the UK 12 August 1976; Accepted for publication 14 September 1976) Abstract Molecular orbital calculations are presented for the parent sulfine and some mono- and dihalogen-

substituted sulfines, using ab initio and INDO methods. A partial geometry optimization was performed for nine different sulfines. Charge distributions, potential surfaces and dipole moments were calculated from the wave functions of the optimized geometries. Cis-trans interconversion barriers and electronic spectra are also given. The atomic charges of the S and O atoms are insensitive to substitutions at carbon, and substituents greatly influence the potential in the environment of the molecule. The implications of the results for the chemical behavior of sulfine derivates are briefly discussed and compared with experimental data.

Sulfines, which are defined as the S-monoxides of thiocarbonyl derivates, have attracted considerable attention in the recent literature. Although the first sulfine was reported as early as 1923) more general synthetic routes to this class of compounds were developed some 40 years later. Dehydrohalogenation of sulflnyl chlorides 2-4 and oxidation of thiocarbonyl containing substrates such as aromatic thioketones, 5 thioacid chlorides, 6'7 dithiocarboxylic esters, s'9 and non-enethiolizable aliphatic thiones ~° can be utilized to prepare a wide variety of substituted sulfines. The parent sulfine, H2C=S--O, has recently been generated by flash vacuum pyrolysis of thietane S-oxide and 1,3-dithietane 1-0xide. 1~'t2 The bent structure of these heterocumulenes has been well established by dipole moment measurements, 9'13'14 NMR spectroscopy, s~:5 X-ray analysis, ~6 and by the isolation of stable geometrical isomers, 7-9,~3 of sulflnes of the type XYC--S---O with X # Y. The chemical behavior is presently under active investigation. Cycloaddition reactions were observed with dienes ~7 and 1,3-dipoles) 8 The course of the reaction with nucleophilic species depends on the nature of the reagent as well as on the substituents attached to the sulfine carbon atom: For instance, alkyl lithium reacts exclusively at the sulfine sulfur atom to give alkyl sulfoxides) 9 chlorosulfines show a displacement of the chlorine upon reaction with aniline) 3 p-toluenethiolate, 7 potassium thiocyanate, 7 and p-toluenesulfinate, 2° while aryl arylthiosulfines 7 react with arenethiolates primarily at the sulfine sulfur atom. Initial attack on that atom was also observed during the reaction of aryl arylsulfonyl sulfines with p-toluenesulfinate, 2° cyanide, 2° and hydroxide ions. 2° Electrophilic reagents preferably react at the sulfine oxygen atom. The acid-catalyzed hydrolysis 21 of sulfines is assumed to proceed via initial oxygen protonation. Electrophilic oxygen alkylation has recently been accomplished with triethyloxonium tetrafluoroborateY Theoretical calculations of sulfines reported thus far only deal with the parent system (simple Hiickel,5b

CNDO/2, 23'24ab initio) 2'25) The aim of the present study is to gain insight in the influence of substituents at the sulfine carbon atom on the charge distribution and some molecular properties, and also to compare the results of ab initio calculations with those obtained by less sophisticated methods. To this end, a set of nine simple halogen substituted sulfines was studied. The substituents on the carbon atom are all possible combinations of H, F and CI atoms, viz. H2, F2, C12, cis and trans HF, HC1 and FCI. Molecular potential maps were computed in order to shed some light on the factors governing reactions of sulflnes with nucleophiles and electrophiles. COMPUTATIONAL ASPECTS

The wave functions of the nine molecules mentioned above were calculated using Roothaan's MO-LCAO-SCF method,26 both ab initio and by means of the semi-empirical INDO method (Intermediate Neglect of Differential Overlape7-29). If the results of the INDO calculations satisfactorily match those obtained by the ab initio method, one could use this faster method also for calculating wave functions for larger sulfines or related compounds. For the ab initio calculations the IBMOL5A program by Clementi et al. was used; 3°the INDO program was written by Van der Lugt.29 Information on the ab initio Gaussian Type Orbital (GTO) bases is presented in Table 1. The INDO calculations used a minimal Slater Type Orbital (STO) basis, with exponents taken from Clementi and Ralmondi?3 The ionisation potentials, also required as input to the INDO program, were calculated from spectral data. 34 From the wave functions several interesting properties could be calculated. The ab initio wave functions were used for population analysis and for dipole moment and molecular potentials calculations. From the INDO wave functions atomic populations and dipole moments were computed. Also the electronic spectra were calculated, by adding a singly excited Configuration Interaction (CI) to the INDO calculations. For the smaller molecules all possible single excitations were considered; for the others only the 60 single excitations lowest in energy were taken into account. For the ab initio population analyses and dipole moment calculations a molecular properties program, written by Van Duijneveldt et al? 5 was applied. The molecular potentials

539

540

JAN VAN LIEROP et al.

were computed by use of the IBMOL5Aproperties package. For calculating the INDO populations and dipole moments a new subroutine was added to the program by Van der Lugt. All calculations were performed on an IBM 370/158 computer, average integral plus SCF times being 10 and 1 min for H2CSOand 32 and 3 min for CI2CSO in the ab initio and INDO cases, respectively.

for H2CSO compared with those obtained in other theoretical studies and with the experimental values.* From columns b and c in this table it is easily seen that in the INDO calculations the inclusion of a 3d orbital on sulfur results in a CSO angle which agrees very well with the experimental value. This result emphasizes the importance of polarization functions in geometry optimization. The barrier to cis-trans interconversion can be calculated as the difference between the energies of the equilibrium geometry and a suitable transition state. Since there are two possible extremes to the mechanism of interconversion (rotation around the C-S bond or inversion with the oxygen remaining in the molecular plane) two transition states of H2CSO were calculated: (a) the CSO-plane perpendicular to the H2CS-plane; all bond lengths and angles are fixed to optimized ground state geometry values; (b) the CSO-grouping in a linear configuration; all atoms in one plane; all bond lengths

RESULTS AND DISCUSSION

Geometries

In order to obtain accurate wave functions it is necessary to start with reliable geometries. The experimental data in this field are rather scarce and have only very recently been extended to include the geometrical parameters of the parent sulfine (thioformaldehyde-Soxide). 11From an analysis of the existing crystallographic data on substituted sulfines 16'36-39it appears that the C-S and S-O bond distances do not vary much between the different sulfines in which there is no inter- or intramolecular hydrogen bonding. The C-S bond length was taken to he 1.63 A, r(S--O) was fixed at 1.47 .~. The C-H, C-F and C-C1 bond lengths were taken equal to the corresponding values in substituted ethenefl Since the angles around carbon will be approximately 120° and slight deviations from this angle will not cause important variations in the calculated properties, these three angles were fixed at the ideal sp ~ hybrid angles. The CSO angle seems to be rather sensitive to the exact molecular environment of the sulfine grouping (104°-115°)3 Therefore this angle had to be optimized for all nine molecules separately. The results of this optimization are summarized in Table 2. Table 3 shows the resulting geometry

Table 1. GTO basis functions used in ~e ab mitre calculations Atom

Ref.

Uncontraeted

Contracted

H C,O,F S,CI

31 31 32

(3) (6;3) (9;5)

<2> <4,1,1;2,1> <4,2,2,1;3,1,1 >

Table 2. Optimized values of the CSO angle

tin some sulfmes small CSO angles are caused by intra- and intermolecular hydrogen bridges. *Notethat inthe calculationsby Floodand Boggsa 3d-functionon sulfur was used.

Rc

Rt

H F C1 F H C1 H F C1

H F C1 H F H C1 el F

Ab

Initio 107.9 104.6 105.5 107.1 103.8 106.5 106.1 105.9 104.2

108.9 106.5 107.6 111.8 103.9 109.1 107.4 110.2 104.0

Table 3. Geometry of H2CSO a

c

b

d

e

f

g

C-S

1.63

1.63

1.63

1.63.

1.63.

1.59.

1.61

S-O

1.47

1.47

1.47

1.62-

1.59-

1.48.

1.47

C-H t

1.08

1.08

1.08

1.08

1.08

1.07.

1.09

C-H c

1.08

1.08

1.08

1.08

1.08

1.07.

1.08

LHtCS

120.0

120.0

120.0

121.1.

--

117.2.

115.6

LHcCS

120.0

120.0

120.0

121.4-

--

122.1-

122.5

L H t C H c 120.0

120.0

120.0

117.5.

111.6.

120.7.

121.9

LCSO

107.9-

108.9-

115.7.

109.1.

I08.6-

114.5.

114.7

a - ab i n i t i o

results

b - INDO

results

c - INDO

results

d - Bernardi basis

and Harpp

(7;3) on

on

sulfur

calculations

using

(ref.23):

and W h i t e h e a d ' s

optimization f - Flood

a d function

ab i n i t i o

a 4-31G

(ref.25~

e - Snyder Boyd

including

et al.:

CNDO/2

parametrization

calculations

Gaussian

(ref.42): Lobe

basis

ab i n i t i o on C

calculations

and O and

S

g - experimental

geometry

* - these

result

values

using

for geometry

(ref.41}

and Boggs

(ref.ll)

from

an o p t i m i z a t i o n

a

using

(10;6;1)

INDO

a

set

Geometries and electronic structures of substituted sulfines and angles (except/-CSO) fixed to ground state geometry values. The resulting barriers are compared with those obtained by other authors in Table 4. In the INDO scheme the influence of a 3d (polarization) function on sulfur is investigated. From Table 4 it is obvious that this influence should not be neglected. The fixed geometries probably cause the high transition state energies. In the future these barriers should be calculated including d functions o n sulfur and relaxing the geometry of the transition states. Electronic charge distribution

Using the optimized geometriest the gross atomic populations have been calculated from the ab initio wave functions by means of Mulliken's population analysis.44 The INDO atomic charges were calculated according to L6wdin's definition, which holds for an orthogonalized atomic orbital basis.45The results are presented in Tables 5 and 6. The ab initio H2CSO charges compare rather well with those obtained by Bernardi et al. using an extended 4-31G basis set25 and with the results of Block et all 2 who included a 3d orbital on sulfur. The large numerical differences between the ab initio and INDO charges can be caused by the different charge definitions used or by the INDO approximations. In spite of these deviations several interesting trends are noticed in both tables. Both the ab initio and INDO results show rather constant charges on sulfur and on oxygen, the S-O tGeometriesoptimizedwithouta 3d orbitalon S;the effectof this on the charge distribution is rather small, however.

541

grouping as a whole being almost neutral. The latter conclusion also agrees with the results of Block et al) 2 and of Bernardi et al. 25 for H2CSO. Apparently the electron distribution in this part of the molecule is not appreciably affected by the nature of the substituents on carbon. Another property observed is that the substituent atoms show a characteristic charge which is independent of the other substituent and also is not affected by changing from a cis to a trans position. The wide range of charges on the substituent atoms is almost completely compensated by the carbon atom. Consequently the carbon charge is very much influenced by the nature of the substituents. From experiments it is known, that nucleophiles may attack on either carbon or sulfur, but that the actual position is determined by the substituents on carbon. It has been proposed to use substituents as tools to direct nucleophilic attacks to a specific location.~ The cause of these experimental results may well be found in the sensitivity of the carbon charge. Molecular potentials

To gain more insight into this matter, the potential surfaces of some of the model molecules were calculated using the ab initio wave functions. The molecular potential is defined as the energy of a particle of unit charge in the field of the nuclei and the electron distribution of the molecule. This field causes nucleophilic species to move towards areas of high positive potentials along paths with the highest possible potential. Electrophilic reagents, on the contrary, search for negative wells in the potential surface, following routes of potentials as negative as possible. Matters are complicated because both the

Table 4. Os-trans in~rconversion barriers for H2CSO (kcal/mol) a Inversion Rotation a b c d e

b

57.7 35.9

c

57.4 26.5

d

82.7 69.5

26.6 23.9

e 18 23

- INDO results, no d-function on sulfur - INDO results, including a d-function on sulfur - Ab initio results, no d-function on sulfur - ref.23, no d-functlon on sulfur - experimental results (ref.13,23,43)

Table 5. Mullikengross atomic charges from ab initio wave functions Rc

Rt

C

S

O

H

H F C1 H F H C1 C1 F

-0.57 0.77 -0.16 0.14 0.14 -0.37 -0.36 0.31 0.30

0.67 0.60 0.61 0.61 0.63 0.64 0.65 0.60 0.60

-0.68 -0.68 -0.70 -0.67 -0.69 -0.69 -0.70 -0.68 -0.70

Cl F H C1 H F C1

Hc

Ht

gc

Ft

C1 c

C1 t

0.29 0.28 . . . . . . . . . . . . . 0.34 -0.36 . . . . . . . . . . . . 0.15 0.11 -0.28i-0.36 . . . . . . 0.30 . . . . . 0.38 . . . . -0.29 . . . . 0.13 -0.31 . . . . . . . . 0.09 . . . . 0.35 . . . . 0.13 . . . . . . . . 0.37 0.17 --

Table 6. L6wdin charges from INDO wave functions Rc Rt H F C1 F H C1 n F C1

TETRA Vol. 33, No. 5--F

H F C1 H F H C1 C1 F

C

S

O

-0.40 0.62 0.05 0.14 0.15 -0.15 -0.16 0.32 0.32

1.57 1.44 1.29 1.50 1.49 1.43 1.42 1.36 1.36

-1.18 -1.17 -1.21 -1.16 -1.17 -1.20 -1.20 -I .19 -1.19

Hc

Ht

Fc

Ft

C1 c !

C1 t

0.00 0.01 . . . . . . . . . . . . . 0.45 -0.44 . . . . -. . . . . 0.07 -0.05 0 0.00 -0.48 . . . . . . -0 . 1 . . . . . . 0.46 ----0.01 . . . . . 0.07 --0.02 . . . . . . . . . 0.06 . . . . . 0.48 . . . . . 0.01 . . . . . . . 0.46 -0.03 --

542

JAN VAN LIEROP et

sulfine molecule as well as the attacking agent will be polarized while approaching each other; as a result the potential surfaces will be distorted. So the path of the attacking species would have to be adapted continuously to the dynamically changing molecular potential. Although this polarization effect is not accounted for in the approach used here, it is known that at least up to distances of about 2.5 A (a relatively short distance) this approximation in many cases still gives reliable predictions of the reaction pathY Figures 1-7 show cross sections through the isopotential surfaces of a number of selected molecules. From the Figs. 1 and 2 it may be concluded that the environment of the parent molecule is divided into two distinct halves: the zero-potential surface appears to be nearly perpendicular to the molecular plane and from calculations of the molecular potential at distances far beyond the frame of these pictures, it is obvious that this surface is not closed within any interesting distance. Hence, nucleophilic substances are likely to attack carbon or sulfur, while electrophilic species direct themselves towards the oxygen atom. Both conclusions agree with the experimental experience. Several examples of nucleophilic attacks have been mentioned in the introduction; protonation and alkylation with (C2Hs)30+BF4- have been found to occur at oxygen.2~2 From Figs. 3-7 and from pictures similar to Fig. 2, but not reproduced here, it appears that the same conclusion may be drawn for the chlorine substituted sulfines, but that a fluorine atom, causing an additional negative

aL

-Z0

Fig. 3. Isopotential curves in the molecular plane, units are kcal/mol.

-5 ~

- 1 1 ] ~

Fig. 4. Isopotential curves in the molecular plane, units are kcal/mol.

Fig. 1. Isopotential curves in the molecular plane, units are kcal/mol.

H

/0

\

',

H

5

10

20

Fig. 2. Isopotential curves in the plane through the C-S bond, perpendicular to the molecular plane, units are kcal/mol.

potential area, constitutes another possible site for electrophilic attack (e.g. protonation). If the fluorine atom is introduced at the position cis to oxygen (Figs. 3-5) the direction of the zero potential surface changes. On the other hand, trans substitution of fluorine hardly affects the appearance of the negative side of the molecule, but from Figs. 4, 6 and 7 it is easily seen that it influences greatly the shape of the potential surface in the positive region. Although we expect in both cases that fluorine substitution favors the sulfur atom over the carbon atom for nucleophilic attack, we expect this effect to be somewhat stronger for the trans substituted compounds. The smaller negative area at the trans fluorine atom in F2CSO may be explained by the high deficiency of electrons on carbon in this example. Figures 5 and 7 show that chlorine behaves very differently from fluorine: if hydrogen is substituted by

Geometries and electronic structures of substituted sulfines

543

",,

. 5 ~

-10

""",,

-20

Fig. 5. Isopotential curves in the molecular plane, units are kcal/mol,

""'".

-5

"". . . . .

10

""

-20

Fig. 7. Isopotential curves in the molecular plane, units are kcal/mol. Dipole moments The dipole moments have been calculated from both the ab initio and the INDO wave functions. The results of these calculations are collected in Tables 7 and 8. As was the case with the atomic charges, the INDO dipole moments differ largely from those obtained from the ab initio wave functions, both in magnitude and direction. The only experimental dipole moments measured up to now are those of H2CSO and CI2CSO; their values are 2.994 '1 and 2.0414 Debyes, respectively. The first dipole moment is directed 25.50 ° away from the C-S bond, towards the oxygen atom. The large difference between Table 7. Ab initio dipole moments (Debyes)

Fig. 6. Isopotential curves in the molecular plane, units are

kcal/mol. chlorine the pictures show qualitatively the same features as they do for the hydrogen species. From these results the conclusion is justified that indeed the substituents at carbon must play a very important role in the outcome of nucleophilic or electrophilic substitution reactions of saltines. It must be emphasized that in the approach used here only electrostatic effects are included. Factors such as polarization, rehybridization, steric hinderance, the presence and quality of a leaving group, stability of transition states, etc. should be examined before the behavior of sulfines in substitution reactions can be understood completely. An interesting feature that should be mentioned here is the fact that potential surfaces calculated on the basis of point charges resemble Figs. 1-7 rather closely. This means that at least in this specific case, the Mulliken definition of gross atomic charges seems to represent the charge distribution in the molecules rather well.

Rc

Rt

B

A

H F Cl F H C1 H F C1

H F Cl H F H C1 C1 F

4.094 3.611 4.271 5.257 2.493 4.522 3.867 5.130 2.791

A

is the angle between the moment vector dipole (+ -) and the C ~ S bond

58.7 77.8 55.3 73.8 56.0 59.0 55.6 71.6 51.9

Table 8. INDO dipole moments (Debyes) A

RC ! R t H F Cl F H Cl H F Cl

H F Cl H F H C1 C1 F

8.379 6.972 7.554 10.416 6.175 8.775 6.661 8.297 6.036

91.7 92.5 72.9 95.2 108.7 76.9 89.6 80.6 83.1

is the angle between the dipole moment vector (+ ~ -) and the C ~ S bond

544

JAN VAN LIEROP et al.

calculated and experimental dipole moments is most likely to be caused by the limited atomic orbital bases chosen for these calculations. Ab initio multipole moments are known to be very sensitive to the choice of basis functions; especially the addition of polarization functions may cause much improvementfl'~9

surroundings of the SO grouping remain almost constant irrespective of the nature of substituents at the carbon atom. Also the substituent atoms carry a characteristic charge which is not influenced by the other substituent or by changing from a cis to trans position. For the parent sulfine it would have been possible to perform a more sophisticated calculation, using a larger basis, including polarization functions on some or all Electronic spectra The INDO wave functions from the CI calculations atoms and optimizing the geometry in more detail. The were used to calculate the electronic transition energies numerical results for some of the calculated properties and transition probabilities of the nine sulfines. As a (in particular the dipole moments) would then probably typical example the spectrum of CI2CSO is reproduced have agreed better with experimental results. Our goal in here as Fig. 8. All calculated sulfine spectra show a this investigation, however, has been to concentrate on a rather amorphous cluster of lines in the region between comparison of some substituted molecules in order to 250 and 30 nm. A second characteristic feature is the point out the most important factors that cause the strong line at 414 nm. This line also is common to all substituent effects observed in substitution reactions of spectra, but its position varies (Table 9). It can be sulfines. identified as resulting from a ~r-~r* excitation. The exThe very different properties of fluorine and chlorine perimental spectrum of C12CSO shows two bands at 252 are striking. This result may be a stimulus for trying to and 288 nm. 6 Comparing these values with Fig. 8 it is synthesize fluoro-substituted sulfines and test their beobvious that at least the particular INDO parametrization havior in substitution reactions, in order to find out we used is not very useful for calculating the electronic whether this theoretical prediction may be verified by spectra of sulfines. Still, when further experimental data experimental facts. The difference between the cis and become available, it is perhaps possible to recognize the trans isomers of a substituted sulfine is expected to be much more subtle. trends that are predicted by the results in Table 9. To learn still more about the reaction mechanisms of Table 9. ~r-~r* excitation energies in nm (INDO Calculations) substitution reactions, it is necessary to perform calculations of transition states (supermolecule calculations, Rc Rt including the attacking reagents), for instance using model nucleophiles such as H-, or model electrophiles as 303 H H F

P

CI F H C1

Cl II F H CI

It F C1

345 414 324 329 380 382 387 388

C1 F

H +.

To settle the question of the preferred mechanism of the cis to trans interconversion reaction, the computations of the transition states would have to be repeated, relaxing the geometry and including a 3d-function on sulfur. Acknowledgements--We are very grateful to Dr. E. Clementi,

Dr. F. B. van Duijneveldt and Dr. W. van der Lugt for making available their computer programs. We also like to thank Prof. James E. Boggs for communicating his results prior to publication.

CONCLUSION

We have found that the S and O atoms in sulfines show very localized properties. Charges and potential

, l

q00

'

'

, '

'

l

350

. . . .

1

300

'

'

'

'

l

250

,I ,,It i

I '"

"

'

200

'

'

150

Fig. 8. The electronic spectrum of CI~CSO.

'

'

I

I00

'

'

'

'

50

I

'

NM

'

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