Topological electron density analysis of organosulfur compounds

THEO CHEM ELSEVIER

Journal of Molecular Structure (Theochem) 337 (1995) 201-207

Topological electron density analysis of organosulfur compounds Steven M. Bachrach*,

Ulrike Salzner

Department of Chemistry, Northern Illinois University. DeKalb, IL 60115. USA

Received 14 December 1994; accepted 16 December 1994

Abstract The geometries and topological electron density analysis of 19 organosulfur compounds were determined at the HF/631G* level. This computational method provides geometries in excellent agreement with experiment. We provide a database of topological electron density values for a variety of C-S bonds which can be used for comparisons with

other molecules. An exponential relationship between bond order and the value of the electron density at the C-S bond critical point is developed and used to investigate the degree of delocalization in prop-2-enethial, thiophene and the conjugate base of thioacetaldehyde. Keywords: Ab initio calculation; Organosulphur compound; Topological electron density analysis

1. Introduction

Extracting chemical information from the wavefunction has been an active research area since the initial development of quantum mechanics. Over the past decade, the topological electron density method has shown promise towards meeting many of these aims [ 11. Making use of the gradient (VP) and the Laplacian (V2p) of the total electron density, ridges of maximum electron density between atoms that correspond with chemical bonds, lone pairs, and regions of electrophilic and nucleophilic attack are identified. Of particular interest in terms of relating chemical properties to the density are bond critical points [2]. Bond critical points occur between bonding atoms where the gradient of the total electron density vanishes. The value of the electron density at the bond critical

point has been shown to scale empirically with bond length and bond order [3]. For example, the exponential relationship Eq. (1) has been fit for the C-C [4], C-N [5], and C-P [6] bonds. We have made extensive use of this relationship to evaluate the extent of bonding in transition states and to understand the chemistry of organophosphorus species [5-l 51. We are beginning a long-term investigation of the chemistry of organosulfur compounds and wish to use the topological method to analyze the density distributions in these systems. Towards that end, we report here the results of the topological analysis of a series of simple organosulfur compounds. The bond order can be expressed as Bond order = exp[A(p(r,) where

for

0166-1280/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI

0166-1280(94)04117-2

(1)

bond type C-C [4], A = 6.458 and for C-N [5], A = 5.12 and B = 0.27; and for C-P [6], A = 19.628 and B = 0.153. B = 0.252;

* Corresponding author.

- B)]

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337 (1995)

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12 H

f-i

‘::I1.317

C

H

C

127.8

10

1.768

98.0

S

1.326

H

Fig. 1.

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337 (1995) 201-207

S

OL 1.597

C

H

H

12

Y s

1.6172

P 129.3

C

C

geometries

of 1-19 at HF/6-31G*.

H

1.482

H

Fig. 1. Optimized

lll.2

All distances

19 perp

arc in A and all angles arc in deg

203

S.M. Bachrach, U. Sal.zner/Journal of Molecular Structure (Theochem)

204 Table 1 Comparison organosulfur

of experimental compounds

and calculated

geometries

of some

Table 2 Topological

values at the CS

Compound Compound

Parameter

Expt.

1 CHsSH

c-s

1.819 1.335 96.5 1.820 1.529 1.322 108.6 96.2 1.802 98.87 1.804 1.804 1.530 99.0 109.5 1.815 1.484 48.3 1.611 116.9 1.610 1.506 125.3 1.714 1.369 I.423 92.1 111.5 112.5

2 CHsCH>SH

5 CHsSCHs 6 CHsSCH&Hs

8 (CH,)zS

12 H2C=S 13 CH,CH=S

18 (CH),S

S-H C-S-H c-s CC S-H c-CS C-S-H CS CS-c C,,-S CA c-c CSC CC-S c-s CC c-s-c c-s H-C-H CS c-c CCS c-s C:-CI c3-c4 C~SXI s-cc GGC

2. Computational

337 (1995) 201-207

bond critical points

P (C-S)

t

r (C-S)

0.1850 0.1841 0.1821 0.1919 0.1895 0.1892 (S-Me) 0.1881 (S-Et) 0.1880 0.1699 0.1562 0.2040 0.2105 0.2413 0.2383 0.2350 0.238 1 0.2363 0.2408 0.2205 0.1944 (planar) 0.2309 (perp)

0.0880 0.0774 0.0683 0.1258 0.1043 0.1014 0.1111 0.1082 0.3932 0.6439 0.1690 0.5069 0.1208 0.1449 0.1607 0.1435 0.1074 0.1008 0.2341 0.0750 0.1937

1.818 1.824 1.833 1.800 1.809 I .809 1.817 1.817 1.811 1.840 1.768 1.706 I.597 1.606 1.617 1.607 1.613 1.557 1.726 1.751 1.654

HF/6-31G* [20]

[20]

[20] [21]

[20]

[20] [20]

[20]

1.818 1.327 91.9 1.824 1.524 1.328 114.2 97.8 1.809 100.0 1.809 1.817 1.526 100.2 110.5 1.811 1.473 48.0 1.597 115.5 1.606 1.491 126.6 1.726 1.345 1.437 91.3 111.8 112.5

methods

The geometries of the organosulfur compounds 1-19 were completely optimized at the HF/6-31G* level using o.4uss1A~-92 [15]. Some structures were obtained from the Quantum Chemistry Archive at Erlangen. The optimized structures are drawn in Fig. 1 and some selected geometrical parameters are listed in Table 1. Topological electron density analysis was performed using a locally modified version of EXTREME [16]. Topological values for the C-S bond critical points are listed in Table 2.

3. Results and discussion A number of computational studies have appeared on organosulfur compounds. The general

1 2 3 4 5 6

CH$H CH,CH2SH (CH,)2CHSH (CH&CHSH CH,SCHI CHsSCH2CHs

7 CH,CH2SCH2CHs 8 (CH&S 9 (CH)?S 10 CH,=CHSH 11 HCKSH 12 H2C=S 13 CHsCH=S 14 (CH,)&=S 15 CHICH2CH=S 16CH2=CHCH=S 17 H&Y=C=S 18 (CH)4S 19 [CH>CH=S]-

conclusion concerning appropriate basis sets is that d functions are necessary to properly describe the geometry, though they play a polarization role solely [ 17- 191. Examination of Table 1, where we compare our HF/6-31G* geometries with some literature structures clearly indicates that HF/631G* geometries agree remarkably well with experiment. The HF/6-31G* differ from the experimental gas-phase structures by less than 0.01 A for bond lengths and less than 1” for most bond angles. As we found for a series of phosphorus compounds [5], the HF/6-31G* level is quite satisfactory for determining ground-state structures of secondrow systems, including multiple bonds. After having evaluated the efficacy of the HF/6-3 1G* wavefunction, we now turn to the main purpose of this paper, establishing a database of topological electron density values for the C-S bond. In Table 2 we present the values of the electron density P(Y~) and ellipticity at the C-S bond critical points for the 19 compounds examined herein. We will analyze the values of p(r,) by first examining a selected group of the compounds. We first select those compounds for which there

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205

1.9

B0 = ,l 3.43(p(r,)

- 0.1 870)

1.8

z J z

1.7

16

1.5 70.125

0.150

0175

0.200

0.225

0250

(114 1

016

018

0.20

0.22

0.24

0.26

p(rd p(rc)

Fig. 2. Plot of p(rJ (e aum3) versus bond order for the C-S bond. The heavy line is the least-squares fit for Eq. (1). The estimated value for thiophene is shown as the empty box.

is no unusual bonding situation, i.e. we avoid any strained rings (8 and 9) molecules with conjugation or hyperconjugation (10, 11, 16, 18, and 19) and cumulene systems (17). Using the remaining compounds which represent “normal” bonding environments, we determine the best least-squares fit to Eq. (1). This exponential equation allows for extrapolation to the limit that as p + 0, the bond order approaches nil. We find a value of A = 13.43 and B = 0.1870, with r2 = 0.987. The plot of this relationship is shown in Fig. 2. Next, we examine the relationship between p(r,) and bond length. A plot of this relationship for all 19 compounds is shown in Fig. 3, along with the least-squares fit. While the correlation is not ideal, there is a strong general trend that shorter bonds will have greater values of p(rc). This trend has been observed for C-C and C-P bonds. Knop et al. [22] have argued that an exponential relationship between bond length and p(r,) should hold. For our data, this leads to a fit with r2 = 0.891, not a significant improvement over the linear fit. The values for the ellipticity e are listed in Table 2. The ellipticity is a measure of the ratio of the rate of density decrease in the two directions perpendicular to the bond path at the bond critical point [23]. A value of zero indicates a symmetrical distri-

Fig. 3. Plot ofp(r,)

(e aum3) versus r(C-S)

(A) for the C-S bond.

bution of density about the bond path, such as found in standard single and triple bonds, while large values indicate a preference for density buildup in a particular orientation, as in the direction of the n-bond in a double bond. The ellipticities do not fall into neat categories for the C-S bonds; this is probably due to the large ionic component of the C-S bond. The very large ellipticities found for the three-membered rings are due to the delocalization of charge about the surface of the ring, as found for many other threemembered ring compounds [24]. The vinyl and ethynyl thiols have very large ellipticities. We have observed similar large ellipticities for vinyl and ethynyl phosphines [5]. Finally, we examine the density distribution of some of the organosulfur compounds. As stated above, the ellipticities of vinyl and ethynyl thiol (10 and 11) are unusual and large. The value of p(r,) for these is also large. Clearly, there is an interaction of the r-system with the lone pair electrons on sulfur. The three-membered ring systems, thiirane 8 and thiirene 9, are strained and the density distribution reflects this strain. The values of p(rc) for the C-S bonds are much smaller than for typical C-S bonds. The bond path, which traces the maximum electron density between bonded atoms [25], for the C-S and C-C bonds are much longer than the

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206

H

B

A Form

1.

internuclear separation, reflecting highly bent bonds. For 8, the C-S bond path is 0.0159 w longer (0.9%) than the C-S distance, while the C-C bond path is 0.065 A longer (0.4%) than th: C-C distance. For 9, the C-S bond path is 0.!3 16 A longer (1.7%) and the C-C path is 0.0076 A longer (0.6%) than their respective internuclear distances. The large ellipticity indicates the more gradual drop-off of density as one approaches the center of the ring relative to movement perpendicular to the ring from the bond critical point. This feature has been termed “surface delocalization” [24]. The C=S bond in thioketene 17 is shorter than typical C=S bonds, as expected since the C is formally sp-hybridized. However, the value of the density for this bond is not much higher than the other C=S bonds. Compounds 16, 18, and 19 all express varying degrees of 7r-delocalization, resonance, and aromaticity. The C=S bond in 16 is not longer than usual, and its value of p(r,) is within the normal range. Delocalization of the 7r-electrons in prop-2ene-thial 16, if any, is not reflected in the electron distribution. Thiophene 18 has been extensively studied [26-281 with the general conclusion that it is the most aromatic of the five-membered ring heterocycles [29]. We find the C-S bond length is between those of typical C-S single and double bonds. The value of p(r,) for the C-S bond is also intermediate between those of single and double bonds. Using our calculated values of A and B and Eq. (l), we estimate that the bond order for the C-S bond in 18 is 1.57 (this estimate is indicated by the box in Fig. 1). The estimated bond orders using Eq. (1) for the C2-C3 and Cj-C4 bonds are 1.84 and 1.34, respectively. The bond orders are consistent with thiophene possessing significant aromatic character. The thioacetaldehyde conjugate base 19 can involve two resonance structures that formally

337 (1995) 201-207

delocalize the charge between the terminal carbon and sulfur atoms (A and B). This resonance is active in the planar form. The C-S distance is long while the C-C bond is short. Using Eq. (l), the value of p(r,) for the C-S and C-C bonds give bond orders of 1.10 and 1.91, respectively. These bond orders suggest a dominance of the resonance structure B. Rotation to the perpendicular conformer lengthens the C-C bond while contracting the C-S distance, consistent with removing resonance. The estimated bond orders for the C-S and C-C bonds in the perpendicular structure are 1.80 and 1.06, respectively. In the perpendicular form of 19, resonance structure A dominates.

4. Conclusion We have shown that HF/6-31G* geometries are in excellent agreement with experimental structures. An exponential relationship between bond order and the value of the electron density at the C-S bond critical point has been verified with parameters A = 13.43 and B = 0.1870. This relationship is used to interpret the bonding in a number of conjugated systems. There is little electron delocalization in prop-2-ene-thial. Electron delocalization in thiophene is consistent with it being an aromatic compound. The thioacetaldehyde conjugate base can best be represented by a single resonance structure B with the anion residing on sulfur.

Acknowledgment The authors thank the National Science Foundation and the Petroleum Research Foundation, administered by the American Chemical Society, for their support of this research.

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