A comparative ab initio study of methyl formate, methyl thiolformate and methyl thionoformate

Journal of Molecular Structure (Theochem), 150 (1987) 381-389 Elsevler Science Publishers B V , Amsterdam - Prmted m The Netherlands

A COMPARATIVE AB INITIO STUDY OF METHYL FORMATE, METHYL THIOLFORMATE AND METHYL THIONOFORMATE

R FAUST0 Departamento

and J. J C TEIXEIRA-DIAS* de Qufmwa da Unwerszdade, 3000 Colmbra (Portugal)

(Received 7 July 1986, m final form 22 September 1986)

ABSTRACT Ab mltlo SCF-MO calculations have been carried out for HCOOCH,, HC(=O)SCH, and HC(=S)OCH, Relative stabilities of s-trans/s-czs conformers are reported and discussed m terms of speclflc mtramolecular mteractlons The energy difference between the s-trans and the s-czs form increases m the order methyl thlolformate < methyl thlonoformate < methyl formate The major stablhzmg factors of the s-czs forms are the bond dlpolar mteraction and the mesomerlc delocahzatlon through the five member rmg mvolvmg both the X=C-Y-C (X, Y = 0, S) skeleton and the out-of-plane hydrogen atoms These effects are used to explain the trends mentioned The non-plananty previously proposed for the thlonoester 1s reinvestigated Our calculations show that this molecule 1s planar Molecular atomic charges, dipole moments and lomsatlon potentials are determined and compared with available experimental values

INTRODUCTION

The most stable molecular conformation of simple carboxyhc esters which have been experimentally studied was shown to be planar, with the alkyl group czs to the carbonyl bond [l-5] (Fig 1) This seems to be also the most stable conformation for thlolesters [6--81. This general trend has been explamed consldermg (1) stenc repulsion between alkyl chains, (u) lone electron pairs repulsion, (111)bond dlpolar interaction, and (iv) mesomerlc delocahzatlon [It111. For thlonoesters, expenment indicates that the most stable conformation may be non-planar [12, 131. In particular, the S=C-O-C d1hedra.langle of methyl thlonoformate has been found to be 15.8” [ 121. The blologlcal importance of esters where one or both the oxygens are replaced by sulfur [ 141 Justifies further study of their molecular conformations In this paper, we present ab mltlo calculations on the 4-31G level for simple sulfur containing esters (HC(=O)SCH, and HC(=S)OCH,) The results

*Author to whom correspondence 0166-1280/87/$03

50

should be sent

0 1987 Elsevler Science Pubhshers B V

382

S-CIS

S-TRANS

Fig 1 The S-CDand s-tmns conformations

of ester molecules

are compared with those of methyl formate to assess the effects of oxygenby-sulfur substltutlon on molecular properties COMPUTATIONAL

METHODS

The ab mltlo SCF-MO calculations reported here were carned out using the POLYATOM (version 2) program package [ 151 adapted to a DG/Echpse MV8000 computer, with the standard 4-31G basis set of Pople and coworkers [ 16,171. Calculations were performed for experimental geometnes shown m Table 1. The ngld rotor approxlmatlon was used to consider different molecular conformations RESULTS AND DISCUSSION

Detailed microwave studies and a few theoretical calculations have been reported on methyl formate [ 1, 18-201 and methyl thlolformate [6]. Their structures and dipole moments are known On the other hand, only the electron dlffractlon structure of methyl thlonoformate had been determined [ 121, and there are no theoretlcal calculations reported on this molecule. The expenmental geometry (see Table 1) was found to be “s-czs and nonplanar” [ 121. Our calculations indicate that the most stable conformation of HC(=S)OCH3 IS planar The calculated S=C-O-C torslonal potential 1sgiven m Fig. 2. Our results are further supported by other studies on similar molecules reaching identical conclusions about the planarity of the S=C-O-C skeleton [ 22-241. In Table 2 we present the calculated SCF energies for the molecules studied herem The total energy ISgwen for the most stable conformers, and relative energies are presented for other conformations At this point we remark that the accuracy of total energies is obviously hrmted by, (1) the size of the basis set and the s = p exponent constramt, and (u) by the neglect of geometry optlmlzatlon Therefore, absolute values of energies ~11 not be emphasised much, since we are plnmanly interested m relative energies of different conformations. In addition, the neglect of geometry optimization introduces some dlfflcultles when comparison 1s made with expenment. Experimental geometnes of the s-czs conformers were

333 TABLE 1 Geometries used m this work Parametera

Bond lengths (pm) C-H( formyl) C-H c=x c-x X-CH, Angles (degrees) H-C=X o=c-x s=c-0 C-X-C H-C-H

HCOOCH, MWb [l]

HC =O)SCH, 6 MW [61

HC(=S)OCH, EDb [12]

110 1 108 6 120 0 133.4 143 7

110 0 109 0 120.0 177 0 180 0

1114 1114 1612 136 9 136.9

124 8 125 9

125.0 126 0

126 7

114 7 110 7

110 0 109 5

C-O-C-H s=c-O-C

126 6 115 5 108 1 29 2 15 8

aX = 0 or S bMW = microwave: ED = electron dlffractlon

4%63[211

a S-C/S

30'

60'

SO

120

150'

160'

s-Mans

Fig 2 Ab nutlo S=C-O-C torsional potential of HC(=S)OCH, obtained usmg the experlmental geometry and rIgId-rotor approxlmatlon

used for all conformations considered, leadmg to an overestunatlon of the calculated energy differences (for example, see calculated versus expenmental (s-trans)-(s-w) energy differences for methyl formate m Table 2). Our conclusions assume that the errors are of the same magmtude for all the molecules studled.

384 TABLE 2 Expenmental

and calculated energies for HCOOCH,, HC(=S)OCH, and HC(=O)SCHsa

Es cw/Eh

-E,.,,) (J’%-tmns

(kJ mol-I)

Calculated HCOOCH,

-227

43518

HC(=S)OCH, HC(=O)SCH,

-549 -549

70583 72461

31 8b 32 33 34 24 3b 17 gb 20

Experimental 6 [201c 5 [181d 7 [61e

19 9 8 4-12 16.1

(IR m Ar matrix) [21] 6 (US relaxation) [25] (IR temp vanatlon) [26]

9 [61e

aEh = 2625 5 kJ mol-’ , IR = Infrared, US = ultrasomc bThls work CCalculatlon performed with a double-zeta basis set using the expenmental geometry [l] without optlmlzatlon d4-21G Calculation using the expenmental geometry [l] without optlmlzatlon e4-31G Calculation usmg a partially optlmlzed STO-3G geometry

The calculated energy differences between s-truns and s-czs conformations increase m the following order methyl thlolformate < methyl thlonoformate < methyl formate The factors which determine the greater stablllty of the s-czs conformation should also Justify the above mentioned energy differences. In explaining the trends several effects were considered: (1) stenc repulsion between the carboxyhc hydrogen atom and the methoxy group (Fig 3(a)), (u) lone electron pour repulsion (Fig. 3(b)), (111) bond dlpolar or electrostatic mteractlons (Fig 3(c)), and (iv) mesomenc delocallzatlon through the five-member rmg mvolvmg both the X=C-Y-C (X, Y = 0, S) skeleton and the out-of-plane hydrogen atoms (Fig 3(d)) The Importance of the first factor could be easily questioned a pnorl consldermg the small size of the hydrogen atom In fact, our calculations clearly show that the stenc repulsion is not important, as the electromc densities on the carboxyhc hydrogen atom and on the nearest out-of-plane methoxy hydrogen atoms increase m the s-trans conformation for all the molecules considered (see Fig. 4) If stenc repulsion were dominant, the reverse trend would be observed On the other hand, if lone electron pair interactions were important, one would expect charactenstlc changes m the energies of the lone-pau molecular orbltals between the s-trans and the S-CMforms This 1s not the case, as can be seen m Table 3 In addition, changes m total overlap populations between the oxygen or the oxygen and sulfur atoms m the lone pan- molecular orbltals are not important (see Fig 4) We now consider bond dlpolar or electrostatic mteractlons (111).In the s-czs conformation, there is a well-balanced dlstnbutlon of charges m the molecule (Fig 3(c)) By contrast, the positively charged carboxyhc hydrogen atom and the out-of-plane hydrogen atoms are close and repel each other m the s-trans form As the distances between these atoms in the same conformer

385

(a) X a ,c-Y

/

C”3

X

a

,c-Y

H _J43

H

(b) @‘x,7

CH3 /

@‘“$?Q C-Y

C-Y H’

@

H’

\ C”3

\,+ ;;-n +i

C”3

Cd)

Fig 3 Factors consldered to analyse the s-cm/s-transconformatlonal equlhbrmm (a) stew repulsion, (b) lone-pan repulsion, (c) electrostatic or bond dlpolar interaction, (d) mesomerlc delocahzatlon

have slmllar values m all three molecules, the relative importance of this interaction can be assessed by consldermg the flow of negative charge towards these hydrogen atoms, from the s-czs form to the s-tram conformer (Fig 4 and Table 4) This migration of electronic charge is clearly greater m methyl formate than m the oxygen-by-sulfur substituted esters (Table 4), thus contnbutmg to the observed trend m the (s-tram)--(s-as) energy differences. Interaction (iv) occurs only m the S-W conformation. The overlap populations between the doubly bonded oxygen (or sulfur) and the out-of-plane hydrogen atoms for the orbltals of the 7~system whose mteractlons are posltlve (qo=..n(HCOOCH3) = 0.011, qo=..H(HC(=O)SCH3) = 0.005, qs=..H(HC(=S)OCH,) = 0.017) suggest an increased stablhzatlon of the s-CIS form m the order methyl thlonoformate > methyl formate > methyl thlolformate. Thus, the greater stabtizatlon by this effect of the thlono s-czs form, when compared with the s-czs thlol conformer, 1s the prmclpal cause leading to a larger (s-tram)-(s-m) energy difference m the former molecule. On the other hand,

386

.0514

a202

s-trans

-0469

s-trans

HCkSlOCH~

00;*;59

Oil8

0088

cs;173

0135

s-c/s

Fig 4 Results of the Mulhken population

s trans analy818

the largest (s-truns)-(s-czs) energy drfference m methyl formate is determined mamly by the electrostatic mteractlons m this molecule. Results of ab mrtlo calculations can be used to mterpret and predict photoelectromc spectra wrth the help of the Koopmans’ approxlmatron. On the other hand, reproducibility of photoelectromc spectra la a good test to assess the general quality of molecular orb&al calculations Table 5 presents the calculated lomzatlon potentials for the highest energy molecular orbrtals of HCOOCH3, HC(=O)SCH3 and HC(=S)OCH3. The expenmental values for

387 TABLE

3

Ab mltlo

molecular

orbital

energies

for HCOOCH,,

E(,-cz&Eh HCOOCH,

HC(=O)SCH,

and HC(ZS)OCH,~

Ewtm.dEh

Orbital

0 167 462 466 -0 530 -0 563

-0 -0 -0 -0

0 153 457 460 541 576

4a” 13a’

C=O* o=

3a” 12a’

0 0

-0 -0 -0 -0

0 129 380 418 482 562

-0 -0 -0 -0

0 124 381 414 491 565

5a” 4a” 16a’ 15a’ 3a”

c=o* S o= S c=o

LUMO HOMO

-0 -0 -0 -0

0 079 363 390 496 510

-0 -0 -0 -0

0073 356 379 514 522

5a” 16a’ 4a” 15a’ 3a”

c=s* S= c=s 0 0

LUMO HOMO

-0 -0

HC(=O)SCH,

HC(=S)OCH,

2a”

LUMO HOMO

c=o

aEh = 2625 5 kJ mol-’ TABLE

4

Calculated changes hydrogen atomsa

m charges

HCOOCH, HC(=S)OCH, HC(=O)SCH, aAq = q(s-czs) -

q(s-tram),

of the formyl

hydrogen

and of the out-of-plane

A4H(fcmmyl)/e

AqH(oop)/e

0 091 0 036 0 024

0 008 0 012 0 012

e = 1 6021892

X lo-l9

methoxy

C

methyl formate [27] are given for comparison. The agreement can be considered as good. There are no reported data for the remammg molecules, but the existmg data for methyl thiolacetate and methyl thionoacetate [28] are quahtatively comparable with our results. In fact, it is well known that iomzation potentials of acetates are lower than those of the formates [27]. Therefore, our calculated values for thioesters should be close to the expenmental values. Accordmg to PES [27], the HOMO m methyl formate is the carbonyl lone-pair orbital with a’ symmetry. Our calculations agree with this conclusion. In fact, the approximately nonbondmg antisymmetnc a” orbital, which is mamly localized on the methoxy oxygen atom, has a lower energy than the a’ orbital. The inverse situation was found for HC(=O)SCH3. In this case,

388 TABLE 5 Calculated and expenmental lomsatlon potent& Orbital

Iomsatlon potentials (eV)” Calc

HCOOCH,

13~’ 3a” 12a’ 20”

(HOMO)

12 12 14 15

57 68 42 32

HC(=O)SCH,

4a” 16a’ 15a’ 3a”

(HOMO)

10 11 13 15

33 39 13 30

HC(=S)OCH,

16a’ 4a” 15a’ 3a’

(HOMO)

9 10 13 13

88 62 50 88

*l eV = 16021892

X

Ekp. (PES) [271 11 02 11 55

CH,COSCH, [ 28 ] 9 65 9 83 12 15 13 27 CH,CSOCH, [ 281 8 82 9 78

lo-l9 J

TABLE 6 Calculated and expenmental dipole moments Molecule

HCOOCH,

Dipole moment ( D)a

S-ClS

s-trans HC(=O)SCH,

s-CIS s-trans

HC( =S)OCH,

S-CD s-trans

Calc

Exp.

188 (40 So) 5 14 (14 3”)

177 [l] (39”)

1 54 (35 4”) 4 38 (17 5”)

158 [6] (43”)

150 (62 0”) 4 16 (3 go)

‘1 D = l/3 x 1O-‘9 Cm, Values m parentheses are the angles between dipole moments and the C=O or C=S bond

389

the a” orbital, which ongmates mainly from the sulfur 3p, orbltal, has higher energy than the carbonyl lone-pan orbital. Although both a’ and a” orbltals are stabilized by electron-releasmg groups [27], the a’ orbital 1s more stablhzed due to a greater contrlbutlon of those groups m this orbital Thus, when the mesomenc electron releasmg power of the substltuent 1s sufflclently large, as m the amides, the a” orbital comes above the a’ orbital. In this respect the -SCH3 substltuent resembles the amide more than the ester group Table 6 presents the calculated dipole moments for the molecules studled herem. Expenmental values, when avalable, are also presented for companson The high dipole moments predicted for s-tram conformers might initiate expenmental research on molecules of considerable interest REFERENCES 1R F Curl, J Chem Phys, 30(1959) 1529 2 G I L Jones and N L Owen, J Mol Struct , 18 (1973) 1 3 J M Rlveros and E B W&on, J Chem. Phys , 46 (1967) 4605 4 R P Muller, H Hollenstem and J R Huber, J Mol Spectrosc , 100 (1983) 95 5 M Ok1 and H Nakamshl, Bull Chem Sot Jpn , 43 (1970) 2558 6 G I L Jones, D G Llster, N. L Owen, M C L Gerry and P Palmlerl, J Mol Spectrosc ,60 (1976) 348 7 E A Noe, T Sanders, M Garahl, H Hosseml and R Young, J Am Chem. Sot , 107 (1985) 4785 8 N S True, C J Sdvla and R K Bohn, J Phys Chem ,85 (1981) 1132 9 N L Owen and N Sheppard, Proc Chem. Sot , London, (1963) 264 10 J Bayley and A M North, Trans Faraday Sot , 64 (1968) 1499 11 J E Plercy and S V Subrahmanyan, J Chem. Phys , 42 (1965) 1475 12 J ROOIJ,F C Mljlhoff and G Renes, J Mol Struct , 25 (1975) 169 13 0 Exner, V JehhEka and A Ohno, Collect Czech. Chem. Commun , 36 (1971) 2157 14A C Storer, W F Murphy and P R Carey, J B1o1 Chem , 254 (1979) 3163 15 D B Neumann, H Basch, R L KorneJay and L C Snyder, POLYATOM (version 2), Program No 238, Q C P E , Indlana Unlverslty, Bloommgton, IN, U S A 16R Ditchfield, W J Hehre and J A Pople, J Chem Phys, 54 (1971) 724 17 W J Hehre and W A Lathan, J Chem Phys, 56 (1972) 5255 18 H Wennerstrom, S For&n and B Roos, J Phys Chem., 76 (1972) 2430 19 D G Llster and P Palmlerl, J Mol Struct , 32 (1976) 355 20 C Alsenoy, J N Scardsdade and L Schafer, J Mol Struct (Theochem), 90 (1982) 297 21 C E Blom and Hs H Gunthard, Chem Phys Lett ,84 (1981) 267 22A L Verma, Y Ozakl, A C Storer and P R Carey, J Raman Spectrosc , 11 (1981) 390 23 R Mattes, L Zhengyan, M Thunemann and H Schnockel, J Mol Struct , 99 (1983) 119 24 P R Carey, Dwlslon of Blologlcal Sciences, NRCC, Ottawa, personal commumcatlon 25 S V Subrahmanyan and J E Plercy, J Acoust Sot Am, 37 (1965) 340 26 S Ruschm and S H Bauer, J Phys Chem., 84 (1980) 3061 27 D A Swelgart and D W Turner, J Am Chem. Sot , 94 (1972) 5592 28 A Flamml, E Semprml and G Condorelh, Chem Phys Lett , 32 (1975) 365