16. 17. 18. 19. 20. 21. 22.

L. Korecz, H. KShler, L. Neef, et al., M~ssbauer-Untesuchungen an einigen Organozinnderivaten nichtlinearer Pseudohalogenide," J. Organomet. Chem., 69, No. i, 105-109 (1974). V. V. Skopenko and R. D. Lampeka, "Type of coordination of nitrosocarbamylcyanomethanide," Zh. Neorg. Khim., 27, No. 12, 3117-3119 (1982). V. F. Bolelyi, V. S. Kuts, and V. V. Skopenko, "Quantum-chemical calculation of the nitrosodicyanomethanide ion," Dokl. Akad. Nauk Ukr. SSR, Ser. B, No. 8, 32-34 (1980). D. A. Zhogolev and V. B. Volkov, Methods, Algorithms, and Programs for the QuantumChemical Calculation of Molecules [in Russian], Naukova Dumka, Kiev (1976). W. Clack, N. S. Hush, and J. P. Yangle, "All-valence-electron CNDO calculation on transition metal complexes," J. Chem. Phys., 57, No. 8, 3503-3510 (1972). J. A. Pople and G. A. Segal, "Approximate self-consistent molecular orbital theory. 3. CNDO results for AB2 and AB3 systems," J. Chem. Phys., 44, No. 9, 3289-3296 (1966). W. Beck, K. Shorpp, C. Oetker, et al., "Komplexe der dS--~delmetalle sowie des Mangans und Rheniums mit Tricyan- und Tris(gthoxycarbonyl)methanid Liganden," Chem. Ber., 106, No. 7, 2144-2155 (1973).

THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE FRAGMENTATION OF THE POSITIVE MOLECULAR IONS OF O-ALKYL THIOACETATES V. V. Lobanov, L. I. Fileleeva, I. P. Samchenko, and M. M. Aleksankin

UDC 539.19.08

The mechanisms of the formation of rearrangement ions upon the fragmentation of the positive molecular ions of esters were studied by quantum-chemical methods in [i, 2]. The calculation results are consistent with the one of the two a priori possible mechanisms according to which restructuring of the nuclear configuration occurs in the molecular ion formed immediately after the ionization of the original ester molecule. In addition, it was shown that the equilibrium conformation of the radical cation of methyl acetate is characterized by the presence of a four-membered ring (I) with equivalent oxygen atoms, between which the methyl group is symmetrically oriented, and that the molecular ion of ethyl acetate has an equilibrium conformation with a five-membered ring (II), in which the properties of both oxygen atoms are also almost identical:

cH -c

/o

+.

"" cH

~" o

Z

/o--Hz~. ]

L:

\

+.

"'H I

O--H~C .

/7

This is attributed to the equalization of the electron densities on the two oxygen atoms upon ionization, which subsequently results in the formation of symmetric cyclic structures. In this context it would be interesting to consider the equilibrium conformation of the molecular ions formed upon the ionization of the alkyl thioacetates corresponding to the esters just mentioned. In the present work we studied the equilibrium spatial and electronic structure of the molecules of O-methyl thioacetate (OM) and O-ethylthioacetate (OE), as well as the corresponding positive molecular ions OM+. and OE +- . The calculations were carried out by the SCF--MO--LCAO method in the all-valence-electron CNDO/2 approximation [3] ; the systems with an open shell were calculated by the unrestricted Hartree--Frock method. The method for optimizing the spatial structure was described in [4]. The OM molecule and the corresponding molecular ion OM+- were studied in a single conformation (see Fig. i), and the OE molecule and the OE +" radical cation were studied in two conformations, which are distinguished from one another by a 60 ~ rotation of the methyl group in the alcohol residue around the Cs--C11 bond. The calculations showed that both in the L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 20, No. 5, pp. 525-532, September-October, 1984. Original article submitted January 27, 1984.

494

0040-5760/84/2005-0494508.50

9 1985 Plenum Publishing Corporation

v"8

a O,~~H,~

'

b

d~)H"

Fig. 1. Equilibrium configuration and numbering of the atoms in the molecule (a) and the positive molecular ion (b) of O-methyl thioacetate.

TABLE i. Equilibrium Bond Lengths and Bond Angles of the O-Methyl Thioacetate and O-Ethyl Thioacetate Molecules and the Corresponding Molecular Ions S~uct~al paramet~

O-Methyl thioacetate molecule

I

ion

O-Ethyl t ~ o a c e t a t e molecNe

I

ion

Bond length, nm

R(C1--C=) R(CI--S) R(C~--O) R~O--Cs) ,~(S--C~) -~(C5--C11) R(S--Cn)

0,1435 0,1672 0,1356 0,1377

0,1410 0,1686 0,1363 0,1402 0,1909

0,1435 0,1667 0,1344 0,1383

0,1412 0,1680 0,1376 0,1416

0,1468 2,1220

0,1506 1,8420

110~ ' 121043' 108~ , 119o14'

158~ ' 125~ ' 131o7' 127~ '

Angle

/_~ClS /C~CxO ./C~0C6 /_OC6Cn

123039 ' 120~ ' 10304 '

157020' 124057' 126025 '

case of the OE molecule and in the case of the OE+" ion, the conformation shown in Fig. 2 is much more stable than the conformation in which one of the hydrogen atoms of the methyl group is in the plane of the molecule and is located between the S and CII atoms. For this reason, in this communication we shall consider only the conformations of the OE molecule and the OE+- ion depicted in Fig. 2. There is no experimental data in the literature on the spatial structure of the OM and OE molecules; however, the bond lengths and bond angles calculated by us, which are presented in Table I, are in good agreement with the values for these parameters for other compounds of similar structure [5]. Table 2 presents the distribution of the electron density in the molecules and ions studied. The electronic structure of each of the ions was calculated for two configurations of the nuclear skeleton: for the configuration of the original molecule and for the equilibrium conformation. A comparison of the populations of the atomic orbitals in the molecule and in the ion formed from it makes it possible to establish how the electron density is redistributed upon ionization, since the latter may be considered vertical. From Table 2 it is seen that ionization results mainly in a decrease in the electron density on the sulfur atom. In the case of O-methyl thioacetate, the electron density decreases to the greatest extent in the 3py and 3pz orbitals of the sulfur atom, which participate in the molecular bonding; the population of the 3Px orbital, which is oriented perpendicularly to the molecular plane, on the other hand, increases significantly. In the case of O-ethyl thioacetate, there is a decrease in the electron density in the 3pz orbital upon ionization, while populations of the 3px and 3py orbitals increase. The decrease in the population of the sulfur 3pz orbital upon vertical ionization should apparently promote the formation of an additional bond between the sulfur atom and the carbon atom of the alcohol residue during the acquisition of the equilibrium configuration by the molecular ion. This is evinced by the increase in the population of this orbital during relaxation of the nuclear skeleton. In the molecules studied the populations of the d orbitals of the sulfur atom were low,

495

TABLE 2. Populations of Atomic Orbitals of the Sulfur and Oxygen Atoms in the O-Methyl Thioacetate and O-Ethyl Thioacetate Molecules and the Corresponding Molecular Ions

Atom

Orbital

' !Mole cule lupon Ion vertical (p~,) lionization

lion with equiliIbrium c o+nfigu-

, M - P .+

Iation

O-Methyl thioacetate

3s 3Px 3py 3Pz 3dzz 3dx~ 3dyz 3dx_y 3dxy Z 2s 2Px 2py 2pz 7~ A tom

Orbital

l, 867 1,669 0,946 1,032 0,065 0,062 0,143 0,048 0,037 5,869 1,613 1,756 1,361 1,347

t, 875 1,299 1,393 1,371 0,065 0,084 0,141 0,050 0,049 6,327 1,612

1,827

1,407 1,315 6,161

O,008 --0,370 O,447 0,339 0,000 0,022 --0,002 O,002 0,012 O,458 --0,001 0,071 1,047 --0,032 0,085

6,077

0,211 --0,188 0,382 O,166 --0,120 --0,016 O,006 --0,089 --0,006 0,345 0,044 0,051

--0,040 0,060 0,115

Ion with equi- I libdum config4PM-P~M uration (P1~) PM--Peq

formed " Molecule !Ion upon vertical

(PM)

1,664 1,487 1,011 1,205 0,185 0, I00 0,135 0,139 0,055 5,981 1,568 1,776 1,447 1,255 6,046

liOnization

(PM)

O-Ethyl thioacetate 3s

3p.

3py 3Pz 3dz~ 3dx~ 3dyz 3ax- u 3dxy E O

2s 2p~

2py 2Pz E

1,868 1,288 0,905 1,849 0,091 0,071 0,157 0,055 0,082 6,366 1,602 1,836 1,443 1,300

6,181

"8 "~

1,857 1,680 0,917 l, 085 O, 100 0,072 0,149 0,055 0,044 5,960 1,606 1,747 1,397 1,351 6,101

-"

0,011 --0,392 --0,01 l 0,763 --0,009 --0,001

1,680 1,317 1,093 1,248 0,239 O, 182 0,157 0,196 0,039 6,152 1,575 1,826 1,461 1,218

0.008 0,000 --0,037 0,332 --0,004 0,089 0,046 --0,051 .0,080

Hsq~:'H~

~Ha

6,o8o

O, 188 --0,029 --0,188 0,600 --0,148 --0,110 0,000 --0, 141 --0,043 O, 129 O,027 O,OlO --0,018 0,082 O, 137

~5, ,,zQ.;Hf~

n'l.).__..
YJ' b -..H,odfi% Fig. 2. Equilibrium configuration and numbering of t h e atoms in the molecule (a) and positive molecular ion (b) of Oethyl thioacetate. attesting to their insignificant role in the molecular bonding. Upon ionization their populations remain practically unchanged. After relaxation of the nuclear skeleton of the molecular ions, the population of the d orbitals increases significantly, and this is one of the factors causing the formation of cyclic structures. From Fig. 1 and Table 1 it is seen that the S--C5 distance is shorter in the equilibrium configuration of the OM +" ion than in the original molecule and that it differs only slightly

496

TABLE 3. Total Energies of Charged and Neutral Fragments Formed upon the Fragmentation of the Positive Molecular Ions of 0-Methyl and O-Ethyl Thioacetate (au)

Charge of particle Formula of particle 0

CHs CHaCH~ CH3CO SCH8

--9,117412 --17,863275 --34,634083 --20,247270

--8,671799 --17,581278 --34,345890 --19,842995

CH,CiSo

--45.759974

--45,441348

CH3CS OCHs

--27,124045 --27,695966

--26.809962 --27.269996

--45,517574

--45,210706

--45,733599

--45,182505

--29,006148 --36,393846

--28,682395 --35,918983

--46,482232

--46,111419

--17,073185

--16,546037

s

\OCH3 SCH~CH8 OCH~CH3

CH~CH~

TABLE 4. Possible Fragmentation Schemes of the Molecular Radical Cation of O-Methyl Thioacetate

Number 1.1 1.2

Reaction scheme

Sum of enersi~ of reaction proaucts, au

ICH3CO]+ + SCH~ CH3CO" + [SCH3]+

--54,593160 --54A77078

2.1

--54,558760

2.2

--54,431773

3.1 3.2 4.1 4.2

[CH3CS]+ + OCH; CH3CS" + [OCHa]+ CH3 + [OCSCH3]+ CH~ + SCOCH~

--54,505928 --54,394041 --54,328118 --54,405398

from the length of the S--C bond in neutral organic molecules with a cyclic structure [5]. The approach of the methyl group to the sulfur atom observed in the equilibrium c o n f i g u ration of the OM +- molecular ion may be attributed to the appearance of an additional bond between the $3 and C5 atoms. This is supported by the tendency for a change in the twocenter contribution E(S3Cs) to the total energy of the system under consideration: In the case of the OM molecule, it is equal to--401.7 kJ/mole, in the case of the OM +- ion formed upon vertical ionization, it is equal to --509.3 kJ/mole, and in the case of the same ion after the acquisition of the equilibrium geometry, it reaches --858.5 kJ/mole. This should naturally have some effect on the nature of the fragmentation of OM +" Table 3 presents the energies of the charged and neutral fragments formed upon the fragmentation of the OM+" and OE-- molecular ions. The energies were calculated with the use of the equilibrium values for the bond lengths and bond angles. The values of these energies were used to determine the sum of the energies of the reaction products (Tables 4 and 5) formed as a result of the fragmentation of the molecular ions. Tables 4 and 5 also present the possible fragmentation schemes of the OM+" and OE +" ions. As is seen from Table 4, reaction i.I, which results in the formation of the [CH3CO] + ion and the neutral radical SCH3", is the energetically most favorable reaction. Following 497

TABLE 5. Possible Fragmentation Schemes of the Molecular Radical Cation of O-Ethyl Acetate Reaction number 1.1

1.2

Reaction scheme

Sum of energie~ of reaction products, au

[CHACO] r -- SCH~CHa CHACO' q- [SCH2CH.~]+

--63,352038 --63,316478

2.1

--63,341252

2.2

--63,304623

3.1 3.2 4.1 4.2

[CHaCS] + t [CHaCH20]' CHaCS" + [CHaCH~O]+

--63.203810 --63,043028

+ CH2CHe O SH CHaC
--63,184604

[CHaC

+

--63,028269

the elimination of the methyl group in the alcoholic residue, the positive charge should be localized on the CHa--C~/~ _

fragment, since the sum of the energies of the products of re-

action 2.1 is lower than that for reaction 2.2. The decomposition of the OM+" ion into the [CH3CS] + and OCH3" fragments is associated with a greater expenditure of energy than are reactions i.i and 2.1. If we start out from the structure of the original molecule, just this reaction is most preferable, since the C-O bond is considered to be the weakest bond in the O-methyl thioacetate molecule, and its energy is equal to ~293 kJ/mole. However, the data from the calculation attest to the fact that fragmentation at the C-O bond is energetically less favorable in comparison to the fragmentation schemes of the OM+" ion already considered. Unfortunately, we do not know of any experimental data on the fragmentation of the OM +" molecular ion which could be compared with the results obtained. Such a comparison can be made for O-ethyl thioacetate, whose experimental field-ionization mass spectrum was studied in detail by us (see Table 6).* The experimental measurements showed that the mass spectrum of O-ethyl thioacetate contains narrow lines at 61 and 43 amu, which correspond to the rearrangement ions [SCzHs] + and [CH3CO]+:

s

|

~

~Jamu

+

9

The intensity of the line for [SC2Hs] + is small and amounts to 0.05%, while that of the [CH3CO] + ion reaches 11.4% of the total ion current. Fragmentation scheme (I) is confirmed by the presence in the mass spectrum of a line of a metastable ion with an apparent mass of 35.78* amu, which corresponds to an ion with a true mass of 61 amu. It should be noted that another possible path for the formation of the [CH3CO] + ion upon the fragmentation of O-ethyl thioacetate might involve the fragmentation of the

[CH3-- c-X S H "J]§ ion,

which is formed as a result of a McLafferty rearrangement :

*O-Ethyl thioacetate was synthesized according to the method described in [6]; the IR and N-MR spectra of the product obtained correspond to the literature data [6, 7].

498

TABLE 6. Intensities of Lines of Ions in the Field-Ionization Mass Spectrum of O-Ethyl Thioacetate* Strength of field m/*

low

mean

106 105 104 89 88 82 81 8O 66 65 64 61 60 59 56 50 48 47 46 45 44 43 34 33 32 31 3O 29 28 15 35,78* (61) 34,6* (60)

3,0 2,9 69,3

2,1 2,1 49,0 0,3 0,8 O, I 0,3 0,4 0,4 0,9 1,5 0,1 0,4 0,2 0,1 O,l 0,6 0,6 3,0

0,4 O,1 0,3 0,4 O,l 1,7 0,! 0,3

2,7 0,3 O,l 5,2 0,3 0,2 ll,l 0,7 0,8

Empirical formula of ion

high

0,2 13,6 0,2 O,I 0,5 0,6 21,0

3,3 3,3 58,7 0,1 0,6 0,2 0,4 0,4

M+(S~); (M+2) + M+(saa); (M+I) +

M+(Sa'-) CaHsOS+ CaH~OS+ C4H2S+ C4HS+ C~HsOS+ C4H20+ C4HO+ C~HsS+ C2H~S+ C2H4S+ C2HaS+ CaH~O+ SCH~+ HSCHa+ CHsS+ C~HsOH+ OC~H5+ OC~H4+ CHACO+ H~S+ HS+ S+ or 02+ OCHa§ C2H6+ C~Hs+ C~H4+ CHa+ M+--qSCoHs]++CHaCO9 M+~C2H~S++C2HaO"

0,05 0,4 0,05 0,2 0,25 0,4 1,3 0,2 0,4 11,4 0,2 0,2 0,8 0,2 0,5 16,3

0,8 0,05 0,1

*Percentage of total ion current

c.z.j

+ s,"

43amu

However, this path, being a two-step path, is less probable under the conditions of field ionization. The migration of the alkyl groups from oxygen to sulfur has also been observed in molecular ions of thione esters obtained as a result of ionization by electron impact [8, 9]. From Table i, which presents information on the calculated equilibrium bond lengths and bond angles of the molecule and the O-ethyl t~ioacetate radical cation, it is seen that the S--CII distance decreases from 2.122 to 1.842 A following ionization. It may be postulated that an additional S--C11 bond appears in the molecular ion in the equilibrium configuration. In fact, as in the case of OM, the ionization of the OE molecule with the formation of the OE +- ion results in the appearance of a two-center contribution E(S3CII) to the total energy of the system. In the case of the OE molecule, it is equal to --469.9 kJ/mole, in the case of OE +" ion, which has the structure of the original molecule, it is equal to--669.5 kJ/mole, and in the case of the ion with the equilibrium structure, it is equal to --1055.4 kJ/mole. These data attest to the formation of a cyclic structure. The decomposition of the structure results in the appearance of fragment ions, whose lines are detected in the mass spectrum.

499

From the possible fragmentation schemes for the OE +" ion presented in Table 5 it follows that the energetically most favorable process involves the simultaneous cleavage of two bonds, viz., CI--S and O-Cs~ with the formation of the [CH30] + ion and the [SCHzCHa]" radical. As in the case of OM ", the positive charge is localized with a high probability in the oxygen-containing fragment, as is evinced by the lower value of the sum of the energies of the products of reaction i.i in comparison to reaction 1.2. The simultaneous cleavage of the S--CII and O-Cs bonds results in the formation of the [CH3CH2] + ion and t h e " --~ICH3_C<~" radical according to reaction 2.1.

I

The cleavage of the

-oJ

S-C11 and Ci-O bonds with the formation of the [CH3CS] + ion according to reaction 3.1 requires more energy than do the fragmentation processes already considered. The probability of the formation of the [CH3CO] + ion according to scheme (2) is also determined by the sum of the energies of the CH3COSH and C2H~ fragments. From Table 5 it is seen that reactions 4.1 and 4.2 are energetically less advantageous. This allows us to conclude that the [CH3CO] + ion forms mainly according to scheme (i). TSus, from a theoretical consideration of the possible fragmentation schemes of the OM +" and OE " radical cations it follows that the formation of the positively charged [CH3CO] + fragment is energetically less favorable than the formation of the [CH3CS] + ion (see Tables 4 and 5, reactions I.I and 3.1). If we start out from the probable structures of the molecular ions of O-methyl and O-ethyl thioacetate, on the other hand, the formation of the [CH3CS] + ion is more likely. Nevertheless, the intensity of this ion in the experimental spectrum is insignificant (See Table 6). Such a fragmentation pattern may be attributed to the formation of an additional bond between the sulfur atom and the terminal carbon atom of the alkyl residue in the OE +- molecular ions: CHj--~ " . 3 - . . 9 0-..

The c l e a v a g e all the ions

c ~ j 7 +" cMz

o f a n y two b o n d s i n s u c h a c y c l i c s t r u c t u r e may a c c o u n t comprising the mass spectrum of O-ethyl thioacetate.

for

the

formation

of

LITERATURE CITED i.

2.

3. 4. 5. 6. 7. 8. 9.

500

V . V . Lobanov, I. P. Samchenko, L. I. Fileleeva, and M. M. Aleksankin, "Molecular positive ions in gas phase. Electronic and geometric structures of HCOOH +, HCOOCH3 +', CHaCOOH +, and CHaCOOCHa+-,"Int. J. Mass Spectrom. Ion Phys., 42, No. 2, 101-113 (1981). V . V . Lobanov, I. P. Samchenko, L. I. Fileleeva, and M. M. Aleksankin, "Equilibrium structure of the molecular cation-radical of ethyl acetate in the gas phase. Mechanism of formation of rearrangement ions," Int. J. Mass Spectrom. Ion Phys., 43, No. i, 5362 (1982). J . A . Pople and D. L. Beveridge, Approximate Molecular Orbital Theory, McGraw-Hill, New York (1970). V . V . Lobanov, "Electronic and spatial structure of the positive and negative molecular ions of acetaldehyde," Teor. Eksp. Khim., i_~8, No. 5, 612-617 (1982). L . E . Sutton (editor), Tables of Interatomic Distances and Configuration in Molecules and Ions, Chemical Society, London (1958). A. Ohno, T. Koizumi, and G. Tsuchihashi, "A novel method to synthesize O-esters of thiocarboxylic acids," Tetrahedron Lett., No. 17, 2083-2085 (1968). R. Radeglia, S. Scheithauer, and R. Mayer, "IH-NMR chemische Verschiebungen von Thiocarbonsaureathylestern," Z. Naturforsch., 24, No. 3, 283-289 (1969). A. Ohno, Y. Ohnishi, T. Koizumi, and G. Tsuchihashi, "A novel rearrangement in thioesters upon electron impact," Tetrahedron Lett., No. 37, 4031-4034 (1968). A. Ohno, T. Koizumi, Y. Ohnishi, and G. Tsuchhach i, "Mass spectral studies on thiobenzoates and thioacetates," Org. Mass Spectrom., No. 3, 261-270 (1970).