Study of methyl chalcogen compounds with ultraviolet photoelectron spectroscopy

Journal of Electron Spectroscopy and Related Phenomena, 40 (1986) 363-383 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

STUDY OF METHYL CHALCOGEN COMPOUNDS WITH ULTRAVIOLET PH~~EL~~RON SPE~RO~PY*

F.C. CHANG’, Department (U.S.A.)

V.Y. YOUNGb, JOHN W. PRATHER’

of Chemistry,

and EL. CHENGd

University of Missouri-Kansas City, Kansas City, MO 64110

(First received 26 January 1986; in final form 5 April 1986)

ABSTRACT Dimethyl sulfide, dhnethyl selenide, dimethyi telluride, dimethyl disulfide, dimethyl diselenide and dimethyl trisulfide have been studied by ultraviolet photoelectron spectroscopy. For the fiit tie, the high resolution He(I) spectrum of dimethyl trisulfide is reported. However, the main thrust of this work is to examine trends among related compounds. We report the results of correlations of the first three ionization energies of (CHs )zX, X = 0, S, Se, and Te and of the correlations of the firzt five ionization energies of (CHs )2X2, X = 0, S, and Se with the ionization energies of oxygen, sulfur, selenium, and tellurium. Where possible, orbital trends of isoelectronic molecules were also examined, Finally, we report the results of correlations of the FWHM of the fist three bands of CHs(S),CH3 with the number of sulfur atoms and investigate convergence of the first two orbital bandwidths to the bandwidths of the two highest occupied bands in /&sulfur.

INTRODUCTION

In this paper, we report the results of ultraviolet photoelectron spectroscopy studies on methyl chalcogen compounds. The high resolution spectrum of dimethyl trisulfide is reported for the first time. Compounds previously studied by other investigators and reexamined here are dimethyl sulfide, dimethyl disulfide, dimethyl selenide, dimethyl diselenide, and dimethyl telluride. Dimethyl sulfide has been studied extensively [l-10]. The most complete spectral analyses have been reported by Frost et al. (33 * Psrt of “Present China bPresent U.S.A. ‘Present dAuthor

the dissertation of F.C. Chang, University of Missouri-Kansas City, 1979. address: Institute of Nuclear Energy Research, Lungtan Taiwan, Republic

of

address: Department of Chemistry, University of Florida, Gainsville, FL 326 11, address: 2152 Pebble Beach, La Place, LA 70668, U.S.A. to whom correspondence should be addressed.

0368-20~8186~~03.50

0 1986 Elsevier Science Publishers B.V.

364

and Aue et al. [ 81. The theoretical electronic structure of this compound has also been studied extensively by empirical molecular-orbital methods [6], semiempirical molecular-orbital methods [3,5,9,11-171, and ab initio molecular-orbital methods [8,14,18,19]. The various methods do not predict the same ordering for the orbitals. Only Cradock et al. [l] have reported photoelectron spectra for dimethyl selenide and dimethyl telluride. Lehn et al. [ 181 have performed ab initio calculations on dimethyl selenide, but they published only the first two calculated ionization energies and made no orbital assignments. No theoretical studies on the electronic structure of dimethyl telluride could be found. McGlynn et al. [20] have shown that the first ionization energies of the compounds (CH3)2X, X = 0, S, Se, and Te, are linearly correlated with the ionization energies of the rare gases, which are isoelectronic with the corresponding hydrides. In this paper, we investigate the correlation of the first three ionization energies of these compounds with the ionization energies of oxygen, sulfur, selenium, and tellurium. Further insight into the orbital ordering is obtained by comparing experimental orbital levels of isoelectronic compounds. Dimethyl disulfide has been studied both in the vapor phase [ 2,6,7,21-241 and as a condensed solid [25]. Its theoretical electronic structure has been studied by empirical molecular-orbital methods [6,26,27], semiempirical molecular orbital methods [26], and ab initio methods [24,26,18]. Chmutova and Bock [21] have reported the only photoelectron spectroscopy study of dimethyl diselenide and the only molecular-orbital study on its electronic structure, an extended Htickel study. To the authors’ knowledge, trends for the ionization energies of the compounds (CHs )pX2, X = 0, S, and Se have not been investigated. In this paper, we investigate the correlation of the first five ionization energies of these compounds with the ionization energies of oxygen, sulfur, and selenium. We also make a comparison of isoelectronic molecules for those cases in which data can be obtained from the literature. It is interesting to compare the results obtained for dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide. These compounds are molecular precursors which can be used to study the initial changes which occur for sulfur-sulfur bonding in the band evolution of ~-sulfur, a pseudo onedimensional helical polymer. We have examined the full width at half maximum (FWHM) of the first three bands as a function of the number of sulfur atoms. The bandwidth convergence of the S-S lone pair orbitals and the S-S bonding orbit&s to the calculated bandwidths of the two highest occupied bands in p-sulfur is investigated.

365

EXPERIMENTAL

Dimethyl sulfide, dimethyl selenide, dimethyl telluride (Ventron Corporation, Alfa Division), dimethyl disulfide (Koch-Light Laboratories Ltd.), dimethyl diselenide and dimethyl trisulfide (PCR, Incorporated) were used as received without further purification. The spectra were recorded using the He(I) (21.22 eV) line as the excitation source on an instrument described previously [29]. Liquid samples were dried by placing Molecular Sieves Type 4A in the sample holders. These samples were degassed prior to obtaining spectra. Calibrations of the spectra were made using xenon or nitrogen. Spectra were not recorded in the 19-21 eV region because of the low signal to noise ratio. The ionization energies showed a precision of 0.02 eV. CNDO/B molecular-orbital calculations were performed on the compounds which contain no elements beyond the first two rows of the periodic table; otherwise, extended Hiickel molecular orbital calculations were performed. Bond lengths, bond angles, and dihedral angles for the compounds were obtained either from microwave or electron-diffraction studies. The parameters for dimethyl sulfide have been obtained from the microwave data of Pierce and Hayashi [30] and those for dimethyl disulfide from the microwave data of Sutter et al. [31]. The parameters of dimethyl trisulfide have been obtained from the electron diffraction data of Donohue [ 321. The parameters of dimethyl selenide have been obtained from the microwave data of Beecher [ 331. The parameters of dimethyl diselenide and dimethyl telluride were obtained from the work of Green and Harvey [34] and Scott [35], respectively. These parameters were used to calculate the coordinates of the atoms, which are necessary to calculate the overlap matrix needed to determine the offdiagonal elements of the Hartree-Fock matrix [36]. Planar molecules were assumed to lie in the X2 plane.

RESULTS

AND DISCUSSION

Dimethyl sulfide Dimethyl sulfide has CZv symmetry and so there are no symmetry degenerate orbitals. In Table 1 our experimental results (vertical ionization energies) are compared with those reported in the literature. Our results differ from those of Frost et al. [ 31 and Aue et al. [8] only because these authors resolved the 13.2-16.5 eV region into 4 bands, whereas only two bands can be identified on the basis of vertical ionization energies. In order to deconvolute this region into 4 bands, one must be able to evaluate the photoionization cross sections for the orbitals. We do not have that information,

366 TABLE 1 IONIZATION ENERGIES

FOR DIMETHYL SULFIDE’

111

Mb

r31

[41

[51

[61

171

161

r91

[lo]

This work

8.71 11.28 12.68 14.5

8.68 10.96 12.16 13.68

8.65 11.2 12.6 14.2 14.8 15.4 15.7 19.7

8.7 11.2 -

8.65 11.2 12.6 -

8.67 12.76 14.91 -

8.71 -

8.68 11.35 12.75 14.25 14.90 15.5 15.5

8.69 11.20 -

8.70 -

8.67 11.28 12.67 14.5 15.4

aColumn numbers correspond to reference number. bAdiabatic values.

so no attempt has been made to deconvolute any of the spectra reported here. The fourth ionization potential is in good agreement with the average of the fourth and fifth ionization potentials reported by Frost et al. [3] (14.5 eV) and Aue et al. [8] (14.58 eV). Likewise, the fifth ionization potential is in good agreement with the averages of their sixth and seventh ionization potentials (15.6 eV and 15.5 eV, respectively). The assignment of the orbital ordering in the neutral molecule is not trivial. Usually Koopmans’ theorem is applied using molecular-orbital calculations of various levels of sophistication. Koopmans’ theorem says that the ith ionization potential is equal to the negative of the ith one-electron Hartree-Fock energy. Thus, less sophisticated molecular-orbital calculations will give the correct orbital ordering only when they predict the same ordering as a Hartree-Fock calculation, and a Hartree-Fock calculation will give the correct orbital ordering only when correlation and relativistic effects cancel. Table 2 shows the Walsh prediction [37] along with the predictions of various molecular-orbital calculations, including our own CNDO/B calculations. In all cases, except for the CNDO/B calculation of Frost et al. [3], the calculations give the same ordering for the first five orbitals. The ab initio results predict that the 3a, and lbz orbit& are accidentally degenerate which is in agreement with the empirical EHMO calculations. One limitation of the CND0/2 calculations is that they force non-degeneracy even in those cases where symmetry degeneracy is expected. The qualitative Walsh prediction cannot predict accidental degeneracies, but it is significant that it agrees in substance with the ab initio calculations, since it can be applied without resorting to calculations. However, even with the good agreement among the various methods, one must still be cautious, because Hartree-Fock calculations often give orbital orderings which are

367

TABLE 2 PREDICTED

Walsh [37]

ORDERING

EHMO [6]

OF THE ORBITALS

CNDO [3]

CNDO/S

IN DIMETHYL

[this work]

SULFIDE’

NMDO [ 171

Ab initio [8]

2b2

2b2

262

2b2

2b2

2b2

401

4al

401

401

401

4al

3bl

3bl

3h

3h

3h

3bl

Jo2

la2

2bl

102

102

102

2bl

2h

102

2bl

2bl

2bl

3at

3al, 201

301 lb2 2al

3al lb2 -

3al,

lb2 201

lb2 3al 201

a Reference

numbers are shown in brackets.

lb2

lb2

-

different from those obtained by ab initio calculations. While Hartree-Fock calculations have not been done on dimethyl sulfide, they have been done on sulfur difluoride [ 38 1, which is isoelectronic with dimethyl sulfide. It is instructive to consider how the Walsh prediction and the less sophisticated calculations compare with these results. On the basis of the Walsh prediction, it is expected that the orbital ordering for dimethyl sulfide and sulfur difluoride should be identical, since the two molecules have almost identical bond angles, 98.9” [30] and 98.3” [39], respectively. However, as shown in Table 3, the Walsh prediction and the CND0/2 calculation are in disagreement with the Hartree-Fock-Slater (HFS) results concerning the ordering of the lb2, 3a, , and 2b, orbit&. The ab initio and HFS results predict different accidental degeneracies. If the prediction of accidental degeneracies is regarded as being of minor importance, Tables 2 and 3 taken together indicate that we can unequivocally conclude that the four higher lying molecular orbitals are 2bz, 4a1, 3b,, and la*, in the order least tightly bound to more tightly bound. Figure 1 shows the 2bz, 4a1, 3b 1, and la? orbitals of sulfur difluoride and dimethyl sulfide. It can be seen that the shifts between localized orbitals (2b2 and la2 ) are smaller (2 2.0 eV) than the shifts between delocalized orbit& (4al and 3b,), > 3.0eV. To a zeroth order approximation, the 2b2 orbital is a sulfur 3p orbital. The Herman-Skillman eigenvalue [41] for a sulfur atom 3p orbital is - 10.27 eV, from which we see that fluorine only minimally stabilizes the orbital, while methyl causes a large destabilization. Inductively, the effect of fluorine should be to stabilize and the effect of methyl to destabilize the sulfur lone pair, since the difference between the Gordy electronegativities [42] of S and F and of S and the effective Gordy electronegativity [43] of CHs are - 1.42 and i- 0.19, respectively. However, F should cause a large stabilization and CHs a much smaller destabilization,

368

TABLE 3 PREDICTED ORDERING OF THE ORBITALS IN SULFUR DIFLUORIDE’ Walsh [37]

CNDO/S [ 381

Ab initio [ 38 ]

HFS [38]

GF [401b

2b2 4ai 3h la2 2b1

262 401 3bl

2bz 4al 3bl

2bz 4al 3bl, 102

401

la2

la2

261 lb2 301 %

lb,

,301

lb2 201 ‘Reference

lb2 301 2h 2al

numbers are shown in brackets. bGreen’s function calculations.

SKH3)2

/

/

/

/

,

/

/

/

,

3bl la2

301

2bl 201

sF2

2b2

/

2b2

/

401

2b2

Fig. 1. Partial level comparison of sulfur difluoride and dimethyl sulfide.

lb2 301 2bl 2al

369

TABLE 4 IONIZATION POTENTIALS FOR DIMETHYL SELENIDE AND DIMETHYL TELLURIDE Dimethyl selenide Ref. 1

This work

Dimethyl telluride Ref. 1

This work

8.40 11.0 12.0 14.0

8.36 10.98 12.03 14.4

7.89 10.35 11.32 14.0

7.86 10.39 11.36 14.3

which is exactly opposite to the observed effect. The overlap of empty sulfur 3cEorbitals with fluorine 2p or methyl pseudo-x would give rise to a mesomerit effect, in which electron density would be transferred from fluorine or methyl to sulfur. This would lead to a more negative charge on sulfur and a destabilization of the sulfur lone pair. However, HFS calculations [38] reveal insignificant S 3d participation in the 2bz orbital of sulfur difluoride, therefore this is also likely to be the case for dimethyl sulfide. Steric effects would also destabilize the sulfur lone pair in both cases, and there should be a somewhat greater destabilization by methyl, since the spatial extent of the electron cloud of the methyl group should be much larger than that for fluorine. Inductive effects and steric effects are in opposite directions for sulfur difluoride and almost cancel. They are in the same direction for dimethyl sulfide and are thus additive. The laz orbital is an antisymmetric combination of F 2p orbit& in sulfur difluoride and of CH3 pseudo-n orbit& in dimethyl sulfide. To a zeroth order approximation, the difference between the la2 level in dimethyl sulfide and the la* level in sulfur difluoride should be equal to the difference in energy between a methyl pseudo-r orbital and a fluorine 2p orbital. Using one-center Coulomb integrals equal to the Herman-Skillman eigenvalues and two-center Coulomb integrals calculated by using the equations of Harrison [41], we obtain - 15.53 eV for a methyl pseudo-n orbital. The Herman-Skillman eigenvalue for the F 2p orbital is - 16.99 eV, giving a difference of 1.46 eV. Experimentally, the corresponding la* orbitals differ by 1.98 eV, a reasonable agreement. Dimethyl selenide and dimethyl telluride Table 4 compares the results of this work with those reported by Cradock and White [l]. The FWHM of the broad band in the region 13-17 eV is 2.1 eV for dimethyl selenide and 2.0 eV for dimethyl telluride. This compares with a FWHM of 2.4eV for dimethyl sulfide, where two groups of

“$

4

9I

10 1

11 I

12 I

13 I

A

IE(EV) OF X

Fig. 2. Correlation of ionization

energies of MezX and ionization

energies of X in MezX.

researchers reported four overlapping bands. The EHMO calculations give an orbital ordering identical to the EHMO results for dimethyl sulfide [6 J and predict that the la2,2b 1, 3~2, and lb2 orbitals should occur in this region. An ab initio calculation has been performed on dimethyl selenide [ 181, but only the first two calculated ionization potentials were reported. HFS calculations [44] on the isoelectronic molecule selenium difluoride gives the ordering 2bz, 1~2, 4al, 3bl, lb,, 3a2, 2bl, 2ul, which is rather unexpected, since the la2 orbital should have an ionization potential identical to that of the la2 orbital of sulfur difluoride, 16.4eV. No other calculations could be found for these two molecules and no calculations at all could be found for dimethyl telluride or tellurium difluoride. The fact that a one-to-one correspondence exists between the spectra of dimethyl sulfide, dimethyl selenide, and dimethyl telluride for the first four bands, as well as the fact that we have observed the fourth ionization potential at almost identical values (14.5 eV, 14.4 eV, and 14.3 eV, respectively) as expected for the la2 orbital, supports the orbital ordering 2b2, 4~2,, 3 b 1 , and la2 for the first four bands. The first ionization energy for selenium difluoride, 10.20 eV [44], has been measured. The Herman-Skillman eigenvalue for a selenium 4p is - 9.53 eV. Thus the Se lone pair in selenium difluoride is more stabilized than the S lone pair in sulfur difluoride and the Se lone pair in dimethyl selenide is less destabilized than the S lone pair in dimethyl sulfide. This reflects a decrease in the steric effects which occurs because the bond length between the central atom and the substituent increases by 0.1-0.2 A in going from S to Se, but in,going from 3p to 4p, the electron distribution in 2b2 increases predominately along the axis through the central atom which is perpendicular to the plane of the molecule. According to the literature, tellurium difluoride has never been synthesized.

371

Correlation As one substitutes selenium for sulfur and tellurium for selenium, the ionization energies of the three highest occupied orbit& all decrease. This parallels the trend in the atomic ionization energies of the elements - 10.36 eV for sulfur, 9.75 eV for selenium, and 9.01eV for tellurium. Plotting ionization energies of dimethyl sulfide, dimethyl selenide, and dimethyl telluride against the ionization energies of the elements gives a set of straight lines as shown in Fig. 2. The slopes are 0.476 for line 1, 0.606 for line 2 and 0.833 for line 3. From such relationships, one can predict some of the ionization energies of an unavailable compound, such as dimethyl polonide in this case. The ionization energy of polonium is 8.43 eV. So the first three ionization energies of dimethyl polonide are estimated to be 7.51,10.03 and 10.55 eV, respectively. Even though oxygen is in the same group as sulfur, and tellurium, only the first ionization energy of dimethylether fits the observed relationship. An interesting phenomenon is that straight lines are formed after plotting the third ionization energy of dimethyl ether to the second line and its fourth ionization energy to the third line, 1, as shown in the dashed lines of Fig. 2. A linear correlation for the first ionization energies of dimethyl oxide, dimethyl sulfide, dimethyl selenide, and dimethyl telluride versus the ionization energies of the rare gases isoelectronic within the corresponding hydrides was also obtained by McGlynn et al. [20]. Within the framwork of Koopmans’ theorem, the lack of complete correlation for the second- and third-band ionization energies must be due to a difference in the electron distribution in the corresponding orbit&. For 4a 1, the orbital is through-space bonding between the methyl groups. Hence, the closer the methyl groups, the more stable the molecular orbital. However, this will be true only if the electron density is equally distributed between X and methyl. The electronegativity differences for X-CH3 are - 0.24, 0.06, and 0.19 for X = Te, Se, and S, respectively. This means that they have less than 5% ionic character. For X = 0, the electronegativity difference is 1.11 indicating ca. 30% ionic character [45]. Thus the electron distribution is significantly distorted towards oxygen. The stability gained from bringing methyl groups closer together is offset by the greater repulsion between the G-C bonding electrons. Thus the ionization energy of band 2 is lower than that predicted by extrapolation. Since 3bl is predominantly through-space a-antibonding between the methyl groups, we cannot explain the increasing ionization energy in going from Te to S using a simple argument. Our correlation curves may also be interpreted within the restricted HF-SCF approach developed by McGlynn et al. [20]. In this approach, it is assumed that a molecule can be divided into a chromophoric segment X and a substituent part S.

372

It is shown that 1 (XS, ti) = I (X/S, 9 II)-- wx VI31 mscl,

cc) -

-m

v,o

ms,,

cc) - (2 -

1)

v (f4 PI

(1)

where 230 designates the nearest neighbor segment, X designates the average chromophore, m is the number of electrons on the average chromophore after ionization, N, is the number of electrons on the actual chromophore after ionization, 2 is the charge on the ion produced by ionization, p is the state of the ion produced by ionization, VI,, @/So, p) is the leading correction of the electrostatic interaction energy between an outer electron and p) is the spin-orbital interaction the average chromophore core, V ,,1 (X/S,, energy between an outer electron and the average chromophore core, and V(S, p) is the substituent potential. For the compounds under discussion here, S = S,, and, since 2 = 1, we have 1 (XSo, cc) = 1 @US,, cc) - Wx -m

VI,

o-m,

PFc)_ v (f%, PI

(2)

If we let XSO = XS,, then 1 (XS,, cc) = 1 (X/S,, Equation

p)--

2 then rearranges

I(XS,,c1)

= UXS,JJ)

(3)

v (S,, p) to

+ (m-Nx)

(4)

Via (X/So+)

For p = 2B, (the 2b2 orbital of the closed shell neutral molecule eqn. (4) has the same form as our empirical correlation (line 1) I(XS0,

is ionized),

2B,) = b + ml (X)

(5)

Thus we may use this data to generate a coordinate axes transformation x’, y’, where x’ = I (XSo, p) and y’ = (n -N, ) VI0 (w/S,, p) as follows: 1(XSo,2B2)

+I(SeSo,2B2)

= 1/5{I(OSo,2B2)+l(SS0,2B2) I (TeSo, 2B2) + I (PoS,,

I(X)

= (8.37 - 3.50)/0.476

=

t

2B2)} = 8.37 eV

where I (XSo, ’ B2 ) are the values obtained of Fig. 2. The origin will occur at 10.23 eV

from

to

the line 1 correlation

(6) fit (7)

The units on the transformed abscissa are thus directly proportional to I (X) - 10.23 eV, but to convert to units of (fl- N,) VI0 (%/So, p), we must scale by a proportionality constant c. Numerical values for c can be determined by solving eqn. (4) for (m - N, ) VI0 (XS, , 2 B2 ) using the values for I (XSO, 2B2 ) used in eqn. (6). The calculations are shown in Table 5; the transformed correlation is shown in Fig. 3. Because R is a constant and VI0 @So, p) is a constant for any fixed JA,the fact that the second and third ionization energies of dimethylether show a negative deviation means that

373 TABLE 5 TRANSFORMATION X

I (Xl

1 wo,

0 S SC Te PO

13.61 10.35 9.75 9.01 8.43

9.98 8.43 8.14 7.79 7.51

‘From

OF THE ABSCISSA

2B2

1 m

)”

--I

(X)

3.38 0.12 - 0.48 - 1.22 - 1.80

(Z-N,)

1.61 0.06 - 0.23 - 0.58 - 0.86

v,,

(X/S,,

2B2)

c

0.48 0.50 0.48 0.48 0.48

eqn. 1 for Fig. 2.

Te

16-

Se

S

14 -

6

, -1.5

-10

-0.5

0.0

0.5

1.0

1.5

2.0

(N-N,)V.,O(XISo,ti)

Fig. 3. The correlation in the McGlynn HF-SCF approach.

N,, is larger than it should be. Since No is the number of electrons on 0 after ionization, the results predict a large electron reorganization in the ion states 2A 1 and 2B 1, which does not occur when X = S, Se or Te. Qualitatively, we could not have made such an argument on the basis of electron distributions in the neutral molecule. Dimethyl disulfide Evidence shows that (CH3M)2 molecules possess neither cis (C,) nor truns (C,) symmetry, but are skewed (C, ) between the cis and trans [30] extremes at dihedral angles ranging from 80”--120’. M can be oxygen, sulfur, selenium or tellurium. Thus, there are only two types of symmetry orbit&, i.e., a and b. In Table 6, we compare our results for dimethyl

374

KH312S2

F2S2

s2 Fig. 4. Lone-pair orbital comparison of difluorodisulfide, Sz , and dimethyl disulfide.

disulfide with results obtained from the literature; overall there is excellent agreement. The results of Cullen et al. [2] show poor agreement with the adiabatic results of Colton and Rabalais [23], but these measurements were made from photoelectron stopping curves. An ideal photoelectron stopping curve is a monotonically increasing step function curve with increasing ionization energy. The adiabatic ionization energies are identified from the position of the steps. Instead of sharp steps, continuous sigmoidal sections are obtained in an actual photoelectron stopping curve. Thus, the beginning of steps cannot be identified with high precision. A further impediment is that the step height decreases with increasing ionization energy, so that it becomes even more difficult to extract higher ionization energies from such curves. Very closely spaced steps cannot be resolved, which is why the 2nd ionization energy observed in all the high resolution UPS spectra was not observed. In the XPS spectrum [25], the first two ionization energies are not resolved because the instrumental function is too broad. Extended Hiickel [6,23], CNDO/B [23,24, this work] and ab initio [25] calculations give the orbital ordering as 7a, 65, 5u, 55, 5cr, 4b, 4q 3b, 3a, 2b, 2a, la. Only the MNDO calculation [ 171 gives a different orbital ordering. Dimethyl disulfide is isoelectronic with difluorodisulfide, which also exhibits Cz symmetry. The UPS spectrum of the latter compound has been determined by Wagner et al. [46]. The three lowest bands in the UPS spectrum of difluorodisulfide are similar in profile to those observed in the UPS spectrum of dimethyl disulfide, however the bands of difluorodisulfide have higher ionization energies. EHMO [ 461, CNDO/B [46], SCFMO [47], MNDO [17], and ab initio [48, 611 molecular-orbital calculations predict

375 TABLE 6 IONIZATION ENERGIES

FOR DIMETHYL DISULFIDE’

[21b

[%=I

I71

t221

[25id

8.71 11.08 12.71 13.19 i4.13

8.97 9.21 11.28 12.30 13.42 (15)c

8.97 9.27 -

9.00 9.25 -

3.6 6.0 6.8 7.5 9.4

(8.8) (11.2) (12.0) (12.7) (14.6)

12.9 (18.1) 15.4 (20.6) 18.1(23.3) 19.6 (24.8)

1231

~241

9.01, 8.3b 9.28 11.3, 10.gb 12.32 13.5 14.8

8.96 9.26 11.26 12.31 13.42 14.35 to 15.55

This work 8.97 9.23 11.23 12.36 13.38 14.5

18.3 21.4

a Column heading corresponds to reference number. bAdiabatic ionization energies. ‘Estimated from figure. dData for condensed solid values in parentheses obtained by applying a work function correction of 5.2 eV.

an orbital ordering identical to that predicted for dimethyl disulfide by EHMO, CNDO/B, and ab initio calculations. It is instructive to examine the sulfur lone pair comparison of difluorodisulfide, SZ , and dimethyl disulfide. The ionization energies of SZ have been determined by UPS [49,50] and electron impact [ 511. The sulfur lone-pair orbitals (7~ and 6b) of difluorodisulfide and dimethyl disulfide, derive from the In, orbit& of SZ . Since SZ is an open-shell diatomic molecule, ionization of the In, orbital gives rise to four bands in the UPS spectrum: 4x,, 2~u, 4Z;, and 2Z;. In order to obtain a value for the ionization energy of the In, orbit&, it is standard procedure to take a statistical weight of the energies of the bands which arise from their ionization [ 521; this process gives a value of 12.58 eV. The level comparison is shown in Fig. 4. Both fluorine substitution and methyl substitution give an apparent destabilization of these orbitals. Using interatomic Coulomb integrals calculated by the method of Harrison [41], the loss of ppr stabilization due to the C2 geometry is 1.73 eV for difluorodisulfide and 1.43 eV for dimethyl disulfide. This gives calculated centroid lone-pair energies of 10.85 eV and 11.15 eV compared to experimental centroid lone-pair energies of 11.04 eV and 9.10 eV for difluorodisulfide and dimethyl disulfide, respectively. Thus overall, fluorine is slightly stabilizing and methyl is very destabilizing. As in the case for sulfur difluoride and dimethyl disulfide, the important factors are inductive effects and steric effects. The steric effects are somewhat lessened in difluorodisulfide due to an abnormally long SF bond [47], therefore the inductive effect dominates to gives an overall stabilization.

376

TABLE 7 IONIZATION ENERGIES AND ORBITAL ORDERING FOR DIMETHYL DISELENIDE

1211

This work

EHMO [21]

8.56 8.79 10.67 11.68 12.42 13.6 14.3 15.7 19.4

8.52 8.84 10.62 11.73 12.48 13.7 14.4 15.4 -

7a 6b 6a 5b

EHMO (This work) 7a

6b 6a 5b 5a 4b 4a 3b

5a

46 4a

3b 3a

3a

Se S

15--

141312-

ll-

IO-

9-

I

8

9

Fig. 6. Correlation MezXz.

10

11 I(X)

of ionization

12

13

14

energies of Me 2X Z and ionization

energies of X in

Dimethyl diselenide and correlations Dimethyl diselenide, like dimethyl disulfide, has Cz symmetry. Its UPS spectrum has previously been reported by Chmutova and Bock [21]; the results of the present work are compared to those of ref. 27 in Table 7 which

also shows the orbital ordering predicted by EHMO calculations. More sophisticated calculations such as CNDO/B [53] and ab initio [53,54] have been performed on dimethyl diselenide, but only the rotational barrier was studied. The isoelectronic compound, difluorodiselenium, occurs as one component of a mixture of five products formed when heated selenium is reacted with very dilute fluorine in argon gas mixtures [55]. It has not been isolated as a pure compound. Moreover, since its more stable and commercially available analogs, SezClz and SezBr,, were reported to severely decompose when attempts were made to record their UPS spectra [23], it is unlikely that difluorodiselenium’s UPS spectrum will be recorded in the near future. The synthesis of dimethyl ditelluride was first reported in 1968 [56], but it is not commercially available and its UPS spectrum has not been reported. Difluoroditellurium has not yet been synthesized. Nevertheless, correlations of the ionization energies of dimethyl peroxide, dimethyl sulfide, and dimethyl diselenide can be explored. In Fig. 5 the first five ionization energies of MezXz , X = 0, S, and Se are plotted versus the ionization energies of X. Ionization energies for dimethyl peroxide are those reported by Kimura and Osafune [24]. The first ionization energies correspond to the ionization of the antisymmetric combination of X lone pairs and the second ionization energies correspond to the ionization of the symmetric combination of X lone pairs in the molecules. If no change in geometry occurs on ionization, the ionization of either lone-pair combination should lead to a hole localized on Z and hence both lines 1 and 2 should be linearly correlated with the ionization energy of X. Figure 5 shows that only the second ionization energies are linearly correlated with I (X). We must first examine if the statement “if no change in geometry occurs, linear correlation with I (X) should occur” is a valid statement. It will only be valid if the geometries are similar, which would imply that the same factors are dominant in defining the geometry. Both dimethyl sulfide and dimethyl selenide have dihedral angles which are close to 90” (84.7’ and 87.5”, respectively). For dimethyl peroxide, Kimura and Osafune [24] proposed a dihedral angle of 180°, but a recent electron diffraction study by Haas and Oberhammer [57] gives a dihedral angle of 119 f 10’. Previous microwave, Raman, and dipole moment studies give strong support to the latter. This means that all three molecules have Cz symmetry and that electron interactions dominate steric effects in determining the geometry. This lends validity to our expectation. In fact, the straight line formed by the first ionization energies of dimethyl disulfide and dimethyl diselenide has a slope of 0.683, which is very close to that obtained for line 2 (0.730). Thus the first. ionization energy of dimethyl peroxide is smaller than expected and, by analogy with the McGlynn analysis discussed for Me2X, we suggest that there are large reorganization effects occurring when the antisymmetric lone-pair combination is ionized in this molecule. Unfortunately, there is insufficient data to carry out a transformation like that performed for Me,X, because we need data for dimethyl-

378

14

12

,

IO

8

IP (eV)

Fig, 6. The He (I) spectrum of dimethyl trisulfide.

TABLE 8 IONIZATION SULFIDE

ENERGIES

AND

ORBITAL

ASSIGNMENTS

FOR DIMETHYL

TRI-

PIa

This workb

CNDO/ 2

MO description

8.73 8.14 11.08 11.59 13.61

8.86 9.20 9.54 11.31 11.86 12.72 13.73

7.85 9.41 9.53 10.67 11.88 11.90 13.71 15.68 18.12 18.23 18.27 18.33 19.62

antibonding S lone pairs nonbonding S lone &rs S-S bonding S-S bonding bonding S lone pairs S-C bonding S-S, S-C bonding S-C bonding C-H bonding C-H bonding C-H bonding C-H bonding S-C, S-S, and DH bonding

14.6

17.9

aAdiabatic

8b So 7b 7a 6b 6a 5b 5a 4b 4a 3b 3a 2b

ionization energies. bVertical ionization energies.

ditelluride. We also find that the third ionization energies of. dimethyl disulfide and dimethyl diselenide and the fourth ionization energy of dimethyl peroxide are linearly correlated with I (X). Thus the peculiar correlations observed for Me,S are not unique to that set of compounds.

379

Number Fig,

7.

of sulfur otoms

FWHM vs. the number of sulfur atoms in CHa(S),CH3.

Dimethyl trisulfide The spectrum of dimethyl trisulfide is shown in Fig. 6. In Table 8, the results of the present work are compared with those obtained from photoelectron stopping curves [2] . The present authors are able to resolve 9 bands in the spectrum, whereas only the adiabatic ionization energies of 5 bands were determined previously. Dewar and McKee [17] performed MNDO calculations on dimethyl trisulfide, but they only reported the optimized geometrical parameters and not the eigenvalues. Therefore, CNDO/2 calculations were done on this molecule in order to facilitate band assignments. As for dimethyl disulfide, dimethyl tr~u~ide has Cz symmet~; there are 16 occupied molecular orbit& (8 of b symmetry and 8 of a symmetry). The calculation predicts 12 orbitals with ionization energies less than 19eV. These calculated ionization energies are shown in Table 8. Comparison with dimethyl disulfide shows that the third band should be the bonding lone-pair orbitals on sulfur, contrary to the predictions of the CNDO/Z calculation. In dimethyl disulfide, the S-S bonding band occurs at 11.23eV. Thus the bands at 11.31 and 11.86eV must be due to S-S bonding in dimethyl trisulfide. Methyl bands are expected to occur around 15 eV, but they are too closely spaced to be resolved. The broad band with a vertical ionization energy of 14.6 eV consists of these four orbitals, but also the 5c S-C bonding orbital.

380 Ah

-5b

wk -

7a

Fig. 8. Lone-pair orbitals in CH3(S),CH3,

n = 1,2,3.

>.53 r 4.20

oto0:s

110

1’5

2:o

2:5

3:c

Fig. 9. EHCO band structure curve for p-sulfur (o = 90”).

FWHM vs. number of sulfur in CH3 (S),CH, The correlation of full width half maximum vs. number of sulfur atoms in CHs(S),CHs for the first three ionization energies is shown in Fig. 7. The FWHM of the band is proportional to the number of sulfur atoms in the compound. The molecular orbital diagrams for the sulfur lone pairs of CH3 (S),CHs, n = 1,2, and 3 are shown in Fig. 8 under Cz, the conserving symmetry for all three molecules. As n + 00, the lone-pair orbitals of CHs (S), CHs converge into the highest occupied band of ~-sulfur, a metastable, polymeric form of sulfur which occurs as helical chains of indefinite length. The helical unit cell of E.c-sulfurcontains a single atom of sulfur. Band-structure calculations have shown that energy-dispersion curves can often be predicted by

381

considering only nearest-neighbor interactions and at most second nearestneighbor interactions. Thus the bandwidths of the lone-pair orbitals and of the S-S bonding orbitals of dimethyl trisulfide might be good approximations to the band widths of the highest occupied bands of Cc-sulfur.Figure 9 shows a band-structure curve calculated for p-sulfur using an extended Hiickel crystal orbital program, which has been previously described in the literature [58,59,61]. The S-S bond length used was that for elemental sulfur, Ss, 2.07 f 0.02 A [60]. No improvement in total energy was observed beyond second nearest-neighbors interactions. Calculations were performed for values of w ranging from 80” to 180°, where o is the angle of rotation about the helix axis. Successive rotations by w followed by a translation d parallel to the helix axis gives the positions of S along the helix. o = 90” corresponds to the Cz geometry observed for CHs (S),CHs . Three occupied bands are observed in the dispersion curve, i.e., a very broad sp band and two narrow p bands. The upper p band has a bandwidth of 2.53 eV which corresponds very well with 2 x FWHM for the Sp bands of dimethyl trisulfide (2.42 eV). The second p band corresponds to the S-S bonding orbitals which occur at ca. 11.3 eV in dimethyl trisulfide. The calculated bandwidth is 4.20 eV, compared with 2 x FWHM = 3.06 eV for the S-S bonding orbitals of dimethyl trisulfide. Thus, the band structure evolution of the two highest occupied bands of polymeric sulfur can be studied by investigating the UPS spectra of the series of compounds CHs (S),CHs . This is so because CHs and S-C orbitals do not contribute to the electron distribution of S-S lone-pair and S-S bonding orbit& in these molecules.

REFERENCES 1 S. Cradock and R.A. White, J. Chem. Sot., Faraday Tram II., (1972) 281. 2 W.R. Cullen, D.C. Frost, and D.A. Vroom, Inorg. Chem., 8 (1969) 1803. 3 D.C. Frost, F.G. Herring, A. Katrih, C.A. McDowell, and R.A.N. McLean, J. Phys. Chem., 76 (1972) 1030. W. Schaefer and A. Schweig, J. Chem. Sot., Chem. Commun., 14 (1972) 824. A. Schweig and W. Thiel, Mol. Phys., 27 (1974) 265. G. Wagner and H. Bock, Chem. Ber., 107 (1974) 68. T. Kobayashi, Phys. Lett. A., 69 (1978) 31. D.H. Aue, H.M. Wells, W.R. Davidson, M. VidaI, M.T. Bowers, H. Goldwhite, L.E. Vertzd, J.E. Douglas, P.A. KoIIman, and G.L. Kenyon, J. Am. Chem. Sot., 102 (1980) 5161. 9 M.A. Weiner and M. Lattman, Enorg. Chem., 17 (1978) 1084. 10 J.C. Buenzli, D.C. Frost, and L. Weiier, J. Am. Chem. Sot., 95 (1973) 7880. 11 D.R. WiIiiams and L.T. Kontnik, J. Chem. Sot. B., (1971) 312. 12 W.A. Lathan, L. Radom, P-C._ Hariharan, W.J. Hehre and J.A. Pople, Forts&r. Chem. Forsch., 40 (1973) 1. 13 V.I. Khaustova, Kvantovaya Khimya, (1976) 231. 14 P.D. Mollere and K.N. Houk, J.,Am. Chem. Sot., 99 (1977) 3226. J. Chim. Phys. Phys. -Chim. Biol., 73 (1976) 16 D. Gorbeau and G. Pfiiter-Guilouzo, 787.

382 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 66 57 58

S.G. Gagarin and R.Z. Zakharyan, Izv. Akad. Nauk SSSR, Ser. Khim,, 1(1979) 31. M.J.S. Dewar and M.L. McKee, J. Comput. Chem., 4 (1983) 84. J.M. Lehn, G. Wipff, and J. Demuynck, Helv. Chim. Acta, 60 (1977) 1239. G. Glideweii and C. Thomson, J. Comput. Chem., 4 (1983) 9. S.P. McGiynn, A.M. Langsjoen, P. Hochmann, P. Brint, and G.L. Findley, J. Phys. Chem., 89 (1985) 1157. G. Chmutova and H. Bock, Z. Naturforsch, Teil B: Anorg. Chem. Org. Chem., 31B (1976) 1616. M.F. Guimon, C. Guimon, and G. Pfister-Guillouzo, Tetrahedron Lett., 7 (1975) 441. R.J. Colton and J.W. Rabalais, J. Electron Spectrosc. Reiat. Phenom., 3 (1974) 345. K. Kimura and K. Osafune, J. Chem. Sot. Jpn., 48 (1975) 2421. J. Riga, J.J. Verbist, C. Lamotte, and J.M. Andre, Bull. Sot. Chim. Belg., 87 (1978) 163. J.P. Snyder and L. Carlsen, J. Am. Chem. Sot., 99 (1977) 2931. L.S. Levitt and C. Parkanyi, Int. J. Sulfur Chem., 8 (1973) 329. D. Demoulin, I. Fischer-Hjalmars, A. Henriksson-Enflo, J.A. Pappas, and M. Sundbom, Int. J. Quantum Chem., 12 (1) (1978) 351. V.Y. Young and K.L. Cheng, J. Chem. Phys., 65 (1976) 3187. L. Pierce and M. Hayashi, J. Chem. Phys., 36 (1961) 479. D. Sutter, H. Dreizler, and H.D. Rudolph, Z. Naturforsch., Teil A: (1965) 1676. J. Donohue, J. Chem. Phys., 16 (1948) 92. J.F. Beecher, J. Mol. Spectrosc., 4 (1966) 414. W.H. Green and A.B. Harvey, J. Chem. Phys., 49 (1968) 3586. J.D. Scott, J. Chem. Phys., 59 (1973) 6677. A.G. Segai, Electronic Structure Calculation, Plenum Press, New York, 1977. A.D. Walsh, J. Chem. Sot., (1953) 2266. D.M. DeLeeuw, R. Mooyman, and C.A. DeLange, Chem. Phys., 34 (1978) 287. D.R. Johnson and F.K. Powell, Science 164 (1969) 950. W. Von Niessen, J. Electron Spectrosc. Relat. Phenom., 17 (1979) 197. W.A. Harrison, Electronic Structure and the Properties of Solids, W.H. Freeman and Company, San Francisco, 1980. W. Gordy, J. Chem. Phys., 14 (1946) 305. R.E. Kagarise, J. Am. Chem. Sot., 77 (1955) 1377. D.M. DeLeeuw, R. Mooyman, and C.A. DeLange, Chem. Phys., 38 (1979) 21. R.H. Petrucci, General Chemistry: Principles and Modern Applications, Macmillan New York, 1982, p. 220. G. Wagner, H. Bock, R. Budenz, and F. Secl. Chem. Ber., 106 (1973) 1285. A. Hincliffe, J. Mol. Struct., 55 (1979) 127. B. Souloucki and H. Bock, Inorg. Chem., 16 (1977) 665. J. Berkowitz, J. Chem. Phys., 62 (1975) 4074. J. Dyke, L. Golob, M. Jonathan, and A. Morris, J. Chem. Sot. Faraday Trans II, 71 (1975) 1026. W. Rosinger, M. Grade, and W. Hirschwaid, Ber. Buneenges, Phys. Chem., 87 (1983) 536. J.H.D. Eland, Photoelectron Spectroscopy, Wiley, New York, 1974, p. 878. V. Renugopaiakrishnan and H.R. Wyssbrod, J. Chem. Phys., 77 (1982) 1362. V. Renugopaiakrishnan and R. Waiter, J. Am. Chem. Sot., 106 (1984) 3413. A. Haas and H. Willner, Z. Anorg. Ailg. Chem., 454 (1979) 17. M.T. Chen and J.W. Goerge J. Organometai. Chem., 12 (1968) 401. B. Haas and H. Oberhammer, J. Am. Chem. Sot., 106 (1984) 6146. V. Young, S.H. Suck, and E.W. Helimuth, J. Appl. Phys., 60 (1979) 6088.

363 69 60 61

V. Young, Solid State Commun., 35 (1980) 715. R.C. Weast (Ed.), Handbook of Chemistry and Physics, The Chemical Rubber Co., Cleveland, 1968, p. F-155. K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, and S. Iwata, Handbook of He1 Photoelectron Spectra of Fundamental Organic Compounds, H&ted, New York, 1981.