The photoelectron spectra of the oxalyl halides (COX)2, [X = F, Cl and Br]

Journal of Electron Spectroscopy and Related Phenomena, 10 (1977) 273-292 @ Elsevier Scientsc PublishingCompany, Amsterdam- Printed in The Netherlands

THE PHOTOELECTRON [X = F, Cl AND Br]

SPECTRA

OF THE

OXALYL

HALIDES

(COX)2,

D. C. FROST, C. A. MCDOWELL, G. POUZARD* and N. P. C. WESTWOOD Department of Chemistry, University of British Columbia, Vancouver, British Cobmbia (Canada)

V6T I W5

(Received 2 August 1976)

ABSTRACT

spectra of the oxalyl halides We have examined the He1 photoelectron (COX),, (X = F, Cl, and Br). We assume the presence of the trans-isomer only, however there is some evidence that more than one species is responsible for some aspects of the spectra. We have used semi-empirical calculations (SCF MO), and Koopmans’ Theorem, as aids in assigning the spectral bands. INTRODUCTION The structure of the simple dicarbonyl compounds, the oxalyl halides, (COX)z, has involved a great deal of investigation_ The solid is known to exist as the transconformerr- 4, whereas the liquid or solution phases, and the vapour phase have been shown by a consideration of the vibrational and electronic spectra, to consist of a mixture of cis- and truns-conformers in equilibrium; however, considerable controversy 5 - ” exists on this point. More recent studies using vibrational spectroscopy ” 3* 4 indicate that some &-isomer exists in the gas phase for the fluoro, chloro and bromo species. This has been further confirmed for oxalyl chlotidezl, and for 23). The first electron oxalyl fluoride2’, (but not for oxalyl chloride-fluoride22* diffraction results indicated only a planar tram form24, but more recent electron diffraction work on oxalyl chloride2 5, and oxalyl bromide26, indicates that these molecules exist as a mixture of truns- and gauche-isomers in the gas phase, and the presence of a cis form is not required in order to interpret the experimental data. There is, therefore, much confusion regarding the nature of the species in the gas phase. The photoelectron (PE) spectra of dicarbonyl compounds is of interest in * On leave from the Universityof Provence, Marseille, France.

274 order to investigate orbital interactions between the two carbonyl groups. In addition, under the appropriate experimental conditions, we may attempt to observe any equilibrium existing between tru~ls-, gauche- and/or &-isomers. EXPERIMENTAL

Oxalyl chloride and oxalyl bromide were commercial samples (Aldrich Chemical Co. Inc.). Initially the hydrolysis products, CO2 and HX (X = Cl, Br) were observable, but over a period of time these decreased to non-detectable limits, until the PE spectra were entirely reproduceable and assignable to pure compound only. Oxalyl fluoride was prepared by the method of Tullock et a12’. This involved fluorination of oxalyl chloride with sodium fluoride in tetramethylene sulfone. It was purified by trap-to-trap distillation on a vacuum line, and its purity confirmed by IR4’ 11’ 22. No detectable impurities were observed. The sample was stored at - 78 “C before use. The 584 A He1 photoelectron spectra were recorded on two separate instruments. The first, described previously2*, was used to record room temperature spectra under high resolution (22 meV FWHM on Ar 2P3,2). Calibration was effected by admission of Ar and/or Xe to the ionization chamber together with the sample. The second instrument is a fast pumping spectrometer specifically designed to study short-lived and reactive species, and is described in another communication2’. This system has facilities for variable temperatures PES, involving a speciahy designed cell with a temperature range of - 160 to + 160°C. RESULTS

The PE spectra of (COBr) 2, (COCl)2 and (COF), recorded at room temperature are shown in Figs. 1, 2 and 3. Certain bands in all the spectra, in particular for (COF),, have vibrational structure, and these are shown in expanded scale in Figs. 4 and 5. The corresponding experimental IP’s are listed in Tables 1, 2 and 3, together with any associated vibrational structure. Interpretation of the experimental data is assisted by the results of several semi-empirical SCF MO calculations, viz. CNDO/S of Del Bene and Jaffe3’* 31, including parametrisation for chlorine32, CNDO/BW of Boyd and Whitehead33, and CND0/2 of Pople 34 . In all cases, Koopmans’ theorem is assumed to be validJ5. Two different geometries are used for the calculations. One is taken from Hedberg’ 5, 26 where it was shown that a change in X (X = Cl or Br) does not affect the geometry of the rest of the molecule. The other is taken from the work of Groth et al.’ on solid trans oxalyl bromide, and extended to the other moIecules by changing the C-X bond lengths. Both sets of geometries give virtually the same results for the caIculations. Apart from the CND0/2 calculations, the total energy curve (Fig. 6) shows a minimum for a gauche conformation, and indicates that the tram form is favoured

275

L

I

II

1

I

13

IONIZATION

1. The photoelectron

spectrum

of oxalyl bromide,

(COB&.

Figure 2. The photoelectron

Figure

spectrum

of oxalyl chloride,

(COCI)Z.

Figure 3. The photoelectron

spectrum

of oxalyl fluoride,

(COF)Z.

I

15

,

I

I

17

POTENTIAL

I

19

(eV)

16

IONIZATION

POTEN:AL

(eV)

Figure 4. (A) Expansion of the 14-16 eV region in the photoelectron spectrum of oxalyl bromide. (B) Expansion of the 15-17 eV region in the photoelectron spectrum of oxalyl chloride.

276

/

14.0

IONIZATION

POTENTIAL

I

15.0 .6

.B

18.0 2

.4

(eV)

Figure 5. (A) Expansion of the 12-13 eV region (first band) in the photoelectron spectrum of oxalyl fluoride. (B) Expansion of the 14-l 5 eV region (second and third bands) in the photoelectron spectrum of oxalyl fluoride. (C) Expansion of the 17_5-18.5 eV region in the photoelectron spectrum of oxalyl fluoride. with respect

to the cis. However,

the existence

of only one calculated

minimum

does

not obviate the possibility of a conformational equilibrium. Nagata et a1.36 have pointed out a large difference between semi-empirical calculations including d orbitals in the basis set, and calculations using only s and p atomic orbitals, e.g. for the carbonyl thiophene compounds. Thus, CNDO/Z and CNDO/S both using d orbitals for Cl atoms and the same core repulsion approximation give quite different results (Fig. 6), whereas CNDO/BW which includes only s andp orbitals, but uses a different expression for the core repulsion gives energy curves similar to CNDO/S. A similar TABLE

1

IONIZATION

POTENTIALS

1 2 3 4 5 6 7 8 9 10 11 B *0.02

eV; b ho.05

OF OXALYL

BROMIDE

(COBr)z

Vertical (e V)

Adiabatic

10.7X& 11.35a 11.64b 11.86” 12.06b 12.87& 15.04~ 15.418 16.77” 17.13” 18.34”

10.49c

eV; e ho.1 eV; d f60

14.198

cm-l; e *40

cm-l.

(eV)

Structure ( CIX-~)

1650,1230, 800d 1500, 1500,400~ 1730, 1730, 5OW 1450e 1430e

277 TABLE

2

IONIZATION

1 2 3 4 5 6 7 8 9 10 11 12

POTENTIALS

OF OXALYL

CHLORIDE

(COCl)a

VerticaE (e V)

Adiabatic (e V)

11.29a 12.3gb 12.64b 12.89b 13.04’) 13.61a 15.258 16.04” 16.87b 17.168 18.57b 19.25c

10.91b

14.s5a

Structure (cm-l)

1660, 600d 1650, 1650, 8206 1225e 14QOe 1270e

a 40.02 eV; b +0.05 eV; C ho.1 eV; d &SO cm-l; e &70 cm-l.

TABLE

3

IONIZATION

L f0.02

POTENTIALS

OF OXALYL

FLUORIDE(COF)s

Vertical (e V)

Adiabatic (e V)

Structure (cm-l)

12.638 14.148 14.84b 15.26” 15.55b 15.97b 17.05” 18.01a 18.6gb

12.20” 13.94a 14.33% 15.26”

570, 600, 300” 1600, 1600, 380c 1410, 1280d 11306

eV; b kO.04 eV; c f30

560d

cm-l; d *40 cm-l.

result was found for glyoxa13’. Koopmans’ theorem3 5, applied to the molecular orbitals obtained by these two methods (CNDO/S and CNDO/BW) gives results in better agreement with the experimental data. This is illustrated in Fig. 7 where CND0/2, CNDO/BW and CNDO/S results are shown for the cis-, gauche- and transisomers of oxalyl chloride. Since the CNDO/BW and CNDO/S methods provide similar results, we show in Figs. 8 and 9 respectively, the CNDO/BW calculations for the cis-, gauche- and trans-isomers of oxalyl bromide and oxalyl fluoride. The

Figure 6. Potential

CNDO/BW method.

from 0 to 1 SO o using the CNDO/Z , and oxalyl fluoride using the CNDO/BW

energy curves for oxalyl chloride calculated

and CNDOjS

methods,

and for oxalyl bromide

OXALY L CHLORIDE cis

-EM4

gauche

trans

4

II-

w

Cm 2

CNDO S -

BW

CNDO 2

CNDO Bw

0,

,’ I’

12-

CYO

_,-’ ,*’

,’

I’

,’

,-

CNDO 2

CNDO BW

0 ,*’

CNDO S ,*.’ 09

,’ _I’

138’ #’ _:

14-

h

-’

15-

02

.:&

,’;,--I, b, >’’ *I .I ! ,!

,

-

(-J

*-_

: -

b,’

02

--__

aI

I

b2 :’ ;:

b2

*-

L

3; ‘

;‘#I

_s:

16-

___ 02

02 :;‘bTt -‘-_ ,‘,’

.‘$

/ !

17-

aI

l81 Figure 7. Orbital

the CND0/2,

energies for the cis, gauche and rrans rotamers

CNDO/BW

and CNDO/S

methods.

of oxalyl chloride

calculated

using

279

OXALYL

-E(eV)

BROMIDE

I

IO-

cis 4 7

II-

gauche -. t.*. ..

a, --

trans

_________--------

----a:--I. s. T. ‘.

IZ-

OXALYL 13-

14-

1%

16-

a, d a2 b, 01 bz -

17-

cis

-E(eV)

IS-

01 -

14-

-

1

01

-

FLUORIDE gauche

trans

-

-

%I

IS-

-

-2 -

au 0;

4 -

-

b, cq

a2 aI -

=

2”

-

au aq

16-

17-

1%

IS-

l9-

19-

b, -

b2 0, -

Figure 8. Orbital energies for the cis, gauche and tram rotamers the CNDO/BW method.

-

of oxalyl bromide calculated

using

energies for the cis, gauche and trnns rotamers of oxalyl fluoride calculated using the CNDO/BW method.

Figure 9. Orbital

calculations show a significant difference between the cis- and trans-conformations, but the MO scheme for the gauche-conformers is predicted to be close to that for the trans-isomer for a dihedral angle within the range 60’ to 180” (tuans). This may make the resulting interpretation somewhat difficult, and so, in our assignments and discussion, we will initially discuss the results from the point of view of only the transisomer, and then later we will discuss the possibility of other isomers being present as a result of careful study of the variable temperature spectra. The important point to note here before beginning a discussion of the assignments is that from the calculations the gauche- and trans-isomers are predicted to have a single IP followed by a group of five IP’s, which are, in turn, well separated from two higher IP’s. ASSIGNMENTS

Oxalyl

chIooride

(COCl)

2

Of the three molecules discussed here, oxalyl chloride presents the most easily interpreted spectrum. We will therefore use it as a starting point, drawing upon

280 the results of our calculations, and other work including CI,CO 3* and (COI!Q2 39. Using a simple molecular orbital approach, we anticipate six bands below ca. 14 eV deriving from the two oxygen lone pair combinations [n +(a,) and n_(b,)] and the four chlorine lone pairs (czg, a,, b, and b,). It is noted here that a high degree of mixing between the chlorine and oxygen orbitals undoubtedly occurs, and so this notation is only used for the convenience of describing the molecular orbitals. Above ca. 14.5 eV one may expect two ionizations (b, and a,) resulting from the two combinations of z orbitals (mainly C = 0 7~), and in addition, C-Cl cz orbitals. The first band in the PE spectrum of (COCl)2 (Fig. 2) is well separated, broad and symmetric (FWHM, 0.39 ev) with no evidence for any vibrational struG ture. It has an adiabatic IP of 10.91 eV and a vertical IP of 11.29 eV. This band may easily be assigned to a symmetric combination (a,) of oxygen and chlorine in plane p orbitals, incorporated with a substantial degree of C-C d bonding. This band parallels in band profile the first IP’s of BzF4 4o and N,04 41 which involve mainly B-B and N-N cr character. The calculated eigenvalue for this band (Fig. 7) does not vary with rotation about the C-C bond, and therefore cannot be used to indicate the presence of more than one rotamer. An alternative MO nomenclature for dicarbonyl compounds, to which we have alluded above, and much used by McGlynn and co-workers42-46, consists of terming the first IP n, i.e. the symmetric combination of lone pair orbitals, where the + sign strictly refers to the phasing in the C-C region, i.e. bonding. The corresponding antisymmetric combination (n_) is expected to be clearly separated from the n, combination by a through-bond interaction, as predicted by the work of Hoffman4’, and Ha4’, and as demonstrated by PES39, 49 for glyoxal, (COH)z where the n+/n_ separation is 1.6 eV. Bearing this in mind, we anticipate the n- MO to be ca. 2 eV removed from this first IP. In addition, we also expect ionizations associated with the four halogen lone pair orbitals in this region. From a careful study of our experimental data, we assign five bands to the region 12-14 eV, with maxima at 12.38, 12.64, 12.89, 13.04 and 13.61 eV, although there appear to be other subsidiary maxima in this region. (This point will be discussed later). The last band is somewhat better separated, and appears to contain some vibrational fine structure. The other four bands are closely spaced and relatively intense, and are therefore assigned to the four halogen “lone pairs” (a,, a,,, b,, b,), which, we reiterate, contain some 0 p character. The sixth IP with a maximum at 13.61 eV is thus assigned to the n_ combination (b,), with a separation from the first IP(n,) of 2.32 eV. This is corroborated by the weak, and somewhat confusing vibrational structure on this band, which indicates a primary vibration of 1660 cm- ‘. The uncertainty here derives from the presence of an underlying vibration of ca. 600 cm- ‘. The primary excitation undoubtedly corresponds to a symmetric C= 0 stretch (1778 cm- ’ in the neutral molecule), and is in accord with the general observation42s 44p 46 for an n- ionization, that it is structured (usually in the C = 0 stretch), compared to the first ionization (n+), which is usually symmetric

281 in shape and only structured (usually in the -C-Cskeletal mode) in a few cases (see (COF)2 later). Because of the small separation of the four halogen lone pairs (spread over 0.66 eV), we cannot give an exact ordering. There is an alternative assignment, which we have rejected for various reasons. This concerns the possibility that the intense band between 12.0 and 13.3 eV consists of only three IP’s, and that the less intense band, centred at 13.61 eV contains two IP’s. This possibility occurs because of the somewhat anomalous structure on &is particular band. However, we are fairly certain in rejecting this notion, since we do anticipate structure on this band, and the PE spectrum of oxalyl bromide (see later), does show ionizations arising from four separate halogen lone pair orbitals, plus an additional band analogous to this particular band under discussion. Above 14.5 eV a structured band occurs with an adiabatic IP of 14.85 eV and a vertical IP of 15.25 eV (See Fig. 4B). This band consists of at least four, and possibly five major components with a separation of 1650 cm-‘. Overlaid on this is another series of 1650 cm-l separation, starting at VA+ 820 cm- ‘. In this region we anticipate a band due to the 7c1-n2(b& combination of carbonyl a ionizations (hereafter termed n-) which compares with the unperturbed Z- level in glyoxal which lies at 14.02 eV39 . Thus the X- orbital in (COCl), is shifted by 1.23 eV due to interaction with the z chlorine lone pair of the same symmetry (bJ. The corollary may indicate that this chlorine lone pair of b, symmetry may also be pushed to higher energy, possibly giving this as the first of the chlorine lone pairs. The assignment of the 7~_ ionization is confirmed by the series in 1650 cm- ’ which undoubtedly corresponds to the symmetric C= 0 stretch, and parallels the value of 1570 cm-’ found on the 7~ionization of oxamide43. It is interesting to note that the n_ ionization band of glyoxa13’ exhibits well defined structure of interval 1360 cm- ‘, which is a considerable reduction from the corresponding frequency in the neutral molecule, and may indicate some delocalisation of the 71electron density. This assignment to the Z- ionization is confirmed by our CNDO/S calculation (Fig. 7). The CNDO/BW method interposes a v ionization (a& between the n_(b,) and I_ orbitals, a not uncommon occurrence in this type of calculation. Above 16.0 eV we expect the other n ionization, 7c1 + 7c2(7c+) of symmetry and in addition, various G ionizations. From the experimental PE spectrum TEg. 2) we observe broad bands centred at 16.04, 17.16 and 18.45 eV, and a possible band at 19.25 eV. In addition, there is a pronounced shoulder on the low side of the 17.16 eV band, estimated to have a maximum at 16.87 eV, From available data on dicarbonyl compounds4’44y 46, the n-/n., separation is of the order of 2 eV, but this may be modified by interaction with other orbitals of the same symmetry. Since the unperturbed carbonyl YZorbitals in glyoxal are assigned to 14.02 eV and between 15.5-16.0 eVj9, then we may anticipate a value of ca. 16-17 eV for the x+. orbital in (COCl)2, bearing in mind that the Z_ orbital is slightly stabilised to 15.25 eV by interaction with the n chlorine lone pair. In (COCl), the bands at 16.04, 16.87 and 17.16 are therefore all contenders for the n+ assignment. We prefer the assignment of

282 the 16.87 eV band to the n+ orbital, a n-/z+ separation of 1.62 eV (the 16.04 eV band would give a much too small separation). This is in accord with the calculations, and results for other dicarbonyl compounds. The 16.04 and 17.16 eV bands are therefore assigned to u orbitals, although it is possible that the 17.16 eV band is the ?t.+ ionization, with two TVionizations interposed between the carbonyl x ionizations. Some weak structure (See Fig. 4B) with a vibrational separation of 1400 cm-’ is evident on the 16.87 eV band, implying some delocalization if this is the n+ MO. Weak structure (1225 cm- ‘) is evident on the 16.04 eV band (Fig. 4B), and some structure (1270 cm-l) is also present on the 17.16 eV band. It is worth noting at this point that all the bands that we observe in (COCl), that have vibrational structure, manifest it in an ambigous fashion. The vibrational structure on all the bands we observe is not entirely clear (see Fig. 4B), which implies either the excitation of many vibrational modes, or the possible existence of another species, i.e. another isomer in the PE spectrum. We address ourselves to this point later in this paper. We have thus established the pattern for the first S-10 IP’s of (COCI), to be due to ionizations from n, , four “lone pairs”, n_, X_ and X+ , with the possible interspersal of one or two 0 ionizations between the last two mentioned orbitals, This sequence is generally observed for 1,2-dicarbonyls42-46, and appears to hold true here. We note that McGlynn et al. include (COC1)2 in a correlation diagram4’, and what little information is discernible indicates some accord with the above discussion. We will now proceed to discuss the assignments for oxalyl bromide since these follow most readily from those of oxalyl chloride, bearing in mind some of the points raised above, and, in addition, taking account of the resulting shifts to be incurred upon substituting a less electronegative halogen atom. Uxalyl bromide (COBr)

2

The first band in the PE spectrum of this compound (Fig. 1) has an adiabatic IP of 10.49 eV and a vertical IP of 10.78 eV. It is analogous to the first band of (COCI),, retaining the same band shape, with no discernible fine structure. This is again assigned to n+(aJ, in agreement with the calculations (Fig. 8) which again indicate a high proportion of C-C 0 bonding in the coefficients, The four halogen lone pairs (again we must draw attention to the fact that these are far from pure, since they have considerable 0 p character included in their wavefunctions), occur in the region 11-12.5 eV, and unlike the corresponding bands of (COCl), are better separated so that we may easily ascribe four peaks to 11.35, 11.64, 11.86 and 12.06 eV. These bands are relatively intense, show no obvious fine structure, and it is therefore impossible to assign them definitively. The calculations (Fig. 8) give the ordering ag, a,, b, and b,. The next band in (COBr), has a maximum at 12.87 eV, and we suggest that this is analogous to the 13.61 eV band of (COC1)2, i.e. the n- ionization. The relative shift of this n- ionization on moving from (COCl), to (COBr), is greater

283 than the corresponding shift of the n, ionization. This indicates a lesser involvement of the C-C 0 coefficients in the n_ ionization, compared to the n, ionization, i.e. the n_ shift almost parallels the shift of the lone pairs, indicating a large percentage of halogen incorporated in the MO. We will, however, continue to refer to this orbital as n_ for the sake of convenience. The puzzling aspect of this band is the vibrational structure which is extremely unusual, although to some degree it parallels that observed in the corresponding band of (COC1)2. We have recorded this spectrum many times, with freshly distilled samples, and observe no change in the relative intensities of the structure_ As we have suggested for (COCl)2 it may be that this band consists of two overlapping IP’s, with only three halogen lone pair orbit& having energies in the region 11.2-12.5 eV, although relative intensity measurements argue against this thesis. We therefore remain with our assignment of the 12.87 eV band as being due to the n_ ionization. The major discernible vibrational series appears to extend over 3 to 4 peaks with an average spacing of 1650 cm-i. Superimposed upon this are various intervals of ca. 1230 cm-’ and 800 cm-‘. We take it that the major progression of 1650 cm-’ corresponds to the carbonyl stretching frequency which is anticipated to be excited in the ionization process. The other vibrations overlap too much to consider at length. The vertical nature of this ionization is as expected for an n_ ionization. It is interesting to note that the magnitude of the n+/n_ separation is only 1.6 eV in (COH),, whereas for (COCl), and (COBr)2 the respective values are 2.3 and 2.1 eV. This increased separation for the halo-substituted species probably derives from a stabilization of the n-i&) orbital incurring by interaction with the corresponding halogen lone pair (b,). The positions of the rc_. and Z+ ionizations may be anticipated to lie in the range 13.0-16.4 eV46. In this range we observe one broad band extending from 14.2 to ca, 16.0 eV. (Fig. 4A), with an adiabatic value of 14.19 eV. We find it hard to envisage a single Franck-Condon envelope of this magnitude, and the integrated area certainly argues for at least two ionizations to be present in this region. Again there is some complex structure on this band, but upon careful investigation this appears to be marginally altered beyond 15.04 eV. We therefore tentatively suggest the presence of two overlapping IP’s one with a maximum at 15.04 eV, at v’ = 5( + l), and the other with a maximum at 15.41 eV with at least three vibrational components. The first band has a vibrational series of 1500 cm- ‘, which is paralleled by another series of 1500 cm-i starting at v; + 400 cm -’ (Fig. 4A). The second band also has two overlapping series both with vibrational intervals of 1730 cm-l, separated by ca. 500 cm- ‘. There is some additional structure beyond the maximum of this band. The first of these two bands is assigned to the x_ ionization with the main excitation corresponding to the carbonyl stretching frequency. The question remains as to whether the second band here (15.41 ev) comprises the other carbonyl z ionization (n,). Two other possibilities present themselves, the possibility of the second band being a c ionization interposed between the two 7c ionizations (as we have surmised for the case of (COCI),), or the possibility that we have a band resulting from the

284 presence of another isomer. From our knowledge of rc-/rc+ splittings it would appear unlikely that the second band is rc+, we do not expect any dramatic change from (COC1)2 to (COBr),, and the calculations (Fig. 8) indicate a separation of at least 2 eV between the carbonyl 7c ionizations. We therefore rest with the last two possibilities, in particular the suggestion of another isomer. This will be treated more fully later. The next band again poses similar problems since it is broad (16.0 to 18.0 eV) with resolvable fine structure. For similar reasons we feel that there are two overlapping ionizations here, one with a maximum at 16.77 eV, and a primary vibrational series of 1450 cm- ‘, and the other with a maximum at 17.13 and a primary vibrational separation of 1430 cm- ‘. One of these two bands probably corresponds to the 7c+ ionization, since the separation of the mean of these two bands from the mean of the preceding two bands is ca. 1.7 eV, a reasonable value for the rc_/rc+ separation. Above 17.5 eV there is an additional ionization process at 18.34 eV, which probably corresponds to a u ionization. Despite the complexity of some of the bands in the PE spectrum of (COBr)2, the trends of the first six IP’s are as anticipated upon replacing Cl by Br. It is interesting to note that the seventh IP, the n- ionization is relatively invariant to the nature of the halogen substituent, which is analogous to the situation in Cl,CO and Br,CO”. Oxalyl fluoride (COF)

z

Of the three oxalyl halides, the fluoro derivative presents the most difficulty in obtaining a definitive assignment, since the fluorine lone pair orbital energies have now moved into the same region as the carbonyl 71ionizations. As evidenced from the spectrum (Fig. 3), the only clear-cut IP is the first with an adiabatic IP of 12.20 eV, and a vertical IP of 12.63 eV. Now that X is the lightest halogen atom we observe extensive structure on the first band (Fig. 5A). The primary series consists of at least 15 components, with an even vibrational interval of 570 cm-‘, and the maximum occurring at v’ = 6. A second series, possibly starting at v; + 300 cm-l (depending upon the electronic origin) has an interval of 600 cm- ‘. There is, in addition, evidence for other complex structure, which is impossible to discern. The extensive structure indicates a large change in geometry upon ionization, the main vibrational mode excited corresponding to the C-C skeletal mode (vJ) reduced from 809 cm-l in the neutral molecule, indicating the C-C bonding nature of the orbital. Although there is considerable incorporation of 0 p orbitals in the wavefunction, we cannot unequivocally identify a vibration in the C = 0 stretching mode, although there is still some unexplained structure. We note that McGlynn43, optimistically indicates two progressions of 1600 cm- ‘, in the PE spectrum of oxamide, in addition to a skeletal mode of 440 cm-i. This band, by virtue of its shape, structure and position, is again assigned to the n, ionization (a&, and in all respects parallels the first IP of the isoelectronic molecules B2Fb4’ and N,0,4’.

285 In (COC1)Z and (COBr),, the next four IP’s correspond to the halogen lone pairs, but in the fluoro derivative we anticipate the F lone pairs to lie above 15-16 eV. Consequently the next two IP’s in (COF), should correspond to the n_ and the carbonyl x (7c-) ionizations. Between 13.3 and 13.9 eV there is a slight rise in the background (Fig. 3), which we notice to vary in intensity from one spectrum to another. We attribute this to either a decomposition product or an impurity, although the IR spectrum indicated pure (COF) 2. The next IP is therefore a well resolved band with an adiabatic IP of 13.94 cV, and a vertical IP (v’ = 1) of 14.14 eV. (Fig. 5s). This band extends over at least five components, with a separation of 1600 cm-l. In addition, there is a parallel series of 1600 cm-’ at vl, + 380 cm- ‘. It remains to be decided whether this band is due to the n_ ionization (b,) or to the Z- ionization (b,). We prefer the latter, i.e. rc_, for the following reasons. The separation of 1600 cm-l undoubtedly corresponds to vl, the symmetric C = 0 stretch which is 1872 cm-l in the neutral molecule. The weak vibration of 380 cm-’ may correspond to v4, the symmetric in-plane bend (565 cm-’ in the neutral molecule). We anticipate the n+/n- separation to be at least 2 eV in this molecule, in view of the separations observed for (COBr), and (COCl), of 2.1 and 2.3 eV, and the separations for oxamide and oxalic acid43 of 1.9 and 2.05 eV respectively. The assignment of the second band in the photoelectron spectrum of (COF)2 as n_ thus appears unreasonable since the magnitude of the separation would only be 1.51 eV. The stabilisation of the n_ ionization is expected on the basis of the perfluoro effect ‘l (comparing (COF), with (COH),), where, upon perfluorination the n: ionizations remain relatively unperturbed, and d ionizations shift by 2-3 eV. It is therefore quite conceivable that the n_ ionization is stabilised below the 7~_ ionization, which is itself only shifted by + 0.12 eV from the 71ionization in (COH), 39. In addition, from the known data on FzCO so* ‘I, it is observed that the ;R:C = 0 ionization in F&O is destabilized with respect to the x C =0 ionizations in Br,CO and Cl&O. This may be taken as due to interaction of the n: ionization with a F lone pair of the same symmetry. A similar effect undoubtedly occurs in (COF),, pushing the X_ ionization above the n_ . The balance of evidence is thus in favour of the second IP being due to the X_ ionization, This is also corroborated by the CNDOjBW calculation (Fig. 9). The third 1P is overlapped by the second to the extent that its adiabatic IP is estimated to be either 14.33 or 14.52 eV depending upon the position of the electronic origin. The vertical IP occurs at 14.84 eV (v’ = 2 or 3), with at least five components (Fig. 5B). The structure on this band is extremely complex, but a series of interval 1410 cm-l may be distinguished. There is an underlying weaker series of 1280 cm-’ together with other unresolved structure. As elucidated above, we assign this band to the n_ combination, with the excited vibration corresponding to a reduced C = 0 vibration. The separation from n, is 2.21 eV, in accord with expectations. It is to be noted here that we find no evidence in this spectrum for F,CO, a possible side product of the preparative procedure.

286

The next band in the PE spectrum of (COF), has a maximum at 15.55 eV, and structure at 15.26, 15.40 and 15.97 eV. It is uncertain whether one, two or three IP’s comprise this band, although the latter two suggestions are more appropriate judging from the overall intensity of the band, It is felt that the sharp peak at 15.26 eV is the adiabatic (and vertical) IP of a structured band with the peak at 15.40 eV comprising some vibrational structure viz. 1130 cm-l. This may well correspond to the 5~+ ionization (a,), the n_/rc+ separation then being of the order of 1.1 eV, a not unreasonable value. The maxima at 15.55 and 15.97 eV have enough individual intensity to be considered as separate bands, and these may therefore be assigned to (T ionizations or F lone pairs. Some unidentified structure occurs on these bands. Above 16.5 eV there are three more distinct bands with maxima of 17.05, 18.01, and 18.69 eV. These are tentatively assigned to ionizations from the F lone pairs, although Q ionizations may also be present depending upon the number of ionizations in the 15.55 eV region. The assignment of the band at 18.01 eV can be put on a stronger footing since it is sharp and intense, and is therefore undoubtedly associated with ionization from a F lone pair orbital. Structure on this band (Fig. 5C) indicates an excitation frequency of 560 cm- I, which may correspond to the symmetric in-plane bending, vq (565 cm-’ in the neutral molecule). Once again the structure is complex, and we cannot rule out the possibility that there is an excitation frequency of 1120 cm-l, corresponding to v 2, the C-F stretching mode (1286 cm-’ in the neutral molecule), superimposed upon some weaker structure. Because of the sharpness of this band we assign it arising from ionization from an a,, orbital, the n F lone pair orbital. One of the sharp bands at 15.55 and 15.97 eV may correspond to the other rc F lone pair combination, b,. The second of these, 15.97 eV is more likely since we anticipate a stabilization of this orbital by interaction with the J-c_(~,) orbital. The broader band at 17.05 eV probably corresponds to the b, ionization (C-F bonding). The CNDO/BW calculations (Fig. 9) indicate an ordering ap, b,, b, and a, for the F lone pair orbital, which is in accord with our assignment based on the band shapes. A 0 ionization may be interposed in here, such that the band at 18.69 eV may be associated with ionization from a fluorine lone pair orbital. DISCUSSION From the preceding assignments of the PE spectra of the oxalyl halides, we are now considering a correlation across the whole series, and in addition, comparison with other isoelectronic systems. This will illustrate an internal consistency in our assignments. We have already mentioned in passing, the perfluoro effect 51, and Fig. 10, which shows the correlation of the three oxalyl halides, also indicates the comparison of the PE spectrum of oxalyl fluoride with that of its perhydro derivative, glyoxa139. The shifts in the energies of the halogen lone pair orbitals, and the n +/n_ ionizations are all as anticipated upon successive replacement of H by Br, Cl and F (Fig. 10). The separation of the n+/n_ ionizations for (COBr),, (COCl), and (COF)z

287 (CO!=),

(COH),

(COCI),

KOBr),

n, ‘.

Figure 10. Correlation diagram of the experimental photoelectron results for glyoxal. oxalyl fluoride. oxalyl chloride and oxalyl bromide. Unmarked levels are c ionizations.

are 2.09,2.32 and 2.21 eV respectively. This splitting depends upon several competing factors including, the magnitude of the through-bond interaction, the magnitude of the n_@J interaction with the appropriate halogen lone pair (stabilizing), and the magnitude of the interaction of the n+(a,) orbital with the ag lone pair combination (destabilizing). This complicated situation is illustrated by the positions of the n, and n_ ionizations with respect to the oxygen lone pair ionizations (no) in the X&O molecules3’, “7 5 ‘. In (COF), the barycentre of the n+/n_ ionizations corresponds well to the n0 ionization in F&O. In comparison, the n0 ionization of Cl,CO is a little destabilized with respect to the barycentre, and in Br,CO the n0 ionization is in approximately the same position as the n, ionization. This may be taken as a measure of the destabilizing effect of the halogen lone pairs in the X,CO (X=Cl, Br) molecules, coupled with the stabilizing effect (on n_) of the halogen lone pairs in the (COX), molecules. Similar perturbations exist for the zCO ionizations of (COX), (especially n_), when they are compared with the zCO ionizations of X,C03*, sop 51. These have been mentioned in the text, but we particularly note the destabilization of the n_ in (COF),, which is pushed up to 14.14 eV by interaction with the appropriate fluorine lone pair orbital, This is analogous to the situation in F,CO 50, 51. A further comment is necessary on the ordering of the halogen lone pairs. In (COF). the fluorine lone pair orbitals are more widely separated in energy, and we have indicated a tentative assignment based on band shapes, although additional 0 ionizations may be interposed in this region (15.5-19.0 eV). In the case of (COBr), and (COCl), the halogen lone pair orbitals are so close in energy that we have not given an ordering, The CNDO/BW calculations (Figs. 7 and 8) give a relative ordering

288 of ag, a,, b,, b,, although the separations are so small as to make Koopmans’ theorem useless here. On an orbital interaction basis we would not anticipate the first halogen lone pair to be of ag symmetry since the first IP (n,) is of the same symmetry, and we would therefore anticipate a stabilization of the former. Similarly, the sixth IP (n_) is of b, symmetry and will destabilize the corresponding (and according to the calculations, adjacent halogen lone pair orbital). The ordering of the halogen lone pair orbital is therefore problematical, The oxalyl halides complete a series of symmetric a-dicarbonyls, (COM),, where M is a first row atom. Thus we have the isoelectronic series (COCH3)2 45. 49, 52, (CONH,), 43* 44, (COOH), 433 53, and (COF),. This correlation is illustrated in Fig. 11, since it indicates interesting features due to substituents of varying inductive strength. Thus the n, and n_ ionizations follow the expected trend. The ionizations due to the N, 0 and F substituents are also in accord with expectations, in fact they progressively encroach upon the n- ionization until at fluorine there is a switch. The correlation again indicates the destabilization of the rc_ (and x,) ionizations in (COF),, leading to the change in the Z-, n_ ordering. The correlation does not allow us to exactly locate the 7c+ ionization in (COF),, but does indicate that it should occur in the region 15.0-15.5 eV. We have assigned it to the band at 15.26 eV. The final comment concerns the first band of the oxalyl halides. This is indicated to have a high degree of C-C 0 bonding, and yet the position of the IP is quite low for an ionization with such character. For the series of isoelectronic compounds BZF440, (COF), and N,0441 we anticipate the increasing nuclear charge to move all the MO’s to higher IP. This is contrary to the experimental observation, and indicates either a modification due to replacement of F by 0, or a destabilization incurred by the C-O antibonding character of this orbital.

KOCH,),

ICONH,),

A.-_.___

n,

Figure 11. Correlation molecules,

biacetyl,

(COOH),

ICOF),

diagram of the experimental photoelectron results for the isoelectronic oxamide, oxalic acid and oxalyl fluoride. Unmarked levels are CTionizations.

289 US/GA UCHE/TRANS

ISOMERISM

IN THE PE SPECTRA OF THE OXALYL

HALIDES

Throughout the previous assignments and discussion we have assumed the presence of only one isomer, the trans-, in order to account for all the features of the PE spectra. However, careful observation shows that there is some evidence for more than one species in some of our spectra. There are many instances of unexplained vibrational structure on several bands, and the possibility of completely new bands altogether. These have been mentioned in the text. We have, in addition the results of the electron diffraction work on (COCl)* and (COBr), 25* 26, which indicates that two isomers, the trans- and the gauche-, may predominate in the gas phase at room temperature. Using the expression in ref. 25, and appropriate values for AS and AE, we calculate that for (COCI), over the temperature range - 20°C to + 130°C (the range used here), the percentage of tvans-isomer will drop from ca. 7 1% to 44 %. For (COBr), over the same temperature range, we expect the percentage of tram to fall from ca. 50% to 39 %. At room temperature we expect to have ca. 61% of the chloro, and 45 oAof the bromo compound in the truns configuration. The temperature effects are therefore not large and we are limited at the low temperature end by freezing of the sample, and at the high temperature end by the construction of the cold cell. As mentioned in the introduction, the presence of tram- and gauche-isomers in equilibrium is difficult to discern by PES, especially since the MO results indicate that the FE spectra of these two isomers are more or less superimposable, within the limits of Koopmans’ theorem. Were the c&isomer present, it would have bands in a different energy region from those of the gauche and tram (Fig. 7), and would therefore be more identifiable. We may therefore exclude the c&isomer from our spectra. We note recent work by Turner et al. ’ 4 on the variable temperature PES of the haloethanes where it was shown that the variations in the PE spectra with temperature are very subtle. Therefore, throughout this paper we have discussed the spectra from the point of view of the trans-isomer, although it is quite clear from the above figures that a substantial proportion of the gauche form may be present. Our variable temperature work tends to confirin this suggestion, but because of the relatively small change in percentage of the two isomers over our limited temperature range we are not able to definitively assign bands due to each isomer. Figure 12 shows PE spectra B, 30°C; C, 125°C. Despite the presence of a recorded for (COCI’J~, A, -20°C; trace of CO and CO2 in the low temperature spectra it is fairly clear that there are changes in the PE spectra over this range of temperatures. These spectra are entirely reproduceable by alternately raising and lowering the temperature. In particular, the sharp band at 13.61 eV becomes more pronounced at low temperature, and at - 30 a is quite clear. Also the chlorine lone pairs between 12 and 13 eV, alter their shape, becoming more asymmetric with increasing temperature. Not shown is the fact that the band with an adiabatic IP of 14.85 eV, loses the vibrational structure upon

I

1

II 12 IONIZATION

I

13 POTENTIAL

,

14 (eV)

Figure 12. The 1 l-14 eV region of the photoelectron temperatures.

spectrum of oxalyl chloride recorded at various

cooling below 0°C. (This is not a resolution effect). The change in shape of the first band is uncertain. Because these changes are slight the evidence is not unequivocal, but we feel it sufficient to merit study of other systems where the effects may be more pronounced. Under the present conditions we are unable to estimate the percentage of gauche- or trczns-isomers, but in a more ideal system this may be feasible. We feel that our analysis of the PE spectra from the point of view of only the truns-isomer is justifiable, since the orbital energies of the gauche- and tvuns-isomers are so close, and discussion of the trans-isomer facilitates the MO scheme. Our observation of complicated vibrational structure undoubtedly derives from the superposition of bands in the PE spectra resulting from two isomers. It is interesting to note that recent preliminary work on the electron diffraction of [COF), 55 indicates that in this case the main species in the gas phase are the cisand trans-isomers, as compared to the gauche- and trans-isomers in the case of

291

(COCI), and (COBr),. The CNDOjBW calculations for (COF),, (Fig. 9), do actually indicate much less of a difference between the cis- and rrans-isomers compared to the other two molecules. In this case, on the basis of the calculations, we cannot rule out the c&isomer, as we did for (COCl)2 and (COBr),. ACKNOWLEDGEMENTS

The generous financial support of the National Research Council of Canada is gratefully acknowledged. One of us (G.P.) thanks the Minis&e des Affaires &rang&es FranCais for a grant.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

P. Groth and 0. Hassel, Acta. Chem. Stand., 16 (1962) 2311. J. R. Durig and S. E. Hannum, J. Chcm. Phys., 52 (1970) 6089. J. R. Durig and S. E. Hannum, J. Chcm. Whys., 54 (1971) 2367. J. R. Durig and S. E. Hannum, J. Chem. Phys., 54 (1971) 4428. R. E. Kagarise, J. Chem. Phys., 21 (1953) 1615. H. Shimada, R. Shimada and Y. Kanda, Spectrochim. Actu, Part A, 23 (1967) 2821. H. Shimada, R. Shimada and Y. Kanda, BUN. Chem. Sot. Japan, 41 (1968) 1289. B. D. Saksena and R. E. Kagarise, J. Chem. Phys., 19 (1951) 987. J. S. Ziomek, A. G. Meister, F. F. Cleveland and C. E. Decker, J. Chem. Phys., 21 (1953) 90. B. D. Saksena, R. E. Kagarise and D. H. Rank, J. Chem. Phys., 21 (1953) 1613. J. L. Hencher and G. W. King, J. Mol. Spectrosc., 16 (1965) 158. J. L. Hencher and G. W. King, J. Mol. Spectrosc., 16 (1965) 168. K. G. Kidd and G. W. King, J. Mol. Specfrosc., 28 (1968) 411. B. D. Saksena and R. E. Kagarise, J. Chem. Phys., 19 (1951) 999. J. W. Sidman, J. Am. Chem. Sot., 78 (1956) 1527. B. D. Saksena and G. S. Jauhri, J. Chem. Phys., 36 (1962) 2233. W. J. Balfour and G. W. King, J. MOE. Spectrosc., 25 (1968) 130. W. J. Balfour and G. W. King, J. Mol. Spectrosc., 26 (1968) 384. W. J. Balfour and G. W. King, J. Moi. Spectrosc., 27 (1968) 432. W. J. Balfour and G. W. King, J. MoI. Spectrosc., 28 (1968) 497. G. Fleury and A. Chapput, J. Mol. Spectrosc., 46 (1973) 513. J. Goubeau and M. Adelhelm, Spectrochim. Acta, Part A, 28 (1972) 2471. K. G. Kidd and G. W. King, J. Mol. Spectrosc., 48 (1973) 592. K. Hjortaas, Acta Chem. Scund., 21 (1967) 1379. K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 1003. K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 4796. C. W. TulIock and D. D. Coffman, J. Org. Chem., 25 (1960) 2016. A. Katrib, Ph. D. Thesis, University of British Columbia, 1972. D. C. Frost, S. T. Lee, C. A. McDowell and N. P. C. Westwood, submitted for publication. J. Del Bene and H. H. Jaffe, J. Chem. Phys., 48 (1968) 1807. J. Del Bene and H. H. Jaffe, J. Chem. Phys., 48 (1968) 4050. M. Rajzmann, G. Pouzard and J. L. Bouscasse, C. R. Acud. Sci., Ser. C, 273 (1971) 595. R. J. Boyd and M. A. Whitehead, J. Chem. Sot., Dalton Trans., 73 (1972). J. A. Pople and D. L. Beveridge, Approximate MoZecular Orbital Theory, McGraw-Hill, New York, 1970. T. Koopmans, Physica, 1 (1934) 104. S. Nagata, T. Yamabe, K. Yoshikawa and H. Kato, Tetrahedron 29 (1973) 2545. W. Hug and G. Wagniere, H&v. Chim. Acta, 54 (1971) 633.

292 38 39 40 41 42

43 44 45 46 47 48

49 50

51 52 53 54 55

D. Chadwick, Can. J. Chem., 50 (1972) 737. D. W. Turner, C. 3aker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, London, 1970. N. Lynaugh, D. R. Lloyd, M. F. Guest, M. B. Hall and I. H. Hillier, J. Chem. Sot., Faraday Trans. 2, (1972) 2192. D. C. Frost, C. A. McDowell and N. P. C. Westwood, J. Electron Spectrosc. Relat. Phenom., 10 (1977) 293. J. L. Meeks, J. F. Arnett, D. Larson and S. P. McGlynn, Chem. Phys. Lett., 30 (1975) 190. J. L. Meeks, J. F. Arnett, D. B. Larson and S. P. McGlynn, J. Am. Chem. Sot., 97 (1975) 3905. J. L. Meeks and S. P. McGlynn, J. Am. Chem. Sot., 97 (1975) 5079. S. P. McGlynn and J. L. Meeks, J. Eiectron Spectrosc. Relat. Phenom., 6, (1975) 269. J. L. Meeks, H. J. Maria, P. Brint and S. P. McGlynn, Chem. Rev., 75 (1975) 603. J. R. Swenson and R. Hoffmann, Nelv. Chim. Acta, 53 (1970) 2331. T. K. Ha and W. Hug, Welv. Chim. Acfa, 54 (1971) 2278. D. 0. Cowan, R. Gleiter, J. A. Hashmall, E. Heilbronner and V. Hornung, Angew. Chem., 10 (1971) 401. R. K. Thomas and H. Thompson, Proc. Roy. Sot. Lund. A, 327 (1972) 13. C. R. Brundle, M. B. Robin, N. A. Kuebler and H. Basch, J. Am. Chem. SOL, 94 (1972) 1451. J. Kelder, H. Cerfontain, B. R. Higginson and D. R. Lloyd, Tetrahedron Lett., 9 (1974) 739. S. P. McGlynn and J. L. Meeks, J. Electron Spectrosc. Relar. Phenom., 8 (1976) 85. D. L. Ames and D. W. Turner, Chem. Commun., 179 (1975). K. W. Hedberg, private communication.