Distinguishing Keto and Enol Structures by Inner-Shell Spectroscopy

Mar 1, 1994 - that 3 is 93% enol in ethanol but only 26% enol in dichlo- romethane.1° Therefore ... 0 1 s spectra of gas-phase cyclohexanone (1) and ...
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3650

J. Phys. Chem. 1994,98, 3650-3657

Distinguishing Keto and Enol Structures by Inner-Shell Spectroscopy J. T. Francis and A. P. Hitchcock' Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4Ml Received: December 6, 1993" Oscillator strengths for C 1s and 0 1s excitation of gas-phase 1,2-~yclohexanedione,1,3-cyclohexanedione, 1,4-~yclohexanedione,and cyclohexanone have been derived from electron energy loss spectra recorded under conditions of electric dipole scattering. Spectral assignments have been aided by extended HDckel (EHMO) and H A M / 3 calculations. Spectral features characteristic of diketo and monoenol structures are identified. The C 1s and 0 1s spectra clearly indicate that 1,2-cyclohexanedione has a monoenol structure in the gas phase while 1,3-cyclohexanedione is predominantly in the diketonic structure in the gas phase. The latter result resolves a n existing literature discrepancy concerning the preferred gas-phase tautomer of 1,3-~yclohexanedione.

1. Introduction

In compounds containing one or more,carbonyl groups, the interconversion between a keto and an enol form ("tautomerism") is well known. A comprehensive treatment of tautomerism has been given by Ingold,' while a review of enolization has been presented by Forsen and Nilsson.2 Ketc-enol interconversion is exploited by synthetic chemists to produce derivatives via the enolate ion intermediate. At equilibrium, monocarbonyl compounds like cyclohexanone (1) do not display any enolic characteristics; indeed, cyclohexanone exists 98.8% in the keto form in solution.3 However, the introduction of a secondcarbonyl group into the molecule can have a profound effect on the enolizability of the first carbonyl. Thus, while 1 itself shows no enolic properties, 1,2-~yclohexanedione(2)exists completely in the monoenolic form in solution and shows weakly acidic propertie~.~.5 This is largely due to the fact that the monoenol tautomer 2a is stabilized relative to the a-diketo form 2b by formation of a five-membered, intramolecularly hydrogen bonded ring. Interestingly, in the solid phase 2 is exclusively d i k e t ~ n i c , ~ indicating that intermolecular interactions can have a profound influence on the position of the ketc-enol equilibrium. The preferred tautomeric form in solution of the 0-dicarbonyl isomer, 1,3-~yclohexanedione(3),is also monoenolic (structure 3a,4see Scheme 1). Due to conformational limitations, 3 does not form any intramolecular hydrogen bonds; rather, it forms intermolecular hydrogen bonds in the solid and concentrated liquid phases. The crystal structure of 3 has been determined by Etter et al.697The monoenolic molecules are linked into chains by short hydrogen bonds; in turn, the chains form sheets which interact mainly via van der Waals forces. Hydrogen bonds are usually the strongest intermolecular forces in molecular crystals. The strong H-bonds in the solid phase of 3 explain why it has the highest melting point of the three isomeric cyclohexanediones. However, there is currently conflicting evidence as to whether the diketo or the monoenol structure is the preferred tautomer of 3 in the gas phase (see below). 1,4-Cyclohexanedione (4) is in the diketo form in the solid state* and remains exclusively diketonic in s o l ~ t i o nfavoring ,~ an extended skew form in both the solid and liquid phases. In light of the above discussion, it is clear that the positions of ketc-enol equilibria are very sensitive to the environment; e.g., Moriyasu et al. found in their HPLC study that 3 is 93% enol in ethanol but only 26% enol in dichloromethane.1° Therefore, techniques are needed to determine the dominant form and position of the equilibrium in both the gas and condensed phases. The preferred tautomeric form at equilibrium has been determined by several spectroscopic techniques. Spectroscopic a Abstract

published in Aduance ACS Abstracts, March 1, 1994.

0022-3654/94/2098-3650$04.50/0

studies have an advantage over chemical methods (specifically Br titration") because they do not interfere with the tautomeric equilibrium.12 UV, Raman, and IR techniquesyielded some of the earliest measurements of tautomeric equilibrium via the relative intensities of bands assigned to individual tautomers.13 In addition to liquid and solution measurements, N M R has also provided a means of determining which tautomer is preferred in the solid state.14J5 More recently, electron diffraction,16 mass spectrometry,17ultraviolet photoelectron spectroscopy (UPS),*2 and thermochemical methods15 have provided means for investigating gas-phase tautomerization. However, even with the support of increasingly high-level a b initio calculations, there is often disagreement among these latter techniques regarding which tautomer is favored in the gas phase. For example, an ion cyclotron resonancestudy by G.Bouchouxet al.17predicts that themonoenol tautomer 3a is the preferred form of 3 in the gas phase. In contrast, the thermochemical study by Pilcher et al.15 predicts that the diketo form, 3b,dominates in the gas phase. UPS studies'C20 alsosuggest that 3b is the preferred gas-phase tautomer, although no firm conclusions were drawn. The purpose of the present work is to demonstrate that core excitation spectroscopy can be an effective means of distinguishing diketo, monoenol, and dienol forms (should the latter exist). We have examined the C 1s and 0 1s spectra of gas-phase cyclohexanone (1) and the three isomeric cyclohexanediones (2,3, and 4) in order to demonstrate the spectroscopic basis for correlating specific spectral features with a particular tautomeric form and to evaluate the keto-enol preference of 2 and 3 in the gas phase. The 0 1s spectrum of 4 was used as part of a recent core excitation study of p-benzoquinone, hydroquinone, and phenol.2l To the best of our knowledge, the spectra of all other species are being reported for the first time. To assist spectral assignments, extended Htickel (EHMO) and HAM13 calculations have been carried out within the equivalent ionic core virtual orbital model (EICVOM).22 Molecular geometries have been optimized using high-level SCF ab initio calculations. The ability of EHMO calculations to reproduce the discrete and, to some extent, the continuum spectral structure has been demonstrated in recent studies of the core excitation spectra of transition metal organometallic complexes,23benzenoid and quinoid species,21and organic compounds related to polyethylene t e r e ~ h t h a l a t e . Another ~~ goal of this work was to evaluate the merits of HAM13 relative to EHMO calculations with regard to the quality of spectral prediction. The most significant difference between these two semiempirical methods is that EHMO employs a Hamiltonian involving a sum of oneelectron terms,25 while HAM13 (hydrogenic atoms in molecules, version 3)26-27utilizes a Hamiltonian involving both one- and two-electron repulsion terms. HAM/3 is a density functional method using Khon-Sham orbitals as LCAO expansions of 0 1994 American Chemical Society

The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3651

Distinguishing Keto and Enol Structures

SCHEME 1

0 8

1

2a

2b

hydrogenic atomic orbitals. Since HAM13 has been used routinely in the past to interpret UV-PES spectra and ESCA measurements, it is a reasonable choice for calculating core excitation spectra. Certainly, much more sophisticated ab initio algorithms exist which may provide more accurate quantitative results. However, the ease and minimum computing power required for EHMO and HAM13 calculations combined with their reasonable predictive capability make them convenient and useful tools to assist spectral assignment.

2. Experimental Section The inner-shellelectron energy loss spectrometer and operating conditions have been described previously.28 The spectra were recorded by inelasticelectronscattering of a high-velocityelectron beam (incident energy of 2.5 keV plus energy loss) through a small scattering angle ( 2 eV. For 0 1s: 1.0-eV fwhm for t < -4 eV; 8-eV fwhm for 4< t < 2 eV; IO-eV fwhm for 2 < c < 12 eV; 30-eV fwhm for c > 12 eV. e Calibration: -4.17(8) eV relative to C 1s T* transition on C02 (290.74 eV). /IPS estimated from propanal and acetone: C1 293.7 eV, Cz3.4 291.32 eV.31 x Calibration: -4.22(6) evrelative t o 0 ls-r* transitioninC02 (535.4 eV). Read as 3.0 X lo-'.

-

*

I

7

I

I

I

I

I

I

I

I

I

I

I

'

EHMO assignment C-O C=O

ener y T(eV) (ev' C-O c=+ 5309 534.4

4.4

6.3 2.9

**K*-/-

r*-/cr 535.9 2.9 537.3b 537.6 1.2 538.86 539.4 -0.6 545.3(6) -8.0

**c-c/M

V :(I

HAM13 v:()

I

-9.8 -7.6 -6.1 -4.1

0.05 1.8ESd 0.01 0.01

I

-10.4 -5.7 -5.1 -2.6

0.04 0.01 0.01 0.02

IP RYd IP -0.9 0.19

U*C-O

4.23 0.48

0.0-0

a Calibration: -4.54(2) eV relative to C02. IPSestimated from XPS IPS of the enol of 2,3-butanedione and 2,4-pentanedione: (21292.94 eV, C2 291.43 eV, C3 290.7 eV; C4.5 290.9 eV; c6 291.45 eV.31 Calibration: -4.46(6) eV relative to C02. Read as 1.8 X 10-5.

TABLE 4 Energies, Term Values, and Proposed

ents for Features in the C 1s and 0 1s Spectra of

c 1s energ (+o.le!) 286.720 288.4 291.P 292.7 293.P 297 302

2

ener y (evf 531.1' 533.9 535.9 537.9 546

&OH

c- 0 0

I

I

I

I

I

I

I

290

I

-1.7

U*cC

-9

U*M

1.5

0.32

3.7

0.63

~ ( e v ) assignment 6.7 4.0 2.0

-8

e

EHMO (ev) I

e

HAM13 (ev) I

**-(+)

-9.2

*OM(-)

-8.8

0.07 1.5E-3'

-11.1 -7.3

2.1

4.OE-3

5.8

0.05 1.4E-3

Ryd IP u*-

0.02

Calibration: -4.02(4) eV relative to C02. IPSestimated from the XPX IPS of acetone and 2,4 pemtanedione: C1 292.94 eV, C & ~290.9 J eve3' Calibration: -4.28(7) eV relative to C02. Read as 1.5 X le3. a

-

280

6.3 2.6

assignment EHMO HAM13 C-H C--O c(ev) I t(ev) I **M -9.0 0.62 -8.7 0.53 3pIC-H. IP U*W C*W -1.2 0.36 0.3 0.36 IP

0 1s

p

i '

T(eV) C-H c=+

l

l

l

300

l

l

l

l

l

310

Energy (eV)

Figure 1. C 1s oscillator strength spectra of 1,4-~yclohexanedione(4), 1,3-~yclohexanedione(3), cyclohexanone (l),and 1,2-cyclohexanedione (2) derived from electron energy loss spectra recorded under electric dipole scattering conditions (&,,I = 2500 eV, 0 = 2O, 0.7-eV fwhm). Hatched lines indicate ionization limits (IPS) as estimated from XPS values for similar molecules.3~

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structure. The lowest energy feature in the C 1s spectrum of 1, 3, and 4 at 286.7 eV is attributed to C ls(C=O) a*transitions. The 50% reduction in the **- intensity in 1 is consistent with the fact that 1 contains only a single carbonyl

moiety. TheC lsspectrumof2containsaC ls(C=O)-a*feature at 286.2 eV of intensity similar to that in 1 (i.e., corresponding toone rather than two C=O groups). The unique feature of the C 1s spectrum of 2 is the well-isolated low-energy peak at 284.3 eV which is attributed to C ls(C=C) ?T*transitions. The presence of the u*peak and the reduced T * C intensity ~ in the C 1s spectrum of 2 strongly suggests 2 has a monoenolic structure in the gas phase. As was noted in our previous work on quinoid versus aromatic structures,21the lowest energy a*resonance is almost invariably the most intense feature in core excitationspectra, and thus it is unusual for a lower energy a* feature to be less intense than a higher energy ?T* feature. In this case, the low-energy position of the 284.3-eV a*- peak is

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Distinguishing Keto and Enol Structures

The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3653

TABLE 5 Energies, Term Values, and Proposed Assignments for Features in the C 1s and 0 1s Spectra of 1,4-Cyclohexanedione T(eV) C-H C=O

energy (*O.leV)

286.640 288.5 289.7 29 1.Ob 292.5 293.W 301

c 1s assignment EHMO C-H C = O c(eV) I

6.3 2.5 1.3

3.3

-2.5

r*-

3p/C-H* Ryd IP

HAM/3 c(eV) I

-9.2

0.64

-8.1

0.54

-1.8

0.41

1.0

0.40

0.31

4.1

0.66

3p

a*-

IP -8

1.1

a*-

0 1s

EHMO energy (eV)

T (eV)

531.140 535.9 531.9b 545

6.8 2.0 -7

assignment u*-

c

HAM13

(eV) -9.6

0.05

-11.3

I 0.09

1.5

0.01

5.5

0.03

I

t

(eV)

Ryd IP a*-

Calibration: -4.10(4) eV relative to C02. IPSassumed similar to acetone: CI293.71 eV,C2291.23eV.31CCalibration: -4.27(3)evrelative 0

to c02.

chiefly associated with the C ls(C=C) chemical shift, while the reduced intensity is associated with the characteristically lower oscillator strength of C ls(C=C) A*as compared to C ls(C=O) A * C transitions. ~ A C-H* resonance feature is found around 288 eV in the C 1s spectra of all four compounds. In the continuum, there are features at ca. 293 and 302 eV in the spectra of 1, 3, and 4 corresponding to U*CX and u*resonances. The positions of these features are in agreement with predictions of 292 and 300 eV based on the bond length correlation.32 In the spectrum of 2 the feature at 291.5 eV is attributed to O*C+ while that at 298 eV is attributed to O*C< and O*C- resonances, consistent with an enol structure. Other spectral features assigned in Tables 2-5 are described in greater detail in section 3.2. B. 0 1 s Spectra. The 0 1s spectra of 1-4 are presented in Figure 2. The energies, term values, and proposed assignments of the spectral features are given in Tables 2-5. The hatched lines indicate the ionization limits estimated from XPS results for similar m0lecules.3~Since there are six carbon atoms in each species,the relative C 1s spectral intensitiesthrough the molecular series are the same on both total and per-carbon-atom oscillator strength intensity scales. This is not the case for the 0 1s spectra since 1 has only one oxygen atom whereas 2-4 each contain two oxygen atoms. We have chosen to plot the 0 1s spectra on a per-oxygen-atom oscillator strength scale in order to emphasize the similarities of the 0 1s spectra of 1, 3, and 4. The 0 1s spectra of 1,3, and 4 are nearly identical, whereas the 0 1s spectrum of 2 is markedly different. Consistent with the C 1s spectra, we conclude 2 must have a different structure. The lowest energy feature in the 0 1s spectrum of each species is attributed to 0 ls(C=O) A*transitions. The 0 ls(C-0) A * C transition ~ in 2 is only half as intense as the A*C+ features in the 0 1s spectra of 3 and 4, clearly indicating that 2 has one less carbonyl group than the other three species. The broad feature in the 0 1s spectra of 1,3, and 4 peaking at 545 eV is attributed to 0 ls(C=O) U * C resonances. ~ The corresponding u*cpo feature in 2 is much weaker, but 2 exhibits an intense feature at 539 eV which is attributed to 0 ls(C-0) U*C- transitions. Clearly, both the A* and u* regions of the 0 1s spectrum of 2 are consistent with a monoenol structure, as opposed to a diketo structure. Further 0 1s spectral features and their assignments, including the weak features at ca. 534 and 536 eV in 3, are discussed,in conjunction with the calculationalresults, in the following section. 3.2, EHMO and W M / 3 Results: Comparisonto Experiment. A . C 1s Spectra. The EHMO and HAM/3 predictions of the

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-

-

-

-

J

"

I

"

l

~

l

530

"

1

1

540

1

1

'

,

550

Energy (eV)

Figure 2. 0 1s oscillator strength spectra of cyclohexanone (l), 1,4(3),and 1,Zcyclohexanedione cyclohexanedione(4), 1,3-~yclohexanedione (2) derived from electron energy loss spectra. See caption to Figure 1 for further details.

C 1s spectra of 1-4 are presented in Figures 3 and 4, respectively. The energy eigenvalues and relative intensities (charge densities or &:values) derived from the EHMO and HAM/3 for major spectral features are presented in Tables 2-5. In constructing each of the predicted C 1s spectra, the carbon component spectrum with the lowest estimated IP is taken as the absolute eigenvalue energy scale and the other components are displaced relative to it by thecoreionization chemical shift. Thus, thelower horizontal scale in the plots of the EHMO and HAM/3 C 1s predicted spectra corresponds to the eigenvalues of virtual orbitals in the equivalent core molecule correspondingto C 1s(C-H) excitation. An approximate absolute energy scale suitable for comparison to experiment (upper horizontal axis) has been derived by matching the C l s ( C 4 ) A*feature in the predicted spectrum of 4 to its experimental value. The intensities of the predicted spectra for each species are proportional to the calculated charge densities (EHMO) or orbital coefficients(HAM/3) (Le., they have not been normalized individually to experiment). The overall scaling of the vertical axes has been chosen so as to obtain the best overall agreement with the experimentalintensities. Thus, comparison of the experimental spectra with each calculation over the series of four molecules is a valid means of evaluating the relative merits of the two calculations. Overall in the discrete region (below C 1s IPS)HAM/3 eigenvaluesare somewhatcloser than the EHMO eigenvalues to the experimental term values. However, in the continuumregions the EHMO eigenvalues display a better relative agreement than HAM/3 with the experimental term values. The relative positions of the main features in both the EHMO and HAM/3 spectra are in good agreement with the main experimental features, and thus the calculations provide a reliable basis for spectral assignments. An exception to this is features corresponding to excited states with a large Rydberg character. These are not reproduced by the EHMO or HAM/3 calculations since the basis sets do not include n L 3 atomic orbitals which correspond to Rydberg molecular orbitals. The shapes of the

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3654

Francis and Hitchcock

The Journal of Physical Chemistry, Vol. 98, No. 14. 1994 280 I

J

Approximate Transition Energy (eV) 290 300 310 "

l

I

I

l

~

I

l

-10

'

I

l

'

l

I

l

I

~

I

~

I

l

0

I

'

l

280

I

l

~

i

l

I

J

C-H EHMO Eigenvalue (eV)

proximately to experiment.

-

EHMO and HAM/3 spectra of 1,3, and 4 are very similar. The C ls(C=O) ? r * c d feature in the predicted spectrum of 1and 2 is approximately half as intense as the corresponding ?r*feature in 3 and 4. Note that the ?r*intensity ratios for different molecules predicted by HAM/3 are in somewhat better agreement with experiment than the EHMO results (e.g., the ?r*cd features in 1 and 2 are almost exactly 50% as intense as the same features in 3 and 4). Both EHMO and HAM/3 predict that the C 1s spectrum of 2 is markedly different from those of 1,3, and 4, consistent with the experimental results and in support of our interpretation that 2 has a monoenol structure in the gas phase. Both semiempirical methods reproduce the main features in the discrete region, in particular the fact that the C ls(C=C) r*~< transition lies ca. 2 eV below and is weaker than the C l s ( C 4 ) x*transition. Two weak features at 281.6 and 289.5 eV in the spectrum of 2 are assigned to C ls(C=O) ?r*w and C ls(C=C) ? r * c , transitions ~ on the basis of the EHMO and HAM/3 orbital coefficients. The MO calculationsindicate there is some interaction (delocalization) between ?r*and T*M, and thus there is some density on all three C atoms in each r* orbital, although the dominant character of each x* orbital is unambiguous. The features at 288 eV in the experimental spectra of 1,3, and 4 can be attributed to C ls(C-H) excitations to upper levels of 3p/C-H* mixed Rydberg-valencecharacter. The experimental C 1s spectrum of 2 also displays a similar feature at 288.4 eV which is comparatively less discernible owing to overlap with adjacent preionization features. The absence of these features in both the HAM/3- and EHMO-predicted C 1s spectra is consistent with a significant Rydberg character. Similar

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1

I

I

I

I

I

I

0

I

I

I

'

I

I

I

l

10

C-H HAM/3 Eigenvalue (eV)

Figure 3. C 1sspectra of 1,4-~yclohexanedione (4), 1,3-~yclohexanedione (3), cyclohexanone (l),and 1,2-~yclohexanedione (2) derived from EHMO calculations. Each spectrum is the sum of component spectra calculated for each symmetry-inequivalentcarbon,with appropriate chemicalshifts introduced prior to summation. The upper scale corresponds ap-

-

I

-10

10

-

Approximate Transition Energy (eV) 280 300 310

-

Figure4. C 1sspectra of 1,4-cyclohexanedione(4), 1,3-cyclohexantdionc (3), cyclohexanone (l), and 1,2-cyclohexanedione (2) derived from HAM/3 calculations. See caption to Figure 3 for further details. 3p/C-H* resonances are observed at 288 eV in the C 1s spectra of saturated hydrocarbon^,^^ which further supports our assignment. Both the HAM/3 and EHMO spectra of 1,3, and 4 display U*GC resonancesin the near continuum. EHMO tends to "bunch up" the u* intensity toward the discrete while HAM/3 tends to spread out the u* intensity and to provide more intensity in the higher energy continuum. Both the EHMO- and HAM/3predicted spectra of 2 display a g * c a resonance in the near continuum, while HAM/3 also reproduces the higher energy u*feature in 2 which is observed experimentally in all four species at 302 eV. B. 0 Is Spectra. The EHMO and HAM/3 predictionsof the 0 1s spectra of 1-4 are presented in Figures 5 and 6,respectively. The lower horizontal axes correspond to the energy eigenvalues of the equivalent core molecule corresponding to 0 l s ( C 4 ) excitation while the upper horizontal axes correspond to an approximate transition energy scale obtained by aligning the 0 ls(C=O) r*- feature in 1with its experimental value. The vertical axes correspond to a per-oxygen-atom intensity scale, i.e., the 0 1s spectra of 2 are constructed as ((S01,cc-o) + S o l ~ ~ ) / 2inJorder ) to match the per-oxygen-atomnormalization used in presenting the experimental 0 1s spectra. The EHMO and HAM/3 eigenvaluesand intensities (charge densitiesor EC$ values) are given in Tables 2-5 for the major features. As for the C 1s spectra, the EHMO and HAM/3 0 1s spectra reproduce the main spectral features and experimental trends. The predicted spectra of 1, 4, and 3 derived from both sets of calculations are very similar, consisting of an intense, low-energy x*feature and a broad u*resonance in the continuum. The EHMO and HAM/3 spectra of 2 are markedly different from thoseof 1,3, and 4, as observed experimentally. The intensity of the lowest energy x * c 4 feature in 2 is much less than that of the corresponding features in the spectra of 1,4, and 3. This is consistent with experiment, although the intensity is somewhat +

The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3655

Distinguishing Keto and Enol Structures

Approx. Energy (eV)

Approximate Transition Energy (eV) ,

,

,

,

530

550

540

530 , I ,

,

I

,

,

,

,

/

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550

540 ~

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-A I

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1

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,

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l

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n

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- 10

0 10 C = O EHMO Eigenvalue (eV)

-10

I

I

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0

I

,

,

10

C = O HAM/3 Eigenvalue (eV)

Figure 5. 0 1s spectra of cyclohexanone, 1,4-cyclohexanedione, 1,3cyclohexanedione, and 1,2-~yclohexanedionederived from EHMO calculations. See caption to Figure 3 for further details.

Figure 6. 0 1s spectra of cyclohexanone (l),1,4-cyclohexanedione(4), 1,3-~yclohexanedione (3), and 1,2-cyclohexanedione(2) derived from HAM/3 calculations. See caption to Figure 3 for further details.

underestimated by HAM/3. There are two weak T* features in the preionization region of 2 predicted by both EHMO and HAM/3 calculations. We believe these correspond to the 534.4and 535.9-eV experimental features. In 2 the 0 ls(C-0) and 0 ls(C-0) core levels are separated by 1.5 eV. In addition there are two virtual T* orbitals, one predominantly T*and the other predominantly u*- in character, but with each orbital having some admixture of the other. Thus, there should be four 0 1s T * transitions in 2. However, in the context of our preferred assignment, it would appear that two of these 0 1s T * transitions overlap to form the 534.4-eV feature and hence three T * features are observed in the experimental 0 1s spectrum of 2, with the two higher energy ones of mixed A*C,O/A*C+ character. HAM/3 predicts a weak second P* feature in 3, but its counterpart in the EHMO spectrum is very weak. This may correspond to the weak feature detected experimentally at 534 eV in 3. Based on the MO calculations we attribute the 534-eV feature in 3 to 0 ls(C=O) u*-(-) transitions, i.e., an 0 1s excitation to the higher energy of two T*-(+)/T*-(-) levels which are split by interaction between the two carbonyl moieties in 3. If this is correct, the absence of a corresponding feature in 4 would then reflect the larger distance between the carbonyl moieties and thus a weaker interaction and smaller ~ * c d ( + ) / u * - ( - ) splitting in 4. Both the EHMO and HAM/3 spectra of 2 reproduce the intense, near continuum u*ca resonance observed experimentally at 540 eV. In general, the continuum structure is weaker in 0 1s spectra derived from EHMO than from HAM/3. Note that only resonant continuum features are predicted by the semiempirical EICVOM approach. For more precise comparisons to experiment, a direct ionization continuum such as that predicted for atomic oxygen should be added, as has been the case in some previous presentations of EHMO core excitation studies.*' To further substantiate our interpretations of these spectra concerning the preferred gas-phase structures of 2 and 3, C 1s

and 0 1s spectra were derived from EHMO calculations carried out on optimized geometries of the diketo form of 2 (2b)and the monoenol form of 3 (3a). The predicted C 1s spectrum of 2b is very similar to the spectra calculated for the diketo forms of 3 and 4, displaying one prominent T*feature in the discrete and an absence of any u*c4 features in the continuum. Similarly, the predicted 0 1s spectrum of 2b closely resembles those of the diketo forms of 3 and 4. The predicted C 1s spectrum of 3a exhibits a reduced A*intensity and a low-lying C ls(C=C) T*transition, as is found in the predicted spectra of the monoenol form of 2 and the experimental spectrum of 2. The predicted 0 1s spectrum of 3a is markedly different than that of 4, exhibiting additional T* features and a prominent U * C ~ resonance in the near continuum much like those observed in the experimental 0 1s spectrum of 2 and the predicted spectrum of 2a. Thus, thecalculations fully support the deductions concerning the preferred tautomer of 2 and 3 in the gas phase and demonstrate that core excitation is sensitive to the differences between keto and enol structures.

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4. Core Excitation Spectra Can Distinguish Keto and Enol Structures This work demonstrates that there are core excitation features characteristic of keto and enol structures. Specifically, a lowenergy C ls(C=C) T*feature and a C ls(C-0) U*near continuum resonance along with 0 l s ( 0 H ) U * C ~and 0 l s ( 0 H ) T*C-C/C-O features are highly indicativeof an enol structure. The presence of these features in the C 1s and 0 1s spectra of 2 firmly supports a monoenolic form of 2 in the gas phase, in agreement with the results of calculation^^^ and the prediction made by Bouchoux et al. based on their gas-phase basicity re~u1ts.l~The dominant gas-phase structure of 2 is thus identical to that of 2 in solution.35 Conversely, the absence of enolic features in the C 1s and 0 1s spectra of 3 is strong evidence that 3 prefers a diketonic structure in the gas phase. In addition,

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Francis and Hitchcock

3656 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 since 4 shows no enolic pr0perty3s-3~and molecular beam studies by Dowd et al.37 indicate that a chair-like diketone form is favored in the gas phase, the strong spectral similarities between 3 and 4 also firmly support a dominant diketone form for 3 in the gas phase. Thus, our results for 3 are in agreement with those of Pilcher et al.15 but do not support the prediction of Bouchoux et al.17 Pilcher has calculated thediketo formof 3 to be energetically more stable than the monoenol form by 18.0 kJ/mol.lS Ignoring entropy considerations, this corresponds to an equilibrium constant of 7 X 10-4 for the reaction 3b 3a at 298 K, indicating that negligible amounts of the monoenol form of 3 are expected. Although our results clearly demonstrate that 3prefers a diketo form in the gas phase, the equilibrium estimate based on Pilcher’s calculation~smay be in error, and thus the presencs of a small amount of the enol form of 3 cannot be discounted on these grounds. The small feature at 534 eV (which we currently attribute to a split ? r * c d feature), in addition to the slight difference in the experimental 0 1s spectrum of 3 around 536 eV compared to the spectra of 1 and 4, could be associated with a small amount of the mono-enol tautomer present at equilibrium. The position of keto-enol equilibria is known to be temperature sensitive, as has been demonstrated for 2,4-pentanedione by Hush et al.12 Recently Oichi et al.41have reported that the N 1s and 0 1s specta of solid bis(2-hydroxy- 1-naphthy1idene)-p-phenylenediamine displays a temperature dependence whereby the changes in the near edge spectral features track a known tautomeric transition. The energy loss spectrometer used in this work can be operated at sample temperatures ranging from 30 to 300 OC. Thus, it would be possible torecord inner-shell energy loss spectra of 3 at variable temperatures to search for signals associated with the monoenol tautomer which should be increasingly important at higher temperatures. If the spectrum is found to be temperature dependent, then it should be possible to deconvolute the results to estimate the relative amounts of 3a and 3b and thus experimentally measure the equilibrium constant for the gas-phase tautomerization reaction. In the absence of definitive-temperature-dependent measurements, we prefer to assign the 0 1s spectrum of 3 solely in terms of excited states of the diketo form. 4.1. Comparison of Core Excitation to Other Techniques. W a l z P et al. have reported the valence-shell EELS spectra of 2 and 4. This data shows that it would be difficult to identify the molecular structure as being diketo or monoenol on the basis of the spectral features alone. This is due to the fact that relatively large molecules such as these have associated with them a large number of possible ?r ?r* transitions in a narrow energy range. Thevariable temperature work by demonstrates that UPS can distinguish keto and enol structures. However, the UPS studiesl8J9~20of 3, which were interpreted without the aid of ab initio calculations, could not provide unambiguous identification of the dominant tautomer in the gas phase. This is in distinct contrast to the core excitation results, where even a qualitative spectral interpretation (section 3.1) suffices to distinguish diketo and monoenol structures. Clark and Haris~on3~ recorded the C 1s and 0 1s X-ray photoelectron spectra (XPS) of 2,4-pentanedione (related to 3) in an attempt to use XPS to determine the preferred tautomeric form. No firm conclusions could be made. This is consistent with other XPS results, which show there is very little difference between the C ls(C=C) and C ls(C-C) or between the C ls(C=O) and C ls(C-0) IPS. Bouchoux et al.I7 used ion cyclotron resonance (ICR) mass spectral results to predict that 1,3-~yclohexanedioneis predominately monoenolic, 3a, in the gas phase, even though they had also noted that semiempirical estimation of the relative heats of formation using Benson’s increment40 predicts that the diketo tautomer, 3b, is more stable. As the present results clearly demonstrate, 3b is the more stable tautomer; thus the prediction of Bouchoux et al. is incorrect. Their interpretation was based on the assumption that the structure of the most stable protonated

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product reflects the structure of a monoenolic neutral species. In light of our core excitation results, we suggest that isomerization of 3b occurs upon protonation to yield the monoenolic tautomer. Therefore, whereas the protonated monoenol species 3a may be the most stable species in a gas-phase basicity measurement,” it is the diketo species 3b which is in fact the most stable neutral structure.

5. Summary Core excitation spectroscopy has been demonstrated to be a useful tool for distinguishing diketo and monoenol structures in the gas phase. We have shown that 1,2-cyclohexanedione has a dominant monoenolic structure while 1,3-~yclohexanedione prefers a diketonic structure. EHMO and HAM/3 semiempirical calculations within the EICVOM approximation reproduce the main features of the experimental spectra and fully support the proposed spectral assignments. Quantitative comparison to experiment shows (perhaps surprisingly) that the EHMO results are somewhat superior to those from HAM/3, in regards to relative intensities and energies of the spectral features. However, both calculations provide a similar level of useful assistance to the assignment of these core excitation spectra.

Acknowledgment. Financial support for this research has been provided by NSERC (Canada). J.T.F. gratefully acknowledges the support of an NSERC postgraduate scholarship. Special thanks are due to Prof. J. K. Terlouw for his helpful discussions, to Dr. Ian Bytheway and Dr. Todd Keith for their invaluable assistance with the geometry optimization calculations, and to Mr. Daren LeBlanc for assistance with NMR characterization. The EHMO calculations were carried out using a package provided by C. Meali. References and Notes (1) Ingold, C. K. Structure and Mechanism in Organic Chemistry;Cornell University Press; New York, 1953. (2) Forsen, S.;Nilsson, M. The Chemistry of the Carbonyl Group, Vol. 2;John Wiley: London, 1970. (3) Solomons, T. W. G. Fundamentals of Organic Chemistry, 2nd 4.; J. Wiley and Sons: New York, 1986. (4) Green, M. In Roddk Chemistry of Carbon Compounds; Coffey, S., Ed.; Elsewer: Amsterdam, 1968;Vol, 2, p 108. (5) Katrusiak, A. Acta Crvsr. 1990,847,398. (6) Etter, C. E.;Urbanczik-Lipkowska, Z.; Jahn, D. A.; Frye, J. S.J. Am. Chem. Soc. 1986,108,5871. (7) Etter, M. C.; Parker, D. L.; Ruberu, S.R.; Panuto, T.W.; Britton, D. J. J . Inclusion Phenom. Mol. Recoanit. Chem. 1990.8. 395. (8) Allinger, N. L.; Frieberg, L. J . Am. Chem. SOC.1961,83,5028. (9) Mossel, A.; Romers, C.; Havinga, E. Tetrahedron Lett. 1963,1247. (10) Moriyasu, M.; Atsushi, K.; Hashimoto, Y. Bull. Chem. Soc. Jpn. 1988,61,2955. (11) Meyer, K. H.Justus Liebigs Ann. Chem. 1911,380,212. (12) Hush, N. S.;Livett, M. K.; Peel, B. J.; Willett, G. D. Aust. J. Chem. 1987, 40, 599. (13) References 2 through 6 cited in ref 11. (14) Duus, F. J . Am. Chem. Soc. 1986, 108,630. (15) Pilcher, G.; Parchment, 0. G.; Hillier, I. H.; Heatley, F.; Fletcher, D.; Ribeiro da Silva, M. A. V.; Ferrao, M. L. C. C. H.; Monte, M. J. S.;Jiye, F. J . Phys. Chem. 1993,97,243. (16) Lowrey, A. H.; George, C.; D’Antonio, P.; Karle, J. J . Am. Chem. SOC.1971,93,6399. (17) Bouchoux, G.; Hoppilliard, Y.; Houriet, R. New J . Chem. 1987,IZ, 225. (18) Houk, K. N.; Davis, L. P.; Newkome, G. R.; Duke, R. E. Nauman, R. V. J . Am. Chem. SOC.1973,95, 8364. (19) Dougherty, D.; Brint, P.; McGlynn, S.P. J. Am. Chem. Soc. 1978,

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100. _ _ 5597. _,

(20) Gleiter, R.; Hofmann, P.; Schang, P.; Sieber, A. Tetrahedron 1980, 36,655. (21) Francis, J. T.;Hitchcock, A. P. J . Phys. Chem. 1992,96,6598. (22) Schwarz, W. H. E. Chem. Phys. 1975,II,217. Tossell, J. A. Chem. Phys. 1991, 154, 211. (23) Wen, A. T.;Riihl, E.; Hitchcock, A. P. Organometallics 1992,11, 2559. -...

(24) Hitchcock, A. P.; Urquhart, S.G.; Rightor, E. G. J . Pkys. Chem. 1992,96,8736. (25) Meali. C.: Proserdo, D. M. J . Chem. Educ. 1990,67. 399. (26) Asbrink, L.; Fridh, C.; Lindholm, E. Chem. Phys. Lett. 1977,52,63, 69,72.

Distinguishing Keto and Enol Structures (27) Lindholm, E.; Asbrink, L. Molecular Orbitals and Their Energies, Studied by the Semiempirical HAM Method; Lecture Notes in Chemistry, Vol. 38; Springer-Verlag: Heidelberg, 1985. (28) Hitchcock, A. P. Phys. Scr. 1990, T31, 159. (29) Standard Spectra Collection;Sadtler Research Laboratories Division, Bio-Rad Laboratories, USA.,1980. Proton NMR spectra numbers: 10208 (I), 23185 (2). 22766 (3), and 559 (4). (30) Hitchcock, A. P.; Mancini, D. C . J . Electron Spectrosc., in press. (31) Jolly, W. L.; Bomben, K. D.; Eyermann, C. J. At. Data. Nucl. Data Tables 1984, 31, 433. (32) Sette, F.; Stbhr, J.; Hitchcock, A. P. Chem. Phys. Lett 1984, 110, 517. (33) Stbhr, J.; Outka, D. A.; Baberschke, K.; Arvanitis, D.; Horsley, J. A. Phys. Rev. B 1987, 36, 2976.

The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3657 (34) Naock, W. E. Theor. Chem. Acta 1979,531, 101. (35) Olah, G. A,; Grant, J. L.; Westerman, P.J . Org. Chem. 1975, 40, 2102. (36) Hesse, G. InMethodender OrganischenChemie,Vol. 6-1d;HoubenWeyl, G.; Ed.; Theime Verlag: Stuttgart, 1978. (37) Dowd, P.;Dyke, T.; Klemperer, W. J. J. Am. Chem. SOC.1970,92, 6327. (38) Walzl, K.N.; Xavier, I. M.; Kupperman, A. J . Chem. Phys. 1987, 86, 6701. (39) Clark, D. T.; Harrison, A. 0. J . Elec. Spec. 1981, 23, 39. (40) Cohen, N.; Benson, S.W. Chem. Rev. 1993, 93, 2419. (41) Oichi,K.;Ito,E.;Seki,K.;Araki,T.;Narioka,S.;Ishii,H.;Okajima, T.; Yokoyama, T.; Ohta, T.; Inabe, T.; Maruyama, Y.Jpn. J . Appl. Phys. 1993, 32 (Suppl. 32-2), 818.