A study of the molecular structure and spectroscopic properties of tetrahydro[4]beltene and related compounds

Chemical Physics 242 Ž1999. 203–216

A study of the molecular structure and spectroscopic properties of tetrahydrow4xbeltene and related compounds V. Galasso

a,)

, W. Grimme b, J. Lex b, D. Jones c , A. Modelli

c

a Dipartimento di Scienze Chimiche, UniÕersita` di Trieste, I-34127 Trieste, Italy UniÕersitat ¨ zu Koln, ¨ Institut fur ¨ Organische Chemie, Greinstraße 4, D-50939 Koln, ¨ Germany Istituto dei Composti del Carbonio contenenti Eteroatomi e loro Applicazioni, C.N.R., Õia Gobetti 101, I-40129 Bologna, Italy b

c

Received 6 November 1998

Abstract The equilibrium structure of hexacyclow8.7.0.0 3,8.0 5,15.0 6,13.0 12,16 xheptadeca-1Ž10.,5-diene-3,8-dicarboxylic anhydride, a tetrahydrow4xbeltene with two s-conjugated double bonds, and of two derivatives, the first with only one double bond and the second with a closed annular belt, was calculated at the RHFr6-31G UU ab initio level of theory. The results, consistent with the available X-ray experimental data, account for an efficient homoconjugation in the two compounds with an open belt. The NMR 13 C chemical shifts were analyzed by means of ab initio CSGT Žcontinuous set of gauge transformations. calculations performed with the B3LYPr6-311q GŽ2d, p. DFT-HF hybrid functional model. The HeŽI. photoelectron spectra were measured and interpreted by means of ab initio OVGF Žouter valence Green function. calculations, which give an overall consistent reproduction of the energies and splittings of the uppermost bands. These are associated with the p ŽC5C. andror Walsh orbitals of the equatorial belt and with the nŽCO. lone pair orbitals of the anhydridic moiety. Electron transmission spectroscopy was employed to characterize the low-lying temporary anion states of tetrahydrow4xbeltene. The single-crystal X-ray structure was also determined for the trisŽdemethano.pagodane derivative. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Carbocycles with molecular belts formed by ring closure of a ladder-type array of n linearly condensed cyclohexa-1,4-diene rings display peculiar structural and spectroscopic properties w1,2x. Of these novel systems, referred to as w n xbeltenes by Alder and Sessions w2x, the hexacyclow8.7.0.0 3,8.0 5,15.0 6,13.0 12,16 xheptadeca-1Ž10.,5diene-3,8-dicarboxylic anhydride ŽTB. has been recently synthesized and fully characterized w3x. The core of this molecule is tetrahydrow4xbeltene, in which the two p bonds are fixed face-to-face at a close distance in a very rigid s-framework. Also, two related compounds have been prepared by Grimme et al. w3x. Indeed, catalytic hydrogenation of TB under a pressure of 5 bar yielded hexahydrow4xbeltene ŽHB., while UV irradiation of TB in the presence of xanthone gave rise to trisŽdemethano.pagodane ŽTDP..

)

Corresponding author. E-mail: [email protected]

0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 9 . 0 0 0 1 1 - 7

204

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

Given their different arrangements of the molecular belt, but having the anhydridic moiety in common, a combined investigation of the structural and spectroscopic properties of all three compounds therefore seemed timely. In the present paper, we report on the ab initio calculation of the equilibrium molecular structure at the HFr6-31G UU level of theory. Since NMR chemical shifts are very efficient monitors of the unique stereochemistry of these molecules, their d Ž13 C. spectroscopic parameters were recorded and studied using the ab initio continuous set of gauge transformations ŽCSGT. method, implemented with the B3LYP exchange-correlation DFT-HF hybrid functional. The electronic structures were also investigated by measuring the HeŽI. photoelectron ŽPE. spectra of TB and TDP, which were interpreted by means of ab initio many-body calculations using the outer valence Green function ŽOVGF. method. Electron transmission spectroscopy ŽETS. is complementary to PES, providing the various electron affinities ŽEAs. of a sample gas. This technique was employed to locate the temporary anion states associated with electron capture into the empty molecular orbitals ŽMOs. of TB and related simple compounds.

2. Computational and experimental details The equilibrium structures were completely optimized at the RHFr6-31G UU level of theory using the Gaussian-94 suite of programs w4x. This polarized basis set is a good compromise between the size of the calculations and the accuracy of the theoretical results. The 13 C NMR shielding constants were calculated at the ab initio level with the CSGT method w5x, using the 6-311 q GŽ2d, p. basis set and the B3LYP DFT-HF hybrid functional w6x, which takes into account the electron exchange-correlation effects. The calculated shieldings were converted to the chemical shifts by noting that at the same level of theory the 13 C shielding in TMS is 179.59 ppm. The vertical ionization energies ŽIEs. were calculated at the ab initio level according to Cederbaum’s OVGF method w7x, which includes the effects of electron correlation and reorganization beyond the Hartree–Fock approximation. The self-energy part was expanded up to third order and the contributions of higher orders were estimated by means of a renormalization procedure. In order to calculate the self-energy part, the 45 highest occupied valence MOs and the 80 lowest virtual MOs were considered, by using the 6-31G basis set. The compounds were synthesized as previously described w3x. 13 C NMR spectra were measured in CDCl 3 solution at 75.5 MHz on a Bruker AM 300 spectrometer. The HeŽI. spectra of TB and TDP were recorded on a Perkin Elmer PS-18 photoelectron spectrometer connected to a Datalab DL4000 signal analysis system. The bands, calibrated against rare-gas lines, were located using the position of their maxima, which were taken as corresponding to the vertical ionization energy ŽIE. values Ž"0.05 eV.. The vertical electron attachment energy ŽAE. values, i.e. the negative of EAs, of TB and some related simple dicarbonyls were measured by means of ETS. Our apparatus is in the format devised by Sanche and Schulz w8x and has been previously described w9x. To

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

205

enhance the visibility of the variations in the electron scattering cross-section due to resonance processes, the impact energy of the electron beam is modulated with a small ac voltage, and the derivative of the electron current transmitted through the gas sample is measured directly by a synchronous lock-in amplifier. The spectra were obtained in the ‘high-rejection’ mode w10x, and are therefore related to the nearly total scattering cross-section. The electron beam resolution was ; 50 meV ŽFWHM.. The energy scales were calibrated with reference to the Ž1s 2 2s1 . 2 S anion state of He. The estimated accuracy is "0.05 or "0.1 eV, depending on the number of decimal digits reported.

3. Results and discussion 3.1. Equilibrium geometries A selection of the most relevant geometrical parameters of all three molecules are presented in Table 1 Ža full listing may be obtained from the authors on request.. As expected, there is a close resemblance in the patterns of most of the structural data. For TB and TDP the calculated structural parameters are in satisfactory agreement with the experimental X-ray data obtained by Grimme et al. w3x; lack of experimental data, however, precludes a similar comparison for HB. All molecules display a rigid cage structure of C s symmetry, where the fused anhydridic moiety forms a five-membered ring which is essentially planar. The principal structural feature that characterizes the equilibrium conformation of these anhydrides is the arrangement of their equatorial belt, whose most representative parameters are calculated to be the following:

˚ Žin full agreement with the X-ray data w3x. It is worth stressing that in TB the transannular distance of 2.92 A between the two ethene units stacked face-to-face is remarkably smaller than the sum of the van der Waals radii ˚ This tightening of the belt is alleviated to some extent by the pyramidalization of the double bonds by 3.40 A. 16.58 and 12.58, respectively. Efficient homoconjugation through the cage skeleton of TB is manifested in a long-wavelength UV absorption and in the formation of a stable radical cation at low potential w3x.

206

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

Table 1 ˚ angles in 8. Theoretical HFr6-31GUU geometries Žbond lengths in A,

a b

Ref. w3x. Present work.

On passing from TB to HB, with formation of an ethano group the planes 1–2 and 2–3 become more tilted ˚ and but they do not reach the normal dihedral angle of 1208. As a consequence, the belt is widened by 0.35 A the homoconjugation between the central units is weakened. On the other hand, in TDP the regular ring closure of the belt reduces the distance between the two opposite central aC–C b units to a normal single-bond value. The covalent belt is nearly square and the tilting of the planes is inverted to 211.98 and 216.58, respectively,

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

207

; 258 less than the normal value of 2408. For the corresponding pagodane-type hydrocarbons the theoretical ˚ and parameters of the equatorial belt, as defined above, are predicted at the same level of theory to be 2.905 A ˚ 146.718 and 173.638 for ‘pagoda-ene’, and 1.555 A˚ and 209.218 Žexp. 1.55 174.038 for ‘pagoda-diene’, 3.286 A, ˚ and 208.68 w11x. for pagodane itself. Therefore, compared with the beltene-type molecules, the transannular A distances are similar, while the bending angles are less acute in the unsaturated pagodanes. As a consequence of their peculiar stereochemistry, the patterns of the spectroscopic properties of these molecules exhibit important similarities and differences. Among the various spectroscopic observables, NMR chemical shifts, IEs and EAs are very efficient monitors of the complex interplay of structural and electronic effects operating in a molecule. Therefore, we report here on these properties. 3.2. Chemical shifts The results of the ab initio CSGT calculations are reported in Table 2. It must be noted that highly accurate predictions of the d Ž13 C. observables require very large basis sets and sophisticated treatment of electron correlation effects, but for the present medium-sized molecules these requirements are computationally prohibitive. However, for all three molecules, the present theoretical results are in substantial accord with the spectroscopic data. A comprehensive reproduction of the absolute values and main trends in the 13 C-NMR spectra has been obtained. In particular, for TB, the high deshielding of the carbonyl carbon atom and the difference of 6.6 ppm in the chemical shifts between the two ethenic carbons are accounted for. Compared with the relevant chemical shift of 155.4 ppm of disecododecahedradiene w12x, a diene system derived from pagodane, both ethenic chemical shifts of TB Ž130.5 and 137.1 ppm. are lower, as a result of less transannular p, p interaction. With regard to TB turning to HB, the most remarkable aspect of the NMR data is the large upfield displacement Ž107.7 ppm. of the C 1 resonance, which is due to the saturation of the original double bond. In TDP, the higher resonance belongs to the more distorted carbon in the belt ŽC 5 .; it falls 5.3 ppm upfield relative to the related pivotal carbons of pagodane Ž62.9 ppm w13x.. The carbonyl resonance along the series of molecules exhibits the variation HB ) TB ) TDP. The small range of only 3.4 ppm reflects a comparatively efficient stereoelectronic interaction between the cage and the anhydridic moiety. In this connection, it is worth mentioning that the d ŽCO. values are very similar to those reported for the related simple systems: succinic anhydride Ž172.5 ppm w14x. and hexahydro-1,3-isobenzofurandione Ž174.3 ppm w15x.. As expected, a sizeable deshielding instead occurs for the bridgehead carbon C 3 in the

Table 2 NMR chemical shifts relative to TMS Žppm. Nucleus

C1 C2 C3 C4 C5 C11 C12 C13 C14 C18 a b

Ref. w3x. Present work.

TB

HB

theor.

expt.

a

136.0 41.9 55.4 34.9 141.4 32.5 40.4 52.4 46.4 173.3

130.5 42.8 54.9 35.9 137.1 33.4 38.4 51.5 48.2 175.1

TDP

theor.

expt.

a

theor.

expt.b

25.7 39.2 49.1 33.3 142.4 29.0 36.7 51.7 47.1 174.6

22.8 40.0 48.6 34.6 138.4 29.6 34.9 51.0 48.5 176.3

49.8 33.3 56.6 29.6 57.0 26.3 49.3 46.2 31.2 170.8

49.7 34.5 56.8 30.7 57.6 27.7 48.9 45.3 33.2 172.9

208

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

beltenes relative to the latter two molecules Ž29.2 w14x and 39.8 ppm w15x, respectively., as a result of polycyclization. 3.3. Ionization energies In the region of the top MOs, the following aspects of the electronic structure of the present molecules are of special interest: Ž1. the nature and energy values of the MOs associated with the molecular belt; Ž2. the splitting of the two n MOs due to the carbonyl lone pairs; and Ž3. the behaviour of the p MO localized mainly on the bridgehead oxygen atom. Accordingly, corresponding bands are expected in the low-energy region of the PE spectra. All three molecules have one important feature in common: their HOMO and HOMO-1 represent the bonding within the belt, but by means of different combinations of orbitals semilocalized in the two opposite aC–C b units. A schematic picture of these MOs is the following:

Specifically, in TB, they can be labelled pq ŽC5C. ŽHOMO-1, in-phase combination of the two double bonds with bonding homoconjugation. and py ŽC5C. ŽHOMO, out-of-phase combination with antibonding homoconjugation.. In HB, the HOMO is essentially localized on the unique ethenic bond p ŽC5C., while the HOMO-1 involves some bonding p-conjugation between the two central units. By contrast, in TDP, the nearly degenerate HOMO-1 and HOMO give rise to the s-bonding through the square belt. On the other hand, the 1,3-dicarbonyl moiety generates the two semilocalized MOs nq ŽCO. Žsymmetric combination of the two CO lone pair orbitals. and ny ŽCO. Žantisymmetric combination.. In the low-energy region of the PE spectrum ŽFig. 1., TB shows three bands peaked at 7.90, 9.20 and 9.65 eV. In addition, the onset of a prominent, overlapping band system starts at 10.2 eV, with a first component centred at ; 10.5 eV. The ab initio Koopmans’ theorem ŽKT. values, i.e. orbital energies, OVGF IEs, pole strengths ŽPSs. and assignments for all three compounds are presented in Table 3. The PSs for all photoionizations are ; 0.9, which

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

209

Fig. 1. Ultraviolet HeŽI. photoelectron spectrum of TB.

indicate the reasonable validity of the one-particle model for these processes. It should be remembered that the search for highly accurate agreement between calculated and experimental IEs requires the use of very large basis set and exhaustive treatment of the particle space. These two requirements are, however, computationally too demanding for the present medium-sized molecules. As a yardstick for assessing the quality of the theoretical reproduction of the compounds of interest, the quite satisfactory results obtained for the related simple succinic anhydride system are also reported in Table 3. In the case of TB, the first two bands are associated with the p " ŽC5C. photoionizations. Their splitting of 1.30 eV is a measure of an effective homoconjugation between the two bridged p bonds. This splitting is, however, smaller than the 1.9 eV value of disecododecahedradiene w13x, a related molecule derived from ˚ . of the two pagodane which has a perfect synperiplanar arrangement and closer distance Ž2.77 vs. 2.92 A opposing p bonds. With respect to disecododecahedradiene, the p " MOs of TB are inductively stabilized by 0.8 eV ŽHOMO. and 0.2 eV ŽHOMO-1.. The next two uppermost bands, attributed to the n " ŽCO. photoemissions, are split by 0.85 eV. The photoionization originating from the p ŽO. MO is connected with the unresolved band centred at ; 11.5 eV. However, a congested srp orbital manifold is responsible for the complex broad-band system starting from 10.2 eV. The most salient aspect in the PE spectrum of HB, which has not been measured, as compared with TB is the uppermost p ŽC5C. band predicted at ; 8.4 eV. This is approximately midway between the two related uppermost bands of the precursor TB, but slightly higher than the 8.1 eV of disecoene w18x, a related molecule

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

210 Table 3 Vertical ionization energies ŽeV. Molecule

MO

KT

IE

PS

IE exp

TB

X 46a Ž pq . X 45a Ž py . Y 32a Žny . X 44a Žnq . Y 31a Y 30a X 43a Y 29a X 42a X 41a Ž p O . X 40a Y 28a X 39a Y 27a X 38a Y 26a X 37a Y 25a X 36a Y 24a X 35a Y 23a Y 22a X 34a X 33a Y 21a

8.45 10.08 11.16 12.12 12.34 12.41 12.51 12.76 12.77 12.98 13.26 13.47 13.75 13.79 14.36 14.70 14.81 14.95 15.15 15.34 15.47 15.85 16.08 16.51 16.75 17.10

7.85 9.30 9.70 10.74 11.04 10.83 10.95 11.39 11.22 11.42 11.94 12.03 12.11 12.37 12.80 13.16 13.15 13.37 13.36 13.83 13.77 14.40 14.50 14.58 15.04 15.15

0.91 0.91 0.91 0.91 0.91 0.90 0.91 0.91 0.91 0.90 0.91 0.91 0.91 0.91 0.91 0.91 0.90 0.91 0.90 0.91 0.90 0.89 0.91 0.89 0.90 0.90

7.90 a 9.20 9.65 10.5 10.7 . . . . . . . . 12.3 12.5 . . . . . 13.7

9.04 11.06 11.36 11.62 12.21 12.39 12.43 12.76 12.87 12.91 12.93 13.36 13.52 13.70 14.04 14.15 14.57 14.62 14.89 15.16 15.47 15.86 16.15 16.27 16.41 16.64

8.38 9.71 9.99 10.35 10.76 10.72 11.02 11.36 11.53 11.38 11.54 11.90 12.15 12.22 12.57 12.61 13.15 12.95 13.34 13.48 13.96 14.35 14.61 14.71 14.51 14.96

0.91 0.92 0.92 0.92 0.91 0.91 0.92 0.91 0.92 0.90 0.91 0.91 0.92 0.92 0.91 0.91 0.92 0.90 0.91 0.91 0.91 0.90 0.91 0.91 0.90 0.91

HB

X

45a Ž p . Y 34a Y 33a Žny . X 44a Y 32a X 43a Žnq . X 42a Y 31a X 41a X 40a Ž p O . Y 30a X 39a Y 29a X 38a Y 28a X 37a Y 27a X 36a Y 26a X 35a Y 25a Y 24a X 34a Y 23a X 33a X 32a

14.2

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

211

Table 3 Žcontinued. Molecule TDP

Succinic anhydride

MO

KT

IE

PS

34a X 44a Y 33a Žny . Y 32a X 43a X 42a X 41a Žnq . Y 31a Y 30a X 40a Ž p O . X 39a X 38a Y 29a Y 28a X 37a Y 27a X 36a X 35a Y 26a X 34a Y 25a Y 24a Y 23a X 33a X 32a X 31a

10.73 10.74 11.32 11.69 11.86 12.16 12.44 12.54 12.63 12.84 12.98 13.11 13.21 13.66 13.88 14.61 14.67 14.96 15.00 15.42 15.58 15.70 16.12 16.38 16.61 16.71

9.60 9.57 9.80 10.47 10.63 11.01 10.73 11.35 10.97 11.40 11.56 11.82 11.87 12.26 12.36 13.07 13.06 13.31 13.58 13.75 13.89 14.29 14.50 14.87 14.82 14.90

0.92 0.92 0.91 0.91 0.92 0.92 0.91 0.91 0.90 0.90 0.91 0.91 0.92 0.92 0.91 0.92 0.92 0.90 0.92 0.90 0.91 0.88 0.91 0.91 0.90 0.90

9b 2 Žny. 12a 1Žnq . 3b1Ž p O . 2a 2 Ž p . 11a 1 1a 2 Ž p . 10a 1 8b 2 2b1Ž p . 7b 2 9a 1 1b1Ž p .

12.53 b 13.29 13.45 14.38 15.88 16.72 17.05 17.20 17.45 18.90 19.48 19.90

10.96 11.67 12.35 13.27 13.96 15.46 15.03 15.22 15.64 16.81 17.25 18.01

0.90 0.90 0.89 0.91 0.91 0.91 0.90 0.90 0.91 0.89 0.89 0.90

Y

IE exp 9.3 a 9.75 10.5sh 10.85 . . 11.1

10.8 c 11.6 12.1 13.1 13.8

15.3 16.4 17.0 17.7

a

Present work. DZP basis set w16x, present work. c Ref. w17x. b

derived from pagodane. The second belt IE should be ; 9.7 eV. The position and splitting of the n " ŽCO. photoemissions of HB are anticipated to resemble those of TB. Owing to their relative intensity, the first band at 9.3 eV in the PE spectrum of TDP ŽFig. 2. is associated with one of the two uppermost Walsh orbitals of the cyclobutane fragment and the second band peaked at 9.75 eV is attributed to two photoemissions, originating from the other Walsh orbital and the ny component. The present theoretical results for these two Walsh levels ŽTable 3., instead, predict a virtually degenerate pair IE 1 and IE 2 at 9.6 eV. This discrepancy may likely be traced to the limited basis set and truncation of the particle space in the OVGF treatment. However, with reference to the experimental data Žaverage value 9.5 eV and

212

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

Fig. 2. Ultraviolet HeŽI. spectrum of TDP.

splitting 0.45 eV., this pair is destabilized by 1.8 eV compared with the e u pair of cyclobutane itself Ž11.3 eV w19x.. Also, it differs from the corresponding bands of pagodane, which are displaced towards lower binding energy Ž8.17 and 9.05 eV. and split by 0.9 eV w18x. According to the present ab initio results, the average n and the splitting D of the n " IEs along this sample of 1,3-dicarbonylic systems exhibit a rather uniform pattern: 10.22 and 1.04 eV ŽTB., 10.35 and 0.73 eV ŽHB., 10.26 and 0.93 eV ŽTDP.; thereby indicating that the electronic influence exerted by the cage skeleton on the anhydride moiety is nearly similar. This situation resembles the aforementioned small variation of the carbonyl chemical shifts in the 13 C NMR spectra. However, correlation of n of the present molecules with that of the related simple succinic anhydride system Ž11.2 eV w17x. shows a nearly constant destabilization by ; 1 eV, as a result of cage formation. On the other hand, the magnitude of D is similar to that of succinic anhydride Ž0.8 eV w15x. and is just of the order found for other cyclic 1,3-dicarbonyls w20,21x. Finally, compared with the corresponding value for succinic anhydride Ž12.1 eV w17x., p ŽO. reveals an overall lowering by ; 0.7 eV, which is again consistent with a similar balance of competing inductive and mesomeric effects. 3.4. Electron attachment energies The ET spectra of TB, succinic anhydride, succinimide and glutaric anhydride are reported in Fig. 3. The most probable AE values are given in the diagram of Fig. 4. The three latter reference molecules possess two empty MOs: the out-of-phase py ŽCO.U and in-phase pq ŽCO.U combinations of the carbonyl p U orbitals. The

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

213

Fig. 3. Electron transmission spectra of succinimide, succinic anhydride, glutaric anhydride and TB.

two resonances displayed by their ET spectra are easily assigned to the py ŽCO.U and pq ŽCO.U MOs, in order of increasing energy. Previous ETS data demonstrated that, owing to lone pairrp U mixing, an adjacent oxygen or nitrogen atom increases the AE Ž1.15 eV. of the p ŽCO.U MO of cyclopentanone by ; 1 eV w22x. Thus, the lower-lying resonance is associated with the out-of-phase combination, which cannot interact for symmetry reasons, and the higher-lying resonance with the in-phase combination, destabilized by mixing with the lone pair orbital. It can be noticed that this destabilising effect is somewhat lower in succinic anhydride than in the hexacyclic or in the nitrogen analogue. This is probably to be ascribed to a reduced lone pairrp U overlap for conformational reasons. In fact, the geometric arrangement of the CO groups in the five-membered rings also leads to a smaller p ŽCO.U splitting Ž0.6 eV in succinic anhydride and 0.75 eV in TB. compared to the six-membered ring Ž1.08 eV in glutaric anhydride. in line with the very low activity of the IR symmetric Žin-phase. stretching mode in succinic compared to glutaric anhydride w23x, ascribed to the smaller angle between the CO groups in the latter compound. The broad resonances centred above 5 eV could be due to electron capture into the lowest s U MOs, but in this energy region core-excited resonances can also occur, i.e. two-particle processes where electron capture, for instance into the LUMO, is accompanied by a simultaneous valence transition. In addition to the p ŽCO.U MOs, TB possesses the in-phase pq ŽCC.U and out-of-phase py ŽCC.U combinations of the two ethenic empty p U MOs. Because of the electron-releasing inductive effect of the large alkenyl substituent, the p ŽCO.U MOs of TB are expected to be destabilized with respect to their succinic anhydride counterparts. Therefore, the two resonances observed at 1.4 and 2.15 eV in the ET spectrum of TB are assigned to the py ŽCO.U and pq ŽCO.U MOs, respectively Žsee the diagram of Fig. 4.. At variance, the strongly electron-withdrawing inductive effect exerted by the CO groups and the oxygen atom is expected to stabilize the p ŽCC.U MOs ŽAE s 1.73 eV in ethylene w24x.. Moreover, the 1.4 eV resonance is narrower ŽFWHMs 0.3 eV.

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

214

Fig. 4. Correlation diagram of the attachment energies ŽAEs. of succinimide, succinic anhydride, glutaric anhydride and TB.

than the corresponding resonance in the reference molecules, so it is unlikely that this signal has contributions from two MOs. Thus, the only remaining signal which can be associated with the empty ethenic p U MOs is the resonance centred at 0.5 eV, whose low-energy wing overlaps with the intense electron beam signal. Whether this resonance is due to the contribution of both the py ŽCC.U and pq ŽCC.U combinations or the first anion state of TB lies at lower energy cannot be determined from the ET spectrum. On the other hand, the negative AE of maleic anhydride w22,25,26x, which however has additional p ŽCC.rp ŽCO. conjugation, would tend to favour the hypothesis of a negative first AE for TB.

Acknowledgements Financial support from MURST of Italy is gratefully acknowledged by VG.

Appendix A. Crystal and molecular data of TDP The X-ray experimental data for single crystal of TDP were measured at 293 K using a CAD4 diffractometer with graphite monochromated Mo-K a radiation. A total of 5408 X-ray reflections were accumulated Ž2.968 - u - 26.408.. After data reduction, the structure was solved by direct methods using the MolEN program package w27x and refined against F02 for all unique reflections using SHELXL-93 w28x ŽC, O atoms with anisotropic temperature factors and H atoms with isotropic ones.. Final R 1 and wR 2 values for reflections I ) 2 s Ž I . are

V. Galasso et al.r Chemical Physics 242 (1999) 203–216 Table 4 ˚ 2 . of TDP The coordinates of atoms and their equivalent thermal parameters Ueq ŽA

215

V. Galasso et al.r Chemical Physics 242 (1999) 203–216

216

R 1 s 0.0690 and wR 2 s 0.1168. Ž R 1 s wÝ 5 F0 < y < Fc 5rÝ < F0
References w1x w2x w3x w4x

w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x

O. Ermer, Aspekte von Kraftfeldrechnungen, W. Bauer, Munchen, 1981, p.311. ¨ R.W. Alder, R.B. Sessions, J. Chem. Soc., Perkin Trans. 2 Ž1985. 1849. W. Grimme, H. Geich, J. Lex, J., Heinze, J. Chem. Soc., Perkin Trans. 2 Ž1997. 1955. M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Peterson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomberts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. DeFrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzales, J.A. Pople, Gaussian-94, Revision C.2, Gaussian, Inc., Pittsburgh, PA, 1995. T.A. Keith, R.F.W. Bader, Chem. Phys. Lett. 210 Ž1993. 223. A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. L.S. Cederbaum, J. Phys. B 8 Ž1975. 290. L. Sanche, G.J. Schulz, J. Phys. Rev. A 5 Ž1972. 1672. A. Modelli, D. Jones, G. Distefano, Chem. Phys. Lett. 86 Ž1982. 434. A.R. Johnston, P.D. Burrow, J. Electron Spectrosc. Relat. Phenom. 25 Ž1982. 119. G.K.S. Prakash, V.V. Krishnamurthy, R. Herges, R. Bau, H. Yuan, G.A. Olah, W.-D. Fessner, H. Prinzbach, J. Am. Chem. Soc. 110 Ž1988. 7764. P.R. Spurr, B.A.R.C. Murty, W.-D. Fessner, H. Fritz, H. Prinzbach, Angew. Chem., Int. Ed. Engl. 26 Ž1987. 455. W.-D. Fessner, H. Prinzbach, G. Rihs, Tetrahedron Lett. 24 Ž1983. 5857. H.-O. Kalinowski, S. Bugh, S. Braun, Carbon-13 NMR Spectroscopy, Wiley, Chichester, 1988. T. Kunitake, M. Tsukino, J. Polym. Sci., Polym. Chem. Ed. 17 Ž1979. 877. T.H. Dunning Jr., J. Chem. Phys. 53 Ž1970. 2823. ˚ ˚ M. Almemark, J.E. Backvall, C. Moberg, B. Akermark, L. Asbrink, B. Roos, Tetrahedron 30 Ž1974. 2503. ¨ D. Elsasser, K. Hassenruck, ¨ ¨ H.-D. Martin, B. Mayer, G. Lutz, H. Prinzbach, Chem. Ber. 124 Ž1991. 2863. P. Bischof, E. Haselbach, E. Heilbronner, Angew. Chem., Int. Ed. Engl. 9 Ž1970. 953. K.N. Houk, L.P. Davis, G.R. Newkome, R.E. Duke Jr., R.V. Nauman, J. Am. Chem. Soc. 95 Ž1973. 8364. V. Galasso, F. Colonna, G. Distefano, J. Electron Spectrosc. Relat. Phenom. 10 Ž1977. 227. A. Modelli, G. Distefano, D. Jones, Chem. Phys. 73 Ž1982. 395. F.-H. Marquardt, J. Chem. Soc. B Ž1966. 1242. P.D. Burrow, A. Modelli, N.S. Chiu, K.D. Jordan, Chem. Phys. Lett. 82 Ž1981. 270. C.D. Cooper, R.N. Compton, J. Chem. Phys. 59 Ž1973. 3550. C.D. Cooper, R.N. Compton, J. Chem. Phys. 63 Ž1975. 598. MolEN is an interactive structure solution procedure, Nonius, Delft. SHELXL-93: Program for the refinement of crystal structures by G.M. Sheldrick, University of Gottingen, Gottingen, 1993. ¨ ¨