Electronic structure of aryl-and carbonyl-containing diazomethane ...

Russian Chemical Bulletin, VoL 42, No. 2, February, 1993

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Fig. 2. Molecular orbitals of o symmetry for PhOPF 2 and

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Of the three boundary MO, the o MO is most affected by the halogen. The structure of this MO is given in Fig. 2 (the dotted line shows the tr orbital of phosphorus). When X = Cl, this MO embraces the whole of the OPX 2 fragment. At the same time, with X = F, it is more localized on the P--O fragment (as shown by the maximum MO LCAO coefficients). It is the otructure and the energy position of this region that may be responsible for the change in the reactivity of phenylhalophosphites towards the chlorophosphonium derivatives.

PhOPC12. References

According to M N D O data, there is no substantial difference in the spatial orientations of the PX 2 fragments: angle C - - O - - P - - C I ( C 1 ) = 36 ~ (146~ angle C - - O - - P - - F ( F ) = 37 ~ (146~

l. B. V. Timokhin, M. V. Kazantseva, V. I. Donskikh, D. D. Chuvashev, and G. V. Ratovsky, Zh. Obshch. Khim., 1991, 61, 2161 [J.Gen.Chem, 1991, 61, No. 10 (Engl.Transl.)] 2. M. I. S. Dewar and W. Thiel, J. Am. Chem. Sot., 1977, 99, 4899.

Received April 6, 1992

Electronic structure of aryl- and carbonyl-containing diazomethane derivatives, as disclosed by photoelectron spectroscopy A. S. Vorob'ev*, L L Furlei, Yu. Z. Ekov, A. M. Nazarov, G. A. Yamilova, U. M. Dzhemilev, and V. A. Dokichev Institute of Organic Chemistry, Ural Branch of Russian Academy of Sciences, 71 prosp. Oktyabrya, 450054 Ufa, Russian Federation. Fax: +7(347) 234 2914 Vertical ionization energies (IE) of aryl and carbonyl derivatives of diazomethane have been determined by photoelectron spectroscopy. The types of the highest occupied molecular orbitals (HOMO) in these molecules are identified. Key words: photoelectron spectroscopy; electronic structure; aryl and carbonyl derivatives of diazomethane.

Up to now only the photoelectron spectra (PES) for diazomethane,1, z 2-diazopropane, 3 diazocyclopentadiene, 3 and for some of a-diazoketones 4 have been reported. The family of diazo compounds studied is thus limited. This may be due to the low stability of this class of compounds, especially of their low-molecular representatives, which decompose readily even at -20*C. The central problem with recording the PES of such compounds is the choice of a suitable temperature of vaporization, which would prevent thermal decomposition while securing a sufficiently high saturated vapor pressure. We managed to overcome this dilemma by exploiting the

characteristic property of diazo compounds to evolve nitrogen upon thermolysis. R2RICN 2 + R~R2CN2 ~ R1R2C=N--N=C=R~R2 + N 2 The appearance of characteristic ionization bands of nitrogen s announces the beginning of the thermolysis of RIRZCN2 and hence the necessity to decrease the vaporization temperature until the disappearance of these diagnostic peaks is complete. This procedure enabled us to adopt appropriate experimental conditions and to measure the PES correctly. In this work photoelectron spectroscopy is used to investigate the following

Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 325--328, February, 1993. 1066-5285/93/4202-0281512.50

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Plenum Publishing Corporation

Russ. Chem.Bull., VoL 42, No. 2, February, 1993

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diazomethane derivatives of general formula R l R2RCN2 (1--10) as well as diazodimedone (11) and 9-diazofluorene(12). R1 R2NzC=N=N 1 -lO

1: R l = H , R 2 = P h C H 2 C O (30~ 2 W = H , R 2 = P h 2 C O C H (50~ 3: R l = H , R 2 = P h C O (30~

4: R 1 = H, R2 = CO (46~

5: R l = H, R 2 = CO2Me (0~ Ph, R2 = Ph (40~ 7: R 1 = Ph, R2 = 4-MeC6H 4 (45~ 8: R 1 = Ph, R2 = 4-CIC6H 4 (45~

6: R l =

9: R 1 = H , R 2 = Ph (0~ 10: R 1 = Ph, R 2 = M e (0~ Me

11: O

Me

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(50~

12: ~

N2

(40~ N2

Diphenylmethane (13) was used as a model compound for the PES measurement (20~

13 Experimental

All the PES were recorded with an ES-3201 electron spectrometer. The photoelectron kinetic energy scale was calibrated with Xe (IE l 12.13 eV, IE2 13.43 eV) and Ar (IE 1 15.76 eV, IE2 15.94 eV) lines. Maxima of the ionization bands were determined with an accuracy better than 0.03 eV. Molecular orbital calculations (the MNDO method) for the interpretation of experimental data comprised full geometry optimization in the sp-basis. The correlation coefficient between the calculated orbital energies and the maxima of the first three ionization bands was no less than 0.03. Results and Discussion

The PES of diazomethane (14), the simplest representative of the class studied, has been reported earlier)

* The temperatures of PES measurement are specified in brackets.

Vorob'ev et al.

Its first ionization band with a maximum at 9.0 eV corresponds to the removal of an electron from the ncN occupied molecular orbital (MO) localized mainly on the carbon atom and the terminal nitrogen atom. The IEs of low-lying MOs exceed those of ncN by 5 eV. In practice, this means that in the range of 8--10 eV all the characteristic features in the PES of substituted diazomethanes are controlled by the HOMOs of the functional groups in substituents R 1 and R 2, which interact with each other and with the 7tCN orbital. For the carbonyl-containing compounds 1--4 the combined inductive and mesomeric effects of the R 2 substituent on the position of the ncN MO is relatively small and remains within ~0.1--0.4 eV (Fig. 1). This results from the weakness of (S--nCN, rtco--rtCN , as well as 7tph--TtCN conjugations and is due to the large energy gap between the interacting orbitals in the former case and to the inefficiency of their spatial overlap (which is inhibited by the CO-group) in the latter case. The inductive effect of R 2 is also insignificant since R 2 is represented by functional groups with moderate donoracceptor properties; moreover, in some cases their effects are oppositely directed. For example, in adamantanoyldiazomethane (4) the positive inductive effect of the adamantyl group is largely counterbalanced by the negative effect of the carbonyl, since the IEs of 7tCN in this compound (8.92 eV) and in 14 (9.00 eV) practically coincide. The determination of MO ordering in the series 1--12 presents no problems except for the case of diphenyldiazoacetone (2). There the difficulties are due to the presence of an extra phenyl ring in the molecular structure of 2 as compared with that of phenyldiazoacetone (1). In 1 the observed MO ordering 7tCN < ~ph(bl)* < nPh (a2) < no is easily justified by the comparison between the IEs of these occupied MOs and the IEs of the corresponding model compound, phenylacetic aldehyde (15) (see Fig. !)- The presence of the extra phenyl ring in compound 2 not only stabilizes the rcCy MO by ~0.4 eV (probably by an inductive mechanism), but also triggers the and