Specfrochimica Acla, Vol. 44A, No. 3, pp. 289-292, 1988. 0584-8539/88 53.00 + 0.00 Printed in Great Britain. Q 1988 Pcrgamon Press plc

Electronic structure of hydantoin studied by He I photoelectron spectroscopy and MNDO quantum-chemical calculations

TOMAS VONDR~* and CARLA CAULETTI~

*ESCA Centre, The J. Heyrovskjr Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Vla?&ksks 9, 118 40 Prague 1, Czechoslovakia; and ‘?Dipartimento di Chimica,

Universita “La Sapienza” di Roma, Piazzale A. Moro 5, 00185 Rome, Italy

(Received 23 April 1987; in final form 21 July 1987; accepted 21 July 1987)

Abstract-The He I photoelectron spectra of hydantoin, I-methyl-hydantoin, 3-methylhydantoin and 1,3- dimethylhydantoin are reported. Displacement of the bands upon the N-methylsubstitution indicates that HOMO and the third highest occupied MO are localized on the nitrogen atoms, the former on the amidic nitrogen and the latter on the imidic one. The second and the fourth highest occupied MO’s are oxygen lone pairs. The MNDO method provides the same picture.

INTRODUCTION

The knowledge of the electronic structure, i.e. the energy and the composition of the outermost molecu- lar orbitals (MO’s) is of a great importance for the understanding of the structure-biological activity re- lationships. The gas phase U.V. photoelectron (PE) spectroscopy provides unique information on the electronic structure of molecules in terms of ionic state energies which can be translated into the MO energies [l]. This work was carried out in the framework of an extended research program on the electronic pro- perties of pentatomic heterocyclic compounds of bio- logical interest-it is well known that hydantoin derivatives exhibit diverse pharmacological activities [2-61. Compounds containing the X, /z group

ii ‘i

(X, Z=O, S, NR, CH,; Y=O, S, Se, R=CHs) have already been studied by X-ray and U.V. PE spectros- copy [7-103, and an investigation on the electronic structure of thio- and seleno-derivatives of hydantoin is on the way [l 11. To have an unambiguous reference point for the discussion of the effects of the third and fourth row element we decided to reinvestigate the electronic structure of the parent hydantoin. The recent study by AJO et al. [ 121, where the assignment of the PE spectra was based on semiempirical INDO/S calculations, evidenced that the HOMO is the amidic nitrogen lone pair. However, the assignment of the following bands was not clearcut. The N- alkylsubstitution proved useful for the assignment of the U.V. PE spectra of compounds containing the amidic group [13-161. We recorded the He I PE spectra of hydantoin (I), 1-methylhydantoin (II), 3- methylhydantoin (III) and 1,3_dimethylhydantoin (IV). The PE spectra are discussed on the basis of the validity of the Koopmans theorem [l]. The assign- ment of the spectra is compared with results of the MNDO [17] and HAM/3 [18] calculations.

EXPERIMENTAL AND COMPUTATIONAL DETAILS

Resublimed commercially available hydantoin (Fluka) was used. The N-methylderivatives were prepared by known methods [19,20] and the purity of samples was checked by mass spectrometry. The He I PE spectra were recorded on the VG Scientific UVG 3 instrument. A sufficient signal/noise ratio was obtained at temperatures from 120 to 180°C. Spectra were calibrated by the simultaneous addition of an Ar/Xe mixture. The resolution (FWHM) was 30-40meV throughout the whole spectrum. The MNDO calculation was carried out by the program written by W. THIEL (Fachbereich Chemie, Philips -Un&ersit& Marburg) and ihe HAM/3 program available from Q.C.P.E. was used [21]. The geometry determined by X-ray diffraction was used [22,23].

RESULTS AND DISCUSSION

The He I photoelectron spectra are shown in Fig. 1 and the vertical IE’s are summarized in Table 1. The broad band envelope beginning at approximately 12.5 eV originates in ionizations of u orbitals. The spectral features corresponding to the carbonyl n levels fall within this region [24, 251. The most interesting part of the spectra ( < 12.5 eV ) involves ionizations of oxygen lone pairs (no) and of nitrogen z type orbitals [24,26]. Four ionization events (14) are observed in this region. Band 4 exhibits a weaker peak or a shoulder on the high energy side in all the spectra under study. The distance from the band maximum is equal to 1260 & 90 cm-‘. This value is very close to the ionic frequency observed on the carbonyl h band [27]. Thus this fine structure allows us to assign band 4 in the spectra of both the parent compound and the N- methylderivatives to the n, ionizations.

The discussion of the observed trends of IFS in a series of related molecules can be complicated by the ambiguity steming from changes in the sequence of MO’s. For this reason we mention alternative assign- ments of the bands. The decrease of IE’s observed generally on alkylation [ZS] can serve as a clue for elimination of this uncertainty.

The substitution of the Nl-hydrogen atom by methyl brings about a decrease of the first IE by

289

290 TOMAS VONDRM and CARLA CAULETTI

I 1 I I I I I I I I I I I I

9 10 11 12 13 16 15 9 10 11 12 13 14 15

IE, eV IE,eV

m 0 r-z 2 H-N

Y

N-CH3

0

2

-,-- I I I I I I I I I I I I I I

9 10 11 12 13 14 15 9 10 11 12 13 14 15

IE,eV IE,eV

Fig. 1. He I photoelectron spectra of hydantoin (I), 1-methylhydantoin (II), 3-methylhydantoin (III), and 1,3_dimethylhydantoin (IV).

Table 1. Experimental vertical ionization energies, eV

Compound band Assignment I II III IV

1 nN1 10.20 9.51 9.33 2 10.50 10.17 10.05 3 10.87 10.61 10.16 4 11.18 (1170) (1200) 10.91 10.74 (1350)

in parentheses are vibrational frequencies, cm- Estimated error 0.05

0.63 eV. N-methyl destabilizes nN level stabilization of Thus we 0.65 eV N-methylamides If the that the of the highest MO’s

band (1) the spectrum of corresponds to the (2) in spectrum of the destabilization would

amount 0.93 eV. other change the ordering of notations HOMO-l, HOMO-2 HOMO-3 refer

MO’s in comparison I would an un- the second, third and fourth highest

molecular

Electronic structure of hydantoin 291

Table 2. HAM/3 and MNDO results on hydantoin:upper valence orbitals

IEHAM/3 -%NDO Localization, “/, Rand Assignment eV Nl 02 N3 04

1 “NI 10.33 11.27 54 30 1 2 2 no 10.16 11.50 2 26 19 43 3 “N3 11.04 11.56 1 6 44 34 4 no’ 10.37 12.16 5 55 7 23

identical with the parent 1. The destabilization of HOMO-l, HOMO-2 and HOMO-3 equals to 0.26, 0.13 and 0.19eV. Thus the rtN, orbital is the main component of HOMO since it exhibits the highest sensitivity to the Nl substitution.

The bands (l-3) considerably overlap in the spec- trum of III. The assignment of the band (1) to the rrN, orbital implies its destabilization by 0.13 eV. The assignment of the rrN, to the band (2) or (3) is unreasonable since this orbital would be destabilized to a negligible extent or stabilized by the N3 methyl. The difference between the third IE in I and III equals 0.59 eV. If the HOMO-2 in I corresponds to the HOMO-l in III this destabilization is 0.71 eV. The participation of the xN, orbital in HOMO of I is larger than that of the rrN3 in HOMO-2 (vide infra). Since the destabilization of an MO is approximately propor- tional to the square of the atomic orbital coefficient in the point of substitution [29], the lower value 0.59 eV is more reasonable. Then the HOMO-1 and HOMO-3 are shifted by 0.34 and 0.27 eV, respectively, to lower IE. Thus adopting the sequence of MO’s identical with the parent I consistent changes of IE’s are obtained. It follows that HOMO-2 is mainly localized on the N3 nitrogen. The IE of HOMO-l is not, within the experimental error, inihrenced by the position of the substituent. This agrees with its oxygen lone pair character.

In IV the IE of HOMO and HOMO-2 is 9.33 and 10.16 eV, respectively. If it is assumed that the effects of the methyl-substitution on the two rrN orbital energies are additive, values of 9.24 eV and 10.05 eV are calculated for IV, in very good agreement with their observed values. This can be an argument that the HOMO-2 is the nN3 orbital. Then band (2) must come from ionization of the oxygen lone pairs. But due to the proximity of bands (2) and (3) this assignment can only be tentative.

The results of the MNDO calculation supports the interpretation of the spectra (Table 2, Fig. 2). The nN, orbital is the main contribution to HOMO (54 %). The HOMO-2 is mainly the rrN3 orbital (44 %). The mutual mixing of the IIN orbitals is negligible. The lower localization of HOMO-2 on the N3 atom explains the lower sensitivity to the N3-substitution in comparison to that of the HOMO to the Nl-substitution. The oxygen lone pairs interact with each other giving rise to the symmetric and antisymmetric combination. The calculations suggest that the antisymmetric combi-

9 -

C?V

10 -

11 -

12 -

HAM13 MN00 Exp.

Fig. 2. Correlation of calculated and experimental IE’s of hydantoin (I), 1-methylhydantoin (II), 3-methylhydantoin

(III), and 1,3_dimethylhydantoin (IV).

nation (c) is above the symmetric one (no’). The HAM/3 method suggests the ordering of the ioniz- ations which is at variance with the observed substi- tution effects and vibrational structure. The calculated IE’s increase in the order 6 < rrNt < no+ < rrN3. But there is a better agreement between the experimental and calculated splitting of nN orbitals than the MNDO method provides.

CONCLUSION

The PE spectroscopic investigation and the MNDO alculation show that there is not a significant mixing between the nitrogen rt orbitals. The HOMO is located mainly on the Nl atom. The rrN3 is the third occupied MO. The oxygen lone pairs interact strongly with each other. The antisymmetric and symmetric combination is the second and fourth occupied MO, respectively. The HAM/3 method failed to predict the ordering of the ionizations consistent with the experimental data

Acknowledgement-The authors thank F. TURE~EK and V. HANG (The J. Heyrovsky Institute of Physical chemistry and Electrochemistry) for carrying out the mass spectromet- rical analyses of the samples.

PI PI

[33

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