A semiempirical - SCF approach to the protonation of urea and thiourea

468

Journal of Molectdar Elsevier Publishing

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dompany,

Amsterdam.

Structure

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A semiempirical

- SCF approach to the protonation

of urea and thionrea

There are two possible sites of protonation in the ureas (A, B). +XH

X

II

II

H2N

\

/

x=o,s

C

C

/

\+ NH3

\

\

HJ’J

NH2

(A)

@I

Evidence has been presented ’ - 3 for either mode of protonation, but more weight is given to protonation on X. A qualitative explanation for the preference of protonation as in B based on resonance theory has been given by Davies and Hopkin4_ This work presents a more quantitative approach to the problem. The SCF method in the Pariser-Parr-Pople approximation was used to calculate the total energy and the electronic transition energies for botb forms. First we treated form (A) as a four-electron system, isoelectronic with formamide, with the perturbation of one unit charge placed at the nitrogen atom. The parameters used were the same as previously’. The results (in eV) are given in Table 1. TABLE

I x=0

protonated AE(Z Eo E, El

+ ‘z*)

Et

x=s

free

4.55 - 101.594 75.226 0.936

6.93 -189.113 104.803 -

-25.432

-84.310

protonated 2.318 -92.806 69.811 1.613 -21.382

free 5.23 -178.885 98.770 -80.115

The total energy (I&) is composed of three parts: the energy of the z electrons (I?,,), the repulsion energy (I$) of the nuclei, and the corrections (,?Z,)due to the perturbation effect of (NH,)+ on the n electrons. The last value was calculated in the electrostatic approximation using fust order perturbation theory. The total energy rises with protonation as expected, although changes in the electrons were neglected. The second factor that can be compared with experiment Gis the change of the electronic transition energy. The calculation gives for urea and thiourea red shifts which are in contradiction with the experimental results’. J. Mol. Structure,

4 (1969) 468-469

SHORT

469

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For the calcuIation on form (B) we took X as an atom charged with one positive unit. The one center integral of X was approx~ated by: ‘yxx = I+ * -Ii where If * is the second ionization energy and I+ the first. The values were taken from ref. 6 as: = 31.761- 14.617 = 17.14 for 0’ and Yxx Yxx = 21.684- 10.548 = 11.14 for Sf. TABLE

AE(n

2

+ “z*)

x=0

X=S

6.94 -219.617 132.163 -87.459

- 198.374 128.765 -69.609

6.97

As can be seen in Table 2, the total energy is slightly lower in the protonated ureas than in the unprotonated ones. This is perhaps a consequence of the neglect of changes in the (r electrons with protonation. The electronic transition energies in both species are blue shifted and this is now in agreement with the experiments’. Both the energy differences and rhe spectral shifts indicate that protonation on X is more likely than on nitrogen. In view of the negIect of the G eIectron interactions, not too much weight should be attached to the total energy differences but the direction of the spectral shifts given by the calculation is certainly of importance.

ACKNOWLEDGEMENT

The financial assistance of the Boris KidriE Fund is gratefulfy acknowIedged.

1 hf. J, JANSSEN, Spectrochim. Acta, 17 (1961) 475. 2 ‘I’. BIRCHALL AFJDR. J. GILLESPIE,Cm. L Chem., 41 (1963) 2643. 3 4 5 6

N. N. G mmwoo~ AND B. H. KOBIX~ON, .T. Cizem. Sm., A (1967) 511. M. DAVIFS AND L. HOPKIN, Trans. Furaday Sac., 53 (1957) 1563. A. A&AN, M. DRO~NIK, D. HA& AND B. LUKMAN, J. Mol. Structure, J, HINZE AND H. H. JAFFE,f. Chem. Whys_.. 38 (1963) 1835.

1 (1967-681 181.

Received June 8th, 1969

J, Mii~~Strucrure, 4 (1969) 468-469