Electronic structure and spectra of organic molecules

Journal of Molectdar

Stracture

245

Elsevier Publishing Company, Amsterdam. Printed in the Netherlands

ELECTRONIC PART AND

XIV.

STRUCTURE ON

THE

AND

SPECTRA

LACTAM

AND

Copernicus

University,

OF ORGANIC

THIONE

FORMS

MOLECULES OF PYRIDINES

PYRIMIDINES*

J. S. ICWIATKOWSKL Institute of Physics, Nicholas

Ton&

(Poland)

(Received December 4th, 1970; in revised form April 21st, 1971)

ABSTRACT

Electronic structures of both lactam and thione forms monomercapto derivatives of pyridines and pyrimidines are basis of experimental data, 3-0~0 and 3-mercapto derivatives oxopyrimidine are thought to exist as tautomers with a proton

of monooxo and discussed. On the of pyridine and 5at the ring nitrogen

(dipolar ionic forms). Parker-Parr-Pople type of calculations are carried out for both neutral and dipolar ionic (zwitterionic) structures. In general, calculated singlet-singlet transition energies in both cases agree well with experimental data.

DISCUSSION OF THE STRUCTURES OF TAUTOMERS

0x0 and mercapto derivatives of N-heteroaromatic compounds are capable of lactim-lactam or thiol-thione tautomerism, respectively’ -4. It is generally accepted that both pyridine and pyrimidine substituted at position 2 or 4 by potentially tautomeric groups -XH (X is oxygen or sulfur) exist in the lactam or thione forms (lb) rather than in the lactim or thiol forms (Ia). The tautomeric ratios of forms having a hydrogen atom attached to nitrogen to those having hydrogen at oxygen (or sulfur) are equal to 340 and 2200 for 2- and 4-oxopyridine, respectively5, while in the case of 2- and 4mercaptopyridine they are much larger being 104-’ and 104, respectively6. However, it is impossible for valency reasons to write classical formulae similar to (Ib) for 3-XH-pyridine and 5-XH-pyrimidine. Hence these molecules are said to exist in lactim or thiol forms of the (Ha) type. It is accepted that these * For Part XIII (Electronic Spectra of Protonated Derivatives of Pyridines and Pyrimidines), see J. S. KWIATKOWSKI, Acta Pizys. Polotz., A39(1971)695. J. Mol.

Structure,

10 (1971) 245-231

246

J. S. KWIATKOWSKI

X

XH (10)

(II01

(Ib)

(Id

tItb,

(EC)

compounds cannot exist as tautomers with a proton at a ring nitrogen atom. In our opinion, however, several experimental results indicate that 3-0~0 and 3-mercapto substituted pyridine and 5-oxopyrimidine may exist as tautomers of some sort (there is a lack of experimental evidence for 5-mercaptopyrimidine). However, the problem of the electronic structures of these tautomers still remains unsolved. Paoloni et al.‘** observed that the idea that there is no tautomeric form of 3-oxopyridine, corresponding to type (Ib), is merely based on the fact that the traditional concept of atomic valence state is usually applied when describing structures of molecules. They suggest that 3-oxopyridine may exist as a tautomer with a proton at the nitrogen atom corresponding to structure (Hb) because the valence states of pyrrolic nitrogen >N-H

and that of nitrogen >N-H

in (Hb)

are the same’*‘. However, some researchers’* lo* r1 postulate ionic structures for the molecules considered. 3-Oxopyridine, its l-methyl derivative and 5-oxopyrimidine, dissolved under appropriate conditions, can exist as so-called dipolar ions (or zwitterions), having forms of the (Hc) type. Metzler and Snelll’ have measured the electronic absorption spectra of spectrally distinct charged species of 3-oxopyridine and its derivatives (cf. refs. 11 and 12). This experimental evidence shows that 3-0x0pyridine exists in neutral aqueous solution as a mixture of equal quantities of the lactim (Ha) and zwitterionic (HG) forms, but in an alcoholic solution the ratio of the (11~) forms to the (Ha) forms is very small. Similarly, 5-oxopyrimidine in aqueous solution can exist as zwitterion”. However, the ratio of the zwitterionic forms to lactim forms is small when comparing the observed intensities of the absorption bands of aqueous soIutions of the compound (cf. ref. 11). Ultraviolet evidence also shows that 3-mercaptopyridine exists as the zwitterion rather than as the thiol form. 3-Mercaptopyridine absorbs in a longer wavelength region than either 3-methylthiopyridine or thiophenol, and its absorption spectrum is very similar* to that of its l-methyl derivative’ 3. The tautomeric ratio4 * It is worth notingthat the calculated transition energies (in eV) for the thiol form of 3-mercaptopyridine are 4.53 (f = O-085), 5.52 cf = 0.234) and 6.38 (f = 0.434). These figures are different from those for the zwitterionic form of the compound (see Table 2). J. Mol.

Structure,

10 (1971) 245-251

ELEGTRONIC

STRUCTURE

AND

SPECTRA

OF ORGANIC

MOLECULES.

XIV.

247

of the zwitterionic forms to the thiof forms for &mercaptopyridine is f 50, indicating that in aqueous solution the molecule exists mainly in the form with hydrogen attached to the nitrogen atom. Similar zwitterionic formulae may be written for other oxo- and mereaptosubstituted pyridines and pyrimidines existing as &tams or thiones (forms of the (Ic) type), and some workers3* Ii prefer to use the name “zwitterion’” when describing these molecules. It is worth noting that the word “‘zwitterion” was first introduced into chemistry to describe substances showing salt-like behaviour (internal salts), i.e. substances having one fully ionized acidic; group (here -X’) and one fully ionized basic group (here NC-H) in each molecule. Experimental evidence shows that > the lactams considered have only partially ionized acidic and basic groups- However, we will ~ont~~u~using the name zwitterion in sush Gases,because the polarization of the C-X-

and >N+-H

bonds is very strong.

A question arises, namely whether the neutral or diionic structure more adequately represents the properties of oxo- and mercapto-pyridines and pyrimidines. The paper of Bliznyukov and Reznikov*4 gives an answer in part to this question. The authors, examining the spectra of 2- and 4-oxopyridine in different solvents, tried to draw some conclusions concerning the real structures of the compounds. They found that: (i) the proton is attached to nitrogen; (ii) the electronic configurations of 2- and Coxopyridine and their N-alkyl derivatives do not correspond to the structures of unsaturated ketones; (iii) the ring nitrogen has an electron acceptor character while the oxygen has an electron donor property; (iv) there is partial polarization of the bonds between carbon and oxygen as well as between nitrogen and hydrogen, Under these circumstances the question arises as to whether the degree of polarization is of the size recorded in the C-O” bond (e.g. in the phenoxide anion” “) and in the >Nf-$5 group (e.g. in protonated N-heterocycles). An answer to this question can be found by considering the spectra of Z-oxo-, I-methyl+oxo-, and 4ooxopyridine compared to those of the corresponding protonated aminopyridines. It is well known that the influence of the -Osubstituent on the spectrum of a molecule is insignificantly larger than that of the -NH2 group (cf. ref. 15) and that the effect of attaching a proton to nitrogen under protonation should be similar to the case of partial polarization of the >Nc-H bond recorded in oxopyridines. The comparison, made on the basis of experimental spectra available in the literature, indicates that the spectra of oxopyridines are insignificantly shifted towards the blue region with respect to those of the protonated a~~opyridi~s (cf. ref. 16). This fact suggests that in the case of monooxopyridines the partial polarization of the bonds is very similar to the polarization of the bonds of C-O- and >N’- IT. We conclude that the n-electron systems of oxopyridines are very similar to those of the corresponding protonated aminopyridines. .J, Mol. Sfrta&ire, 10 (1971) 245-251

248

J. S. KWIATKOWSKI

1

TABLE ELECTRONIC

SPECIRA

OF

OXOPYRIDINES

AND

OXOPYRIMIDINEeS=

Theoretical

Ekperimental

Neutral Iactam

Zwifrerion

AE

ub

AE

15 -52 -43 -42

4.14 0.278 5.38 0.204 6.37 0.722 6.50 0.121

f

f

tcb

AE(log E)

2 -61 -60 54

4.20(3.77)" 4.17(3.8)'8 4_13(3.7)'g*20 5.51(33.86) 5.46(4-O) 5.4-5.5(3.9) 6.42"

.?-oxqoyridine 4.43 5.56 6.64 6.92

0.277 0.215 0.500 0.208

3-Oxopyridine (cf. ref. 12) 4.01 5.06 5.86

0.179 0.097 0.691

-26 26 48 6.68 0.397 2

koxopyridine 4.84 0.569 90 4.93 0.056 0 5.96 0.163 0 7.31 0.640 90

4.67 0.028 4.90 0.363 5.86 0.503

90

6.60

0.902

9:

4.22 0.205 5.53 0.374 6.40 0.213 6.72 0.433

33 -79 -66 -59

0

5.06(3.95)

4.98(3.9) 5.8

5.04 -

I

4.86(4.2)=

2-Oxopyrimidine 4.04 5.80 6.56 6.75

0.192 0.162 0.429 0.656

31 81 -58 -85

boxopyrimidine

4.47 5.18 6.23 7.21

4.15(3.6)25 X77(3.66)

4.1 I (3.73)25*c 4.16(3.68)” 5.85(4.03) 5.77(4-O)

(I -NH)

0.138 -10 0.376 30 0.229 72 0.749 43

4.75 5.14 6.02 6.76

0.131 0.253 0.504 0.724

4.35 5.37 6.52 6.57

0.262 0.223 0.216 0.649

47 -15 -82 10

4.63(3.56)27* = 4.61(3.59)=* 5.61(3.83) 5.53(3.75)

0.173 0.306 0.299 0.406

-16 87 78 82

3.81”

16 6 87 22

4-Oxopyrimidine O-NH) 4.41 5.35 6.56 7.03

0.212 0.285 0.499 0.215

85 11 28 30

*

5-Oxopyrimidine (cf. ref. 12) 3.94 5.02 6.13 6.89

a AE = transition energy (in &V); f = oscillator strength; .e = molar extinction coefficient. b The polarization direction (angle a) is measured positive towards C, with respect to an axis from Cz-Cs (in pyridines) or N,-NJ (in pyrimidines). c Data for l-methyl derivative. d Absorption linaximlltnat ~215 nm (AE w 5.8 eV) evaluated.from the absorption curve. c Data for 3,6-dimethyl derivative. r Data for 3-methyl derivative. J; Mol. Structure. 10 (1971) 245-251

ELECTRONIC TABLE

STRUCTURE

AND

SPECTRA

MOLECULES.

XIV.

249

2

ELECl-RONIC

SPECTRA

OF MERCAPTOPYRIDINES

AND

Theoretical Zwitterion

ab

f

MERCAPTOPYRIMIDINES3

Experimental

Netural Iactam mz

OF ORGANIC

AE

ab

f

AE(log E)

2-Mercaptopyridine 3.64 4.55 5.80 4.23

0.300 0.459 0.156 0.203

- 6 -37 -71 -45

3.54 4.53 5.41 6.12

0.313 0.299 0.039 0.214

-10 -50 -89 -61

3.35 4.37 5.13 6.14

0.099 0.295 0.288 0.170

4.02 4.41 5.19 6.22

3.59(3.87)13

3.65(3.96)28

3.65(3.94)6

4.54(4.03)

4.57(4.04)

4.57(4.08)

-20 37 23 42

3.42(3.37)13 4.27(4.09) 5.34(4.15)

3.36(3.34)‘3*c 4.22(4.07) 5.25(4.12)

0.487 0.001 0.279 0.288

90 0 0 90

3.79(4.34)‘3 ~4.5 sh(3.12)

3-Mercaptopyridine

CMercaptopyridine 4.01 4.09 5.56

0.941 0.013 0.113

90 0 0

6.27

0.002

90

3.85(4.38)=

3.83(4.37)6

5.37(4.02)

5.4 l(3.99)

5.59(4-O)

3.58(3.42)2* 4.46(4.33)

3.67(3.51)+7*d 4.48(4.28) 5.77(4.01)

3.73(3.67)“*= 4.49(4.38)

3.79(3.91P 4.35(4.03)

3_85(4_05)*7*f 4.30(4.0)

_?-Mercaptopyrinzidine 3.06

0.128

58

4.68

0.597

87

3.50 4.47

0.142 0.486

51 -84

5.58 6.25

0.352 0.099

-45 81

5.39 6.39

0.125 0.093

-23 -58

0.080 0.241 0.279 0.369

20 -87 61

4-Mercaptopyrinzidine (I-NH) 3.50 4.17 5.75 6.24

0.090 0.776 0.098 0.032

12 28 -87 18

3.74 4.51 5.44 5.96

5

I

, 4-Mercaptopyrimidine (3-NH) 3.49 4.41 5.89

0.139 0.656 0.037

66 23 -39

3.70 4.45 5.56

0.338 0.280 0.007

6.18

0.288

37

6.21

0.210

40 5 77 28

/

J AE = transition energy (in eV); f = oscillator strength; E = molar extinction coefficient; sh = shoulder. b The pol arization direction (angle a) is measured positive towards C& with respect to ZHIaxis from C2-Cs (in pyridines) or N1-N3 (in pyrimidines). c Data for I-methyl derivative. d Data for bmethyl derivative. c Data for 4,Cdimethyl derivative. f Data for 6-methyl derivative. J. Mol. Structure; 10 (1971)

245-251

J. S. KWIATKOWSKI

250

As far as mercapto-substituted pyridines and pyrimidines are concerned, this kind of comparison cannot be made because of the lack of the appropriate experimental data for protonated species.

CALCULATION

REZSULTS

To check our discussion based on the analysis of experimental facts we have

performed calculations

by means of the Pariser-Parr-Pople

ionic forms of molecules, assuming that the polarization as that in the -O-

substituent (or -S-j

\Nf-H and in the /

method for zwitter-

of the bonds is the same group in the case of

protonated ring nitrogen. Thus, in the calculations for zwitterions, a11the semiempirical parameters are identical with those of our previous calculations for

anions and cations of 0x0 and mercapto compound?-“‘. In the calculations for the neutral forms of the type (Xb) we used the integral values which can be found in our previous papers or in the literature*. The experimental data and the results of the calculations** are given in Tables 1 and 2. The calculations for both forms are, in general, in a good agreement with experimental data. The theoretical results for zwitterions of 2- and 4-mercapto-

pyrimidine are slightly better, but these results can be treated as an exception rather than as a rule. We have performed some calculations for the neutral form of 3-oxopyridine (of the type (IXb)), assuming different values of the resonance integrals &e. The differences between the resulting transition energies and the experimental values were of the order of 1 eV. Similar calculations for the thione form of 3-mercaptopyridine gave values of the transition energies about 1.5-2.0 eV Iower than those indicated by experiment. In conclusion, the tautomers of oxo- and mercaptopyridines and pyrimidines with a proton at the ring nitrogen are compounds having strongly polarized bonds C-X-

and >N’-H,

and may be described by the zwitterionic

structures

even though the neutral structures can be successfully used for 2- and 4-substituted molecules. This conclusion does not necessarily concern polysubstituted systems. In the case of such systems the influence of different substituents changes the character of polarization of the particular bond. Therefore, the polarization of the C-X-

and

>

N”-I3

bonds will not be as strong as that recorded in mono-

substituted pyridines and pyrimidines * Here we give the valuesof the resonance integrals(@ in eV, r in A): /Lc(r = 1.38)= -2.6; &_c(r= 1.41) = -2.2; &,&= 1.24) = -2.8; &,&= 1.7) = -1.9; &Jr= 1.33) = -2.5: &_~(r = 1.34) = -2.4; #?c_&r = 1.37) = -2.1. ** The presentresults’for the-.spectra of zwitterions of 3-oxopyridine and 3-oxopyrimidine are

differentwhen comparedto those reportedpreviously’2.as we usedsomewhatdiffefent parametersfor proton&ed nitrozen.

% -Mol. Slrucrure, 10

(1971).245-251

ELECTRONIC

STRUCTURE

AND

SPECTRA

OF ORGANIC

MOLECULES.XIV.

251

ACKNOWLEDGMENT

This work was supported in part by the Committee (KNiT) through Research Grant 09.3.1.

on Science and Technics

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J. Mol. Srrucfure, 10 (1971) 245-251