Indian Journal of ChemistryVol. 25A, January 1986, pp. 7-14

Molecular Orbital Study of Some Aromatic N-Oxide Systems

RITA CHADHADepartment of Chemistry, University of Delhi, Delhi 110007

Received 6 October 1983; revised and accepted 26 July 1985

The effect of substituents on the 7l-electronic charge distributions in a set of monosubstituted pyridine N-oxides hasbeen analyzed by means of Pariser-Parr-Pople (PPP) calculations. The electronic charge distribution in the ring has been used topredict the electrophilic and nucleophilic reactivities of these systems. The UV spectra of some N-oxide systems have also beencalculated and compared with the experimental ones. The 7l-electroniccharges, bond orders and energies of molecular orbitalshave been correlated with experimental quantities such as the proton magnetic resonance chemical shifts, infrared stretchingfrequencies and polarographic half-wave reduction potentials. The unrestricted-Hartree-Fock method of Amos and Snyderhas been used to calculate the spin density distributions in some N-oxide radical anions. Empirical relations between the spindensities and experimental ESR hyperfine splitting constants have been derived.

The N-oxides form a class of compounds which notonly find applications in synthetic organic chemistry,but are known for their biological activity'. Manypotent antibiotics contain the N-oxide group. PyridineN-oxides show marked catalytic activity in a numberof diverse reactions; their reactivity pattern is also ofconsiderable theoretical importance. In view of this,we have considered it worthwhile to theoreticallycalculate the electronic properties of aromatic N-oxides with a view to understanding the structure andreactivity of these compounds. Some molecular orbitalcalculations on N-oxides are available".

Method of CalculationThe Pariser-Parr-Pople (PPP) method.' -5 was used

for all n-electron calculations on ground state neutralmolecules. Nine configurations obtained by singleelectron excitation from three HOMOs to threeLUMOs were considered in configuration interactioncalculations. Allring distances were assumed to' be1.40 A. TheC-CH3' C-OCH3,C-OH,C-NH2' C- Cl and N + - 0 - bond distances were taken as 1.51,1.36,1.36,1.46, 1.69 and 1.24A, respectively. Incyanosubstituted N-oxides, the C' - C bond distance in the3 C - C' == N fragment was taken as 1.42 A, while theC' - N distance in - C' == N was taken as 1.16 A.

The one-centre two-electron repulsion integrals (Ypp)were calculated using the Pariser" approximation.Two-centre electron repulsion integrals were calcu-lated by Hie method of Mataga and Nishimoto", Allcore resonance integrals (fJrJ were estimated usingLinderberg's relation. Orbital exponents (Zr) neededfor the evaluation of overlap gradients were takenfrom the literature". Hetero-atom (X) model? was usedfor the methyl group and parameters for the pseudo-hetero atom X were taken from the work of Ray and

Narasimhan! 0. VESCF calculations!! were alsoperformed on some representative systems.

The unrestricted Hartree-Fock (UHF) method ofAmos and Snyder! 2 was used for calculating the spindensity distributions. The parameters employed forthe calculation13 were the same as those used in thePPP calculations.

Results and DiscussionThe dipolar N-oxide group (3 N+ -0 -) can act

both as an electron donor and as an electron acceptor.The positively charged nitrogen and the negativelycharged oxygen have tendencies to withdraw electronsfrom and donate to the n-electron system respectively.The extent to which both these effects operate dependson the nature of the other substituents. Thus, the netelectron density at the dipolar N-oxide group, i.e. (q N+

+qo~ gives an indication of the electron withdrawingor electron accepting ability of the group.

If a substituent (X) is present in the ring, the rr-electronic charge (q,J on the substituent gives ameasure of the electron donating or accepting abilityof the substituent. In Table 1, the quantities (q N+

+qo~, qringand qx are given for a number ofN-oxidesystems. The N-oxide group contributes threeelectrons towards conjugation: one from N + and twofrom 0-. In the parent pyridine N-oxide (qw+qo~> 3, indicating that in the parent compound the N-oxide group acts as a better electron acceptor than anelectron donor, since it accepts 0.07 units of electrondensity from the carbon atoms in the ring.

A comparison of substituent electronic charges (qJfor amino, methoxy and chloro groups indicates theorder of electron donating abilities of thesesubstituents as OCH3 > NH 2> Cl. For each of thesesubstituents, the efficiency of electron donation to the

7

INDIAN J. CHEM .• VOL. 25A. JANUARY 1986

Table I-Calculated Total PPP n-electron Densities at N-Oxide Group (qN' +~ ),Ring (qrinJ and Substituent (qJ in

Pyridine N-Oxide DerivativesN-Oxide qw+qo- qrin. qxPyridine 3.071 4.9292-Methoxypyridine 3.124 4.971 1.9053-Methoxypyridine 3.074 5.011 1.9154-Methoxypyridine 3.100 4.990 1.9102-Aminopyridine 3.105 4.946 1.9493-Aminopyridine 3.069 4.977 1.9544-Aminopyridine 3.090 4.959 1.9512-Chloropyridine 3.087 4.940 1.9733-Chloropyridine 3.070 4.953 1.9774-Chloropyridine 3.080 4.946 1.9742-Cyanopyridine 3.000 4.873 2.1273-Cyanopyridine 3.069 4.898 2.0334-Cyanopyridine 3.054 4.914 2.032

Table 2-Partitioning of Electronic Charge in Pyridine N-Oxide Derivatives having Electron Donating Substituents

N-Oxide % Electronic chargeaccepted by

N-Oxide Ringgroup

2-Methoxypyridine3-Methoxypyridine4-Methoxypyridine2-Aminopyridine3-Aminopyridine4-Aminopyridine2-Chloropyridine3-Chloropyridine4-Chloropyridine

55.83.5

32.266.7

-4.338.859.3

-4.334.6

44.296.567.833.3

104.361.240.7

104.365.4

rest of the n-system is greatest when the substituent isat the 2-position and varies in the order 2> 4 > 3. Sucha conclusion on the electron donating abilities ofsubstituents in various positions in pyridine N-oxide issupported by the work of Katritzky et a114. on theinfrared spectra of pyridine N-oxides. It is also clearfrom Table I that the electron withdrawing cyanogroup withdraws charge most effectively when it issubstituted at the 2-position.

Itis interesting to examine the partitioning of theelectronic charge, withdrawn from or donated to asubstituent. between the N-oxide group and the ring.For electron donating substituents, Table 2 gives thepercentage of the total donated charge that is acceptedby the N-oxide group and the percentage accepted bythe ring. The electron accepting ability of the N-oxidegroup from electron donating substituents (amino andmethoxy substituents) is dependent on the position ofthe substituent in the ring and varies in the order 2> 4> 3. If the substituent is in the 2-position, most ofthe donated charge is accepted by the N-oxide group,whereas the ring accepts most of the charge donated by

8

Table 3-Partitioning of Electronic Charge in Pyridine N-Oxide Derivatives having Electron Withdrawing Sub-

stituentsN-Oxide % Electronic charge

donated by

2-Cyanopyridine3-Cyanopyridine4-Cyanopyridine

N-Oxide Ringgroup

55.9 44.16.1 93.9

53.1 46.9

Table 4-n-Electronic Charge Density (qa) at N-OxideOxygen in Some Pyridine N-Oxides in Ground and Excited

StatesN-Oxide n-Electronic charge density

Ground Excited Statestate

Lowest Lowestsinglet triplet

1.866 1.758 1.6711.873 1.776 1.6891.866 1.775 1.6681.870 1.741 1.6611.870 1.753 1.6721.866 1.756 1.6661.868 1.731 1.6601.877 1.786 1.6941.866 1.793 1.6791.872 l.'l36 1.6741.878 1.789 1.6971.866 1.797 1.6821.873 1.735 1.6781.874 1.777 1.6871.866 1.778 1.6701.871 1.741 1.6641.832 1.687 1.7701.865 1.723 1.6601.860 1.772 1.645

Pyridine2-Methylpyridine3-Methylpyridine4-Methylpyridine2-Chloropyridine3-Chloropyridine4-Chloropyridine2-Hydroxypyridine3-Hydroxypyridine4-Hydroxypyridine2-Methoxypyridine3-Methoxypyridine4-Methoxypyridine2-Aminopyridine3-Aminopyridine4-Aminopyridine2-Cyanopyridine3-Cyanopyridine4-Cyanopyridine

a substituent in the 3- or 4-position. The percentages ofelectronic charge donated by the N-oxide group andthe ring to the cyano group in 2-, 3- and 4-cyanopyridine N-oxides (see Table 3) follow the order2> 4 > 3. In 2- and 4-cyanopyridine N-oxides, most ofthe charge accepted by the cyano group comes fromthe N-oxide group, while in 3-cyanopyridine N-oxide,the cyano group withdraws charge mostly from thering. Hence it is expected that the ring carbon atoms of3-cyanopyridine N-oxide should be most resistant toelectrophilic substitution. It has been observed expe-rimentally!" that 3-cyanopyridine N-oxide is resistantto nitration.

A comparison of the z-electronic charges on the N-oxide oxygen in a number of pyridine N-oxides (seeTable 4) reveals that electron withdrawing substituentsin the ring tend to lower the z-electronic charge at the

CHADHA: M.O. STUDY OF AROMATIC N-OXIDE SYSTEMS

Table 5-Charge Density Distributions in Pyridine N-Oxideand 4- and 2-Hydroxypyridine N-Oxides

N-Oxide Position q~

Pyridine 2,6 0.965 (0.962)3,5 1.006 (0.996)4 0.988 (0.983)

4-Hydroxypyridine 2,6 0.960 (0.964)3,5 1.046 (1.006)4 0.972 (0.973)

2-Hydroxypyridine 2 0.948 (0.952)3 1.047 (1.007)4 0.983 (0.985)5 1.027 (1.007)6 0.960 (0.964)

(a)Va\uesin parentheses were obtained from VESCF calculations.

N-oxide oxygen. Since it is expected that the pKa ofN-oxide conjugate acids should decreaseas the electroniccharge on the oxygen decreases, electron withdrawingsubstituents should lower the pKa, while electrondonating substituents should increase the pK; of N-oxide conjugate acids. This expectation is in accordwith experimental results obtained for a series of N-oxide conjugate acids. In the lowest singlet excitedstate, qo decreases, indicating that N-oxides becomeweaker bases in the excited state. A similar trend is alsoobserved in the lowest triplet excited state.

The electrophilic and nucleophilic reactivities ofpyridine N-oxide have drawn a great deal of interest.Table 5 gives the charge density distributions at thering carbons in pyridine N-bxide and its 2- and 4-hydroxy derivatives. In pyridine N-oxide, the n-electronic charge is the largest at the 3-position,thereby rendering the 3-position most susceptible toelectrophilic attack, in accord with the experimentalobservation 17 that sulfonation of pyridine N-oxidegives a good yield of the3-sulfonic acid. The n-electronic charges at the 2- and 4-positions are lessthan unity, indicating that nucleophilic substitutionshould be facile at these positions. It has been foundexperimentally+" that 2- and 4-chloro derivatives ofheterocyclic N-oxides are easily formed by the actionof phosphorus oxychloride, phosphorus pen tach-loride, or sulfuryl chloride. These reactions are knownto be nucleophilic substitution reactions. The chargedensity distributions indicate that the ease ofnucleophilic attack should follow the order 2> 4 > 3.In the event of the 2-position being blocked,nucleophilic substitution preferentially occurs at the 4-position, as confirmed by experimental observations.

In 4-hydroxypyridine N-oxide, the x-electroniccharge is maximum at the 3-position,indicating thatelectrophilic substitution should occur at 3-position,again in accord with the experimental obser-

vations 19,20. The charge density distribution in 2-hydroxy pyridine N-oxide indicates that 3- and 5-positions are the preferred sites for electrophilicsubstitution. It has been found experimentally that 2-hydroxypyridine N-oxide undergoes nitration at 5-positiorr", or at both 3- and 5-positions22

. Anexamination of the charge densities obtained fromVESCF calculations leads to the same conclusionsregarding the relative reactivities of various positionsin the ring.

All these results lead to the conclusion that thesemiempirical Pariser-Parr-Pople method is quitesuitable for studying the z-electronic structures of N-oxide systems. To prove this point further, we havealso correlated various calculated quantities with theexperimental data, and have found good agreementbetween the two.

Calculated transition energies for pyridine Nsoxide,along with available experimental data23,2\ are givenin Table 6. The calculated results are in goodagreement with the experimental data. Introduction ofanother nitrogen, as in pyrazine mono- and pyrazinedi-N-oxides leads to a bathochromic shift (see Table 6).The calculated spectrum for quinoxaline N-oxide alsoagrees well with the experimental data25,26.

It is known that greater the strength. of a bond,higher is its stretching frequency. The bond order ofthe N + - 0 - bond is a rough measure of the forceconstant, and hence the strength, of this bond. A leastsquares fit analysis gives a linear relation of the form(1)

vN+ -0 - (em -1)=667.Pw_o-+ 1053 ... (1)

The vN + - 0 - modes of some derivatives of pyridineN-oxide, calculated on the basis ofEq. (1), are given inTable 7. The calculated results are in good agreementwith available experimental data14,23.

It has been recognized for some time that protonchemical shifts (r.J in aromatic molecules can becorrelated with the n-electron densities (q.) in thesemolecules. Proton magnetic resonance data areavailable28.29 for some N-oxides. A least squares fitanalysis has been carried out and the relation (2) isfound to be quite adequate for predicting the protonchemical shifts in N-oxides. Table 8 lists the results ofthis study. Good agreement between predicted andexperimental shifts can be observed.rH= 1l.96 (1-q,,)+2.95 ... (2)

Further, a linear relationship is expected betweenthe polarographic half-wave potentials and the energyof the LUMOs. A least squares fit analysis yieldsEq.(3)E1/2= -O.626ELUMO-3.037 ... (3)

9

INDIAN J. CHEM., VOL. 25A, JANUARY 1986

with ELUMO expressed in eV. Here El/2 is assumed to bedetermined by the LUMO energy of the neutralmolecules and not of the anion radicals. Table 9 givesthe observed and calculated values of E1/2• Goodagreement between the two quantities can be seen. A

comparison of the energies of LUMOs in the set ofcompounds listed in Table 9 indicates that the ease ofreduction of a N-oxide group increases as the numberof condensed rings and/or the number of nitrogenatoms in the molecule increases'.

Table 6-CalcuIated x-x· Singlet-Singlet Transition Energies (L\E) and Oscillator Strengthsfor Pyridine N-Oxide and Its Derivatives

N-Oxide AE(eV) Oscillator log £

strength (Exptl.")Calc.4.195.124.145.054.155.1I5.964.155.036.034.114.984.095.034.104.984.125.014.084.934.01-5.()65.844.084.944.135.105.954.135.024.144.984.155.094.164.954.044.975.953.904.675.673.713.753.684.085.14

5.41}5.605.87

("'Taken from references 23-27; (b'mono-N:.oxide; ("di-N-oxide; and (d'3-oxide.

Pyridine

2-Methylpyridine

3-Methylpyridine

4-Methylpyridine

2,4-Dimethylpyridine

2.5-Dimethylpyridine

2.6-Dimethylpyridine

3.4-Dimethylpyridine

2.4,6-Trimethylpyridine

3-Hydroxypyridine

4-Hydroxypyridine

3-Aminopyridine

4-Aminopyridine

2-Chloropyridine

3-Chloropyridine

4-Chloropyridine

QuinoxaJine

Quinazolined

Obs."

4.88

4.79

4.885.93

4.846.02

4.75

4.79

4.80

4.70

4.79

4.715.51

4.733.954.925.30

4.49

4.68

4.61

4.54

4.715.79

4.105.583.593.77

3.60-3.694.134.79

5.39-5.79

0.000.290.010.220.010.240.890.000.340.860.000.270.020.180.010.170.000.290.000.220.020.180.880.010.390.010.230.890.000.350.000.170.000.200.000.370.060.300.750.000.480.820.010.390.200.130.67

0.06}1.040.25

4.07

4.06

4.11

4.22

3.944.03

3.60-3.613.754.27

4.15-4.21

10

CHADHA: M.O. STUDY OF AROMATIC N-OXIDE SYSTEMS

The results of UHF studies are given in Tables 10-13. It is interesting to note that the value of <S2 > for theUHF wavefunction is always greater than 0.75 andbecomes much closer to this value after annihilation ofthe quartet spin component (see Table 10).

The proton hyperfine splitting constants, aH, forprotons attached to aromatic carbons are assumed tobe proportional to the spin density (pc) on the attachedcarbons (see Eq.4)

caH = QCHpC +K ... (4)

where K is a constant.A least squares fit analysis of the data given in Table 10indicates a linear relation of the form (5)

aH= -23.04Pc-0.90 ... (5)

between Pc and aH' The calculated ring protonsplittings are in good agreement with the experimen-tal? values (see Table 10).

Table 7-Comparison of Calculated and Observed InfraredvN + - 0 - of Some Derivatives of Pyridine N-Oxide

N-Oxide PW -0- vN+ -0 -(em -I)

Pyridine 0.32462-Methylpyridine 0.31124-Methylpyridine 0.31683-Chloropyridine 0.32464-Chloropyridine 0.32062-Cyanopyridine 0.36854-Cyanopyridine 0.33394-Nitropyridine 0.33962,4-Dimethylpyridine 0.30372,6-Dimethylpyridine 0.29803,4-Dimethylpyridine 0.31732,4,6-Trimethylpyridine 0.2908(')Obtained from references 14 and 23.

Calc(Eq.1)127012611265127012671299127612801256125212651247

Obs'

126512641262126412731295128212831250125912591249

Table 8-Calculated Proton Chemical Shifts (rn> for SomeN-Oxides

N-Oxide Position (l-q,J

Calc" Obs"

3,5 -l'.0060 2.88 2.724 0.0122 3.09 2.922 0.0103 3.07 3.242,6 0.0381 3.40 3.333,5 -0.0309 2.58 2.553,4,5 -0.0449 2.47 2.412,6 0.0404 3.43 3.343,5 -0.0489 2.36 2.302,6 0.0366 3.39 3.463,5 -0.0178 2.74 2.81

(''Calculated from the relation: tH = 11.96(I -qJ +2.95;and (b)takenfrom references 28 and 29.

Pyridine

3-Methylpyridine4-Methylpyridine

2,6-Dimethy1pyridine4-Methoxypyridine

4-Chloropyridine

The constant Kin Eqs. (4) and (5), which may beinterpreted as a deviation from the McConneiPorelation,aH;=: Q~HPC ., .(6)

is small and can be neglected. A least squares fitanalysis of the data given in Table 10 to Eq. (4) yieldsEq.(7)

... (7)

The Q~H value of - 28.62 G is reasonable inmagnitude. Although Q~H depends on a number offactors including the structure of the radical, bondangles, steric factors, whether the radical is an anion ora cation etc., all previous calculations " have resultedin Q~Hvalues which lie in the range - 22.5 to - 30 G.Table 10 gives the calculated values of aH obtainedusing Eq. (7).

According to the Karplus and Fraenkel theory ", wecan write aN, the nitrogen hyperfine splitting constant,as

aN=(SN+~ Q~X)PN+L Q~;NPx;1 1

... (8)

where SN is the a-n interaction parameter contributedby the nitrogen IS2 inner shell and Q~x; and Q~;Narethe interaction parameters contributed by the N - Xiand Xi - N a-bonds, respectively. Carrington andSantos-Viega " have suggested that the contribution

Table 9-Predicted Polarographic Half-Wave Potentials forN-Oxides

N-Oxide ELUMo

(eV)Calc. Obs'

(Eq.3)Pyridine - 1.3736 - 2.178 - 2.2974-Methylpyridine -1.2081 -2.281 -2.3754-Chloropyridine - 1.2926 - 2.228 - 1.8894-Cyanopyridine -2.0536 -1.751 -1.5573-Cyanopyridine -1.8318 -1.890 -1.667Pyrazine" - 2.0758 -1.738 -1.809Pyridazine" -1.8999 - 1.848 -1.898Pyrimidine" -1.7845 -1.921 -1.949Pyrazine" -1.6837 -1.984 -1.616Quinoline - 2.2457 - 1.632 - 1.809Isoquinoline -2.1217 -1.710 -1.946Phthalazine" -2.3027 -1.596 -1.716Quinoxaiine" -2.7417 -1.322 -1.419Quinoxaline" -2.4152 -1.526 -1.241Phenanthridine -2.3071 -1.594 -1.774Acridine -2.8638 -1.245 -1.300Phenazineb -3.3202 -0.960 -0.972Phenazine" -3.0134 -1.152 -0.8333,5-Lutidine - 1.1907 - 2.292 - 2.365(''Obtained from reference 2; (b)mono-N-oxides;and (c)di-N-oxides.

11

INDIAN J. CHEM., VOL. 25A, JANUARY 1986

Table IO-Calculated Spin Densities (pi) and Ring Proton Splitting Constants (aM) in N-Qxide Radical Anions'N-Oxide Position (S2)bo (S2).. P; laHI(G)

Calc.b Calc.c Obs,d

2,6 0.791 0.750 0.092 3.02 2.63 3.013,5 0.015 1.24 0.43 0.444 0.303 7.88 8.67 8.512,6 0.816 0.751 0.012 1.18 0.34 0.613,5 0.086 2.88 2.46 2.362,6 0.880 0.757 -0.035 0.09 1.00 1.263,5 0.106 3.34 3.03 3.322,6 0.800 0.751 0.019 1.34 0.54 1.623,5 0.097 3.13 2.78 2.132,3,5,6 0.811 0.750 0.046 1.96 1.32 1.373 0.849 0.753 0.005 1.01 0.14 1.414 0.083 2.81 2.38 4.585 0.207 5.67 5.92 5.916 -0.080 0.94 2.29 0.422,3 0.841 0.754 0.053 2.12 1.52 1.815,8 0.103 3.27 2.95 2.446,7 0.021 1.38 0.60 1.444,5 0.841 0.756 0.082 2.79 2.35 2.893,6 0.022 1.40 0.63 1.432,7 0.016 1.27 0.46 0.491.,8 0.075 2.63 2.15 2.589 0.172 4.86 4.92 5.341,8 0.816 0.753 0.079 2.72 2.26 1.804,5 0.025 1.47 2.72 1.803,6 0.032 1.65 0.92 1.652,7 -0.004 0.81 0.11 1.231,4,5,8 0.870 0.759 0.068 2.46 0.95 1.752,3,6,7 0.014 1.22 0.40 1.36

(a'(S2)bo and (S2) •• are the expectation valuesof the S2 operator before and after annihilation of the quartet spin component from the UHFwavefunctions.Pc is the spin density obtained after annihilation of the quartet spin component; (b'Theoreticalproton splitting constants laHIwereobtained by using the relation aH = - 23.04Pc - 0.90;«'Obtained from McConnell's relation aH = - 28.62Pc; (dlObtainedfrom reference2; (O'mono-N-oxides;and (fJdi-N-oxides.

Pyridine

4-Cyanopyridine

4-Nitropyridine

Pyrazine"

Pyrazine"Pyridazine"

Quinoxaline'

Acridine

Phenazine"

Phenazlne'

from the term 7 Q~;NPX. is small. Neglecting this term,

Eq. (8) reduces toaN=(SN+~Q~X)PN ... (9)

I

oraN=Q~PN ... (10)

The best value. of Q~, determined by the leastsquares fit method using UHF spin densities andexperimental splitting constants, is 25.74G. Thenitrogen splittings (a~, calculated from relation (10)using the above value of Q~, are given in Table 11andgood agreement-with experiment can be observed(correlation coefficient=0.9l). A value of21 G for Q~was obtained by Hinchliffe34, using relation (10).Leastsquares fitting to the equationaN=Q~PN+KN ... (11)where KN is a constant, was also carried out. The valueobtained for Q~ is 32.31 G and KN= -2.27G, and thecorrelation coefficient is 0.91. In Table 11, acomparison is made between the experimental values

12

of aN and those calculated from Eq. (11) using theabove parameters.

The fact that the value of KN in Eq. (11) is not

negligible, suggests that the term I: Q~-NPX- in Eq. (8)i I I

should be taken into consideration. For the case of the

fragment ~~N -0 -Eq.(8) can be written as

aN=Q~PN+Q~NPO+Q~N(PC+pd ... (12)

A least squares fit of Eq. (12) with experimental laNIvalues yields the following values for the a-tt

interaction terms: Q~=41.33G; Q~N= -9.33G; andQ~N= -41.77G. The results of this calculation,together with the experimental nitrogen hyperfinesplitting constants, are given in Table 12. Use ofEq. (12)predicts splitting constants which are in betteragreement with experiment and the correlationcoefficient increases to 0.98.

Inclusion ofa constant in Eq. (12)and a least squaresfit analysis yields Eq. (13)

CHADHA: M.O. STUDY OF AROMATIC N-OXIDE SYSTEMS

Table l l=-Calculated Spin Densities (p~ and NitrogenSplitting Constants in the Radical Anions of Some N-OxidesN-Oxide PN IUN!(G)

Calc.' Calc.b Obs."

Pyridine 0.388 9.99 10.27 10.914-Cyanopyridine 0.346 8.91 8,91 9.614-Nitropyridine 0.206 5.30 4.39 4.73Pyrazine" 0.423 10.89 11.40 12.09Pyrazine' 0.326 8.39 8.26 9.49Pyridazine" 0.412 10.60 11.04 10.03Quinoline 0.277 7.13 6.68 6.00Quinoxaline' 0.258 6.64 6.07 6.72Acridine 0.330 8.49 8.39 6.82Pbenazine 0.252 6.49 5.87 5.91

I. 'Calculated from the relation aN = 25.74 PN; Ib)calculated from therelation aN = 32.31 PN- 2.27; I<)obtained from reference 2; (d)mono-Nsoxide; and I"di-N-oxide.

Table 12-Calculated Nitrogen Splitting Constants <laNI) inN-Oxide Radical Anions

N-Oxide Po

Calc." Calc. b Obs.<

Pyridine4-CyanopyridinePyrazine"Pyrazine"Pyridazine"QuinolineQuinoxaline'AcridinePhenazine"Phenazine'

0.3880.3460.4230.3260.4120.2770.2580.3300.4290.252

0.0940.1060.1220.0810.1170.1020.0910.1730.2620.121

0.1850.0240.0380.0920.1750.0260.027

-0.043-0.069-0.037

10.389.65

12.039.23

10.516.956.616.817.435.71

10.389.69

12.159.21

10.536.876.516.797.495.61

10.919.61

12.099.49

10.036.006.726.827.585.91

""Calculated from the relation: aN=41.33 p,,-41.77 Po-9.33 (Pc+ P, ); '·'calculated from the relation: a,; =42.84 p,; -42.71 Po -9.97(I', +P, ) -0.38; '''obtained from reference 2; 'dlmono-N-oxide; and'''di-N-oxide.

a-; =42.84 fiN -42.71 Po-9.96(pc+ pd-0.38 ... (13)Theconstant term. -0.38 G. is small in magnitude andcan be neglected. The value of Q~N agrees withpreviously reported vatues+'". The value of Q~ alsoagrees with previous calculations".

Talcott and Myers:" obtained the followingeq uation for the hyperfine splitting constants of thering nitrogen atoms in the heterocyclic compounds:a,,=27.3p,,-1.7(p{+pc) ... (14)

Using this equation. a value of 5.73 G has beenobtained for the hypcrfine splitting constant of thenitrogen atom in pyrazine mono-N-oxide. Theexperimental value is 5.55 G. Table 13 gives the resultsof the calculation for pyrazine, phenazine andpyridazine mono- N -oxides.

For the nitrogen atom or the cyano group. Eq. (15)was shown:" to hold

(1,,=23.1 p,,-6.Xpc ... (15)

Table I3-Calculated Nitrogen Hyperfine Splitting Con-stants (laNI) of Ring Nitrogen in N-Oxide Radical AnionsMono-N-oxide PN PC+Pc laNI(G)

Calc.' Obs."

Pyrazine 0.222 0.195 5.73 5.55(4)Phenazine 0.074 0.040 1.95 3.60(10)Pyridazine 0.255 0.417 6.25 5.26«2)

(')Calculated from the equation: aN=27.3 PN-1.7 (Pc+ Pc); and(b)obtained from reference 2.

Using this equation for the nitrogen atom of the cyanogroup in 4-cyanopyridine Nsoxide, aN was calculatedas 1.83 G The experimental value is 1.76 G.

From the present study, it appears that thesemiempirical Pariser-Parr-Pople method can explainthe electronic structures of N-oxides quite satisfac-torily, while the unrestricted Hartree-Fock method ofAmos and Snyder, in conjunction with the Karplus-Fraenkel relation, can explain nitrogen hyperfinesplittings in aromatic N-oxide radical anions quitesatisfactorily.

AcknowledgementThe authoress is grateful to Prof. N K Ray for his

interest in the work and to the CSIR, New Delhi for theward of a junior research felIowship.

ReferencesI Ochiai E. Aromatic Amine Oxides (Elsevier, Amsterdam), 1967.2 Kubota T. Nishikida K. Miyazaki H, Iwatani K & Oishi Y. J Am

chern Sac, 90(1968) 5080; Elveth EM, Theoret Chim Acta, 11(1968) 145; Saison R & Jaffe H H. Compt Rend. 256(1964)667.

3 Pariser R & Parr R G. J chem Phys, 21 (1953) 466. 767.4 Pople J A. Trans Faraday Soc. 49 (1953) 1375.5 Pariser R. J chem Phys, 21 (\ 953) 568.6 Mataga N & Nishimoto K. Z phys Chern. 13 (1957) 140.7 Linderberg J. Chern phys ieu. I (1967) 39.8 Mulliken R S. Rieke C A. Orloff D & Orloff H, J chem Phys, 17

(1949) 1248.9 Matsen FA, J Am chem Soc. 72 (1950) 5243.

10 Ray N K & Narasimhan PT. Chemphys Leu, 2(1968) 101,634.11 Ray N K & Rastogi R C Indian J Chern. 13 (1974) 355.12 Amos A T & Snyder L C. J chem Phys, 41 (1964) 1773.13 Hinze J & Jaffe'H H. J Am chem Soc. 84 (1962) 540.14 Katritzky A R & Gardner J N. J chem Soc. (1958) 2192.15 Jujo R. J Pharm Soc Japan. ~ (1946) 21.16 Jaffe H H & Doak G O. J Am chem Soc. 77(1955) 4441, 4451.17 Mosher H S & Welch F J. J Am chem Soc. 77 (1955) 2902.18 Murakami M & Matsumara E. Nippon Kagaku Zasshi. 70( 1949)

393.19 Hayashi E. J pharm Soc Japan. 70 (1950) 145.20 Ochiai E & Futaki K. J pharm Sac Japan. 72 (1952) 274.21 Loll W A & Shaw E. J Am chem Soc. 71 (1949) 70.22 VanAmmers M & den Hertrog H J. Reel Trov Chim. 75(1956)

1259.23 Gherseui S. Maccagnani G. Mangini A & Montanari F. J

Heterocyclic Chern, 6 (\969) 859.

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INDIAN J. CHEM., VOL. 25A, JANUARY 1986

24 Jaffe H H & Doak G 0, JAm chem s«. 77 (1955) 4441, 4451.2S Klein D & Berkowitz J, JAm chem Soc, 81 (1959) 5160.26 Otomasu H, Yamaguchi R, Ishigooka K & Takamashi N, J

pharm Soc Japan, 82 (1962) 1434.27 Armarego W L F, J chem Soc, (1962) 5030.28 Abramovitch R A & Davis J D, J chem Soc (8), (1966) 1137.29 Katritzky A R & Lagowski J M, J chem Soc, (1961) 43.

14

30 McConnell H M, J chem Phys, 14 (1956) 632, 764.31 Carrington A, Quart Re» (London), 17 (1963) 67.32 Karplus M & Fraenkel G K, J chem Phys, 35 (1961) 1312.33 Carrington A & Dos Santos-Viega, J Moree Phys, 5 (1962) 21.34 Hinchliffe A, Theoret chim Acta, 5 (1966) 208.35 Talcott C L & Myers R J, Molec Phys, 12 (1967) 549.36 Rieger P H & Fraenkel G K, J chem Phys, 37 (1962) 2795.