NUCLEAR QUADRUPOLE RESONANCE O F NITROQEN-14
2501
Nuclear Quadrupole Resonance of Nitrogen-14 in Some Pyridine Derivatives by Ryuichi Ikeda, Shinzaburo Onda, Daiyu Nakamura, and Masaji Kubo Department of Chemistry, Nagoya University, Chikusa, Nagoya, Japan
(Received January 8, 1968)
The pure quadrupole resonance frequencies of 14N in chloro-, cyano-, amino-, and methylpyridines and in aniline were determined at liquid nitrogen temperature. From the observed V I and v11 frequencies, the quadrupole coupling constant and the asymmetry parameter were evaluated and assigned to ring nitrogen and nitrogen in substituents. The u- and *-bond ionicities of ring N-C bonds were evaluated and were discussed in relation to the data of molecular orbital calculations.
Introduction The electronic structure of heterocyclic compounds is complicated by the role of heteroatoms, as contrasted with the case of homocyclic compounds. I n view of the ever increasing importance of nitrogen heterocycles, a number of experimental studies have been made on the electronic state of these compounds in the field of electronic absorption spectra, dipole moments, electron spin resonance, etc. The results have been used to check the adequacy of various parameters to be used in molecular orbital calculations. 14Nhaving a nuclear spin equal to unity is amenable to nuclear quadrupole resonance studies and permits the experimental determination of both the quadrupole coupling constant and the asymmetry parameter. Accordingly, from the observed data, one can obtain valuable information about the electronic states of various nitrogen-containing compounds of chemical interest. In fact, since Guib@ and others2v3succeeded for the first time in observing 14Nnuclear quadrupole resonance, a number of article^^-^ have been published on the nuclear quadrupole resonance of aromatic nitrogen heterocycles. One of the characteristic features of 14N quadrupole resonance data is that the asymmetry parameter is very high, as in pyridine (39%),I pyrazine (54%),6 and sym-triazine (44%).K However, cyanuric chloride shows a small asymmetry parameter (1.7%).* This suggests the possibility of investigating the effect of substituents on ring nitrogen by nuclear quadrupole resonance. Since the quadrupole coupling constant and the asymmetry parameter can be correlated with the ionicity of N-C bonds regarding both u- and a-bond character, one can evaluate the electronic population of u and T electrons about a nitrogen nucleus and compare the results with those of molecular orbital calculations. The present investigation has been undertaken in order to elucidate the electronic state about nitrogen atoms in pyridine and its derivatives from the pure quadrupole resonance spectroscopy of 14N.
Experimental Section Apparatus. A modified
Pound-Watkins-type
spectrometer, already described,' was used for the observation of I4N quadrupole resonance signals. Frequency modulation was employed for the determination of resonance frequencies, while Zeeman modulation was used for the assignment of observed lines to V I and $I. Field modulation was performed at a strength of about 100 G or less. Materials. All the materials used were procured from commercial sources and were purified by distillation or recrystallization from organic solvents. Since 2- and 3-chloropyridines, 2- and 3-methylpyridines, and aniline are liquids a t room temperature, they were frozen slowly in glass tubes and cooled with liquid nitrogen. 2-, 3-, and 4-aminopyridines, 2-, 3-, and 4-cyanopyridines, and 2,6-dichloropyridine, which are solids a t room temperature, were melted in sample tubes to increase the filling factor and were allowed to solidify by gradual cooling. 2-Aminopyridine was dissolved in an approximately equal weight of heavy water and then water was allowed to evaporate in a vacuum desiccator. This process was repeated several times until the extent of deuteration was estimated to amount to about 90% of hydrogen in amino groups.
Results Since I4N has a nuclear spin equal to unity, one can usually observe two resonance frequencies, V I and YII, when the asymmetry parameter q is finite, VI
= 1 -e&q(3
+d
4 YII =
-e&p(3 1 4
- 7)
(1) L.Guib6, Compt. Rend., 250, 3014 (1960). (2) H.Negita and P. J. Bray, J. Chem. Phys., 33, 1876 (1960). (3) 8. Kojima and M. Minematsu, J . Phys. 800. Jap., 15, 356 (1960). (4) L.Guib6, Ann. Phys. (Paris), 7, 177 (1962). (5) L.Guib6 and E. A. C . Lucken, Mol. Phys., 10, 273 (1966). (6) E. Schempp and P. J. Bray, J . Chem. Phys., 46, 1186 (1967). (7) R. Ikeda, D. Nakamura, and M. Kubo, J . Phys. Chem., 70, 3626 (1966). Volume 79,Number 7 July 1968
R. IKEDA, S. ONDA,D. NAKAMURA, AND M. KUBO
2502 Here eQq denotes the quadrupole coupling constant in frequency units. Table I shows the resonance frequencies observed at liquid nitrogen temperature. Figure 1 shows the spectra of 2,6-dichloropyridine as an example. The absorption derivative curves taken by frequency modulation are not quite symmetric, owing to the saturation effect, the negative peak being stronger than the positive peak when scanning is made with increasing frequency. One can obtain more symmetric derivative curves by decreasing the power level of radiofrequency oscillation and/or increasing the sweep rate. For the accurate determination of resonance frequencies, it is desirable to employ the frequency modulation in the absence of saturation. I n absorption curves recorded by Zceman modulation, VI has a negative wing on the high-frequency side, while vI1 is accompanied by a negative wing on the low-frequency side.8 Thus resonance lines can be assigned to VIand vI1 with the aid of line shapes. I n general, VIwas found to be stronger than vII in Zeeman m o d u l a t i ~ n . ~ This rule does not necessarily hold when powder crystals are oriented in space.
SI 3’
I
-
-1, --! -..>
.:---r
...... . .. . ..... -__ _.,_--.--.I. . ..... ...... .....
I
I,1 --
”-’-‘T’
,-... .... ._._.-.,,........ ._. . -. .. !j . ‘ ’ .. . d
,
3 increasing-> Table I : Quadrupole Resonance Frequencies, V I and P, of 14N in Some Pyridine Derivatives and Aniline at Liquid Nitrogen Temperature Compound
V I ,kcps
2-Chlorop yridine 3-Chlorop yridine 2,6-Dichloropyridine 2-Cyanopyridine
3-Cyanopyridine 4Cyanopyridine
2-Aminop yridine 3-Aminopyridine 4-Aminopyridine Aniline
2-Methylp yridine 3-illethylpyridine a
Reference 2.
41,
3626.7 f 0 . 1 3891.3i.0.1 3304.8fO.l 3947.6 f 0 . 1 3039.8i.O.l 3039.6 i.0.2‘ 3899.7 i 0 . 1 3010.8f0.1 4099.4i.O.l 2935.2iO.l 2935.3 =t0.2“ 2841.8rtO.l 2842b 2970.3h0.1 3810.4h0.1 3138.2i.0.1 2967.9i.O.l 2915.5i.0.1 3243.2 f 0 . 2 \ 3183.7 i 0 . 2 3712.6f0.1 3920.5 f 0 . l
t
kcps
3050.7 f 0 . 1 3062.8i0.1 3079.1i.O.l 3129.8fO.l 2898.4hO.l 2897.8 ==! 0.2& 3050.2i0.1 2816.8-10.1 3057.6 f 0 . l 2907.1f0.1 2907.3 f 0.2a 2776.1 f 0.1 2776b 2355.2rtO.l 2934.4i.O.l 2426.9f0.3 2291.7 h 0 . 1 .
;E:;
:Oo:t
\ \ 2649.8 i0 . 2 2975.010.1 3009.9i.0.1
Reference 4.
All chloro- and methylpyridines studied show a pair of resonance lines attributable to VI and VII. On the other hand, both cyano and amino derivatives of pyridine show two pairs of VI and vI1 lines. These facts indicate that the number of chemically nonequivalent nitrogen atoms in a molecule is equal to that The Journal of Physical Chemistry
Figure 1. Nuclear quadrupole resonance spectra of 2,6-dichloropyridine recorded by frequency modulation (top) and Zeeman modulation (bottom).
of nonequivalent atomic sites of nitrogen in crystals at liquid nitrogen temperature. However, aniline shows two pairs of VI and vlI, in spite of the presence of a single nitrogen atom in a molecule. This indicates that there are two crystallographically nonequivalent aniline molecules in crystals. Unfortunately, no crystallographic data are available for any of these compounds. Previously, only one pair of lines has been detected for 2-cyano, 4-cyano, and 2-amino derivatives of ~ y r i d i n e . ~Our ? ~ results are in good agreement with these. From observed frequencies, VI and vII, quadrupole coupling constants and asymmetry parameters were evaluated, as shown in Table 11. For the chloro, cyano, and methyl derivatives of pyridine, they could be calculated in a straightforward manner. For the amino derivatives and aniline, however, an ambiguity remains as to whether VI of higher frequency should be combined with vI1 of higher frequency or whether the alternative correspondence should be made. The choice can be settled as follows. I n the case of 2aminopyridine, VI at 2970.3 kcps and vI1 at 2355.2 kcps were observable only under a high field strength of Zeeman modulation. The signals were sharp and were (8) P. A. Casabella and P. J. Bray, J . Chcm. Phys., 28, 1182 (1958) ; 2 9 , 1105 (1958).
(9) H. Negita, ibid., 44, 1734 (1966).
NUCLEAR
QUADRUPOLE RESONANCE OF NITROGEN-14
2503
Table I1 : Quadrupole Coupling Constants, e&q, and Asymmetry Parameters, 7, of 14N in Some Pyridine Derivatives and Related Compounds at Liquid Nitrogen Temperature Compound
e&,
2-Chlorop yridine 3-Chlorop yridine 2,6-Dichloropyridine 2-Cyanopyridine 3-Cyanopyridine 4-Cyanopyridine Cyanobenzene" 2-Aminop yridine
3-Aminop yridine
4-Aminop yridine Aniline 2-Methylpyridine 3-Rkthylpyridine
4-Meth ylp yridind Pyridine* a
Reference 2.
7, %
kops
4451.4f0.2 4836.110.2 4255.9 1 0 . 2
25.87 3 ~ 0 . 0 2 36.74 10.02 10.61i0.02
4718.2 f 0 . 2 3958.8 f 0 . 2 4633.310.2 3885.0 f 0 . 2 4771.310.2 3894.9 f 0 . 2 3885.4f0.3 3745.310.2 3550.3f0.2 4496.510.2 3710.010.3 3780.7 f 0 . 2 3506.4f0.2 3932 4458.4f0.2 4620.3f0.2 4414.0 4600
34.66 f 0.02 (ring) 7 . 1 4 f 0.02 (cyano) 36.67 i 0.02 (ring) 9 . 9 9 f 0.02 (cyano) 43.67 f 0.02 (ring) 1 44 i 0.02 (cyano) 1O.73fO0.02 3 . 5 1 f 0.02 (ring) 34.65 =k 0.02 (amino) 38.96 f 0 . 0 2 (ring) 38.35 f 0 . 0 3 (amino) 8 . 4 6 f 0.02 (ring) 38.57 f 0.02 (amino) 26.9 (av) 33.09 f 0 . 0 2 39.42f0.02 34.2 39 (av) I
* Reference 1.
accompanied by a sharp negative wing, indicative of a large asymmetery ~ a r a m e t e r . ~On the other hand, V I at 2541,s kcps and v I I at 2776.1 kcps could be observed even when the modulation field was weak. The accompanying negative wings were broad, suggesting that these lines correspond to each other to yield a small asymmetry parameter (Figure 2). I n the case of 3-aminopyridine1 V I lines at 3810.4 and 3138.2 kcps and a v I 1 line at 2934.3 kcps were not detectable unless the Zeeman-modulation field was increased. Negative wings were sharp, indicating a fairly large asymmetry parameter. The line at 2426.9 kcps could not be observed by Zeeman modulation, although it could be located by frequency modulation. If V I of lower frequency is combined with v I 1 of higher frequency, the resulting asymmetry parameter is small and contradicts the aforementioned line shapes recorded by Zeeman modulation. Therefore, V I of higher frequency must be combined with Y I I of higher frequency, as shown in Table I. I n the case of 4-aminopyridine, V I at 2967.9 kcps and v I 1 at 2291.7 kcps were not detected unless the Zeeman-modulation field was strong. They showed a sharp negative wing. On the other hand, V I at 2915.5 kcps and v I 1 at 2755.5 kcps could be observed even under a weak modulation field and showed a broad negative wing. Accordingly, the correspondence between V I and VI' is obvious. The same method is not applicable to aniline, because two V I lines as well as two VI' lines are close to each other. Therefore,
3 i ncreasingl-> Figure 2. Yuclear quadrupole resonance spectra of ring nitrogens in 2-aminopyridine (top) and 2-cyanopyridine (bottom) recorded by Zeeman modulation with modulation fields of 60 and 100 G, respectively. When the asymmetry parameter is small, negative wings are broad as in the spectrum of the former compound. Sharp negative wings observed for the latter compound are indicative of a large asymmetry parameter.
the averages were taken for the calculation of the quadrupole coupling constant and the asymmetry parameter, errors being about 1.1% for e&q and 12% for 7. The two quadrupole coupling constants and the corresponding asymmetry parameters observed for cyanopyridines arise from ring nitrogen and nitrogen in cyano groups. By taking into account the data of pyridine' and cyanobenzene,2 the higher values of eQq and 7 are attributed to ring nitrogen atoms, while the lower ones are assigned to nitrogen atoms in cyano groups. Two sets of the quadrupole coupling constant and the asymmetry parameter observed for each of aminopyridine isomers surely correspond to the existence of two kinds of nitrogen atoms in a molecule. However, the assignment of the two sets to two different nitrogen atoms is not easy, because the observed resonance lines deviate from those of pyridine and aniline to a considerable extent, owing to a strong electronic interaction between the amino nitrogen and the ring nitrogen. Therefore, me have studied N-deuterated 2-aminopyridine in an attempt to obtain some clue to the solution of this problem. At liquid Volume 72, Number 7
July 1968
R. IKEDA, S. ONDA,D. NAKAMURA, AND &KUBO !I,
2504 nitrogen temperature, three weak lines were observed at 2992.5, 2843.2, and 2841.0 kcps within an accuracy of ~ t 0 . 5kcps. The last line was the weakest among the three lines and appeared as a shoulder on the second line. The lowest frequency agrees excellently with YI (2841.8 kcps) of the undeuterated compound within experimental errors, indicating the partial deuteration of amino groups. The highest frequency is about 22 kcps higher than one of the V I lines of'normal compound at 2970.3 kcps, while the second frequency is very close to the other VI line (2841.8 kcps) of the undeuterated compound. It is expected that the resonance lines of deuterated amino groups shift to some extent, owing to the secondary hydrogen isotope effect,10-12 whereas those of ring nitrogen atoms are affected to a lesser extent. Accordingly, the line of the highest frequency is attributed to nitrogen atoms in amino groups and the second line is presumed to be due to ring nitrogen atoms. Although VI lines could be observed, vI1 lines were undetectable, owing to their low intensity. Since 4-aminopyridine is analogous to 2-aminopyridine in many respects as a 7r-electron system, assignment can be made for the former compound on the basis of results for the latter by taking into account the relative magnitude of quadrupole coupling constants and especially asymmetry parameters. The two sets of eQq and 17 of 3-aminopyridine are close to each other; therefore, it is very difficult to make a conclusive assignment. However, since the eQq values of aniline and aliphatic amines are about 3.9 and 4.0 ;\Icps,4113 respectively, while that of pyridine is about 4.6 llcps, it seems to be reasonable to assign eQq = 3.710 I\Icps to amino nitrogen and eQy = 4.497 Alcps to ring nitrogen.
Discussion I n order to discuss the electronic structure of a nitrogen atom in a pyridine ring, let a coordinate system be taken with its origin at the nitrogen nucleus, the z axis being chosen along the negative direction of the bisector of LCniC equal to 28. Let the x and y axes be perpendicular and parallel to the ring plane, respectively. As calculated by Guib6 and Luckenj5 the hybridized valency orbitals and the lone-pair orbital of nitrogen are given by
#*
=
$PX
Let the number of electrons occupying the two ubond orbitals and the a-bond orbital be denoted by 1 i,,, 1 i,t,, and 1 i,, respectively. Theionic character of the nitrogen atom is i = i,! i,!~ i,. The lone-pair orbital is simply assumed to accommodate two electron^.'^ Then the occupation numbers, N,, N,, and N , of the px, py, and p, orbitals are given by N , = 1 i,
+
+
+
+ +
+ 1 1 + -(i,+ + i,),) = 1 + i, N, 2 N , = 2(1 - Cot2 e) + (1 + i,)CotZ e
Since the field gradient at the nitrogen nucleus originates mostly from electrons in these orbitals, let contributions from other electrons and nuclei be disregarded.15"6 This approximation immediately leads to the coincidence of the principal axes of the field-gradient tensor with the geometric coordinate axes and the identity of q,, with the principal value q of the field-gradient tensor having the largest absolute value. Accordingly, one has
[1 - cot2
0
+ (cot2 e)i, - i(i, 1 + i,) ]l""pe -
Here, qp is the field gradient at a nitrogen nucleus formed by the charge distribution of a single 2p electron in a neutral nitrogen atom along the symmetry axis. The denominator for leQq,] takes into account a decrease in the electrostatic interaction between a valency p electron and the nucleus due to the expansion of the charge cloud in a negatively charged nitrogen, the screening constant e being estimated by Townes and Schawlow at 0.3.l' The valency angle LCNC of nitrogen in a pyridine molecule has been determined as 20 = 116" 15' from microwave rotational spectra.'* (10) P. Love, J . Chem. Phys., 39, 3044 (1963). (11) S.S.Lehrer and C. T. O'Konski, ibid., 43, 1941 (1965).
R. H. Widman, ibid., 43,2922 (1965). Y . Abe, J . Phys. SOC.Jap., 18, 1804 (1963). R. Hoffmann, J . Chem. Phus., 40,2745 (1964). C. H. Townes and B. P. Dailey, ibid., 17, 782 (1949). T. P. Das and E. L. Hahn, "Solid State Physics," Suppl. 1, Academic Press Inc., New York, N. Y., 1958, p 119. (17) C. H. Townes and A . L. Schawlow, "Microwave Spectroscopy," McGraw-Hill Book Co., Inc., New York, N. Y., 1955, p 225. (18) B. Bak, L. Hansen-Nygaard, and J. Rastrup-Andersen, J. Mol. Spectrose., 2 , 361 (1958). (12) (13) (14) (16) (16)
The Journal of Physical Chemistry
XUCLEAR QUADRUPOLE RESONANCE O F XITROGEN-14
It is assumed that this value is unaltered in pyridine derivatives even in the solid state. The value of leQy,] has been estimated at about 12 RIcps in our previous paper.6Jg1g With these values, the bond ionicities, i, and i,, can be calculated from the observed quadrupole coupling constants and the corresponding asymmetry parameters for two possible cases of i, > i, and i, < i,. Table I11 shows the results obtained for the former choice as well as the alternative assumption. The latter is unreasonable in many respects. For instance, i, increases in the order of pyridine, 3chloropyrindine, 2-chloropyridine, and 2,6-dichloropyridine, whereas the reverse order is expected because substituted chlorine atoms attract electrons from a pyridine ring through u bonds. Therefore, one can rule out the second choice in agreement with the result of microwave measurement on pyridinez0 and with an anticipation presented by Guib6 and Lucken6*21 for nitrogen heterocycles. We have calculated i, by the Pariser-Parr-Pople molecular orbital method also. Although the agreement is not excellent, relative magnitudes are quite satisfactory, in view of the different methods employed and the various assumptions made in the calculations. This and subsequent molecular orbital calculations have been performed by use of a Hitac 5020 computer. The following exchangeintegral parameters have been used: Pc-c = -2.39 eV (ring), PC-N = -2.58 eV (ring), P C ~ N= -3.50 eV, Pc-c = -2.14 eV (ring CN), and PC-N = -3.00 eV (ring NHz). Table I11 : Calculated Values of i, and i, io
Pyridine derivatives
(q,l $0
Pyridine 2-Chloro3-Chloro2,6-Dichlor‘02-Cyano3-Cyano&Cyano2-Amino3-Amino4-Amino2-Methyl3-Methyl4-Methyl-
0.320 0.308 0.310 0.298 0.299 0.312 0.314 0.343 0.330 0.348 0.322 0.319 0.329
> in >I d )
iu is,
in
(MO)
0.195 0.227 0.195 0.266 0.187 0.194 0.170 0.334 0.207 0.325 0.219 0.193 0.223
0.176
0.165 0.170 0.163 0.257 0.145 0,221
(IQYYl io
0.164 0.206 0.167 0.257 0.159 0.165 0.160 0.331 0.176 0.317 0.192 0.161 0.195
quite different from that of pyridine. The same conclusion has been reached by Luckenzz for some methylpyridines. Thus the n-electron distribution about ring nitrogen is practically unaffected by substitution at position 3, regardless of the kind of substituents. In fact, the observed pure quadrupole resonance frequency of Wl in 3-chloropyridine is higher than those in 2- and 4-chloro derivative^,^^ indicating that the C-C1 ?r-bond character is the smallest for 3-chlor0pyridine.~~ The data of cyano groups in cyanopyridines lead to the same conclusion. As in a previous article17let the orbitals of nitrogen in a cyano group involvedin bonding be expressed by #u
#Ip
+
=
(1 - s)1/zr+5p,
=
(1 - s)1’*48 - s1’*4,, *rx
=
4PX
#?ill
=
4PY
S1/*&
The sp-hybridized a-bond orbital, #,, accommodates 1 i, electrons, while the lone-pair orbital, #I,, is fully occupied by a pair of electrons. The extent of s character has been estimated to be 0.2-0.3,7 because s = 26.1% for a nitrogen molecule in S ~ h e r obtained r~~ his molecular orbital calculations. The average value, 25%, was taken. The n-bond orbitals, #,x and having their axes perpendicular and parallel to the plane of the pyridine ring, respectively, are populated i,, and 1 i,, electrons, respectively. with 1 Calculations along the same line as for the foregoing ones yield
+
+
+
< i, < lsazl) t,
0.282 0.283 0.276 0.288 0.266 0.277 0.281 0.340 0.292 0.340 0.288 0.281 0.296
Data shown in Table I11 indicate that the donation of P electrons from various substituents to ring nitrogen decreases in the order NH2 > C1 > CHI H > CN, whereas that of u electrons decreases in the order NH, > CH3 H > C1- CN. It is interesting to note that i, of 3-substituted pyridines is almost identical with that of pyridine, whereas those of 2- and 4-substituted pyridines are N
N
2505
Since there are three unknowns, i,, i,,, and i,,, whereas only two observables, eQqzz and 7, are available, let it be assumed that i,, is equal to 10% by taking into account the electronegativity difference vs. ioniccharacter curve of Townes and Dailey.z6 The calculated values of i,, are shown in Table IV, along with those from molecular orbital calculations. Again, (19) L. Guib6 and E. A. C. Lucken, Compt. Rend., 263, 815 (1966). (20) G. Sgirensen, J . Mol. Spectrosc., 22, 325 (1967). (21) E. A. C. Lucken, Trans. Faraday SOC.,57, 729 (1961). (22) E. A. C. Lucken, “Nuclear Quadrupole Resonance Spectroscopy,” International Summer School on Theoretical Chemistry, Frascati, Italy, Oct 1967. (23) P. J. Bray, S. Moskowitz, H. 0. Hooper, R. G. Barnes, and S. L. Segel, J . Chem. Phys., 28, 99 (1958); M.J. 8. Dewar and E. A. C. Lucken, J . Chem. Soc., 2653 (1958). (24) M. J. S. Dewar and E. A. C. Lucken, Special Publication No. 12, The Chemical Society, London, 1958, p 223. (25) C. W. Scherr, J . Chem. Phys., 23, 569 (1955). (26) B. P. Dailey and C. H. Townes, ibid., 23, 118 (1955). Volume 72, Number 7 July 1968
R. IKEDA, S. ONDA,D. NAKAMURA, AND M. KUBO
2506
i,, of the cyano nitrogen in 3-cyanopyridine is close to that of cyanobenzene, indicating the absence of the effect of the ring nitrogen as contrasted with the case of 2- and 4-cyanopyridines. It should be mentioned that this conclusion is unaltered by the choice of i,, in the vicinity of 10%.
Table IV :
T
Ionicity of CN Bonds
molecular orbital calculation to evaluate the electron population in the u and T orbitals of pyridine and methylpryidines. The results are compared in Table V with i = 2i, i, of the present investigation. Although the calculated values are greater than the observed values by about lo%, qualitative agreement is excellent.
+
~~
~
Table V : Total Ionic Character i of a Nitrogen Atom in a Pyridine Ring Compound
2-Cyanopyridine 3-Cyanopyridine 4-Cyanopyridine Cyanobenzene
The Journal of Physical Chemistry
0.119 0,126 0.103 0.127
0.142 0.150 0.142 0.149
i Compound
Pyridine 2-Methylpyridine 3-Methylpyridine 4Methylpyridine
i
0.835 0.863 0.831 0.881
(Hoff mann)
0.915 0.987 0.912 0.947
