1644
Lichter et al.
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
Natural-Abundance Nitrogen-I 5 Nuclear Magnetic Resonance Spectroscopy. Nitrogen Chemical Shifts of Alkylpyridines, Picolinium and Lutidinium Ions, and Picoline N-Oxides' Alice J. DiGioia, George T. Furst, Linda Psota, and Robert L. Lichter" The City University of New York, Hunter College, Department of Chemistry, New York, New York 10021 (Received February 6, 1978) Publication costs assisted by the Petroleum Research Fund, the Research Corporation, and the Research Foundation of CUNY
15Nchemical shifts of the title compounds have been obtained at the natural-abundance level. Changes in the nitrogen resonance positions of the methylpyridines are small except for the para-substituted compounds. This is rationalized qualitatively in terms of competitive inductive vs. hyperconjugative interactions between the substituent and the nitrogen. tert-Butyl substitutionat the ortho position induces upfield shifts characteristic of y-gauche interactions. Protonation or N-oxide formation has little effect on the manner in which methyl groups influence the shielding, suggesting that the lone-pair electrons in the free base play only a minor role in the response of the nitrogen nucleus to alkyl substitution. The pyridinium ion resonance positions are markedly influenced by solvent and concentration. Picoline and lutidine chemical shifts do not correlate with electron densities calculated by the INDO method, but chemical shifts calculated by the Karplus-Pople method or the Witanowski modification thereof qualitatively reproduce trends. The validity of these types of calculations is assessed.
Introduction With natural-abundance 15Nnuclear magnetic resonance (NMR) spectroscopy now technologically feasible, it has become appropriate to characterize the manner in which nitrogen chemical shifts are influenced by structural changes. Among the various different types of aromatic nitrogen nuclei, the azine nitrogen exemplified by that in pyridine has been examined extensively by 14NNMR.2i3 However, the larger error inherent in the measurement of 14Nchemical shifts can obscure substituent effects on the nitrogen resonance positions. Indeed, the experimental error often encompasses the expected range of absorption for a particular substituent, so that 14N chemical shift measurement by and large can account only for gross structural changes. Hence, it is clear that in order to obtain more insight into electronic and structural factors affecting pyridine nitrogen chemical shifts, measurement of the more precise 15Nchemical shifts is warranted. This paper reports the results of a study of variously substituted alkylpyridines (1-3) and some of their ions and N-oxides.
la, R = 2-CH, l b , R = 3-CH; IC, R = 4-CH,
2a, R , = 2-CH3, R, = 3-CH, 2b, R, = 2-CH3, R, = 4-CH3 2c, R, = 2-CH3, R, = 5-CH, 2d, R , = 2-CH3, R, = 6-CH, 2e, R, = 3-CH3,R, = 4-CH, 2f, R , = 3-CH3,R, = 5-CH3
3a, R, = C,H,, R , = R , = H 3b, R, = 2'-C,H,, R, = R, = H 3c, R, = t-C,H,, R, = R , = H 3d, R, = R, = H,R , = C,H, 3e, R, = R, = H, R, = i-C,H, 3f, R, = R , = H, R, = t-C,H, 3g, R, = R , = t-C,H,, R, = H 3h, R , = R, = R, = CH, 3i, R, = R, = R , = t-C,H, 0022-3654/78/2082-1644$01 .OO/O
A future manuscript will describe the effects of substituents other than alkyl, because these affect the nitrogen resonance positions in a different manner.'^^ Experimental Section The 15Nspectra were obtained at a frequency of 10.09 MHz on a JEOL PS/PFT-lOOspectrometer equipped with the JEOL EC-100 data system. The probe was fitted with a receiver insert modified to improve ~ensitivity.~ Samples were contained in 10-mm 0.d. tubes. In general free induction decays were accumulated over a 5-kHz range using 8K words of memory (1.22 Hz/address). Exponential multiplication of the FID prior to Fourier transformation induced a calculated line broadening of 0.8 Hz. The resonance position of fresh samples of pyridine was checked periodically; reproducibility was within 0.1 ppm. For the free bases, a coaxial 3-mm capillary of deuteriobenzene provided the field/frequency lock. Nitromethane (20 vol %) was used as an internal reference in most cases, while the remaining compounds were referenced to a 3-mm capillary containing I5N enriched nitromethane in deuteriobenzene acting as a combination lock and reference capillary. Tris(acety1acetonato)chromium, Cr(acac)3,was used to shorten the T I values; the effects of methods of referencing6 and of paramagnetic relaxation reagents1,' on the resonance positions have been discussed separately. The normal operating conditions employed a 30' flip angle and a pulse delay of 2 s. With these conditions useful spectra could be obtained on neat liquids within 2.5 h. For the pyridinium ions, a coaxial 2-mm capillary of 2.9 M 15N-enrichedammonium chloride in 1M HC1 provided the reference. Deuteriobenzene (10 ~ 0 1 %was ) used as the internal lock substance. Spectra were obtained within 1 h using a 30° flip angle a t a repetition rate of 1.1s. No relaxation reagents were employed. The pyridines used in this study were all commercially available (Reilly Tar and Chemical Co. and Aldrich Chemical Co.) with the exception of 2- and 4-isopropylpyridine, which were prepared by known methodsS8The liquid samples were distilled from potassium hydroxide pellets after storage over this material. All solvents were distilled prior to use. Substituted pyridine trifluoro0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1645
Natural-Abundance 15N NMR Spectroscopy
TABLE I: Nitrogen Shifts of Some Alkyl Substituted Pyridines solvent 6 15Na 81sNb pyridine (py) 2-Mepy 3-Mepy 4-Mepy 29 Z-MezPY 2,4-Me2py 2, ~ - M ~ , P Y 2 96-Me~ P Y 3,4-Me2py 3,5-Me2py 2,4,6-Me,py 2-Etpy
neat neat neat neat neat neat neat neat neat neat neat neat 2-i-Prp y neat 2-t-Bupy neat 4-Etpy neat 4-i-Prpy neat 4-t-Bupy neat 2,6-(t-Bu), py neat 2,4,6-( t- Bu ) ~ P Y C, He Py N - 0 Me,SO Me,SO 2-Mepy N-0 water 3-Mepy N-0 Me,SO water Me,SO 4-Mepy N-0 water
-62.2
-58.9
- 62.6
- 61.7
-70.2 - 62.3 - 71.0 - 62.7 - 62.4 - 68.8 - 61.7
-66.7
-68.0 - 64.0
-67.3 - 64.7
-65.5 -64.8 -64.7 -70.4 - 76.4
-87.5 - 89.6 -102.7 - 87.5 -101.0 -96.3 -110.2
0.0
- 0.4
0.3 -7.8 -0.1 -8.8 - 0.5 - 0.2 - 6.6 0.5 -9.1 - 1.8 - 5.1 - 2.5 -6.6 -5.9 -5.8 -8.2 - 14.2 - 25.3 - 27.4 -25.3
- 34.1
a Chemical shifts reported in ppm from 20% internal nitromethane; excluding the N-oxides all samples contain Chemical shifts reported in ppm 0.0250 M Cr(acac),. from 20% enriched nitromethane in psrdeuterated Chemical shift benzene in a 3-mm coaxial capillary. relative to pyridine.
acetates were made directly in the sample tube by volumetric addition of trifluoroacetic acid (TFA) to the free bases in chloroform.
Results Chemical Shift Measurements. The I5N chemical shifts of the alkylpyridines and of the picoline N-oxides are summarized in Table I. Because of the differences in measurement conditions among the different samples, the values to be focussed upon are the chemical shift differences from pyridine, given in the last column. Ortho and para substitution induce upfield shifts while meta substitution induces little change relative to pyridine. Both ortho and meta effects are small, with the exception of the 2-isopropyl compound, while para substitution consistently induces much larger changes. The trend observed in the free bases is maintained in the N-oxide series, with the 4-substituted derivatives displaying the largest upfield shifts. This accords with recently reported I4N result^.^ The effect of substituents on the nitrogen resonance
position of the pyridinium ion was expected to be useful in elucidating the role of the lone-pair orbital as a factor in determining the nuclear screening of the pyridine nitrogen atom. Pyridinium ions were formed directly in trifluoroacetic acid (TFA) where the mole ratio of the substrate to TFA could be maintained constant for the entire series. From pK, values protonation at a 1:l mole ratio of pyridine:TFA is expected to be complete,ll but to ensure solubility, a 1mol excess of TFA was used initially as a solvent. The 16Nchemical shifts so obtained are given in Table 11. Because of the possibility that excess TFA as solvent might influence the chemical shifts, the resonance positions were examined in chloroform as the most inert solvent in which sufficiently high solubilities of the trifluoroacetates could be obtained. Under these conditions, the nitrogen resonances of several of the substrates were displaced substantially to lower field. In order to determine the appropriate conditions, the I6N shifts of 4-picoline and 3,4-lutidine were obtained as a function of the mole ratios of TFA, substrate, and solvent. Both ion pairing12aand solvent12bare expected to influence the protonation shifts. Our detailed studied suggest that a mole ratio of substrate:TFA:solvent = 1:2:2 best reflects substituent effects, although the overall trends were not markedly affected by the specific conditions used. As indicated in Table I1 the substituent effects were found to parallel those in the free bases except for the 3methylpyridine trifluoroacetates. In the latter, the nitrogen nuclei are shielded relative to pyridine trifluoroacetate, whereas in the free bases 3-substitution deshields the pyridine nitrogen. In both cases, the differences are very small. Electron Density Calculations. To ascertain their utility, molecular orbital calculations were carried out at the INDO level of approximation. Experimental geometrical parameters of pyridine listed in Table I11 were used. The geometry of the ring was taken to be that of pyridine for the entire series. Any deviation from this geometry upon methyl substitution was assumed to be constant throughout the series and not to affect trends. Preliminary calculations on the methylbenzenes indicated that rotation of ortho methyl groups perturbed the electron densities both of the ring carbon directly bonded to the methyl group and the methyl carbon itself. Rotation of the methyl groups was treated in the calculations as successive 90' variations of the dihedral angle between a methyl C-H bond and the plane of the ring. Only when substituents were ortho to each other and bonded to the a-carbon was it necessary to consider the effect of the methyl conformation on the excess electron densities in the methylpyridines. In these cases, the lowest energy conformation was taken.
TABLE 11: Nitrogen Chemical Shifts (6 ~ S Nppma) , of Methylpyridinium Ions 1:1:1
1:210
1:2:2
substrate:TFA:CHCl, (mole ratios) 1:3:3 1:4:4 1:5:5
1:6:6
1:lO:lO
1:2:5
183.2 179.9 (O.O)b 184.3 179.5 (-0.4) 195.0 173.4 179.6 (-0.3) 170.3 173.1 (-6.8) 171.6 169.1 168.2 167.7 180.9 192.0 178.7 178.9 (-1.0) 194.3 172.8 173.0 (-6.9) 204.9 183.2 179.8 (-0.1) 180.1 179.3 (-0.6j 173.1 (-6.8) 171.8 169.7 168.5 167.7 166.5 176.9 176.2 177.7 178.7 (-1.2) a Chemical shift with respect to external l5NH,C1, see Experimental Section. Positive values denote downfield shifts. Values in parentheses are the differences between the chemical shift of each substituted pyridinium ion and the parent pyridinium ion itself.
1646
The Journal of Physical Chemistty, Vol. 82, No. 14, 1978
Lichter et ai.
TABLE 111: Input Geometrical Parameters for INDO Calculationsa bond bond angles, lengths, a deg N-C 1.3402 HC,C, 118.058888 c,=c, 1.3945 HC,C, 120.133333 c,=c, 1.3944 HC,C, 120.833333 C, -H 1.0843 HC,C, 121.300000 C,-H 1.0805 HCH(methy1) 109.47125 C-CH, 1.51 CNC 116.83333 C-H(methy1) 1.09 a L. E. Sutton, Ed., "Tables of Interatomic Distances and Configurations in Molecules and Ions", The Chemical Society, London, 1958.
TABLE IV: Nitrogen Electron Densities (X Methylpytidinesa
a
3,4 I
b-
5-
A6'k
4-
3t
lo4)of
Wtot
*go
A4n
182 - 17 77 140 27 1 150 37 4 49 - 40 436
- 15
197 -88 124 95 324 99 404 29 - 187 508
71 - 46 45 - 53 51 - 30 20 147 -70
.4
'1,.~
'1
0
,
-1
-200
0
200
,
;2,b
,
4 00
Aq,, X IO'
Figure 2. Plot of T INDO-MO electron density differences vs. methylpyridine I5N chemical shifts.
Relative to pyridine: A q i = qi - qpy.
9t
A 2$,6
A2,4
't
2,4,6 b2.4
03.4
6 -
5 -
A6"N
4
-
i/ 2
-1
I
A3
I
-200
i
0
200
400
Aqtotx io4 Figure 1. Plot of total INDO-MO electron density differences vs. methylpyridine "N chemical shifts.
The calculated electron densities of the nitrogen in the various pyridines are given in Table IV. There appear to be no obvious relationships with the 15Nchemical shifts. Plots of ASN vs. Aqtot, Aq,, and Aqo are shown in Figures 1-3. The relative changes in the electron densities are much greater than those in the chemical shifts, and there
Flgure 3. Plot of cr INDO-MO electron density differences vs. methylpyridine I5N chemical shifts.
is no evident correlation between the two sets of values. However, it is apparent that those compounds bearing methyl groups para to the pyridine nitrogen cluster separately from the remaining compounds. The same result has been noted without explanation for the meth~1anilines.l~ Furthermore the slopes evident in Figures 1-3 are in the same directions as those for the corre-
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1647
Natural-Abundance 15N NMR Spectroscopy
TABLE V: Approximate Calculations of Pyridine 5NChemical Shiftsa A%arab
a
In ppm.
0.0 3.42 - 0.80 1.84 2.12 5.43 2.32 7.00 0.84 - 2.08 8.41 Karplus-Pople method. Witanowski
AO&C
A0ParaC
AQtotC
0.0 0.0 0.0 0.0 0.12 13.63 13.75 - 0.40 - 0.04 - 2.71 - 2.75 0.30 6.44 6.54 - 7.80 0.10 9.40 9.43 -0.10 0.03 20.59 21.19 -8.80 0.60 - 0.50 9.81 8.98 - 0.83 33.05 33.29 0.24 - 0.20 -6.60 27.85 27.90 0.05 0.50 -2.95 - 0.02 - 2.93 -9.10 7.45 7.18 0.27 method (ref 15). Experimental values, this work,
sponding plots for the m e t h y l a n i l i n e ~ ,which ~ ~ ~ , suggests ~ a slight direct contribution to the chemical shifts from changes in sigma and total electron density. For pyridine, 3-picoline, and 3,5-lutidine, the directions and the magnitudes of the excess total and u electron densities compare favorably with the experimental 16Nchemical shifts, but because of the small experimental differences it is not clear that this is significant. However, the order of the nuclear shieldings for the other methylpyridines is not consistent with the calculated electron densities. The most obvious discrepancy is in the 2,6-lutidine which exhibits the largest changes in total and a electron densities of the series, but whose chemical shift is only 0.4 ppm upfield of pyridine. Using the results of the INDO calculations we have calculated the paramagnetic contribution to the nitrogen chemical shifts of the picolines and the lutidines within the framework of the Karplus-Pople f0rmu1ation.l~The results are summarized in Table V. While numerical agreement is not satisfactory, the range of the calculated shifts is the same order as that of the experimental values, and the direction of the changes is reproduced. However, a detailed interpretation of the calculations is not warranted. have reported reasonable agreement Witanowski et between the experimental 14Nchemical shifts of several polyazines and nuclear screening constants evaluated from electron densities calculated at the INDO level of approximation. The essential difference between this approach and that of the Karplus-Pople method lies in the evaluation of the effective nuclear charge, which enters into the orbital expansion term of the paramagnetic contribution to the chemical shift (see Discussion). The theoretical values obtained via this method are compared with the experimental values in Table V. Although uniformly larger in magnitude, the values exhibit a parallel trend to the Karplus-Pople approach.
Discussion Alkylpyridines. Previously reported 14N data on the m e t h y l p y r i d i n e ~ ~failed ~ i ~ ~to ~ detect any substantial changes in resonance position as a function of substitution. At the time this was attributed in part to the uncertainties arising from the large line widths, but our results confirm that the changes are small indeed. The 14Nvalues do not compare well with those reported here. Thus, Witanowski et found -4,0, and -6 ppm for pure liquid 2-, 3-, and 4-picoline, respectively, while Evans and Richards report -7, -3, and -14 ppm for the same substances in ether. Furthermore, in contrast to the results for methylanilines13bthe effects of methyl substitution are not additive, although the trends are reasonable for the small observed range. In the former case, additivity could be correlated with the polarization of the ring electron distribution by the methyl groups, presumably by influencing
the way in which the lone-pair electrons on the amino nitrogen interacted with the ring. Consequently the nitrogen shifts of the anilines appeared to be dominated by the changes in electron density at nitrogen. The absence of such a correlation for the pyridines would seem to exclude this kind of mechanism, which is probably related to the fact that the pyridine lone-pair orbital is orthogonal to the a system and incapable of direct mesomeric interaction. On the other hand, this orbital may interact with the u skeleton, as has been discussed in connection with photoelectron data on substituted pyridines.16 While it is not apparent how an interaction of this type may affect electron density, it certainly influences the relative ordering of the molecular orbitals, which may in turn affect the chemical shifts. This point will be discussed further below. The fact remains, however, that in general methyl substitution at C-4 shields the nitrogen significantly more than at other positions. Bose et al. have noted this for 4-picoline itself.3b Indeed, the response of a nucleus in an aromatic molecule to a para substituent often has been used as a measure of the electronic nature of that substituent; this applies especially to lH, 13C, and 19F.17J8 Nitrogen nuclei of para-substituted anilines behave ~ i m i l a r l y , las ~ ~do J ~those of other substituted pyridines.'~~ In the 4-substituted methylpyridines (4-picoline, 2,4lutidine, 3,4-lutidine), the nitrogen nucleus is analogous to the appropriate para carbon in the methylbenzenes, so similar behavior might be expected. In lieu of a comprehensive theoretical framework within which to discuss these results, a more qualitative treatment, decomposing the substituent effect into inductive and conjugative influences, seems in order. In the picolines and lutidines, the methyl group may be considered to interact hyperconjugatively via the a and inductively via the u system. Since there are no lone-pair electron orbitals on the methyl groups, the molecular orbitals of a symmetry interact with the a system of the ring. This hyperconjugative interaction, which has been used to account for the electron-donating properties of a methyl group in benzene ring,20may also be used to rationalize the experimental 15N chemical shifts of the methylpyridines. The largest influence is expected at the nuclei ortho and para to the substituent. However, ortho substituents may be considered to exert a deshielding P effect acting in the opposite direction. The net result appears to be a cancellation of effects at the ortho but a net shielding at the para position. These trends parallel those in the correspondingly substituted benzenes, but there is no quantitative relationship.l7J8 Qualitatively the tert-butylpyridines behave similarly to the methylpyridines. The upfield shifts displayed by 3a-c and 3i are consistent with the presence of y substitutents, which are known to shield nitrogen nuclei by
1648
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
several ppmq2JlbThe additional shielding on para substitution parallels the effect induced by the methyl group. The trend observed for the 2- and 4-alkyl substituents is in accord with that observed in the 13Cchemical shifts with the exception of the large upfield shift (5.1 ppm) found for the nitrogen resonance of 2-isopropylpyridine. At present we are unable to explain this anomaly. Methylpyridinium Ions. It is well known that protonation of pyridine shifts the nitrogen resonance position more than 100 ppm upfield from its value in the free base.z2 This has been discussed in terms of removal of the contribution of the n--H* transition to the paramagnetic term in the chemical shift expression, as well as in terms of changes in the C-N P bond order on protonation. The latter especially has been invoked t o explain the changes in 13C resonance positions.ll As expected, shifts of the same order of magnitude were experienced by all the methylpyridines reported here. More significant, however, is the fact that the methyl substituent effect on the pyridinium nitrogen resonance position is essentially the same as that in the pyridines. Thus, only with methyl in the 4 position is the nitrogen shielded. While unknown effects of solvent, ion pairing, and concentration prevent a detailed discussion, a larger change in resonance position than in the free bases was anticipated as a function of substitution because of the larger electronegativity of the positively charged nitrogen. Somewhat surprisingly, there is no quantitative correlation with the corresponding 13C shifts in the isoelectronic rnethylbenzenes.lsb This might be considered striking in view of the relationship between the 14N chemical shifts of a series of ortho-substituted N-ethylpyridinium ions and the 13C shifts of correspondingly substituted benzenesaZ3In that case, however, the nitrogen shifts spanned a range of about 30 ppm and included largely electron-withdrawingsubstituents; hence the likelihood of correlation is enhanced. Finally, there is no relationship between the changes each pyridine displays on protonation and corresponding pK, values. Again, these influences may be masked by solvent effects, nonetheless, the parallel behavior exhibited by the pyridines and pyridinium ions suggests that the lone-pair orbital plays only a minor role in the way in which the methyl group affects the behavior of the free bases. Pyridine N-Oxides. The upfield shift in resonance position exhibited by the pyridine nitrogen nuclei upon oxide formation is consistent with the removal of the lone-pair orbital contribution to the chemical shift. Compared to the values in the pyridinium ions, these shifts are attenuated because of the larger electronegativity of the oxygen compared to hydrogen. For pyridine N-oxide itself, the difference from pyridinium ion, 88.7 ppm, is more than twice the difference (39.6 ppm) between C-1 of the phenoxide ion and benzene,24 which serve as isoelectronic models. This may be a reflection of the somewhat higher a bond character in the >N+-O- bond compared to the >C-0- bond. The upfield displacement of the N-oxide of 4-picoline relative to that of pyridine (-8.8 ppm) compares favorably with the behavior in the free bases and ions. The additional diamagnetic shifts experienced in aqueous solution (-13.1 to -13.9 ppm) relative to the values in Me2S0 or as neat liquids must reflect a higher contribution from the dipolar form 4a relative to 4b. This solvent effect com-
0-0 \t
b-N
1
4a
4b
Lichter et al.
pares to that of 10 ppm which the 14Nresonances display in m e t h a n ~ l . ~ ~ ~ ~ ~ Theoretical Considerations. Although previous reports indicate that linear relationships do exist between calculated electron densities and chemical shifts of azine,3e and halopyridine26nitrogens, such a relationship is not observed for the methylpyridines. However, the observed ranges (118, 255, and 54 ppm, respectively) of chemical shifts in the former systems are large compared with that of the methylpyridines, which makes correlation with electron densities easier to generate. That apparent correlations of chemical shifts with calculated electron densities have been obtained might be fortuitous. Nuclear screening is a complex function of electron densityn so that the expectation of a linear relationship rests on questionable grounds.27b In assessing the validity of the approximate calculations we have carried out, the various components of the equation for the paramagnetic contribution to the chemical shift, up, must be considered (eq 1). In this equation, as
has been amply d i s c u ~ s e dAE, , ~ ~is~ the ~ average excitation energy (AEE) chosen to allow integrations to be carried out over ground-state wave functions only (closure theorem); ( r 3 j Z pis an orbital expansion term representing the mean inverse cube of the distance of an electron in a 2p orbital from the nucleus; and QABare the elements of the charge density-bond order matrix representing the extent to which electrons in A are shared with other atoms B. It is the orbital expansion term, related to the effective nuclear charge Z*, on which the relationship to electron density is based. As noted above, Witanowski's calculation of Z* differs from that of Karplus and Pople. First Burns' rulesz8are used instead of the more commonly employed Slater's rules to provide the screening constant for the various atomic orbital electrons. Secondly, fractional calculated electron densities at nitrogen are used in the determination of Z*; Witanowski's Z* values thus indicate that the nitrogen nuclei are more screened by the intervening electrons than the Karplus-Pople Z* values. Witanowski et al. have ' s ~ g g e s t e dthat ~ ~ , ~the broad features of the azine chemical shifts are reflected in the CQAB term or in the product of this term and 2". We find no such relationship with our data, using either calculational method. The telling feature of the approximate calculations carried out here lies in the use of the AEE approximation. The risks inherent in its use have been discussed frequently in the l i t e r a t ~ r e . ' ~ *In*particular, ~ two important limitations, which appear to have remained unnoticed, should be pointed out. First, McLachlan30 noted the possible result of the mathematics employed in simplifying the second-order perturbation sum. As a consequence of the closure theorem, the possibility exists that the sign of the perturbation sum may not be correctly predicted unless the product of integrals over the ground and excited states has the same sign for every excited state. This would lead to a smaller value of the paramagnetic term than expected. Secondly, experimental determinations of AJ3 are usually not available, and the value of 10 eV suggested by P ~ p l e l ~ ~ ~ ~ for homocyclic systems has been widely used by others in calculations on dissimilar systems. It should be noted that the AE value of 10 eV does not represent a weighted average of the contributions from the different excited states, and is an arbitrary rather than an approximate value, If chemical shift calculations are performed on a
Natural-Abundance I5N NMR Spectroscopy
series of closely related molecules where the excited state energies could be expected to correspond to the same transitions, then by using an arbitrary AE one might expect to obtain calculated nuclear shieldings which maintain the order of the experimentally observed chemical shifts. Indeed, we have used a value of 2.2 eV for AEavto obtain upara using the Witanowski formulation. This value was obtained by Witanowski in a least-squares fit of experimentally determined 14Nchemical shifts vs. the calculated shifts for a series of azines which include pyridine.31 When the highest occupied molecular orbital (HOMO) is not the same for each molecule in the series, then the AEE's will contain unequally weighted contributions from the various excited states. Use of the same AE for all the molecules becomes even more tenuous in this instance. The photoelectron spectroscopic results of Heilbronner et a1.16indicate that the methyl substitution in the pyridine ring in some cases can raise the energy of the raorbital above the nonbonding orbital. Hence, the HOMO varies even within the methylpyridine series. Although it is expected that the lowest energy excited state will make a major contribution to the AEE, the inclusion of the other excited states might not be negligible. For example, a change in AE of 0.1 eV produces a change in the calculated chemical shift of 2,4-lutidine of 2.3 ppm. In the substituted pyridines, there are three low energy bands lying between approximately 9 and 11 eV. The problem then remains as to what percentage of these other contributions should be included to more accurately represent the excitation energy of a molecule. These problems can be circumvented by avoiding the AEE using other computational methods such as finite perturbation theory.27a A more comprehensive theoretical discussion must await such a treatment. Nonetheless, our results do emphasize that the chemical shifts of pyridine and, presumably, azine nitrogens in general, should not be expected to reflect electron density alone. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. The work also was supported in part by the U. S. Public Health Service, Grant No. GM-21148 from the Division of General Medical Sciences, by CUNY Faculty Research Award Program Grant No. 10588, by the Research Corporation, and by Eli Lilly and Company. We are grateful to the Computing Center of the City University of New York for a generous grant of computer time, to Dr. P. R. Srinivasan for determining several of the pyridine N-oxides in Me,SO, and to Drs. M. Albright and K. Goto of JEOL for determining the spectra of the tert-butylpyridines on an FX-100 spectrometer. References and Notes (1) Taken in part from the R . D dissertation of A. J. DGioia, City University of New York, 1976. (2) M. Witanowski, L. Stefanlak, and H. Januszewski, in "NRrogen NMR", M. Witanowski and G. A. Webb, Ed., Plenum Press, London, 1973, pp 218-229.
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
1649
(3) (a) D. H. Evans and R. E. Richards, Mol. Phys., 8, 19 (1964); (b) M. Bose, N. Das, and N. Chatterjee, J. Mol. Specfrosc.,18,32 (1965); (c) A. Mathias, Mol. Phys., 12, 381 (1967); (d) P. Hampson and A. Mathias, Chem. Commun., 371 (1967); (e) M. Witanowski, L. Stefaniak, H. Januszewski, and G. A. Webb, Tetrahedron, 27, 3129 (1971); (fJ M. Wttanowski, L. Stefaniak, H. Januszewski, Z. Grabowski, and 0. A. Webb, /bid.,28,637 (1972); (9) M. Witanowski, T. Salwere, L. Stefaniak, H. Januszewski, and G. A Webb, Mol. Phys., 23, 1071 (1972). (4) A. J. DiGioia, 0. T. Furst, and R. L. Lichter, manuscript in preparation. (5) A. J. DiGioia, E. H. Cole, and R. L. Llchter, 15th Annual Experimental NMR Conference, Raleigh, N. C., April 29, 1974. (6) (a) R. L. Lichter, J. Magn. Reson., 18,367 (1975); (b) P. R. Srinivasan and R. L. Lichter, ibid., 28, 227 (1977); (c) M. Witanowski, L. Stefaniak, S. Srymanski, and H. Januzewski, ibid., 28, 217 (1977). (7) A. J. DiGioia and R. L. Lichter, J . Magn. Reson., 27, 431 (1977). (8) (a) W. Sobecki, Chem. Ber., 41, 4103 (1908); (b) G. R. Ciemo, J. Chem. Soc., 41 (1941). (9) L. Stefaniak, Spectrochim. Acta, Part A , 32, 345 (1976). 10) A. Flscher, W. J. Galloway, and J. Vaughan, J. Chem. Soc., 3591 (1964). 11) R. J. Pugmire and D. M. Grant, J. Am. Chem. Soc., 90, 697 (1968). 12) (a) J. M. Briggs and E. W. Randall, Mol. Phys., 26, 699 (1973); (b) J. D. Roberts, Abstracts of the Centennial Meeting of the American Chemical Society, New York, N.Y., April 18, 1975, ORGN 28. 13) (a) G. Adler and R. L. Lichter, J. Org. Chem., 39, 3547 (1974); (b) L. Psota, M. Franzen-Sieveking, J. Turnier, and R. L. Lichter, Org. Magn. Reson., in press; (c) R. L. Lichter and J. D. Roberts, ibid., 6, 636 (1974). 14) (a) M. Karplus and J. A. Pople, J. Chem. Phys., 38, 2803 (1963); (b) J. A. Pople, ibid., 37, 53, 60 (1962); (c) bid., 7, 301 (1963). 15) M. Witanowski, L. Stefaniak, and H. Januszewski, J. Magn. Reson., 16, 69 (1974). 16) E. Heilbronner, V. Hornung, F. H. Pinkerton, and S. F. Thames, Helv. Chim. Acta. 55. 289 119721. 17) J. B. Stothers, "Carbon-13 NMR Spectroscopy", Academic Press, hia.., I.-"
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(18) (a) G. L. Nelson, G. Levy, and J. D. Cargioli, J. Am. Chem. Soc., 94, 3089 (1972), and references therein. (b) After this work was completed, Dalling et at. reported revised values of '%chemical shifts of methylbenzenes. The aromatic carbons display small changes on ortho substitution, and rather larger effects upon para substiution. A rationalization very much the same as ours was offered. While the trends in these l3C shifts qualitatively parallel the effects dlsplayed by the similarly constituted nitrogen nuclei of both the methylpyridines and pyridinium ions, no direct relationship is apparent. (D. K. Dalling,
and 0. W. A. Milne, J. Am. Chem. Soc., 93, 6536 (1971). (20) L. Libit and R. Hoffmann, J. Am. Chem. Soc., 96, 1370 (1974). (21) (a) R. L. Lichter and J. D. Roberts, J . Am. Chem. Soc., 94, 2495 (1972); (b) S. N. Y. Fanso-Free, R. L. Lichter, R. Nelson, G. W. Grlbble, G. T. Furst, and P. R. Srinivasan, submitted for publication. (22) (a) J. D. Baldeschwieler and E. W. Randall, Proc. Chem. Soc., 303 (1961); (b) V. M. S. Gii and J. N. Murrell, Trans. Faraday Soc.,60, 248 (1964); (c) R. L. Lichter and J. D. Roberts, J. Am. Chem. Soc., 93, 5218 (1971); (d) M. Witanowski, ibid., 90, 5683 (1968). (23) W. Giger, P. Schauwecker, and W. Simon, Helv. Chim. Acta, 58, 2488 (1971). (24) Reference 16, p 197. (25) L. Stefaniak and A. Grabowska, Bull. Akad. Polon. Sci., Ser. Sci. Chim., 22, 267 (1974). (26) K. Hensen and K. P. Messer, Chem. Ber., 102, 957 (1969). (27) (a) R. Ditchfield and P. Ellis, Top. Carbon- 13 NMR Spectrosc., 1, 1 (1974). (b) For a cogent, critical discussion of the relationship between chemical shifts and electron densities, see G. E. Maclel, ibid., 1, 54 (1974). (28) 0. Burns, J . Chem. Phys., 41, 1521 (1964). (29) (a) T. K. Wu, J. Chem. Phys., 49, 1139 (1969); (b) W. A. Adams, A. Grimison, and G. Rodriguez, J. Chem. Phys., 50, 645 (1969); (c) T. K. Wu and B. P. Dailey, J. Chem. Phys., 41, 2796 (1964). (30) A. D. McLachlan, J. Chem. Phys., 32, 1263 (1960). (31) For additional comments on the arbitrariness of the AEE, see N. C. Baird and K. C. Teo, J. Magn. Reson., 24, 87 (1976).
