- Email: [email protected]
NMR studies of pyridine-N-oxide. Determination of spectroscopic constants from [15N]-, [4-2H], and the parent species
JOURNAL
NMR
OF MAGNETIC
RESONANCE
31,177-186
(1978)
Studies of Pyridine-N-Oxide. Determination of Spectroscopic Constants from [lsN]-, [4-2H]-, and the Parent Species
TOREEN WAMSLER,JPIRGENTORMOD NIELSEN, ERIK JONAS PEDERSEN,AND KJELD SCHAIJMBURG* Department
of Chemical
Physics,
H.C.
0rsted
Institute, DK-2100
The University
of Copenhagen, Denmark
ReceivedApril 11, 1977;revisedDecember27, 1977 Pyridine-N-oxide, [‘5Nlpyridine-N-oxide, and [4-2Hlpyridine-N-oxide have been investigatedby NMR spectroscopy.‘H, 2H, and lgC spectra have been obtained and analyzed,in terms of chemicalshifts and spin-spin couplingconstants.Relaxationtimes havebeenmeasuredfor ‘H and “C and havebeenusedto determinethe correlationtime r, under the assumption of isotropic molecular diffusion. On the basis of experimental data and observed linewidth variations the nitrogen quadrupole coupling constant has been evaluated. The experimental data are correlated with semiempirical calculations within the azine series.
INTRODUCTION Pyridine and its derivatives have been the subject of numerous NMR investigations. In the present paper we report a study of pyridine-N-oxide (Ia), [lSNlpyridine-N-oxide (Ib), and [4-ZHlpyridine-N-oxide (1~): lo
Ib
IC
Earlier studies of the ‘H spectrum of Ia have been published (I, 2). In the work by Castellano and Kostelnik (I) accurate values for all the interproton coupling constants have been determined using 14N decoupling. The geometry of the N-oxide group, and in particular the geometry of I, has not been particularly well known. Viigtle and Risler (3) have tried to argue for a more reliable picture of the N-O bond in I on the basis of NMR data. In our laboratory a microwave spectroscopic determination of the structure of I was initiated (4, 5). We found it of interest parallel to this work to undertake an investigation of the NMR spectra of a series of enriched isotopic substituted species Ib to Ic. It is well known that in pyridine derivatives a significant variation in linewidth can be observed in the proton spectra (1, 6). This is related to the scalar coupling to nitrogen. With access to the labeled species Ib and Ic, the relaxation mechanism can be * Author to whom correspondence should be addressed. 177
OOZZ-2364178/03 IZ-0177$02.00/0 Copyright 0 1978 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
178
WAMSLER
ET
AL.
investigated in some detail and a value of the nuclear quadrupole coupling constant (NQCC) can be deduced. Having a reliable geometry of I, we calculated the spin-spin coupling constants within the CND0/2 and INDO semiempirical approximations. Experimentally large changes in magnitude of individual spin-spin coupling constants occur upon N-oxidation of pyridine. We felt that determining whether these changes are reflected properly in the calculations would be of interest. EXPERIMENTAL
[15Nlpyridine-N-oxide was prepared from [i5Nlpyridine 95% enriched in 15N by Snerling et al. (4). [4-*Hlpyridine was prepared according to the method of (7) from 4bromopyridine by zinc powder reduction in acetic anhydride. The oxide was prepared as above (4, 5). Samples were prepared by dissolving I in CS, (0.2 M) or in CDCl, at several concentrations (see below). TMS was added as the internal standard and the samples were sealed after several freeze-pump-thaw cycles. For 2H spectra a 0.5 M solution in CHCl, was prepared. 13C spectra were obtained in the FT mode using a Varian XL-100 spectrometer operating at 25.12 MHz at 32OC. Proton-coupled spectra were recorded with i4N decoupling using 8K data points and a spectral range of 600 Hz. The repetition time was 6.7 set and the pulse width 75 psec, corresponding to 70°C. Similar conditions were used for linewidth measurements in ‘H-decoupled 13C spectra. i3C T, measurements were performed on a Varian XL-100 and a Bruker WH 90 spectrometer. In both cases the inversion-recovery method was used. The fitting of relaxation data to a relaxation time was accomplished by means of a nonlinear three-parameter regression analysis. 2H T, measurements were performed on a Varian XL-100 spectrometer using the inversion-recovery technique as above. ‘H spectra were recorded on a Varian HA-100 spectrometer operating at 32”C, modified for digital sweep (8) under the control of a Varian 620/i computer. Hetero-INDOR spectra were obtained by driving a Schlumberger FSD 120 synthesizer with a DAC ramp from the computer. In the heterodecoupling experiments an Electronic Navigation Industries power amplifier model 3 1OL was used to boost the output of the FSD 120. The V 4333 probe was modified for heterodecoupling according to McFarlane’s method (9). ‘H spectra were also obtained using a Bruker HX 60 spectrometer coupled to a Varian C 1024 CAT. This instrument was used for homotickling and homo-INDOR experiments. Weak lines in the wings of the spectra were also observed using this instrument. The ‘H spectra used in determination of coupling constants were recorded with a sweep expansion of 0.2 Hz/cm and with sweep rates not exceeding 6 x 1O-3 Hzlsec. The analysis of the spectra was performed using the programs LAOCOON III, HOMO, and HETERO (10). The latter two were used in tickling and INDOR calculations. CNDO/Z and INDO calculations were carried out using the sum-over-state method (SOS). The parametrization has been discussed by Tow1 and Schaumburg (II). RESULTS
Analysis of the 60-MHz proton spectra of Ib results in the values in Table 1. For comparison the coupling constants published by Castellano and Kostelnik (1) have
NMR
OF
PYRIDINE-N-OXIDE
179
been included. Analysis of the lOO-MHz proton spectra of Ia produced values identical to those reported. Ic was similarly analyzed, and the parameter values were within the uncertainties coinciding with the data in Table 1. The indicated uncertainties have been taken as three times the probable errors. Experimentally it was found that the inclusion of weak outer lines (only observable after data accumulation) reduced the correlation among various parameters decisively and improved the rms error somewhat. TABLE ANALYSES
OF 60.MHz
SPECTRA
1
OF Ib AND lO@MHz
SPECTRA
OF Ia
Species lb
Ib
la
Resonance
Coupling
constant
“-‘HA Hg (Hz)
“J,
&‘W
Ha Wd
(ppm)
a Data
n
A
B
3 4 5 4 3 4 2 3 4
2 2 2 2 3 3 1 1 1
3 4 5 6 4 5 2 3 4
s, = s, s, = s, 4 44.,-J,) from
Ref.
la”
frequency
60 MHz
60 MHz
100 MHz
60 MHz
0.2 Min -1
l.OMin CDCl,
1OMin CDCl,
1.0 M in (CH,),CO
6.530 1.108 0.625 1.820 7.606 2.129 0.35 1 -5.174 1.026
+ f f f k + f f f
0.009 0.009 0.009 0.006 0.006 0.006 0.009 0.009 0.012
7.91095 f 0.00005 7.13348 f 0.00010 7.05490 + 0.00010 0.07858
6.53 1.12 0.64 1.90 7.69 2.07 0.47 -5.32 1.11
f f f + f + f f +
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02
6.5 1 + 1.12 f 0.63 + 1.89 f 7.67 f 2.11 f
0.02 0.02 0.02 0.01 0.01 0.01
6.47 1.12 0.63 1.88 7.65 2.13
-t i + k i k
0.08 0.08 0.08 0.08 0.08 0.08
-
8.2854 f O.OCOl 7.3408 f 0.0002 7.3427 + 0.0002 -0.0019
8.2921 k O.OGOl 7.3804 5 0.0001 7.3321 + 0.0002 0.0583
(I).
Using INDOR, the sign of the JNHc3)coupling constant’ was determined relative to the assumption that the ortho coupling JH(2jw(3j was positive. The signs of the two smaller JNH are, in the iterative fitting of all the spectra analyzed, found to be opposite in sign to JNwc3). In Table 2, JCN and JCH values are reported as well as carbon chemical shifts. The JCH values have been determined from spectra of a 10 M CDCl, solution of Ia. This is due to the solvent effect that makes &c(j) - &c(d)eq ual to -0.1 ppm for a 0.1 M solution, but +0.6 ppm for a 10 M solution. Only in the last case was it possible to analyze the coupled carbon spectra unambiguously. To remove nitrogen broadening, decoupling of nitrogen was necessary, to obtain the JCH values. The quoted values of JCN are observations on a 1.0 M solution of Ib. Spin-spin coupling constants involving hydrogen have been calculated in the CND0/2 approximation (II), and the values are listed in Table 3. Spin-spin coupling constants between two nonhydrogen nuclei are calculated, and the INDO results are shown in Table 4. In these cases the contact, spin-orbit, and dipole-dipole terms may all contribute to the coupling constants, and they have been separately listed. ‘Throughout the paper coupling constants between nitrogen comparison has made it necessary, 14N data have been converted.
and
other
nuclei
refer
to
15N.
Where
180
WAMSLER TABLE “C
Gb J CH(2) b J CH(3) b J CH(4) b J CH(5) b J CH(6) b c
J iiCd
NMR
DATA
ET
AL.
2
FOR PYRIDINE-N-0x1~~~
13.3903
k 0.0008
0.5478
k 0.0008
187.42 4.26 8.12 -1.54 4.10
k + 2 + f
4.19 167.16 1.5 1 8.90 -0.83
& + k It k
“H(2) - “H(2) ’ “H(3) - “HOI ’ “H(4) - “H(4) ’ + 15.24
0.05 0.007 0.06 0.06 0.03
0.05 0.04 0.08 0.08 0.05
” H(2) - “H(Z) ‘-0.11 “H(3) - “H(3) ’ = 0.26 “H(4) - “H(4) ’ -- 0.04
-- 0.21 + 0.06 -- 0 11 & 0.06 -- 0 02 & 0.07 f 0.05
0
1.32 f 0.10
6.75 + 0.03 -0.35 k 0.03 169.58 & 0.03 -0.35 f 0.03 6.75 k 0.03 kO.06 & 0.07 f 0.08 -5.13
a The 61, values are reported relative to a 6,(,, value of 126.30 ppm. The JCH values in hertz. 13C isotope effects on ‘H are reported in hertz at a proton frequency of 100 MHz. carbon coupling constants are given for 15N species (see text). b 10 M in CDCl,. c Prime indicates the ‘“C position. d 1.0 Min CDCI,.
+ 0.10 are reported The nitrogen
Table 5 includes the relaxation data obtained from ‘H and *H spectra. To evaluate the scalar contribution to the linewidth we have used one of the nuclei in the molecule as the internal standard, choosing a nucleus for which the actual values of the coupling constants make it likely that the scalar broadening is negligible. From this, the excess broadening (A* - d) is measured. The nitrogen chemical shift was determined from Ia and Ib, by observing ‘H and irradiating at the nitrogen frequencies. It was found earlier by Becker (12) that a 0.1 A4 MeNO, solution in CDCl, has a resonance frequency of 22,300,783 Hz provided ‘H in TMS is at rtkonance at 220 MHz. We have used a TMS frequency of 100,002,500 Hz and 8, = 0 corresponding to 10,136,973 Hz. On this scale we find that SN is 80 ppm in Ia in a 10 A4 solution in CDCl, and 78 ppm in Ib in a 0.2 A4 solution in CS,. Previously Stefaniak (13) reported 85 ppm for solutions in acetone and in CDCl,. DISCUSSION
Our experimental values of proton-proton coupling constants are seen from Table 1 to be almost identical to the data reported earlier (I). The minor changes are of the same order as that of the solvent dependence reflected within our own data. The largest change is present for JNH.
NMR
OF
TABLE CALCULATED COUPLING STATE METHOD
Coupling constant
3
A
J HH
3 4 5 4 3 4
2 2 2 2 3 3
J NH
2 3 4
1 1 1
J CHkV
1 2 3 4 3
CND0/2 calculation
B
2 3 4
Calculation term
0.35 -5.17 1.03
2 2 2 2 2
81.05 2.89 3.06 0.14 1.62
187.42 4.26 8.12 -1.54 4.10
2 1 2 3 4
3 3 3 3 3
1.58 68.69 0.95 3.61 0.00
4.19 167.16 1.51 8.90 -0.83
3 2 1
4 4 4
2.91 0.59 72.74
6.75 -0.35 169.58
2 3 4 data
CONSTANTS
B
J NC
1 2 3
1 1 1
2 3 4
-5.19 0.27 -1.94
1.62 -0.10 -0.18
J cc
1 1
2 3
3 4
24.90 19.92
-1.99 -2.03
the three terms
from
10 M CDCl,,
4
IN INDO SUM-OVER-STATE ELECTRON DENSITIES~
A
from
Contact
are taken
“JAB
n
a Contributions
6.53 1.11 0.63 1.82 7.6 1 2.13
-1.89 -2.59 -0.39
TABLE COUPLING
Experimental
4.56 1.23 0.53 0.65 6.14 1.76
a Carbon hydrogen experimental all other data from 0.2 M CS,.
CALCULATED
“JAB IN CNDO/Z SUM-OVERELECTRON DENSITIES
CONSTANTS USING FIXED
n -
181
PYRIDINE-N-OXIDE
term
Spin-orbit
in the total value
Dipole+lipole
METHOD
Total
-0.11 0.01 -0.33
-3.67 0.18 -2.45
0.23 0.15
23.14 18.04
are individually
given.
USING
VARIABLE
Experimental kl5.24 1.32 -5.13 -
182
WAMSLER
ET
TABLE RELAXATION
DATA
A
2 3 4
5
0.5 M SOLUTION
(A* - A) (‘T) (Hz)
Position
4
FOR
AL.
OF [4-ZH~PYRIDINE-~-OXIDE (A* -A) (‘H) (Hz)
4.2 &- 0.2 0.5
T (1OGc)
0 0.4 -
r, = 1.0 f 0.2 x 10-r]
IN CDClan
4.3 f 0.2 3.5 4.46
seed
e’qQ/h W-W 1.26b 1.39' 1.23b
180 + 2od
a (A* - A) indicates the scalar broadening b Determined from carbon linewidth. c Determined from proton linewidth. d *H relaxation measurements.
of lines.
The calculated 3JHHvalues (Table 3) are in accordance with the previously published correlation between experimental and calculated coupling constants (14) as seen in Fig. 1. Monotonic trends for the coupling constants are observed through the series pyridine, pyridinium ion, alkyl pyridinium salts, and pyridine-N-oxide (see below).
HZ t
15 -
10 -
,I
III
I 5
I
III
I1 10
III 15
IHz
FIG. 1. Correlation diagram between experimental (ordinate) and CNDO/Z calculated (abscissa) 3Jrm coupling constants. Cl, acetylene, ethylene (cis), ethane; 0, pyridazine, thiazole, pyridine, pyrazole; x, benzene; & pyridine-l\r-oxide. The calculated values are taken from Table 3. In the calculations experimentally determined geometries are used.
NMR
OF
183
PYRIDINE-N-OXIDE
For the couphng constant 4Ju(znnaj and the analogous 4JHH in other nitrogencontaining six-membered heteroaromatic molecules, one can propose the following sequence with increasing values of ‘J,,:
HANAH
HANAH
HANAH
HANAH
HANAH I OH
H
The following literature data support this proposal: Compound Pyridine Pyridinium ion Pyridine N-oxide Pyridine N-oxide (protonated) Pyrimidine Pyrimidine N-oxide Pyrazine Pyrazine N-oxide
Coupling constant (Hz)
4JW(*)Ii(a) = -0.15 4JH(*)H(a) = 0.8 4JHmI(a) = 1.90 4JIi(2)H(a) = 2.05 4JH~~~H~6~ = +O.15
4JH(2)H(a) = 2.07 4JH(z)H(4)= -0.3 l
4JFi(2)H(6) = -0.2 4J H(Z)H(6)
=
1.75
Ref.
(15) UhI7)
(This work) (1) (18) (19) (19) W) (19)
An arrangement similar to that shown above has also been found by Tori et al. (21) to characterize the monotonic increase/decrease of 2JNH(2J3JNwc2,in the equivalent series of quinoline derivatives. It is noted that the 4JHHvalue in benzene is 1.38 Hz (22) and that it falls between the values in pyridine and the N-substituted species of pyridine. value in pyridinium ion has been reported as -0.55 Hz by Lichter and The 4JH(2)H(6) Roberts (15). Earlier papers quoted values of +0.99 (16) and +0.69 Hz (17). Taking the corresponding coupling constant in pyridine, -0.15 Hz, as a starting point, one may deduce from the positive value in I that the 4JH(2jH(6bvalue in pyridinium ion should be positive. A similar argument was used in Ref. (I) to relate a positive value of 1.0 Hz to the electronegativity of the nitrogen substituents in pyridines. When the sigma lone pair of nitrogen is involved in bond formation, the 2JNH increases. Thus we find the values: in pyridine, -10.76 Hz (15); in pyridinium ion, -3.01 Hz (15); in alkyl pyridinium salts, -1.12 to -1.47 Hz (23); and in I, 0.4 Hz. For 3JNH the opposite trend can be observed. From a value of -1.4 Hz (15) in pyridine it decreases to -3.98 Hz in pyridinium ion (IS). The alkyl pyridinium salts show a further decrease, to -3.5 to -4.3 Hz (2.?), and in I we find a value of -5.32 Hz. Values of 4JNH are kO.211 and 0.69 Hz in pyridine and pyridinium ion (15). With a positive value of 1.11 Hz in I it would be reasonable to expect a value of 0.8 Hz in alkyl pyridinium salts. On the basis of changes in linewidth upon 14N decoupling, McFarlane and McFarlane (23) have estimated the coupling constant to be 0.01 to 0.1 Hz. This value is obviously too small. In Table 2 some isotope effects on ‘H due to carbon are reported. These data emerge from the iteration on the proton-coupled carbon spectra. The signs and magnitudes of the effect are similar to those of data published earlier (24) by Goldstein on paradisubstituted benzenes.
184
ET AL.
WAMSLER
The changes in the experimental lJCH values (Table 3) compared with the values in pyridine (25) are in the same directions and are almost of the same magnitudes as those observed by Gill and Pinto (18) in pyridinium ion. This is in accordance with the small geometrical changes that take place on N-oxidation of the ring (4) and the minor changes in charge distribution. Values are given in Table 3. As usual (II) the calculated values are much smaller than the experimental values. The ‘JCCH and 3JcccH values are positive and their magnitudes correspond to the inequality suggested by Tarpley and Goldstein (26) on the basis of substituted benzenes: I 2JCCHI < I 3JcccHI. The 4JCHvalue is small and negative, as in benzenes. For we find a value of 4.10 Hz in I; in pyridine this value is 11.16 Hz (25). A 3JcccH 3JCNCH value of 7.58 Hz has been found for benzene (27). The ‘JCN value is - 15.24 Hz, where the sign is chosen from the INDO calculations (Table 4). As in the previous examples it can be regarded as an extension of the series consisting of pyridine and pyridinium ion (15), where the numerical values 0.45 and 12 Hz were found. A negative sign would be the choice for the pyridinium ion, whereas nothing can be deduced concerning the smaller value in pyridine. The *JCN value is small and positive and the 3JCN value is larger and negative in 1. These findings are in accordance with data for pyridine and pyridinium ion (28). RELAXATION
DATA
AND
LINESHAPES
McFarlane and McFarlane (23), in a paper on alkyl pyridinium salts, noted that the ‘H lineshapes were unequally broadened in the presence of 14N. In pyridine-N-oxide this effect is very pronounced. The origin of this broadening is scalar coupling (29) to a fairly rapidly relaxing 14N nucleus. The nuclear quadrupole coupling constant (NQCC) in I is too small to be determined from MW spectroscopy. Recently Hsieh (30) obtained a value of NQCC (1.23 MHz). In the following section a value of NQCC in the liquid phase is estimated on the basis of the observed line broadenings. Under conditions where the multiplet structure due to nitrogen coupling J,, has disappeared the scalar contribution l/T,(sc) to the linewidths of ‘H and 13C is given by (29) [ll 1
1
1
z=-+-) T,W
T:
Dl
where T,, is the quadrupolar relaxation time for nitrogen. Equation 111assumes that it is possible to assign lines in the spectrum to individual nuclei or equivalent groups of nuclei. In I it is possible to observe i3C spectra under proton decoupling. The proton spectra are considerably more complex, since (see Table 1) H,, H,, and H, are closely coupled and produce mixed transitions where Eq. 111loses its precise meaning. In Ic this problem does not arise. When the scalar contribution to the linewidth in Eq. 121 dominates and the proton nitrogen coupling constants to the protons in the ring differ sufficiently, a sizable variation in linewidth is possible. To dominate the linewidth according to Eqs. 111 and
NMR
OF
PYRIDINE-N-OXIDE
185
121, T,, must be of a reasonable magnitude. Long Tqs values would invalidate Eq. [ll and very short T,, values make the effect unobservable. In general (29), T,, is given by
The asymmetry parameter r7has been found (31) in many heterocyclic compounds to be less than 0.5. Hsieh (30) has found v to be equal to 0.62. We assume that q2/3 can be neglected in the following discussion. The correlation time r, in Eq. [31 can be determined experimentally by observation of the 2H spectrum of Ic. Deuterium relaxation is exclusively quadrupolar (32) and Eq. [31 is valid also for deuterium. The NQCC of deuterium is not sensitive to molecular structure, since the predominant field gradient has its origin in the C-2H bond (33). In Table 5, T, = T, measured for 2H is reported. It is about five times shorter than values reported (33, 34) in perdeuterated pyridine, but similar to the value found for the para position in nitrobenzene (33). In I we are assuming a NQCC for ‘H of 195 kHz. This guess is based on the reported values of 199 kHz in 14-2Hlpyridine (34) and 193 kHz in benzene (35). With this value of the NQCC, Eq. 131 is consistent with a rc of 1.0 x lo-i1 sec. This result can be tested by calculation of the T, relaxation times for carbon. Assuming pure dipolar relaxation and using the experimentally determined CH distance of 1.080 A (4), Eq. 141 predicts a T, value of 4.4 ? 1.0 set for a 0.5 M solution of Ib in CHCl,. -
The actual experimental T, data for a 1.0 M solution in CDCl, are: C(2) = 5.2, C(3) = 5.5, and C(4) = 5.0, in reasonable agreement with predictions. Using Eqs. 111 to 131, we can now turn our attention to the proton and carbon spectra of I. Table 5 lists the observed line broadenings and the resulting NQCC values. The carbon spectra form the better starting point for the calculations. The large value of JNC(2) results in a broadening of C(2) and C(6) signals. The NQCC is calculated assuming that the effect was absent in the (3,5) positions of Ic. It was possible to identify lines from H(3) and H(5) because the molecule is deuterated in the 4-position. The linewidth was measured under 2H decoupling to remove the triplet structure due to the deuteron and relative to the linewidth of H(2). The resulting NQCC does not differ significantly from the values obtained on the basis of carbon data, and it is in reasonable agreement with the unpublished value found by Hsieh (30). In their experiments on alkyl pyridinium salts McFarlane and McFarlane (23) observed water solutions at 70 to 9O’C. Assuming proportionality to the molecular weight and Debye dependence on viscosity, the r, values should be similar in water solutions and our chloroform solutions. Comparing the effects in the spectra with our observation on Ia, we found this to be the case. It can therefore be predicted that the NQCC for nitrogen in the alkyl pyridinium salts will be of the order of 1.5 MHz. CONCLUSION
The combination of 2H, 13C, and ‘H NMR data allows an interpretation of the lineshapes and relaxation behavior that relies on fewer assumptions than is generally the
186
ET AL.
WAMSLER
case. Coupling constants can usefully be related within the series pyridine, pyridinium ion, alkyl pyridinium salts, and pyridine-N-oxide, and this provides guidance in the choice of proper signs in some dubious cases. ACKNOWLEDGMENTS The authors are grateful to NEUCC (Northern Europe University Computing Centre), free computation time. We also acknowledge the gift of [lsNIpyridine from Professor University of Kiel, and the gift of [4-ZH]pyridine-N-oxide from A. Tang-Pedersen. The obtained in part at the University measurements and *H data were
Copenhagen, for H. Dreizler, the ‘“C spectrawere
of Aarhus owing to the cooperation of Dr. H. K. Bildsee. Relaxation obtained by one of us (K.S.) in the laboratory of Dr. I. C. P. Smith,
National Research Council, Ottawa, during a period as Visiting Professor in 1974. Finally, we acknowledge the help of the Danish Scientific Research Council in placing at our disposal and in providing the FSD 120 synthesizer used in this work.
a Bruker
WH
90
REFERENCES I. 2. 3. 4.
S. P. F. 0.
CASTELLANO AND R. KOSTELNIK,J. Am. Chem. Sot. 90,141 (1968). HAMM AND W. VON PHILIPSBORN, Helv. Chim. Acta 54,2363 (1971). VGGTLE AND H. RISLER, Angew. Chem. 84,770 (1972). SNERLING, C. J. NIELSEN, L. NYGAARD, E. J. PEDERSEN, AND G. 0. SORENSEN, 205 (1975).
J. Mol. Struct. 27,
5. A. TANG-PEDERSEN, M. SC. Thesis, University of Copenhagen, 1975. 6. W. G. SCHNEIDER, H. J. BERNSTEIN, AND J. A. POPLE, Ann. N.Y. Acad. Sci. 70,806 7. B. BAK, L. HANSEN, AND J. RASTRUP-ANDERSEN, J. Chem. Phys. 22,2013 (1954).
(1958).
8. 9. 10.
E. J. PEDERSEN, A. TARP@, AND K. SCHAUMBURG, to appear. A. CHARLES AND W. MCFARLANE, Mol. Phys. 14,299 (1968). 0. H. MANSCHER, M.Sc. Thesis, University of Copenhagen, 1971.
II. 12.
A. D. C. TOWL AND K. SCHAUMBURG, Mol. Phys. E. D. Becker, in “Nitrogen NMR” (M. Witanowski York, 1973.
13. 14. 16. 17.
L. J. R. J. T.
18. 19. 20. 21.
V. A. R. K.
22. 23.
J. M. READ, R. E. MAYO, AND J. H. GOLDSTEIN, H. C. E. MCFARLANE AND W. MCFARLANE, Org.
24. 25. 26.
J. M. READ, JR., R. W. CRECELY, AND J. H. J. B. STOTHERS, “Carbon-13 NMR Spectroscopy,” Academic Press, A. R. TARPLEY AND J. H. GOLDSTEIN, Mol. Phys. 21,549 (1971).
27. 28. 29.
A. R. TARPLEY AND J. H. GOLDSTEIN, J. Phys. Chem. 76,515 (1972). T. BUNDGAARD AND H. J. JAKOBSEN, Tetrahedron Left. 19,162l (1976). T. C. FARRAR AND E. D. BECKER, “Pulse and Fourier Transform NMR,” Academic Press, New York, 1971. Y. N. HSIEH, unpublished data cited in C. P. CHENG AND T. L. BROWN, J. Chem. Phys. 67, 1797 (1977). M. WITANOWSKI AND G. A. WEBB (Eds.), “Nitrogen NMR,” Plenum, New York, 1973. A. ABRAGAM, “The Principles of Nuclear Magnetism,” Chap. 8, Oxford, Univ. Press, New York/London, 196 1.
15.
30.
31. 32. 33. 34. 35.
22,49 and
(1971). G. A. Webb,
Eds.),
STEFANIAK, unpublished data cited in Ref. (12). P. JACOBSEN, K. SCHAUMBURG, AND J. T. NIELSEN, J. Magn. Reson. 13,372 L. LICHTER AND J. D. ROBERTS, J. Am. Chem. Sot. 93,5218 (1971). B. MERRY AND J. H. GOLDSTEIN, J. Am. Chem. Sot. 88,556O (1966). TOKUHIRO, N. K. WILSON, AND G. FRAENKEL, J. Am. Chem. Sot. 90,3622
Chap.
4, Plenum,
(1974).
(1968).
M. S. GIL AND A. J. L. PINTO, Mol. Phys. 19,573 (1970). G. MORITZ AND D. B. PAUL, Aust. J. Chem. 22,130s (1969). H. Cox AND A. A. BOTHNER-BY, J. Phys. Chem. 72,1646 (1968). TORI, M. OHTSIJRU, K. AONO, Y. KAWAZOE, AND M. OHNISHI, J. Am.
Chem. Sot. 89,2765 J. Mol. Spectrosc. 21,235 (1966). Magn. Reson. 4,161 (1972). GOLDSTEIN, J. Mol. Spectrosc. 25,107 (1968).
H. MANTSCH, H. SAITO, I. C. LEITCH, AND I. C. P. SMITH, J. Am. J. P. KINTZINGER AND J. M. LEHN, Mol. Phys. 22,273 (1971). J. C. ROWELL, W. D. PHILLIPS, L. R. MALBY, AND M. PANAR, J.
New
York,
New
(1967).
1972.
Chem. Sot. 96,256 Chem. Phys. 43,3442
(1974). (1965).
NMR
OF MAGNETIC
RESONANCE
31,177-186
(1978)
Studies of Pyridine-N-Oxide. Determination of Spectroscopic Constants from [lsN]-, [4-2H]-, and the Parent Species
TOREEN WAMSLER,JPIRGENTORMOD NIELSEN, ERIK JONAS PEDERSEN,AND KJELD SCHAIJMBURG* Department
of Chemical
Physics,
H.C.
0rsted
Institute, DK-2100
The University
of Copenhagen, Denmark
ReceivedApril 11, 1977;revisedDecember27, 1977 Pyridine-N-oxide, [‘5Nlpyridine-N-oxide, and [4-2Hlpyridine-N-oxide have been investigatedby NMR spectroscopy.‘H, 2H, and lgC spectra have been obtained and analyzed,in terms of chemicalshifts and spin-spin couplingconstants.Relaxationtimes havebeenmeasuredfor ‘H and “C and havebeenusedto determinethe correlationtime r, under the assumption of isotropic molecular diffusion. On the basis of experimental data and observed linewidth variations the nitrogen quadrupole coupling constant has been evaluated. The experimental data are correlated with semiempirical calculations within the azine series.
INTRODUCTION Pyridine and its derivatives have been the subject of numerous NMR investigations. In the present paper we report a study of pyridine-N-oxide (Ia), [lSNlpyridine-N-oxide (Ib), and [4-ZHlpyridine-N-oxide (1~): lo
Ib
IC
Earlier studies of the ‘H spectrum of Ia have been published (I, 2). In the work by Castellano and Kostelnik (I) accurate values for all the interproton coupling constants have been determined using 14N decoupling. The geometry of the N-oxide group, and in particular the geometry of I, has not been particularly well known. Viigtle and Risler (3) have tried to argue for a more reliable picture of the N-O bond in I on the basis of NMR data. In our laboratory a microwave spectroscopic determination of the structure of I was initiated (4, 5). We found it of interest parallel to this work to undertake an investigation of the NMR spectra of a series of enriched isotopic substituted species Ib to Ic. It is well known that in pyridine derivatives a significant variation in linewidth can be observed in the proton spectra (1, 6). This is related to the scalar coupling to nitrogen. With access to the labeled species Ib and Ic, the relaxation mechanism can be * Author to whom correspondence should be addressed. 177
OOZZ-2364178/03 IZ-0177$02.00/0 Copyright 0 1978 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
178
WAMSLER
ET
AL.
investigated in some detail and a value of the nuclear quadrupole coupling constant (NQCC) can be deduced. Having a reliable geometry of I, we calculated the spin-spin coupling constants within the CND0/2 and INDO semiempirical approximations. Experimentally large changes in magnitude of individual spin-spin coupling constants occur upon N-oxidation of pyridine. We felt that determining whether these changes are reflected properly in the calculations would be of interest. EXPERIMENTAL
[15Nlpyridine-N-oxide was prepared from [i5Nlpyridine 95% enriched in 15N by Snerling et al. (4). [4-*Hlpyridine was prepared according to the method of (7) from 4bromopyridine by zinc powder reduction in acetic anhydride. The oxide was prepared as above (4, 5). Samples were prepared by dissolving I in CS, (0.2 M) or in CDCl, at several concentrations (see below). TMS was added as the internal standard and the samples were sealed after several freeze-pump-thaw cycles. For 2H spectra a 0.5 M solution in CHCl, was prepared. 13C spectra were obtained in the FT mode using a Varian XL-100 spectrometer operating at 25.12 MHz at 32OC. Proton-coupled spectra were recorded with i4N decoupling using 8K data points and a spectral range of 600 Hz. The repetition time was 6.7 set and the pulse width 75 psec, corresponding to 70°C. Similar conditions were used for linewidth measurements in ‘H-decoupled 13C spectra. i3C T, measurements were performed on a Varian XL-100 and a Bruker WH 90 spectrometer. In both cases the inversion-recovery method was used. The fitting of relaxation data to a relaxation time was accomplished by means of a nonlinear three-parameter regression analysis. 2H T, measurements were performed on a Varian XL-100 spectrometer using the inversion-recovery technique as above. ‘H spectra were recorded on a Varian HA-100 spectrometer operating at 32”C, modified for digital sweep (8) under the control of a Varian 620/i computer. Hetero-INDOR spectra were obtained by driving a Schlumberger FSD 120 synthesizer with a DAC ramp from the computer. In the heterodecoupling experiments an Electronic Navigation Industries power amplifier model 3 1OL was used to boost the output of the FSD 120. The V 4333 probe was modified for heterodecoupling according to McFarlane’s method (9). ‘H spectra were also obtained using a Bruker HX 60 spectrometer coupled to a Varian C 1024 CAT. This instrument was used for homotickling and homo-INDOR experiments. Weak lines in the wings of the spectra were also observed using this instrument. The ‘H spectra used in determination of coupling constants were recorded with a sweep expansion of 0.2 Hz/cm and with sweep rates not exceeding 6 x 1O-3 Hzlsec. The analysis of the spectra was performed using the programs LAOCOON III, HOMO, and HETERO (10). The latter two were used in tickling and INDOR calculations. CNDO/Z and INDO calculations were carried out using the sum-over-state method (SOS). The parametrization has been discussed by Tow1 and Schaumburg (II). RESULTS
Analysis of the 60-MHz proton spectra of Ib results in the values in Table 1. For comparison the coupling constants published by Castellano and Kostelnik (1) have
NMR
OF
PYRIDINE-N-OXIDE
179
been included. Analysis of the lOO-MHz proton spectra of Ia produced values identical to those reported. Ic was similarly analyzed, and the parameter values were within the uncertainties coinciding with the data in Table 1. The indicated uncertainties have been taken as three times the probable errors. Experimentally it was found that the inclusion of weak outer lines (only observable after data accumulation) reduced the correlation among various parameters decisively and improved the rms error somewhat. TABLE ANALYSES
OF 60.MHz
SPECTRA
1
OF Ib AND lO@MHz
SPECTRA
OF Ia
Species lb
Ib
la
Resonance
Coupling
constant
“-‘HA Hg (Hz)
“J,
&‘W
Ha Wd
(ppm)
a Data
n
A
B
3 4 5 4 3 4 2 3 4
2 2 2 2 3 3 1 1 1
3 4 5 6 4 5 2 3 4
s, = s, s, = s, 4 44.,-J,) from
Ref.
la”
frequency
60 MHz
60 MHz
100 MHz
60 MHz
0.2 Min -1
l.OMin CDCl,
1OMin CDCl,
1.0 M in (CH,),CO
6.530 1.108 0.625 1.820 7.606 2.129 0.35 1 -5.174 1.026
+ f f f k + f f f
0.009 0.009 0.009 0.006 0.006 0.006 0.009 0.009 0.012
7.91095 f 0.00005 7.13348 f 0.00010 7.05490 + 0.00010 0.07858
6.53 1.12 0.64 1.90 7.69 2.07 0.47 -5.32 1.11
f f f + f + f f +
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02
6.5 1 + 1.12 f 0.63 + 1.89 f 7.67 f 2.11 f
0.02 0.02 0.02 0.01 0.01 0.01
6.47 1.12 0.63 1.88 7.65 2.13
-t i + k i k
0.08 0.08 0.08 0.08 0.08 0.08
-
8.2854 f O.OCOl 7.3408 f 0.0002 7.3427 + 0.0002 -0.0019
8.2921 k O.OGOl 7.3804 5 0.0001 7.3321 + 0.0002 0.0583
(I).
Using INDOR, the sign of the JNHc3)coupling constant’ was determined relative to the assumption that the ortho coupling JH(2jw(3j was positive. The signs of the two smaller JNH are, in the iterative fitting of all the spectra analyzed, found to be opposite in sign to JNwc3). In Table 2, JCN and JCH values are reported as well as carbon chemical shifts. The JCH values have been determined from spectra of a 10 M CDCl, solution of Ia. This is due to the solvent effect that makes &c(j) - &c(d)eq ual to -0.1 ppm for a 0.1 M solution, but +0.6 ppm for a 10 M solution. Only in the last case was it possible to analyze the coupled carbon spectra unambiguously. To remove nitrogen broadening, decoupling of nitrogen was necessary, to obtain the JCH values. The quoted values of JCN are observations on a 1.0 M solution of Ib. Spin-spin coupling constants involving hydrogen have been calculated in the CND0/2 approximation (II), and the values are listed in Table 3. Spin-spin coupling constants between two nonhydrogen nuclei are calculated, and the INDO results are shown in Table 4. In these cases the contact, spin-orbit, and dipole-dipole terms may all contribute to the coupling constants, and they have been separately listed. ‘Throughout the paper coupling constants between nitrogen comparison has made it necessary, 14N data have been converted.
and
other
nuclei
refer
to
15N.
Where
180
WAMSLER TABLE “C
Gb J CH(2) b J CH(3) b J CH(4) b J CH(5) b J CH(6) b c
J iiCd
NMR
DATA
ET
AL.
2
FOR PYRIDINE-N-0x1~~~
13.3903
k 0.0008
0.5478
k 0.0008
187.42 4.26 8.12 -1.54 4.10
k + 2 + f
4.19 167.16 1.5 1 8.90 -0.83
& + k It k
“H(2) - “H(2) ’ “H(3) - “HOI ’ “H(4) - “H(4) ’ + 15.24
0.05 0.007 0.06 0.06 0.03
0.05 0.04 0.08 0.08 0.05
” H(2) - “H(Z) ‘-0.11 “H(3) - “H(3) ’ = 0.26 “H(4) - “H(4) ’ -- 0.04
-- 0.21 + 0.06 -- 0 11 & 0.06 -- 0 02 & 0.07 f 0.05
0
1.32 f 0.10
6.75 + 0.03 -0.35 k 0.03 169.58 & 0.03 -0.35 f 0.03 6.75 k 0.03 kO.06 & 0.07 f 0.08 -5.13
a The 61, values are reported relative to a 6,(,, value of 126.30 ppm. The JCH values in hertz. 13C isotope effects on ‘H are reported in hertz at a proton frequency of 100 MHz. carbon coupling constants are given for 15N species (see text). b 10 M in CDCl,. c Prime indicates the ‘“C position. d 1.0 Min CDCI,.
+ 0.10 are reported The nitrogen
Table 5 includes the relaxation data obtained from ‘H and *H spectra. To evaluate the scalar contribution to the linewidth we have used one of the nuclei in the molecule as the internal standard, choosing a nucleus for which the actual values of the coupling constants make it likely that the scalar broadening is negligible. From this, the excess broadening (A* - d) is measured. The nitrogen chemical shift was determined from Ia and Ib, by observing ‘H and irradiating at the nitrogen frequencies. It was found earlier by Becker (12) that a 0.1 A4 MeNO, solution in CDCl, has a resonance frequency of 22,300,783 Hz provided ‘H in TMS is at rtkonance at 220 MHz. We have used a TMS frequency of 100,002,500 Hz and 8, = 0 corresponding to 10,136,973 Hz. On this scale we find that SN is 80 ppm in Ia in a 10 A4 solution in CDCl, and 78 ppm in Ib in a 0.2 A4 solution in CS,. Previously Stefaniak (13) reported 85 ppm for solutions in acetone and in CDCl,. DISCUSSION
Our experimental values of proton-proton coupling constants are seen from Table 1 to be almost identical to the data reported earlier (I). The minor changes are of the same order as that of the solvent dependence reflected within our own data. The largest change is present for JNH.
NMR
OF
TABLE CALCULATED COUPLING STATE METHOD
Coupling constant
3
A
J HH
3 4 5 4 3 4
2 2 2 2 3 3
J NH
2 3 4
1 1 1
J CHkV
1 2 3 4 3
CND0/2 calculation
B
2 3 4
Calculation term
0.35 -5.17 1.03
2 2 2 2 2
81.05 2.89 3.06 0.14 1.62
187.42 4.26 8.12 -1.54 4.10
2 1 2 3 4
3 3 3 3 3
1.58 68.69 0.95 3.61 0.00
4.19 167.16 1.51 8.90 -0.83
3 2 1
4 4 4
2.91 0.59 72.74
6.75 -0.35 169.58
2 3 4 data
CONSTANTS
B
J NC
1 2 3
1 1 1
2 3 4
-5.19 0.27 -1.94
1.62 -0.10 -0.18
J cc
1 1
2 3
3 4
24.90 19.92
-1.99 -2.03
the three terms
from
10 M CDCl,,
4
IN INDO SUM-OVER-STATE ELECTRON DENSITIES~
A
from
Contact
are taken
“JAB
n
a Contributions
6.53 1.11 0.63 1.82 7.6 1 2.13
-1.89 -2.59 -0.39
TABLE COUPLING
Experimental
4.56 1.23 0.53 0.65 6.14 1.76
a Carbon hydrogen experimental all other data from 0.2 M CS,.
CALCULATED
“JAB IN CNDO/Z SUM-OVERELECTRON DENSITIES
CONSTANTS USING FIXED
n -
181
PYRIDINE-N-OXIDE
term
Spin-orbit
in the total value
Dipole+lipole
METHOD
Total
-0.11 0.01 -0.33
-3.67 0.18 -2.45
0.23 0.15
23.14 18.04
are individually
given.
USING
VARIABLE
Experimental kl5.24 1.32 -5.13 -
182
WAMSLER
ET
TABLE RELAXATION
DATA
A
2 3 4
5
0.5 M SOLUTION
(A* - A) (‘T) (Hz)
Position
4
FOR
AL.
OF [4-ZH~PYRIDINE-~-OXIDE (A* -A) (‘H) (Hz)
4.2 &- 0.2 0.5
T (1OGc)
0 0.4 -
r, = 1.0 f 0.2 x 10-r]
IN CDClan
4.3 f 0.2 3.5 4.46
seed
e’qQ/h W-W 1.26b 1.39' 1.23b
180 + 2od
a (A* - A) indicates the scalar broadening b Determined from carbon linewidth. c Determined from proton linewidth. d *H relaxation measurements.
of lines.
The calculated 3JHHvalues (Table 3) are in accordance with the previously published correlation between experimental and calculated coupling constants (14) as seen in Fig. 1. Monotonic trends for the coupling constants are observed through the series pyridine, pyridinium ion, alkyl pyridinium salts, and pyridine-N-oxide (see below).
HZ t
15 -
10 -
,I
III
I 5
I
III
I1 10
III 15
IHz
FIG. 1. Correlation diagram between experimental (ordinate) and CNDO/Z calculated (abscissa) 3Jrm coupling constants. Cl, acetylene, ethylene (cis), ethane; 0, pyridazine, thiazole, pyridine, pyrazole; x, benzene; & pyridine-l\r-oxide. The calculated values are taken from Table 3. In the calculations experimentally determined geometries are used.
NMR
OF
183
PYRIDINE-N-OXIDE
For the couphng constant 4Ju(znnaj and the analogous 4JHH in other nitrogencontaining six-membered heteroaromatic molecules, one can propose the following sequence with increasing values of ‘J,,:
HANAH
HANAH
HANAH
HANAH
HANAH I OH
H
The following literature data support this proposal: Compound Pyridine Pyridinium ion Pyridine N-oxide Pyridine N-oxide (protonated) Pyrimidine Pyrimidine N-oxide Pyrazine Pyrazine N-oxide
Coupling constant (Hz)
4JW(*)Ii(a) = -0.15 4JH(*)H(a) = 0.8 4JHmI(a) = 1.90 4JIi(2)H(a) = 2.05 4JH~~~H~6~ = +O.15
4JH(2)H(a) = 2.07 4JH(z)H(4)= -0.3 l
4JFi(2)H(6) = -0.2 4J H(Z)H(6)
=
1.75
Ref.
(15) UhI7)
(This work) (1) (18) (19) (19) W) (19)
An arrangement similar to that shown above has also been found by Tori et al. (21) to characterize the monotonic increase/decrease of 2JNH(2J3JNwc2,in the equivalent series of quinoline derivatives. It is noted that the 4JHHvalue in benzene is 1.38 Hz (22) and that it falls between the values in pyridine and the N-substituted species of pyridine. value in pyridinium ion has been reported as -0.55 Hz by Lichter and The 4JH(2)H(6) Roberts (15). Earlier papers quoted values of +0.99 (16) and +0.69 Hz (17). Taking the corresponding coupling constant in pyridine, -0.15 Hz, as a starting point, one may deduce from the positive value in I that the 4JH(2jH(6bvalue in pyridinium ion should be positive. A similar argument was used in Ref. (I) to relate a positive value of 1.0 Hz to the electronegativity of the nitrogen substituents in pyridines. When the sigma lone pair of nitrogen is involved in bond formation, the 2JNH increases. Thus we find the values: in pyridine, -10.76 Hz (15); in pyridinium ion, -3.01 Hz (15); in alkyl pyridinium salts, -1.12 to -1.47 Hz (23); and in I, 0.4 Hz. For 3JNH the opposite trend can be observed. From a value of -1.4 Hz (15) in pyridine it decreases to -3.98 Hz in pyridinium ion (IS). The alkyl pyridinium salts show a further decrease, to -3.5 to -4.3 Hz (2.?), and in I we find a value of -5.32 Hz. Values of 4JNH are kO.211 and 0.69 Hz in pyridine and pyridinium ion (15). With a positive value of 1.11 Hz in I it would be reasonable to expect a value of 0.8 Hz in alkyl pyridinium salts. On the basis of changes in linewidth upon 14N decoupling, McFarlane and McFarlane (23) have estimated the coupling constant to be 0.01 to 0.1 Hz. This value is obviously too small. In Table 2 some isotope effects on ‘H due to carbon are reported. These data emerge from the iteration on the proton-coupled carbon spectra. The signs and magnitudes of the effect are similar to those of data published earlier (24) by Goldstein on paradisubstituted benzenes.
184
ET AL.
WAMSLER
The changes in the experimental lJCH values (Table 3) compared with the values in pyridine (25) are in the same directions and are almost of the same magnitudes as those observed by Gill and Pinto (18) in pyridinium ion. This is in accordance with the small geometrical changes that take place on N-oxidation of the ring (4) and the minor changes in charge distribution. Values are given in Table 3. As usual (II) the calculated values are much smaller than the experimental values. The ‘JCCH and 3JcccH values are positive and their magnitudes correspond to the inequality suggested by Tarpley and Goldstein (26) on the basis of substituted benzenes: I 2JCCHI < I 3JcccHI. The 4JCHvalue is small and negative, as in benzenes. For we find a value of 4.10 Hz in I; in pyridine this value is 11.16 Hz (25). A 3JcccH 3JCNCH value of 7.58 Hz has been found for benzene (27). The ‘JCN value is - 15.24 Hz, where the sign is chosen from the INDO calculations (Table 4). As in the previous examples it can be regarded as an extension of the series consisting of pyridine and pyridinium ion (15), where the numerical values 0.45 and 12 Hz were found. A negative sign would be the choice for the pyridinium ion, whereas nothing can be deduced concerning the smaller value in pyridine. The *JCN value is small and positive and the 3JCN value is larger and negative in 1. These findings are in accordance with data for pyridine and pyridinium ion (28). RELAXATION
DATA
AND
LINESHAPES
McFarlane and McFarlane (23), in a paper on alkyl pyridinium salts, noted that the ‘H lineshapes were unequally broadened in the presence of 14N. In pyridine-N-oxide this effect is very pronounced. The origin of this broadening is scalar coupling (29) to a fairly rapidly relaxing 14N nucleus. The nuclear quadrupole coupling constant (NQCC) in I is too small to be determined from MW spectroscopy. Recently Hsieh (30) obtained a value of NQCC (1.23 MHz). In the following section a value of NQCC in the liquid phase is estimated on the basis of the observed line broadenings. Under conditions where the multiplet structure due to nitrogen coupling J,, has disappeared the scalar contribution l/T,(sc) to the linewidths of ‘H and 13C is given by (29) [ll 1
1
1
z=-+-) T,W
T:
Dl
where T,, is the quadrupolar relaxation time for nitrogen. Equation 111assumes that it is possible to assign lines in the spectrum to individual nuclei or equivalent groups of nuclei. In I it is possible to observe i3C spectra under proton decoupling. The proton spectra are considerably more complex, since (see Table 1) H,, H,, and H, are closely coupled and produce mixed transitions where Eq. 111loses its precise meaning. In Ic this problem does not arise. When the scalar contribution to the linewidth in Eq. 121 dominates and the proton nitrogen coupling constants to the protons in the ring differ sufficiently, a sizable variation in linewidth is possible. To dominate the linewidth according to Eqs. 111 and
NMR
OF
PYRIDINE-N-OXIDE
185
121, T,, must be of a reasonable magnitude. Long Tqs values would invalidate Eq. [ll and very short T,, values make the effect unobservable. In general (29), T,, is given by
The asymmetry parameter r7has been found (31) in many heterocyclic compounds to be less than 0.5. Hsieh (30) has found v to be equal to 0.62. We assume that q2/3 can be neglected in the following discussion. The correlation time r, in Eq. [31 can be determined experimentally by observation of the 2H spectrum of Ic. Deuterium relaxation is exclusively quadrupolar (32) and Eq. [31 is valid also for deuterium. The NQCC of deuterium is not sensitive to molecular structure, since the predominant field gradient has its origin in the C-2H bond (33). In Table 5, T, = T, measured for 2H is reported. It is about five times shorter than values reported (33, 34) in perdeuterated pyridine, but similar to the value found for the para position in nitrobenzene (33). In I we are assuming a NQCC for ‘H of 195 kHz. This guess is based on the reported values of 199 kHz in 14-2Hlpyridine (34) and 193 kHz in benzene (35). With this value of the NQCC, Eq. 131 is consistent with a rc of 1.0 x lo-i1 sec. This result can be tested by calculation of the T, relaxation times for carbon. Assuming pure dipolar relaxation and using the experimentally determined CH distance of 1.080 A (4), Eq. 141 predicts a T, value of 4.4 ? 1.0 set for a 0.5 M solution of Ib in CHCl,. -
The actual experimental T, data for a 1.0 M solution in CDCl, are: C(2) = 5.2, C(3) = 5.5, and C(4) = 5.0, in reasonable agreement with predictions. Using Eqs. 111 to 131, we can now turn our attention to the proton and carbon spectra of I. Table 5 lists the observed line broadenings and the resulting NQCC values. The carbon spectra form the better starting point for the calculations. The large value of JNC(2) results in a broadening of C(2) and C(6) signals. The NQCC is calculated assuming that the effect was absent in the (3,5) positions of Ic. It was possible to identify lines from H(3) and H(5) because the molecule is deuterated in the 4-position. The linewidth was measured under 2H decoupling to remove the triplet structure due to the deuteron and relative to the linewidth of H(2). The resulting NQCC does not differ significantly from the values obtained on the basis of carbon data, and it is in reasonable agreement with the unpublished value found by Hsieh (30). In their experiments on alkyl pyridinium salts McFarlane and McFarlane (23) observed water solutions at 70 to 9O’C. Assuming proportionality to the molecular weight and Debye dependence on viscosity, the r, values should be similar in water solutions and our chloroform solutions. Comparing the effects in the spectra with our observation on Ia, we found this to be the case. It can therefore be predicted that the NQCC for nitrogen in the alkyl pyridinium salts will be of the order of 1.5 MHz. CONCLUSION
The combination of 2H, 13C, and ‘H NMR data allows an interpretation of the lineshapes and relaxation behavior that relies on fewer assumptions than is generally the
186
ET AL.
WAMSLER
case. Coupling constants can usefully be related within the series pyridine, pyridinium ion, alkyl pyridinium salts, and pyridine-N-oxide, and this provides guidance in the choice of proper signs in some dubious cases. ACKNOWLEDGMENTS The authors are grateful to NEUCC (Northern Europe University Computing Centre), free computation time. We also acknowledge the gift of [lsNIpyridine from Professor University of Kiel, and the gift of [4-ZH]pyridine-N-oxide from A. Tang-Pedersen. The obtained in part at the University measurements and *H data were
Copenhagen, for H. Dreizler, the ‘“C spectrawere
of Aarhus owing to the cooperation of Dr. H. K. Bildsee. Relaxation obtained by one of us (K.S.) in the laboratory of Dr. I. C. P. Smith,
National Research Council, Ottawa, during a period as Visiting Professor in 1974. Finally, we acknowledge the help of the Danish Scientific Research Council in placing at our disposal and in providing the FSD 120 synthesizer used in this work.
a Bruker
WH
90
REFERENCES I. 2. 3. 4.
S. P. F. 0.
CASTELLANO AND R. KOSTELNIK,J. Am. Chem. Sot. 90,141 (1968). HAMM AND W. VON PHILIPSBORN, Helv. Chim. Acta 54,2363 (1971). VGGTLE AND H. RISLER, Angew. Chem. 84,770 (1972). SNERLING, C. J. NIELSEN, L. NYGAARD, E. J. PEDERSEN, AND G. 0. SORENSEN, 205 (1975).
J. Mol. Struct. 27,
5. A. TANG-PEDERSEN, M. SC. Thesis, University of Copenhagen, 1975. 6. W. G. SCHNEIDER, H. J. BERNSTEIN, AND J. A. POPLE, Ann. N.Y. Acad. Sci. 70,806 7. B. BAK, L. HANSEN, AND J. RASTRUP-ANDERSEN, J. Chem. Phys. 22,2013 (1954).
(1958).
8. 9. 10.
E. J. PEDERSEN, A. TARP@, AND K. SCHAUMBURG, to appear. A. CHARLES AND W. MCFARLANE, Mol. Phys. 14,299 (1968). 0. H. MANSCHER, M.Sc. Thesis, University of Copenhagen, 1971.
II. 12.
A. D. C. TOWL AND K. SCHAUMBURG, Mol. Phys. E. D. Becker, in “Nitrogen NMR” (M. Witanowski York, 1973.
13. 14. 16. 17.
L. J. R. J. T.
18. 19. 20. 21.
V. A. R. K.
22. 23.
J. M. READ, R. E. MAYO, AND J. H. GOLDSTEIN, H. C. E. MCFARLANE AND W. MCFARLANE, Org.
24. 25. 26.
J. M. READ, JR., R. W. CRECELY, AND J. H. J. B. STOTHERS, “Carbon-13 NMR Spectroscopy,” Academic Press, A. R. TARPLEY AND J. H. GOLDSTEIN, Mol. Phys. 21,549 (1971).
27. 28. 29.
A. R. TARPLEY AND J. H. GOLDSTEIN, J. Phys. Chem. 76,515 (1972). T. BUNDGAARD AND H. J. JAKOBSEN, Tetrahedron Left. 19,162l (1976). T. C. FARRAR AND E. D. BECKER, “Pulse and Fourier Transform NMR,” Academic Press, New York, 1971. Y. N. HSIEH, unpublished data cited in C. P. CHENG AND T. L. BROWN, J. Chem. Phys. 67, 1797 (1977). M. WITANOWSKI AND G. A. WEBB (Eds.), “Nitrogen NMR,” Plenum, New York, 1973. A. ABRAGAM, “The Principles of Nuclear Magnetism,” Chap. 8, Oxford, Univ. Press, New York/London, 196 1.
15.
30.
31. 32. 33. 34. 35.
22,49 and
(1971). G. A. Webb,
Eds.),
STEFANIAK, unpublished data cited in Ref. (12). P. JACOBSEN, K. SCHAUMBURG, AND J. T. NIELSEN, J. Magn. Reson. 13,372 L. LICHTER AND J. D. ROBERTS, J. Am. Chem. Sot. 93,5218 (1971). B. MERRY AND J. H. GOLDSTEIN, J. Am. Chem. Sot. 88,556O (1966). TOKUHIRO, N. K. WILSON, AND G. FRAENKEL, J. Am. Chem. Sot. 90,3622
Chap.
4, Plenum,
(1974).
(1968).
M. S. GIL AND A. J. L. PINTO, Mol. Phys. 19,573 (1970). G. MORITZ AND D. B. PAUL, Aust. J. Chem. 22,130s (1969). H. Cox AND A. A. BOTHNER-BY, J. Phys. Chem. 72,1646 (1968). TORI, M. OHTSIJRU, K. AONO, Y. KAWAZOE, AND M. OHNISHI, J. Am.
Chem. Sot. 89,2765 J. Mol. Spectrosc. 21,235 (1966). Magn. Reson. 4,161 (1972). GOLDSTEIN, J. Mol. Spectrosc. 25,107 (1968).
H. MANTSCH, H. SAITO, I. C. LEITCH, AND I. C. P. SMITH, J. Am. J. P. KINTZINGER AND J. M. LEHN, Mol. Phys. 22,273 (1971). J. C. ROWELL, W. D. PHILLIPS, L. R. MALBY, AND M. PANAR, J.
New
York,
New
(1967).
1972.
Chem. Sot. 96,256 Chem. Phys. 43,3442
(1974). (1965).