1H NMR spectra of diazomethane and diazocyclopentadiene containing 15N

JOURNAL

OF MAGNETIC

‘H NMR

RESONANCE

13,372-378

(1974)

Spectra of Diazomethane and Diazocyclopentadiene Containing 15N

JENSPETERJACOBSEN,KJELD SCHAUMBURG,AND JP~RGEN TORMOD NIELSEN Chemical Laboratory V, The H.C. 0rsted Institute, Universitetsparken 5 DK-2100 Copenhagen El., Denmark

ReceivedAugust 20, 1973 Preparation and ‘H NMR spectra are described for [D]-, [l-15N]-, [2-15N]diazomethane,and [l-15N]diazocyclopentadiene.CNDO/Z and INDO calculations of spin-spin coupling constants in diazomethane and diazocyclopentadieneare reported. Correlations betweenthe calculatedand experimentalvaluesare discussed. INTRODUCTION

As an extension of our earlier work on spin-spin coupling constants in 15N-containing molecules (I), we wish to report the NMR data of diazomethane and diazocyclopentadiene.

H\ ti’ For this purpose [D]-, [l-lJN]-, [2-15N]diazomethane, and [I-15N]diazocyclopentadiene have been synthesized and the ‘H NMR spectra analyzed. Except for H-H coupling constants in diazocyclopentadiene (2), none of these coupling constants have been published before. Within the CND0/2-INDO formalism (3), we have calculated semiempirical values for the coupling constants in [l J5N]diazocyclopentadiene. These values are reported together with the calculated values in diazomethane partly given before (4). The correlation between the calculated and experimental values will be discussed. EXPERIMENTAL

‘H NMR spectra were obtained on a Varian HA-100 spectrometer modified for sweep expansion (5). Hetero tickling of 15N was carried out using a Schlumberger FSD 120 frequency synthesizer with yH,/2n of about 0.1 Hz. Spectra were recorded at 3 1 “C in frequency sweep mode using TMS as internal standard. Data accumulation was performed by using a Varian Spectra System 100. Copyright 6 1974 by Academic Press, Inc. All ri&s of reproduction in any form reserved. Printed in Great Britain

372

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15 ENRICHED

DIAZOCOMPOUNDS

37.3

The microwave (MW) and infrared (IR) spectra described later were obtained on a conventional Stark modulated spectrograph and a Perkin IElmer model 125 spectrometer, respectively. The samples of diazomethane for NMR purposes were prepared by distilling a solution of the compound in CD&l2 into an NMR tube containing TMS. The solute concentrations were about 10 mole%. The sample of diazocyclopentadiene was prepared by distilling the pure compound into an NMR tube containing CC& and TMS. The solute concentration was about 25% v/v. After several freeze-pump-thaw cycles, the tubes were sealed. In the case of [ lJ5N]diazocyclopentadiene, chemical shifts and coupling constants were obtained from the spectral data using the iterative program LAOCOON III (6). Simulations of double resonance spectra were made by use of the programs HOMO and HETERO (6). These programs are derived from LAOCOON III and based upon the formalism described by Govil and Whiffen (7). Two programs have been developed for homo and hetero decoupling, respectively. Theoretical values of coupling constants were obtained us.ing the CNDOJ2-INDO program described elsewhere (4,8). PREPARATION

Diuzomethane

The isotopic substituted diazomethanes described here were prepared by adding a solution of KOH in a mixture of water and diethyleneglycol monoethyl ether to a solution of the corresponding isotopic substituted N-methyl-N-nitroso-p-toluenesulfonylamide (A) (9) in CD&l,. A mixture of diazomethane and CD&l2 was then distilled from the reaction mixture. The pure diazomethanes used in the IR and MW investigation were obtained by substituting CD& with 2-butoxyethanol(10). The diazomethanes were then purified by distillations (-90 to -143°C followed by -125 to -143°C) in an Nz atmosphere. Replacing water by DzO, a mixture of CD,N2, CHDN,, and CH,N, was obtained. By preparing A from Na15N02 (95.6 % enriched in lsN), CHZNiSN (95.6 % enriched in lsN) was synthesized. Using CH31sNH,CI (12) (33 % enriched in lsN) as starting material for A, CH2i5NN (33 % enriched in 15N) was obtained. Diazocyclopentadiene

[I-15N]Diazocyclopentadiene (47.8 % enriched in 15N) was prepared according to Weil and Cais (22) using potassium[l-15N]azid (23) as starting material. RESULTS

In Table 1 the experimental values of the coupling constants in the various isotopic substituted diazomethanes are reported and compared to the calculated values partly reported by Tow1 and Schaumburg earlier (4). The lJcH coupling constant was found from the 13C satellites obtained by accumulation of 185 scans. The spectrum of [1-i5N]diazocyclopentadiene shown in Fig. 1 was anaIyzed as an AA’BB’X system. Iterations on 38 lines with LAOCOON III gave the values listed

314

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TABLE 1 EXPERIMENTAL

AND THEORETKAL

VALUES OF COUPLING

CONSTANTS

(Hz)

IN DIAZOMETHANE’

Theoretical CNDO/Z Coupling constants

2J”D ‘JEW ‘JCH ‘JNH 3JNH

INDO

Experimental

without CT

with CI

without CI

with CI

+0.70 + 0.02 k4.56 + 0.13b k195.1 + 0.1 M.14 Ifr0.1 +1 .lO + 0.02

+11.15 +109.7 -2.11 -0.45

+14.94 +138.7 -1.97 +0.05

+11.63 +112.3 -3.32 +0.14

+13.64 +148.0 -1.28 -0.22

a Chemical shift 6, = 3.28 ppm. b Calculated using yH/yD = 6.51.

2.50 Hz -1

FIG. 1. Experimental ‘H NMR spectrum of [l-15N)diazocyclopentadiene.The spectrum of H(2) and H(3) is shown in the upper trace, The spectrumof H(1) and H(4) is shown in the lower trace.

in Table 2 with a rms error of 0.041 Hz. The relative values of J1, and JZ3were confirmed from analysis of the 13C satellites obtained by accumulation of 120 scans. All line positions were measured as a mean of several expanded spectra with the scale 0.13 Hz/cm. The relative signs of H-H coupling constants were confirmed by homo tickling

37c

NMR OF NITROGEN-l 5 ENRICHED DIAZOCOMPOUNDS TABLE

2

EXPERIMENTAL AND CALCULATED VALUES OF COUPLING CONSTANTS(Hz) IN [1-i5N]~~~~~~~~~~~~~~~~~~~E"

-

Coupling constants ~~ -.-____ J 12 J 13 J 14 J 23

3JNH -‘JNH

Experimental Ref. (2) This work” +4.68 i-1.88 $2.14 +2.73 -0.34

4.93 1.87 2.20 2.70

-1.11

0 Chemical shift S,,(,) = &Cd) = 6.76 ppm. &, ’ All data better than kO.O.5 Hz.

Theoretical CNDO/Z INDO without CI without CI -t-2.20 +3.42 +I .24 +4.10 -0.43 -1.06

t1.70 13.78 “b1.53 +3.32 -0.27 -1.53

= BHc3,= 5.99 ppm.

experiments. The signs of 15N-H coupling constants have been established relative to J12 > 0 by hetero tickling experiments irradiating 15N while observing ‘H. The calculated double resonance spectra corresponded completely tat the experimental ones, when the final parameters in Table 2 were used. The theoretical values of coupling constants in [I -‘5N]diazocyclopentadiene given in Table 2 were obtained using the parameterization described earlier (4). Since our earlier work (4) indicated that data based on variable electron density at the nucleus are somewhat superior to fixed values, only the former values are quoted here. Calculations with configuration interaction (CI) were not possible because of limitations in computer storage. The geometry used as input parameters was made by superposition of the geometry of cyclopentadiene (14) with the geometry of diazomethane (15). DISCUSSION

The signs of coupling constants in diazomethane have not been established experimentally. However, if experimental 2JnH coupling constants for various compounds are plotted against the theoretical values obtained in the INDO approximation using CI (Fig. 2), only the positive sign of 2JHHseems to agree with the correlation. Two values of zJNH are obtainable from the spectrum show,n in Fig. 3. If 2JNH is taken as 0.14 +_0.1 Hz, the isotopic effect on chemical shift can be measured to 6 (diazomethane) - 6 ([l-lSN]diazomethane) = 0.07 &-0.05 Hz. If 2JNHis chosen to be 0.00 + 0.05 Hz, an isotopic effect of 0.14 + 0.05 Hz is obtained. In both cases the isotopic effect is in agreement with similar isotopic effects found elsewhere (1, 8, 16-18). From intensity arguments, however, the former seems to be the more likely value. On basis of the previous data for [i5N]pyridazine (I), the value of *JNH and 3JW,, in diazomethane might seem surprising. In determining the zJ,n and jJNH coupling constants, the position of 15N in the molecule is tacitly assumed to be known. A rearrangement mechanism in the synthetic path would, however, reverse the labeling position and the arguments above. Since no detailed investigations have been reported concerning the reaction involved in the preparation, we have examined the microwave

376

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NIELSEN

I /

* 30

LO Theoretical values of coupling constants

FIG. 2. The correlation between the experimental values (Hz) and the theoretical values (Hz) obtained in INDO with CI of the geminal H-H coupling constantsin various compounds: (1) H&O (4); (2) Cd-L (4); (3) W==CHz (SW; (4) C&CJW (8); (5) C&F Q; W’CH, (4); (7) Cf3DW (8) C,Hb (4); (9) CHjCFO (26).

FIG.

3. ‘H NMR spectrumof [l-lSN]diazomethane.

spectra of the two 15N isotopic species. By measuring the isotopic effect on the OoO--f loI transition, it has been established that the isotopic substitution takes place as expected. A solid-state IR measurement of the isotopic effect on the fundamental frequency lying at 2075 cm-l (29, 20) agreed with this conclusion. This assignment gives the best agreement with the correlation of the experimental value of 3JNHto the theoretical values obtained in the CND0/2 approximation shown in Fig. 3 in Ref. (1). This correlation also indicates that 3JNHin diazomethane probably is negative.

NMR

OF NITROGEN-1

5 ENRICHED

37 :’

DIAZOCOMHXJNDS

Some difference is observed between calculations with and without CI. In principle, inclusion of CI leads to improved description of the excited states used in the sum over state method provided a proper Hamiltonian is used. With the approximate parameterized Hamiltonian used here, it is a question open for empirical testing whether CI provides a better basis for establishment of correlations between experiments and theory. Our view, based upon a limited number of cases, is that a small but significant improvement can be found (I, 4, 8). In the present case, both calculation schemes provide poor correlation for 2&, with experiment when compared to the *JNH correlation shown in Fig. 3 in Ref. (I). Such deviations might occur in a few cases by using semiempirical calculations. By changing the parameterization, discrepancies of this kind might disappear. The relative chemical shifts of the protons in [ 1-15N]diazocyclopentadiene have been chosen in accordance with an excellent work by Cram and Partos (21). On the basis of the NMR spectra of 1- and 2-nitrodiazocyclopentadiene, they showed that H( 1) and H (4) give rise to NMR signals at lower field than H(2) and H (3). Based upon a comparison with five-membered heteroaromatic systems, Smith et ai. (2) suggested JZ3 > J,,. We have recorded the spectrum of the 13C satellites. A complete analysis has not been performed since all 13C-H coupling constants are not known. Model calculations show that the separation of the two strong outer lines in the satellite spectrum is identical to the sum of the numerical values of the coupling constants, i.e., IJI~I + lJ131+ IJ141r viz., (J12[ + [Jr51 + (JtJ[. Since JIZ and J13 have been determined from the spectrum of the parent molecule, we have verified the prediction by Smith et al. The analysis of the spectrum of [1-15N]diazocyclopentadiene shows that f4JlriHl> j 3JNHl. The relationship of the values of the coupling constants differs from other data presently known. In pyridine (22), pyridazine (I), aniline (23), and acetanilide (23). the values of j3JNHI have been found to be larger than those of 14JNHl.From studies of 15N-containing compounds (24, 29, it is known that the geometry of the coupling path and the availability of a lone pair on nitrogen plays a decisive role for the magnitude of the observed coupling constants. Geometric similarity exists between aniline and diazocyclopentadiene, whereas ring-nitrogen atoms do not exhibit the same arrangement. The nitrogen hybridization is evidently different in aniline and diazocyclopentadiene and this invalidates any direct comparison. The correlation between experimental and theoretical ‘&n coupling constants described in Fig. 3 in Ref. (I) suggests an experimental value for 3JNH in [1-‘“N]diazocyclopentadiene of about -1.0 Hz. This is in excellent agreement with the value found from analysis of the NMR spectra. Lack of data has prevented the establishment of a correlation for 4JNH coupling constants. However, both CNDO/Z and INDO calculations suggest that 14JNHI> I’JNH(. ACKNOWLEDGMENT

We thank Northern free computing time.

1. J. P. JACOBSEN,

Europe University Computing

0.

SNERLING,

Resonance 10, 130 (1973). 14

E. J.

Centre (NEUCC),

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PEDERSEN,

NIELSEN,

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NIELSEN

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