MAGNETIC RESONANCE IN CHEMISTRY, VOL. 34, 233-236 (1996)

Solvent Effects on the Nitrogen NMR Shielding of 2-Methyl-2-nitrosopropane and its Azodioxy Dimer

M. Witanowski and Z. Biedrzycka Institute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland

G. A. Webb* Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, UK

High-precision 14N NMR shielding data are reported for 2-methyl-2-nitrosopropane and its azodioxy dimer in a variety of solvents. A range of about 30 ppm, as a function of solvent, is observed for the nitroso nitrogen atom. This contrasts with the corresponding shielding range for the dimer, which is about 6 ppm. The effects of solvents on the nitrogen shielding of the nitroso group are largely controlled by the polarity of the solvents. In conjunction with molecular orbital calculations incorporating the solvaton model, the increasing polarity of the solvents is found to enhance the migration of electric charge from the nitrogen to the oxygen atom of the nitroso group. The reverse trend was observed for nitrogen atoms doubly bonded to carbon rather than to oxygen. Hydrogen bond effects on the nitrogen shielding are surprisingly small for the nitroso group in comparison with those found for 0-nitroso groups in covalent nitrites. This difference is probably due to steric effects caused by the bulky alkyl group directly attached to the nitrogen atom of the monomer studied. In the majority of solvents used, the monomer-dimer equilibrium is displaced heavily towards the monomer form. Aqueous solutions are a notable exception, where the dimer still largely predominates a t low concentrations.

KEY WORDS I4N NMR shielding; 2-methyl-2-nitrosopropane; solvent effects on shielding; solvation model

INTRODUCTION RESULTS AND DISCUSSION

It has recently been shown that nitrogen NMR shield- ings ACT, (which is equivalent to -A6 on the frequency scale of chemicals shifts) of the NO group in nitro- soalkanes are a sensitive probe of electron-withdrawing effects of substituents and of steric strain effects.’ So far there have been no reports of how molecular inter- actions in solution influence the nitrogen shielding of the nitroso group and those of the corresponding dimers, which are in equilibrium with the nitroso mono- mers in room temperature solutions.

Comparison of solvent-induced nitrogen shielding variations for the nitroso group with those for close structural analogues is also of interest. Such analogues include the NO, group, which is formally an N-oxide of the nitroso group, and covalent nitrites which contain an 0-nitroso moiety.

The model compound studied in this work was 2- methyl-2-nitrosopropane, (Scheme 1) and its dioxy dimer. The geometry of the dimer in the present case is trans,’ as depicted in Scheme 1. In our work we use the sign convention such that a plus sign corresponds to an increase in nuclear hi el ding.^.^ Consequently, we employ the term ‘nitrogen shielding,’ ACT, rather than ‘nitrogen chemical shift,’ Ad. These two terms are of opposite sign but equivalent in magnitude.

Author to whom correspondence should be addressed.

14N high-presision NMR data for the compounds studied are presented in Table 1. The range of solvent effects on the nitrogen shielding of the nitroso monomer is large, about 30 ppm, which contrasts with that for the dimer, where it is about 6 ppm. This is intuitively correct owing to the symmetry of the dimer and the fact that protic solvents can form hydrogen bonds directly to the nitrogen in the case of the monomer.

With a view to unravelling the various specific and non-specific contributions to the solvent-induced nitro- gen shielding variations, we employed the empirical scheme based on the following master equation : 5 * 6

o(i, j ) = a,(i) + a(i)a(j) + b(i)B(j)

+ s(i)Cn*(j) + d ( i ) W I (1) where i and j denote the solute and solvent, respectively, CT is the nitrogen shielding, a corresponds to the hydro- gen bond donor strength of the solvent, B represents its hydrogen bond acceptor strength, x* is its polarity/

CH3 CH3 - + A CH O-N=N. C H ~

3 y +o- CH3 CH3

CH - + = A = O-N=N. C H ~ 3 y +o-

CH3 CH3

1 2 Scheme 1. Structures of the compounds studied.

CCC 0749-1 58 1/96/030233-04 0 1996 by John Wiley & Sons, Ltd.

Received 24 August 1995 Accepted (revised) 24 October 1995

234 M. WITANOWSKI, Z . BIEDRZYCKA AND G. A. WEBB

Table 1. Nitrogen NMR shieldings of the compounds studied (0.2 M solu- tions assuming 100% monomer, at +35 "C, unless specified other- wise; referenced to neat liquid nitromethane).

Nitroso monomer 1 Azodioxy dimer 2

14N "N resonance resonance

Nitrogen half-height Nitrogen half-height Monomer shielding width shielding width content

Solvent (ppm) (W bpm) (W (%. w/w)

Cyclohexane -580.86 78 +64.17 100 95 Et,O (30°C) -594.47 82 +64.23 80 96 CCI, -590.76 100 +64.06 149 94 Benzene -596.03 102 +63.32 106 95 Dioxane -597.27 124 +63.12 161 95 Acetone -599.40 95 +63.26 91 96 D M S O -601.31 158 +62.92 257 90 CH,CI, -596.1 6 121 +62.48 160 94 CHCI, -593.44 158 +62.15 205 95 Ethanol -594.64 110 +62.00 168 90 Methanol -595.62 104 +61.78 149 89 Water (0.02 M) -580.0 150 +58.0 247 19 CF,CH,OH -575.7 61 5 +59.0 855 40

'The data were obtained from a Lorentzian lineshape fitting procedure which included the relevant frequencies, linewidths, intensities, phase and baseline distortions and the concomitant standard deviations; the shieldings are bulk- susceptibility corrected, and the linewidths are corrected for the use of a matched filter (i.e. exponential multiplication of the free induction decays); the precision of the data is such that the last digit reported is uncertain.

polarizability and 6 is a correction for polychlorinated present study, as reported e l~ewhere ,~ .~ together with solvents (6 = 0.5) and aromatic solvents (6 = 1). The the least-squares-fitted estimates of the solute nitrogen solute terms a, b, s and d represent the corresponding shielding responses and the linear correlation coefi- responses of the nitrogen shielding to a given property cients for the experimental shieldings with respect to of the solvent employed. The nitrogen shielding in the those produced by means of Eqn (1) in the present reference state, taken to be cyclohexane, is given by g o . work. Owing to some uncertainty in the relevant Table 2 lists the solvent parameter set employed in the solvent parameters, the data for water and tri-

Table 2. Solvent parameters used and least-squares-fitted solute parameters for a set of master lEqn w1

Solvent

Cyclohexane Et,O

Benzene Dioxane Acetone DMSO CH,CI, CHCI, Ethanol Methanol Water CF,CH,OH

CCI,

a

0 0 0 0 0 0.07 0 0.22 0.34 0.86 0.98 1.13 1.51

B

0 0.47 0 0.1 0.37 0.48 0.76 0 0 0.77 0.62 0.1 8 0

T'

0 0.27 0.29 0.59 0.55 0.72 1 .oo 0.80 0.76 0.54 0.60 1.09 0.73

6

0 0 0.5 1 0 0 0 0.5 0.5 0 0 0 0

a b S d

scale) scalej scale) less j (dimension- (pprnlunit (ppmlunit (ppm/unit

1 -590.3 f 1 .O +3.7 f 1.2 -5.1 *2.3 -7.9 f 1.8 0.0 f 0.2 2 +64.3 f 0.2 -2.3 f0.2 +1.3 t0.4 -2.2 f 0.3 -0.3 f 0.2

a The constants were recalculated for a temperature of 35 "C from the data available in Ref. 7

equations

Dielectric constant'

1.87 3.89 2.21 2.25 2.1 9 19.75 45.8 8.54 4.55 24.2 30.71 76.7

Correlation coefficient

I

0.950 0.989

NITROGEN NMR SHIELDING OF 2-METHY L-2-NITROSOPROPANE 235

fluoroethanol, as solvents, are not included in the linear regression.

For compound 1, the value of the s term given in Table 2, clearly dominates the solvent effects on the nitrogen shielding of the nitroso group. This represents the effect of a change in solvent polarity on the nitrogen shielding of the nitroso compound 1. The negative sign for s implies that an increase in solvent polarity results in a decrease in the nitrogen shielding. This is an inter- esting observation for comparison with some analogous results on related nitrogen molecular environments. When nitrogen is doubly bonded to a carbon, such as in open-chain imines' and nitrogenous heteroaromatic system^,^-'^ the s term is invariably positive in sign. In this case an increase in solvent polarity favours the movement of electronic charge towards the nitrogen atom concerned. The situation in the nitroso group is that the nitrogen atom is doubly bonded to an oxygen atom, which is more electronegative.

Hence an increase in solvent polarity tends to encour- age the migration of electronic charge from the nitrogen to the oxygen atom. Support for this conclusion is given by the influence of electron-attracting substituents, in the vicinity of the nitroso group, which result in signifi- cant increases in the nitrogen shielding of the nitroso moiety. Thus a comprehensive view of electron delocal- ization effects in a solute molecule, due to solvent polar- tity effects, is available.

Further support for this description is provided by the results of the calculations of nitrogen shielding based upon the INDO/S-SOS parameterized solvaton method, given in Table 3. The solvaton calculations of nuclear shielding' 3,14 produce results which are depen- dent upon the dielectric constant ( E ) of the solvent. Cal- culations were performed for various values of E and in Table 3 we report the changes in the calculated nitrogen shieldings for various values of E with respect to that for the case of E = 2. The solvaton calculations correctly predict the signs of the changes in the nitrogen shield- ings of the nitroso, open-chain imines and hetero- aromatic systems as the solvent polarity increases. For the nitro group the value of s is also found to be nega- tive and the magnitude reportedi5 for nitromethane is approximately the same as that observed in the present study on the nitroso group.

The values given for the a term in Table 2 are small, indicating that solvent to solute hydrogen bonding does not have a large influence on the nitrogen shieldings of the compounds studied. For the monomer, a has a posi- tive sign, showing that an increase occurs in nitrogen shielding, albeit a small one, upon hydrogen bond for- mation with the solvent molecules. The sign found is

Table 3. Nitrogen shielding increments induced by varying the dielectric constant (E) of the medium as calculated by the solvaton model

Nitrogen shielding increment (in ppm) with respect to B - 2 Compound c - 4 c - 8 c - 1 0 c - 4 0

1 -2.5 -3.7 -4.0 -4.2 2 -1.8 -2.4 -2.8 -3.0

typical of that for doubly bonded nitrogen atoms having a lone pair of electrons. However, the magnitude observed is surprisingly small, particularly when com- pared with that for the 0-nitroso group in tert-butyl nitrite, about + 20 ppm/unit scale.I6 A possible reason for this large difference in a values is that the bulky tert- butyl group is directly attached to the nitrogen atom in 1 and hinders the approach of hydrogen bond donor molecules to the nitrogen atom concerned.

In the case of the nitroso dimer 2, the range of solvent effects on the nitrogen shielding is small. Thus the cor- responding solute interaction terms, given in Table 2, are also small and none of them is predominant. However, the s term is also negative for the dimer, as it is for the monomer, and is correctly predicted by the solvaton calculations given in Table 3.

A detailed study of the equilibrium between the monomer and dimer has not been pursued in detail. Nonetheless, integration of the I4N NMR signals for the two forms has enabled us to estimate the equi- librium concentrations, in the various solvents given in Table 1. Some points of interest arise. At the concentra- tions and temperature employed, the monomer form clearly predominates in most of the solvents used and accounts for about 90% by weight of the solute. Excep- tions are found when strong hydrogen bond donor sol- vents are used. In this case the dimer predominates even at the low concentrations used. A good example is that for aqueous solutions (Table l), where at a total concen- tration of 0.02 M, assuming 100% monomer, the monomer accounts for only 19% by weight of the solute.

EXPERIMENTAL ~ ~ ~ ~~~ ~ ~ ~~

The compound studied was prepared by a previously published procedure. l 7 Particular care was taken in the NMR measurements to use very pure and dry solvents as reported previously.8-12 All solutions were prepared and handled under a dry argon atmosphere in glove- bags. The I4N shielding measurements were taken on a Bruker AM500 spectrometer at 35 & 0.2 "C, as main- tained by a VT unit, at a frequency of 36.14 MHz. Random and systematic errors were reduced to below 0.1 ppm for the solute nitrogen shieldings in different solvents. External neat liquid nitromethane was used as a reference by means of 10 mm x 4 mm 0.d. coaxial tubes. The inner tube contained 0.3 M nitromethane in acetone-d, ; the nitrogen shielding of this solution is + 0.77 ppm from that of neat liquid n i t r~methane .~ .~ This value is obtained from measurements using con- centric spherical sample/reference containers in order to eliminate bulk susceptibility effects. The value of + 0.77 pprn is used as a conversion constant. Thus the contents of the inner tube act both as a reference, with respect to neat nitromethane as standard, and as a deuterium lock for the NMR spectrometer. The exact resonance fre- quency of the 14N signal of neat nitromethane is 36.141 524 MHz, from which a value of 36.136 826 MHz is obtained for the bare nitrogen n u ~ l e u s . ~ . ~ This latter value is used in conjunction with the relevant resonance

236 M. WITANOWSKI, Z. BIEDRZYCKA AND G. A. WEBB

frequency differences to calculate the nitrogen shieldings constant' 3*14 were performed on the University of relative to that of neat nitromethane. Lorentzian line- Surrey HP Central system using INDO-optimized shape fitting of the 14N signals was used to produce geometries based upon standard structures.'8 values for the precise resonance frequencies of both the samples used and of the external standard. Dilute solu- tions were used in the present study, hence their suscep- tibilities are assumed to be equal to those of the

We gratefully acknowledge support from the Polish Committee for Research Advancement, KBN, grant No. 2P303 12207, and support

corresponding solvent at 35 "C.

shieldings as a function of solvent dielectric from NATO with a collaborative research grant, No. 921286. The INDO/S solvaton calcdations of the nitrogen

REFERENCES

1 . M . Witanowski, Z. Biedrzycka, L. Konopski and K. Grela,

2. B. G. Gowenlock, K. J. McCullough and R . B. Manson, J .

3. M. Witanowski, L. Stefaniak and G. A. Webb, Annu. Rep.

4. M. Witanowski, L. Stefaniak and G. A. Webb, Annu. Rep.

5. M. H. Abraham, P. L. Grellier, J. L. M. Abboud, R. M. Doherty

6. Y. Marcus, Chem. SOC. Rev. 409 (1 993). 7 . R . C. Weast (Ed.), Handbook of Chemistry and Physics, 64th

ed., p. E-49. Chemical Rubber Co., Cleveland, OH (1984). 8 . M. Witanowski, W. Sicinska and G. A. Webb, Spectrosc. Int.

J . 10, 25 ( 1 992). 9. M . Witanowski, W. Sicinska, Z. Biedrzycka and G. A. Webb. J .

Magn. Reson. A109,177 (1994).

Magn. Reson. Chem. 33,674 (1995).

Chem. Soc.. Perkin Trans.2 701 (1988).

NMR Spectrosc. 18, 1 (1 986).

NMR Spectrosc. 25, 1 (1 993).

and R . W. Taft, Can. J. Chem. 66,2673 (1 988).

10. M. Witanowski, W. Sicinska, S. Biernat and G. A. Webb, J. Magn.Reson.91.289 (1991).

1 1 . M. Witanowski, W. Sicinska and G. A. Webb, J. Magn. Reson. 98, 109 (1 992).

12. M. Witanowski, W. Sicinska, Z. Biedrzycka, Z. Grabowski and G. A. Webb, J . Magn. Reson. A112 66 (1 995).

13 . G. Klopman, Chem. Phys. Left. 1 , 200 (1 967). 14. I . Ando and G. A. Webb, Theory of NMR Parameters. Aca-

demic Press, London (1 983). 15. M . Witanowski W. Sicinska and S. Biernat, Spectrosc. Int. J.

7, 305 (1 989). 16. M . Witanowski, J . Sitkowski, S. Biernat, B. Kamienski, 6. T.

Hamdi and G. A. Webb, Magn. Reson. Chem. 23,748 (1 985). 17. J . C . Stowell, J . Org. Chem. 36, 3055 (1 971 ). 18. J . A. Pople and M. S. Gordon, J. Am. Chem. SOC. 89, 4233

(1967).