Isotope effects in the nmr spectra of nitromethane

89,123-128

JOURNALOFMAGNETICRESONANCE

(1990)

Isotope Effects in the NMR Spectra of Nitromethane Yu . A. STRELENKO Institute of Organic Chemistry, Academy of Sciences, Moscow, USSR AND

V. N. TOROCHESHNIKOV

AND N. M. SERGEYEV

*

NMR Lab@‘atory, Department of Chemistry, Moscow State University. I 19899, Moscow. L!WR Received December 5, 1989 The high-resolution ‘H, 13C, “‘N, “N, and “0 NMR spectra of some isotopomeric nitromethanes have been obtained and all possible coupling constants have been measured. The one-bond “0-15N coupling constant has been found for the first time. The program QUADR has been applied to analyze the 13C- 14N multiplets in the ‘)C NMR lineshapes modified by the 14N quadrupole effects. *D/‘H isotope effects on some coupling constants and 2D/‘H, “C/‘*C, and “N/“‘N isotope shifts for ‘H. 13C, and 15N nuclei have been also estabhshed. o 1990 Academic PXSS. I~C.

In this paper we report the results of NMR measurements of ‘H, 13C, 15N, 14N, and “0 nuclei in isotopomeric nitromethanes, performed in order to find isotope effects on chemical shifts and spin-spin coupling constants. Deuterium isotope effects on the one-bond 13C-lH coupling constant in nitromethane were first measured by Everett ( I ) . Using data on 13C- ‘H and ’ 3C- ‘D coupling constant in CH3N02 and CD3N02 (‘J( “C-‘H) = 146.29 f 0.27 Hz and ‘J( ‘3C-2D) = 22.42 + 0.29 Hz, respectively) he found a small negative deuterium isotope effect on ’ J( 13C- ‘H) defined as AJ(D/H)

= J*(13C-‘H)

- J(13C-‘H)

= YH J( ‘3C-2D) - J( 13C-‘H) = -0.26 k 2.0 Hz. YD

We suppose that the mainly due to a very NMR spectra of both quadrupole relaxation

large experimental error obtained in these experiments was large linewidth (approximately 8 Hz according to ( 1)) in 13C CH3N02 and CD3N02. This large broadening is a result of the of 14N nuclei (2). Such broadenings are often observed in ‘H

* To whom all correspondence should be addressed. 123

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124

STRELENKO,

TOROCHESHNIKOV,

AND SERGEYEV

a 1 87

I 66

I

65

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61

I

PPI(

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63

FIG. 1. (a) The 13CNMR spectrum of CH3N02/CD3N02 same with 14Ndecoupling.

62

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mixture without 14N decoupling. (b) The

NMR spectra of nitrogen-containing organic compounds (e.g., for the nitro compounds(3)). We have developed an iterative method to analyze the quadrupole effects in the NMR spectra of nuclei with spin-j coupled to spin-l and applied it previously to analyze the 13C NMR spectra of benzaldehyde-d, (4). In this paper we report the data of more precise measurements of 13C- ‘H / 13C- *D coupling constants in nitromethane using 14N spin decoupling. EXPERIMENTAL

The ‘H, 13C, 14N, 15N, and “0 NMR spectra have been recorded on a Bruker AM300 spectrometer. A sample containing a 1: 1 mixture of CH3N02 and CD3N02 has been used. The spectra were analyzed using the iterative computer program QUADR (5) on an ASPECT 3000 computer. RESULTS

AND DISCUSSION

Figure 1a presents the proton-coupled 13C NMR spectrum of the CH3N02/ CD3N02 mixture, where both the 1:3:3: 1 quartet of CH3N02 and the 1:3:6:7:6:3: 1 septet of CD3N02 are clearly visible. One can see that each component of the spectrum is a noticeably broadened triplet due to the spin-spin coupling with the 14N nucleus. We have also measured the 13C NMR spectrum with 14N decoupling (Fig. lb), which resulted in very narrow 13C NMR lines and vanishing of the 13C NMR line overlap. This spectrum has been analyzed statistically and the values of the J( 13C- ‘H ) and J( ’ 3C- *D ) coupling constants have been obtained (Table 1) . Thus, the isotope effect on J( 13C- ‘H) proved to be equal to -0.97 + 0.07 Hz, which is substantially different from the value reported by Everett ( I ) .

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60

ISOTOPE

TABLE

I

Spin-Spin Couplings and Isotope Effects AJ(D/H)

J(“N-*D) J(‘4N-‘3C)‘+

J(‘4N-‘3C)’ .I( ‘5N-‘3C) J( “0-“N)

146.376 22.322 2.33 0.35 6.79 6.34 9.57 41.4

in Nitromethanen hl(D/H)

JW-4 J(“C-‘H) J(‘3C-2D) J(j5N-‘H)

125

EFFECTS IN NITROMETHANE

+ 0.01 f 0.0 I +O.Ol r 0.05 ~0.05 f 0.05 Iko.01 to.5

(Hz)

-0.97 It 0.07 -0.05 * 0.3 m~o.45k 0.05

’ The positive signs of the coupling constants are assumed. ’ Measured in CH,NO*. ’ Measured in CD3N02.

Note that the observed value is the sum of two effects: the primary, secondary ‘-2AJ ones (see discussion of these quantities in Ref. (6)) AJ= ‘,‘AJ+

130AJ, and the

21,2AJ.

Using the QUADR progam we have determined also the 14N- 13C coupling constants. As the individual 13C NMR components are poorly resolved triplets it is desirable to know the spin-lattice relaxation time of the 14N nucleus to perform iineshape analysis. The 14N NMR spectrum of nitromethane CH3NOz is a singlet with a halfwidth of 18.2 Hz. Supposing T, = T2, we obtain T, ( 14N) = 17.5 ms in accord with the other data ( 7). We have also measured T, ( 14N) experimentally using a standard inversion-recovery sequence and obtained the value of 22.0 ms. We used this likely more exact value in the QUADR calculations. The halfwidth Av, ,2 ( 14N) decreases up to 11.0 Hz at 323 K with an increase of temperature. T, at 323 K is 28.9 ms. Simultaneously the 13CNMR lines become more clear triplets. Almost all components of the 13CNMR multiplets of CH3 NOz and CD3NOz have been analyzed by the QUADR program with approximately 100 points per line. For the extreme and rather weak lines of the multiplets the coupling constant v&es have been obtained with a rather large error of about 0.3 Hz. Retaining only the strongest and most exact central lines, two for CD3 NOz and three for CH3 NOz , we have established the values of J( 14N- 13C) in both isotopomers given in Table 1. The data obtained indicate a small deuterium isotope effect on the ’ J( 14N- 13C) coupling constants. According to the definitions given in Ref. (6) this isotope effect is the secondary ‘,2A effect. The reduced (per one D/H substitution) ‘.*A effect can be determined as ‘s2AJ= 1/3[J(14N-‘3C)(CD3N02)-

J(‘4N-‘3C)(CH3NOz)],

and hence it is equal to -0.15 f 0.02 Hz. We have also measured the 13C NMR spectrum with both ‘H and 14N decoupling, leading to the very accurate value of the deuterium isotope shift of the 13C nudeus, equal to 685 ppb (very close to the literature data 684 ppb in ( I)).

126

STRELENKO,

TOROCHESHNIKOV,

AND SERGEYEV

b

I

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I 0.06

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FIG. 2. (a)The ‘SNNMRspectrumofpureCH3N02. mixture.

I -0.06

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(b)The’SNNMRspectrum

I

I -0.12

pm

ofCD3N02/CH3N02

Further we have measured the 15N NMR spectra, at natural abundance, of two samples: pure CH3 NO* and a mixture of CH3N02 / CD3 NO2 ( shown in Figs. 2a and 2b, respectively). The quartet of CH3NOz is easily observed in the 15N NMR spectrum of the pure CH3NOz while two multiplets from both CH3N02 and CD3NOz overlap in the spectrum of the mixture (Fig. 2b) where the central line of the septet coincides with one of the components of the quartet. The coupling constants J( “N2D)and J(“N-‘H)aregi ven in Table 1. Deuterium isotope shift of the “N nucleus 2A 15N (D/H) equal to 38.0 ppb (kO.3) is close to the literature data (8). It is worth noting that the corresponding 2J( 14N- ‘H) coupling constant should be equal to approximately 1.8 Hz. Using this value of 2J( 14N- ‘H) and the value of r, ( 14N) equal to 17 ms we may estimate the halfwidth of the ‘H NMR signal of nitromethane by (2)

A~,,~= $?r2Tl~~ This gives a value of about 1.7 Hz. We have confirmed this estimate through the direct observation of the ‘H NMR singlet of CH3N02. The singlet became substantially narrower upon decoupling from 14N nuclei. The 13C satellites in the ‘H NMR spectrum of CH3N02 have been also observed giving a 12C/ r3C isotope shift for ‘H nuclei equal to 2.2 ppb. Several NMR measurements have been performed for the “N-enriched sample and for the mixture of CD3N02 and CH3N02. Using the 13C NMR spectrum of CH3”N02 we have determined the ’ J( 15N-13C) coupling constant of 9.57 Hz (Table 1) which differs substantially from the data known from the literature (9)). Thus, the primary ‘*‘A “N / 14N isotope effect on J( “N- 13C), defined as

ISOTOPE

117

EFFECTS IN NITROMETHANE TABLE 2

Isotope Shifts in Nitromethane au (ppb) Isotopic substitution

‘D/‘H

9’H A’C A’5N

‘Tpc

0 685.0 + 0.2 38.0 f 0.2

15N/14N

2.2 t 0.2 19. I f 0.2

0.7 1 t 0.2 20.1 kO.2 -

” Not measured.

AJ( 15Nf4N)

- -Y(‘~N) J( 14N- “C) Y( 14N)

= J( ‘5N-‘3C)

is about 0.05 Hz, which is comparable with the error. We have also found the 13C satellites in the “N NMR spectrum of CHj1’N02 resulting in a ‘2C/‘3C isotope shift of the ‘jN nucleus equal to 20.1 I~I 0.2 ppb which is substantially less than the values of the corresponding isotope shifts in aqueous solutions of CN- ions ( 10). From the 13C NMR spectrum of the mixture CH3 NOz / CD3 NO2 the isotope shift 2A 13C( “N/ 14N) has been measured while the ‘H NMR spectrum of the same sample gives the isotope shift 2A’H ( “N/14N) (Table 2). Finally, we have measured the “0 NMR spectrum of CH315N02 (Fig. 3b), where the 170- 15N coupling constant is readily visible. In the “0 NMR spectrum of pure

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616 FIG.

617 3.

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615

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Prll

(a) The “0 NMR spectrum ofCH3N02.

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610

606

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607

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(b) The “0 NMR spectrum of CH315NOZ

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128

STRELENKO,

TOROCHESHNIKOV,

AND

SERGEYEV

CH3NOz (Fig. 3a) the hidden 170- 14N multiplet structure is revealed from the halfwidth of the I70 NMR signal (95 Hz). Using the QUADR program we have found that a substantial part of the halfwidth can be attributed to the 14N quadrupole relaxation contribution. Using the experimental value of T, ( 14N) equal to 22 ms and the value of J( 170- 14N) calculated from J( 170- 15N) (neglecting the possible “N/ 14N isotope effect), we have obtained the quadrupole contribution to the halfwidth of about 57 Hz, thus supporting the idea that a substantial part of the 170 NMR signal halfwidth is due to the 14N quadrupole effects. It is worthy noting that the ’ J( 170- 15N) coupling constant is reported here for the first time and we hope that it may be useful in the future characterization of nitro compounds. REFERENCES 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

J. A. J. 1.

R. EVERETT, Org. Magn. Reson. 19,169 (1982). V. CUNLIFFE AND R. K. HARRIS, Mol. Phys. 15,413 ( 1968). P. KINTZINGER, J. M. LEHN, AND R. L. WILLIAMS, Mol. Phys. 17, 135 ( 1969). F. LESHCHEVA, V. N. TOROCHESHNIKOV, V. A. CHERTKOV, V. N. KHLOPKOV, AND N. M. SERGEYEV, submitted. V. N. TOROCHESHNIKOV, A. V. BUEVICH, 1. A. BOLKUNOV, V. A. CHERTKOV, V. 1. MSTISLAVSKY, A. G. SHAKHATUNI, V. N. KHLOPKOV, Yu. M. DYOMIN, AND N. M. SERGEYEV, “NMR Software for ASPECT 3000,” Abstracts, IX Ampere Summer School, p. 22 1, Novosibirsk, USSR, September 1987. N. M. SERGEYEV, in “NMR Basic Principles and Progress” (P. Diehl, E. Fluck, and R. Kosfeld, Eds.), Vol. 22, p. 3 1, Springer-Verlag, Berlin, 1990. W. SUCHANSKY ANDP. C. CANEPA, J. Magn. Reson. 33,389 ( 1979). A. LYCKA ANDP. E. HANSEN, Magn. Reson. Chem. 23,973 ( 1985). G. J. MARTIN, M. L. MARTIN, AND J. D. GOUESNARD, in “NMR Basic Principles and Progress” (P. Diehl, E. Fluck, and R. Kosfeld, Eds.), Vol. 18, p. 299, Springer-Verlag, Berlin, 198 1. R. E. WASYLISHEN, Can. J. Chem. 60,2194 (1982).