Isotope effect due to 15N14N substitution on 14N14N coupling constants in nitrous oxide

J o u r n a l of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 378 (1996) 61-65

Isotope effect due to 15N/14N substitution on 14N-14N coupling constants in nitrous oxide 1 Y u . A . S t r e l e n k o a'*, N . M .

Sergeyev b

aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, 117913 Moscow, Russia bDepartment of Chemistry, Moscow State University, 119899 Moscow, Russia

Received 23 June 1995; accepted 30 August 1995

Abstract 14N and 170 N M R spectra of a solution of nitrous oxide, NNO in acetonitrile were measured on a Bruker AM 300 spectrometer. The laN N M R spectrum reveals two narrow lines at about -147 and -231 ppm relative to external nitromethane assigned to the central and terminal nitrogen atoms, respectively. Both lines exhibit mutual spin-spin splittings equal to 4.233 (+0.006) Hz. The 170 N M R spectrum also reveals a rather narrow line with a chemical shift of 110 ppm relative to external water with a clearly seen 1 : 1 : 1 triplet structure due to a 1j(IaN-170) coupling constants of 35.8 (4-0.3) Hz. Accurate experimental values were obtained using the QUADRprogram. Comparison of the l j(laN_14N) coupling constant measured in the present study with 1j(15N-15N) coupling constant equal to -9.16 (4-0.3) Hz (or -4.65 (4-0.2) Hz on the basis of 14N-14N coupling constants) obtained previously by Bhattacharrya and Dailey reveals a strong, double primary isotope effect of about 10%. This can be considered as one of the largest isotope effects on coupling constants ever measured. Keywords: Isotope effect; NMR spectroscopy; Coupling constant; Nitrous oxide

1. Introduction Study o f isotope effects on N M R chemical shifts and coupling constants remains in the focus o f interest o f m a n y investigators. The data on isotope shifts were summarised in two recent reviews [1,2] while the data on coupling constants were analysed separately [3]. One o f the central ideas behind the study o f * Corresponding author. 1 Presented at the Summer School on Isotope Effects as Tools in Basic and Environmental Research, Roskilde, Denmark, 24-28 June 1995.

isotope effects is the design o f multidimensional surfaces for N M R parameters. This a p p r o a c h was first widely a n n o u n c e d at the special N A T O meeting in 1992 [4]. In m a n y cases variations along the internal coordinates do not really affect the energy, although they do strongly affect some N M R parameters. P r o b a b l y the best example is the ethane molecule where rotation a r o u n d the H-C-C-H dihedral angle is traditionally considered as almost free, while the vicinal 1 H - 1 H coupling constant reveals very specific and the well-known Karplus-type dependence [5]. Isotope effects on coupling constants are n o w well d o c u m e n t e d in cases o f 1 3 C - 1 H and 3 1 p - I H

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Yu.A. Strelenko, N.M. Sergeyev/Journal of Molecular Structure 378 (1996) 61-65

62

b)

c)

i

, .........

-15

, .........

-10

J .........

-5

J .........

0 l-/z

, .........

5

4 .........

10

,

15

a) I

-130

-150

-170

-190

-210

-230

ppm

Fig. 1. 14N NMR spectrum of nitrous oxide measured on a Bruker AM 300 spectrometer at room temperature. (a) The spectrum with a very broad line for the solvent CD3CN and two narrow signals for nitrous oxide at -147 and -231 ppm relative to the external nitromethane. The two insets shown above are the signals of two nitrogen atoms at -147 ppm (b) and -231 ppm (c). In both insets the upper trace is the best calculated spectrum obtained with the QUADRprogram (see text), the middle trace is the experimental spectrum and the lower trace is the difference between the experimental and calculated spectrum.

coupling constants where D/H-induced primary and secondary effects were found [3]. It is worth noting that the primary isotope effects for 13C-1H coupling constants in methane [6] and toluene [7] were reported to be very small (less than 0.1 Hz). This has been recently supported by ab initio calculations at the high level of electron correlation approximation [8]. As a rule D/H-induced isotope effects on coupling constants do not exceed 1%0 of the magnitude. Only in the cases of PH 3 [9] and Sell2 [10] were the primary isotope effects of the order of 6 and 4%, respectively. It is expected that the isotope effects on coupling constants due to heavier isotope substitution (i.e. 13C/12C, 15N/14N etc.) should be

much smaller. In this paper we present an unexpectedly large isotope effect on coupling constant between nitrogen isotopes in nitrous oxide, NNO.

2. Experimental The 14N and 170 N M R spectra of a solution of gaseous NNO in acetonitrile CD3CN saturated at room temperature were measured on a Bruker AM 300 spectrometer at 21.7 and 40.7 MHz respectively. The 14N N M R spectrum reveals broadened signal of the solvent at -137 ppm and two narrow signals at - 1 4 7 and -231 ppm relative to external nitromethane (Fig. la) which can be attributed to

Yu.A. Strelenko, N.M. Sergeyev/Journal of Molecular Structure 378 (1996) 61-65

63

Table 1 Data on the spin-lattice relaxation times Tl, the half-widths (Avl/2) and the calculated values of the 14N-14N coupling constant in nitrous oxide Parameter

Signal at -147 ppm

Signal at -231 ppm

Relaxation time Tt (s) (experimental)a Relaxation time Tt (s) (calculated)b Linewidth (Avl/2) (Hz) tj(14N-14N) (Hz) b

2.21 (4- 0.02) 0.147 ( t 0.002)¢ 0.71 (4-0.02) 4.233 (4-0.005)

0.164 (4- 0.003) ¢,d 2.70 (4-0.01) 4.221 (4-0.004)

a Inversion-recovery experiments. b Using the QUADRprogram. ¢ Referred to the adjacent nitrogen atom. d Not calculated, fixed value at 2.21 s.

nitrous oxide. The IaN N M R signals of NNO are clearly seen 1 : 1 : 1 multiplets. Both signals reveal the same splitting of 4.2 Hz which should be assigned to the 1j(14N-14N) coupling constant. This assignment was independently verified by a 14N-inN COSY (homonuclear chemical shift correlation spectroscopy) experiment, where intensive cross-peaks between two narrow 14N signals were observed. The signals at -147 and -231 ppm were assigned to the central (NNO) and to the terminal (NNO) nitrogen atoms, respectively, on the basis of literature data [11,12]. It is worth noting that both signals have different lineshapes. According to the general theory of quadrupole effects in N M R spectra of nuclei spin 1/2 interacting with spin 1 [13], it is well known that quadrupole effects should be revealed in more broadened side components of 1:1:1 triplets. This indeed is clearly seen for the central nitrogen atom at -147 ppm and is almost completely obscured for the terminal nitrogen atom at -231 ppm. This means that the additional quadrupole broadening proportional to the factor j 2 T l is more effective for the central nitrogen than for the terminal one. As the coupling constant is the same for both signals it simultaneously means that the following relationship is valid for the relaxation times TI(Q): Tl(Q)terminal < Tl(Q)cent~al. This was indeed proved in the inversion-recovery measurements of T1 values (see Table 1) assuming T1 (experimental) to be equal to T 1 (quadrupole). In general it seems reasonable to apply the QUADR program [14] for total lineshape analysis of N M R spectra with quadrupole broadenings

for more accurate calculations of 1 4 N - 1 4 N coupling constants. We first applied the QUADR program to the low-field signal at -147 ppm. The calculations led to the results given in Table 1. First, it is interesting to comment on the linewidths. The total linewidth is the sum of two contributions (A//1/2) = ( m v l / 2 ) q + (AVl/2)f.inh.

(1)

where (Avu2)q is the quadrupole contribution a n d (m/Jl/2)f.inh. is the magnetic field inhomogeneity (neglecting other mechanisms of relaxation). The calculations with QUAOR led to an experimental total linewidth (AUl/2) equal to 0.71 Hz (Table 1). The quadrupole contribution can be estimated using the assumption T1 = T2. It gives (Aul/2)q = 1/(~-T2) = 1/(TrTl) = 0.14 Hz. This means that (Avl/2)f.inh. was equal to about 0.6 Hz. At the same time the QUADR calculations for Ncentral gave the value of Tl for the adjacent nitrogen Nterminal equal to 0.147 s which is in excellent accord with the experimental result of 0.16 s obtained in the inversion-recovery experiments. For the terminal nitrogen we performed QUADR calculations with a fixed value of time Tl of the central nitrogen equal to 2.21 s. This gave a total linewidth of about 2.70 Hz. The quadrupole contribution can be estimated as above using the T1 value. It gives (A//l/2) q = 2.0 Hz. This means that the field inhomogeneity contribution was about 0.7 Hz, again in excellent agreement with the estimates obtained from the signal of Ncentral. In both calculations we obtained very close values of Ij(14N-14N) equal to 4.227 (+ 0.005) Hz.

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Yu.A, Strelenko, N.M. Sergeyev/Journal of Molecular Structure 378 (1996) 61-65

3. Discussion

a)

We can now pay attention to the value of Ij(14N-14N) measured in the present study as it may be compared with the analogous coupling constant j(15N-15N) measured by Bhattacharyya and Dailey [12] and is equal to 9.6 (4-0.3) Hz. Using nematic phase data the authors [12] also proved that the 15N-15N coupling constant is negative. The 15N-15N coupling constant can now be converted onto the basis of 14N-14N coupling constants by using the relationship: J**(14N-14N) = [7(14N)/7(15N)]2j(15N-15N)

(2) b)

IIrlllllllllllllllllll'lllllllllIFIll~Ellr~llllllll

-200

-100

0 Hz

100

In order to extract the isotope effects on the coupling constant we consider four possible isotopomerle N N O species, namely: 14N14NO

14NlSNO

15NI4NO

15N15NO

a

b

c

d

200

Fig. 2. 170 NMR spectrum of nitrous oxide measured on a Bruker AM 300 spectrometer at the resonance frequency 40.69 MHz. (a) The calculated spectrum obtained with the QUADRprogram, (b) the experimental spectrum and (c) their difference. We have also measured the 170 N M R spectrum of N N O (solution in CD3CN ) on a Bruker AM 300 spectrometer at a resonance frequency of 40.69 MHz (Fig. 2). The spectrum consists of only one line with a chemical shift of 110 ppm relative to external water with a clearly seen 1 : 1 : 1 triplet structure. The exact value of the coupling constant between 170 and 14N was also obtained using the QUADR program. It was extremely time consuming to measure the T1 relaxation time of the 170 nucleus in nitrous oxide, so the natural linewidth was found iteratively by QUADR. It is reasonable to suggest that the observed splitting for the 170 N M R signal was due to the direct 1j(170-14N) coupling constant with the central nitrogen atom. That is why we used the fixed spin-lattice relaxation time T1 equal to 2.21 s to calculate the best theoretical spectrum (see Fig. 2(a)) which gives an 170 linewidth equal to 34.8 (-4-0.7) Hz with a 1j(170-14N) coupling constant equal to 35.5 (+0.2) Hz.

with several types of nitrogen-nitrogen coupling constants: (a) j(14NIgN) = Jo (b) j(14NISN) = Jo + PAJI (c) J(15N14N) = Jo + VAJ2 (d) j(15NI5N) = Jo + PAJI + PAJ2

(3) (4) (5) (6)

where P A J 1 and P A J 2 are the primary isotope effects on the 14N-14N coupling constants due to 15N/14N substitution. In principle PAJ 1 and P A J 2 are not equal as they refer to non-equivalent chemical positions. However as they are not measured so far we can suggest that they equal each other. Thus J**(14N-14N) = j(14N-14N) + 2 P A J

(7)

Using the known values of 7(14N)/7(15N)= -0.71288 we obtain the J** value in 14N14NO equal to 4.655 (+0.15) with a negative sign as well. Thus the primary isotope effect can be estimated to be equal to -0.21 (4-0.07) Hz or about 5% of the magnitude. This can be considered as the first measured primary isotope effect induced by 15N/14N substitution. The value seems to be

Yu.A. Strelenko, N.M. Sergeyev/Journal of Molecular Structure 378 (1996) 61-65

anomalously large if compared with D/H-induced isotope effects [3]. However one should take into account possible solvent effects since the values to be compared are referred to different solvents. It is also worth noting that the measured coupling constant o f 1 7 0 - 1 4 N , equal to 35.5 (+0.02) Hz, can be compared with the data on 170-15N coupling constants recently measured by us for nitromethane, 41.4 (+0.5) Hz [15]. This additionally supports our assumption that we have indeed measured the one bond coupling c o n s t a n t 1 7 0 - 1 4 N in nitrous oxide. New data on isotope effects for the direct 1J(I4N-laN) coupling constant due to 14N/15N substitution can be of interest from the theoretical point of view since the NNO molecule is linear and ab initio calculations of spin-spin coupling constants are possible.

65

[5] R.K. Harris, Nuclear Magnetic Resonance Spectroscopy, Pitman Books, London, 1983. [6] B. Bennett, W.T.Raynes and C.W. Anderson, Spectrochim. Acta, Part A, 45 (1989) 821. [7] I.F. Leshcheva, V.N. Torocheshnikov, N.M. Sergeyev, V.A. Chertkov and V.A. Khlopkov, J. Magn. Reson., 94 (1991) 9. [8] W.T.Raynes, J. Geertsen and J. Oddershede, Chem. Phys. Lett., 197 (1992) 516. [9] C.J. Jameson and A.K. Jameson, J. Magn. Reson., 32 (1978) 455. [10] H.J. Jakobsen, A.J. Zozulin and P.D. Ellis, J. Magn. Reson., 38 (1980) 219. [11] A. Lowenstein and M. Brenman, J. Magn. Reson., 34 (1979) 193. [12] P.K. Bhattacharyya and B.P. Dailey, J. Chem. Phys., 59 (1973) 5820. [13] J.A. Pople, Mol. Phys., 1 (1958) 168. [14] I.F. Leshcheva, V.N. Torocheshnikov, N.M. Sergeyev, V.A. Chertkov and V.A. Khlopkov, J. Magn. Reson., 94 (1991) 1. [15] Yu. A. Strelenko, V.N. Torocheshnikov and N.M. Sergeyev, J. Magn. Reson., 89 (1991) 123.

References [1] P.E. Hansen, Prog. Nucl. Magn. Reson., 20 (1988) 207. [2] C.J. Jameson and H.-J. Osten, Annu. Rep. NMR Spectrosc., 17 (1986) 1. [3] N.M. Sergeyev, in P. Diehl, E. Fluck, H. Gunther, R. Kosfeld and J. Seelig (Eds.), NMR - Basic Principles and Progress, Vol. 22, Springer, Berlin, 1990, p. 31. [4] C.J. Jameson and A.C. de Dios, Nuclear Magnetic Shielding and Molecular Structure, NATO meeting 1992, Kluwer Academic, Dordrecht, 1993, p. 95.

Note added in proof

The value of lj (15N 15N) coupling constant in NNO was later confirmed by more precise measurements (C.J. Jameson, A.K. Jameson, H. Parker, S.M. Cohen and C.-L. Lee, J. Chem. Phys., 68 (1978) 2861) where J(15N-15N) equal to 8.9 + 0.1 Hz was obtained.