Investigations of the 1J(13C1H) spin–spin coupling and 13C magnetic shielding of gaseous benzene-13C

Investigations of the 1J(13C1H) spin–spin coupling and 13C magnetic shielding of gaseous benzene-13C

Journal of Molecular Structure 744–747 (2005) 101–105 www.elsevier.com/locate/molstruc Investigations of the 1J(13C1H) spin–spin coupling and magneti...

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Journal of Molecular Structure 744–747 (2005) 101–105 www.elsevier.com/locate/molstruc

Investigations of the 1J(13C1H) spin–spin coupling and magnetic shielding of gaseous benzene-13C

13

C

Karol Jackowski*, Edyta Macia˛ga, Marcin Wilczek Laboratory of NMR Spectroscopy, Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warszawa, Poland Received 6 September 2004; accepted 4 October 2004 Available online 28 November 2004

Abstract 13

C NMR spectral parameters have been studied for benzene-13C in the gas phase. The solute compound was observed at low constant pressure (approx. 300 Torr) and the temperature of 298 K while xenon and carbon dioxide were used as the gaseous solvents. Complex ABB 0 CC 0 DX spectra were analyzed with the use of a PERCH program. It was found that the 13C chemical shift of benzene was linearly dependent on the solvent density. Similar dependence was also detected for the one-bond spin–spin coupling between proton and carbon nuclei, 1J(CH). For the first time the two NMR parameters for an isolated benzene molecule were obtained together with the coefficients responsible for the solute-solvent molecular interactions. q 2004 Elsevier B.V. All rights reserved. Keywords: Gas phase; Intermolecular interactions; 13C magnetic shielding; Spin–spin coupling

1. Introduction A molecule of benzene (C6H6) is one of the most interesting objects in chemistry. Its aromatic structure is well known and often makes benzene less reactive than cyclohexane (c-C6H12) [1]. In 1H NMR spectroscopy, magnetic shielding of aromatic protons is usually explained as due to ring currents of six mobile p electrons [2] but modern theoretical studies reveal that the contribution of s electrons is also very important. Recently, Ferraro et al. [3] have shown that s and p electrons deshield benzene protons via different mechanisms and a novel model has been proposed for interpreting of shielding of aromatic protons. Carbon-13 nuclei of benzene are also less shielded than 13C nuclei in other hydrocarbons but it is mostly due to the sp2 hybridization of carbon atoms in a C6H6 molecule. The sp2 hybridization is even more important when the magnitude of 1 13 J( C–1H) spin–spin coupling is considered [4]. The first approximate measurement of 1J(13C–1H) coupling in liquid benzene (159 Hz) has been performed * Corresponding author. Tel.: C48 22 822 02 11x315; fax: C48 22 822 59 96. E-mail address: [email protected] (K. Jackowski). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.10.026

exploring 13C NMR spectra under rapid passage conditions [5]. Two 13C satellites are associated with a strong 1H NMR signal of benzene and as shown the satellites also permit a similar estimation of the 1J(CH) spin–spin coupling constant [6]. However, any precise measurement of the 1 J(CH) coupling constant cannot be done straightforward because at least one 13C nucleus is needed in a benzene molecule for the observation of this spin–spin coupling and it means that the ABB 0 CC 0 DX spin system must be analyzed. Modern 13C NMR spectra of benzene have sufficient sensitivity and selectivity for the precise determination of the 1J(CH) coupling constant. Chertkov and Sergeyev [7] have measured this coupling constant in neat liquid benzene as 158.345(1) Hz. Later the same 1J(CH) coupling was also observed for few liquid solutions of benzene [8]. Schaefer et al. [9] used the latter results and the solvaton model of Ando and Webb [10] in order to estimate the 1J(CH) value in gaseous benzene as equal to 156.99(2) Hz. In our opinion this estimation cannot be satisfactory and direct measurements in the gas phase are obviously required. In this paper we present 13C NMR investigations of the 1 J(CH) spin–spin coupling and 13C magnetic shielding of gaseous benzene. In our laboratory we have developed

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special experimental techniques which permit us to monitor micrograms of compounds in the gas phase. NMR parameters of many medium-size molecules like acetonitrile [11,12], methylamine [13], iodomethane [14] and many others [15] were already investigated this way. A 13C NMR spectrum of benzene is rather complex so we have used 13C-enriched benzene in order to control all components of the spectrum. Benzene-13C was observed in gaseous solutions where xenon (Xe) and carbon dioxide were applied as the solvents. The 13C NMR spectra were carefully analyzed using a PERCH program [16]. On the other hand the 13C chemical shift of benzene can be easily measured using proton-decoupled 13C NMR spectra. Jameson et al. [17] probably used this approach for the determination of carbon shielding constant of gaseous benzene but their experiment was not described with details. The present measurements of 13C shielding of benzene were also performed using proton-decoupled carbon spectra but the carbon shielding of benzene was observed as a function of solvent density and extrapolated to the zero-density point. It permitted us to measure the 13C shielding constant of benzene-13C with better accuracy.

3. Results and discussion The nuclear magnetic shielding of a nucleus in a molecule is affected by both intermolecular interactions and intramolecular motion. In the gas phase, these effects are observed as a dependence of the shielding constant (s(T,r)) on density [20] and temperature: sðTÞ Z so ðTÞ C s1 ðTÞr C s2 ðTÞr2 C .

where so(T) is the shielding for an isolated molecule and the higher terms (s1(T), s2(T).) are dependent on the density r and describe the intermolecular interactions in gases. For most gaseous compounds at constant temperature the shielding s(T) varies linearly with density if the pressure of gas is not too large. In such a case the s2(T) and higherorder coefficients in Eq. (1) can be safely ignored and the remaining parameters, i.e. so(T) and s1(T), are precisely determined. Gas mixtures can also be used for the determination of the latter shielding parameters. For a binary mixture of gaseous compound A, containing the nucleus X whose shielding s(X) is of interest, and gas B as the solvent, Eq. (2) can be rewritten as follows sðXÞ Z so ðXÞ C sAA ðXÞrA C sAB ðXÞrB C .

2. Experimental Standard one-dimensional NMR spectra were acquired on a Varian UNITYplus-500 FT spectrometer at the 125.88 and 500.62 MHz transmitter frequency for the 13C and 1H nuclei, respectively. The FID acquisition time was set to 2 s and the spectral width from 400 to 800 Hz. Both the 13C and 1H NMR spectra were used for the observation of coupling constants in benzene-13C but the X part of ABB 0 CC 0 DX spin system (i.e. 13C NMR spectrum) was more suitable for analyses preformed with a PERCH program [16], the analyses were based on the total line shape fitting method. 13C NMR chemical shifts were measured relatively to external liquid TMS as described earlier [11]. The absolute 13C magnetic shielding of TMS (186.37 ppm in a cylindrical tube parallel to external magnetic field) [18,19] was used to convert the NMR chemical shifts into the absolute shielding constant of 13C nuclei in benzene-13C. Gas samples were prepared by condensation of benzene vapor and pure solvent gas (Xe or CO2) from the calibrated part of vacuum line as described in our previous paper [18]. Benzene-13C (99% 13C, Aldrich) from a glass container and xenon (99.9%, Messer Duisburg, Germany) or carbon dioxide (99.8%, Aldrich) from lecture bottles was used to prepare samples without further purification. In gaseous mixtures the solute gas (benzene-13C) has been used at a low constant density (w0.0065 mol/L, pressure w300 Torr) and mixed with various quantities of the gaseous solvents: Xe or CO2 (approx. from 0.90 to 3.80 mol/L).

(1)

(2)

where rA and rB are the densities of A and B, respectively, and so(X) is the shielding at the zero-density limit. The coefficients sAA(X) and sAB(X) contain the bulk susceptibility corrections ((sA)b and (sB)b) and the terms taking account of intermolecular interactions during the binary collisions of A–A and A–B molecules are (s1(A–A)(X) and s1(A–B)(X)), respectively. The shielding parameters in Eq. (2) are obviously temperature dependent and for this reason the measurements for various densities must be performed at constant temperature. In the gas phase, nuclear spin–spin coupling is also modified by interactions in pairs of molecules and multiple interactions, the appropriate equation for spin–spin coupling in the binary mixtures of gases (A and B) is similar to Eq. (2) at constant temperature JðXYÞ Z Jo ðXYÞ C JAA ðXYÞrA C JAB ðXYÞrB C .

(3)

where Jo(XY) is the spin–spin coupling between X and Y nuclei at the zero-density limit and JAA(XY), JAB(XY) are solely due to intermolecular effects in the binary collisions of A–A and A–B molecules, respectively. Usually, the density of A is kept sufficiently low for Eqs. (2) and (3) to simplify to sðXÞ Z so ðXÞ C sAB ðXÞrB

(4)

JðXYÞ Z Jo ðXYÞ C JAB ðXYÞrB

(5)

and the terms sAA(X) and JAA(XY) can safely be ignored if micrograms of gas A are diluted in gaseous solutions. The latter approximation is additionally verified when at least two different gaseous solvents are used and the so(X) and Jo(XY) parameters obtained by extrapolation remain the same within an experimental error.

K. Jackowski et al. / Journal of Molecular Structure 744–747 (2005) 101–105

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Fig. 1. Calculated and experimental proton-coupled 125.88 MHz 13C NMR spectrum of gaseous benzene-13C.

Fig. 1 presents an example of 13C NMR spectra of gaseous benzene-13C: the upper picture shows a simulated spectrum with the use of the PERCH program and the lower one displays an experimental 13C NMR spectrum of benzene-13C in the gas phase. The theoretical spectrum was obtained assuming values of all the 13C–1H coupling constants present in the ABB 0 CC 0 DX spin system. As shown this procedure is of high quality and permits the determination of all carbon-proton and proton-proton coupling constants. In the present study the 13C spectra and appropriate simulations were obtained for two sets of gaseous samples containing benzene-13C in different solvents (Xe and CO2). Unfortunately, 13C NMR signals observed in the gas phase were slightly broadened due to the spin-rotation mechanism of relaxation, especially when a density of gas solvent was low. The large 1J(CH) spin–spin coupling could be still measured with excellent accuracy while the other carbon–proton and proton–proton coupling

constants were much smaller and their determination was made with relatively lower precision. We have found that the 13C magnetic shielding of benzene, and the 1J(CH) spin– spin coupling as well, are linearly dependent on the density of gaseous solvents as shown in Figs. 2 and 3. As seen in Fig. 2 xenon gas changes more the 13C magnetic shielding of benzene than carbon dioxide does it. On the other hand the 1J(CH) spin–spin coupling is more affected by CO2 molecules than by Xe atoms, cf. Fig. 3. It is clear that different mechanisms are responsible for the variations of 13C shielding and spin–spin coupling in the investigated gaseous solutions. According to Eqs. (4) and (5) the extrapolation of experimental points to the zero-density limit allows the determination of appropriate parameters free from intermolecular interactions, so and Jo. Results of our investigations are summarized in Table 1 where the intermolecular effects (s(A–B)(C) and 1JAB(CH)) are also presented. It is worth noting that only the so(13C) shielding

Fig. 2. The dependence of 13C magnetic shielding in benzene-13C on the density of solvent gases (xenon, Xe (B) and carbon dioxide, CO2 (&)) at 298 K. The solute compound has been observed at low and constant pressure (approx. 300 Torr). The results are corrected for bulk susceptibility effects.

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K. Jackowski et al. / Journal of Molecular Structure 744–747 (2005) 101–105 Table 2 13 C–1H and 1H–1H 0 spin–spin coupling constants [Hz] in a molecule of benzene-13C Coupling

Experimental in c-C6H12a

J0 b

1

158.607 1.1595 7.5974 K1.2882 7.5401 7.5400 7.5418 1.3811 1.3838 1.3803 1.3848 0.6553 0.6594

157.77(2) 153.0 1.10(4) 1.5 7.66(4) 7.2 K1.36(8) K1.7 7.55(4) 7.1 7.58(8) 7.1 7.63(4) 7.1 1.40(12) 1.2 1.28(4) 1.1 1.34(2) 1.1 1.30(8) 1.1 0.76(12) 0.7 0.64(8) 0.5

JCH JCH 3 JCH 4 JCH 3 J12 3 J23 3 J34 4 J13 4 J24 4 J26 4 J35 5 J14 5 J25 2

1

13

Fig. 3. The J(CH) spin–spin coupling in benzene- C is presented as a function of solvent density (xenon, Xe (B) and carbon dioxide, CO2 (&)) in the gas phase at 298 K.

constant can be compared with one previous measurement (57.2 ppm [17] vs. 57.11 ppm obtained in our case), the other parameters in Table 1 are determined for the first time. In this study we have also measured the rest of spin–spin coupling constants available from 13C NMR spectra of gaseous benzene-13C and we have found these parameters independent of density. Their values (J0) are presented in Table 2 together with some previous results obtained for benzene-13C dissolved in liquid cyclohexane (c-C6H12) [8], the latter data are cited saving their original form with extraordinary precision (too optimistic in our opinion). As seen the intermolecular interactions in liquid cyclohexane have unexpectedly small influence on all the spin–spin coupling constants in benzene. The largest coupling, 1 J(CH), is increased less than 0.85 Hz (a little more 0.5% than the total value) when a molecule of benzene-13C is placed in liquid cyclohexane. It is important because the measurements of spin–spin coupling constants performed in this solvent can be really used as good estimations of the coupling constants for isolated molecules. The approximation of 1J(CH) made by Schaefer et al. [9] (156.99(2) Hz) is not really much better than the direct measurement obtained from benzene dissolved in liquid cycloxehane (158.61 Hz [8]) if the real value of 1J0(CH) is equal to 157.77(2) Hz as shown in Table 1. Table 1 13 C nuclear magnetic shielding and 1J(13C–1H) spin–spin coupling of gaseous benzene-13C and their dependence on density in binary mixtures with CO2 and Xe at 298 K Parameter Nuclear magnetic shielding s0(C) (ppm) (s1)b (ppm ml molK1)a s(A–B)(C) (ppm ml molK1) Spin–spin coupling 1 J0(CH) (Hz) 1 JAB(CH) (Hz ml molK1) a

As described in Ref. [18].

Gas solvent (B) CO2

Xe

57,113(4) 88 K99(2)

57,105(9) 191 K150(3)

157,77(2) 67(7)

157,78(2) 3(7)

a b c d

Theoretical Jeb,c

Refs. [21]

[22]

[23]

164.636 166.3 176.7 1.798 2.0 K7.4 7.877 8.0 11.7 K1.329 K1.2 K4.6 8.571d 8.7d

0.962d

1.3d

0.661d

0.8d

As given in Ref. [8]. This work. Including vibrational corrections calculated in Ref. [22]. In the original papers described as 3JHH 0 , 4JHH 0 and 5JHH 0 , respectively.

Recently, the vibration corrections to spin–spin coupling constants of benzene have been calculated by Ruden et al. [22] and we have used the corrections in order to determine the experimental coupling constants for a benzene molecule free from internal vibrations. This way we could estimate the temperature effect on spin–spin coupling constants. Let us add that this procedure is not perfect because the vibrations are responsible only for the main part of temperature effects. Nevertheless, it is very helpful because the new values of coupling constants (Je) can be directly compared with ab initio calculations performed for a molecule with optimized geometry. The last three columns in Table 2 show some resent results of calculations [21–23] which can be compared with the present experimental data. As shown the calculations performed in 2003 [21,22] are more accurate than the previous theoretical results obtained in 1996 [23]. The progress made in the calculations during few years is impressive but still not satisfactory if both the experimental and theoretical data are compared.

4. Conclusions The present study yields experimental results for the 13C magnetic shielding and isotropic spin–spin coupling constants of benzene-13C in the gas phase. We have found distinct density dependences of nuclear magnetic shielding and one-bond 13C-1H spin–spin coupling for gaseous solution of benzene in xenon and carbon dioxide. The appropriate parameters for an isolated molecule of benzene-13C have been determined with high accuracy. The other 13 C–1H and 1H–1H spin–spin coupling constant of benzene were also determined but they were found rather independent of density. Our new results permit us to point out

K. Jackowski et al. / Journal of Molecular Structure 744–747 (2005) 101–105

the progress in the ab initio calculations of spin–spin coupling constants for benzene. The present study also gives one more prove that intermolecular interactions can not be a priori neglected if precise measurements of spin–spin coupling constants are required.

Acknowledgements This work was partly supported by the Polish State Committee for Scientific Research (to K.J. and E.M.) as the research grant number 4 T09A 035 23 available in years 2002–2005.

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