- Email: [email protected]
15N, 13C and 1H nuclear magnetic shielding and spin–spin coupling in gaseous 15N-enriched methylamine
Journal of Molecular Structure 704 (2004) 305–309 www.elsevier.com/locate/molstruc
15
N,
13
C and 1H nuclear magnetic shielding and spin –spin coupling in gaseous 15N-enriched methylamine
Edyta Wielogo´rska, Włodzimierz Makulski, Wiktor Koz´min´ski, Karol Jackowski* Laboratory of NMR Spectroscopy, Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warszawa, Poland Received 12 September 2003; revised 13 November 2003; accepted 13 November 2003
Abstract New measurements of 15N, 13C and 1H nuclear magnetic resonance (NMR) spectral parameters have been performed for 15N-enriched methylamine (CH15 3 NH2) in the gas phase. The solute compound was observed at low constant pressure (approx. 50 Torr) and the temperature of 300 K, while sulfur hexafluoride (SF6) and xenon (Xe) were used as the gaseous solvents. It was found that the 15N and 13C chemical shifts of CH15 3 NH2 were linearly dependent on the solvent density and the appropriate shielding parameters for an isolated methylamine molecule were obtained, together with the coefficients responsible for the solute – solvent molecular interactions. In contrast the 1H chemical shifts and spin– spin coupling constants of CH15 3 NH2 were found to be independent of density within experimental error. For the first time, the present measurements reveal the majority of NMR spectral parameters of methylamine free from intermolecular effects. q 2004 Elsevier B.V. All rights reserved. Keywords: 15N; 13C and 1H NMR chemical shifts; Spin –spin coupling; Gas phase; Intermolecular effects
1. Introduction Methylamine (CH3NH2) is very important compound in organic chemistry, biochemistry and chemical industry. Molecular spectroscopy, and specially nuclear magnetic resonance (NMR) spectroscopy, is frequently used for the studies of CH3NH2. In particular, 1H and 15N NMR methods have been applied to monitor the influence of hydrogen bonds on chemical shifts and spin –spin coupling constants in the liquid state [1,2]. Alei et al. [3] have monitored the 15 N gas-to-liquid shift of CH15 3 NH2 as a function of temperature. It was found that the nitrogen shielding of methylamine was diminished upon condensation, 2 9.4 ppm at 180 K. Paolillo and Becker [4] have investigated the relative signs of the spin – spin coupling constants of liquid and gaseous CH15 3 NH2 using double resonance methods and they have determined the magnitudes and signs of five coupling constants. Jameson et al. [5] have given the 13C shielding constant of an isolated CH3NH2 molecule, s0 ð300 KÞ ¼ 158:3 ppm: The latter * Corresponding author. Tel.: þ 48-22-822-0211x315; fax: þ 48-22-82259-96. 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.2003.11.062
result is rather unique in literature, all the other shielding and spin –spin coupling parameters remain unknown for an isolated methylamine molecule. This small molecule is often used as the molecular model when new methods of quantum chemical calculations are tested. Calculations of nuclear magnetic shielding constants of methylamine have been performed using various methods and different approximation levels [6]. The influence of molecular association on the shielding parameters has also been studied by GIAO-CHF calculations [7]. Recently, Pecul and Helgaker [8] have given a comparison of three ab initio methods (DFT, MCSCF and CCSD) applied to the determination of spin – spin coupling constants in methylamine. The present work has been designed for the investigation of NMR spectral parameters of CH3NH2 in the gas phase. All the shielding constants of methylamine have been monitored as functions of density in order to obtain the parameters free from intermolecular interactions. The same procedure has been applied for the investigation of four different coupling constants: 1J(NH), 1J(CH), 1J(NC) and 3 J(HH), which could be observed in NMR spectra of 15 N-enriched methylamine. We have also monitored the influence of intermolecular interactions on the 15N and 13C
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309
306
nuclear magnetic shielding constants in liquid solvents. It has been shown that hydrogen bonds can effectively change the 15N shielding of methylamine while the 13C shielding constant remains almost insensitive on the formation of hydrogen bonds.
2. Experimental 2.1. Preparation of gaseous samples 15
N-enriched methylamine (CH15 3 NH2) was recovered 15 N) by from crystalline CH15 3 NH3Cl (Aldrich, 99 atom % adding an excessive amount of saturated aqueous solution of NaOH. The experiment was carried out in a vacuum line where gaseous CH15 3 NH2 was carefully dried over solid BaO and collected in a glass container. Gas samples were prepared by condensation of gaseous CH15 3 NH2 and pure solvent gas (SF6 or Xe) from the calibrated part of vacuum line into the 4.0 mm o.d. glass tubes (approx. 5.5 cm long) which were then sealed. The volumes of sample tubes and the vacuum line were measured using mercury. Sulfur hexafluoride (99.75%, Aldrich) or xenon (99.8, 99.9%, Messer Duisburg, Germany) from lecture bottles was used to prepare samples without further purification. In the gaseous mixtures, the solute gas (CH15 3 NH2) was used at a low constant density (, 0.004 mol l21, pressure , 50 Torr) and mixed with various quantities of the gaseous solvents: SF6 or Xe (from 0.08 to 1.3 mol l21). 2.2. Measurements of nuclear magnetic shielding The gas samples were fitted into the standard 5 mm o.d. thin-walled NMR tubes (Wilmad 528-PP) with liquid toluene-d8 in the annular space. The 1H and 13C NMR chemical shifts were measured relative to neat liquid TMS as the external reference standard. For this purpose, the absolute frequencies of the TMS resonance was determined in the conditions of lock system tuned to the CD3 signal of external toluene-d8. The constant frequency of the lock system (76.8464 MHz) allows one to preserve the same B0 for all measurements. The absolute magnetic shielding of TMS (32.775 ppm for protons and 186.37 ppm for 13C nuclei in a cylindrical tube parallel to an external magnetic field) [9,10] was used to convert the NMR chemical shifts into the shielding parameters of CH15 3 NH2. Liquid nitromethane (CH3NO2) was used in the same way for the measurements of 15N nuclear magnetic shielding. Its own shielding ðsðCH3 NO2;liq: ; kB0 ) ¼ 2 134.14 ppm) was determined from the 15N chemical shift measured relative to gaseous ammonia at the zero-density limit [11] and assuming the value of s0 ðNH3 Þ ¼ 264:54 ppm [12] as the absolute 15N shielding of ammonia.
2.3. Measurements of spin – spin coupling constants The modified PFG-HSQC technique, with PFG selection of doubly longitudinal two spin 2Iz Sz coherence during both INEPT steps was employed [13] in order to avoid diffusion and convection effects. The refocusing period before t2 data acquisition was omitted and consequently purely absorptive correlation signals appear with the active coupling in antiphase along the F2 domain. Sixteen scans were coherently added for each data set for 512 t1 increments recorded in the states—TPPI manner [14]. The maximum t1 and t2 times were set to 0.256 and 0.99 s. The corresponding spectral width parameters were set to 2000 Hz in F1 and 1000 Hz in F2 : A relaxation delay of 1 s was used. The initial polarization transfer was tuned to the active coupling 1 13 J( C – 1H) of 136 Hz for 1H – 13C and to 1J(15N – 1H) of 70 Hz for 1H – 15N experiments. The complex data matrix was multiplied by cosine function in both time domains and zero-filled to prior to Fourier transformation to achieve the digital resolution of ca. 0.2 Hz/point in both frequency dimensions, respectively. 1 J(NC) coupling constant was achieved from standard 13 C NMR 1H decoupled spectrum, the sample of pure CH3C15N at approx. 4 atm was used. Due to not fully resolved doublet splitting (Dn1=2 , 4 Hz) deconvolution procedure was applied in order to avoid underestimation of 1 J(NC) due to line overlapping. All spectra were acquired on a Varian Unity Plus 500 spectrometer equipped with a Performa I z-PFG unit and a standard 5 mm ID_PFG probehead with the actively shielded z-gradient coil. The PFG pulses, followed by 100 ms recovery delay, with the duration of 2 ms and strength of 10 G cm21 were used in HSQC experiments. 7, 12 and 27 ms high power 1H, 13C, and 15N p=2 pulses, respectively, were used.
3. Results and discussion For a binary mixture of gas A, containing the nucleus X whose shielding sðXÞ is of interest, and gas B as the solvent, it can be written according to the RBB approximation [15]:
sðXÞ ¼ s0 ðXÞ þ sAA ðXÞrA þ sAB ðXÞrB þ :::
ð1Þ
where rA and rB are the densities of A (CH3C15N) and B (SF6 or CO2), respectively, and s0 ðXÞ is the shielding at the zero-density limit. The coefficients sAA ðXÞ and sAB ðXÞ contain the bulk susceptibility corrections (ðsA Þb and ðsB Þb Þ and the terms taking account of intermolecular interactions during the binary collisions of A –A and A – B molecules are (s1ðA – AÞ ðXÞ and s1ðA – BÞ ðXÞ), respectively. It is worth noting that the shielding parameters in Eq. (1) are temperature dependent and for this reason all the present measurements have been performed at the constant temperature of 300 K. Moreover, in this work the density of A has been
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309
kept sufficiently low to simplify Eq. (1):
sðXÞ ¼ s0 ðXÞ þ sAB ðXÞrB
ð2Þ
According to Eq. (2), the measurements of nuclear magnetic shielding linearly depend on solvent density and the extrapolation of results to the zero-density limit allows the determination of the s0 ðXÞ parameter. It is certainly important to use at least two different gaseous solvents in order to verify the final result of s0 ðXÞ; within experimental error every chemically inert solvent should give the same value. Fig. 1 shows the 13C nuclear magnetic shielding in CH3 15NH2 as a function of density at 300 K when sulfur hexafluoride (SF6) and xenon (Xe) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results in Fig. 1 are corrected for bulk susceptibility effects and they show the real effects of intermolecular interactions in the gas phase. As shown, the dependence on density is linear for both the solvents and it permits one to determine the 13C magnetic shielding of an isolated CH15 3 NH2 molecule, s0 ðCÞ ¼ 158:259ð2Þ ppm: The latter value means the absolute 13C shielding constant found assuming that s0 ðCOÞ ¼ 0:6ð9Þ ppm [16]. It is in agreement with the previous result given by Jameson et al. ðs0 ðCÞ ¼ 158:3 ppmÞ [5]. The plots in Fig. 1 also show the intermolecular effects in carbon shielding which are due to the binary collisions between solute and solvent molecules. Both the solvents (Xe and SF6) disturb methylamine molecules but xenon does it much stronger, cf. the sðA – BÞ ðCÞ parameter equal to 2 245(3) [ppm ml mol21] for Xe and 2 102(3) [ppm ml mol21] for SF6 in Table 1. Fig. 2 presents the measurements of 15N magnetic shielding of CH15 3 NH2 in the range of solvent density, approx. 0.08– 1.30 [mol l21] for SF6 and Xe. The nitrogen results are qualitatively similar to the carbon data but the magnitude of shielding change with density is considerably larger for the nitrogen nuclei. Xenon atoms disturb the nitrogen shielding of methylamine again more efficiently
Fig. 1. The 13C nuclear magnetic shielding in CH15 3 NH2 as a function of density at 300 K when sulfur hexafluoride (SF6 (O)) and xenon (Xe (B)) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results are corrected for bulk susceptibility effects.
307
Table 1 Nuclear magnetic shielding and spin–spin coupling of gaseous methylamine– 15Na and their dependence on density in binary mixtures with sulfur hexafluoride and xenon at 300 K Parameter
Gas solvent (B) Xe
SF6 1
H, 13C and 15N magnetic shielding s0 ðCH3 Þ (ppm)b,c 28.269(4) s0 ðNH2 Þ (ppm)b,c 30.330(4) s0 ðCÞ (ppm)b 158.259(2) s0 ðNÞ (ppm)b 249.55(1) sðA – BÞ ðCÞ (ppm ml mol21) 2102(3) sðA – BÞ ðNÞ (ppm ml mol21) 2289(12) sAB ðCÞ (ppm ml mol21) 82(3) sAB ðNÞ (ppm ml mol21) 2105(12) ðsÞb ðBÞ (ppm ml mol21)d 184 1
JðNHÞ; 1 JðCHÞ; 1 JðCNÞ and 3 JðHHÞ J0 ðNHÞ (Hz) 1 J0 ðCHÞ [Hz] 1 J0 ðNCÞ (Hz) 3 J0 ðHHÞ (Hz) 1
28.262(3) 30.324(2) 158.260(3) 249.52(2) 2245(3) 2785(32) 254(3) 2594(32) 191
spin–spin coupling 265.4(1)e 132.5(1) e 25.4(5)f 7.0(1)e
265.4(1)e 132.5(1) e 25.4(5)f 7.0(1)e
The solute (A) gas (CH15 3 NH2) has been used at a low constant density (,0.004 mol l21, pressure ,50 Torr). b Absolute shielding assuming: s0 ðCH4 Þ ¼ 30:611ð24Þ ppm [20], s0 ðCOÞ ¼ 0:6ð9Þ ppm [16] and s0 ðNH3 Þ ¼ 264:54ð5Þ ppm [12]. c Independent of density within experimental error. d 2ð4p=3ÞxM ; where xM is the molar susceptibility of solvent gas [21]. e From PFG-HSQC experiments. f For pure gaseous CH3C15N at ,4 atm. a
than sulfur hexafluoride, cf. in Table 1 the sðA – BÞ ðNÞ values equal to 2 289(12) and 2 785(32) [ppm ml mol21] for Xe and SF6, respectively. The extrapolation of nitrogen shielding to the zero-density point gives the s0 ðNÞ shielding constant equal to 249.55(1) ppm for SF6 and 249.52(2) ppm for Xe. In our opinion, the first result (s0 ðNÞ ¼ 249:55ð1Þ ppm) is determined with higher accuracy and therefore it can be accepted as the absolute nitrogen shielding in an isolated CH15 3 NH2 molecule. This value has been obtained assuming the nitrogen shielding of ammonia, s0 ðNÞ; equal to 264.54(5) ppm [12].
Fig. 2. The 15N nuclear magnetic shielding in CH15 3 NH2 as a function of density at 300 K when sulfur hexafluoride (SF6 (O)) and xenon (Xe (B)) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results are corrected for bulk susceptibility effects.
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309
308 Table 2 15 N and
13
C NMR gas-to-liquid shifts for methylamine
Solvent
DsðNÞ (ppm)
DsðCÞ (ppm)
c-C6H12 CH3OH CH3OD C2H5OH C2H5OD (CH3)2CHCH2OH (CH3)2CHCH2OD (CH3)2CHOH (CH3)2CHOD (CH3)3COH (CH3)3COD
27.719 28.710 27.109 210.872 29.560 211.380 210.300 212.362 211.012 213.619 212.220
23.047 21.595 21.346 21.879 21.630 22.719 22.564 22.101 21.915 22.652 22.466
Ds ¼ sliq 2 s0 ; where sliq means the shielding constant of methylamine observed in liquid solvents, bulk susceptibility corrections are included.
Our present NMR measurements have been carried out with high precision using PFG-HSQC spectra as it is described in the experimental part of this paper. Nevertheless, we could not find any density dependence for proton shielding and spin – spin coupling parameters of CH15 3 NH2. In the gas phase, nuclear spin – spin coupling is also modified by interactions between pairs of molecules and multiple interactions and the appropriate equations for the binary mixtures of gases (A and B) are similar to Eqs. (1) and (2). It means that all the following parameters: sðA – BÞ ðNH2 Þ; sðA – BÞ ðCH3 Þ; 1 JAB ðCHÞ; 1 JAB ðNHÞ; 1 JAB ðNCÞ and 3 JAB ðHHÞ were equal to zero within the experimental error. The latter fact has had no influence on the precision of s0 ðHÞ and J0 values. The measurements were performed in the same wide range of density as it was done for 13C and 15N shielding. All the final results for an isolated CH15 3 NH2 molecule are displayed in Table 1 and they are certainly different from the appropriate parameters measured for liquid methylamine. It is true even for the spin – spin coupling constants (1 JðCHÞ; 1 JðNHÞ; 1 JðNCÞ and 3 JðHHÞ) which are independent of density in the gas phase. One can find: 1 JðCHÞ ¼ 132:2ð2Þ Hz; 1 JðNHÞ ¼ 265:0ð2Þ Hz; 1 JðNCÞ ¼ 24:5ð5Þ Hz; and 3 JðHHÞ ¼ 7:1 Hz for liquid methylamine [4] and these values are slightly different from the J0 results shown in Table 1. It seems that hydrogen bonds in liquid methylamine are mostly responsible for the gas-to-liquid changes of spin –spin coupling. The influence of hydrogen bonds in methylamine is well seen when the nitrogen shielding is monitored. Table 2 shows the 13C and 15N gas-to-solution shifts of methylamine where cyclohexane (c-C6H12) and aliphatic alcohols are used as the liquid solvents. In every case, the shielding of methylamine is diminished in the liquid phase but the change is always more distinguish for nitrogen than for carbon nuclei. It is especially well seen for alcohols, hydrogen bonds mostly disturb the nitrogen shielding
constant and there is no correlation between the effects observed by the two NMR methods (13C and 15N). Moreover, the solvent effects are slightly smaller when hydrogen bonds are formed by deuterium atoms. All the results in Table 2 confirm that hydrogen bonds are responsible for the variation of nuclear magnetic shielding in liquid methylamine.
4. Conclusions The present study yields experimental results for the 15N, C and 1H magnetic shielding and spin – spin coupling constants of 15N-enriched methylamine in the gas phase. We have found distinct density dependences of 15N and 13C nuclear magnetic shielding. The shielding parameters for an isolated CH15 3 NH2 molecule have been determined with high accuracy. In contrast, the 1H shielding and all the spin – spin coupling constants of CH15 3 NH2 were independent of density when xenon and sulfur hexafluoride were used as the gaseous solvents. In the latter case, the spectral parameters could also be precisely measured without intermolecular effects. Our new results can be helpful for the verification of numerous ab initio calculations of shielding and spin – spin coupling if the appropriate rovibrational corrections for methylamine are available, as they are obtained for acetylene [17,18] or methyl fluoride [19]. This study also confirms that the formation of hydrogen bonding system has fairly local influence on nuclear magnetic shielding in a molecule, in methylamine it is limited to – NH2 protons and 15N nuclei. The present work shows that intermolecular interactions have fairly selective influence on the NMR spectral parameters of methylamine. 13
Acknowledgements This work was supported by the Polish State Committee for Scientific Research as the research grant number 4 T09A 035 23 available in years 2003 – 2005. The authors thank Dr hab. J. Mieczkowski (Warsaw University) for helpful discussions.
References [1] J.A. Pople, W.G. Schneider, H.J. Bernstein, High-Resolution Nuclear Magnetic Resonance, McGrow-Hill Book Company Inc., New York, 1959, pp. 403. [2] G.A. Webb, M. Witanowski, Proc. Indian Acad. Sci. (Chem. Sci.) 94 (1985) 241. [3] M. Alei Jr., A.E. Florin, W.M. Lichman, J.F. O’Brien, J. Phys. Chem. 75 (1971) 932. [4] L. Paolillo, E.D. Becker, J. Magn. Reson. 3 (1970) 200. [5] A.K. Jameson, C.J. Jameson, Chem. Phys. Lett. 134 (1987) 461. [6] T. Helgaker, M. Jaszun´ski, K. Ruud, Chem. Rev. 99 (1999) 293. [7] K. Jackowski, A. Les´, P. Bernatowicz, Pol. J. Chem. 72 (1998) 527.
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309 [8] M. Pecul, T. Helgaker, Int. J. Mol. Sci. 4 (2003) 143. [9] K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 9806(E). [10] K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 5955. [11] K. Jackowski, E. Wielogo´rska, W. Koz´min´ski, W. Makulski, to be published. [12] C.J. Jameson, A.K. Jameson, D. Oppusunggu, S. Wille, P.M. Burell, J. Mason, J. Chem. Phys. 74 (1981) 81. [13] M. Wilczek, W. Koz´min´ski, K. Jackowski, Chem. Phys. Lett. 358 (2002) 263. [14] D.J. Marion, M. Ikura, R. Tschudin, A. Bax, J. Magn. Reson. 85 (1989) 393.
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[15] W.T. Raynes, A.D. Buckingham, H.J. Bernstein, J. Chem. Phys. 36 (1962) 3481. [16] W.T. Raynes, R. McVay, S.J. Wright, J. Chem. Soc., Faraday Trans. 2 (85) (1989) 759. [17] R.D. Wigglesworth, W.T. Raynes, S. Kirpekar, J. Oddershede, S.P.A. Sauer, J. Chem. Phys. 112 (2000) 736. [18] R.D. Wigglesworth, W.T. Raynes, S. Kirpekar, J. Oddershede, S.P.A. Sauer, J. Chem. Phys. 112 (2000) 3735. [19] C.H. Gee, W.T. Raynes, Chem. Phys. Lett. 330 (2000) 595. [20] W.T. Raynes, in: R.J. Abraham (Ed.), A Specialist Periodical Report NMR, 7, The Chemical Society, London, 1978, p. 25. [21] Landolt-Bo¨rnstein, Zahlenwerte und Funktionen, Band II, Teil 10, Springer, Berlin, 1967.
15
N,
13
C and 1H nuclear magnetic shielding and spin –spin coupling in gaseous 15N-enriched methylamine
Edyta Wielogo´rska, Włodzimierz Makulski, Wiktor Koz´min´ski, Karol Jackowski* Laboratory of NMR Spectroscopy, Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warszawa, Poland Received 12 September 2003; revised 13 November 2003; accepted 13 November 2003
Abstract New measurements of 15N, 13C and 1H nuclear magnetic resonance (NMR) spectral parameters have been performed for 15N-enriched methylamine (CH15 3 NH2) in the gas phase. The solute compound was observed at low constant pressure (approx. 50 Torr) and the temperature of 300 K, while sulfur hexafluoride (SF6) and xenon (Xe) were used as the gaseous solvents. It was found that the 15N and 13C chemical shifts of CH15 3 NH2 were linearly dependent on the solvent density and the appropriate shielding parameters for an isolated methylamine molecule were obtained, together with the coefficients responsible for the solute – solvent molecular interactions. In contrast the 1H chemical shifts and spin– spin coupling constants of CH15 3 NH2 were found to be independent of density within experimental error. For the first time, the present measurements reveal the majority of NMR spectral parameters of methylamine free from intermolecular effects. q 2004 Elsevier B.V. All rights reserved. Keywords: 15N; 13C and 1H NMR chemical shifts; Spin –spin coupling; Gas phase; Intermolecular effects
1. Introduction Methylamine (CH3NH2) is very important compound in organic chemistry, biochemistry and chemical industry. Molecular spectroscopy, and specially nuclear magnetic resonance (NMR) spectroscopy, is frequently used for the studies of CH3NH2. In particular, 1H and 15N NMR methods have been applied to monitor the influence of hydrogen bonds on chemical shifts and spin –spin coupling constants in the liquid state [1,2]. Alei et al. [3] have monitored the 15 N gas-to-liquid shift of CH15 3 NH2 as a function of temperature. It was found that the nitrogen shielding of methylamine was diminished upon condensation, 2 9.4 ppm at 180 K. Paolillo and Becker [4] have investigated the relative signs of the spin – spin coupling constants of liquid and gaseous CH15 3 NH2 using double resonance methods and they have determined the magnitudes and signs of five coupling constants. Jameson et al. [5] have given the 13C shielding constant of an isolated CH3NH2 molecule, s0 ð300 KÞ ¼ 158:3 ppm: The latter * Corresponding author. Tel.: þ 48-22-822-0211x315; fax: þ 48-22-82259-96. 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.2003.11.062
result is rather unique in literature, all the other shielding and spin –spin coupling parameters remain unknown for an isolated methylamine molecule. This small molecule is often used as the molecular model when new methods of quantum chemical calculations are tested. Calculations of nuclear magnetic shielding constants of methylamine have been performed using various methods and different approximation levels [6]. The influence of molecular association on the shielding parameters has also been studied by GIAO-CHF calculations [7]. Recently, Pecul and Helgaker [8] have given a comparison of three ab initio methods (DFT, MCSCF and CCSD) applied to the determination of spin – spin coupling constants in methylamine. The present work has been designed for the investigation of NMR spectral parameters of CH3NH2 in the gas phase. All the shielding constants of methylamine have been monitored as functions of density in order to obtain the parameters free from intermolecular interactions. The same procedure has been applied for the investigation of four different coupling constants: 1J(NH), 1J(CH), 1J(NC) and 3 J(HH), which could be observed in NMR spectra of 15 N-enriched methylamine. We have also monitored the influence of intermolecular interactions on the 15N and 13C
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309
306
nuclear magnetic shielding constants in liquid solvents. It has been shown that hydrogen bonds can effectively change the 15N shielding of methylamine while the 13C shielding constant remains almost insensitive on the formation of hydrogen bonds.
2. Experimental 2.1. Preparation of gaseous samples 15
N-enriched methylamine (CH15 3 NH2) was recovered 15 N) by from crystalline CH15 3 NH3Cl (Aldrich, 99 atom % adding an excessive amount of saturated aqueous solution of NaOH. The experiment was carried out in a vacuum line where gaseous CH15 3 NH2 was carefully dried over solid BaO and collected in a glass container. Gas samples were prepared by condensation of gaseous CH15 3 NH2 and pure solvent gas (SF6 or Xe) from the calibrated part of vacuum line into the 4.0 mm o.d. glass tubes (approx. 5.5 cm long) which were then sealed. The volumes of sample tubes and the vacuum line were measured using mercury. Sulfur hexafluoride (99.75%, Aldrich) or xenon (99.8, 99.9%, Messer Duisburg, Germany) from lecture bottles was used to prepare samples without further purification. In the gaseous mixtures, the solute gas (CH15 3 NH2) was used at a low constant density (, 0.004 mol l21, pressure , 50 Torr) and mixed with various quantities of the gaseous solvents: SF6 or Xe (from 0.08 to 1.3 mol l21). 2.2. Measurements of nuclear magnetic shielding The gas samples were fitted into the standard 5 mm o.d. thin-walled NMR tubes (Wilmad 528-PP) with liquid toluene-d8 in the annular space. The 1H and 13C NMR chemical shifts were measured relative to neat liquid TMS as the external reference standard. For this purpose, the absolute frequencies of the TMS resonance was determined in the conditions of lock system tuned to the CD3 signal of external toluene-d8. The constant frequency of the lock system (76.8464 MHz) allows one to preserve the same B0 for all measurements. The absolute magnetic shielding of TMS (32.775 ppm for protons and 186.37 ppm for 13C nuclei in a cylindrical tube parallel to an external magnetic field) [9,10] was used to convert the NMR chemical shifts into the shielding parameters of CH15 3 NH2. Liquid nitromethane (CH3NO2) was used in the same way for the measurements of 15N nuclear magnetic shielding. Its own shielding ðsðCH3 NO2;liq: ; kB0 ) ¼ 2 134.14 ppm) was determined from the 15N chemical shift measured relative to gaseous ammonia at the zero-density limit [11] and assuming the value of s0 ðNH3 Þ ¼ 264:54 ppm [12] as the absolute 15N shielding of ammonia.
2.3. Measurements of spin – spin coupling constants The modified PFG-HSQC technique, with PFG selection of doubly longitudinal two spin 2Iz Sz coherence during both INEPT steps was employed [13] in order to avoid diffusion and convection effects. The refocusing period before t2 data acquisition was omitted and consequently purely absorptive correlation signals appear with the active coupling in antiphase along the F2 domain. Sixteen scans were coherently added for each data set for 512 t1 increments recorded in the states—TPPI manner [14]. The maximum t1 and t2 times were set to 0.256 and 0.99 s. The corresponding spectral width parameters were set to 2000 Hz in F1 and 1000 Hz in F2 : A relaxation delay of 1 s was used. The initial polarization transfer was tuned to the active coupling 1 13 J( C – 1H) of 136 Hz for 1H – 13C and to 1J(15N – 1H) of 70 Hz for 1H – 15N experiments. The complex data matrix was multiplied by cosine function in both time domains and zero-filled to prior to Fourier transformation to achieve the digital resolution of ca. 0.2 Hz/point in both frequency dimensions, respectively. 1 J(NC) coupling constant was achieved from standard 13 C NMR 1H decoupled spectrum, the sample of pure CH3C15N at approx. 4 atm was used. Due to not fully resolved doublet splitting (Dn1=2 , 4 Hz) deconvolution procedure was applied in order to avoid underestimation of 1 J(NC) due to line overlapping. All spectra were acquired on a Varian Unity Plus 500 spectrometer equipped with a Performa I z-PFG unit and a standard 5 mm ID_PFG probehead with the actively shielded z-gradient coil. The PFG pulses, followed by 100 ms recovery delay, with the duration of 2 ms and strength of 10 G cm21 were used in HSQC experiments. 7, 12 and 27 ms high power 1H, 13C, and 15N p=2 pulses, respectively, were used.
3. Results and discussion For a binary mixture of gas A, containing the nucleus X whose shielding sðXÞ is of interest, and gas B as the solvent, it can be written according to the RBB approximation [15]:
sðXÞ ¼ s0 ðXÞ þ sAA ðXÞrA þ sAB ðXÞrB þ :::
ð1Þ
where rA and rB are the densities of A (CH3C15N) and B (SF6 or CO2), respectively, and s0 ðXÞ is the shielding at the zero-density limit. The coefficients sAA ðXÞ and sAB ðXÞ contain the bulk susceptibility corrections (ðsA Þb and ðsB Þb Þ and the terms taking account of intermolecular interactions during the binary collisions of A –A and A – B molecules are (s1ðA – AÞ ðXÞ and s1ðA – BÞ ðXÞ), respectively. It is worth noting that the shielding parameters in Eq. (1) are temperature dependent and for this reason all the present measurements have been performed at the constant temperature of 300 K. Moreover, in this work the density of A has been
E. Wielogo´rska et al. / Journal of Molecular Structure 704 (2004) 305–309
kept sufficiently low to simplify Eq. (1):
sðXÞ ¼ s0 ðXÞ þ sAB ðXÞrB
ð2Þ
According to Eq. (2), the measurements of nuclear magnetic shielding linearly depend on solvent density and the extrapolation of results to the zero-density limit allows the determination of the s0 ðXÞ parameter. It is certainly important to use at least two different gaseous solvents in order to verify the final result of s0 ðXÞ; within experimental error every chemically inert solvent should give the same value. Fig. 1 shows the 13C nuclear magnetic shielding in CH3 15NH2 as a function of density at 300 K when sulfur hexafluoride (SF6) and xenon (Xe) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results in Fig. 1 are corrected for bulk susceptibility effects and they show the real effects of intermolecular interactions in the gas phase. As shown, the dependence on density is linear for both the solvents and it permits one to determine the 13C magnetic shielding of an isolated CH15 3 NH2 molecule, s0 ðCÞ ¼ 158:259ð2Þ ppm: The latter value means the absolute 13C shielding constant found assuming that s0 ðCOÞ ¼ 0:6ð9Þ ppm [16]. It is in agreement with the previous result given by Jameson et al. ðs0 ðCÞ ¼ 158:3 ppmÞ [5]. The plots in Fig. 1 also show the intermolecular effects in carbon shielding which are due to the binary collisions between solute and solvent molecules. Both the solvents (Xe and SF6) disturb methylamine molecules but xenon does it much stronger, cf. the sðA – BÞ ðCÞ parameter equal to 2 245(3) [ppm ml mol21] for Xe and 2 102(3) [ppm ml mol21] for SF6 in Table 1. Fig. 2 presents the measurements of 15N magnetic shielding of CH15 3 NH2 in the range of solvent density, approx. 0.08– 1.30 [mol l21] for SF6 and Xe. The nitrogen results are qualitatively similar to the carbon data but the magnitude of shielding change with density is considerably larger for the nitrogen nuclei. Xenon atoms disturb the nitrogen shielding of methylamine again more efficiently
Fig. 1. The 13C nuclear magnetic shielding in CH15 3 NH2 as a function of density at 300 K when sulfur hexafluoride (SF6 (O)) and xenon (Xe (B)) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results are corrected for bulk susceptibility effects.
307
Table 1 Nuclear magnetic shielding and spin–spin coupling of gaseous methylamine– 15Na and their dependence on density in binary mixtures with sulfur hexafluoride and xenon at 300 K Parameter
Gas solvent (B) Xe
SF6 1
H, 13C and 15N magnetic shielding s0 ðCH3 Þ (ppm)b,c 28.269(4) s0 ðNH2 Þ (ppm)b,c 30.330(4) s0 ðCÞ (ppm)b 158.259(2) s0 ðNÞ (ppm)b 249.55(1) sðA – BÞ ðCÞ (ppm ml mol21) 2102(3) sðA – BÞ ðNÞ (ppm ml mol21) 2289(12) sAB ðCÞ (ppm ml mol21) 82(3) sAB ðNÞ (ppm ml mol21) 2105(12) ðsÞb ðBÞ (ppm ml mol21)d 184 1
JðNHÞ; 1 JðCHÞ; 1 JðCNÞ and 3 JðHHÞ J0 ðNHÞ (Hz) 1 J0 ðCHÞ [Hz] 1 J0 ðNCÞ (Hz) 3 J0 ðHHÞ (Hz) 1
28.262(3) 30.324(2) 158.260(3) 249.52(2) 2245(3) 2785(32) 254(3) 2594(32) 191
spin–spin coupling 265.4(1)e 132.5(1) e 25.4(5)f 7.0(1)e
265.4(1)e 132.5(1) e 25.4(5)f 7.0(1)e
The solute (A) gas (CH15 3 NH2) has been used at a low constant density (,0.004 mol l21, pressure ,50 Torr). b Absolute shielding assuming: s0 ðCH4 Þ ¼ 30:611ð24Þ ppm [20], s0 ðCOÞ ¼ 0:6ð9Þ ppm [16] and s0 ðNH3 Þ ¼ 264:54ð5Þ ppm [12]. c Independent of density within experimental error. d 2ð4p=3ÞxM ; where xM is the molar susceptibility of solvent gas [21]. e From PFG-HSQC experiments. f For pure gaseous CH3C15N at ,4 atm. a
than sulfur hexafluoride, cf. in Table 1 the sðA – BÞ ðNÞ values equal to 2 289(12) and 2 785(32) [ppm ml mol21] for Xe and SF6, respectively. The extrapolation of nitrogen shielding to the zero-density point gives the s0 ðNÞ shielding constant equal to 249.55(1) ppm for SF6 and 249.52(2) ppm for Xe. In our opinion, the first result (s0 ðNÞ ¼ 249:55ð1Þ ppm) is determined with higher accuracy and therefore it can be accepted as the absolute nitrogen shielding in an isolated CH15 3 NH2 molecule. This value has been obtained assuming the nitrogen shielding of ammonia, s0 ðNÞ; equal to 264.54(5) ppm [12].
Fig. 2. The 15N nuclear magnetic shielding in CH15 3 NH2 as a function of density at 300 K when sulfur hexafluoride (SF6 (O)) and xenon (Xe (B)) are used as the solvent gases. The solute compound has been observed at constant low pressure (approx. 50 Torr). The results are corrected for bulk susceptibility effects.
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308 Table 2 15 N and
13
C NMR gas-to-liquid shifts for methylamine
Solvent
DsðNÞ (ppm)
DsðCÞ (ppm)
c-C6H12 CH3OH CH3OD C2H5OH C2H5OD (CH3)2CHCH2OH (CH3)2CHCH2OD (CH3)2CHOH (CH3)2CHOD (CH3)3COH (CH3)3COD
27.719 28.710 27.109 210.872 29.560 211.380 210.300 212.362 211.012 213.619 212.220
23.047 21.595 21.346 21.879 21.630 22.719 22.564 22.101 21.915 22.652 22.466
Ds ¼ sliq 2 s0 ; where sliq means the shielding constant of methylamine observed in liquid solvents, bulk susceptibility corrections are included.
Our present NMR measurements have been carried out with high precision using PFG-HSQC spectra as it is described in the experimental part of this paper. Nevertheless, we could not find any density dependence for proton shielding and spin – spin coupling parameters of CH15 3 NH2. In the gas phase, nuclear spin – spin coupling is also modified by interactions between pairs of molecules and multiple interactions and the appropriate equations for the binary mixtures of gases (A and B) are similar to Eqs. (1) and (2). It means that all the following parameters: sðA – BÞ ðNH2 Þ; sðA – BÞ ðCH3 Þ; 1 JAB ðCHÞ; 1 JAB ðNHÞ; 1 JAB ðNCÞ and 3 JAB ðHHÞ were equal to zero within the experimental error. The latter fact has had no influence on the precision of s0 ðHÞ and J0 values. The measurements were performed in the same wide range of density as it was done for 13C and 15N shielding. All the final results for an isolated CH15 3 NH2 molecule are displayed in Table 1 and they are certainly different from the appropriate parameters measured for liquid methylamine. It is true even for the spin – spin coupling constants (1 JðCHÞ; 1 JðNHÞ; 1 JðNCÞ and 3 JðHHÞ) which are independent of density in the gas phase. One can find: 1 JðCHÞ ¼ 132:2ð2Þ Hz; 1 JðNHÞ ¼ 265:0ð2Þ Hz; 1 JðNCÞ ¼ 24:5ð5Þ Hz; and 3 JðHHÞ ¼ 7:1 Hz for liquid methylamine [4] and these values are slightly different from the J0 results shown in Table 1. It seems that hydrogen bonds in liquid methylamine are mostly responsible for the gas-to-liquid changes of spin –spin coupling. The influence of hydrogen bonds in methylamine is well seen when the nitrogen shielding is monitored. Table 2 shows the 13C and 15N gas-to-solution shifts of methylamine where cyclohexane (c-C6H12) and aliphatic alcohols are used as the liquid solvents. In every case, the shielding of methylamine is diminished in the liquid phase but the change is always more distinguish for nitrogen than for carbon nuclei. It is especially well seen for alcohols, hydrogen bonds mostly disturb the nitrogen shielding
constant and there is no correlation between the effects observed by the two NMR methods (13C and 15N). Moreover, the solvent effects are slightly smaller when hydrogen bonds are formed by deuterium atoms. All the results in Table 2 confirm that hydrogen bonds are responsible for the variation of nuclear magnetic shielding in liquid methylamine.
4. Conclusions The present study yields experimental results for the 15N, C and 1H magnetic shielding and spin – spin coupling constants of 15N-enriched methylamine in the gas phase. We have found distinct density dependences of 15N and 13C nuclear magnetic shielding. The shielding parameters for an isolated CH15 3 NH2 molecule have been determined with high accuracy. In contrast, the 1H shielding and all the spin – spin coupling constants of CH15 3 NH2 were independent of density when xenon and sulfur hexafluoride were used as the gaseous solvents. In the latter case, the spectral parameters could also be precisely measured without intermolecular effects. Our new results can be helpful for the verification of numerous ab initio calculations of shielding and spin – spin coupling if the appropriate rovibrational corrections for methylamine are available, as they are obtained for acetylene [17,18] or methyl fluoride [19]. This study also confirms that the formation of hydrogen bonding system has fairly local influence on nuclear magnetic shielding in a molecule, in methylamine it is limited to – NH2 protons and 15N nuclei. The present work shows that intermolecular interactions have fairly selective influence on the NMR spectral parameters of methylamine. 13
Acknowledgements This work was supported by the Polish State Committee for Scientific Research as the research grant number 4 T09A 035 23 available in years 2003 – 2005. The authors thank Dr hab. J. Mieczkowski (Warsaw University) for helpful discussions.
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