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N NQR Study of Polymorphism and Hydrogen Bonding in Molecular Complex Isonicotinamide-Oxalic Acid (2:1) J. Seliger*,†,‡ and V. Zˇagar† Jozef Stefan Institute, JamoVa 39, 1000 Ljubljana, SloVenia, and UniVersity of Ljubljana, Faculty of Mathemathics and Physics, Department of Physics, Jadranska 19, 1000 Ljubljana, SloVenia ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: October 4, 2010
The complete 14N nuclear quadrupole resonance (NQR) spectra have been measured in the two polymorphic crystalline phases of the molecular complex isonicotinamide-oxalic acid (2:1) by nuclear quadrupole double resonance. The observed NQR frequencies, quadrupole coupling constants, and asymmetry parameters (η) have been assigned to the two nitrogen positions (ring and amide) in a molecule on the basis of the intensity and multiplicity of the double resonance signals. The NQR data for the ring nitrogen in both polymorphic phases deviate from the correlation relations observed in substituted pyridines. This deviation is analyzed in a model, where it is assumed that an additional electric charge on the nitrogen atom changes the NQR parameters. The model suggests that this additional electric charge is negative so that the N · · · H-O hydrogen bond seem to be partially ionic, of the type N- · · · H-O. Introduction Hydrogen bonds occur in a variety of organic and inorganic materials. They influence the structure as well as the physical, chemical, and biological properties of materials. The study of short, strong hydrogen bonds in solids is of particular interest because they model the proton transfer process. The position of the proton in a hydrogen bond is not necessary fixed. A displacement of a proton or a two-site exchange maysby decreasing temperaturesresult in a collective ordering and sometimes even in the proton transfer as, for example, observed in organic ferroelectric phenazine-chloranilic acid (1:1).1 The cocrystals of isonicotinamide-oxalic acid (2:1) are composed of chains of hydrogen bonded pairs of isonicotinamide molecules glued by the oxalic acid molecules.2-4 Two polymorphic forms of the isonicotinamide-oxalic acid (2:1) cocrystals are schematically presented in Figure 1. They differ in the cis-trans isomerism of the oxalic acid hydroxyl groups. The chains are crosslinked by further moderate strength N-H · · · O hydrogen bonds, forming a three-dimensional network in form I and a twodimensional layered structure in form II. The O-H · · · N hydrogen bonds between the oxalic acid molecules and the ring nitrogen atoms of the isonicotinamide molecules are very short: 2.559 Å in form I and 2.539 Å in form II.2 Also, the position of hydrogen in a O-H · · · N hydrogen bond is well displaced toward the center of the hydrogen bond in comparison to a isolated hydroxyl group, where the O-H distance is about 0.98 Å. The O-H and H · · · N distances are, in the form I, 1.161 and 1.398 Å, respectively; whereas they are, in the form II, 1.235 and 1.313 Å, respectively.2 The neutron diffraction data indicate no pronounced proton intrabond dynamics in either of the two phases.2,3 Nuclear quadrupole resonance (NQR) of 14N is well suited for the study of the hydrogen bonds formed by the nitrogen atoms. A nuclear electric quadrupole moment eQ interacts with the inhomogeneous electric field at the position of the nucleus produced * To whom correspondence should be addressed. E-mail: janez.seliger@ fmf.uni-lj.si. † Jozef Stefan Institute. ‡ University of Ljubljana.
Figure 1. Two polymorphic forms I and II of isonicotinamide-oxalic acid (2:1). The hydrogen bonds are shown as broken lines.
by the surrounding electric charges, which result in the removal of the degeneracy of the nuclear ground state.5 In case of 14N (I ) 1) we may observe three NQR frequencies ν+ g ν- g ν0, which equal
e2qQ (3 + η) 4h e2qQ (3 - η) ν- ) 4h e2qQ η ν0 ) ν+ - ν- ) 2h
ν+ )
(1)
Here, e2qQ/h is the quadrupole coupling constant, and η is the asymmetry parameter of the traceless electric-field-gradient (EFG) tensor Vik, Vik ) ∂2V/∂xi∂xk, at the position of the 14N nucleus. Here, V is the electrostatic potential in the vicinity of the nucleus produced by the surrounding electric charges. The quadrupole coupling constant is the largest principal value VZZ ) eq of the EFG tensor multiplied by the nuclear quadrupole moment eQ and divided by the Planck’s constant h. The asymmetry parameter η is the difference of the two smaller principal values of the EFG tensor VXX - VYY, |VXX| e |VYY|, divided by VZZ. The asymmetry parameter η ranges between 0 and 1. The elements of the EFG tensor can
10.1021/jp105449w 2010 American Chemical Society Published on Web 10/22/2010
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experimentally not be determined. The experimental methods give the quadrupole coupling tensor qik, qik ) eQVik/h. The 14N NQR frequencies are related to the principal values qZZ, qYY, and qXX (|qZZ| g |qYY| g |qXX|) of the quadrupole coupling tensor as
1 |q - qYY | 2 ZZ 1 ν- ) |qZZ - qXX | 2 1 ν0 ) |qXX - qYY | 2
ν+ )
(2)
Hydrogen bonding changes the inhomogeneous electric field at the donor and at the acceptor position. As a result of this change the quadrupole coupling constant e2qQ/h ) qZZ and the asymmetry parameter η change at the position of the donor and the acceptor atomic nuclei (in the present case N and O) involved in the hydrogen bond. For example, the 14N quadrupole coupling constant is in a noninteracting pyridine ring in the gas phase about 4.9 MHz.6 It decreases with the increasing N · · · H interaction in the hydrogen bonded systems and reaches a value lower than 1 MHz in the case of a complete proton transfer (pyridinium ion).7,8 In the present compound we probe the hydrogen bonds between the oxalic acid and isonicotinamide molecules as well as the hydrogen bonds formed by the amide groups using 14N NQR. The NQR results are related to the results of the structural determination.2 Experimental Section The two polymorphs of isonicotinamide-oxalic acid (2:1) have been prepared by slow crystallization from the water solution of stoichiometric ratios of isonicotinamide and oxalic acid, separated on the basis of their morphology.2 The measurements were performed with approximately 0.5 g of each sample. The 14N NQR frequencies were measured by nuclear quadrupole double resonance using magnetic field cycling. In this experiment the influence of the quadrupole nuclei (14N) on the proton NMR signal is observed. A magnetic field cycle consists of the following steps. The proton spin system is first polarized in a high magnetic field B0, in the present case B0 ) 0.75 T. The sample is then pneumatically transferred into a low magnetic field B, where it is left for a time τ. During this time, which is called the relaxation period, the proton magnetization relaxes toward its equilibrium value, which is for the factor B/B0 smaller than the proton magnetization at the beginning of the relaxation period. After the time τ the sample moves back into the high magnetic field B0, and the proton NMR signal is measured immediately after the sample stops in the high magnetic field. The intensity of the proton NMR signal is proportional to the proton magnetization at the end of the relaxation period. Coupling of protons to the quadrupole nuclei may increase the relaxation rate of the proton magnetization during the relaxation period and, as a consequence, we observe at the end of the relaxation period a smaller proton NMR signal. The coupling of protons to the quadrupole nuclei during the relaxation period requires either the proper choice of the low magnetic field B or the proper frequency ν of a rf magnetic field, applied during the relaxation period, both related to the NQR frequencies. Various double resonance techniques are used depending on the spin of the quadrupole nuclei and on the spin-lattice relaxation rates of protons and the quadrupole nuclei.
Figure 2. 1H-14N solid-effect spectra of crystal polymorphs I and II of isonicotinamide-oxalic acid (2:1) at T ) 170 K. The spectra are measured at the proton Larmor frequency νL ) 200 kHz. The dips at the frequencies 200, 400, and 600 kHz, that are observed in both spectra, are the consequence of the direct absorption of the rf power by the proton spin system at ν ) νL, ν ) 2νL, and ν ) 3νL.
The 14N NQR frequencies were measured using the solid effect technique9,10 and the two-frequency irradiation technique.11 In the solid effect technique the low magnetic field is fixed at, say, B ) 4.7 mT. The proton Larmor frequency is, in this magnetic field, equal to νL ) γHB/2π ) 200 kHz. A strong rf magnetic field with the frequency ν is applied during the relaxation period. The frequency ν is changed between magnetic field cycles in steps of 10 kHz. The range of the 14N NQR frequencies is scanned by the frequency ν. In the ν-dependence of the proton NMR signal, observed at the end of the magnetic field cycle, we observe dips at ν ) νQ (level crossing dip), ν ) νQ ( νL (first-order solid effect dips), and sometimes also at ν ) νQ ( 2νL (second-order solid effect dips). Here, νQ is a 14N NQR frequency. The solid effect dips are associated with the simultaneous transitions between the proton and 14N energy levels, which are allowed in solids due to the presence of the internuclear dipole-dipole interaction. In the two-frequency irradiation technique the protons are in the low magnetic field B set in resonance with the lowest 14N NQR frequency ν0 (γHB/2π ) νL ) ν0). The sample is during the relaxation period irradiated with two rf magnetic fields with the frequencies ν1 and ν2. When ν1 ) ν- and ν2 ) ν+ we observe a fast relaxation of the proton magnetization toward zero value and consequently a very low proton NMR signal at the end of the magnetic field cycle. This technique is used to improve the resolution of the double resonance detection of the 14N NQR frequencies and, in case of a complex 14N NQR spectrum, to determine the triplets of the 14N NQR frequencies (ν+, ν-, and ν0) arising from various nitrogen positions in the crystal. The proton spin-lattice relaxation time in the high magnetic field, which is in both samples several minutes, was reduced as suggested by Rabbani and Edmonds12 by fine grinding of the samples. Results and Discussion The 14N NQR frequencies have first been located by the solid effect technique.9,10 The solid effect spectra of the two polymorphic phases of isonicotinamide-oxalic acid (2:1) are shown in Figure 2. The resolution has been later improved and the NQR triplets (ν+, ν-, and ν0) have been unambiguously determined by the two-frequency irradiation technique.11 The 14 N NQR frequencies, quadrupole coupling constants and asymmetry parameters η are presented in Table 1. The assignment of the NQR lines to the two nitrogen positions (amide
Polymorphism and H-Bonding in Isonicotinamide-Oxalic Acid
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TABLE 1: 14N NQR Frequencies, Quadrupole Coupling Constant (e2qQ/h) and Asymmetry Parameter (η) of the EFG Tensor at Various Nitrogen Positions in the Two Polymorphic Forms of Isonicotinamide-oxalic Acid (2:1) at T ) 170 K nitrogen phase position ν+ [kHz] ν- [kHz] ν0 [kHz] e2qQ/h [kHz] I II
-NH2 ring -NH2 ring
2215 2230 2035 1855
1695 1750 1495 1295
520 480 540 560
2605 2655 2355 2100
η 0.40 0.36 0.46 0.53
and ring) has been done on the basis of the intensity of the solid-effect and level-crossing NQDR lines, which aresdue to the shorter NH distancesmore intense from the amide nitrogen position than from the ring nitrogen position, and on the observation of some second-order solid-effect lines at the frequencies νQ ( 2νL from the amide nitrogen position. The 14N quadrupole coupling constant e2qQ/h and the asymmetry parameter η at the amide nitrogen position are in both phases not very much different from e2qQ/h and η at the ring nitrogen position in contrast to solid isonicotinamide, where e2qQ/h ) 2670 kHz and η ) 0.376 at the amide nitrogen position, whereas e2qQ/h ) 4650 kHz and η ) 0.336 at the ring nitrogen position.13 The 14N quadrupole coupling constant and η at the amide nitrogen position are comparable to the ones observed in pure isonicotinamide, indicating a similar intermolecular bonding via two N-H · · · O bonds between the pairs of molecules. The quadrupole coupling constant of the ring nitrogen is, in isonicotinamide-oxalic acid (2:1), only about one-half of the quadrupole coupling constant observed in pure isonicotinamide. This fact agrees with the presence of a strong N · · · H-O hydrogen bond between the ring nitrogen and oxalic acid. Brown and co-workers7,14 observed a correlation of the principal values of the 14N quadrupole coupling tensor in coordinated pyridine. A plot of the principal values of the quadrupole coupling tensor for isonicotinamide and related amides13 and acids15 involving hydrogen bonds to the pyridine nitrogen is presented in Figure 3, together with the data for isonicotinamide-oxalic acid (2:1). The data for the pyridine in the gas phase6 and for the pyridinium ion7,8 are added for comparison. A strong nearly linear correlation of the principal values of the quadrupole coupling tensor is observed in the whole range from pyridine in the gas phase to the pyridinium
Figure 4. Principal axes of the pyridine.
N quadrupole coupling tensor in
ion. The data for isonicotinamide-oxalic acid (2:1) clearly deviate from the correlation lines. The correlation relations observed in Figure 3 may be for the majority of compounds expressed as follows:
qxx ) 1.0 MHz - 0.088qzz
(3)
qyy ) -1.0 MHz - 0.912qzz
Brown and co-workers7,14 described the variation of the principal values of the 14N quadrupole coupling tensor of coordinated pyridine in terms of a modified Townes-Dailey theory. They assumed that the interaction of pyridine as a Lewis base with an acid displaces the electric charge distribution of the lone electron pair. This influences the electric charge distribution in the σ and π bonds formed by the nitrogen atom. The contributions of the lone electron pair, π bond, and σ bonds to the change of the quadrupole coupling tensor are assumed to be mutually proportional. The correlation relations (3) can be obtained from this theory. The deviations from the correlation relations, as observed in the present compound, may be the consequence of an additional electric charge on the nitrogen atom. A second possibility would be the presence of a strong molecular motion, which causes an averaging of the quadrupole coupling tensor. No such motion has been observed crystallographically.2 The result of a strong molecular motion is a fluctuation of the EFG tensor, which produces a fast spin-lattice relaxation of 14N. In the NQDR experiments we observed no cross relaxation 1H-14N dips. In the solid effect spectra we observed the level crossing dips. These experiments clearly show that the 14 N spin-lattice relaxation rates are of the order of 1 s-1 or lower, excluding the presence of strong molecular motions. In the continuation we will examine the possibility that the nitrogen atom bears an excess electric charge. We assume that the additional electric charge spreads over the σ and π bonds formed by the nitrogen atom. The changes of the quadrupole coupling tensor caused by this additional electric charge are in the coordinate system, presented in Figure 4, expressed as
[
δqπ ) a Figure 3. The correlation of the principal values qXX (open triangles) and qYY (solid triangles) of the quadrupole coupling tensor versus the largest principal value qZZ at the pyridine nitrogen position in pyridine gas,6 various compounds involving hydrogen bonded pyridine nitrogen (several amides13 plus nicotinic acid and related compounds15), and pyridinium ion.7,8 The data for isonicotinamide-oxalic acid (2:1) in both polymorphic forms are shown as squares.
14
-1/2 0 0 0 1 0 0 0 -1/2
]
3 2 1 sin θ 2 2
0
[
δqCN ) 2b
1 2
0
-
0
0
0 0 1 3 2 cos θ 2 2
]
(4)
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Figure 5. The correlation between the principal values of the 14N quadrupole coupling tensor at the amide nitrogen position (a) and the correlation between the 14N quadrupole coupling constant at the amide nitrogen position and the average length of the two N-H · · · O hydrogen bonds formed by a -NH2 group (b) in several solid amides,13 a formamide molecule in the gas phase16 and in the present compound. The data for the present compound are presented as open symbols.
Here δqπ is the change of the quadrupole coupling tensor produced by the electric charge difference in the π bond. The term δqCN is the contribution of the electric charge difference in the C-N σ bonds to the quadrupole coupling tensor. The constants a and b are proportional to the additional electric charge in the π and σ bonds, respectively, and have the sign of the additional electric charge on the nitrogen atom. The angle θ is one-half of the CNC angle. Combining eqs 3 and 4, we obtain the principal values of the quadrupole coupling tensor, which we label as q1xx,q1yyand 1 . qzz
q1xx ) 1.0 MHz - 0.088qzz - 0.5a + b(3sin2 θ - 1) q1yy ) -1.0 MHz - 0.912qzz + a - b 1 qzz ) qzz - 0.5a + b(3cos2 θ - 1)
(5) There are three unknowns (a, b, and qZZ) in eq 5, and there are only two independent principal values of the traceless quadrupole coupling tensor. We will therefore examine two strongly different situations: (i) When the additional electric charge approximately equally spreads over the σ and π bonds (b ) a) and (ii) when the additional electric charge spreads only over the π bonds (b ) 0). For the sake of simplicity we take θ equal 60°. First we consider polymorphic phase II, where the principal values of the quadrupole coupling tensor are -2100, 1605, and 495 kHz. The sign of the quadrupole coupling constant is taken from the modified Townes-Dailey theory.7,14 If we assume that 1 ) -2100 kHz we have two possibilities for q1xx: it is either qzz 495 or 1605 kHz. If q1xx ) 495 kHz, then we obtain in the situation (i) qzz ) -2.9 MHz and a ) b ) -1 MHz, whereas we obtain in the situation (ii) qzz ) -1.5 MHz, a ) 1.27 MHz, and b ) 0. If q1xx ) 1605 kHz then we obtain in the situation (i) qzz ) -1.6 MHz and a ) b ) 0.6 MHz, whereas we obtain in the situation (ii) qzz ) -2.5 MHz, a ) -0.8 MHz, and b ) 0. In the modified Townes-Dailey theory7,14 the quadrupole coupling constants qzz ) -2.9 MHz and qzz ) -2.5 MHz correspond to the donor orbiral occupancy 1.78 and 1.73, respectively, whereas qzz ) -1.6 MHz and qzz ) -1.5 MHz correspond to the donor orbital occupancy 1.61 and 1.60, respectively. The later situation is highly improbable, because such a low donor orbital occupancy (1.61 or 1.60) corresponds to proton transfer from oxygen to nitrogen, which is not observed crystallographically. The parameters a and b associated with the larger by magnitude qZZ are in both situations negative or zero, which means that the nitrogen atom bears an excess negative electric charge.
In the polymorphic phase I the principal values of the quadrupole coupling tensor are -2655, 1805, and 850 kHz. If we take q1xx ) 850 kHz, then we obtain (i)qzz ) -3.1 MHz and a ) b ) -0.6 MHz; and (ii) qzz ) -2.3 MHz, a ) 0.7 MHz, and b ) 0. If we take q1xx ) 1805 kHz, then we obtain (i) qzz ) -2.1 MHz, a ) b ) 0.8 MHz; and (ii) qzz ) -3.2 MHz, a ) -1.1 MHz, and b ) 0. The values qzz ) -2.3 MHz and qzz ) -2.1 MHz are again less probable since they correspond to rather low donor orbital occupancy 1.71 and 1.68, respectively. On the other hand, the more probable values qzz ) -3.1 MHz and qzz ) -3.2 MHz correspond to the donor orbital occupancy 1.81 and 1.82, respectively. A larger by magnitude value of qZZ in the polymorphic phase I as compared to the value of qZZ in the polymorphic phase II is consistent with a weaker hydrogen bond. The parameters a and b are in the more probable cases negative or zero, which means that there is also in phase I an excess of the negative electric charge on the nitrogen atom. We can estimate the excess electric charge on the nitrogen atom by dividing the sum a +2b by q0 ) -9 MHz, where q0 is the largest principal value of the axially symmetric contribution of one electron in the nitrogen 2p orbital to the quadrupole coupling tensor.7,14 The excess electric charge is in case of a stronger hydrogen bond (polymorphic phase II) approximately equal to 30% of the electron electric charge in the model (i) and 9% of the electron electric charge in the model (ii). It is, in the case of a weaker hydrogen bond in polymorphic phase I, approximately equal to 20 and 12% of the electron electric charge in the models (i) and (ii), respectively. The N · · · H-O hydrogen bond thus seems to be partially ionic of the type N- · · · H-O. The excess negative electric charge on the nitrogen atom may be transferred from the N-H bonds of the amide group, which forms two N-H · · · O hydrogen bonds. In this case the amide nitrogen atom becomes electrically positive, what may strengthen the intra- and interchain hydrogen bonds formed by the amide groups. If this is true, the electric charge transferred from the amide group to the pyridine nitrogen must be larger in phase II than in phase I, favoring the assumption that the excess electric charge distributes over the σ and π bonds formed by the pyridine nitrogen atom and not only over the π-bonds. In ref 13 we observed the correlation between the principal values of the 14N quadrupole coupling tensor and a correlation between the 14N quadrupole coupling constant and an average length of the N-H · · · O hydrogen bonds in several amides, where a -NH2 group forms two hydrogen bonds of comparable strength. The data for the present compound in both crystalloghraphic phases seem to obey these correlations, as seen in Figure 5. The data for formamide in the gas phase16 and the data for the present compound are added to the data given in ref 13. A nearly linear correlation of the principal values of the
Polymorphism and H-Bonding in Isonicotinamide-Oxalic Acid quadrupole coupling tensor is also observed in this case. The sign of the quadrupole coupling constant is taken from a theoretical study.17 A lower (by magnitude) quadrupole coupling constant qZZ corresponds to stronger hydrogen bonding. Conclusions The complete 14N NQR spectra have been measured in the two polymorphic phases of isonicotinamide-oxalic acid (2:1). The observed quadrupole coupling constants and asymmetry parameters have been assigned to the two nitrogen positions (ring and amide) in a molecule on the basis of the NQDR intensities and second-order solid effect lines. The NQR data for the ring nitrogen in both crystallographic phases deviate from the correlation relations observed in substituted pyridines. Low 14N spin-lattice relaxation rates indicate that this deviation is not the result of molecular motions (reorientations or librations), but may be caused by an excess electric charge on the nitrogen atom. The experimentally determined quadrupole coupling tensor is analyzed in two strongly different situations, where it is assumed (i) that the excess electric charge spreads uniformly over the σ and π bonds formed by the nitrogen atom and (ii) that the excess electric charge spreads only over the π bonds. The analysis shows that the excess electric charge is in both situations negative. The hydrogen bond is thus of the type N- · · · H-O. The excess negative electric charge on the nitrogen atom may be transferred from the N-H bonds of the amide nitrogen atom, which forms two N-H · · · O hydrogen bonds. In this case the amide nitrogen atom becomes electrically positive, which may strengthen the intra- and interchain hydrogen bonds formed by the amide groups.
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12087 More thorough quantum chemical calculations are needed to prove the suggestions obtained from the simple intuitive model we used. References and Notes (1) Asaji, T.; Seliger, J.; Zˇagar, V.; Sekiguchi, M.; Watanabe, J.; Gotoh, K.; Ishida, H.; Vrtnik, S.; Dolinsˇek, J. J. Phys.: Condens. Matter 2007, 19, 226203. (2) Schmidtmann, M.; Farrugia, L. J.; Middlemiss, D. S.; Gutmann, M. J.; McIntyre, G. J.; Wilson, C. C. J. Phys. Chem. A 2009, 113, 13985– 13997. (3) Schmidtmann, M.; Gutmann, M. J.; Middlemiss, D. S.; Wilson, C. C. Cryst. Eng. Comm. 2007, 9, 743–745. (4) Vishweshwar, P.; Nangia, A.; Lynch, V. M. Cryst. Growth Des. 2003, 3, 783–790. (5) See for example; Seliger, J., Nuclear Quadrupole Resonance: Theory. In Encyclopedia of Spectroscopy and Spectrometry; Lindon, J. C.; Tranter, G. E.; Holmes, J. L., Eds.; Academic Press: San Diego, etc., 2000; pp 1672-1680. (6) Heineking, N.; Dreizler, H.; Schwarz, R. Z. Natuforsch 1986, 41a, 1210–1213. (7) Rubenacker, G. V.; Brown, T. L. Inorg. Chem. 1980, 19, 392– 398. (8) Seliger, J.; Zˇagar, V.; Asaji, T.; Konnai, A. Magn. Reson. Chem. 2008, 46, 756–760. (9) Seliger, J.; Blinc, R.; Mali, M.; Osredkar, R.; Prelesnik, A. Phys. ReV. B 1975, 11, 27–36. (10) Seliger, J.; Zˇagar, V. J. Magn. Reson. 2008, 193, 54–62. (11) Seliger, J.; Zˇagar, V.; Blinc, R. J. Magn. Reson. A 1994, 106, 214– 222. (12) Rabbani, S. R.; Edmonds, D. T. Phys. ReV. B 1994, 50, 6184– 6188. (13) Seliger, J.; Zˇagar, V. Magn. Reson. Chem. 2007, 46, 58–62. (14) Hsieh, Y.-N.; Rubenacker, G. V.; Cheng, C. P.; Brown, T. L. J. Am. Chem. Soc. 1977, 99, 13841389. (15) Seliger, J.; Zˇagar, V.; Zidansˇek, A.; Blinc, R. Chem. Phys. 2006, 331, 131–136. (16) Brown, R. D.; Godfrey, P. D.; Kleibo¨mer, B. J. Mol. Spectrosc. 1987, 124, 34–35. (17) Palmer, M. H. Chem. Phys. 1988, 127, 335–341.
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