1490
J. Phys. Chem. B 2000, 104, 1490-1493
Correlation of Deuterium Quadrupolar Couplings and Carbon-13 Chemical Shifts in Ordered Media by Multiple-Quantum NMR Dick Sandstro1 m* DiVision of Physical Chemistry, Arrhenius Laboratory, Stockholm UniVersity, S-10691 Stockholm, Sweden
Herbert Zimmermann Max-Planck-Institut fu¨ r Medizinische Forschung, AG Moleku¨ lkristalle, Jahnstrasse 29 D-69120 Heidelberg, Germany ReceiVed: NoVember 2, 1999
We report a simple and robust NMR approach capable of correlating deuterium quadrupolar splittings with carbon-13 chemical shifts in oriented materials. The technique, which is based on heteronuclear multiplequantum spectroscopy, results in two-dimensional maps that are very useful for the assignment of deuterium NMR spectra. The experiment has been employed to measure and assign all quadrupolar couplings produced by a perdeuterated liquid crystal.
1. Introduction Nuclear magnetic resonance (NMR) spectroscopy has established itself as a powerful method for investigations of systems possessing long-range orientational order.1-3 In particular, the deuterium quadrupolar interaction has proven to be an excellent probe of local order, structure, and dynamics.4-7 Its popularity derives from the fact that the 2H spins produce simple and intense spectra, and that the extraction of accurate quadrupolar couplings is straightforward. Deuterium NMR experiments are in most cases also easy to implement and perform. Unfortunately, the resonance assignment is often cumbersome for molecules containing many different 2H sites. This is mainly due to the low chemical-shift dispersion of the deuterons. In Figure 1 we illustrate this problem by showing the onedimensional (1D) 2H spectrum of the nematic liquid crystal 4-npentyl-4′-cyanobiphenyl (5CB-d19). Even though the quadrupolar couplings are readily estimated, the assignment of the various doublets to the nine nonequivalent 2H sites is far from trivial. One way to circumvent this dilemma is to employ site-specific labeling, i.e., to introduce only one or a few deuterons at a time. This is, however, synthetically demanding and it is advantageously if we instead can analyze 2H spectra obtained directly from perdeuterated material. Some time ago, Emsley and co-workers reported a twodimensional (2D) heteronuclear NMR technique which offers the potential of assigning the 2H quadrupolar frequencies in ordered media.8 The correlation of 2H resonances with 13C chemical shifts enables 2H-13C connectivities to be traced out and thus allows for the assignment of 2H signals on the basis of the 13C spectrum. This approach is without doubt very useful, but the experiment utilizes cross-polarization (CP) for magnetization transfer between 2H and 13C spins and is therefore not without problems. This is due to the 2H quadrupolar interactions which interfere unfavorably with the 2H-13C CP process. In particular, the broad distribution of 2H quadrupolar coupling constants often encountered in partially ordered systems makes * Corresponding author. E-mail:
[email protected].
Figure 1. (a) Chemical structure of 5CB-d19 showing the atomic labeling. (b) 61.4 MHz deuterium spectrum of 5CB-d19. The spectrum exhibits seven well-resolved quadrupolar splittings and some additional fine structure due to 2H-2H dipolar couplings.
standard Hartmann-Hahn matching difficult.9,10 The coherence transfer is considerably improved by sweeping the amplitude of one of the two radio frequency fields during the CP step.10 However, it is still difficult to obtain a flat magnetization-transfer profile, i.e., a situation in which the magnetization transfer between two nuclei is independent of the strength of the 2H quadrupolar interaction. Furthermore, the relatively weak 2H-13C dipolar couplings in liquid crystals require long contact times (typically tens of milliseconds) which may result in problems with 2H spin diffusion. Another technique for assigning 2H resonances in oriented materials is to correlate the 2H quadrupolar couplings with the 2H chemical shifts.11 However, this approach requires that all 2H shifts can be resolved and is, therefore, only feasible in studies of relatively small and weakly oriented molecules which produce narrow NMR peaks.
10.1021/jp9938765 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
Correlation of Deuterium Couplings and
13C
NMR Shifts
Figure 2. Pulse sequence for 2D 2H-13C heteronuclear multiplequantum spectroscopy in partially ordered media. The experiment correlates 2H quadrupolar frequencies during the evolution period t1 with 13C chemical shifts during the detection period t2.
To avoid the potential problems associated with 2H-13C cross-polarization discussed above, we introduce in this paper an alternative implementation of the carbon-deuterium correlation experiment. The technique uses heteronuclear multiplequantum (MQ) coherences and is demonstrated on a perdeuterated liquid crystal. 2. Heteronuclear Multiple-Quantum Spectroscopy The novel method that we have used is based on the standard HMQC12,13 pulse sequence shown in Figure 2. The 13C transverse magnetization created by the initial π/2 pulse is converted into heteronuclear zero- and double-quantum coherences by the 2H-13C dipolar coupling during τ, followed by the first 2H π/2 pulse. The multiple-quantum coherences evolve during the variable evolution period t1 under the 2H quadrupolar coupling, whereas unwanted 2H-13C dipolar couplings and 13C chemical shifts are refocused by the central 13C π pulse. The quadrupolar-modulated MQ coherences are reconverted into detectable 13C magnetization by means of the last 2H π/2 pulse and the 2H-13C dipolar coupling active during the second delay τ. The signal is finally observed during the detection period t2 as it evolves under the 13C chemical shift interaction and continuous wave 2H decoupling. This implementation is related to a recently introduced NMR technique for 2H-13C correlation spectroscopy of biological systems in the solid state.14 For the simple case of an ensemble of isolated 2H-13C spin pairs, a density operator analysis is straightforward.15 After the basic two-step phase cycle which involves signal subtraction, one obtains the following expression for the observable difference time domain NMR signal
∆S(t1,t2) ) S+(t1,t2) - S-(t1,t2) ∝ sin2 2ωDτ cos ωQt1 exp(iωCSt2) (1) where ωD, ωQ, and ωCS are the 2H-13C dipolar, 2H quadrupolar, and 13C chemical shift frequencies, respectively. Explicit expressions for these quantities can be found elsewhere.1-3 The subscripts + and - refer to the phase of the second 2H π/2 pulse (cf. Figure 2). A 2D signal matrix is built up by repeating the experiment for many different values of t1. Double Fourier transformation gives a spectrum in which the 2H quadrupolar splitting ∆ωQ ) 2|ωQ| in the first dimension (ω1) is correlated with the 13C chemical shift ωCS in the second dimension (ω2). We remark that only 13C spins coupled to 2H spins produce correlation peaks since the amplitude-modulated signal in eq 1 vanishes if ωD is identically zero or negligibly small. Note also that, in contrast to the CP method in ref 8, the magnetization-
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1491 transfer factor sin2 2ωDτ is a simple function of the 2H-13C dipolar coupling. The pulse sequence in Figure 2 does not achieve quadrature detection in the first dimension, i.e., the 2D spectrum will always be symmetric about ω1 ) 0. This is, however, not a problem in our case since the 1D 2H spectra themselves exhibit this symmetry to a very good extent (see Figure 1). The applicability of the HMQC approach relies on two factors. (i) The 1D 13C spectrum of the partially oriented molecules must be assigned. As previously reported, this can be accomplished by natural-abundance double-quantum 13C NMR.16,17 (ii) The 13C-2H coherence transfer during the excitation/reconversion time τ has to be effective over short distances only, i.e., over distances corresponding to one or two chemical bonds. This requirement is usually fulfilled since the dipolar coupling ωD is strongly dependent on the heteronuclear spin-spin distance r through its r-3 dependence. 3. Results and Discussion To demonstrate the usefulness of the method, Figure 3 shows a HMQC spectrum of 5CB-d19 acquired with a τ value of 200 µs. We chose this system because it has been extensively studied in the past.9,17-21 Thus, there exists a considerable body of data to compare our results with. It is tempting to believe that the deuterium-carbon connectivities are most easily traced out from the one-bond correlations. This is certainly true for the aliphatic part of the HMQC spectrum which exhibits five pairs of correlations produced by the five nonequivalent C2Hn groups in the alkyl chain. The assignment of the aliphatic deuterons is thus very simple. It is sometimes assumed that the 2H quadrupolar splittings should decrease monotonically along the chain toward the end methyl group. Clearly, this is not the case for 5CB-d19 since the 2H quadrupolar couplings in the aliphatic chain decrease in the following order: R, γ, β, δ, and ω. As discussed by others,8,9 the assignment of the aromatic region of the 2D 2H-13C correlation spectrum is slightly more complicated than that of the aliphatic part. For example, the cross-section through C3′ exhibits two well-resolved quadrupolar splittings. A similar situation prevails for C2′, which indicates that the one- and two-bond 2H-13C dipolar couplings (and thus the coherence-transfer efficiencies) between C3′-D3′ and C3′-D2′ (and between C2′-D2′ and C2′-D3′) are very similar to each other. This observation is supported by previous measurements of 1H-13C dipolar couplings in 5CB.19,21 In contrast, the cross-sections through the quaternary carbons contain only one quadrupolar splitting (except for the partial overlap in the ω2 dimension from signals produced by C4 and C1′). These peaks can only arise from two-bond correlations and are therefore extremely helpful in the assignment of the ring-deuteron couplings. The slice through C4′ identifies the D3′ quadrupolar signal directly, and the D2′ signal indirectly (by using the C2′ and/or C3′ slices). Hence, the C4′ correlation provides an unambiguous assignment of the two pairs of deuterons on the ring connected to the nitrile group. Similarly, the 2H quadrupolar coupling of D2 can be estimated from the cross-section through C1, and the D3 splitting from the C3 slice. The 2D spectrum in Figure 3 also exhibits a two-bond correlation between C4 and DR (the outer doublet in the C4,1′ slice). Table 1 summarizes the 2H quadrupolar splittings in 5CBd19 that we have measured and assigned by the HMQC approach. Note in particular that the splitting produced by 2H spins at position 3′ (i.e., adjacent to the nitrile group) is appreciably smaller than those from deuterons at 2′, 2, and 3.
1492 J. Phys. Chem. B, Vol. 104, No. 7, 2000
Sandstro¨m and Zimmermann
Figure 3. Two-dimensional 2H-13C HMQC spectrum of 5CB-d19 obtained by the pulse sequence in Figure 2 with τ ) 200 µs. The 2D data set consisted of 80 × 768 points, and for each t1 value 304 transients were recorded (total acquisition time of 24 h). The 2H decoupler field strength during the detection period t2 corresponded to a nutation frequency of 25 kHz.
TABLE 1: Experimental 2H Quadrupolar Splittings in 5CB-d19
b
position
splitting (kHz)a,b
R β γ δ ω 2 3 2′ 3′
51.9 (51.8) 35.4 (35.4) 37.9 (37.9) 25.4 (25.3) 18.4 (18.3) 12.3 (12.3) 11.9 (12.0) 11.4 (11.4) 9.0 (9.0)
a The accuracy of the 2H quadrupolar splittings is around (0.2 kHz. Values in parentheses are from ref 9 and were measured at 26 °C.
As discussed by Emsley et al.18 and Fung et al.,19 this can be explained by a small (
