Trends in the chlorine chemical shift (CS) tensors of amino acid hydrochlorides are investigated in the context of new data obtained at 21.1 T and extensive quantum chemical calculations. The analysis of chlorine-35/37 NMR spectra of solid L-tryptophan hydrochloride obtained at two magnetic field strengths yields the chlorine electric field gradient (EFG) and CS tensors, and their relative orientations. The chlorine CS tensor is also determined for the first time for DL-arginine hydrochloride monohydrate. The drastic influence of 1 H decoupling at 21.1 T on the spectral features of salts with particularly small 35 Cl quadrupolar coupling constants (C Q ) is demonstrated. The chlorine CS tensor spans (Ω) of hydrochloride salts of hydrophobic amino acids are found to be larger than those for salts of hydrophilic amino acids. A new combined experimental-theoretical procedure is described in which quantum chemical geometry optimizations of hydrogen-bonded proton positions around the chloride ions in a series of amino acid hydrochlorides are cross-validated against the experimental chlorine EFG and CS tensor data. The conclusion is reached that the relatively computationally inexpensive B3LYP/ 3-21G* method provides proton positions which are suitable for subsequent higher-level calculations of the chlorine EFG tensors. The computed value of Ω is less sensitive to the proton positions. Following this cross-validation procedure, |C Q ( 35 Cl)| is generally predicted within 15% of the experimental value for a range of HCl salts. The results suggest the applicability of chlorine NMR interaction tensors in the refinement of proton positions in structurally similar compounds, e.g., chloride ion channels, for which neutron diffraction data are unavailable. IntroductionThe availability of high-field solid-state nuclear magnetic resonance (NMR) spectrometers, e.g., those with 1 H resonance frequencies of 800 MHz, 900 MHz, and above, has created new opportunities for the study of quadrupolar nuclei (I > 1/ 2) with low resonance frequencies or large quadrupole moments, and for nuclei with both of these properties. For example, Stebbins et al. have described applications of 18.8 and 21.1 T solid-state 27 Al, 17 O, 39 K, and 35 Cl NMR spectroscopy to study oxide materials. 1-3 Gan et al. used magnetic fields as high as 40 T to achieve chemical shift resolution between four different aluminum environments in aluminoborate. 4 Ellis and colleagues have applied high-field 67 Zn NMR at low temperature to study zinc binding environments of biological relevance. 5,6,7,8 Magicangle-spinning (MAS) NMR in a magnetic field of 19.6 T has been applied by Wu and co-workers to detect potassium cations in guanine-quadruplex structures 9 as well as to explore cation-π interactions in alkali metal tetraphenylborates. 10 Cross and coworkers have demonstrated the utility of 17 O solid-state NMR at 21.1 T for characterizing the ion channel gramicidin. 11,12 The major reason that higher fields are so beneficial for the study of quadrupolar nuclei in ...

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The dipolar-chemical shift (CS) method has been applied to analyze the carboxyl-methylene carbon isolated spin pair in phenylacetic-13 C 2 acid and potassium hydrogen bisphenylacetate-13 C 2 . The span, Ω, of the CS tensor is decreased significantly for both carbon atoms in the potassium acid salt compared to the acid. The orientations of the carboxyl CS tensor and, more notably, the methylene CS tensor, have a marked dependence on the protonation state of the carboxyl group. Ab initio calculations [RHF/6-311G*, RHF/6-311++G(2d,-2p)] support the experimental findings. In addition to these studies, we demonstrate how rotational resonance (RR) NMR spectroscopy complements the dipolar-CS method in a study of the isolated 13 C spin pairs in phenylacetic-13 C 2 acid. In particular, the higher-order RR experiments provide a stringent check on the CS parameters and the dipolar coupling constant, R, derived from the dipolar-CS analysis. The dipolar-CS method, in combination with a two-dimensional spin-echo experiment, yields R ) 2150 ( 30 Hz for phenylacetic acid, whereas RR indicates that R ) 2100 ( 15 Hz. Although n ) 1 RR can be applied reliably to determine internuclear distances in the absence of CS tensor data, these data are critical for simulations of the n ) 2 RR effects. Specifically, longitudinal magnetization exchange curves are shown to be sensitive to slight rotations of the carboxyl carbon CS tensor about an axis perpendicular to the carboxyl plane, a phenomenon observed upon moving from the acid to the acid salt. Simulations indicate that altering the MAS rate by only a few tens of hertz can drastically alter the higher-order RR line shapes. The ab initio calculations of chemical shielding tensors provide data that are useful in the simulation of rotational resonance effects. We propose phenylacetic-13 C 2 acid as a setup sample for rotational resonance and other homonuclear dipolar recoupling experiments. IntroductionThe dipolar-chemical shift (CS) method is an effective NMR technique for studying isolated spin-1 / 2 pairs in stationary powdered solids. [1][2][3][4][5][6] Under favorable circumstances, when symmetry dictates the orientation of at least one component of one of the CS tensors, this method will yield the direct dipolar coupling constant (R DD ), the magnitudes of the principal components of the two CS tensors, the relative orientations of the CS tensors, and the orientations of these tensors with respect to the molecular axis system. 6 The fact that this orientational information is derived from a powdered sample makes the method particularly valuable when single crystals are not available. R DD is of interest because of its simple relationship with the distance, r 12 , between nucleus 1 and nucleus 2 (eq 1) where µ 0 is the permeability of free space and γ 1 and γ 2 are the magnetogyric ratios of the nuclei under consideration. It is important to recognize that r 12 is a motionally averaged distance. Complications in measuring very accurate values of r 12 using solid-state NMR will be di...

Despite the favorable NMR properties of 9 Be (I ) 3 / 2 ), NMR spectroscopy of this nucleus in the solid state remains comparatively unexplored, perhaps owing to the extreme toxicity of beryllium and its compounds. We present here an integrated experimental and theoretical study of the Be chemical shielding (CS) and electric field gradient (EFG) tensors in bis(2,4-pentanedionato-O,O′)beryllium [Be(acac) 2 ]. Interpretation of the 9 Be NMR data was facilitated by crystal X-ray diffraction results, which indicate two crystallographically unique sites (Onuma, S.; Shibata, S. Acta Crystallogr. 1985, C41, 1181). Beryllium-9 NMR spectra acquired at 4.7 and 9.4 T for magic-angle spinning (MAS) and stationary samples have been fitted in order to extract the nuclear quadrupole coupling constant (C Q ), asymmetry parameter (η), and isotropic chemical shift (δ iso ). The best-fit nuclear quadrupole parameters for the two sites were determined to be C Q (1) ) -294 ( 4 kHz, η(1) ) 0.11 ( 0.04; C Q (2) ) -300 ( 4 kHz, η(2) ) 0.15 ( 0.02. Our analyses of the stationary samples also reveal a definite anisotropy in the beryllium CS tensor and allow us to place upper and lower limits on the spans of 7 and 3 ppm. This is the first evidence for anisotropic shielding in beryllium. Ab initio calculations of the beryllium CS tensors in Be(acac) 2 at the RHF level indicate spans ranging from 7 to 9 ppm; this represents a substantial fraction of the total known chemical shift range for Be (<50 ppm). The calculated C Q s are also in good agreement with the experimental results. To put the Be(acac) 2 results in context, calculations of the beryllium CS tensors for a series of compounds encompassing the known range of 9 Be chemical shifts are also presented. The calculations are in outstanding accord with experimental data from the literature. On the basis of calculations for linear molecules, it is shown that the assumption that the 9 Be chemical shift is governed essentially by the diamagnetic term is erroneous. For some of these molecules, the calculated Be CS tensor spans are greater than the total known chemical shift range.

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35/37 Cl NMR spectroscopy studies of organic systems are very rare, with only a few neat liquids having been studied. [1] The lack of chlorine NMR spectroscopy data may be explained by the fact that 35 Cl and 37 Cl are quadrupolar (spin I = 3/2) and low-frequency isotopes. The quadrupole moments of the chlorine nuclei couple with the electric field gradient (EFG) tensor at the nuclei; this phenomenon is known as the quadrupolar interaction (QI). The quadrupolar coupling constant, C Q , and the quadrupolar asymmetry parameter, h Q , describe the magnitude and asymmetry of the QI. In solution, one of the consequences of the QI is fast relaxation, which means that the 35/37 Cl NMR signals for covalently bound chlorines are very broad and are of low intensity. [1] For these reasons, chemically distinct chlorine sites are very difficult to distinguish with solution NMR spectroscopy. However, in the solid state, nuclear spin relaxation is typically slower, thus enabling higher quality 35 Cl NMR spectra to be collected, at least in principle. Unfortunately, the magnitude of the QI for covalently bound chlorines is very large because of the substantial, anisotropic EFG at the Cl atom, owing mainly to its electronic configuration when it forms a chlorine-carbon bond. Conventional wisdom is that such chlorine sites cannot be studied in powders by solid-state NMR spectroscopy as the central transition (CT; m I = 1/2$À1/2) can span tens of megahertz in typical commercially available magnetic fields. For this reason, only ionic chlorides [2] and inorganic chlorides [3] have been studied, as the EFG at these chlorides is often an order of magnitude smaller than at covalently bound chlorine atoms in organic molecules. The bonding environments for these types of chlorine atoms are substantially different from the environments in those chloride-containing molecules that have been studied previously. [2, 3] A partial 35 Cl NMR spectrum for hexachlorophene has been briefly mentioned in the literature. [4] On the other hand, most of the interesting chlorine chemistry occurs when Cl is covalently bound to a carbon atom, where the chlorine atom often acts as a leaving group. Chlorine atoms are also important in many organic pharmaceuticals as well as in crystal design applications where they can form halogen bonds. [5] Recent studies show that covalently bound chlorine is also important in biological chemistry where, for example, the tryptophan 7-halogenase was found to selectively chlorinate tryptophan moieties. [6] Herein, we show that with the combination of an ultrahigh magnetic field (B 0 = 21.1 T) and the state-of-the-art WURST-QCPMG pulse sequence, [7] it is possible to acquire highquality 35 Cl NMR spectra of organic compounds that contain a covalently bound chlorine atom in powder samples in a reasonable amount of time. We have acquired 35 Cl NMR spectra of 5-chlorouracil (1); the pesticide 2-chloroacetamide (2); sodium chloroacetate (3); a,a'-dichloro-o-xylene (4); chlorothiazide (5), a diuretic pharmaceutical also k...

Many scientists seem to be unaware of the relatively simple relationship between nuclear spin-rotation tensors, C, and nuclear magnetic shielding tensors, σ, in linear molecules. The availability of accurate spin-rotation data from high-resolution microwave spectra or molecular beam experiments allows for meaningful comparison between nuclear magnetic shieldings derived from these experimental values and values determined from ab initio calculations, because both results are for "isolated" molecules. Presented here is an undergraduate-graduate-level exercise in the computational quantum chemistry of NMR parameters for a series of fluorine-containing diatomic molecules for which experimental spin-rotation data are available. Calculations carried out at the RHF/6-311++G(2d,2p) level demonstrate the relationship between Ramsey's paramagnetic term, σP, and C. In addition, electric field gradient calculations at this level of theory for Li, Na, K, and Cl in their fluorides qualitatively reproduce experimental trends in the nuclear quadrupolar coupling constants. This exercise will give students an appreciation of the tensorial nature of nuclear magnetic shielding and electric field gradients, an awareness of the relationship between nuclear spin-rotation tensors and nuclear magnetic shielding tensors, insight into the connection between electronic and molecular structure, and hands-on experience in computational chemistry. Implementation of the exercise in a fourth-year undergraduate/first-year graduate course on magnetic resonance was very successful.

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