Characterization of the 19F chemical shielding tensor using cross-correlated spin relaxation measurements and quantum chemical calculations

Chemical Physics Letters 489 (2010) 248–253

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Characterization of the 19 F chemical shielding tensor using cross-correlated spin relaxation measurements and quantum chemical calculations S. Begam Elavarasi a, Kavita Dorai b,* a b

Department of Physics, Indian Institute of Technology-Madras, Chennai 600 036, India Department of Physics, Indian Institute of Science Education and Research (IISER) Mohali, Chandigarh 160 019, India

a r t i c l e

i n f o

Article history: Received 14 January 2010 In final form 28 February 2010 Available online 3 March 2010

a b s t r a c t The 19 F chemical shift anisotropy (CSA) tensor is an indispensable structure estimation tool in the NMR investigations of flourinated biomolecules. This work focuses on the characterization of the 19 F CSA tensor in small molecules, through the combined use of quantum chemical methods and liquid-state NMR cross-correlated spin relaxation experiments. The effect of different basis sets and quantum computational methods on the magnitude and orientation of the 19 F CSA tensor are discussed. The results from ab initio methods and the liquid-state relaxation experiments match well and are comparable to values of the CSA tensor obtained from previous solid-state studies and from theoretical investigations of similar molecules. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Fluorine is becoming increasingly useful in chemical biology as a probe of biomolecular structure, dynamics and interactions [1,2]. The fluorine nucleus has a spin of I ¼ 1=2, a high magnetogyric ratio c (and hence a high NMR sensitivity), a 100% natural isotope abundance, wide chemical shift dispersion and strong dipolar couplings, which makes it an attractive candidate for biological NMR studies [3–5]. Fluorine NMR techniques have been employed in the biotransformation of fluorinated compounds [6], in screening libraries of potential ligands in the field of drug discovery [7] and to quantify rotational diffusion of proteins in cells in vivo [8]. There have been several studies using fluorinated amino acids as NMR probes in protein binding interactions [9–12] and fluorinated nucleoside analogs have been developed to investigate the mechanisms by which enzymes process DNA/RNA substrates [13–15]. 19 F NMR can also be used to obtain structural information about the alignment of antimicrobial peptides in membranes [16–18]. Many recent studies have shown that in addition to the isotropic chemical shifts, the CSA tensor of different nuclei are sensitive indicators of the electronic environment and can hence be used as probes of molecular structure and dynamics [19]. Solid-state 19 F tensor orientations can be obtained for single-crystal and powder samples using multipulse heteronuclear decoupling experiments and there have been several investigations of the fluorine CSA tensor using solid-state multipulse experiments [20–23]. Several liquid state NMR pulse sequences have been developed to measure * Corresponding author. Fax: +91 172 2790188. E-mail addresses: [email protected] (S.B. Elavarasi), kavita@ iisermohali.ac.in, [email protected] (K. Dorai). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.078

the cross-correlated relaxation rate between the CSA of a specific spin and its dipolar interaction with other spins [24–29], and such experiments have been used to extract magnitude and orientational information about the 19 F CSA tensor and to estimate dissociation rate constants of molecules that bind reversibly to protein targets [30–32]. Quantum chemistry calculations have become an important tool in the interpretation of experimental NMR spectra and many researchers have computed 19 F shielding constants using a variety of theoretical approaches [33–42]. This work is intended to develop a fluorine NMR strategy using liquid-state cross-correlated spin relaxation experiments in order to obtain useful information about the magnitude and orientation of the fluorine CSA tensor. The results of the cross-correlated spin relaxation experiments to measure the fluorine CSA tensor are compared with quantum chemical calculations. The 19 F CSA tensor principal axis system with respect to the molecular frame has been calculated with the gauge including atomic orbitals (GIAO) combined with DFT methods and gives tensor values that correlate well with experiment. There have been several investigations of the 19 F CSA tensor using ab initio methods and solid-state experiments however, there have been only a few studies that use liquid-state relaxation methods to compute the fluorine CSA tensor. While solid-state NMR experiments are a useful means of obtaining fluorine CSA tensor information, they are technically challenging to implement. Furthermore, single-crystal solid-state NMR experiments require the design and construction of special goniometer fluorine NMR probes for crystal mounting and improving stepwise rotation. Liquid-state CSA-DD cross-correlated spin relaxation experiments on the other hand, are easy to standardize and do not require special hardware for implementation. While this work focuses on small fluorinated molecules, it is expected that such studies will

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be useful in characterizing the biomolecules.

19

F CSA tensor in fluorinated

2. Theory When a spin is placed in an external magnetic field B0 , induced local magnetic fields lead to a shielding or de-shielding of the net magnetic field at the local site of the spin, resulting in frequency shifts in the NMR spectrum. In general, this interaction is tensorial in nature and hence the chemical shift anisotropy (CSA) can be mathematically represented as a tensor of rank 2. For isotropically tumbling molecules in solution, the CSA is averaged out to give an isotropic chemical shift. The CSA tensor characterizes the magnitude and the orientation dependence of the chemical shift and is represented by a 3  3 matrix r, where each element rij represents the ith component of shielding when B0 is applied along the jth axis. The antisymmetric part of the tensor is not observed experimentally and the symmetric part of the tensor is diagonalized in its principal axis system with the eigenvalues rii ; i ¼ 1; 2; 3 being the principal elements of the CSA tensor where by convention r11 < r22 < r33 , r11 being the least shielded tensor component. Any non-axially symmetric cartesian tensor T P expressed in its principal axis system can be decomposed into two axially symmetric tensors (T X and T Y ) [43,44]

TP ¼ TX þ TY 2 0 T 0 6 T X ¼ 4 0 T 0  DT 0 2

0

0

0 0

T  DT

0

T 00  DT 00 6 TY ¼ 4 0

3

0

0 T 00 0

0

7 5 3

0 0 00

T  DT

00

ð1Þ

7 5

where T 0 and T 00 are not unique but are chosen such that T 0 þ T 00 ¼ r11 þ r22  r33 . For relaxation mechanisms involving a well-defined relaxation axis the spectral density can be expressed in a simple fashion. For dipolar interactions and for axially symmetric CSA tensors the relaxation vector is simply the internuclear vector and the symmetry axis respectively. The above decomposition of a tensor without axial symmetry into two axially symmetric tensors hence leads to simplified calculations. The fluorine CSA tensor in coupled 19 F—1 H spin systems can be measured using liquid-state cross-correlated spin relaxation experiments. The orientations of the dipolar and CSA tensors relative to the external magnetic field are modulated by the overall molecular tumbling. These tensorial orientational fluctuations lead to local fluctuating fields that contribute to spin relaxation. Since the principal axes of the dipolar and CSA tensors transform identically under rotations (these relaxation mechanisms are both tensors of rank 2), their orientational fluctuations are correlated. Such CSA-DD cross-correlation effects manifest as differential relaxation rates for different transitions within a spin multiplet. When looking at the longitudinal relaxation of a fluorine spin F coupled to a proton H, the presence of CSA-DD cross-correlations between the CSA relaxation of the F spin and the F—H dipolar interaction means that multiplet transitions associated with the proton H in the spin up state will have relaxation rates different from those transitions with the proton H in the spin down state. The longitudinal relaxation equations for weakly coupled multi-spin systems of spin-1/2 nuclei in the Redfield relaxation matrix formalism can be recast in the magnetization modes picture [45,46]. The accumulation of longitudinal two-spin order is described by the CSA-DD cross-correlated relaxation of the concerned spins.

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The CSA-DD rate constant DFHF describes the cross-correlation between the CSA of the 19 F spin with the dipolar interaction between the fluorine and proton spins. The rate constant is given by [47,48]

DFHF ¼

2 l0 c2F cH hB0 sc ðDrg ÞF 5 4p r 3HF 1 þ x2F s2c

ð2Þ

where sc is the overall correlation time assuming isotropic molecular tumbling and ðDrg ÞF is the 19 F CSA parameter which contains both magnitude and orientation information (hence the symbol ‘g0 which denotes a geometric factor). All the other parameters in the above rate equation are constants, and the r HF distance is taken from the geometry optimized electronic structure calculation. It has been previously noted that the fluorine CSA tensor is not axially symmetric and can hence be written as a sum of two axially symmetric tensors with their symmetry axes oriented along two orthogonal directions (assumed here to be along x and y). The geometric CSA orientation parameter ðDrg ÞF in the above expression can thus be estimated as

ðDrg ÞF ¼ ðDrx ÞF



   1 1 ð3 cos2 /x  1Þ þ ðDry ÞF ð3 cos2 /y  1Þ 2 2

ð3Þ

where ðDri Þ; i ¼ x; y is the anisotropy of the axially symmetric shielding tensor with its symmetry axis along i and the angle /i refers to the orientation of the HF bond vector with respect to the symmetry axis of the CSA tensor oriented along the i axis. For an axially symmetric CSA tensor the geometric CSA orientation parameter ðDrg ÞF depends only on one angle / subtended by the HF internuclear vector and the symmetry axis of the fluorine CSA tensor. In most molecules the fluorine CSA tensor is highly asymmetric and for aromatic fluorine moieties, the usual model of the orientations of the CSA principal axes follow the solid-state determination of a fluoro-phenyl ring [49]. In this work, the 19 F CSA tensor also turns out to be highly asymmetric, with the most shielded principal axis being normal to the plane of the aromatic ring. The least shielded principal axis lies in the plane of the ring and is nearly perpendicular to the CF bond vector. Most spin relaxation studies use the basis of ‘magnetization modes’, many of which can be directly related to physical observables. For longitudinal relaxation, these modes are essentially linear combinations of the level populations. Various single- and multi-spin modes can be constructed for weakly coupled spins, and are classified as symmetric or antisymmetric depending on their parity under total spin inversion. In a product operator basis, this differential relaxation leads to cross-relaxation between the fluorine Zeeman single-spin order hFz i and longitudinal two-spin order h2Fz Iz i. The evolution of the modes has the same form as the Redfield relaxation matrix equation (which describes evolution of level populations)

d M ¼ CDM dt

ð4Þ

where DM denotes the deviation of the mode M from its equilibrium value and C is the relaxation matrix in the magnetization mode basis (the elements of C are linear combinations of different auto- and cross-correlation spectral densities). Symmetric and antisymmetric modes interconvert solely through cross-correlations between the chemical shift anisotropy and the dipolar relaxation mechanisms (CSA-DD cross-correlations). The single spin order mode hIiz i corresponds to the total magnetization of the ith spin, while the two-spin order mode h2Iiz Ijz i between the ith and jth spins can be observed as antiphase magnetization within a spin multiplet (after a selective inversion pulse on the spin of interest). Since we are interested in accurately measuring the CSA-DD cross-correlation rates between the 19 F CSA and the 19 F—1 H dipolar interaction,

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we use experimental pulse schemes that invert the single-spin order mode (the total fluorine magnetization) and monitor the subsequent buildup of the two-spin order mode. For systems with two magnetically equivalent spins (such as an A2X spin system for instance), the modes of interest are the symmetrized two-spin order mode h2ðAz þ A0z ÞXz i and the symmetrized four-spin order mode h4Az A0z Xz i. Since the A spins are magnetically equivalent, the other modes are not physically observable. 3. Materials and methods All experiments were carried out at 300 K on a Bruker DRX-500 NMR spectrometer operating at a proton resonance frequency of 500 MHz. Studies have been performed on five fluorinated samples, as prototypes of weakly coupled AX, AMX, AX2, A2MX, and A3X2 spin systems, respectively: 2,4-dichloro 5-fluoro benzoic acid (DCFBA, AMX) dissolved in CDCl3, 2,6-dichloro 4-fluoro phenol (DCFP, AX2) dissolved in CDCl3, 1,5-difluoro 2,4-dinitro benzene (DFDNB, A2MX) dissolved in CDCl3, 4-fluoro uracil (FU, AX) dissolved in D2O, and 1,1,1-trifluoro 2-iodo ethane (ITE, A3X2) dissolved in CDCl3. The samples were de-gassed and the NMR tubes sealed, to eliminate the possibility of impurities like dissolved oxygen acting as external sources of relaxation. While de-gassing (and the absence of residual dissolved oxygen in the sample) does not affect the CSA-DD cross-correlation rate, it has a dramatic effect on the T1 relaxation rates. Spectral data processing was done using TopSpin (Bruker Inc.). 3.1. Quantum chemistry Quantum chemical calculations were carried out using GAUSSIAN 03 [50] on a Linux workstation. Input structures were generated and visualized in GAUSSVIEW 3.0. The structures were optimized with Hartree–Fock and DFT levels of theory (with the Lee Yang and Parr correlation functional B3LYP method) using the 6  311 þ þðd; pÞ basis set and the 6  311 þ þð2d; pÞ basis set and the midix basis set. Both geometry optimization of the molecules as well as GIAO shielding calculations of the 19 F CSA tensor were performed at the same level of theory. It has been noted previously that the effects of electron-correlation on the calculation of chemical shielding tensors depend significantly on the level of detail of the calculations [33,35,37]. MP2GIAO methods underestimate the isotropic shift as well as the inplane components of the shielding, and overestimate the tensor element perpendicular to the ring plane. It has been shown that the use of density-functional theory (DFT) is not recommended for fluorine since the inclusion of electron-correlation in general showed an over-estimation of the chemical shielding tensor elements. Further, the estimated shielding constant is larger because of a larger number of surrounding electrons [33,37]. In systems where electron correlation is unimportant such as aminoacids or fluoroaromatics, Hartree–Fock (HF) methods perform better than Moeller–Plesset (MP) or DFT shielding calculations [33]. While the inclusion of dynamic electron-correlation is necessary for accurate geometry optimization, however as noted by previous workers, the MP2-GIAO and DFT-GIAO methods tend to overestimate the experimental 19 F chemical shifts as compared to HF methods. The orientation of the internuclear vector with respect to the x and y symmetry axes and the geometric CSA orientation parameter are obtained from the CSA tensor using a MATHEMATICA program. 3.2. CSA-DD experiment The 19 F magnetization is inverted with a p pulse and detected at different relaxation intervals. The fluorine spectra were recorded at

different recovery times and the evolution of the single- and the two-spin order modes of the weakly coupled spin systems obtained from the spectral analysis. Similar inversion recovery spectra have been obtained for all the five samples, with separate inversion of the fluorine and the proton nuclei (spectra not shown). We focus here on the analysis of the fluorine inversion recovery experiment since we are interested in the characterization of the fluorine CSA tensor. The unequal relaxation of different lines of the fluorine multiplet is an evidence of the emergence of multispin order and is a direct measure of cross correlations in the system. Hence, after an inversion of the 19 F spin magnetization, the buildup of the two-spin mode h2Hz Fz i quantifies the CSA-DD cross-correlation rate by estimating the derivative of the curve with respect to the delay time s, in the limit s ! 0 (the initial rate approximation). Longitudinal spin order is detectable by conversion to antiphase magnetization by a selective pulse on the spin of interest, or for homonuclear spins, by a non-selective pulse of flip angle less than 90°. Since our studies are confined to heteronuclear spin systems, we use detection pulses of flip angle 90°. Crosscorrelation rates were determined from the initial slope of buildup curves of multi-spin modes after a double-exponential fitting of the entire curve, the analytical first derivative at zero delay giving the buildup rate. 3.3. Estimation of correlation times The correlation time sc for each molecule is obtained from an independent experiment to measure the spin–lattice relaxation time (T1) of the carbon nucleus, assuming isotropic tumbling spectral densities. The 13 C spin–lattice relaxation times were obtained from an inversion recovery experiment with different recovery delay times. The values of the correlation times sc and the experimental CSA tensor were calculated using C–H bond lengths of 1.08 Å, obtained by ab initio computations in vacuo for each molecule separately. All experimental and theoretical chemical shifts were computed using hexafluorobenzene as a reference compound, with a value of diso ¼ 337:17 ppm. 4. Results and discussion The experimental spectra corresponding to various two-spin order modes are shown for the set of five fluoroaromatics in Figs. 1– 5. The multiplet effect i.e. differential line relaxation of different lines of the fluorine spin multiplet is clear evidence for the presence of CSA-DD cross-correlations. The spectra shown in Figs. 1– 5 are an illustrative selection as a pictorial depiction of the multiplet effect and do not reflect the entire dataset. The buildup of the two-spin order mode in the initial rate approximation quantifies the CSA (F)-DD (FH) cross-correlation rate. The values of the experimentally determined cross-correlation rates, the correlation times and the geometric CSA parameter thus obtained for all five molecules are summarized in Table 1. Although all samples were vacuum sealed after several freeze pump and thaw cycles, they show different T1s and CSA-DD rate constants. While the T1s do depend on the amount of residual dissolved oxygen in the sample, the cross-correlated spin relaxation rates do not have such a dependence. Therefore the observed differences in the geometric CSA parameter are not due to dissolved oxygen, but reflect real variations in the CSA tensor. The theoretical estimates of the angles subtended between the internuclear vector and the different symmetry axes and the theoretically computed geometric CSA orientation parameter values for all five molecules are summarized in Table 2. As seen from Tables 1 and 2, there is a quantitative agreement between the experimentally measured CSA parameter and the theoretically computed parameters to within a reasonable

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Fig. 1. The 19 F spectra of DCFBA molecule for different recovery times obtained from the spin-selective inversion recovery experiment. The differential relaxation of the fluorine multiplet shows the emergence and buildup of longitudinal two-spin order for this weakly coupled AMX spin system.

Fig. 2. The 19 F spectra of DCFP molecule for different recovery times obtained from the spin-selective inversion recovery experiment. The differential relaxation of the fluorine multiplet shows the emergence and buildup of longitudinal two-spin order for this weakly coupled AX2 spin system.

accuracy, except for the DFDNB molecule. Even for the DFDNB molecule, the trend of geometric CSA parameter being much larger than for the other molecules of similar geometry is captured in both the experiment and the corresponding theoretical calculation.

Table 1 Experimental CSA-DD cross-correlation rate constants (CC rate in sec1 ), correlation times sc in ðpsÞ and experimentally determined geometric CSA parameters ðDrg ÞF (in ppm) for 19 F in different fluoroaromatic molecules. Molecule

CC rate constant (s

FU (AX) DCFBA (AMX) DCFP (AX2) DFDNB (A2MX) ITE (A3X2)

1

)

0.143 ± 0.005 0.306 ± 0.03 0.124 ± 0.02 0.563 ± 0.007 0.050 ± 0.004

sc

Fig. 3. The 19 F spectra of DFDNB molecule for different recovery times obtained from the spin-selective inversion recovery experiment. The differential relaxation of the fluorine multiplet shows the emergence and buildup of longitudinal two-spin order for this weakly coupled A2MX spin system.

Fig. 4. The 19 F spectra of FU molecule for different recovery times obtained from the spin-selective inversion recovery experiment. The differential relaxation of the fluorine multiplet shows the emergence and buildup of longitudinal two-spin order for this weakly coupled AX spin system.

Given that previous studies have noted that DFT calculations tend to differ from experimental values by 10–20 ppm, we believe the present theoretical estimates of the fluorine CSA tensor can be improved upon, using experimental values as benchmarks. Further-

Table 2 Theoretical angles /x ; /y (in radians) between the CSA symmetry axes and the F–H bond vector, and the theoretical CSA parameter ðDrg ÞF (in ppm) obtained from quantum chemistry calculations for 19 F in different fluoroaromatic molecules. The geometry optimization and NMR-GIAO calculations were performed using the GAUSSIAN03 package at the DFT level of theory and with a large 6-311+(d, p) basis set.

ðDrg ÞF ðppmÞ

Molecule

ðpsÞ 15.29 8.39 3.36 6.29 23.09

19.8 73.32 74.23 189.3 4.45

FU (AX) DCFBA (AMX) DCFP (AX2) DFDNB (A2MX) ITE (A3X2)

/x

/y

(radians)

(radians)

ðDrg ÞF (ppm)

3.099 1.048 2.171 0.554 1.117

1.528 2.619 1.569 1.017 0.997

26.4 82.3 60.74 123.7 18.97

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Fig. 5. The 19 F spectra of ITE molecule for different recovery times obtained from the spin-selective inversion recovery experiment. The differential relaxation of the fluorine multiplet shows the emergence and buildup of longitudinal two-spin order for this weakly coupled A3X2 spin system.

more, the CSA-DD cross-correlation rate can be measured using a set of different multi-nuclear, multi-dimensional experiments and the results collated to obtain a more accurate characterization of the fluorine CSA tensor. Such studies are currently in progress in our group. The theoretical estimates of the principal axis elements of the fluorine CSA tensor as well as the computed isotropic chemical shift values for all five molecules are summarized in Table 3. The validation of the experimental measurements using theoretical quantum chemistry computations paves the way for evolving a standard experimental protocol for estimation of the fluorine CSA tensor using easily reproducible liquid-state cross-correlated spin relaxation experiments. For systems with magnetically equivalent fluorine spins, the CSA-DD cross-correlation rate of the two fluorine spins with their dipolar interaction with the nearest proton dominates the relaxation, with the other cross-correlation rates being extremely small. Previous solid-state experimental investigations of the fluorine CSA tensor in fluorobenzenes noted pronounced variations in the magnitude and orientation of the tensors upon chemical substitution as well as the existence of a noticeable ‘ortho effect’ in the principal axis values upon substitution of a phenol, toluene or another fluorine in an ortho position to the fluorine spin [20]. The experimentally determined and theoretF ically computed values of the ðDrg Þ (geometric CSA parameter) for 19 F in this study lie within the range of values found by the prior solid-state multiple pulse experimental studies and reflect similar trends and variations in the fluorine CSA tensor upon chemical substitution. 19 F CSA tensor elements have been computed at different levels of theory using the GAUSSIAN03 package. In general it is to be ex-

Table 3 Theoretical principal axis components of the full 19 F CSA tensor and the isotropic chemical shift values (in ppm) for different fluoraromatic molecules, obtained from quantum chemical calculations. The geometry optimization and NMR-GIAO calculations were performed using the GAUSSIAN03 package at the DFT level of theory and with a large 6-311+(d, p) basis set. Molecule FU (AX) DCFBA (AMX) DCFP (AX2) DFDNB (A2MX) ITE (A3X2)

r11

r22

r33

riso

(ppm)

(ppm)

(ppm)

(ppm)

292.27 219.26 229.34 173.11 227.73

322.15 283.11 314.96 228.37 299.06

409.05 367.8 347.13 408.26 353.93

341.16 290.06 297.15 269.91 293.57

pected that a higher level of theory and/or a denser basis set would yield better estimates of the tensor elements. However, previous studies have shown that the least computationally expensive in vacuo GIAO HF methods give better values of the tensor compared to Moeller–Plesset (MP) and density-functional theory (DFT) calculations and that DFT methods overestimate the 19 F shielding in fluorinated aromatic molecules [33,37]. This implies that the introduction of electron-correlation does not lead to enhanced accuracy in the 19 F CSA tensor computation. We have performed the NMR GIAO calculations using HF and DFT levels of theory and with three different basis sets ranging from less to more dense: 3-21+G*, 6-31+G (d, p) and 6-311++G (d, p) (Supplementary information). In general, our results corroborate previous computations, namely that HF methods lead to fairly good estimates of the 19 F CSA tensor. We also find that increase in density of the basis set for DFT calculations leads to an over-estimation of the values of the individual shielding tensor elements. It has been noted previously that the magnitude of the principal axis element r22 of the 19 F CSA tensor in fluoro substituted benzenes is unexpectedly quite sensitive to small changes in the C–F bond length and it was suggested that this might be due to a strong 19 F p-orbital interaction with the benzene ring. We have computed the fluorine CSA tensor for all the molecules using a range of C–F bond lengths. While we notice variations in the individual tensor elements with increasing bond length, we do not notice a significant correlation between bond length and the on the r22 element. In corroboration with previous results, isotropic chemical shifts do not vary much with varying C–F bond lengths (Supplementary information). 5. Conclusions An experimental protocol was evolved to characterize the fluorine CSA tensor in various weakly coupled 19 F—1 H spin systems. The cross-correlated spin relaxation (CSA-DD) rates between the fluorine spins and various attached protons were experimentally measured and used to obtain information about the magnitude and orientation of the fluorine CSA tensor. Different sets of magnetization modes were used to analyze the relaxation of the fluorine spin and the CSA-DD rate constants were obtained as the crossrelaxation components of the fluorine single-spin order and different longitudinal two-spin order modes. Quantum chemistry calculations of the fluorine CSA tensor were performed using HF and DFT levels of theory and a large basis set, in order to validate the experimental results. The experimental and computed CSA tensors for fluorine agree well to within a reasonable accuracy. Quantum chemical calculations were also performed to observe the effect of optimization of the CF bond length on individual CSA tensor elements and to compute the effect of different levels of theory on the orientation and magnitude of the fluorine CSA tensor. In conclusion, this work seeks to validate a method to compute the 19 F CSA tensor through a combination of quantum chemical calculations and liquid-state relaxation experiments. It is hoped that future investigations will use such a 19 F NMR strategy to explore the role of the fluorine CSA tensor as a reporter of structure and dynamics of fluorinated biomolecules. Acknowledgments This work is supported by the Department of Science and Technology Government of India, under Grant No. SR/FTP/PS-12/2007. The experiments were performed on the Bruker AVIII 500 MHz FT-NMR spectrometer at the SAIF IIT-Madras and on the Bruker DRX 500 MHz FT-NMR spectrometer at the NMRRC IISc Bangalore and the use of these facilities is gratefully acknowledged. We thank

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