The role of ?-type nonbonding orbitals for spin-orbit induced NMR chemical shifts: DFT study of13C...

The Role of p-Type Nonbonding Orbitalsfor Spin]Orbit Induced NMR ChemicalShifts: DFT Study of 13C and 19F Shifts

( )in the Series CF IF n = 0, 2, 4, 63 n

MARTIN KAUPP,1 OLGA L. MALKINA,2 VLADIMIR G. MALKIN3

1 Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany¨ ¨2 Computing Center and Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava,Slovakia3 Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia

Received 19 January 1999; accepted 21 April 1999

ABSTRACT: p-type nonbonding orbitals on heavy halogen or relatedsubstituents are largely responsible for significantly shielding spin]orbit-induced

Ž .heavy-atom effects on nuclear magnetic resonance NMR chemical shifts of theneighboring atoms. This suggestion has been examined and confirmed by

Ž . 13density functional theory DFT calculations on C shifts of trifluoromethylŽ .compounds CF IF n s 0, 2, 4, 6 , including both one- and two-electron3 n

spin]orbit corrections. Indeed, the ‘‘removal of iodine p-type lone pairs’’ uponoxidation leads to a dramatic reduction in the absolute values of the negative13 ŽC spin]orbit shifts along the first three members of the series y57, y29, and

.0 ppm for n s 0, 2, and 4, respectively . The inclusion of the spin]orbit effects ismandatory to reach even qualitative agreement between theoretical andexperimental trends. Analyses of the shift tensors provide further insight intothe spin]orbit effects. In particular, the orientation of the 13C shift tensors forCF I and CF IF is altered dramatically by spin]orbit coupling. Structural and3 3 2chemical shift predictions are made for the as yet unknown CF IF . Spin]orbit3 6

Correspondence to: M. Kaupp; e-mail: [email protected]

Contractrgrant sponsor: Deutsche ForschungsgemeinschaftContractrgrant sponsor: Fonds der Chemischen IndustrieContractrgrant sponsor: Slovak Grant Agency VEGA; con-

tractrgrant number: 2r4012r98Contractrgrant sponsor: REHErESFContractrgrant sponsor: European Commission

( )Journal of Computational Chemistry, Vol. 20, No. 12, 1304]1313 1999Q 1999 John Wiley & Sons, Inc. CCC 0192-8651 / 99 / 121304-10

p-TYPE NONBONDING ORBITALS

effects on 19 F shifts for fluorine atoms bound to iodine are much less pronouncedthan those for carbon shifts. This is related to the low fluorine s-character in thebonds, and thus to a less effective Fermi-contact mechanism. Q 1999 JohnWiley & Sons, Inc. J Comput Chem 20: 1304]1313, 1999

Keywords: density functional theory; NMR chemical shifts; p type nonbondingelectron pairs; spin-orbit coupling; trifluoromethyl-iodine compounds

Introduction

Ž .pin]orbit SO effects on nuclear magneticS Ž .resonance NMR chemical shifts have re-ceived increased interest during the past 4 years.The reason is that the accuracy and efficiency of ab

1 ] 4 5 ] 8 Ž .initio and density functional theory DFTapproaches to calculate NMR chemical shiftshas now arrived at a stage where the applicationto compounds containing heavier atoms is with-in reach. Thus, one has to deal with relativisticeffects—that is, both with scalar relativistic contri-butions and with SO coupling. While early workdealing with SO effects on chemical shifts wasdone at semiempirical levels of theory,9, 10 variousmore quantitative methods are now available tocompute SO corrections to nuclear shieldings,based on Hartree]Fock11, 12 or multiconfigura-tional SCF wave functions,12 or based on DFTapproaches.5, 13 ] 15

Comparison with experiment has clearly shownthat, in many cases involving heavy elements, theobserved shielding trends across a given series ofcompounds are due predominantly to SO effects

Žcaused by the heavy atom substituents see, e.g.,.refs. 5]14, 16]19 . A typical and well-studied ex-

ample is the frequently observed ‘‘normal halogenŽ .dependence’’ NHD of main group chemical shifts;

that is, a decrease of the shift in going from Cl toBr to I substituents.20 The NHD of main groupchemical shifts has been found to be an SO effectin all cases examined computationally.6, 9 ] 18 Be-cause we can now compute such SO contributionsat a quantitative level, it is obvious to ask whetherwe may also understand SO influences qualita-tively. This would enable us to develop simplemodels to be used by investigators for the inter-pretation of spectra. In addition, such qualitativemodels should also help to evaluate, prior to ex-plicit, potentially expensive quantum-chemical cal-culations of nuclear shieldings, whether SO effectshave to be considered for a specific chemical sys-tem of interest.

Toward this end, we recently made use of asimple analogy between the SO contributions to

Ž .chemical shifts SO shifts and the Fermi-contactŽ .FC mechanism of indirect spin]spin couplingconstants.17 Thus, the spin]orbit-induced elec-tronic spin polarization in the presence of an exter-nal magnetic field interacts with the nuclear mag-netic moments in a very similar fashion as this isknown for indirect spin]spin coupling. This anal-ogy had already been suggested implicitly in thelate 1960s by Nakagawa et al.,21 and we couldconfirm the validity of the concept by explicit DFTcalculations on a number of iodosubstituted com-pounds.17 Thus, for example, the 1H and 13C SOshifts in iodobenzene were found to exhibit closesimilarity to the corresponding reduced I]H andI]C spin]spin coupling constants. A Karplus-typerelationship was found to hold for three-bond SOshifts, and the s-character of bonding was found toplay an important role in the FC-based transfer ofthe SO-induced spin polarization to the nucleusobserved by NMR.17

One observation we frequently made in MOanalyses of the SO contributions for halide sub-stituents17 is that it is the p-type nonbonding MOscentered on the halogen that appear to be largelyresponsible for the negative SO contributions tothe shifts of the neighboring atoms. Similar conclu-sions had already been drawn by Pyykko et al.,¨

Ž .based on relativistic extended Huckel REX calcu-¨lations.10 This suggests that the removal or delo-calization of such p-type ‘‘lone pairs’’ should leadto a dramatic decrease in the SO shifts. In thepresent work, we examine this idea by DFT calcu-lations on the 13C and 19 F shifts in the CF IF3 nŽ .n s 0, 2, 4, 6 series of molecules, which differ inthe formal number of p- and s-type nonbondingorbitals on iodine. The particular choice of com-pounds has been motivated by the recent experi-mental work of Tyrra et al.,22 who examined indetail the 13C and 19 F spectra for the first three

Ž .members of the series i.e., for n s 0, 2, 4 . We willexamine the influence of SO effects on chemicalshift anisotropy, noting in particular how the ori-

JOURNAL OF COMPUTATIONAL CHEMISTRY 1305

KAUPP, MALKINA, AND MALKIN

entation of the shift tensor may be changed by SOcorrections.

Computational Methods

SHIELDING CALCULATIONS

The nuclear shielding calculations were carriedout at the sum-over-states density functional per-

Ž .turbation theory SOS-DFPT level in its Loc1 ap-proximation,5, 23 using a modified version of theDeMon-KS program24 augmented by the DeMon-NMR code.5 All calculations of SO corrections tothe nuclear shieldings, following the third-orderperturbation theory approach of refs. 13 and 14,have been obtained with the Perdew]Wang ex-change functional25 and the Perdew correlation

Ž . 26 Žfunctional PP , whereas the uncorrected nonrel-.ativistic chemical shifts were calculated with the

Perdew]Wang exchange-correlation functionalŽ . 27PW91 . Further computational details can beseen in refs. 13, 14, 16]18. This choice of differentexchange-correlation potentials has been our usualprocedure in such calculations.6, 7 It is based onour general experience that the PW91 functionalperforms somewhat better than other gradient-corrected functionals for nonrelativistic main groupchemical shifts, whereas the PP functional is supe-rior to other gradient-corrected functionals forproperties involving Fermi-contact interactions.5

Test calculations on some of the present systemswith the PP functional also for the nonrelativisticshifts indicate that this functional gives systemati-cally larger values by ca. 3]6 ppm for both 13C and19 F shifts and thus would produce a slight deterio-ration of the results with respect to experiment.

We have mainly used the basis set BII of2 ŽKutzelnigg et al. also known as IGLO-II; f-func-

. 13tions on iodine were omitted . For C shifts, whichare our primary focus, we also used, for compari-

Ž .son, a very large, fully uncontracted ‘‘UP’’ basisderived from Partridge’s basis sets,28 with sizes

Ž . Ž . Ž . Ž16s4p H , 18s13p4d C,F , and 27s20p14d I cf..ref. 13 for details . While all six Cartesian compo-

nents of the d-functions were kept in the nonrela-tivistic shielding calculations, the s-like contami-nant was dropped in the calculations of the SOcorrections, for compatibility with the SO integralcodes. Auxiliary basis sets for fit of the chargedensity and exchange-correlation potential were of

Ž . Ž . Ž .the sizes 5,1 for H, 5,2 for C and F, and 5,5 forŽI n,m denotes n s-functions and m spd-shells

24 .with shared exponents . A FINE angular grid

Ž .with 32 for calculation of chemical shifts or 64Ž .for SO corrections points of radial quadra-ture5, 24, 29 was employed. The initial finite pertur-bation for the FC term was taken to be l s 0.001.

We compared calculations in which only theone-electron SO operators were included to thosethat also incorporate the two-electron SO terms viaa recently implemented14 mean-field, one-centerapproximation. The IGLO choice of gauge origin2

was used for the nonrelativistic shielding calcula-tions, whereas a common gauge origin at the io-dine nucleus was used to compute the SO correc-

Ž .tions. We neglected the spin]dipolar SD contri-bution to the hyperfine terms. For light atomshieldings, the SD term has previously been foundto typically contribute less than ca. 2]3% of the FCterm.11, 12 We note in passing that the present cal-culations of the SO terms employ basis sets of up

Ž .to 981 basis functions CF IF with the UP basis ,3 6without any use of symmetry. The calculation ofthe two-electron SO integrals for such large basissets has recently become possible through themean-field and one-center approximation used,which has been shown to perform very well in thiscontext.14

Computed absolute 13C shieldings have beenconverted to chemical shifts relative to tetrameth-

Ž .ylsilane TMS using absolute shieldings computedŽ .for TMS with the BII basis 187.5 ppm and with

Žthe UP basis 182.4 ppm; this value was obtainedvia the computed shielding of 189.4 ppm for the

.secondary standard CH , respectively. Because4the absolute shielding for the usual 19 F standardCFCl appears to be difficult to compute accu-3rately, the conversion of computed absolute fluo-rine shieldings to relative shifts used the experi-mental value of "195.6 ppm for the absolute 19 Fshielding in CFCl .30

3

SPIN]SPIN COUPLING

Ž .The Fermi-contact FC contribution to indirectspin]spin coupling constants has been computedusing the finite-perturbation theory approach ofref. 31. These calculations neglect any relativistic

Žcorrections which are expected to be notable for.couplings to iodine , and we do not consider or-

Žbital or spin]dipolar contributions which may be.important for couplings to fluorine . Therefore, the

results should be viewed as semiquantitative atbest, and are used here only to examine the quali-tative analogy between SO shifts and FC spin]spincoupling as detailed in ref. 17. These calculationswere restricted to the BII basis set. Integration

VOL. 20, NO. 121306


grids and other computational parameters werethe same as those discussed earlier for the calcula-

Žtions of SO shifts including the choice of l s.0.001 , but with the initial finite perturbation on

iodine rather than on the NMR nucleus in questionŽtest calculations showed that the results essen-

.tially do not depend on this choice .

STRUCTURES

As experimentally determined structures wereonly available for CF I and CF IF , all structures3 3 4used for the shielding calculations have, for consis-tency, been optimized at the Hartree]Fock level,32

using quasirelativistic effective-core potentials, to-Ž . w x Ž .gether with 4s4p1d r 2s2p1d and 5s5p1d r

w x3s3p1d valence basis sets for carbon and halogen,33 Ž . w xrespectively. A 4s1p r 2s1p basis was em-

ployed for hydrogen.34 The optimization resultsare shown in Figure 1, together with the availableexperimental data35, 36 and with the atom labels

Žused later optimizations for CF I and CH I were3 3done in C symmetry, those for the other CF IF3v 3 n

.compounds in C . After completion of the presents

work, an X-ray diffraction study of solid CF IF3 2

was reported,37 and thus we may now comparethe calculated structures with experiment for all

Žspecies, except for the as-yet-unknown CF IF Fig.3 6.1a]c, experimental values are in parentheses .

Agreement between theory and experiment is gen-erally good, with the largest deviations in bond

˚ Žlengths at around 0.02 A except for those bondsbelonging to groups that are disordered in the

.X-ray structure, see Fig.1 . Note that the solid-statestructures of both CF IF and CF IF exhibit no-3 2 3 4

ticeable I—F . . . I intermolecular contacts.36, 37 Thiswill be of importance in the discussion of chemicalshifts for the fluorine atoms involved in these

Ž .contacts. The structure shown for CF IF Fig. 1b3 2

is a minimum on the potential energy surface.However, at this computational level, the transi-tion state for CF group rotation, which also has C3 s

symmetry, is only 1.4 kJrmol higher in energy.This indicates almost free rotation, consistent withthe disorder found in the solid-state structure.37

We discuss only averaged chemical shift values forthose fluorine atoms that are made equivalent bythe rotation. For CF IF , the computed rotational3 4

FIGURE 1. Optimized structures and atom labels used in the discussion. Experimental data are given in parentheses( ) ( ) 35 ( )in italics . a CF I. Experimental data obtained from microwave spectroscopy in the gas phase. b CF IF .3 3 2

37 ( )Experimental data from solid-state X-ray diffraction. Experimental C } F bond lengths of disordered CF groups3˚ ˚ ( )range from 1.235 A to 1.352 A, F } C } I angles from 106.28 to 112.98. c CF IF . Experimental data from solid-state3 4

36 ( )X-ray diffraction. Experimental C } F bond lengths with significant thermal motion of the fluorine atoms range from˚ ˚ ( )1.25 A to 1.28 A VF } C } I angles from 1108 to 1128 d CF IF .3 6



barrier is even lower, probably below the numeri-cal accuracy permitted by the numerical integra-tions used in the DFT calculations.

Results and Discussion

13C SHIFTS

Table I summarizes the 13C shifts obtained withboth the UP and BII basis sets, with and withoutSO corrections. The experimental data available for

Žthe first three members of the CF IF series n s 0,3 n.2, 4 are included as well. Figure 2 provides a

graphical comparison of computed and experi-mental data, and it also shows schematically theremoval of iodine lone pairs with increasing n.

Obviously, the experimentally observed in-crease of the 13C shifts with increasing oxidation of

Ž .the IF substituent i.e., with increasing n is duento the diminishing SO effects from n s 0 throughn s 4. The nonrelativistically calculated shifts de-crease somewhat from n s 0 through n s 4 and

Žincrease very slightly from n s 4 to n s 6 a simi-lar trend is found for the fluorine shifts of the CF3groups, and the reasons for this will be discussed

.in the next subsection . The remarkably large SOcorrections invert this trend up to n s 4, and thusthe experimental behavior is recovered. We seeslightly better agreement with experiment whenusing the larger UP basis set. The nonrelativistic

Ž .value for CF IF is improved increased with the3 4Žlarger basis the SO contributions being small in

.this case . In addition, the larger SO contributionsincrease the slope of the curve somewhat and thus

Žalso improve the agreement with experiment thelargest discrepancies still occur for CF I, which3

.also shows the largest SO shifts . However, theoverall trends are already reproduced satisfactorilywith the smaller BII basis.

FIGURE 2. Comparison of computed and experimental13C shifts. SO-corrected data include both one- and

( )two-electron contributions cf. Table I . BII and UP basisresults are compared. Experimental data in acetonitrilesolution.22 At the bottom of the figure, the presence ofiodine lone pairs is shown schematically.

As expected, the two-electron SO contributions13 Žreduce the negative C SO shifts by ca. 7]9% UP

.basis results for those cases with significant SOŽ .effects CH I, CF I, CF IF . Similar results have3 3 3 2

been obtained previously for CH I and HI at both3DFT14 and MC-SCF levels12 of theory. Larger rela-

Žtive two-electron contributions but, of course,.smaller absolute ones are found with lighter halo-

gen substituents.12, 14

At the UP basis level, the SO contributions ofŽy27.4 ppm in CF IF including both one- and3 2

.two-electron SO terms are only half those of CF I3Ž .y57.4 ppm , and those in CF IF and CF IF are3 4 3 6negligible. These results suggest, to a first approxi-mation, that the SO contribution of one p-typelone pair on iodine to the 13C shifts in these sys-tems is roughly ca. y27 ppm, whereas the contri-

Žbutions from the s-type lone pair are small cf..schematic orbital description in Fig. 2 .

TABLE I.Computed and Experimental Isotropic 13C Shifts.a

bd 1-el.-SO 1 + 2-el.-SO d d Exp.nonrel. +1-el.-SO +1+2-el.-SO

( ) ( ) ( ) ( ) ( )CH I +1.3 y1.1 y35.0 y29.0 y32.6 y27.0 y33.7 y30.1 y31.3 y28.3 y21.83( ) ( ) ( ) ( ) ( )CF I +146.9 +141.3 y62.1 y50.0 y57.4 y46.6 +84.8 +91.3 +89.5 +94.7 +79.53( ) ( ) ( ) ( ) ( )CF IF +140.0 134.6 y30.1 y25.9 y27.4 y23.7 +109.9 +108.7 +112.6 +110.9 +106.73 2( ) ( ) ( ) ( ) ( )CF IF +132.9 +128.1 y0.2 y2.0 y0.2 y1.5 +132.7 +126.1 +132.7 +126.6 +131.23 4( ) ( ) ( ) ( ) ( )CF IF +134.8 +129.6 y1.2 y1.1 y0.7 y0.7 +133.6 +128.5 +134.1 +128.9 }3 6

aIn ppm vs. TMS. UP basis results with BII basis results in parentheses.bIn acetonitrile solution.2 2

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This preliminary picture is borne out by ourŽcanonical MO analyses of the SO shifts at the UP

.basis level , with only minor modifications: In CF I,3Žthe degenerate HOMO corresponding to the two

.p-type nonbonding orbitals on iodine accounts forthe entire SO shift of ca. y57 ppm, with severalsmall contributions from other MOs canceling each

Žother. In CF IF , the dominant contribution ca.3 2.y23 ppm of ca. y27 ppm comes again from the

HOMO, which corresponds indeed to the singlep-type nonbonding electron pair on iodine. Amongthe remaining, smaller contributions, one MO withpredominantly p-type lone pair character of thefluorine atoms within the CF group accounts for3

Žca. y9 ppm some still smaller, positive contribu-.tions arise from several other orbitals . For CF IF ,3 4

the HOMO corresponds to the remaining G-typeŽlone pair on iodine with some I]C bonding char-

.acter . This MO accounts for an SO shift of ca."9 ppm. This positive shift is almost exactly com-pensated by negative contributions from two lowerlying MOs with significant fluorine p-type lone

wpair character of the CF group and with some3Ž .additional antisymmetric s I—F bonding charac-

xter within the IF unit . No individual MO contri-4butions of more than ca. 3]4 ppm to the SO shiftsare apparent for CF IF , for which the overall SO3 6shifts are almost negligible.

We note, in passing, that the overall SO contri-Ž .butions in CF I y57.4 ppm are almost twice as3

Ž .large as those in CH I y32.6 ppm , although the3fluorine lone pair contributions can account for, atmost, a few ppm. As discussed in ref. 17, theelectronegative fluorine substituents in CF I with-3draw charge from the central carbon atom andthus increase the carbon 2s character of the I—Cbond. This enhances the FC mechanism for thetransfer of spin polarization and thus the overallSO contributions significantly.

We may also ask about the influence of the SOcontributions on the anisotropy of the shifts. Early

Ž . 10relativistic extended Huckel REX calculations¨of the 1H shifts in hydrogen iodide have shownthat the SO contributions arise entirely via thetensor components perpendicular to the threefold

Ž .symmetry axis d , with no contribution to theHŽ .parallel component d , due to symmetry reasons.5

The shift anisotropy is therefore altered dramati-cally. Similar conclusions were drawn for the 13Cshifts of the methylhalides.10

In the case of CF I, we also have axial symme-3try of the shift tensor and thus may still obtain areasonably straightforward interpretation. Thus, at

Ž .the nonrelativistic level UP basis , the parallel

shift tensor component is d s 129.0 ppm, and the5

Žperpendicular components d s 156.0 ppm withinHa localized MO framework, the paramagneticshielding contributions to d and d arise largely5 H

.from C—F and I—C bonding MOs . This pictureŽis altered significantly by the SO shifts one- and

.two-electron SO contributions included , resultingin d s 126.3 ppm and in d s 71.3 ppm; that is,5 Hthe SO contributions reduce the shifts predomi-

Ž 1nantly but not exlusively as for the H shifts in.the linear molecule HI via a reduction of d byH

84.7 ppm. Qualitatively similar results have alsobeen found at the REX level for CH I,10 and this3behavior is exactly what is expected from an SO-induced coupling between the iodine p-type non-

Ž .bonding MOs and a s I—C antibonding MO, asŽfound in our MO analyses of the SO shifts the

small SO contributions of y2.7 ppm to d arise5

.from fluorine lone-pair-type MOs . The early REXŽ .analyses for HI or for CH I further suggest a3

significantly deshielding SO contribution from theŽ .I—H or I—C s-bonding MO, which partially

compensates the shielding contributions from thep-type lone pairs.10 However, in our MO analysesat the DFT level, such s-type nonbonding MOcontributions are at least one order of magnitudesmaller than those arising from the iodine p-typelone pairs.

The description is already more complicated forŽCF IF , where we have only C symmetry cf. Fig.3 2 s

. Ž .1b . At the nonrelativistic level UP basis , thecomputed anisotropy of the shift is small, withd s 146.7 ppm, d s 136.8 ppm, and d s 136.411 22 33ppm. Figure 3a shows the orientation of the corre-

Ž .sponding absolute shielding tensor s relative tothe molecular framework, and the Cartesian axes

Žused note the different sign conventions for s.and d . s points along the x-direction; that is, it11

is perpendicular to both the p-type lone pair oniodine and to the I—C bond vector. The other twocomponents arise from linear combinations ofcomponents lying in the yz plane. In contrast, theSO contributions are essentially diagonal in theCartesian axes system shown, with the largest con-

Ž .tribution s SO s q79.5 ppm, as well asx xŽ . Ž .s SO s y4.0 ppm and s SO s q6.6 ppm.y y z z

As a result, the SO-corrected shift tensor s hasd s 142.0 ppm, d s 130.1 ppm, and d s 65.911 22 33ppm. The corresponding absolute shielding tensors is shown in Figure 3b. Due to the large SOcontribution to s , the latter now correspondsx xessentially to s . s is now close to the y-direc-33 11tion, and s points almost exactly along the z-22axis. The large, negative SO contribution to s isxx



FIGURE 3. Schematic description of the orientation ofthe 13C absolute shielding tensor in CF IF relative to the3 2molecular framework, with a Cartesian axis system used.

( ) ( )UP basis results: a nonrelativistic calculation; b one-and two-electron SO corrections included.

completely consistent with a coupling between theŽremaining single iodine p-type lone pair oriented

. Ž . Žalong z and a s I—C antibonding MO oriented.along y . This is confirmed by our MO analyses.

Shift anisotropies are small for CF IF and CF IF ,3 4 3 6and they are not influenced very much by SOeffects. Thus, we omit a presentation of detailedanalyses of the shift tensors for the latter twosystems.

In ref. 17, we stressed the usefulness of ananalogy between the FC mechanism for the trans-fer of SO-induced spin polarization and that ofindirect spin]spin coupling constants. It wasshown that structural and electronic changes in theenvironment of the NMR nucleus probed affect SOshifts and reduced spin]spin coupling constants ina similar way. Figure 4 compares the 13C SO shiftsand the FC part of the reduced I—C coupling

1 Ž .constants K I,C for CH I, and for the CF IFFC 3 3 nseries of molecules. In going from CH I to CF I,3 3the two quantities obviously correlate quite well.However, starting with CF IF , we change the3 2direct electronic environment of the heavy atom

Žsubstituent by ‘‘removing’’ iodine nonbonding or-.bitals . Consequently, the analogy ceases to hold.

In particular, while the SO shifts are already negli-gible for both CF IF and CF IF , due to the lack of3 4 3 6p-type lone pairs on iodine, the reduced couplingconstants still change significantly, from q160.71019 NAy2 my3 for CF IF to q743.6 1019 NAy2

3 4

FIGURE 4. Comparison of SO contributions to 13Cshielding constants, s , and Fermi-contactSOcontributions to the reduced coupling constants,1 ( )K I } C . BII basis results.FC

my3 for CF IF . The removal of the remaining3 6s-type nonbonding electron pair in going fromCF IF to CF IF does not affect the SO shifts3 4 3 6much. However, the iodine s-character of the I—Cbond is enhanced significantly. MO analyses clearlyindicate that the iodine s-type nonbonding MOprovides a significant negative contribution to thereduced coupling I—C constants in CF IF but is3 4not much involved in the 13C SO shifts. Obviously,such effects are not covered by the analogy inref. 17.

19F SHIFTS

Table II summarizes computed and experimen-tal 19 F shifts obtained with the BII basis. Com-puted values have been averaged for nuclei thatare made equivalent by rapid rotation of the CF3groups. We start with the CF groups, where SO3corrections are essentially negligible in all cases.The calculations overestimate the measured shiftsby ca. 10]25 ppm, but reproduce the observed

Žtrends reasonably well see Fig. 5; the computed.slope is too negative . The shifts decrease from

CF I to CF IF , but increase very slightly from3 3 4CF IF to CF IF . A similar behavior is exhibited3 4 3 6

13 Žby the nonrelativistically calculated C shifts see.previous section and Fig. 2 .

Part of the trends is due structural changesalong the series. Upon oxidation of the iodinecenter, the coordination number around iodine in-creases in steps of two and, correspondingly, the

Ž .I—C bond expands Fig. 1 . As a consequence, theŽ .C—F bonds contract also Fig. 1 . We can examine

the effect of these structural changes by keepingthe structure of the CF I fragment frozen to that of3

19 Ž .CF I itself. We obtain F CF shifts of q17.9,3 3

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TABLE II.Computed and Experimental Isotropic 19F Shifts.a

bMolecule Group d 1-el.-SO 1 + 2-el.-SO d d Exp.nonrel. +1-el.-SO +1+2-el.-SO

CF I +17.9 y0.1 +0.3 +17.8 +18.2 y7.53CF IF CF y17.6 y0.6 y0.3 y18.2 y17.9 y34.83 2 3

IF y89.0 +4.3 +4.2 y84.7 y84.8 y176.42CF IF CF y49.7 y0.6 y0.3 y50.3 y49.4 y57.93 4 3

IF +33.2 y0.9 y0.8 +32.3 +25.8 y36.54CF IF CF y30.2 +0.3 +0.6 y29.9 y29.6 }3 6 3

( )IF ax. +206.8 y7.7 y6.9 +199.1 +199.9 }6( )IF eq. +92.3 y8.4 y7.6 +83.9 +84.7 }6

aIn ppm vs. CFCl . BII basis results. The shifts for the CF groups, the IF group in CF IF , the IF group in CF IF , and the3 3 2 3 2 4 3 4equatorial atoms of the IF group in CF IF have been averaged.6 3 6bIn acetonitrile solution.2 2

y9.8, y36.8, and y14.0 ppm for n s 0, 2, 4, 6,respectively, compared with q17.9, y17.6, y49.7,and y30.2 ppm for the fully optimized structures.Thus, the changes become less pronounced. Simi-

13 Žlarly, the C shifts nonrelativistic basis BII re-.sults vary only 141.3, 137.6, 131.3, and 133.0 ppm

for the partly frozen structures, compared with141.3, 134.6, 128.1, and 129.6 ppm for the fully

Ž .optimized structures also see Tables I and II .However, further effects beyond these struc-

tural influences are obviously taking place, whichare more difficult to analyze. It appears that it ismainly the destabilization of the C—F s *-typeMOs with increasing n, which reduces paramag-

19 Ž . 13netic contributions to both F CF and C shifts.3Ž . Ž .We analyzed the localized occupied MO LMO

contributions to the nonrelativistic 19 F shieldingsŽ .calculated at the DFT-IGLO level . Of those LMOscoupled by the magnetic-field perturbation to theaforementioned C—F s *-type virtual MOs, it is

FIGURE 5. Comparison of computed and experimental19 ( )F CF shifts. The calculations include both one- and3two-electron SO corrections, using the BII basis.Experimental data in acetonitrile solution.22

mainly the fluorine lone pair LMO contributionsthat are rendered smaller. An inversion of thetrend is found from CF IF to CF IF , where all3 4 3 6

contributions increase slightly. In the case of the13C shifts, the major contributions that decreasewith increasing n, are due to the I—C bondingand iodine lone pair LMOs.

We now turn to the fluorine atoms bound di-rectly to iodine. The computed SO correctionsrange from q4.8 ppm in CF IF to y7.6 ppm in3 2

ŽCF IF two-electron contributions included, see3 6.Table II . These relatively small one-bond SO shifts

contrast markedly with the much larger ones dis-cussed earlier for the 13C shifts. The reason is that,in contrast to the significant carbon s-character ofthe I—C bonds, the I—F bonds have rather littlefluorine s-character, due to the concentration ofs-character in the fluorine s-type lone pair. As

Ž .discussed previously see also Introduction , alarge s-character is necessary to allow an efficientFermi-contact mechanism for the transfer of thespin]orbit-induced spin polarization to the NMRnucleus.17 As fluorine uses relatively little valences-character in most bonding situations, we expectthat fluorine SO shifts will generally not be verypronounced. In contrast, hydrogen shifts have beenfound to be particularly sensitive to SO effects, asthe hydrogen s-character predominates the bond-ing.17, 19

Agreement with experimental shifts is poor forthe fluorine atoms bound to iodine. The computed

Ž .shifts with SO corrections are too large by ca. 91ppm for CF IF , and by ca. 60 ppm for CF IF .3 2 3 4

These discrepancies are outside the expected sys-tematic errors of the computational methods used.We suspect that they are due to specific inter-



molecular interactions of the IF groups, eithernwith other solute molecules or, most likely, withthe acetonitrile solvent. In the solid-state structuresof both compounds, significant intermolecularI—F . . . I interactions are apparent.36, 37 The largerdeviation from experiment for CF IF would then3 2also be consistent with the expected larger I—F

Žbond ionicity as confirmed by our population.analyses , and thus with particularly pronounced

solvent coordination for this molecule. Both thecoordination of solvent nitrogen donor atoms toiodine or of solute IF fluorine donors to the sol-nvent might be envisioned. Computational studieson model complexes may shed light on this ques-tion but are beyond the scope of the present work.It is clear that the 19 F and 13C shifts of the CF3groups will be much less affected by these inter-molecular interactions, as borne out by the results.

Conclusions

This case study has provided further under-standing of the mechanisms of spin]orbit-inducedheavy atom effects on NMR chemical shifts. Theearly suggestion of the importance of p-type lonepairs on heavy halogen or related substituents byPyykko et al.10 has been confirmed. By studying a¨series of closely related molecules that differmainly in the number of p-type and s-type lonepairs on iodine, the dependence of theshiftrshielding tensors on SO-induced couplingsbetween occupied and virtual MOs has been clari-fied. We note, in particular, that the SO correctionsmay be strongly anisotropic and can thus altersignificantly the orientation of the shift tensor, asshown here for CF I and CF IF .3 3 2

We have previously emphasized the importanceof an efficient Fermi-contact-type mechanism forthe transfer of SO-induced spin polarization to thenuclei within the system.17 The present study hashighlighted that the efficient coupling of suitablehigh-lying occupied and low-lying unoccupied

ŽMOs by the other two perturbations involved SO.coupling and external magnetic field is also neces-

sary for significant SO chemical shifts. We alsoshowed that the analogy between SO shifts andindirect spin]spin coupling constants delineatedin ref. 17 will be less useful when changing signifi-cantly the electronic structure of the direct neigh-borhood of the heavy atom ‘‘spin]orbit centers.’’

Although the inclusion of SO effects is obvi-ously mandatory if one intends to reproduce the

experimentally observed trends of 13C shifts fromCF I through CF IF , the calculations indicate only3 3 4minor SO effects for the carbon shifts of the as-

Ž . Žyet-unknown iodine VII compound CF IF and3 6.for CF IF , due to the lack of iodine p-type lone3 4

13 19 Ž .pairs. Both the C and F CF shifts for this3system are predicted to be slightly larger than

Ž .those for the corresponding iodine V species,CF IF . The 13C shifts for CF IF should thus invert3 4 3 6the SO-dominated decrease in the carbon shiftsfound for the first three members of the CF IF3 nŽ .n s 0, 2, 4, 6 series.

Finally, we note that SO effects on 19 F shifts offluorine atoms attached to the iodine substituenthave been found to be much smaller than those onthe 13C shifts. This is due to the small contribu-tions from s-orbitals on fluorine to the I—F bonds.We generally expect that SO shifts are moderate orsmall for fluorine, whereas they tend to be particu-larly large for hydrogen.17

Acknowledgments

V.G.M. thanks the Alexander von HumboldtStiftung for the donation of a HP9000rC160 work-station.

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