Torsional and Electronic Factors Control the C−H⋅⋅⋅O Interaction

Abstract The precise role of non‐conventional hydrogen bonds such as the C−H⋅⋅⋅O interaction in influencing the conformation of small molecules remains unresolved. Here we survey a series of β‐turn mimetics using X‐ray crystallography and NMR spectroscopy in conjunction with quantum calculation, and conclude that favourable torsional and electronic effects are important for the population of states with conformationally influential C−H⋅⋅⋅O interactions. Our results also highlight the challenge in attempting to deconvolute a myriad of interdependent noncovalent interactions in order to focus on the contribution of a single one. Within a small molecule that is designed to resemble the complexity of the environment within peptides and proteins, the interplay of different steric burdens, hydrogen‐acceptor/‐donor properties and rotational profiles illustrate why unambiguous conclusions based solely on NMR chemical shift data are extremely challenging to rationalize.


Introduction
Ac omplex ensemble of noncovalent interactions, including hydrogen bonds, drives the folding of biopolymersi nto functional conformations. Relativelyw eak interactions such as the CÀ H···O hydrogen bond [1] make importantc ontributions to the folding process but their potentialf or directly influencingc onformationis unclear. [2] For instance, the CÀH···O interaction likely contributest ot he stability of the A·T base pair in the Hoogsteen geometry, [3,4] and is also ak ey component governing certain catalytic enantioselective processes. [5] Increasingly, these nonconventional hydrogen bonds are considered to play as ignificant role in molecular recognitioni nvolved in the binding of smallm olecules to large ones. [6] We have previously investigatedw hether an intramolecular CÀH···O interaction can influence the conformation of small moleculesi nt he solidstate through ac ombined crystallographic and computational study. [7] This approachd emonstrated that for certain a,a-difluoroamides the CÀH···O interaction could be ad eterminant of conformation, with bond enthalpies of up to 3.5 kcal mol À1 through increased polarization of the CaÀHb ond. However, distinguishing between an attractiveC ÀH···O hydrogen bond and ac oincidental close contact proved challenging and we subsequently began to investigate the factors that determine whetheraw eak interaction is conformationally influential. [8] The study of noncovalent interactions is most relevant when examined in ad ynamic environment and hence we decided to extendo ur work to encompass the solution-state. We recognized that at horough study would require: 1) ac onformationally well-definedp latform that would allow us to study the CÀ H···O interaction;2 )the ability to vary both acceptora nd donorg roupsa round this scaffold;3 )suitable methods to gauget he influence of the CÀH···O interaction on globalc onformation.C onsequently,w edecided to utilize an established parallel b-turn scaffold which would allow us to systematically probet he influence that of aC ÀH···Od onor on the rotationo f as ingle bond within the confines of ac onformationally welldefined system (Figure 1). We and others have previously extensively investigatedt he conformational properties of this peptidomimetic in the solid and solution state and have shownt hat b-turn like conformationss tabilized by intramolecular hydrogenb onding are well-populated for most derivatives. [9] Our motivation in choosingasmall and dynamic b-turn mimeticwas to perform our investigation in aconstruct resembling an aturalp rotein and peptidee nvironmentr ather than as imple chemical model. Our systemh as am ultitudeo fr otatable bonds and many interdependent noncovalent interactions,w hichs ignificantly complicate analysis but which are necessary to probe the factorst hat make the CÀH···O interaction influential within small molecule models,synthetic foldamers and ultimately biomacromolecules.

Results and Discussion
Solid state studies We rationalized that to probe the influenceo ft he CÀH···O interaction would require the synthesis of as eries of a,a-disubstitutedd erivatives possessing donor and acceptorm oieties of different sizes (and hence different torsional profiles)a nd varying electronic properties.T he conformation of these materials was initially examined in the solid-state using singlec rystal Xray crystallography ( Figure 2). [10] The substrates we examined are sterically and electronicallyd iverse, butt he vast majority were found to populate b-turn like structuresw ith intranuclear distances consistent with aN ÀH···O=Ch ydrogen bond between the aromatic amide proton donor and the carbonyl oxygen acceptor,a se xpected. These distances varied significantly between 2.0 for powerful a-electron withdrawing groups such as the bistrifluoromethyl substrate 8,a nd 2.5 for other groups sucha sp entafluorophenylbenzyld erivative 12. Less polarizeds ubstituents such as alkyl groups (typified by 2-6)g enerally possess longer NÀH···O=Cb ond lengths. [11] We recognizedt hat many substrates contain aromatic rings and were therefore capable of engaging in additional inter-and intramolecular interactions that could potentially affect the global conformation.I no ur search for these additional interactions (par-ticularlyC ÀH···p contacts)w ew ere guided by distance criteria established during surveys of protein X-ray structures and theoreticalstudies. [12] We were initially interested in whether changing the size of a-carbonyl substituents through disubstitution and cyclization withoute xplicitly increasing CaÀHa cidity,w ould affect rotation around the HCCO torsion. Compounds 1-6 explore this question through as eries of simple alkyl derivatives. Although there are no obvious additional interactions in the crystalso f these materials (1-6)t hat appear to be responsible for influencing the rotational states of the a,a-disubstituted amide moiety,i ncidental intermolecularh ydrogen bonds between ad-jacent molecules can be observed within the crystal lattice of these and all other compounds. [10] Close examination of the Xray crystal structure of acyl derivative 1 reveals a b-turn-like motif featuring an NÀH···O=Cd istance of 2.2 ,c onsistent with this hydrogen bond being as tabilizing element. The CÀH···O= Cd istance (2.5 )a nd angle q (1478,C ÀH···O=C) in 1 is also consistentw ith the presenceo faC ÀH···O interaction, but we have previously commented that the shallow rotational energy profile for related materials bearing simple acyl groups is inconsequential to the observed conformation. [7] Therefore we also compared the CÀH···O=Cd istances in 2-6 to determine whether disubstitution and cyclization could be influencing the putative CÀH···O interaction. These internucleard istances are between 2.4 and 2.6 and all are consistent with the presence of aC ÀH···O hydrogen bond. Similarly,e xamination of the angle q(CÀH···O=C) across structures 1-6 reveals ar elatively narrowr ange of values between 1478 and 1558,a ll of which lie within the conventionally accepted angle ranges for identification of aC ÀH···O interaction. [13] For compounds 2-6 the measured anglesa nd distances are largely similar to those observed for 1.
We next decided to examine compounds 7-14,w hich bear substituents designed to probe the effect of changing the CaÀ Ha cidity relative to acyl control compound 1.A ll appear to populate turn-like conformations in the solid-state, but ac loser examination of BocGly derivative 14 shows an intramolecular distance (3.2 )m uch larger than normal; this is consistent with the global conformation in this case being dominated by intermolecular hydrogen bonding interactions in the crystal lattice. [14] In contrast, compounds 7-13 shown oa dditional important intermolecular interactions and have relatively short NÀH···O=Cd istances (between2 .0 and 2.5 )c onsistent with hydrogenb onding. CÀH···O=Cd istances vary between 2.2 (for bistrifluoromethylamide derivative 8)a nd 2.7 (for pentafluorophenylbenzyl derivative 12). Bistrifluoroamide 8 has asignificantlys horter CÀH···O=Cd istance than any other member in this series, which appears to be consistent with its powerful electronwithdrawing ability.Compound 7,which has abenzylic CÀHp roton, and would consequently be expected to be more acidic, does not have as ignificantly shorter CÀH···O=Cd istance than 1.As imilaro bservation can be made for cyclopropane 9, which is somewhat more acidic than an acyclic alkane by virtue of its greater s-orbital character. [15] Substitutedb enzyl derivatives 10, 11 and 12 have relatively long CÀH···O=Cd istances( 2.8, 2.6 and 2.7 ,r espectively)w hen compared to 1, which appears to result from ab alance between the proximity of the sterically demandinga rene and Boc groups and stabilization by the CÀH···O interaction. There is no indication that pentafluorophenyld erivative 12 participates in significant CÀ H···p interactions. BocPro derivative 13 possesses ar elatively short CÀH···O=Cd istance of 2.3 ;t his is shorter than geometrically similarc yclopentane 5 and electronically similar BocGly derivative 14 and likely resultsf rom unique torsional preferences within the turn scaffold.
Bistrifluoromethyl derivative 8 has the shortest CÀH···O=C distance and was therefore used as ab aseline to evaluate other compounds with varioush ydrogen-bond acceptor  . Solid-state conformationso fb-turn mimic bearing different CÀH···O donorswith relevant intramolecular distances and angles (somea toms omitted for clarity;distances []a re indicated between atomsi nbold). [10] Positions of hydrogen atoms are calculated.* :Asymmetric unit contains two conformationally similarb ut crystallographically unique molecules;d istances and hydrogen-bond angles are given for only one molecule. Xanth = 9-xanthene. groups (16-18). As ureas are somewhatb etter hydrogen-bond acceptors than carbamates, [16] we first synthesized urea derivative 16.T his compound has as hort (2.2 )C ÀH···O=Cd istance, which again (see 10-12 above) likely resultsf rom ab alance between the stericd emando ft he bistrifluoromethylaryl group and the increased hydrogen-bond acceptor ability of the urea. We also synthesized amides 17 and 18 with the expectation that the trifluoromethyl amide 18 would be ap oorer hydrogen-bond acceptor than 17.T his is borne out by the observation that the H···O=Cd istances are 2.3 and 2.6 (for 17 and 18,r espectively), although the stericd emands of these two amides are obviously different. We also examined the X-ray structureso f15, 16 and 17 for evidence of interactions involving the aromatic group (particularly H···p contacts) but as in 10-12 no interactions relevant to the globalc onformation were found.
These bistrifluoromethylamides can also be compared with monotrifluoromethylamides 20 and 21,which possess different hydrogen-bond acceptor groups. It was expected that amonotrifluoromethylamide group would be ap oorer H···O donor than ab istrifluoromethylamide group, and this is in fact reflected in the longerC ÀH···O=Cd istance in 21 (2.6 )v ersus 8 (2.2 ). It is relevant to note that several of the compounds we examined (specifically 22 and 23)d id not populate turn-like conformations in the solid-state ( Figure 3).The global conformation of these materials is dominated by intermolecularh ydrogen bonds andc rystal packing forces that preclude observation of the intramolecular interactions of interest;t hese compounds are thus included here for completeness but their solid-state structures are ad eparture from our usual observations.

Solutions tate study:hydrogen-bonddonors
In general, our solid-state survey of CÀH···O donors and acceptors shows that many compounds possessi nternucleard istances and angles consistent with the presence of multiple noncovalenti nteractions. However,i ni solationt hese observations do not constitute ad emonstration that as pecific potential interaction is necessarily important or influential in the overall folding process, as solid-states tudies of crystalsp rovide aw ealth of information about small-molecule geometry but betray little about the energetic or dynamic aspects. Consequently we decided to expand our study and examine the 1 HNMR spectra of ac ross-section of these compounds and relate observable parameters such as 1 Hc hemical shift to the potentials trength of these interactions through quantumc alculation( Figure 4). [17] We chose as tructurally diverses eries of derivatives to investigate the effectsofbothtorsional and electronic factors on the CÀH···O interaction, and also prepared ac orresponding series of control compounds that do not possess the carbamatei ntramolecular hydrogen-bond acceptor. Thesea re imperfect control compounds as their conformational preferences are necessarily different from compounds bearing an intramolecular hydrogen-bond acceptorg roup, but they nonetheless provide av aluable benchmark for comparison. Dilution experiments ruled out intermolecularaggregation at concentrations below 50 mm,a nd as uite of 2D experiments permitted assignment of all spin systems and confirmed that all compounds examined populate at urn-like conformation in solution, similart ot hat observedi nt he solid-state ( Figure 4). Hydrogen bonding is manifested in 1 HNMR spectroscopy through ar eduction in diamagnetic shielding and hence we examined the chemical shifts of amide NÀH (shown in green in all Figures) and CaÀH (showni nr ed in all Figures)p rotons. [18] 1 HNMR chemical shift data appear consistent with the involvement of amide NÀH and CaÀH protons in hydrogen bonds for all compounds (with the possible exception of 15), as both are deshielded relative to their controls. Fluorenyl derivative 15 has only smallc hemical shift differences relative to its control 32,p otentially consistent with relativelyw eak and conformationally inconsequential solution-state interactions. This is as ignificant departure from the solid-state structure of 15,i nw hichc lose CÀH···O and NÀH···O contacts were observed.T his discrepancy may derivef rom differences in the solution and solid-state conformations of 15,b ut is more likely ad emonstration of the limitations intrinsic to our control compounds. In general the amide NÀH and CaÀH proton chemical shift differences vary significantly within the series, particularly for the CaÀH protons. [19] For our nominal controlc ompound 1, the changei nc hemical shift (DdCH)v ersusc ontrol compound 31 is 0.78 ppm. [20] The CaÀH donor in 1 is only very moderately polarized by the adjacent carbonyl;w eh ave previously demonstrated that similar compounds have an almostf lat rotationalp rofile, consistentw ith av ery weak CÀH···O interaction. [7] This suggests that as ignificant contributor to the deshielding observed in 1 is an effect other than hydrogen bonding. It is knownt hat proton chemical shifts are sensitive to magnetic anisotropic effects from carbonyl groups proximal to the CaÀH,a nd also to steric and electric fielde ffects;t hese are likely responsible for the observed DdCH in 1 versus 31. [21] A related study estimated that the deshielding of ap roton involved in CÀH···O hydrogen bondingi nb indonea nalogues was mostly due to these other effects, with only 0.6 ppm (of a1 .8 ppm shift) ascribed to the influenceo fh ydrogen bonding. [22] Examinationo f24 [23] versus 29,a nd 2 versus 28 demonstrated that the chemical shift differences are significantly higher (DdCH = 1.03 and 1.07 ppm, respectively)t han the differenceb etween 1 and 31;t his is counterintuitivea sc ompounds bearing a,a-dialkyl groups sucha s24 or 2 are not significantly better hydrogen-bond donors than 1. [24] This shift differencei si nsteadc onsistent with al arger population of conformers that place the CaÀH in proximity to the carbonyl group, possibly because non-hydrogen bonded conformers are disfavoured through steric or torsional effects, thereby increasing the significance of the CÀH···O interaction across the timeaveragede nsemble. Although this mechanism is well known in peptides containing a,a-dialkyl amino acids, [25] it is difficult to prove here conclusively.A zetidine 3-carboxylic acid derivative 3 and its control 27 demonstrate al arger shiftd ifference than cyclobutanes 2 and 28 (DdCH = 1.34 ppm vs. DdCH = 1.07 ppm, respectively); this appears to be at odds with solidstate data, where CÀH···O=Cd istances for 2 and 3 are 2.4 and 2.5 ,r espectively.T his is likelyaconsequence of the azetidine Boc group functioning as ah ydrogen-bond donor in the solidstate, which obscures the increased hydrogen bond acidity of the azetidine 3 CaÀH versus cyclobutane 2.A ne ven larger chemicals hift difference is apparent in the cyclopropanes 9 and 26 (DdCH = 1.51 ppm), [26] whichi sc onsistentw ith favourable electronic and torsional effects working in concert to enhance cyclopropane CaÀH hydrogen-bond donor ability through increased s-orbital character and restriction around the f(OCCH) torsion by virtue of a,a-disubstitution. Bistrifluoromethyl substituted example 8 exhibitst he largest chemical shift differencev ersus 25 (DdCH = 2.51 ppm). [27] We attribute this predominantly to the electron-withdrawing effect of the trifluoromethyl groups, enhancing the hydrogen-bond donor ability of the CaÀH,b ut also to their increased size versus the alkyl groups (compounds 2-6)d escribed earlier. [28,29] Thus, if 8 is compared to 24,t here is ac lear effect duet ot he increased size of CF 3 relative to CH 3 ,w hich leads to torsional restriction about the OCCH bond and favours conformations that place CaÀH in close proximity to the carbonyl oxygen (as in 9). However,t he largestc ontributor to chemical shift is likely the electron-withdrawing effect of substitution with electronegative fluorine atoms,w hich polarize the CaÀH bond and result in am ore significant CÀH···O interaction. The combination of these favourable torsional and electronic factorsl eads to the large DdCH observed for 8.
In order to probe furtherw hether these chemical-shift changes are consistent with the presence of hydrogen bonds, we examined the temperature dependence of four key compounds( Ta ble 1). [30] The amide temperature coefficients are almostidentical across this series, consistentwith the thermally mediated change in environment being similar.W ei nterpret this as being ar eflection of conserved NÀH···O hydrogen bond strength,a st he length (2.0-2.2 )i sa lmost invariant across 1, 2, 8 and 24.Inc ontrast, examination of the temperature coefficients fort he CaÀH protons shows that:1 )8 has the largest negative coefficient;2 )2 and 24 have very similara nd relatively large negative coefficients;a nd 3) the temperature coefficient for 1 has as ignificantly smaller value than 2, 8,o r24 (Table 1). We interpret these larger CaÀH temperature coefficients as reflecting more drastic environmental changes upon thermalp erturbation that ultimately lead to an increasei nt he average internuclearC aÀH to O=Cd istance. With increasing temperature in an aprotics olvent, the population-weighted average will reflect:1 )a greaterc ontribution from non-hydrogen-bonded conformations leading to reducedd eshieldingo f protons involved in hydrogen bonds, and hence larger negative temperature coefficients, 2) as maller contribution of the magnetic anisotropic effect as proximity to the carbonyl group is reduced. The larger temperature coefficient of CaÀH in 24 and 2 versus 1 could thus be ac onsequence of a" stronger" CÀH···O interaction. However,a lthough torsional restriction could certainly potentiate the CÀH···O interaction in 24 by disfavouring conformations with f(OCCH) significantly different from 1808,w ef eel that the contribution of this effect to the temperature coefficient is likely small compared to that of magnetic anisotropy.C ompound 8 has as ignificantly larger CaÀH temperature coefficient than 2, 24 or 1 and we attribute this to ac ombinationo fs ignificant torsional restriction (placing the CÀHg roup closer to the carbonylo xygen) and considerable CaÀH polarization (increasing the hydrogen-bond donor ability). In the case of 8 we are confident that the large positivec hemical 1 Hs hiftd ifference versus control (2.51 ppm) and large negative temperature coefficient (4.1 ppb K À1 )i ndicate the presenceo faC ÀH···O interaction that influences global conformation in the solution-state.

Solution state study:hydrogen-bondacceptors
We next surveyed the effectso fc hanging the hydrogen-bond acceptorg roup in 8 by comparingt he 1 HNMR chemical shifts of amide NÀH and CaÀH protons to those of controlm olecule 25 ( Figure 5). We chose to examinec ompounds possessing ab istrifluoromethyl group as the putative CÀH···O donor because both solid-state and solution-state data for 8 were consistent with the presence of ac onformationally influential CÀ H···O interaction in this molecule. As discussed earlier,i ti sc hallenging to deconvolute the factors contributingt oac hemical shift and observe directly the effect of hydrogen bonding within an individual compound, but using 1 HNMR spectroscopy data from as hort series of amides, carbamates andu reas, all of which are proficienth ydrogen-bond acceptors, we soughtt od elineateg eneral trends. When analysing compoundss uch as carbamate 8,w here the hydrogen-bond acceptor oxygen has two lone pairs, we must also consider how one hydrogen bond could affect the strength of the other,a s it has been shown that participation in an intramolecular hydrogen bond reduces the propensity of an atom to accept additional intermolecular hydrogen bonds. [31] The largest DdCH shifts are observed for amides 17 and 33 (DdCH 3.18 and 3.01 ppm, respectively), followed by carbamates 34 and 8, whilst ureas 35 and 16 demonstrate smaller shifts. By some measures, ureas may be considered to be more powerful hydrogen-bond acceptors than amides whichm akes this observation difficult to directlye xplain. [16] While ethyl urea 35 has a DdCH value similar to that observedi n34 and 8 it does have as ignificantly higher DdNH value and therefore potentially as tronger NÀH···O bond than the other compounds in this series;t he DdCH value may reflect only electronic factors, as the steric profile of the ethyl substituent is small. Although it is possible that the unexpectedly low DdCH value for 16 results from shielding of the CaÀH by the bistrifluoromethylphenyls ubstituent, there is no indicationo ft his in the X-ray structure, and it may that the solution and solid-state conformationalp references differ substantially (as in fluorenyl derivative 15). In contrast, amides 17 and 33 show relatively large DdCH values, which is consistent with their steric relatively bulk, favouring hydrogen-bonded conformations.T rifluoromethyl substituted amide 18,w hich would be expectedt ob e ap oorer hydrogen-bond acceptor,d oes present as maller DdCH than the other amides.T here are only relatively small differences between carbamates 34 and 8,c onsistent with an increasei nb ulk (for 8)h aving relatively little effect when distant from hydrogen bondings ites. This series demonstrates the challenges in attempting to deconvoluteamyriado fi nterdependentn oncovalent interactions within ac onstruct that resembles the complex environmento fp eptides andp roteins. In particular, the interconnectedness of the NÀH···O and CÀH···O hydrogen bonds coupled with the interplay of different steric burdens,h ydrogen-acceptor/-donorp roperties and rotational profiles make cleara nd unambiguous trends based solelyo n NMR chemical shift data difficult to rationalize. [32] Quantum chemical calculations Our solution-state and solid-state analyses suggest acorrelation between thes terica nd electronic properties of differentC aÀH substituents and the propensity of those substituents to function as donors in CÀH···O interactions. To furthere valuate whether these observations-the close contacts observed in crystal structures and chemicals hift changes seen by 1 HNMR spectroscopy-are ac onsequence of an attractive CÀH···O interaction we applied quantum calculation using the Gaussian 09 package at the M06-2X/6-31 + G** level of theory. [33] The geometry of compounds 8, 9, 2 and 1 and their respective controls 25, 26, 28 and 31 were probedb yr otatingt he amide group about torsion angle f(OCCH);f ully optimized geometries of minima are presented above ( Figure 6). [28] These calculated geometries are in excellent agreement to those observed by X-ray crystallography and exhibit only minor differences. For example:i na,a-bistrifluoromethylamide 8,t he optimized conformation is remarkably similar to that seen in the crystal structure, with as hort CÀH···O distance of 2.18 and  Figure 2). This conformation is significantly different from that of its control 25,w hich possesses a f(OCCH)t orsion angle of 48. [34] The minimai llustrated for 9 and 2,a nd their controls 26 and 28 have f(OCCH)t orsion angles close to 1808;o nly 28 hasa na dditional minimum with f(OCCH) close to 08 within 3kcal mol À1 .F or 1 and 31,r otation about the OCCH torsion from the illustrated minimum is effectively free, with other minimaw ithin 0.1 kcal mol À1 .T his appearst ob ec onsistent with aw eak andi nconsequential CÀ H···O interaction. In order to estimate the strength of these intramolecular interactions, we examined the second-order perturbation energy E(2) for 8, 9, 2,a nd 1 through natural bond orbital( NBO) analysis. [35] This providesa ne stimation of the donor-acceptor interaction energy between the oxygen lonepair acceptora nd the s*o rbital of the CaÀHd onor. E(2)i s equal to 5.08, 1.66, 2.00, and 1.61 kcal mol À1 for molecules 8, 9, 2,and 1,respectively,consistent with values normally observed for CÀH···O hydrogen bonds. [36] As econd metric to estimate the strength of aC ÀH···O interaction is the 1 HNMR deshielding of the bridging CaÀH. [17] The calculated deshielding (DdCH calcd ) in 8, 9, 2,a nd 1 (relative to controls 25, 26, 28 and 31,r espectively) was 1.88, 1.26, 0.75, and 0.45 ppm. These values are smaller than, but consistent with the order observed experimentally in 8, 9, 2,a nd 1 but refer to as ingle, static conformation without thermala veraging. To give an indicationo fh ow much of this deshielding can be attributed to the CÀH···O hydrogen bond, we compared the CaÀH chemical shift when in the optimized geometry ( Figure 6) to that after rotating about the OCNH torsion by 1808.T he deshielding of the CaÀH calculated in this manner is:1 .42, 0.67, 0.50, and 0.38 ppm, for 8, 9, 2,a nd 1,r espectively.I ti sn otable that all of the metrics outlined above suggest that the CÀH···O interaction is strongest for 8 and least significant for 1. [37] Compound 8 appears to provide the clearest evidence for ac onformationally influential solutiona nd solid-state CÀH···O interaction. To furthere xplore the relationship between energy and f(OCCH)t orsion,t he terminal CH(CF 3 ) 2 group was rotated (in ca. 308 increments) aroundt he f(OCCH) torsion,g enerating as eries of structures that were fully optimized, tracing out ap otentiale nergy curve as af unctiono fd ihedral angle (Figure 7). The profile shows ac lear minimuma tf(OCCH) = 1618 and two maxima at f(OCCH) = 428 and f(OCCH) = 2828. In both of the maxima, the amide C=Oi se ffectively transplanar to one of the CF 3 groups,w ith the NÀH···O hydrogen bond intact. In contrast, the minimuma tf(OCCH) = 1618 correspondst ot he optimum geometry of the CÀH···O hydrogen bond.
We subsequently quantified the strength of this interaction using an approach thath as previously been applied to protein b-sheets. [38] Taking 8 andr emoving all atomse xcept those directly relatedt ot he CÀH···O hydrogen bond left two fragments, which were frozen in their precise relative orientations. The energy of this complex was then compared with the sum of the energies of each isolatedm onomer,l eading to av alue of 3.6 kcal mol À1 .T his is ar elatively strong interaction, consistent with the acidifying trifluoromethylgroup, which, in combination witht he torsional effects of geminal Ca-substitution, led to the observed rotational minimum.

Conclusions
Through ac ombined crystallographic, spectroscopic and quantum computational study we have probedt he presence and influenceo ft he CÀH···O interaction in as eries of b-turn mimetics and demonstrated that electronic and torsional effectsa ct in concert to modulate the potentiali nfluence of the CÀH···O hydrogen bond. Considerably polarizedC aÀH donor atoms and conformationalr estriction appear to be necessary fort he CÀH···O interaction to play as ignificant role in conformation. Our results also highlight the challenges in attempting to deconvolute am yriado fi nterdependent noncovalent interactions in order to focus on the contribution of as ingleo ne. Within ab iomimetic construct designed to resemble the complexity of the environment within peptides andp roteins, the interconnectedness of different hydrogen bonds coupled with the subtle interplay of stericb urdens, hydrogen acceptor/ donor properties and rotational profiles illustrate why unambiguousc onclusions based solely on NMR chemical shift data are extremelyc hallenging to rationalize, even for carefullyc alibrated model systems. These conclusions are directly relevant to foldamer and protein conformational preferences, especially b-sheets, [13] where a-electronegative atoms can enhance CaÀH acidity and work in concert with conformational and torsional restriction provided by an extensive network of strong hydrogen bonds to potentiate weaker CÀH···O interactions. In the context of designinga nd developing new folded systems and hydrogen-bonding organocatalysts that exploit the CÀH···O hydrogen bond as as tructural element, it is clearthat ac ombination of favourablee lectronica nd torsional effects is ap rerequisite for such interactions to be conformationally influential. [39] Experimental Section Full synthetic procedures and complete spectroscopic data (including 1 Ha nd 13 CNMR spectra) for all compounds are available in Supporting Information.