Solution-phase NMR study of BDAB

In contrast with the solid-state spectra shown in Figure 6.2 (c) and (d), solution-phase

14/15_{N NMR spectra are commonly referenced to NH}

3 at 0.0 ppm. The 14N (I = 1) NMR

spectrum of BDAB recorded in D2O solution is shown in Figure 6.6. The only resonance

observed (at 47.7 ppm) must be assigned to the two tetra-alkyl nitrogens. The other two

Figure 6.6 14_{N NMR spectrum of the BDAB in D}

2O at 9.4 T. Only one nitrogen environment is

Figure 6.7 The 14N NMR spectrum of the BDAB in acid D2O solution at 9.4 T is shown in (a). In (b)

apical nitrogens are expected, in D2O, to be in equilibrium between the protonated and

the non-protonated forms. These dynamics take place on a much shorter timescale than the solution-phase NMR signal detection. Therefore, both forms are expected to exhibit a single resonance in a water solution. The lack of this resonance in the spectrum indicates the equilibrium is highly shifted in favour of the non-protonated system, for which the neutral apical nitrogens are not easily detected due to their large quadrupolar constant. The acidification of the D2O-BDAB solution should, in principle, shift the equilibrium

towards protonation of the apical nitrogen. This latter state of the system is characterized by a much weaker EFG at the apical nitrogen site therefore resulting in a much narrower lineshape. In order to experimentally test this hypothesis, two equivalents of H+ (HCl/H2O 36% in weight) were added to the BDAB D2O sample. The resulting 14N

spectrum is shown in Figure 6.7 (a). In contrast with the spectrum shown in Figure 6.6, a

Figure 6.8 1_H-15_{N HMQC spectrum of the BDAB in D}

2O at 9.4 T. The expected second nitrogen

second much broader resonance is present at higher field, as shown in Figure 6.7 (b). The fitting of this broader lineshape with a single peak is shown in Figure 6.7 (c) and yields an isotropic shift of 33.1 ppm and a line broadening of 723.2 Hz, in contrast with the 13.4 Hz line broadening obtained for the resonance at 47.7 ppm. The acidification of the solution therefore shifts the equilibrium towards protonation of the apical nitrogen. Furthermore, there is the experimental evidence that in a D2O solution the order of shieldings calculated by the DFT method is correct. The apical 14N resonance, upon acid addition, is expected to progressively become narrower (decrease of the quadrupolar interaction) as the protonated form becomes more representative in the acquisition time and shifts to low field as calculated at all the levels of theory explored. The 14N spectrum (not shown) of this latter basic sample is very similar to the neutral case. This is perhaps expected as the proton content of the neutral solution is already too low to show any

Figure 6.9 Overlay of the 1H-15N HMQC spectra of the BDAB in D2O at 9.4 T. The spectra in acid,

Figure 6.10 15N projections extracted from 1H-15N HMQC spectra recorded under acid (a), neutral (b) and basic conditions.

signal associated with the apical nitrogen and the neutralization of this proton content shifts further the equilibrium towards the undetectable non-protonated trivalent species. Although characterized by a very low natural abundance (0.36%) which compromises enormously the sensitivity of their detection, 15N spectra can be efficiently recorded in the indirect dimension of a 1H-acquired 2D correlation experiment. Furthermore, and in contrast with the 14N isotope, 15N is a I = 1/2 spin. This means that relatively narrow resonances can be acquired regardless the protonation state of this nucleus. As shown in Figure 6.8, the 2\3J long range 1H-15N correlation HMQC 2D spectrum of BDAB reveals the second apical nitrogen at 16.2 ppm. The 3J correlation between 15N at 47.7 ppm and

1_{H at 1.82 ppm (aliphatic linker) confirms this nitrogen is the tetra-alkyl species as the}

apical nitrogen is too many bonds away to correlate with these protons. Further confirmation of a pH-dependent shift is supplied by the 1H-15N HMQC of the BDAB in acid and basic conditions. The overlay of these three spectra is shown in Figure 6.9. The

Figure 6.11 Correlation between experimental and calculated nitrogen chemical shifts in the BDAB. The correlation coefficient R2 is 0.989.

apical 15N nitrogen found at 16.2 ppm in the D2O solution (Figure 5.8) moves downfield

to 30.3 ppm in the D2O/HCl sample, in good agreement with the 33.1 ppm 14N shift

obtained from the lineshape fitting. In contrast, in the D2O/NaOH sample, the same

resonance moves upfield to 11.1 ppm. It is interesting to notice the two CH2

environments of the Dabco moiety experiencing a down-field shift of ~0.5 ppm whereas the aliphatic-linker CH2 resonances remain mainly unaffected. This observation is readily

rationalized by considering the distances between the aliphatic protons in the BDAB linker and the apical-nitrogen protonation site and experimentally justifies the assumption adopted in the computational model that the protonation of one Dabco unit of BDAB leaves the other relatively electronically unperturbed. In Figure 6.10 the 15N projections extracted from the three HMQC spectra are shown. It is clear that, when the I = 1/2 15N spectrum is acquired, the resonance width is not dramatically affected by the changes in EFG caused by the protonation as observed previously for the quadrupolar isotope 14N (I

= 1). Differences in lineshape in the three 15N projections are related to the important change in ionic strength of the solution upon addition of acid or base. This usually, besides causing changes in tuning of the spectrometer, dramatically alters the mobility of the molecules in solution due to the increase of viscosity. This is, of course, related to relaxation processes which control the lineshape breadth.