Frozen Rotation of the Ammonium Group ... - ACS Publications

27 Dec 2016 - crowded molecular environment was frozen on the NMR time scale. ... picture of the rotation process could be obtained at low and moderat...
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Frozen Rotation of the Ammonium Group Observed for the First Time in Liquid-Phase NMR Spectra Artur Szupiluk,† Piotr Bernatowicz,‡ Tomasz Ratajczyk,‡ and Slawomir Szymanski*,† †

Institute of Organic Chemistry and ‡Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland S Supporting Information *

ABSTRACT: Hindered rotation of the −NH3+ group in a sterically crowded molecular environment was frozen on the NMR time scale. These effects were observed for solutions of 1,2,3,4-tetrabromotriptycyl-9-ammonium tetrafluoroborate in tetrahydrofuran with and without excess of HBF4. In the absence of acid, the hindered rotation is accompanied by a number of other effects originating in part from practically nonremovable traces of water in the solvent. Because of these additional effects, an effective rate constant of the hindered rotation could be evaluated at only one temperature for the neutral solution of the salt. For one of the acidified samples, which was prepared with special precautions against moisture, these complications are practically nonexistent at low and intermediate temperatures. For the latter sample, the Arrhenius parameters of the hindered rotation could be determined from the rotation rates evaluated in the range 194−226 K.

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INTRODUCTION

Hindered rotation of ammonium ions of some aromatic amines was studied in nonaqueous solvents, namely, trifluoromethanesulfonic acid and dimethyl sulfoxide.7,8 The impact of solvent on these processes is hardly predictable, and its mechanism is poorly understood. Little help in this regard is offered by model experimental studies of the reorientation dynamics of the NH4+ cation in a series of solvents, where any conceivable hindrance of the motion must come from interactions of the cation with its solvation shell.9,10 The present work involves hindered rotation of the ammonium cation placed in an extremely crowded molecular environment. Inferences from variable-temperature (VT) 1H and 19F spectra of 1,2,3,4-tetrabromotriptycyl-9-ammonium tetrafluoroborate (1) dissolved in perdeuterated tetrahydrofuran are reported. This solvent is an effective acceptor of HBs. The cation 1+ is a structural analogue of 1,2,3,4-tetrabromo-9methyltriptycene (2). The latter compound belongs to the narrow class of 9-methyl-triptycene derivatives for which decoalescence of the methyl proton signals can be observed in liquid-phase NMR spectra.11−15 The present findings point to substantial complications of the dynamics of the ammonium cation in a strained environment, as compared with the methyl group under similar conditions. The steric strain promotes dissociation of the salt. Both the free amine and nonremovable in practice traces of water in the solvent lead to an obscured evidence of the hindered rotation in NMR spectra. For these

−NH3+

Ammonium cation in protonated primary amines and in zwitterionic forms of amino acids is structurally similar to the methyl group, with respect to both the bond lengths and interbond angles. Unlike those of the methyl group, the ammonium protons can engage in strong hydrogen bonds (HBs) with the corresponding counterions and with other proton acceptors in their surroundings. While the torsional barriers of the methyl groups seldom exceed 4 kcal mol−1, the corresponding values reported for the ammonium groups in molecular crystals often exceed 5−6 kcal mol−1. For example, this takes place for a series of proteinogenic amino acids, where for L-alanine and L-leucine the barriers are about 9 and 12 kcal mol−1, respectively.1 For protonated primary amines, the torsional barriers can be also high. In crystalline adamantylammonium choride, the barrier reaches 13 kcal mol−1.2 For ammonium salts in solution, experimental data about the torsional barriers are scarce. Some notable exceptions involve hindered rotation of the side-chain −NH3+ groups of the lysine residuals in water solutions of some proteins at neutral and acidic pH.3,4 At physiological temperatures, the considered dynamics are in a nanoseconds to picoseconds range, depending on the composition of the investigated samples. In water solutions, the hindering effects caused by HBs engaging the ammonium protons can be surprisingly small.5 It is amazing that the torsional barrier calculated with quantum chemical methods for alanine dissolved in water does not exceed 1 kcal mol−1, i.e., it falls below one-ninth of the experimental value in the solid state.6 © XXXX American Chemical Society

Received: November 23, 2016 Revised: December 27, 2016 Published: December 27, 2016 A

DOI: 10.1021/acs.jpca.6b11812 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A reasons, solutions of the salt with added excess of the acid were also investigated. For one of the acidified samples, a clear NMR picture of the rotation process could be obtained at low and moderate temperatures. For the analogous methyl compounds dissolved in organic solvents, the above problems do not occur. Despite the fact that separate signals of nonequivalent methyl protons were observed for these compounds at relatively high temperatures, exceeding 170 K, quantum mechanical phenomena known as effects of damped quantum rotation (DQR)16 could be detected in several cases.17−20 Unlike their counterparts occurring in a solid at cryogenic temperatures,21 the DQR effects are subtle and can be observed only in the spectra in the limit of slow exchange. Under conditions where a hindered ammonium group preserves its integrity in the course of time, the occurrence of the DQR effects cannot be excluded. However, for the presently investigated ammonium salt, fast transverse relaxation processes cause considerable broadenings of the ammonium protons’ resonances such that an unambiguous identification of the possible DQR effects would be unachievable. Therefore, in the herein reported interpretation of exchange broadened spectra, use is made of the conventional NMR line shape theory, based on the picture of random jumps of the hindered rotator between its equivalent orientations.

Figure 1. Structure of 1 obtained from single crystal X-ray diffraction measurements.22 The included solvent molecules are not shown.

C1−Br bond out of the approximate ring plane by about 12°. In the presence of traces of water, the question of the structure of 1 in solution can be complicated. It will be addressed in some detail later on. Unless explicitly stated otherwise, in the rest of this paper the discussion will involve the “neutral” solution of 1, one prepared from crystalline 1 without the addition of acid (i.e., sample A). Overview of VT NMR Spectra of the Ammonium Salt. Examples of VT NMR spectra of the ammonium protons in sample A in the range 298−166 K are shown in Figure 2. The corresponding VT spectra of the nonammonium protons in 1 are shown in Figure S1 in the SI. With decreasing temperature, the broad singlet at 9.5 ppm initially undergoes a downfield shift to about 10.2 ppm at 224 K, and a consistent narrowing

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EXPERIMENTAL DETAILS The synthesis of 1, of its parent amine, 1,2,3,4-tetrabromo-9aminotriptycene, and an X-ray diffraction structure of 1 are reported elsewhere.22,23 Variable-temperature NMR spectra were measured for three samples, A, B, and C, placed in hermetically locked NMR tubes, each containing 3.3 mg of 1 dissolved in 0.8 mL of THF-d8 (6.8 mM), with about 2-fold molar excess of HBF4 in diethyl ether added to samples B and C. The solvent was dried over molecular sieves 3A. Samples A and B were prepared in a drybox, and sample C was prepared in open air without peculiar precautions against moisture. The 1H and proton-decoupled 19F spectra were measured in the range from room temperature to 220 K using a 400 MHz NMR machine. 1H spectra in the full temperature range, down to 166 K, were measured using a 300 MHz NMR facility. 1H spin− lattice relaxation measurements were carried out in the range 170−298 K on the latter instrument. The NMR experiments were carried out on freshly prepared samples. After several months, a partial decomposition of the substance in samples A and C was observed. Fits of the experimental spectra with theoretical models were performed using home written programs. Quantum chemical calculations of the molecular geometry and selected NMR parameters of 1, as well as of the electric field gradient (EFG) at the nitrogen atom were carried out at the B3LYP/6-311++G(2d,p) level of theory using the Gaussian 09 package.24 Partial VT 1H NMR spectra of samples A and B, as well as results of the spin−lattice relaxation measurements are given in the Supporting Information (SI).

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RESULTS The single crystal X-ray structure of 1 is shown in Figure 1. It was refined with the assumed standard geometry of the −NH3+ group. In the crystal lattice, which includes also solvent molecules (THF and diethyl ether), anion BF4− is positioned above the sector between the proton- and bromine-substituted rings. The planar symmetry of the cation is broken. The most significant feature of the broken symmetry is a bending of the

Figure 2. Selected VT 1H NMR (300 MHz) spectra of sample A in the region of ammonium protons’ resonances. B

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The decoalesced spectra were fitted with the AlexanderBinsch28,29 line shape function for an A2B system undergoing cyclic exchange of spins. (The rate constant k1 entering the presently used version of this function is defined as 3/2 of the inverse mean residence time of the rotating group in any of its three orientations.) Two versions of such fits were performed, with the same simplifying assumption that in the hypothetic situation of completely stopped rotation, all resonance lines would have Lorentzian line shapes of the same HWHH. This parameter, to be further denoted by W, acts as an approximate measure of the combined effect of the rate processes other than the hindered rotation, as well as contributions of the transverse DD and scalar second kind relaxation to the signal widths. In one version of the fits, the coupling constant between the nonequivalent ammonium protons, JAB, was set equal to zero, and in the other to −13 Hz, the theoretical value calculated using the Gausssian 09 package24 (see Table 1). The variant

down to about 240 K. Below 240 K, it broadens back to finally decoalesce, at about 215−210 K, into two differently broadened signals of the intensity ratio 1:2. Over the whole temperature range, the total intensity of the signals occurring in the region 8.5−11.5 ppm corresponds strictly to the molecular structure. The overall changes of the spectra, illustrated in Figure 2, evidence unequivocally the decreasing with temperature rotation rate of the ammonium group. However, the above process is not the only origin of the displayed line shape effects. Discordant with such a simple picture is the above-mentioned narrowing of the signal, continued from room temperature down to about 240 K. A complicated origin of these effects is also evidenced in a progressive broadening with decreasing temperature, occurring from 190 to 166 K, of the high-field component of the decoalesced signal, accompanied by a continued narrowing of its low-field partner. The initial narrowing of the signal down to about 240 K could at least in part be due to the modulation by spin−lattice relaxation of the 14N nucleus of the ammonium protonnitrogen J-couplings (of about 50 Hz in methylammonium cation25,26). To shed some light on this problem, spin−lattice relaxation rates for the ammonium protons were determined in the range 298−166 K. The results are shown in Figure S2 of the SI. These relaxation processes are caused predominantly by the dipole−dipole (DD) interactions between the ammonium protons. Hence, from the location of the minimum of the relaxation time, T1, at about 220 K an effective correlation time of the molecular tumbling, τc, could be estimated at this temperature to be approximately equal to 0.5 ns (the inverse of the applied proton Larmor frequency, ω0H = 2π × 300 MHz). Moreover, while the DD relaxation leaves the extreme narrowing regime about the temperature of the T1 minimum, the longitudinal quadrupolar relaxation of the 14N nucleus will likely remain in this regime down to the lowest temperatures explored (because ω0H/ω0N‑14 ≈ 15). With decreasing temperature, the contribution of the transverse DD relaxation to the line widths of the ammonium protons’ signals will thus consistently increase while the influence of the N−H Jcouplings on these signals will decrease. As will be shown later on, the fine structure due to these couplings is collapsed already at room temperature. At lower temperatures, they can only give rise to some slight broadenings of the ammonium protons’ resonances, by the transverse scalar second kind relaxation mechanism driven by fast longitudinal relaxation of the 14N nucleus.27 Despite the complications mentioned in the foregoing, attempts at fitting the considered spectral patterns with simple theoretical models were undertaken in the hope to evaluate the rate constant of the hindered rotation in the dissolved salt without extra additions. Above the decoalescence temperature, the spectra are featureless and can be fitted with a single Lorentzian curve. The evaluated signal width, i.e., half-width-athalf height (HWHH), of the Lorentzian function could hardly give faithful estimates of the rotation rates in this temperature range. If the rotation was the only process controlling the line shape, the width of this signal should consistently increase with decreasing temperature down to the decoalescence temperature, which is not observed (see above). On the other hand, the decoalesced spectra appear to be shaped mainly by the hindered rotation, except for the lowest temperatures, where below about 190 K the narrowing trend of the high-field component of the decoalesced signal is reverted.

Table 1. Selected Structure and NMR Parameters Calculated at a DFT Level of Theory atoms 14

N, H1 14 N, H2 14 N, H3 H1, H2 H1, H3 H2, H3 H1, F4 H2, F5 H3, F6

distance/pm

J-coupling/Hz

104.3 102.9 102.1 162.4 165.4 162.8 170.4 197.0 234.0

47.62 50.49 50.25 −14.32 −12.65 −13.10 -

with JAB = 0 corresponds roughly to the situation where these other rate processes, likely involving intermolecular exchange of the ammonium protons, are fast enough to cause collapses of the fine structures due to the considered spin−spin couplings but are ineffective in bringing about a dynamic averaging of the chemical shifts. That with JAB = −13 Hz modeled the opposite case where the considered processes would cause only some extra broadenings of the individual lines in the resolved spin multipletes. In the fits, apart from k1 and W, the values of δA and δB were optimized. (For cyclic exchange in the slow exchange limit, uncorrelated values of k1 and W can be extracted from any experimental spectrum of an A2B system because these parameters differently influence the line shape function.) In both their variants, the fits to the experimental spectra at and above 194 K proved fairly good, while for those below 194 K noticeable misfits were obtained. An example of such a good fit is shown in Figure 3. The values of line shape parameters, obtained at convergence, including the rate constant k1, are listed in the caption of the figure. These fit results will be commented upon in the discussion section. VT 19F NMR spectra of sample A, measured in the range 303−233 K, will be considered in the Discussion section. VT NMR Spectra of the Ammonium Salt with an Excess of Acid. The experiments reported below were performed in the hope that cross-referencing of observations made for all three investigated samples can shed some light onto those line shape effects in sample A that have a different origin than the hindered rotation. VT NMR spectra in the range 295−180 K of the ammonium protons in the acidified solution of 1 in sample B are shown in Figure 4. The corresponding spectra of the nonammonium protons, shown in C

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Figure 3. Experimental spectrum of the ammonium protons in sample A at 200 K (300 MHz) superposed with a best fit theoretical spectrum (red) calculated using the Alexander-Binsch line shape equation for cyclic exchange of these protons, measured by rate constant k1. The A2B pattern was assumed, with JAB = −13 Hz, and with the natural signal shape modeled by a Lorentzian function of the same HWHH = W for all resonances. At convergence, the optimized values of k1, W, δA, and δB are 288 ± 3 s−1, 14.1 ± 0.2 Hz, 10.048 ppm, and 10.702 ppm, respectively. In the fit with the assumed value JAB = 0 Hz, k1 and W are 290 ± 3 s−1 and 15.0 ± 0.2 Hz, respectively.

Figure S3 in the SI, are strictly similar to those of sample A (Figure S1 in the SI). The ammonium protons’ spectra in sample C at temperatures above about 230 K are shown in Figure 5. Those below 200 K will be considered later on. At the intermediate temperatures, the considered signals in sample C are heavily overlapped with some foreign signals coming probably from various solvates of the acid. Like those of the neutral sample, the VT spectra of both of these acidified samples are dominated by effects of progressive freezing of the hindered rotation. However, only in sample B does the NMR picture appear to evidence the hindered rotation without admixture of other effects, at least at temperatures below 250 K. Before discussing these spectra of sample B, the role of the N−H couplings in shaping the observed spectra will be commented upon. Because the dynamics of the molecular tumbling in samples A, B, and C should be not much different, the spin relaxation effects should be similar in all these samples. In the spectra of samples B and C, the ammonium proton signals are relatively narrow already at room temperature. The possible causes that in sample C they occur as closely spaced (by about 0.04 ppm), nonsymmetric doublets will be considered in the Discussion section. It is seen from these spectra that the fine structure due to the proton−nitrogen couplings measured by 1JNH ≈ 50 Hz25,26 is collapsed already at room temperature. This means that either the ammonium protons undergo rapid intermolecular exchange or the spin−lattice relaxation rate constant, 1/ T1Q (with Q standing for quadrupole mechanism), of the 14N nucleus exceeds 2π(1JNH) by at least 1 order of magnitude.27 The former possibility is discordant with chemical commonsense, given the observed signal narrowing on passing from the “neutral” to the acidified solutions. In conclusion, in the investigated temperature range, the influence of the N−H Jcouplings on the considered spectra resigns itself to an extra broadening of the resonance lines by the process of scalar relaxation of the second kind.27 Its significance will decrease because, according to the predictions of the preceding subsection, 1/T1Q will consistently increase with decreasing

Figure 4. Same as Figure 2, but for sample B. The signal marked with X probably comes from a solvate of HBF4.

temperature. Therefore, the substantial broadenings of the corresponding signals in sample A at temperatures above the decoalescence temperature must be due to some rate processes, which undergo quenching by an excess of HBF 4 . A consideration of their possible nature is deferred to the Discussion section. Note only that the initial narrowing of the ammonium protons’ signals, observed also in the acidified solutions, may indicate an incomplete quenching of these processes at ambient temperatures even in the presence of an excess of acid. In sample B, which, unlike C, was prepared with special precautions against moisture, the dynamically averaged ammonium protons’ resonances come as singlets. Below about 230 K, the signal shapes perfectly fit with the Alexander−Binsch line-shape function for an A2B spin system undergoing cyclic exchange of spins. Examples of these lineshape fits are shown in Figure 6. The details thereof as well as some relevant spectral parameters evaluated in them are given D

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Figure 5. VT NMR spectra of the ammonium protons (400 MHz) in the acidified solution of 1 in sample C. Foreign signals are marked with X. Total integral intensities of the nonsymmetric doublets at 10.0−10.2 ppm are strictly consistent with the molecular structure of 1.

Figure 6. Selected experimental spectra of the ammonium protons (300 MHz) in sample B superposed with best-fit theoretical spectra (red) calculated assuming the A2B pattern. The spectrum at 181.9 K was fitted assuming completely frozen rotation. The remaining spectra were fitted with the Alexander−Binsch line shape function for cyclic exchange of three protons, measured by rate constant k1 (see caption of Figure 3 for details). The quantities δA and δB, W, and JAB were treated as adjustable parameters in the ranges from 181.9 K to 210.7, 204.2, and 198.4 K, respectively. The fitted values of W and JAB oscillate in the respective ranges of 6.0−7.7 Hz, and −11.2−11.0 Hz. Those of δB−δA linearly drift from 0.566 ppm at 181.9 K to 0.514 at 210.7 K. At higher temperatures, the corresponding linearly extrapolated values were used. At 181.9 K, δA and δB are 10.052 and 10.618 ppm, respectively.

Figure 7. Arrhenius plot of the values of rate constant k1 obtained for sample B. The Arrhenius activation energy, Ea, and preexponential factor, A, are 11.37 ± 0.10 kcal mol−1 and (3.1 ± 0.4) × 1014 s−1, respectively.

above the decoalescence temperature. As shown in Figure 8, a numerical deconvolution affords to identify two dominating components in these spectra, in the approximate proportion of 2:1. The more intense and less intense components fit with the A2B and nonsymmetric CDE patterns, respectively, where the chemical shifts δE and δB are practically identical and the scatter of the δA−δC values does not exceed 0.1 ppm. The composite structure of the spectra of the ammonium protons in sample C will be commented upon in the Discussion section. Structure and NMR Parameters from DFT Calculations. The structure of 1 and NMR parameters of the NH3+group, as well as the EFG tensor at the N nucleus, were calculated at a DFT level of theory using the Gaussian 09 package.24 Unfortunately, solvent effects could not be taken

in the caption of the figure. The obtained values of rate constant k1 are represented in the Arrhenius coordinates in Figure 7. The Arrhenius parameters of the hindered rotation for the solution of 1 in sample B are given in the figure caption. As already mentioned, the spectra of the ammonium protons in sample C from the most interesting temperature range are severely distorted by foreign signals. At low temperatures where the rate processes are frozen, the foreign signals assume definite form. Under such circumstances a composite structure of the ammonium protons’ signals is revealed, like in the spectra E

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Figure 8. Selected VT 1H NMR (300 MHz) spectra of the ammonium protons in sample C at temperatures where rotation of the ammonium group is slow or completely frozen. The signals identified as foreign are marked with X. The experimental spectrum at 181 K is superposed with the best fit theoretical spectrum (red) involving a superposition of an A2B and CDE patterns of relative intensities x and 1−x, and a Lorentzian singlet with adjustable shape parameters to describe the impurity signal at 10.35 ppm. All spectral lines in the A2B and CDE patterns were assumed to have a Lorentzian shape of the same HWHH = W. To avoid overparametrization, it was also assumed that JCD = JCE = JDE ≡ JCDE. The parameter values obtained at convergence are x = 0.64 ± 0.01, δA = 10.0617 ± 0.0001 ppm, δB = 10.6383 ± 0.0003 ppm, δC = 10.0783 ± 0.0006 ppm, δD = 10.1583 ± 0.0004 ppm, δE = 10.6217 ± 0.0006 ppm, JAB = −12.76 ± 0.06 Hz, JCDE = −14.3 ± 0.2 Hz, and HWHH = 9.09 ± 0.09 Hz.

Figure 9. Minimum energy structure of 1 calculated at a DFT level of theory. The calculated shielding constants for the ammonium protons 1, 2, and 3 are 21.04, 23.61, and 24.69 ppm, respectively.

these calculations is that the BF4− anion can form more than one HB with the ammonium group. The calculated shielding constants are consistent with the general, well-documented observation for protons engaged in HBs that the stronger the bond, the stronger deshielding of the proton. For the calculated EFG tensor at the 14N nucleus, the quadrupole coupling constant, Q, and the asymmetry parameter, η, amount to 1.21 MHz, and 0.714, respectively. The value of Q is large as for a (distorted) tetrahedral arrangement of the substituents at the ammonium nitrogen. Using these values of Q and η, the spin−lattice relaxation rate constant of 12700 s−1 at 220 K can be calculated for the 14N nucleus in sample A, assuming isotropic tumbling with the correlation time of 0.5 ns evaluated from the 1H spin−lattice relaxation rates (see Figure S2 in the SI). The calculated value of Q is therefore consistent with the observations for samples B and C of a complete collapse of the fine structure due to the N−H J-couplings.

into account even in the Polarizable Continuum Model approximation because of convergence problems. The obtained results should therefore be treated with caution. In particular, in contrast to the observations for sample B, the calculated lowestenergy structure of the ion pair in 1, shown in Figure 9, is asymmetric. The asymmetry is introduced by the location of the BF4− ion out of the approximate symmetry plane of the cationic moiety. Selected structure parameters and J-coupling values calculated for 1 are collected in Table 1. The calculated shielding constants for the ammonium protons are given in the caption of Figure 9. The general ordering of the calculated values is consistent with the experimental data, although the differences between these values are about 5 times too large. The calculated J-coupling values between the ammonium protons are in a rather poor agreement with the data evaluated from the low temperature spectra of sample B (see Figure 6). The fact that they somewhat better fit with the values extracted from a spectrum of sample C (see Figure 8) should not be overrated, because the latter may suffer a systematic bias as having been extracted from strongly overlapping spectral patterns. On the other hand, the 14N−H coupling values are in a fair correspondence with the reported data for ammonium groups.25,26 Like for the experimental and calculated C−H couplings in the methyl analog of 1,30 the smallest of the calculated coupling constants involves the N−H bond disposed anti to the peri Br atom. In the optimized geometry, of interest is a qualitative, inverse correlation between the N−H bond lengths and the distances of the corresponding ammonium protons to their neighboring fluorine atoms in the anion. The H−F distances measure the strengths of the corresponding HBs. The most striking result of

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DISCUSSION In 9-methyltriptycenes with one benzene ring substituted with halogen atoms, the methyl proton disposed anti to the halogenated ring is shielded stronger than the two remaining protons.12−20 The reverse ordering observed herein may indicate that the analogously positioned ammonium proton forms stronger HBs with the counterion and/or solvent molecules than the remaining ammonium protons. Solvation Effects. In the acidified solution of 1 prepared in open air (sample C), the splittings of the ammonium protons’ signals into two slightly different patterns are observed in the motionally averaged and low temperature spectra. It is natural to assume that also at the intermediate temperatures the considered signals are similarly split, but the effects are masked by their overall broadenings. The occurrence of these splittings in sample C and their absence in sample B appear to be connected with different content of the residual water in these F

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Actually, at room temperature, apart from an intense, narrow signal at −149.1 ppm, two weak signals, at −156.9 and −152.6 ppm, are observed. (The chemical shift values are given for the 11 B-isotopomers of the abundance of about 80%; the signals of the 10B-isotopomers are shifted downfield by about 0.05 ppm.) The combined intensity of these two weak signals amounts to about 12% of that of the dominating signal. With decreasing temperature, the total intensity of these weak signals consistently decreases, down to about 3.5% at 220 K. Below 220 K they become undetectable. Both of these low-intensity signals can be ascribed to two differently solvated complexes of HBF4 with THF-d8, and the intense signal to the BF4− anion. Thus, the dynamic equilibrium in eq 1 is probably slow. However, apart from it, a degenerate process of proton exchange between the salt and the amine is likely to occur,

samples. It is probable that the weaker of the two dominating components in the ammonium protons’ spectra of sample C comes from such molecules of 1 whose solvation shells contain molecule(s) of water. The stronger component would thus correspond to the solvation shells not containing water molecules. The observations for sample B may indicate that with a reduced content of water, as compared to sample C, the excess of acid is sufficient to engage a large fraction of these residuals in its own solvation shells. Then, those of the salt will be practically unaffected. In the neutral solution (sample A), the dissolved salt is probably the strongest hydrophilic agent in the system and even tiny amounts of water, practically non removable from THF, will “infect” some fraction of the solvation shells of the salt. This simplified picture can explain the broadening with decreasing temperature of the high-field component of the decoalesced signal in sample A. This effect could be due to a slowing down with decreasing temperature migration of water molecules between the infected and noninfected solvation shells of the salt. A complete freezing of this process could not be achieved even at 166 K. If it were achieved, the decoalesced spectrum would presumably resemble the low temperature pattern observed in sample C (see Figure 7). However, it remains a puzzle why in sample C the exchange of water molecules between the solvation spheres of the salt is frozen even at room temperature. If the above picture is realistic, it is imaginable that a permanent presence of water molecules in the solvation shells of the salt could cause a slight diversification of the chemical shifts of the otherwise equivalent ammonium protons disposed syn to the peri-Br, leaving the remaining ammonium and nonammonium protons unaffected. The theoretical structure of 1, shown in Figure 9, was calculated without an account of interactions with the environment. Thus, speculations about structures of the considered hydrated and nonhydrated solvation shells of 1 would be groundless at the present stage. Intermolecular Proton Exchanges in the Neutral Solution. The VT spectra of the neutral and acidified solutions of 1 differ most strongly above the decoalescence temperature. Both the considerable broadenings of the resonances of the ammonium protons in the neutral solution (i.e., in sample A) and their consistent downfield shift with temperature decrease are evidently concerned with some intermolecular proton exchange processes. Among these, of a principal significance is the solvent-assisted dissociation of 1,

(RNH 2H)+ (BF4 )− + RNH 2 k3

⇄ RNH 2 + (RNH 2H)+ (BF4)−

If it was sufficiently fast, it could cause the observed dynamical averaging of the ammonium and amine protons’ resonances, leaving the discussed fluorine resonances unaffected. It is conceivable that a similar degenerate process involving the residual water can occur in the system, k4

̲ + (BF4 )− + HOH ⇄ (RNH 2H)+ (BF4)− + HOH ̲ (RNH 2H) k4

(3)

In the exchange schemes in eqs 1−3, the singled out ammonium proton can come from any of the structurally nonequivalent sites in 1+. Each of the kinetic rate constants k2−4 would be in fact a combination of different rate constants for the different proton sites and differently solvated salt molecules involved in the exchange. From our experience with only cursorily dried samples of THF, it follows that an enhanced presence of dissolved water causes a shift of the equilibrium in eq 1 toward the amine. Because the solubility of 1 drops with decrease of temperature while the concentration of water remains unchanged, an exact evaluation of temperature effects on the equilibrium in eq 1 was not possible. Approximate assessments are given below. From the already discussed 19F spectra, it follows that the dissociation degree of the salt,

k2

(RNH 2H)+ (BF4 )− + THF ⇄ RNH 2 + HBF4 ·THF k2

(2)

k3

α = C B/(CS + C B) (1)

(4)

consistently drops from the value of about 0.10 at room temperature to 0.033 at 223 K. In eq 4, CS and CB = CA are equilibrium molar concentrations of the salt, the base (i.e., the amine), and the acid, respectively. These assessments should be taken with caution because of a progressive precipitation of 1 with decreasing temperature. At 303 K, the equilibrium constant

where k2 and k2̅ are the kinetic rate constants concerned. The extent of the signal broadenings observed above 240−250 K is consistent with a non-negligible shift of the dynamic equilibrium in eq 1 toward the free amine. At a first glance, this dissociation-recombination process appears to be fast because no signs of the amine protons’ signal in its characteristic range of 3.3−3.5 ppm (as determined from the spectra of the parent amine in THF-d8) can be observed. However, if this was really so, a single, dynamically averaged signal should occur in the VT 19F spectra of sample A at the considered temperatures. This might be expected because in the investigated solution the observed chemical shift differences on the frequency scale between the 19F signals of the BF4− anion and various solvates of HBF4 are similar to the differences between the amine and ammonium protons’ chemical shifts.

K = k 2/k′2̅ = CAC B/CS = C B 2/CS −5

(5)

−1

is about 9 × 10 mol L , where the concentration of the solvent, CTHF ≈ 12.3 M, was included in k′2̅ = k2̅CTHF. For less strained ammonium salts of primary amines in the same solvent, the corresponding equilibrium constants would be by at least 1 order of magnitude smaller. The intermolecular processes discussed above could facilitate interconversions of the postulated, differently solvated forms of G

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includes energies of the HBs with the counterion and, possibly, with solvent molecules. These HBs stabilize the equilibrium orientation of the ammonium groups in its torsional potential while they are most probably broken in the transition state.

the salt. The collected experimental evidence do not resolve whether and how the former and the latter are interrelated. It can only be stated that the processes of both these kinds are quenched by an excess of acid. It is likely that below 200 K the intermolecular exchange processes engaging the free amine become practically frozen while those involving water are still active. This is suggested by the observation that, unlike for samples B and C, the fine structure due to the 2JHH couplings remains completely obliterated even in the spectra at the lowest temperatures. The relative insensitivity of these processes to decreasing temperature may point to a participation in them of some quantum tunneling processes of protons engaged in HBs.31 Because a complete removal of water from solvents capable of dissolving 1 is hardly possible, obtaining a distortion-free NMR picture of the hindered rotation in a neutral solution of 1 may be unachievable. Energy Barriers of the Hindered Rotation in the Neutral and Acidified Solutions. As has already been argued, from spectra of the neutral solution of 1 (sample A) potentially meaningful estimates of the rotation rates could be obtained only in a narrow temperature range of 207−194 K. However, for all these temperatures, the estimated values of the corresponding rate constant k1 were similar while those of W were substantially different. The nonphysical temperature behavior of k1 may stem from the fact that the single parameter W is a simple but rather poor measure of the effects due to the remaining rate processes still occurring in sample A. Actually, they can differently affect the individual resonances in an A2B system. In view of the lack of an adequate model of these effects, the use of W can be a reasonable approximation, but only as long as its values obtained in the fits are significantly smaller than those of k1. At 194, 200, and 207 K, these values are 5.6, 14.1, and 39 Hz, respectively. That at 194 K appears to be too small in comparison with the corresponding values of about 6−7 Hz, obtained for sample B (see caption of Figure 6), because, apart from the DD and scalar relaxation, it should also include effects of these remaining rate processes being still not completely frozen at this temperature. The value at 207 K is comparable to that of k1 estimated at this temperature. Such a large value of W may indicate a substantial impact of these interfering processes on the line widths such that the employed approximate description thereof may be inaccurate. In this context, the results of the fits to the spectrum at 200 K (see Figure 3) appear to be most credible. Calculated from the values of k1 = 288−290 s−1, obtained in these fits, the Eyring free energy of activation, ΔG‡200K, amounts to 9.1 kcal mol−1. Without the knowledge of the corresponding activation entropy, a comparison of this value with the Arrhenius activation energy evaluated for sample B would be meaningless. However, in sample B the value of k1, calculated for 200 K from the Arrhenius parameters given in the caption of Figure 7, is 119 s−1. Considering that the corresponding value for sample A is probably a dynamic average over the hydrated and nonhydrated solute molecules, the difference between these values can be understood. For the methyl group in 2, the close structural analog of 1+, the Arrhenius parameters of A = 5.0 × 1013 s−1 and Ea = 10.3 kcal mol−1 were obtained for one of the DQR rate constants,20 where Ea gives a reasonable approximations of the corresponding torsional barrier. The value of Ea = 11.37 kcal mol−1 obtained presently for sample B is somewhat higher, probably because of the fact that, apart from the torsional barrier, it

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CONCLUSIONS It is shown for the first time that, in solution, the hindered rotation of the −NH3+ group in a sterically crowded molecular environment can be completely frozen on the NMR time scale. These effects were observed for solutions of 1,2,3,4tetrabromotriptycyl-9-ammonium tetrafluoroborate in tetrahydrofuran with and without excess of the acid. In the neutral and one of the acidified solutions, that prepared in open air, a part of the solvates of the ammonium salt includes water molecules present in trace amount in the samples. In the absence of an excess of HBF4, these structures generally undergo fast interconversions as well as intermolecular exchanges of the ammonium protons. With added excess of HBF4, the intermolecular exchanges are quenched and the individual solvates, with and without participation of water molecules, give separate NMR signals of the ammonium protons already at room temperature. For the acidified solution prepared in a drybox, the solvation shells probably do not contain water. For this sample, a clear NMR picture of the hindered rotation of the ammonium group was observed at low and intermediate temperatures, and the Arrhenius parameters of this process were evaluated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11812. Partial VT 1H NMR (300 MHz) spectra of 1 in samples A and B; spin-lattice relaxation data for the ammonium protons in 1 in sample A (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Slawomir Szymanski: 0000-0002-5510-4726 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Center (NCN Grant OPUS-2012/05/B/ST4/00103). REFERENCES

(1) Gu, Z.; Ebisawa, K.; McDermott, A. Hydrogen bonding effects on amine rotation rates in crystalline amino acids. Solid State Nucl. Magn. Reson. 1996, 7, 161−172. (2) Craven, S. M. Far infrared spectra of 1-adamantanol, 1adamantanamine and 1-adamantanamine hydrochloride torsional frequencies and barriers to internal rotation. Spetrochim. Acta 1973, 29, 679−685. (3) Esadze, A.; Li, D. W.; Wang, T.; Bruschweiler, R.; Iwahara, J. Dynamics of lysine side-chain amino groups in a protein studied by heteronuclear 1H−15N NMR spectroscopy. J. Am. Chem. Soc. 2011, 133, 909−919. (4) Zandarashvili, L.; Iwahara, J. Temperature dependence of internal motions of protein side-chain NH3+ groups: Insight into energy

H

DOI: 10.1021/acs.jpca.6b11812 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A barriers for transient breakage of hydrogen bonds. Biochemistry 2015, 54, 538−545. (5) Perrin, C. L.; Gipe, R. K. Rotation, solvation, and hydrogen bonding of aqueous ammonium ion. J. Am. Chem. Soc. 1986, 108, 1088−1089. (6) Kikuchi, O.; Watanabe, T.; Ogawa, Y.; Takase, H.; Takahashi, O. Ab initio MO and Monte Carlo simulation study on the conformation of L-alanine zwitterion in aqueous solution. J. Phys. Org. Chem. 1997, 10, 145−151. (7) Levy, G. C.; Godwin, A. D.; Hewitt, J. M.; Sutcliffe, C. Natural abundance 15N and 13C spectroscopy. Aminobenzoic acids, substituted anilines, and related compounds. J. Magn. Reson. 1978, 29, 553−552. (8) Lambert, J. B.; Stec, D., III Effect of molecular motion and solvent interactions on nitrogen-15 relaxation in anilines. Org. Magn. Reson. 1984, 22, 301−307. (9) Perrin, C. L.; Gipe, R. K. Rotation and solvation of ammonium ion. Science 1987, 238, 1393−1394. (10) Masuda, Y. Solvent effect on rotational relaxation time of ammonium ion. J. Phys. Chem. A 2001, 105, 2989−2996. (11) Anderson, J. E.; Rawson, D. I. Nuclear magnetic resonance. Slow rotation of a methyl group. J. Chem. Soc., Chem. Commun. 1973, 830−831. (12) Nakamura, M.; Oki, M.; Nakanishi, H. Restricted rotation involving the tetrahedral carbon. V. Direct observation of the hindered rotation of a methyl group by high resolution nuclear magnetic resonance spectroscopy. J. Am. Chem. Soc. 1973, 95, 7169−7171. (13) Nakamura, M.; Oki, M.; Nakanishi, H.; Yamamoto, O. Restricted rotation involving the tetrahedral carbon. X. Barriers to rotation of methyl groups in 9-methyltriptycene derivatives. Bull. Chem. Soc. Jpn. 1974, 47, 2415−2419. (14) Imashiro, F.; Hirayama, K.; Takegoshi, K.; Terao, T.; Saika, A.; Taira, Z. Crystal structure of 1,4,9,10-tetramethyltriptycene and barriers to rotation of bridgehead-methyl groups in 1,4,9,10tetramethyl- and 1,4-dichloro-9,10-dimethyl-triptycene. J. Chem. Soc., Perkin Trans. 2 1988, 1401−1408. (15) Oki, M. The Chemistry of Rotational Isomers; Springer-Verlag: Berlin, 1993. (16) Szymanski, S. Nuclear magnetic resonance line shapes of methyl-like quantum rotors in low-temperature solids. J. Chem. Phys. 1999, 111, 288−299. (17) Bernatowicz, P.; Szymanski, S. Wave properties of a methyl group under ambient conditions. Phys. Rev. Lett. 2002, 89, 023004. (18) Czerski, I.; Bernatowicz, P.; Jazwinski, J.; Szymanski, S. Nonclassical effects in liquid-phase nuclear magnetic resonance spectra of 9-methyltriptycene derivatives. J. Chem. Phys. 2003, 118, 7157− 7160. (19) Bernatowicz, P.; Czerski, I.; Jazwinski, J.; Szymanski, S. Carr− Purcell echo spectra in the studies of lineshape effects. Nonclassical hindered rotation of methyl groups in 1,2,3,4-tetrachloro-9,10dimethyltriptycene. J. Magn. Reson. 2004, 169, 284−292. (20) Czerski, I.; Szymanski, S. Damped quantum rotation of the methyl group in liquid-phase NMR spectra of 9-methyltriptycene derivatives. Pol. J. Chem. 2006, 80, 1233−1257. (21) Gutsche, P.; Schmitt, H.; Haeberlen, U.; Ratajczyk, T.; Szymanski, S. Fingerprints of damped quantum rotation observed in solid-state proton NMR spectra. ChemPhysChem 2006, 7, 886−893. (22) CCDC 1448445 (1) (23) Szupiluk, A. Synthesis of sterically crowded 9-nitrotriptycenes by the Diels-Alder cycloaddition reaction. Tetrahedron Lett. 2016, 57, 5251−5253. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision A1; Gaussian, Inc.: Wallingford CT, 2009. (25) Roberts, J. D. Temperature effects on nuclear magnetic resonance absorption of hydrogens attached to nitrogen. J. Am. Chem. Soc. 1956, 78, 4495−4496.

(26) Lehn, J. M.; Kintzinger, J. P. Nitrogen NMR; Witanowski, M., Webb, G. A., Eds.; Plenum Publishing Company Ltd.: Berlin, 1973; p 146. (27) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: London, 1961; Chapter 8. (28) Alexander, S. Exchange of interacting nuclear spins in nuclear magnetic resonance. II. Chemical exchange. J. Chem. Phys. 1962, 37, 974−980. (29) Kleier, D. A.; Binsch, G. General theory of exchange-broadened NMR line shapes II. Exploitation of invariance properties. J. Magn. Reson. 1970, 3, 146−160. (30) Ratajczyk, T.; Czerski, I.; Kamienska-Trela, K.; Szymanski, S.; Wojcik, J. 1J(C,H) couplings to the individual protons in a methyl group: Evidence of the methyl protons’ engagement in hydrogen bonds. Angew. Chem., Int. Ed. 2005, 44, 1230−1232. (31) Limbach, H. -H. In Hydrogen-Transfer Reactions; Hynes, J. T., Klinman, J. P., Limbach, H.-H, Schowen, R. L., Eds.; Wiley-VCH: Weinheim, 2007; Vol. 1, pp 135−221, and references therein.

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DOI: 10.1021/acs.jpca.6b11812 J. Phys. Chem. A XXXX, XXX, XXX−XXX