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NMR and XRD Study of Hydrogen Bonding in Picolinic Acid N-Oxide in Crystalline State and Solutions: Media and Temperature Effects on Potential Energy Surface Vytautas Balevicius, Arunas Marsalka, Vytautas Klimavicius, Laurynas Dagys, Maria Gdaniec, Ingrid Svoboda, and Hartmut Fuess J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05421 • Publication Date (Web): 05 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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The Journal of Physical Chemistry

NMR and XRD Study of Hydrogen Bonding in Picolinic Acid N-Oxide in Crystalline State and Solutions: Media and Temperature Effects on Potential Energy Surface

Vytautas Balevicius1*, Arūnas Maršalka1, Vytautas Klimavičius1,2, Laurynas Dagys1, Maria Gdaniec3, Ingrid Svoboda4, Hartmut Fuess4

1

Institute of Chemical Physics, Vilnius University, Sauletekio av. 3, LT-10257 Vilnius, Lithuania 2

Eduard-Zintl Institute for Inorganic and Physical Chemistry, University of Technology Darmstadt, Alarich-Weiss-Str. 8, D-64287 Darmstadt, Germany

3

Faculty of Chemistry, Adam Mickiewicz University, Umultowska ul. 89b, PL-61614 Poznań, Poland

4

Institute for Materials Science, University of Technology Darmstadt, Alarich-Weiss-Str. 2, D64287 Darmstadt, Germany

ACS Paragon Plus Environment

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ABSTRACT: Solvent and temperature effects on H-bond in crystalline picolinic acid N-oxide (PANO) and in solutions were studied by NMR (1H MAS and 1H–13C CP/MAS) and XRD methods. The single-crystal XRD experiments on β-polymorph were carried out at 105 K and 299 K.

13

C chemical shifts of PANO pyridine ring carbons were chosen as an effective

diagnostic tool for the H-bond sensing. The crystal field in PANO forces the proton displacement from donor to acceptor atoms much stronger than the solvent reaction field, including that created by the most polar solvents. NMR and XRD data for crystalline PANO do not confirm any H-bond geometry changes in the studied temperature range. On the contrary, a considerable contraction of r(O–H) bond was observed for PANO in acetonitrile (ACN) solution upon heating. The relative contraction of r(O–H) bond respect to R(O···O) perfectly fits to the global dielectric scheme deduced for a vast set of common solvents and the dependence of dielectric permittivity of ACN on temperature. The subtle H-bond changes can be explained by the temperature dependence of the shape of potential energy surface in the liquid state. Both factors, temperature and dielectric permittivity are comparable in triggering this effect.


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Introduction "Hydrogen bonds are quite special bonds: they are strong enough to significantly modify properties of molecules, yet weak enough to be distorted or broken fairly easily by an external stimulus, an incoming reactant or by-passing molecule" - this felicitous sentence was brought out by Hermansson at the XXII International Conference on Horizons in Hydrogen Bond Research.1 Indeed, a crucial role of media in H-bonding is stressed almost in all studies on this topic. Due to its abundance and high importance in the molecular world covering biochemistry, biological systems and life processes, the need of better understanding of subtle details of H-bonding stimulate research and unabated discussions. Among the important issues are the proton location and motion pathways, correlating them with the strength of the bonds and with the external factors, quantum origin of hydrogen delocalization, and proton transfer reactions in various phases including the features to monitor it.2-7 Direct information on H-bond arrangement and atom displacements in the solid state is obtained by single-crystal neutron diffraction (ND).8-11 Divers aspects of H-bonding can be studied with different experimental techniques, such as NMR, vibrational (FTIR/Raman) and dielectric spectroscopy, nevertheless ND provides the most accurate geometric structural parameters of this interaction.10 In contrast with X-ray diffraction (XRD), the hydrogen-atom parameters in ND experiments are determined with approximately the same precision as those for other atoms. One of the drawbacks of this technique is that it requires relatively large single crystals, what is not feasible in many cases. For this reason, and due to a better accessibility, XRD is still used to provide geometric information about the arrangement of atoms in H-bond, even though it does not give precise information on the H-atom position. Therefore, the most


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effective way to study H-bond is to merge spectroscopic and diffraction data. Parameters, which can be very precisely obtained from spectroscopic measurements, like vibrational frequencies, chemical shifts, coupling constants, have to be correlated with structural features obtained by ND or XRD. In the present work a combined study by NMR and XRD has been undertaken on the picolinic acid N-oxide (PANO). The solid-state NMR applying cross polarization/magic angle spinning (CP MAS) techniques together with the high resolution liquid state NMR spectroscopy allow to follow the evolution of H-bond with temperature changes, both in the crystalline state and in solutions with solvents of varying polarity. There were several reasons for choosing PANO for this study. Complexes between carboxylic acids and pyridine N-oxide or its derivatives are promising benchmark systems for short O– H···O H-bonding. These systems exhibit in most cases asymmetric and flat single well proton potentials, that results in large amplitude proton dynamics with occurrences of frequent proton transfer.4 PANO is probably the most prominent member of this family with a very short O···O distance that only slightly exceeds 2.40 Å. The data of high level quantum chemistry calculations,12-15 XRD,16,17 inelastic neutron scattering (INS), FTIR and Raman spectra13,18 are available for this system. The choice of PANO was also dictated by our earlier NMR research and the experience in studying H-bonded systems such as complexes between divers acids and pyridine N-oxide (PyO) and PANO in various solvents.5,15,19-21 The main focus was on Hbonding in the crystalline state and in solution and the effect of dielectric media and temperature on these systems. The knowledge of precise values of NMR parameters can provide new insight into the temperature evolution of the potential energy surface (PES) and thus contribute to the polemic around the differences between H-bonded structures at low and high temperatures.8,9


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Experimental Samples. Commercial picolinic acid N-oxide (PANO), acetonitrile (ACN), acetone (AC), chloroform (CLF), dimethylsulphoxide (DMSO) (all from Sigma – Aldrich) and water were preliminary purified by standard methods.22 The non-aqueous solutions were prepared in the nitrogen filled dry box. In addition, molecular sieves, degassing, argon bubbling, vacuum flushing were applied for drying the chemicals. NMR spectroscopy. Solid-state 1H MAS and

13

C CP/MAS NMR experiments were carried

out on BRUKER AVANCE III HD 400 MHz NMR spectrometer operating at 400 and 100 MHz for 1H and 13C, respectively, using MAS 4 BL CP BB DVT probe and 4 mm zirconia rotor. For MAS spectra the sample-spinning rate was used up to 15 kHz. The solid state NMR measurements were performed on a powder sample at 230 K to 340 K. The temperature in the MAS rotor was calibrated using temperature dependence of

207

Pb NMR isotropic chemical shift

of Pb(NO3)2.23 For variable temperature 1H–13C CP MAS experiments a ramped 2 ms contact time pulse was employed and 8-256 scans were accumulated using repetition delay equal to 3·T1 (4-100 s). To assign the

13

C peaks unambiguously, a 1H–13C HETCOR measurement was

performed with the following parameters: 50 µs contact pulse, 32 scans, 128 increments, 5 s of repetition delay. Adamantane was used as an external 1H and 13C chemical shift reference (1.85 ppm – 1H; 29.47/38.52 ppm – 13C). Liquid-state 1H and

13

C NMR experiments were carried out on the same NMR spectrometer

using 5 mm BBO probe. The variable-temperature measurements in the range of 230 K - 350 K were controlled with the accuracy of ± 0.5 K. The signal of TSPSA (3-(trimethylsilyl)propane sulfonic acid sodium salt) in ACN-d3 solution in capillary insert was used as the reference for 1

H NMR spectra and for lock. When necessary, 1H and

13

C chemical shifts of PANO were


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confirmed by the analysis of 2D homo- and hetero-nuclear NMR experiments (1H–1H COSY, 1

H–13C HSQC and 1H–13C HMBC). NMR spectra were processed using Topsin 3.2 software. Some additional processing was

carried out using Microcal Origin 9 package. X-ray crystallography. A single crystal of β-PANO was obtained from water solution at the National Institute of Chemistry (Ljubljana, Slovenia). Diffraction data were collected at 105(2) K and 293(2) K with an Oxford Diffraction Xcalibur CCD diffractometer (TU Darmstadt) using Mo Kα radiation. The structure was solved with SHELXS24 and refined with SHELXL-2016.25 All H atoms were located in an electron-density difference map, and their atomic parameters were freely refined. Crystallographic data have been deposited in CIF format at the Cambridge Crystallographic Data Centre with CCDC accession codes 1845196 and 1845197.

Results and discussion XRD studies of β-PANO and H-bond geometries. The crystals of PANO are known to exhibit dimorphism.13 The crystal structure of α-polymorph of PANO was reported by Steiner et al.16 (XRD at 125 K) and Shishkina et al.17 (XRD, charge density, 100 K). For β-polymorph some limited structural information was given in ref. 12, however full structural report is missing. In order to evaluate the temperature effect on the arrangement of atoms forming OH···O H-bond, we decided to redetermine the crystal structure of β-PANO at 105 K and 299 K. Crystallographic data and some details of XRD analysis are listed in Table 1. The ORTEP presentation of the molecules in β-PANO for 105 and 299 K is given in Figure 1. The geometric parameters of the intramolecular H-bond for α-16,17 and β-PANO crystals are listed in Table 2.


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TABLE 1. Crystal data and details of structure determination of β-PANO at 105 and 299 K. C6H5NO3

Empirical formula Temperature, K

105 (2)

299 (2)

Wavelength, Ǻ

0.71073

Crystal system, space group

Orthorhombic, Pbcm

Unit cell dimensions: a, Ǻ

6.7693(2)

6.823(1)

b, Ǻ

14.1543(3)

14.234(2)

c, Ǻ

6.0467(1)

6.218(1)

90.0, 90.0, 90.0

α, β, γ; ° Volume, Ǻ3

579.36 (2)

603.88 (16)

Z; Calculated density, Mg m–3

4; 1.595

4; 1.530

Absorption coefficient, mm–1

0.131

0.125

Crystal size, mm

0.38 x 0.36 x 0.34

2Θ range for data collection, °

6.02 to 52.65

8.26 to 52.70

Refls collected/unique; R(int)

5456/653; 0.0091

3962/676; 0.0181

Completeness to Θmax

99.5%

99.3%

Absorption correction

Semi-empirical from equivalents

Max. and min. transmission

0.971 and 0.942

0.975 and 0.940

Full-matrix least-squares on F2

Refinement method Data/restraints/parameters

653/0/77

676/0/76

1.106

1.134

Final R indices (I > 2σ(I))

R1 = 0.0280; wR2 = 0.0877

R1 = 0.0663; wR2 = 0.1782

R indices (all data)

R1 = 0.0292; wR2 = 0.0884

R1 = 0.0790; wR2 = 0.1982

0.32 and – 0.20

0.32 and – 0.39

Goodness-of-fit on F2

Largest diff. peak and hole, e⋅Ǻ–3


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Figure 1. ORTEP presentation of the PANO molecule in β-polymorph at 299 K (bottom) and 105 K (top).


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Table 2. Geometry of intramolecular O2–H···O1 hydrogen bond in PANO D–H (Å)

H···O1 (Å)

O2···O1(Å)

∠O2–H···O1 (°)

T (K)

β-PANO

1.00 (6)

1.48 (7)

2.409 (4)

153(5)

299 (2)

β-PANO

1.01 (3)

1.45 (3)

2.4137 (14)

157 (2)

105 (2)

α-PANO16

1.04(3)

1.42 (3)

2.425 (2)

159 (3)

125

α-PANO17

1.01

1.44

2.424

161.3

100

The observed changes in the H-bond geometry for PANO polymorphs at different temperatures are smaller than the e.s.d.s of the determined parameters (Table 2), a situation already discussed in ref. 9. The authors stressed that changes in donor-acceptor distances of Hbond are smaller than, or only slightly exceed, the margin of the experimental error, what leads to some pessimism and polemic.8,9 Figure 2 shows electron density peaks corresponding to the carboxylic H-atoms on electrondensity difference maps for β-PANO (299 and 105 K) and α-PANO (100 K).16 It is evident, that in the studied temperature range the bridging H-atom is located much closer to the carboxylic oxygen than to the N-oxide O atom. The difference Fourier maps for the low-temperature structures of the two polymorphs are generally similar, however, the peak corresponding to the carboxylic H-atom in α-PANO is noticeably more elongated in the direction of the acceptor atom O1 than that in β-PANO (Fig. 2b-c). The observed changes of the O1···O2 and O-H distances with temperature for β-PANO are negligible as they are within the range of one e.s.d. Hence, the XRD data indicate that H-bond in β-PANO can be considered as a relatively ‘stiff’. Due to the fact that any plausible alterations in the atomic arrangement of the O-H···O hydrogen bond in PANO are beyond the detection limit of the XRD method, we decided to study this system, both


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in the solid state and in solutions, with NMR spectroscopy hoping to reveal some fine details in the H-bond geometry changes.

Figure 2. Electron density difference maps of H-bond region showing carboxylic H-atom peaks: a – β-PANO at 299 K; b – β-PANO at 100 K; c – α-PANO at 125 K.16

1

H and

13

C solid state NMR of dry and wet PANO. Carrying out NMR experiments it is

quite logical to expect that the most sensitive source of information on the state of H-bond should be the 1H chemical shift of the bridge proton. The relationship between experimental 1H chemical shifts and hydrogen position in the O–H…O bridge can be calibrated using available structural data sets and then can be used in further studies. However, the precise measurements of the chemical shift of O–H···O proton often fail because of difficulties of drying and purification of the systems under investigation. In the present work an extremely large 1H chemical shift of H-bond proton of solid PANO was observed (20.5 ± 0.5 ppm, Fig. 3) that was never found in its liquid-state NMR spectra in various solvents.15 This indicates that crystalline PANO definitely represents a class of short- and very short H-bonds, even with certain


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symptoms the incipient proton transfer can occur. Also note, the observed chemical shift

δ(COOH) fits well into various correlation schemes, e.g. δ vs. O–H...O geometry,15 or δ vs. pK and ∆PA(proton affinity),26 getting in the range between values obtained for the series of H-bond complexes of strong carboxylic acids with pyridine N-oxides (∼ 17 - 19 ppm)15,26 and the symmetric Zundel cation (H5O2+, 20.93 (calculated) and 21.3 ppm (experimental), respectively).27 However, the 1H chemical shifts are strongly affected by the presence of water. The sample of PANO powder was intentionally contaminated for the next experiments by ∼ 20 µl of H2O added to 280 mg of the main substance. Even such small amount of moisture influences the bridge proton signal position in 1H NMR spectrum very drastically (Figure 3). Hence, this spectral parameter cannot be considered as very reliable to build the correlation with structural details.

Figure 3. 1H MAS and 13C CP/MAS spectra of solid PANO at 293 K. The 1H chemical shift of the bridging proton is strongly affected by contamination with water: blue line corresponds to a ‘dry’ crystal, red dashed line – powder sample with added water (for more comments see text).


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The water impurities have less significant effect on the

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13

C chemical shifts (Figure 3). This

suggests that instead of the bridge proton chemical shift, the chemical shifts of the pyridine ring carbons in PANO can be used as an effective and more reliable diagnostic tool monitoring the state of H-bond.

Correlation of

13

C liquid and solid state NMR data with the calculated H-bond

geometries and the polarity of the environment. The chemical shifts of C2, C6 and C3, C5 carbon atoms are only slightly affected by increasing H-bond strength and the polarity of the solvent. In contrast, the chemical shift of C4 shows a strong tendency towards higher values. This was noticed in the earlier study on the series of pyridine N-oxide (PyO) complexes with acids of increasing strength.15 Thus, the pattern of the

13

C chemical shifts of pyridine ring

carbons can be used for monitoring the H-bond geometry changes. For this purpose the parameter ξ is constructed:

ξ =

∆ ∆ᇲ

,

where ∆= ߜሺC4ሻ − ሺߜሺC3ሻ + ߜሺC5ሻሻ/2 ,

ሺ1ሻ

∆ᇱ = ሺߜሺC2ሻ + ߜሺC6ሻሻ/2 − ሺߜሺC3ሻ + ߜሺC5ሻሻ/2 .

Its definition is presented graphically in Figure 4. What is most important – such internal referencing of chemical shift δ(C4) using the signals of intramolecular carbons instead of those from the external reference compound (usually placed in a capillary insert) allows to avoid the


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problems to evaluate the changes of the magnetic susceptibility of the sample. These changes contribute to the measured chemical shifts and thus mask the 'pure' effect due to the changes in the H-bond arrangement. The correlation between ξ (calculated using experimental

13

C NMR

data) and the relative proton position r(O–H)/R(O···O) is fairly good (Figure 5).

Figure 4. The definition of parameter ξ (see eq 1) used for the experimental monitoring of the geometry changes of H-bond.


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Figure 5. The correlation between ξ determined using experimental 13C NMR data (all at room temperature) and the relative proton position r(O–H)/R(O···O) in the H-bond of PANO in solutions and crystalline phase. The distances were determined using high level calculations: DFT (full geometry optimization, PBE1PBE/6-311++G** with PCM-UFF) for PANO dissolved in various solvents15 and CPMD for the crystalline PANO, where two points on the graph correspond to the Γ-point approximation and to the k-point optimization, respectively.12 The solid line was drawn as a guideline. The left-right arrow symbolizes the SRF (solvent reaction field) versus CFF (crystal force field) effect on r(O–H)/R(O…O).

The electric field created by the electric dipoles of neighboring particles at the H-bond site can be considered as one of the most significant factors driving the proton along H-bond.28 The strength of this field depends on the geometric details as well as a degree of coherence in the motion of individual dipoles. In a series of common organic liquids as solvents, where the dielectric relaxation processes are fast enough, this can be correlated via static dielectric


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constants (ε). However, the ε-presentation is not proper if the data for nano-structured, porous or powder samples have to be included into the joint consideration as the dielectric constants are too difficult to determine for such systems precisely. In such case, only the comparative matching of NMR data (the

13

C chemical shifts in the present case) can be realized in the ε-

presentation (Figure 6). This mapping allows to state whether the crystal force field (CFF) in PANO forces the proton from donor to acceptor atoms much stronger than the solvent reaction field (SRF) created even by most polar molecules (DMSO and H2O). The jump due to SRF ↔ CFF effect in the calculated values of r(O–H)/R(O…O) is also very clearly seen at the liquidcrystal transition (Figure 5).


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Figure 6. The matching of solid-state 13C NMR (CP MAS) data of PANO to the global dielectric scheme that was built using the dielectric constant values for a wide set of traditional solvents.

Discussion on the observed temperature and solvent polarity effects. As it was noted in ref 29, for any discussion concerning the mobility of bridging protons in short hydrogen bonds, the nature of the potential energy surface (PES) should be discussed. Strictly speaking, PES is by definition temperature independent. However, this holds for the complete surface involving all the degrees of freedom. Therefore the term 'effective' PES is more appropriate, but it should be stressed that the effective PES is a projection of the full PES on the selected degrees of freedom.


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The projected/effective PES can exhibit temperature effects originating from thermal fluctuations. The high-level computations12-14 predict that the potential surface for PANO has to be short asymmetric and flat single well. For proton migration to occur, two scenarios are possible: (i) the effective potential surface is unresponsive to temperature change, and thus thermal excitations bring the proton to the higher-energy levels; (ii) the shape of the potential surface is dependent on temperature, and the protons occupy the lowest-energy state appropriate for that temperature.29 In order to establish, which of these two scenarios is more appropriate, in other words - does the effective PES slightly change in the local environment depending on the temperature, variable temperature NMR experiments were carried out for PANO in the crystalline state and in acetonitrile (ACN) solution. ACN was chosen due to its middle range value of the global dielectric scheme (Figure 6). In addition, the liquid phase ACN extends over a wide temperature range from 228 K (melting point) to 355 K (boiling point). The dependence of static dielectric constant of ACN on temperature was studied in several works (Figure 7).30-32


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Figure 7. Temperature dependence of the static dielectric constant of ACN. The data have been taken from Refs 30-32. The linear fit covers the temperature range of the present NMR experiments.

The experimental

13

C chemical shifts of solid PANO have been measured applying CP MAS

technique. They depend very weakly on temperature over the whole studied range 230 K - 340 K. The largest changes in

13

C NMR signal position was less than 0.6 ppm, and therefore this

graph is not shown. However, the temperature effect for PANO in ACN solution is much more pronounced. The C4 chemical shift decreases with increasing temperature whereas an opposite tendency is observed for all other chemical shifts (C2 – C7) (Figure 8).


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Figure 8. Temperature dependencies of

13

C chemical shifts of PANO in ACN solution. For

better visualization the upper graph was transformed referencing the chemical shifts for each carbons with respect to their values at the lowest temperature (Tmin).


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In order to see the changes in PANO H-bond arrangement upon varying temperature the parameter ξ was calculated (eq 1) using the experimental

13

C chemical shifts for C2 – C6

carbons in the solid state and in ACN solution. The most important finding is that ξ in the crystalline PANO looks to be temperatureindependent in the margins of error, although a week tendency of ξ to decrease upon heating can be envisaged (Figure 9). It would mean that the intramolecular H-bond in solid PANO can be considered as a rather rigid framed structure. This correlates with the XRD data (Table 2). Also the high level Car - Parrinello solid-state calculations, carried out by Stare et al.,13 have shown that the present system is characterized by an asymmetric single well potential with no large amplitudes in the bridge proton motion. Hence, it can be stated that the collected NMR and XRD data sets for PANO crystal do not allow rigorously to confirm any geometry changes in the hydrogen bond and the temperature evolution of its PES, at least in the range of ∼ 100 - 300 K. This failure may be caused by the large uncertainties of solid-state NMR experiments, which arise due to significant broadening and partial overlapping of NMR signals (see the insert in Figure 6). This means that PANO is quite different from the other systems of short H-bonds in crystals, like pyridine-3,5-dicarboxylic acid, urea - phosphoric acid, the series of complexes of derivatives of pyridine with carboxylic acids, etc,9,11,29 where the PES obtained from experimental ND data were found to be temperature-dependent. Probably such ‘softness’ in the atomic replacements is due to intermolecular H-bonds in the mentioned systems, whereas in PANO it is intramolecular.


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0.88

ξ

Crystal

0.86 0.84 0.38 0.36

ACN solution

0.34 0.32 0.30 250

300

350

T, K

Figure 9. Temperature dependencies of parameter ξ of PANO in crystal and ACN solution.

A different situation was met in solution. The parameter ξ is definitely temperature-dependent in the liquid environment (Figure 9). Hence, the H-bond in PANO in the liquid state (ACN solution) can be expected to be much softer and more flexible. As the static dielectric constant of ACN decreases with increasing temperature, the behavior of 13C NMR signals (Figure 8) can be mapped on the global dielectric scheme using the ε values taken from Figure 7. The variation of

ξ fits well to this scheme (Figure 10). This means that upon heating the relative contraction of r(O–H) bond with respect to R(O···O), that reflects in the ξ values. Here it is worthy to note a much stronger effect of temperature on dielectric constant and thus on the state of H-bond was deduced for the collidine (2,4,6-trimethylpyridine)/H–F complex in CDF3/CDClF2 solution.33 A strong increase of ε from 14 at 190 K to 38 at 103 K was observed for the mixtures used in the NMR measurements. Upon cooling, hence, increasing the dielectric constant, the NMR spectra have revealed a gradual transformation of an asymmetric molecular


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complex F–H...N to a quasi-symmetric complex Fδ−...H...Nδ+ and eventually to a more or less zwitterionic species F−...H–N+.

Figure 10. The mapping of temperature dependence of parameter ξ of PANO in ACN (blue rhombs) on the global dielectric scheme build using the dielectric constants of acetone (AC), chloroform (CLF), dimethylsulphoxide (DMSO) and H2O at fixed room temperature (red circles).

The effect of the relative contraction of r(O–H) in PANO can be explained in terms of dielectric media effect on the shape of PES. The potential energy surface of PANO in two dimensions (2D PES) using r(O–H) and R(O···O) as coordinates was determined by various computational approaches and levels of theory.14,34 The shape of 2D PES, obtained therein, is approximately reproduced in the contour map in Figure 11 without matching the exact numerical values of energy and distances. As PANO can be considered being a rather rigid H-bond system


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where the O…O distance does not change with temperature, the 2D PES can be projected to 1D PES. The PES in 1D, i.e. only r(O–H) varies at fixed R(O··· O), which corresponds to the fully optimized structure, and its evolution upon changing polarity of the solvent was studied applying PCM approach. The effect of increasing polarity on PES was found to be very significant for various H-bond systems, with- and without proton transfer.18,35 The solvent effect on 1D PES of PANO was approximately reproduced taking the view of the proton potential functions from ref 18. This is shown in the upper part in Figure 11.

Figure 11. On the thermal evolution of effective PES of PANO. More comments in text.

It is often conceivable that the observed structural changes in the H-bond systems with proton transfer can be explained in the frame of a static PES.9 However, the shape of PES must be


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sufficiently flat, so that the first excited state may become populated at realistic temperatures, i. e. those are available working with organic materials and not to destroy them. In PANO, according to the proton dynamics quantum mechanical simulation by Car-Parrinello method,13 the bridge proton is located about 99% of time in the energy minimum near the carboxylic oxygen (O2) and the jumps to the N–O(1) acceptor are rare. Moreover, the O–H stretching frequencies (0 → 1) in PANO and its derivatives in solution and matrix isolation observed in FTIR experiments are within ∼ 1300 – 2600 cm–1,6,18 and thus corresponding to temperatures ∼ 1000 K and higher. Therefore, it seems to be very unlikely that the population of the first excited state could be changed at the temperatures covered in the present work. The observed contraction of r(O–H) bond at increasing temperature is far out the NMR experimental error in the case of PANO in ACN solution (Figure 9). This observation as well as the arguments given just above rule out the concept of static PES. The contraction in the ground state (denoted as ∆r(O–H), Figure 11) can be explained via dielectric media factor that is temperature-dependent and influences the shape of PES, as it follows from quantum chemistry calculations.18,34 Thus, the concept of temperature-dependent shape of PES in the liquid state can be accepted. Moreover, temperature and dielectric media effects are equivalent procuring such subtle geometry changes of H-bond.

Concluding remarks The spectral feature for monitoring the relative proton position in the H-bond based on the experimentally measured 13C NMR signal shifts (eq 1) was found; it can be applied in the solid state as well as in solutions for a wide range of molecular systems containing a pyridine ring. However, its correlation with H-bond arrangement and atomic displacements has to be checked


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and calibrated using either proper level quantum chemistry calculations or experimental geometries when such are available. The single-crystal XRD and solid-state NMR data have shown that the intramolecular Hbond in solid PANO can be considered as a rigidly framed structure that is characterized by an asymmetric single well potential with no large amplitude motion of atoms in the O–H···O bridge in the temperature range 100 K – 300 K. This correlates with the high level Car - Parrinello solid-state calculations, carried out by Stare et al.13 The crystal field in PANO forces the proton displacement from donor to acceptor atoms much stronger than the solvent reaction field created by most polar molecules (DMSO and H2O). In the case of PANO the shape of effective PES in solution must change due to slight changes in the local environment with temperature, as it was suggested by Wilson for crystalline systems.11 In the fast relaxing liquid solutions the variation of the static dielectric constant with temperature properly and completely characterizes the media effect on H-bonding (Fig. 10). Unfortunately, the uncertainties of solid-state NMR experiments, which arise from significant broadening and partial overlapping of NMR signals, did not rigorously confirm or reject the temperature evolution of PES in the crystalline state. However, the solid-state NMR looks very promising and stimulating for further experiments approaching at much lower temperatures.

Supporting Information 1. The data of the liquid state

13

C NMR experiments and ξ values used in Figures 9 and 10

(PDF file). 2. The dependencies of dielectric constant of ACN on temperature (PDF file).


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Other data of this study are available from the corresponding author on reasonable request.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Tel: +370 5 223 4588.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge Center of Spectroscopic Characterization of Materials and Electronic/Molecular

Processes

(Scientific

infrastructure

"Spectroversum"

www.spectroversum.ff.vu.lt) at Lithuanian National Center for Physical Sciences and Technology for use of spectroscopic equipment. The special thanks to Professor Dušan Hadži for single-crystal and valuable discussions.

REFERENCES (1) Hermansson, K. Book of Abstracts, XXII Horizons in Hydrogen Bond Research, Jyväskylä, Finland, Sept. 11–14, 2017; p.8.


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