Density Functional Study of the Proton Transfer Effect on Vibrations of Strong (Short) Intermolecular O−H···N/O⁻···H−N⁺ Hydrogen Bonds in Aprotic Solvents

The structure and spectroscopic properties of the 1:1 complexes of substituted pyridines with benzoic acid and phenol derivatives in aprotic solvents are studied using B3LYP functional combined with the polarizable continuum model approximation. Two extreme structures are investigated: the state without (HB) and with proton transfer (PT). In the presence of an external electric field the O···N distance is contracted and the PT state does appear. The PT state of both the pyridine−benzoic and the pyridine−phenol complexes displays the only IR-active band in the 2800−1800 frequency region, which is located around 2000 cm−1. However, the nature of the band is different for these two complexes. In the pyridine−benzoic acid complex it is practically a pure stretching vibration of the HN+ group, while in the pyridine−phenol complex it is the mixed vibration of the bridging proton. A specific feature of the PT state in the pyridine−phenol complex is an IR-intensive band near 600 cm−1, associated with the asymmetric stretching vibrations of the O−···HN+ fragment. Its intensity is reciprocally proportional to the O···N distance. The appearance of this band provides an efficient criterion to differentiate between the HB and PT states of the 1:1 complexes of phenols with pyridines in aprotic solvents

Low-Temperature NMR Studies of the Structure and Dynamics of a Novel Series of Acid−Base Complexes of HF with Collidine Exhibiting Scalar Couplings Across Hydrogen Bonds^†

The low-temperature 1H, 19F, and 15N NMR spectra of mixtures of collidine-15N (2,4,6-trimethylpyridine-15N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the temperature range 94−170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19F1H19F and 19F1H15N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex [ColHFHCol]+[FHF]- is a contact ion pair. Collidinium hydrogen difluoride [ColH]+[FHF]- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the [FHF]- anion. In this complex, the anion [FHF]- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride [ColH]+[F(HF)2]- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the [F(HF)n]- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant 1JFH when the F···H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1JFH = 0, the heavy atom coupling constant 2JFN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries

Nuclear Magnetic Resonance and ab Initio Studies of Small Complexes Formed between Water and Pyridine Derivatives in Solid and Liquid Phases

The structure and geometry of hydrogen-bonded complexes formed between heterocyclic bases, namely, pyridine and 2,4,6-trimethylpyridine (collidine), and water were experimentally studied by NMR spectroscopy in frozen phase and in highly polar aprotic liquefied freon mixtures and theoretically modeled for gas phase. Hydrogen-bonded species in frozen heterocycle−water mixtures were characterized experimentally using 15N NMR. When base was in excess, one water molecule was symmetrically bonded to two heterocyclic molecules. This complex was characterized by the rHN distances of 1.82 Å for pyridine and 1.92 Å for collidine. The proton-donating ability of water in such complexes was affected by an anticooperative interaction between the two coupled hydrogen bonds and exhibited an apparent pKa value of about 6.0. When water was in excess, it formed water clusters hydrogen bonded to base. Theoretical analysis of binding energies of small model heterocycle−water clusters indicated that water in such clusters was oriented as a chain. The NMR estimated rHN distances in these species were 1.69 Å for pyridine and 1.64 Å for collidine. Here, the proton-donating ability of the hydroxyl group bonded to the heterocycle was affected by a mutual cooperative interaction with other water molecules in the chain and became comparable to the proton-donating ability of a fictitious acid, exhibiting an apparent pKa value of about 4.9. This value seems to depend only slightly on the length of the water chain and on the presence of another base at the other end of the chain if more than two water molecules are involved. Thus, the proton-donating ability of the outer hydroxyl groups of biologically relevant water bridges should be comparable to the proton-donating ability of a fictitious acid exhibiting a pKa value of about 4.9 in water. Driven by the mixing entropy, monomeric water presented in the aprotic freonic mixtures above 170 K but completely precipitated upon further cooling. Traces of water could be suspended in the mixtures down to 130 K in the presence of about 20-fold excess of heterocyclic bases. The obtained experimental data indicated that at these conditions water trended to form the symmetric 2:1 heterocycle−water complexes, whose bridge protons resonated around 6.7 ppm

Nuclear Scalar Spin−Spin Couplings and Geometries of Hydrogen Bonds

Ab initio calculations of the scalar coupling constants 1J15N-1H ≡ JNH and 2J15N···15N ≡ JNN of the N−H···N hydrogen bonds in the anion [C⋮15N···L···15N⋮C]- (1), L = H, D, and of the cyclic hydrogen-bonded formamidine dimer (HCNHNH2)2 (2) have been performed using the density functional formalism as a function of the hydrogen bond and molecular geometries. The coupling constants are discussed in comparison with the experimental and calculated constants 1J19F-1H ≡ JFH and 2J19F-19F ≡ JFF reported previously as first set of examples of scalar couplings across hydrogen bonds for the hydrogen-bonded clusters of [F(HF)n]-, n = 1−4 by Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Gindin, V. A.; Golubev, N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.; Limbach, H. H. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 422. Using the valence bond order model, which has been successfully applied previously to explain hydrogen bond correlations in crystallography and solid-state NMR of hydrogen-bonded systems, the coupling constants are related to the hydrogen bond geometries and NMR chemical shifts. In terms of this model, there is no principal difference between FHF- and NHN hydrogen-bonded systems. Whereas the coupling constant values calculated using the DFT method for the fluorine case only reproduce the experimental trends, the agreement between theory and experiment is much better in the nitrogen cases, which allows one to determine the hydrogen bond geometries including the hydrogen bond angle from a full set of experimental coupling constants. It is found that the coupling constants JAB in A−H···B are proportional to the product of valence bond orders (pAHpHB)m, where m is an empirical parameter equal to 2 in the case of fluorine bridge atoms and close to 1 in the case of nitrogen bridge atoms. The coupling constants JAH depend on two terms, a positive term proportional to pAH and a negative term proportional to pAH(pHB)2 leading to vanishing or even negative values of JAH at larger A···H distances; in this region the constants JAB are larger than the absolute values of JAH. As a consequence, vanishing couplings between a hydrogen-bonded proton to a heavy nucleus across the hydrogen bond cannot be taken as an indication for a noncovalent character of this hydrogen bond. The existence of JAB is taken as a strong evidence for the covalent character of the hydrogen bonds studied. This is corroborated by a analysis of the molecular orbitals of (1) and their individual contributions to the coupling constants

Solid-State NMR Studies of Aminocarboxylic Salt Bridges in l-Lysine Modified Cellulose

LysCel is a cellulose-based material in which l-lysine molecules are grafted with their amino side chains to the cellulose hydroxyl groups. This modification increases considerably the mechanical strength and resistance of cellulosic structures toward water. It has been attributed to the formation of double salt bridges between lysine aminocarboxyl groups in the zwitterionic state. In order to characterize this unusual structure, we have performed high-resolution solid-state 15N and 13C CPMAS NMR experiments on LysCel samples labeled with 15N in the α-position or ε-position. Furthermore, 13C−15N REDOR experiments were performed on LysCel where half of the aminocarboxyl groups were labeled in 1-position with 13C and the other half in α-position with 15N. The comparison with the 13C and 15N chemical shifts of l-leucine lyophilized at different pH shows that the aminocarboxyl groups of LysCel are indeed zwitterionic. The REDOR experiments indicate distances of about 3.5 Å between the carboxyl carbon and the nitrogen atoms of different aminocarboxyl groups, indicating that the latter are in close contact with each other. However, the data are not compatible with isolated aminocarboxyl dimers but indicate the assembly of zwitterionic aminocarboxyl dimers either in a flat ribbon or as tetramers, exhibiting similar intra- and interdimer 13C···15N distances. This interaction of several aminocarboxyl groups is responsible for the zwitterionic state, in contrast to the gas phase, where amino acid dimers exhibiting two OHN hydrogen bonds are neutral

Photoinduced Proton Transfer in a Pyridine Based Polymer Gel

We describe an experimental and theoretical consideration of photoexcited proton transfer in a poly(4-vinyl pyridine)/pyridine gel. Evidence was found for two states of a multiple state process analyzed by DFT modeling. According to the latter, following irradiation at 385 nm, the proton donor is the CH group of the polymer main chain and the proton acceptor is the nitrogen of the polymeric pyridine side chain. Proton transfer is made possible through the assistance of a mobile pyridine solvent molecule acting as a transfer vehicle. Proton transfer promotes both a geometrical rearrangement of the vinyl side chain as well as electronic density redistribution. The photoproduct intermediatethe hydrogen-bonded complex between the protonated solvent pyridine molecule and the deprotonated polymeric pyridine side chainis identified by its Curie law magnetic susceptibility, ESR spectrum, and fluorescence lifetime measurements. The proton transfer from the nitrogen of the solvent pyridine molecule to the pyridine side chain nitrogen, producing pyridinium, is a thermodynamically favorable relaxation process and occurs without an energy barrier. The protonation of nitrogen on the polymeric side chain was detected by solid state NMR spectroscopy performed on a 15N-polymer enriched gel. The calculations and experimental data suggest a central role for the gel solvent molecule as a catalytic agent and proton transfer vehicle. The process suggested by DFT modeling may have relevance for photosensitive devices in part due to the fact that we have been able to show that long-lived paramagnetism may be included among the inducible properties of soft polymer gels