Reaction Pathways of Proton Transfer in Hydrogen-Bonded Phenol–Carboxylate Complexes Explored by Combined UV–Vis and NMR Spectroscopy

Combined low-temperature NMR/UV–vis spectroscopy (UVNMR), where optical and NMR spectra are measured in the NMR spectrometer under the same conditions, has been set up and applied to the study of H-bonded anions A··H··X– (AH = 1-13C-2-chloro-4-nitrophenol, X– = 15 carboxylic acid anions, 5 phenolates, Cl–, Br–, I–, and BF4–). In this series, H is shifted from A to X, modeling the proton-transfer pathway. The 1H and 13C chemical shifts and the H/D isotope effects on the latter provide information about averaged H-bond geometries. At the same time, red shifts of the π–π* UV–vis absorption bands are observed which correlate with the averaged H-bond geometries. However, on the UV–vis time scale, different tautomeric states and solvent configurations are in slow exchange. The combined data sets indicate that the proton transfer starts with a H-bond compression and a displacement of the proton toward the H-bond center, involving single-well configurations A–H···X–. In the strong H-bond regime, coexisting tautomers A··H···X– and A–···H··X are observed by UV. Their geometries and statistical weights change continuously when the basicity of X– is increased. Finally, again a series of single-well structures of the type A–···H–X is observed. Interestingly, the UV–vis absorption bands are broadened inhomogeneously because of a distribution of H-bond geometries arising from different solvent configurations

Solvent and H/D Isotope Effects on the Proton Transfer Pathways in Heteroconjugated Hydrogen-Bonded Phenol-Carboxylic Acid Anions Observed by Combined UV–vis and NMR Spectroscopy

Heteroconjugated hydrogen-bonded anions A···H···X^– of phenols (AH) and carboxylic/inorganic acids (HX) dissolved in CD₂Cl₂ and CDF₃/CDF₂Cl have been studied by combined low-temperature UV–vis and ¹H/¹³C NMR spectroscopy (UVNMR). The systems constitute small molecular models of hydrogen-bonded cofactors in proteins such as the photoactive yellow protein (PYP). Thus, the phenols studied include the PYP cofactor 4-hydroxycinnamic acid methyl thioester, and the more acidic 4-nitrophenol and 2-chloro-4-nitrophenol which mimic electronically excited cofactor states. It is shown that the ¹³C chemical shifts of the phenolic residues of A···H···X^–, referenced to the corresponding values of A···H···A^–, constitute excellent probes for the average proton positions. These shifts correlate with those of the H-bonded protons, as well as with the H/D isotope effects on the ¹³C chemical shifts. A combined analysis of UV–vis and NMR data was employed to elucidate the proton transfer pathways in a qualitative way. Dual absorption bands of the phenolic moiety indicate a double-well situation for the shortest OHO hydrogen bonds studied. Surprisingly, when the solvent polarity is low the carboxylates are protonated whereas the proton shifts toward the phenolic oxygens when the polarity is increased. This finding indicates that because of stronger ion-dipole interactions small anions are stabilized at high solvent polarity and large anions exhibiting delocalized charges at low solvent polarities. It also explains the large acidity difference of phenols and carboxylic acids in water, and the observation that this difference is strongly reduced in the interior of proteins when both partners form mutual hydrogen bonds

Two-Dimensional UV–vis/NMR Correlation Spectroscopy: A Heterospectral Signal Assignment of Hydrogen-Bonded Complexes

We explore in a combined UV–vis/NMR approach the hydrogen-bonded complexes of 2-chloro-4-nitrophenol (CNP) with acetate anion as a model for amino acid side-chain interactions. For the first time, we present two-dimensional UV–vis/NMR correlation spectra, measured simultaneously for the same sample inside of the magnet of a NMR spectrometer. Synchronous and asynchronous 2D plots allow us to monitor the formation of hydrogen-bonded complexes, assign the signals to specific species, and finally estimate the geometry of a hydrogen-bonded 1:1 heteroconjugated anion coexisting with four other anionic species formed in CD₂Cl₂ solution at 180 K. Combined analysis of NMR and UV–vis spectra with the help of previously published hydrogen-bond correlations shows that the hydrogen bond in the heteroconjugate is of the CNP–O^–···HOOCCH₃ type with <i>r</i>(O···O) ≈ 2.48 Å and the average bridging proton displacement from the hydrogen bond center of about 0.18 Å

Geometries and Tautomerism of OHN Hydrogen Bonds in Aprotic Solution Probed by H/D Isotope Effects on ¹³C NMR Chemical Shifts

The 1H and 13C NMR spectra of 17 OHN hydrogen-bonded complexes formed by CH313COOH(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF3/CDF2Cl mixture as solvent. The slow proton and hydrogen bond exchange regime was reached, and the H/D isotope effects on the 13C chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding 1H chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the 13C chemical shifts of the carboxyl group with the OHN hydrogen bond geometries

Correlating Photoacidity to Hydrogen-Bond Structure by Using the Local O–H Stretching Probe in Hydrogen-Bonded Complexes of Aromatic Alcohols

To assess the potential use of O–H stretching modes of aromatic alcohols as ultrafast local probes of transient structures and photoacidity, we analyze the response of the O–H stretching mode in the 2-naphthol-acetonitrile (2N–CH₃CN) 1:1 complex after UV photoexcitation. We combine femtosecond UV-infrared pump–probe spectroscopy and a theoretical treatment of vibrational solvatochromic effects based on the Pullin perturbative approach, parametrized at the density functional theory (DFT) level. We analyze the effect of hydrogen bonding on the vibrational properties of the photoacid–base complex in the S₀ state, as compared to O–H stretching vibrations in a wide range of substituted phenols and naphthols covering the 3000–3650 cm^–1 frequency range. Ground state vibrational properties of these phenols and naphthols with various substituent functional groups are analyzed in solvents of different polarity and compared to the vibrational frequency shift of 2N induced by UV photoexcitation to the ¹L_b electronic excited state. We find that the O–H stretching frequency shifts follow a linear relationship with the solvent polarity function <i>F</i>₀ = (2ε₀ – 2)/(2ε₀ + 1), where ε₀ is the static dielectric constant of the solvent. These changes are directly correlated with photoacidity trends determined by reported p<i>K</i>_<i>a</i> values and with structural changes in the O···N and O–H hydrogen-bond distances induced by solvation or photoexcitation of the hydrogen-bonded complexes

N–H Stretching Excitations in Adenosine-Thymidine Base Pairs in Solution: Pair Geometries, Infrared Line Shapes, and Ultrafast Vibrational Dynamics

We explore the N–H stretching vibrations of adenosine-thymidine base pairs in chloroform solution with linear and nonlinear infrared spectroscopy. Based on estimates from NMR measurements and ab initio calculations, we conclude that adenosine and thymidine form hydrogen bonded base pairs in Watson–Crick, reverse Watson–Crick, Hoogsteen, and reverse Hoogsteen configurations with similar probability. Steady-state concentration and temperature dependent linear FT-IR studies, including H/D exchange experiments, reveal that these hydrogen-bonded base pairs have complex N–H/N–D stretching spectra with a multitude of spectral components. Nonlinear 2D-IR spectroscopic results, together with IR-pump-IR-probe measurements, as also corroborated by ab initio calculations, reveal that the number of N–H stretching transitions is larger than the total number of N–H stretching modes. This is explained by couplings to other modes, such as an underdamped low-frequency hydrogen-bond mode, and a Fermi resonance with NH₂ bending overtone levels of the adenosine amino-group. Our results demonstrate that modeling based on local N–H stretching vibrations only is not sufficient and call for further refinement of the description of the N–H stretching manifolds of nucleic acid base pairs of adenosine and thymidine, incorporating a multitude of couplings with fingerprint and low-frequency modes