Intermolecular Clamping by Hydrogen Bonds: 2‐Pyridone⋅NH₃

Abstract

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH(3) has been performed. The S(1)←S(0) spectrum extends up to ≈1200 cm(-1) above the 0(0) (0) band, close to twice the range observed for 2PY. The S(1) state nonradiative decay for vibrations above ≈300 cm(-1) in the NH(3) complex is dramatically slowed down relative to bare 2PY. Also, the Δv=2, 4,… overtone bands of the ν(1) ' and ν(2) ' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY⋅NH3, which implies that the bridging (2PY)NH⋅⋅⋅NH(3) and H(2) NH⋅⋅⋅O=C H-bonds clamp the 2PY at a planar geometry in the S(1) state. The mass-resolved UV vibronic spectra of jet-cooled 2PY⋅NH(3) and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S(0) and S(1) equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S(1)←S(0) vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C(S) symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H(2) NH⋅⋅⋅O=C H-bond distance is ≈0.7 Å shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H(2) NH⋅⋅⋅O distance to increase by 0.2 Å upon S(1)←S(0) electronic excitation, while the (2PY)NH⋅⋅⋅NH(3) H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

Transition from Water Wires to Bifurcated H-Bond Networks in 2-Pyridone·(H2O)n, n = 1–4 Clusters

“…11,12,32,44−46 To account for the long-range dispersive interactions, we performed additional M06-2X/6-311++G(d,p) and B97-D/ def2-TZVPP calculations. 24,47 Correlated ab initio calculations were also carried out using the spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) method and the def2-TZVPP basis set.…”

Gas-phase Lifetimes of Nucleobase Analogues by Picosecond Pumpionization and Streak Techniques

“…In the trimers and tetramer the H-bond arrangements are increasingly strained and twisted, [19] which leads to a weaker clamping and allowing easier out-of-plane twisting of the H-N-C=O group and more rapid radiationless relaxation. [20] The ps streak camera setup has the advantage that: i) the local density in the beam is higher in front of the pulsed nozzle, ii) the entire lifetime range (0.2-50 ns) is measured for every laser shot, iii) the measurement accuracy for lifetimes >500 ps is good and becomes increasingly better with increasing lifetime. Disadvantages are: i) Fluorescence measurements are not intrinsically mass-or species-specific and may contain contributions from other fluorescent species.…”

“…This 70% increase in lifetime relative to 2PY is in agreement with previous observations that bridging the cisamide H-N-C=O group of 2PY with two or more H-bonds prevents out-of-plane twisting of the NH and C=O bonds in the 1 ττ* state, thereby blocking the access to the lowest conical intersection of 2PY. [20] Held et al determined the (2PY) 2 lifetime via Lorentzian line-broadening as 9 ±1 ns. [18] Given that this lifetime is 4 ns shorter while the 2PY lifetime is 2.2 ns longer than the ps streak camera measurements, and given the much smaller fit errors of the latter, we suggest that the Lorentzian broadening lifetime determinations have be increased without deteriorating the time resolution.…”

Stepwise vs concerted excited state tautomerization of 2-hydroxypyridine: Ammonia dimer wire mediated hydrogen/proton transfer

“…In this way, several previous studies on clusters of 2HP/2PY with solvent molecules have been investigated with different theoretical and experimental methods such as one-color resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed fluorescence spectra. 8,10,[12][13][14][15][16][17][18][19][20][21][22][23][24] 2HP/2PY clustered with ammonia molecules has particularly received much attention, especially because of an attempt to study the environmental influence on the proton or hydrogen transfer to facilitate tautomerization reaction of 2HP/ 2PY. 5,[12][13][14][15][16]18 In this respect, chromophore to ammonia excited state proton transfer has been found to compete with a reaction classified as a hydrogen or coupled electron-proton transfer, where the formation of a bi-radical is considered as the primary step in these types of reactions.…”