Intermolecular Clamping by Hydrogen Bonds: 2-Pyridone center dot NH3
| Autor: | Blaser S, Ottiger P, Lobsiger S, Frey HM, Leutwyler S |
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| Rok vydání: | 2011 |
| Zdroj: | CHEMPHYSCHEM |
| DOI: | 10.1002/cphc.201100037 |
| Popis: | A combined spectroscopic and ab initio theoretical study of the doubly hydrogen bonded complex of 2 pyridone (2PY) with NH_3 was performed. The S1\leftarrow S0 spectrum extends up to 1200 cm 1 above the 0_0^0 band close to twice the range observed for 2 pyridone. The S_1 state non radiative decay for vibrations above 300 cm 1 in the NH3 complex is dramatically slowed down relative to bare 2PY. Also the Delta v=2 4.. overtone bands of the nu_1' and nu_2' out of plane vibrations that dominate the low energy spectral region of 2PY are much weaker or missing for 2PY ... NH3 implying that the bridging (2PY)NH...NH3 and H2NH....O=C hydrogen bonds clamp the 2PY at a planar geometry in the S1 state. We measured the mass resolved UV vibronic spectra of jet cooled 2 pyridone...NH3 and its H/D mixed isotopomers using two color resonant two photon ionization spectroscopy. The S0 and S1 equilibrium structures and normal mode frequencies were calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second order coupled cluster CC2). The S_1\leftarrow S_0 vibronic assignments are based on CI singles (CIS) and CC2 calculations. A doubly H bonded bridged structure of Cs symmetry is predicted in agreement with that of Held and Pratt [J. Am. Chem. Soc. {\bf 115} 9718 (1993)]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment the calculated H_2NH ... O=C hydrogen bond distance is 0.7 Ang. shorter than derived by Held and Pratt. On the hand this underlines their observation that ammonia can act as a strong H bond donor when built into a hydrogen bonded bridge. The CC2 calculations predict the H_2NH...O distance to increase by 0.2 Ang. upon S1 < S0 electronic excitation while the (2PY)NH ...NH3 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. |
| Databáze: | OpenAIRE |
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