Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, 3012 Bern, Switzerland.
Chemphyschem. 2011 Jul 11;12(10):1841-50. doi: 10.1002/cphc.201100037. Epub 2011 May 23.
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.
我们对 2-吡啶酮（2PY）与 NH3 的氢键复合物进行了组合光谱和从头算理论研究。S1←S0 谱延伸到 0（0）（0）带上方约 1200 cm-1，接近 2PY 观察到的范围的两倍。在 NH3 复合物中，NH3 复合物中高于约 300 cm-1 的振动的 S1 状态非辐射衰减相对于裸露的 2PY 大大减慢。此外，占据 2PY 低能光谱区域的 ν1'和 ν2'面外振动的 Δv=2,4，…泛音带对于 2PY⋅NH3 很弱或缺失，这意味着桥接（2PY）NH⋅⋅⋅NH3 和 H2NH⋅⋅⋅O=C H 键在 S1 状态下将 2PY 固定在平面几何形状上。使用双色共振双光子电离光谱法测量了喷射冷却的 2PY⋅NH3 及其 H/D 混合同位素的质量分辨紫外振动态谱。通过密度泛函（B3LYP）和相关的从头算方法（MP2 和近似二阶耦合簇，CC2）计算了 S0 和 S1 平衡结构和正则模态频率。S1←S0 振动态分配基于组态相互作用单重态（CIS）和 CC2 计算。预测了 C（S）对称的氢键桥接结构，与 Held 和 Pratt [J. Am. Chem. Soc. 1993, 115, 9718] 的预测一致。尽管 B3LYP 和 MP2 计算的旋转常数与实验非常吻合，但计算得到的 H2NH⋅⋅⋅O=C H 键距离比 Held 和 Pratt 得出的距离短约 0.7 Å。另一方面，这强调了他们的观察结果，即当氨构建到氢键桥中时，氨可以作为强氢键供体。CC2 计算预测，在 S1←S0 电子激发时，H2NH⋅⋅⋅O 距离增加 0.2 Å，而（2PY）NH⋅⋅⋅NH3 氢键几乎保持不变。因此，激发态中氢键桥的扩展是不对称的，几乎完全归因于氨与酮受体基团相互作用的减弱。
