Vaupel Sascha, Brutschy Bernhard, Tarakeshwar Pilarisetty, Kim Kwang S
Institut für Physikalische und Theoretische Chemie, J. W. Goethe-Universität Frankfurt, Marie-Curie-Str. 11, D-60439 Frankfurt/Main, Germany.
J Am Chem Soc. 2006 Apr 26;128(16):5416-26. doi: 10.1021/ja056454j.
Among the several weak intermolecular interactions pervading chemistry and biology, the NH-pi interaction is one of the most widely known. Nevertheless its weak nature makes it one of the most poorly understood and characterized interactions. The present study details the results obtained on gas-phase complexes of ammonia with various substituted pi systems using both laser vibrational spectroscopy and ab initio calculations. The spectroscopic measurements carried out by applying one-color resonant two-photon ionization (R2PI) and IR-vibrational predissociation spectroscopy in the region of the NH stretches yield the first experimental NH stretching shifts of ammonia upon its interaction with various kinds of pi-systems. The experiments were complemented by ab initio calculations and energy decompositions, carried out at the second-order Møller-Plesset (MP2) level of theory. The observed complexes show characteristic vibrational spectra which are very similar to the calculated ones, thereby allowing an in-depth analysis of the interaction forces and energies. The interaction energy of the conformers responsible for the observed vibrational spectra has the maximum contribution from dispersion energies. This implies that polarizabilities of the pi-electron systems play a very important role in governing the nature and geometry of the NH-pi interaction. The larger polarizability of ammonia as compared to water and the tendency to maximize the dispersion energy implies that the characteristics of the NH-pi interactions are markedly different from that of the corresponding OH-pi interactions.
在化学和生物学中普遍存在的几种弱分子间相互作用中，N-H-π相互作用是最为人熟知的相互作用之一。然而，其弱相互作用的性质使其成为理解和表征最为不足的相互作用之一。本研究详细介绍了使用激光振动光谱和从头算计算方法，对氨与各种取代π体系的气相配合物所获得的结果。通过应用单色共振双光子电离（R2PI）和NH伸缩区域的红外振动预解离光谱进行的光谱测量，得出了氨与各种π体系相互作用时首次实验测定的NH伸缩位移。实验得到了在二级Møller-Plesset（MP2）理论水平上进行的从头算计算和能量分解的补充。观察到的配合物显示出与计算结果非常相似的特征振动光谱，从而能够对相互作用力和能量进行深入分析。对观察到的振动光谱负责的构象异构体的相互作用能，色散能的贡献最大。这意味着π电子体系的极化率在控制N-H-π相互作用的性质和几何结构方面起着非常重要的作用。与水相比，氨具有更大的极化率，以及使色散能最大化的趋势，这意味着N-H-π相互作用的特征与相应的O-H-π相互作用明显不同。
