Departement für Chemie und Biochemie, Universität Bern, Switzerland.
Phys Chem Chem Phys. 2011 Aug 21;13(31):14110-8. doi: 10.1039/c1cp20793h. Epub 2011 Jun 23.
The N-H···π hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr·Bz, Pyr·Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H···π hydrogen bond to the benzene ring. The pyrrole is tipped by ω(S(0)) = ±13° relative to the surface normal of Bz. The N···ring distance is 3.13 Å. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle ω(S(1)) = ±21°. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H···π interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1)←S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr·Bz in the same region exhibits a weak 0 band that is red-shifted by 58 cm(-1) relative to that of Bz (38 086 cm(-1)). The origin appears due to symmetry-breaking of the π-electron system of Bz by the asymmetric pyrrole NH···π hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0 band. The Bz moiety in Pyr·Bz exhibits a 6a band at 0 + 518 cm(-1) that is about 20× more intense than the origin band. The symmetry breaking by the NH···π hydrogen bond splits the degeneracy of the ν(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by ∼6 cm(-1). Both the 0 and 6 bands of Pyr·Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration ω', in agreement with the change of the ω tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.
N-H···π 氢键是许多生物体系中重要的分子间相互作用。我们已经研究了超声速射流冷却的吡咯与苯和苯-d(6)复合物（Pyr·Bz，Pyr·Bz-d(6)）的红外（IR）和紫外（UV）光谱。DFT-D 密度泛函、SCS-MP2 和 SCS-CC2 计算预测了一个 T 形和（几乎）C(s)对称结构，其中存在一个 N-H···π 氢键与苯环结合。吡咯相对于 Bz 的表面法线倾斜ω(S(0))=±13°。N···环距离为 3.13 Å。在 S(1)激发态下，SCS-CC2 计算预测倾斜角ω(S(1))增加到±21°。IR 消耗光谱支持 T 形几何形状：NH 伸缩振动红移-59 cm(-1)，相对于 3531 cm(-1)处“自由”吡咯 NH 伸缩振动，表明存在中等强度的 N-H···π 相互作用。这种相互作用比（Pyr）(2)二聚体中的弱，其中 NH 供体位移为-87 cm(-1)[Dauster 等人，Phys. Chem. Chem. Phys.，2008，10，2827]。Bz 亚基的 IR C-H 伸缩频率和强度与（Bz）(2)二聚体中的接受体非常相似，证实 Bz 作为接受体。虽然 Bz 的 S(1)←S(0)电子起源在气相中是禁止的，并且不可观察，但 Pyr·Bz 在同一区域的 UV 光谱在 58 cm(-1)处显示出一个较弱的 0 带，相对于 Bz（38086 cm(-1)）红移。该起源是由于不对称吡咯 NH···π 氢键破坏了 Bz 的π电子系统的对称性。这与（Bz）(2)形成对比，（Bz）(2)不表现出 0 带。Pyr·Bz 中的 Bz 部分在 0+518 cm(-1)处表现出 6a 带，其强度比起源带大约高 20 倍。NH···π 氢键的对称性破坏使 ν(6)(e(2g))振动的简并性分裂，产生 6a'和 6b'亚带，间隔约 6 cm(-1)。Pyr·Bz 的 0 和 6 带都在低频（10 cm(-1)）激发态倾斜振动ω'中表现出进展，这与 SCS-MP2 和 SCS-CC2 计算预测的ω倾斜角变化一致。
