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.

The 10 nJ pulses from a Kerr-lens mode-locked Ti:sapphire oscillator (Mai Tai, SpectraPhysics) are amplified in a Ti:sapphire multipass chirped-pulse amplifier system (ODIN DQC, Quantronix), which is pumped by a 14 W pulsed frequency- The output of the fs laser system is reduced to a laser pulse energy between 75 − 120 µJ and split into three equally intense pump, dump and probe beams, which are aligned in a forward BOXCARS degenerate four-wave mixing (DFWM) arrangement, and focused by an f = 1000 mm achromatic lens into the center of either a 1.0 m long stainless-steel gas-cell or into the supersonic jet vacuum chamber. The probe beam runs over a retroreflector that is mounted on a high-precision 1000 mm long delay stage, housed in a vacuum tank with 800 nm AR coated windows that is evacuated to < 10 −3 mbar. The time-delay of the probe pulse is by measuring the retroreflector displacement with a two-axis He-Ne laser interferometric system (SP2000 D; SIOS GmbH) with an accuracy of ±30 nm. After the output window of gas-cell or jet vacuum chamber, the three input beams are blocked by a mask, while the DFWM signal beam is collimated, spatially filtered and detected by a thermoelectrically cooled GaAs photomultiplier (H7422-50, Hamamatsu). The signals are recorded with a 1 GHz/8-bit oscilloscope at a sweep rate of 8 GHz/s. The experiment is controlled and the data acquired by a PC using LabView.