N–H⋯π hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole–benzene

Abstract

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-d₆ (Pyr·Bz, Pyr·Bz-d₆). 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(S₀) = ±13° relative to the surface normal of Bz. The N⋯ring distance is 3.13 Å. In the S₁ excited state, SCS–CC2 calculations predict an increased tipping angle ω(S₁) = ±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 S₁ ← S₀ 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 0⁰₀ 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 0⁰₀ band. The Bz moiety in Pyr·Bz exhibits a 6a¹₀ band at 0⁰₀ + 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 ν₆(e_2g) vibration, giving rise to 6a′ and 6b′ sub-bands that are spaced by ∼6 cm⁻¹. Both the 0⁰₀ and 6¹₀ 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.