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Issue 31, 2011
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N–H⋯π hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrolebenzene

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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) Cs 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 S1S0 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(e2g) 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.

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

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The article was received on 16 Mar 2011, accepted on 25 May 2011 and first published on 23 Jun 2011

Article type: Paper
DOI: 10.1039/C1CP20793H
Citation: Phys. Chem. Chem. Phys., 2011,13, 14110-14118

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

    C. Pfaffen, D. Infanger, P. Ottiger, H. Frey and S. Leutwyler, Phys. Chem. Chem. Phys., 2011, 13, 14110
    DOI: 10.1039/C1CP20793H

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