Probing the Structure, Pseudorotation, and Radial Vibrations of Cyclopentane by Femtosecond Rotational Raman Coherence Spectroscopy.

Femtosecond time-resolved Raman rotational coherence spectroscopy (RCS) is employed to determine accurate rotational, vibration­rotation coupling constants, and centrifugal distortion constants of cyclopentane (C5H10). Its lowest-frequency vibration is a pseudorotating ring deformation that interconverts 10 permutationally distinct but energetically degenerate "twist" minima interspersed by 10 "bent" conformers. While the individual twist and bent structures are polar asymmetric tops, the pseudorotation is fast on the time scale of external rotation, rendering cyclopentane a fluxionally nonpolar symmetric top molecule. The pseudorotational level pattern corresponds to a one-dimensional internal rotor with a pseudorotation constant Bps ≈ 2.8 cm(-1). The pseudorotational levels are significantly populated up to l = ± 13 at 298 K; <10% of the molecules are in the l = 0 level. The next-higher vibration is the "radial" ν23 ring deformation mode at 273 cm­1, which is far above the pseudorotational fundamental. Femtosecond Raman RCS measurements were performed in a gas cell at T = 293 K and in a pulsed supersonic jet at T ≈ 90 K. The jet cooling reduces the pseudorotational distribution to l < ±8 and eliminates the population of ν23, allowing one to determine the rotational constant as A0 = B0 = 6484.930(11) MHz. This value is ∼300 times more precise than the previous value. The fit of the RCS transients reveals that the rotation­pseudorotation coupling constant αe,psB = −0.00070(1) MHz is diminutive, implying that excitation of the pseudorotation has virtually no effect on the B0 rotational constant of cyclopentane. The smallness of αe,psB can be realized when comparing to the vibration­rotation coupling constant of the ν23 vibration, αe,23B = -9.547(1) MHz, which is about 104 times larger.

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

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.