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, vibration-rotation coupling constants, and centrifugal distortion constants of cydopentane (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 cydopentane a fluxionally nonpolar symmetric top molecule. The pseudorotational level pattern corresponds to a one-dimensional internal rotor with a pseudorotation constant B-ps, approximate to 2.8 cm(-1). The pseudorotational levels are significantly populated up to 1 = +/- 13 at 298 K; <10% of the molecules are in the 1 = 0 level. The next-higher vibration is the "radial" nu(23) ring deformation mode at 273 cm(-1), 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 approximate to 90 K. The jet cooling reduces the pseudorotational distribution to 1 < +/- 8 and eliminates the population of nu(23), allowing one to determine the rotational constant as A(0) = B-0 = 6484.930(11) MHz. This value is similar to 300 times more precise than the previous value. The fit of the RCS transients reveals that the rotation-pseudorotation coupling constant alpha(B)(e,ps), = -0.00070(1) MHz is diminutive, implying that excitation of the pseudorotation has virtually no effect on the B-0 rotational constant of cydopentane. The smallness of alpha(B)(e,ps), can be realized when comparing to the vibration-rotation coupling constant of the nu(23) vibration, alpha(B)(a,23) = -9.547(1) MHz, which is about 104 times larger.