Rate and mechanism of intramolecular vibrational redistribution in the ν16 asymmetric methyl stretch band of 1-butyne

Abstract

The spectrum of the ν16 asymmetric methyl stretch vibration of 1-butyne near 2991 cm−1 has been studied via direct absorption infrared spectroscopy at a resolution of 35 MHz. Analysis by ground state combination difference indicates that the ν16 band is extensively perturbed by dark vibrational bath states. All of the transitions appear as multiplets of about five eigenstates in a window of about 0.017 cm−1. A detailed analysis is presented for the upper state levels K′a = 0–2 and J′=0–6. A lack of J′ dependence implies anharmonic coupling is dominant and that b- and c-type Coriolis interactions are not important at these low J′ values. However, the average dilution factor goes from 0.72 at the K′a = 0 to 0.46 at the K′a = 2 suggesting weak a-type Coriolis interactions. For the K′a = 0 levels, the measured average level density of 17 states/cm−1/symmetry species is comparable to the value of 14 vibrational states/cm−1/symmetry species obtained from a symmetry specific direct count. This is an indication that the dynamics explore all of the energetically available vibrational phase space. The nearly Gaussian distribution of matrix elements suggests that there is significant coupling among the bath states. At the K′a = 0 level, the rms anharmonic coupling matrix element is <vsj2≳1/2=0.0125 cm−1. From the frequency-resolved data, a coherently prepared asymmetric methyl stretch in 1-butyne is deduced to decay with a 276 ps time constant to the asymptotic probability of 0.6.