RATE AND MECHANISM OF INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION IN THE NU(16) ASYMMETRIC METHYL STRETCH BAND OF 1-BUTYNE

The spectrum of the nu16 asymmetric methyl stretch vibration of 1-buty ne near 2991 cm-1 has been studied via direct absorption infrared spec troscopy at a resolution of 35 MHz. Analysis by ground state combinati on difference indicates that the nu16 band is extensively perturbed by dark vibrational bath states. All of the transitions appear as multip lets of about five eigenstates in a window of about 0.0 1 7 cm-1. A de tailed analysis is presented for the upper state levels K(a)' = 0-2 an d J' = 0-6. A lack of J' dependence implies anharmonic coupling is dom inant and that b- and c-type Coriolis interactions are not important a t 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 s ymmetry specific direct count. This is an indication that the dynamics explore all of the energetically available vibrational phase space. T he 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 [v(sj)2]1/2 = 0.0125 c m-1. From the frequency-resolved data, a coherently prepared asymmetri c methyl stretch in 1-butyne is deduced to decay with a 276 ps time co nstant to the asymptotic probability of 0.6.