Knowledge Bank

A molecular beam/optothermal detection technique and F-center laser system have been used to obtain the rotationally resolved, near infrared spectrum of the argon-methylacetylene van der Waals complex in the vicinity of the $v=1$, C - H stretch of the methylacetylene monomer at 3335 $cm-1$. Of the ca. 140 transitions observed (linewidths 15 - 20 MHz fwhm), 59 have been assigned to an A internal rotor state, b-type spectrum of the complex. These transitions have been fit using a Watson A reduction Hamiltonian in a $I1$ representation. The fit constants for the A state are: $A\prime$ = 0.2910(2) $cm-1$, $B\prime =0.06109(7)$, $C\prime =0.05000(4)$, $A\prime \prime =0.2939(4)$, $B\prime \prime =0.06127(8)$, $C\prime \prime =0.04991(6)$, $v0=3334.5220(5)$ $cm-1$. The A state hand center is red shifted from the monomer band center by 0.5374(8) $cm-1$. A preliminary estimate of the complex’s structure (estimate based on (B + C) / 2 and B - C values of trial structures) has the angle between the symmetry axis of the methylacetylene and a line connecting the triple bond center to the argon at $900$ and the length of this line as 3.60A.