JOURNAL OF MOLECULAR SPECTROSCOPY 182, 132–147 (1997) ARTICLE NO. MS967193 Infrared and Microwave Spectra of the N 2 –Propyne van der Waals Cluster Shao-Hui Tseng, * ,1 David F. Eggers,* ,2 Thomas A. Blake,* ,3 Rainer Beck,* ,4 R. O. Watts, * ,5 F. J. Lovas,² and N. Zobov² ,6 * Department of Chemistry, University of Washington, Mail Stop BG-10, Seattle, Washington 98195-2145; and ² National Institute of Standards and Technology, Optical Technology Division, Building 221, Room B208, Gaithersburg, Maryland 20899 Received August 5, 1996; in revised form September 24, 1996 The infrared spectrum of 14 N 2 –CH 3 CCH in the 3.0-mm region has been observed and the rovibrational transitions have been assigned for the A ( m Å 0) and E ( m Å{1) methyl top internal rotor states of this near slipped parallel van der Waals molecule. The E-state transitions were used to estimate the barrier to internal rotation as 10 cm 01 . The observed A-state microwave transitions of 14 N 2 –CH 3 CCH, 15 N 2 –CH 3 CCH, and 15 N 2 –CH 3 CCD are split into doublets. This doublet splitting, which is too small to be observed in the infrared spectrum, is attributed to the internal motion of the N 2 moiety about an axis connecting the centers-of-mass of the molecules in the cluster; i.e., the N 2 molecule rotates out of the heavy atom plane of the cluster. The splittings in the microwave spectra were fit to a barrier height of 71 cm 01 for the hindered N 2 motion.  1997 Academic Press INTRODUCTION nearly free internal rotation of the methyl top, and the micro- wave spectra of the A-state for the 14 N 2 –propyne and 15 N 2 – In a recent paper we reported the structures of the N 2 – propyne complexes also show splittings which we interpret CH 3 CCH and CO–CH 3 CCH van der Waals clusters based as arising from the internal rotation of the nitrogen molecule on a preliminary analysis of the infrared and microwave in the complex. spectra ( 1 ) . Sufficient microwave data for isotopic variants In the present paper we provide a detailed analysis of of the clusters allowed for the fitting of the parameters re- the infrared and microwave spectra of N 2 –propyne with a quired to uniquely determine their structures. The experi- treatment of the barriers to internal rotation of each subunit mental geometries of the complexes have the heavy atoms in the complex. Values of barriers to methyl and N 2 internal of the clusters lying in a plane and the N 2 and CO center- rotation are also determined. of-mass vectors approximately perpendicular to the symme- try axis of the propyne unit with center-of-mass separations EXPERIMENT of 3.708(2) and 3.756(1) A ˚ , respectively. These structures are analogous to the T-shaped structures of the rare gas Infrared spectra of N 2 –CH 3 CCH were measured using a complexes Ar–CH 3 CCH ( 2 ), Ar–CH 3 Cl ( 3 ), and Ar– computer-controlled F-center laser and a differentially CH 3 CN ( 4 ). Each of these rare gas complexes exhibit A pumped molecular beam apparatus with optothermal detec- and E symmetry states, which arise from nearly free internal tion. Specific details of this apparatus are given elsewhere rotation of the methyl symmetric top unit with barriers on ( 2, 5 ) . A brief description of the current study is given here. the order of 20 cm 01 or less. The infrared spectrum of N 2 – A mixture consisting of about 1% by volume CH 3 CCH and CH 3 CCH also exhibits internal rotation splitting from the 5% by volume N 2 in helium was expanded through a 50- mm, room-temperature nozzle at a backing pressure of 1000 1 Present address: Department of Molecular Biotechnology, Fluke Hall, kPa. The molecular beam was collimated using a 190-mm University of Washington, Box 357730, Seattle, WA 98195-2145. skimmer. The molecular beam then passed between two par- 2 Deceased. allel, gold-coated mirrors, placed about 25 cm in front of the 3 Permanent address: Battelle / Pacific Northwest National Laboratory, bolometer detector. The mirrors allow the molecular beam to P.O. Box 999, Mail Stop K3-58, Richland, WA 99352. 4 Permanent address: Institute de Chimie Physique Moleculaire, Ecole be multiply crossed with the infrared radiation from the F- Polytechnique Federal de Lausanne, CH-1015 Lausanne, Switzerland. center laser. A silicon-on-sapphire bolometer cooled to 1.8 5 Permanent address: School of Chemistry, The University of Melbourne, K was used to monitor the energy flux of the molecular Parkville, Victoria 3052, Australia. beam. The bolometer has an NEP of Ç2 1 10 014 W/  Hz 6 Permanent address: Institute of Applied Physics, Nizhny, Novgorod, Russia. and a time constant of 3 ms. The laser radiation was chopped 132 0022-2852/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.