Perturbations of the Fully Resolved Electronic Spectra of Large Molecules by the Internal Rotation of Attached Methyl Groups. Influence of Complex Formation † Timothy M. Korter and David W. Pratt* Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: December 11, 2000; In Final Form: February 23, 2001 Rotationally resolved S 1 rS 0 fluorescence excitation spectra of three methylindoles and their single atom van der Waals complexes with argon have been obtained. Each spectrum is extensively perturbed by the hindered internal rotation of the methyl group. Analyses of these perturbations show that the barriers to such motion are substantially increased by complex formation. Barriers of this type are primarily electronic in origin. Thus, even at the relatively large distances (3.5 Å) found in van der Waals complexes, the attachment of a weakly bound argon atom has a significant effect on the electron distribution in the indole ring. Introduction The field of electronic spectroscopy, like that of surface science, has advanced immensely in the last few years. The recording of the near-UV spectra of naphthalene and perdeu- terated naphthalene with fully resolved rotational structure 1 can be considered as marking the beginning of the modern era in this field. This era is characterized by studies of such spectra at eigenstate resolution, exposing for the first time all of the underlying quantum states in the system and the interactions that are responsible for them, using molecular beam machines, high resolution lasers, and state-of-the-art computer software. Subsequent to its discovery, the technique has been widely employed for the determination of the structures of many large molecules and their complexes in the gas phase, in different electronic states, and for exploring their dynamical behavior following the absorption of light. 2 We focus in this report on the special sensitivity of the fully resolved electronic spectrum of a large molecule to the torsional motion of an attached methyl group, and on the changes in the potential energy surfaces that govern such motions that are produced when a rare gas atom is attached to the molecule by a weak van der Waals “bond”. Rotationally resolved studies are sensitive to torsional motions because of their intrinsic angular momenta, which couple to overall molecular rotation and produce detectable splittings and/or perturbations in the spectra. 3 Here, we exploit this sensitivity in a study of a series of methyl-substituted indoles and their single Ar atom com- plexes, in their S 0 and S 1 electronic states. Somewhat surpris- ingly, we find that the methyl groups attached to the bare molecules and their complexes exhibit different barriers to internal rotation, despite the fact that the Ar atom is located at some distance (∼3.5 Å) away from the aromatic plane. Apart from the methyl group, and the weakly bound Ar atom, the molecules investigated here all are planar molecules, encouraging comparison with analogous studies in surface science. A number of groups have used vibrational spectroscopy to study the rotation of physisorbed and chemisorbed species on well-defined structures. Additionally, adsorbates such as NH 3 and PF 3 have been extensively studied by ESDIAD, a method that images chemical bond directions. 4 An interesting question is whether such motions are influenced by the surfaces to which they are attached. There also is a need to understand the coupling between adsorbate vibrations and the surface electrons, which may influence the rates by which such vibrations relax. 5 Thus, there are significant conceptual links between the fields of electronic spectroscopy and surface science, especially as practiced at the University of Pittsburgh. Experimental Section 1-Methylindole (97 + %), 3-methylindole (98%), and 5-me- thylindole (99%) were purchased from Aldrich and used as received. Dry argon gas (99.999%) was used for the van der Waals complex experiments. High resolution data were obtained using the molecular beam laser spectrometer described in detail elsewhere. 6 The molecular beam was formed by the expansion of the molecule of interest (typically heated to ∼350 K) in an Ar carrier gas (∼500 Torr) through a 240 μm quartz nozzle into a differentially pumped vacuum system. Van der Waals complexes were formed by expansions of the bare molecule in ∼100 Torr of Ar. The expansion was skimmed 2 cm downstream of the nozzle with a 1 mm skimmer and crossed 13 cm further downstream by a CW ring dye laser operating with R590 and intracavity frequency doubled in BBO, yielding ∼300 μW of ultraviolet radiation. Fluorescence was collected using spatially selective optics, detected by a photomultiplier tube and photon counting system, and processed by a computerized data acquisition system. Relative frequency calibrations of the excitation spectra were performed with a near-confocal interferometer having a mode-matched FSR of 299.7520 ( 0.0005 MHz at the fundamental frequency of the dye laser. Absolute transition frequencies were determined by comparison to transition frequencies in the iodine absorption spectrum and are accurate to ( 30 MHz. 7 Results and Interpretation A. 1-Methylindole (1MI). Figure 1 shows the rotationally resolved fluorescence excitation spectrum of the 0 0 0 band of 1MI. Though not apparent at first glance, a close examination † Part of the special issue “John T. Yates, Jr. Festschrift”. We dedicate this paper to our friend and colleague, John T. Yates, Jr., on the occasion of his 65 th birthday. 4010 J. Phys. Chem. B 2001, 105, 4010-4017 10.1021/jp004451h CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001