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An Analysis of the Methyl Rotation and Aldehyde Wagging Dynamics in the S0

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An Analysis of the Methyl Rotation and Aldehyde Wagging Dynamics in the S0 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Digital.CSIC An analysis of the methyl rotation and aldehyde wagging dynamics in 1 3 the S0 (X̃ A’) and T1 (ã A‘) states of thioacetaldehyde from pyrolysis jet spectra Cite as: J. Chem. Phys. 97, 3964 (1992); https://doi.org/10.1063/1.462935 Submitted: 02 January 1992 . Accepted: 02 June 1992 . Published Online: 31 August 1998 D. C. Moule, H. A. Bascal, Y. G. Smeyers, D. J. Clouthier, J. Karolczak, and A. Niño J. Chem. Phys. 97, 3964 (1992); https://doi.org/10.1063/1.462935 97, 3964 © 1992 American Institute of Physics. An analysi~ of the methyl rotation and aldehyde wagging dynamics in the So (X 1A') and T1 ( rA") states of thioacetaldehyde from pyrolysis jet spectra D. C. Moulea) and H. A. 8ascal Department of Chemistry, Brock University, St Catharines, Ontario, L2S3AJ, Canada Y. G. Smeyers Instituto de Estructura de la Materia, CSIC, c/Serrano, 119, 28006 Madrid, Spain b D. J. Clouthier and J. Karolczak ) Department of Chemistry, The University of Kentucky, Lexington, Kentucky 40506-0055 A. Nii'l'o Escuela Universitaria de Informatica, Universidad de Castilla-La Mancha, Ciudad Real, 13071, Spain (Received 2 January 1992; accepted 2 June 1992) Jet-cooled, laser induced phosphorescence (LIP) excitation spectra of thioacetaldehyde (CH3CHS, CH3CDS, CD3CHS, and CD3CDS) have been observed in the 15800-17 300 cm - 1 region in a continuous pyrolysis jet. The responsible electronic transition, T1 +- So, (j3A" +- i lA', results from an n--+7r* electron promotion and gives rise to a pattern of vibronic bands that can be attributed to activity of the methyl torsion and the aldehyde hydrogen out-of-plane wagging modes. Potential and kinetic energy surfaces were mapped out for the aldehyde wagging (a) and the torsional (e) internal coordinates by using 6-31G* Hartree-Fock calculations in which the structural parameters were fully relaxed. The potential and kinetic energy data points were fitted to double Fourier expansions in a and e and were incorporated into a two-dimensional Hamiltonian operator. The spectrum was simulated from the transition energies and the Franck-Condon factors and was compared to the observed jet cooled LIP spectra. It was concluded that while the RHF procedure gives a good description to the ground state dynamics, the triplet state surface generated by the UHF method is too bumpy and undulating. INTRODUCTION Additional large amplitude motion is introduced into these systems when more complex structures are attached In general, the higher electronic states of molecules to the carbonyl group. In the case of acetaldehyde, exhibit a greater structural flexibility than do the corre­ CH3CHO, the CCHO frame is rigidly planar in the So state sponding ground electronic states. This nonrigidity is a and the rotation of the methyl group constitutes the single direct consequence of the excitation process which pro­ large amplitude mode. As would be expected, the molecu­ motes an electron from a bonding or nonbonding orbital to lar frame in SI acetaldehyde is pyramidal and the structure a molecular orbital which is antibonding. Thus the n--+7r* is highly flexible along both the aldehyde wagging and me­ electronic excitation in the carbonyl chromophore results thyl torsion coordinates. Thus the low frequency dynamics in a reduction of the C==O bond order from 2.0 to 1.5, of the excited state are governed by two large amplitude while at the same time the bond length increases by 0.08- modes: torsion of the methyl group; and a wagging motion 0.11 A. Even more dramatic changes occur in the bond of the aldehyde hydrogen. An even more complex case angle relationships. In the case of the molecular proto­ would be acetone, (CH hCO, where the low frequency type,1 formaldehyde CH 0, the rigid planar conformation 3 2 dynamics of the ground state are described by interacting of the So ground electronic state converts into a pyramidal methyl groups. Additional flexibility is introduced into the structure on excitation. The out-of-plane motion that in­ upper excited states from the wagging displacement of the verts the pyramidal S 1 singlet excited state structure is described by a double minimum potential which contains a carbonyl group. Thus it would require three large ampli­ central barrier of 350 cm -1. The heights of the barriers to tude coordinates to describe the dynamics of excited states molecular inversion are sensitive to the nature of the at­ of this simple molecule. tached group. For example, the first singlet and triplet It is the Franck-Condon principle that makes elec­ tronic spectroscopy an ideal tool for investigating large states of the sulphur analog, thioformaldehyde CH2S, are amplitude motions in polyatomic molecules. This principle pseudoplanar, while formyl fluoride CF20, on the other hand, has a barrier of 3100 cm -1 to molecular inversion. relates the assignment of the observed band progressions to the normal coordinates that most closely correspond to the changes in molecular structure which occurs on electronic ')To whom correspondence should be addressed. b)Perrnanent address: Quantum Electronics Laboratory, Institute ofPhys­ excitation. Thus we anticipate that the CH wagging and ics, A. Mickiewicz University, Grunwaldzka 6,60-780 Poznan, Poland. the CH3-torsion modes would be active in forming progres- 3964 J. Chem. Phys. 97 (6).15 September 1992 0021-9606/92/183964-09$006.00 © 1992 American Institute of Physics Moule et al.: Dynamics in thioacetaldehyde 3965 sions in the So --+ SI electronic spectrum of acetaldehyde. 2 The visible spectrum of thioacetaldehyde ,3 lies in the 630-560 nm region and results from an n --+ 1T* electron promotion. Both the spin allowed So --+ SI and spin forbid­ den So --+ TI transitions are observed in absorption and have about the same strength. As a result of the structural dif­ ferences between the ground and excited electronic states, ~ (f) the vibronic fine structure contained in the absorption z W spectrum is found to be highly complex. In the case of the I­ Z lower So --+ TI system,4,5 the spin-orbit selection rules limit W the number of active vibronic transitions, and thereby re­ > i= duce the complexity of the spectra. At room temperature, :5 this system consists of two clusters of bands that are sep­ w arated by a quantum of the C S stretching mode, Q9' The 0::: dense cluster that is located in the region of tht: system origin has been attributed to the activity of the methyl torsion mode, Q15' Franck-Condon arguments suggest the appearance of this mode in the spectrum would result from a displacement of the equilibrium conformations in the two electronic states. The lower Tl state is observed to be ra­ diative and under collision free conditions has a lifetime4 of 16200 16400 16600 16800 about 10 Jls. WAVENUMBER (cm-1) FIG. 1. The phosphorescence excitation spectra of CH3CHS and EXPERIMENT CH3CDS recorded under jet conditions. Thioacetaldehyde is unstable at room temperature. It may be prepared by pyrolysis of the trimer 1,3,5,­ RESULTS AND ASSIGNMENTS trimethyl-s-trithiane and detected in a flow system. In this Figures 1 and 2 show the phosphorescence excitation case, the trimer was prepared by the method of Kroto et spectra of thioacetaldehyde and its deuterated isotopomers 7 al. by passing H2S gas through an ice cold mixture ofHCl and acetaldehyde. The CH3CDO, CD3CHO, and CD3CDO samples were supplied by MSD isotopes. For CD3CHS and CD3CDS, which have exchangeable hydro­ gens DCI and D2S, were used. Jet-cooled phosphorescence excitation spectra of thio­ acetaldehyde and its deuterated isotopomers were obtained using the pyrolysis jet spectroscopic technique. 8 The crys­ talline trimer was warmed to 50 'C, the vapor entrained in ~ 1 atm of argon, and the mixture pyrolyzed at 700'C just (f) z prior to expansion through a 150 Jl nozzle into vacuum. W I­ From past experience we estimate that rotational temper­ Z atures of 5-10 K were obtained under these conditions. In W order to accentuate the weak hot bands, warm jet spectra > i= also were recorded by altering the expansion conditions to :5 prevent complete cooling of molecules in higher vibrational w 0::: levels populated in the high temperature pyrolysis zone. The phosphorescence of thioacetaldehyde was excited with a Nd:YAG pumped dye laser system (Lumonics HY 750+HyperDye 300) using rhodamine 6G, rhodamine 610, and coumarin 540A laser dyes (Exciton). Laser pow­ ers of 1-5 mJ per pulse and linewidths of approximately 0.1 cm -I were employed. The emission was detected by 16200 16400 16600 16800 imaging the cold portion of the supersonic expansion 1 through a suitable cutoff filter to reject scattered light and WAVENUMBER (cm- ) onto a high gain, red sensitive photomultiplier (EMI 9816 FIG. 2. The phosphorescence excitation spectra of CD CHS and 9 3 QB). The excitation spectra were wavelength calibrated CD3CDS recorded under jet conditions. by simultaneously observing the optogalvanic effect in a neon-filled hollow cathode lamp. J. Chem. Phys., Vol. 97, No.6, 15 September 1992 3966 Moule et at.: Dynamics in thioacetaldehyde TABLE I. Observed band maxima" in the excitation spectrum of (a) CH3CHS, (b) CH3CDS, (c) CD3CHS, and (d) CD3CDS. 000. Ditr. Assign.b Obs. Ditr. Assign. Obs. Ditr. Assign. Obs. Ditr. Assign. (a) Warm jet spectrum (b) Warm jet spectrum (c) Warm jet spectrum (d) Warm jet spectrum 15987.2 -307.7 15~al 15847.1 -51\.6 159 55.7 - 344.0 15~e 15867.2 -500.0 14?e 160 35.3 -259.6 15le 15943.6 -415.1 15~e 15046.0
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