Dispersed emission and time of flight mass spectra are presented for jet-cooled toluene, and 0-, m-, and p-xylene. The spectra exhibit features, typically within loo cm-I of the SI-.:t.S o origins, which are assigned to transitions associated with the internal rotation of the ring methyl groups. A model is developed which treats this methyl motion as that of a one-dimensional rigid rotor. The spacings of the peaks in the spectra are used to solve for the rotational constant B of the methyl rotor, and for the size and shape ofthe n-fold barrier to rotation (i.e., V 3 , V 6 , etc.) within this model. For toluene and p-xylene, the barrier is found to be small in both the ground (So) (V 6-10 cm-I) and excited (SI) (V 6-25 cm-I) electronic states. For m-xylene, the ground state is again found to have a low barrier (V6-25 cm-I), but the excited state has a potential barrier of V3 = 81 cm-I, V6 =-30 cm-I. The barrier to rotation of the ring methyl groups is observed to be the highest for a-xylene. In this case the ground state is found to have a rather large barrier V3 = 425 cm-I, V6 = 18 cm-I which changes to V3 _ 166 cm-I, V;-25 cm-I, and V6-0 cm-I in the excited state. The V; term represents a potential cross term between the two methyl rotors. The use of a kinetic energy cross term with a weighting coefficient of 0.72 in the Hamiltonian is also required for an accurate description of the excited state of this isomer. Empirical force field (EFF) calculations are performed for toluene and the three xylenes using a molecular orbital-molecular mechanics (MOMM) algorithm. The EFF-MOMM calculations are in essential agreement with the spectroscopic results and the one-dimensional rigid rotor model. II. EXPERIMENTAL PROCEDURES The time-of-flight mass spectrometer (TOFMS) chamber has been described previously.s TOFMS experiments utilize an R. M. Jordan pulsed valve. Helium is used

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Two-color time-of-flight mass spectra of 3-and 4-dimethylaminobenzonitrile (3-and 4-DMABN) bare molecules and clusters with methane, water, acetone, dichloromethane, and acetonitrile are reported and discussed. The clusters and molecules are isolated and cooled in a supersonic expansion. Both 3-and 4-DMABN bare molecules display significant changes in geometry, associated with rotation-inversion coordinates of the dimethylamino group, upon excitation from the ground electronic state So to the first excited singlet state S\. Cluster spectra for the monosolvates [CH 4 , H 2 0, (CH3)2CO, CH 2 CI 2 , CH 3 CN] of3-and 4-DMABN evidence two general types of behavior. (1) Cluster spectra have both red and blue shifts from their respective bare molecule origins which are relatively small (in general less than 200 cm-\). These cluster spectra are nearly identical with the bare molecule spectra; the solvation process seems to have little effect on the DMABN molecule, especially the-N(CH 3)2 moiety, for these clusters or cluster states. (2) Cluster spectra have shifts that are large (-500-1000 cm-\) and to low energy of the bare molecule spectra. These cluster spectra are composed of both sharp and broad features which bear little resemblance to those observed in the respective bare DMABN molecule spectra. Results of calculations of cluster geometry suggest that thedifferent cluster spectra may be associated with clusters of different geometry. These data are interpreted in terms of solution behavior of 3-and 4-DMABN. The relationship between the twisted intramolecular charge transfer (TICT) state model proposed for 4-DMABN in dipolar aprotic solvents and these cluster results is explored.

One-color time-of-flight mass spectra (mass resolved excitation spectra) for jet-cooled 4dimethylaminobenzonitrile (4-DMABN) and some of its chemical analogs, dimethylaniline (DMA), 3-dimethylaminobenzonitrile (3-DMABN), N, N-dimethyl-4-(trifluoromethyl)aniline (4-CF3-DMA), and 4-(d 6 -dimethylamino)benzonitrile (4-d 6 -DMABN), are presented and analyzed. Near the origin of the SI ,-So transition the low frequency modes can be assigned to motions of the dimethylamino group for this series of mol~cules. The inversion motion of the dimethylamino group and the dimethylamino group torsion about the Cjpso-N bond (the twist coordinate) in SI give rise to the most prominent peaks in this spectrum. The potential parameters for the twist coordinate of 4-DMABN and DMA are quite similar in SI: B = 0.546, V 2 = 175, and V 4 = 525 cm -I for 4-DMABN and B = 0.546, V 2 = 175, and V 4 = 515 cm-I for DMA. The V 2 and V 4 terms are slightly larger for 3-DMABN and 4-CF3-DMA. The inversion motion is also similar for these molecules but is more anharmonic for the para-substituted dimethylanilines, 4-DMABN and 4-CF3-DMA, than for the meta and unsubstituted molecules. A Franck-Condon intensity analysis for the dimethylamino twist in these molecules suggests that this group in 4-DMABN is displaced in the excited state by -30° with respect to its planar orientation in the ground state. In both solutions and monosolvate clusters of 4-DMABN with polar aprotic solvents, a low lying charge transfer (CT) state is identified in addition to the usual1T1T* excited state of the bare molecule. The relation between the bare molecule 4-DMABN twisting displacement upon excitation and the low lying CT state is discussed.

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