Lithium parameters have been optimized for Stewart's standard PM3 method. The average deviation of the heats of formation calculated for 18 reference compounds is 6.2 kcal/mol from the experimental or high-level ab initzo data; the average deviation with Li/MNDO is 18.9 kcal/mol. The average error in bond lengths is also reduced by a factor of two to three. Ionization potentials and dipole moments are reproduced with comparable accuracy than Li/MNDO. However, the mean deviation for the heats of formation of both methods increases when being applied to other systems, especially to small inorganic molecules. The applicability of the new parameter set is demonstrated further for various compounds not included in the reference set, for the calculation of the activation barriers of several lithiation reactions, as well as for the estimation of oligomerization energies of methyl lithium (including the tetramer). Li/PM3 gives reliable results even for large dimeric complexes, like [{4-(CH3CR)C&N}LiI2, containing TMEDA or THF as coligands and reproduces the haptotropic interaction between Li' and n-systems (e.g., in benzyl lithium) as well as the relative energies and structural features of compounds with "hypervalent" atoms (e.g., in lithiated sulfones). 0 1993 by John Wiley & Sons, Inc.

Vibrational spectra of two dyes, malachite green and crystal violet are presented. Raman and hyper-Raman spectra are recorded as surface-enhanced (hyper-) Raman spectra on silver colloids. These data are compared to the respective IR spectra with respect to the point group of the molecules (C, and C3). Theoretical calculations (PM3 semiempirical) and polarized Raman spectra are used to give an assignment of some vibrational bands. The advantage of SERS (surface-enhanced Raman scattering) and SEHRS (surface-enhanced hyper-Raman scattering) is discussed for the analysis of vibrational spectra.Schwingungsspektren der beiden Farbstoffe Malachitgrun und Kristallviolett werden vorgestellt. Die Raman-und Hyperramanspektren werden als oberflachenverstirkte Spektren auf Silberkolloiden aufgenommen. Diese Daten werden mit den entsprechenden lnfrarotspektren unter Beriicksichtigung der Punktgruppen (C, und C,) verglichen. Die Zuordnung einiger Banden wird mittels polarisierter Ramanspektren und quantenmechanischer Rechnung (semiempirisch PM3) durchgefiihrt. Es werden die Vorziige von SERS (surface-enhanced Raman scattering) und SEHRS (surface-enhanced hyper-Raman scattering) fur die Interpretation von Schwingungsspektren gezeigt.

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In addition to IR spectra, polarized Raman spectra of p-aminobenzonitrile and p-dimethylaminobenzonitrile in acetonitrile solution were recorded with di †erent excitation wavelengths. Normal mode coordinates were calculated using a modiÐed PM3 program package. Instead of only scaling the calculated wavenumbers of the resulting normal modes, a scaling factor was introduced into the force Ðeld. Since the normal coordinate pictures of several modes change owing to di †erent mixing of local modes, a modiÐed assignment for several modes is proposed. Figure 8. Comparison of the IR (upper part) and polarized Raman spectra (lower part) of ABN (left panel) and DMABN (right panel).