No abstract

lower O+ component are allowed and result in type C bands whereas those that terminate on the upper inversion component 0-give rise to forbidden Herzberg-Teller bands of AB mixed band type. A similar situation exists for the transitions to the syn G and G* isomers. As the O+-O-inversion splitting is small for both anti and syn isomers (less than 1 cm-I), the major bands in the spectrum are expected to be of mixed A, B, and C character. ConclusionsThe vapor-phase absorption spectrum of formic acid which arises from a n -a* electron promotion consists of a complex of bands that are built onto the side of a strong absorption continuum. The fluorescence excitation spectrum of formic acid in this region is greatly simplified and consists of a group of bands that display a well-defined rotational fine structure. A comparison of the deuterium-hydrogen frequency shifts among the four isotopomers of formic acid allowed the origin to be assigned to the band at 37431.5 cm-I. The modes u3(C=O), u,(O-C=O), u8(CH), and u,(OH) were observed to form intervals in the spectrum. The activity of the ug torsional mode is of particular interest, as it must result from a SI equilibrium configurationwhere the O H group is twisted from the 0-C=O frame of the molecule. The activity of v8 mode demonstrates that the CH bond is also displaced from the molecular plane. Both the aldehyde and the hydroxy groups would be distorted from the 0-C=O molecular frame, which would result in a complex equilibrium structure for SI formic acid.The equilibrium structure and the dynamics of the So and TI states of formic acid were evaluated by ab initio SCF theory using a 6-31G* basis set. The calculations correctly showed that the anti was more stable than the syn conformer in the lower electronic state, although the calculated 2 142.9-cm-I energy difference was substantially larger than the experimental value of 1365 cm-'. The structures calculated for the the TI state were more complex. The stable equilibrium configuration was calculated to have the O H and CH bonds twisted from the 0-C=O frame by 67.99' and 45.87', respectively. The calculations predicted a second conformation with torsional and wagging angles of -58.29' and 40.71'. This form is less stable by 464.1 cm-I. Acknowledgment. F.I. and D.C.M. thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support. We express our gratitude to Dr. J. D. Goddard and R. B. Ogawa for their assistance in the ab initio calculations and Dr. J. Karolczak for his help with the experiments. This paper presents extensive investigations of state-selective vibrational excitation induced by an IR picosecond laser pulse, including the first simulation of such processes by means of fast Fourier transform propagation of a molecular wave packet e(t). For simplicity, we consider a model Morse oscillator with potential V, and semiclassical dipole interaction -pLE(f).A selective resonant laser field E(t) with sin2 shape is chosen and compared with several alternatives, including much...

Model simulations for the inversion of ligands (H) in the double-well potential of [(C5H5)(C0)2FePH2] yield selective isomerizations controlled by a series of intense picosecond infrared (IR) laser pulses. Essentially, these pulses induce a corresponding series of selective molecular transitions, from the vibrational ground state to more excited states of the reactant isomer, via a delocalized state above the potential barrier, to a highly excited state of the product isomer. Simple superposition of the individual laser pulses with analytical, e.g., Gaussian, shapes yield the effective overall pulse which may have a rather complex structure. Optimal laser parameters are tailored to maximum population of the product isomers. Repeated application of these overall pulses produces pure product isomers. This rather general strategy for selective isomerization is compared with inefficient alternative ones. The simulations involve various theoretical techniques, from quantum chemical ab initio calculations of the relevant double-well potential energy surface and electric dipole function, to calculations of vibrational states and dipole matrix elements, to evaluation of the laser-induced molecular dynamics by propagation of the algebraic version of the time-dependent Schrijdinger equation.

No abstract

No abstract