No abstract

No abstract

Articles you may be interested inThe vibrational spectra (100-1600 cm− 1) and scaled a b i n i t i o STO3G and 321G harmonic force fields for norbornane, norbornene, and norbornadiene Theoretical harmonic force fields are reported for 16 symmetric tops H3MX (M = C, Si, Ge, Sn; X = F, CI, Br, I). Based on the evaluation of systematic test calculations for H 3 CF, H 3 CCI, H 3 SiF, and H 3 SiCI, all molecules are treated uniformly at the Hartree-Fock level using effective core potentials and polarized double-zeta basis sets for the valence electrons. For each molecule the calculated geometries, rotational constants, frequencies, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with the available experimental data. The agreement is satisfactory and generally of similar quality as in analogous all-electron Hartree-Fock calculations of molecules with first-row atoms. It is stressed that theory may reliably provide data which are hard to derive from experiment, e.g., the off-diagonal symmetry force constants and the signs of the dipole moment derivatives. The effects of scaling the theoretical force fields are investigated carefully including the transferability of scaling factors between related molecules, and the vibrational spectrum of the as yet unknown H3SnF molecule is predicted. Our present results indicate that the chosen theoretical approach is generally suitable for studying the harmonic force fields of inorganic molecules with heavy main-group elements.

Infrared spectra of the short-lived difluoroethyne molecule have been recorded in neon and argon matrices between 200 and 5000 cm−1. Fourier transform infrared spectra with a resolution of 0.004 cm−1 have been measured in the gas phase around 1350 cm−1 (ν3, ν2+ν4+ν5, hot bands) and 2150 cm−1 (ν2+ν3, ν1−ν5, hot bands). The high resolution study yields rotational parameters of the ground and all singly excited vibrational states. The interpretation of the experimental data has been guided by ab initio calculations at the SCF (self-consistent-field) level and the correlated MP2 level (Moller–Plesset second order perturbation theory) employing three different large basis sets. The theoretical calculations provide the SCF and MP2 harmonic fields as well as the SCF anharmonic force field of FCCF. The agreement between the available theoretical and experimental results is generally quite good, with the exception of the spectroscopic constants involving the trans-bending mode ν4 where more theoretical work is required. The combined use of theoretical and experimental information leads to an estimate of the equilibrium structure [D∞h, re(CC)=1.1865 Å, re(CF)=1.2832 Å] and to recommended ‘‘best’’ values for the wave numbers of all fundamental vibrations based on the matrix and high resolution infrared data and some ab initio anharmonicity constants. The present study demonstrates the advantages of a combined theoretical and experimental approach to the spectroscopy of short-lived molecules.

we can in. each case specify the optical density x55 that corresponds to a particular spectrum (see legends).A comparison between the PH5 spectra in Figures 4 and 5 reveals that improvements in the basis set beyond the 6-31G** level change the predicted spectrum only very slightly. The vt and v6 bands, which overlap strongly around 1200 cm'1, are coupled by a second-order Coriolis interaction (|f46| = 0.35, 6-31G**); including this interaction in the simulation leads to a reduced intensity of the Q branch of v4 and to some changes in the fine structure that are not shown presently. With regard to the dipole-forbidden transitions in PH5, the a/ bands cannot gain intensity by second-order Coriolis effects being coupled to e" only,