Theoretical interpretation of the vibrational spectrum of bicyclo[1.1.0]butane in terms of an ab initio anharmonic model

Abstract

The anharmonic vibrational IR and Raman spectra of the bicyclo[1.1.0]butane molecule have been calculated in the range of up to 4000 cm -1 using a numerical and analytical realization of the van Vleck second order operator perturbation theory. Cubic and quartic force constants in normal coordinates, as well as cubic surfaces of the dipole moment and polarizability, have been found by numerical differentiation of the corresponding first and second derivatives calculated by the MP2/cc pVTZ quantum mechanical method. In order to increase the prediction accuracy of vibrational transitions, corresponding harmonic frequencies have been obtained by the CCSD(T)/cc pVTZ high precision quantum mechanical method. The anhar monic intensities of the IR and Raman spectra have been calculated using canonical transformations of the operators of the dipole moment and polarizability expanded into a Taylor series around the equilibrium con figuration. The assignment of experimental vibrational bands in the IR and Raman spectra has been analyzed. It has been shown that the anharmonic calculation based on the above described procedure of combining more exact harmonic frequencies with the anharmonic force field obtained with a more economical method makes possible the reliable interpretation of the majority of spectral bands, including Fermi and DarlingDennison resonances.

Tracing the Fingerprint of Chemical Bonds within the Electron Densities of Hydrocarbons: A Comparative Analysis of the Optimized and the Promolecule Densities

“…Interestingly, this subset includes C−C bonds that have been classified as “exotic” bonds previously, and a large number of studies have been conducted in the last decades to elucidate the detailed nature of these bonds. The members are the naked “formally” triple C−C bond in ortho ‐benzyne ( 73 ), the central single C−C bond in the highly strained bicyclo‐1,1,0‐butane ( 77 ), the strongly deformed central double bond in the strained bicyclo‐2,2,0‐hex‐1,4‐ene ( 81 ), and finally the central inverted C−C bond in 988 . The computed % ρ (%▿ 2 ρ ) are 34, 12, and 35 (72, 121, and 82) for 73 , 77 , and 81 , respectively.…”

Ab Initio Anharmonic Analysis of Vibrational Spectra of Uracil Using the Numerical-Analytic Implementation of Operator Van Vleck Perturbation Theory

“…We have chosen CVPT2+ WK as the main method of our calculation of the anharmonic spectrum of uracil. The detailed methodology of CVPT2+ WK has been previously described in several publications. − In this section, we describe a number of practical matters that are more specific for uracil because of its size and provide additional essential specifications.…”

“…In the present study we employed our systematic implementation of the anharmonic vibrational canonical Van Vleck perturbation theory in an essentially analytic operator form, − as well as PES and MPS obtained by a purely ab initio method (MP2). This approach is enhanced by harmonic frequencies obtained at higher theoretical level.…”

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

“…4 Interestingly, notwithstanding the significant amount of strain in BCB, the compound contains two essentially undistorted cyclopropane rings 5 in which the bridgehead and side C–C bonds are almost identical in length at approximately 1.50 Å. 6 These C–C bond lengths have been determined experimentally by vibrational 7 and microwave spectroscopy, 8 as well as electron diffraction studies. 4 a Earlier molecular orbital descriptions contrasted with these experimental findings by calculating more significant differences between the lengths of the two types of bonds (bridgehead shorter than side “bridging” bonds) 6 a ,9 as well as other discrepancies leading to shorter calculated bond lengths for both types of bonds.…”