A reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP3 to ionisation of the inner π-level (1b1) is correct and that most other assignments to ionisation of a σ-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a2−1 1 ππ ∗ states of symmetry 1A1 and 1B2. The range of calculated valence states of ππ ∗ -type has been extended to include those from 1b1.

The electronic states of pyrrole studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

The electronic states of furan studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

Equilibrium twist angles, rotational barriers around essential single bonds, and preferred conformations for over 60 conjugated organic molecules were calculated using the semiempirical AM1 (Austin model 1) method. Comparison with ab initio and experimental data shows that AM1 can be applied quite successfully to conformational problems of this type. Relatively large errors are, however, found for compounds in which lone pair–hydrogen and especially lone pair–lone pair interactions are decisive for their conformational behavior. AM1-calculated rotational barriers in conjugated molecules, however, are found to be much too low. Moreover, AM1 does not seem to even correctly reproduce the trend of rotational barriers within a series of structurally related compounds. E/Z energy differences obtained by AM1 are also frequently considerably too low. In contrast to rotational barriers, their trends and thus conformational preferences, however, are quite satisfactorily calculated by this method. Ionization potentials obtained by AM1 are too high by about 0.5 eV. However, trends are quite well predicted.

AM1 calculations of rotation around essential single bonds and preferred conformations in conjugated molecules

Molecular structure and infrared spectra of furan, thiophene, selenophene and their 2,5-N and 3,4-N derivatives: density functional theory and conventional post-Hartree-Fock MP2 studies

The shape of urea

Microwave spectrum, barrier height to internal rotation of methyl group of 3-methylthiophene, and dipole moments of 3-methylthiophene and thiophene

The microwave spectrum of cyclohexene has been investigated in the frequency region from 7.3 to 27 Gc/sec with a conventional Stark modulation spectrometer, and the spectrum in the ground vibrational state has been assigned. The dipole moment lies entirely along the b-principal axis; its value, as obtained from the Stark displacements, is 0.332±0.012D. From the rotational constants and the orientation of the dipole moment, it can be concluded that the equilibrium conformation is a “half-chair” form. Its most probable structure has been determined.

Microwave Spectrum of Cyclohexene

Microwave spectra, barrier heights to internal rotation of methyl group, and dipole moments for conjugated chain compounds: Methacrolein

The quadrupole hyperfine structure was resolved and analyzed for the vibrationally-ground state of the NH2 species. The quadrupole coupling constants are: χaa=2.34, χbb=1.86, and χcc=−4.20 MHz. The rotational constants obtained from the quadrupole corrected frequencies are as follows: A=5617.67, B=2593.88, and C=1777.04 MHz. The rotational constants of the ND2 species in the vibrationally-ground state were also determined. By the use of the moments of inertia of both species, the non-planarity of this molecule was ascertained. The dipole moments of the NH2 and the ND2 species were determined to be 1.15±0.02 D and 1.13±0.01 D respectively.

Nuclear Quadrupole Effects in the Microwave Spectrum and Dipole Moment of Aniline

The microwave spectrum of 3-methylfuran was measured in the frequency region from 8.2 to 30 GHz. By observing the A-E splittings due to the hindering rotation of the methyl group, the barrier height was determined to be 1.09kcal/mol. The value is nearly the same as that of 2-methylfuran. The dipole moment was determined to be 1.03 D, which is a little larger than that of furan.

Kunio Kozima

Papers

Microwave spectrum, barrier height to internal rotation of methyl group of 3-methylthiophene, and dipole moments of 3-methylthiophene and thiophene

Microwave Spectrum of Cyclohexene

Microwave spectra, barrier heights to internal rotation of methyl group, and dipole moments for conjugated chain compounds: Methacrolein

Nuclear Quadrupole Effects in the Microwave Spectrum and Dipole Moment of Aniline

Microwave Spectrum, Barrier Height to Internal Rotation of Methyl Group, and Dipole Moment of 3-Methylfuran