Volker Dyczmons

Combined jet relaxation and quantum chemical study of the pairing preferences of ethanol

The subtle trans-gauche equilibrium in the ethanol molecule is affected by hydrogen bonding. The resulting conformational complexity in ethanol dimer manifests itself in three hydrogen-bonded OH stretching bands of comparable infrared intensity in supersonic helium expansions. Admixture of argon or nitrogen promotes collisional relaxation and is shown to enhance the lowest frequency transition. Global and local harmonic frequency shift calculations at MP2 level indicate that this transition is due to a gauche-gauche dimer, but the predictions are sensitive to basis set and correlation level. Energetically, the homochiral gauche-gauche dimer is predicted to be the most stable ethanol dimer conformation. The harmonic MP2 predictions are corroborated by perturbative anharmonicity contributions and CCSD(T) energies. Thus, a consistent picture of the subtle hydrogen bond energetics and vibrational dynamics of the ethanol dimer is starting to emerge for the first time.

No N4-dependence in the calculation of large molecules

It is shown that the time needed for computing large molecules is proportional to N2 · (lnN)2, when Gaussian functions are used and the molecule is larger than a well-definable limit. Some proposals to overcome this limit are made.

Rotational isomerism and charge density properties of vinylcyclopropane (VCP)

Abstract Ab initio SCF calculations of more than double zeta quality have been carried out for nearly fully optimizing the s-trans, gauche and s-cis structures of VCP. The potential curve of internal rotation, which is obtained under (partial) consideration of the structural parameter relaxation, confirms the three minimum curve of preceding experimental and theoretical investigations. As was conjectured in a microwave study,] the dipole moment of gauche VCP (0.29 D) proves to be considerably smaller than the s-trans value (0.48 D). The different CC bonds (the ring bonds, the double bond, and the bond between ring and vinyl group) are mutually compared on the basis of bond lengths, force constants, population analyses (Mulliken and Davidson—Roby analyses), and bond path considerations. Especially, a π-conjugative interaction between the cyclopropyl ring and the vinyl group is thoroughly studied as a function of internal rotation. The occurrence of two gauche structures is interpreted as a compromise between the attempt to achieve maximal π-Conjugative interaction (which favours the s-cis structure) and to avoid steric interaction of the endstanding hydrogen atoms (which disfavours the s-cis structure).

Equilibrium structures and chemical shifts of closo-1,5-C2B3H5,C2B3H3X2 and C2B3X3H2 with X = Li, F, Cl and NH2

Abstract The equilibrium structure of closo-1,5-C 2 B 3 H 5 was determined by SCF, DFT, MP2, CCSD and CCSD(T) calculations with three different basis sets. The three parameter hybrid exchange-correlation functional of Becke (B3LYP) was employed in the DFT calculations. The force constants and the harmonic wavenumbers of the A 1 ′-modes were calculated with the CCSD(T) and DFT methods. For an investigation of the deformability of the polyhedron of C 2 B 3 H 5 , the equilibrium structures together with the BB bond strengths and the chemical shifts of the compounds C 2 B 3 H 3 Li 2 , C 2 B 3 H 2 Li 3 , C 2 B 3 Li 5 , C 2 B 3 H 3 F 2 , C 2 B 3 H 2 F 3 , C 2 B 3 F 5 , C 2 B 3 H 3 Cl 2 , C 2 B 3 H 2 Cl 3 , C 2 B 3 H 3 (NH 2 ) 2 and C 2 B 3 H 2 (NH 2 ) 3 were calculated. The BB distances vary from 177.5 pm (C 2 B 3 H 3 Li 2 ) to 195.2 pm (C 2 B 3 F 5 ). The molecules C 2 B 3 H 3 F 2 , C 2 B 3 H 2 F 3 , C 2 B 3 F 5 , C 2 B 3 H 2 Cl 3 , C 2 B 3 H 2 (NH 2 ) 3 are classically bonded, while C 2 B 3 H 3 Li 2 and C 2 B 3 Li 5 have a non-classical structure. The bonding situation of all other investigated molecules lie between classical and non-classical.