Achim Mehlhorn

Stacking interactions: ab initio SCF and MP2 study on (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2

Abstract The stacking complexes (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2 were studied at the SCF and MP2 levels with different basis sets. The SCF interaction energies are only slightly basis set dependent, with the exception of STO-3G, for which they are underestimated. MP2 interaction energies are, on the other hand, strongly basis set dependent. Minimal and split-valence basis sets give small values of this energy. A modified 6–31 G* basis set (diffuse polarization functions) provides reasonable values of SCF interaction energies, correlation interaction energies and total interaction energies of stacking complexes. The dipole—dipole electrostatic energy is comparable with the SCF interaction energy at large distances only. The dispersion energy agrees satisfactorily with the correlation interaction energy obtained with a basis set containing diffuse polarization functions.

THE ELECTRONIC STRUCTURE AND PHYSICAL PROPERTIES OF THIONITROSO COMPOUNDS—A QUANTUM CHEMICAL STUDY

Abstract Semiempirical and nonempirical quantum chemical methods have been used to describe the electronic structure and physical properties of thionitroso compounds unknown so far. Predictions on geometry, electron distribution, ionization potential, electron affinity, UV-VIS spectrum and the S 0−T 1−energy splitting of these compounds have been made. First experimental results on thionitroso compounds are critically examined.

Theoretical study of molecular structure and spectra of HNS

Abstract Using complete active space self-consistent field electronic wavefunctions the three-dimensional potential energy and electric dipole functions for the lowest two singlet and triplet states of HNS have been generated and used for the evaluation of the spectroscopic constants, anharmonic frequencies and radiative transition probabilities. The electronic excitation energies have been calculated by a multi-reference configuration interaction approach.

MO-LCAO calculations on polymethines: Part XIX. Ground and excited state geometries of closed and open shell [N + 1] π-streptopolymethines

Abstract Molecular geometries of simple cyanines and merocyanines and their radical congeners are predicted by MINDO/3 calculations. A high degree of carbon—carbon bond length equalization is found for cyanines both in the ground and lowest excited ππ states, whereas carbon—carbon bond length alternation dominates in the molecular ground state of merocyanines. This result coincides with experimentally found bond lengths of closed shell cyanines and merocyanines in the molecular ground state. In agreement with experiment, the Stokes shift between the maxima of absorption and fluorescence decreases for cyanines with ascending chain length, but increases for longer chain merocyanines. The polymethinic structure of the cyanines and the polyenic structure of the merocyanines are also reflected in the distinct bond strength and energetic stabilization effect of donor and acceptor substituents at different sites of the trimethine chain. The bond lengths of simple violene radicals in the ground state are closely related to those of the non-radical cyanines and similarities are found in the substituent effects as well.

MRD-CI study of the low-energy s-s and s-t transitions of thionitroso hydrogen (HNS)

Abstract The results of MRD-CI calculations support former estimations that the unknown HNS molecule should have a singlet ground state and exhibit a weak absorption in the near infrared region (ca. 1300 nm) occurring from an nπ∗ transition. The colour of the compound (i.e. the predicted absorption in the visible region) will be determined by the second S-S transition, which results mainly from a doubly excited configuration (nπ∗)2. The S1-T1 splitting is estimated to be larger than the S0-T1 energy difference. All results are relatively insensitive to the molecular geometry of the system, which was recalculated at an improved theoretical level.

The molecular structure of substituted cyclopentadienyl cations

Abstract Internal or external substitution of the cyclopentadienyl cation brings about hetero- or carbocyclic compounds that differ greatly in their electronic and molecular structure. Depending on the substitution pattern and the nature of the heteroatoms the compounds constitute two series. Each series is related to one of the two Jahn—Teller distorted structures of the parent cyclopentadienyl cation. This classification follows from qualitative molecular orbital interaction schemes which are fully consistent with correlation diagrams obtained by molecular orbital analysis. Semiempirical MINDO/3, MNDO and STO-2G ab initio calculations of the molecular geometry of nitrogen and boron substituted cyclopentadienyl cations furnished evidence in favour of their classification into two series of distorted structures.

MO-LCAO calculations on polymethines: Part XX. The effect of cyclization on the molecular geometry of closed shell [N + 1] π polymethines

Abstract The bond lengths of cyclized cyanines and merocyanines are predicted by MINDO/3 and QCFF/Pi calculations. Extremely long carbon bonds ( r CC > 151.7 pm) are found for some compounds that belong to Dahnes non-alternant polymethines and fulfill Kulpes charge repulsion rule. According to the calculations, shortening of the long bond occurs upon excitation from the electronic ground to the lowest excited ππ* state. Since this excited state is stabilized the energy gap to the ground state is low and relatively long wavelength absorptions are predicted for such compounds. Alternatively, these cyclized polymethines may be considered as derivatives of antiaromatics that bear nitrogen or oxygen containing substituents. Experimental data available are in accordance with the theoretical results.

Properties and reactivity of first and second row hydrides: Part 2. Reactions between the hydride cations and H2 and related processes

Abstract The reaction energies of the reactions between the first row cations of the XHn−1+ type (Li+, BeH+, BH+2, CH+3, NH+2, OH+ and F+) and the H2 molecule have been calculated for three alternative reaction channels at the MP4(CCD)/6-31G∗∗//SCF/6-31G∗∗ level. In addition, calculations for van der Waals ions related to the reactants have been performed. The preference for the individual reaction channels of the cations under study is discussed based on reaction energies. These energies are expressed in terms of the adiabatic electron attachment/detachment processes of the reactants and products. Simultaneously, this approach permits one to generalize the archetype of electrophilic substitution and to extend the description to reactions between XH+n-1 and molecules such as LiH, CH4 and HF.

Properties and reactivity of first- and second-row hydrides: Part 31. Reactions between the first-row hydride anions and H2 - interrelationships between nucleophilic and electrophilic elementary reactions

Abstract Ab initio MP4/6-311++G ∗∗ //HF/6-311++G ∗∗ calculations of the first-row hydride anions XH − n -1 and XH − n and the corresponding uncharged species (with X = Li to F) are used to systematize the course of reaction energies for the reaction of the anions with molecular hydrogen leading to three different pairs of reaction products. The equations for the reaction energies are attributed to the electron detachment/attachment properties of the reactants and products and the energy of those bonds being split and formed. The CH − 3 anion undergoes the most exoergonic reaction with H 2 leading to CH 4 and H − , whereas Li − and F − have strongly endoergonic balances. The parabolic shape of the anionic reaction energy relationship dependent on X contrasts sharply with that of the corresponding cations. The reasons for this difference are revealed.

Measures of partial distance for characterization of electronic states in conjugated molecules

Abstract Within the framework of similarity analysis, measures of partial distance (MDP) are proposed (with respect to both closed-shell and open-shell reference systems) in order to interpret the different electronic states of organic π systems in terms of molecular building blocks. The partial distance measures allow locally excited, charge-transfer and exciton-like excited states to be distinguished. Furthermore, selected ground-state molecules can be characterized in terms of weakly coupled fragments.