Hm Henk Buck

A bactracking algorithm for exact counting of internal molecular energy levels

An algorithm is described for counting exactly internal molecular energy levels. Difficulties in the concept of the density of states are reinvestigated. The influence of anharmonicity is shortly discussed and it is shown that reliable densities can only be obtained for low energies if anharmonicity constants are used.

Ab initio CI calculation of the vibrational structure of the 1(nπ∗) transition in formaldehyde

This paper reports an ab initio CI calculation of the radiative 1A1→1A2 transition of H2CO and D2CO. Throughout the calculation the electronic wavefunctions and transition moments are explicitly calculated as functions of the nuclear geometry, contrary to the conventional Herzberg–Teller approach. The evaluation of the vibrational wavefunctions and integrals was made numerically. The results show that the excited state frequency for mode 3 has to be reassigned and that the calculated vibrational structure agrees well with the experimental intensities.

Different rotamer populations around the C-5C-6 bond for α- and β-d-galactopyranosides through the combined interaction of the gauche and anomeric effects: A 300-MHz 1H-n.m.r. and mndo study

Abstract A 300-MHz 1 H-n.m.r. study of methyl 2,3,4-tri- O -methyl-ga- ( 1 ) and β- d -galactopyranoside-6-(dimethyl phosphate) ( 3 ), using various solvents, shows that the gauche (gg) rotamer populations about the C-5C-6 bond are are the same in all solvents, whereas those of the gauche(trans) (gt) and trans(gauche) (tg, O-5 and O-6 trans ) rotamers are solvent dependent. The tg population increases with decreasing polarity of the solvent, which is attributed to an increased electrostatic repulsion between O-5 and O-6 in apolar solvents. The tg population of 3 is larger than that of 1 and the same difference is observed in the corresponding compounds ( 2 and 4 ) which have a trigonal-bipyramidal five-coordinated phosphorus (P v ) at position 6 and which have a higher electron density at O-6. These differences in rotamer populations are due to an effect additional to that of the coulombic effect between O-5 and O-6. That these differences are caused by a combination of the gauche and anomeric effects is supported by the finding that the tg population increases with increasing p K a of the group at C-1. The results of the n.m.r. measurements (in CCl 4 ) are reproduced fairly accurately by MNDO calculations on model systems. The solvent dependence of the rotamer population around the C-5ẋC-6 bond is a good criterion for the assignment of the H-6 S ,6 R resonances since, for galactopyranosides, J 5,6 S increases and J 5,6 R decreases as the polarity of the solvent decreases.

Stereoselective hydride uptake in modelsystems related to the redox-couple nad+/nadh

The present work deals with the mechanistic investigations of the hydride transfer reactions concerning the redox couple NAD+/NADH. Based on the theoretical and experimental investigatins of NAD(H) model compounds as 3-carbamoyl pyridinium cations (3-carbamoyl-1,4-dihydropyridine) it was found that the out-of-plane rotation of the carbonyl function controls the stereo-and regiospecificity of the introduced hydride anion. It was found that the hydride anion transfered in the reaction, is always syn-positioned with respect to the carbonyl group. The unique stereoselectivity exhibits a strong coherence with the recent crystallographic 3D-data for the ternary complex of NAD bonded horse liver alcohol dehydrogenase. The results show that the amide group is 30° out of the plane with the carbonyl directed toward the A side. There are observations that the absolute configuration of the introduced chirality in the 3-carbamoyl pyridinium cations selects between the hydride uptake coresponding with the enzymatic A or B specificity.

Dynamical calculations on the photoisomerization of small polyenes in a nonadiabatic formalism

The dynamics of the cis–trans isomerization in 10° monopyramidalized ethylene, butadiene, and hexatriene has been studied in a nonadiabatic treatment including explicitly the twisting motion. The electronic energy curves, (transition) dipole moments, and nonadiabatic couplings have been obtained from ab initio CI calculations. An initially (Franck–Condon) excited wave function is constructed from the final vibrational eigenfunctions and energies. The appearance of the calculated prepared state is discussed in relation to the increasing line broadening vs decreasing chain length of the primary vibrational bands in the 1 1B+u absorption spectra of polyenes. It is found that this feature can be explained from the increasing tendency of shorter polyenes to be nonplanar in the excited state. The time evolution of the molecules is monitored by calculating the radiationless decay, the dipole moment, and the contributions of the electronic states to the total wave function. The role of the nonabsorbing 2 1A−g sta...

Ab initio CI calculation of the radiationless transition of the 1(nπ) state of formaldehyde

This paper reports an ab initio CI calculation of the radiationless decay of the formaldehyde 1A2 state. First we derive quantitative conditions, which a basis set must satisfy if it is to be used for describing radiationless decay. We checked these conditions for the formaldehyde molecule and found them to be satisfied for the adiabatic Born–Oppenheimer set used in the calculation. We then derive a general equation for the coupling elements resulting from this basis set. With the method used, rotational coupling could be treated completely equivalent with vibrational coupling; this rotational coupling turned out to be not important in formaldehyde however. The coupling elements for D2CO are a factor 10 smaller than the corresponding ones in H2CO. The results of the calculation show, that the internal conversion in formaldehyde is an example of the so‐called resonance case. Therefore the decay cannot be described by the model proposed by Yeung and Moore, where S1→S0 internal conversion is the rate determi...

The different influences of ether and ester phospholipids on the conformation of gramicidin A. A molecular modelling study

With AMBER (assisted model building with energy refinement) molecular modelling techniques, the interactions between lipids which differ in the type of chain linkage (e.g., ether or ester) and gramicidin were approximated. It was found, theoretically, that replacement of the ester function in dipalmitoylphosphatidylcholine (DPPC) by an ether moiety induces a shift in the rotameric distribution of the Trp-15 side-chain in gramicidin A. Concomitantly, the channel entrance is contracted by approx. 0.4 A. The perturbation can be related to the strong hydrogen-bond formed between the lipid carbonyl group and the indole proton of the Trp-15 residue of gramicidin. In the ether lipid-gramicidin assembly a weaker H-bond is formed between Trp-15 and the phosphate moiety. To obtain a first indication of the influence of the strength of this H-bond on gramicidin A, a preliminary experimental study was set up. The transport properties of gramicidin A were studied using efflux measurements through vesicle walls containing ether and ester lipids, respectively. A change in the permeability of gramicidin A was found when ether lipids were added to a bilayer composed of the ester lipid dioleoylphosphatidylcholine (DOPC).

A comparative study of theoretical methods for calculating forbidden transitions

Abstract In this study simple calculational methods for forbidden transitions are tested. Using different methods we calculated vibrationaly induced electronic transition moments. These D ( Q ) functions are, to a very good approximation, linear functions of the normal coordinates. A separate calculation of ground and excited state with a 4-31G basis set is the best alternative way to reproduce large basis set+CI results on formaldehyde. Contrary to older results, we show that in formaldehyde mode 5 (and in a less degree mode 6) contributes largely to the total n π* oscillator strength. It is argued that the experimental determination of f -values should be reconsidered.

Ab initio CI calculation of single vibronic level fluorescence emission spectra and absolute radiative lifetimes of H2CO (1A2)

Overlap integrals and dipole transition moments which were obtained by an ab initio CI calculation are used for the calculation of fluorescence emission spectra and absolute radiative lifetimes of single vibronic levels of H2CO (1A2). The agreement betwen calculation and experiment is fairly good. The analysis of the results shows that a large fraction of the total emission intensity is due to low‐intensity transitions and bands at wavelengths larger than 460 nm. The implications of this result for the determination of fluorescence quantum yields and radiative lifetimes are discussed.

Solving the discretized time-independent Schrödinger equation with the Lanczos procedure

A new method is presented to find bound state solutions of the one‐, two‐, or three‐dimensional Schrodinger equation. The equation is turned into a sparse matrix eigenvalue problem by representing the potential energy surface and the wave function on a grid. The Laplacian is represented by a high (10th) order finite difference formula. Eigenvalues are found by the Lanczos procedure [J. Cullum and R. A. Willoughby, J. Comp. Phys. 44, 329 (1981)] and transition probabilities (Franck–Condon factors) are found by the recursive residue generation method [A. Nauts and R. E. Wyatt, Phys. Rev. Lett. 51, 2238 (1983)]. Examples are given for the 1D Morse oscillator and the 2D Henon‐Heiles potential. Numerical convergence can be checked easily and highly accurate results can be obtained. The algorithm is fast, easy to implement, and vectorizable.