Mjh Martin Kemper

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.

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...

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.

A theoretical study on the reactivity and spectra of H2CO and HCOH. A dimeric model for nonzero pressure formaldehyde photochemistry

The reactivity and spectra of formaldehyde isomers and dimeric complexes between them are studied with ab initio methods. A large number of complexes between H2CO, trans‐HCOH, cis‐HCOH is calculated. Infrared and Raman spectra of (H2CO)2 are calculated with relatively simple methods using spectroscopic masses and scaled force constants. In this way, the structure of dimers in matrices can be deduced. Hydroxycarbene (HCOH) plays a key role in a model that explains a large number of experimental facts of the nonzero pressure photochemistry. Hydroxycarbene forms complexes with H2CO; the stabilization is due to classical hydrogen bonds. HCOH is a new example of an ambiphilic carbene. Addition products are formed from HCOH⋅⋅⋅H2CO complexes. The calculations show that, in agreement with matrix experiments, glycoaldehyde and methanol are easily formed. The formation of trans‐HCOH occurs through a dimeric interaction with the shifting hydrogen. This bimolecular process is 9.6 kcal/mol (6–31G*) in favor of the uni...

AB-initio Cl calculation of radiative and non-radiative decay of formaldehyde (1A2) with application to its photochemical decomposition

A method is described for an ab-initio Cl calculation of the decay of H2CO(1A2). The normal modes 4, 5 and 6 turn out to be about equally important for inducing the radiative transition, while the non-radiative decay is an example of the resonance case. So the experimentally determined exponential decay must be due to a coupling with the dissociative continuum.

On the comparison between crude and adiabatic Born-Oppenheimer coupling elements

A comparison is made between the coupling elements arising from the crude and the adiabatic Born-Oppenheimer approximation. It is shown, that if one includes the differences between the vibrational states in these approximation, it is possible, within the harmonic approximation, that the crude coupling elements are equal to, or even less than, the adiabatic ones. Further, it is shown, which quantities should be known to decide for the better basis set.

A method for determining a basis set suitable for the description of radiationless decay

Quantitative conditions are derived which a basis set must satisfy if it is to be used for describing radiationless decay. The derivation is valid for both internal conversion and intersystem crossing.