Application of genetic algorithms in automated assignments of high-resolution spectra

Abstract

This paper describes an automated assignment and fitting procedure for high-resolution rotationally resolved spectra. The method is based on the application of genetic algorithms (GA) and both frequency and intensity information of these spectra is used. The basic ideas behind the GA technique is introduced and the particular fitness function critical for the success of the GA evaluation is discussed. The meta-optimization of the internal GA parameters for an optimal exploration of the error landscape of the spectrum fitting is investigated. A number of typical examples are given in which the use of automated spectrum assignments with the GA method is of crucial importance in the analysis. Examples are given for fits of very dense spectra due to overlap of a number of rovibronic spectra in conformers and isotopomers and strongly congested spectra in dimer systems. It is also shown that since the GA performs an overall fit of the complete spectrum very good information on the orientation of the transition dipole moments is gathered. Contents PAGE 1. Introduction 354 2. The genetic algorithm 357 2.1. Introduction 357 2.2. The fitness function 358 2.2.1. Definition 358 2.2.2. Numerical evaluation of the fitness function 359 3. Optimization of the GA fits 360 3.1. Effect of the width (Δ w) of the weight function w(r) on the convergence of the GA 360 3.2. Meta-optimization of the internal GA parameters 361 3.3. Uncertainties in and correlations between the parameters from a GA fit 362 4. Methods 363 4.1. Experimental 363 4.2. The calculated spectra 364 4.2.1. The asymmetric rotor Hamiltonian 364 4.2.2. Internal rotation Hamiltonian 365 4.2.3. Intensities 366 4.3. The structure determination 367 4.4. The software used 368 5. Examples of the success of GA automatic assignments 368 5.1. GA fits of very dense rovibronic spectra 368 5.1.1. [7-D]phenol-N2 368 5.1.2. Benzonitrile-Ar 369 5.1.3. 4-methylphenol and its water complex 371 5.2. Simultaneous GA fits of a number of overlapping rovibronic spectra 373 5.2.1. 7-azaindole (Pyrrolo[2,3-b]pyridine) 374 5.2.2. Resorcinol (1,3-dihydroxybenzene) 375 5.2.3. Conformers and isotopomers of tryptamine 377 5.3. Strongly congested spectra in dimer systems 379 5.3.1. Benzoic acid dimer revisited 380 5.3.2. Benzonitrile dimer 385 5.3.3. Phenol dimer 387 5.4. Orientation of the transition dipole moment 389 5.5. GA fit of rotational contours 391 5.5.1. The FTIR spectrum of benzotriazole 391 5.5.2. The low-resolution LIF spectrum of azaindole-water 393 6. Summary 394 Acknowledgements 395 Appendix A 395 Appendix B 396 References 400 Supporting online material 403