1. ASSIGNING OF VIBRATION-ROTATION SPECTRA USING THE LWW PROGRAM PACKAGE
Wiesław Łodyga, Marek Kręglewski

2. Introduction
The Loomis-Wood for Windows program package (LWW program) is designed for assigning vibration-rotation spectra from Fourier-transform (FT) infrared (IR) spectra. It exists currently in three versions for:
Symmetric top molecules
Asymmetric top molecules
Molecules with large amplitude motions
It integrates several approaches used previously by computer programs of this type, giving in separate windows of this application simultaneous access to:
The plot of the IR spectrum with labels of predicted/assigned. The plot of the spectrum is directly linked with the table of peak wavenumbers and intensities.
Listings of wavenumbers in individual spectral branches.
Symbolic representation of sequences of transitions in Loomis-Wood diagrams, which translates the numerical information from the wavenumber listings of branches into a graphical representation.The program allows simultaneous displaying of several LW diagrams that are mutually linked by lower state combination difference (LSCD) relations. This link makes the series of transitions terminating in common upper levels appear as visually recognizable patterns of the same shape.

3. Basic windows of the program
Peaklist
window
Spectrum window
LW spectrum viewer
Loomis-Wood windows
GC Loomis-Wood window
Assignments window
Branches window

4. The concept of working with branches
Assignment procedures of the LWW program are based on working with branches which are sets of transitions with just one rotational quantum number changing and the remaining ones kept fixed. In our case, a spectral branch is to be understood as a series of transitions with the total angular momentum quantum number J being increased by 1 between two subsequent members of this series. All other quantum numbers (vibrational and rotational) of both lower and upper states remain the unchanged in one spectral branch. These remaining fixed QN, common to the whole branch, are the lower/upper state {K, l} in case of symmetric top molecules or {Ka, Kc} in case of asymmetric top molecules. The difference between these {QN} in the lower and the upper state is also given by the dipole moment selection rules. The idea of making the spectral branches logical units of assignment procedures is used because in a spectrum without resonances the transition wavenumbers in each such branch series are smooth, continuous functions of the J quantum number. In a graphical representation of the spectrum (e.g. the Loomis-Wood diagram), these series form visually recognizable patterns among other spectral peaks that do not belong the particular branch. Once several transitions in the branch are assigned and aligned, further assignments can be propagated following the continuous branch pattern.

5. Energy file
E Vib J K Sym
Predictions of vibration-rotation wavenumbers are calculated from a table of vibration-rotation energies. Every branch used in the assignment procedures of the LWW program has to be first defined by the user.
A1G+
A1U-
A2G-
A2U+
B1G+
B1U-
B2G-
B2U+
E+U
E+L
E-U
E-L
A1G+
A1U-
A2G-
A2U+
B1G+
B1U-
B2G-
B2U+
E+U
E+L
E-U
E-L

6. Branches and assignments windows
The Branches window is a control center of the LWW program. The most powerful feature of the LWW program consists in an interactive linking of the Branches window with the Assignments, Loomis-Wood, and Spectrum windows. This enables performing assignments either in the Assignments window or in the Loomis-Wood window.

7. Loomis-Wood diagrams combined with LSCD checking
The manipulation with branches is made easy in case of incorrect assignments detected
by LSCDs.
Whole branches can be given another K-assignment by moving them as one object.
This may be also followed by shifting their J-assignments up or down by the utilities in the Branches window.

8. Loomis-Wood diagrams combined with LSCD checking
The LWW diagrams display visually discernible series which are guiding the assignments effectively. Their similar shape is due to the LSCD binding of the three diagrams. After fitting the correction function, all three series should be aligned along the central vertical lines of the LW diagrams. When this alignment does not occur, the K-assignments are obviously incorrect !
rP1
rQ1
rR1
After the fitting

9. Loomis-Wood diagrams combined with LSCD checking
The three rows from Loomis-Wood diagrams in LW Spectra Viewer
rP1
rQ1
rR1

10. The Giessen/Cologne Loomis-Wood diagram without LSCD checking
This window is designed for making assignments in cases when the LSCD are not yet available. It follows the 'classical' approach of the Giessen/Cologne Loomis-Wood program.
[B.P. Winnewisser et al., J.Mol.Specctrosc. 136 (1989) 12-16]
rP1
rR1

11. Tools of the program Simulated Spectrum
The predicted spectrum can be displayed as a second trace of the Spectrum window, below the experimental one.

12. Tools of the program Reduced energy plot
Representation of energies of vibration-rotation levels with the purely J-dependent part of the energy subtracted from the total vibration-rotation energies.
Useful for locating resonant crossings and easy identification
of perturbed spectral patterns.
Reduced energies are calculated from vibration-rotation energies read from the file of energies.

13. Tools of the program
The tools designed for easier manipulation with data
Import of energies and Export of assigned wavenumbers.
Converting the FT spectrum from external sources into the internal representation of the LWW program. This tool is used both for importing the plot of the spectrum and the peak list.
There is also an option for merging two spectra from two different regions into one internal spectrum file. These tools are available only in the step of defining a new project and become disabled once the new project is defined and saved.
Assembling the energy file from several data sources, which cannot be handled by using the user-defined import template, by the Merge Energy Files command in the Tools menu.

14. The LWW program web page