RESUMEN
The H2O/D2O/HDO isotopomers of water are presented in terms that enable bond-flexing, bond-twist and bond-anharmonicity to be quantified during the bending (Q1), symmetric-stretch (Q2) and anti-symmetric-stretch (Q3) normal modes of vibration. Bond-flexing was detected by the presence of curved bonding and the bond-anharmonicity was detected by the presence of motion of the bond critical point (BCP) relative to the oxygen atom. These 2-D scalar measures are unable to fully describe the 3-D nature of the normal modes of vibration and therefore any susceptibility towards normal mode coupling, or to fully distinguish the three isotopomers. To detect bond-twist a vector-based measure was used, in the form of the bond critical point (BCP) trajectory, constructed in terms of preferred directions of electronic motion, defined by the variation of the position of the BCP during the normal modes of vibration. The BCP trajectories describe the coupling of the intramolecular bending and symmetric-stretch normal modes as well as distinguishing all three isotopomers within the harmonic approximation. The coupling of the bending and symmetric-stretch normal modes are suggested to be facilitated by the absence of bond-twist that would disrupt the coupling between Sigma O-H bonds and hydrogen-bonding. Partial coupling was found for the mixed isotopomer HDO.
RESUMEN
We show that combining vibrational spectroscopy with signal processing can result in a scheme for ultrasensitive detection of molecules. We consider the vibrational spectrum as a signal on the energy axis and apply a matched filter on that axis. On the example of a nerve agent molecule, we show that this allows detection of a molecule by its vibrational spectrum, even when the recorded spectrum is completely buried in noise when conventional spectroscopic detection is impossible. Detection is predicted to be possible with signal-to-noise ratios in the recorded spectra as low as 0.1. We have studied the importance of the spectral range used for detection as well as of the quality of the computed spectrum used to program the filter, specifically, the role of anharmonicity, of the exchange correlation functional, and of the basis set. The use of the full spectral range rather than of a narrow spectral window with key vibrations is shown to be advantageous, as well as accounting for anharmonicity.
