Resonant IR Photon Induced Dissociation in Organic Molecules with Chain-like Substructures

Abstract

IntroductionPhoton induced dissociation (PID) of molecules due to action of low intensity IR radiation is the subject of several experimental investigations [1][2][3][4][5][6][7][8][9][10][11][12]. The interest in this field arose, as information of molecular dissociation by IR photons is important not only for the development of quantum chemistry of organic molecules and for the theory of laser beam interactions with large molecules, but also for practical applications. It was shown experimentally that the PID of molecules consisting of chain-like substructures can be induced resonantly by IR radiation with frequencies which do not coincide with the vibrational frequencies of the diatomic valence groups contained in these molecules [4,5]. Also, it was shown that IR photon induced dissociation (IR-PID) leads with high probability to fragments consisting of small atomic groups. These groups are mainly the end groups of the chain-like substructures inside the irradiated molecules [6][7][8][9][10][11][12]. The abundance of different fragments depends on the photon fluence, on the number of diatomic groups in the chains and on the corresponding dissociation energies [6][7][8][9][10][11][12].In the present work a model for the dissociation of organic molecules with chain-like substructures by IR-PID is suggested. The model is based on the excimol theory for excitation and fragmentation of organic molecules, which was worked out previously [13]. In the frame of the excimol theory the collection of internal molecular energy occurs in chain-like molecular substructures consisting of identical diatomic groups with significant dipole momenta. In these chains many collective vibrational states (excimols) with frequency w ex are induced by a resonant external periodic field, e.g. IR photons. The excimol energy E ex = h w ex is less than the energy E 01 = h w 01 of the first vibrational state of an isolated diatomic quantum oscillator. This energy difference is caused by the energy E tr required for energy transitions within the chain from one diatomic oscillator to another. These tansitions are due to resonance dipole-dipole interactions between identical molecular dipoles and the transition energy E tr which strongly depends on the dipole moment of the diatomic groups and on the three-dimensional structure of the chain [14,15]. The excimol lifetime t ex is larger than the lifetime t 01 of the excitation of an isolated diatomic oscillator in the first vibrational state, because the resonance interaction of the diatomic dipoles within the chain decreases the probability of vibrational relaxation in each dipole. For example, the excimol lifetime t ex in a (CH 2 ) n chain is two orders of magnitude higher than the lifetime t 01 of the excitation of an isolated CH-group.Resonant excimol excitation can be induced by periodic electromagnetic fields including IR-fields, if the frequency of the field is equal to the excimol frequency. Several excimols can be excited independently in a chain. They are not accumulated in a single diat...

Grazing Molecule Excitation as a Tool to Analyse the Amino Acid Sequence in Oligopeptides

“…The proposed sequencing method uses the grazing molecule excitation (GME) and dissociation (GMD) effect, which is able to produce cations of the N-terminal amino acid in an oligopeptide. This effect is explainable by the excimol theory worked out by us earlier [6][7][8][9]. In the frame of this theory we obtain (8), to define the grazing velocity υ gr max for which the N-terminal fragment peak in the dissociation spectrum appears with maximum probability (see Table 1).…”

“…Our calculations using 7and (8) show that υ gr max is varying for different residues R 1 j in fragment X 1 j depending on the number of real CH dipoles in the IR antenna. For at least four CH dipoles in the antenna of R 1 j , the optimal grazing velocity υ gr max should be found in the intervals [0.5-0.7] • 10 4 m/s or [1.5-1.9] • 10 4 m/s.…”

“…A method to determine sequences of amino acid residues in oligopeptides is suggested here, which is a practical application of GMD induced by IR photons [6][7][8][9]. The basis of the method is a special vibrational excitation process, which occurs to peptide molecules irradiated by IR photons.…”