Intermolecular Processes in Excited Electronic States in the Gas Phase (Review)

Abstract

A brief review of works on the temperature dependences of the rate constants k q of the intermolecular processes proceeding in the excited electronic states in the gas phase is given. The dependences k q (T) for such biomolecular processes as intermolecular vibrational energy transfer in the triplet state vibrational quasi-continuum, triplet-triplet electron excitation energy transfer, and intermolecular photoinduced electron transfer have been compared. The experimental data have shown that in the gas phase for all analyzed intermolecular processes both an increase and a decrease in k q with increasing temperature (T) is observed, which is not associated with the specific intermolecular interactions leading to the formation of long-lived components. The change in the type of temperature dependence is due to the change in the mechanisms of the radiationless transitions with increasing density of vibrational levels in the final electronic state. The applicability of the known models based on the theory of radiationless transitions for predicting the temperature dependences k q (T) is discussed.Keywords: temperature dependence of the rate constants of intermolecular processes, intermolecular vibrational energy transfer, triplet-triplet energy transfer, photoinduced intermolecular electron transfer.Introduction. Biomolecular processes play an important part in gaseous media such as the atmosphere, gaseous mixtures resulting from combustion and explosions, working mixtures of gas lasers, gaseous mixtures of isotopes, etc. In real molecular mixtures, the yield of photochemical reactions depends on the efficiency of the electron and vibrational energy exchange and transfer. The temperature dependences of the probability of collisional processes are of fundamental interest both for studying the nature of intermolecular interactions deactivating excited molecules and testing theoretical models. The practical interest in such studies is primarily due to the necessity of estimating the yield of gas-phase reactions in a wide range of temperatures.In analyzing the intermolecular processes in the excited electronic states, it is usually assumed that their rates increase with increasing temperature. Negative temperature dependences are considered as a result of specific intermolecular interactions leading to the formation of stable complexes of various nature. The most detailed theoretical studies of the temperature dependence of the energy transfer probability have been made for the intermolecular vibrational relaxation of small molecules in the system of discrete vibrational levels [1]. It has been shown that for this intermolecular process both positive and negative temperature dependences of the rate constants and the energy transfer efficiencies are possible. For the triplet-triplet (T-T) electron excitation energy transfer and the intermolecular photoinduced electron transfer (PET), the possibilities of modeling the temperature changes in the process probability were discussed only for some special cases in [2] and [3, 4...

Effect of intravenous laser irradiation on the molecular structure of blood and blood components

“…As shown in [6,15,16], upon absorption of emission from a He-Ne laser, the porphyrin within the Hb goes to the excited state, with partial dissociation of the heme-ligand bonds. As a result of the reversible process of photoconversion of oxyhemoglobin (HbO 2 ) to deoxyhemoglobin (Hb) initiated by laser radiation, HbO 2 +H + ^ H + Hb+O 2 , the oxygen transport function of blood is changed.…”

“…The IR absorption bands for NH stretching vibrations and also amide I, amide II, and amide III bands are of interest for analysis of the effect of exposure to laser radiation, since their position and intensity are sensitive to changes in the secondary and tertiary structure of proteins. In [6], it was noted that when blood is exposed to laser radiation, changes are observed in the intensity and shape of the amide I and amide II bands for the erythrocytes, due to conformational changes in the hemoglobin tetramer in the reversible reaction of photodissociation of the oxygen ligand.…”

“…The primary photoacceptors absorbing the emission from a He-Ne laser in blood and the molecular mechanisms for the interaction between laser radiation and blood remain a subject of vigorous debate [4,7]. Based on study of the absorption spectra in the UV-visible and IR ranges of the spectrum, it has been shown [6] that the primary photoacceptor for emission from a He-Ne laser in blood is hemoglobin (Hb). Accordingly, it was of interest to perform a detailed comparison of the changes in the IR spectra of blood, erythrocytes, and plasma that are initiated by intravenous irradiation of blood with emission from a He-Ne laser.…”

“…The process of separation and addition of the oxygen ligand is accompanied by conformational and structural changes in the Hb tetramer. The spectral manifestations of some of these changes have been demonstrated in [6,15,16]. Despite the relatively low absorption coefficient of Hb at the emission frequency of a He-Ne laser (0.115⋅10 -4 mol/L for deoxyhemoglobin), it is several orders of magnitude higher than the absorption coefficient for O 2 , which was considered as a direct photoacceptor for laser radiation in [13].…”