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PubMed2005

Vibrational Dynamics of Biological Molecules: Multi-quantum Contributions.

High-resolution X-ray measurements near a nuclear resonance reveal the complete vibrational spectrum of the probe nucleus. Because of this, nuclear resonance vibrational spectroscopy (NRVS) is a uniquely quantitative probe of the vibrational dynamics of reactive iron sites in proteins and other complex molecules. Our measurements of vibrational fundamentals have revealed both frequencies and amplitudes of (57)Fe vibrations in proteins and model compounds. Information on the direction of Fe motion has also been obtained from measurements on oriented single crystals, and provides an essential test of normal mode predictions. Here, we report the observation of weaker two-quantum vibrational excitations (overtones and combinations) for compounds that mimic the active site of heme proteins. The predicted intensities depend strongly on the direction of Fe motion. We compare the observed features with predictions based on the observed fundamentals, using information on the direction of Fe motion obtained either from DFT predictions or from single crystal measurements. Two-quantum excitations may become a useful tool to identify the directions of the Fe oscillations when single crystals are not available.

Journal Article↗
PubMed2004

Theoretical study of the structures, stability and vibrational spectra of the nitrous acid complexes with CH4.

The structures, stability and vibrational spectra of the binary complexes CH4...HONO-trans and CH4...HONO-cis have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimization was made for the complexes studied. It was established that the complex CH4...HONO-trans is more stable by 0.41 kcal mol(-1) than the complex CH4...HONO-cis. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The changes in the vibrational characteristics of methane and trans-, cis-nitrous acid upon formation of the hydrogen bond show that the complexes CH4...HONO-trans and CH4...HONO-cis have geometry in which the OH group interacts with a methane molecule forming a single hydrogen bond. This fact is confirmed by relatively strong perturbation of the OH stretching vibration to lower frequencies and an increase of the infrared intensity of this vibration up to three times upon hydrogen bonding.

Drug Stability↗
PubMed2004

Characteristics of coated copper wire specimens using high frequency ultrasonic complex vibration welding equipments.

Welding characteristic of thin coated copper wires were studied using 40, 60, 100 kHz ultrasonic complex vibration welding equipments with elliptical to circular vibration locus. The complex vibration systems consisted of a longitudinal-torsional vibration converter and a driving longitudinal vibration system. Polyurethane coated copper wires of 0.036 mm outer diameter and copper plates of 0.3 mm thickness and the other dimension wires were used as welding specimens. The copper wire part is completely welded on the copper substrate and the insulated coating material is driven from welded area to outsides of the wire specimens by high frequency complex vibration.

Journal Article↗
PubMed1991

Collective vibrations of an alpha-helix. A molecular dynamics study.

The internal dynamics of a 20-residue polyalanine helix was investigated by molecular dynamics simulations. Special attention was paid to the collective vibrations of the helix backbone. The stretch and bend vibrations could be assigned unambiguously to oscillations with periods of 1.4 and 4.3 ps, respectively. The influence of the environment on the dynamics of the collective vibrations was studied by coupling the helix to a heat bath and by adding water molecules. In the presence of water, the stretch vibration becomes more strongly dampened, but still exists as a vibration , while the bend vibration becomes overdamped and degenerates into a relaxation process. The results are compared with available experimental data.

Elasticity↗
PubMed2001

The aversion of broiler chickens to concurrent vibrational and thermal stressors.

The requirement for assessing the effects of multiple concurrent stressors in improving the welfare of broiler chickens during transport has not been widely recognised. A discrete-choice technique was used to investigate the aversion of broiler chickens to concurrent vibrational and thermal transport stressors. In experiment 1, 12 female broiler chickens, aged 42+/-3 days were studied individually using two choice-chambers. Each chamber had four compartments connected via a central zone and offered four treatments; thermal (T: air temperature; 40 degrees C, 21% RH), vibrational (V: frequency; 2Hz, acceleration; 1ms(-2)), concurrent vibrational and thermal (VT) and no applied stressors (N). Coloured compartment wall panels, allocated at random, assisted chickens' identification of compartments. Birds were fasted overnight and were required to make five consecutive choices on each of four consecutive days. A choice was defined as entering a compartment and feeding (5g pellets), whereupon confinement for 60min was initiated. Choices were totalled over all birds and analysed using a log-linear generalised linear model. The vibration was significantly avoided (V and VT versus N and T; P<0.05) but the thermal stressor was not (T and VT versus N and V; P>0.05) and there was no interaction. In experiment 2, the procedure was repeated with 12 more birds and modifications to increase method sensitivity and maximise bird learning. Choices were more disparate than before with vibration avoided to a greater extent (V and VT versus T and N; P<0.01) but there was still no main effect of the thermal treatment or a significant interaction. Substantial differences between individuals were observed in both experiments. The overall response to vibration supported previous findings for short-term exposure, however, non-avoidance of the thermal treatment was unexpected. Possibly, the birds were unable to associate the delayed heat stress with the compartment. Alternatively, the thermal conditions were not perceived as aversive either initially or throughout the 60min confinement. The preference method provides a useful starting point for assessing combinations of stressors which affect broiler welfare, allowing relative ranking of treatments from an animal-centred perspective.

Journal Article↗
PubMed1997

The vibrator mutation causes neurodegeneration via reduced expression of PITP alpha: positional complementation cloning and extragenic suppression.

The mouse vibrator mutation causes an early-onset progressive action tremor, degeneration of brain stem and spinal cord neurons, and juvenile death. We cloned the vibrator mutation using an in vivo positional complementation approach and complete resequencing of the resulting 76 kb critical region from vibrator and its parental chromosome. The mutation is an intracisternal A particle retroposon insertion in intron 4 of the phosphatidylinositol transfer protein alpha gene, causing a 5-fold reduction in RNA and protein levels. Expression of neurofilament light chain is also reduced in vibrator, suggesting one signaling pathway that may underlie vibrator pathology. The vibrator phenotype is suppressed in one intercross. We performed a complete genome scan and mapped a major suppressor locus (Mvb-1) to proximal chromosome 19.

Alleles↗
PubMed1998

Postural responses to vibration of neck muscles in patients with uni- and bilateral vestibular loss.

Postural responses to vibration applied unilaterally to the dorsal neck muscles were recorded with a sway platform in 11 patients with bilateral vestibular loss (BLD), 13 patients with unilateral vestibular lesions (ULD) and 19 normal subjects. In the normals, the vibration induced a forward postural deviation. Vibration failed to induce postural sway in the BLD patients but induced a backwards head movement in 7 patients. In the ULD patients, vibration contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway directed to the side of the lesion and backwards. The findings demonstrate the importance of concurrent vestibular signals in determining the behavioural response to neck afferent input. We propose that in normal subjects the intact vestibular signal gives no confirmation that a head movement has occurred so it is assumed that the lower body has tilted forwards which provokes a compensatory sway. In the total absence of vestibular function the neck signal may represent a real head movement so the preferential response is a head tilt to restore upright posture. The vestibular imbalance in the ULD patients is roughly equivalent to the asymmetrical signals obtained in a normal subject during head rotation to the intact side. The stretch signal induced by ipsi-lesional vibration confirms possible head rotation, thus provoking a compensatory postural sway. Copyright 1998 Elsevier Science B.V. All rights reserved

Journal Article↗
PubMed2004

Structure, stability and vibrational spectrum of the hydrogen-bonded complex between HNO3 and H2O. Ab initio and DFT studies.

The structure, stability and vibrational spectrum of the binary complex between HONO2 and H2O have been investigated using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complex studied. It was established that the hydrogen-bonded H2O...HONO2 complex has a planar structure. The corrected values of the dissociation energy at the SCF and MP2 levels and B3LYP calculations are indicative of relatively strong OH...O hydrogen-bonded interaction. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding between HONO2 and H2O have been estimated by using the ab initio calculations at SCF and MP2 levels and B3LYP/6-31G(d,p) calculations. It was established that the most sensitive to the complexation is the stretching O-H vibration from HONO2. In agreement with the experiment, its vibrational frequency in the complex is shifted to lower wavenumbers. The predicted frequency shift with the B3LYP/6-31G(d,p) calculations (-439 cm(-1)) is in the best agreement with the experimentally measured (-498 cm(-1)). The intensity of this vibration increases dramatically upon hydrogen bonding. The ab initio calculations at the SCF level predict an increase up to five times; at the MP2 level up to 10 times and the B3LYP/6-31G(d,p) predicted increase is up to 17 times. The good agreement between the predicted values of the frequency shifts and those experimentally observed show that the structure of the hydrogen-bonded complex H2O...HONO2 is reliable.

Hydrogen↗
PubMed2006

Assignment of the hydrogen-out-of-plane and -in-plane vibrations of the retinal chromophore in the K intermediate of pharaonis phoborhodopsin.

pharaonis phoborhodopsin (ppR; also called pharaonis sensory rhodopsin II, psR-II) is a photoreceptor protein for negative phototaxis in Natronomonas pharaonis. Photoisomerization of the retinal chromophore from all-trans to 13-cis initiates conformational changes of the protein leading to activation of the cognate transducer protein (pHtrII). Elucidation of the initial photoreaction, formation of the K intermediate of ppR, is important for understanding the mechanism of storage of photon energy. We have reported the K minus ppR Fourier transform infrared (FTIR) spectra, including several vibrational bands of the retinal, the protein, and internal water molecules. It is interesting that more vibrational bands were observed in the hydrogen-out-of-plane (HOOP) region than for the light-driven proton pump, bacteriorhodopsin. This result implied that the steric constraints on the retinal chromophore in the binding pocket of ppR are distributed more widely upon formation of the initial intermediate. In this study, we assigned the HOOP and hydrogen-in-plane vibrations by means of low-temperature FTIR spectroscopy applied to ppR reconstituted with retinal deuterated at C7, C8, C10-C12, C14, and C15. As a result, the 966 (+)/971 (-) and 958 (+)/961 (-) cm(-1) bands were assigned to the C7=C8 and C11=C12 Au HOOP modes, respectively, suggesting that the structural changes spread to the middle part of the retinal. The positive bands at 1001, 994, 987, and 979 cm(-1) were assigned to the C15-HOOP vibrations of the K intermediate, whose frequencies are similar to those of the K(L) intermediate of bacteriorhodopsin trapped at 135 K. Another positive band at 864 cm(-1) was assigned to the C14-HOOP vibration. Relatively many positive bands of hydrogen-in-plane vibrations supported the wide distribution of structural changes of the retinal as well. These results imply that the light energy was stored mainly in the distortions around the Schiff base region while some part of the energy was transferred to the distal part of the retinal.

Deuterium↗
PubMed2006

H2CO3 and its oligomers: structures, stabilities, vibrational and NMR spectra, and acidities.

Although the molecular form of carbonic acid, H2CO3, is of very limited stability in aqueous solution, some form of H2CO3 is apparently stable as a solid and may be a stable species on acid-treated carbonate mineral surfaces. Experimental vibrational spectra that have been assigned to solid H2CO3 have been obtained by several research groups, although there is no information on either the local or long-range structure of this phase and calculated vibrational frequencies for monomeric H2CO3 show significant discrepancies with the experimental IR data. Previous calculations have also indicated that H-bonded H2CO3oligomers are more stable than the monomer and have significantly different vibrational spectra, but the accuracy of the spectral calculations was not considered sufficient to reassign the experimental data. We have now calculated the harmonic vibrational spectrum of monomeric H2CO3 at the 6-311+G(2d,p) CCSD level and have calculated anharmonic corrections at the CBSB7 B3LYP level (used in CBS-QB3 calculations), combining the two to obtain an accurate description of the fundamental vibrations of gas phase H2CO3, which disagree significantly and systematically with the experimental data for solid H2CO3. For the H-bonded dimer, H2CO3(2), we have calculated both the harmonic spectrum and anharmonicity corrections at the CBSB7 B3LYP level, finding much better agreement with the experimental spectrum of solid H2CO3 than for the monomer, particularly for the C=O and O-H stretching vibrations, which are strongly red-shifted by both H bonding and anharmonic effects in the dimer. The free-energy changes for the formation of the H2CO3(n) (n = 2 and 3) oligomers and for the formation of a 1D chain structure of H-bonded monomers are negative in the gas phase, despite an unfavorable entropic contribution. However, in aqueous solution, the free-energy change for the formation of the n = 2 and 3 oligomers becomes positive because of the loss of hydration free energy, since the -OH groups of H2CO3 are removed from H bonding with the solvent. We have also calculated 13C NMR shieldings for the H2CO3 oligomers and some other related molecules, finding that the central C is systematically deshielded by oligomerization.

Journal Article↗
PubMed2001

Vibrational structure of titanium silicate catalysts. A spectroscopic and theoretical study.

A thorough analysis of the vibrational features of the titanium silicalite-1 (TS-1) catalyst is presented, based on quantitative IR measurements, Raman and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm(-1) associated with titanium insertion into the zeolite framework. The enhancement of the intensity of the 1125 cm(-1) feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of resonant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theory provide the basis for the assignment of the main vibrational contributions and for the understanding of Raman enhancement. The resonance-enhanced 1125 cm(-1) mode is unambiguously associated with a totally symmetric vibration of the TiO(4) tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as a totally symmetric stretching of the four Si-O bonds pointing toward Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO(4) units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxygen atom belongs to an OH group, such as in terminal tetrahedra of cluster models and in real defective zeolites. Asymmetric SiO(4) and TiO(4) stretching modes appear above and below 1000 cm(-1), respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm(-1) (in both SiO(4) and TiO(4)) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features associated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO(4) modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO(4) vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti caused by water adsorption quenches the UV-Raman enhancement of the 1125 cm(-1) band.

Journal Article↗
PubMed2006

Cytochrome c552 mutants: structure and dynamics at the active site probed by multidimensional NMR and vibration echo spectroscopy.

Spectrally resolved infrared stimulated vibrational echo experiments are used to measure the vibrational dephasing of a CO ligand bound to the heme cofactor in two mutated forms of the cytochrome c552 from Hydrogenobacter thermophilus. The first mutant (Ht-M61A) is characterized by a single mutation of Met61 to an Ala (Ht-M61A), while the second variant is doubly modified to have Gln64 replaced by an Asn in addition to the M61A mutation (Ht-M61A/Q64N). Multidimensional NMR experiments determined that the geometry of residue 64 in the two mutants is consistent with a non-hydrogen-bonding and hydrogen-bonding interaction with the CO ligand for Ht-M61A and Ht-M61A/Q64N, respectively. The vibrational echo experiments reveal that the shortest time scale vibrational dephasing of the CO is faster in the Ht-M61A/Q64N mutant than that in Ht-M61A. Longer time scale dynamics, measured as spectral diffusion, are unchanged by the Q64N modification. Frequency-frequency correlation functions (FFCFs) of the CO are extracted from the vibrational echo data to confirm that the dynamical difference induced by the Q64N mutation is primarily an increase in the fast (hundreds of femtoseconds) frequency fluctuations, while the slower (tens of picoseconds) dynamics are nearly unaffected. We conclude that the faster dynamics in Ht-M61A/Q64N are due to the location of Asn64, which is a hydrogen bond donor, above the heme-bound CO. A similar difference in CO ligand dynamics has been observed in the comparison of the CO derivative of myoglobin (MbCO) and its H64V variant, which is caused by the difference in axial residue interactions with the CO ligand. The results suggest a general trend for rapid ligand vibrational dynamics in the presence of a hydrogen bond donor.

Bacterial Proteins↗
PubMed2004

Rotational state-dependent mixings between resonance states of vibrationally highly excited DCO (X2A').

Rotational state-dependent mixings between highly excited resonance states of DCO (X (2)A(')) were investigated by stimulated emission pumping spectroscopy via a series of intermediate rotational levels in the B (2)A(') electronic state of the radical. Two examples for such interactions, between pairs of accidentally nearly degenerate vibrational states at energies of E(v) approximately 6450 and E(v) approximately 10 060 cm(-1), respectively, were analyzed in detail. Deperturbations of the measured spectra provided the zeroth-order vibration-rotation term energies, widths, and rotational constants of the states and the absolute values of the vibrational coupling matrix elements. The coupled states turned out to have very different A rotational constants so that their mixings switch on or off as they are tuned relative to each other as function of the K(a) rotational quantum number. The respective zeroth-order states could be assigned to different interlaced vibrational polyads. Thus, when two states belonging to different polyads are accidentally nearly isoenergetic, even very weak interpolyad interactions may start to play important roles. The derived interpolyad coupling elements are small compared to the typical intrapolyad coupling terms so that their influences on the vibrational term energies are small. However, large effects on the widths (i.e., decay rates) of the states can be observed even from weak coupling terms when a narrow, long-lived state is perturbed by a broad, highly dissociative state. This influence contributes to the previously observed strong state-to-state fluctuations of the unimolecular decay rates of the DCO radical as function of vibrational excitation. Similar mechanisms are likely to promote the transition to "statistical" rates in many larger molecules.

Journal Article↗
PubMed2004

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy.

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.

Journal Article↗
PubMed2004

Classical trajectory calculations of intramolecular vibrational energy redistribution. I. Methanol-water complex.

Intramolecular vibrational energy redistributions of the O-H stretching (nuOH) vibration for the methanol monomer and its water complex, the methanol-water dimer, are investigated by using ab initio full-dimensional classical trajectory calculations. For the methanol monomer, in the high-energy regime of the 5nuOH overtone, the time dependence of the normal-mode energies indicates that energy flowed from the initial excited O-H stretching mode to the C-H stretching mode. This result confirms the experimental observation of energy redistribution between the O-H and C-H stretching vibrations [L. Lubich et al., Faraday Discuss. 102, 167 (1995)]. Furthermore, a lot of dynamical information in the time domain is contained in the power spectra, whose density is given by the Fourier transformation of the total momentum obtained from trajectory calculations. For the methanol-water hydrogen-bonded complex, at the high-energy level of the 5nuOH overtone, the calculated power spectrum shows considerable splitting and broadening, indicating significant energy redistribution through strong coupling between the O-H stretching vibration and other vibrations. It is thus clear that the A-H...B hydrogen-bond formation facilitates energy redistribution subsequent to the vibrational excitation of the hydrogen-bonded A-H stretching mode.

Journal Article↗
PubMed2004

B(2)A(')-X(2)A(') detection of vibrationally excited HCO produced by the O((3)P)+C(2)H(4) reaction.

The distribution of rotational and vibrational energy in HCO produced by the O((3)P)+C(2)H(4) reaction has been measured using laser-induced fluorescence detection via the B(2)A(')-X(2)A(') transition. Over a detection wavelength range of 248-290 nm, our experiments have shown that HCO is formed in both the ground state and in at least six vibrationally excited states with up to two quanta of energy in the C-O stretch and the bending mode. Dispersed fluorescence experiments were conducted to positively assign all of the HCO vibrational bands. The experiments confirmed that many bands, including the B(000)-X(000) band, are affected by overlap with other HCO bands. Spectral modeling was used to separate the contributions of overlapping HCO B-X bands and to determine a nascent HCO rotational temperature of approximately 600 K, corresponding to approximately 6% of the total energy from the O((3)P)+C(2)H(4) reaction. HCO vibrational distributions were determined for two different average collision energies and were fit with vibrational temperatures of 1850+/-80 K and 2000+/-100 K, corresponding to approximately 15% of the total energy. The observed Boltzmann distribution of vibrational energy in HCO indicates that HCO and CH(3) are formed by the dissociation of an energized intermediate complex.

Journal Article↗
PubMed2004

Time resolved infrared absorption studies of geminate recombination and vibrational relaxation in OClO photochemistry.

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm(-1) is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent-solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl-O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed.

Journal Article↗
PubMed2004

The workings of a molecular thermometer: the vibrational excitation of carbon tetrachloride by a solvent.

An intriguing energy-transfer experiment was recently carried out in methanol/carbon tetrachloride solutions. It turned out to be possible to watch vibrational energy accumulating in three of carbon tetrachloride's modes following initial excitation of O-H and C-H stretches in methanol, in effect making those CCl(4) modes "molecular thermometers" reporting on methanol's relaxation. In this paper, we use the example of a CCl(4) molecule dissolved in liquid argon to examine, on a microscopic level, just how this kind of thermal activation occurs in liquid solutions. The fact that even the lowest CCl(4) mode has a relatively high frequency compared to the intermolecular vibrational band of the solvent means that the only solute-solvent dynamics relevant to the vibrational energy transfer will be extraordinarily local, so much so that it is only the force between the instantaneously most prominent Cl and solvent atoms that will significantly contribute to the vibrational friction. We use this observation, within the context of a classical instantaneous-pair Landau-Teller calculation, to show that energy flows into CCl(4) primarily via one component of the nominally degenerate, lowest frequency, E mode and does so fast enough to make CCl(4) an excellent choice for monitoring methanol relaxation. Remarkably, within this theory, the different symmetries and appearances of the different CCl(4) modes have little bearing on how well they take up energy from their surroundings--it is only how high their vibrational frequencies are relative to the solvent intermolecular vibrational band edge that substantially favors one mode over another.

Journal Article↗