The calculation of vibrational intensities in forbidden electronic transitions

A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained.     

Response calculations of electronic and vibrational transitions in molecular oxygen induced by interaction with noble gases

The Einstein coefficient for the singlet oxygen emission a1Deltag-->X3Sigmag- at lambda=1270 nm and b1Sigmag+-->X3Sigmag- emission at lambda=750 nm were calculated by quadratic response (QR) multiconfiguration self-consisted field (MCSCF) method for a number of collision complexes O2+M, where M=He, Ne, Ar. Interaction with He clusters was studied in order to simulate cooperative effect of the environment on the oxygen emission. Calculations of the dipole transition moment for the Noxon band, b1Sigmag+-a1Deltag, by linear response (LR) MCSCF method were also performed for a number of collision complexes. Spin-orbit coupling (SOC) between the b1Sigmag+ and X3Sigmag- (MS=0) states does not change much upon collisions, thus the a-X transition borrows intensity mostly from the collision-induced Noxon band b-a. The a-X intensity borrowing from the Schumann-Runge transition is negligible. The calculations show that the b-a and a-X transition probabilities are enhanced approximately by 10(5) and 10(3) times by O2+M collisions. An order of magnitude differences occur for both transitions for noble gases with large difference in polarizability. A strong cooperative effect is obtained when few He atoms perturb the oxygen molecule. Depending on mutual orientation of the partners it can be a complete quenching of the a-->X emission or strong non-additive enhancement. Collision-induced infrared vibrational transitions in a number of molecular oxygen excited states were studied and shown to be state selective.     

Vibrational amplitude profile of molecular vibrational modes for vibrational mode assignment

Ultrashort pulse lasers with 6- and 20-fs durations were utilized for phthalocyanine thin film sample to induce several vibrational modes and vibration amplitude spectra were determined by multi-wavelength measurement technique. From the spectra we could identify the electronic states, which couple to two vibrational modes with frequencies of 670 and 750 cm⁻¹. It was shown that the vibrational amplitude profile obtained by the method can be used for providing information for the assignment of the vibrational mode.     

Using vibrational modes in the search for global minima of atomic and molecular clusters

The reaction between dimethyl sulfide (CH₃SCH₃) and nitrate radical (NO₃) is studied using density functional theory and ab initio methods. The transition state for this reaction is optimized at different levels of theory and basis sets, and then used for kinetics calculations of the rate constant. The CH₃SCH₃ + NO₃ reaction leading to CH₃SCH₂ and HNO₃ is shown to have a negative activation energy and thus negative temperature dependence. The study confirms that the NO₃ radical is a significant contributor to the oxidation of DMS in the troposphere.     

Novel spectroscopic effects in molecular transitions involving degenerate electronic states

We discuss the intricate Jahn—Teller-like and non-Jahn—Teller spectroscopic patterns which result via the recently revealed cross-quadratic terms involves modes of different symmetry which arise in the nuclear potential of degenerate electronic states of non-linear molecules.     

Detection of electronic and vibrational coherences in molecular systems by 2D electronic photon echo spectroscopy

Two-dimensional optical photon echo spectra are simulated for model systems which exhibit vibrational, electronic and a combination of electronic and vibrational coherent dynamics. The coherent motion manifests itself as periodic beatings of the spectrum cross-peak intensity with the population time. The intensity modulations are compared to evolution of the excited-state population and coordinate expectation value. The advantageous capabilities of the technique as well as possible difficulties in spectra interpretations are outlined. Possibilities for distinguishing electronic and vibrational coherences are discussed.     

Striking dependence of the rate of electronic radiationless transitions on the size of the molecular system

We analyze electronic radiationless transitions in molecular aggregates in the limit where the coupling between the monomeric units is strong. We show that in this limit the rate at a given electronic energy gap decreases drastically with the increase in the size of the system. The decrease becomes more striking for larger electronic energy gaps. Extrapolation to the very strong limit prevailing in polyconjugated systems of major biological importance is suggested.     

Temperature-induced structural transitions and vibrational properties of microclusters: ab-initio molecular dynamics studies

We present the results of ab-initio molecular dynamics studies of selected microclusters of sodium, silicon and magnesium at finite temperatures, and especially discuss those obtained around room temperature. In particular, from the analysis of the atomic trajectories we can identify in some cases the existence of different isomers and the isomerization pathways. We have also calculated vibrational spectra at low temperatures and find that they can be used as a very sensitive structural probe also in sodium clusters, where the electronic properties are quite insensitive to the geometry.     

Relating normal vibrational modes to local vibrational modes with the help of an adiabatic connection scheme

Information on the electronic structure of a molecule and its chemical bonds is encoded in the molecular normal vibrational modes. However, normal vibrational modes result from a coupling of local vibrational modes, which means that only the latter can provide detailed insight into bonding and other structural features. In this work, it is proven that the adiabatic internal coordinate vibrational modes of Konkoli and Cremer [Int. J. Quantum Chem. 67, 29 (1998)] represent a unique set of local modes that is directly related to the normal vibrational modes. The missing link between these two sets of modes are the compliance constants of Decius, which turn out to be the reciprocals of the local mode force constants of Konkoli and Cremer. Using the compliance constants matrix, the local mode frequencies of any molecule can be converted into its normal mode frequencies with the help of an adiabatic connection scheme that defines the coupling of the local modes in terms of coupling frequencies and reveals how avoided crossings between the local modes lead to changes in the character of the normal modes.     

Experimental and theoretical studies on molecular structures and vibrational modes of novel compounds containing silver

The two novel silver containing complexes of N-(pyridin-2-yl)pyridin-2-amine (NDPA) molecule has been synthesized and characterized by elemental analysis, FT-Raman and FT-IR (at far and mid regions) spectroscopies. The optimized geometries of the compounds were obtained by using DFT/B3LYP method with LANL2DZ basis set for the (C₁₀H₉N₃) · AgNO₃ and B3LYP method with DGDZVP basis set for the (C₁₀H₉N₃) · AgClO₄. The vibrational frequencies and their IR and Raman intensities were determined by theoretical methods. In addition, the HOMO-LUMO energies, thermochemical properties and atomic charges for the complexes were obtained in same level of theory. The experimental and theoretical results show that the silver atoms are coordinated to ligands in bidentate fashion. In addition, in the structures of complexes there are intra-molecular interactions. The oxygen atoms of nitrate and perchlorate are bonded to ligand via hydrogen bonds.     

Modelling vibrational modes in diacetylene polymers

High-frequency vibrational modes in polydiacetylene polymers have been simulated using a simple point-mass and harmonic- force-constant model. Excellent agreement has been found between the frequencies of these modes and those observed in resonant Raman scattering from crystals of polydiacetylene polymers with four radically different sidegroups. The atomic displacement vectors for each mode have also been calculated and related to backbone-sidegroup interactions on the polymer chain.     

Nonequilibrium molecular dynamics simulations with a backward-forward trajectories sampling for multidimensional infrared spectroscopy of molecular vibrational modes

A full molecular dynamics (MD) simulation approach to calculate multidimensional third-order infrared (IR) signals of molecular vibrational modes is proposed. Third-order IR spectroscopy involves three-time intervals between three excitation and one probe pulses. The nonequilibrium MD (NEMD) simulation allows us to calculate molecular dipoles from nonequilibrium MD trajectories for different pulse configurations and sequences. While the conventional NEMD approach utilizes MD trajectories started from the initial equilibrium state, our approach does from the intermediate state of the third-order optical process, which leads to the doorway-window decomposition of nonlinear response functions. The decomposition is made before the second pump excitation for a two-dimensional case of IR photon echo measurement, while it is made after the second pump excitation for a three-dimensional case of three-pulse IR photon echo measurement. We show that the three-dimensional IR signals are efficiently calculated by using the MD trajectories backward and forward in time for the doorway and window functions, respectively. We examined the capability of the present approach by evaluating the signals of two- and three-dimensional IR vibrational spectroscopies for liquid hydrogen fluoride. The calculated signals might be explained by anharmonic Brownian model with the linear-linear and square-linear system-bath couplings which was used to discuss the inhomogeneous broadening and dephasing mechanism of vibrational motions. The predicted intermolecular librational spectra clearly reveal the unusually narrow inhomogeneous linewidth due to the one-dimensional character of HF molecule and the strong hydrogen bond network.     

Bias-controlled selective excitation of vibrational modes in molecular junctions: a route towards mode-selective chemistry

We show that individual vibrational modes in single-molecule junctions with asymmetric molecule-lead coupling can be selectively excited by applying an external bias voltage. Thereby, a non-statistical distribution of vibrational energy can be generated, that is, a mode with a higher frequency can be stronger excited than a mode with a lower frequency. This is of particular interest in the context of mode-selective chemistry, where one aims to break specific (not necessarily the weakest) chemical bond in a molecule. Such mode-selective vibrational excitation is demonstrated for two generic model systems representing asymmetric molecular junctions and/or scanning tunneling microscopy experiments. To this end, we employ two complementary theoretical approaches, a nonequilibrium Green's function approach and a master equation approach. The comparison of both methods reveals good agreement in describing resonant electron transport through a single-molecule contact, where differences between the approaches highlight the role of non-resonant transport processes, in particular co-tunneling and off-resonant electron-hole pair creation processes.     

Rotational—vibrational dipole transitions in HD

The transition moments are computed for several lines in the X¹Σ⁺_g state of HD. The results are obtained by taking into account interactions with Σ_u and Π_u states. It is shown that the latter influence appreciably the transition probabilities in the 00 band. The permanent dipole moment is also computed for several vibrational and rotational states.     

Assignment of the electronic transitions in benzene

The problem of the assignment of π-electronic transitions in benzene is discussed using all criteria presently available. It is shown that based on this information a few different assignments are impossible to exclude. The correct assignment ³B_1u < ³E_1u < ¹B_2u < ³B_2u < ¹B_1u < ³E⁺_2g < ¹E_1u < ¹E^?_2g < ³E^?_2g has been selected as result of theoretical considerations based on a new approach to the semi-empirical π-electron theory. The results confirm the adequacy of the π-electron model for energy level calculations, and emphasise the fundamental importance of multi-excited configurations.     

Triethylsilanol: molecular conformations and role of the hydrogen-bonding oligomerization in its vibrational spectra

We report a theoretical study of the molecular structure of the triethylsilanol molecule and a thorough conformational analysis of the species following the Boltzmann's distribution law. The vibrational spectra of the title molecule have been assigned by means of the combined use of experimental data obtained from IR and Raman spectra and theoretical DFT calculations with the subsequent implementation of the SQMFF methodology. The role of hydrogen bonding in the shifting of the vibrational bands of the silanol group in the spectra of the liquid phase is discussed using a model of triethylsilanol dimer.     

Influence of substitution pattern on the benzenoid transitions: Quantitative evaluation of the electronic and vibrational components from new experimental polarization data

The influence of substitution on the benzenoid ¹L_b, ¹L_a and ¹B transitions is studied by the technique of linear dichroism in stretched polyethylene films.The total intensity of the ¹L_b system is resolved into induced and vibrational components and the influence of various substitution patterns on the magnitude and polarization of both components is analyzed quantitatively. As expected from theoretical considerations, the vibrational components like the induced intensity are found to be strongly affected by the substituent strength and the symmetry of substitution pattern. The previously accepted assumption, according to which the vibrational intensity is regarded as a constant, appears to be completely unrealistic when dealing with cases of strong substituents. A satisfactory quantitative treatment of both weak and strong substituents using the same simple theoretical approach seems possible only if the systematic variations observed in the vibrational intensity are not neglected.A qualitative analysis is given for the ¹L_a, and ¹B systems. The results are compared with available theoretical predictions and previous experimental data.Practical implications of the present new experimental information to the study of aromatic optical activity are discussed.     

Quantum effect of intramolecular high-frequency vibrational modes on diffusion-controlled electron transfer rate: from the weak to the strong electronic coupling regions

The Sumi-Marcus theory is extended by introducing two approaches to investigate electron transfer reactions from weak-to-strong electronic coupling regime. One of these approaches is the quantum R-matrix theory, useful for dealing with the intramolecular vibrational motions in the whole electronic coupling domain. The other is the split operator approach that is employed to solve the reaction-diffusion equation. The approaches are then applied to electron transfer in the Marcus inverted regime to investigate the nuclear tunneling effect on the long time rate and the survival probabilities. The numerical results illustrate that the adiabatic suppression obtained from the R-matrix approach is much smaller than that from the Landau-Zener theory whereas it cannot be predicted by the perturbation theory. The jointed effects of the electronic coupling and solvent relaxation time on the rates are also explored.