On vibrational bonding of IHI

The vibrational bond of IHI is described in analogy to the covalent bond of H₂⁺ by a Born—Oppenheimer-type separation of light (“hydrogenic” ≡ “electronic”) and heavy (“iodines” ≡ “protons”) particle degrees of freedom. Competing potential energy decreases and hydrogen zero-point energy increases (for the IHI antisymmetric stretching and bending modes) yield a minimum in the iodine symmetric stretching mode's potential energy which supports one bound vibrational IHI state.     

Bond types of molecular orbitals and the photoelectron spectrum

The vibrational structures of the photoelectron spectra for diatomic molecules can be accounted for in terms of the slope of the orbital energy curve in the conventional correlation diagram with respect to internuclear distance. The vibrational structures of the photoelectron spectra for simple polyatomic molecules HCN, C₂H₂, and AH₂ type of hydrides can also be accounted for in terms of the slopes of the orbital energy curves in the correlation diagrams with respect to angles, as well as distances. Among all correlation diagrams, the slopes in the distance correlation diagram are related to the criterion for bond type—the positive for “bonding,” the negative for “antibonding,” while slopes with small magnitudes for “nonbonding.” The Fock matrix elements within the bond orbital basis provide heuristic and systematic rationalization of the slopes for the orbital energy curves. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 53–65, 2001     

Determination of intermolecular energy transfer by time resolved CARS measurements

Energy transfer in methyl isocyanide following i.r. multiphoton excitation has been examined with a CARS probe. Results show that CARS is an effective technique both for measuring the fraction of molecules receiving energy from the laser and for monitoring intermolecular energy transfer following laser irradiation. In addition, the Raman transition at 2169 cm⁻¹ shows a vibrational progression due to hot bands of the ν₈ bend. Since the ν₈, bend is the only low energy mode of the molecule, it is an effective vibrational “thermometer”. Utilizing this thermometer, results show that the vibrational state distribution does not fit a Boltzmann distribution for 7 μs following i.r. multiphoton excitation.     

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol^?1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m_A = 20 u, m_B = 1 u, m_C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r_BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.     

Thermal-energy charge transfer of Ar+ with H2O: Internal and kinetic energy of the product H2O+

The reaction of Ar⁺ with H₂O has been investigated at near-thermal energy. The product ions H₂O⁺ and ArH⁺ account for 90 and 10% of the total reaction rate, respectively. Kinetic energy measurements and emission spectroscopy of the H₂O⁺ product ions are reported. It is concluded that at least 60% of H₂O⁺ ions are in the X? state with ≈2.4 eV vibrational energy while up to 40% are in the à state with a mean vibrational energy of 1.4 eV; the à state vibrational distribution has been determined. It is shown that both H₂O⁺ states are populated via an energetically “non-resonant” charge transfer process.     

Competition between intersystem crossing and internal conversion in isolated large molecules

The excess energy and deuteration dependence of the radiationless decay rate in “isolated” aromatic hydrocarbons (anthracene, 9,10-dimethylanthracene, phenanthrene and fluorene) suggest that S₁→S₀ internal conversion dominates over S₁→T intersystem crossing for molecules with very large excess vibrational energies.     

On the excess-energy dependence of radiationless decay rate constants

The results of calculations of the dependence of the radiationless rate constant on the excess of excitation energy within the two-electronic states model under the weak coupling and statistical limits are presented. It is assumed that the exact molecular states for a given electronic configuration are global in character containing equal contributions from all degenerated vibrational levels at a given excitation energy due to intramolecular vibrational relaxation (IVR). The results of calculations indicate an important role of the low-frequency vibrational modes, the potential energy surfaces of which cross between the two electronic states involved into the radiationless process. The sharp increase of the rate constant is predicted for the excitation energy below the diabatic crossing point, followed by saturation at higher energies. The calculated rate constants for the T₁→S₀ intersystem crossing in pyrazine and benzene are in good agreement with experimental observations. Some comments concerning the “channel-three” phenomenon in benzene are presented.     

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h₈ and d₈ were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B_3u, A_u and B_1u vibrational exciton bands were found to be in the separated band limit. The lowest energy B_3u “butterfly” mode with its large (15 cm^?1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm^?1 was also assigned as a peak in the vibrational exciton density-of-states of the B_3u mode.     

On the “third decay channel” and vibrational redistribution problems in benzene derivatives

Fluorescence lifetimes and yields have been measured for toluene, aniline-h₇ and aniline-d₅ vapors in the spectral range corresponding to a rapid increase of the non-radiative rates. Non-exponential decays and discrepancies between lifetime and yield dependence on ν_exc are observed. Results are discussed on the ground of a model assuming: (a) strong anharmonic coupling of “active” and “non-active” modes, (b) specific “third channel” threshold for each vibrational progression and (c) absence of rapid vibrational redistribution.     

Double resonance observation of the (2ν3, E) state in methane

By using the double resonance technique we have observed the infrared inactive (2ν₃, E) level in methane. The experimental value of the vibrational energy (6043.8 cm^?1) is in good agreement with recent calculations based on the “local mode” model.     

Vibrational relaxation of compressed nD2 fluid

Experimental observations of the vibrational population relaxation time of ⁿD₂ fluid under pressures of up to 500 atm in the 25–85 K range are presented and described in terms of a semi-classical model for energy transfer in liquids. For comparison with the parameters of this model, a classical equivalent potential for quantum systems is derived from the “real” intermolecular potential.     

Classical trajectory study of three-body direct photofragmentation of Cd(CH3)2. Comparison of sampling methods

The classical trajectory method has been used to investigate product-state symmetry in the three-body direct photofragmentation of Cd(CH₃)₂. Photon absorption was assumed to satisfy the Franck principle and relative weights for trajectory initial conditions were assigned by either the Wigner or the classical (300 K) distribution function. In qualitative agreement with earlier “quasi-classical” sampling studies of Kellman et al. for photolysis of the Cd(CH₃)₂ vibrational ground state, product-state asymmetry is found to increase with increasing photolysis wavelength. Contrary to the “quasi-classical” sampling results, however, sampling with the classical distribution function indicates that product-state symmetry is unaffected by vibrational excitation of Cd(CH₃)₂ prior to photolysis. For sampling with the Wigner distribution function, in agreement with the “quasi-classical” sampling studies, excitation of the Cd(CH₃)₂ asymmetric stretch prior to photolysis is found to increase product-state asymmetry.     

In-plane vibrational modes of cytosine from an ab initio MO calculation

Harmonic force constants, in-plane vibrational frequencies, and in-plane vibrational modes of cytosine were calculated by an ab initio Hartree—Fock SCF MO method. The force contants were calculated by the use of an energy gardient method with the STO-3G basis set, and then they were corrected into “4-31G force constants” by the scaling factors given by us previously for the case of uracil. The corrected set of force constants can produce a calculated vibrational spectra of cytosine and cytosine-1,amino-d₃, that can be well corrected with the observed Raman and infrared spectra of these compounds, with little ambiguity. Thus, the assignments of all the in-plane vibrations are now practically established. The calculated vibrational modes, in addition, can account for the recently published resonance Raman effects of cytosine residue.     

Experimental evidence for “cascading”; rapid vibrational relaxation with retention of rotational quantum number

In previous work we have postulated that highly rotationally-excited hydrides, AH, can relax by way of “cascading”, i.e., rapid vibrational exchange in which AH is vibrationally deactivated with retention of rotational quantum number (Δv_AH = ?1, ΔJ_AH ≈ 0). Infrared chemiluminescence studies reported here provide evidence of cascading in OH2 and HCl2 (2 symbolises vibrational excitation in the ground electronic state). The collision partners, BC, responsible for the cascading are thought to be (i) OH2 + NO₂, (ii) HCl2 + NOCl and (iii) HCl2 + ICl. Extremely rapid vibrational exchange is indicative of a resonant process. Resonance must occur for a wide range of v_AH and J_AH, and a variety of collision partners BC. A plausible basis for such a process involves multiquantum vibrational and rotational transitions in the recipient molecule, BC, which “tune” the energy-exchange to resonance.     

Vibrational levels and Franck–Condon factors of diatomic molecules via Morse potentials in a box

This work deals with two shortcomings in the use of Morse potentials to describe energy spectra and transitions of diatomic molecules: (1) Morse's well-known “exact” solution for purely vibrational states includes the unphysical region – ∞ < r < 0 of the internuclear separation, and (2) Franck-Condon factors are evaluated in harmonic and anharmonic approximations to the Morse potentials. The method of confining the molecule in a spherical box is developed to obtain (1) purely vibrational energy spectra and eigenvectors of Morse potentials in the physical region 0 ? r < ∞, and (2) the corresponding Franck-Condon factors without any additional approximations. The method is applied to Li₂, N₂, CN, and CO molecules. © 1995 John Wiley & Sons, Inc.     

Electronic relaxation of biacetyl in a supersonic jet

Emission spectra have been recorded and decay times measured for biacetyl selectively excited in the 0⁰₀ band of the S₁S₀ (¹A_u¹A_g) transition. The spectrum of the “long emission” corresponds to a superposition of the fluorescence and of the “hot” phosphorescence. The results may be treated in terms of a uniform distribution of the singlet oscillator strength among the quasi-stationary levels, in the absence of vibrational redistribution on a microsecond scale.     

Ab initio calculations on (SiH3)2F+: stability in the gas phase and model for bridging fluorine atom in ion-implanted amorphous silicon

Ab initio calculations have been performed on model molecular clusters simulating bridging fluorine configurations in fluorinated amorphous silicon. Optimized geometries, total energies and vibrational frequencies have been computed for (SiH₃)₂F⁺ clusters with the terminal SiH₃ groups eclipsed or staggered. The stable minimum on the potential energy surface corresponds to a linear, but very flexible, Si-F-Si bridging configuration. (SiH₃)₂F⁺ appears to be stable with respect to unimolecular decomposition. The calculated vibrational frequencies include a strongly infrared-active antisymmetric stretch mode at 740 cm⁻¹, similar to the metastable “Bband” experimentally observed at 750 cm⁻¹ in the ion-implanted samples. These results are compared with calculated geometries and vibrational frequencies of SiH₃F, SiH₃F⁺, SiH₂F⁺ and Si₂H₅F.     

Laser photodissociation of s-tetrazine: Product vibrational excitation

The photosensitive and thermally unstable molecule s-tetrazine decomposes to yield one nitrogen molecule and two HCN molecules. Following pulsed irradiation of tetrazine vapor at 492.3 nm, we have observed time resolved infrared fluorescence from HCN(001). In a similar experiment, small quantities of CO₂ were added to the sample cell, and we observed infrared fluorescence from CO₂ (001) populated by VV energy transfer. From fluorescence intensity measurements, we have been able to estimate the amount of excitation in certain product vibrations. We conclude that ≈ 1% of the HCN is produced in the (001) state, and the “equivalent” of ≈ 0.1 quantum of N₂ vibrational excitation is excited. This latter figure may include some excitation of HCN (ν₁), but the measured energy transfer rate coefficients are consistent with N₂ excitation. The small amount of HCN(ν₃) and N₂ vibrational excitation is surprising, as the photodissociation is exothermic by more than 100kcal/mole.     

A theoretical study of the force field for carbon trioxide

Geometries force constants and harmonic frequencies are calculated for the cyclic (dioxirane) structure of CO₃ by ab initio SCF methods using the 3-21G, 3-21G(*). 6-31G and 6-31G* bases calculated frequency shifts for isotopomeric species agree with experiment and lend support to the vibrational assignments of Jacox and Milligan. Earlier arguments for and against the cyclic, cyclic and open structures of CO₃, based upon the “physical reasonableness” of valence force constants are shown to be invalid owing to the dependence of conventional “rigid” force constants upon the choice of valence coordinates. The use of “relaxed” force constants as a basis for meaningful comparison of force fields for molecules described by different sets of redundant valence coordinates is demonstrated.