The nature of the vibronic coupling in a metalloporphyrin and its Raman scattering

It has been shown that a Raman line of a nontotally symmetric vibration (b_1g, b_2g, or a_2g) of a metalloporphyrin (D_4h) can be caused, not only by a vibronic coupling between the first ($\text{A}\text{?}$) and second ($\text{B}\text{?}$) excited electronic states but also by a vibronic coupling within one of the electronic states (let us call the former the QS mechanism and the latter the QQ and/or SS mechanism). A simple formulation has been made for each of those different mechanisms, so that a numerical calculation can be made of the excitation profile of a Raman scattering for a given set of coupling constants and damping factors. Next, the result of an examination is given of the excitation profile of the Raman scatterings of some of the b_1g, b_2g, and a_2g vibrations of nickel octaethylporphyrin and nickel octaethyltetrachloroporphryin. Most of the Raman lines of these nontotally symmetric vibrations show a resonance effect only in the Q-band ($\text{A}\text{?}\text{←}\text{X}\text{?}$) region but not in the Soret-band ($\text{B}\text{?}\text{←}\text{X}\text{?}$) region. On the basis of this fact, it has been suggested that those Raman lines are caused mainly by the QQ mechanism rather than the QS mechanism. The observed vibrational structure of an excitation profile also seems to support this suggestion.     

Resonance Raman effect in thiourea

Raman spectra of thiourea have been observed in H₂O and D₂O solutions with the exciting laser beams of 514.5, 488.0, 457.9, 363.8, 325.0, and 257.3 nm. The resonance Raman excitation profile of the 729-cm^?1 line has been examined in the region of the 237-nm absorption band ($\text{π}{}_{\mathrm{<mtext>CS</mtext>}}{}^1\text{← π}{}_{\mathrm{<mtext>CS</mtext>}}$) by use of a solvent shift of the absorption band instead of by changing the wavelength of the exciting beam. The depolarization degree of this line was measured and its overtone Raman line was also observed. On the basis of the results of these experiments, it has been concluded that the 729-cm^?1 Raman line, assignable to the CS stretching vibration, derives its intensity solely from the 237-nm band when it is excited at 257.3 or 325.0 nm. On exciting in the region 363.8–514.5 nm, however, contributions of the higher-frequency bands are predominant rather than the contribution from the 237-nm band. The Raman line at 1520 cm^?1 of thiourea-d₄ is assignable to the NCN antisymmetric stretching vibration. From its excitation profile, its intensity has been considered to come from a vibronic coupling between the excited electronic states of the 220-nm ($\text{π}{}_{\mathrm{<mtext>CS</mtext>}}{}^1\text{← π}{}_{\mathrm{<mtext>N</mtext>}}\text{? π}{}_{\mathrm{<mtext>N</mtext>}}$) and the 197-nm ($\text{π}{}_{\mathrm{<mtext>CS</mtext>}}{}^1\text{← π}{}_{\mathrm{<mtext>N</mtext>}}\text{+ π}{}_{\mathrm{<mtext>N</mtext>}}$) bands.     

Correspondences of the vibrational frequencies of the ground and electronic excited states of quinoxaline as revealed by the Raman intensities

The intensities of the Raman lines of quinoxaline were measured at different excitation wavelengths. The matrix element ratios of the vibronic couplings between the two lowest electronic excited states of the molecule were evaluated from the Raman intensities of the b₁ vibrations, and they were compared with the matrix element ratios obtained from the vibrational frequencies of the ground and electronic excited states of the molecule. It was suggested that the ground state frequency of the b₁ vibration at 867 cm^?1 decreases greatly to 425 cm^?1 in the lowest $\text{nπ}{}^1$ excited state.     

Vibronic coupling as a major cause of the anharmonicity of antisymmetric stretching in the ground state of NO2

Approximate experimental and theoretical information about vibronic coupling of the $\text{X}\text{?}{}^2\text{A}{}_1$ (ground) and $\text{A}\text{?}{}^2\text{B}{}_2$ electronic states of NO₂—by its antisymmetric vibration ν₃(b₂)—is tested in model calculations of the accurately known ground-state levels ν₃ = 0, 1, 2, 3. The test is positive and it is estimated that 64% of the very large observed anharmonic constant χ₃₃ has its origin in vibronic coupling. In this model, ν₃ in the à state is predicted at about 1200 cm^?1.     

Why are formaldehyde and (possibly) propynal nonplanar in their S1(π1, n) electronic states?

Abnormally low frequencies observed for the out-of-plane vibration (b₁) of the $\text{A}\text{?}{}^1\text{A}{}_2$ electronic state of formaldehyde (H₂CO) and for the analogous carbonyl hydrogen vibration (a″) of $\text{A}\text{?}{}^1\text{A″}$ propynal (HCCCHO) are modeled by means of two-state calculations of vibronic coupling with higher singlet states, ¹B₂ and ¹A′, respectively. In each case, the active vibration is the out-of-plane hydrogen motion. The same vibronic calculations reproduce also the large positive anharmonicities of the active vibrations in the $\text{A}\text{?}\text{(π}{}^1\text{, n)}$ states; for H₂CO the calculated vibrational spacing alternates as observed, consistent with the known nonplanar structure, while in propynal the calculated spacing increases regularly, thus predicting an effectively planar structure. The nonplanarity of H₂CO is caused mainly by a vibronic coupling constant nearly twice that of propynal. The H₂CO coupling constant is near the value estimated independently by means of the intensity “borrowed” by the S₁-S₀ transition from the much stronger S₂-S₀ transition. Brief consideration is given to analogous vibrational levels of the ¹A₂ state of H₂CS and of the ³A₂ state of D₂CO in the vibronic context of this paper.     

Selection rules and Raman spectrum of a molecule with a degenerate ground state and zero spin-orbit coupling: VCl4

Two interesting features can normally be expected in the Raman spectrum of a molecule with a degenerate electronic ground state (i) the contribution of asymmetric or even antisymmetric tensors to the Raman intensity (ii) lifting of the electronic degeneracy due to Jahn-Teller distortion. The experimental data and discussion on VCl₄, presented here, show that this molecule does not show any deviations in the Raman selection rules. Further, its Raman spectrum closely resembles that of TiCl₄ which also has T_d geometry but a nondegenerate singlet ground state. This suggests the absence of any static Jahn-Teller distortion in VCl₄. However, we have observed a broadening of the ν₂ band of VCl₄ over TiCl₄ and we attribute this to a possible dynamic Jahn-Teller splitting.     

The 370-nm electronic spectrum of tropolone: Evidence from single vibronic level fluorescence spectra regarding the assignment of some vibrational fundamentals in the X̃ and à states

Single vibronic level fluorescence (SVLF) spectra of tropolone from vibronic levels in the $\text{A}\text{?}{}^1\text{B}{}_2$ electronic state, in combination with recently reported supersonic jet spectra, have enabled the assigning of many absorption bands in the region of 0₀⁰ which had previously been impossible. Some of the complexity in these bands has been shown to be due to a large Duschinsky effect involving the two lowest b₁ vibrations, ν₂₅ and ν₂₆. It has been shown that these vibrations have wavenumbers of 176 and 110 cm^?1, respectively, in the $\text{X}\text{?}$ state, and 172 and 39 cm^?1 in the $\text{A}\text{?}$ state. This last value shows how unresistent the molecule is in the $\text{A}\text{?}$ state to out-of-plane bending in the region of the five-membered ring. Other aspects of the vibrational complexity are due to the effect of ν′₂₆ in increasing the barrier to tunnelling of the hydrogen-bonding proton in the $\text{A}\text{?}$ state contrasting with very little effect of ν″₂₆ in the $\text{X}\text{?}$ state.     

Successive Jahn-Teller transitions and the canted pseudospin state in K2PbCu(NO2)6

The successive phase transitions in K₂PbCu(NO₂)₆ has been studied by precise X-ray measurements. The unit cell in Phase III is doubled along each of the cubic principal axes. The experimental results can be interpreted as sequencial cooperative Jahn-Teller transitions where the higher transition is due to the coupling with the uniform distortion, while the lower transition is caused by the coupling with the uniform distortion, while the lower transition is caused by the coupling with the zone boundary phonon mode with $\text{k}{}_0\text{= (}\text{1}\text{2}\text{,}\text{1}\text{2}\text{,}\text{1}\text{2}$. The electronic state in Phase III is characterized as the “canted pseudospin” state.     

具有D4h对称性构型的C2+4分子的Jahn-Teller效应与能级分裂

依据量子理论与配位场理论,利用群论和对称性分析的方法探讨了C²⁺₄分子在具有D_4h对称性构型时,E×(b_1g+b_2g)系统的Jahn-Teller效应中的相关问题.研究了C²⁺₄分子的电子态与声子态的对称性及其活跃声子态,讨论了系统声子间的耦合与CG系数,构建了E×(b_1g+b_{2g 关键词： 2+4分子')" href="#">C²⁺4分子 对称性 能级分裂 Jahn-Teller畸变}     

Rotational analysis of the 8000–9000 Å bands of nitrogen dioxide

The rotational structure of bands of NO₂ vapor in the region 8300–9000 Å has been partially analyzed and the absorption assigned to the (000)-(000) and (000)-(010) vibronic bands of the $\text{A}\text{?}{}^2\text{B}{}_2\text{←}\text{X}\text{?}{}^2\text{A}{}_1$ electronic transition. Irregular weak perturbations in the N-structure of the upper-state manifold are accompanied by larger resonance-type crossings in the K-structure. The larger perturbation is attributed to vibronic coupling between the à state and excited vibrational levels of the ground state, characterized by a low density of ground state levels and a large vibronic coupling matrix element between the à and X? states. The reconstituted, deperturbed bands have blue-degraded N-structure and strongly red-degraded K-structure, indicating that the bond angle decreases sharply in the excited state. The physical structure of the ²B₂ state is uncertain but some suggestions are made. The electronic energy of the ²B₂ state is T₀ = 11 962.9 cm^?1.     

Preparation and Mössbauer spectra of CoV2O4 spinel doped with Fe2+ on tetrahedral sites

Co²⁺_0.98Fe²⁺_0.02Fe³⁺_?V³⁺_2-? 04 (?? 0.001) was obtained from a sample containing the iron dope mainly in the trivalent state, by reducing it at 910°C in a CO₂/CO atmosphere. Mössbauer spectra of ⁵⁷Fe at the tetrahedral site were taken between 5 and 300 K. Below the Curie temperature (143 K) there is a magnetically induced electric field gradient eQV_zz and a reduction of the hyperfine field H_hf. Values at 5 K are H_hf=141 kG and eQV_zz=5.74 mm/s. The effective splitting of the Fe²⁺⁵E ground-state doublet, as estimated from the temperature dependence of eQV_zz and H_hf, is $\text{2q}\text{δ}\text{/k = 28 K}$. It is derived that in these oxides the coupling of the electronic ⁵E states to local Jahn-Teller active E-modes of vibration is stronger than in comparable sulphides.     

The characterization of the complete set of bound vibronic states of HNO in its excited Ã1A″ electronic state

Laser-induced fluorescence excitation spectra of the HNO $\text{A}\text{?}{}^1\text{A″-}\text{X}\text{?}{}^1\text{A′}$ band system have been recorded with high sensitivity. This has enabled detection of the Franck-Condon unfavored vibronic bands (002)-(000) and (003)-(000), thereby completing the set of fully bound vibronic levels in the $\text{A}\text{?}$ state. Extensions have also been made to other bands. A strong Coriolis resonance between the K_a¹ = 8 levels of the excited (010) vibronic state and the K_a¹ = 9 levels of the (001) state leads to rotational perturbations of up to 9 cm^?1. The (100-000) band includes weak axis-tilting branches. It is concluded from the vibrational energy level spacings that vibronic interaction makes an important contribution to the energies of the higher bending levels, consistent with the correlation of the $\text{A}\text{?}{}^1\text{A″}$ state with a component of a ¹Δ state for linear HNO.     

Electric field effects in EPR of the SrTiO3 :Cr5+ Jahn-Teller system

It is shown that a static electric field affects the intensity ratio of the EPR lines of Cr⁵⁺ (d₁) in SrTiO₃ at 77K. The experimental results can be accounted for quantitatively by considering for the ${}^2\text{T}{}_{\mathrm{2g}}$ electronic ground state of Cr⁵⁺ a strong Jahn-Teller interaction with localized ρ_g mode. The degeneracy of the resulting vibronic ground state is lifted appreciably by a macroscopic strain which, as a result of the large electrostrictive coupling in SrTiO₃, is induced at moderate electric field strengths.     

Microwave optical double resonance and fluorescence spectrum of NO2 excited by the 4579 Å line of the argon-ion laser

A microwave-optical double resonance (MODR) involving the $\text{J =}\text{21}\text{2}\text{←}\text{19}\text{2}$ and $\text{J =}\text{21}\text{2}\text{←}\text{17}\text{2}$ components of the 10_{0, 10} ← 9_{1, 9} rotational transition of the ground vibronic state of ¹⁴N¹⁶O₂ has been observed using the 4579 Å Ar⁺ laser line for excitation. By means of high resolution study of the laser excited fluorescence, the optical transition has been assigned to the $\text{J =}\text{19}\text{2}\text{←}\text{21}\text{2}$ spin component of the 9_{0, 9} ← 10_{0, 10} transition from the ground vibronic state to an unknown vibrational state of A₁ symmetry of the ²B₂ electronic state.     

Analysis of the perturbed NO22B2 ← 2A1 system in the 591.4- to 592.9-nm region based on sub-Doppler laser spectroscopy

Sub-Doppler excitation spectra of NO₂, covering four vibronic bands within the spectral range from 16 861 to 16 903 cm^?1, were measured with a resolution of down to 10 MHz in a collimated supersonic molecular beam. Unambiguous assignment of all prominent lines in the 42-cm^?1-wide interval of the ${}^2\text{B}{}_2\text{←}{}^2\text{A}{}_1$ excitation spectrum was achieved by recording for each excitation line at least four vibrational bands of the corresponding fluorescence spectrum with completely resolved rotational lines. From least-squares fits to the line positions in the excitation spectra the rotational, fine, and hyperfine structure of the ²B₂ state was analyzed. A perturbation analysis, based on information from both types of spectra, confirms earlier models of vibronic coupling with high-lying vibrational levels of the ²A₁ ground state and gives evidence for spin-orbit coupling. Possible models are discussed which may explain the observed perturbations.     

Doping of polyacetylene with tin chloride - Mössbauer,EPR and Raman spectroscopic studies

Polyacetylene film, containing varying amounts of trans isomer, has been chemically oxidized with SnCl₄. The reaction product has been studied by Mössbauer spectroscopy. The results suggest the existence of SnCl₅^? as the inserted species. From the temperature dependence of the recoil-free fraction the Mössbauer lattice temperature, θ, was estimated as 95K. This low value of θ indicates a significant mobility of SnCl₅^? ions at RT. Raman and EPR results are consistent with the Mössbauer effect measurements. Upon doping a broad ($\text{Δ}\text{Hpp}\text{= 5}\text{G}$) EPR line, which is a super-position of the lines characteristics of all trans polyacetylene and cis-rich polyacetylene, is transformed into a narrow ($\text{Δ}\text{Hpp}\text{= 0.8}\text{G}$), slightly assymetric ($\text{A}\text{/}\text{B}\text{= 1.23}$) line characteristic of partially oxidized trans-(CH)_x chain of high conductivity. The same doping induced isomerization can also be evidenced by Raman spectroscopy showing the disappearance of cis-(CH)_x modes after the oxidation.     

Electronic spectrum of 1,5-naphthyridine: Vapor spectra

Analyses of the vapor spectra of 1,5-naphthyridine-d₀ and -d₆ are presented. The spectra are characterized by two principal origins, one the true electronic origin, magnetic dipole allowed, ${}^1\text{B}{}_{\mathrm{g}}\text{←}{}^1\text{A}{}_{\mathrm{g}}$, 27 598.5 cm^?1 (-d₆ 27 676.9), and the other a vibronic origin, electricd-dipole allowed, corresponding to the activity of a low-frequency vibration 6a_u, 183 cm^?1 (-d₆ 163). Extensive sequence structure is evident and the relative intensities of the sequence bands associated with the two origins provide the strongest argument for their assignments. The absence of 6a_u as a major source of intensity in the hot bands is in agreement with vibronic coupling calculations which propose that in absorption intensity “flows” to the lower-frequency vibrations.     

Doppler-limited dye laser excitation spectroscopy of HCCl

The cw dye laser excitation spectrum of the $\text{A}\text{?}{}^1\text{A″(050) ←}\text{X}\text{?}{}^1\text{A′(000)}$ vibronic band of HCCl was observed between 16 539 and 16 656 cm^?1 with the Doppler-limited resolution, 0.03 cm^?1. The HCCl molecule was generated by the reaction of discharged CF₄ with CH₃Cl. The observed spectra were assigned to c-type transitions with ΔK_a = ±1 and also to axis-switching transitions with ΔK_a = 0 or ?2, but all with K′_a = 0, both for HC³⁵Cl and HC³⁷Cl. A rotational analysis yielded the rotational constants and quartic centrifugal distortion constants for the ground vibronic state and the band origin. A weak vibronic band, about one-third as intense as the main band, was found at about 57 cm^?1 to the violet of the main band for both isotopic species, and was ascribed to a transition from the ground vibronic state to a vibrational level, possibly (041), of the à state. The rotational levels of HC³⁵Cl in the à state showed a large perturbation; the J′ = 8, 9, and 10 levels were found to be split into two components. A normal coordinate analysis was carried out to calculate the centrifugal distortion constants and the inertia defect, which were in fair agreement with the observed values. The molecular structure of HCCl in the ground vibronic state was recalculated from the rotational constants of the two isotopic species combined with the 0.75B₀ + 0.25C₀ value previously reported for DC³⁵Cl.     

The lower excited states of CS: A study of extensive spin-orbit perturbations

We present previously unpublished data on the A¹Π, e³Σ^?, d³Δ_i, $\text{a′}{}^3\text{Σ}{}^{\mathrm{+}}$, and a³Π states of CS from uv emission and absorption transitions to the X¹Σ⁺ ground state. Term values obtained from d³Δ_i ← a³Π ir emission bands are also included. Rotational analyses are presented for about 50 new fine-structure components in some 30 new vibrational levels, together with extended data and analyses for many of the previously observed levels. The data now availabe for these five electronic states more than triples that previously published. Vibrational numbering for the e, d, and a′ states is established by data for minor isotopes. In a Hund's case a-b basis, off-diagonal spin-orbit elements (incipient case c effects) produce extensive coupling among these levels, not only for perturbation crossings but also between levels widely separated in energy. A systematic deperturbation requires two stages, which are iterated. Term values computed from the spectral lines are used to fit parameters of Hamiltonian matrices for groups of nearby, coupled levels. Additional shifts are computed by second-order perturbation theory from the electronic interactions deduced from vibronic coupling elements. The resultant parameters satisfy certain tests for self-consistency; they conform to low-order polynomials in $\text{v +}\text{1}\text{2}$, and vibrational overlap factors from wave-functions computed with RKR potential curves are proportional to the vibronic coupling elements, to within experimental error in most cases. To obtain this self-consistency, we have computed and applied normally neglected centrifugal distortion effects in the off-diagonal coupling elements and in the second-order perturbation sums. We also present and interpret the diagonal spin-orbit fine structure in the a and d states, including the centrifugal distortion parameters, A_D, for the latter, and values for several fitted second-order elements. Possible assignments for three additional perturbations of the A¹Π state and one faint band are discussed in view of ¹Δ, ¹Σ^?, and ⁵Π states that are also expected to occur in the region studied. Tentative parameters for the D¹Δ state are obtained from one possible set of assignments.