The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

Structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane

In this work, we present a complete structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane (BF₂OH) at the HF/aug‐cc‐pVTZ, MP2(full)/aug‐cc‐pVTZ, and CCSD/aug‐cc‐pVTZ theory levels. After full relaxation of the geometry, the equilibrium structure is found in a planar conformation of C_s symmetry. The difference in the two BF distances suggests the existence of a nonbonded interaction between the fluorine and oxygen atoms. The structural and energetic variation of BF₂OH as a function of the OH torsional angle is considered. The torsional barrier, at the CCSD/aug‐cc‐pVTZ level, and including the effect of the zero‐point energy of the remaining vibrations, is found 2,728 cm^?1. In addition, an anharmonic Hamiltonian for the OH torsional mode is presented and variationally solved. To simplify the treatment and to classify the energy levels, BF₂OH is classified under a G₄ nonrigid group accounting for the inversion symmetry of the molecule and the interchange of the fluorine atoms. The computed torsional energy levels exhibit a very small inversion splitting. The torsional spectrum is simulated considering the dipole moment components along the principal axes of inertia as an explicit function of the torsional coordinate. We observe three dominant bands in the spectrum formed by doublets corresponding to ν₉ = 0 → 1, 2 transitions. The fundamental is an a‐type, Franck–Condon, transition. This is the strongest and appears at 466.80 cm^?1 with relative intensity 0.4312. The ν₉ = 0 → 2 bands correspond to doublets of b‐ and c‐type, i.e., Herzberg–Teller transitions. These are two overlapping bands found at 890.92 and 890.94 cm^?1 with intensity 0.2207 for the b‐type band and 0.2193 for the c‐type band. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Polarization and assignment of the two-photon excitation spectrum of benzene vapor

The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm^?1 to 42 441 cm^?1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b_1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ₁₄(b_2u) vibronic origin at 1570 cm^?1 (above the electronic origin of the ^IB_2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A_2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ₁₄ band may be due to vibronic borrowing by the b_2u mode from the ground electronic state (A_1g).     

Theoretical prediction on HBeN− and HNBe− anions using multiconfiguration second‐order perturbation theory

The HBeN^? and HNBe^? anions have been investigated for the first time using the CASSCF, CASPT2, and DFT/B3LYP methods with the contracted atomic natural orbital (ANO) and cc‐pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces were optimized at the CASSCF/ANO and B3LYP/cc‐pVTZ levels. The ground and the first excited states of HBeN^? are predicted to be X²Π and A²Σ⁺ states, respectively. It was predicted that the ground state of HNBe^? is X²Σ⁺ state. The A²Π state of HNBe^? has unique imaginary frequency. A bend local minimum M1 was found along the 1²A″ potential energy surface and the A²Π state of HNBe^? should be the transition state of the isomerization reactions for M1 ? M1. The CASPT2/ANO potential energy curves of isomerization reactions were calculated as a function of HBeN bond angle. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Theoretical computation of low‐lying electronic states of HCNS: A CASPT2 study

Complete active space self‐consistent field (CASSCF) and complete active space second‐order perturbation theory (CASPT2) calculations in conjunction with the aug‐cc‐pVTZ basis set have been used to investigate the low‐lying electronic states of thiofulminic acid (HCNS), HCNS⁺, and HCNS^?. The result of geometry optimization using CASPT2/aug‐cc‐pVTZ shows that theoretically determined geometric parameters and harmonic vibrational frequencies for the HCNS ground state X¹Σ⁺(X¹A′) are in agreement with previous studies. The ionization energies, the electron affinity energies, the adiabatic excitation energies, and vertical excitation energies have been calculated and the corresponding cation and anion states are identified. By calculating adiabatic electron affinity, the states of HCNS^? have been identified to contain both π orbital states (X²A′ and 1²A″) and dipole‐bond states (1⁴A′ and 1⁴A″). © 2012 Wiley Periodicals, Inc.     

Theoretical study on HBC− and HCB− anions using multiconfiguration second‐order perturbation theory

The HBC^? and HCB^? anions have been studied using the complete active space self‐consistent field, CASPT2, and DFT/B3LYP methods with the contracted atomic natural orbital (ANO) and cc‐pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces were optimized at the CASSCF/ANO and B3LYP/cc‐pVTZ levels. The ground state of HBC^? is predicted to be X²? state, which is different from the previously published results. The CASPT2/ANO potential energy curves (PECs) of isomerization reactions were calculated as a function of HBC bond angle and the PECs also show the ²? state is the ground state of HBC^? anion. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Vibronic activity in the two-photon spectrum of pyrene

A CNDO/S Cl calculation of the two-photon vibronic activity of some b_1u and b_2u vibrational modes of pyrene is presented. The two-photon amplitude tensor is discussed in terms of vibronic coupling coefficients calculated by means of the orbital-following method. The results are in good agreement with the experimental data taken from a recently observed two-photon spectrum of pyrene in a Shpolskii matrix. The role of the ground-state coupling with the first excited B_1u state has been investigated and is shown to be responsible for most of the vibronically induced two-photon intensity of the spectrum. The calculations also show that the 1310 cm^?1 (b_1u; observed ground-state value) mode is associated with the largest vibronic coupling coefficients and the strongest two-photon amplitude tensor and therefore must be correlated with the most intense B_1u X b_1u false origin ≈ 1496 cm^?1 from the pure (0-0) line.     

Moderate-resolution two-photon spectrum of the naphthalene crystal and naphthalene in durene

The two-photon absorption spectra at ～ 5 cm^?1 resolution are presented for the naphthalene crystal and naphthalene in durene at 2°K in polarized light. A rich vibronic spectrum of the ¹B_2u(3200 Å) naphthalene state is observed and some of the strongest vibronic origins are from in-plane vibrations.     

Theoretical study of spectroscopic and molecular properties of several low‐lying electronic states of CO molecule

The potential energy curves (PECs) of eight low‐lying electronic states (X¹Σ⁺, a³Π, a′³Σ⁺, d³Δ, e³Σ^?, A¹Π, I¹Σ^?, and D¹Δ) of the carbon monoxide molecule have been studied by an ab initio quantum chemical method. The calculations have been performed using the complete active space self‐consistent field method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation‐consistent aug‐cc‐pV5Z basis set. The effects on the PECs by the core‐valence correlation and relativistic corrections are included. The way to consider the relativistic corrections is to use the third‐order Douglas–Kroll Hamiltonian approximation at the level of a cc‐pV5Z basis set. Core‐valence correlation corrections are performed using the cc‐pCVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are corrected for size‐extensivity errors by means of the Davidson modification (MRCI+Q). The spectroscopic parameters (D_e, T_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e, α_e, and γ_e) of these electronic states are calculated using these PECs. The spectroscopic parameters are compared with those reported in the literature. Using the Breit–Pauli operator, the spin–orbit coupling effect on the spectroscopic parameters is discussed for the a³Π electronic state. With the PECs obtained by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations, the complete vibrational states of each electronic state have been determined. The vibrational manifolds have been calculated for each vibrational state of each electronic state. The vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν of the first 20 vibrational states when the rotational quantum number J equals zero are reported and compared with the experimental data. Comparison with the measurements demonstrates that the present spectroscopic parameters and molecular constants determined by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations are both reliable and accurate. © 2012 Wiley Periodicals, Inc.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

ChemInform Abstract: Smallest Deltahedra Silicon Dicarbide: C2Si32‐.

A total of 20 singlet and 36 triplet C₂Si₃^2‐ isomers are obtained by quantum chemical calculations at the B3LYP/aug‐cc‐pVTZ level.     

Further evidence of the vibronic coupling of pyrazine as revealed by preresonance raman effect

The excitation wavelength dependences of the intensities of the Raman lines of pyrazine have been measured. The intensity enhancement of the non-totally symmetric Raman line at 925 cm^?1 provided firm evidence of the vibronic coupling between the lowest ¹B_3u(n,π^*) and second lowest ¹B_2u(π,π^*) states. The excitation wavelength dependences of other non-totally symmetric Raman lines suggest also the various vibronic coupling schemes.     

Excited state geometry of and radiationless processes in the lowest B3u(nπ*) singlet state of s-tetrazine

High resolution absorption and laser induced emission spectra of the lowest B_3u(nπ^*) singlet state of s-tetrazine-h₂ and -d₂ in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm^?1 (h₂) is dominated by a progression in ν_6a and a Herzberg-Teller origin which has been assigned as ν₁. The absence of ν₁ in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν_6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h₂ and -d₂ to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h₂ and dimethyl s-tetrazine we calculate for s-tetrazine-h₂ a fluorescence lifetime of 1.5 ± 0.2 ns and a fluoresence quantum yield of 1.8 × 10^?3. The ratio of the emissive lifetimes of s-tetrazine-d₂ and -h₂ was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h₂, -d₂ and dimethyl for excitation into the origin of the ¹B_3u(nπ^*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the ¹B_3u state. This finding is in agreement with earlier measurements by Vemulapalli and Cassen, but the report by these authors that such quantum yield variations also occurred in the rovibronic structure is not confirmed.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

IR Low‐Temperature Matrix and ab Initio Study on β‐Alanine Conformers

For the first time the argon‐matrix low‐temperature FTIR spectra of β‐alanine are recorded. They reveal a quite complicated spectral pattern which suggests the presence of several β‐alanine conformers in the matrix. To interpret the spectra, the eighteen β‐alanine conformers, stable in the gas phase, are estimated at the B3LYP and MP2 levels combined with the aug‐cc‐pVDZ. Ten low‐energy structures are reoptimized at the QCISD/aug‐cc‐pVDZ and B3LYP and MP2 levels by using the aug‐cc‐pVTZ basis sets. Assignment of the experimental spectra is undertaken on the basis of the calculated B3LYP/aug‐cc‐pVDZ anharmonic IR frequencies as well as careful estimation of the conformer population. The presence of at least three β‐alanine conformers is demonstrated. The detailed analysis of IR spectra points to the possible presence of five additional β‐alanine conformers.     

Synthesis and Characterization of Acetylene‐Linked Bisphenalenyl and Metallic‐Like Behavior in Its Charge‐Transfer Complex

We prepared and isolated a phenalenyl‐based neutral hydrocarbon ( 1 b ) with a biradical index of 14 %, as well as its charge‐transfer (CT) complex 1 b –F₄‐TCNQ. The crystal structure and the small HOMO–LUMO gap assessed by electrochemical and optical methods support the singlet‐biradical contribution to the ground state of the neutral 1 b . This biradical character suggests that 1 b has the electronic structure of phenalenyl radicals coupled weakly through an acetylene linker, that is, some independence of the two phenalenyl moieties. The monocationic species 1 b^{. +} was obtained by reaction with the organic electron acceptor F₄‐TCNQ. The cationic species has a small disproportionation energy ΔE for the reaction 2× 1 b^{. +}? 1 b + 1 b ²⁺, which presumably originates from the independence of the phenalenyl moieties. The small ΔE led to a small on‐site Coulombic repulsion U_eff=0.61 eV in the CT complex. Moreover, a very effective orbital overlap of the phenalenyl rings between molecules afforded a relatively large transfer integral t=0.09 eV. The small U_eff/4t ratio (=1.7) resulted in a metallic‐like conductive behavior at around room temperature. Below 280 K, the CT complex showed a transition into a semiconductive state as a result of bond formation between phenalenyl and F₄‐TCNQ carbon atoms.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

An analytical approach for computing Franck‐Condon integrals of harmonic oscillators with arbitrary dimensions

We have developed an analytical approach for computing Franck‐Condon integrals (FCIs) of harmonic oscillators (HOs) with arbitrary dimensions in which the mode‐mixing Duschinsky effect is taken into account. A general formula of FCIs of HOs was obtained and was applied to study the photoelectron spectroscopy of vinyl alcohol and ovalene (C₃₂H₁₄). The equilibrium geometries, harmonic vibrational frequencies and normal modes of vinyl alcohol, ovalene, and their cations were computed at the B3LYP/aug‐cc‐pVTZ or the B3LYP/6‐31G(d) level, from which Franck‐Condon factors were calculated and photoelectron spectra were simulated. The adiabatic ionization energies of vinyl alcohol were also computed by extrapolating the CCSD(T) energies to the complete basis set limit with aug‐cc‐pVXZ (X = D, T, Q, 5). The simulated photoelectron spectra of both vinyl alcohol and ovalene are in agreement with the experiments. The computed adiabatic ionization energies of syn‐ and anti‐vinyl alcohol are in consistent with the experiment within 0.008 eV and 0.014 eV, respectively. We show, for the first time, that the analytical approach of computing FCIs is also efficient and promising for the studies of vibronic spectra of macrosystems. © 2012 Wiley Periodicals, Inc.