Experiments and quantum-chemical calculations on Rydberg states of H2CS in the region 5.6-9.5 eV

Absorption spectrum of H(2)CS in the region 5.6-9.5 eV was recorded with a continuously tunable light source of synchrotron radiation. After we subtracted absorption bands of CS(2), our spectrum clearly shows vibrational progressions associated with transitions (1)A(1)(pi,pi*)-X (1)A(1) and (1)B(2)(n,4s)-X (1)A(1) in the region 5.6-6.7 eV. A spectrum from which absorption of C(2)H(4) and CS(2) are subtracted shows several discrete bands in the region 6.9-9.5 eV. A Rydberg state (1)B(2)(n,4p(z)) lying below Rydberg state (1)A(1)(n,4p(y)) is confirmed, and the C-H symmetric stretching (nu(1)) and CH out-of-plane bending (nu(4)) modes for a transition (1)B(2)(n,4s)-X (1)A(1) are identified. New transitions to Rydberg states associated with excitation to 5s-11s, 5p(z)-7p(z), 5p(y)-7p(y), and 3d-6d are identified based on quantum defects and comparison with vertical excitation energies predicted with time-dependent density-functional theory (TD-DFT) and outer-valence Green's-function (OVGF) methods. For lower excited states predictions from these TD-DFT6-31+G calculations agree satisfactorily with experimental values, but for higher Rydberg states the OVGF method using aug-cc-pVTZ basis set augmented with extra diffuse functions yields more accurate predictions of excitation energies.     

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.     

IR spectrum and normal mode analysis of the antiAlzheimer''''s disease natural product Huperzine A: A quantum chemistry density-functional theory (DFT) investigation

ＨｕｐｅｒｚｉｎｅＡ(ＨｕｐＡ),ａｎａｌｋａｌｏｉｄｉｓｏｌａｔｅｄｆｒｏｍＣｈｉｎｅｓｅｈｅｒｂＨｕｐｅｒｚｉａｓｅｒｒａｔａＴｈｕｎｂ[1],ｉｓａｐｏｔｅｎｔｒｅｖｅｒｓｉｂｌｅａｃｅｔｙｌｃｈｏｌｉｎｅｓｔｅｒａｓｅ(ＡＣｈＥ)ｉｎｈｉｂｉｔｏｒ[2]ｗｉｔｈｈｉｇｈｅｆｆｉｃａｃｙａｎｄｌｏｗｔｏｘｉｃｉｔｙ(ｆｉｇ.1).Ａｃｅｔｙｌｃｈｏｌｉｎｅ(ＡＣｈ)ｉｓａｃｈｅｍｉｃａｌｓｕｂｓｔａｎｃｅ,ｗｈｉｃｈｃａｎｔｒａｎｓｍｉｔｔｈｅｓｉｇｎａｌｏｆｎｅｒｖｅｉｍｐｕｌｓｅ.Ｔｈｅｒｅａｒｅｍａｎｙ…     

Modelling the circularly and linearly polarised spectrum of [Ni(en)3]2+

The relative intensity and band shapes of the low energy spin-allowed transitions in the linearly polarised and circular dichroism spectrum of [Ni(en)(3)](2+) have been calculated using a time-dependent density functional theory approach. The effect of the trigonal ligand-field is minimal and no splitting of the bands is predicted by the simulations or observed experimentally. The 'd-d' transitions of the [Ni(en)(3)](2+) ion are electric dipole allowed but gain much of their intensity through Herzberg-Teller vibronic coupling. Its CD spectrum is dominated by the low energy band, which gains its rotatory strength through the magnetic dipole-allowed character of the parent octahedral transition and the electric dipole character due to the trigonal field. The simulation of the spectrum incorporates the contribution from all inducing vibrational modes with significant involvement of the {NiN(6)} unit. Vibrations which are centred on the chelate rings are not important in generating intensity, reflecting the localised d-d' character of the transitions. Simulated linearly polarised and circular dichroism spectra of such an open-shell system are presented for the first time and predict the essential elements of the experimental spectra.     

Accurate prediction for electron affinities of the radicals derived from the halide benzene

The molecular structures and electron affinities of eight radicals derived from the halide benzene by removing a hydrogen atom have been determined using seven hybrid Hartree-Fock/density-functional methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted as DZP++. These methods have been carefully calibrated [J. C. Rienstra-Kiracofe, G. S. Tschumper, H. F. Schaefer, S. Nandi, and G. B. Ellison, Chem. Rev. (Washington, D. C.) 102, 231 (2002)]. The geometries are fully optimized with each density-functional theory method and discussed, respectively. The three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The most reliable adiabatic electron affinities (with zero-point vibrational energy correction), obtained at the DZP++ B3LYP level of theory, are 1.74 eV (o-C6H4F), 1.39 eV (m-C6H4F), 1.34 eV (p-C6H4F), 1.78 eV (o-C6H4Cl), 1.53 eV (m-C6H4Cl), 1.45 eV (p-C6H4Cl), 2.06 eV (o-C6H3F2), and 2.04 eV (p-C6H3F2), respectively. Compared with the experimental values, the average absolute error of the B3LYP method is 0.03 eV. The BLYP, BP86, and BPW91 functionals also gave excellent predictions, with average absolute errors of 0.05, 0.08, and 0.08 eV, respectively.     

Photoelectron spectroscopy of phosphorus hydride anions

Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1).     

Coupled-cluster studies of the lowest excited states of the 11-cis-retinal chromophore

The first few excited states of the 11-cis-retinal (PSB11) chromophore have been studied at the coupled-cluster approximative singles and doubles (CC2) level using triple-zeta quality basis sets augmented with double sets of polarisation functions. The two lowest vertical excitation energies of 2.14 and 3.21 eV are in good agreement with recently reported experimental values of 2.03 and 3.18 eV obtained in molecular beam measurements. Calculations at the time-dependent density functional theory (TDDFT) level using the B3LYP hybrid functional yield vertical excitation energies of 2.34 and 3.10 eV for the two lowest states. Zero-point vibrational energy (ZPVE) corrections of -0.09 and -0.17 eV were deduced from the harmonic vibrational frequencies for the ground and excited states calculated at the density functional theory (DFT) and TDDFT level, respectively, using the B3LYP hybrid functional.     

IR spectrum and normal mode analysis of the antiAlzheimer’s disease natural product Huperzine A: A quantum chemistry density-functional theory (DFT) investigation

Quantum chemistry density-functional theory (DFT)B3LYP method with 6-31G basis set has been empolyed to study the electronic structure and IR spectrum of Huperzine A. The calculation result showed that the characteristic of the predicted IR bands was in general consistent with the experimental spectrum. 45 vibration modes were assigned clearly from the total of 102 vibration bands. The strongest IR-intensive band corresponds to the stretching vibration of the C=O bond of the pyridone ring, and the highest frequency band belongs to the pyridone N-H stretch. The investigation showed that the obvious differences between the calculated bands and the experimental spectrum existed at the bands involving the hydrogen atoms of amino and pyridone amide groups, which could form intermolecular hydrogen bond with other Huperzine A in the crystal structure. The hydrogen bonds can not only affect the orientation of these hydrogen atoms, but also can affect the force property of the chemical bond, which can change the vibrational frequencies. Project supported by the “863” High Technology Program of China (No. 863-103-04-01) and the National Natural Science Foundation of China (Grant No. 29403027).     

Threshold photoelectron spectroscopy of ozone

Threshold photoelectron spectrum of ozone is presented for the first time at a resolution of 21-38 meV using synchrotron radiation in the energy region of 12-21 eV. The ionization energies of the first ionized states were determined and an interpretation of the O3 spectrum with respect to its first three ionic states, 1 2A1, 1 2B2, and 1 2A2, is presented. Above 16 eV the enhancement of the intensities of the 2 2B1, 3 2A1, and 4 2B2 band systems due to the contribution of indirect processes was observed, not accessible by conventional photoelectron spectroscopy. It was also resolved and assigned the extensive vibrational structures of ozone. Between 15.5 and 18.5 eV the main band contours are similar to those found in conventional photoelectron spectroscopy, except that our threshold photoelectron spectrum reveals extensive additional vibrational structures. The band 2 2B1 was found to present an irregular vibrational spacing DeltaE, with a minimum value of 80 meV at approximately 16.47 eV.     

The lowest singlet states of octatetraene revisited

The two lowest excited singlet states of all-trans-1,3,5,7-octatetraene, 2?(1)A(-)(g) and 1?(1)B(+)(u), are studied by means of high level ab initio methods computing the vertical and adiabatic excitation energies for both states and the vertical emission energy for the 1 (1)A(g)(-)←2?(1)A(-)(g) transition. The results confirm the known assignment of two energies, the 2?(1)A(-)(g) adiabatic excitation energy and the 2?(1)A(-)(g) vertical emission energy, for which well defined experimental values are available, with an excellent agreement between theory and experiment. In the experimental absorption spectrum, the maximum of the band describing the 1?(1)B(+)(u)←1?(1)A(g)(-) excitation is the first peak and it has been assigned to the (0-0) vibrational transition, but in literature it is normally compared with the theoretical vertical excitation energy. This comparison has been questioned in the past, but a conclusive demonstration of its lack of foundation has not been given. The analysis reported here, while confirming the assignment of the highest peak in the experimental spectrum to the (0-0) adiabatic transition, indicates that it cannot be used as a reference for the vertical excitation energy. The theoretical vertical excitation energies for the 2?(1)A(-)(g) and 1?(1)B(+)(u) states are found to be almost degenerate, with a value, ? 4.8 eV, higher than that normally accepted in the literature, 4.4 eV. The motivations which have induced in the past other authors to consider this a correct value are discussed and the origin of their feebleness are analyzed.     

Improving convergence of experimental and computed vertical ionization energies using the ionization potential equation-of-motion coupled-cluster with singles and doubles method

Comparison of statistically evaluated experimental vertical ionization energies (IEs) for 53 medium-sized molecules (6-34 atoms) with ionization potential equation-of-motion coupled-cluster with singles and doubles (IP-EOMCCSD) computations shows that discrepancies between computed and experimental results can be accounted for with a combination of experimental and theoretical contributions. Discrepancies can be minimized by extrapolating computations to the complete basis set limit and correcting for vibrational zero-point energy (ZPE) while comparing with experimental IEs calculated as the intensity-weighted mean band position to account for band asymmetries. This procedure reduced the average discrepancy for ethylene, (E)-2-butene, 2,5-dihydrofuran, and pyrrole from 0.25 to 0.05 eV. Agreement between reported vertical IEs and computations without either making adjustments as described in this paper or using complete simulation of the ionization spectrum should be considered fortuitous. The comparisons made in this work show that estimates of vertical and adiabatic IE made using IP-EOMCCSD extrapolated to the complete basis set limit and corrected for vibrational ZPE can be used with reasonable confidence when experimental values are not available.     

Vibronic interactions in the photodetachment spectroscopy of phenide anion

Photodetachment spectroscopy of phenide anion C6H5- is theoretically studied with the aid of electronic structure calculations and quantum dynamical simulations of nuclear motion. The theoretical results are compared with the available experimental data. The vibronic structure of the first, second, and third photoelectron bands associated with the ground X 2A1 and low-lying excited A 2B1 and B 2A2 electronic states of the phenyl radical C6H5 is examined at length. While the X state of the radical is energetically well separated and its interaction is found to be rather weak with the rest, the A and B electronic states are found to be only approximately 0.57 eV apart in energy at the vertical configuration. Low-energy conical intersections between the latter two states are discovered and their impact on the nuclear dynamics underlying the second and third photoelectron bands is delineated. The nuclear dynamics in the X state solely proceeds through the adiabatic path and the theoretically calculated vibrational level structure of this state compares well with the experimental result. Two Condon active totally symmetric (a1) vibrational modes of ring deformation type form the most dominant progression in the first photoelectron band. The existing ambiguity in the assignment of these two vibrational modes is resolved here. The A-B conical intersections drive the nuclear dynamics via nonadiabatic paths, and as a result the second and third photoelectron bands overlap and particularly the third band due to the B state of C6H5 becomes highly diffused and structureless. Experimental photodetachment spectroscopy results are not available for these bands. However, the second band has been detected in electronic absorption spectroscopy measurements. The present theoretical results are compared with these absorption spectroscopy data to establish the nonadiabatic interactions between the A and B electronic states of C6H5.     

Characterization of the molecular parameters determining charge transport in anthradithiophene

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene.     

Combined study of vibrational spectra of α-carboline by theoretical and experimental IR methods

This paper reports the application of a scaled ab initio density-functional theory (DFT) at B3LYP/ccpvtz level to calculate the vibrational spectrum of α-carboline (9H-pyrido[2,3-b]indole), αCB, as well as the comparison of theoretical results with the experimental spectra. They have been recorded in Cl₃CD solution and in solid KBr pellets in the 4000-700 cm⁻¹ range. To test the adequacy of the computational method to reproduce the experimental vibrational spectra of αCB, this computational method has also been applied to the related and simpler molecules indole, Ind, and α-azaindole, αAInd. Previously reported assignments for the last compounds have been taken as reference for the subsequent assignments of the αCB vibrational bands. The results show that the hydrogen bonding interactions mainly affect the high frequency region while the skeletal vibration region keeps rather unchanged with the physical state of the sample. Moreover, apart from the vibrations involving the whole carboline nucleus, most of the experimental bands retain their original Ind and/or αAInd frequencies, thus allowing an easy assignment of the computed modes to the αCB vibrational bands.     

Exploring the Jahn-Teller and pseudo-Jahn-Teller conical intersections in the ethane radical cation

We report a theoretical account on the static and dynamic aspects of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the ground and first excited electronic states of the ethane radical cation. The findings are compared with the experimental photoionization spectrum of ethane. The present theoretical approach is based on a model diabatic Hamiltonian and with the parameters derived from ab initio calculations. The optimized geometry of ethane in its electronic ground state (1A1g) revealed an equilibrium staggered conformation belonging to the D3d symmetry point group. At the vertical configuration, the ethane radical cation belongs to this symmetry point group. The ground and low-lying electronic states of this radical cation are of 2Eg, 2A1g, 2Eu, and 2A2u symmetries. Elementary symmetry selection rule suggests that the degenerate electronic states of the radical cation are prone to the JT distortion when perturbed along the degenerate vibrational modes of eg symmetry. The 2A1g state is estimated to be approximately 0.345 eV above the 2Eg state and approximately 2.405 eV below the 2Eu state at the vertical configuration. The symmetry selection rule also suggests PJT crossings of the 2A1g and the 2Eg electronic states of the radical cation along the vibrational modes of eg symmetry and such crossings appear to be energetically favorable also. The irregular vibrational progressions, with numerous shoulders and small peaks, observed below 12.55 eV in the experimental recording are manifestations of the dynamic (E x e)-JT effect. Our findings revealed that the PJT activity of the degenerate vibrational modes is particularly strong in the 2Eg-2A1g electronic manifold which leads to a broad and diffuse structure of the observed photoelectron band.     

One-photon mass-analyzed threshold ionization spectroscopy of CH2BrI: extensive bending progression, reduced steric effect, and spin-orbit effect in the cation

One-photon mass-analyzed threshold ionization (MATI) spectrum of CH2BrI was obtained using coherent vacuum-ultraviolet radiation generated by four-wave difference-frequency mixing in Kr. Unlike CH2ClI investigated previously, a very extensive bending (Br-C-I) progression was observed. Vibrational frequencies of CH2BrI+ were measured from the spectra and the vibrational assignments were made by utilizing frequencies calculated by the density-functional-theory (DFT) method using relativistic effective core potentials with and without the spin-orbit terms. A noticeable spin-orbit effect on the vibrational frequencies was observed from the DFT calculations, even though its influence was not so dramatic as in CH2ClI+. A simple explanation based on the bonding characteristics of the molecular orbitals involved in the ionization is presented to account for the above differences between the MATI spectra of CH2BrI and CH2ClI. The 0-0 band of the CH2BrI spectrum could be identified through the use of combined data from calculations and experiments. The adiabatic ionization energy determined from the position of this band was 9.5944+/-0.0006 eV, which was significantly smaller than the vertical ionization energy reported previously.     

Broad photoelectron spectrum and lowered electron affinity due to hydrogen in ZnOH: a joint experimental and theoretical study

We report a combined experimental and theoretical photoelectron spectroscopy study of ZnOH(-). We find that the electron binding energy spectrum of ZnOH(-) reveals a broad and featureless peak between 1.4 and 2.4 eV in energy. The vertical detachment energy (VDE) of ZnOH(-) is determined to be 1.78 eV, which is lower than the 2.08 eV VDE of ZnO(-). Our theoretical calculations match the VDE of ZnOH(-) accurately, but we find that the broadness of the peak cannot be explained by rotational or vibrational state excitation. The broadness of this peak is in strong contrast to the narrow and easily understood first peak of the ZnO spectrum, which features a well-resolved vibrational progression that can be readily explained by calculating the Franck-Condon transition factors. This study provides spectroscopic evidence of the effect of hydrogen on diatomic ZnO.     

Vibronic structure of the permanganate absorption spectrum from time-dependent density functional calculations

The UV absorption spectrum of the permanganate anion is a prototype transition-metal complex spectrum. Despite this being a simple d0 Td system, for which a beautiful spectrum with detailed vibrational structure has been available since 1967, the assignment of the second and third bands is still very controversial. The issue can be resolved only by an elucidation of the intricate vibronic structure of the spectrum. We investigate the vibronic coupling by means of linear-response time-dependent density functional calculations. By means of a diabatizing scheme that employs the transition densities obtained in the TDDFT calculations in many geometries around Re, we construct a Taylor series expansion in the normal coordinates of a diabatic potential energy matrix, coupling 24 excited states. The simulated vibronic structure is in good agreement with the experimental absorption spectrum after the adjustment of some of the calculated vertical excitation energies. The peculiar blurred vibronic structure of the second band, which is a very distinctive feature of the experimental spectrum, is fully reproduced in the calculations. It is caused by the double-well shape of the adiabatic energy surface along the Jahn-Teller active e mode of the allowed 1E state arising from the second 1T2 state, which exhibits a Jahn-Teller splitting into 1B2 and 1E states. We trace the double-well shape to an avoided crossing between two diabatic states with different orbital-excitation character. The crossing can be explained at the molecular orbital level from the Jahn-Teller splitting of the set of 7t2{3d(xy), 3d(xz), 3d(yz)} orbitals (the LUMO + 1), to which the excitations characterizing the diabatic states take place. In contrast to its character in the two well regions, at Re the 2(1)T2 state is not predominantly an excitation to the LUMO + 1, but has more HOMO - 1 --> LUMO (2e = {3d(x2-y2), 3d(z2)}) character. The changing character of the 2(1)T2 - 1E state along the e mode implies that the assignment of the experimental bands to single orbital transitions is too simplistic intrinsically. This spectrum, and notably the blurring of the vibronic structure in the second band, can be understood only from the extensive configurational mixing and vibronic coupling between the excited states. This solves the long-standing assignment problem of these bands.     

Vibrational and electronic absorption spectroscopy of dibenzo[b,def]chrysene and its ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).