TZVP Basis Set Research Articles

Computational Study on the Coordinate Systems of Proline with Cu, Cu<sup>+</sup> and Cu<sup>2+</sup>

运用M06-2X和ωB97XD方法分别在6-311＋＋G（2d, p）和TZVP基组水平上,对脯氨酸（Pro）的15种构象与Cu、Cu＋和Cu2＋形成的多种配合物的几何结构、能量学特征、振动光谱和电子结构等进行计算研究.四种水平得到20种[Pro-Cu]、16种[Pro-Cu]＋和16种[Pro-Cu]2＋稳定结构.[Pro-Cu]和[Pro-Cu]＋体系中出现12种Pro构象,而[Pro-Cu]2＋体系中出现11种Pro构象,三种体系中最稳定的结构都不是由能量最低的Pro构象生成的.在结构CI3、CI4、CII7和CII8中, Pro的羧基氢转移到亚氨基氮形成两性离子与Cu双配位结合.[Pro-Cu]0/1＋/2＋体系四种水平计算相对能差范围逐渐增加,结合能分别在-60.0--5.0 kJ·mol-1、-340.0--170.0 kJ·mol-1和-1100.0--860.0 kJ·mol-1范围,配位体系中Pro的变形能逐渐增加. N-H和O-H键伸缩振动频率普遍发生红移,配位体系中部分电荷从Pro转移到Cu上,在[Pro-Cu]2＋体系中单配位结构中电荷转移最多,约为单位负电荷.

Density Functional Theoretical Modeling of Selective Ligand for the Separation of Zr and Hf Metal Oxycations (ZrO2+ and HfO2+)

The structure and energetics of TBP (tri-butyl phosphate), THPO (Tri-hexyl phosphine oxide), and CYANEX925 (bis (2, 4, 4-trimethylpentyl) octylphosphine oxide) and their complexes with Hf and Zr oxycation (ZrO2+ and HfO2+) with BP86 density functional employing TZVP basis set have been presented. The calculated gas phase binding energy was shown to follow the order CYANEX925 > THPO > TBP >. The calculated gas phase binding energy for Hf oxycation is found to be higher than that of Zr oxycation for all the ligands which is contradictory to the observed experimental trend. The experimental selectivity trend was recovered by inclusion of the second solvation sphere of the metal ion both in gas and aqueous phase for a particular ligand. Both the implicit and explicit solvation model fail to predict the high selectivity of ZrO2+ with CYANEX925 over TBP in the absence of nitrate anion. The presence of nitrate anion along with the second solvation shell (n = 24 water molecules) around the oxycation, either in the gas or solvent phase shows consistent results for selectivity as observed in the solvent extraction experiments. Further, the preferential selectivity of Hf4+ cation over Zr4+ cation using liquid MIBK (methyl isobutyl ketone) in the thiocyanate medium is also captured through DFT calculation. The experimental separation factor [SF(Zr/Hf or Hf/Zr)] was found to be well correlated with the extraction energy difference between two metal ions for a particular ligand.

Assessment of n-Electron Valence State Perturbation Theory for Vertical Excitation Energies.

The multireference n-electron Valence State Perturbation Theory is applied to a benchmark set of 28 organic molecules compiled by Schreiber et al. J. Chem. Phys. (2008) 128, 13. Different types of low-lying vertical excitation energies are computed using the same geometries and TZVP basis set as in the original work. The previously published coupled cluster CC3 results are used as a reference. The complete active space second order perturbation theory (CASPT2) results, as well as the results of second order N-electron valence perturbation theory (NEVPT2) (both in their single-state variants) are evaluated against this reference set, which includes 153 singlet and 72 triplet vertical transition energies. NEVPT2 calculations are carried out in two variants: the partially contracted (PC) and the strongly contracted (SC) scheme. The statistical evaluation with respect to CC3 is found to be similar for both: the mean unsigned deviations is 0.28 eV for singlets and 0.16 eV for triplets for PC-NEVPT2, while it is 0.23 and 0.17 eV for SC-NEVPT2, respectively. Further analysis has shown that deficiencies in the zeroth-order wave functions, in particular for the subset of π → π* singlet excitations, are responsible for the largest deviations from CC3. Those states have either a charge transfer or an ionic character. For the remaining singlet and all triplet excitations the general trend was established that NEVPT2 tends to slightly overestimate excitation energies while CASPT2 slightly underestimates them. However, overall, both methods are of very similar accuracy provided that the IPEA shift is used in the CASPT2 method. Interestingly, the conclusions reached in this study are independent of the orbital canonicalization scheme used in the NEVPT2 calculation.

Adsorption of quaternary amine surfactants and their penetration into the intracrystalline cavities of sepiolite

Organoclays are very important composite materials used in many diverse industries including plastics and paint. In this study, an organoclay was formed using sepiolite and homologs of quaternary amine surfactants of tetradecyl dimethyl ethylbenzyl ammonium chloride (TDEBAC), hexadecyl dimethyl ethylbenzyl ammonium chloride (HDEBAC) and octadecyl dimethyl ethylbenzyl ammonium chloride (ODEBAC). Adsorption isotherms of sepiolite/quaternary amine surfactants show that the surfactant with the longest chain, ODEBAC, is adsorbed at most onto the surface of sepiolite. FTIR spectroscopy of organosepiolite indicated that spectroscopic features related to the surface and tunnel of sepiolite are notably modified in the presence of surfactants. In addition to these experiments, high level ab initio counter-poise corrected binding energy computations at MP2, SCS-MP2, B97-D and B2PLYP-D levels of theory employing the TZVP basis set were also performed. At all levels, basal interaction of the surfactant at the surface of sepiolite was found to exhibit the most favourable configuration. Furthermore, penetration of the surfactant into the intra-crystalline cavities of sepiolite was also found to be plausible.

A quantum chemical investigation of the electronic structure of thionine

We have examined the electronic and molecular structure of 3,7-diaminophenothiazin-5-ium dye (thionine) in the electronic ground state and in the lowest excited states. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Equilibrium geometries were optimized employing (time-dependent) density functional theory (B3LYP functional) combined with the TZVP basis set. Solvent effects were estimated using the COSMO model and micro-hydration with up to five explicit water molecules. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet and triplet states at the ground state geometry to be of π→π* (S(1), S(2), T(1), T(2)) and n→π* (S(3), T(3)) character. This order changes when the molecular structure in the electronically excited states is relaxed. Geometry relaxation has almost no effect on the energy of the S(1) and T(1) states (~0.02 eV). The relaxation effects on the energies of S(2) and T(2) are moderate (0.14-0.20 eV). The very small emission energy results in a very low fluorescence rate. While we were not able to locate the energetic minimum of the S(3) state, we found a non-planar minimum for the T(3) state with an energy which is very close to the energy of the S(1) minimum in the gas phase (0.04 eV above). When hydration effects are taken into account, the n→π* states S(3) and T(3) are strongly blueshifted (0.33 and 0.46 eV), while the π→π* states are only slightly affected (<0.06 eV).

Fifth Stereoactive Orbital on Silicon: Relaxation of the Lowest Singlet Excited State of Octamethyltrisilane

We address relaxation pathways in the excited singlet states S(1) of saturated molecules, specifically alkylated oligosilanes. Unlike their longer peralkylated homologues, disilanes and trisilanes do not fluoresce even at low temperatures. An examination of the S(1) potential energy surface of Si(3)Me(8) with density functional (TDDFT, LC-TDDFT), and ab initio (RICC2, RIADC(2)) methods with TZVP basis sets revealed only extremely shallow minima in the vicinity of funnels, accounting for the absence of fluorescence, rapid internal conversion, and photoproducts. Relaxed singlet excited state structures either contain one approximately trigonal bipyramidal Si atom or two that are halfway between tetrahedral and trigonal bipyramidal. Four of the ligands are those that the Si atom had in the ground state. Natural bond orbital analysis suggests that the fifth one is a nonbonding hybrid orbital of the lone-pair type and size intermediate between valence and Rydberg, with an only very small occupancy, yet stereochemically active. The fifth natural hybrid orbital is composed primarily of 4s, 4p, and usually to a lesser degree, also 3d atomic orbitals. The trigonal bipyramidal structure allows an optimal accommodation of the presence of both a negative and a positive charge in the Lewis structures. The excess negative charge on the distorted Si atom is shared between the nonbonding fifth hybrid orbital and σ* antibonding orbitals associated with its bonds. The positive charge resides in an adjacent σ SiSi bond orbital. A Rydberg minimum also occurs on the S(1) surface at the geometry of the radical cation.

Structure, spectroscopy, and theory calculations on 1,3-bis((5,6-dimethyl-1H-benzo[d]imidazol-1-yl)methyl)benzene

The molecule, 1,3-bis((5,6-dimethyl-1H-benzo[d]imidazol-1-yl)methyl)benzene (short for T, C26H26N4), was synthesized and characterized by UV–Vis spectroscopy, infrared spectroscopy and single crystal X-ray diffraction. The molecular geometry, vibrational frequencies, absorption spectrum of the compound T have been calculated by using the density functional theory (DFT/B3LYP) method with TZVP basis sets, and have been compared with the experimental data. The calculated results showed that the optimized geometries can well reproduce the crystal structural parameters, and the theoretical absorption spectra show good agreement with experimental UV–Vis values. Besides, frontier molecular orbitals analysis of the title compound was investigated by theoretical calculations.

Density‐functional‐theory study of α‐cyclodextrin inclusion complexes

AbstractWe apply the density functional theory on the description of the α‐cyclodextrin (α‐CD) and the analysis of the molecular electrostatic potential (MEP) for complexes of this system with Na+, F−, CO, and N,N‐dimethyl formamide, as substrates. Four exchange‐correlation functionals were considered: one of the local density approximation, two of the generalized gradient approximation (BLYP and PBE), and one of the hybrid family. These exchange‐correlation functionals were coupled with the DZVP/A1, DZVP, and TZVP basis set functions. All complexes were fully optimized by all methods. The experimental molecular structure of the α‐CD compares better with that described by the PBE exchange‐correlation functional. The MEP analysis shows that the electrostatic is quite relevant on the intermolecular interaction for the α‐CD complexes in gas phase. Some of our results are in disagreement with those published previously with the PM3 semiempirical method and the B3LYP//PM3 method. © 2012 Wiley Periodicals, Inc.

A novel barbituric acid-based azo dye and its derived polyamides: Synthesis, spectroscopic investigation and computational calculations

Abstract 5-[(3,5-dicarboxyphenyl)azo]-barbituric acid, an azo-based aromatic diacid was prepared and fully characterized by FT IR, 1 H and 13 C NMR along with UV–vis absorption spectroscopy. The effects of varying pH and solvent upon the absorption spectrum of the azo dye were investigated. The optimized molecular structures of possible tautomeric forms and HOMO–LUMO energies of expecting preferred forms of the synthesized heterocyclic azo dye have been calculated using density functional theory, (B3LYP) methods with TZVP basis set level and Polarizable Continuum Model (PCM) model for solvation effect. The calculations showed that the most probably preferred form of the azo dye in gas phase and solution was a hydrazone-keto form. FT IR and UV–vis spectroscopy, DSC, TGA and XRD techniques were used to obtain the characteristics and structural features of the monomer-synthesized polyamides. The influence of organic solvents on the absorption spectra of the polymers along with viscosity measurements (0.19–0.44 dl/g), solubility and film formability of the synthesized polyamides were also investigated.

Mechanistic DFT studies – helicate-type complexes with different alcylic spacers

Metal-controlled self-assembly of complexes is of high interest in the field of Supramolecular Chemistry [1,2]. In the current study, we synthesized binuclear complexes with different spacers and study the influence of chain length on their relative energy. The considered complexes prefer the zigzag conformation. Thus a bridge with an odd number of methylene units forms a meso-Helicate (ΔΛ or ΛΔ) and one with an even number leads to a Helicate (ΔΔ or ΛΛ) (figure ​(figure1)1) [3,4]. Figure 1 Geometrically optimized complexes with Ti(IV); left: energies of the complexes with different alcylic spacers; right: ΔΔ and ΛΔ form of the complex with a propylene spacer. Comparison of the calculated transition energies for the non-dissociative interconversions of the diastereomeres with experimental results provides inside into the isomerization process. Moreover, insertion of different cations (templates) into the cavities of the binuclear complexes and corresponding calculations allow prediction of their influence on the isomerization. Enlargement of the studied system results in binuclear complexes with imino-bridged ligands. The obtained computational results provide a possible explanation for the experimentally observed high diastereoselectivity. As the DFT functionals like B3LYP do not describe long-range interactions properly, we chose the coulomb-attenuating method CAM-B3LYP [5] which corrects the exchange interaction at long range. The complexes with Ti(IV) in their helical or meso form have been geometrically optimized at the CAM-B3LYP level of theory with the TZVP basis set and MDF10 as ECP for Ti(IV) as implemented in the program package Gaussian09 [6].

Structural differences in eight- and ten-membered heterocyclic tin compounds displaying transannular interactions O⋯Sn: An experimental and theoretical study

Two series of heterocyclic tin compounds of general formula [(S{C6H3(CH2)nS}2O)SnR1R2] with different central ring sizes were prepared. The ten-membered series includes the compounds with n=1 and R1=R2=Ph (5); R1=Cl, R2=Ph (6); R1=Cl, R2=n-Bu (7); R1=R2=Cl (8); the eight-membered series includes the compounds with n=0 and R1=R2=Ph (10); R1=Cl, R2=n-Bu (11) and R1=R2=Cl (12). The compounds 5, 7, 8, 10, and 11 were investigated by single-crystal X-ray diffraction. The chloro compounds 7, 8, and 11 displayed a bipyramidal geometry at the tin atom with different degrees of distortion ranging from 57% to 62%. The diphenyl compounds 5 and 10 displayed a tetrahedral geometry at Sn. The conformation of the central ring in the ten-membered series is similar and is described as boat; the other series displayed two different conformations described as boat–chair and boat–boat. The possible conformers in the gas state of compounds 5, 7, 8, 10, 11, and 12 were investigated by MMFF, LSDA, BLYP, B3LYP, and M06 functionals using the DGDZVP and TZVP basis sets. The structural data of the total optimization agreed with the experimental results. The topological analysis indicated that bond critical points are present along the O⋯Sn direction in the compounds 7, 8, 11, and 12.

DFT calculations of the redox potentials for the nitrosyl complexes [Fe2(μ‐RS)2(NO)4] with R = alkyl

AbstractThe redox potentials of alkyl substituted Roussin's red esters in tetrahydrofuran and acetonitrile were studied using the density functional theory. The geometry and electronic structures of monoanions of Roussin's red esters [Fe2(μ‐RS)2(NO)4]− (R = Me, Et, i‐Pr, t‐Bu) together with their redox potentials in solutions have been computed using GGA (BP86,PBE, BLYP), meta‐GGA (TPSS), hybrid‐GGA (B3LYP), and meta‐hybrid GGA (BMK, B1B95) combined with 6‐311++G**//6‐31G* and tzvp basis set. The BP86 functional provides excellent agreement with experiment, whereas B3LYP requires some correlation, which makes it possible to estimate the redox potentials of related complexes, intermediate, or unstable. In particular, a redox potential of the complex with R = Ph, a perspective anticancer drug, has been evaluated for several solvents. © 2012 Wiley Periodicals, Inc.

Ligand Effects on Optical Properties of Small Gold Clusters: A TDDFT Study

Ligand influence on the excited state structure of small neutral gold clusters (Au2 and Au4) has been investigated using Time Dependent Density Functional Theory. We study in detail the absorption profile of bare and ligated small gold clusters in solution modeled with Polarizable Continuum Model. Performance of CAM-B3LYP and TPSS DFT functionals combined with TZVP basis set has been assessed. We found that ligands substantially modify the excited state structure of clusters by eliminating low-lying optically inactive excited states. Depending on the ligand environment, the cluster may gain significant fluorescence efficiency. Our results suggest that small gold clusters ligated with amines will have better fluorescence potential compared to those ligated with phosphine or thiol ligands, in agreement with preliminary experimental data. TPSS fails to describe excited state structure of ligated clusters due to spurious charge-transfer states, thus highlighting the necessity of choosing appropriate quantum-chemistry model for correct excited state description.

Conformational and vibrational studies of isomeric hydrogen cyanide tetramers by quantum chemical methods

The results of structural studies and detailed harmonic and anharmonic vibrational analysis on two hydrogen cyanide (HCN) tetramers diaminomaleonitrile (DAMN) and diaminofumaronitrile (DAFN), which are important molecules for understanding the chemistry of interstellar space and nitrile rich environments, are being reported on the basis of density functional theory using second-order perturbation theory. Both the molecules are found to have C1 symmetry. While all the heavy atoms of DAMN lie in the same plane (maximum deviation 6°), the two nitrogen atoms in DAFN are out of plane by about 15°. The two amino groups are tetrahedral and do not have significant bond angle anisotropy. Detailed conformational studies are reported on the two molecules and their possible rotational isomers are identified. Complete vibrational analysis based on harmonic and anharmonic frequencies, intensity of infrared and activity of Raman bands and potential energy distribution over the internal coordinates has been provided for the two molecules. Affect of hydrogen bonding on molecular geometry and frequencies of the NH stretch modes has been studied by calculations on the dimers of the two molecules. A close agreement has been observed between the experimental and calculated frequencies. Vibrational–rotational constants such as rotational constants in the ground vibrational state (A0, B0, C0) and the effective rotational constants (Ae, Be and Ce), including terms due to quartic centrifugal distortion constants, rotation–vibration coupling constants, Wilson and Nielsen's centrifugal distortion constants have been calculated using B3LYP and B97-1 functionals and 6-31G**, 6-311+G** and TZVP basis sets.

Magnetic properties of single crystal alpha-benzoin oxime: An EPR study

The electron paramagnetic resonance (EPR) spectra of gamma irradiated single crystals of alpha-benzoinoxime (ABO) have been examined between 120 and 440 K. Considering the dependence on temperature and the orientation of the spectra of single crystals in the magnetic field, we identified two different radicals formed in irradiated ABO single crystals. To theoretically determine the types of radicals, the most stable structure of ABO was obtained by molecular mechanic and B3LYP/6-31G(d,p) calculations. Four possible radicals were modeled and EPR parameters were calculated for the modeled radicals using the B3LYP method and the TZVP basis set. Calculated values of two modeled radicals were in strong agreement with experimental EPR parameters determined from the spectra. Additional simulated spectra of the modeled radicals, where calculated hyperfine coupling constants were used as starting points for simulations, were well matched with experimental spectra.

Time-dependent density functional theory study on the hydrogen bonding in electronic excited states of 6-amino-3-((thiophen-2-yl) methylene)-phthalide in methanol solution

In this work, we have applied the time-dependent density functional theory (TDDFT) method to investigate the excited-state hydrogen bonding dynamics of 6-amino-3-((thiophen-2-yl) methylene)-phthalide (6-ATMPH) in methanol (MeOH) solution. In the hydrogen-bonded complex, the intermolecular hydrogen bond (C O⋯H O) can be formed between the 6-ATMPH and the MeOH molecules. The 6-ATMPH monomer and hydrogen-bonded dimer can be photoexcited initially to the S 1 state. We calculated the geometric structures and energies of the hydrogen-bonded complex and the isolated 6-ATMPH in different electronic states at the level of B3LYP with the TZVP basis sets. We found that the bond lengths of the C O and O H groups increased after formation of the intermolecular hydrogen bond in the ground state. Furthermore, the calculated hydrogen bond binding energy increased to 31.5 kJ/mol in the electronically excited state from 23.6 kJ/mol in the ground state. These results clearly indicate the intermolecular hydrogen bond of the hydrogen-bonded complex is strengthened in the electronically excited state. The hydrogen bond of the hydrogen-bonded 6-ATMPH–MeOH complex strengthening in the electronically excited state was also confirmed by theoretically monitoring the spectra shift of the stretching vibrational modes of the C O and O H groups. Our theoretical study results have clarified the dispute regarding the intermolecular hydrogen bond cleavage or strengthening in the electronically excited state.

Theoretical study of donor-acceptor ability of borazine, alumazine, and boraphosphinine

Donor-acceptor complexes of borazine, alumazine, and boraphosphinine were studied by a quantum-chemical method. Structural and thermodynamic characteristics of complexes with Lewis acids (BCl3 and AlCl3) and bases (NH3 and pyridine Py) were calculated by the B3LYP method with the TZVP basis set. Energies of donor-acceptor bonds and energies of reorganization of donors, acceptors, and heterocycles upon the complex formation were found. Analysis of the energy variations occurring at the complex formation has shown that the reorganization energies of acceptors (BCl3 and AlCl3) and heterocycles play a key role in the complex stabilizations, whereas the reorganization energies of donors (NH3 and Py) are small and do not bring essential contribution to the complex-formation energy. The stability of donor-acceptor complexes decreases in the sequence alumazine > boraphosphinine > borazine. High alumazine reactivity toward chlorine atoms of the acceptor molecules BCl3 and AlCl3 was noted.

Spectroscopic characterization of aminoacetonitrile, its ions and protonated aminoacetonitrile using quantum chemical methods

We present theoretical vibrational and absorption spectra of aminoacetonitrile, its cation, anion, cyanoprotonated, and aminoprotonated aminoacetonitrile. We used second-order Moller–Plesset perturbation method (MP2) with TZVP basis set to obtain ground state geometries and vibrational spectra. Time dependent density functional theory method was used to obtain absorption spectra. Shifts in vibrational modes for aminoacetonitrile upon ionization and protonation are determined. The C≡N stretching mode which is the most important mode in detection of nitriles in space is more intense in aminoacetonitrile ions and its two protonated form and is less IR active for neutral aminoacetonitrile. The nature of electronic transition for these molecules is identified. All the electronic transitions for neutral aminoacetonitrile and its cation are the σ → σ* electronic transitions, whereas its anion and protonated aminoacetonitrile display the σ → σ* as well as π → π* transitions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Effect of Ionization on Infrared and Electronic Absorption Spectra of Methyl and Ethyl Formate in the Gas Phase and in Astrophysical H2O Ice: A Computational Study

This work reports infrared and electronic absorption spectra of trans and gauche conformers of neutral ethyl formate, trans and cis conformers of neutral methyl formate, their ions in the gas phase, and neutral ethyl and methyl formate in astrophysical H(2)O ice. The second-order Møller-Plesset perturbation (MP2) method with TZVP basis set has been used to obtain ground-state geometries. An influence of ice on vibrational frequencies of neutral ethyl and methyl formate was obtained using integral equation formalism polarizable continnum model (IEFPCM). Significant shift in vibrational frequencies for neutral methyl and ethyl formate when studied in H(2)O ice and upon ionization is observed. Rotational and distortion constants for neutral ethyl and methyl formate from this work are in excellent agreement with the available experimental values. Electronic absorption spectra of conformers of ethyl and methyl formate and their ions are obtained using time-dependent density functional method (TDDFT). The nature of electronic transitions is also identified. We suggested lines especially good to detect these molecules in interstellar medium. Using these lines, we can identify the conformers of ethyl and methyl formate in gas phase and H(2)O ice in interstellar medium. This comparative study should provide useful guidelines to detect conformers of ethyl and methyl formate and their ions in gas phase and neutral molecules in H(2)O ice in different astronomical environment.

Vibronic Spectra of Jet-Cooled 2-Aminopurine·H2O Clusters Studied by UV Resonant Two-Photon Ionization Spectroscopy and Quantum Chemical Calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine·H(2)O monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift δν of the S(1) ← S(0) transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm(-1)), while that of isomer B is much larger (δν = -889 cm(-1)). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H(2)O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy D(e) = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest (1)ππ* states of A and B in excellent agreement with the observed 0(0)(0) bands; also, the relative intensities of the A and B origin bands agree well with the calculated S(0) state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited (1)ππ* state to the lower-lying (1)nπ* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the (1)ππ* state of B is planar and decoupled from the (1)nπ* state. These observations agree with the calculations, which predict the (1)nπ* above the (1)ππ* state for isomer B but below the (1)ππ* for both 9H-2AP and isomer A.