Study of 7-azaindole in its first four singlet states

The molecular structure and properties of 7-azaindole in its first four singlet states were studied with a view to improving current understanding of the photophysical behavior of its C(2h) dimer. This dimer, which exhibits a double proton transfer via its two hydrogen bonds upon electronic excitation, has for 35 years been used as a model for the photophysical behavior of DNA base pairs. Electronic excitation of 7-azaindole simultaneously increases its acidity and basicity; these changes facilitate a concerted mechanism for the double proton transfer in the dimer. In this work, we found the acidity and basicity changes to occur only in its first pi,pi(*) excited singlet state.     

Electronically excited states of water clusters of 7-azaindole: structures, relative energies, and electronic nature of the excited states

The geometries of 1H-7-azaindole and the 1H-7-azaindole(H(2)O)(1-2) complexes and the respective 7H tautomers in their ground and two lowest electronically excited pi-pi(*) singlet states have been optimized by using the second-order approximated coupled cluster model within the resolution-of-the-identity approximation. Based on these optimized structures, adiabatic excitation spectra were computed by using the combined density functional theory/multireference configuration interaction method. Special attention was paid to comparison of the orientation of transition dipole moments and excited state permanent dipole moments, which can be determined accurately with rotationally resolved electronic Stark spectroscopy. The electronic nature of the lowest excited state is shown to change from L(b) to L(a) upon water complexation.     

A theoretical and optical spectroscopic study of the mechanism of a tautomeric transformation in the 7-azaindole dimer and the 7-azaindole complex with a water molecule

Electronic, vibrational, and electronic vibrational spectra of the 7-azaindole dimer, the 7-azaindole complex with a water molecule, and their tautomers are calculated. Transition states are considered based on the analysis of frequencies and shapes of low-frequency vibrations and the Mulliken charge redistribution. The performed quantum chemical calculation of chemical reactions enabled the determination of the structure of transition states and proton transfer conditions. It is shown that in the 7-AzI dimer the proton transfer has a character consistent with the formation of a zwitterionic form. The structure of excited states is calculated and the fluorescence spectra of the first electronic transitions that can be used as a criterion of the formation of 7-AzI tautomers as a result of chemical reactions proceeding through a proton transfer in the 7-azaindole dimer and the 7-azaindole complex with a water molecule, are interpreted.     

Excited state proton transfer in 3-methyl-7-azaindole dimer. Symmetry control

The concerted double proton transfer undergone by the C(2)(h) dimer of 7-azaindole upon electronic excitation has also been reported to occur in 3-methyl-7-azaindole monocrystals and in dimers of this compound under free-jet conditions. However, the results obtained in this work for the 3-methyl-7-azaindole dimer formed in a 10(-4) M solution of the compound in 2-methylbutane suggest that the dimer produces no fluorescent signal consistent with a double proton transfer in the liquid phase or in a matrix. In this paper, the spectroscopic behavior of the doubly hydrogen bonded dimer of 3-methyl-7-azaindole is shown to provide a prominent example of molecular symmetry control over the spectroscopy of a substance. This interpretation opens up a new, interesting research avenue for exploring the ability of molecular symmetry to switch between proton-transfer mechanisms. It should be noted that symmetry changes in the 3-methyl-7-azaindole dimer are caused by an out-of-phase internal rotation of the two methyl groups.     

On the ordering of the first two excited electronic states in all-trans linear polyenes

Reported experimental evidence of the relative position of the first two excited electronic states in linear polyenes was carefully examined and compared with that derived from time dependent density functional theory (TDDFT) theoretical calculations performed at the B3LYP level on optimized geometries. The energy values for the first two triplet states 3Bu and 3Ag, obtained from TDDFT calculations, were found to be highly strongly correlated with the experimental values. Also, the theoretical calculations for the electronic transition 1 1Ag --> 1 1Bu were also extremely well correlated with their experimental counterparts; even more important, the three reported experimental data for 1 1Ag --> 2 1Ag transitions in these systems conformed to the correlation for the TDDFT 1 1Ag --> 1 1Bu transition. The first excited electronic state in the linear polyenes studied (from ethene to the compound consisting of 40 ethene units, P40) was found to be 1Bu. The energy gap between the excited states 2 1Ag and 1 1Bu decreased with increasing length of the polyene chain, but not to the extent required to cause inversion, at least up to P40. In the all-trans linear polyenes studied, the widely analyzed energy gap from the ground electronic state to the first excited singlet state for infinitely long chains may be meaningless as, even in P40, it is uncertain whether the ground electronic state continues to be a singlet.     

Charge-transfer pipi* excited state in the 7-azaindole dimer. A hybrid configuration interactions singles/time-dependent density functional theory description

The hybrid configuration interaction singles/time dependent density functional theory approach of Dreuw and Head-Gordon [Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007] has been applied to study the potential energy landscape and accessibility of the charge-transfer pipi* excited state in the dimer of 7-azaindole, which has been traditionally considered a model for DNA base pairing. It is found that the charge-transfer pipi* excited state preferentially stabilizes the product of a single proton transfer. In this situation, the crossing between this state and the photoactive electronic state of the dimer is accessible. It is found that the charge-transfer pipi* excited state has a very steep potential energy profile with respect to any single proton-transfer coordinate and, in contrast, an extremely flat potential energy profile with respect to the stretch of the single proton-transfer complex. This is predicted to bring about a pair of rare fragments of the 7-azaindole dimer, physically separated and hence having very long lifetimes. This could have implications in the DNA base pairs of which the system is an analogue, in the form of replication errors.     

First principles investigation of the electronic structure of the iron carbide cation, FeC+

We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.     

Theoretical studies on the photochemical reaction mechanisms of o-xylylene. Effects of phenyl group for electrocyclic reaction mechanism

The potential energy surfaces (PESs) of the electrocyclic reactions of o-xylylene at the ground and the lowest excited states are calculated by CASSCF molecular orbital and MRMP2 methods. The lowest excited state geometry of o-xylylene has C(2v) symmetry and is about 65 kcal mol(-1) in energy above the ground state. The PESs in the vicinity of the conical intersection are different from those of the electrocyclic reaction of cis-butadiene. In the vicinity of the conical intersection, the transition state at the ground state relating to methylene-cycloheptadienyl carbene is located. The transition state is only 4.3 kcal mol(-1) lower in energy than the conical intersection at the CASSCF(10,10)/6-31G(d) level and 0.5 kcal mol(-1) lower at the MRMP2/6-311+G(d,p) level. The transition state corresponding to benzocyclobutene does not locate in the vicinity of the conical intersection because of the resonance energy between benzene ring and methylene group.     

Theoretical investigation on the electronic and geometric structure of GaN2+ and GaN4+

The electronic and geometric structures of gallium dinitride cation, GaN2+ and gallium tetranitride cation, GaN4+ were systematically studied by employing density functional theory (DFT-B3LYP) and perturbation theory (MP2, MP4) in conjunction with large basis sets, (aug-)cc-pVxZ, x = T, Q. A total of 7 structures for GaN2+ and 24 for GaN4+ were identified, corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces, and bonding mechanisms for some low-lying electronic states. The calculated dissociation energy (De) of the ground state of GaN2+, X1Sigma+, is 5.6 kcal/mol with respect to Ga+(1S) + N2(X1Sigmag+) and that of the excited state, ?3Pi, is 24.8 kcal/mol with respect to Ga+(3P) + N2(X1Sigmag+). The ground state and the first excited minimum of GaN4+ are of 1A1(C2v) and 3B1(C2v) symmetry with corresponding De of 11.0 and 43.7 kcal/mol with respect to Ga+(1S) + 2N2(X1Sigmag+) for X1A1 and Ga+(3P) + 2N2(X1Sigmag+) for 3B1.     

7—氮吲哚二体激发态双质子转移的量子化学研究

用AMl和INDO/CI方法研究了7-氮吲哚二体激发态双质子转移反应的位能面和机理,异构二体虽存在较强的分子内氢键,但基态时正常二体的能量仍比异构二体低,光照时正常二体可通过激发态质子转移变为异构二体,这是其荧光产生反常Stokes位移的原因。     

Excited state tautomerization of azaindole

Fluorescent tryptophan analogs, like azatryptophan, offer an advantage for exploring protein and peptide structure and dynamics. The chromophoric moieties, azaindole, of the azatryptophan analogs are investigated for their potential as fluorescent probes. The photophysical properties of 4-azaindole (4AI) and 5-azaindole (5AI) and their tautomers are characterized through computational and experimental methods. Both 4AI and 5AI undergo excited state tautomerization in the presence of 1 M NaOH. The protonated forms of 4AI and 5AI have a fluorescence emission of 415 and 410 nm, respectively, while the tautomers of 4AI and 5AI have a fluorescent emission of 480 and 450 nm, respectively. Gas phase computations (B3LYP/6-31+G**) show that the N1H azaindole tautomer is lower in energy in the ground state by as much as 12.5 kcal mol(-1), while the N(n)H azaindole tautomer is lower in energy in the excited state by as much as 18.1 kcal mol(-1). Solvent effects on the tautomer energy differences were computed using the isodensity polarized continuum model (IPCM). The polarity of the solvent helps to reduce the energy difference between the tautomers in the ground state by as much as 5.8 kcal mol(-1), but not enough to reverse the ground state tautomer preference.     

Proton transfer of excited 7-azaindole in reverse-micellar methanol nanopools: even faster than in bulk methanol

The methanol-catalyzed double-proton transfer of photoexcited 7-azaindole in the free cores of solvation-restricted reverse micelles takes place on the time scale of 90 ps, even shorter than in bulk methanol. This anomalous rate increase with a large kinetic isotope effect of 5 experimentally proves the widely discussed two-step model for the overall reaction of solvent-mediated proton transfer. On the other hand, the molecules in the bound layers and in the headgroup layers relax in 900 and 6000 ps, respectively, without going through proton transfer. The tautomerization and the relaxation of excited 7-azaindole can be exploited to probe the nanopools of methanol reverse micelles.     

Excited-state double-proton transfer in a model DNA base pair: Resolution for stepwise and concerted mechanism controversy in the 7-azaindole dimer revealed by frequency- and time-resolved spectroscopy

The excited-state double-proton transfer (ESDPT) reaction in the dual hydrogen-bonded 7-azaindole dimer (7AI₂) in a supersonic jet expansion has been extensively studied with various laser spectroscopic methods and quantum chemistry calculations by many groups. This article reviews the results and discussions associated with stepwise and concerted mechanism controversy on ESDPT of 7AI₂ together with the excited-state dynamics associated with ESDPT.     

Electronic structure of the benzene dimer cation

The benzene and benzene dimer cations are studied using the equation-of-motion coupled-cluster model with single and double substitutions for ionized systems. The ten lowest electronic states of the dimer at t-shaped, sandwich, and displaced sandwich configurations are described and cataloged based on the character of the constituent fragment molecular orbitals. The character of the states, bonding patterns, and important features of the electronic spectrum are explained using qualitative dimer molecular orbital linear combination of fragment molecular orbital framework. Relaxed ground state geometries are obtained for all isomers. Calculations reveal that the lowest energy structure of the cation has a displaced sandwich structure and a binding energy of 20 kcal/mol, while the t-shaped isomer is 6 kcal/mol higher. The calculated electronic spectra agree well with experimental gas phase action spectra and femtosecond transient absorption in liquid benzene. Both sandwich and t-shaped structures feature intense charge resonance bands, whose location is very sensitive to the interfragment distance. Change in the electronic state ordering was observed between sigma and piu states, which correlate to the B and C bands of the monomer, suggesting a reassignment of the local excitation peaks in the gas phase experimental spectrum.     

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4sigma- symmetry featuring three bonds, with a recommended dissociation energy of D0 = 70.0 kcal/mol and r(e) = 1.640 angstroms. The first and second excited states, which also correlate to ground state atoms, are of 6sigma- and 8sigma- symmetry, respectively, and lie 17.7 and 28.2 kcal/mol above the X state at the MRCI level of theory.     

Computational study on the structures of the [H, Si, N, C, O] isomers: possible species of interstellar interest

Ab initio methods have been used to study the lowest lying [H, Si, N, C, O] isomers, which are of astrochemical interest. Over 20 [H, Si, N, C, O] isomers in the 1A' electronic state have been investigated at the MP2/aug-cc-pVTZ level of theory. Of these, the seven lowest isomers have been further investigated using different levels of theory, including B3LYP and QCISD(T). It has been found that the relative energies of the isomers in their ground electronic state (1A') are very dependent on the level of theory used with either the cis-HOSiCN or cis-HOSiNC isomers being the lowest in energy. Overall, the four lowest isomers are within 6 kcal/mol of each other, and a further three isomers are less than 15 kcal/mol higher in energy than the lowest lying isomer, including HSiNCO, which has recently been detected spectroscopically. Natural bond analysis has been carried out on the ground electronic states of the seven lowest lying isomers to examine their electronic structure. The enthalpies of formation of the seven lowest isomers have also been evaluated using the G3MP2 and G3B3 multilevel methods and show that the isomers are relatively thermodynamically stable. The structures and energies of lowest lying 1A' and 3A' electronic states of these isomers have also been investigated and show that for most of the isomers the optimized structures in these excited electronic states correspond to a transition state structure.     

A theoretical study of the ground and first excited singlet state proton transfer reaction in isolated 7-azaindole–water complexes

A systematic study of the proton transfer in the 7-azaindole–water clusters (7-AI(H₂O)_n; n=1–4) in both the ground and first excited singlet electronic states is undertaken. DFT(B3LYP) calculations for the ground electronic state shows that the more stable geometry of the initial normal tautomer presents a cyclic set of hydrogen bonds that links the two nitrogen atoms of the base across the waters. For the n=4 cluster the water molecules adopt a double ring structure so that two cycles of hydrogen bonds are found there. From this structure full tautomerization implies only one transition state so that a concerted but non-synchronous process is predicted by our theoretical calculations. This behavior is found both in the ground and the excited states where CIS geometry optimizations and TD(B3LYP) energy calculations are performed. The difference between both states is the height of the energy barrier that is much lower in the excited state. Another clear difference between both electronic states is that full tautomerization is an endergonic process in the ground state whereas it is clearly exergonic (then favorable) in the excited state. This is so because electronic excitation implies a charge transfer from the five-member cycle to the six-member one of 7-azaindole so that the proton transfer from the pyrrolic side to the pyridinic one is favored. These results clearly indicate that full tautomerization will not likely occur in the ground state but it will be quite easy (and fast) in the excited state. Reaction is already feasible in the S₁ 1:1 complex but it is faster in the 1:2 complex. However the reaction slows again for the 1:3 complex and, finally, reaches a new maximum for the largest cluster studied here, the n=4 case. These results, which are in agreement with experimental data, are explained in terms of the number of hydrogen bonds that are involved in the transfer. The proton transfer through a ring formed by the substrate and two water molecules is found to be the more efficient one, at least in this system.     

The electronic structure of vanadium carbide, VC

Within an energy range of 2.4 eV, we have explored 29 of the 36 states of the diatomic molecule VC that arise from the atoms in their ground state, V(4s23d3;4F)+C(2s2 2p2;3P). We use multireference methods with large atomic natural orbital basis sets. The ground state is of 2Delta symmetry with the first two excited states, 4Delta and 2Sigma+, located 4.2 and 7.0 kcal/mol above the X state. All the states examined in this work are relatively strongly bound and show significant charge transfer from V to C. The binding energy of the X 2Delta state is estimated to be 95.3 kcal/mol in good agreement with the experimental value.     

Coupled cluster investigation on the low-lying electronic states of CuCN and CuNC and the ground state barrier to isomerization

The observation of several metal cyanides and isocyanides in interstellar space has raised much interest these molecules. Optimum molecular structures, harmonic vibrational frequencies, and dipole moments of the ground electronic states (X1Sigma+), triplet excited states, and open shell singlet excited states of CuCN and CuNC were determined using different levels of nonrelativistic and scalar relativistic (Douglas-Kroll) [Ann. Phys. 82, 89 (1979)] coupled cluster theory in conjunction with atomic natural orbital basis sets and correlation consistent basis sets. For the relativistic computations the specially contracted correlation consistent Douglas-Kroll (DK) basis sets were used. Moreover, barriers to isomerization from CuCN to CuNC were computed. The predicted structures of the X1Sigma+ state for CuCN are re(Cu-C)=1.826 A and re(C-N)=1.167 A, at the most sophisticated level of theory, the scalar relativistic DK-CCSD(T)/cc-pVQZ(DK) method. These results are in excellent agreement with the experimentally determined Cu-C bond length of 1.829 A and C-N bond distance of 1.162 A. At the same level of theory, the zero-point corrected barrier to isomerization from CuCN to CuNC is estimated to be 14.7 kcal mol(-1), and the cyanide is more stable than the isocyanide by 11.5 kcal mol(-1). For both CuCN and CuNC the 3Sigma+ state is the lowest lying excited electronic state. At the DK-CCSD/cc-pVQZ(DK) level of theory, the energetic ordering of excited states of CuCN and CuNC is X1Sigma+相似文献   

Application of equation-of-motion coupled-cluster methods to low-lying singlet and triplet electronic states of HBO and BOH

The equilibrium structures and physical properties of the X (1)sigma(+) linear electronic states, linear excited singlet and triplet electronic states of hydroboron monoxide (HBO) (A (1)sigma(-), B (1)delta, a (3)sigma(+), and b (3)delta) and boron hydroxide (BOH) (A (1)sigma(+), B (1)Pi, and b (3)Pi), and their bent counterparts (HBO a (3)A('), b (3)A("), A (1)A("), B (1)A(') and BOH X (1)A('), b (3)A('), c (3)A("), A (1)A('), B (1)A('), C (1)A(")) are investigated using excited electronic state ab initio equation-of-motion coupled-cluster (EOM-CC) methods. A new implementation of open-shell EOM-CC including iterative partial triple excitations (EOM-CC3) was tested. Coupled-cluster wave functions with single and double excitations (CCSD), single, double, and iterative partial triple excitations (CC3), and single, double, and full triple excitations (CCSDT) are employed with the correlation-consistent quadruple and quintuple zeta basis sets. The linear HBO X (1)sigma(+) state is predicted to lie 48.3 kcal mol(-1) (2.09 eV) lower in energy than the BOH X (1)sigma(+) linear stationary point at the CCSDT level of theory. The CCSDT BOH barrier to linearity is predicted to lie 3.7 kcal mol(-1) (0.16 eV). With a harmonic zero-point vibrational energy correction, the HBO X (1)sigma(+)-BOH X (1)A(') energy difference is 45.2 kcal mol(-1) (1.96 eV). The lowest triplet excited electronic state of HBO, a (3)A('), has a predicted excitation energy (T(e)) of 115 kcal mol(-1) (4.97 eV) from the HBO ground state minimum, while the lowest-bound BOH excited electronic state, b (3)A('), has a T(e) of 70.2 kcal mol(-1) (3.04 eV) with respect to BOH X (1)A('). The T(e) values predicted for the lowest singlet excited states are A (1)A(")<--X (1)sigma(+)=139 kcal mol(-1) (6.01 eV) for HBO and A (1)A(')<--X (1)A(')=102 kcal mol(-1) (4.42 eV) for BOH. Also for BOH, the triplet vertical transition energies are b (3)A(')<--X (1)A(')=71.4 kcal mol(-1) (3.10 eV) and c (3)A(")<--X (1)A(')=87.2 kcal mol(-1) (3.78 eV).