Phospholes with Reduced Pyramidal Character from Steric Crowding. 1. Synthesis and NMR Characterization of 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole

The sterically crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was synthesized by dehydrohalogenation of the corresponding 3,4-dibromophospholane, in order to probe the possibility that the steric congestion would cause some flattening of the phosphorus pyramid and an increase in electron delocalization. The phosphole was a recrystallizable solid with (31)P NMR delta 1.8. Semiempirical calculations indicated that the pyramidal shape was retained but was noticeably flatter than in 1-phenylphosphole. In the low energy conformation, the phosphole and phenyl ring planes are approximately orthogonal, with the 2-tert-butyl group in the less crowded position that is syn to the lone pair on phosphorus. The 6-methyl group is positioned under the phosphole ring. This conformational prediction was amply confirmed by several chemical shift and coupling effects in the (13)C NMR spectrum. The (1)H NMR spectrum displayed an unusually large four-bond coupling (6 Hz) of (31)P to the m-phenyl proton syn to the lone pair (and none to the anti-meta proton), consistent with the orthogonal conformation. The oxide of the phosphole showed more stability than that of less crowded phospholes and gave a (31)P NMR signal that was detectable over a several hour period at room temperature. The oxide proceeded to give the usual Diels-Alder dimer and also formed a cycloadduct with N-phenylmaleimide. The phosphoryl group of the latter was reduced with trichlorosilane to give the phosphine. This new 7-phosphanorbornene derivative gave the most downfield (31)P NMR shift (delta 153.3) of any member of this family, all of which are characterized by remarkable deshielding in the syn isomer.     

SYNTHESIS AND STEREOCHEMISTRY OF PENTACARBONYL(PHOSPHOLE)METAL(0)- AND TETRACARBONYLBIS(PHOSPHOLE)METAL(0) COMPLEXES OF 6B ELEMENTS

Abstract

Pentacarbonylphospholemetal(0) and cis-tetracarbonylbis(phosphole)metal(0) complexes were synthesized from the thermal reaction of M(CO)₃(THF) and M(CO)₄(COD) (M: Cr, Mo, W) with corresponding phosphole (1-phenyl-3,4-dimethylphosphole, 1-phenyl-3-methylphosphole, and 1-phenylphosphole). These complexes were isolated as orange crystals by column chromatography on silicagel at 253 K and crystallization from n-hexane at 223 K and characterized by means of IR and NMR (¹H, ¹³C, and ³¹P). Spectroscopic data shows that the phosphole is coordinated to the transition metal through its phosphorus atom rather than through the conjugated diene unit in the both types of complexes. The tetracarbonylbis(phosphole)metal(0) complexes were found to have cis-arrangement of two phosphole ligands. Comparing ¹³C-NMR chemical shifts of the complexes with the free ligands, one can deduce that the involvement of the phosphorus atom in the ring π-electron delocalization is drastically reduced upon coordination. This is attributed to the stronger [sgrave]-donation but weaker π-accepting ability of the phosphorus atom in the phosphole ligands compared to the carbonyl groups.     

Theoretical study of the structure-property relationship in phosphole monomers

Use of the DFT method with the B3LYP functional offers an efficient way for determining geometries of phosphole structures. It also provides inversion barriers in good agreement with experiment. The molecular orbital MP2 method is better suited to calculate the relative stability of the considered isomers. Combination of geometric (Julg index) and magnetic (NICS) criteria leads to an interesting approach to analyze the pi-electron delocalization and/or aromaticity in conjugated heterocyclic systems and can be used for investigation of much larger oligomers. It thus allows us to confirm that planarizing the phosphorus atom in phosphole monomers leads to a greater conjugation over the butadienic pi-system and that substituents, characterized by a strongly pronounced pi-system, exhibit a large extent of conjugation with the pi-diene moiety of the heterocyclic system. In push-pull systems such as 2-BH2-5-NH2-1H-phosphole, the electron delocalization along the pi-diene system is even more pronounced than in phospholes with strongly pronounced pi-systems. The employed approach is also used to analyze the relationship between electron conjugation and the phosphorus inversion barrier. More relevant for chemical applications is the result that the computational study provides a method of fine-tuning of phosphole building blocks, which may enable us to obtain higher pi-conjugation along the polymer backbone.     

On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+* are formed for A = Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+*(S)1,2 are formed for A = Arg, His, Met, Tyr, and Trp. The stability of the hydrogen-deficient AH2+* in the "microsolution" depends on the energetics of the electron transfer reaction AH2+* +S --> AH++S+*, the hydrogen abstraction reaction AH2+*+S --> AH2(2+)+[S-H]*, and the proton transfer reaction AH2+* + S --> A+*+SH+. Using B3LYP/ 6-311+G(2d,p)//B3LYP/6-31+G(d) model chemistry, we describe these three reactions in detail for A=Tyr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr+* and SH+ from TyrH2+*(S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 micros. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1+/-2.1 eV ([IE(CH3CN)+IE(Tyr)]/ 2); hydration further lowers the IE by 0.3 eV [IE(TyrH+(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH+, HisH+, and MetH+ to be 10.1+/-2.1 eV, 12.4+/-0.2 eV, and 12.4+/-0.2 eV, and that of PheH+ to be larger than 12.6 eV.     

Persistent oxidation dications from twisted fluoranthenes, benzo[k]fluoranthene and dimethyldibenzo[j.l]fluoranthene: charge delocalization mode, tropicity, and formation of novel 8,8'-bifluoranthenyls. An NMR and theoretical study.

First examples of persistent oxidation dications from fluoranthene-PAHs namely 1,3,4,6,7,10-hexamethyl- 2 and 3,4-dichloro-1,6,7,10-tetramethylfluoranthene 3, benzo[k]fluoranthene 6, and 3,6-dimethyldibenzo[j,l]fluoranthene 9 are reported. Charge delocalization mode and tropicity in the resulting nonalternant dications are examined. Quenching of the superacid solutions of the dications resulted in the formation of novel 8,8'-bifluoranthenyls 2a-4a. AM1 was used as an initial guide for dication generation (DeltaDeltaH(f) degrees and ionization potentials) and for probing the structures of the crowded fluoranthene substrates and their bifluoranthenyls. In selected cases, the dications and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level. Charge delocalization mode (difference in NPA charges) and DFT/GIAO-derived NMR chemical shifts were obtained for comparison with experiment; aromaticity was assessed via nucleus independent chemical shift (NICS) calculations.     

Molecular structure of 6-(N,N-dimethylamino)fulvene from a joint gas-phase electron diffraction and quantum-chemical study

The molecular structure of 6-(N,N-dimethylamino)fulvene was studied by gas-phase electron diffraction and quantum-chemical methods (HF/6-31G(d), MP2/3Z, MP2/4Z, density functional theory with the B3LYP/6-31G(d) and PBE/3Z functionals). Pronounced flattening of the nitrogen atom and equalization of the intracyclic C—C bonds were found to be a consequence of the electron delocalization in the molecule.     

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.     

NH0-1+2-3离解能等的高级ab initio计算与评价

A recent experimental determination[1] of the dissociation energies (D0) for H2N-H, H2N+-H and H2N-H+, the ionization energies for NH3 and NH2 resulted in large deviations when compared with those of the earlier values and the QCISD(T)/6-311+G(3df,2p) ab initio calculations. We have performed some higher level ab initio calculations on these data by utilizing the Gaussian 92/DFT and Gaussian 94 pakages of programs and have assessed the available experimental values. Our calculations were carried out at the QCISD (TQ)/aug-cc-pVDZ, G2(QCI), QCISD(T)/6-311 ++G(3df,3pd) and QCISD(T)/aug-cc-pVTZ levels of theory. Geometries were optimized at both of the MP2(full)/6-31G(d) and the MP2(full)/6-31(d,p) levels, and were compared with those of the experiments if available. The MP2(full)/6-31G(d,p) tight-optimized geometries for the neutrals are closer to those of the experiments than those of the MP2 (full)/6-31G(d), and are in excellent agreement with the experimental results as shown in Table 1. In this case, we assumed that the optimized geometries for the cations would be better if p polarization functions are added to the hydrogen atoms. We firstly noted that the symmetry of the NH3+ cation was D3h, other than Cs. as reported in ref.[1]. All of the zero-point energies and the final geometries are calculated at the MP2(full)/6-31G(d,p) level of theory. We have also repeated the QCISD(T )/6-311 + G(3df,2p) calculations of ref. [1], because we could not identify their level of goemetry optimization. It is found that the total energy, -55.244 19 Hartrees, for NH2+(1A1 ) in ref.[1] might be in error. Our result is -55.336 29 Hartrees at the same level of theory. At our highest level [QCISD(T)/aug-cc-pVTZ] of calculations as shown in Table 3, the D0 (temperature at zero Kelvin) values of H2N-H, H2N+-H(3B1for NH2+ ) and H2N- H+ are 4.51, 5.49 and 8.00 eV, respectively. These data reported in re f.[1] were 4.97, 5.59 and 8.41 eV, respectively. Our result on D0(H2N-H) supports the work of ref.[2,3,5,6]. The ionization energies (IE) for NH3 and NH2 (3B1 for NH2+) at our highest level are 10.11 and 11.09 eV while in ref.[1] were 10.16 and 10.78 eV, respectively. For the latter, our result supports the experiment of ref.[3]. Our predicted D0 for HN2+-H and IE for NH2 (1A1 for each NH2+) are 6.80 and 12.39 eV, respectively. These values differ greatly from the predicted values (9.29 and 14.88 eV) of ref.[1] where the total energy of NH2+(1A1) might be in error. The D0 value for HN-H has not been found in ref.[1]. Our result supports the work of ref.[3]. We have also derived all of these values at the temperature of 298K and under the pressure of 101kPa at several levels of thoery as shown in Table 3. On examining the experiment of ref.[1] in detail, it is easy to find that all of the larger deviations might be from a too high value of the appearance potential of proton AP(H+). Indeed, ref.[1] has mentioned that the determintion of AP(H+), due to kinetic shift, would lead to a hihger value for the dissociation energy as has been pointed out by Berkowitz and Ruscic. In this work, we concluded that, besides some mistakes in the theoretical calculations of ref.[1], the dissociation energies for H2N-H and H2N-H+,the IE for NH2 (3B1 for NH2+) might also be unreliable and need to be re-examined.
     

NH_(2-3)~(0-1 )离解能等的高级ab initio计算与评价

FeiQi等人[1]最近对H2N－H、H2N －H的离解能以及NH3、NH2的电离能，用真空紫外光电离实验进行了重新测定，并同时得到了H2N－H 的离解能．他们还在QCISD（T）／6－311＋G（3df，ZP）／／MPZ（full）／6刁IG（d）水平（GZ理论的参考水平）上对这些数据及其它相关分子的某些性质     

Spectroscopic analysis of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitro-phenyl) acetonitrile

The optimized geometry and vibrational frequencies of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitro-phenyl) acetonitrile (DOPNA) were obtained by ab initio DFT/B3LYP level with complete relaxation in the potential energy surface using 6-31G and 6-311G basis sets. The Fourier-transform infrared (FT-IR) spectrum of DOPNA has been recorded in the region 4000-400 cm(-1). The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR spectrum. The calculated frequencies are in comparable agreement with the experimental frequencies. The calculated energy span between the HOMO and the LUMO of DOPNA is 2.94 and 2.87eV by B3LYP/6-31G and B3LYP/6-311G, respectively.     

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.     

氰基苯阴离子与CO2间的内球电子转移

用半经验AM1方法以及从头算方法在3-21G和6－31G*基组水平上研究了从氰基苯阴离子到CO2的电子转移过程．结果表明，对于先驱物(precursor),三种计算方法得出的给体、受体间的距离分别为0.2728nm(AM1)、0.2479nm（UHF/3-21G）和0.2769nm(UHF/6-31G*)．在这样短的距离内给体的HOMO与受体的LUMO轨道具有相当程度的重叠，应产生较强的相互作用，说明此反应是内球电子转移反应，从而解释了此类体系的电子转移反应不符合Marcus理论的原因．计算给出先驱物的束缚能为0.19eV(AM1)和0.26eV(6-31G*)     

Cyclopropyl- and Allyl-substituted Arenes in Reaction with Dinitrogen Tetroxide. Effect of Substrate Oxidation Potential on Reaction Direction

A correlation was found between oxidation potentials of acylcyclopropanes in solution (in CH₂Cl₂ and CH₃CN) and their HOMO energies calculated by semiempirical (AM1) and nonempirical (HF/6-31G and HF/6-31G**) methods. The correlation provides a possibility to forecast the reaction direction of the mentioned substrates and N₂O₄. The correlation possesses a general character. It was established for instance that arylcyclo-propanes, cyclopropylmethyl- and allylbenzenes oxidized at more positive potentials than reduction potential of NO⁺ and having more positive e_HOMOthan 9.0 eV (AM1), 8.4 eV (HF/6-31G), and 8.3 eV (HF/6-31 G**) reacted with N₂O₄ following the mechanism "electron transfer radical pair recombination" affording nitroaromatic derivatives retaining the cyclopropane (or allyl) fragments. Substrates of the same type where the electron transfer to NO⁺ should be endothermic process and whose HOMO values are less than the above critical numbers react with N₂O₄ by the mechanism of electrophilic cyclopropane ring opening (with aryl and benzylcyclopropanes) or by electrophilic addition across the double C=C bond (with allylbenzenes).     

DFT能量分解和氮苄叉基苯胺分子π电子离域能的研究

为了探索密度泛函理论(DFT)方法中氮苄叉基苯胺分子π电子离域的本质, 介绍了将非平面分子氮苄叉基苯胺分子的DFT能量分成π和σ的方法, 并将π和σ电子能量分成单电子能部分: 动能ΔEπK(θ), ΔEσK(θ)和位能ΔEπP(θ), ΔEσP(θ); 双电子相互作用部分: 库仑作用ΔEππJ(θ), ΔEσσJ(θ), ΔEπσJ(θ)和交换相关作用ΔEππXC(θ), ΔEσσXC(Δ)以及ΔEπσXC(θ), 分析了垂直离域能ΔEV的稳定性及π电子离域对π和σ体系的影响. 在B3LYP/6-31G*, 6-311G*, 6-31G(2d), 6-311G(2d)水平下的计算结果表明, 与经典观点不同, π电子的离域是失稳定的, 且平面时失稳定性最强; 分析各个能量分量表明, 在π电子的离域过程中, π和σ体系均对基组较敏感, π体系本身单电子能的影响大于σ体系, π电子离域对双电子部分作用的影响主要体现在π-σ的耦合作用上.     

Ionization energies of the nucleotides

The vertical ionization energies of the four nucleotides have been computed. Geometries have been chosen to mimic orientations as they appear in B-DNA. The negative charge on the phosphate was neutralized by protonation, and also by the inclusion of counterions. Calculations have been performed with electron propagator methods (P3), M?ller-Plesset second-order perturbation theory, and density functional theory to determine the nature of the orbitals associated with the highest lying ionization energies. Calculations at the MP2/6-311G(d,p)//P3/6-311G(d,p) level of theory yield vertical ionization energies for 5'-dTMP 9.05 eV, for 5'-dCMP 8.40 eV, for 5'-dAMP 8.16 eV and for 5'-dGMP 7.96 eV. In all cases the highest occupied molecular orbital resides on the base moieties.     

Environmental photochemistry of nitro-PAHs: direct observation of ultrafast intersystem crossing in 1-nitropyrene

Femtosecond broadband transient absorption experiments of 1-nitropyrene, a nitro-polycyclic aromatic hydrocarbon of environmental concern are presented in cyclohexane and hexane solutions. The transient absorption spectra show the presence of three species that are assigned to the Franck-Condon excited lowest singlet (S1) state, the structurally relaxed S1 state, and the lowest excited triplet state. The spectral changes at early times are interpreted in terms of conformational dynamics; primarily due to an ultrafast rotation of the nitro group in the S1 state. This excited state relaxation is followed by intersystem crossing with a time constant of 7 ps. CIS/6-31G(d,p) calculations predict planarization of the nitro-aromatic torsional angle as the major nuclear relaxation coordinate, from 32.8 degrees at the HF/6-31G(d,p) level of theory in the ground state (27.46 degrees at B3LYP/6-31++G(d,p)) to 0.07 degrees in the S1 state. Vertical excitation energies at the TDDFT/6-31++G(d,p) and TDDFT/IEFPCM/6-31++G(d,p) levels of theory predict a small energy gap (<0.12 eV) between the S1(pipi*) state and the third excited triplet state T3(npi*) in the gas phase and in cyclohexane, respectively. The small energy gap suggests a large spin-orbit coupling between the S1(pipi*) and T3(npi*) states, which explains the ultrafast intersystem crossing of 1-nitropyrene in nonpolar solvents.     

Conformational studies in the cyclohexane series. 2. Phenylcyclohexane and 1-methyl-1-phenylcyclohexane

The structures and relative energies of the conformers of phenylcyclohexane, and 1-methyl-1-phenylcyclohexane have been calculated at theoretical levels including HF/6-31G, B3LYP/6-311G, MP2/6-311G, MP2/6-311(2df,p), QCISD/6-311G, and QCISD/6-311G(2df,p). The latter gives conformational enthalpy (DeltaH degrees ), entropy (DeltaS degrees ), and free energy (DeltaG degrees ) values for phenylcyclohexane that are in excellent agreement with the experimental data. The calculations for 1-methyl-1-phenylcyclohexane find a free energy difference of 1.0 kcal/mol at -100 degrees C, favoring the conformation having an axial phenyl group, that is in only modest agreement with the experimental value of 0.32 +/- 0.04 kcal/mol. The origin of the phenyl rotational profiles for the conformers of phenylcyclohexane and 1-methyl-1-phenylcyclohexane is discussed.     

Enhancing the dienic reactivity of phospholes: an improved access to trivalent 7-phosphanorbornenes.

A structural comparison of 1-cyano- and 1-alkoxy-3,4-dimethylphospholes with 1-benzylphosphole has led to some unexpected conclusions. There is no univocal relationship between phosphole aromaticity and pyramidality at phosphorus. It has been found that both the highly pyramidal 1-cyanophosphole 1 (sigma(CPC angles) = 290 degrees), and the much less pyramidal 1-alkoxyphosphole 6 (sigma(CPC angles) = 310 degrees) have a low Bird aromaticity index (27 for both molecules), when compared to 1-benzylphosphole (sigma(CPC angles) = 303 degrees, BI = 35.5). This low aromaticity is correlated with a high reactivity of the diene in both 1 and 7 (similar to 6) toward acrylonitrile. Good stereochemical control is observed with 7, which gives exclusively the anti,endo [4 + 2] cycloadducts with acrylonitrile and diethyl vinylphosphonate.     

Predictions of nitrogen isotropic hyperfine coupling constants in the nitroxide radicals with the aid of DF/HF calculations

Nitrogen isotropic hyperfine coupling constants in different nitroxide radicals calculated via ab initio hybrid density functional/Hartree-Fock methods (UB3LYP and UB1LYP) with 6-31G(d) basis set of Gaussian 98 were found to be in a good agreement with the experimental EPR results. UB3LYP/6-31G(d) and UBLYP/6-31G(d) calculated atomic spin populations and spin density maps in the gas phase correspond to the general features of the experimentally obtained data by polarized neutron diffraction studies in the solid state. The results were analyzed in terms of unpaired electron delocalization and the influence of the surrounding on the radical centers.     

A computational study of the ground and excited state structure and absorption spectra of free-base N-confused porphine and free-base N-confused tetraphenylporphyrin

Computational investigations into the ground and singlet excited-state structures and the experimental ground-state absorption spectra of N-confused tetraphenylporphyrin tautomers 1e and 1i and N-confused porphines (NCP) 2e and 2i have been performed. Structural data for the ground state, performed at the B3LYP/6-31G(d), B3LYP/6-31+G(d)//B3LYP/6-31G(d), and B3LYP/6-311+G(d)//B3LYP/6-31G(d) levels, are consistent with those performed at lower levels of theory. Calculations of the gas-phase, ground-state absorption spectrum are qualitatively consistent with condensed phase experiments for predicting the relative intensities of the Q(0,0) and Soret bands. Inclusion of implicit solvation in the calculations substantially improves the correlation of the energy of the Soret band with experiment for both tautomers (1e, 435 nm predicted, 442 nm observed in DMAc; 1i, 435 nm predicted, 437 nm observed in CH2Cl2). The x- and y-polarized Q-band transitions were qualitatively reproduced for 1e in both the gas phase and with solvation, although the low-energy absorption band in 1i was predicted at substantially higher energy (646 nm in the gas phase and 655 nm with solvation) than observed experimentally (724 nm in CH2Cl2). Franck-Condon state and equilibrated singlet excited-state geometries were calculated for unsubstituted NCP tautomers 2e and 2i at the TD-B3LYP/SVP and TD-B3LYP/TZVP//TD-B3LYP/SVP levels. Electronic difference density plots were calculated from these geometries, thereby indicating the change of electron density in the singlet excited states. Adiabatic S1 and S2 geometries of these compounds were also calculated at the TD-B3LYP/SVP level, and the results indicate that while 2i is a more stable ground-state molecule by approximately 7.0 kcal mol-1, the energy difference for the S1 excited states is only approximately 1.0 kcal mol-1 and is 6.1 kcal mol-1 for the S2 excited states.