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Ground-state geometries of carbenes H2Cn (n = 3-10) have been fully optimized with the C2ν-symmetry constraint at the density functional theory and restricted-spin coupled-cluster single-double plus perturbative triple excitation levels of theory, respectively. Comparison of structures corresponding to the X1A1 and B1B1 electronic states has been made by the complete active space self-consistent field calculations. Parity alternation effect on various properties of the ground-state geometries has been discovered in the present study, which generally gives illustration for the relative stabilities of the titled carbon chains. Further calculations on their electronic spectra have been carried out by means of the complete active space second-order perturbation theory method along with the cc-pVTZ basis set. It is found that the vertical excitation energies of the dipole-allowed B1B1 ← X1A1 transition in the gas phase are 2.28, 4.75, 1.69, 3.66, 1.30, 2.94, 1.12, and 2.49 eV, respectively, which agree very well with the available experimental result for H2C3 (2.27 eV). In addition, the vertical excitation energies for both transitions B1B1 ← X1A1 and A1A2 ← X1A1 are found to obey a nonlinear ΔE-n relationship as a function of chain size by performing curves fitting. [ABSTRACT FROM AUTHOR]

A theoretical study of the linear nitrogen-terminated carbon chains CN ( n = 3-12) is carried out with density functional theories and ab initio methods. All species in the ground state are fully optimized with the B3LYP, CAM-B3LYP, and RCCSD(T) calculations. The present results reveal that the carbon radicals under study possess stable structures with the XΣ ground-state symmetry for CN ( n = 3, 5) or XΠ for the rest members in the series. According to the trends of odd/even alternation in energy differences and incremental binding energies, it is clear that the n-odd CN chains are relatively more stable than n-even ones. Furthermore, the electronic spectra of CN ( n = 3-12) are investigated by means of CASPT2 method. The predicted vertical excitation energies from the ground state to the low-lying excited states basically agree well with the available experimental observations. In addition, by performing curve fittings, the vertical excitation energies of the transitions from the ground states ( XΣ or XΠ) to the low-lying excited state 2Π for the odd members are found to obey a nonlinear Δ E- n relationship as a function of chain size, matching the experimental findings well. Moreover, the strongest absorption band for the even series shows a nonlinear trend of biexponential decay. [ABSTRACT FROM AUTHOR]

The ground-state equilibrium geometries of the linear carbon chain cations NCN ( n = 1-7) have been investigated with B3LYP, CAM-B3LYP, and RCCSD(T) calculations. The ground state (XП) and excited state (1П) have been optimized by using the complete active space self-consistent field method. The present study reveals that these linear cations generally have the characteristic of bond length alternation in both electronic states. The vertical excited energies for the dipole-allowed (1, 2, 3)П ← XП transitions as well as the dipole-forbidden 1Φ ← XП transitions have been computed with the complete active space second-order perturbation theory. The calculated transition energies of 1П ← XП for NCN ( n = 1-6) in the gas phase are 2.26, 2.09, 1.91, 1.72, 1.56, and 1.39 eV, respectively, which mutually agree well with the available experimental values of 2.11, 2.07, 1.88, 1.67, 1.49, and 1.34 eV. Moreover, the corresponding absorption wavelengths are predicted to have the significant nonlinear size dependence, which is different from the bands origin in NCN ( n = 1-7). [ABSTRACT FROM AUTHOR]

Abstract: In this work, the odd-numbered linear hydrocarbon cations HC2n+1H+ (n =2–7) have been investigated with the B3LYP, CAM-B3LYP, and RCCSD(T) calculations focusing on the ground-state geometries, as well as with the CASSCF calculation for the structural optimizations of the ground and first excited states. The present studies reveal that these cation radicals possess stable structures with the ground state of X2Πu when n is even or X2Πg when n is odd, featuring some sort of cumulenic character for the middle carbon chains. Consistent with the previous studies of HC n Si+ clusters, the odd-numbered HC n H+ chains are less stable than the even-numbered ones. The vertical excitation energies for the dipole-allowed 12Πg/u ←X2Πu/g transitions of HC2n+1H+ (n =2–7), obtained by the CASPT2/cc-pVTZ level, are 2.59, 2.11, 1.87, 1.65, 1.49, and 1.35eV, respectively, which mutually agree with the available experimental data of 2.48, 2.07, 1.78, 1.57, 1.42, and 1.29eV. Particularly the corresponding absorption wavelengths are predicted to have the remarkably linear size dependence, as experimentally observed. In addition, the higher excited electronic transitions of HC2n+1H+ (n =2–7) are also calculated, indicating that the absorption wavelengths for the 32Πg/u ←X2Πu/g transitions also exhibit similar linear relationship and the largest oscillator strengths make them accessible more easily in the further experiments. [Copyright &y& Elsevier]

The density functional theory (DFT) and the complete active space self-consistent-field (CASSCF) method have been used for full geometry optimization of carbon chains C2nH+ (n = 1–5) in their ground states and selected excited states, respectively. Calculations show that C2nH+ (n = 1–5) have stable linear structures with the ground state of X3Π for C2H+ or X3Σ- for other species. The excited-state properties of C2nH+ have been investigated by the multiconfigurational second-order perturbation theory (CASPT2), and predicted vertical excitation energies show good agreement with the available experimental values. On the basis of our calculations, the unsolved observed bands in previous experiments have been interpreted. CASSCF/CASPT2 calculations also have been used to explore the vertical emission energy of selected low-lying states in C2nH+ (n = 1–5). Present results indicate that the predicted vertical excitation and emission energies of C2nH+ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Abstract: Structures of linear valence isoelectronic carbon chain anions C4O−, C4S− and C4Se− in their ground states have been investigated by density functional theory (DFT-B3LYP) and coupled cluster with single–double substitution (CCSD) approach. Complete-active-space-self-consistent-field (CASSCF) method has been used for geometry optimization of selected low-lying states. The vertical excitation energies from the ground state to selected low-lying excited states have been carried out with complete-active-space-second-order-perturbation-theory (CASPT2). All of them have a similar set of excited states. In comparison with available experimental observations, the predicted excitation energies for the allowed transitions have an accuracy of no more than 0.12eV. Moreover, the calculations here confirm that the previous assignments of the observed electronic absorption bands of C4O− and C4S− are reliable. The excitation energies of 22Π←X2Π and 12Σ−←X2Π transitions in C4Se− are predicted to be 2.09 and 3.90eV, with oscillator strengths 0.1941 and 0.0105, respectively, which may be observed in future experiment. CASPT2 also evaluated the vertical electron detachment energies for C4O−, C4S− and C4Se− to be 3.55, 3.40 and 3.40eV, respectively. [Copyright &y& Elsevier]

Abstract: Extensive theoretical studies on a series of linear phosphaalkynes HC2n+1P and NC2n P have been reported for the first time up to n=10. Density functional theory has been used to explore their equilibrium geometries. It shows a single and triple bond alternation structure for their ground states, in good agreement with the literature values for simple chains. Using time-dependent density functional theory, the vertical excitation energies for the X1Σ+→11Σ+ transition in HC2n+1P and NC2n P have been investigated. The first adiabatic and vertical ionization energies have been obtained by B3LYP method. On the basis of present calculations, the explicit nonlinear expressions for the size dependence of the electronic absorption wavelength (λ) and the first ionization energy (IE) have been suggested, from which we can obtain the values of arbitrary n and the limits for enough large n. Compared HC2n+1P with NC2n P and PC2n P, λ of HC2n+1P is longer than that of NC2n P, but shorter than that of PC2n P; while IE of HC2n+1P is lower than that of NC2n P, but higher than that of PC2n P. According to our study, the former may be caused by the electronic delocalization effect, and the latter may be caused by electronegativity. [Copyright &y& Elsevier]

Abstract: The B3LYP, CAM-B3LYP, and RCCSD(T) theories have been used to calculate the ground state equilibrium geometries of the linear cationic chains NC2 n +1N+ (n =1–6). Compared with the system NC2 n N+, the odd-n cationic chains are more susceptible to fragmentation than the even-n cationic chains. The complete active space self-consistent-field method has been utilized to determine the stationary structure of the ground state (X2Пg/u) and first excited state (12Пu/g). The complete active space second-order perturbation theory has been used to compute the vertical excitation energies for the dipole-allowed (1,2,3)2Пu/g ←X2Пg/u transitions as well as the dipole-forbidden 12Φu/g ←X2Пg/u transitions. The calculated transition energies of 12Пu/g ←X2Пg/u in the gas phase are 2.61, 2.37, 2.07, 1.88, 1.64, and 1.34eV, respectively, which accord well with the available experimental values. Moreover, the absorption spectra of 22Пu/g ←X2Пg/u may be detected more easily among the selected four transitions. [Copyright &y& Elsevier]

Highlights: [•] The ground state of the two groups compounds were optimizated based on 6-31G (d) basis set of B3LYP method. [•] UV absorption spectra are simulated through TD-DFT method with PCM model. [•] Explained the red shift behavior in the electronic spectra of products. [•] The reaction products of two sets of experiments are confirmed reasonably with theoretical calculations. [Copyright &y& Elsevier]

Abstract: The DFT-B3LYP method has been used for the ground-state (S0) geometry optimization of two novel compounds, 2-dicyanovinyldithieno[2,3-b:3′,2′-d]thiophene (DCST) and 2-dicyanovinyldithieno[3,2-b:2′,3′-d]thiophene (DCTT). The CASSCF approach has also been used to optimize the S0 and S1 electronic states of their more stable isomers, DCST 1 and DCTT 2. The absorption spectra of DCST and DCTT have been investigated at the TD-B3LYP/cc-pVTZ level, compared with the experimental results in the hexane solution. The studies indicate that the two notable absorption bands of DCST 1 occur at 387.83 and 265.04nm, respectively, which agree very well with the experimental data of 404 and 270nm, respectively; the two major absorption peaks of DCTT 2 appear at 403.42 and 257.84nm, respectively, also in good agreement with the observed values of 427 and 256nm, respectively. In addition, the emission wavelengths of DCST 1 and DCTT 2, under the S1 geometries, are 454.17 and 470.79nm, respectively, showing the remarkable red-shift in comparison with the corresponding absorption wavelengths. [Copyright &y& Elsevier]

Abstract: Geometries and stabilities of the linear aluminum-bearing carbon chains AlC2 n H (n =1–5) in their ground states have been explored by the DFT-B3LYP and RCCSD(T) methods. Structures of the X1Σ+ and 11Π electronic states have also been optimized by the CASSCF approach. The studies indicate that these species have single–triple bond alternate pattern, Alh name="tbnd" />Ch name="tbnd" />C⋯Ch name="sbnd" />H, and the electronic excitation from X1Σ+ to 11Π leads to the shortening of the AlThe vertical excitation energies of the 11Π←X1Σ+ and 21Π←X1Σ+ transitions for AlC2 n H (n =1–5) have been investigated by the CASPT2, EOM-CCSD, and TD-B3LYP levels of theory with the cc-pVTZ basis set, respectively. CASPT2-predicted 11Π←X1Σ+ transition energies are 3.57, 3.44, 3.33, 3.26, and 3.21 eV, respectively. For AlC2H, our estimate agrees very well with the experimental value of 3.57eV. In addition, the Alssociation energies and the exponential-decay curves for these vertical excitation energies are also discussed. [Copyright &y& Elsevier]

Abstract: Density functional theory (DFT) has been used to determine the equilibrium geometries, IR, total energies and NMR chemical shifts of various kinds of lactides. According to the calculations, besides l-lactide and d-lactide, three kinds of meso-lactides have been obtained: meso-i-lactide, meso-α-lactide and meso-β-lactide. The latter two are enantiomers. It indicates that meso-i-lactide with a symmetrical center is a transition state connecting meso-α-lactide and meso-β-lactide, with activation energy only about 6.77kJ/mol. It is assumed that meso-lactide prepared by previous experiments may be a racemate of meso-α-lactide and meso-β-lactide. The energy of l(or d)-lactide is predicted to be about 4.83kJ/mol lower than meso-α(or β)-lactide. Time-dependent DFT calculations show that the strongest absorption peaks of l(or d)-lactide and meso-α(or β)-lactide are at about 155 and 158nm, respectively. [Copyright &y& Elsevier]