Recasting wave functions into valence bond structures: A simple projection method to describe excited states

A method is proposed to obtain coefficients and weights of valence bond (VB) determinants from multi configurational wave functions. This reading of the wave functions can apply to ground states as well as excited states. The method is based on projection operators. Both energetic and overlap‐based criteria are used to assess the quality of the resulting VB wave function. The approach gives a simple access to a VB rewriting for low‐lying states, and it is applied to the allyl cation, to the allyl radical and to the ethene (notably to the V‐state). For these states, large overlap between VB and multi reference wave functions are easily obtained. The approach proves to be useful to propose an interpretation of the nature of the V‐state of ethene. © 2015 Wiley Periodicals, Inc.     

Identity SN2 reactions X- + CH3X --> XCH3 + X- (X=F, Cl, Br, and I) in vacuum and in aqueous solution: a valence bond study

The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004, 108, 6017) valence bond method coupled with a polarized continuum model (VBPCM) has been applied to the identity SN2 reaction of halides in the gas phase and in aqueous solution. The barriers computed at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimentally derived barriers in solution (W. J. Albery, M. M. Kreevoy, Adv. Phys. Org. Chem. 1978, 16, 85). The reactivity parameters needed to apply the valence bond state correlation diagram (VBSCD) method (S. Shaik, J. Am. Chem. Soc. 1984, 106, 1227), were also determined by VB calculations. It has been shown that the reactivity parameters along with their semiempirical derivations provide a satisfactory qualitative and quantitative account of the barriers.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

Factors determining bond angles from a classical valence bond perspective. Covalent structure of H2O

We identify the energy contributions that govern the interorbital and internuclear angles in the classical covalent structure of H₂O. The central atom valence state term plays a primary role in H₂O and other AH₂ molecules as well. Lone pair interactions of three different types are also of major significance.On leave 1977–78 at the University of California, Santa Barbara     

The valence and Rydberg excited states of CH2: A theoretical exploration

Using the completed active space second‐order perturbation (CASPT2) method, valence and Rydberg excited states of CH₂ molecule are probed with the large atomic natural orbital (ANO‐L) basis set. Five states are optimized and the geometric parameters are in good agreement with the available data in literatures, furthermore, the state of 2¹B₁ is obtained for the first time. Valence and Rydberg excited states of CH₂ are also calculated for the vertical transitions with the ANO‐L+ basis set that is constructed by adding a set of 1s1p1d Rydberg orbitals into the ANO‐L basis set. Two Rydberg states of the p?³A₂ and r?³B₁ at 9.88 and 10.50 eV are obtained for the first time, and the 3a₁ → 3d_yz nature of the state p?³A₂ and the 3a₁ → d_x2?y2 nature of the state r?³B₁ are confirmed. © 2012 Wiley Periodicals, Inc.     

Potential functions of inversion of R2CO (R=H, F, Cl) molecules in the lowest excited electronic states

The inversion potentials of R₂CO (R=H, F, Cl) molecules in the lowest excited electronic states were determined from experimental data using various model potential functions and approximations for the kinetic energy operator of inversion motion. The estimates of the heights of the barriers to inversion and the equilibrium values of the inversion coordinate for the H₂CO molecule in the S₁ and T₁ states are fairly stable. The results for the F₂CO and Cl₂CO molecules are strongly dependent on the approximation used; for these molecules, the most reliable parameters of the potential functions were chosen. The problem of qualitative characteristics of the shape of inversion potentials is discussed using the results ofab initio quantum-chemical calculations of the molecules under study. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 645–651, April, 1999.     

The structure of CX2YNO (X, Y=F, Cl) molecules in the ground and lowest excited singlet electronic states

Ab initio quantum-chemical calculations of equilibrium geometric parameters, vibrational frequencies, and potentials of internal rotation for CCIF₂NO and CCl₂FNO molecules in the ground (S₀) and lowest excited singlet (S₁) electronic states were performed. The results of calculations were compared with experimental data. A new interpretation of experimental spectra of the CCIF₂NO molecule was suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1453–1458, August, 1999.     

Theoretical studies of proton-transfer reactions of 2-hydroxypyridine--(H2O)n (n = 0-2) in the ground and excited states

The potential energy profiles for proton-transfer reactions of 2-hydroxypyridine and its complexes with water were determined by MP2, CASSCF and MR-CI calculations with the 6-31G** basis set. The tautomerization reaction between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) does not take place at room temperature because of a barrier of approximately 35 kcal/mol for the ground-state pathway. The water-catalyzed enol-keto tautomerization reactions in the ground state proceed easily through the concerted proton transfer, especially for the two-water complex. The S1 tautomerization between the 2HP and 2PY monomers has a barrier of 18.4 kcal/mol, which is reduced to 5.6 kcal/mol for the one-water complex and 6.4 kcal/mol for the two-water complex. The results reported here predict that the photoinduced tautomerization reaction between the enol and keto forms involves a cyclic transition state having one or two water molecules as a bridge.     

乙醇-水分子团簇C2H5OH(H2O)n (n=1-9)稳定结构的量子化学研究

采用密度泛函理论B3LYP方法, 在B3LYP/6-311++G(2d,2p)//B3LYP/6-311++G(d,p)基组水平上对乙醇-水分子团簇(C₂H₅OH(H₂O)_n (n=1-9))的各种性质进行研究, 如: 优化的几何构型、结构参数、氢键、结合能、平均氢键强度、自然键轨道(NBO)电荷分布、团簇的生长规律等. 结果表明, 从二维(2-D)环状结构到三维(3-D)笼状结构的过渡出现在n=5的乙醇-水分子团簇中. 此外, 利用团簇结合能的二阶差分、形成能、能隙等性质, 发现在n=6时乙醇-水分子团簇的最低能量结构稳定性较好, 可能为幻数结构. 最后, 为了进一步探讨氢键本质, 将C₂H₅OH(H₂O)_n (n=2-9)最低能量结构的各种性质与纯水分子团簇(H₂O)_n (n=3-10)比较, 结果表明前者与后者中的水分子之间氢键相似.     

Theoretical study of structures of the X<Subscript>2</Subscript>CO and XYCO molecules (X and Y = H,F, or Cl) in the ground and lowest excited triplet electronic states

Systematic calculations of the structures of the H₂CO, F₂CO, Cl₂CO, HClCO, HFCO, and FClCO molecules in the S₀ and T₁ states were performed using the B3LYP and MP2 methods with different AO basis sets and also at the CCSD(T)/cc-pV(T+d)Z level of theory. The saturation of the correlation consistent sequence of basis sets cc-pV(N+d)Z (N = D, T, Q, and 5) and aug-cc-pV(N+d)Z (N = D, T, and Q) was studied. Recommendations for choosing the calculation method are given. The relativistic corrections were estimated. The influence of the number and type of halogen atoms on the geometric parameters of the molecules in the S₀ and T₁ states and the heights of inversion barriers in the T₁ state was investigated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2625–2635, December, 2005.     

Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X,Y = H,F, Cl) in the ground and lowest excited electronic states,part 1: A test of ab initio methods

Geometrical parameters of tetraatomic carbonyl molecules X₂CO and XYCO (X, Y = H, F, Cl) in the ground (S₀) and lowest excited singlet (S₁) and triplet (T₁) electronic states as well as values of barriers to inversion in S₁ and T₁ states and S₁ ← S₀ and T₁ ← S₀ adiabatic transition energies were systematically investigated by means of various quantum‐chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM‐CCSD, CCSD(T), CR‐EOM‐CCSD(T), CASSCF, MR‐MP2, CASPT2, CASPT3, NEVPT2, MR‐CISD, and MR‐AQCC within cc‐pVTZ and cc‐pVQZ basis sets. The accuracy of quantum‐chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR‐EOM‐CCSD(T), and MR‐AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar in S₁ and T₁ electronic states with CO out‐of‐plane angle ranging from 34° (H₂CO, S₁) to 52° (F₂CO, T₁), and height of barrier to inversion varying from 300 (H₂CO, S₁) to 11,000 (F₂CO, T₁) cm^?1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical study of the desorption of neutral and ionic alkali metal atoms from the excited Li+(H2O)n = 1-4 and Na+(H2O)n = 1-4 cluster models: Electronic excitation charge transfer

In this work, the potential energy curves of several low-lying excited states of M⁺(H₂O)_{n = 1-4} (M = Li and Na) clusters with one M─O bond, related to the stretching of their M─O bond, were calculated in the gas phase. The time-dependent density functional theory and direct-symmetry-adapted cluster-configuration interaction were used in this study separately. Theoretical calculations showed that the charge transfer occurred between M⁺ and (H₂O)_n in the excited clusters so that the neutral metal atom was obtained at the dissociation limit of the potential curves. The excited potential curves of clusters were also calculated in the presence of the electrostatic field of water (EFW), and it was found that the charge transfer was blocked in the presence of EFW. The effect of the size of the (H₂O)_n cluster on the shape of the excited potential curves was investigated to observe how the M─O bond was affected in the excited states depending on the (H₂O)_n size. It was found that the increase in the size of the (H₂O)_n cluster increased the number of bonding excited potential curves. The difference between the electron density of the excited and ground electronic states was calculated to see how the charge transfer was affected by the size of the (H₂O)_n cluster.     

A theoretical study on the low-lying excited states of 2,2':5',2'-terthiophene and 2,2':5',2':5',2'-quaterthiophene.

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2'-terthiophene (terthiophene) and 2,2':5',2':5',2'-quaterthiophene (tetrathiophene) are discussed on the basis of high-level ab initio computations. The spectroscopic features determined experimentally for short alpha-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S1 state of terthiophene, the S1 and T2 states become degenerate, which leads to a favourable situation for the occurrence of ISC. The parallel process is expected to be less favoured in tetrathiophene because of the less efficient spin-orbit coupling and the increase of the S1-T2 energy gap.     

A challenge to chemical intuition: donor-acceptor interactions in H3B-L and H2B+-L (L=CO; EC5H5, E=N-Bi)

The equilibrium geometries and bond energies of the complexes H(3)B-L and H(2)B(+)-L (L=CO; EC(5)H(5): E=N, P, As, Sb, Bi) have been calculated at the BP86/TZ2P level of theory. The nature of the donor-acceptor bonds was investigated by energy decomposition analysis (EDA). The bond strengths of H(3)B-L have the order CO>N>P>As>Sb>Bi. The calculated values are between D(e)=37.1 kcal mol(-1) for H(3)B-CO and D(e)=6.9 kcal mol(-1) for H(3)B-BiC(5)H(5). The bond dissociation energies of the cations H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) are larger than for H(3)B--L, particularly for complexes of the heterobenzene ligands. The calculated values are between D(e)=51.9 kcal mol(-1) for H(2)B(+)-CO and D(e)=122.1 kcal mol(-1) for H(2)B(+)-NC(5)H(5). The trend of the BDE of H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) is N>P>As>Sb>Bi>CO. A surprising result is found for H(2)B(+)-CO, which has a significantly stronger and yet substantially longer bond than H(3)B-CO. The reason for the longer but stronger bond in H(2)B(+)-CO compared with that in H(3)B-CO comes mainly from the change in electrostatic attraction and pi bonding at shorter distances, which increases more in the neutral system than in the cation, and to a lesser extent from the deformation energy of the fragments. The H(2)B(+)<--NC(5)H(5) pi( perpendicular) donation plays an important role for the stronger interactions at shorter distances compared with those in H(3)B-NC(5)H(5). The attractive interaction in H(2)B(+)--CO further increases at bond lengths that are shorter than the equilibrium value, but this is compensated by the energy which is necessary to deform BH(2) (+) from its linear equilibrium geometry to the bent form in the complex. The EDA shows that the contributions of the orbital interactions to the donor-acceptor bonds are always larger than the classical electrostatic contributions, but the latter term plays an important role for the trend in bond strength. The largest contributions to the orbital interactions come from the sigma orbitals. The EDA calculations suggest that heterobenzene ligands may become moderately strong pi donors in complexes with strong Lewis acids, while CO is only a weak pi donor. The much stronger interaction energies in H(2)B(+)-EC(5)H(5) compared with those in H(3)B-EC(5)H(5) are caused by the significantly larger contribution of the pi(perpendicular) orbitals in H(2)B(+)-EC(5)H(5) and by the increase of the binding interactions of the sigma+pi( parallel) orbitals.     

Theoretical studies on structures and electronic spectra of linear carbon chains C2nH+ (n = 1−5)

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 C_2nH⁺ (n = 1–5) in their ground states and selected excited states, respectively. Calculations show that C_2nH⁺ (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 C_2nH⁺ 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 C_2nH⁺ (n = 1–5). Present results indicate that the predicted vertical excitation and emission energies of C_2nH⁺ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Structure and stability of Al13H(n) (n=1-13) clusters: exceptional stability of Al13H13

We have performed density functional calculations for the structure and stability of Al(13)H(n) (n=1-13) clusters. Population analysis has shown significant charge transfer occurring from the Al cluster to the H atoms. The population for Al(13) varies from 0.24 (Al(13)H) to 2.83 (Al(13)H(13)). The shape of Al(13) moieties in the Al(13)H(n) (n>/=8) clusters is significantly distorted from the icosahedral structure of Al(13) and is a "cagelike" form. Al(13)H(13) has a capped icosahedron as the ground-state structure, similar to B(13)H(13), while the shape of B(13) (planar) is different from Al(13) (icosahedral). The Al(13)H(13) cluster is predicted to be exceptionally stable on the basis of the high stabilization energy and the negative nucleus independent chemical shift value.     

Collision-induced dissociation studies of protonated ether--(H2O)n (n = 1-3) clusters

We have studied the protonated ether-(H2O)n (n = 1-3) complexes containing tetrahydrofuran, dimethyl, diethyl, dibutyl, and butylmethyl ethers using a flowing afterglow triple-quadrupole mass spectrometer. Collision-induced dissociation, CID, of all clusters with n = 1, 2 shows sequential water loss. The n = 3 cluster of dimethyl ether shows sequential water loss, while all other ether clusters display selective product formation. The CID spectra are interpreted based on known energetics, and theoretical studies of the dimethyl and diethyl ether systems.